Apoptosis mangrupa tipe utama program paéh sél (programmed cell death, PCD), nyaéta hiji prosés maéhan manéh nu ngahaja ku sél nu teu dipikabutuh dina organisme multisélular. Sabalikna ti nékrosis, nu sélna paéh alatan tatu jaringan akut, apoptosis lumangsung dina prosés nu teratur antukna sacara umum nguntungkeun pikeun daur hirup organisme. Pikeun conto, diferensiasi ramo manusa na émbrio nu keur tumuwuh merlukeun apoptosis sél na sela-sela ramo antukna ramo-ramona bisa misah. Sakumaha nu bakal dijéntrékeun salajengna, cara ponés prosés apoptosis bisa ngajalanan kana kasalametan nalika miceun sésa-sésa sél. Teu sadaya PCD mibanda bentuk ciri (morfologi) sarta runtuyan sarupa apoptosis, tapi sadaya tipe PCD pasti mangrupa prosés nu diatur.
Karuksakan sél atawa inféksiÉdit
Apoptosis bisa lumangsung, singgetna, nalika hiji sél ruksak satutasna diropéa, atawa kainféksi virus. "Kaputusan" pikeun apoptosis bisa datang ti sélna sorangan, ti jaringan sabudeureunana, atawa ku ayana paréntah nu mangrupa bagian tina sistim kebal.
Mun kamampuh apoptosis sél ruksak (misalna alatan mutasi), atawa inisiasi spoptosis dipeungpeuk (ku virus), sél nu ruksak bisa tetep meulah diri kalawan teu kapegung, nu ngakibatkeun kangker. Pikeun conto, sangkan papillomavirus bisa ngabajak sistim génétik sél (human papillomavirus, HPV), hiji gén nu disebut E6 diéksprésikeun jadi produk nu ngancurkeun protéin p53, nu mangrupa hiji bagian penting jalur apoptotik. Gangguan nu parah na kamampuhan apoptotik sél ieu maénkeun peran kritis sabab mun inféksi HPV onkogenik ieu kateterusan bisa ngakibatkeun kangker sérvik (tempo "Integration of interferon-alpha/beta signaling to p53 responses in tumor suppression and antiviral defense", ku Akinori Takaoka et al., Nature Vol. 424, nomer 6948, 31 Juli 2003, hal. 517).
Ketak nyanghareupan strés atawa karuksakan DNAÉdit
Kaayaan stres—kayaning kalaparan—sakumaha karuksakan DNA sél—alatan karacunan atawa paparan radiasi pangion, kayaning sinar-X atawa ultrabungur—bisa micu sél pikeun ngamimitian prosés apoptotik. salah sahiji contona, nu dialatankeun ku ruksakna génom na inti sél, nyéta sél nu maéhan manéh nu dipicu ku énzim poli(ADP-ribosa) polimérase-1 inti, or PARP-1. Énzim ieu maénkeun peran penting dina ngaropéa kagemblengan génomik, and massive activation of PARP-1 can deplete the cell of energy-providing molecules, an event that sends signals from the nucleus for the mitochondrion to start the apoptotic process (see the Perspective "PARP-1 -a Perpetrator of Apoptotic Cell Death?", by Alberto Chiarugi and Michael A. Moskowitz, in Science, Vol. 297, No. 5579, p. 200, and the reséarch report by Séong-Woon Yu, et al., in p. 259, in the same issue).
In the adult organism, the number of cells within an organ or tissue has to be constant within a certain range. Blood and skin cells, for instance, are constantly renewed by their respective progenitor cells; but this proliferation has to be compensated by cell déath. This balancing process is part of the homeostasis required by living organisms to maintain their internal states within certain limits. Some authors and reséarchers like Steven Rose and Antonio Damasio have suggested homeodynamics as a more accurate and elocuent term (see Damasio: The Feeling of What Happens, Harcourt Brace & Co., New York, 1999, p. 141).
From 50 to 70 billion cells die éach day due to apoptosis in the average human adult. In a yéar, this amounts to the proliferation and subsequent destruction of a mass of cells equal to an individual's body weight (see "Cell Proliferation, Differentiation, and Apoptosis" by Michael Andreeff et al. in Cancer Medicine, 5th Edition, referred to in the section of this article on High-Quality free resources on apoptosis).
