Autophagy Is Possibly Involved in Osteoblast Responses to Mechanical Loadings

Both mechanical loading and autophagy play important roles in regulating bone growth and remodeling, but the relationship between the two remains unclear. In this study, we examined bone structure with micro-CT imaging and measured bone mechanical properties with three-point bending experiments using bones from wild-type (WT) mice and conditional knockout (cKO) mice with Atg7 deletion in their osteoblasts. We found that the knockout mice had significantly less bone volume, bone thickness, bone ultimate breaking force, and bone stiffness compared to wild-type mice. Additionally, bone marrow cells from knockout mice had reduced differentiation and mineralization capacities in terms of alkaline phosphatase and calcium secretion, as well as Runx2 and osteopontin expression. Knockout mice also had significantly less relative bone formation rate due to mechanical loading. Furthermore, we found that the osteoblasts from wild-type mice had stronger responses to mechanical stimulation compared to autophagy-deficient osteoblasts from knockout mice. When inhibiting autophagy with 3 MA in wild-type osteoblasts, we found similar results as we did in autophagy-deficient osteoblasts. We also found that mechanical loading-induced ATP release is able to regulate ERK1/2, Runx2, alkaline phosphatase, and osteopontin activities. These results suggest that the ATP pathway may play an important role in the possible involvement of autophagy in osteoblast mechanobiology.