Increased osteoclast-mediated bone resorption is characteristic of osteoporosis, malignant bone disease and inflammatory arthritis. Targeted deletion of Sirtuin1 (Sirt1), a key player in aging and metabolism, in osteoclasts results in increased osteoclast-mediated bone resorption in vivo, making it a potential novel therapeutic target to block bone resorption. Sirt1 activating compounds (STACs) were generated and were investigated in animal disease models and in humans however their mechanism of action was a source of controversy. We studied the effect of SRT2183 and SRT3025 on osteoclastogenesis in bone-marrow derived macrophages (BMMs) in vitro, and discovered that these STACs inhibit RANKL-induced osteoclast differentiation, fusion and resorptive capacity without affecting osteoclast survival. SRT2183 and SRT3025 activated AMPK, increased Sirt1 expression and decreased RelA/p65 lysine310 acetylation, critical for NF-κB activation, and an established Sirt1 target. However, inhibition of osteoclastogenesis by these STACs was also observed in BMMs derived from sirt1 knock out (sirt1-/-) mice lacking the Sirt1 protein, in which neither AMPK nor RelA/p65 lysine 310 acetylation was affected, confirming that these effects require Sirt1, but suggesting that Sirt1 is not essential for inhibition of osteoclastogenesis by these STACs under these conditions. In sirt1 null osteoclasts treated with SRT2183 or SRT3025 Sirt3 was found to be down-regulated. Our findings suggest that SRT2183 and SRT3025 activate Sirt1 and inhibit RANKL-induced osteoclastogenesis in vitro however under conditions of Sirt1 deficiency can affect Sirt3. As aging is associated with reduced Sirt1 level and activity, the influence of STACs on Sirt3 needs to be investigated in vivo in animal and human disease models of aging and osteoporosis.

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