Hongkang Zhou, America B. Newnum, Joseph R. Martin, Ping Li, Mark T. Nelson, Akira Moh, Xin-Yuan Fu, Hiroki Yokota and Jiliang Li
Signal transducers and activators of transcription 3 (Stat3) is a transcription factor expressed in many cell types including osteoblasts, osteocytes, and osteoclasts. STAT3 mutations cause a rare human immunodeficiency disease that presents reduced bone mineral density and recurrent pathological fractures. To investigate the role of Stat3 in load-driven bone metabolism, two strains of osteoblast/osteocyte-selective Stat3 knockout (KO) mice were generated. Compared to age-matched littermate controls, this selective inactivation of Stat3 significantly lowered bone mineral density (7–12%, p < 0.05) as well as ultimate force (21–34%, p < 0.01). In ulna loading (2.50–2.75 N with 120 cycles/day at 2 Hz for 3 consecutive days), Stat3 KO mice were less responsive than littermate controls as indicated by reduction in relative mineralizing surface (rMS/BS, 47–59%, p < 0.05) and relative bone formation rate (rBFR/BS, 64–75%, p < 0.001). Furthermore, inactivation of Stat3 suppressed load-driven mitochondrial activity, which led to an elevated level of reactive oxygen species (ROS) in cultured primary osteoblasts. Taken together, the results support the notion that the loss-of-function mutation of Stat3 in osteoblasts and osteocytes diminishes load-driven bone formation and impairs the regulation of oxidative stress in mitochondria.