Diabetes mellitus (DM) is characterized by hyperglycemia resulting from defects in insulin secretion and/or action. The chronic hyperglycemia of DM can lead to long-term dysfunction, damage or failure of various tissues and organs such as the eyes, kidneys, heart, vascular tissue, and nerves. The DM is classified into two types, based on the primary mode of onset and pathobiology of the disease process. The younger onset insulin dependent diabetes mellitus (IDDM) is called type 1 diabetes, and the older onset insulin non-dependent diabetes mellitus (NIDDM) is called type 2 diabetes. In type 1 diabetes mellitus, the development of DM is due to autoimmune destruction of pancreatic β cells with consequent insulin deficiency. These patients are dependent on insulin and must be administered from external sources. The patients with type 2 diabetes mellitus are not insulin deficient but show peripheral insulin resistance and consequent hyperinsulinemia. But, over a period of time, these patients also become insulin deficient due to exhaustion of pancreatic β cells and hence, may eventually become insulin dependent. Recently, it was reported that some patients may have both type 1 and type 2 DM and are termed mixed DM or type 3 DM. These patients have evidences of both type 1 and type 2 DM. Due to presence of peripheral insulin resistance in those with type 1 DM, their requirement of insulin would be high.
The basis of abnormalities in carbohydrate, protein, and lipid metabolism seen in diabetes is ascribed to the deficiencies in the action(s) of insulin on target tissues. Deficient insulin action results from inadequate insulin secretion and/or diminished tissue responses to insulin at one or more points in the complex pathways of insulin action. In majority of the patients, impairment of insulin secretion and defects in insulin action frequently coexist.
Common Clinical Manifestations
In majority of subjects with type 2 DM, no symptoms could be present at the time of detection of the disease. Many a times, type 2 DM is detected during a routine general check-up or when the subject is evaluated for yet another illness. Thus, the type 2 DM could be asymptomatic for long period of time. In an occasional instance the type 2 DM is detected due to presence of a complication secondary to long standing diabetes; yet the subject could be unaware of the presence of diabetes. Hence a high degree of suspicion is necessary on the part of the physician to detect and diagnose type 2 diabetes. Contrarily, type 1 DM, in general, shows a more dramatic presentation such as diabetic ketoacidosis.
The symptoms of marked hyperglycemia include polyuria, polydipsia, polyphagia, weight loss, and blurred vision. In children with type 1 DM impairment of growth is common. Susceptibility to certain infections may also accompany hyperglycemia. Acute life-threatening complications of uncontrolled diabetes include hyperglycemia with ketoacidosis or the nonketotic hyperosmolar syndrome. Long-term complications of diabetes include retinopathy with potential loss of vision, nephropathy leading to renal failure, peripheral neuropathy with risk of foot ulcers, amputations and Charcot joints, autonomic neuropathy causing gastrointestinal, genitourinary, and cardiovascular symptoms and sexual dysfunction. The incidence of atherosclerosis is common in diabetes. Thus, atherosclerotic cardiovascular, peripheral arterial, and cerebrovascular diseases are common in these patients. Subjects with diabetes also have a high incidence of hypertension and abnormalities of lipoprotein metabolism.
The degree of hyperglycemia may vary depending on the underlying disease process. The severity of the metabolic abnormality can progress, regress, or remain unchanged. The degree of hyperglycemia reflects the severity of the underlying metabolic process and factors that can modulate it. Hence, the treatment of diabetes should take into consideration not only the underlying pathobiology but also various factors that have the potential to modify it.
Assigning a type of diabetes to an individual depends to a large extent on the circumstances present at the time of diagnosis. It should be understood that many diabetics do not necessarily fit into a single class. For instance, a person with gestational diabetes mellitus (GDM) may continue to have hyperglycemia even after delivery and may be determined to have, in fact, type 2 DM. In contrast, a person who received large doses of corticosteroids may be mistakenly diagnosed to have type 2 DM; yet such an individual may become normoglycemic once the corticosteroids are withdrawn. Another example would be a person who was given thiazides and develops diabetes many years later. Thiazides by themselves would not cause significant hyperglycemia; such individuals probably had type 2 diabetes that is exacerbated by thiazides. Thus, for both the patient and physician and for all practical purposes, it is less important to label a particular type of diabetes than it is to understand the significance and implications of hyperglycemia and to treat it as effectively as possible.
Type 1 Diabetes Mellitus
The vast majority of cases of diabetes fall into two broad etiopathogenetic categories: type 1 diabetes and type 2 diabetes. Type 1 diabetes accounts for only 5–10% of those with diabetes; in this category of diabetes there is an absolute deficiency of insulin secretion due to immune-mediated destruction of pancreatic β cells. These individuals are at increased risk of developing diabetic ketoacidosis if the hyperglycemia is not detected early and treated appropriately. Subjects who are at high risk of type 1 diabetes can often be identified by serological evidence of an autoimmune pathologic process occurring in the pancreatic β cells and by genetic markers. Markers of the immune destruction of the β cells include the following: islet cell autoantibodies, autoantibodies to insulin, glutamic acid decarboxylase (GAD65), and tyrosine phosphatases 1A-2 and 1A-2β. In majority of the subjects (in about 85–90%) usually more than one of these autoantibodies is present at initial detection of fasting hyperglycemia. A strong association is known to exist between some HLA markers such as HLADQA and DQB genes. These HLA-DR/DQ alleles can be either predisposing or protective.
It is important to note that the rate of β-cell destruction is variable. In infants and children, β-cell destruction could be rapid whereas in adults it could be slow. In children and adolescents, the first manifestation could indeed be ketoacidosis, while in others modest hyperglycemia present in the beginning rapidly progresses to severe hyperglycemia and/or ketoacidosis. In these individuals and others, ketoacidosis is precipitated in the presence of a stressful event such as infections. On the other hand, in few subjects some residual β cell function may be present that prevents precipitation of ketoacidosis. But over a period of time these subjects become dependent on insulin for survival and are at equal risk to develop ketoacidosis. At this point of time these subjects may have little or no insulin secretion (detected by little or no detectable levels of C-peptide in their plasma). Although type 1 diabetes is more common in children and adolescents, it should be understood that it could occur at any age. Patients with type 1 diabetes may also have other autoimmune diseases such as Graves’ disease, Hashimoto's disease, Addison's disease, autoimmune hepatitis, myasthenia gravis, pernicious anemia, vitiligo, and celiac sprue.
