Novel mechanism of steroid action in skin through glucocorticoid receptor monomers

Glucocorticoids (GCs), important regulators of epidermal growth, differenti ation, and homeostasis, are used extensively in the treatment of skin disea ses. Using keratin gene expression as a paradigm of epidermal physiology an d pathology we have developed a model system to study the molecular mechani sm of GCs action in skin. Here we describe a novel mechanism of suppression of transcription by the glucocorticoid receptor (GR) that represents an ex ample of customizing a device for transcriptional regulation to target a sp ecific group of genes within the target tissue, in our case, epidermis. We have shown that GCs repress the expression of the basal-cell-specific kerat ins K5 and K14 and disease-associated keratins K6, K16, and K17 but not the differentiation-specific keratins K3 and K10 or the simple epithelium-spec ific keratins K8, K18, and K19. We have identified the negative recognition elements (nGREs) in all five regulated keratin gene promoters. Detailed fo otprinting revealed that the function of nGREs is to instruct the GR to bin d as four monomers. Furthermore, using cotransfection and antisense technol ogy we have found that, unlike SRC-1 and GRIP-1, which are not involved in the GR complex: that suppresses keratin genes, histone acetyltransferase an d CBP are. In addition, we have found that GR, independently from GREs, blo cks the induction of keratin gene expression by AP1. We conclude that GR su ppresses keratin gene expression through two independent mechanisms: direct ly, through interactions of keratin nGREs with four GR monomers, as well as indirectly, by blocking the AP1 induction of keratin gene expression.