SUPRABASAL EXPRESSION OF A DOMINANT-NEGATIVE RXR-ALPHA MUTANT IN TRANSGENIC MOUSE EPIDERMIS IMPAIRS REGULATION OF GENE-TRANSCRIPTION AND BASAL KERATINOCYTE PROLIFERATION BY RAR-SELECTIVE RETINOIDS

To determine whether 9-cis retinoic acid receptors (RXRs) regulate the biological activity of all-trans retinoic acid (tRA) and its receptor s (RARs) in skin, we have targeted a dominant-negative RXR alpha (dnRX R alpha) lacking transactivation function AF-2 to differentiated supra basal keratinocytes in the epidermis of transgenic mice. Driven by the suprabasal-specific keratin-10 gene promoter, expression of dnRXR alp ha severely reduced the ability of RAR-selective ligands tRA and CD367 to induce epidermal mRNA levels of the CRABPII, CRBPI, and CRBPII gen es, which contain RA-responsive elements (RAREs) DR1 and/or DR2. It al so reduced gene-specific, synergistic induction of CRBPI mRNA by a com bination of CD367 and RXR-selective SR11237. Like endogenous RXR alpha , dnRXRa in epidermal nuclear extracts from the transgenic mice compet itively formed heterodimers with endogenous RAR gamma on RAREs, sugges ting that dnRXR alpha impairs retinoid signaling by competing with end ogenous RAR gamma-RXR alpha heterodimers. Histologically, the epidermi s of dnRXR alpha. mice showed no detectable developmental abnormalitie s. Surprisingly, in adult animals, the suprabasal expression of dnRXR alpha significantly reduced the ability of topically applied tRA to st imulate proliferation of undifferentiated keratinocytes in the basal l ayer of epidermis. RXR-selective ligands alone had no detectable effec ts on both normal and transgenic mouse epidermis. Accordingly, we sugg est that in vivo: (1) in suprabasal keratinocytes, retinoids regulate gene transcription via RAR-RXR heterodimers in which RAR confers a pre dominant ligand response, whereas RXR AF-2 is required for liganded RA R AE-2 to efficiently trans-activate target genes, and (2) this suprab asal RXR-assisted mechanism indirectly regulates proliferation of basa l keratinocytes likely via intercellular signaling.