Pathobiology of Type 1 Diabetes
The exact cause(s) and mechanism(s) of destruction of pancreatic β cells is not clear. Several studies suggested that pro-inflammatory cytokines such as interleukin-1 (IL-1), IL-2, IL-6, tumor necrosis factor-α (TNF-α) and macrophage migration inhibitory factor (MIF), nitric oxide (NO), superoxide anion and other free radicals play an important role in this disease process. Damage to pancreatic β cells due to the release of TNF-α and IL-1 produced by infiltrating macrophages, lymphocytes and monocytes leads to development of type 1 DM.1, 2 In the model of multiple lowdose streptozotocin-induced diabetes in rats and mice, it was observed that high levels of IL-2, interferon-γ (IFN-γ), and TNF-α produced by TH1 lymphocytes activate macrophages and promote destruction of β cells both by nitric oxide (NO) and non-NO-mediated mechanisms.3 It was reported that human duct cells, which are in a close topographic relationship with the β cells, are a source of TNF-α that has been implicated in the development of autoimmune diabetes in mice.4 Another cytokine produced by macrophages known as macrophage migration inhibitory factor (MIF) has also been reported to play a significant role in the development of type 1 diabetes mellitus. MIF-mRNA expression in splenic lymphocytes was upregulated during the development of cell-mediated diabetes (type 1 diabetes mellitus) in non-NOD (non-obese diabetic) mice.5 Further, treatment of NOD mice with recombinant MIF-protein at 25 μg twice a week, from age 6 to 11 weeks, led to an increased diabetes incidence compared with untreated control groups at week 34 suggesting a role of MIF in autoimmune-inflammatory type-1 diabetes.5 TNF-α6, 7 could also stimulate MIF production; TNF-α and MIF may act in concert with each other to produce damage to pancreatic β cells and induce type 1 diabetes.
MIF, TNF-α, and ILs modulate the production of various prostaglandins (PGs). For instance, prostacyclin synthase (PGI2S) mRNA expression was suppressed by high concentrations of TNF-α and MIF, while COX-2 mRNA was induced by MIF, TNF-α and IL-1β in pulmonary artery smooth muscle cells. TNF-α paradoxically decreased the PGI2 production at low concentrations. IL-1β increased PGI2 production in a dose-dependent manner, whereas at low concentrations TNF-α and MIF also increased PGI2 production, but to a far lesser degree. These results imply that there is a close interaction between IL-1, TNF-α, MIF and prostaglandins.8–10
Prostaglandin E2 (PGE2), derived from arachidonic acid (AA, 20:4 ω-6), suppresses TNF-α and IL-1 production and is an immuno-suppressor.11 This could suggest that inhibition of TNF-α, IL-1 and enhancement of the production of PGE2 may limit the insult and in turn might inhibit the development of type 1 DM. This is supported by the observation that peroxisome proliferator-activated receptor-γ (PPAR-γ) activators: conjugated linoleic acid (CLA) and troglitazone,12 prevented development of diabetes in the Zucker diabetic fatty fa/fa rat13, 14 by suppressing the production of free radicals and TNF-α and IL-2.
We have earlier reported that oral supplementation of polyunsaturated fatty acid (PUFA)-rich oils prevent development of alloxan-induced DM in experimental animals.15–18 PUFAs serve as endogenous ligands of PPARs.12
Free radicals induced DNA strand breaks activate poly (ADP-ribose) polymerase (PARP) synthase;19 this enhances NAD+ utilization leading to NAD+ depletion, cessation of NAD+ dependent energy and protein metabolism, events that cause pancreatic β cell death.19,20 This is supported by the observation that nicotinamide supplementation and free radical removal protect against chemical-induced diabetes mellitus. The mechanism by which PUFAs prevented damage to pancreatic β cells could be attributed to its ability to revert the altered concentrations of lipid peroxides, NO, SOD, ceruloplasmin, glutathione peroxidase, glutathione-S-transferase and catalase. NO quenches superoxide anion,21,22 whereas SOD inactivates superoxide anion. Treatment with PUFA restored both SOD and NO concentrations to normal. This could be one mechanism by which PUFAs prevented alloxan-induced damage to β cells.
NO has a wide range of biological functions. Induction of iNOS (inducible nitric oxide synthase that leads to production of supraphysiological amounts of NO) is toxic to pancreatic islet cells. NO produced by β cells23 is an important effector molecule involved in macrophage-induced islet cell lysis.24 NO-induced cell death is due to damage to DNA.25 NO donors induced the activity of ADP-ribose polymerase in islet cell nuclei26 with concomitant depletion of intracellular NAD+. This causes insufficient energy generation in the cell that ultimately leads to cell death. The fact that mice lacking PARP (poly-ADP-ribose polymerase) gene are resistant to development of diabetes induced by streptozotocin lends support to this view.27 Since PUFAs-prevented alloxan-induced destruction of β cells in vivo, it is likely that this could be due to their ability to preserve NAD+ content of β cells by blocking the enzyme PARP. These results are supported by the observation that oral supplementation of cod liver oil, a rich source of ω-3 eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), during pregnancy decreased the incidence of type 1 diabetes.28 Thus, one environmental factor that could contribute to development of type 1 diabetes is decreased intake of PUFAs during pregnancy by the mother and during infancy and/or sub-clinical deficiency of PUFAs during the perinatal period. It remains to be seen whether perinatal supplementation of PUFAs could be a relevant strategy to prevent type 1 diabetes.
Type 2 Diabetes Mellitus
Type 2 diabetes mellitus is more prevalent than type 1 diabetes. It is characterized by a combination of resistance to insulin action and an inadequate compensatory insulin secretory response. This form of diabetes accounts for more than 90% of those with diabetes. This leads to hyperglycemia that is sufficient to cause pathologic and functional changes in various target tissues. These features may be present for a long period before diabetes is detected. During this asymptomatic period, it is possible to demonstrate an abnormality in carbohydrate metabolism by measuring plasma glucose in fasting state or after challenge with an oral glucose load. During this period, the disease process could cause impaired fasting glucose (IFG) and/or impaired glucose tolerance (IGT) without fulfilling the criteria for diagnosis of diabetes. In many instances, in individuals with diabetes, adequate glycemic control can be achieved with weight reduction, exercise, and/or oral glucose lowering medicines. These individuals therefore do not require insulin. In some individuals with type 2 diabetes, some residual insulin secretion could be present but require exogenous insulin for optimal glycemic control, but are able to survive without it. On the other hand, individuals with extensive β cell destruction and therefore have very little endogenous insulin secretion require exogenous insulin for survival. These individuals are like those with type 1 diabetes.
The progress of diabetes is hard to predict. The severity of metabolic abnormality can progress, regress or remain stable and unchanged. Thus, the degree of hyperglycemia reflects the severity of underlying metabolic process/abnorma-lity. The degree of hyperglycemia and the underlying etiopathogenesis also determines response to the treatment employed. In majority of instances, it is quite possible that, at least initially and often throughout their lifetime, these subjects do not require insulin for survival. Hence, these patients should be impressed upon that they should follow the recommended dietary regimen and exercise regularly so that normoglycemia can be achieved and maintained for a long period of their lifetime.
