Osteopontin (OPN) is an osteoblast-derived secretory protein that plays a role in bone remodeling, osteoblast responsiveness, and inflammation. We recently found that osteoblast differentiation is type-specific, with conditions of JNK inactivation inducing osteoblasts that preferentially express OPN (OPN-type). Since OPN-type osteoblasts highly express osteogenesis-inhibiting proteins and Rankl, an important inducer of osteoclastogenesis, an increased appearance of OPN-type osteoblasts may be associated with inefficient and poor-quality bone regeneration. However, whether specific osteogenic inducers can modulate OPN-type osteoblast differentiation is completely unknown. Here, we demonstrate that bone morphogenic protein 9 (BMP9) prevents induction of OPN-type osteoblast differentiation under conditions of JNK inhibition. Although JNK inactivation suppressed both BMP2- and BMP9-induced matrix mineralization and osteocalcin expression, the expression of Rankl and specific cytokines such as Gpha2, Esm1, and Sfrp1 under similar conditions was increased in all cells except those treated with BMP9. Increased expression of Id4, a critical transcriptional regulator of OPN-type osteoblast differentiation, was similarly prevented only in BMP9-treated cells. We also found that BMP9 specifically induces the expression of Hey1, a bHLH transcriptional repressor, and that Id4 inhibits the suppressive effects of Hey1 on Opn promoter activity by forming Id4-Hey1 complexes in osteoblasts. Using site-direct mutagenesis, ChIP, and immunoprecipitation, we elucidated that BMP9-induced overexpression of Hey1 can overcome the effects of Id4 and suppress OPN expression. We further found that p38 activation and JNK inactivation are involved in BMP9-induced Hey1 expression. Collectively, these data suggest that BMP9 is a unique osteogenic inducer that regulates OPN-type osteoblast differentiation.
Keywords: BMP9; Differentiation; Hey1; Id4; Osteoblast; Osteocalcin; Osteopontin.
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