Sp1, but not Sp3, functions to mediate promoter activation by TGF-beta through canonical Sp1 binding sites.

Transforming growth factor beta (TGF-beta) causes growth arrest at the G1 phase of the cell cycle in most cell types. Both the cyclin dependent kinase inhibitor p15(INK4B) and p21(Cip1/WAF1) genes have been found to be induced by TGF-beta in human keratinocyte HaCaT cells. Analyses of the human p15 and p21 promoters have led to the identification of GC-rich sequences capable of binding to Sp1 transcription factors as necessary elements for the TGF-beta induction of both promoters. We report here that canonical Sp1 binding sites derived from the SV40 21 bp repeat could also support promoter induction by TGF-beta when placed upstream of a minimal luciferase reporter construct containing only the TATA and Inr elements. Gel retardation assays identified Sp1, Sp3 and DeltaSp3 as major factors binding to the canonical Sp1 sites in HaCaT cells and that TGF-beta treatment did not change their binding activities over a 24 h period. More importantly, GAL4-Sp1, but not GAL4-Sp3, chimeric protein supported TGF-beta mediated gene induction from a luciferase reporter construct driven by five GAL4 DNA binding sites. Our results suggest that Sp1 binding site can function as a distinct TGF-beta responsive element for TGF-beta mediated promoter expression and Sp1 per se can mediate this response.