Differential and bi-directional regulation between TR2/TR4 orphan nuclear receptors and a specific ligand mediated-peroxisome proliferator-activated receptor α in human HaCaT keratinocytes

Background: We have reported that human TR2 orphan nuclear receptor (TR2) can modulate the transcriptional activity of the reporter gene containing an AGGTCA direct repeat-hormone response element. Objective: The aim of this study is to investigate the potential role and regulation of TR2 in human HaCaT keratinocytes. Methods: We performed mainly chloramphenicol acetyltransferase reporter gene assays (CAT assays), and Western blot analysis. Results: From CAT assays, TR2 can suppress retinoic acid (RA)-induced transactivation by 44.7% in HaCaT keratinocytes. This suppression is similar to our previous report showing TR4 orphan nuclear receptor (TR4) can suppress RA-induced transactivation. However, TR4 but not TR2 can significantly repress Wy-14643-mediated peroxisome proliferator-activated receptor α (PPARα) transactivation by 95%. Western blot analysis suggested that Wy-14643 can differentially regulate the expression of TR2 and TR4 (by increasing the expression of TR4 protein and decreasing that of TR2) in HaCaT keratinocytes. Conclusion: Our data not only provides the first evidence to demonstrate that close members of orphan nuclear receptors group, such as TR2 and TR4, can have distinct functions, but also suggests the existence of differential and bi-directional regulation between PPARα and TR2/TR4, that may play some important roles in the PPARα signaling pathway in human keratinocytes.