The Foxo1 Regulate Srebp-1c Promoter Transcriptional Activity In Preliminary Study And Pkc¦Æ In Insulin-induced Glut4 Translocation In Functional Studies

Sterol regulatiry element-binding protein-1c(SREBP-1c) has been established as lipid synthetic transcription factor for fatty acid,triglyceride synthesis and glucose metabolism. SREBP-1c seems to be a mediator for insulin/glucose signaling to lipogenesis,and could be involved in insulin resistance and fatty livers.The regulation of SREBP-1c expression is mainly in the transcriptional level.Insulin and LXRαcan up-regulate the transcription of SREBP-1c.FoxO1,a transcript factor suppressed by insulin through phosphorylation,has an important function in liver metabolism.By dual luciferase reporter gene system,FoxO1 inhibits the basal transcription of SREBP-1c-2.6kb promoter.To measure promoter activities SREBP-1c genomic 5'-flanking fragments with different sizes,we find that FoxO1 surpressses the transcriptional activity 3-fold and 2.6-fold in the-500 and-250bp, respectively,but 1.7-fold in the-148bp.The domain between -250 and -148bp is the LXRE element.Further,FoxO1 can suppress the transcription of SREBP-1c promoter mediated by LXRαand its ligand agonist,TO901317.Furthermore,transfection with increasing amounts of expression vectors encoding WTFoxO1 results in a dose-dependent inhibition of LXRα-driven SREBP-1c reporter activation.Interesting,when the SREBP-1c promoter-500bp short of LXRE element,FoxO1 has still the surpress effect.In addition, FoxO1 inhibits the LXRα-induced transcriptional activity of SREBP-1c promoter by RXRαand PGC-1αsynergistically.FoxO1A3H215R,a mutant not binding the DNA element,has the same effect on SREBP-1c promoter.FoxO1 suppresses LXRαbinding to LXRE element by CHIP.Moreover,the effect of FoxO1 surpression LXRα-induced the transcriptional activity of SREBP-1c promoter can be restored by insulin.These results show that FoxO1 can srepress the transcriptional activity of SREBP-1c promoter by inhibition LXRαbinding to LXRE element,but this is not only a way.And insulin can block the FoxO1 surpression effect on SREBP-1c promoter. Actin remodeling plays a crucial role in insulin-induced translocation of glucose transporter 4(GLUT4) from cytoplasm to plasma membrane and subsequent glucose transport.Protein kinase C(PKC)ζhas been implicated in this translocation process, though the exact mechanism remains unknown.In this study,we investigated the effect of PKCζon actin cytoskeleton and GLUT4 translocation in CHO-K1 cells expressing myctagged GLUT4.Insulin stimulated the phosphorylation of PKCζat Thr410 with no apparent effect on its expression.Moreover,insulin promoted co-localization of PKCζand actin,which could be abolished by Latrunculin B.The over-expression of PKCζmimicked the insulin-induced change in actin cytoskeleton and translocation of GLUT4,and these effects were also completely abrogated by Latrunculin B.Using cell-permeable pseudosubstrate(PS) of PKCζ,the effect of insulin can be alleviated.Our results suggest that PKCζmediates the effect of insulin on GLUT4 translocation through its interaction with actin cytoskeleton.