Unraveling distinct intracellular signals that promote survival and proliferation: study of erythropoietin, stem cell factor, and constitutive signaling in leukemic cells.

This review summarizes selected recent studies of the intracellular signals that allow erythroid cells to survive and proliferate under the control of erythropoietin (EPO) and alteration in signals that contribute to EPO-independent survival and proliferation. The hypothesis explored is that the proliferation and survival signals are distinct and can be separately studied with the proper cell lines and growth factor stimulation. The anti- and pro-apoptotic proteins Bcl-XL and BAD are highly implicated in EPO-dependent survival of erythroid cells. Stat5 activity appears to be upstream of Bcl-XL expression such that pathologic, constitutive activation of Stat5 may be a common event in leukemic cells that become resistant to apoptosis by constitutive expression of Bcl-XL. Other signals apparently also control the expression of Bcl-XL, such as the expression of JunB which seem to be required to suppress Bcl-XL expression when EPO is withdrawn. Apoptosis may also be triggered by inactivation of Bcl-XL by BAD. Dephosphorylation of BAD as a result of withdrawal of survival factors converts prosurvival BAD to proapoptotic BAD. Phosphorylation of BAD at the serine 112 residue seems critical to promoting survival. Constitutive activation of a kinase that phosphorylates BAD serine 112 may, therefore, contribute to resistance to apoptosis in leukemic cells. We describe the resistance of erythroleukemic cells to apoptosis induced by EPO withdrawal apparently caused by constitutive BAD phosphorylation. The resistance to apoptosis in these cells is reversed by treatment with the PI3-kinase inhibitor, LY294002, suggesting that resistance to apoptosis in these cells likely results from constitutive P13-kinase that is an upstream activator of an S-112 BAD kinase. The MAP kinase cascade is apparently active in EPO-dependent and stem cell factor (SCF)-dependent proliferation but not survival. In addition, autocrine tumor necrosis factor-a! (TNF-alpha) may also be a proliferation factor not affecting survival. P13-kinase seems to be required for full EPO-dependent proliferation but is not required for EPO-dependent survival (but it can promote survival when activated).