Molecular effects of gallium on osteoclastic differentiation of mouse and human monocytes

We had previously reported that gallium (Ga) inhibited both the differentiation and resorbing activity of osteoclasts in a dose-dependent manner. To provide new insights into Ga impact on osteoclastogenesis, we investigated here the molecular mechanisms of Ga action on osteoclastic differentiation of monocytes upon Rankl treatment. We first observed that Ga treatment inhibited the expression of Rankl-induced early differentiation marker genes, while the same treatment performed subsequently did not modify the expression of late differentiation marker genes. Focusing on the early stages of osteoclast differentiation, we observed that Ga considerably disturbed both the initial induction as well as the autoamplification step of Nfatc1 gene. We next demonstrated that Ga strongly up-regulated the expression of Traf6, p62 and Cyld genes, and we observed concomitantly an inhibition of IκB degradation and a blockade of NFκB nuclear translocation, which regulates the initial induction of Nfatc1 gene expression. In addition, Ga inhibited c-Fos gene expression, and subsequently the auto-amplification stage of Nfatc1 gene expression. Lastly, considering calcium signaling, we observed upon Ga treatment an inhibition of calcium-induced Creb phosphorylation, as well as a blockade of gadolinium-induced calcium entry through TRPV-5 calcium channels. We identify for the first time Traf6, p62, Cyld, IκB, NFκB, c-Fos, and the calcium-induced Creb phosphorylation as molecular targets of Ga, this tremendously impacting the expression of the master transcription factor Nfatc1. In addition, our results strongly suggest that the TRPV-5 calcium channel, which is located within the plasma membrane, is a target of Ga action on human osteoclast progenitor cells.