Most patients with type 2 diabetes are obese and obesity by itself is known to cause insulin resistance. In South East Asians (especially in persons of Indian subcontinent), type 2 diabetes may occur who are not obese by traditional weight criteria. But these subjects have an increased percentage of body fat distributed predominantly in the abdominal region (abdominal obesity). It is also known that intra-and inter-myocellular and β cell triglyceride accumulation causes insulin resistance, though the mechanism of insulin resistance is not clear. It is likely that decreased food intake may reduce oxidative stress, plasma TNF-α and IL-6 concentrations, and myocellular triglyceride content that in turn leads to amelioration of type 2 DM.
Patients with type 2 diabetes rarely develop ketoacidosis unless and otherwise an additional factor in the form of stress of another illness such as infection is also present. Since mild to moderate degree of hyperglycemia is asymptomatic, in majority of the instances diabetes goes undiagnosed for several years. In these patients, hyperglycemia develops gradually and at earlier stages is often not severe enough to cause classic symptoms of diabetes (polyuria, polydipsia, polyphagia, and weight loss). Despite the lack of the symptoms these patients are at increased risk of developing macrovascular and microvascular complications. Although they have normal or elevated levels of insulin, the higher blood glucose levels in type 2 diabetes would be expected to result even higher insulin values had their β-cell function been normal. On the other hand, if the peripheral insulin resistance seen in these patients is overcome or decreased they are expected to have much lower insulin levels. Thus, in these patients there are two abnormalities: one of defective insulin secretion (insufficient to compensate for insulin resistance); and the other one of insulin resistance (rendering β cell function inappropriate).
Both weight reduction coupled with appropriate and adequate exercise may improve insulin resistance but is seldom restored to normal. It is known that the incidence of type 2 diabetes increases with age, obesity, lack of physical activity, and increase in the consumption of energy dense food. It is also more frequent in women who had gestational diabetes mellitus (GDM) and those with hypertension, dyslipidemia, and coronary heart disease.
Type 2 diabetes often shows strong genetic predisposition compared to type 1 diabetes but, the genetics of this form of diabetes is more complex and not well defined. It is generally well accepted that type 2 diabetes may occur one decade earlier with each passing generation, if their parents (one or both) have the disease. Thus, eventually, it is expected that type 2 diabetes may be seen in much younger people compared to a decade ago. This may explain why type 2 diabetes is detected, at present, in subjects who are in their second and third decade of life.
The exact cause for type 2 diabetes is not clear. Since, there is no insulin deficiency, at least in the early stage of the disease, it is obvious that pancreatic β cells are not at fault. But this implies that islet cells are not able to secrete enough insulin to overcome peripheral insulin resistance in these patients. In other words, if peripheral insulin resistance is corrected, then in all probability insulin secreted by β cells is adequate to maintain normoglycemia. Recent studies suggested that low-grade systemic inflammation plays a significant role in pathogenesis of type 2 diabetes.29, 30 This is based on the observation that the plasma concentrations of C-reactive protein (CRP), TNF-α, IL-6, and resistin, which are markers of inflammation, are elevated whereas the concentrations of adiponectin that shows anti-inflammatory actions are reduced in type 2 diabetes.31–33
Low-grade Systemic Inflammation and Type 2 Diabetes Mellitus
Elevated plasma concentrations of CRP, TNF-α and IL-6 may produce their harmful effects in type 2 diabetes mellitus, hypertension, and obesity by inducing endothelial dysfunction. TNF-α and IL-6 damage endothelial cells, cause apoptosis of endothelial cells, and trigger procoagulant activity and fibrin deposition.29–34 It was shown that forearm blood flow responses to acetylcholine (ACh) were inversely correlated with CRP serum levels indicative of endothelial dysfunction.35 High CRP concentrations were associated with decreased endothelial nitric oxide (eNO) generation.36 We have demonstrated that eNO levels were low in patients with diabetes mellitus.37 Elevated CRP, IL-6 and TNF-α concentrations in patients with type 2 diabetes may lead to decrease in eNO production and consequently endothelial dysfunction. Since eNO is a potent vasodilator and platelet anti-aggregator, low eNO may, in turn, lead to increase in peripheral vascular resistance and higher incidence of thrombosis and atheroslcerosis.
It is not clear whether inflammation is the primary event or is secondary to the development of type 2 diabetes. For instance, CRP levels do not correlate with the extent of atherosclerosis thus suggesting that CRP levels reflect body's response to inflammation elsewhere. On the other hand, CRP functions as a chemoattractant, increases the expression of adhesion molecules, and activates complement proteins, which are important mediators of inflammation. Furthermore, CRP binds to low density lipoprotein (LDL) cholesterol and increases the uptake of LDL by macrophages. Experimental studies have demonstrated that CRP enhances the size of myocardial infarction.38 These results point to the possible role of inflammation in the pathobiology of type 2 diabetes and diseases associated with it.
Both IL-6 and TNF-α increase neutrophil superoxide anion generation.39,40 Superoxide anion (O2–) inactivates eNO and prostacyclin (PGI2) and thus causes endothelial dysfunction, and enhances thrombosis and atherosclerosis,41,42 which are common in type 2 diabetes. On the other hand, optimal production of eNO inactivates O2– and thus prevents/arrests thrombosis and atherosclerosis.42,43 Thus, an increase in oxidative stress could yet be another factor contributing to development of type 2 diabetes.
Adipose tissue produces several biologically active molecules that have important actions on immune response, and inflammation. Three of these molecules are adiponectin, resistin, and corticosterone. Adiponectin has anti-inflammatory actions and its plasma concentrations are inversely related to insulin resistance and the severity of type 2 diabetes whereas resistin induces insulin resistance and has pro-inflammatory actions.44,45 Transgenic mice over expressing 11 β-hydroxysteroid dehydrogenase types 1 (11β-HSD-1) selectively in adipose tissue is shown to develop abdominal obesity and exhibited insulin-resistant diabetes (type 2 diabetes), hyperlipidemia, and hyperphagia.46 Thus, type 2 diabetes could be termed as “localized Cushing's syndrome”.
Several other studies have also shown that elevated plasma concentrations of CRP and possibly, IL-6 and TNF-α predict the future progression of type 2 diabetes mellitus, and occurrence of hypertension, and coronary heart disease.47–49 Furthermore, reductions in the levels of CRP, IL-6 and TNF-α achieved by diet control, exercise, and statin therapy predicted a better outcome to these patients. One should consider measurement of these inflammatory markers to predict development of type 2 diabetes and its response to various therapies.
Nitric Oxide and Type 2 Diabetes Mellitus
Nitric oxide (NO) was originally discovered as a factor released from endothelial cells that caused vasodilatation and hence was called as endothelium-derived relaxing factor.50 NO is a soluble gas that is produced not only by endothelial cells, but a variety of cells such as macrophages and neurons in the brain. It is now evident that many cells (if not all) produce NO and that it also participates in inflammation. NO acts in a paracrine manner on target cells through induction of cyclic guanosine monophosphate (cGMP) that, in turn, initiates a series of intracellular events leading to the desired response such as relaxation of vascular smooth muscle cells, neurotransmission, tumoricidal, cytotoxic, and bactericidal actions. The half-life of NO is only few seconds and hence, it has to be produced in close proximity to where it is needed.
