Upregulated Osterix expression elicited by Runx2 and Dlx5 is required for the accelerated osteoblast differentiation in PP2A Cα-knockdown cells.

Serine/threonine protein phosphatase 2A (PP2A) is involved in regulating various physiological processes including cell cycle, growth, apoptosis, and signal transduction. Osteoblast differentiation is controlled by main bone specific transcription factors including Osterix, distal-less homeobox 5 (Dlx5), and Runt-related transcription factor 2 (Runx2). We previously reported that knockdown of PP2A C increases the expression of Osterix, leading to the accelerated osteoblast differentiation through the upregulation of bone-related genes. In this study, we examined whether Dlx5 and Runx2 are involved in the upregulated Osterix expression in PP2A C-knockdown osteoblasts (shPP2A cells). The expression of Dlx5 as well as Osterix was significantly higher in shPP2A cells in the initial stage of osteoblast differentiation compared with the control cells (shCont). The expression of Runx2 protein was also higher in shPP2A cells compared with shCont cells although its mRNA level was comparable. Reduction of Dlx5 or Runx2 decreased Osterix expression and alkaline phosphatase activity in shPP2A cells. Luciferase assay showed that Osterix promoter activity was drastically elevated in shPP2A cells compared with that in shCont cells. The deletion or mutation of the Dlx5 and Runx2 binding sites significantly suppressed Osterix promoter activity in shPP2A cells. These results indicate that Dlx5 and Runx2 are critical factors for the upregulated Osterix expression in shPP2A cells, which is considered to be important for the accelerated osteoblast differentiation in these cells.