INDUCIBLE AND CELL-TYPE-SPECIFIC EXPRESSION OF VL30 U3 SUBGROUPS CORRELATE WITH THEIR ENHANCER DESIGN

The murine VL30 elements constitute one family of retrotransposons rep resented in 100 to 200 copies that are dispersed among the mouse chrom osomes. On the basis of sequence homology, we have subdivided mouse VL 30 members into four distinct U3 subgroups. The use of subgroup-specif ic probes in Northern (RNA) blot analyses shows that individual VL30 U 3 subgroups are expressed in a tissue-specific manner. We show by in s itu hybridization of mouse skin treated with 12-O-tetradecanoylphorbol -13-acetate (TPA) that VL30 expression is induced in epidermal keratin ocytes but not in dermal fibroblasts. Transient transfections of repor ter gene plasmids together with in vitro binding analysis indicate tha t TPA-induced VL30 transcription specific for keratinocytes is mediate d by two cooperating sequence motifs in juxtaposed position. One seque nce motif is shown to constitutively bind CREB- and Jun-related protei ns in both keratinocytes and fibroblasts, whereas the other is a targe t for TPA-induced c-Rel/p65(NF-kappa B)-binding activity specifically in keratinocytes. These binding sites are found to be conserved within U3 subgroups and individual U3 regions showing induced expression in TPA-treated mouse epidermis. These results together with a sequence co mparison between different U3 subgroups indicate that cell type-specif ic activity of transcription factors known to regulate VL30 transcript ion and the presence or absence of their cognate binding sites within individual U3 regions determine inducible and cell type-specific VL30 expression. The variable VL30 U3 regions might thus be useful tools to study inducible and cell type-specific transcription in many differen t cell systems.