Overexpression of Insig-1 protects β cell against glucolipotoxicity via SREBP-1c.

High glucose induced lipid synthesis leads to β cell glucolipotoxicity. Sterol regulatory element binding protein-1c (SREBP-1c) is reported to be partially involved in this process. Insulin induced gene-1 (Insig-1) is an important upstream regulator of Insig-1-SREBPs cleavage activating protein (SCAP)-SREBP-1c pathway. Insig-1 effectively blocks the transcription of SREBP-1c, preventing the activation of the genes for lipid biosynthesis. In this study, we aimed to investigate whether Insig-1 protects β cells against glucolipotoxicity. An Insig-1 stable cell line was generated by overexpression of Insig-1 in INS-1 cells. The expression of Insig-1 was evaluated by RT-PCR and Western blotting, then, cells were then treated with standard (11.2 mM) or high (25.0 mM) glucose for 0 h, 24 h and 72 h. Cell viability, apoptosis, glucose stimulated insulin secretion (GSIS), lipid metabolism and mRNA expression of insulin secretion relevant genes such as IRS-2, PDX-1, GLUT-2, Insulin and UCP-2 were evaluated. We found that Insig-1 suppressed the high glucose induced SREBP-1c mRNA and protein expression. Our results also showed that Insig-1 overexpression protected β cells from ER stress-induced apoptosis by regulating the proteins expressed in the IRE1α pathway, such as p-IRE1α, p-JNK, CHOP and BCL-2. In addition, Insig-1 up-regulated the expression of IRS-2, PDX-1, GLUT-2 and Insulin, down-regulated the expression of UCP-2 and improved glucose stimulated insulin secretion (GSIS). Finally, we found that Insig-1 inhibited the lipid accumulation and free fatty acid (FFA) synthesis in a time-dependent manner. There results suggest that Insig-1 may play a critical role in protecting β cells against glucolipotoxicity by regulating the expression of SREBP-1c.