The transcriptional modulator Ifrd1 is a negative regulator of BMP-2-dependent osteoblastogenesis

We previously demonstrated that the transcriptional coactivator/repressor interferon-related developmental regulator 1 (Ifrd1) was expressed in osteoblasts and participated in the regulation of bone homeostasis. However, it remains unclear how Ifrd1 expression itself is regulated in osteoblasts. In the present study, we investigated the upstream regulatory mechanisms of Ifrd1 in osteoblasts during osteoblastogenesis. Ifrd1 protein expression and runt-related transcription factor 2, the master regulator of osteoblastogenesis, were markedly upregulated by bone morphogenetic protein 2 (BMP-2) stimulation in primary osteoblasts. Moreover, BMP-2 stimulation significantly induced Ifrd1 mRNA expression and promoter activity in osteoblasts. LDN193189, an inhibitor of activin-like kinase 2/3, almost completely inhibited the BMP-2-induced increase in Ifrd1 protein expression. There were at least two putative Smad-binding elements in the 5'-flanking region, which was highly conserved between mouse and human Ifrd1 genes. Co-introduction of both Smad4 and Smad1 significantly increased Ifrd1 promoter activity in osteoblasts. In addition, BMP-2 induced the recruitment of Smad1 to the Ifrd1 promoter in osteoblasts. Moreover, BMP-2-dependent osteoblastogenesis was further enhanced in Ifrd1 knocked-down osteoblasts, as determined by the intensity of Alizarin red stain and marker gene expression. These results suggest that BMP-2 directly induces Ifrd1 expression at the transcriptional level in osteoblasts via the Smad pathway, and Ifrd1 negatively regulates BMP-2-dependent osteoblastogenesis.