Current Protein and Peptide Science - Volume 10, Issue 1, 2009

Diabetes mellitus remains a hot research topic. More than 10,000 articles related to it have been published in the last twelve months (PubMed). In the U.S., 1.5 million new cases were diagnosed in 2005 and 6.2% of Americans are diabetic. According to the World Health Organization, approximately 197 million people worldwide have impaired glucose tolerance, most commonly attributed to obesity and metabolic syndrome. This number is expected to increase to an astounding 420 million by 2025. As the incidence of the disease continues to increase, so do the incidences of diabetes-related cardiovascular and renal complications. A great deal of work remains in improving our understanding of diabetes mellitus. A deeper knowledge of the pancreatic and peripheral mechanisms involved in hyperglycemia, insulin resistance and insulin insufficiency should facilitate therapeutic improvements. Therefore in this special issue, we have concentrated on novel peptides and proteins that affect insulin sensitivity and glucose metabolism or pancreatic islet differentiation, growth and function. The growth hormone releasing factor ghrelin is thought to affect energy balance and to regulate appetite and body weight maintenance. The presence of low plasma levels of ghrelin has been linked to metabolic syndrome. Hence ghrelin regulation represents a promising target for the development of new drugs for the treatment of obesity and diabetes. The non-protein sulfur amino acid taurine, which is the most abundant free amino-acid in the body, plays an important role in several essential biological processes and has anti-oxidant properties. Taurine-deficiency during pregnancy can lead to impaired glucose tolerance and vascular dysfunction in the adult offspring. Meanwhile, taurine supplementation can improve glucose metabolism and insulin action and taurine can decrease glycation and thus advanced glycation endproduct formation. The recently discovered hormone resistin is secreted by adipose tissue and strongly associated with insulin resistance. Its effects are counterbalanced by adiponectin. The excess of resistin that accompanies excessive adipose tissue mass underscores the deleterious consequences of visceral obesity and the associated risk of developing cardiometabolic syndrome. Furthermore, the endocannabinoid system is overactive in the presence of abdominal obesity or diabetes, and thus contributes to disturbances of energy balance and metabolism. If improving the effects of insulin is a clear goal for treatment, one should not forget the pancreas, its development and its function. Oxidative stress is a critical factor, not only because of the peripheral dysfunction it causes but also because of its deleterious effects on the pancreas itself. High angiotensin II levels and low heat shock protein concentrations appear to be critically involved in the pathogenesis of diabetes mellitus. An ultimate cure for diabetic patients will require development of therapies to improve pancreatic cell growth and function. Pancreatic function may be improved by pharmacological blockade of the renin-angiotensin system (RAS).

Ghrelin is a gut-brain peptide that has somatotropic, food intake increasing and adipogenic effects. Ghrelin is involved in modulating insulin and glucose metabolism in rodents according to recent studies. In humans acylated ghrelin reduces insulin sensitivity while unacylated ghrelin has opposite effects. In general, ghrelin seems to have diabetogenic effects. Obese, in particular abdominally obese, subjects have low ghrelin levels and decreased total ghrelin levels have been associated with metabolic syndrome and Type 2 diabetes. Most of the human studies in Type 1 diabetes have reported low ghrelin levels probably as a compensatory mechanism against hyperglycaemia. The data on obestatin in the regulation of energy balance is extremely contradictory. Interestingly, ghrelin receptor antagonists may improve glucose tolerance in rats without inducing weight gain by increasing insulin secretion. Antagonism of ghrelin function to treat diabetes is thus a fascinating idea. This review concentrates on recent findings on the orexigenic peptide ghrelin and its derivatives in metabolic disorders with emphasis put on human studies.

Emerging evidence suggests that amino acids may be potentially important in the prevention of diabetes and diabetes-associated complications. The pathways involved in the pathogenesis of diabetic complications include increased polyol pathway flux, increased advanced glycation end products formation, activation of protein kinase C and oxidative and carbonyl stress. This review will discuss the modulatory effects of amino acids on insulin secretion and their action in concert with insulin as signaling molecules. Evidences for the role of some amino acids in controlling glycemia and glucose- triggered pathological pathways are also included. Individual amino acids, especially the ones bestowed with antioxidant property like N-acetyl cysteine and taurine seem to have beneficial effects by their ability to reduce intracellular oxidative stress generation and glycooxidation. Other amino acids like glycine and lysine may be good candidates for the prevention of glycation. Nutritional intervention with taurine, phenyl alanine or branched chain amino acids can improve insulin sensitivity and post-prandial glucose disposal. Deficiency of one or more amino acids has been observed in diabetes and the beneficial effects of amino acids in some studies are positively correlated with the increase in plasma levels of these amino acids. Inclusion of individual amino acids/mixture, perhaps as a combinational therapy with conventional treatment protocols could be of therapeutic interest.

