Regulation of the catalytic subunit (p34PSK-J3/cdk4) for the major D-type cyclin in mature B lymphocytes.
We examined the expression of the cyclin-dependent kinase 4, p34PSK-J3/cdk4 protein, in small dense, activated, and proliferating primary B lymphocytes. A small steady state level of cdk4 synthesis was detected in resting B cells. Stimulation of resting B cells with mitogenic amounts of F(ab')2 fragments of goat anti-mouse IgM (anti-Ig) resulted in increased synthesis of cdk4 protein during the mid to late G1 phase of the cell cycle; LPS or the combination of phorbol ester and calcium ionophore also elevated cdk4 levels. Resting B cells that we rendered competent by treatment with IL 4 or low doses of anti-Ig or, alternatively, were activated by phorbol ester or ionomycin alone also exhibited heightened cdk4 protein levels. Subsequent analysis of potential cdk4 regulatory subunit D-type cyclins revealed that cyclin D2, not cyclin D1 or D3, is expressed in primary mature B lymphocytes. The induction of cyclin D2 synthesis in response to mitogenic anti-Ig paralleled cdk4 expression; however, IL-4 or low dose anti-Ig alone did not increase the rate of de novo cyclin D2 synthesis above that of resting B cells. The significance of the lack of cyclin D2 regulation by competence-inducing growth factors was demonstrated, in that only mitogenic factors that stimulated DNA synthesis 1) led to the formation of stable cyclin D2/cdk4 holoenzyme complexes during G1 phase progression, and 2) afforded the isolation of anti-cyclin D2 or anti-cdk4 immunoprecipitates that phosphorylated retinoblastoma. These findings suggest a role for these proteins during the mid to late G1 phase progression and possibly the G1/S phase transition in primary mature B lymphocytes.
Department of Biology, Boston College, Chestnut Hill, MA 02167, USA.
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Chiles
D000818:Animals; D000888:Antibodies, Anti-Idiotypic / Q000276:immunology; D001402:B-Lymphocytes / Q000201:enzymology*; D001665:Binding Sites; D002384:Catalysis; D002478:Cells, Cultured; D056742:Cyclin D2; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases / Q000737:chemistry / Q000378:metabolism*; D016213:Cyclins / Q000096:biosynthesis; D004789:Enzyme Activation / Q000187:drug effects; D004790:Enzyme Induction / Q000187:drug effects; D016193:G1 Phase / Q000187:drug effects; D006041:Goats; D007152:Immunologic Capping; D015847:Interleukin-4 / Q000494:pharmacology; D015759:Ionomycin / Q000494:pharmacology; D008213:Lymphocyte Activation; D051379:Mice; D008807:Mice, Inbred BALB C; D008934:Mitogens / Q000494:pharmacology; D009097:Multienzyme Complexes / Q000378:metabolism; D011518:Proto-Oncogene Proteins; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D000888:Antibodies, Anti-Idiotypic; C486196:Ccnd2 protein, mouse; D056742:Cyclin D2; D016213:Cyclins; D008934:Mitogens; D009097:Multienzyme Complexes; D011518:Proto-Oncogene Proteins; C044203:anti-IgM; D015847:Interleukin-4; D015759:Ionomycin; C495902:Cdk4 protein, mouse; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases; D013755:Tetradecanoylphorbol Acetate