title: Inhibition of phorbol ester-induced T cell proliferation by bryostatin is associated with rapid degradation of protein kinase C.
abstract: The bryostatins (Bryo) represent a group of immunomodulators that counteract the tumor-promoting effects of PMA. In contrast to the mitogenic effect of PMA on human peripheral blood T lymphocytes, Bryo was nonmitogenic and, furthermore, it inhibited PMA-induced T cell proliferation in a dose-dependent manner when added up to 2 days after PMA. Because both Bryo and PMA bind to, and activate, protein kinase C (PKC), we compared their effects on PKC expression and activity in human PBL or leukemic T cells (Jurkat). After treatment for 5 to 60 min, both Bryo and PMA were found to: a) activate PKC in vitro with similar dose-response curves; b) induce a nearly complete cytosol-to-membrane translocation of enzymatically active, Ca(2+)-dependent PKC and of distinct immunoreactive PKC isoforms in intact PBL; and c) stimulate similar patterns of protein phosphorylation. After a longer, 20-h treatment with PMA (20 nM), a considerable portion of PKC was still membrane-associated, and the total amount of immunoreactive PKC was not reduced considerably. In contrast, Bryo induced a marked loss of cellular immunoreactive PKC, including PKC-alpha and -beta. These results were paralleled by measurements of total cytosol- or membrane-associated PKC enzymatic activity. Thus, substantial PKC activity was associated with the particulate fraction of PMA-, but not Bryo-stimulated PBL. Furthermore, inhibition of PMA-induced T cell proliferation by Bryo also correlated with a reduction in the amount of cytosolic and membrane-bound immunoreactive PKC and enzymatic activity, demonstrating the dominance of Bryo over PMA. We propose that Bryo inhibits PMA-induced T cell proliferation by causing rapid degradation of PKC, reflecting a requirement of persistent PKC stimulation (lasting approximately 48 h) for the activation of human T cells and progression through the cell cycle.
authors.affiliation: Department of Microbiology and Immunology, Ben Gurion University of the Negev, Beer Sheva, Israel.
authors.forename: N
authors.initials: N
authors.lastname: Isakov
authors.forename: D
authors.initials: D
authors.lastname: Galron
authors.forename: T
authors.initials: T
authors.lastname: Mustelin
authors.forename: G R
authors.initials: GR
authors.lastname: Pettit
authors.forename: A
authors.initials: A
authors.lastname: Altman
mesh_terms: D054713:Bryostatins; D002451:Cell Compartmentation / Q000187:drug effects; D002478:Cells, Cultured; D006801:Humans; D066298:In Vitro Techniques; D007783:Lactones / Q000494:pharmacology*; D008213:Lymphocyte Activation / Q000187:drug effects*; D008214:Lymphocytes / Q000276:immunology*; D018942:Macrolides; D010703:Phorbol Esters / Q000494:pharmacology*; D010750:Phosphoproteins / Q000378:metabolism; D011493:Protein Kinase C / Q000378:metabolism*; D013997:Time Factors
chemical_list: D054713:Bryostatins; D007783:Lactones; D018942:Macrolides; D010703:Phorbol Esters; D010750:Phosphoproteins; C046785:bryostatin 1; D011493:Protein Kinase C