Evidence for different mechanisms of growth inhibition of T-cell lymphoma by phorbol esters and concanavalin A.
Stimuli that are mitogenic for mature T-cells induce cell cycle arrest in some T-cell tumors and T-cell hybridomas. The molecular mechanism of this growth inhibition is poorly understood. In this report, we show that in EL4, a murine T-lymphoma cell line, stimulation with concanavalin A or treatment with phorbol 13-myristate 12-acetate (PMA) inhibit growth, due to cell cycle arrest at both the G1 and the G2/M phases. The block at the G1 phase is accompanied by the appearance of a hypophosphorylated form of the retinoblastoma protein (pRb), due to the inhibition of G1 cyclin-Cdk complexes. However, the molecular mechanisms leading to this G1 cell cycle arrest differ between concanavalin A and PMA: concanavalin A inhibits both cyclin E-Cdk2 and cyclin D-Cdk4 complexes, while PMA inhibits only cyclin E-Cdk2. We demonstrate that concanavalin A inhibits cyclin D-Cdk4 activity by decreasing the amount of cyclin D. The inhibition of cyclin E-Cdk2 by both concanavalin A and PMA is due to increased binding of the Cdk inhibitor p21 to this complex. However, while stimulation of the cells with concanavalin A did not result in an evident increase of the total level of p21, treatment of the cells with PMA increased p21 levels significantly. Our results indicate, furthermore, that the G2/M block results from the inhibition of cyclin A- and cyclin B1-associated kinase activities. As for cyclin E-Cdk2, the inhibition of the cyclin A-Cdk2 complex is due to increased binding of the p21 inhibitor.
Desrivières
S
S
Institute for Experimental Cancer Research, Tumor Biology Center, Freiburg 79106, Germany.
Volarević
S
S
Merćep
L
L
Ferrari
S
S
D000818:Animals; D003208:Concanavalin A / Q000494:pharmacology*; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D016193:G1 Phase / Q000187:drug effects; D016195:G2 Phase / Q000187:drug effects; D006801:Humans; D016399:Lymphoma, T-Cell / Q000473:pathology*; D051379:Mice; D008938:Mitosis / Q000187:drug effects; D019319:Okadaic Acid / Q000494:pharmacology; D010703:Phorbol Esters / Q000494:pharmacology*; D010766:Phosphorylation; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014407:Tumor Cells, Cultured
D010703:Phorbol Esters; D003208:Concanavalin A; D019319:Okadaic Acid; D018844:Cyclin-Dependent Kinases; D013755:Tetradecanoylphorbol Acetate