Diabetes and the related metabolic syndrome are multi system disorders that result from improper interactions between various cell types. Even though the underlying mechanism remains to be fully understood, it is most likely that both the long and the short distance range cell interactions, which normally ensure the physiologic functioning of the pancreas, and its relationships with the insulin-targeted organs, are altered. This review focuses on the short-range type of interactions that depend on the contact between adjacent cells and, specifically, on the interactions that are dependent on connexins. The widespread distribution of these membrane proteins, their multiple modes of action, and their interactions with conditions/molecules associated to both the pathogenesis and the treatment of the 2 main forms of diabetes and the metabolic syndrome, make connexins an essential part of the chain of events that leads to metabolic diseases. Here, we review the present state of knowledge about the molecular and cell biology of the connexin genes and proteins, their general mechanisms of action, the roles specific connexin species play in the endocrine pancreas and the major insulintargeted organs, under physiological and patho-physiological conditions.

PDZ domains are versatile protein interaction modules with the ability to dimerize and to recognize internal and carboxy-terminal peptide motifs. Their function in mediating the formation of multi-molecular signaling complexes is best understood at neuronal and epithelial membranes. In a screen for interactors that regulate transcription factor function in pancreatic beta cells, we isolated two PDZ-containing proteins Bridge-1 (PSMD9) and PDZD2, which contain one and six PDZ domains, respectively. Here, we review their functions in the regulation of pancreatic beta cells as a nuclear coactivator or extracellular signaling molecule. Bridge-1 interacts with both E12 and PDX-1 to stimulate insulin promoter activity. Recent gain-of-function analysis in both cell and transgenic models has revealed its functions to regulate both insulin gene expression and pancreatic beta-cell survival. Little is known about the intracellular function of PDZD2 that is predominantly localized to the endoplasmic reticulum of INS-1E cells. Interestingly, PDZD2 is proteolytically processed by caspase-3 to generate a carboxy-terminal secreted protein (sPDZD2) containing two PDZ domains. Expressed in fetal pancreatic progenitor and INS-1E cells, sPDZD2 when added as recombinant protein exerts concentration-dependent mitogenic effects on beta-like cells. We propose that the PDZ domain proteins Bridge-1 and PDZD2 likely transduce signals that regulate insulin production, proliferation, and survival of pancreatic beta cells.

Efforts to cure diabetes are now focused on restoring a physiologically-regulated population of insulinproducing cells to the patient. A number of animal models of β cell regeneration have been employed to study the mechanisms of the process. Islet neogenesis, the regeneration of pancreatic islets from pancreatic stem cells, is arguably the least fraught with barriers to widespread use as a therapy for diabetes. These animal models have led to the description of the reg family of proteins that appear to be related to islet regeneration. Islet neogenesis-associated protein (INGAP) is an initiator of islet neogenesis in animal models and a peptide sequence from INGAP carries the biological activity. INGAP peptide has been shown to stimulate an increase in β cell mass in mice, rats, hamsters and dogs. INGAP is also found in the pancreas in human pathological states involving islet neogenesis. The peptide has been tested in human clinical trials, with success being reported. The evidence points to INGAP as a major factor in stimulating islet neogenesis, and, therefore, may play a significant therapeutic role in diabetes.

This review article focuses on the therapeutic potential of the incretin hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), in treating type 2 diabetes mellitus (T2DM). T2DM is characterized by insulin resistance, impaired glucose-induced insulin secretion and inappropriately regulated glucagon secretion which in combination eventually result in hyperglycemia and in the longer term microvascular and macrovascular diabetic complications. Traditional treatment modalities - even multidrug approaches - for T2DM are often unsatisfactory at getting patients to glycemic goals as the disease progresses due to a steady, relentless decline in pancreatic beta-cell function. Furthermore, current treatment modalities are often limited by inconvenient dosing regimens, safety and tolerability issues, the latter including hypoglycemia, body weight gain, edema and gastrointestinal side effects. Therefore, the actions of GLP-1 and GIP, which include potentation of meal-induced insulin secretion and trophic effects on the betacell, have attracted a lot of interest. GLP-1 also inhibits glucagon secretion, and suppresses food intake and appetite. Two new drug classes based on the actions of the incretin hormones have recently been approved for therapy of T2DM; injectable long-acting stable analogues of GLP-1, incretin mimetics, and orally available inhibitors of dipeptidyl peptidase 4 (DPP4; the enzyme responsible for the rapid degradation of GLP-1 and GIP), the so-called incretin enhancers. This review article focuses on these two new classes of antidiabetic agents and will outline the scientific basis for the development of incretin mimetics and incretin enhancers, review clinical experience gathered so far and discuss future expectations for incretin-based therapy.

