Modulation of vitronectin receptor-mediated osteoclast adhesion by Arg-Gly-Asp peptide analogs: A structure-function analysis

<jats:title>Abstract</jats:title> <jats:p>This study details the investigation of induction of retractile shape change in the osteoclast through inhibition of adhesion between osteoclasts and matrix with (1) peptide analogs bearing an Arg-Gly-Asp (RGD) sequence, (2) antibodies to the integrin αvβ3 vitronectin receptor, and (3) the RGD-containing snake venom peptide echistatin. Osteoclast retraction on dentin has been demonstrated for GRGDSP peptide, in contrast to the inactivity of the analog containing the conservative RGE sequence modification. An osteoclast adhesion assay employing rat or chick bone cells and serum-coated glass coverslips as substrate was developed for routine evaluation of inhibition of adhesion. Antibodies F4 and F11 to the β3 chain of rat vitronectin receptor were effective at submicromolar concentrations in rat osteoclasts (IC50 0.29 and 0.05 μM, respectively), whereas MAb 23C6 to human/chick vitronectin receptor was somewhat less effective against chick osteoclasts (ICso 1.6 μM). A rank order of RGD analog activity (mean IC50, μM) in the serum-coated glass adhesion assay was derived for the linear peptides GRGDSP (201 μM), GRGDTP (180 μM), Ac-RGDS-NH2 (84 μM), Ac-RGDV-NH2 (68 μM), RGDV (43 μM), GRGDS (38 μM), and RGDS (26 μM). The two most potent short peptides were the cyclic analog SK&F 106760 Ac-S,S-cyclo-(Cys-(NαMe)Arg-Gly-Asp-Pen)-NH2 (IC50 7.0 μM), and the Telios peptide H-Gly-S,S-cyclo-(Pen-Gly-Arg-Gly-Asp-Ser-Pro-Cys)-Ala-OH (IC50 6.6 μM). The snake venom peptide echistatin was the most potent substance evaluated in the serum-coated glass assay (IC50 0.78 nM) employing either rat or chick osteoclasts. Specificity control peptides fibronectin CS1 (ligand for VLA-4), fibrinogen H12 (alternate ligand for gpIIb/IIIa), and Iaminin cell binding fragment YIGSR were inactive up to 800 μM. Comparison of SK&F 106760 and the Telios peptide as inhibitors of platelet aggregation (IC50 0.36 and 10.1 μM, respectively) and inhibitors of L8 skeletal muscle cell adhesion to vitronectin (IC50 67.2 and 12.3 μM, respectively) suggests that the Telios peptide is nonselective whereas SK&F 106760 may be selective with regard to β3 integrins. This study demonstrates that structural modification in RGD peptides and the use of antireceptor antibodies or the venom peptide echistatin yields potent inhibitors of vitronectin receptor-mediated adhesion in isolated rat and chick osteoclasts. It is hoped that further peptide modification will yield improved specificity and thus selective inhibitory effects upon bone resorption.</jats:p>