Nitric oxide is synthesized from L-arginine by the action of nitric oxide synthase (NOS) enzyme.51 There are three different types of NOS- the endothelial (eNOS), the neuronal (nNOS), and the inducible (iNOS) nitric oxide synthase. NOS exhibit two patterns of expression: eNOS and nNOS are constitutively expressed at low levels and can be activated rapidly by an increase in cytoplasmic calcium ions. Influx of calcium into cells leads to a rapid production of nitric oxide. In contrast, iNOS is induced in macrophages and other cells when activated by cytokines such as TNF-α and IFN-γ. It is paradoxical to know that eNO and nNO have many beneficial properties whereas iNO shows pro-inflammatory actions.
Nitric oxide plays an important role in the vascular and cellular components of inflammatory responses. NO is a potent vasodilator and prevents platelet aggregation. NO inhibits vascular smooth muscle cell proliferation. NO reduces platelet adhesion and inhibits several features of mast cell-induced inflammation, and serves as an endogenous regulator of leukocyte recruitment. Inhibition of endogenous NO production promotes leukocyte rolling and adhesion in postcapillary venules. On the other hand, delivery of exogenous NO reduces leukocyte recruitment. Thus, under normal physiological conditions NO is an inhibitor of inflammatory response and possibly, increased production of NO in inflammatory conditions could be a compensatory mechanism to block inflammatory responses.52 But, it should be understood that increased production of NO seen in response to various inflammatory stimuli might itself perpetuate inflammation. This is so since in these situations NO may get converted to peroxynitrite radical that has potent pro-inflammatory actions (Fig. 1.1). Decreased production of eNO occurs in insulin resistance, obesity, atherosclerosis, diabetes, and hypertension.53–56 NO and its derivatives have microbicidal actions and thus, NO functions as an endogenous mediator of host defense against infections.57
Although NO is unstable, its concentrations in the plasma and various cells in vitro could be measured using various colorimetric techniques and specific NO probes. NO is measured as its stable metabolites, nitrite and nitrate in the plasma, give an indication to the concentrations of NO released by endothelial cells. Highly sensitive NO probes are commercially available to measure intracellular concentrations of NO and NO released by cells in in vitro cultures.
Fig. 1.1: Scheme showing generation of ROS and NO and formation of RNI (reactive nitrogen intermediates)
Reactive Oxygen Species (ROS) and Interactions with NO vis-a-vis Type 2 Diabetes
ROS or oxygen-derived free radicals are released by leukocytes, macrophages and other similar cells present in various organs into the extracellular compartment on exposure to various noxious agents such as microbes, foreign objects, and in response to chemokines, ingestion of immune complexes, or following a phagocytic challenge.58 The production of ROS is due to activation of the NADPH oxidative system. Known ROS species are mainly: superoxide anion (O2–), hydrogen peroxide (H2O2), and hydroxyl radical (OH). ROS are produced mainly within the cell, and are capable of reacting with NO to form reactive nitrogen intermediates that are cytotoxic to various organelles of cells.59 Since ROS and reactive nitrogen intermediates are highly toxic, their release into the extracellular space even in low concentrations may be harmful. Furthermore, even at very low concentrations they are capable of increasing the expression of chemokines (e.g., IL-8), cytokines, and endothelial leukocyte adhesion molecules, events that are capable of amplifying the inflammatory cascade.60 The physiological function of both ROS and reactive nitrogen species is to destroy bacteria, viruses, fungi, and cancer cells. At the other end of the spectrum, increased production of ROS and reactive nitrogen intermediates are potentially harmful and could cause acute and chronic inflammation. Thus, ROS and reactive nitrogen intermediates (RNI) can cause endothelial cell damage that results in increased vascular permeability, insulin resistance, and thrombosis. In this context, it is important to note that activated adherent neutrophils not only produce ROS and RNI but also stimulate xanthine oxidase in endothelial cells that, in turn elaborates further generation of superoxide anion. ROS and RNI inactivate antiproteases such as α1-antitrypsin. This in turn leads to unopposed protease activity, which could result in increased destruction of extracellular matrix. ROS by themselves damage many cells and tissues. Excess production of ROS in endothelial cells (or close to endothelial cells) produce damage to these cells that results in endothelial dysfunction typically seen in type 2 diabetes mellitus. Increased generation of ROS is seen in type 2 diabetes mellitus though the reasons and mechanisms of increased generation of ROS are unclear. Once these mechanisms are identified, it could be possible to develop reasonable therapeutic approaches to prevent or even treat these conditions. It is equally important to know the time the increase in ROS generation is triggered so that appropriate timing of preventive or therapeutic measures can be implemented.
Production of appropriate amounts of eNO is possible only when endothelial cells are healthy. Hence, plasma concentrations or endothelial production of NO can be used as a marker of endothelial cell integrity and health. In type 2 diabetes mellitus, plasma concentrations of eNO are low indicating endothelial dysfunction. NO levels revert to normal following weight loss. Thus, measurement of plasma eNO could be used as a marker not only for endothelial function but also to judge adequacy of treatment given to patients.
Insulin Resistance and Type 2 DM are Common in Indians: Why and How?
Although genetics could play an important role in the pathobiology of type 2 DM, it is not clear as to how the so-called genetic factors interact with environmental and dietary factors and if so, how such an interaction(s) reflects on the onset of the disease. Furthermore, the incidence of type 2 DM and insulin resistance is more common in Indians compared to Western population but it is unclear why it should occur frequently in Indians.
Indians have a higher incidence of abdominal obesity, high prevalence of type 2 diabetes, hypertension, low concentrations of high-density lipoprotein (HDL) cholesterol, hypertriglyceridemia, hypercholesterolemia, and lead a sedentary lifestyle. Insulin resistance is common in all these conditions. Hyperinsulinemia may be a consequence of this. It should be noted that insulin resistance may not always be present in these conditions and even when present it may not occur in all tissues of the body at the same time. The distribution of body fat in Indians differs from the Caucasians. Indians have higher body fat or abdominal obesity even at normal range of body mass index (BMI). This abdominal obesity or increased visceral fat in Indians is a marker of the presence of insulin resistance and hyperinsulinemia, which are risk factors for presence or development of type 2 diabetes. It is not clear, however, why and how the Indians develop abdominal obesity.
