Palmitate impairs and eicosapentaenoate restores insulin secretion through regulation of SREBP-1c in pancreatic islets

Abstract

OBJECTIVE-Chronic exposure to fatty acids causes (β-cell failure, often referred to as lipotoxicity. We investigated its mechanisms, focusing on contribution of SREBP-1c, a key transcription factor for lipogenesis. RESEARCH DESIGN AND METHODS-We studied in vitro and in vivo effects of saturated and polyunsaturated acids on insulin secretion, insulin signaling, and expression of genes involved in (β-cell functions. Pancreatic islets isolated from C57BL/6 control and SREBP-1-null mice and adenoviral gene delivery or knockdown systems of related genes were used. RESULTS-Incubation of C57BL/6 islets with palmitate caused inhibition of both glucose- and potassium-stimulated insulin secretion, but addition of eicosapentaenoate (EPA) restored both inhibitions. Concomitantly, palmitate activated and EPA abolished both mRNA and nuclear protein of SREBP-lc, accompanied by reciprocal changes of SREBP-lc target genes such as insulin receptor substrate-2 (IRS-2) and granuphilin. These palmitate-EPA effects on insulin secretion were abolished in SREBP-l-null islets. Suppression of IRS-2/Akt pathway could be a part of the downstream mechanism for the SREBP-lc-mediated insulin secretion defect because adenoviral constitutively active Akt compensated it. Uncoupling protein-2 (UCP-2) also plays a crucial role in the palmitate inhibition of insulin secretion, as confirmed by knockdown experiments, but SREBP-lc contribution to UCP-2 regulation was partial. The palmitate-EPA regulation of insulin secretion was similarly observed in islets from C57BL/6 mice pretreated with dietary manipulations. Furthermore, administration of EPA to diabetic KK-Ay mice ameliorated impairment of insulin secretion in their islets. CONCLUSIONS-SREBP-lc plays a dominant role in palmitate- mediated insulin secretion defect, and EPA prevents it through SREBP-lc inhibition, implicating a therapeutic potential for treating diabetes related to lipotoxicity. © 2008 by the American Diabetes Association.