Augmentation of TNF-induced osteoclast differentiation by inhibition of ERK and activation of p38: Similar intracellular signaling between RANKL- and TNF-induced osteoclast differentiation

Research in osteoclast differentiation has been greatly advanced since the identification of receptor activator of nuclear factor-κB ligand (RANKL) as osteoclast differentiation factor. The mechanisms of RANKL-induced osteoclast differentiation have been extensively investigated. Mitogen-activated protein kinases (MAPKs) were shown to play crucial roles in RANKL-induced osteoclast differentiation. RANKL-induced osteoclast differentiation was enhanced by inhibition of extracellular signal-regulated kinase (ERK), whereas it was suppressed by inhibition of p38 MAPK. It was reported that tumor necrosis factor (TNF), a major proinflammatory cytokine, induced osteoclast differentiation independently of RANKL. A report showed that inhibition of p38 suppressed TNF-induced osteoclast differentiation, whereas inhibition of ERK did not augment TNF-induced osteoclast differentiation. In this study we reevaluated the roles for MAPKs in TNF-induced osteoclast differentiation. In contrast with the previous report, pretreatment of mouse monocytic RAW264 cells with MAPK/ERK kinase (MEK) inhibitors including PD98059 and U-0126 augmented TNF-induced osteoclast differentiation. Furthermore, we found that U-0126 was more effective in augmentation of osteoclast differentiation than PD98059. Western blot analysis showed that U-0126 inhibited ERK phosphorylation and enhanced p38 phosphorylation, whereas PD98059 inhibited both ERK and p38 phosphorylation. SB203580, a p38 inhibitor, suppressed TNF-induced osteoclast differentiation, and inhibited p38 phosphorylation whereas it augmented ERK phosphorylation. These results demonstrate that ERK inhibition and p38 activation play crucial roles in both RANKL- and TNF-induced osteoclast differentiation.