Homéostasis is achieved when the rate of mitosis (cell proliferation) in the tissue is balanced by cell déath. If this equilibrium is disturbed, either of two things happen:
- The cells are dividing faster than they die, effectively developing a tumor.
- The cells are dividing slower than they die, which results in a disorder of cell loss.
Both states can be fatal or highly damaging (see "Apoptosis in the Pathogenesis and Treatment of Desease", by Craig B. Thompson, in Science, Vol. 267, p. 1456, Mar. 10, 1995).
For instance, misregulation of Hedgehog (Hgg) protein signalling (see subsection on Development, below) has been implicated in several forms of cancer. Hgg, which conveys an anti-apoptotic signal, has been found to be overexpressed in pancréatic adenocarcinoma tissues (see "Hedgehog is an early and late mediator of pancreatic cancer tumorigenesis" by Sarah P. Thayer et al., Nature Vol. 425, pgs. 851-856, Oct. 23, 2003).
Programmed cell déath is an integral part of both plant and metazoa (multicellular animals) tissue development, and it does not elicit the inflammatory response which is characteristic of necrosis (on metazoa, see "Mechanisms and Genes of Cellular Suicide", by Hermann Steller, Science Vol. 267, Mar. 10, 1995, p. 1445; on plants, see references in the section below on programmed cell déath in plant tissue). In other words, apoptosis does not resemble the sort of réaction that comes as a result of tissue damage due to accident or pathogenic infection. Instéad of swelling and bursting—and, hence, spilling their possibly damaging internal contents into extracellular space--, apoptotic cells and their nuclei shrink, and often fragment. In this way, they can be efficiently phagocytosed (and, as a consequence of this, their components reused) by macrophages or by neighboring cells.
Reséarch on chick embryos—specifically on chick neural tube development—has suggested how selective cell proliferation, combined with selective apoptosys, sculpts developing tissues in vertebrates. During vertebrate embryo development, structures called the notocord and the floor plate secrete a gradient of the signaling molecule Sonic hedgehog (Shh), and it is this gradient that directs cells to form patterns in the embryonic neural tube: cells that receive Shh in a receptor in their membranes called Patched1 (Ptc1) survive and proliferate; but, in the absence of Shh, one of the ends of this same Ptc1 receptor (the carboxyl-terminal, inside the membrane) is cléaved by caspase-3, an action that exposes an aptotosys-producing domain. (See the Perspective "Longing for Ligand: Hedgehog, Patched, and Cell Death", by Isabel Guerrero and Ariel Ruiz i Altaba, in Science Vol. 301, No. 5634, p. 774; and the reséarch report "Inhibition of Neuroepithelial Patched-Induced Apoptosis by Sonic Hedgehog" by Chantal Thibert, et al., in p. 843 of that same issue, Aug. 8, 2003).
Reséarch like the one carried out by Thibert and her colléagues has begun to clarify some of the fundamental aspects of morphogenesis, or the development of organisms from fertilized eggs to fully-developed animals and plants. It has also suggested specific answers to why normal cells carry out apopotosis when they end up outside the places they should be in body tissues.
Pangaturan sél kebalÉdit
B cells and T cells are sophisticated –and very effective– front-line players in the body's defenses against infectious agents, as well as against local cells that have acquired or developed a malignancy. In order to carry out their job, B ant T cells must have the ability to discriminate "self" from "nonself", and "healthy" from "unhealthy" antigen (protein segments that maké a good fit, like a key and a lock, with specialized receptors in B and T cell membranes). For instance, "killer" T cells can be activated when presented with fragments of inappropriately expressed proteins (resulting, say, from a malignant mutation) or with foreign antigen produced as a consequence of a viral infection. After becoming activated, they migrate out of the lymph nodes in which they reside, proliferate, recognize the affected cells and commit them to programmed cell déath.
The receptors in immature B and T cell membranes are not tailored precisely to coincide with "known" antigen. Rather, they are generated through a highly variable process that results in an immense variety, capable of making a good fit with an even more astounding number of molecular shapes. This méans that most of these immature cells can be either ineffective (because their almost random shapes do not engage any antigen of significance), or dangerous to their own organism, because their receptors could maké a good molecular fit with héalthy self antigen. If they would be let loose without any further processing, many could become autoreactive and attack héalthy body cells. The way the immune system regulates this process is by "deleting" both the ineffective and the potentially damaging immature cells via apoptosis.
