Sp1 and Smad Proteins Cooperate to Mediate Transforming Growth Factor-b1-induced a2(I) Collagen Expression in Human Glomerular Mesangial Cells*

The mechanism(s) by which Smads mediate and modulate the transforming growth factor (TGF)-b signal transduction pathway in fibrogenesis are not well characterized. We previously showed that Smad3 promotes a2(I) collagen gene (COL1A2) activation in human glomerular mesangial cells, potentially contributing to glomerulosclerosis. Here, we report that Sp1 binding is necessary for TGF-b1-induced type I collagen mRNA expression. Deletion of three Sp1 sites (GC box) between 2376 and 2268 or mutation of a CAGA box at 2268/2260 inhibited TGF-b1-induced a2(I) collagen promoter activity. TGF-b1 inducibility was also blocked by a Smad3 dominant negative mutant. Chemical inhibition of Sp1 binding with mithramycin A, or deletion of the GC boxes, inhibited COL1A2 activation by Smad3, suggesting cooperation between Smad3 and Sp1 in the TGF-b1 response. Electrophoretic mobility shift assay showed that Sp1 and Smads form complexes with 2283/2250 promoter sequences. Coimmunoprecipitation experiments demonstrate that endogenous Sp1, Smad3, and Smad4 form complexes in mesangial cells. In a Gal4-LUC reporter assay system, Sp1 stimulated the TGF-b1-induced transcriptional activity of Gal4-Smad3, Gal4Smad4 (266–552), or both. Using the transactivation domain B of Sp1 fused to the Gal4 DNA binding domain, we show that, in our system, the transcriptional activity of this Sp1 domain is not regulated by TGF-b1, but it becomes responsive to this factor when Smad3 is coexpressed. Finally, combined Sp1 and Smad3 overexpression induces marked ligand-independent and liganddependent promoter activity of COL1A2. Thus, Sp1 and Smad proteins form complexes and their synergy plays an important role in mediating TGF-b1-induced a2(I) collagen expression in human mesangial cells.