Extracellular calcium increases bisphosphonate-induced growth inhibition of breast cancer cells.
BACKGROUND
Bisphosphonates have become standard therapy for the treatment of skeletal complications related to breast cancer. Although their therapeutic effects mainly result from an inhibition of osteoclastic bone resorption, in vitro data indicate that they also act directly on breast cancer cells, inhibiting proliferation and inducing apoptosis.


METHODS
The present study examined the effects of calcium (from 0.6 to 2.0 mmol/l) on the antitumour activity of the bisphosphonate ibandronate (1 to 1,000 nmol/l) on MDA-MB-231 and MCF-7 breast cancer cells. Cell culture densities were determined using crystal violet staining assay. Apoptotic cell death was assessed by annexin V-phycoerythrin and 7-amino-actinomycin double staining.


RESULTS
At low calcium concentration, 30 micromol/l ibandronate had no effect on MDA-MB-231 cells growth and only slightly inhibited MCF-7 cells growth. Higher calcium levels significantly increased growth inhibition as well as cell apoptosis induced by ibandronate. We observed similar effects with zoledronic acid. Of note, enhancement of ibandronate-induced growth inhibition was also observed in other breast cancer cell lines (T-47D, ZR-75, Hs-578T and BT-549 cells). The growth inhibitory effect of ibandronate in the presence of high concentrations of calcium was partly suppressed by the calcium chelator EGTA (ethylene glycol tetra-acetic acid). In addition, in the presence of calcium at high concentrations, cells accumulated more [14C]ibandronate than at low calcium concentrations. We obtained further evidence of enhancement of cellular ibandronate accumulation by calcium by demonstrating that high calcium levels increased the inhibition of protein prenylation induced by the bisphosphonate.


CONCLUSIONS
Altogether, our data suggest that extracellular calcium, probably through its binding to ibandronate, markedly increased its cellular accumulation and its inhibitory activity on breast tumour cells. Thus, calcium released during the process of tumour-induced osteolysis might enhance the antitumour effects of bisphosphonates and contribute to their therapeutic efficacy.
Journé
Fabrice
F
Laboratory of Endocrinology and Bone Diseases, Institut Jules Bordet, Université Libre de Bruxelles, rue Héger-Bordet, B-1000, Brussels, Belgium. fabrice.journe@bordet.be
Kheddoumi
Naïma
N
Chaboteaux
Carole
C
Duvillier
Hugues
H
Laurent
Guy
G
Body
Jean-Jacques
JJ
D000970:Antineoplastic Agents / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects*; D001842:Bone and Bones / Q000187:drug effects / Q000378:metabolism; D001943:Breast Neoplasms / Q000188:drug therapy*; D002118:Calcium / Q000494:pharmacology*; D045744:Cell Line, Tumor; D049109:Cell Proliferation / Q000187:drug effects*; D004164:Diphosphonates / Q000494:pharmacology*; D004357:Drug Synergism; D005260:Female; D006801:Humans; D000077557:Ibandronic Acid; D007093:Imidazoles / Q000494:pharmacology; D000077211:Zoledronic Acid
D000970:Antineoplastic Agents; D004164:Diphosphonates; D007093:Imidazoles; D000077211:Zoledronic Acid; D002118:Calcium; D000077557:Ibandronic Acid