Carcinogenesis Advance Access published online on May 9, 2003
Carcinogenesis, doi:10.1093/carcin/bgg066
© 2003 by Oxford University Press
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CARCINOGENESIS
1 Department of Environmental and Occupational Cancer, Institute for Cancer Research, The Norwegian Radium Hospital, N-0310 Oslo, Norway
* Corresponding author. E-mail: eleithe{at}klinmed.uio.no.
Received 17 January 2003
; revised 13 April 2003
; accepted 15 April 2003
Reversible downregulation 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
gap junctional intercellular communication, connexin43, phorbol ester, protein kinase C, proteasome
Recovery of gap junctional intercellular communication after phorbol ester treatment requires proteasomal degradation of protein kinase C
, 
, and 
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 downregulation of PKC, , and , and reduction in the total PKC activity, which was associated with recovery of GJIC. Cotreatment of IAR20 cells with TPA and the proteasomal inhibitor MG132 suppressed downregulation of PKC, , and 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.
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