Peroxisome proliferator-activated receptor alpha physically interacts with CCAAT/enhancer binding protein (C/EBPbeta) to inhibit C/EBPbeta-responsive alpha1-acid glycoprotein gene expression

Recently, the role of the peroxisome proliferator-activated receptor alpha (PPARalpha) in the hepatic inflammatory response has been associated to the decrease of acute phase protein transcription, although the molecular mechanisms are still to be elucidated. Here, we were interested in the regulation by Wy-14643 (PPARalpha agonist) of alpha1-acid glycoprotein (AGP), a positive acute phase protein, after stimulation by Dexamethasone (Dex), a major modulator of the inflammatory response. In cultured rat hepatocytes, we demonstrate that PPARalpha inhibits at the transcriptional level the Dex-induced AGP gene expression. PPARalpha exerts this inhibitory effect by antagonizing the CCAAT/enhancer binding protein (C/EBPbeta) transcription factor that is involved in Dex-dependent up-regulation of AGP gene expression. Overexpression of C/EBPbeta alleviates the repressive effect of PPARalpha, thus restoring the Dex-stimulated AGP promoter activity. Furthermore, glutathione-S-transferase GST pull-down and coimmunoprecipitation experiments evidenced, for the first time, a physical interaction between PPARalpha and the C-terminal DNA binding region of C/EBPbeta, thus preventing it from binding to specific sequence elements of the AGP promoter. Altogether, these results provide an additional molecular mechanism of negative regulation of acute phase protein gene expression by sequestration of the C/EBPbeta transcription factor by PPARalpha and reveal the high potency of the latter in controlling inflammation.

Retinoids increase alpha-1 acid glycoprotein expression at the transcriptional level through two distinct DR1 retinoic acid responsive elements

In the present study, we analyzed the influence of retinoic acids on the expression of alpha-1 acid glycoprotein (AGP). We show that in rat primary hepatocytes, 9-cis retinoic acid and all-trans retinoic acid increase AGP gene expression at the transcriptional level. Transient transfections of rat primary hepatocytes with a reporter construct driven by the rat AGP gene promoter indicated that retinoids regulate AGP gene expression via the -763/-138 region of the AGP promoter. Furthermore, cotransfection experiments with retinoic acid receptor alpha (RARalpha) and retinoid X receptor alpha (RXRalpha) expression vectors in NIH3T3 cells demonstrated that both RXRalpha/RXRalpha homodimer and RXRalpha/RARalpha heterodimer are competent for ligand-induced transactivation of the AGP promoter. Unilateral deletion and site-directed mutagenesis identified two retinoic-acid responsive elements (RARE), RARE-I and RARE-II, which interestingly correspond to a direct repeat of two TGACCT-related hexanucleotides separated by a single bp only (DR1-type response element). Cotransfection assays showed that RXRalpha and RARalpha activate AGP gene transcription through these two elements either as a homodimer (RXRalpha/RXRalpha) or as a heterodimer (RXRalpha/RARalpha). The RXRalpha/RXRalpha homodimer acts most efficiently through the RARE-I response element to promote AGP transactivation, whereas the RXRalpha/RARalpha heterodimer mediates transactivation better via the RARE-II responsive element.