A GIP Receptor Agonist Exhibits β-Cell Anti-Apoptotic Actions in Rat Models of Diabetes Resulting in Improved β-Cell Function and Glycemic Control

The gastrointestinal hormone GIP promotes pancreatic islet function and exerts pro-survival actions on cultured beta-cells. However, GIP also promotes lipogenesis, thus potentially restricting its therapeutic use. The current studies evaluated the effects of a truncated GIP analog, D-Ala(2)-GIP(1-30) (D-GIP(1-30)), on glucose homeostasis and beta-cell mass in rat models of diabetes.The insulinotropic and pro-survival potency of D-GIP(1-30) was evaluated in perfused pancreas preparations and cultured INS-1 beta-cells, respectively, and receptor selectivity evaluated using wild type and GIP receptor knockout mice. Effects of D-GIP(1-30) on beta-cell function and glucose homeostasis, in vivo, were determined using Lean Zucker rats, obese Vancouver diabetic fatty rats, streptozotocin treated rats, and obese Zucker diabetic fatty rats, with effects on beta-cell mass determined in histological studies of pancreatic tissue. Lipogenic effects of D-GIP(1-30) were evaluated on cultured 3T3-L1 adipocytes.Acutely, D-GIP(1-30) improved glucose tolerance and insulin secretion. Chronic treatment with D-GIP(1-30) reduced levels of islet pro-apoptotic proteins in Vancouver diabetic fatty rats and preserved beta-cell mass in streptozotocin treated rats and Zucker diabetic fatty rats, resulting in improved insulin responses and glycemic control in each animal model, with no change in body weight. In in vitro studies, D-GIP(1-30) exhibited equivalent potency to GIP(1-42) on beta-cell function and survival, but greatly reduced action on lipoprotein lipase activity in 3T3-L1 adipocytes.These findings demonstrate that truncated forms of GIP exhibit potent anti-diabetic actions, without pro-obesity effects, and that the C-terminus contributes to the lipogenic actions of GIP

The pro-survival effects of glucose-dependent insulinotropic polypeptide receptor signalling on beta cells

Gastrointestinal incretin hormones glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) have been targeted for the treatment of diabetes due, in large part, to their physiological actions on pancreatic β-cells in potentiating glucose-stimulated insulin secretion and increasing insulin biosynthesis. In recent years, incretins have also been revealed to exert potent actions on β-cell proliferation and survival. However, most studies have focused on GLP-1, and so there is a relative paucity of evidence regarding GIP. In the current thesis, it was hypothesized that GIP receptor (GIPR) activation in β-cells enhances critical anti-apoptotic signalling networks and promotes β-cell survival, which in rodent models of type 2 diabetes mellitus results in an elevation in β-cell mass and improvement in glycaemic control. This hypothesis was initially tested by examining the effects of GIPR activation on the protein-serine/threonine kinase Akt, which is known to exert potent pro-survival actions in β-cells. It was revealed that GIPR stimulation activates Akt signalling through both canonical and non-canonical mechanisms. Next, the effects of GIPR activation on pro- and anti-apoptotic Bcl-2 family proteins were examined on β-cells under pro-apoptotic stress. GIPR activation was found to suppress the onset of stress-induced apoptosis by preventing mitochondrial translocation of pro-apoptotic proteins Bad and Bim as well as activation of the key pro-apoptotic protein Bax, and this effect was due to Akt-mediated inhibition of the apoptosis signal regulating kinase-1. Lastly, the effects of GIPR activation on β-cell survival were examined in vivo by administering a long acting GIP analogue to multiple rodent models of diabetes that exhibit elevated β-cell apoptosis. In all models the GIP analogue promoted β-cell survival and improved glycaemic control. Overall, these studies supported the hypothesis proposed. In conclusion, similar to the signalling actions of the GLP-1 receptor, GIPR signalling exerts potent actions on β-cell survival, and therefore, therapeutics that enhance GIPR signalling in β-cells merit consideration for the treatment of type 2 diabetes mellitus.Medicine, Faculty ofCellular and Physiological Sciences, Department ofGraduat

Brown adipose tissue thermogenic adaptation requires Nrf1-mediated proteasomal activity

