Abstract
Bone metastases represent common long term complications of patients with breast cancer. Zoledronic acid, an amino-bisphosphonate and mevalonate pathway inhibitor, is an established agent for the treatment of bone metastases. Direct antitumor effects of zoledronic acid have been proposed in breast cancer. Statins are another group of mevalonate pathway inhibitors that have been repeatedly discussed for potential anti-tumor activity. In this study, we tested the hypothesis, whether these agents regulate adhesion of breast cancer cells to extracellular matrix components. Treatment of breast cancer cells with zoledronic acid and atorvastatin, significantly impaired MDA-MB-231 breast cancer cell adhesion on the αvβ3 ligands gelatin and vitronectin, but had no effect on collagen type 1 (α2β1-ligand) and fibronectin (α5β1-ligand). Anti-adhesive effects of zoledronic acid were fully reversed by geranylgeranyl pyrophosphate (GGPP), but not by farnesylpyrophosphate (FPP). Furthermore, effects of zoledronic acid and atorvastatin were mimicked by a specific inhibitor of geranylgeranylation GGTI-298. Functional (using integrin array) and quantitative (using FACS) integrin analyses on MDA-231 cells following zoledronic acid exposure revealed decreased levels of αv and αvβ3 expression. In addition to its effects on integrin mediated adhesion of breast cancer cells, the presence of zoledronic acid caused pronounced morphological changes in MDA-231 cells as seen by F-actin and vinculin rearrangement. Furthermore, phosphorylation of the focal adhesion kinase was inhibited by zoledronic acid. In both cases, changes were fully reversed by GGPP. These results emphasize the role of mevalonate pathway mediated impairment of geranylgeranylation in the anti-adhesive effects of zoledronic acid in breast cancer cells.