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 Akt (protein kinase B; PKB) is downstream of PI3K and activated by growth factors. This study found that erythropoietin (EPO) induced tyrosine phosphorylation of Akt in a time- and dose-dependent manner in EPO-dependent human leukemia cell line UT-7/EPO. In vitro kinase assay using histone H2B and glucose synthase kinase as substrates demonstrated that Akt was actually activated by EPO. EPO-induced phosphorylation of Akt was completely blocked by a PI3K-specific inhibitor, LY294002, at 10 micromol/L, indicating that activation of Akt by EPO is dependent on PI3K activity. In addition, overexpression of the constitutively active form of Akt on UT-7/EPO cells partially blocked apoptosis induced by withdrawal of EPO from the culture medium. This finding suggested that the PI3K-Akt activation pathway plays some role in the antiapoptotic effect of EPO. EPO induced phosphorylation of a member of the trancription factor Forkhead family, FKHRL1, at threonine 32 and serine 253 in a dose- and time-dependent manner in UT-7/EPO cells. Moreover, results showed that Akt kinase activated by EPO directly phosphorylated FKHRL1 protein and that FKHRL1 phosphorylation was completely dependent on PI3K activity as is the case for Akt. In conjunction with the evidence that FKHRL1 is expressed in normal human erythroid progenitor cells and erythroblasts, the results suggest that FKHRL1 plays an important role in erythropoiesis as one of the downstream target molecules of PI3K-Akt.

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