Suprabasal expression of a dominant-negative RXRα mutant in transgenic mouse epidermis impairs regulation of gene transcription and basal keratinocyte proliferation by RAR-selective retinoids

Abstract

To determine whether 9-cis retinoic acid receptors (RXRs) regulate the biological activity of all-trans retinoic acid (tRA) and its receptors (RARs) in skin, we have targeted a dominant-negative RXRα (dnRXRα) lacking transactivation function AF-2 to differentiated suprabasal keratinocytes in the epidermis of transgenic mice. Driven by the suprabasal-specific keratin- 10 gene promoter, expression of dnRXRα severely reduced the ability of RAR- selective ligands IRA and CD367 to induce epidermal mRNA levels of the CRABPII, CRBPI, and CRBPII genes, which contain RA-responsive elements (RAREs) DR1 and/or DR2. It also reduced gene-specific, synergistic induction of CRBPI mRNA by a combination of CD367 and RXR-selective SR11237. Like endogenous RXRα, dnRXRα in epidermal nuclear extracts from the transgenic mice competitively formed heterodimers with endogenous RARγ on RAREs, suggesting that dnRXRα impairs retinoid signaling by competing with endogenous RARγ-RXRα heterodimers. Histologically, the epidermis of dnRXRα mice showed no detectable developmental abnormalities. Surprisingly, in adult animals, the suprabasal expression of dnRXRα significantly reduced the ability of topically applied tRA to stimulate proliferation of undifferentiated keratinocytes in the basal layer of epidermis. RXR-selective ligands alone had no detectable effects on both normal and transgenic mouse epidermis. Accordingly, we suggest that in vivo: (1) in suprabasal keratinocytes, retinoids regulate gene transcription via RAR-RXR heterodimers in which RAR confers a predominant ligand response, whereas RXR AF-2 is required for liganded RAR AF-2 to efficiently trans-activate target genes, and (2) this suprabasal RXR-assisted mechanism indirectly regulates proliferation of basal keratinocytes likely via intercellular signaling.