Long-term osteoblast-specific Plekho1 silencing by RNA interference prevents bone formation reduction during aging in safety in both genders (Abstract)

Osteoporosis is considered as a serious public health concern due to its high prevalence among the elderly population. However, the only FDA-approved agent stimulating new bone formation, parathyroid hormone (PTH), has the great concern of potential risk of osteosarcoma after long-term administration. Smad-dependent BMP signaling is responsible for bone formation. Pleckstrin homology domain-containing family O member 1 (PLEKHO1) specifically targets Smad ubiquitination regulatory factor 1 (Smurf1) to promote the poly-ubiquitylation and proteasomal degradation of Smad1/5. In the bone samples from both human and rats, the Plekho1 mRNA level within osteoblasts is negatively correlated with both the p-Smad1/5 protein level in osteoblasts and bone formation rate during aging in both genders. We found that the osteoblast-specific Plekho1 knockout mice demonstrated higher Smad-dependent BMP signaling and bone formation compared to wildtype mice during aging in both genders. Osteoblast-specific Smad1 overexpression attenuated the age-related bone formation reduction, while overexpression of Plekho1 in osteoblasts counteracted the above beneficial effect. Treating aging rats with weekly intravenous injection of Plekho1 siRNA conjugated with an osteoblast delivery system for six weeks significantly enhanced Smad-dependent BMP signaling and promoted bone formation in both genders. Furthermore, we evaluated the effect of long-term osteoblast-specific Plekho1 silencing on bone formation in aging rats of both genders. For male, the 16-month old rats were intravenously injected with osteoblast-targeting Plekho1 siRNA weekly for six months. For female, the rats were ovariectomized at 4-month old and the treatment started at 13-month old and lasted for six months. For both genders, the animals with the treatment showed enhanced Smad-dependent BMP signaling, promoted bone formation, and elevated bone mass compared to the vehicle control group. Moreover, the treatment group didn’t show any sign of side effects. Taken together, the study established the critical role of intraosteoblast Plekho1 during age-related bone formation reduction. Targeting Plekho1 in osteoblasts may be a potential bone anabolic strategy to reverse osteoporosis. Long-term osteoblast-specific Plekho1 silencing by RNA interference could prevent osteoporosis during aging in safety in both genders.Acknowledgments: This study was supported by the Hong Kong General Research Fund (14108816)