Fibrates upregulate TRB3 in lymphocytes independent of PPAR alpha by augmenting CCAAT/enhancer-binding protein beta (C/EBP beta) expression.

Fibrates, which function by binding and activating peroxisome proliferator-activated receptor alpha (PPARalpha), have been used successfully to treat hyperlipidemia and atherosclerosis. Increasing evidence suggests that in addition to their lipid lowering activities these medications also function as immunosuppressive agents. Tribbles is a Drosophila protein that slows cell cycle progression, and its mammalian homolog, TRB3 interferes with insulin-induced activation of AKT. In these studies we demonstrate that fibrates upregulate TRB3 expression in mitogen-activated lymphocytes. Interestingly, in lymphocytes fibrates augment TRB3 expression in both PPARalpha wildtype and knockout mice, suggesting that upregulation of this protein occurs in a PPARalpha-independent manner. Fibrates activate a proximal TRB3 promoter construct and mutation or partial deletion of a potential PPAR response element does not alter the ability of fibrates to drive TRB3 expression. Subsequent studies reveal that fibrates upregulate C/EBPbeta and CHOP in lymphocytes and mutation of potential C/EBPbeta and CHOP consensus sequences abrogates the ability of fibrates to upregulate TRB3 promoter activity. Accordingly, fibrates enhance the recruitment of C/EBPbeta and CHOP to the proximal TRB3 promoter. Finally, TRB3 expression in lymphocytes induces G2 cell cycle delay and cellular depletion. These studies outline a novel PPARalpha-independent mechanism of action of fibrates and document for the first time the expression of TRB3 in activated lymphocytes.