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P. Amati - Role of the early phases of infection in determining virus permissivity
85% of the animals. Interestingly, a crucial difference
was observed in kidney sarcomas induction
ability. Indeed, the mutant induced this neoplasia in
only 25% of mice in contrast to 65% of the PyWtinoculated
mice (p=0.02).
These results suggest that the interaction of Py VP1
and α4β1 integrin is involved in tissue-tropism, during
primary infection, and in kidney tumorigenesis.
The meaning of these findings and the correlation
between the Py replication and Py tumour induction
will be further investigated.
Role of PARP-1 the induction of growthregulated
genes during cell cycle re-activation
The interaction of Py with the cell membrane promotes,
through the VP1 protein, cell cycle progression
of quiescent fibroblast cells, by stimulating
transcription of several early-response genes such as
c-myc, c-fos, and c-jun. On the basis that i) an early
event in VP1-host cell interaction, occurring in concomitance
with the induction of early growthresponse
genes, is the stimulation of PARP-1 activity;
and that ii) PARP-1 has been recently shown to
be involved in controlling cell cycle, we decided to
investigate the possible role of PARP-1 in the exit
from quiescence, a potentially important regulatory
step, not only in the context of viral infections but
also in the abnormal cell cycle of transformed cells.
ADP-ribose polymerases (PARPs) lead to the transfer
of ADP-ribose units from NAD + to target proteins,
forming homopolymers of different sizes.
This modification alters their functional and physico-chemical
properties. The modified proteins loose
their affinity for DNA and dissociate from it increasing
the accessibility of protein complexes to chromatin.
More recently it has been discovered that
poly(ADP-ribosyl)ation is implicated in a rich variety
of physiological processes, including mitotic
functions, cell death, transcriptional regulation, differentiation
and aging.
The exit of cells from a quiescent state (G0 phase) is
a multistep process that begins with the immediate
early response to mitogens and extends into a specialized
G1 phase. We have demonstrated that
PARP-1 is required in G0-G1 transition of resting
cells. Indeed, the examination of early times following
stimulation of quiescent cells in the absence of
PARP activity confirmed that poly(ADP-ribosyl)ation
is necessary for G0 exit. We showed that
PARP activity is involved in this step through the
regulation of immediate early response genes, such
4
as c-Fos and c-Myc. This was supported by the finding
that exogenous Myc expression substantially
restores cell cycle re-activation in the absence of
polymer synthesis. Furthermore, using siRNAs, we
demonstrated that PARP-1 is the PARP family member
involved in the initial step of cell cycle re-activation
(Carbone et al., 2008).
It is reasonable that PARP-1 activity participates in
the regulation of chromatin organization modulating
the accessibility of transcription factors. In line
with this function we plan to study the direct implication
of PARP-1 in the chromatin structure at the
IEGs promoters.
Interplay between muscle regulatory factors
and cell cycle inhibitors
The appropriately timed induction of cdk inhibitors
and their sustained expression are critical for the
induction and the maintenance of the postmitotic
state in muscle cells as well as in other differentiating
cell types. This research has been focused on the
mechanisms regulating the expression of p57kip2, a
cdk inhibitor with unique properties, devoting particular
attention to the epigenetic modifications participating
in the transcriptional control of this gene at
the onset of differentiation. Our recent results showed
that in muscle cells p57 is subject to a complex regulation
involving a DNA demethylation process besides
the induction of trans-acting factors (Figliola et al.,
2008). Work is in progress to further investigate the
mechanisms regulating the methylation status of p57
promoter during muscle differentiation and their relationship
with transcriptional activation.
Selected publications
Caruso M, Busanello A, Sthandier O, Cavaldesi M,
Gentile M, Garcia MI, Amati P. Mutation in the
VP1-LDV motif of the murine polyomavirus affects
viral infectivity and conditions virus tissue tropism
in vivo. J Mol Biol. 2007, 367:54-64.
Carbone M, Rossi MN, Cavaldesi M, Notari A,
Amati P, Maione R. Poly(ADP-ribosyl)ation is implicated
in the G0-G1 transition of resting cells.
Oncogene 2008, 27:6083-92.
Figliola R, Busanello A, Vaccarello G, Maione R.
Regulation of p57(KIP2) during muscle differentiation:
role of Egr1, Sp1 and DNA hypomethylation. J
Mol Biol. 2008, 380:265-77.