The expression of and insulin binding to cellular thyroid hormone binding protein, but not insulin degrading enzyme, is increased during 3T3-L1 adipocytes differentiation

Specific insulin binding to several proteins (cytosolic insulin binding proteins; CIBPs) in the isolated cytoplasm of numerous cell types has been demonstrated. CIBPs include insulin degrading enzyme ODE), CIBP p55 (identified as cellular thyroid hormone binding protein (CTHBP), which is also known as protein disulfide isomerase, or glutathione insulin transhydrogenase). To assess the possible role of CIBP p55\CTHBP in insulin action, we compared ¹²⁵I-insulin binding to CIBP in cytosol isolated from 3T3-L1 cells at various time points during differentiation of the adipocytes. Insulin did not bind to CTHBP in fibroblasts, but the labeling was markedly increased during adipocyte differentiation. In contrast, insulin binding to IDE did not change during differentiation. Protein expression level of CTHBP in the cytosol fraction increased gradually during the differentiation of adipocytes, whereas that of IDE did not change throughout the period. These data indicate that CTHBP, but not IDE, was up-regulated during differentiation of the adipocytes, suggesting that CIBP p55\CTHBP may play a role in regulating some insulin action, especially the counter regulation between insulin and other hormones during adipocyte differentiation.