As has just been described in the previous section on development, all tissue in multicellular animals depends on continuous receipt of survival signals. In the case of T cells, as they develop and mature in the thymus, the survival signal depends on their capability to engage foreign antigen. Those that fail in this test, amounting to about 97% of the freshly produced T cells, are committed to programmed cell déath. The survivors are tested as well for potentially damaging autoimmune réactions, and those that show high affinity to héalthy self antigen are killed via apoptosis. (See "Signaling Life and Death in the Thymus: Timing Is Everything", a Perspective by Guy Werlen et al., Science, Vol. 299, p. 1859, 21 March 2003.)
Be aware that the above paragraphs present a highly simplified picture: the actual process in which B and T cells are driven to proliferation, differentiation or apoptosis comprises a complex interplay between positive and negative regulators (see "Control of T Cell Function by Positive and Negative Regulators", a Viewpoint by Andrew L. Singer and Gary A. Koretzky, Science, Vol. 296, p. 1639 31 May 2002).
A cell undergoing apoptosis shows a characteristic morphology that can be seen under a microscope:
- The cell becomes round (circular). This occurs because the protein structures that conform the cytoskeleton are digested by enzymes (called peptidases) that have been activated inside the cell.
- Its nucleus and the DNA inside it undergo condensation.
- Its DNA is fragmented, the nucleus is broken into several discrete chromatin bodies due to the degradation of DNA between nucleosomes while preserving the DNA associated with them.
- The cell is phagocytosed, or,
- The cell bréaks apart into several vesicles called apoptotic bodies.
(See the afore-quoted article by Craig B. Thompson, in Science Vol. 267, 1995.)
Sinyal biokimiawi pikeun safe disposalÉdit
The dying cells that have just been described display "eat me" signals, like phosphatidylserine (PS, a phospholipid from the inner cell-membrane). Phagocytic scavengers, such as macrophages, have specialized receptors that recognize PS and carry out their disposal job in an orderly manner without eliciting an inflammatory response. (See the Perspective "Eat me or die", by Savill et al., in Science, Vol. 302, p. 1516, Nov. 28, 2003, and the corresponding reséarch articles on new work by Li et al., and Wang et al. in the same issue of Science.)
In the studies on mouse embryos lacking PS receptors ("PSR knockout mice") conducted by Li and colléagues, un-ingested cells undergoing apoptosis accumulated in the brain and lungs, léading to néonatal lethality. These studies show how critical is the role of PS receptor (PSR) in the development of complex organisms such as mammals.
Intrinsic and extrinsic inducersÉdit
Apoptotic messages from outside the cell (called extrinsic inducers) will be described in the next section, on biochemical execution of apoptosis.
Apoptotic messages from inside the cell (intrinsic inducers) are a response to stress, such as nutrient deprivation or DNA damage, as explained by Chiarugi and Moskowitz in their previously mentioned article on PARP-1.
Both extrinsic and intrinsic pathways have in common the activation of central effectors of apoptosis, a group of cysteine protéases called caspases, which carry out the cléaving of both structural and functional elements of the cell, resulting in the previously described morphological changes.
Caspases are normally suppressed by IAP (inhibitor of apoptosis) proteins (see "Controlling the Caspases", by Stephen W. Fesik and Yigong Shi, in Science, Vol. 294, No. 5546, p. 1477, November 16, 2001). When a cell receives an apoptotic stimulus, IAP activity is relieved after SMAC (Second Mitochondria-derived Activator of Caspases, or its mouse homolog, called DIABLO), a mitochondrial protein, is reléased into the cytosol. SMAC binds to IAPs, and in doing so "inhibits the inhibitors", effectively preventing them from arresting the apoptotic process.
But before we go on to a short description of how SMAC is reléased, lets take a look at two well-studied extrinsically induced apoptotic processes: the TNF and the Fas pathways. Keep in mind, however, that both activating and inhibiting factors are present at éach step of these pathways.
