Induction of TR4 orphan receptor by retinoic acid in human HaCaT keratinocytes

Human TR4 orphan receptor (TR4) can modulate the transcriptional activity o f the reporter gene containing an AGGTCA direct repeat-hormone response ele ment. Here we studied the potential role of TR4 in human HaCaT keratinocyte s. Using a chloramphenicol acetyltransferase reporter gene assay, it was sh own that TR4 can suppress retinoic acid-induced transactivation by 47.3% in human HaCaT keratinocytes. Electrophoretic mobility shift assay indicated that this suppression may be due to TR4 binding with higher affinity to the retinoic acid response element than retinoid receptors, Western blot analy sis further suggested that retinoic acid can increase: the expression of TR 4 protein in human HaCaT keratinocytes, indicating that TR4 acts as a negat ive feedback modulator for retinoic acid action. Interestingly, TR4 express ion is increased in normal human keratinocytes when substituting a low calc ium-medium with a high calcium medium. Together, our data suggested, for th e first time, that an orphan receptor, such as TR4, may play an important p art in retinoid-mediated signaling pathways in human keratinocytes, providi ng a new insight into keratinocyte biology.