(Circulation. 1999;99:1656-1659.)
© 1999 American Heart Association, Inc.
Brief Rapid Communications |
From the Cardiology Branch (K. Tanaka, T.M.J., T.F.) and the Pathology Branch (K. Takeda, V.J.F.), National Heart, Lung, and Blood Institute, and the Experimental Immunology Branch (J.P.Z.), National Cancer Institute, National Institutes of Health, Bethesda, Md; the Medical College of Wisconsin, Madison (G.R.S.); and the Cardiovascular Research Foundation, Washington, DC (S.E.E.).
Correspondence to Stephen E. Epstein, MD, Cardiovascular Research Foundation, Washington Hospital Center, 110 Irving St, NW, Suite 4B-1, Washington, DC 20010.
| Abstract |
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Methods and ResultsApoptosis of SMCs was induced either by use of an adenovirus vector encoding human wild-type p53 protein or by treatment with doxorubicin. HCMV IE1-72 and IE2-84, the major IE proteins of HCMV, were overexpressed separately with adenovirus vectors encoding each protein, and the effects on p53-induced apoptosis were examined by both nick end-labeling (TUNEL) assay and flow cytometry. Expression of IE2-84, but not IE1-72, protected SMCs from p53-mediated apoptosis.
ConclusionsThese data indicate that an HCMV IE protein antagonizes p53-mediated apoptosis, suggesting a pathway by which HCMV infection predisposes to SMC accumulation and thereby contributes to restenosis and atherosclerosis.
Key Words: viruses proteins apoptosis restenosis
| Introduction |
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In the present investigation, we further elucidated the role of HCMV in modulating apoptosis. Specifically, we determined whether IE1-72 and IE2-84, the major HCMV IE proteins, inhibit p53-induced apoptosis in human coronary artery SMCs.
| Methods |
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Adenovirus Vectors
The adenoviruses Ad.IE1 and Ad.IE2 contain the HCMV IE1-72 and
IE2-84 genes, respectively, under control of the HCMV major IE promoter
(MIEP) and upstream of a polyadenylation signal, inserted into an Ad5
vector. During transgene insertion, homologous recombination results in
deletion of the entire E1A region and
90% of the E1B coding
sequence of wild-type adenovirus. The adenovirus Ad.p53 (a gift from
Silvia Bacchetti, McMaster University, Hamilton, Ontario, Canada)
contains an expression cassette consisting of the HCMV MIEP promoter
and SV40 T-antigen polyadenylation signal flanking coding sequences for
human wild-type p53 protein, inserted into an E1-deleted Ad5 vector.
Ad.ßGal, used as a control adenovirus, is an E1-deleted Ad5 vector
containing the Escherichia coli LacZ gene controlled by
MIEP.
Apoptosis Induced by p53 Through Adenoviral Gene Transfer
in Coronary Artery SMCs
Twenty-four hours after plating, cells were infected with
Ad.IE1, Ad.IE2, or Ad.ßGal at 200 multiplicity of infection (MOI).
Viruses were removed from the medium 24 hours after infection, and then
cells were superinfected with 100 MOI of Ad.p53 to induce
apoptosis. As a control, cells were superinfected with 100 MOI
of Ad.ßGal instead of Ad.p53. The total amount of adenovirus
infection in each group was 300 MOI. Virus was removed from the medium
24 hours after superinfection. Apoptosis was assessed by nick
end-labeling (TUNEL) assay at 48 hours or by flow cytometry at 72 hours
after superinfection.
Doxorubicin-Induced Apoptosis in Coronary
Artery SMCs
Coronary artery SMCs were infected with Ad.IE1, Ad.IE2,
or Ad.ßGal at 200 MOI 24 hours after plating. Virus was removed from
the medium 24 hours after infection, and cells were cultured in regular
medium for an additional 24 hours before addition of 1.0 µmol/L
doxorubicin (Sigma Chemical Co). Adherent cells were harvested 24 hours
after addition of doxorubicin for the expression of
endogenous p53 and 48 hours after for quantification
of the proportion of apoptotic cells by flow cytometry.
Assays for Apoptosis
For demonstration and quantification of apoptosis, TUNEL
assay was performed with an ApopTag Direct In Situ Apoptosis
Detection Kit-Fluorescein (Oncor) according to the
manufacturer's recommendations. Apoptotic nuclei were labeled
with fluorescein, and all nuclei were counterstained with
4,6-diamidino-2-phenylindole (DAPI, Sigma). Samples were examined with
a laser scanning confocal fluorescence microscope.
