Regulation of retinoic acid receptor expression in dermal fibroblasts

Retinoic acid (RA) is known to exert profound effects on growth and differentiation in a variety of cell types in the skin. In vitro studies have also shown that RA modulates gene expression in both fibroblasts and keratinocytes. Recently, three nuclear receptors specific for retinoic acid (RAR alpha, RAR beta, and RAR gamma) have been cloned and all are members of a large multigene family of ligand-inducible transcription enhancer factors. As a first step in defining the role of each receptor in the retinoid response of the skin, we examined the regulation of RAR alpha, RAR beta, and RAR gamma gene expression in human dermal fibroblasts by all-trans-retinoic acid. We demonstrate that human dermal fibroblasts express modest basal levels of RAR alpha and RAR gamma, but not RAR beta. When treated with 1 microM RA, the messenger RNAs for both RAR beta and RAR gamma are induced. In contrast, RAR alpha remains unchanged. The induction of RAR gamma is attenuated by the protein synthesis inhibitor, cycloheximide, while the induction of RAR beta increases slightly. Studies with actinomycin D and cycloheximide show that all three receptors have different half-lives, with RAR gamma having the longest half-life at 8 h. Gel shift analysis of known retinoic acid response elements (RAREs) in the RAR beta and RAR gamma genes demonstrates that the upregulation of these by genes by RA involves increased binding of complexes to the individual RAREs. In summary, these data demonstrate that fibroblasts express all three receptor types. Moreover, striking differences exist in the regulation of RAR gene expression in skin-derived fibroblasts and suggest that each receptor may well have a separate and discrete function in the retinoid response of the skin.