Identification of a novel enhancer element mediating calcium-dependent induction of gene expression in response to either epidermal growth factor or activation of protein kinase C

The VL30 family of defective murine retroviruses consists of 100 to 200 members, of which fewer than 5% appear to be transcriptionally active. A genomic clone of the transcriptionally active VL30 element RVL-3 was identified and sequenced. Genetic analysis indicated that a triple-repeat sequence within the RVL-3 long terminal repeat is capable of functioning as an inducible enhancer element responding to a variety of agonists. In Rat-1 flbroblasts, the ability of the RVL-3 enhancer to mediate induction of gene expression from a heterologous promoter in response to either epidermal growth factor (EGF) or phorbol ester treatment required coelevation of intracellular calcium. Two CArG boxes present in the triple-repeat sequence appeared to exert a negative effect on gene expression, as mutation of these sequences elevated the basal level of expression observed without altering the fold induction in response to either EGF or protein kinase C activation. In the presence of these CArG elements, mutation of AP-1-like sites adjacent to the CArG elements significantly inhibited the ability of either EGF or phorbol esters to induce gene expression. The effect of mutating these AP-1-like sites was relieved by simultaneous mutation of the CArG sites, indicating that interactions among these sites modulate RVL-3 expression. Mutational analysis and gel mobility shift experiments have identified a third sequence within the VL30 triple-repeat element that is required for the induction of gene expression and serves as a binding site for nuclear proteins. Sequence comparisons indicate that this enhancer element has not been described previously.