Carcinogenesis, Vol 18, 731-738, Copyright © 1997 by Oxford University Press
SH Zeisel, CD Albright, OH Shin, MH Mar, RI Salganik and KA da Costa
The mechanisms which drive initiated cells to progress to form carcinomas
are poorly understood. CWSV-1 rat hepatocytes, in which p53 protein is
inactivated by SV40 large T antigen, respond by inducing p53- independent
apoptosis when acutely switched to medium containing low choline (16%
apoptotic at 48 h in 5 microM choline) as compared with controls (1%
apoptotic at 48 h in 70 microM choline). The rate of apoptosis was
inversely correlated with cellular phosphatidylcholine content. Choline
deficiency (CD)-induced apoptosis is probably mediated by TGFbeta1 and
reactive oxygen species, since immunoneutralization of TGFbeta1 in the
medium or treatment with N-acetylcysteine (an antioxidant) or addition of
neocuproine (a transition metal chelator) prevented CD-induced apoptosis.
CWSV-1 hepatocytes could be gradually adapted to survive in 5 microM
choline. CD-adapted cells had increased membrane phosphatidylcholine
concentrations (compared with acute CD cells). Adapted cells acquired
relative resistance to CD-induced apoptosis (7% of adapted cells compared
with 19% of non-adapted cells were apoptotic at 48 h in 5 microM choline).
They also became relatively resistant to another p53-independent form of
apoptosis (TGFbeta1-induced). CD-adapted hepatocytes developed increased
capability for anchorage-independent growth and formed tumors when
transplanted into nude mice; passage-matched control hepatocytes did not
possess these properties. Cell transformation was dependent on exposure to
the selective pressure of CD apoptosis, as we observed that when CD
apoptosis was inhibited with an antioxidant during adaptation, cells did
not become anchorage independent. Acquisition by p53- deficient cells of
resistance to p53-independent inducers of apoptosis (CD, TGFbeta1 and
reactive oxygen species) may leave cells without another important
apoptotic defensive barrier and may be responsible for the progression of
initiated cells to frank carcinomas.
ARTICLES
Choline deficiency selects for resistance to p53-independent apoptosis and causes tumorigenic transformation of rat hepatocytes
Department of Nutrition, School of Public Health and School of Medicine, University of North Carolina, Chapel Hill 27599-7400, USA.
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