In this context, it is interesting to note that mice over expressing 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD-1) enzyme selectively in adipose tissue develop abdominal obesity and exhibit insulin-resistant diabetes, hyperlipidemia, and hyperphagia despite hyperleptinemia.61, 62 These features are similar to those seen in Indians with obesity and type 2 DM. We had suggested earlier that the activity of 11β-HSD-1 in the abdominal adipose tissue of Indians could be higher compared to the Caucasians. It is also likely that the activity of 11β-HSD-1 (and so the levels of its product corticosterone) is high in the abdominal adipose tissue compared to that seen in the subcutaneous adipose tissue63 which may explain abdominal obesity, insulin resistance and type 2 DM in Indians. It will be important to know the factors that regulate the activity of 11β-HSD-1 in adipose tissue since an understanding may lead to develop strategies to prevent or treat abdominal obesity effectively.
Indians have higher CRP concentrations than do the Caucasians.64,65 It is likely that Indians also have higher levels of TNF-α and IL-666, 67 (because IL-6 induces CRP production, whereas TNF-α stimulates IL-6 synthesis). Elevated plasma TNF-α levels have been associated with obesity and insulin resistance. It is positively correlated with hypertriglyceridemia, glucose intolerance, and hyperleptinemia, and negatively correlated with HDL cholesterol.68–70 This indicates that plasma levels of TNF-α and in turn those of CRP and IL-6 are closely related to the biochemical parameters that are known risk factors for development of type 2 diabetes mellitus. HDL stimulates endothelial nitric oxide (eNO) synthesis71 and NO in turn inhibits LDL oxidation.72 Both oxidized LDL and reduced levels of eNO enhance the risk of atheroslcerosis and thrombosis. This may explain the high incidence of coronary heart disease in Indians. This is supported by our study wherein we demonstrated high plasma lipid peroxides and low NO concentrations in Indians with type 2 diabetes.73 TNF-α and IL-6 augment whereas insulin-like growth factor-I (IGF-I) and insulin suppress the activity of 11β-HSD-1.74 Insulin and IGFs suppress the synthesis of TNF-α and IL-6, enhance the production of eNO, and show anti-inflammatory actions.75–79
Insulin has anti-inflammatory actions.75,77 Insulin suppresses the production of TNF-α, IL-6, IL-1, IL-2, and macrophage migration inhibitory factor (MIF), which are pro-inflammatory molecules. Insulin enhances the production of IL-4 and IL-10 that are anti-inflammatory cytokines. Thus, the presence and purpose of hyperinsulinemia in normal Indians is probably to prevent or abrogate the low-grade systemic inflammation, evidenced by elevated levels of CRP, and possibly, TNF-α and IL-6. On the other hand, leptin has pro-inflammatory actions.67 Since hyperinsulinemia and hyperleptinemia are evident in Indian children compared with their Caucasian counterpart,80,81 the features of low-grade systemic inflammation and type 2 DM are initiated very early in life.
The distribution of body fat in Indians is different. Indians have abdominal obesity even at normal range of BMI. Earlier we proposed that the activity of 11β-HSD-1 could be high in the adipose tissue of Indians secondary to enhanced plasma and tissue levels of CRP, IL-6, and TNF-α compared to the Caucasian population.82 11β-HSD-1 activities in adipose tissue are regulated by insulin, IGFs, TNF-α, and IL-6. The final expression of 11β-HSD-1 in adipose tissue, especially in the abdominal fat, depends on the balance between pro-inflammatory cytokines TNF-α and IL-6 and insulin and IGFs that have anti-inflammatory action.82 When this balance tilts more towards TNF-α and IL-6, the activity of 11β-HSD-1 is increased and in turn contributes to abdominal obesity. In other words, the presence of abdominal obesity can be considered an indication of the presence of elevated plasma/tissue levels of CRP, TNF-α, and IL-6, insulin resistance and hyperinsulinemia and decreased levels/activity of IGFs and insulin, and increased expression and activity of 11β-HSD-1 in abdominal adipose tissue. Elevated TNF-α and IL-6 are associated with low plasma HDL and elevated LDL levels, hypertriglyceridemia, hyperleptinemia, and glucose intolerance; these abnormalities are common in subjects with abdominal obesity. These events ultimately lead to low-grade systemic inflammation and development of type 2 DM in Indians.
If this is true, what is the initial event/stimulus that triggers the development of metabolic syndrome X? As insulin resistance is seen even in healthy Indian children80 is it possible that the trigger for the development of metabolic syndrome X is present since childhood or even much earlier?
Is type 2 DM has its origins in the perinatal period?
Type 2 diabetes may have its origin early in life. Low birth weight has been associated with high prevalence of type 2 DM in later life.83,84 Indian babies are small. Type 2 DM and metabolic syndrome X were 10 times greater in those who were 2.95 kg or less at birth compared to those with birth weight more than 4.31 kg. This has been disputed, and instead been suggested that much of what was claimed to be fetal in origin might, in fact, relate to postnatal nutrition and growth.85 Early nutrition could have a bearing on the development of type 2 DM in later life. If it is true that type 2 DM has its origins in fetal life, improved obstetric care, general increase in the standard of living, and better nutrition during pregnancy are expected to decrease its incidence. Contrary to this, the incidence of obesity and type 2 DM has increased.86,87 The babies born to second-generation Asian women (babies born in the United Kingdom with mean birth weight of 3196 gm) are heavier than those born to first generation Asian women (babies born in the Indian subcontinent but residing in the UK; mean birth weight 2946 gm).88 The mean birth weight for babies of second-generation women was 249 gm more than the mean birth weight of babies of first generation women (P<0.001). It is possible that a similar trend towards an increase in the birth weight of babies born after 1970s is seen even in India. Thus, low birth weight could no longer account for the rapidly increasing incidence of type 2 DM in Indians. If under nutrition is no longer responsible for the increasing incidence of type 2 DM, does over nutrition has a role in its pathobiology in Indians?
ω-3 and ω-6 PUFAs in type 2 DM and Insulin Resistance
ω-3 and ω-6 fatty acids are essential for fetal growth and development.89–91 Dietary linoleic acid (LA) and α-linolenic acid (ALA) are essential fatty acids (EFAs). They are desaturated and elongated to form their respective long-chain metabolites.92–94 (see Fig. 1.2 for metabolism of EFAs). Newborn infants, especially pre-term infants, have limited capacity to form EPA, DHA and AA. The AA status correlated with one or more measures of normalized growth through 12 months in infants. Dietary AA improves first year growth of pre-term infants95 possibly by stimulating glucose uptake by cells.90 EPA and DHA increase birth weight by prolonging gestation and/or by increasing the fetal growth rate.96,97 Some of the actions of LCPUFAs that are relevant to the present discussion are the following.
- EPA and DHA inhibit TNF-α and IL-6 production that accounts for their anti-inflammatory actions.
- EPA, DHA and AA enhance eNO generation.
