Analysis of the Runx2 promoter in osseous and non-osseous cells and identification of HIF2A as a potent transcription activator

Abstract

Little is known about the upstream regulator of Runx2, a master regulator of osteoblast differentiation in bone tissues. To elucidate the molecular mechanism of Runx2 gene expression, we analyzed Runx2 promoter activity in osseous (MC3T3-E1, KS483, Kusa) and non-osseous (NIH3T3, C3H10T1/2, mouse embryonic fibroblasts) cells and also identified Runx2 upstream regulator using a Runx2 promoter-derived luciferase reporter system. After cloning 15 serial deletion constructs from − 6832 bp/+ 390 bp to − 37 bp/+ 390 bp of the Runx2-P1 promoter, we performed a transient transfection assay in osseous and non-osseous cells. A reduction in Runx2 promoter activity was observed in two regions; one was between − 3 kb and − 1 kb, and the other was between − 155 bp and − 75 bp. The step-down pattern in promoter activity between − 3 kb and − 1 kb was observed only in osseous cells. Interestingly, the step-down pattern between − 155 bp and − 75 bp was revealed in both cell types. Consistently, β-galactosidase staining in axial skeleton of − 3 kb-Runx2-P1- LacZ transgenic mice was positive, but that of all skeletal tissues of − 1 kb-Runx2-P1- LacZ transgenic mice was negative. To identify upstream regulators of the Runx2-P1 promoter, we screened 100 transcription factors using Runx2-P1-luciferase reporter constructs in NIH3T3 fibroblasts and HeLa cells. Among them, HIF2A was identified as the strongest activator of Runx2-P1 promoter activity. A HIF2A-responsive site on the Runx2 promoter was identified between − 106 bp and − 104 bp by mutation analysis. An electrophoretic mobility shift assay and chromatin immunoprecipitation assay confirmed the binding of HIF2A to the Runx2-P1 promoter in vitro and in vivo, respectively. Knock-down using siRNA against HIF2A confirmed that HIF2A is an important regulator of Runx2 gene expression. Collectively, these results suggest that the region between − 3 kb and − 1 kb is required for the minimal skeletal tissue-specific expression of Runx2, and that the region between − 155 bp and − 75 bp is important for its basal transcription, which may be in part mediated by HIF2A in bone tissues.