Protein kinase B (c-Akt), phosphatidylinositol 3-kinase, and STAT5 are activated by erythropoietin (EPO) in HCD57 erythroid cells but are constitutively active in an EPO-independent, apoptosis-resistant subclone (HCD57-SREI cells)

We found that erythropoietin (EPO) and stem cell factor (SCF) activated pro tein kinase B (PKB/Akt) in EPO-dependent HCD57 erythroid cells. To better u nderstand signals controlling proliferation and viability, erythroid cells that resist apoptosis in the absence of EPO were subcloned and characterize d (HCD57-SREI cells). Constitutive activations of PKB/Akt, STAT5a, and STAT 5b were noted in these EPO-independent cells. PIS-kinase activity was an up stream activator of PKB/Akt because the PI3-kinase inhibitor LY294002 block ed both constitutive PKB/Akt and factor-dependent PKB/Akt activity. The LY2 94002 study showed that proliferation and viability of both HCD57-SREI and HCD57 cells correlated with the activity of PKB/Akt; however, PKB/Akt activ ity alone did not protect these cells from apoptosis, Treatment of HCD57 ce lls with SCF also activated PKB/Akt, but did not protect from apoptosis, Th is result suggested that PKB/PI3-kinase activity is necessary but not suffi cient to promote viability and/or proliferation. Constitutive STAT5 activit y, activated through an unknown pathway not including JAK2 or EPOR, may act in concert with the constitutive PI3-kinase/PKB/Akt pathway to protect the EPO-independent HCD57-SREI cells from apoptosis and promote limited prolif eration. (C) 1999 by The American Society of Hematology.