Osteoblast/osteocyte-specific inactivation of Stat3 decreases load-driven bone formation and accumulates reactive oxygen species.

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 (2.50-2.75N with 120 cycles/day at 2Hz 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.