Interaction between GC Box Binding Factors and Smad Proteins Modulates Cell Lineage-specific α2(I) Collagen Gene Transcription*

Type I collagen is produced predominantly in mesenchymal cells, but molecular mechanisms responsible for cell type-specific expression are virtually unknown. During fibrogenic process in the liver, activated hepatic stellate cells (HSC) are the main producers of type I collagen, whereas parenchymal hepatocytes produce little, if any, of this protein. We have previously reported that Sp1 and an interacting unknown factor(s) bind to the −313 to −255 sequence of the α2(I) collagen gene (COL1A2) and play essential roles for basal and TGF-β-stimulated transcription in skin fibroblasts and HSC. Recently, Smad3 has been shown to bind to this region, and its interaction with Sp1 has been implicated in TGF-β-elicited COL1A2 stimulation. The present study demonstrates predominant binding of Sp3 rather than Sp1 to this regulatory element in parenchymal hepatocytes. In these cells, this region did not exhibit strong enhancer activity or mediate the effect of TGF-β. Transfection of HSC with an Sp3 expression plasmid abolished the COL1A2 response to TGF-β, whereas overexpression of Sp1 in hepatocytes increased basal COL1A2transcription and conferred TGF-β responsiveness. Functional and physical interactions between Sp1 and Smad3, but not between Sp3 and Smad3, were demonstrated using the bacterial GAL4 system and immunoprecipitation-Western blot analyses. These results indicate that cell lineage-specific interactions between GC box binding factors and Smad protein(s) may account, at least in part, for differential COL1A2 transcription and TGF-β responsiveness in HSC and parenchymal hepatocytes.