Biology Reference
In-Depth Information
degradation, through the use of a proteasome inhibitor Z-L
3
VS, induced a large
proportion of cells to exhibit centriole multiplication (Duensing et al.
2007
). This
observation led to functional studies to discover what cellular factors were
necessary for this phenotype to occur and it was discovered that CDK2, cyclin E,
and PLK4 were necessary factors for Z-L
3
VS-induced centriole multiplication
(Duensing et al.
2007
). Further experiments revealed that ectopic expression of
cyclin E/CDK2 alone was not sufficient to induce centriole multiplication
(Korzeniewski et al.
2009
). Deregulation of cyclin E/CDK2 complexes by
themselves was found to promote the aberrant recruitment of PLK4 to maternal
centrioles but endogenous levels of PLK4 were insufficient to induce centriole
multiplication. Centriole multiplication occurred when CDK2/cyclin E complexes
and PLK4 were upregulated, suggesting that endogenous levels of PLK4 are not
sufficient to induce centriole multiplication and that PLK4 protein levels are
rate-limiting for centriole multiplication (Korzeniewski et al.
2009
).
PLK4 is an essential regulator of both centriole duplication and cell viability. In
vitro studies have demonstrated that PLK4 overexpression in tissue culture results
in an increase in supernumerary centrosomes and when PLK4 is depleted via RNA
interference, centriole numbers are reduced with progressive loss of centrioles and
the subsequent development of monopolar spindles (Habedanck et al.
2005
). In
vivo, PLK4 knockout is embryonic lethal; however, heterozygous mice develop
normally (Ko et al.
2005
; Hudson et al.
2001
). Aged PLK4 heterozygous mice
have an increased risk for development of spontaneous malignancies than do their
littermates and exhibit abnormal chromosomal segregation and alignment defects,
cytokinesis failure, and multinucleation (Ko et al.
2005
). These observations
suggest that a strict control of PLK4 transcript and protein levels is necessary to
maintain cell viability and prevent malignant progression. Although the threshold
level of PLK4 protein which induces centriole multiplication is not known, our
own experiments have shown that very small changes in PLK4 protein level
induces a small but significant percentage of cells to exhibit centriole multipli-
cation (Korzeniewski et al.
2009
) which may ultimately promote a tolerable level
of chromosomal instability leading to malignant progression.
12.11 Cellular Proteolysis and Centriole Multiplication
The discovery that inhibition of cellular protein degradation mechanisms strongly
induced centriole multiplication suggested that proteolysis was important in
maintaining normal centriole duplication control (Duensing et al.
2007
). PLK4 is a
very unstable protein whose stability is known to be controlled by both ubiquitin
and non-ubiquitin-mediated proteolysis (Korzeniewski et al.
2009
; Cunha-Ferreira
et al.
2009
; Rogers et al.
2009
). The PLK4 protein coding sequence has been
reported to contain a PEST domain implicated in the mediation of rapid protein
degradation by intracellular proteases and is also known to be targeted for
E3-ubiquitin ligase-mediated degradation (Rechsteiner and Rogers
1996
).
