Inhibition of αvβ3 integrin reduces angiogenesis, bone turnover, and tumor cell proliferation in experimental prostate cancer bone metastases

The growth of metastatic prostate cancer cells in the bone involves an intimate interaction between the tumor cells and various elements of the bone microenvironment, resulting in increased rate of bone turnover and rapid tumor growth. The alphavbeta3 integrin has been shown to play an important role in tumor growth and angiogenesis, and is known to be critical to osteoclast formation and activity. This study was designed to examine the role of alphavbeta3 expressed by cells native to the bone in the growth and pathogenesis of prostate cancer bone metastases. Human prostate cancer cells which do not express alphavbeta3 or betaIIbbeta3 integrins were injected directly into human bone fragments previously implanted subcutaneously in SCID mice (SCID-human-bone model). At the same time treatment with anti-beta3 antibody fragment (m7E3 F(ab')(2)) i.p. at 300 mug/dose 3 x per week was initiated and continued for 2 weeks. In this system, m7E3 F(ab')(2) only recognizes human bone-derived alphavbeta3. Antibody inhibition of alphavbeta3 integrin in vivo resulted in a specific reduction in the proportion of antigenically-human blood vessels within tumor-bearing bone implants ( from 73.5% +/- 3.93 in controls to 17.74% +/- 5.64 in treated animals). Proliferation of the alphavbeta3-negative tumor cells was also reduced, although the overall vessel density was maintained by compensating mouse vasculature. Blockage of human bone-derived alphavbeta3 also significantly reduced the recruitment of osteoclasts in response to tumor cells, as well as degradation of calcified bone tissue. Together these observations confirm the importance of alphavbeta3 in bone metabolism and angiogenesis, and point to the role of these processes in controlling growth of metastatic prostate cancer cells in the bone.