Noncanonical CDK4 signaling rescues diabetes in a mouse model by promoting ß cell differentiation.

Expanding ß cell mass is a critical goal in the fight against diabetes. CDK4, an extensively characterized cell cycle activator, is required to establish and maintain ß cell number. ß cell failure in the IRS2-deletion mouse type 2 diabetes model is, in part, due to loss of CDK4 regulator cyclin D2. We set out to determine whether replacement of endogenous CDK4 with the inhibitor-resistant mutant CDK4-R24C rescued the loss of ß cell mass in IRS2-deficient mice. Surprisingly, not only ß cell mass but also ß cell dedifferentiation was effectively rescued, despite no improvement in whole body insulin sensitivity. Ex vivo studies in primary islet cells revealed a mechanism in which CDK4 intervened downstream in the insulin signaling pathway to prevent FOXO1-mediated transcriptional repression of critical ß cell transcription factor Pdx1. FOXO1 inhibition was not related to E2F1 activity, to FOXO1 phosphorylation, or even to FOXO1 subcellular localization, but rather was related to deacetylation and reduced FOXO1 abundance. Taken together, these results demonstrate a differentiation-promoting activity of the classical cell cycle activator CDK4 and support the concept that ß cell mass can be expanded without compromising function.

Angiotensin II Inhibits Insulin Receptor Signaling in Adipose Cells.

Angiotensin II (Ang II) is a critical regulator of insulin signaling in the cardiovascular system and metabolic tissues. However, in adipose cells, the regulatory role of Ang II on insulin actions remains to be elucidated. The effect of Ang II on insulin-induced insulin receptor (IR) phosphorylation, Akt activation, and glucose uptake was examined in 3T3-L1 adipocytes. In these cells, Ang II specifically inhibited insulin-stimulated IR and insulin receptor substrate-1 (IRS-1) tyrosine-phosphorylation, Akt activation, and glucose uptake in a time-dependent manner. These inhibitory actions were associated with increased phosphorylation of the IR at serine residues. Interestingly, Ang II-induced serine-phosphorylation of IRS was not detected, suggesting that Ang II-induced desensitization begins from IR regulation itself. PKC inhibition by BIM I restored the inhibitory effect of Ang II on insulin actions. We also found that Ang II promoted activation of several PKC isoforms, including PKCα/ßI/ßII/δ, and its association with the IR, particularly PKCßII, showed the highest interaction. Finally, we also found a similar regulatory effect of Ang II in isolated adipocytes, where insulin-induced Akt phosphorylation was inhibited by Ang II, an effect that was prevented by PKC inhibitors. These results suggest that Ang II may lead to insulin resistance through PKC activation in adipocytes.

Living Dangerously: Protective and Harmful ER Stress Responses in Pancreatic ß-Cells.

Type 2 diabetes (T2D) is a growing cause of poor health, psychosocial burden, and economic costs worldwide. The pancreatic ß-cell is a cornerstone of metabolic physiology. Insulin deficiency leads to hyperglycemia, which was fatal before the availability of therapeutic insulins; even partial deficiency of insulin leads to diabetes in the context of insulin resistance. Comprising only an estimated 1 g or <1/500th of a percent of the human body mass, pancreatic ß-cells of the islets of Langerhans are a vulnerable link in metabolism. Proinsulin production constitutes a major load on ß-cell endoplasmic reticulum (ER), and decompensated ER stress is a cause of ß-cell failure and loss in both type 1 diabetes (T1D) and T2D. The unfolded protein response (UPR), the principal ER stress response system, is critical for maintenance of ß-cell health. Successful UPR guides expansion of ER protein folding capacity and increased ß-cell number through survival pathways and cell replication. However, in some cases the ER stress response can cause collateral ß-cell damage and may even contribute to diabetes pathogenesis. Here we review the known beneficial and harmful effects of UPR pathways in pancreatic ß-cells. Improved understanding of this stress response tipping point may lead to approaches to maintain ß-cell health and function.

Autonomous activation of CaMKII exacerbates diastolic calcium leak during beta-adrenergic stimulation in cardiomyocytes of metabolic syndrome rats.

Autonomous Ca2+/calmodulin-dependent protein kinase II (CaMKII) activation induces abnormal diastolic Ca2+ leak, which leads to triggered arrhythmias in a wide range of cardiovascular diseases, including diabetic cardiomyopathy. In hyperglycemia, Ca2+ handling alterations can be aggravated under stress conditions via the ß-adrenergic signaling pathway, which also involves CaMKII activation. However, little is known about intracellular Ca2+ handling disturbances under ß-adrenergic stimulation in cardiomyocytes of the prediabetic metabolic syndrome (MetS) model with obesity, and the participation of CaMKII in these alterations. MetS was induced in male Wistar rats by administering 30 % sucrose in drinking water for 16 weeks. Fluo 3-loaded MetS cardiomyocytes exhibited augmented diastolic Ca2+ leak (in the form of spontaneous Ca2+ waves) under basal conditions and that Ca2+ leakage was exacerbated by isoproterenol (ISO, 100 nM). At the molecular level, [3H]-ryanodine binding and basal phosphorylation of cardiac ryanodine receptor (RyR2) at Ser2814, a CaMKII site, were increased in heart homogenates of MetS rats with no changes in RyR2 expression. These alterations were not further augmented by Isoproterenol. SERCA pump activity was augmented 48 % in MetS hearts before ß-adrenergic stimuli, which is associated to augmented PLN phosphorylation at T17, a target of CaMKII. In MetS hearts. CaMKII auto-phosphorylation (T287) was increased by 80 %. The augmented diastolic Ca2+ leak was prevented by CaMKII inhibition with AIP. In conclusion, CaMKII autonomous activation in cardiomyocytes of MetS rats with central obesity significantly contributes to abnormal diastolic Ca2+ leak, increasing the propensity for ß-adrenergic receptor-driven lethal arrhythmias.

Metabolic syndrome diminishes insulin-induced Akt activation and causes a redistribution of Akt-interacting proteins in cardiomyocytes.

Metabolic syndrome (MetS) is a cluster of cardiometabolic risk factors, with insulin resistance as a critical component for its development. Insulin signaling in the heart leads to Akt (also known as PKB) activation, a serine/threonine protein kinase, which regulates cardiac glucose metabolism and growth. Cardiac metabolic inflexibility, characterized by impaired insulin-induced glucose uptake and oxidation, has been reported as an early and consistent change in the heart of different models of MetS and diabetes; however, the evaluation of Akt activation has yielded variable results. Here we report in cardiomyocytes of MetS rats, diminished insulin-induced glucose uptake and Akt activation, evaluated by its impaired mobilization towards the plasma membrane and phosphorylation, and reflected in a re-distribution of its interacting proteins, assessed by label-free mass spectrometry (data are available via ProteomeXchange with identifier PXD013260). We report 45 proteins with diminished abundance in Akt complex of MetS cardiomyocytes, mainly represented by energy metabolism-related proteins, and also, 31 Akt-interacting proteins with increased abundance, which were mainly related to contraction, endoplasmic reticulum stress, and Akt negative regulation. These results emphasize the relevance of Akt in the regulation of energy metabolism in the heart and highlight Akt-interacting proteins that could be involved in the detrimental effects of MetS in the heart.