Runx2 Can Induce The Gene Expression Of Osterix In The Non-osteoblastic Cells

The skeleton is a very complicated and continuously changing tissue, whose process of development involves many factors. Transcription factors play a vital role in the developing process. Runx2 and Osterix are both key transcription factors involved in osteoblast differentiation and function and thus bone formation. Osterix acts downstream of Runx2 in the pathway of osteoblastic lineage. Runx2 has an essential role in the stage that multipotential mesenchymal preosteoblasts differentiating into functional osteoblasts, whether it is targeted by Runx2, however, is still unclear. By using RT-PCR, we found that transient expression of Runx2 in a number of non-osteoblastic cell lines, either pluripotent or differentiated, was able to induce the expression of Osterix. We cloned the 3.2 kb 5'-flanking region of human Osterix gene ,and functionally analyzed the promoter linked to a firefly luciferase reporter gene. We found that Runx2 could also upregulate the promoter activity in NIH3T3 and 293T cells. The fragment between 1.1 kb and 0.5 kb upstream of the first translation start codon was needed for both basal promoter activity and Runx2 responsiveness in 293T cells. A main functional Runx2 binding site "AGTGGTT" was then identified within the promoter, whereas, another Runx2 binding site "TGTGGT" was only a very weak regulatory site. Our results suggested that Osterix is a target of Runx2 and may be directly induced in the Runx2-directed commitment of osteoblast differentiation from multipotential mesenchymal cells.