In 1960, immunoassays of insulin first demonstrated significant quantities of circulating hormone in non-insulin-dependent (type II) diabetes and for 30 yr have fostered debate as to whether a beta-cell abnormality plays an etiological role in this syndrome. Early efforts to determine the adequacy of islet beta-cell function showed that obesity and its associated insulin resistance were major confounding variables. Subsequently, it was recognized that glucose not only directly regulated insulin synthesis and secretion but moderated all other islet signals, including other substrates, hormones, and neural factors. When both obesity and glucose are taken into account, it becomes clear that patients with fasting hyperglycemia all have abnormal islet function. Type II diabetes is characterized by a defect in first-phase or acute glucose-induced insulin secretion and a deficiency in the ability of glucose to potentiate other islet nonglucose beta-cell secretagogues. The resulting hyperglycemia compensates for the defective glucose potentiation and maintains nearly normal basal insulin levels and insulin responses to nonglucose secretagogues but does not correct the defect in first-phase glucose-induced insulin release. Before the development of fasting hyperglycemia, only first-phase glucose-induced insulin secretion is obviously defective. This is because progressive islet failure is matched by rising glucose levels to maintain basal and second-phase insulin output. The relationship between islet function and fasting plasma glucose is steeply curvilinear, so that there is a 75% loss of beta-cell function by the time the diagnostic level of 140 mg/dl is exceeded. This new steady state is characterized by glucose overproduction and inefficient utilization. Insulin resistance is also present in most patients and contributes to the hyperglycemia by augmenting the glucose levels needed for compensation. Decompensation and absolute hypoinsulinemia occur when the renal threshold for glucose is exceeded and prevents further elevation of circulating glucose. The etiology of the islet beta-cell lesion is not known, but a hypothesis based on basal hyperproinsulinemia and islet amyloid deposits in the pancreas of type II diabetes is reviewed. The recent discovery of the islet amyloid polypeptide (IAPP) or amylin, which is the major constituent of islet amyloid deposits, is integrated into this hypothesis. It is suggested that pro-IAPP and proinsulin processing and mature peptide secretion normally occur together and that abnormal processing, secondary to or in conjunction with defects in hormone secretion, lead to progressive accumulation of intracellular IAPP and pro-IAPP, which in cats, monkeys, and humans form intracellular fibrils and amyloid deposits with a loss of beta-cell mass.(ABSTRACT TRUNCATED AT 400 WORDS)

Guanine nucleotide-binding proteins (G proteins) are critically important mediators of many signal-transduction systems. Several important sites regulating stimulus-secretion coupling and release of insulin from pancreatic beta-cells are modulated by G proteins. Gs mediates increases in intracellular cAMP associated with hormone-induced stimulation of insulin secretin. Gi mediates decreases in intracellular cAMP caused by inhibitors of insulin secretion, e.g., epinephrine, somatostatin, prostaglandin E2, and galanin. G proteins also regulate ion channels, phospholipases, and distal sites in exocytosis. Cholera and pertussis toxins irreversibly ADP ribosylate G proteins and are important tools that can be used both to manipulate G-protein-dependent modulators of insulin secretion and detect and quantify G proteins by electrophoretic techniques. The stage is set to pursue these initial observations in greater depth and ascertain whether G-protein research will provide important new insights into normal and abnormal regulation of insulin secretion.

The beta-cell is unique because its major agonists, i.e., insulin secretagogues, undergo metabolism instead of interacting with a receptor. This perspectives presents the hypothesis that the first part of a metabolic signal of a secretagogue is specific to the secretagogue and the beta-cell and can be envisioned as proximal. The second part, which occurs after transduction to more universal signaling mechanisms, is viewed as distal. Distal signaling and exocytosis in the beta-cell operate the same as in other cells. Aerobic glycolysis is required for glucose-induced insulin release. Because glyceraldehyde, which enters metabolism at the triose phosphates in the glycolytic pathway, is a potent insulin secretagogue but pyruvate, which is metabolized in the mitochondrion, is not an insulin secretagogue, the proximal signal for glucose-induced insulin release originates with an interaction between the central part of the glycolytic pathway and mitochondrial metabolism. The proximal message in leucine-induced insulin release originates with leucine allosterically activating glutamate dehydrogenase, which activates endogenous glutamate metabolism, and by the metabolism of leucine itself. The methyl ester of succinate is a potent experimental insulin secretagogue. It is puzzling why the glucose signal requires the interplay of glycolysis and mitochondrial metabolism, whereas the signals from leucine and succinate originate entirely from within the mitochondrion. Leucine-induced insulin release is suppressed and glucose-induced insulin release is activated in islets cultured at a high concentration of glucose. Conversely, leucine-induced insulin release is activated and glucose-induced insulin release is suppressed in islets cultured at low glucose.(ABSTRACT TRUNCATED AT 250 WORDS)

