Plasminogen activator inhibitor‐1 deficiency suppresses osteoblastic differentiation of mesenchymal stem cells in mice

Abstract Plasminogen activator inhibitor‐1 (PAI‐1) is known as an inhibitor of fibrinolytic system. Previous studies suggest that PAI‐1 is involved in the pathogenesis of osteoporosis induced by ovariectomy, diabetes, and glucocorticoid excess in mice. However, the roles of PAI‐1 in early‐stage osteogenic differentiation have remained unknown. In the current study, we investigated the roles of PAI‐1 in osteoblastic differentiation of mesenchymal stem cells (MSCs) using wild‐type (WT) and PAI‐1‐deficient (PAI‐1 KO) mice. PAI‐1 mRNA levels were increased with time during osteoblastic differentiation of MSCs or mesenchymal ST‐2 cells. However, the increased PAI‐1 levels declined at the mineralization phase in the experiment using MC3T3‐E1 cells. PAI‐1 deficiency significantly blunted the expression of osteogenic gene, such as osterix and alkaline phosphatase enhanced by bone morphogenetic protein (BMP)‐2 in bone marrow‐derived MSCs (BM‐MSCs), adipose‐tissue‐derived MSCs (AD‐MSCs), and bone marrow stromal cells of mice. Moreover, a reduction in endogenous PAI‐1 levels by small interfering RNA significantly suppressed the expression of osteogenic gene in ST‐2 cells. Plasmin did not affect osteoblastic differentiation of AD‐MSCs induced by BMP‐2 with or without PAI‐1 deficiency. PAI‐1 deficiency and a reduction in endogenous PAI‐1 levels did not affect the phosphorylations of receptor‐specific Smads by BMP‐2 and transforming growth factor‐β in AD‐MSCs and ST‐2 cells, respectively. In conclusion, we first showed that PAI‐1 is crucial for the differentiation of MSCs into osteoblasts in mice.