Repression of apoptosis in human B-lymphoma cells by CD40-ligand and Bcl-2: relationship to the cell-cycle and role of the retinoblastoma protein
Wang, H.; Grand, R.J.; Milner, A.E.; Armitage, R.J.; Gordon, J.; Gregory, C.D.
Oncogene 13(2): 373-379
1996
ISSN/ISBN: 0950-9232 PMID: 8710376 Document Number: 459436
Using a Burkitt lymphoma cell line to model human B-cell apoptosis in vitro, we observed that crosslinking, by antibody, of cell surface immunoglobulin induced G-1 growth-arrest followed by apoptosis. By contrast, cells treated with the Ca-2+-ionophore, ionomycin, generated apoptotic signals in G-2/M as well as in G-1. Both ionomycin and anti-immunoglobulin treatment induced rapid dephosphorylation of Rb prior to apoptosis. Apoptosis was repressed following exposure to CD40-ligand and was accompanied by hyperphosphorylation of Rb and cell-cycle progression but not Bcl-2 expression. Expression of Bcl-2 protein in stable bcl-2-transfectants, also resulted in repression of apoptosis and anti-immunoglobulin-treated cells no longer underwent growth-arrest. In Bcl-2-expressing cells in which apoptosis was repressed, Rb remained hyperphosphorylated, even during G-1-arrest induced by ionomycin. TGF-beta treatment of Bcl-2-expressing cells induced G-1-arrest, de-phosphorylation of Rb and apoptosis. These results suggest that the functional activity of Bcl-2 in B-lymphoma cells is dependent upon, or leads to, sustained hyperphosphorylation of Rb and that Rb hyperphosphorylation can be uncoupled from cell-cycle progression.