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.
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 G1 growth-arrest followed by apoptosis. By contrast, cells treated with the Ca(2+)-ionophore, ionomycin, generated apoptotic signals in G2/M as well as in G1. 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 G1-arrest induced by ionomycin. TGF beta treatment of Bcl-2-expressing cells induced G1-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.
Wang
H
H
Department of Immunology, University of Birmingham Medical School, Edgbaston, UK.
Grand
R J
RJ
Milner
A E
AE
Armitage
R J
RJ
Gordon
J
J
Gregory
C D
CD
D000888:Antibodies, Anti-Idiotypic / Q000494:pharmacology; D017209:Apoptosis / Q000187:drug effects* / Q000502:physiology*; D001402:B-Lymphocytes / Q000473:pathology*; D002051:Burkitt Lymphoma / Q000473:pathology*; D023201:CD40 Ligand; D002453:Cell Cycle / Q000502:physiology; D016193:G1 Phase / Q000187:drug effects / Q000502:physiology; D006801:Humans; D015759:Ionomycin / Q000494:pharmacology; D007476:Ionophores / Q000494:pharmacology; D008562:Membrane Glycoproteins / Q000494:pharmacology*; D010766:Phosphorylation; D011518:Proto-Oncogene Proteins / Q000494:pharmacology* / Q000502:physiology; D019253:Proto-Oncogene Proteins c-bcl-2; D016160:Retinoblastoma Protein / Q000378:metabolism / Q000502:physiology*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D014407:Tumor Cells, Cultured
D000888:Antibodies, Anti-Idiotypic; D007476:Ionophores; D008562:Membrane Glycoproteins; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D016160:Retinoblastoma Protein; D023201:CD40 Ligand; D015759:Ionomycin