Erythropoietin receptor in growth control and anti-estrogen resistance of estrogen receptor positive breast cancer

Abstract: The main function of Erythropoietin (EPO) and its receptor (EPOR) is the stimulation of erythropoiesis. Therefore, recombinant human EPO (rhEPO) is used to treat anemia in cancer patients, but some studies have reported that rhEPO treatment might promote tumor progression. Additionally, EPOR expression has been detected in various cancer forms. Here, we confirm EPOR expression in several breast cancer cell lines with a newly produced EPOR antibody. Functional studies revealed that EPOR knockdown reduced proliferation in estrogen receptor alpha (ERα) positive breast cancer cells and decreased ERα activity. Our data also show that EPOR knockdown may improve the tamoxifen response. These effects were not dependent on rhEPO, as stimulation had no effect on proliferation and did not result in activation of the EPOR itself or signaling pathway mediators. We further investigated the link between EPOR and ERα and show that EPOR signaling affects ERα transcriptional activity and phosphorylation status via the PI3K/AKT pathway. Supporting the knockdown experiments, overexpression of EPOR in the breast cancer cells resulted in increased proliferation and in increased phosphorylation/activity of AKT. In a continuative study, FACS analysis of EPOR knockdown breast cancer cells showed cell cycle arrest in the G0/G1-phase. Microarrays were analyzed to further investigate the effect of EPOR on proliferation. Expression of the mitotic checkpoint and mitotic progression modules were considerably reduced in EPOR knockdown ERα positive breast cancer cells. Furthermore, estrogen-induced expression of mitotic genes was shut down under reduced EPOR levels. The data of study I, II and III suggest a constitutively active EPOR that acts EPO-independently in ERα positive breast cancer cells, affecting proliferation and regulating ERα activity via an AKT-mediated pathway. In study IV, we investigated a possible relation between tumor hypoxia and anti-estrogen therapy resistance in breast cancer. Anti-estrogen therapeutics such as tamoxifen and fulvestrant block ERα function, and are important adjuvant therapeutics for ERα positive breast tumors. However, resistance to anti-estrogen treatment is a major complication. Insufficient oxygen supply (hypoxia) occurs frequently in solid tumors due to rapid tumor growth leading to insufficient circulation. The transcription factors HIF1 and HIF2 regulate the cellular hypoxic response. We used three anti-estrogen resistant MCF-7-derived cell lines, being resistant to tamoxifen (TAMR1) or to fulvestrant (FUR1 and FUR2), respectively. All three showed significantly increased HIF2α protein and mRNA levels and exposure to tamoxifen or fulvestrant further augmented HIF2α levels. HIF-inhibition could restore anti-estrogen sensitivity. Our data suggest that HIF signaling may be involved in anti-estrogen resistance in ERα positive breast cancer. In summary this thesis contributes to an increased understanding of cellular mechanisms behind breast cancer cell proliferation and resistance to anti-estrogen therapy.