Erythropoietin (Epo), a glycoprotein hormone that is the principal regulator of erythropoiesis, is known to act also on nonhematopoietic cell types. Epo receptors have been reported on several normal and neoplastic human cells and tissues, including ovarian cancer cells. We found that long-term Epo treatment of A2780 cells resulted in the development of a phenotype exhibiting both enhanced Epo signaling, evidenced by increased peak levels of phospho-Erk1/2 and increased paclitaxel resistance. This phenotypic effect was specific for paclitaxel, since no change in cisplatin or carboplatin sensitivity was observed. In addition, the change in phenotype was stable, even after the removal of Epo. Measurement of mono-and oligonucleosome formation revealed that longterm Epo treated A2780 cells exhibited markedly less apoptosis than nonerythropoietin treated cells at essentially all concentrations of paclitaxel tested. Western blot analyses revealed that the long-term Epo treated cells had significantly reduced expression of apoptosis-related proteins Bcl-2 and Bcl-10. These findings may have implications for the clinical use of recombinant human Epo and other erythropoiesis stimulating agents to correct anemia in paclitaxel-treated cancer patients. ' 2007 Wiley-Liss, Inc.Key words: erythropoietin; signaling; ovarian cancer; paclitaxel; chemoresistance; apoptosis Erythropoietin (Epo) is the prime regulator of erythroid cell growth and differentiation. Additionally, a wide variety of normal and malignant nonhematopoietic cells have been reported to express the Epo receptor (EpoR) and in some cases, Epo has been shown to act on these cells, sometimes as a survival/antiapoptotic factor (reviewed in Ref. 1).Reports of erythropoietin receptor (EpoR) expression by a variety of cancer cell types and the use of recombinant human erythropoietin (epoetin) and darbepoetin to treat anemia in cancer patients have raised concern regarding the potential for antiapoptotic effects of these erythropoiesis stimulating agents on the cancer itself. [2][3][4][5][6] Some clinical studies have proven especially worrisome. The Breast Cancer Erythropoietin Trial (BEST) was terminated early because of increased mortality in patients receiving Epo. 6 There was an increase in early disease progression. A trial of Epo in head and neck cancer patients treated with radiotherapy resulted in an increase in locoregional recurrence and a decrease in survival in the Epo-treated group. 5 Recently, a trial of darbepoetin to enhance radiosensitivity of head and neck cancer was stopped due to a significantly poorer outcome in the darbepoetin treatment group. 7,8 In an accompanying study, we show that each of 4 human ovarian cancer cell lines, including A2780, expresses both the EpoR and Epo at the mRNA and protein levels. Additionally, we show that these EpoR are functional in initiating Epo-dependent intracellular signaling and that an Epo/EpoR autocrine/ paracrine mechanism may be operative in enhancing ovarian cancer cell growth.In the present report, we ha...