Flow cytometric quantification of apoptotic cells was performed as described by Nicoletti et al.10 Adherent cells were harvested and stained in hypotonic fluorochrome solution (propidium iodide 50 µg/mL in sodium citrate plus 0.1% Triton X-100, Sigma). Cells were analyzed by a fluorescence-activated cell sorter (FACScan) flow cytometer (Becton Dickinson). Apoptotic nuclei were identified as a subgenomic DNA peak and were distinguished from cell debris on the basis of both forward light scatter and fluorescence of propidium iodide.
Western Blot Analysis
Western blot was performed with 10 µg of cell lysate with
mouse monoclonal antibodies against IE1-72 and IE2-84 (Vancouver
Biotech), p53 (DO-1; Santa Cruz Biotechnology), or p21 (Santa Cruz
Biotechnology). A monoclonal antibody against
-tubulin (Calbiochem)
was used to confirm equal protein loading. Immune complexes were
detected by enhanced chemiluminescence (Amersham).
Statistical Analysis
Differences between groups were analyzed by 1-way ANOVA.
A value of P<0.05 was considered statistically
significant.
| Results |
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The ability of p53 to induce apoptosis may depend on its
function as a transactivator of gene
expression.11 Although p53 regulates several gene
products, the best-characterized product is the
cyclin-dependent kinase inhibitor p21.5 Using
p21 levels as an indicator of p53 transcriptional activity in SMCs, we
assessed the effects of IE gene expression on p53 transcriptional
activity. P53 expression significantly increased p21 levels (Figure 3
). Coexpression of IE1-72 potentiated
this increase, whereas expression of IE2-84 inhibited it. The degree of
inhibition by IE2-84 was dependent on the level of IE2-84 expression.
These results are consistent with the notion that IE gene
products modulate p53-dependent apoptosis by regulating p53
transcriptional activity.
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To assess whether other agents that induce p53-mediated
apoptosis are also altered by IE gene products, we treated
SMCs with doxorubicin. When added to SMCs at a concentration of
1.0 µmol/L for 24 hours, doxorubicin increased p53 levels
(Figure 4
) and induced apoptosis
in Ad.ßGal-infected cells (the number of apoptotic cells
increased
4-fold above control values). This is consistent
with results in other cell types demonstrating that doxorubicin can
induce apoptosis through a p53-dependent
pathway.12 Consistent with our results with direct
p53-mediated apoptosis, IE1-72 increased and IE2-84 suppressed
doxorubicin-induced apoptosis (Figure 5
).
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| Discussion |
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That these observations are not limited to experimental conditions in which p53 is overexpressed through adenoviral gene transfer is indicated by our experiments using doxorubicin-induced apoptosis. This model uses, at least in part, the intrinsic p53-modulated apoptotic pathway.12 Exposure of SMCs to doxorubicin increased p53 levels and led to apoptosis. As in the model of apoptosis caused by exogenous p53 gene transfer, IE1-72 increased and IE2-84 decreased apoptosis.
Although the proapoptotic effect of IE1-72 and the functionally homologous proteins of other DNA tumor viruses is difficult to understand from the point of view of viral evolution, a reasonable explanation derives from an evolutionary context of competing strategies for host versus pathogen survival. To survive, a virus infects a cell and forces it to replicate; the virus uses the cell's replicative machinery to drive its own replication. HCMV accomplishes this, in part, through expression of IE1-72, which interacts with the Rb gene product, eliminating its inhibition of E2F.15 E2F in turn activates genes required for cell cycling. The resulting proliferation signal drives the cell to replicate in an inappropriate context (inadequate growth factors present or other influences in play that signal the cell to remain quiescent), leading the cell to activate p53 and initiate its apoptotic cascade. This is analogous to what occurs with c-myc overexpression, which induces cellular proliferation in the presence of growth factors but induces p53-dependent apoptosis when the cell is deprived of growth factors.13
In summary, p53-induced apoptosis in coronary artery SMCs is inhibited by HCMV IE2-84 and stimulated by IE1-72. These data, together with those of other studies, provide mechanistic support for a role of HCMV in restenosis and atherosclerosis.
Received December 8, 1998; revision received January 27, 1999; accepted February 1, 1999.
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