BS2-1 Sterol regulatory element-binding protein-1c activation is implicated in pancreatic β-cell lipotoxicity

Abstract

Objective: To elucidate the underlying molecular mechanisms of chronic free fatty acid (FFA) exposure on impaired pancreatic β-cell function and β-cell apoptosis. Methods: A INS-1E pancreatic β-cell line was exposed to palmitate or oleate, and glucose stimulated insulin secretion (GSIS) then measured. The effect of FFA on sterol regulatory element-binding protein-1c (SREBP-1c) lipogenic pathway, AMPK, UCP-2, IRS-2, PDX-1, GLUT-2 and Bcl-2 gene expression were investigated. Apoptosis of the exposed cells was determined by MitoCapture, Annexin V-Cy3 and TUNEL assay. Cell lipid accumulation was measured by oil red O staining and TG extraction. Results: 1) FFA treatment resulted in lipid accumulation, impaired GSIS, apoptosis, and SREBP-1c over expression in INS-1E cells. SREBP-1c over expression induced lipid accumulation and damaged GSIS of INS-1E cells. 2) In addition, the expression of lipogenic genes and UCP-2 were up-regulated, but AMPK, IRS-2, PDX-1, GLUT-2 and Bcl-2 were down-regulated in the exposed cells. These lipotoxic effects of FFA were largely prevented by induction of a SREBP-1c siRNA. 3) The induction of a dominantnegative mutant of SREBP-1c into INS-1E cells resulted in maintenance of normal GSIS and TG content. Conclusion: These data suggest a strong correlation between FFA treatment and SREBP-1c activation in INS-1E cells. SREBP-1c might be a major factor responsible for β-cell lipotoxicity. Part II It has been demonstrated in the previous study that there is a strong correlation between FFA exposure and SREBP-1c activation in INS-1E cells. To illuminate the relationship between SREBP-1c over expression and β-cell lipotoxicity, pSPORT-SREBP1c plasmid over expressing SREBP-1c was transfected transiently into INS-1E cells. Then the expression of ACC, FAS, UCP-2, IRS-2, PDX-1 and GLUT-2 were determined. TG content and GSIS were measured in these cells. SREBP-1c over expression induced lipid accumulation and damaged GSIS. The expression of ACC, FAS and UCP-2 were up-regulated, but the expression of IRS-2 and GLUT-2 were down-regulated. These manifestations resembled those induced by chronic FFA exposure. However, the induction of a dominant-negative mutant of SREBP-1c into INS-1E cells resulted in normal GSIS and TG content being retained. Thus, SREBP-1c over expressing INS-1E cells are a candidate for a lipotoxicity β-cell model, and SREBP-1c knockdown is a candidate for gene manipulation in lipotoxicity therapy.