Bisphosphonates (BP) are used for the treatment of
osteoporosis and bone
metastases due to breast and
prostate cancer. Recent clinical studies indicated a benefit in survival and
tumor relapse with the supportive treatment of
breast cancer using
zoledronic acid (ZA), thus stimulating the debate about its putative anti-
tumor activity in vivo. MCF-7
breast cancer cells were treated for 3 h (pulse treatment) and 72 h (permanent treatment) with ZA, and apoptosis rates and cell viability, defined as
ATP content, were determined after 72 h. Permanent and pulse stimulation with ZA inhibited the viability of MCF-7 cells, which could partly be rescued by
atorvastatin (Ator) pre-treatment but not by
geranylgeranyl pyrophosphate (GGPP) co-treatment. Microarray analysis of ZA treated MCF-7 cells identified genes of the
mevalonate pathway as significantly upregulated, which was verified by qPCR. Additionally the putative
tumor suppressors krüppel-like factor 2 and 6 (KLF2 and KLF6) were markedly upregulated, while the classical proliferation marker Ki-67 was clearly downregulated. The expression of all three genes was confirmed by qPCR on
mRNA level and by immunocytochemistry or Western blot staining. Expression of target genes were also analyzed in other breast (MDA-MB-231, BT-20, ZR75-1, T47D) and prostate (LNCaP, PC3)
cancer cell lines by qPCR. ZA responsiveness of KLF2, KLF6 and Ki-67 could be verified in PC3 and T47D cells, KLF6 responsiveness in LNCaP and KLF2 responsiveness in MDA-MB-231 and BT-20 cells. Here we demonstrate in the apoptosis insensitive MCF-7 cell line a remarkable impact of ZA exposure on cell viability and on the regulation of putative
tumor suppressors of the KLF family. The molecular mechanism involved might be the accumulation of
isopentenyl pyrophosphate (
IPP) and ApppI, since we could partly rescue the ZA effect by Ator pre-treatment and GGPP co-treatment. These data should stimulate further research into both the role of the
mevalonate pathway and the accumulation of
pyrophosphate compounds like ApppI in
tumorigenesis and differentiation and their potential apart from the inhibition of mitochondrial
ADP/
ATP translocase and apoptosis, since such effects might well be responsible for the adjuvant ZA treatment benefit of patients suffering from
breast cancer.