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Abstract
B chronic lymphocytic leukemia (B-CLL) and hairy cell leukemia (HCL) cells are refractory to many of the signals which activate normal B cells but are stimulated to proliferate by tumor necrosis factor (TNF). Cell signalling by TNF is mediated in part by the induction of the transcription factor families AP-1 and NF-kB. In some cellular contexts, these factors play a role in regulating cell cycle transit. AP-1 binds DNA as dimers of jun and fos family proteins and is regulated by a cascade of protein kinases which eventually activate a mitogen-activated protein kinase (MAP kinase) and also by protein kinase C. Three pathways have been implicated in the activation of NF-kB by extracellular ligands. 1, the activation of protein kinase C by diacylglycerol generated by ligand-mediated activation of phosphatidylcholine hydrolysis, 2, stimulation of specific protein kinases by ceramide generated following activation of a sphingomyelinase by diacylglycerol and 3, a novel pathway involving ligand-induced generation of free radical species. In B-CLL and HCL cells, the generation of nuclear-localized c-jun and c-fos proteins (components of AP-1) in response to TNF or PMA appears to be blocked. Whereas PMA failed to induce NF-kB in these cells, this factor was readily induced by TNF. TNF induction of NF-kB was abolished by antioxidants, suggesting involvement of the free radical pathway. The data discussed here suggest defects in coupling of some protein kinase C-dependent pathways in B-CLL and HCL cells and that TNF is able to bypass these blocks by the activation of NF-kB via a free radical-dependent pathway which is independent of protein kinase C. Furthermore, the ability of low dose PMA to generate a proliferative reponse in synergy with TNF suggests that the pharmacological activation of protein kinase C may trigger pathways which collaborate with TNF-induced pathways in the promotion of cell cycle transit.