VEGF enhancement of osteoclast survival and bone resorption involves VEGF receptor-2 signaling and beta3-integrin

VEGF dependent angiogenesis is required for normal bone development and has been implicated in cancer metastasis to bone. These processes, while dependent on osteoclastic bone resorption, are reportedly mediated by endothelial cells, stromal osteoblasts, chondrocytes, and/or tumor cells. We demonstrate here that VEGF treatment of purified murine bone marrow osteoclast precursors directly enhances their survival, differentiation into mature osteoclasts, and resorptive activity. The actions of VEGF on mature osteoclasts principally involve the receptor VEGFR2 (Flk1, KDR), and the receptor signaling utilizes both the PI3-kinase-->Akt and MEK-->ERK pathways. Increased osteoclast survival and resorptive activity is correlated with VEGF-dependent phosphorylation of multiple downstream targets of activated Akt [glycogen synthase kinase, GSK-3beta; forkhead transcription factor, FKHR; and the Bcl-2 antagonist of cell death, Bad (Ser136)] and activated ERK1/2 [ribosomal S6 kinase, p90RSK; and Bad (Ser112)]. Expression of the VEGFR2 gene increases 20-fold during the 6 day in vitro differentiation of mature osteoclasts from mononuclear precursors, while alternate receptors VEGFR1 and neuropilin-1, decrease 30- and 3-fold respectively. Additionally, VEGF enhancement of osteoclast survival is diminished in cells prepared from beta3 integrin-deficient mice, thus associating VEGF signaling in osteoclasts with their attachment to extracellular matrix. Our results indicate that VEGF directly targets osteoclasts, thereby playing a novel role in bone development, angiogenesis, and tumor metastasis.