Chemosensitization by erythropoietin through inhibition of the NF-κB rescue pathway

Two cell lines that exemplify erythropoietin (EPO) receptor-positive tumors, human renal carcinoma cell lines RCC and the myelomonocytic leukemia cell line U937, were investigated for the apoptosis-modulatory potential of EPO. Cells cultured in the presence of EPO exhibited an elevated apoptotic response to cancer chemotherapeutic agents such as daunorubicin (Dauno) and vinblastine (VBL). Chemosensitization by EPO did not involve an increase in p53 activation, yet correlated with enhanced Bax/Bak-dependent mitochondrial membrane perturbation and caspase maturation. In vitro monotherapy with Dauno or VBL induced the degradation of IκBα, provoked the translocation of NF-κB p65/50 to the nucleus and stimulated the expression of an NF-κB-activatable reporter gene. All these signs of NF-κB activation were perturbed in the presence of EPO. Inhibition of JAK2, one of the receptor-proximal elements of EPO-mediated signal transduction, greatly diminished the EPO-mediated chemosensitization and NF-κB inhibition. EPO lost its death-facilitating effects in the presence of an NF-κB inhibitor, underscoring the cause-effect relationship between EPO-mediated chemosensitization and NF-κB inhibition. Altogether, these results suggest that, at least in a specific subset of tumors, EPO receptor agonists can prevent activation of the NF-κB pathway, thereby enhancing the propensity of EPO receptor-positive tumor cells to undergo apoptosis.