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.

Most studies dealing with the pathogenesis of IDDM have emphasized the immune assault against beta-cells. In this perspective, we review the data that suggest that the beta-cell destruction of IDDM depends on a balance between beta-cell damage and repair. The progressive beta-cell damage leading to IDDM seems to follow markedly different temporal courses in individual patients. Some individuals at high risk for developing IDDM, and presenting with impaired beta-cell function, appear to recover beta-cell function when followed prospectively. Moreover, after the clinical onset of IDDM, most patients experience a transitory period of improved insulin secretion. In vitro and in vivo experimental data suggest that beta-cells are indeed able to repair themselves after damage. Dispersed beta-cells or whole islets can survive and regain their function after a toxic assault. Furthermore, the abnormal insulin release and glucose oxidation of islets isolated from NOD mice during the prediabetic period is completely restored after 1 wk in tissue culture. Finally, treatment of NOD mice with monoclonal antibodies directed against infiltrating T-cells reverses the altered glucose metabolism of beta-cells. Note that beta-cell repair after exposure to different toxic agents can be enhanced both in vivo and in vitro. Potential enhancers of beta-cell repair are nicotinamide, glucose, protein-rich diets, and branched chain amino acids. A basic question that remains to be answered is the nature of the repair mechanisms triggered by beta-cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Potentiation of glucose-induced insulin secretion by intestinal factors has been described for many years. Today, two major peptides with potent insulinotropic action have been recognized: gastric inhibitory peptide and truncated forms of glucagon-like peptide I, GLP-I(7-37) or the related GLP-I(7-36)amide. These hormones have specific beta-cell receptors that are coupled to production of cAMP and activation of cAMP-dependent protein kinase. Elevation in intracellular cAMP levels is required to mediate the glucoincretin effect of these hormones: the potentiation of insulin secretion in the presence of stimulatory concentrations of glucose. In addition, circulating glucoincretins maintain basal levels of cAMP, which are necessary to keep beta-cells in a glucose-competent state. Interactions between glucoincretin signaling and glucose-induced insulin secretion may result from the phosphorylation of key elements of the glucose signaling pathway by cAMP-dependent protein kinase. These include the ATP-dependent K+ channel, the Ca++ channel, or elements of the secretory machinery itself. In NIDDM, the glucoincretin effect is reduced. However, basal or stimulated gastric inhibitory peptide and glucagon-like peptide I levels are normal or even elevated, suggesting that signals induced by these hormones on the beta-cells are probably altered. At pharmacological doses, infusion of glucagon-like peptide I but not gastric inhibitory peptide, can ameliorate postprandial insulin secretory response in NIDDM patients. Agonists of the glucagon-like peptide I receptor have been proposed as new therapeutic agents in NIDDM.

Vasodilation and increased blood flow are characteristic early vascular responses to acute hyperglycemia and tissue hypoxia. In hypoxic tissues these vascular changes are linked to metabolic imbalances associated with impaired oxidation of NADH to NAD+ and the resulting increased ratio of NADH/NAD+. In hyperglycemic tissues these vascular changes also are linked to an increased ratio of NADH/NAD+, in this case because of an increased rate of reduction of NAD+ to NADH. Several lines of evidence support the likelihood that the increased cytosolic ratio of free NADH/NAD+ caused by hyperglycemia, referred to as pseudohypoxia because tissue partial pressure oxygen is normal, is a characteristic feature of poorly controlled diabetes that mimics the effects of true hypoxia on vascular and neural function and plays an important role in the pathogenesis of diabetic complications. These effects of hypoxia and hyperglycemia-induced pseudohypoxia on vascular and neural function are mediated by a branching cascade of imbalances in lipid metabolism, increased production of superoxide anion, and possibly increased nitric oxide formation.

Since the discovery of insulin and its receptor, the downstream elements responsible for the pleiotropic insulin signal have been difficult to define. The recently discovered insulin receptor substrate, IRS-1, provides an innovative and simple way to think about this problem: IRS-1 may mediate the control of various cellular processes by insulin. Overexpression of IRS-1 enhances insulin-stimulated DNA synthesis in Chinese hamster ovary cells, and microinjection of IRS-1 protein potentiates the maturation of Xenopus oocytes. We suspect that insulin signals are enabled when the activated insulin receptor kinase phosphorylates specific tyrosine residues in IRS-1. These phosphorylated sites associate with high affinity to cellular proteins that contain SH2 (src homology-2) domains. This association is specific and depends on the amino acid sequence surrounding the phosphotyrosine residue and the isoform of the SH2 domain. A growing number of SH2 domain-containing proteins have been identified, and we suspect that IRS-1 has the potential to simultaneously regulate many of them. We have only begun to identify the specific proteins that associate with phosphorylated IRS-1. One of them, the phosphatidylinositol 3'-kinase, is activated when the SH2 domains in its 85,000-M(r) regulatory subunit bind to phosphorylated IRS-1. IRS-1 also interacts with other proteins such as SHPTP2, a novel SH2 domain-containing Tyr phosphatase, and GRB-2/sem-5, a protein that is implicated in p21ras signaling. The interaction between phosphorylated IRS-1 and multiple SH2 domain-containing proteins may ultimately explain the pleiotropic effects of insulin.