Research on mapping diabetes-susceptibility genes is dependent on several factors, including the existence of a single major gene for susceptibility, genetic homogeneity, and the existence of appropriate clinical material. The power to detect susceptibility genes is dependent on the risks in relatives and the distance of genetic markers from the susceptibility genes. For insulin-dependent diabetes mellitus (IDDM), the best-fitting risk models are those with a single major locus with residual polygenic factors. The major locus effect is likely represented by genes in the HLA complex, because specific genotypes have been found to affect IDDM risk significantly. Thus, mapping the remaining polygenic IDDM susceptibility factors--each of small effect--is a difficult and long task. For non-insulin-dependent diabetes mellitus (NIDDM), the likely risk models result in few genes with moderate effect. Models of NIDDM have significant residual polygenic variation remaining, reflecting the importance of multiple loci with small effect, environmental effects, or genetic heterogeneity; however, the prospects for mapping genes that provide at least moderate susceptibility for NIDDM now appear promising.

Our understanding of the role of HLA genes associated with insulin-dependent diabetes mellitus (IDDM) is in disarray, despite recent improvements in the definition of specific alleles and haplotypes. Some genes are highly associated with IDDM, other genes are associated with resistance to IDDM, and some highly associated susceptibility genes are markedly influenced by trans-associated synergistic effects (DR3/4 heterozygotes) or protective effects (DR2/4 heterozygotes). This plethora of genetic associations has spawned the notion that there are many contributing susceptibility genes, which, in turn, has led to the search for shared structural features among different genes on IDDM-associated haplotypes. From a more mechanistic point of view, however, the wide range of variable IDDM associations, with both cis- and trans-encoded protective and/or synergistic effects, suggests a different approach. This article proposes a hypothesis in which the different HLA associations with IDDM can be simply explained by a single unifying concept: a hierarchy of affinities determines the interaction between a diabetogenic peptide and different class II molecules, and an individual is susceptible to IDDM if the class II molecule in that individual with the highest affinity for such a peptide is a DQ beta susceptibility gene. The explicit formulation of this proposal and its genetic implications provide an explanation for HLA-encoded dominant "protection" and for some of the more subtle genetic observations related to cis and trans influences in IDDM susceptibility.

The insulin receptor is a multifunctional protein encoded by a modular gene. Certain discrete domains within the insulin-receptor structure subserve specific functional properties. In some instances, these discrete domains are encoded by individual exons. This organizational model of the insulin receptor predicts the existence of divergent signaling pathways facilitating specific bioeffects. Some of these signaling pathways are shared with the closely related insulinlike growth factor I receptor (convergent pathways), whereas others are different (divergent). The concept of discrete functional domains also provides several mechanisms whereby inactive insulin receptors (no kinase activity) can inhibit the function of normal receptors. The ability of kinase-inactive insulin receptors to inhibit the signaling function of normal insulin receptors may be an operative mechanism in certain insulin-resistant states.

Transgenic mouse technology has gained recognition as an important tool for examining many fundamental biological questions in vivo. Recently, transgenic mouse techniques have been applied to the study of type I (insulin-dependent) diabetes. These studies have been particularly informative in elucidating 1) mechanisms whereby immune tolerance is maintained to antigens on rare specialized cells such as the pancreatic beta-cell, 2) disease susceptibility and resistance genes, and 3) potentially important immune effector mechanisms. In this article, we discuss these studies, their impact on understanding of the pathogenesis of type I diabetes, and the potential of the transgenic mouse approach for future research.

