VEGF-induced phosphorylation of Bcl-2 influences B lineage leukemic cell response to apoptotic stimuli

Post- translational modification of Bcl- 2 protein has been described in a variety of cell models with effects varying from enhanced to abrogated function. In this study, we demonstrated that Bcl- 2 was constitutively phosphorylated in several hematopoietic tumor cell lines and in primary ALL cells. Increased phosphorylation of Bcl- 2 protein in the JM1 ALL cell line, achieved by expression of the phosphomimetic Bcl- 2 construct S70E, enhanced JM1 cell chemoresistance. In contrast, initiation of JM1 cell apoptosis was coincident with dephosphorylation of Bcl- 2 and elevated protein phosphatase 2A activity. S70E expression also diminished tBid- mediated cytochrome c release and blunted chemotherapy- induced activation of caspases- 9 and - 3 in JM1 cells. To determine whether soluble factors produced by stromal cells in the bone marrow influence phosphorylation of Bcl- 2 protein, a panel of recombinant cytokines was evaluated. Of those tested, vascular endothelial growth factor ( VEGF) induced phosphorylation of Bcl- 2 protein and blunted cytochrome c release during chemotherapy or tBid treatment of ALL cells. In contrast, JM1 cells transfected with S70A, resulting in expression of Bcl- 2 protein that cannot be phosphorylated, were not efficiently rescued from apoptosis by VEGF. These observations suggest that optimal protection of leukemic cells by VEGF may require activation of a pathway that includes Bcl- 2 phosphorylation.