ABSTRACT:Mechanisms regulating CYP4F genes remain under investigation, although characterization of CYP4F regulatory modalities would facilitate the discovery of new drug targets. This present study shows that all-trans-and 9-cis-retinoic acids can inhibit CYP4F11 expression in human keratinocyte-derived HaCaT cells. Transrepression of many genes by retinoic acids is mediated by interactions between retinoid receptors and the activator protein 1 (AP-1) complex. Proinflammatory cytokines tumor necrosis factor ␣ Cytochrome P450 4F enzymes are involved in cellular protection and metabolism of numerous small molecules, including drugs, toxins, and eicosanoids. To date, studies have been focused on CYP4F functions rather than transcriptional regulatory mechanisms. This approach is partially because of the apparent complexity of CYP4F gene regulation and differences observed among the various model systems studied.Despite the seeming complexity of CYP4F regulation, some research groups have made substantive contributions to this puzzle. Zhang and colleagues showed that retinoic acids and peroxisome proliferators can regulate CYP4F2 gene activities in HepG2 cells, and that retinoid X receptor (RXR) ␣ heterodimers stimulated and retinoic acid receptor (RAR) ␣ repressed CYP4F2 expression Hsu et al., 2007) showed that statins induced CYP4F2 in primary human hepatocytes and HepG2 cells. We recently showed that retinoic acids induced the expression of CYP4F2, CYP4F3A, CYP4F3B, and CYP4F11 in primary human epidermal keratinocytes, and that RXR (but not RAR) nuclear receptors mediate retinoic acid-induced up-regulation of CYP4F2 and CYP4F3A (Kalsotra et al., 2008). In a preliminary study (data not shown) using a transformed immortal human keratinocyte cell line (HaCaT), a different story unfolded. Lovastatin did not affect HaCaT cell CYP4F2 expression, which is in contrast to its up-regulatory effects on CYP4F2 in human hepatic cells (Hsu et al., 2007). In addition, 9-cis-retinoic acid failed to induce CYP4F11 in HaCaT cells; rather, it significantly decreased CYP4F11 transcript levels.Retinoic acids are derived from retinol, the animal form of vitamin A. Retinoic acids play a role in gene regulation via two groups of nuclear receptors (NRs), RARs and RXRs (Mangelsdorf et al., 1995