This perspective deals with prediction of overt diabetic nephropathy in patients with insulin-dependent diabetes mellitus (IDDM). The role of elevated urinary albumin excretion rate (microalbuminuria) in predicting diabetic nephropathy has been emphasized by new follow-up studies. Development of severe kidney impairment was seen in a large percentage of patients with microalbuminuria, but with more intensive care for diabetic patients, this percentage may be falling. Herein, I analyzed alternatives to microalbuminuria in predicting kidney disease in diabetes. 1) Parental predisposition to hypertension is not seen in all studies and therefore may not be a decisive factor, and it cannot be used in prediction of nephropathy. 2) Prediabetic blood pressure may predict nephropathy in certain non-insulin-dependent diabetic patients, but elevated blood pressure seems to develop after early microalbuminuria and is likely to be an aggravating factor in established microalbuminuria in IDDM patients. 3) At the clinical diagnosis of IDDM, diabetic nephropathy cannot be predicted. 4) Glycemic control is poor in normoalbuminuric patients with later development of microalbuminuria, and multiple glycosylated hemoglobin measurements are therefore important. 5) In diabetes, glomerular hyperfiltration is associated with late nephropathy, but it alone cannot be the decisive factor, because hyperfiltration in nondiabetic individuals does not produce kidney disease, according to new long-term follow-up studies. 6) Studies of glomerular structure and ultrastructure have not yet documented predictive values for overt nephropathy, but further studies are in progress. 7) Isolated blood pressure elevation without microabuminuria (probably representing essential hypertension in diabetes) has not been predictive. 8) It is clear that elevation of serum creatinine is a very late and insensitive parameter, occurring only with pronounced proteinuria.(ABSTRACT TRUNCATED AT 250 WORDS)

This article reviews evidence for a pivotal role of glucokinase as glucose sensor of the pancreatic beta-cells. Glucokinase explains the capacity, hexose specificity, affinities, sigmoidicity, and anomeric preference of pancreatic islet glycolysis, and because stimulation of glucose metabolism is a prerequisite of glucose stimulation of insulin release, glucokinase also explains many characteristics of this beta-cell function. Glucokinase of the beta-cell is induced or activated by glucose in contrast to liver glucokinase, which is regulated by insulin. Tissue-specific regulation corresponds with observations that liver and pancreatic beta-cell glucokinase are structurally distinct. Glucokinase could play a glucose-sensor role in hepatocytes as well, and certain forms of diabetes mellitus might be due to glucokinase deficiencies in pancreatic beta-cells, hepatocytes, or both.

Glucokinase is expressed in both the liver and the pancreatic beta-cell and plays a key role in the metabolism of glucose by both tissues. Expression of this enzyme is differentially regulated; hepatic glucokinase is stimulated by insulin and repressed by cAMP, whereas beta-cell glucokinase activity is increased by glucose. Recently, the glucokinase gene has been characterized and was found to contain two different transcription control regions. One region regulates transcription of the gene in the liver, whereas the other region, which lies at least 12 kilobases further upstream, controls transcription in the pancreatic beta-cell. The finding of two different transcription control regions in a single glucokinase gene provides a genetic basis for the tissue-specific differential regulation of glucokinase and will serve as the basis for further studies to identify and characterize the different regulatory elements and factors in the liver and beta-cell, which are presumably involved. Comparison of different glucokinase cDNAs isolated from hepatic, insulinoma, and islet cDNA libraries indicates that at least three glucokinase isoforms are generated by differential RNA processing of the glucokinase gene transcripts. Whether any of these glucokinase isoforms are functionally unique remains to be determined.

The concept of pancreatic beta-cell mass is fundamental to the understanding of normal metabolism, the pathogenesis of diabetes, and the transplantation of beta-cell tissue. The amount of beta-cell tissue present in the pancreas is a major determinant of the quantity of insulin that can be secreted, and its mass will vary according to the size of the individual and the degree of insulin resistance present. Not all insulin-producing cells are the same, and the dimensions of this heterogeneity remain to be defined. Pancreatic beta-cell mass is markedly reduced in insulin-dependent diabetes mellitus and moderately reduced in non-insulin-dependent diabetes mellitus. In both forms of diabetes, there are qualitative and quantitative abnormalities of insulin secretion that cannot be explained entirely by changes in beta-cell mass. The amount of beta-cell tissue needed for successful transplantation has only been partially defined. Segmental (approximately 50% of the pancreas) transplantation can normalize plasma glucose levels in humans. Difficulty obtaining sufficient amounts of beta-cell tissue is expected to remain a barrier to successful islet transplantation for the immediate future. More should be learned about the function and fate of grafted islet cells.

The hyperglycemic patient remains persistently at risk for infectious complications. Whether ascribable to diabetes mellitus, to the administration of glucocorticoids, or to the infusion of hyperalimentation fluids, hyperglycemia may impair several mechanisms of humoral host defense, including such varied neutrophil functions as adhesion, chemotaxis, and phagocytosis. In addition, binding of glucose to the biochemically active site of the third component of complement C3 inhibits the attachment of this protein to the microbial surface and thereby impairs opsonization. Last, several pathogens frequently encountered in hyperglycemic patients possess unique mechanisms of virulence that flourish in the hyperglycemic environment. Most notable in this regard is the yeast Candida albicans, which expresses a glucose-inducible protein that is structurally and functionally homologous to a complement receptor on mammalian phagocytes. This protein promotes adhesion in the yeast and subverts phagocytosis by the host. Thus, hyperglycemia serves as a central mechanism in the predisposition of hyperglycemic patients to infection.

