STRUCTURAL DETERMINANTS OF CALCIUM SIGNALING BY RGD PEPTIDES IN RAT OSTEOCLASTS - INTEGRIN-DEPENDENT AND-INDEPENDENT ACTIONS

Extensive characterization of the vitronectin receptor (VNR), a member of the integrin group of cell adhesion molecules, which is abundantly expressed in osteoclasts, has revealed a role for this receptor in os teoclast adhesion as well as bone resorption. Earlier evidence from ou r laboratory suggests that VNR is also capable of transducing intracel lular signals following receptor ligand interaction, although this fun ction is poorly understood. Thus, addition of peptides containing the minimal tripeptide Arg-Gly-Asp (RGD) integrin recognition sequence eli cits transient increases in intracellular free calcium ions, with maxi mal responses seen with a bone sialoprotein peptide, BSP-IIA. In the p resent study we have attempted to determine some of the structural req uirements for calcium signaling in osteoclasts using derivatives of th e peptide PRGDN/T sequence found in bone sialoprotein. While some pept ides, such as the parent sequence PRGDN, can induce both signaling and retractile events, it was found that minor structural modifications y ielded peptides such as PRADN which elicited a transient increase in i ntracellular free calcium ions without promoting a reduction in osteoc last spread area (retraction). Conversely, certain other modifications resulted in peptides, such as PrGDN and benzoyl-RGDN, which effect os teoclast retraction, while having minimal Ca2+ signaling capabilities. Osteoclast adhesion, and hence retraction, are known to be RGD-depend ent and integrin-dependent events. However, intracellular Ca2+ signali ng is RGD-independent and, based on lack of inhibition by an anti-beta (3) integrin antibody F11 and echistatin, very Likely integrin-indepen dent, These data suggest that signaling is not always via VNR and as y et unknown receptors on the osteoclast membrane play a role in osteocl ast signaling and hence function. (C) 1995 Academic Press, Inc.