VEGF-C, a lymphatic growth factor, is a RANKL target gene in osteoclasts that enhances osteoclastic bone resorption through an autocrine mechanism

Osteoclasts are bone-resorbing cells, but they also secrete and respond to cytokines. Here, we test the hypothesis that osteoclasts secrete the lymphatic growth factor, VEGF-C, to increase their resorptive activity. Osteoclasts and osteoclast precursors were generated by culturing splenocytes with macrophage colony-stimulating factor and RANKL from wild-type, NF-kappa Bp50(-/-)/p52(-/-), and Src(-/-) mice. Expression of VEGFs was measured by real time reverse transcription-PCR, Western blotting, and immunostaining. The effect of VEGF-C signaling on osteoclast function was determined by osteoclastogenesis and pit assays. RANKL increased the expression of VEGF-C but not of other VEGFs in osteoclasts and their precursors. RANKL-induced VEGF-C expression was reduced in NF-kappa Bp50(-/-)/p52(-/-) precursors or wild- type cells treated with an NF-kappa B inhibitor. VEGF-C directly stimulated RANKL-mediated bone resorption, which was reduced by the VEGF-C-specific receptor blocker, VEGFR3:Fc. Osteoclasts express VEGFR3, and VEGF-C stimulated Src phosphorylation in osteoclasts. VEGF-C-mediated bone resorption was abolished in Src(-/-) osteoclasts or cells treated with an Src inhibitor. We conclude that RANKL stimulates osteoclasts and their precursors to release VEGF-C through an NF-kappa B-dependent mechanism, indicating that VEGF-C is a new RANKL target gene in osteoclasts and functions as an autocrine factor regulating osteoclast activity.