Contortrostatin, a Homodimeric Snake Venom Disintegrin, Is a Potent Inhibitor of Osteoclast Attachment

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Abstract
Disintegrins are small disulfide-rich proteins containing an Arg-Gly-Asp (RGD) sequence near their carboxyl terminus. These polypeptides inhibit binding of adhesion molecules to their receptors (integrins) on the surface of cells. Osteoclasts express integrins, heterodimeric cell surface adhesion receptors, that have been shown to be involved in interactions with the extracellular matrix (ECM), including attachment to bone and bone resorption. It has recently been shown that disintegrins effectively inhibit attachment of osteoclasts to components of the ECM and also disrupt osteoclast-mediated bone resorption. Here we characterize the effects of contortrostatin (CTS), a novel homodimeric snake venom disintegrin, on osteoclast attachment. Plastic dishes coated with CTS were able to support osteoclast attachment with a high affinity (EC50,CTS = 86 ± 6.7 nM) similar to that of vitronectin (VTN; EC50,VTN = 80 ± 20 nM). Further, CTS was observed to inhibit completely osteoclast attachment to fetal bovine serum (FBS; IC50,FBS = 0.36 ± 0.04 nM) and VTN (IC50,VTN = 0.85 ± 0.13 nM). We used monoclonal antibodies directed against the β1 (monoclonal antibody [MAb] CD29) and β3 (MAb F11) integrin subunits to explore the mechanism of osteoclast attachment to immobilized CTS. Only MAb F11 inhibited attachment to immobilized CTS (IC50 = 0.41 ± 0.12 μg/ml), suggesting that binding to CTS is mediated in part by a β3 integrin, presumably the αvβ3 VTN receptor. In further support of an integrin-mediated mechanism, binding of osteoclasts to CTS is inhibited by the RGD peptide, GRGDSP. CTS was also more potent (IC50,FBS = 0.36 ± 0.04 nM) at inhibiting osteoclast attachment to FBS-coated wells than the monomeric snake venom disintegrin echistatin (IC50,FBS = 8.9 ± 1.5 nM) or VTN (IC50,FBS = 97.5 ± 25.5 nM). Taken together, these data suggest that the snake venom disintegrin CTS is a potent inhibitor of β3 integrin-mediated osteoclast attachment, presumably involving the VTN receptor (an αvβ3 integrin). Further studies of the mechanism of CTS–osteoclast interactions may aid in the design of peptide mimetics to act as antiresorptive agents for the treatment of osteoporosis and other skeletal pathology.