Pancreatic insulin secretion rates can be accurately derived by mathematical deconvolution of peripheral C-peptide concentrations either by using individual C-peptide kinetic parameters obtained by analysis of the decay curve of biosynthetic human C-peptide or by using published group parameters with appropriate adjustments for age and degree of obesity. Since the cross-reactivity of proinsulin and related peptides is low (< 10%) in many C-peptide assays, this experimental approach avoids the spurious increase in insulin immunoreactivity resulting from cross-reactivity with proinsulin and related peptides in the insulin assay. Application of this technique has demonstrated that the phenotypic expression of beta-cell dysfunction differs in subjects with different genetic mechanisms of non-insulin-dependent diabetes mellitus (NIDDM). Subjects who have maturity-onset diabetes of the young (MODY) due to mutations in the glucokinase gene demonstrate different patterns of altered insulin secretion when compared with subjects who have mutations in the MODY1 gene on chromosome 20. Glucokinase mutations affect the ability of the beta-cell to detect and respond to small increases in glucose above the basal level. However, compensatory mechanisms operative in vivo, which include a priming effect of glucose on insulin secretion, limit the severity of the observed insulin secretory defect, resulting in a generally mild clinical course in these subjects. In contrast, mutations in the MODY1 gene are associated with an inability to increase insulin secretion as the plasma glucose concentration increases above 7-8 mmol/l and the normal priming effect of glucose on insulin secretion is lost. These characteristics of the dose-response relationships between glucose and insulin secretion result in a more severe degree of hyperglycemia than observed in subjects with glucokinase mutations, and these subjects more frequently need insulin treatment. These alterations are evident in prediabetic subjects with normal glucose levels who carry the MODY1 mutation, suggesting that defective beta-cell function is the primary pathogenetic defect in the diabetic syndrome in these subjects. Studies performed in the classic form of NIDDM demonstrate that subjects with mild glucose intolerance and normal fasting glucose concentrations and glycosylated hemoglobin levels consistently demonstrate defective beta-cell function. These results are consistent with studies in the Zucker diabetic fatty rat, an animal model of NIDDM in which prediabetic animals demonstrate extensive alterations in expression of multiple genes involved in the regulation of insulin secretion. It thus appears that abnormal beta-cell function is present at a relatively early stage in the evolution of NIDDM, even before the onset of overt hyperglycemia.

Several mechanisms are implicated in the pathogenesis of diabetic retinopathy. They include biochemical, hemodynamic, and hormonal factors, all of which have an important role in the development of diabetic retinopathy. These factors are not independent of each other, but rather they interact and together are responsible for the well-known lesions of vascular occlusion, microaneurysms, hemorrhages' hard exudates, and eventually new vessel formation.

Many risk factors for non-insulin-dependent diabetes mellitus (NIDDM), such as obesity, physical inactivity, and high-fat diet, can potentially be modified. Furthermore, some of the metabolic abnormalities, such as insulin resistance and impaired glucose tolerance, that predict diabetes can be improved by behavior modification and drug treatment. Thus, at least to some extent, NIDDM may be preventable. Several small clinical trials have addressed the hypothesis that NIDDM can be prevented by dietary modification, physical activity, or drug treatment. Some studies suggest a preventive effect, but the conclusions are limited by considerations of sample size, randomization, or intensity of the interventions. Consequently, the hypothesis that NIDDM is preventable requires further testing.

Unlike classical microvascular complications, large-vessel atherosclerosis can precede the development of diabetes, suggesting that rather than atherosclerosis being a complication of diabetes, both conditions have common genetic and environmental antecedents, i.e., they spring from a "common soil." It is now known that adverse environmental conditions, perhaps related to less-than-optimal nutrition, in fetal and early life are associated with an enhanced risk of both diabetes and cardiovascular disease many decades later. These same adverse environmental conditions are also associated with the development in adult life of abdominal obesity and the insulin-resistance syndrome (IRS). The IRS consists of glucose intolerance, hyperinsulinemia, dyslipidemia (high triglyceride and low high-density lipoprotein [HDL] cholesterol levels), and hypertension. Although the mechanism underlying this cluster is controversial, the statistical association is well established. All of the elements of the IRS have been documented as risk factors for type II diabetes. Some, but not all, of these elements are also cardiovascular disease risk factors, in particular, hypertension and low HDL cholesterol. Other factors associated with the IRS that may enhance cardiovascular disease risk are plasminogen activator inhibitor 1 and small, dense low-density lipoprotein particles. Whether insulin itself is a risk factor remains controversial, but recent epidemiological evidence has been mostly negative. This question has marked clinical relevance because if the IRS enhances cardiovascular disease risk by virtue of its concomitant factors and not the hyperinsulinemia per se, this would tend to alleviate concerns that intensive insulin management of type II diabetic subjects could enhance the risk of large-vessel atherosclerosis. Clinical trials are urgently needed to settle this point.

