Liver X receptor α (LXRα), an oxysterol-activated nuclear hormone receptor, regulates the expression of genes involved in lipid and cholesterol homeostasis and inflammation. We show here that transactivation by LXRα in monkey kidney COS-1 (Cos-1) cells is decreased by activation of the protein kinase C (PKC) signaling pathway. In transient co-transfection assays, phorbol myristate acetate (PMA) suppressed LXR-dependent transactivation of LXR-responsive reporter genes or the natural promoter of the human ATP-binding cassette (ABC),
ABCA1gene. The decrease in LXR transactivation after PMA treatment was also observed in human embryonic kidney (HEK) 293 and human hepatocellular carcinoma (HepG2) cells. Moreover, endogenous LXR target genes, ABCA1and sterol response element-binding protein-1c, were also decreased by PMA treatment in HEK293 cells as assessed by real-time PCR. The PMA-mediated decrease of LXR activity was blocked by the PKC inhibitor bisindolylmaleimide and mimicked by constitutively active PKCα. Nuclear extracts treated with PMA show no decrease in LXRα DNA binding as assessed by mobility shift and chromatin immunoprecipitation assays. Additionally, in vitrokinase assays demonstrate that PKCα can phosphorylate LXRα. Our findings reveal a mode of regulation of LXRα that may be relevant to disease conditions where aberrant PKC signaling is observed, such as diabetes.