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Endocrinology. 2014 Feb;155(2):380-91. doi: 10.1210/en.2013-1773. Epub 2013 Nov 21.

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

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  • 1Beta-Cell Biochemistry Laboratory (D.K.A., A.K.), John D. Dingell Veterans Affairs Medical Center, and Department of Pharmaceutical Sciences (D.K.A., A.K.), Eugene Applebaum College of Pharmacy and Health Sciences, Wayne State University, Detroit, Michigan 48201; Section of Endocrinology, Diabetes and Metabolism (B.M.), University of Illinois at Chicago, Chicago, Illinois 60612; Department of Engineering and Science (A.M.), University of Detroit Mercy, Detroit, Michigan 48221; Department of Pharmacology (B.E.W.), Vanderbilt University Medical Center, Nashville, Tennessee 37232; and Department of Cell, Developmental, and Integrative Biology (S.R.), University of Alabama at Birmingham, Birmingham, Alabama 35294.

Abstract

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 β-cell to HG leads to accelerated PP2A signaling pathway, leading to loss in glucose-induced insulin secretion.

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