Objective Brown adipose tissue (BAT) generates heat in response to cold, and low BAT activity has been linked to obesity. However, recent studies were inconclusive as to whether BAT is involved in diet‐induced thermogenesis and mitigates weight gain from prolonged overeating. Therefore, this study investigated whether BAT activity is related to metabolic adaptation arising from 8 weeks of overfeeding in humans. Methods Fourteen men (aged 24 ± 3 years, BMI 24.5 ± 1.6 kg/m2) were overfed by 40% for 8 weeks. Before and after, energy expenditure and metabolic adaptation were measured by whole‐room respiratory calorimetry. A marker of BAT activity was measured using infrared imaging of the supraclavicular BAT depot. Results At the end of 8 weeks of overfeeding, metabolic adaptation—defined as the percent increase in sleeping energy expenditure beyond that expected from weight gain—rose from −0.9 ± 3.9% to 4.7 ± 5.6% (P = 0.001). However, BAT thermal activity was unchanged (P = 0.81). Moreover, BAT thermal activity did not correlate with the degree of metabolic adaptation (P = 0.32) or with the change in body weight (P = 0.51). Conclusions BAT thermal activity does not change in response to overfeeding, nor does it correlate with adaptive thermogenesis. Our data suggest that BAT does not mediate metabolic adaptation to overeating in humans

Suppression of p38 MAPK and JNK via Akt-mediated Inhibition of Apoptosis Signal-regulating Kinase 1 Constitutes a Core Component of the β-Cell Pro-survival Effects of Glucose-dependent Insulinotropic Polypeptide*

Glucose-dependent insulinotropic polypeptide (GIP) potentiates glucose-stimulated insulin secretion, insulin biosynthesis, and β-cell proliferation and survival. In previous studies GIP was shown to promote β-cell survival by modulating the activity of multiple signaling modules and regulating gene transcription of pro- and anti-apoptotic bcl-2 family proteins. We have now evaluated the mechanisms by which GIP regulates the dynamic interactions between cytoplasmic bcl-2 family members and the mitochondria in INS-1 cells during apoptosis induced by treatment with staurosporine (STS), an activator of the mitochondria-mediated apoptotic pathway. STS induced translocation of bad and bimEL, activation of mitochondrial bax, release of mitochondrial cytochrome c, cleavage of caspase-3, and apoptosis. Each response was significantly diminished by GIP. Using selective enzyme inhibitors, overexpression of dominant-negative Akt, and Akt siRNA, it was demonstrated that GIP promoted β-cell survival via Akt-dependent suppression of p38 MAPK and JNK and that combined inhibition was sufficient to explain the entire pro-survival responses to GIP during STS treatment. This signaling pathway also explained the pro-survival effects of GIP on INS-1 cells exposed to two other promoters of stress: thapsigargin (endoplasmic reticulum stress) and etoposide (genotoxic stress). Importantly, we discovered that GIP suppressed p38 MAPK and JNK via Akt-mediated changes in the phosphorylation state of the apoptosis signal-regulating kinase 1 in INS-1 cells and human islets, resulting in inhibition of its activity. Inhibition of apoptosis by GIP is therefore mediated via a key pathway involving Akt-dependent inhibition of apoptosis signal-regulating kinase 1, which subsequently prevents the pro-apoptotic actions of p38 MAPK and JNK

Noncanonical Activation of Akt/Protein Kinase B in β-Cells by the Incretin Hormone Glucose-dependent Insulinotropic Polypeptide*

Therapeutics based on the actions of the incretin hormones, glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), have recently been introduced for the treatment of type 2 diabetes mellitus. The serine/threonine kinase Akt is a major mediator of incretin action on the pancreatic islet, increasing β-cell mass and function and promoting β-cell survival. The mechanisms underlying incretin activation of Akt are thought to involve an essential phosphoinositide 3-kinase-mediated phosphorylation of threonine 308, similar to the prototypical Akt activator, insulin-like growth factor-I (IGF-I). In this study, using activity assays on immunoprecipitated Akt, we discovered that GIP and GLP-1 were capable of stimulating Akt in the INS-1 β-cell line and isolated mouse islets via a mechanism that did not require phosphoinositide 3-kinase or phosphorylation of Thr308 and Ser473, and this pathway involved the production of cAMP. Furthermore, we found that GIP stimulated anti-apoptotic signaling via this alternate mode of Akt activation. We conclude that incretins can activate Akt via a novel noncanonical mechanism that may provide an alternative therapeutic target for the treatment of type 2 diabetes mellitus and have broader implications for Akt physiology in human health and disease