Type 2 diabetes is closely related to abdominal obesity and is generally associated with other cardiometabolic risk factors, resulting in a high incidence of cardiovascular complications. Several animal and human observations suggest that the endocannabinoid (EC) system is overactivated in presence of abdominal obesity and/or diabetes, and contributes to disturbances of energy balance and metabolism. Not only it regulates the intake of nutrients through central mechanisms located within the hypothalamus and limbic area, but it also intervenes in transport, metabolism and deposit of the nutrients in the digestive tract, liver, adipose tissue, skeletal muscle, and possibly pancreas. Activation of both central and peripheral CB1 receptors promotes weight gain and associated metabolic changes. Conversely, rimonabant, the first selective CB1 receptor antagonist in clinical use, has been shown to reduce body weight, waist circumference, triglycerides, blood pressure, insulin resistance and C-reactive protein levels, and to increase HDL cholesterol and adiponectin concentrations in both non-diabetic and diabetic overweight/obese patients. In addition, a 0.5-0.7% reduction in glycated hemoglobin (HbA1c) levels was observed in metformin- or sulfonylurea-treated patients with type 2 diabetes and in drug-naive or insulin-treated diabetic patients. Almost half of metabolic changes occurred beyond weight loss, in agreement with direct peripheral effects. Rimonabant was generally well-tolerated, but with a slightly higher incidence of depressed mood disorders, anxiety, nausea and dizziness compared to placebo. New trials are supposed to confirm the potential role of rimonabant (and other CB1 neutral antagonists or inverse agonists) in overweight/obese patients with type 2 diabetes and high risk cardiovascular disease.

Angiotensin II (Ang II) is well-known as a systemic vasoconstrictor but recently a novel role for the peptide in endocrine function has been suggested and it has been linked to the pathophysiology of type 2 diabetes mellitus. According to several large-scale clinical studies, blocking Ang II prevented the onset of type 2 diabetes in potential patients. Type 2 diabetes is a complicated disease that is primarily characterized by insulin resistance and relative insulin deficiency mediated by numerous organs. Among these organs, the pancreas, adipose tissue, skeletal muscle and liver are the most prominent in maintaining glucose homeostasis. Interestingly, locally generated Ang II has been identified in these organs, where it plays different physiological roles and is produced in relatively high amounts with significant function. In type 2 diabetic human patients or animal models, Ang II, its generating enzymes and receptors are upregulated and trigger detrimental effects. Moreover, Ang II seems to play roles in the regulation of insulin secretion by the pancreatic β-cell and insulin sensitivity by peripheral tissues, which are two critical factors contributing to the development of type 2 diabetes. Accordingly, inhibiting Ang II produced beneficial effects on individual organs and throughout the body. Therefore, the present review discusses the role of Ang II in particular organs during normal physiological conditions as well as in type 2 diabetes.

The heat shock proteins (HSPs), originally identified as heat-inducible gene products, are a highly conserved family of proteins that respond to a wide variety of stress. Although HSPs are among the most abundant intracellular proteins, they are expressed at low levels under normal physiological conditions, and show marked induction in response to various stressors. HSPs function primarily as molecular chaperones, facilitating the folding of other cellular proteins, preventing protein aggregation, or targeting improperly folded proteins to specific pathways for degradation. By modulating inflammation, wound debris clearance, cell proliferation, migration and collagen synthesis, HSPs are essential for normal wound healing of the skin. In this review, our goal is to discuss the role and clinical implications of HSP with respect to skin wound healing and diabetes. The numerous defects in the function of HSPs associated with diabetes could contribute to the commonly observed complications and delayed wound healing in diabetics. Several physical, pharmacological and genetic approaches may be considered to address HSP-directed therapies both in the laboratory and in the clinics.

Resistin is a potential link between obesity and insulin resistance or type 2 diabetes. In rodents, resistin is primarily expressed in and secreted from mature adipocytes, with some expression in pancreatic islets and portions of the pituitary and hypothalamus. Its secretion can be up-regulated by several factors, including insulin and glucose. The exposure of rodents, or their cells, to resistin results in decreased response to insulin. This is likely in part due to an upregulation of suppressor of cytokine signaling (SOCS)-3, which interferes with the activation of insulin receptor substrate (IRS)-1. However, in humans resistin is expressed primarily by macrophages and seems to be involved in the recruitment of other immune cells and the secretion of pro-inflammatory factors, including tumor necrosis factor (TNF)α. Human resistin may interfere with insulin signaling by stimulating the expression of phosphatase and tensin homolog deleted on chromosome ten (PTEN), which dephosphorylates 3-phosphorylated phosphoinositide (PIP3). Resistin also seems to be involved in the development of atherosclerosis in humans by promoting the formation of foam cells and the proliferation and migration of vascular endothelial and smooth muscle cells. Many of the inflammatory related functions of human resistin appear to be regulated by activation of the nuclear factor (NF)κB transcription factor. The divergent roles of resistin in humans and rodents are evident by the data presented in this review but they will not be able to be fully understood until the resistin receptor is identified.