Sp7/Osterix, a transcription factor whose expression is restricted in osteoblasts, belongs to the Sp family of transcription factor that bind to G/C-rich sequences. Previous studies have identified a Sp1binding site in the proximal promoter region of the mouse Col5a1 gene, but it did not activate or repress this gene in a mouse fibroblast cell line and a human rhabdomyosarcoma cell line. The purpose of the present study was to clarify the involvement of Sp7/Osterix in the mouse Col5a1 gene. A functional analysis revealed that mutation of the Sp1 binding site specifically decreased the promoter activity in osteoblastic cells. An overexpression of Sp7/Osterix significantly increased the promoter activity and the endogenous mRNA levels of the Col5a1 gene in osteoblastic cells. Conversely, siRNA-mediated knockdown of Sp7/Osterix decreased the promoter activity and the endogenous mRNA levels of the Col5a1 gene. These effects on promoter activity were canceled when the mutant construct of Sp1 binding site was introduced. Consistent with these data, the experiments using an osteoblast differentiation model showed increased promoter activity and endogenous mRNA levels, along with increased Sp7/Osterix during differentiation. Therefore, type V collagen appears to be involved in bone formation.
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