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prematurely triggers initiation of centrosome duplication through phosphorylation
of NPM/B23 and consequential superactivation of ROCK II. However, because
initiation of DNA replication requires many CDK2-cyclin E-independent events,
CDK2-cyclin E can trigger DNA replication only after those events are completed,
and thus the presence of active CDK2-cyclin E shortens the G1 duration for only
few hours (Dulic et al.
1992
; Koff et al.
1992
). Thus, loss or mutational inacti-
vation of p53 leads to uncoupling of initiation of centrosome duplication and DNA
replication (Fig.
10.2
). However, because uncoupling of initiation of centrosome
duplication and DNA replication in cells lacking functional p53 depend on
occurrence of ''accidental'' premature activation of CDK2-cyclin E, apparently the
cells lacking functional p53 do not always experience the uncoupling of these two
events, but in a long term, the majority of the p53-negative cells in a given
population
will
experience
uncoupling
of
centrosome
duplication
and
DNA
replication.
10.3 Loss of p53 and Centrosome Amplification
As described in other chapters, centrosome amplification leads to a high frequency
of mitotic spindle defects and consequentially chromosome segregation errors.
Centrosome amplification occurs frequently in various types of cancers, and is
thought to be the major cause of chromosome instability in cancer cells (D'Assoro
et al.
2002
; Fukasawa
2005
). Initially, induction of centrosome amplification by
loss of p53 was identified in cells and tissues of p53-deficient mouse (Fukasawa
et al.
1996
,
1997
). The mechanism of how loss of p53 leads to centrosome
amplification was explored by the experimental system often referred to as
''centrosome amplification (reduplication) assay'', in which centrosomes undergo
multiple rounds of duplication exposed to DNA synthesis inhibitors such as the
DNA polymerase inhibitor (i.e., aphidicolin) and ribonucleotide reductase inhib-
itors (i.e., hydroxyurea (HU)), resulting in generation of amplified centrosomes.
However, centrosome reduplication in the cell cycle-arrested cells occurs effi-
ciently only when p53 is either inactivated or lost (Tarapore et al.
2001
). In normal
cells, p53 is upregulated in response to the physiological stress associated with the
prolonged arrest by the ARF-mediated inhibition of Mdm2 (Sherr
2006
) as well as
DNA damages inflicted by the inhibitors by ATM/ATR- as well as Chk1/Chk2-
mediated phosphorylation (Taylor and Stark
2001
), leading to an increase in the
intracellular level of p21, which in turn inhibits CDK2. Without the activity of
CDK2, centrosome reduplication cannot be initiated. In contrast, in cells lacking
functional p53, there will be no inhibitory mechanism for the CDK2 activity in
response to the physiological and genotoxic stresses, and fortuitous activation of
CDK2 leads to centrosome reduplication. This observation helped understanding
the mechanism of how loss of p53 could lead to centrosome amplification
(Fig.
10.3
). Even under a normal growth condition/environment, cells are con-
stantly subjected to internal as well as external stresses that temporarily halt cell