- EPA, DHA and AA inhibit 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity and thus, regulate cholesterol metabolism, indicating that LCPUFAs function as endogenous statins.98
- LCPUFAs are endogenous ligands for PPAR-α and PPAR-γ and thus they have actions similar to thiazolidinediones. PPARs have anti-inflammatory actions (they suppress TNF-α and IL-6 production and inhibit free radical generation), and enhance the production of adiponectin. By functioning as ligands of PPARs, LCPUFAs augment adiponectin production and prevent/arrest atheroslcerosis.
- EPA, DHA, and AA ameliorate insulin resistance. EPA and DHA ameliorated insulin resistance and hypertension in experimental animals. EPA reduced insulin resistance and decreased the incidence of type 2 DM in experimental animals.Fig. 1.2: Metabolism of essential fatty acids. Prostaglandins of 3 series are less pro-inflammatory compared to prostaglandins of 2 series. Resolvins are formed from both EPA and DHA and are known to have anti-inflammatory actions and participate in the resolution of inflammation. EPA can be converted to DHA. DHA can be retroconverted to EPA. It is estimated that about 30–40% of DHA can be retroconverted to EPA. The biochemical and/or clinical significance of this retroconversion of DHA to EPA are not known
- EPA and DHA suppress leptin gene expression.103 Leptin has pro-inflammatory actions. EPA/DHA functions as endogenous anti-inflammatory molecule by inhibiting leptin production.
- Normal Indians have significantly lower concentrations of AA, EPA, and DHA compared to healthy Canadians and Americans.104
- Incorporation of LCPUFAs into cell membranes increases the number of insulin receptors on the membrane and their affinity to insulin by altering membrane fluidity and thus, decreases insulin resistance. In contrast, saturated fatty acids increase insulin resistance.105
These facts suggest that type 2 DM occurs in Indians due to perinatal deficiency of EPA, DHA and AA. For instance, maternal protein restriction or increased consumption of saturated and/or trans-fatty acids (increasing with the rapid Westernization of dietary habits in the Indian subcontinent) and energy rich diet during pregnancy decreases the activity of enzymes Δ6 and Δ5 desaturases that are essential for conversion of dietary EFAs LA and ALA to their respective LCPUFAs. This leads to both maternal and fetal deficiency of EPA, DHA and AA. Perinatal protein depletion leads to almost complete absence of measurable activities of Δ6 and Δ5 desaturases in fetal liver and placenta.106
EPA, DHA, and AA have a negative feedback control on TNF-α and IL-6 synthesis. Hence, EPA, DHA, and AA deficiency increases the generation of TNF-α and IL-6 that in turn induce insulin resistance. Thus, maternal and fetal sub-clinical deficiency of EPA, DHA, and AA increase the levels of TNF-α and IL-6 in the fetus. This is supported by the observation that that prenatal exposure to TNF-α produces obesity,107 and obese children and adults have high levels of TNF-α and IL-6.108,109 Furthermore, low plasma and tissue concentrations of EPA, DHA, and AA decrease production of adiponectin that in turn aggravates insulin resistance and enhances the chances of development of type 2 DM. Increased concentrations of TNF-α and IL-6 enhance the activity of 11β-HSD-1 that leads to an increase in production of corticosterone in the adipose tissue. This causes accumulation of adipose tissue in the abdomen, resulting in characteristic abdominal obesity seen in Indians and induces insulin resistance and type 2 DM.
Diagnostic Criteria for Diabetes Mellitus
The criteria for the diagnosis of diabetes are shown in Table 1.1. Three ways to diagnose diabetes are possible, and each, in the absence of unequivocal hyperglycemia, must be confirmed, on a subsequent day, by any one of the three methods given in Table 1.1.
Impaired Glucose Tolerance (IGT) and Impaired Fasting Glucose (IFG)
Studies revealed that there is an intermediate group of subjects, who are neither normal nor diabetic. They do not meet the criteria for diabetes though, nevertheless have blood glucose levels too high to be considered normal.
This group is defined as having fasting plasma glucose (FPG) levels between ≥ 100 mg/dl and ≤ 126 mg/dl or 2-hour values in the oral glucose tolerance test between ≥ 140 mg/dl and < 200 mg/dl. The categories of FPG and OGTT are shown in Table 1.2.
Subjects with IFG and IGT are considered to have pre-diabetes suggesting that they are at high risk for development of diabetes. They need close follow-up and should be advised appropriate diet modification and exercise, and if obese, additionally reduce weight. Both IFG and IGT may be associated with other known features of metabolic syndrome X such as abdominal obesity, dyslipidemia of the high-triglyceride and/or low HDL type, and hypertension.
Gestational Diabetes Mellitus (GDM)
GDM is defined as any degree of glucose intolerance with onset or first recognition during pregnancy.110 GDM may persist even after pregnancy and could respond to diet modification and exercise; insulin treatment may not be essential for all patients. It is likely that GDM may have antedated or begun concomitantly with pregnancy. Approximately 4% of all pregnancies are complicated by GDM (a total of 540,000 cases could occur annually in India). It should be noted that deterioration of glucose tolerance occurs normally during pregnancy, particularly in the third trimester.
Diagnostic Criteria for GDM
Although it is ideal to test/screen all pregnant women for the presence of GDM, this may not be practical and cost effective. The low and high risk of GDM in pregnancy is depicted in Table 1.3.
Those who need to be assessed for presence of GDM should be screened during the first prenatal visit. They should undergo glucose testing as soon as possible. If they are found not to have GDM at the initial screening, they should be retested between 24–28 weeks of gestation. A fasting plasma glucose level >126 mg/dl or a random and/or casual plasma glucose >200 mg/dl meets the threshold for the diagnosis of diabetes. In absence of unequivocal hyperglycemia, the diagnosis must be confirmed on a subsequent day. Oral glucose tolerance test generally confirms presence of GDM.
Criteria for the diagnosis of GDM based on OGTT are given in Table 1.4. The diagnosis and treatment of GDM is important since GDM is associated with higher incidence of fetal malformations and the mother is at high risk of obstetric complications.
Management of Diabetes
It is not the purpose of this article to detail all the drugs that are available for the treatment of diabetes and their side effects/ complications. We provide here only general guidelines with more emphasis on some of the recent developments that offer new hope in management of diabetes.
Management of diabetes mellitus may include lifestyle modifications such as achieving and maintaining proper weight, diet, exercise and foot care. The most important is the hypoglycemic treatment with either oral hypoglycemics and/or insulin therapy. Nowadays, the goal for diabetics is to avoid or minimize chronic diabetic complications, as well as to avoid acute problems of hyper or hypoglycemia.
Adequate control of diabetes leads to a lower risk of the complications of uncontrolled diabetes which include kidney failure (requiring dialysis or transplant), blindness, heart disease and limb amputation. Recent studies show that statins might be needed in primary and secondary prevention of cardiovascular complications and mortality.
Diet and exercise delay the onset of type 2 diabetes in persons at risk. Weight loss, restricted diets, and exercise have all been advocated for the treatment of type 2 diabetes.
