Type V collagen is a quantitatively minor collagen, but acts as critical regulator of fibril formation in the extracellular matrix The purpose of this study is to clarify the mechanism responsible for the transcriptional regulation of the mouse Col5a3 gene in osteoblastic cells Sp7/Osterix is a transcription factor specifically expressed by osteoblasts and is important for osteoblast differentiation The vet expression of Sp7/Osterix significantly increased the promoter activity and the endogenous mRNA level of the Col5a3 gene in osteoblastic cells Conversely, a reduction of Sp7/Osterix by siRNA treatment decreased the promoter activity and the endogenous mRNA level of the Col5a3 gene A CHIP assay confirmed that Sp7/Osterix interacted with the Col5a3 core promoter in vivo at the Sp1 binding site The data from the experiments using the osteoblast differentiation model and the co-overexpression of Sp7/Osterix with Sp1 suggest that Sp7/Osterix promotes the expression of the collagen gene. Col5a3, and thereby playing a role in bone formation (C) 2010 Elsevier Inc All rights reserved