Bone sialoprotein stimulates cancer cell adhesion through the RGD motif and the αvβ3 and αvβ5 integrin receptors

Abstract

Being implicated in bone metastasis development, bone sialoprotein (BSP) expression is upregulated in patients with cancer. While BSP regulates cancer cell adhesion to the extracellular matrix, to the best of our knowledge, the specific adhesive molecular interactions in metastatic bone disease remain unclear. The present study aimed to improve the understanding of the arginine-glycine-aspartic acid (RGD) sequence of BSP and the integrin receptors αvβ3 and αvβ5 in BSP-mediated cancer cell adhesion. Human breast cancer (MDA-MB-231), prostate cancer (PC-3) and non-small cell lung cancer (NSCLC; NCI-H460) cell lines were cultured on BSP-coated plates. Adhesion assays with varying BSP concentrations were performed to evaluate the effect of exogenous glycine-arginine-glycine-aspartic acid-serine-proline (GRGDSP) peptide and anti-integrin antibodies on the attachment of cancer cells to BSP. Cell attachment was assessed using the alamarBlue^® assay. The present results indicated that BSP supported the adhesion of cancer cells. The RGD counterpart GRGDSP peptide reduced the attachment of all tested cancer cell lines to BSP by ≤98.4%. Experiments with anti-integrin antibodies demonstrated differences among integrin receptors and cancer cell types. The αvβ5 antibody decreased NSCLC cell adhesion to BSP by 84.3%, while the αvβ3 antibody decreased adhesion by 14%. The αvβ3 antibody decreased PC-3 cell adhesion to BSP by 46.4%, while the αvβ5 antibody decreased adhesion by 9.5%. Adhesion of MDA-MB-231 cells to BSP was inhibited by 54.7% with αvβ5 antibody. The present results demonstrated that BSP-induced cancer cell adhesion occurs through the binding of the RGD sequence of BSP to the cell integrin receptors αvβ3 and αvβ5. Differences between cancer types were found regarding the mediation via αvβ3 or αvβ5 receptors. The present findings may explain why certain cancer cells preferentially spread to the bone tissue, suggesting that targeting the RGD-integrin binding interaction could be a promising novel cancer treatment option.

Keywords: BSP; RGD; cell adhesion; integrin receptors; metastasis.

Figure 1.

Adhesion of cancer cell lines to various surface coatings, as determined by the alamarBlue^® assay. (A) MDA-MB-231 (n=5), (B) PC-3 (n=5) and (C) NCI-H460 (n=5) cancer cells attached to PBS, BSP and FN. Error bars represent the standard deviation. ****P<0.0001 vs. negative control (PBS). PBS, phosphate-buffered saline; BSP, bone sialoprotein; FN, fibronectin.

Figure 2.

Increased attachment of human cancer cells to BSP with increasing protein concentrations, as determined by the alamarBlue^® assay. (A) MDA-MB-231 (n=9), (B) PC-3 (n=9) and (C) NCI-H460 (n=9) cancer cells attached to BSP after 2 h. Error bars represent the standard deviation. BSP, bone sialoprotein.

Figure 3.

Effects of exogenous GRGDSP peptide on cancer cell adhesion to BSP, as determined by the alamarBlue^® assay. (A) MDA-MB-231 (n=9), (B) PC-3 (n=9) and (C) NCI-H460 (n=9) cells were incubated for 30 min at 37°C in the presence of increasing concentrations of GRGDSP peptide. Afterwards, cells were cultured for 2 h on BSP-coated plates and attached cells were examined. Data are reported as percentages compared with 0 µM GRGDSP (100%). Error bars represent the standard deviation. ****P<0.0001 vs. negative control (0 µM GRGDSP). GRGDSP, glycine-arginine-glycine-aspartic acid-serine-proline; BSP, bone sialoprotein.

Figure 4.

Effect of incubation with (A) 0, (B) 7.5, (C) 15, (D) 30 or (E) 60 µM glycine-arginine-glycine-aspartic acid-serine-proline peptide for 30 min and 2 h culturing on BSP-coated plates on the adhesion to BSP and morphology of PC-3 cells (magnification, ×20). BSP, bone sialoprotein.

Figure 5.

Effect of anti-αvβ3 or anti-αvβ5 antibody on cancer cell adhesion to BSP, as determined by the alamarBlue^® assay. (A) MDA-MB-231 (n=4), (B) PC-3 (n=4) and (C) NCI-H460 (n=4) cells were incubated for 60 min at 37°C in the presence of 10 µg/ml isotype control IgG1, 10 µg/ml anti-αvβ3 antibody or 10 µg/ml anti-αvβ5 antibody. Afterwards, cells were cultured for 2 h on BSP-coated plates and attached cells were examined. Data are reported as percentages compared with 10 µg/ml isotype control IgG1 (100%). Error bars represent the standard deviation. **P<0.01; ****P<0.0001 vs. negative control IgG1. BSP, bone sialoprotein; ns, not significant.