Two distinct coactivators, DRIP/mediator and SRC/p160, are differentially involved in vitamin D receptor transactivation during keratinocyte differentiation.

Cell programs such as proliferation and differentiation involve the sequential activation and repression of gene expression. Vitamin D, via its active metabolite 1,25-dihydroxyvitamin D [1,25-(OH)2D3)], controls the proliferation and differentiation of a number of cell types, including keratinocytes, by directly regulating transcription. Two classes of coactivators, the vitamin D receptor (VDR)-interacting proteins (DRIP/mediator) and the p160 steroid receptor coactivator family (SRC/p160), control the actions of nuclear hormone receptors, including the VDR. However, the relationship between these two classes of coactivators is not clear. Using glutathione-S-transferase-VDR affinity beads, we have identified the DRIP/mediator complex as the major VDR binding complex in proliferating keratinocytes. After the cells differentiated, members of the SRC/p160 family were identified in the complex but not major DRIP subunits. Both DRIP and SRC proteins were expressed in keratinocytes. DRIP205 expression decreased during differentiation, although SRC-3 levels increased. Both DRIP205 and SRC-3 potentiated vitamin D-induced transcription in proliferating cells, but during differentiation, DRIP205 was no longer effective. These results indicate that these two distinct coactivators are sequentially involved in vitamin D regulation of gene transcription during keratinocyte differentiation, suggesting that these coactivators are part of the means by which the temporal sequence of gene expression is regulated during the differentiation process.