It generally is accepted that a diet high in fiber, particularly soluble fiber, is useful in the management of the plasma glucose concentration in individuals with diabetes. This is one of the reasons several national diabetes associations have recommended that diabetic individuals ingest a diet high in fiber-containing foods. However, more recent data obtained in carefully controlled studies with more definitive end points, indicate this may not be the case. It has been shown clearly that addition of water-soluble, gel-forming fiber in the form of guar gum and perhaps gum tragacanth to an ingested glucose solution or to a mixed meal will reduce the expected rise in glucose concentration. This has been demonstrated in both normal subjects and subjects with IDDM and NIDDM. However, it is only observed when large amounts of fiber are added. The fiber also must be mixed with the administered glucose or food. Other less viscous soluble fiber sources such as the pectins and psyllium powder are not effective. In long-term, well-controlled trials, guar gum, pectin, beet fiber, or cereal bran fiber ingested with meals has been of little or no value in controlling the plasma glucose concentration in individuals with NIDDM. Several studies have been conducted in which a high-carbohydrate diet has been reported to reduce the plasma glucose concentration. In these diets, foods with a high fiber content have been emphasized. In general, they were not well controlled, and several confounding variables such as weight loss, decreased food energy intake, different food sources with potential for differences in starch digestibility, and decreased dietary fat content were present.(ABSTRACT TRUNCATED AT 250 WORDS)

D-glucose induces a rise in pancreatic islet beta-cell cytosolic [Ca2+] by processes requiring both glucose metabolism and Ca2+ entry from the extracellular space, and this Ca2+ signal is thought to be critical to the induction of insulin secretion. Insulin secretagogues also induce phospholipid hydrolysis and accumulation of phospholipid-derived mediators in islets, including the lipid messengers DAG, nonesterified arachidonic acid, and arachidonate 12-LO products. This study offers the following viewpoints on potential roles of these lipid messengers in insulin secretion as working hypotheses: 1) the Ca2+ signal provided to the beta-cell by D-glucose induces insulin secretion only in the context of amplifying background signals provided by the beta-cell content of messengers including DAG; 2) muscarinic receptor agonists amplify glucose-induced insulin secretion in part by altering the beta-cell content of DAG; 3) the Ca2+ signal provided by metabolism of D-glucose is amplified by the level of nonesterified arachidonic acid in beta-cell membranes, which acts to facilitate Ca2+ entry; 4) metabolism of glucose induces accumulation of nonesterified arachidonate in beta-cells via activation of a recently identified ASCI-PLA2 enzyme, which may be a component of the beta-cell fuel sensor apparatus; and 5) arachidonate 12-LO metabolites are potential candidates as adjunctive modulators of beta-cell K(+)-channel activity.

Risk of progression to IDDM has been assessed extensively in first-degree relatives of IDDM patients, and highly specific prediction is possible within a small subset of this population. Because approximately 90% of future cases will come from those who have no close relative with IDDM, prediction and intervention within the general population will become the main priority for the future. This review presents a decision tree analysis of risk of progression to IDDM, highlights the different prognosis of markers when applied to those with and without a family history of the disease, and proposes a strategy for disease prediction in the latter. Large collaborative studies in well-characterized populations will allow new predictive markers and models to be evaluated, and strategies of intervention to be tested with maximum efficiency and minimal delay.

Patients with hyperinsulinemia, defined by increased concentrations of IRI in plasma, experience increased cardiovascular mortality. In type II diabetic patients, the increase in IRI may reflect, in part, not only insulin but also proinsulinemia as a result of impaired conversion of proinsulin to insulin by pancreatic beta-cells. High IRI is accompanied by attenuation of endogenous fibrinolytic activity and increased plasma PAI-1, the primary physiological inhibitor of t-PA. Concordant increases of plasma PAI-1 and plasma IRI appear to reflect direct effects of insulin and proinsulin on the synthesis and secretion of PAI-1 by endothelial and liver cells as judged from results of studies in vitro. Because attenuated fibrinolysis may predispose to thrombosis, the increased exposure of luminal surfaces of vessels to atherogenic, clot-associated mitogens and chemoattractants may activate macrophages and potentiate proliferation of vascular smooth muscle cells. Accordingly, increased concentrations of plasma IRI may contribute to macrovascular disease in diabetic patients by impairing endogenous fibrinolysis.

Immunoisolation is a potentially important approach to transplanting islets without need for immunosuppressive drugs. Immunoisolation systems have been conceived in which the transplanted tissue is separated from the immune system of the host by an artificial barrier. These systems offer a solution to the problem of human islet procurement by permitting use of islets isolated from animal pancreases. The devices used are referred to as biohybrid artificial organs because they combine synthetic, selectively permeable membranes that block immune rejection with living transplants. Three major types of biohybrid pancreas devices have been studied. These include devices anastomosed to the vascular system as AV shunts, diffusion chambers, and microcapsules. Results in diabetic rodents and dogs indicate that biohybrid pancreas devices significantly improve glucose homeostasis and can function for more than a year. Recent progress made with this approach is discussed, and some of the remaining problems that must be resolved to bring this technology to clinical reality are addressed.

Glucokinase, the major enzyme that phosphorylates glucose upon entry into liver and islet beta-cells, has been considered a prime candidate for inherited defects predisposing to NIDDM. Now that the human gene has been isolated, this question has been addressed directly. Polymorphic markers flanking the gene were identified. These markers (microsatellites) are composed of variable numbers of dinucleotide repeats that vary in size, resulting in different alleles. Variably sized alleles can be typed rapidly from genomic DNA of individuals by the PCR. Studies of inheritance of glucokinase genes have revealed significant linkage in families with early-onset NIDDM, or MODY, and mutations have been identified within the coding region of the gene in some families. These studies are extremely encouraging, as they indicate that genes can be identified even in this heterogeneous genetic disorder. This study considers the phenotypes that result from glucokinase defects and the relationship of MODY to NIDDM, and it estimates the role of glucokinase defects in NIDDM in general.