The effect of the multi-kinase inhibitor Sorafenib was investigated in an in vitro model of human osteoclastogenesis, represented by peripheral blood mononuclear cells (PBMCs) induced to differentiate into osteoclast-like cells in presence of receptor activator of nuclear factor kappa B ligand (RANKL) plus macrophage-colony stimulating factor (M-CSF). Sorafenib significantly inhibited osteoclastic formation at clinically achievable concentrations (1–3 μM) and promoted autophagia with minimal induction of apoptosis. At the molecular levels, the M-CSF + RANKL combination increased the expression level of the Bcl-2 family member Mcl-1 protein, which is known to play a key role in the control of both cell survival and autophagia. The simultaneous treatment with Sorafenib significantly down-regulated endogenous Mcl-1 expression. Conversely, over-expression of Mcl-1 in primary human macrophages significantly counteracted the anti-osteoclastic activity of Sorafenib, strongly suggesting that Mcl-1 down-regulation played a major role in mediating the inhibitory activity of Sorafenib in cells of the osteoclastic lineage.
