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Mol. Cell. Biol., Jan 1998, 629-643, Vol 18, No. 1
AB Niculescu 3rd, X Chen, M Smeets, L Hengst, C Prives and SI Reed
It has been proposed that the functions of the cyclin-dependent kinase
inhibitors p21(Cip1/Waf1) and p27Kip1 are limited to cell cycle control at
the G1/S-phase transition and in the maintenance of cellular quiescence. To
test the validity of this hypothesis, p21 was expressed in a diverse panel
of cell lines, thus isolating the effects of p21 activity from the
pleiotropic effects of upstream signaling pathways that normally induce p21
expression. The data show that at physiological levels of accumulation,
p21, in addition to its role in negatively regulating the G1/S transition,
contributes to regulation of the G2/M transition. Both G1- and G2-arrested
cells were observed in all cell types, with different preponderances.
Preponderant G1 arrest in response to p21 expression correlated with the
presence of functional pRb. G2 arrest was more prominent in pRb-negative
cells. The arrest distribution did not correlate with the p53 status, and
proliferating-cell nuclear antigen (PCNA) binding activity of p21 did not
appear to be involved, since p27, which lacks a PCNA binding domain,
produced similar arrest Bs. In addition, DNA endoreduplication occurred in
pRb-negative but not in pRb-positive cells, suggesting that functional pRb
is necessary to prevent DNA replication in p21 G2- arrested cells. These
results suggest that the primary target of the Cip/Kip family of inhibitors
leading to efficient G1 arrest as well as to blockade of DNA replication
from either G1 or G2 phase is the pRb regulatory system. Finally, the
tendency of Rb-negative cells to undergo endoreduplication cycles when p21
is expressed may have negative implications in the therapy of Rb-negative
cancers with genotoxic agents that activate the p53/p21 pathway.
Copyright © 1998, American Society for Microbiology
Effects of p21(Cip1/Waf1) at both the G1/S and the G2/M cell cycle transitions: pRb is a critical determinant in blocking DNA replication and in preventing endoreduplication
Department of Molecular Biology, The Scripps Research Institute, La Jolla, California 92037, USA.
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