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Enhanced C/EBPβ‐LIP production by oltipraz leads to inhibition of preadipocyte differentiation as a result of CUGBP1 activation

The C/EBPβ isoforms, “full‐length” LAP and truncated dominant‐negative LIP, differentially regulate adipogenesis. We previously demonstrated that oltipraz, a cancer chemopreventive agent, promotes LAP activation in hepatocytes. In this study, we investigated the effect of oltipraz on adipocyte differentiation and its molecular basis. Oltipraz markedly inhibited 3T3‐L1 preadipocyte differentiation. The expression of LIP notably increased after oltipraz treatment of 3T3‐L1 preadipocytes, whereas that of LAP was minimally changed. Oltipraz treatment significantly elevated the ratio of LIP to LAP, enhancing the levels of nuclear LIP and LAP and their binding to the C/EBP binding site. Cotransfection with LIP interfered with LAP‐mediated luciferase expression, confirming the role of LIP in the gene repression. Similarly, LAP‐mediated luciferase gene transactivation was inhibited by oltipraz, as was observed by cotransfection of AC/EBP. Oltipraz enhanced cytoplasmic translocation and RNA binding of CUGBP1, but not calreticulin, another RNA binding protein that interacts with C/EBPβ mRNA. Oltipraz‐mediated LIP production, accompanying inhibition of hormone‐induced adipocyte differentiation, was also observed in primary‐cultured rat preadipocytes. In conclusion, oltipraz inhibits adipogenesis by promoting LIP production and activation, and the enhanced LIP production accompanies cytoplasmic translocation of CUGBP1 and its binding to the GC rich region of C/EBPβ mRNA. Our finding holds significance that adipogenesis can be pharmacologically controlled by LIP production.