Expression of Kit ligand (KL) mRNA is induced in primary prepuberal Sertoli cells by FSH and by other agents that increase cAMP levels. The cAMP effect is exerted at the transcriptional level and appears to be cell type specific, since it is not observed in other KL-expressing primary cells or cell lines. Deletion analysis of the 5′-flanking region of the mouse KL gene shows that the proximal promoter sequence between −88 and 8 from the transcriptional start site is necessary and sufficient to obtain the full cAMP responsiveness of the promoter in primary mouse Sertoli cells. In the −88/ 8 promoter region, several cis-acting elements play a role in cAMP response. The −88/−56 sequence is necessary for full induction of the gene, since its removal causes a drastic decrease in cAMP responsiveness; however, cAMP-stimulated expression is still observed with the minimal promoter region between −55 and 8. A more detailed mutational analysis of the minimal promoter region shows that mutations in the canonical TATA box sequence and in an immediately downstream GC-rich element completely abolish cAMP responsiveness. DNA-binding experiments show that transcription factor Sp1 binds to the −88/−56 fragment of the KL proximal promoter in both control and cAMP-stimulated cells, whereas a new cAMP-induced complex is observed when the −55/ 8 minimal promoter region is used as probe. The canonical TATA box sequence is essential for formation of the latter complex. We also show that the binding of an unknown nuclear factor (different from Sp1, Egr-1, Rnf6, and AP-2) to a GC-rich element between −19 and 8 increases after cAMP treatment, and this effect seems to be specific of primary Sertoli cells. Thus, cAMP-induced transcription from the KL gene promoter in primary mouse Sertoli cells is mediated by a complex interaction among a Sp1-binding region, factors recognizing the canonical TATA box sequence, and a not yet identified cAMP-induced factor binding a GC-rich sequence just downstream from it.
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