Biology Reference
In-Depth Information
PLK4 protein level is also necessary for centriole overduplication to occur since it
is known to be rate-limiting for this process.
The PLK4 promoter has been shown to contain E2F-responsive elements and be
repressed by HDACs, two important cellular transcriptional control pathways that
are deregulated following HPV-16 E7 oncoprotein expression (Li et al.
2005
;
McLaughlin-Drubin and Munger
2009
). PLK4 promoter activation assays and
real-time quantitative reverse transcriptase (qRT-PCR) analysis of PLK4 mRNA
level were performed on cells expressing either wild-type or mutant HPV-16 E7
and it was found that wild-type HPV-16 E7 could both activate the PLK4 promoter
and upregulate PLK4 mRNA levels (Korzeniewski et al.
2011
).
Conversely, an HPV-16 E7 mutant with deletion of the amino acid region
21-24 which contains the LCXCE motif (HPV-16 E7 D21-24), and is incapable of
pRB-binding was unable to activate the PLK4 promoter or upregulate PLK4
mRNA and, in accordance with previous studies, unable to promote centriole
overduplication (Korzeniewski et al.
2011
; Duensing and Munger
2003
). Although
the HPV-16 E7 D21-24 mutant is unable to bind and degrade pRB, it is still
capable of interacting with HDACs (Phelps et al.
1992
).
Like the HPV-16 E7 D21-24 mutant construct, an HDAC-interaction-deficient
mutant L67R was also unable to activate the PLK4 promoter or upregulate PLK4
mRNA (Korzeniewski et al.
2011
). The HPV-16 L67R mutant is still capable of
interacting with pRB although it does so less efficiently and has a reduced capacity
to activate E2F-dependent transcription (Avvakumov et al.
2003
). This defect
complicates analysis of the role of HDACs in the HPV-16 E7-induced modulation
of PLK4 transcription. Analyzing PLK4 mRNA abundance in HPV-16 E7
expressing cell which is deficient in HDACs or pRB-family members would clarify
the role of HDACs in the HPV-16 E7-mediated modulation of PLK4 mRNA.
A mechanism for the rapid induction of centriole multiplication by the HPV-16
E7 oncoprotein can now be postulated (Fig.
12.2
). It has been previously shown
that cyclin E/CDK2 complexes mediate the aberrant recruitment of PLK4 to
maternal centrioles (Korzeniewski et al.
2009
). However, a concurrent increase in
PLK4 protein is necessary for centriole multiplication to occur (Korzeniewski
et al.
2009
). The aberrant recruitment of PLK4 to maternal centrioles is recapit-
ulated in cells expressing HPV-16 E7 and these cells also contain increased PLK4
mRNA transcript levels (Korzeniewski et al.
2011
). This increase in PLK4 mRNA,
albeit only modest, may be necessary to promote the aberrant recruitment of
excess PLK4 to maternal centrioles in the form of multiple PLK4 dots and ulti-
mately centriole multiplication. Support for this notion comes from a previous
study which determined that ongoing RNA polymerase II transcription is neces-
sary for HPV-16 E7-induced centriole overduplication but dispensable for normal
centriole duplication (Duensing et al.
2007
). This is in line with our finding that
increased PLK4 mRNA transcripts play a role in HPV-16 E7-induced centriole
multiplication (Korzeniewski et al.
2011
).
Therefore, HPV-16 E7 can interfere with two steps of centriole biogenesis in
order to stimulate centriole multiplication: increase PLK4 at the gene expression
level and enhance the recruitment of PLK4 to maternal centrioles. However, PLK4
