Tumor necrosis factor-alpha supports the survival of osteoclasts through the activation of Akt and ERK.
Differentiated osteoclasts have a short life span. We tested various cytokines and growth factors for the effects on the survival of purified mature osteoclasts. In the absence of any added factors, osteoclasts exhibited the survival rate of less than 25% after a 24-h incubation. Among the tested factors, tumor necrosis factor-alpha (TNF-alpha) was found to increase the survival rate to approximately 80%. The TNF-alpha-enhanced survival of osteoclasts appeared to be associated with reduction in apoptosis and suppression of caspase activation. The antiapoptotic signaling pathways involved in the TNF-alpha-induced osteoclast survival were investigated. TNF-alpha treatment increased the phosphorylation of Akt in osteoclasts, which was suppressed by a phosphatidylinositol 3-kinase inhibitor LY294002 and an Src family kinase-selective inhibitor PP1. These inhibitors also attenuated the TNF-alpha stimulation of osteoclast survival. In addition an increase in the phosphorylation of ERK was observed upon TNF-alpha stimulation. PD98059, a specific inhibitor of the ERK-activating kinase MEK-1, abolished the TNF-alpha-induced ERK phosphorylation and osteoclast survival, and in these responses the involvement of Grb2 and ceramide was observed. These results suggest that TNF-alpha promotes the survival of osteoclasts by engaging the phosphatidylinositol 3-kinase Akt and MEK/ERK signaling pathways.