(Circulation. 1997;95:782-786.)
© 1997 American Heart Association, Inc.
Articles |
the Divisions of Molecular Medicine and Cardiology, Department of Internal Medicine, and Cardiovascular Research Center, Institute of Molecular Medicine for the Prevention of Human Diseases, University of TexasHouston Health Science Center.
Correspondence to Edward T.H. Yeh, MD, Division of Molecular Medicine, University of TexasHouston Health Science Center, 6431 Fannin, Suite 4200, Houston, TX 77030.
Key Words: Editorials apoptosis free radicals molecular biology
| Introduction |
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Ecclesiastes 3:2
In recent years, the concept of apoptosis1 has swept through the fields of biology and medicine, creating a cultlike following in many disciplines, including cardiovascular research. This editorial is written explicitly to update the readers of Circulation on this fascinating and rapidly expanding field. The focus is on the concept of apoptosis relevant to cardiovascular diseases. Not all of the research areas in apoptosis can be covered in great depth in this editorial. Interested readers should consult several other excellent reviews for details.2 3 4 5 6 7 8 9 10 11
| Apoptosis: Definition and Detection |
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Although the term apoptosis was introduced only 30 years ago, typical apoptotic morphology has been described by others as far back as the late 1880s.5 Early scholars recognized the need for some mechanism to counterbalance cellular proliferation, especially during the development of organs. For example, during development of the limb, cells in the interdigital zone undergo massive apoptotic death to allow for formation of the shape of digits. These cells are obviously programmed to die, and their deaths are considered a normal physiological process. If the interdigital cells do not die, webbed toes or fingers will be the unsightly result. Apoptosis, however, is not limited to cell death during embryonic development. In recent years, apoptosis has been implicated in cell deaths caused by ionized radiation, steroid treatment, chemotherapy, and ischemia-reperfusion injury.3
The initial description of apoptosis was based on morphological features (Fig 1A
). Several useful biochemical and immunohistochemical detection methods were proposed later. Andrew Wyllie12 described fragmentation of nuclear DNA into multiples of 180 bp as the result of endogenous endonuclease activation in a classic paper published in 1980. When fragmented DNAs were electrophoresed in an agarose gel, they separated into a characteristic DNA ladder pattern. Gavrieli et al13 described another widely used method in which DNA breaks in apoptotic cells were marked by dUTP-biotin transferred to the free 3'-end of cleaved DNA. Because terminal deoxynucleotidyl transferase was used to transfer dUTP-biotin by nick end-labeling, a more convenient acronym, TUNEL, was used to describe this procedure. Examples of TUNEL staining are shown in the article by Perlman et al in this issue of Circulation.14
| More Than One Way to Die: Apoptosis Versus Necrosis |
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The main differences between apoptosis and necrosis are listed in the Table
. The distinction between apoptosis and necrosis should be self-evident at a quick glance. However, there are gray areas in which the distinction may not be clear. Readers interested in the classification of cell death mechanisms and terminology should consult the excellent review by Majno and Joris.5
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| Genetics of Apoptosis: Of Worm and Man |
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| Assembly of the Death Complex: Fas Signaling as a Paradigm |
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Fas, also called APO-1, is a member of the TNF receptor (TNFR) family.7 Fas and TNFR1 share a common cytoplasmic signaling motif called the death domain. Deletion or mutation in the death domain abolishes the ability of these receptors to transduce apoptosis signal. Because the death domain does not contain any obvious kinase or phosphatase motif, its signaling function must be dependent on other associated proteins.
As shown in Fig 1B
, Fas signals by assembling a complex that contains Fas and at least two other molecules: FADD and FLICE. FADD contains a cell death domain (D) in the C-terminus and uses it to interact with the death domain of Fas.18 The N-terminus of FADD contains another novel motif, called the death-effector domain (E), which is used for binding to the third protein, FLICE. FLICE and FADD interact via their respective death-effector domains. Thus, FADD is simply an adapter molecule that serves to recruit FLICE to the complex. Most interestingly, FLICE contains an ICE-like domain (I) that may function as an initiator of the cysteine protease cascade.10 Both FADD and FLICE have also been shown to play a critical role in TNF-induced apoptosis. However, another adapter molecule, TRADD, is needed to recruit FADD to the death domain of TNFR1. The elegant apoptosis signaling pathway of Fas and TNFR1 was discovered only very recently, demonstrating the rapid progress of our understanding of the biochemistry of apoptosis. Currently, we do not know whether other apoptosis triggers also use similar signaling pathways.
