Smad-interacting protein 1 is a repressor of liver/bone/kidney alkaline phosphatase transcription in bone morphogenetic protein-induced osteogenic differentiation of C2C12 cells

Up-regulation of liver/bone/kidney alkaline phosphatase (LBK-ALP) has been associated with the onset of osteogenesis in vitro. Its transcription can b e up-regulated by bone morphogenetic proteins (BMPs), constitutively active forms of their cognate receptors, or appropriate Smads. The promoter of LB K-ALP has been characterized partially, but not much is known about its tra nscriptional modulation by BMPs. A few Smad-interacting transcriptional fac tors have been isolated to date. One of them, Smad-interacting protein 1 (S IP1), belongs to the family of two-handed zinc finger proteins binding to E 2-box sequences present, among others, in the promoter of mouse LBK-ALP. In the present study we investigated whether SIP1 could be a candidate regula tor of LBK-ALP transcription in C2C12 cells. We demonstrate that SIP1 can r epress LBK-ALP promoter activity induced by constitutively active Alk2-Smad 1/Smad5 and that this repression depends on the binding of SIP1 to the CACC T/CACCTG cluster present in this promoter. Interestingly, SIP1 and alkaline phosphatase expression domains in developing mouse limb are mutually exclu sive, suggesting the possibility that SIP1 could also be involved in the tr anscriptional regulation of LBK-ALP in vivo. Taken together, these results offer an intriguing possibility that ALP up-regulation at the onset of BMP- induced osteogenesis could involve Smad/SIP1 interactions, resulting in the derepression of that gene.