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Insulin Regulates TRB3 and Other Stress-Responsive Gene Expression through Induction of C/EBPβ

Molecular Oncology Research Institute, Tufts Medical Center, Boston, Massachusetts 02111

Address all correspondence and requests for reprints to: Keyong Du, Molecular Oncology Research Institute, Tufts Medical Center, Boston, Massachusetts 02111. E-mail: kdu{at}tufts-nemc.org.

Pseudokinase TRB3 is an inducible gene whose expression is regulated by stress response and insulin and associated with insulin resistance and metabolic syndrome. In this report, we have investigated the mechanism under which insulin regulates TRB3 gene expression and demonstrated that insulin induces TRB3 expression via C/EBPβ. We found that in Fao hepatoma and 3T3-L1 adipocytes, C/EBPβ expression induced by insulin preceded that of TRB3 and that mutation of the C/EBPβ binding site in TRB3 promoter abolished the responsiveness of the TRB3 gene to insulin. We further showed that ectopic expression of C/EBPβ augmented, whereas knockdown of C/EBPβ reduced, TRB3 expression induced by insulin. In addition, we presented data to show that insulin, through a similar mechanism under which insulin induces TRB3 expression, promotes the expression of genes such as ANAS, ATF3, BIP, and CHOP, which are typical stress-responsive genes. We also examined the impact of C/EBPβ expression on Akt activation and found that inaction of C/EBPβ not only augmented Akt activation but also obliterated the suppression of Akt activation due to prolonged insulin stimulation. We suggest, through induction of C/EBPβ in hepatic cells and adipocytes, that insulin induces the expression of stress-responsive genes, which may represent a novel insulin action.