Estrogens protect bone mass by inhibiting NAD+ metabolism in osteoclasts

Estrogens protect against bone loss by reducing osteoclast number and bone resorption, primarily via direct actions on osteoclast precursors. In these cells, estrogens attenuate RANKL-induced stimulation of mitochondrial complex I, which is crucial for ATP generation through NADH oxidation. NAD+ promotes redox reactions and activates NAD+-dependent enzymes, including the mitochondrial deacetylase SIRT3. However, the contribution of NAD+ to the skeletal effects of estrogens remains unknown. We show that NAD+ levels and SIRT3 activity are upregulated by RANKL and inhibited by 17{beta}-estradiol (E2) in mouse and human osteoclast precursors. Increasing NAD+ or the mitochondrial NAD+/NADH ratio reverses the inhibitory effects of E2 on SIRT3 activity and osteoclastogenesis in vitro. Deletion of Nampt, a key NAD salvage enzyme, reduces NAD+ and prevents bone loss in ovariectomized mice. Similarly, deletion of Sirt3 in osteoclast precursors mitigates estrogen deficiency-induced bone resorption. These findings indicate that suppression of NAD+ levels and mitochondrial redox metabolism by estrogens contributes to their anti-resorptive effects via inhibition of SIRT3.