Inhibition of the mevalonate pathway and activation of p38 MAP kinase are independently regulated by nitrogen-containing bisphosphonates in breast cancer cells

Bisphosphonates are widely used inhibitors of bone resorption. They also inhibit the growth of various cancer cells in vitro, but the clinical significance of this effect is unclear. The cancer growth inhibitory effects of nitrogen-containing bisphosphonates, (i.e. zoledronate) have been attributed to their ability to inhibit the mevalonate pathway. We have shown that bisphosphonates also induce p38 activation, which signals resistance against the drug-induced growth inhibition through an unknown mechanism. We show here that zoledronate induces a G1/S cell cycle arrest in human MDA-MB-231 breast cancer cells. Furthermore, p38 inhibitor augments bisphosphonate-induced growth inhibition by inducing an additional G2-phase cell cycle arrest. We also show that the nitrogen-containing bisphosphonate-induced effects on p38 phosphorylation occur before accumulation of unprenylated Rap1A or Rac1 activation. Geranylgeranyl pyrophosphate, an end-product of the mevalonate pathway, reversed the accumulation of unprenylated Rap1A but not phosphorylation of p38. Geranylgeranyl pyrophosphate also reversed n-BP induced growth inhibition, but the completeness of this reversal was nitrogen-containing bisphosphonate concentration dependent. Also mevastatin induced the accumulation of unprenylated Rap1A, but it did not induce p38 phosphorylation. In conclusion, our results suggest that in addition to the previously reported effects on apoptosis, nitrogen-containing bisphosphonates also inhibit the growth of MDA-MB-231 breast cancer cells by inducing G1/S cell cycle arrest. The bisphosphonate-induced p38 activation signals for resistance against these drugs, by promoting progression through the G2/M-checkpoint. Of these pathways only growth inhibition is mediated via inhibition of the mevalonate pathway in MDA-MB-231 cells. Combining p38 inhibitors with bisphosphonates may result in increased anti-cancer efficacy.