Download Death-Defying Pathways Linking Cell Cycle and Apoptosis

See related article, pages 1004 –1011
Death-Defying Pathways Linking Cell Cycle and Apoptosis
Lorrie A. Kirshenbaum
O
Downloaded from http://circres.ahajournals.org/ by guest on April 30, 2017
stage heart failure, given the limited and meager ability of the
adult myocardium for regeneration after injury. The fact that
apoptosis is a genetically regulated event suggests that
components of the apoptotic pathway may be targeted to
modulate or prevent the inappropriate cell death in disease
conditions.2
Because it is generally believed that the number of available ventricular myocytes can directly influence cardiac
performance, the ultimate therapeutic objective in reducing
morbidity and mortality in patients with diminished pump
function would be to either prevent excessive cell loss from
occurring or, alternatively, increase the number of functionally active myocytes through a regenerative process. Toward
this goal, several innovative and genetic strategies have been
devised to increase the number of functionally active ventricular myocytes in the myocardium. These include the phenotypic conversion of fibroblasts to cells of the muscle lineage,3
exogenous grafting of muscle cells directly into the myocardium,4 the generation of myocardium de novo with bone
marrow stem cells,5 and manipulation of cellular factors to
promote myocyte cell-cycle reentry and cell proliferation.6 – 8
The latter approach has received considerable attention over
the last several years given the identification of key cell-cycle
regulatory proteins and the ability to reactivate DNA synthesis in vivo and in vitro in postmitotic ventricular myocytes.6
Transition through the cell cycle is a tightly regulated event
involving multiple checkpoint proteins that ensure genetic
stability of the host-cell DNA. The retinoblastoma gene
product Rb is the prototypic member of a family of cellular
proteins that also includes p107 and p130. Rb was first
identified as a putative negative regulator of cell growth by
the observation that functional loss or inactivation of Rb
correlated with the development of a variety of human
malignancies.9 These include retinoblastoma and prostate
carcinomas as well as glioblastoma and leukemias. Equivalent human malignancies dependent on functional loss of
p107 or p130 have not yet been identified. The biochemical
mechanisms by which Rb family members suppress tumor
growth are ascribed to their ability to interact with the E2F
cellular factors through a binding domain that classically has
been referred to as the large pocket that retains the cell in
G0/G1 of the cell cycle.10 Several cellular genes that are
required for cell-cycle progression and DNA synthesis contain consensus E2F binding sites within their promoters,
supporting a role for E2F in transactivation of S phase–
dependent genes.11 To date, at least five E2F homologues
(E2F-1 to E2F-5) together with their dimerization partners
DP1 and DP2 have been identified, each with preferential
binding affinities for Rb family members. This may account
for the differing abilities of E2F factors to regulate cell cycle
and growth, because E2F-1, E2F-2, and E2F-3 can override
ne of the most intriguing and perplexing issues to
impact contemporary cardiology to date is the concept of myocyte regeneration after injury. In contrast
to other cells of the body that retain the ability to regenerate
throughout life, cardiac muscle cells lose this inherent property soon after birth. Consequently, growth of the neonatal
myocardium occurs by myocyte hypertrophy, as manifested
by an increase in cell volume and myofibrillar protein content
rather than by cell number. The molecular mechanisms that
dissociate cardiac growth from hyperplasia remain unknown.
This basic biological fact has significant clinical implications
for patients, because loss of viable cardiac cells after acute or
chronic myocardial injury or during conditions of increased
hemodynamic loading, such as those imposed by uncontrolled hypertension, can have detrimental effects on ventricular performance. In these cases, adaptive cellular growth of
adjacent myocytes compensates for the cellular loss and
impending contractile load to normalize wall stress. However, in some patients, for reasons unknown, these adaptive
physiological processes become inadequate and deteriorate
into dilatation and overt heart failure. Despite the substantial
progress that has been made during the last two decades of
cardiac research, heart failure still remains a prominent cause
of death in North America. In particular, chronic congestive
heart failure represents a major financial and socioeconomic
burden, because patients diagnosed with this form of heart
disease require costly medical treatments and chronic longterm care.
At the cellular level, defects in contractile proteins and,
more recently, loss of cells through an apoptotic process are
considered independent contributing factors to heart failure.
