Htra1 is a Novel Transcriptional Target of RUNX2 That Promotes Osteogenic Differentiation

Runt-related transcription factor 2 (Runx2) is a master regulator of osteogenic differentiation, but most of the direct downstream targets of RUNX2 during osteogenesis are unknown. Likewise, High-temperature requirement factor A1 (HTRA1) is a serine protease expressed in bone, yet the role of Htra1 during osteoblast differentiation remains elusive. We investigated the role of Htra1 in osteogenic differentiation and the transcriptional regulation of Htra1 by RUNX2 in primary mouse mesenchymal progenitor cells.

Overexpression of Htra1 was carried out in primary mouse mesenchymal progenitor cells to evaluate the extent of osteoblast differentiation. Streptavidin agarose pulldown assay, chromatin immunoprecipitation assay, and dual luciferase assay were carried out to investigate the interaction of RUNX2 protein at the Htra1 promoter during osteoblast differentiation.

Overexpression of Htra1 increased the production of mineralized bone matrix, upregulating several osteoblast genes, such as Sp7 transcription factor (Sp7) and Alkaline phosphatase, liver/bone/kidney (Alpl). In addition, Htra1 upregulated osteogenesis-related signalling genes, such as Fibroblast growth factor 9 (Fgf9) and Vascular endothelial growth factor A (Vegfa). A series of experiments confirmed Htra1 as a direct RUNX2 transcriptional target. Overexpression of Runx2 resulted in the upregulation of Htra1 mRNA and protein. Chromatin immunoprecipitation and streptavidin agarose pull-down assays showed that RUNX2 binds a proximal -400 bp region of the Htra1 promoter during osteogenic differentiation. Dual luciferase assays confirmed that RUNX2 activates the proximal Htra1 promoter during osteogenic differentiation. Mutation of putative RUNX2 binding sites revealed that RUNX2 interacts with the Htra1 promoter at -252 bp and -84 bp to induce Htra1 expression.

We demonstrate that Htra1 is a positive regulator of osteogenic differentiation, showing for the first time that Htra1 is a direct downstream target of RUNX2.