Regulation of expression of plasminogen activator inhibitor-1 in cultured rat osteoblastic cells by osteogenic protein-1 (BMP-7)

Osteogenic Protein-1 (OP-1), a member of the bone morphogenetic protein (BMP) family that belongs to the TGF-β superfamily, induces bone formation in vivo and stimulates the synthesis of biochemical markers characteristic of osteoblast phenotypes in vitro. In the present study, effects of OP-1 on the expression of the plasminogen activator inhibitor-1 (PAI-1) in fetal rat calvaria (FRC) cells were examined. The PAI-1 protein levels in conditioned media of FRC cells treated with OP-1 or solvent control were determined by quantitative 2-dimensional polyacrylamide gel electrophoresis. The identity of PAI-1 was confirmed by mass spectroscopy. OP-1 increased the PAI-1 protein level by about 5-fold after 48 h. Northern blot analysis showed that the PAI-1 mRNA level was elevated by OP-1 by about 25% compared to the control. The observed increase in the PAI-1 mRNA and protein level was regulated post-transcriptionally as supported by the following observations: (a) OP-1 did not stimulate the cloned PAI-1 promoter-reporter gene activity in transient transfection studies, (b) inhibition of transcription by actinomycin D did not change the PAI-1 mRNA level in the OP-1-treated FRC cells, and (c) the stability of the PAI-1 mRNA in FRC cells treated with OP-1 was increased by about 28% compared to that in the control cells. Hence, the present study shows that primary cultures of rat osteoblastic cells synthesize and secrete PAI-1 protein and that OP-1 elevates the PAI-1 protein level. At least, one of the regulatory mechanism is by stabilizing the PAI-1 mRNA.