Although cell death can occur by apoptosis or necrosis,
apoptosis is an energy-requiring process that is genetically
regulated.1 Cell death by necrosis involves depletion of
cellular ATP stores, cell swelling, and disruption of the cell
membrane. In contrast, apoptotic cell death occurs in the
absence of membrane rupture and is devoid of an inflammatory response. Therefore, inappropriate or excessive cell loss
through an apoptotic program may underlie ventricular remodeling and ventricular dysfunction associated with end-
The opinions expressed in this editorial are not necessarily those of the
editors or of the American Heart Association.
From the Institute of Cardiovascular Sciences, St Boniface General
Hospital Research Centre and the Department of Physiology, Faculty of
Medicine, University of Manitoba, Winnipeg, Manitoba, Canada.
Correspondence to Dr Lorrie A. Kirshenbaum, Institute of Cardiovascular Sciences, St Boniface General Hospital, Research Centre Room
3016, 351 Taché Ave, Winnipeg, Manitoba, Canada R2H 2A6. E-mail
[email protected]
(Cir Res. 2001;88:978-980.)
© 2001 American Heart Association, Inc.
Circulation Research is available at http://www.circresaha.org
978
Kirshenbaum
Downloaded from http://circres.ahajournals.org/ by guest on April 30, 2017
G1-mediated growth arrest in quiescent cells whereas E2F-4
and E2F-5 only marginally promote S-phase entry.
Rb family members are postulated to be downstream
targets for mitogen-activated G1 cyclins, which include
cyclin D1, D2, D3, and E as well as the cyclin-dependent
protein kinases (cdk2, cdk4, and cdk6). In response to
positive growth signals, cyclin D-cdk complexes are activated
by the cdk-activating kinase MO15 and cdk7-cyclin H,
resulting in Rb phosphorylation and the release of E2F
factors.12 Other cyclins include cyclin A and B, which are
predominantly expressed in S and G2/M. Inappropriate cellcycle activation leads to apoptosis largely through the activation of another tumor suppressor and G1 checkpoint
protein, p53.13
Specific proteins of the DNA tumor viruses, including
adenovirus early region 1 (E1A), human papilloma virus E6
protein, and simian virus 40 large T antigen (T-Ag) can drive
quiescent nonproliferating cells into S phase.14 Ostensibly,
viral oncoproteins physically interact with members of the Rb
family and inactivate their function by displacing E2F factors
from the pocket domain, promoting cell-cycle entry. Hence,
the interaction of viral oncoproteins with negative regulators
of cell-cycle provides an attractive mechanistic explanation
for virally induced cellular transformation and a potential
mode for reactivating cell-cycle progression in postmitotic
ventricular myocytes. This is substantiated by transgenic
studies in which atrial and ventricular muscle growth was
augmented by expression of T-Ag. 15 Furthermore,
adenovirus-mediated delivery of E1A proteins to neonatal or
adult ventricular myocytes in vivo and in vitro resulted in
S-phase entry with the cells arresting in G2/M6. Moreover,
expression of E1A or the cellular transcription factor E2F-1,
which is presumably liberated by Rb inactivation, was sufficient to provoke cell-cycle reentry, illustrating a key role for
Rb-like molecules in cell-cycle control in cardiac tissue.6,7
However, one caveat to these experiments is that whereas
E1A and E2F-1 proteins promoted cell-cycle reentry, an
increased incidence of apoptosis was evident when either
protein was expressed alone in ventricular myocytes. Apoptosis triggered by E1A or E2F-1 could readily be abrogated
by coexpression of antiapoptotic proteins, adenovirus E1B, or
Bcl-2.6,16 Whereas T-Ag and E1A functionally share the
ability to bind to Rb, p107, and p130 equivalently, differences
in cellular binding partners, such as p300 in the case of E1A
and p53 in the case of T-Ag, clearly may account for the
biochemical and functional differences in these viral proteins,
such as the observed lack of apoptosis with T-Ag (Figure).
Difference in cellular binding targets may provide important
clues into the molecular mechanisms that govern cell-cycle
progression and its relation to apoptosis.
