The mechanism of phorbol ester-induced tumor promotion.
Item
-
Title
-
The mechanism of phorbol ester-induced tumor promotion.
-
Identifier
-
AAI9908344
-
identifier
-
9908344
-
Creator
-
Lu, Zhimin.
-
Contributor
-
Adviser: David A. Foster
-
Date
-
1998
-
Language
-
English
-
Publisher
-
City University of New York.
-
Subject
-
Biology, Cell | Biology, Molecular | Biology, Genetics | Health Sciences, Oncology
-
Abstract
-
Tumor promoting phorbol esters activate, but then deplete cells of protein kinase C (PKC) with prolonged treatment. It is not known whether phopbol ester-induced tumor promotion is due to activation or depletion of PKC. In rat fibroblasts overexpressing the c-Src protooncogene, the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) induced anchorage-independent growth and other transformation-related phenotypes. The appearance of transformed phenotypes induced by TPA in these cells correlated not with activation, but rather with depletion of expressed PKC isoforms. Consistent with this observation, PKC inhibitors also induced transformed phenotypes in the c-Src-overexpressing cells. Bryostatin 1, which inhibited the TPA-induced downregulation of the {dollar}\delta{dollar} PKC isoform specifically, blocked the tumor-promoting effects of TPA, implicating PKC {dollar}\delta{dollar} as the target of the tumor promoting phorbol esters. Consistent with this hypothesis, expression of a dominant negative PKC {dollar}\delta{dollar} mutant in cells expressing c-Src caused transformation of these cells; and rottlerin, a protein kinase inhibitor with specificity for PKC {dollar}\delta{dollar}, like TPA, caused transformation of the c-Src-overexpressing cells. These data implicate that the downregulation of PKC {dollar}\delta{dollar} results in phorbol ester-induced tumor promotion effect.;To look for the mechanism of phorbol ester-induced downregulation of PKC, we investigated the role of the ubiquitin-proteasome pathway in the downregulation of PKC isoforms in response to the tumor promoting phorbol ester TPA. In 3Y1 rat fibroblasts, proteasome inhibitors prevent the depletion of PKC isoforms {dollar}\rm\alpha,\ \delta,\ and\ \epsilon.{dollar} in response to the TPA. Consistent with the involvement of the ubiquitin-proteasome pathway in the degradation of PKC isoforms, ubiquitinated PKC {dollar}\rm\alpha,\ \delta,\ and\ \epsilon{dollar} was detected within 30 min of TPA treatment. Diacylglycerol, the physiological activator of PKC, also stimulated ubiquitination and degradation of PKC suggesting that ubiquitination is a physiological response to PKC activation. Compounds that inhibit activation of PKC prevented both TPA-and diacylglycerol-induced PKC depletion and ubiquitination. Moreover, a kinase-dead ATP-binding mutant of PKC a could not be depleted by TPA treatment. These data are consistent with a suicide model whereby activation of PKC triggers its own degradation via the ubiquitin-proteasome pathway. Proteasome inhibitors also blocked the tumor promoting effects of TPA on 3Y1 cells overexpressing c-Src, which results from the depletion of PKC {dollar}\delta{dollar}, this provides further evidence to support that the tumor promoting effect of phorbol esters is due to depletion of PKC {dollar}\delta{dollar}, which has an apparent tumor suppressor function.
-
Type
-
dissertation
-
Source
-
PQT Legacy CUNY.xlsx
-
degree
-
Ph.D.
