Activation of peroxisome proliferator-activated receptor-γ and retinoid X receptor inhibits aromatase transcription via nuclear factor-κB

Our previous studies demonstrated that a peroxisome proliferator-activated-receptor( PPAR)-gamma ligand, troglitazone(TGZ), and/or a retinoid X receptor (RXR) ligand, LG100268 (LG), decreased the aromatase activity in both cultured human ovarian granulosa cells andhumangranulosa-like tumor KGN cells. In the present study, we further found that a combined treatment of TGZ + LG decreased aromatase promoter II (ArPII) activity in both ovarian KGN cells and fibroblast NIH-3T3 cells in a PPARgamma-dependent manner. Furthermore, the inhibition of both aromatase activity and the transcription of ArPII by TGZ + LG was completely eliminated when nuclear factor-kappaB (NF-kappaB) signaling was blocked by specific inhibitors, suggesting NF-kappaB, which is endogenously expressed in both fibroblast and granulosa cells, might be a mediator of this inhibition. Interestingly, activation of NF-kappaB by either forced expression of the p65 subunit or NF-kappaB-inducing kinase upregulated ArPII activity. Positive regulation of aromatase by endogenous NF-kappaB was also suggested by the fact that NF-kappaB-specific inhibitors suppress basal activity of the aromatase gene. A concomitant formation of high-order complex between NF-kappaB p65 and ArPII was also observed by chromatin immunoprecipitation assay. Although activation of PPARgamma and RXR affected endogenous expression levels of neither inhibitory kappaBalpha nor p65, it impaired the interaction between NF-kappaB and ArPII and the p65 based transcription as well. Altogether, these results indicate that activation of a nuclear receptor system, constituted by PPARgamma and RXR, downregulates aromatase expression through the suppression of NF-kappaB-dependent aromatase activation and thus provide a new insight in the mechanism of regulation of the aromatase gene.