Osteoclast differentiation is dependent on the activities of receptor activator 
NF-kB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF). Given 
that RANKL plays a critical role in osteoclast formation and bone resorption, 
any new compounds found to alter its activity would be predicted to have 
therapeutic potential for disorders associated with bone loss. 
Methylsulfonylmethane (MSM) is a naturally occurring sulfur compound with 
well-documented anti-oxidant and anti-inflammatory properties; currently its 
effects on osteoclast differentiation are unknown. We sought to investigate 
whether MSM could regulate osteoclastogenesis, and if so, its mechanism of 
action. In this study, we investigated the effects of MSM on RANKL-induced 
osteoclast differentiation, together with STAT3's involvement in the expression 
of osteoclastic gene markers. These experiments were conducted using bone marrow 
derived macrophages (BMMs) and cell line material, together with analyses that 
interrogated both protein and mRNA levels, as well as signaling pathway 
activity. Although MSM was not toxic to osteoclast precursors, MSM markedly 
inhibited RANKL-induced TRAP activity, multinucleated osteoclast formation, and 
bone resorptive activity. Additionally, the expression of several 
osteoclastogenesis-related marker genes, including TRAF6, c-Fos, NFATc1, 
cathepsin K, and OSCAR were suppressed by MSM. MSM mediated suppression of 
RANKL-induced osteoclastogenesis involved inhibition of ITAM signaling effectors 
such as PLCγ and Syk, with a blockade of NF-kB rather than MAPK activity. 
Furthermore, MSM inhibited RANKL-induced phosphorylation of STAT3 Ser727. 
Knockdown of STAT3 using shRNAs resulted in reduced RANKL-mediated 
phosphorylation of Ser727 STAT3, and TRAF6 in cells for which depletion of STAT3 
was confirmed. Additionally, the expression of RANKL-induced osteoclastogenic 
marker genes were significantly decreased by MSM and STAT3 knockdown. Taken 
together, these results indicate that STAT3 plays a pivotal role in 
RANKL-induced osteoclast formation, and that MSM can attenuate RANKL-induced 
osteoclastogenesis by blocking both NF-kB and STAT3 activity.