In this issue of Circulation Research, Pasumarthi et al17
provide evidence for the existence of a novel proapoptotic
protein p193, a member of the BH3-only family of proapoptotic Bcl-2 that binds to T-Ag, that may account for
phenotypic differences observed with E1A and T-Ag binding
targets. Structure-function analysis of p193 indicates that the
full-length protein exhibits proapoptotic properties when
overexpressed in cells. Importantly, a large C-terminal truncation mutation of the p193 protein was found to promote cell
Cell Cycle and Apoptosis of Ventricular Myocytes
979
Dual inactivation of cell-cycle proteins and proapoptotic factors
is a requirement for cell proliferation without apoptosis. E1A and
T-Ag preferentially interact with cellular proteins that regulate
the cell cycle and apoptosis. T-Ag binds to and inactivates the
cell-cycle proteins Rb and related members p107 and p130 in
addition to the proapoptotic proteins p53 and p193. This permits T-Ag to initiate cell proliferation without provoking apoptosis. E1A preferentially binds to Rb and related family members
p107 and p130 but not to the proapoptotic proteins p53 and
p193, thereby provoking G1 exit and apoptosis.
growth and protection from proapoptotic signals mediated
through both p53-dependent and p53-independent pathways.
The fact that T-Ag can evoke a proliferative response in
transgenic mice without provoking apoptosis illustrates the
existence of multiple points of regulation that must be
overcome by T-Ag to provoke cell-cycle reentry without
triggering apoptosis. This implies the existence of cellular
proteins that coordinately link cell-cycle progression to apoptosis. In these elegant studies, Pasumarthi et al17 test the
possibility that the function of proapoptotic proteins p53 and
p193 must be inhibited to successfully promote cell proliferation in the absence of cell death. Using embryonic stem (ES)
cell– derived cardiac myocytes and a genetic approach of
dominant-negative mutants of p53 and p193, the authors
demonstrate that expression of a mutant p53 alone is not
sufficient to promote cell growth or overcome the apoptosis
triggered by adenovirus E1A. Moreover, expression of the
mutant p193 alone also failed to suppress E1A-mediated
apoptosis. Interestingly, codelivery of mutant p53 and mutant
p193 in cardiac ES cells was sufficient to support E1Ainduced cell proliferation in the absence of apoptosis. These
experiments illustrate that for viral protein–induced cell-cycle
progression and proliferation to be successful, the proapoptotic pathways mediated through p53 and p193 must be
compromised. The differential interaction of E1A and T-Ag
with alternative cellular targets explains the apparent biochemical differences in the two proteins. In the case of T-Ag,
the E1A protein has the ability to promote S-phase entry but
provokes apoptosis because of an inability to override the
p193 checkpoint. Whether ablation of these pathways alone
will be sufficient to globally promote myocyte proliferation
by mitogenic agents or is solely restricted to viral oncoproteins like E1A and T-Ag is undetermined and awaits additional investigation. Moreover, the role played by other
death-promoting pathways, such as caspases and mitochondria, is equivalently unknown.
Although these elegant and important studies point toward
potential cellular targets that could be genetically manipulated to overcome G1 exit, the experiments must be inter-
980
Circulation Research
May 25, 2001
preted with caution. These studies were conducted in cardiac
embryonic stem cells, which may have a greater propensity
for cell-cycle exit and proliferation than neonatal or differentiated adult ventricular myocytes. Furthermore, whether
equivalent biochemical pathways are functionally operational
in normal or diseased adult myocardium will have to be
addressed. Although there are some reports that adult ventricular myocytes may still retain the ability to reenter the cell
cycle and proliferate under certain conditions,18 –20 the proof
of concept of reactivating cardiac myocyte proliferation by
manipulating putative cellular regulators or conversion of
pluripotent stem cells to cardiac myocytes5 holds promise for
the design of novel therapeutic interventions to enhance
ventricular performance during disease conditions.
References
Downloaded from http://circres.ahajournals.org/ by guest on April 30, 2017
1. Wyllie AH. Death from inside out: an overview. Philos Trans R Soc Lond
B Biol Sci. 1994;345:237–241.
2. Yuan J. Molecular control of life and death. Curr Opin Cell Biol. 1995;
7:211–214.
3. Koh GY, Soonpaa MH, Klug MG, Pride HP, Cooper BJ, Zipes DP, Field
LJ. Stable fetal cardiomyocyte grafts in the hearts of dystrophic mice and
dogs. J Clin Invest. 1995;96:2034 –2042.
4. Murry CE, Wiseman RW, Schwartz SM, Hauschka SD. Skeletal
myoblast transplantation for repair of myocardial necrosis. J Clin Invest.
1996;98:2512–2523.
5. Orlic D, Kajstura J, Chimenti S, Jakoniuk I, Anderson SM, Li B, Pickel
J, McKay R, Nadal-Ginard B, Bodine DM, Leri A, Anversa P. Bone
marrow cells regenerate infarcted myocardium. Nature. 2001;410:
701–705.
