The long terminal repeat of VL30 retrotransposons contains sequences that determine retinoic acid-induced transcription in cultured keratinocytes.

The characterization of retinoic acid (RA)-regulated gene transcription in keratinocytes has important implications as to the function of retinoids in epidermal homeostasis and to the central role retinoids play in the pharmaco-therapy of a variety of skin disorders. We show that cultured mouse keratinocytes (Balb/MK) responded to RA with induced expression of VL30 retrotransposons. The induction was rapid, was present at nanomolar concentrations of RA, was independent of new protein synthesis, and occurred both in proliferating and differentiated keratinocytes. The long terminal repeat of a VL30 retrotransposon, expressed in mouse epidermis in vivo, was found to contain two RA-responsive elements (RREs) that independently conferred RA responsiveness on a heterologous promoter in both cultured Balb/MK cells and normal human keratinocytes. Functional characterization and in vitro binding analysis showed that the sequence requirement for binding of retinoid X receptor alpha (RXR alpha) and retinoic acid receptor (RAR, either alpha, beta, or gamma) heterodimers, correlated with the sequence requirement for RA-induced transcription in keratinocytes. The VL30 RREs differed functionally from the RA response element present in the RAR-beta 2 promoter, in that the VL30 RREs were non-responsive in fibroblasts cultured from human skin. The non-responsiveness correlated with a reduced complex formation between a VL30 RRE and endogenously expressed nuclear factors present in skin fibroblasts. The data suggest that a direct repeat of two half-sites, spaced by two base pairs, with the consensus sequence T(A/G)AACTTTT(T/C)ACC(T/C), bound RAR-RXR heterodimers and mediated, at constitutive receptor levels, a primary RA response on gene transcription specifically in keratinocytes.