Cholesterol accumulation and diabetes in pancreatic b-cell-specific SREBP-2 transgenic mice: a new model for lipotoxicity

To determine the role of cholesterol synthesis in pancreatic b-cells, a transgenic model of in vivo activation of sterol-regulatory element binding protein 2 (SREBP-2) specifically in b-cells (TgRIP-SREBP-2) was developed and analyzed. Expression of nuclear human SREBP-2 in b-cells resulted in severe diabetes as evidenced by greater than 5-fold elevations in glycohemoglobin compared with C57BL/6 controls. Diabetes in TgRIP-SREBP-2 mice was primarily due to defects in glucoseand potassium-stimulated insulin secretion as determined by glucose tolerance test. Isolated islets of TgSREBP-2 mice were fewer in number, smaller, deformed, and had decreased insulin content. SREBP-2-expressing islets also contained increased esterified cholesterol and unchanged triglycerides with reduced ATP levels. Consistently, these islets exhibited elevated expression of HMG-CoA synthase and reductase and LDL receptor, with suppression of endogenous SREBPs. Genes involved in b-cell differentiation, such as PDX1 and BETA2, were suppressed, explaining loss of b-cell mass, whereas IRS2 expression was not affected. These phenotypes were dependent on the transgene expression. Taken together, these results indicate that activation of SREBP-2 in b-cells caused severe diabetes by loss of b-cell mass with accumulation of cholesterol, providing a new lipotoxic model and a potential link of disturbed cholesterol metabolism to impairment of b-cell function.—Ishikawa, M., Y. Iwasaki, S. Yatoh, T. Kato, S. Kumadaki, N. Inoue, T. Yamamoto, T. Matsuzaka, Y. Nakagawa, N. Yahagi, K. Kobayashi, A. Takahashi, N. Yamada, and H. Shimano. Cholesterol accumulation and diabetes in pancreatic b-cell-specific SREBP-2 transgenic mice: a new model for lipotoxicity. J. Lipid Res. 2008. 49: 2524–2534. Supplementary key words transcription factors • sterol-regulatory element binding protein • triglycerides Molecular mechanisms of pancreatic islet b-cell failure, a crucial pathological contributor to the development of diabetes mellitus, have been extensively explored (1). Impairment of glucose-stimulated insulin secretion (GSIS) is an early feature of type 2 diabetes. Chronic influx of FAs into b-cells, i.e., b-cell lipotoxicity, has been thought to be involved in its pathogenesis (2). Sterol-regulatory element binding protein 1c (SREBP-1c) is a membrane-bound transcription factor of the basic HLH (bHLH) leucine zipper family and has been established as a nutritional regulator of lipogenic enzymes in the liver (3, 4). Expression of SREBP-1c is highly upregulated by dietary intake of carbohydrates, sugars, and saturated FAs, whereas PUFAs, such as eicosapentaenoic acid, have been shown to inhibit hepatic SREBP-1c through multiple mechanisms (5, 6). These nutritional regulations of SREBP-1c are also observed in a cultured b-cell line and in isolated islets of mice (7, 8). SREBP-1c also plays a role in insulin signaling by inhibiting insulin receptor substrate 2 (IRS-2), the major insulin-signaling mediator in the liver and in b-cells (9, 10). As a model for lipotoxicity by endogenous FAs in pancreatic b-cells, we previously developed transgenic mice overexpressing the active form of SREBP-1c under the insulin promoter expression (10). These mice exhibited impaired glucose tolerance in vivo due to both decreased b-cell mass and impaired insulin secretion estimated in isolated islets, which was enhanced by feeding the mice a high-fat, high-sucrose diet. The SREBP-1c-overexpressing islets had ATP depletion caused by enhanced lipogenesis and increased uncoupling protein 2 (UCP-2). Explaining the loss of b-cell mass, these islets had decreased expression of IRS-2 and PDX1. In addition to inhibition of GSIS, This work was supported by grants-in-aid from the Ministry of Science, Education, Culture, and Technology of Japan. Manuscript received 8 May 2008 and in revised form 15 July 2008 and in re-revised form 1 August 2008. Published, JLR Papers in Press, August 8, 2008. DOI 10.1194/jlr.M800238-JLR200 1 To whom correspondence should be addressed. e-mail: [email protected] M. Ishikawa and Y. Iwasaki contributed equally to this work. Copyright © 2008 by the American Society for Biochemistry and Molecular Biology, Inc. 2524 Journal of Lipid Research Volume 49, 2008 This article is available online at http://www.jlr.org by gest, on O cber 1, 2017 w w w .j.org D ow nladed fom SREBP-1c transgenic islets exhibited decreased potassiumstimulated insulin secretion (KSIS), which could indicate dysfunction in a process of insulin secretion following ATP production. We have recently found that granuphilin, an effecter of Rab27 involving exocytosis of insulin granules, is an SREBP target and contributes at least partially to the impaired insulin secretion of islets with SREBP-1c overexpression and to diabetes (11). Conversely, islets isolated from SREBP-1-null mice exhibited increased basal insulin secretion and immunity to the impairment in insulin secretion caused by saturated FAs, establishing the contribution of SREBP-1c to lipotoxicity (7). SREBP-2 is another member of the membrane-bound transcription factor SREBP family, and plays a crucial role in the regulation of cholesterol metabolism (12). Cellular sterol levels are controlled by feedback regulation of cholesterol biosynthetic and LDL receptor pathways. Genes in these pathways are transcriptionally regulated by SREBP-2 activation, which requires cleavage of its N-terminal bHLH portion for nuclear translocation to activate promoters of genes such as HMG-CoA reductase and synthase as well as the LDL receptor gene (13–15). FA metabolism has been well-studied with regard to its contribution to b-cell lipotoxicity; however, cholesterol metabolism and the role of SREBP-2 in b-cells have not been fully investigated. Our current study investigated whether perturbation in sterol regulation by SREBP-2 could be involved in impaired insulin secretion and b-cell function.