Effect of hypoxia on caveolin-1 expression, caveolae structure and insulin signaling

Abstract

Obesity courses with the hypertrophy and hyperplasia of adipose tissue, which has been related to the development of hypoxia and, as consequence, insulin resistance. In this sense, insulin receptor is enriched in a specialized invagination of the plasma membrane named caveolae. Caveolin-1 (Cav-1) is one of the main proteins of caveolae and promotes insulin receptor (IR) phosphorylation and the subsequent activation of insulin signaling. In this study we have demonstrated that hypoxia inhibits 3T3-L1 adipocyte differentiation process and insulin signaling. Furthermore, 48 hours of hypoxia reduced the glucose uptake induced by insulin whereas the basal uptake was increased. This result was consistent with the upregulation of GLUT1, a facilitative glucose transporter independent of insulin, and the downregulation of the insulin-dependent transporter GLUT4, which also showed defective translocation to plasma membrane when adipocytes are stimulated with this hormone. On the other hand, hypoxia, whether continuous (48 hours) or intermittent, produced alterations in caveolae structure in 3T3-L1 cells and in mouse adipose tissue respectively, suggesting that caveolae disruption is implicated in the obesity-related insulin resistance. Furthermore, we also showed that caveolae density in plasma membrane of endothelial cells was reduced and that Cav-1 is accumulated in the nucleus of these cells by hypoxia. The role of HIF-1 in all these effects was addressed using echinomycin, which inhibits HIF-1 binding to Hypoxia Response Elements (HRE), indicating that the expression of GLUT4, Cav-1, Cav-2 and SDPR/Cavin-2 genes is regulated by HIF-1 in hypoxic 3T3-L1 adipocytes. Moreover, a Chromatin immunoprecipitation (ChIP) assay demonstrated that HIF-1 directly binds to the hypoxia response element identified in mouse Cav-1 promoter (-442 TSS) by bioinformatics analysis. A microarray of gene expression was also performed to explore new genes implicated in hypoxia response of 3T3-L1 adipocytes. The results of this analysis confirmed that pathways related to caveolae structure and function, insulin response, glucose transport and lipid biosynthesis are affected by hypoxia and highlighted the involvement of other genes such as Sort1, Ghrh and Igf in the effects of hypoxia on adipocytes.