The human insulin-receptor (hINSR) gene spans a region of greater than 120,000 base pairs (bp) on the short arm of chromosome 19. It is comprised of 22 exons or coding regions that vary in size from 36 to greater than 2500 bp. To a large degree, the introns appear to divide the hINSR gene into segments that encode structural and/or functional elements of the hINSR protein. The exon-intron organization of the hINSR gene provides a clue to the evolutionary history of this gene and suggests that it is a mosaic constructed from protein-coding regions recruited from other genes. Eight mutations in the hINSR gene that result in expression of structurally abnormal proteins have been described. These mutations are associated with insulin resistance and provide insight into the role of the hINSR gene in the development of diabetes mellitus.

Glucose transport by facilitated diffusion is mediated by a family of tissue-specific membrane glycoproteins. At least four members of this gene family have been identified by cDNA cloning. The HepG2-type transporter is the most widely distributed of these proteins. It provides many cells with their basal glucose requirement for ATP production and the biosynthesis of sugar-containing macromolecules. The liver-type transporter is expressed in tissues from which a net release of glucose can occur and in beta-cells of pancreatic islets. A genetic defect resulting in reduced activity of this transporter could hypothetically lead to the two principal features of non-insulin-dependent diabetes mellitus, insulin resistance and relative hypoinsulinemia. The adipocyte/muscle transporter is expressed exclusively in tissues that are insulin sensitive with respect to glucose uptake. This protein is an excellent candidate for a highly specific genetic defect predisposing to insulin resistance.

Vanadate ions, low-molecular-weight phosphate analogues, mimic most of the rapid actions of insulin in various cell types. When administered orally to diabetic hyperglycemic rats, vanadate reaches the circulation, mimics insulin stimulation of glucose uptake and metabolism, and leads to normoglycemic and partial anabolic states. In addition, vanadate restores tissue responsiveness to insulin and hepatic glycogen levels and activates new synthesis of key enzymes for carbohydrate metabolism. This suggests that correcting hyperglycemia is sufficient to correct the typical metabolic alterations found in streptozocin-induced diabetic rats. Several weeks of oral administration of vanadate to diabetic rats has not produced detectable liver or kidney toxicity. The mechanism by which vanadate mimics the actions of insulin is still obscure. Unlike insulin, vanadate does not seem to stimulate the autophosphorylation and endogenous tyrosine phosphorylation of insulin-receptor kinase or other intracellular proteins either directly or by virtue of its known inhibitory effect on protein phosphotyrosine phosphatase. Results from many studies support a model in which vanadate activates glucose metabolism by either utilizing an alternative (insulin-independent) cascade or bypassing the early events of the insulin-dependent cascade. Either of these possibilities is of clinical importance, because early insulin events may become defective, as a result of severe hyperinsulinemia, and may contribute to insulin resistance. Alternative pathways by which vanadate may stimulate glucose metabolism, e.g., by increasing intracellular Ca2+ levels and/or regulating intracellular and intravesicular pH, are discussed. From a clinical perspective, studies should be continued in evaluating the level of vanadate toxicity after prolonged treatment and searching for agents that potentiate its insulin mimetic actions in vitro and in vivo.

Glucose tolerance depends on a complex interaction among insulin secretion from the beta-cells, clearance of the hormone, and the actions of insulin to accelerate glucose disappearance and inhibit endogenous glucose production. An additional factor, less well recognized, is the ability of glucose per se, independent of changes in insulin, to increase glucose uptake and suppress endogenous output (glucose effectiveness). These factors can be measured in the intact organism with physiologically based minimal models of glucose utilization and insulin kinetics. With the glucose minimal model, insulin sensitivity (SI) and glucose effectiveness (SG) are measured by computer analysis of the frequently sampled intravenous glucose tolerance test. The test involves intravenous injection of glucose followed by tolbutamide or insulin and frequent blood sampling. SI varied from a high of 7.6 x 10(-4) min-1.microU-1.ml-1 in young Whites to 2.3 x 10(-4) min-1.microU-1.ml-1 in obese nondiabetic subjects; in all of the nondiabetic subjects, SG was normal. In subjects with non-insulin-dependent diabetes mellitus (NIDDM), not only was SI reduced 90% below normal (0.61 +/- 0.16 x 10(-4) min-1.microU-1.ml-1), but in this group alone, SG was reduced (from 0.026 +/- 0.008 to 0.014 +/- 0.002 min-1); thus, defects in SI and SG are synergistic in causing glucose intolerance in NIDDM. One assumption of the minimal model is that the time delay in insulin action on glucose utilization in vivo is due to sluggish insulin transport across the capillary endothelium. This was tested by comparing insulin concentrations in plasma with those in lymph (representing interstitial fluid) during euglycemic-hyperinsulinemic glucose clamps. Lymph insulin was lower than plasma insulin at basal (12 vs. 18 microU/ml) and at steady state, indicating significant loss of insulin from the interstitial space, presumably due to cellular uptake of the insulin-receptor complex. Additionally, during clamps, lymph insulin changed more slowly than plasma insulin, but the rate of glucose utilization followed a time course identical with that of lymph (r = .96) rather than plasma (r = .71). Thus, lymph insulin, which may be reflective of interstitial fluid, is the signal to which insulin-sensitive tissues are responding. These studies support the concept that, at physiological insulin levels, the time for insulin to cross the capillary endothelium is the process that determines the rate of insulin action in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