Type II diabetes remains a genetic nightmare. The major problem is identifying suitable pedigrees, sib-pairs, and populations for study. Segregation analysis data suggest that type II diabetes is likely to be polygenic, although one or more major genes could also be involved. This and the high prevalence of diabetes affect the strategies for searching for genetic mutations. Linkage analysis in classical type II diabetes pedigrees is unlikely to be successful. In addition, affected sib-pair analysis is limited because both parents are often affected, leading to bilineal inheritance. Sib-pairs with both parents alive are unusual, so identity by descent analysis is rarely feasible. Strategies to reduce bilineal inheritance by identifying sib-pairs with one known nondiabetic parent or with the second sibling having mild subclinical diabetes may be worthwhile. Identification of individuals or pedigrees with an unusual phenotype that suggests a single gene disorder, such as maturity-onset diabetes of the young, will continue to be important, for this allows linkage analysis with markers near candidate genes and exclusion mapping of chromosomal regions using highly polymorphic markers. Population association studies with candidate genes can detect mutations that have a minor role in the majority proportion of diabetic subjects, but large numbers are required and great care must be taken to exclude ethnic group differences between the diabetic and normoglycemic populations. The study of small inbred communities might be helpful because they may have fewer diabetogenic genes than outbred populations, and this would increase the power of sib-pair and population association studies. Direct screening for mutations in candidate genes (with single-strand conformation polymorphism or heteroduplex screening or with direct sequencing) in patients with the appropriate pathophysiological abnormality can be a successful strategy. The identification of well-defined diabetic pedigrees, sib-pairs, and suitable matched diabetic and nondiabetic populations will be key to the discovery of the genes for diabetes.

The pathogenesis of non-insulin-dependent diabetes mellitus (NIDDM) involves complex interactions between multiple physiological defects, both genetic and acquired. The application of transgenic technology to create animal models that address questions concerning NIDDM (and obesity) is a very recent development that is now gaining rapid momentum and receiving deserved attention. In general, transgenic methods afford new opportunities to alter the site or level of expression of functional genes in vivo, to transfer novel foreign genes into animals, to prevent the expression of specific genes, or to replace genes with specific genetic variants. Two general approaches can be applied: 1) conventional transgenics, the transfer to and expression of new genetic information in animals; and 2) gene targeting, the disruption or replacement of specific endogenous genes. Recent transgenic initiatives have provided important insights into 1) the mechanism of glucose-stimulated insulin secretion and the role of potential defects in this system, 2) the regulated expression of genes that control hepatic glucose production, 3) the role of specific molecules that mediate the actions of insulin, and 4) the elucidation of factors that contribute to in vivo regulation of energy balance and body composition. Emerging transgenic strategies should have a dramatic impact on future efforts to assess the function of newly identified molecules implicated in the regulation of in vivo glucose homeostasis and to determine the roles of candidate loci or specific mutations uncovered during the search for new NIDDM susceptibility genes.

Iatrogenic hypoglycemia is the limiting factor in the management of insulin-dependent diabetes mellitus (IDDM). It causes recurrent physical morbidity, some mortality, and recurrent or even persistent psychosocial morbidity. The principles of glucose counterregulation, the physiological mechanisms that normally very effectively prevent or correct hypoglycemia, are now known. Decrements in insulin, increments in glucagon, and, in the absence of the latter, increments in epinephrine stand high in the hierarchy of redundant glucose counterregulatory factors. Iatrogenic hypoglycemia in IDDM is the result of the interplay of absolute or relative therapeutic insulin excess and compromised glucose counterregulation. Syndromes of compromised glucose counterregulation include defective glucose counterregulation (the result of combined deficiencies of the glucagon and epinephrine responses to falling glucose levels), hypoglycemia unawareness (loss of the warning, neurogenic symptoms of developing hypoglycemia), and elevated glycemic thresholds (lower glucose levels required) for autonomic activation and symptoms during effective intensive therapy. These have been conceptualized as examples of hypoglycemia-associated autonomic failure, a functional disorder distinct from classical diabetic autonomic neuropathy, in IDDM. Recent antecedent iatrogenic hypoglycemia appears to be a major factor in the pathogenesis of hypoglycemia unawareness; there is increasing evidence that this syndrome is reversible with scrupulous avoidance of hypoglycemia. It probably also contributes substantially to the syndrome of elevated glycemic thresholds during intensive therapy. However, factors in addition to recent antecedent hypoglycemia play an important role in the pathogenesis of the syndrome of defective glucose counterregulation. Pending the prevention and cure of IDDM, we need to learn to replace insulin in a much more physiological fashion and/or to prevent, correct, or compensate for compromised glucose counterregulation if we are to eliminate hypoglycemia from the lives of people with IDDM without compromising glycemic control. In the meantime, we must continue to seek better insight into the fundamental mechanisms of compromised glucose counterregulation and to develop practical preventive clinical strategies and practice hypoglycemia risk factor reduction with our patients.

