Selective ET_AR antagonist atrasentan inhibits hypoxia-induced breast cancer cell invasion

Tumour hypoxia, being widespread in solid tumours, is related to an increased risk of invasion and metastasis as well as to resistance to chemotherapy and radiotherapy. Hypoxia-inducible factor-1α (HIF-1α) has emerged as the key regulator of the cellular response to hypoxia. In primary breast cancers, HIF-1α is overexpressed, and high levels of HIF-1α predict for early relapse and increased metastasis. The endothelin (ET) axis, comprising the peptides ET-1, -2, -3 and their receptors A (ET_AR) and B (ET_BR), is another regulatory system of major relevance in human breast cancer. However, little is known about the interaction of HIF-1α and the ET axis in breast carcinomas. Therefore, we analysed expression of HIF-1α and the ET axis in 600 breast cancer tissue samples by immunohistochemistry, observing a significant correlation between expression of HIF-1α and ET-1 (P < 0.001). In vitro, hypoxia was found to double ET-1 secretion of MCF-7 breast cancer cells (203.5% of controls; P < 0.001), thereby promoting an invasive phenotype. Of note, real-time PCR analysis revealed that the increase of ET-1 was not due to enhanced transcription of the ET-1 gene. In invasion assays, breast cancer cell invasiveness was strongly increased by hypoxia (150.0% of controls; P = 0.007). Most important, this increase was completely inhibited by the selective ET_AR antagonist atrasentan. In conclusion, we provide evidence for a relevant interaction between hypoxia and the ET axis in breast cancer cells. Our data suggest that tumour hypoxia induces breast carcinoma invasiveness by releasing intracellularly stored ET-1. However, induction of invasiveness may be inhibited by selective ET_AR antagonism, thus emphasising the promising status of the ET axis as a therapeutic target in breast cancer.