BMP9 directly induces rapid GSK3‐β phosphorylation in a Wnt‐independent manner through class I PI3K‐Akt axis in osteoblasts

Bone morphogenetic protein (BMP)9 has been reported to be the most potent BMP to induce bone formation. However, the details of BMP9‐transduced intracellular signaling remain ambiguous. Here, we have investigated signal transduction mechanisms of BMP9 in comparison to BMP2, another potent inducer of bone formation, in osteoblasts. In a mouse osteoblast cell line, BMP9 induced higher mRNA levels of alkaline phosphatase (ALP) and runt‐related transcription factor 2 (Runx2) than BMP2 within 2 h. Unlike BMP2, BMP9 induced rapid phosphorylation of glycogen synthase kinase 3‐β (GSK3‐β) and protein kinase B (Akt) and increased the cellular protein content of β‐catenin. BMP9 moderately increased mRNA levels of several canonical Wingless‐related integration site to lower degrees than BMP2. Furthermore, BMP9‐induced GSK3‐β phosphorylation was not inhibited by pretreatment with actinomycin D, cycloheximide, or Brefeldin A, indicating it is independent of Wnt protein secretion. BMP9‐induced GSK3‐β phosphorylation was abrogated by Akt or class I PI3K‐specific inhibitors. Moreover, inactivation of GSK3‐β by LiCl did not further promote ALP and Runx2 mRNA induction by BMP9 as significantly as that by BMP2. Notably, BMP9‐induced GSK3‐β phosphorylation was inhibited by small interfering RNA against endoglin and GIPC PDZ domain‐containing family, member 1. Taken together, our present findings have indicated that BMP9 directly activates GSK3β‐β‐catenin signaling pathway through class I PI3K‐Akt Axis in osteoblasts, which may be essential for the potent osteoinductive activity of BMP9.—Eiraku, N., Chiba, N., Nakamura, T., Amir, M. S., Seong, C.‐H., Ohnishi, T., Kusuyama, J., Noguchi, K., Matsuguchi, T. BMP9 directly induces rapid GSK3‐β phosphorylation in a Wnt‐independent manner through class I PI3K‐Akt axis in osteoblasts. FASEB J. 33, 12124‐12134 (2019). www.fasebj.org