Function of STAT5A in osteogenic differentiation and bone formation

Abstract

The regulation of osteogenesis in human bone marrow-derived stromal cells (hBMSCs) is important for bone formation. Despite advances in understanding the molecular mechanism of osteogenesis, crucial modulators in a fracture healing are not well investigated. Signal transducer and activator of transcription 5 (STAT5A) is vital for proliferation, differentiation, and survival of various cells. To investigate the role of STAT5A in osteogenesis, I performed knockdown of Stat5a using small interfering RNA and STAT5 inhibitor. The suppression of STAT5A increased the osteogenic differentiation and distal-less homeobox 5 (DLX5) protein expression, whereas the inhibition of STAT5B only partially increased differentiation. Subsequent gene expression profiling and RT-qPCR analyses of STAT5A inhibition led to transcriptional activity of DLX5, osteogenic transcription factor, in osteogenesis of hBMSCs. Chromatin immune-precipitation (ChIP) assays analyzing the corresponding gene loci identified STAT5A binding sites on the promoter region of DLX5. I propose that STAT5A control osteogenesis throughout regulating transcription level of DLX5. To better characterize the contribution of STAT5A in bone formation, I generated mice with deletion of Stat5a (Stat5a -/- mice) and analyzed their bone phenotype. Stat5-/- mice exhibited increased bone-formation compared to wild type mice. And, the expression level of DLX5 protein was increased in mBMSCs of Stat5-/- compared to wild type mice. To evaluate the role of osteoblast activity of Stat5-/- mice in bone repair, femoral fracture healing was compared with wild-type control mice. With respect to fracture healing, total callus of Stat5-/- mice was larger than wild-type mice fractures at post fracture 2 weeks. Surprisingly, soft callus of Stat5-/- mice was organized rapidly to hard bone at post fracture 4 weeks. In summary, I show that the suppression of STAT5A activates distal-less homeobox 5 (DLX5) and enhances osteogenesis in vitro and in vivo. Stat5a knockout (Stat5a -/-) mice are protected from age-related osteoporosis, caused by loss of bone density and strength. In a murine model of fracture repair, lack of Stat5a leads to significant enhancement in a bone healing process by stimulating the formation of new bones. Cartilaginous callus of Stat5a -/- mice organized to bony callus faster than that of wild type mice. Taken together, these findings suggest that STAT5A plays a role in osteogenesis and bone formation via modulation of DLX5.