Bone Abstracts

Introduction: Bone metastases represent a frequent complication of breast cancer and are characterized by increased tumour-driven activation of osteoclasts and subsequent bone loss. Aminobisphosphonates inhibit osteoclast function and are established therapies of skeletal metastases. Similar to statins, they block the mevalonate pathway and are thought to have direct anti-tumour effects. Here, we report on the anti-tumour potential of a sequential inhibition of the mevalonate pathway by combining atorvastatin and zoledronic acid in human breast cancer cell lines.

Materials and methods: Successful inhibition of the mevalonate pathway using zoledronic acid and atorvastatin was assessed by detection of unprenylated Ras and Rap1A in the hormone-receptor negative MDA-MB-231 breast cancer cell line. Caspase 3/7 activation assay, annexin V/PI staining and detection of cleaved caspase 3 and poly ADP ribose polymerase (PARP) were used to quantify apoptosis.

Results: Atorvastatin and zoledronic acid in concentrations ranging from 1 to 100 μM led to dose-dependent increase of apoptosis (up to sixfold). The observed effects could be reversed by geranylgeranylpyrophosphate, but not by farnesylpyrophosphate. Concordantly, treatment with geranylgeranyl transferase I inhibitor GGTI-298 but not farnesyl transferase I inhibitor FTI-277 evoked apoptosis highlighting that geranylation of proteins is the main affected process. Interestingly, the combination of 10 μM zoledronic acid and 1 μM atorvastatin induced a threefold increase of apoptosis (P< 0.01) and accumulation of cleaved caspase 3 and PARP after 48 h, whereas only mild effects were observed with individual treatment (1.2-fold each). Annexin V/PI staining revealed 4.58% of apoptotic cells upon combination treatment in comparison to single treatment with atorvastatin and zoledronic acid (1.76 and 2.59%).

Conclusion: Our results indicate the mevalonate pathway as a potential therapeutic target that is amenable to a combination of commonly available and approved drugs. Such strategy could be useful to treat breast cancer-induced bone metastases.