Abstract
Response gene to complement 32 (RGC-32) is activated by transforming growth factor- β (TGF-β) and plays an important role in smooth muscle cell (SMC) differentiation from neural crest Monc-1 cells. The molecular mechanism governing TGF-β activation of RGC-32, however, remains to be determined. The present studies indicate that TGF-β regulates RGC-32 gene transcription. Sequence analysis revealed a Smad binding element (SBE) located in the region from -1344 to -1337 bp upstream of the transcription start site of RGC-32 gene. A polyomavirus enhancer activator (PEA3) binding site is adjacent to the SBE. Mutation at either SBE or PEA3 site significantly inhibited RGC-32 promoter activity. Mutations at both sites completely abolished TGF-β-induced promoter activity. Biochemically, TGF-β stimulated recruitment of Smad2, Smad4, and PEA3 to the RGC-32 promoter, as revealed by gel shift and chromatin immunoprecipitation analyses. Functionally, Smad2, but not Smad3, activated RGC-32 promoter. PEA3 appeared to enhance Smad2 activity. In agreement with their function, Smad2, but not Smad3, physically interacted with PEA3. In TGF-β-induced SMC differentiation of Monc-1 cells, knockdown of Smad2 by short hairpin RNA resulted in downregulation of RGC-32 and SMC marker genes. The downregulation of SMC markers, however, was rescued by exogenously introduced RGC-32. These results demonstrate that Smad2 regulation of RGC-32 transcription is essential for SMC differentiation from neural crest cells.