Recent data have suggested a key role for tumor necrosis factor (TNF)-alpha in the insulin resistance of obesity and non-insulin-dependent diabetes mellitus (NIDDM). TNF-alpha expression is elevated in the adipose tissue of multiple experimental models of obesity. Neutralization of TNF-alpha in one of these models improves insulin sensitivity by increasing the activity of the insulin receptor tyrosine kinase, specifically in muscle and fat tissues. On a cellular level, TNF-alpha is a potent inhibitor of the insulin-stimulated tyrosine phosphorylations on the beta-chain of the insulin receptor and insulin receptor substrate-1, suggesting a defect at or near the tyrosine kinase activity of the insulin receptor. Given the clear link between obesity, insulin resistance, and diabetes, these results strongly suggest that TNF-alpha may play a crucial role in the systemic insulin resistance of NIDDM. This may allow for new treatments of disorders involving resistance to insulin.

Our perspective is that the concepts of glucose toxicity and glucose desensitization should be differentiated because they carry very different connotations. The term glucose desensitization most properly refers to a pharmacological event involving a temporary, readily induced, physiological and reversible state of cellular refractoriness because of repeated or prolonged exposure to high concentrations of glucose. The term glucose toxicity should be reserved for nonphysiological, irreversible alterations in cellular function caused by chronic exposure to high glucose concentrations. With regard to the pancreatic islet beta-cell, the mechanism of action for glucose desensitization seems most likely to be expressed at the level of the insulin exocytotic apparatus or insulin stores within the beta-cell, whereas the mechanism of action for glucose toxicity may be at the level of insulin gene transcription. This differentiation raises the possibility that exposure of patients to chronic hyperglycemia may cause glucose toxic effects on the process of insulin gene transcription and/or expression that are irreversible. If so, this may contribute to so-called secondary drug failure and, in any event, reemphasizes the need to intensify therapeutic efforts to better regulate glycemia in type II diabetes.

Enhanced metabolism of glucose via the polyol pathway may play an important role in the pathogenesis of diabetic retinopathy, neuropathy, and nephropathy. Aldose reductase catalyzes the NADPH-dependent conversion of glucose to sorbitol, the first step in the polyol pathway. Interruption of the polyol pathway by inhibition of aldose reductase holds considerable promise as a therapeutic measure to prevent or delay the onset and severity of these late complications of diabetes. Dramatic advances in our understanding of the molecular biology, enzymology, and three-dimensional structure of aldose reductase have occurred in recent years, providing new and challenging insights into the enzyme's catalytic mechanism. Recent developments in structure determination of aldose reductase and the implications for evaluation and development of aldose reductase inhibitors are summarized.

Insulin-dependent diabetes mellitus (IDDM) is caused by destruction of the insulin-secreting beta-cells of the islets of Langerhans. It is proposed that the disease is caused by nongenetic, probably environmental, factors operating in a genetically susceptible host to initiate a destructive immune process. These environmental factors may operate over a limited period in early childhood to induce the immune process that destroys the islet beta-cell. Thereafter, there is a long prodrome before the onset of clinical diabetes, during which clinical, immune, and metabolic changes can be detected. If these proposals are correct, epidemiological studies should focus on environmental events in early childhood that might induce, or accelerate, the disease process. Moreover, it should be possible to identify, from an early age, changes in the prediabetic period that persist to diagnosis and have predictive value. The variable age of presentation of IDDM may, therefore, reflect different rates of disease progression rather than different ages of exposure to the critical environmental events. Those patients in whom the disease process is slow could present with IDDM as adults, develop diabetes that does not require insulin treatment, or even fail to develop diabetes altogether. These proposed features, if confirmed, would have important implications for the prediction of IDDM and raise the possibility that modulation of the destructive immune process could prevent progression to clinical diabetes.