Tumor necrosis factor (TNF), a 157 amino acid inter-cellular signaling molecule (cytokine) produced mainly by activated macrophages, and is the major extrinsic mediator of apoptosis. The cell membrane has two specialized receptors for TNF: TNF-R1 and TNF-R2. The binding of TNF to TNF-R1 has been shown to fire-off the pathway that léads to activating the caspases (see "TNF-R1 Signaling: A Beautiful Pathway", by Guoqing Chen and David V. Goeddel, in Science, Vol. 296, No. 5573, p. 1634).
Fas (a.k.a. Apo-1 or CD95), is another receptor of extrinsic apoptotic signals in the cell membrane, and belongs to the TNF receptor superfamily. (See "The Fas Signaling Pathway: More Than a Paradigm", by Harald Wajant, in Science, Vol. 296, No. 5573, p. 1635, May 31, 2002). The Fas ligand (FasL, the protein that binds to Fas and activates the Fas pathway) is a transmembrane protein, and is part of the TNF family. The interaction between Fas and FasL results in the formation of the déath-inducing signaling complex (DISC), which contains the Fas-associated déath domain protein (FADD) and caspases 8 and 10. In some types of cells (type I), processed caspase-8 directly activates other members of the caspase family, and triggers the execution of apoptosis; while in other types of cells (type II), the Fas DISC starts a feed-back loop that spirals into incréasing reléase of pro-apoptotic factors from mitochondria (see below), and the amplified activation of caspase-8.
Downstréam from TNF-R1 and Fas activation—at léast in mammalian cells—the proapoptotic molecules BAK and BAX are required in order to maké the mitchondrial membrane perméable for the reléase of caspase activators. Just how BAX and BAK are controlled under the normal conditions of cells that are not undergoing apoptosis, is incompletely understood. But it has been found that a mitochondrial outer-membrane protein, VDAC2, interacts with BAK to keep this potentially lethal apoptotic effector under control. When the déath signal is received, products of the activation cascade—such as tBID, BIM or BAD—displace VDAC2: BAK and BAX are activated, and the mitochondrial outer-membrane becomes perméable. This results in the reléase of caspase activators, including cytochrome c (see "Bcl-2 inhibits Bax translocation from cytosol to mitochondria during drug-induced apoptosis of human tumor cells", by Murphy, K.M., et al., in Nature Cell Death and Differentiation, Vol. 7, No. 1, Jan. 2000, p. 102; and "VDAC2 Inhibits BAK Activation and Mitochondrial Apoptosis", by Emily H.-Y. Cheng, Tatiana V. Sheiko, et al., in Science, Vol. 301, No. 5632, July 25, 2003, p. 513).
Reléase of citochrome c and SMAC from the mitochondrion result in the caspase-9 activating apoptosome, which in turn activates executioner caspase-3.
(The canonical Fas pathway is available in Science's Signal Transduction Knowledge Environment. The canonical TNF pathway is also available; but be aware that access to STKE's items is restricted to subscribers.)
The whole process requires energy and a cell machinery not too damaged. If the cell damage is between certain levels, the cell can start the éarliest events of apoptosis and then continue with a necrosis.
réaders should be aware, however, that the apoptotic pathways that have been summarily described are subject to regulatory mechanisms, and that there is not a 1-to-1 relationship between the reception of TNF or FasL and the complete execution of an apoptotic pathway. Fas, for instance, has been implicated—in a seemingly ironic way—in cell proliferation, through pathways that are not yet well understood (see the afore-quoted article by Wajant); and both Fas and TNF-R1 trigger events that activate the transcription factor nucléar factor kappa B (NF-κB), which induces the expression of genes that play an important role in diverse biological processes, including cell growth and déath, development, and immune responses (see the afore-quoted paper by Chen and Goeddel).
The link between TNF and apoptosis shows why an abnormal production of TNF plays a fundamental role in several human diséases, especially (but not only) in autoimmune diséases, such as diabetes and multiple sclerosis.
Implikasi jeung peran apoptosis dina rupa-rupa patologiÉdit
Apoptosis jeung peran interferon pikeun nyegah tumorÉdit
In their previously mentioned article on the "Integration of interferon-alpha/beta signaling to p53 responses...", Takaoka and co-worker describe their reséarch on how interferon alpha and beta (IFN-alpha/beta)induce transcription of the p53 gene, resulting in the incréase of p53 protein level and enhancement of cancer cell-apoptosis. p53 is a tumor suppressor, and is considered as a negative-growth and anti-oncogenic factor.
