High glucose exposure promotes activation of protein phosphatase 2A in rodent islets and INS-1 832/13 ß-cells by increasing the posttranslational carboxylmethylation of its catalytic subunit.

Existing evidence implicates regulatory roles for protein phosphatase 2A (PP2A) in a variety of cellular functions, including cytoskeletal remodeling, hormone secretion, and apoptosis. We report here activation of PP2A in normal rat islets and insulin-secreting INS-1 832/13 cells under the duress of hyperglycemic (HG) conditions. Small interfering RNA-mediated knockdown of the catalytic subunit of PP2A (PP2Ac) markedly attenuated glucose-induced activation of PP2A. HG, but not nonmetabolizable 3-O-methyl glucose or mannitol (osmotic control), significantly stimulated the methylation of PP2Ac at its C-terminal Leu-309, suggesting a novel role for this posttranslational modification in glucose-induced activation of PP2A. Moreover, knockdown of the cytosolic leucine carboxymethyl transferase 1 (LCMT1), which carboxymethylates PP2Ac, significantly attenuated PP2A activation under HG conditions. In addition, HG conditions, but not 3-O-methyl glucose or mannitol, markedly increased the expression of LCMT1. Furthermore, HG conditions significantly increased the expression of B55α, a regulatory subunit of PP2A, which has been implicated in islet dysfunction under conditions of oxidative stress and diabetes. Thapsigargin, a known inducer of endoplasmic reticulum stress, failed to exert any discernible effects on the carboxymethylation of PP2Ac, expression of LCMT1 and B55α, or PP2A activity, suggesting no clear role for endoplasmic reticulum stress in HG-induced activation of PP2A. Based on these findings, we conclude that exposure of the islet ß-cell to HG leads to accelerated PP2A signaling pathway, leading to loss in glucose-induced insulin secretion.

Hyperactivation of protein phosphatase 2A in models of glucolipotoxicity and diabetes: potential mechanisms and functional consequences.

The protein phosphatase 2A [PP2A] family of enzymes has been implicated in the regulation of a variety of cellular functions including hormone secretion, growth, survival and apoptosis. PP2A accounts for ~1% of total cellular protein and ∼ 80% of total serine/threonine phosphatases, thus representing a major class of protein phosphatases in mammalian cells. Despite significant advances in our current understanding of regulation of cellular function by PP2A under physiological conditions, little is understood with regard to its regulation under various pathological conditions, such as diabetes. Emerging evidence suggests hyperactivation of PP2A in liver, muscle, retina and the pancreatic islet under the duress of glucolipotoxicity and diabetes. Interestingly, pharmacological inhibition of PP2A or siRNA-mediated depletion of the catalytic subunit of PP2A [PP2Ac] levels largely restored PP2A activity to near normal levels under these conditions. Herein, we provide an overview of PP2A subunit expression and activity in in vitro and in vivo models of glucolipotoxicity and diabetes, and revisit the existing data, which are suggestive of alterations in post-translational methylation, phosphorylation and nitration of PP2Ac under these conditions. Potential significance of hyperactive PP2A in the context of cell function, survival and apoptosis is also highlighted. It is hoped that this commentary will provide a basis for future studies to explore the potential for PP2Ac as a therapeutic target for the treatment of diabetes and other metabolic disorders.

Protein farnesylation is requisite for mitochondrial fuel-induced insulin release: further evidence to link reactive oxygen species generation to insulin secretion in pancreatic ß-cells.

Several lines of recent evidence implicate regulatory roles for reactive oxygen species (ROS) in islet function and insulin secretion. The phagocyte-like NADPH oxidase (Nox2) has recently been shown to be one of the sources of ROS in the signaling events leading to glucose stimulated insulin secretion (GSIS). We recently reported inhibition of glucose- or mitochondrial fuel-induced Nox2-derived ROS by a specific inhibitor of protein farnesyl transferse (FTase; FTI-277), suggesting that activation of FTase might represent one of the upstream signaling events to Nox2 activation. Furthermore, FTase inhibitors (FTI-277 and FTI-2628) have also been shown to attenuate GSIS in INS 832/13 cells and normal rodent islets. Herein, we provide further evidence to suggest that inhibition of FTase either by pharmacological (e.g., FTI-277) or gene silencing (siRNA-FTase) approaches markedly attenuates mitochondrial fuel-stimulated insulin secretion (MSIS) in INS 832/13 cells. Together, our findings further establish a link between nutrient-induced Nox2 activation, ROS generation and insulin secretion in the pancreatic ß-cell.