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Mol. Cell. Biol., 01 1998, 536-545, Vol 18, No. 1
OB Chernova, MV Chernov, Y Ishizaka, ML Agarwal and GR Stark
Genomic instability, including the ability to undergo gene amplification,
is a hallmark of neoplastic cells. Similar to normal cells, "nonpermissive"
REF52 cells do not develop resistance to N- (phosphonacetyl)-L-aspartate
(PALA), an inhibitor of the synthesis of pyrimidine nucleotides, through
amplification of cad, the target gene, but instead undergo protective,
long-term, p53-dependent cell cycle arrest. Expression of exogenous MYC
prevents this arrest and allows REF52 cells to proceed to mitosis when
pyrimidine nucleotides are limiting. This results in DNA breaks, leading to
cell death and, rarely, to cad gene amplification and PALA resistance.
Pretreatment of REF52 cells with a low concentration of PALA, which slows
DNA replication but does not trigger cell cycle arrest, followed by
exposure to a high, selective concentration of PALA, promotes the formation
of PALA-resistant cells in which the physically linked cad and endogenous
N-myc genes are coamplified. The activated expression of endogenous N-myc
in these pretreated PALA-resistant cells allows them to bypass the
p53-mediated arrest that is characteristic of untreated REF52 cells. Our
data demonstrate that two distinct events are required to form
PALA-resistant REF52 cells: amplification of cad, whose product overcomes
the action of the drug, and increased expression of N-myc, whose product
overcomes the PALA-induced cell cycle block. These paired events occur at a
detectable frequency only when the genes are physically linked, as cad and
N-myc are. In untreated REF52 cells overexpressing N-MYC, the level of p53
is significantly elevated but there is no induction of p21waf1 expression
or growth arrest. However, after DNA is damaged, the activated p53 executes
rapid apoptosis in these REF52/N-myc cells instead of the long-term
protective arrest seen in REF52 cells. The predominantly cytoplasmic
localization of stabilized p53 in REF52/N-myc cells suggests that
cytoplasmic retention may help to inactivate the growth-suppressing
function of p53.
Copyright © 1998, American Society for Microbiology
MYC abrogates p53-mediated cell cycle arrest in N-(phosphonacetyl)-L- aspartate-treated cells, permitting CAD gene amplification
Department of Molecular Biology, Research Institute, The Cleveland Clinic Foundation, Ohio 44195, USA.
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