Current Diabetes Reviews - Volume 9, Issue 2, 2013

Glucagon like peptide-1 (GLP-1) is one of the gastrointestinal peptides implicated in glycaemic homeostasis. In non-obese individuals with normal glucose tolerance GLP-1 is secreted in response to nutrient intake. However, this GLP- 1 response is generally accepted to be significantly diminished in those with diabetes, obesity or both. Given that GLP-1 is secreted from enteroendocrine L cells in the intestine, it is not surprising that manipulation of the gastro- intestinal tract has been shown to alter GLP-1 secretion; particularly when this intestinal manipulation is designed to aid weight reduction. GLP-1 dynamics are altered by bariatric surgery, with an improved secretory response to nutrient intake. However, there remains debate on the mechanisms responsible for the alterations in GLP-1 dynamics. Here we review the evidence for GLP-1 dynamics after Roux-en-Y gastric bpyass (RYGB), adjustable gastric banding (AGB), biliopancreatic diversion (BPD) and sleeve gastrectomy (SG), and make comparisons between modalities. In addition, we review the potential mechanisms underlying these dynamics, other molecules that may add to the “incretin effect” and other possible roles for GLP-1 following bariatric surgery. Finally, we will offer our critique of the evidence base.

Mainly known for its role in peripheral glucose homeostasis, insulin has also significant impact within the brain, functioning as a key neuromodulator in behavioral, cellular, biochemical and molecular studies. The brain is now regarded as an insulin-sensitive organ with widespread, yet selective, expression of the insulin receptor in the olfactory bulb, hypothalamus, hippocampus, cerebellum, amygdala and cerebral cortex. Insulin receptor signaling in the brain is important for neuronal development, glucoregulation, feeding behavior, body weight, and cognitive processes such as with attention, executive functioning, learning and memory. Emerging evidence has demonstrated insulin receptor signaling to be impaired in several neurological disorders. Moreover, insulin receptor signaling is recognized as important for dendritic outgrowth, neuronal survival, circuit development, synaptic plasticity and postsynaptic neurotransmitter receptor trafficking. We review the multiple roles of insulin in the brain, as well as its endogenous trafficking to the brain or its exogenous intervention. Although insulin can be directly targeted to the brain via intracerebroventricular (ICV) or intraparenchymal delivery, these invasive techniques are with significant risk, necessitating repeated surgical intervention and providing potential for systemic hypoglycemia. Another method, intranasal delivery, is a non-invasive, safe, and alternative approach which rapidly targets delivery of molecules to the brain while minimizing systemic exposure. Over the last decades, the delivery of intranasal insulin in animal models and human patients has evolved and expanded, permitting new hope for associated neurodegenerative and neurovascular disorders.

Transient receptor potential channels, especially the members of the melastatin family (TRPM), participate in insulin secretion. Some of them are substrates for protein kinases, which are involved in several neurotransmitter, incretin and hormonal signaling cascades in β cells. The functional relationships between protein kinases and TRPM channels in systems of heterologous expression and native tissues rise issues about novel regulation pathways of pancreatic β-cell excitability. The aim of the present work is to review the evidences about phosphorylation of TRPM channels in β cells and to discuss the perspectives on insulin secretion.

During periods of extreme heat individuals with diabetes have greater rates of heat-related morbidity and mortality compared to their non-diabetic counterparts. The reason for this discrepancy is currently unknown. Furthermore, there is a lack of information about whether or not individuals with type 1 diabetes are at a thermoregulatory disadvantage during strenuous physical activity especially when performed in the heat. Purpose: This review discusses the current literature pertaining to thermoregulatory responses in individuals with type 1 diabetes. Methods: We included 14 reviews and 95 original research articles identified by searches of PubMed and Google Scholar and deemed relevant to our subject by three independent readers. Results: Individuals with poorly controlled type 1 diabetes may have impaired heat sensation, and a reduced capacity to dissipate heat due to lower skin blood flow and sweating responses and a greater tendency towards dehydration compared to individuals without diabetes. Impairments may be attenuated or absent in those with good blood glucose control. We found no published studies examining thermoregulatory responses to physical activity in the heat in individuals with type 1 diabetes. Conclusions: Type 1 diabetes may cause impairments in heat loss resulting in a greater level of thermal strain. Advancement in our understanding about the effects of type 1 diabetes on the heat stress response, especially during different challenges to human heat balance associated with changes in both environmental heat load and metabolic heat production (physical activity), will help us to determine where the risk of heat-illness/injury actually exists.

Nuclear erythroid factor 2 like 2 (Nrf2) has been described as a transcription factor that serves as a master regulator of the adaptive response to exogenous and endogenous oxidative and electrophilic stresses. Evidence of Nrf2 crosstalk with other molecular pathways is increasing; recent publications have proposed a role of Nrf2 in the development of obesity and in the highly regulated process of adipocyte differentiation through its interaction with other transcription factors and receptors implicated in metabolic regulation. In the present review, we discuss the available data on the possible role of Nrf2 in obesity and metabolic syndrome and the feasibility of using Nrf2 as a therapeutic target in the clinical setting.

Diabetic retinopathy (DR) is the leading cause of blindness amongst the working-age population, and diabetes accelerated cardiovascular disease (CVD) the commonest cause of death in diabetic patients. Although, there is evidence suggesting a close association between DR and CVD, particularly in patients with Type 2 diabetes, the pathophysiology underlying the link is unclear. Here we review common risk factors and pathogenic mechanisms linking DR and CVD, and aim to highlight the need for a more holistic view of the management of diabetes and its complications. The understanding of the link between the two complications could eventually lead to refined management strategies and improved patient outcomes in the expanding diabetes epidemic.

Type 2 diabetes (T2DM) is a disease of epidemic proportion associated with significant morbidity and excess mortality. Optimal glucose control reduces the risk of microvascular and possibly macrovascular complications due to diabetes. However, glycemic control is rarely optimal and several therapeutic interventions for the treatment of diabetes cause hypoglycemia and weight gain; some may exacerbate cardiovascular risk. Exenatide (synthetic exendin-4) is a glucagon- like peptide-1 receptor (GLP-1R) agonist developed as a first-in-class diabetes therapy. This review presents an overview of the evolution of exenatide as a T2DM treatment, beginning with the seminal preclinical discoveries and continuing through to clinical pharmacology investigations and phase 3 clinical trials. In patients with T2DM, exenatide enhanced glucose-dependent insulin secretion, suppressed inappropriately elevated glucagon secretion, slowed gastric emptying, and enhanced satiety. In controlled phase 3 clinical trials ranging from 12 to 52 weeks, 10-mcg exenatide twice daily (ExBID) reduced mean HbA1c by -0.8% to -1.7% as monotherapy or in combination with metformin (MET), sulfonylureas (SFU), and/or thiazolidinediones (TZD); with mean weight losses of -1.2 kg to -8.0 kg. In controlled phase 3 trials ranging from 24 to 30 weeks, a 2-mg once-weekly exenatide formulation (ExQW) reduced mean HbA1c by -1.3% to -1.9%, with mean weight reductions of -2.3 to -3.7 kg. Exenatide was generally well-tolerated. The most common side effects were gastrointestinal in nature, mild, and transient. Nausea was the most prevalent adverse event. The incidence of hypoglycemia was generally low. By building upon early observations exenatide was successfully developed into an effective diabetes therapy.