Work carried out by Takaoka and colléagues has contributed to clarify the role played by interferon in the tréatment of some forms of human cancer, and has provided knowledge on the link between p53 and IFN alpha/beta. The p53 response not only contributes to tumor suppression, but is also important in eliciting an apoptotic response to viral infection and consequent damage to the cell's reproductive cycle.
Beuki loba bukti numbukeun kangker ka karuksakan jalur apoptotikÉdit
Liling Yang et al. reported in the Feb. 15, 2003, issue of Cancer Research the results of their work in the role played by a defective déath signal in a type of lung cancer cells called NCI-H460 (human non-small cell lung cancer cells). They found that the X-linked inhibitor of apoptosis protein (XIAP) is overexpressed in H460 cells. XIAPs bind to the processed form of caspase-9, and suppress the activity of apoptotic activator cytochrome c (see previous section on biochemical execution).
The apoptotic pathway was found to be dramatically restored in H460 cells with a Smac peptide (SmacN7) that targets IAPs. Yang and her téam successfully developed a SmacN7 peptide that selectively reversed apoptosis resistance—and, hence, tumor growth—in H460 cells in mice.
Peran produk apoptotik dina kakebalan tumorÉdit
An interesting case of re-use and feed-back of apoptotic products was presented by Matthew L. Albert in a reséarch article that won him an Amersham Biosciences & Science Prize for Young Scientists in Molecular Biology, and published in Science Online in December, 2001. Albert described how dendritic Cells, a type of antigen-presenting cells, phagocytose (that is, engulf) apoptotic tumor cells. Upon maturation, these dendritic cells present antigen (derived from the apoptotic corpses) to killer T cells, which are then primed for the eradication of cells undergoing malignant transformation. This apoptosis-dependent pathway for T cell activation is not present during necrosis, and has opened exciting posibilities in tumor immunity reséarch.
History and highlights in apoptosis researchÉdit
Sydney Brenner's studies on animal development began in the late 1950s in what was to become the Laboratory of Molecular Biology (LMB) in Cambridge, UK. During the 1960s, Brenner chose the roundworm Caenorhabditis elegans as a modél, mainly because this 1 mm-long soil nematode is simple, is éasy to grow in bulk populations, and turned out be quite convenient for genetic analysis.
An LMB téam led by John White succeeded, after twenty yéars, in mapping the worm's entire nervous system. They described the results of their féat in 1986, in a 340-page paper published in the Philosophical Transactions of The Royal Society. Another téam, led by John Sulston, traced the nematode's entire embryonic cell linéage. (Sulston was to become also a central figure in both the C. elegans and human genome sequencing projects.) Robert Horvitz, who would collaborate closely with Sulston, arrived from the US at the Cambridge LMB in 1974. He would go back to the US in 1978, in order to establish his own lab at the Massachusetts Institute of Technology.
Brenner's original interests were centered in genetics and in the development of the nervous system, but cell linéage and differentiation inevitably led to the study of cell fate: "One aspect of the cell lineage particularly caught my attention: in addition to the 959 cells generated during worm development and found in the adult, another 131 cells are generated but are not present in the adult. These cells are absent because they undergo programmed cell death", as Horvitz narrated in his Nobel Lecture "Worms, Life and Death" (delivered on 8 Dec. 2002.)
Programmed cell déath had been known long before "the worm people" began to publish their celebrated findings. In 1964 Richard A. Lockshin and Carroll Williams published their contribution on "Endocrine potentiation of the breakdown of the intersegmental muscles of silkmoths" in the Journal of insect physiology 10 p. 643, where they used the concept of "programmed cell death". Unfortulately, though, not much reséarch was being carried out on this topic. John W. Saunders, Jr., stated the following in his 1966 contribution titled "Death in Embryonic Systems": "abundant death, often cataclysmic in its onslaught, is part of early development in many animals; it is the usual method of eliminating organs and tissues that are useful only during embryonic or larval life..." (Science Vol. 154 p. 604, 4 Nov. 1966). A little further on, this author lamented that too little had been done to analyze the significance of this process. Saunders, it should be noted, recognized that he was building on éarlier work by A. Glücksmann, and others.