| Executioners: The ICE Family |
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Some ICE family members can also activate themselves or each other in a manner similar to the protease proenzymes of the coagulation or complement cascades. The order in which the ICE family members activate each other either sequentially or in combination in vivo has not been clearly elucidated. Overexpression of several members of the ICE family in cell lines leads to morphological changes in the nucleus typical of apoptosis.10 Prevention of accidental activation of the ICE family is essential for survival. Two viral gene products, p35 derived from baculovirus and CrmA derived from the cowpox virus, are potent inhibitors of some of the ICE family members. Both CrmA and p35 are capable of blocking anti-Fas and TNF-induced apoptosis. Cell-permeable peptide inhibitors have also been shown to be effective blockers of in vivo ICE activities.19 Regulation of the ICE family members is currently under intense investigation because of their potential as drug targets.
| Guardian Angels: The Bcl-2 Family |
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The importance of Bcl-2 and Bcl-x in normal physiology is best illustrated by mice that are deficient in these proteins. Bcl-2deficient mice developed fulminant lymphoid apoptosis, hypopigmented hair, and polycystic kidney.25 Bcl-xdeficient mice died around embryonic day 13. Massive cell death of immature hematopoietic cells and neurons occurred in the Bcl-xdeficient mice.26 A large body of literature is available on the protective effect of Bcl-2 or Bcl-x overexpression on the susceptibility of different cell lines to various apoptosis-inducing signals. However, inconsistencies also exist. For example, Bcl-2 protects against anti-Fasinduced apoptosis in some systems but not others. The variable protective effect may be the result of other prolife or prodeath factors operative at a given cell.
The Bcl-2 family members are also subject to transcription regulation. The tumor suppressor p53 has been shown to be a direct transcriptional activator of the human bax gene.27 The induction of bax message by p53 is consistent with the role of P53 in some forms of apoptosis. In this issue of Circulation, Perlman et al14 show that the Bcl-x protein level in the medial smooth muscle cells dramatically diminished after balloon injury. The mechanism leading to lower Bcl-x levels in the medial smooth muscle cells, however, has not been elucidated.
| Coconspirator: The Mitochondria |
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| Integration of the Apoptosis Pathways |
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The simplest model is shown in Fig 1B
, in which apoptosis signaling through Fas can be accomplished through a three-step process: (1) aggregation of the Fas molecule, (2) recruitment of FADD, and (3) recruitment and activation of FLICE. Activation of FLICE would then lead to activation of the protease cascade and commit the cell to die. Fas signaling could represent a special case in which the Bcl-2 family and mitochondria are not directly involved in apoptosis induction. However, this model probably cannot be generalized to all cases of Fas-mediated apoptosis.
A more complex model is shown in Fig 1C
, in which death signals are delivered to the mitochondria, which is protected by the good members of the Bcl-2 family. However, if the death signals are overwhelming, mitochondrial damage will result and cytochrome c will be released. Cytochrome c plus additional cytosolic factors then initiate the cysteine protease cascade. This model incorporates some of the newer information about apoptosis signal and includes both the Bcl-2 family and mitochondria in the scheme.
The scheme proposed in Fig 1C
does not take into account a large number of newly discovered genes32 33 34 or second messenger systems that may also regulate apoptosis. It is not plausible to place all of these newcomers into the pathway. Several generalizations, however, can be made regarding additional inputs into the system. For example, the mitogen-activated protein (MAP) kinase pathway and the stress-activated kinase pathway appear to have opposing effects in apoptosis signaling. Activation of the MAP kinase pathways prolongs survival, whereas activation of the stress-activated kinase pathways promotes cell death.35 Ceramide promotes cell death, whereas sphingosine 1-phosphate opposes it.36 These modulating signals may feed into different parts of the pathway. In addition, transcription factors, through their effects on gene regulation and cell cycle regulation, could also affect the apoptotic pathway.11
| Life Lessons to Be Learned From Death |
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| Acknowledgments |
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| Footnotes |
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| References |
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