A universal finding in hyperglycemic patients with type II (non-insulin-dependent) diabetes mellitus is that all share a common defect in glucose recognition resulting in abnormal insulin secretion by pancreatic islet beta-cells. This defect is 1) specific for glucose signals rather than global, 2) related to chronic hyperglycemia, and 3) partially reversible after brief treatment with insulin to induce normoglycemia and through use of other pharmacological agents without normalizing glucose levels. My perspective is that an essential component of this defect is secondary and may represent a state of homologous desensitization of the beta-cell secretory apparatus to glucose. Elucidation of the biochemical mechanism(s) of defective recognition of glucose signals by beta-cells--or glucose "non-sense"--in these patients will provide key insights into the pathogenesis of type II diabetes mellitus.

The actions of insulin are mediated by an integral plasma membrane protein, the insulin receptor. The processed receptor is a tetramer composed of two alpha-subunits that bind insulin and two beta-subunits that traverse the plasma membrane and are, in their cytosolic domains, protein tyrosine kinases. The insulin proreceptor cDNA has been cloned and its complete amino acid sequence deduced. The availability of cDNA permitted an analysis of both the role of protein tyrosine kinase activity in insulin action and the autophosphorylation sites that regulate kinase activity. The human cDNA probe has also been used to identify a putative Drosophila insulin receptor. This work is reviewed, and approaches that may be used to identify physiological substrates for the receptor kinase are suggested.

Impaired islet function is a feature of non-insulin-dependent diabetes mellitus (NIDDM), which is manifested in part by disproportionate proinsulin release. A disproportionate increase in proinsulin also occurs in insulinomas, suggesting that enhanced proinsulin release results from an increase in synthesis and premature release of proinsulin-rich immature granules in both conditions. However, recent human and animal studies suggest that normal beta-cells respond to an increase in synthetic demand by enhancing their ability to process proinsulin. Thus, impaired processing of proinsulin is likely in NIDDM. A new point of similarity with insulinoma has been the demonstration of a novel pancreatic peptide isolated from insulinomas and the pancreas of patients with NIDDM. This peptide, named islet amyloid polypeptide or amylin, is also present in normal islets. Because of its association with two apparently dissimilar disease states, we propose a hypothesis that encompasses the observations related to proinsulin and islet amyloid polypeptide and suggest they are manifestations of the same abnormality. In this hypothesis, we suggest that this new pancreatic peptide is a normal participant in the process of proinsulin processing and storage. We also suggest that in the presence of defective proinsulin processing and insulin release, as occurs in NIDDM, hyperglycemia stimulates amylin biosynthesis so that this peptide is deposited in increased quantities in the islet as amyloid. This then further exacerbates the diabetic process, resulting in progressive hyperglycemia and deterioration in islet function.

Diabetic retinopathy involves anatomic changes in retinal vessels and neuroglia. The pathogenetic mechanism responsible for retinopathy is imperfectly understood, but much of the mechanism is apparently reproduced by experimental diabetes in animals and by chronic elevation of blood galactose in nondiabetic animals. The evidence that retinopathy is a consequence of excessive blood sugars and their sequelae is consistent with a demonstrated inhibition of retinopathy by strict glycemic control in diabetic dogs. However, retinopathy in the dog model has shown a tendency to resist intervention by strict control. Biochemical and pathophysiological sequelae of hyperglycemia possibly critical to the development of retinopathy in humans and animal models are being studied in many laboratories. Retinopathy occurs in experimental galactosemia in the absence of the renal hypertrophy, mesangial expansion, and glomerular obliteration typical of diabetes in humans and dogs, implying that retinopathy and nephropathy differ appreciably in pathogenesis.