title: Recovery of gap junctional intercellular communication after phorbol ester treatment requires proteasomal degradation of protein kinase C.
abstract: Reversible down-regulation of gap junctional intercellular communication (GJIC) is proposed to be an important cellular mechanism in tumor promotion. Gap junction function is modified by a variety of tumor promoters, including the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Treatment of cells with TPA results in the activation and subsequent depletion of the TPA-responsive protein kinase C (PKC) isoforms. TPA-induced degradation of the PKC isoforms alpha, delta and epsilon was recently shown to occur via the ubiquitin-proteasome pathway. In the present study we investigated the role of the proteasome in the TPA-induced modification of GJIC in IAR20 rat liver epithelial cells. TPA exposure of IAR20 cells induced hyperphosphorylation of gap junction protein connexin43 and inhibition of GJIC. Prolonged TPA treatment induced down-regulation of PKCalpha, delta and epsilon and a reduction in the total PKC activity, which was associated with recovery of GJIC. Co-treatment of IAR20 cells with TPA and the proteasomal inhibitor MG132 suppressed down-regulation of PKCalpha, delta and epsilon and caused prolonged PKC activity. Under these conditions, the recovery of GJIC was blocked. The general PKC inhibitor GF109203X reversed the effect of MG132, indicating that the prolonged TPA-induced inhibition of GJIC caused by MG132 was due to the prolonged PKC activity. These results indicate that proteasomal degradation of PKC is one mechanism by which the recovery of GJIC after TPA treatment is regulated.
authors.affiliation: Department of Environmental and Occupational Cancer, Institute for Cancer Research, The Norwegian Radium Hospital, N-0310 Oslo, Norway. eliethe@klinmed.uio.no
authors.forename: Edward
authors.initials: E
authors.lastname: Leithe
authors.forename: Véronique
authors.initials: V
authors.lastname: Cruciani
authors.forename: Tore
authors.initials: T
authors.lastname: Sanner
authors.forename: Svein-Ole
authors.initials: SO
authors.lastname: Mikalsen
authors.forename: Edgar
authors.initials: E
authors.lastname: Rivedal
mesh_terms: D000818:Animals; D015153:Blotting, Western; D002450:Cell Communication / Q000187:drug effects; D002478:Cells, Cultured; D018031:Connexin 43 / Q000037:antagonists & inhibitors / Q000378:metabolism; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D015536:Down-Regulation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004847:Epithelial Cells / Q000187:drug effects / Q000201:enzymology; D017629:Gap Junctions / Q000187:drug effects*; D007211:Indoles / Q000494:pharmacology; D007527:Isoenzymes; D007976:Leupeptins / Q000494:pharmacology; D008099:Liver / Q000187:drug effects* / Q000201:enzymology; D008301:Maleimides / Q000494:pharmacology; D009097:Multienzyme Complexes / Q000378:metabolism*; D010447:Peptide Hydrolases / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D011493:Protein Kinase C / Q000378:metabolism*; D051381:Rats; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*
chemical_list: D018031:Connexin 43; D015853:Cysteine Proteinase Inhibitors; D004791:Enzyme Inhibitors; D007211:Indoles; D007527:Isoenzymes; D007976:Leupeptins; D008301:Maleimides; D009097:Multienzyme Complexes; D011493:Protein Kinase C; D010447:Peptide Hydrolases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; C070515:bisindolylmaleimide I; D013755:Tetradecanoylphorbol Acetate; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde