BONE SIALOPROTEIN SUPPORTS BREAST-CANCER CELL-ADHESION PROLIFERATION AND MIGRATION THROUGH DIFFERENTIAL USAGE OF THE ALPHA-V-BETA-3 AND ALPHA-V-BETA-5 INTEGRINS

Bone sialoprotein (BSP), a secreted glycoprotein found in bone matrix, has been implicated in the formation of mammary microcalcifications a nd osteotropic metastasis of human breast cancer (HBC). BSP possesses an integrin-binding RGD (Arg-Gly-Asp) domain, which may promote intera ctions between HBC cells and bone extracellular matrix. Purified BSP, recombinant human BSP fragments and BSP-derived RGD peptides are shown to elicit migratory, adhesive, and proliferative responses in the MDA -MB-231 HBC cell line. Recombinant BSP fragment analysis localized a s ignificant component of these activities to the RCD domain of the prot ein, and synthetic RGD peptides with BSP flanking sequences (BSP-RGD) also conferred these responses. The fibronectin-derived RGD counterpar t, GRGDSP (Gly-Arg-Cly-Asp-Ser-Pro), could not support these cellular responses, emphasizing specificity of the BSP configuration. Although most of the proliferative and adhesive responses could be attributed t o RGD interactions, these interactions were only partly responsible fo r the migrational responses. Experiments with integrin-blocking antibo dies demonstrated that BSP-RGD-induced migration utilizes the alpha v beta 3 vitronectin receptor, whereas adhesion acid proliferation respo nses were alpha v beta 5-mediated. Using fluorescence activated cell s orting, we selected two separate subpopulations of MDA-MB-231 cells en riched for alpha v beta 3 or alpha v beta 5 respectively. Although som e expression of the alternate alpha v integrin was still retained, the alpha v beta 5-enriched MDA-MB-231 cells showed enhanced proliferativ e and adhesive responses, whereas the alpha v beta 3-enriched subpopul ation was suppressed for proliferation and adhesion, but showed enhanc ed migratory responses io BSP-RGD. In addition, similar analysis of tw o other HBC cell lines showed less marked, but similar RGD-dependent t rends in adhesion and proliferation to the BSP fragments. Collectively , these data demonstrate BSP effects on proliferative, migratory, and adhesive functions in HBC cells and that the RGD-mediated component di fferentially employs alpha v beta 3 and alpha v beta 5 integrin recept ors. J. Cell. Physiol. 176:482-494, 1998. (C) 1998 Wiley-Liss, Inc.