Suppression of vitamin d receptor and induction of retinoid X receptor alpha expression during squamous differentiation of cultured keratinocytes

To gain more insight in the role of the vitamin D system in epidermal diffe rentiation, we studied the expression of the vitamin D receptor and its het erodimeric partner retinoid X receptor alpha in cultured normal human kerat inocytes during squamous differentiation, as triggered by different approac hes. Northern and western blot analysis allowed us to investigate mRNA and protein levels of these nuclear receptors and of markers for growth control (c-myc, cyclin D1, p21(WAF1)) and differentiation (keratinocyte transgluta minase, small proline rich proteins). Growing cells to postconfluence was a potent stimulus for growth arrest and differentiation with concomitant sup pression of vitamin D receptor and induction of retinoid X receptor alpha, at both the mRNA and the protein level. These changes could be prevented by concomitant treatment with epidermal growth factor or keratinocyte growth factor. Subjecting the cells to a calcium switch leading to stratification and differentiation lowered vitamin D receptor protein levels without affec ting vitamin D receptor mRNA and induced both retinoid X receptor alpha mRN A and protein. Interferon-gamma and the phorbolester 12-O-tetradecanoyl pho rbol 13-acetate, two well-known inducers of keratinocyte differentiation, b oth inhibited vitamin D receptor expression but only interferon-gamma induc ed retinoid X receptor alpha. The decreased vitamin D receptor expression w as accompanied by reduced vitamin D responsiveness (as assessed by 24-hydro xylase mRNA induction) in postconfluent, high calcium, and 12-O-tetradecano yl phorbol 13-acetate treated keratinocytes but not with interferon-gamma t reatment. Taken together, our results associate vitamin D receptor expressi on with undifferentiated, proliferating keratinocytes, whereas retinoid X r eceptor alpha expression appears to be related to the differentiated phenot ype. Therefore, proliferating and differentiating keratinocytes may be diff erentially targeted by active vitamin D metabolites.