Several studies have compared diet alone and diet plus antiglycemic drugs (chlorpropamide, glyburide, insulin, metformin). All studies found all drugs good at lowering glucose and were better than diet alone.
Modes of Action of Different Oral Hypoglycemic Agents
Sulfonylureas increase insulin secretion and potentiate insulin action on the liver and peripheral tissues. Metformin decreases hepatic glucose production, increases glucose uptake and possibly decreases appetite. Alpha glucosidase inhibitors slow the absorption of carbohydrates. Troglitazone decreases insulin resistance and activate peroxisome proliferator–activated receptor (PPAR) gamma, a nuclear transcription factor that is important in fat cell differentiation and fatty acid metabolism.
Table 1.5 lists the recommended daily dose of various oral hypoglycemic agents, and their advantages and disadvantages.
Patients with type 1 diabetes are dependent on insulin for survival. On the other hand, those with type 2 diabetes can be managed with diet, exercise and oral hypoglycemic agents and in not infrequently need insulin. Patients with GDM are best managed with diet and insulin though recent studies suggest that these patients can also be treated with oral hypoglycemic agents without any significant complications. It is essential that the plasma glucose levels are maintained near normal values in all these patients particularly true for patients with GDM and type 1 diabetes. Since type 1 diabetes frequently occurs in children and adolescents, it is important to ensure that their dietary regimen meets their growth demands without compromising adequate diabetes control. These patients should also be advised adequate exercise (essential for the control of hyperglycemia) and maintain normal physical activity expected of their age. In other words, all these patients should try maintain as normal life as possible despite the presence of diabetes. Wherever it is necessary, insulin is recommended to control hyperglycemia to all these types of patients. Although, it is convenient to give oral hypoglycemic agents for patients with type 2 diabetes, it is now clear that there are some distinct advantages when insulin is used for control of hyperglycemia. One of the long-term strategies is to see that these patients do not develop target organ damage or postpone such damage so that a good quality of life is maintained for the rest of their life. All diabetics should have their lipid profile, cardiac, renal and eye conditions evaluated at least once in a year if not more frequently to ensure that the damage to these target organs is recognized early and managed appropriately.
Treatment of Diabetes under Special Circumstances
Intercurrent medical illness: Patients with intercurrent illness become more insulin resistant because of the effects of increased counter-regulatory hormones. Therefore, despite decreased nutritional intake, glycemia may worsen. Patients on oral agents may need transient therapy with insulin to achieve adequate glycemic control. In patients requiring insulin, scheduled doses of insulin, as opposed to sliding scale insulin, are far more effective in achieving glycemic control.
Recent work suggests improved outcomes in patients with type 2 diabetes with acute myocardial infarctions or strokes who receive constant intravenous insulin during the acute phase of the event to maintain blood glucose values of approximately 100–150 mg/dL. In case of cardiac ischemia, the beneficial effects may be due to reducing free fatty acids with insulin therapy. In patients treated with metformin, any illness leading to dehydration or hypoperfusion should lead to temporary discontinuation of the drug because of possible increased risk of lactic acidosis.
Surgery: Surgical patients may experience worsening of glycemia due to stress and release of counter regulatory hormones similar to those for intercurrent medical illness. Patients on oral agents may need transient therapy with insulin to maintain blood glucose at approximately 100–180 mg/dL. In patients requiring insulin, scheduled doses of insulin, as opposed to sliding scale insulin, are far more effective in controlling glucose. Intensive regulation of glucose (i.e. maintaining glucose approximately <110 mg/dL) in surgical ICU patients on ventilators appears to improve survival and reduce complications.
Patients who can eat soon after surgery: The time-honored approach of administering one half of the usual morning neutral protamine (NPH) insulin dose with 5% dextrose IV is acceptable, with resumption of scheduled insulin (perhaps at reduced doses) within the first 1–2 days. Patients receiving insulin need not receive their usual dose unless and otherwise they are given intravenous glucose during surgery. During and after surgery their plasma glucose levels should be monitored and accordingly should be given appropriate dose of insulin. Recent studies suggest that patients undergoing simple day care surgeries such as cataract can be asked to continue their oral hypoglycemic agents but need careful monitoring of their blood glucose levels.
Patients without oral nutrition for a long period for major surgery (coronary artery bypass grafting and major abdominal surgery): Constant infusion intravenous insulin is preferred. Discontinue metformin temporarily after any major surgery until the patient is hemodynamically stable and normal renal function is documented. Plasma glucose should be monitored 3 to 4 times a day and soluble insulin should be administered depending on the plasma glucose levels. Once these patients start taking oral feeds, every effort should be made to get back to their original schedule of insulin depending on the food intake.
Pregnancy: Insulin is the only acceptable pharmacologic therapy during pregnancy for women with established diabetes mellitus. (Glyburide has been used for gestational diabetes mellitus patients late in the second and third trimesters, but this is not appropriate therapy for patients with established diabetes. Its safety during early gestation is not established). The insulin regimen should result in a smooth glucose profile throughout the day, with no hypoglycemic reactions between meals or at night. Initiate the regimen early enough before pregnancy so that the glycosylated hemoglobin level is maintained within the normal range. In patients with GDM, the goal of dietary therapy is to avoid single large meals and foods with a large percentage of simple carbohydrates. A total of 6 feedings per day is preferred, with 3 major meals and 3 snacks to limit the amount of energy intake at any interval. Examples include foods with complex carbohydrates and cellulose, such as whole grain breads and legumes. Carbohydrates should account for no more than 50% of the diet, with protein and fats equally accounting for the remainder. A 30–33% energy restriction (to ≤105 kJ/kg/d [25 kcal/kg/d] actual weight) has been shown to reduce hyperglycemia and plasma triglyceride levels with no increase in ketonuria in obese (BMI >30 kg/m2) women.
Individualizing the frequency and timing of home glucose monitoring is important. A typical schedule involves capillary glucose checks upon awakening in the morning, 1 hour after breakfast, before and after lunch, before dinner, and at bedtime. Emphasis should be on gaining and sustaining compliance with the target glucose levels around normal range. Meticulous glycemic control requires attention to both preprandial and postprandial glucose levels. The goal of insulin therapy during pregnancy is to achieve glucose profiles similar to those of nondiabetic pregnant women. Given that healthy pregnant women maintain their postprandial blood sugar excursions within a relatively narrow range (70–120 mg/dl), the task of reproducing this profile requires meticulous daily attention by both the patient and physician.
