A member of Forkhead family transcription factor, FKHRL1, is one of the downstream molecules of phosphatidylinositol 3-kinase-Akt activation pathway in erythropoietin signal transduction

The phosphatidylinositol 3-kinase (PI3K) signaling pathway is important for the regulation of a number of cellular responses. Serine/threonine kinase AM (protein kinase B; PKB) Is downstream of PI3K and activated by growth fa ctors. This study found that erythropoietin (EPO) induced tyrosine phosphor ylation of AM in a time- and dose-dependent manner in EPO-dependent human l eukemia cell line UT-7/EPO, In vitro kinase assay using histone H2B and glu cose synthase kinase as substrates demonstrated that AM was actually activa ted by EPO, EPO-induced phosphorylation of AM was completely blocked by a P I3K-specific inhibitor, LY294002, at 10 mu mol/L, indicating that activatio n of AM by EPO is dependent on PI3K activity. In addition, overexpression o f the constitutively active form of AM on UT-7/EPO cells partially blocked apoptosis induced by withdrawal of EPO from the culture medium. This findin g suggested that the PI3K-Akt activation pathway plays some role in the ant iapoptotic effect of EPO, EPO induced phosphorylation of a member of the tr ancription factor Forkhead family, FKHRL1, at threonine 32 and serine 253 i n a dose- and time-dependent manner in UT-7/EPO cells. Moreover, results sh owed that AM kinase activated by EPO directly phosphorylated FKHRL1 protein and that FKHRL1 phosphorylation was completely dependent on PI3K activity as Is the case for AW, In conjunction with the evidence that FKHRL1 is expr essed in normal human erythroid progenitor cells and erythroblasts, the res ults suggest that FKHRL1 plays an important role in erythropoiesis as one o f the downstream target molecules of PI3K-Akt. (C) 2000 by The American Soc iety of Hematology.