RANKL-stimulated TNFα production in osteoclast precursor cells promotes osteoclastogenesis by modulating RANK signaling pathways

Although TNFα is known to be an important factor for bone resorption, particularly in inflammatory bone diseases, the relevance between RANKL and TNFα in osteoclastogenesis remains unclear. In this study we examined the mechanism of TNFα induced osteoclastogenesis and its downstream signaling. We show that osteoclastogenesis is suppressed by anti-TNFα- and anti-TNF receptor type I (TNFRI)-antibodies and in TNFα- and TNFRI-deficient mice using in vitro culture systems: (1) co-culture of mouse spleen derived osteoclast precursor cells (pOCs) with osteoblasts, (2) pure pOC culture and (3) RAW264.7 cells in presence of RANKL. Furthermore, TNFα production in pOCs was stimulated by RANKL. Endogenous TNFα in pOCs induced c-Fos and NFATc1. Expression rates of NFATc1 and c-Fos were significantly decreased in TNFα- and TNFRI-deficient pOCs during osteoclastogenesis. These results indicate that TNFα is induced by RANKL in pOCs and serves as an autocrine factor promoting osteoclastogenesis through c-Fos and NFATc1 activation.