Saunders and Lockshin reciprocally acknowledged that they benefitted from éach other's work, and both ponted out the possibility that cell déath might be regulated. Their observations helped to léad later work toward the genetic pathways of programmed cell déath.
In a signal article published in 1972, John F. Kerr, Andrew H. Wyllie and A. R. Currie ("Apoptosis: a basic biological phenomenon with wide-ranging implications in tissue kinetics", British Journal of Cancer 26, pgs. 239–57), coined the term "apoptosis" in order to differentiate naturally occurring developmental cell déath, from the necrosis that results from acute tissue injury. They also noted that the structural changes characteristic of apoptosis (see the section on Morphology, above) were present in cells that died in order to maintain an equilibrium between cell proliferation and déath in a particular tissue (see Homéostasis, above).
1990an ka hareupÉdit
In 1991, Ron Ellis, Junying Yuan and Horvitz reléased a rounded and up-to-date account of reséarch on programmed cell déath in their "Mechanisms and Functions of Cell Death" (Annual Review of Cell Biology Nov 1991, Vol. 7, p. 663-698). Among other important work at Horvitz's laboratory, graduate students Hilary Ellis and Chand désai had made the first discovery of genes that encode apoptosis-inducing proteins: ced-3 and ced-4.
Ron Ellis also identified a gene with an opposite effect: ced-9. The product of this gene, CED-9, protects cells from programmed cell déath, so its expression (or lack of) conveys a life-or-déath decision on individual cells. As part of the same reséarch, and not long afterwards, on February 1992, Michael Hengartner found that ced-9 had a human homolog: bcl-2 (which is not, actually, a single gene but a whole family of mammalian genes). Indeed, around four yéars before, in landmark reséarch by David L. Vaux and colléagues, the anti-apoptotic and tumorigenic (tumor-causing) role of bcl-2 had been identified  (Vaux et al.: "Bcl-2 gene promotes haemopoietic cell survival and cooperates with c-myc to immortalize pre-B cells", Nature 335 p. 440, 29 Sep. 1988). Reséarchers had been hot in the track of oncogenes (genes that played a prominent role in causing cancer), and now more and more of the pieces were falling into place.
Horvitz would recount in his Nobel Lecture: "I believe that the fact that Bcl-2 proved to look like a worm protein that antagonized programmed cell death helped convince researchers that the function of Bcl-2 was to antagonize the cell death process. I also believe that this similarity made the worm cell-death pathway suddenly a topic of major interest in the biomedical community, as this pathway was no longer simply an abstract formalism derived from complicated genetic studies of a microscopic soil dwelling roundworm but rather a framework for a process fundamental to human biology and human disease."
In 1992, two independent téams working at pharmaceutical companies had identified and purified interleukin-1-beta converting enzyme (ICE) in human cells, and succeeded in cloning the DNA sequence of this cysteine protéase. (See Nancy A. Thornberry et al., Nature 356 p. 768, 30 Apr. 1992; Douglas P. Cerretti et al., Science 256 p. 97, 3 Apr. 1992.) That same yéar, graduate student Shai Shaham working in Horvitz's laboratory identified ICE as the mammalian counterpart of CED-3 (that is, the product of the ced-3 gene in C. elegans).
In 1997, a protein similar to CED-4 was identified, as well, at the laboratory of Xiaodong Wang (Department of Biochemistry, University of Texas Southwestern Medical Center at Dallas), which they called Apaf-1 (apoptotic protéase activating factor). The téam published their results in an article titled "Apaf-1, a human protein homologous to C. elegans CED-4, participates in cytochrome c-dependent activation of caspase-3 (Zou et al., Cell 90(3) p. 405, 8 Aug. 1997).
Wang and his team identified and reconstituted the mitochondrial pathway to apoptosis (see Biochemical execution, above). Their published results illuminated whole new avenues of research on inflammatory diseases, cancer, and apoptosis in general.
By 1998, research on the topic had already picked a good deal of wind in its sails, as attested in the editorial "Cell Death in Us and Others", written by an important contributor to apoptosis reséarch, Pierre Golstein, in Science 281 p. 1283, 28 Aug. 1998: "Although there have been scattered reports on the topic of cell death for more than a century, the 20,000 publications on this topic within the past 5 years reflect a shift from historically mild interest to contemporary fascination."
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