6. Kirshenbaum LA, Schneider MD. Adenovirus E1A represses cardiac
gene transcription and reactivates DNA synthesis in ventricular myocytes,
via alternative pocket protein- and p300-binding domains. J Biol Chem.
1995;270:7791–7794.
7. Kirshenbaum LA, Abdellatif M, Chakraborty S, Schneider MD. Human
E2F-1 reactivates cell-cycle progression in ventricular myocytes and
represses cardiac gene transcription. Dev Biol. 1996;179:402– 411.
8. Hasegawa K, Meyers MB, Kitsis RN. Transcriptional coactivator p300
stimulates cell type-specific gene expression in cardiac myocytes. J Biol
Chem. 1997;272:20049 –20054.
9. Jacks T, Fazeli A, Schmitt EM, Bronson RT, Goodell MA, Weinberg RA.
Effects of an Rb mutation in the mouse. Nature. 1992;359:295–300.
10. Nevins JR. E2F: a link between the Rb tumor suppressor protein and viral
oncoproteins. Science. 1992;258:424 – 429.
11. Schwarz JK, Devoto SH, Smith EJ, Chellappan SP, Jakoi L, Nevins JR.
Interactions of the p107 and Rb proteins with E2F during the cell proliferation response. EMBO J. 1993;12:1013–1020.
12. Sherr CJ. Mammalian G1 cyclins and cell cycle progression. Proc Assoc
Am Physicians. 1995;107:181–186.
13. Lane DP, Lu X, Hupp T, Hall PA. The role of the p53 protein in the
apoptotic response. Philos Trans R Soc Lond B Biol Sci. 1994;345:
277–280.
14. Nevins JR. Cell cycle targets of the DNA tumor viruses. Curr Opin Genet
Dev. 1994;4:130 –134.
15. Steinhelper ME, Lanson NA Jr, Dresdner KP, Delcarpio JB, Wit AL,
Claycomb WC, Field LJ. Proliferation in vivo and in culture of differentiated adult atrial cardiomyocytes from transgenic mice. Am J Physiol.
1990;259:H1826 –H1834.
16. Kirshenbaum LA, de Moissac D. The bcl-2 gene product prevents programmed cell death of ventricular myocytes. Circulation. 1997;96:
1580 –1585.
17. Pasumarthi KBS, Tsai S-C, Field LJ. Coexpression of mutant p53 and
p193 renders embryonic stem cell– derived cardiomyocytes responsive to
the growth-promoting activities of adenoviral E1A. Circ Res. 2001;88:
1004 –1011.
18. Leri A, Malhotra A, Liew CC, Kajstura J, Anversa P. Telomerase activity
in rat cardiac myocytes is age and gender dependent. J Mol Cell Cardiol.
2000;32:385–390.
19. Kajstura J, Pertoldi B, Leri A, Beltrami CA, Deptala A, Darzynkiewicz Z,
Anversa P. Telomere shortening is an in vivo marker of myocyte replication and aging. Am J Pathol. 2000;156:813– 819.
20. Setoguchi M, Leri A, Wang S, Liu Y, De Luca A, Giordano A, Hintze
TH, Kajstura J, Anversa P. Activation of cyclins and cyclin-dependent
kinases, DNA synthesis, and myocyte mitotic division in pacing-induced
heart failure in dogs. Lab Invest. 1999;79:1545–1558.
KEY WORDS: cardiac myocyte apoptosis
䡲
myocardium
䡲
cell cycle
Death-Defying Pathways Linking Cell Cycle and Apoptosis
Lorrie A. Kirshenbaum
Downloaded from http://circres.ahajournals.org/ by guest on April 30, 2017
Circ Res. 2001;88:978-980
doi: 10.1161/hh1001.091865
Circulation Research is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 2001 American Heart Association, Inc. All rights reserved.
Print ISSN: 0009-7330. Online ISSN: 1524-4571
The online version of this article, along with updated information and services, is located on the
World Wide Web at:
http://circres.ahajournals.org/content/88/10/978
Permissions: Requests for permissions to reproduce figures, tables, or portions of articles originally published
in Circulation Research can be obtained via RightsLink, a service of the Copyright Clearance Center, not the
Editorial Office. Once the online version of the published article for which permission is being requested is
located, click Request Permissions in the middle column of the Web page under Services. Further information
about this process is available in the Permissions and Rights Question and Answer document.
Reprints: Information about reprints can be found online at:
http://www.lww.com/reprints
Subscriptions: Information about subscribing to Circulation Research is online at:
http://circres.ahajournals.org//subscriptions/