In myeloid cells, Mcl 1 prevents apoptosis and its expression is highly regulated by survival promoting factors via the PI3K Akt mammlian target of rapamycin S6 kinase as well as mitogen activated protein kinase signalling pathways. The latter are also known to be activated by M CSF, TNF, and RANKL, which promote osteoclast survival. Whilst our work was in progress, Bradley and colleagues PD 0332991 showed that Mcl 1 mRNA and protein are upregulated in osteoclasts by M CSF via activation of MEK ERK and increased expres sion of Egr2. Hence, although the exact mechanism by which RANKL upregulates Mcl 1 in osteoclasts remains to be proven, it Inhibitors,Modulators,Libraries is highly likely that this also involves MAPK and or mTOR signalling pathways.

ALN treatment alone caused a decrease in Mcl 1 levels, perhaps since the activation of mTOR S6 kinase in osteoclasts appears to involve geranylger anylated proteins such as Rac, and ALN is known to effectively prevent protein geranylgeranylation. With myeloma cells, others have also shown that the inhibition of protein geranylgeranylation causes the loss of Mcl 1. In our study, Inhibitors,Modulators,Libraries RANKL restored the level of Mcl 1 to that in control osteoclasts, but this was still less than the level of Mcl 1 Inhibitors,Modulators,Libraries in the presence Inhibitors,Modulators,Libraries of RANKL alone. Hence, ALN and RANKL may have opposing effects on the same signalling pathways that are required for Mcl 1 expression in osteoclasts. In further support of an important role for Mcl 1 in osteoclast survival, we found that the level of Mcl 1 rapidly decreased fol lowing the removal of M CSF and RANKL from cultures of mouse osteoclasts, which are highly dependent on the pres ence of such survival factors.

In this model, the loss of Mcl 1 preceded the appearance of morphological and biochemical features of apoptosis and occurred in the absence of changes in the level of Bcl 2 or Bcl xL. In addition, the loss of Mcl 1 in mouse osteoclasts could be prevented by the addition of fac tors that are known to pro mote osteoclast survival. The loss of Mcl 1 in osteoclasts probably occurred by proteasomal Inhibitors,Modulators,Libraries degradation since we also found that the proteasome inhibitor MG132 caused an increase in the level of Mcl 1 in mouse osteoclasts. This is consistent with a requirement in oste oclasts for Y-27632 solubility continual protein synthesis to prevent apoptosis. Together, these observations indicate that Mcl 1 plays an important role in maintaining osteoclast survival.