Interaction between p21‐activated kinase 4 and β‐catenin as a novel pathway for PTH‐dependent osteoblast activation

Parathyroid hormone (PTH) serves dual roles in bone metabolism, exhibiting both anabolic and catabolic effects. The anabolic properties of PTH have been utilized in the treatment of osteoporosis with proven efficacy in preventing fractures. Despite these benefits, PTH can be administered therapeutically for up to 2 years, and its use in patients with underlying malignancies remains a subject of ongoing debate. These considerations underscore the need for a more comprehensive understanding of the underlying mechanisms. p21‐activated kinase 4 (PAK4) is involved in bone resorption and cancer‐associated osteolysis; however, its role in osteoblast function and PTH action remains unknown. Therefore, in this study, we aimed to clarify the role of PAK4 in osteoblast function and its effects on PTH‐induced anabolic activity. PAK4 enhanced MC3T3‐E1 osteoblast viability and proliferation and upregulated cyclin D1 expression. PAK4 also augmented osteoblast differentiation, as indicated by increased mineralization found by alkaline phosphatase and Alizarin Red staining. Treatment with PTH (1–34), an active PTH fragment, stimulated PAK4 expression and phosphorylation in a protein kinase A‐dependent manner. In addition, bone morphogenetic protein‐2 (which is known to promote bone formation) increased phosphorylated PAK4 (p‐PAK4) and PAK4 levels. PAK4 regulated the expression of both phosphorylated and total β‐catenin, which are critical for osteoblast proliferation and differentiation. Moreover, p‐PAK4 directly interacted with β‐catenin, and disruption of β‐catenin's binding to T‐cell factor impaired PAK4‐ and PTH‐induced osteoblast differentiation. Our findings elucidate the effect of PAK4 on enhancing bone formation in osteoblasts and its pivotal role in the anabolic activity of PTH mediated through its interaction with β‐catenin. These insights improve the understanding of the mechanisms underlying PTH activity and should inform the development of more effective and safer osteoporosis treatments.