The autoimmune response that leads to destruction of pancreatic islet beta-cells and insulin-dependent diabetes mellitus (IDDM) has a genetic basis; however, environmental factors can exert profound modulating effects on the genetic predisposition to this autoimmune response. Recent studies in animal models for human IDDM, the genetically diabetes-prone NOD mouse and BB rat, have revealed that microbial agents--including certain viruses and extracts of bacteria, fungi, and mycobacteria--often have a protective action against diabetes development. Many of these microbial preparations are immune adjuvants, which are agents that stimulate the immune system. The protective effects of these agents against diabetes appear to involve perturbations in the production of cytokines, which are polypeptides produced by and acting on cells of the immune system. Thus, recent studies in NOD mice suggest that the islet beta-cell-directed autoimmune response may be mediated by a T-helper 1 (Th1) subset of T-cells producing the cytokines interleukin-2 (IL-2) and interferon-gamma. These studies also suggest that the diabetes-protective effects of administering microbial agents, adjuvants, and a beta-cell autoantigen (GAD65 [glutamic acid decarboxylase]) may result from activation of a Th2 subset of T-cells that produce the cytokines IL-4 and IL-10 and consequently downregulate the Th1-cell-mediated autoimmune response. The clinical implication of these findings is that the autoimmune response leading to islet beta-cell destruction and IDDM may be amenable to prevention or suppression by therapeutic interventions aimed at stimulating the host's own immunoregulatory mechanisms.

Pancreatic beta-cell dysfunction is a characteristic of non-insulin-dependent diabetes mellitus (NIDDM). An aspect of this dysfunction is that an increased proportion of proinsulin is secreted, but an actual beta-cell defect that leads to hyperproinsulinemia is unknown. Nevertheless, an impairment in beta-cell proinsulin conversion mechanism has been suggested as the most likely cause. Insulin is produced from its precursor molecule, proinsulin, by limited proteolytic cleavage at two dibasic sequences (Arg31, Arg32 and Lys64, Arg65). Two endopeptidase activities catalyze this cleavage: PC2 and PC3. PC2 endopeptidase cleaves predominately at Lys64, Arg65, and PC3 endopeptidase cleaves at Arg31, Arg32. The recent identification and characterization of these endopeptidases has enabled a better understanding of the human proinsulin-processing mechanism. In particular, experimental evidence suggests that the majority of human proinsulin processing is sequential. PC3 cleaves proinsulin first to generate a proinsulin conversion intermediate that is the preferred substrate of PC2. Both PC2 and PC3 activities are influenced by Ca2+ and pH, but the more stringent Ca2+ and pH requirements of PC3 suggest it as the most likely enzyme to regulate proinsulin conversion, as well as initiate it. When an increased demand is placed on the proinsulin-processing mechanism by a glucose-stimulated increase in proinsulin biosynthesis, there is a coordinate increase in PC3 biosynthesis (but not in PC2). This supports PC3 as the key endopeptidase that regulates proinsulin processing. In this perspective, the current concepts of the enzymology and regulation of proinsulin conversion at a molecular level are reviewed.(ABSTRACT TRUNCATED AT 250 WORDS)

In diabetes, insulin secretion is either completely absent (insulin-dependent diabetes mellitus [IDDM]) or inappropriately regulated (non-insulin-dependent diabetes mellitus [NIDDM]). In recent years, new insights into the molecular and biochemical mechanism(s) of fuel-mediated insulin release coupled with advances in gene transfer technology have led to the investigation of molecular strategies for replacement of normal insulin delivery function. Such initiatives have included attempts to engineer glucose-stimulated insulin secretion in cell lines that might serve as surrogates for islets in IDDM. The development of DNA virus gene transfer systems of remarkable efficiency also has suggested ways in which the beta-cell dysfunction of NIDDM might ultimately be repaired by gene therapy. The emerging work in these areas and implications for the future are summarized in this perspective.

Protein kinase C (PKC) is activated in rat renal glomerulus within a week of induction of experimental diabetes. Studies in isolated glomeruli and in cultured endothelial and mesangial cells have demonstrated that high ambient concentrations of glucose activate PKC and thus implicate hyperglycemia per se as a mediator of PKC activation in glomerular cells in diabetes. High glucose concentrations activate PKC by increasing cellular levels of diacylglycerol (DAG), the major endogenous modulator of this signalling system. In contrast to physiological extracellular stimuli of PKC that increase cellular DAG levels by receptor-mediated enhancement of membrane inositol phospholipid hydrolysis, in glomerular cells high concentrations of glucose increase DAG by de novo synthesis from glycolytic intermediates. Activation of PKC by glucose or other agonists increases the permeability of endothelial cells to albumin and stimulates matrix protein synthesis in mesangial cells; it thereby may be involved in the pathogenesis of both the functional and structural alterations of the glomerulus in diabetes. Recent studies in isolated glomeruli from diabetic rats have also implicated activation of PKC in suppression of nitric oxide (NO)-mediated increases in glomerular cGMP generation in response to cholinergic stimuli. In mesangial cells, cGMP suppresses PKC-mediated increases in matrix protein synthesis. Thus, impaired NO-mediated cGMP generation in glomeruli of diabetic individuals may amplify matrix protein synthesis in response to hyperglycemia and other stimuli of PKC. These and other observations suggest that activation of the PKC system by hyperglycemia may represent an important pathway by which glucotoxicity is transduced in susceptible cells in diabetes.