As pregnancy progresses, there is increasing fetal demand for glucose; with concurrent and progressive lowering of fasting and between-meal blood sugar levels the risk of symptomatic hypoglycemia increases. Upward adjustment of short-acting insulin to control postprandial glucose surges within the target band only exacerbates the tendency to interprandial hypoglycemia. Thus, any insulin regimen for pregnant women requires combinations and timing of insulin injections quite different from those that are effective in the non-pregnant state. Further, the regimens must be continually modified as the patient progresses from the first to the third trimester and insulin resistance rises. Hence, one should be aware of the rising need for insulin, and increase insulin dosages preemptively. Recent studies showed that the second-generation oral sulfonylurea, glyburide, minimally transports across the human placenta due to its high plasma protein binding capacity coupled with a short half-life. In a randomized trial comparing glyburide with insulin showed no difference in the mean maternal blood glucose level, the percentage of infants who were large for gestational age, the birth weight, or neonatal complications between the groups. Only 4% of the glyburide study arm required addition of insulin to achieve glucose control. But, glyburide should not be used in the first trimester because its effects on the embryo, if any, are not known.
Recent Advances that may have Implications in Management of Diabetes
VMH (Ventromedial Hypothalamus) Neurons and Type 2 Diabetes
Recent studies suggest that specific areas of hypothalamus may have a significant role in control and maintenance of plasma glucose and regulate insulin secretion by β cells of the pancreas. This indicates that stimuli or insults induced during growth of brain occurring during the perinatal period do play a major role in pathogenesis of diabetes. Hormonal signals or nutritional factors acting during the perinatal period may have lifetime consequences and serve as programming stimuli. For instance, short and thin at birth, continued slow growth in early childhood, followed by acceleration of growth (height and weight approach the population mean) are considered the most unfavorable growth pattern. This can result in development of insulin resistance, obesity and type 2 DM in later life.111 Perinatal nutrition is an important determinant of adult diseases. In experimental animals ventromedial hypothalamic (VMH) lesion induces hyperphagia and excessive weight gain, fasting hyperglycemia, hyperinsuli-nemia, hypertriglyceridemia, and impaired glucose tolerance. Intraventricular administration of antibodies to neuropeptide Y (NPY) abolished hyperphagia in these animals. Streptozotocin-induced diabetic animals showed increase in NPY concentrations in paraventricular, VMH and lateral hypothalamic areas. VMH-lesioned rats showed selectively decreased concentrations of norepinephrine and dopamine in the hypothalamus; whereas long-term infusion of norepinephrine and serotonin into the VMH impaired pancreatic islet cell function. These changes in the hypothalamic neurotransmitters reverted to normal after insulin therapy. This study suggests that dysfunction of VMH impairs pancreatic β-cell function and induces metabolic abnormalities that are seen in type 2 DM. TNF-a decreases the firing rate of the VMH neurons and is neurotoxic, whereas PUFAs are neuroprotective. Since PUFAs decrease TNF-α production, concentration of TNF-α increases whenever local concentrations of PUFAs are low. This may ultimately produce neuronal damage.
The brain is rich in PUFAs, especially AA, EPA, and DHA, which constitute as much as 30 to 50% of total fatty acids in the brain. Hence, if the concentrations of PUFAs are inadequate during the critical period of brain growth from the third trimester of pregnancy to two-year post-term, TNF-α concentration tend to be high. This increase in TNF-α may cause damage to VMH neurons and lead to development of type 2 DM in adult life. Thus, TNF-α may participate in pathogenesis of type 2 DM by two mechanisms: (1) inducing peripheral insulin resistance and (2) damaging or interfering with the action of VMH neurons.
Insulin and Insulin Receptors in the Brain and Type 2 DM
Insulin signaling has a role in the regulation of food intake, neuronal growth, and differentiation by regulating neuro-transmitter release and synaptic plasticity in the central nervous system (CNS). Neuron-specific disruption of the insulin-receptor gene (NIRKO) in mice induces obesity, insulin resistance, hyperinsulinemia, and type 2 diabetes without interfering with brain development.111 This indicates that a decrease in the number of insulin receptors, defects in the function of insulin receptors, and insulin lack or resistance in the brain leads to development of type 2 DM even when pancreatic β-cells are normal. Intraventricular injection of insulin inhibits food intake and the site of insulin action is on the hypothalamic NPY network. Insulin enhances the formation of PUFAs, whereas PUFAs augment the action of insulin and the number of insulin receptors. Further, both insulin and PUFAs augment the formation of eNO, a potent neuro-transmitter that somehow seem to take the message (probably via RBCs that are known to carry NO) from VMH neurons to the pancreatic β-cells and thus control their insulin secretion. This suggests that maintaining adequate amounts of insulin and insulin receptors in the brain controls the appetite, obesity (BMI), and helps maintain normoglycemia.
These results imply that factors that regulate insulin action in the brain could be important in control of type 2 DM; this is especially so since hypothalamus is rich in insulin receptors. This is supported by the observation that drugs that specifically bind to insulin receptors in the brain decreases appetite, reduces obesity and plasma glucose levels. In another study, it was reported that infusion of oleic acid in the third ventricle resulted in marked decline in the plasma insulin concentration and a modest decrease in the plasma glucose concentration.112 These changes were detected within one hour of oleic acid infusion. Oleic acid (one of the PUFAs) did not alter glucose uptake but suppressed the rate of glucose production. It was also reported that oleic acid enhances hepatic insulin action via the activation of KATP channels in the hypothalamus. Oleic acid also decreased the hypothalamic expression of NPY suggesting that PUFAs directly control food intake via their action on hypothalamic centers.
Although, insulin is the drug of choice for treatment of both type 1 and type 2 DM, it is inconvenient to use injectable insulin daily. Recent studies showed that it is possible to deliver insulin via buccal mucosa and orally by preparing insulin complexes with other compounds such that insulin is not degraded in the stomach and intestines prior to its absorption. Phase I and II studies demonstrated that insulin administered via buccal mucosa has significant biological action comparable to the subcutaneously administered insulin. Oral insulin preparations also showed similar potency. These developments, if confirmed could usher in a new era in management of diabetes.113
Other Miscellaneous Advances
Advances in pancreatic β-cell transplantation could prove to be of significant benefit to those with type 1 DM and in type 2 DM who require insulin for control of their hyperglycemia. But, a clinical application of this technique awaits advances in transplantation immunology.
A recent study demonstrated that a combination therapy with epidermal growth factor (EGF) and gastrin increases β-cell mass and reverses hyperglycemia in non-obese diabetic mice.114
Variations in the eNOS, adiponectin, IL-1, IL-2, IL-6, and TNF-α genes (especially in their SNPs, single nucleotide polymorphisms) could be responsible for the increased or decreased susceptibility to diabetes in various populations, and sub-groups of people.115 More studies are needed to develop these indices as reliable markers for the identification of individuals who are at high-risk of diabetes and their complications. It is also not clear how the advances made in the area of PUFAs and insulin action could be used in the clinic to predict and prevent diabetes. Development of perinatal strategies to prevent diabetes may prove exciting but awaits further development. Advances in microarray, proteomics, and metabolomics are expected to have a profound impact on our understanding of several diseases and help in development of newer methods of predicting and preventing diabetes.
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