The
integrin alphavbeta3 mediates cell-matrix interactions.
Vitaxin(R), a humanized
monoclonal antibody that blocks human and rabbit alphavbeta3
integrins, is in clinical trials for metastatic
melanoma and
prostate cancer. alphavbeta3 is the predominant
integrin on osteoclasts, the cells responsible for
bone resorption in health and disease. Here, we report the first investigation of
Vitaxin's effects on osteoclast activity.
Vitaxin (100-300 ng/ml) decreased total resorption by 50%, but did not alter resorptive activity per osteoclast.
Vitaxin (300 ng/ml) decreased osteoclast numbers on
plastic by 35% after 48 h. Similarly, attachment after 2 h was reduced by 30% when osteoclasts were incubated with
Vitaxin (300 ng/ml) for 25 min prior to plating; however, the rate of fusion of osteoclast precursors in
Vitaxin-treated and control groups was equal. Using time-lapse microscopy, we evaluated the effect of
Vitaxin on osteoclast morphology and found a significant reduction in osteoclast planar area only when cells were pretreated with
macrophage colony stimulating factor (
M-CSF). Extracellular Ca(2+) and
M-CSF have opposite effects on alphavbeta3 conformation. Elevation of extracellular Ca(2+) eliminated the inhibitory effect of
Vitaxin on osteoclast attachment. In contrast, the effect of
Vitaxin was enhanced in cells pretreated with
M-CSF. This action of
M-CSF was suppressed by the
phosphatidylinositol 3-kinase (PI3-kinase) inhibitor
wortmannin, suggesting that
M-CSF increases
Vitaxin's inhibitory effect by inside-out activation of alphavbeta3. In conclusion,
Vitaxin decreases resorption by impairing osteoclast attachment, without affecting osteoclast formation and multinucleation. Our data also show that
Vitaxin's inhibitory effects on osteoclasts can be modulated by factors known to alter the conformation of alphavbeta3.