Zoledronic acid sensitizes breast cancer cells to fulvestrant via ERK/HIF-1 pathway inhibition in vivo

Previous studies have reported that hypoxia-inducible factor (HIF)-1 confers endocrine resistance and that zoledronic acid (ZOL) decreases HIF-1 expression in estrogen receptor-positive breast cancer. The present study investigated the effect of the combination treatment with ZOL and fulvestrant and its possible mechanism for HIF-1 inhibition in vitro and in vivo. First, cell proliferation, clonogenic ability and HIF-1 expression by western blotting were determined in MCF-7 breast cancer cells stably expressing HIF-1 in vitro. Next, a mouse xenograft model was established with the HIF-1-overexpressing MCF-7 breast cancer cells, and treated with PBS, fulvestrant, ZOL or fulvestrant plus ZOL. Tumor volumes were compared and animal [F-18]-fluoromisonidazole (FMISO) positron emission tomography-computer tomography (PET-CT) was used to detect the hypoxic status of the xenograft tumors. Protein expression levels of HIF-1 in the xenograft tumors were detected by immunohistochemistry and western blotting. The results demonstrated that the HIF-1-overexpressing xenograft tumors grew faster and larger compared with control tumors. The animal [F-18]-FMISO PET-CT also confirmed these results. [F-18]-FMISO uptake was significantly higher in HIF-1-overexpressing xenograft tumors compared with control tumors. In addition, the combination treatment with ZOL and fulvestrant acted synergistically in the mouse xenograft model in vivo to significantly reduce tumor burden. Similarly, combination of ZOL and fulvestrant significantly reduced tumor cell growth in vitro. ZOL alone did not inhibit the tumor growth of MCF-7 cells stably expressing HIF-1. Furthermore, ZOL significantly inhibited extracellular signal-regulated kinase (ERK) 1/2 phosphorylation, while phosphoinositide 3-kinase/AKT signaling was not affected. In conclusion, the present study demonstrated that ZOL significantly increased the sensitivity of breast cancer cells to fulvestrant through inhibition of the ERK/HIF-1 pathway.