Absence of p21(WAF1/CIP1/SDI1) protects against osteopenia and minimizes bone loss after ovariectomy in a mouse model.

p21(WAF1/CIP1/SDI1) is a critical sentinel of the cell cycle that plays an important role in determining cell fate with respect to proliferation, differentiation and apoptosis. Recent studies have demonstrated that inhibition/loss of p21 promotes osteo-chondro differentiation in progenitor/stem cells, and that p21 knockout (p21-/-) mice demonstrate enhanced bone regeneration compared to wild-type controls after a non-critical size defect. It was therefore hypothesized that the absence of p21 may also protect against bone loss through enhancing bone formation, tilting the balance away from bone resorption, in an ovariectomy-induced osteopenia mouse model, investigated via microCT imaging. While p21-/- mice demonstrated significantly less bone loss after ovariectomy compared to wild-type controls, no increase in the number osteoclasts or osteoblasts in the bone or bone marrow was observed, nor was there an increase in osteoclast activity. Therefore, while the absence of p21 protected mice against estrogen mediated bone loss, the mechanisms/pathways responsible remained elusive. This study demonstrates that p21 may play a significant role in bone remodeling, and a better understanding of how the p21 pathway regulates bone anabolism and catabolism could lead to novel therapies for osteoporosis in the future.