Protein kinase C alpha controls erythropoietin receptor signaling

Protein kinase C (PKC) is implied in the activation of multiple targets of erythropoietin (Epo) signaling, but its exact role in Epo receptor (EpoR) s ignal transduction and in the regulation of erythroid proliferation and dif ferentiation remained elusive. We analyzed the effect of PKC inhibitors wit h distinct modes of action on EpoR signaling in primary human erythroblasts and in a recently established murine erythroid cell line. Active PKC appea red essential for Epo-induced phosphorylation of the Epo receptor itself, S TAT5, Gab1, Erk1/2, AKT, and other downstream targets. Under the same condi tions, stem cell factor-induced signal transduction was not impaired. LY294 002, a specific inhibitor of phosphoinositol 3-kinase, also suppressed Epo- induced signal transduction, which could be partially relieved by activator s of PKC. PKC inhibitors or LY294002 did not affect membrane expression of the EpoR, the association of JAK2 with the EpoR, or the in vitro kinase act ivity of JAK2. The data suggest that PKC controls EpoR signaling instead of being a downstream effector. PKC and phosphoinositol 3-kinase may act in c oncert to regulate association of the EpoR complex such that it is responsi ve to ligand stimulation. Reduced PKC-activity inhibited Epo-dependent diff erentiation, although it did not effect Epo-dependent "renewal divisions" i nduced in the presence of Epo, stem cell factor, and dexamethasone.