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Apoptosis and Necrosis

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Apoptosis and Necrosis RESEARCH UPDATE APOPTOSIS AND NECROSIS Two Types of Cell Death in Alcoholic Normal cell Liver Disease Amin A. Nanji, M.D., Small blebs Small blebs and Susanne Hiller-Sturmhöfel, Ph.D. form; the form. structure of Heavy alcohol consumption over long periods of time the nucleus can result in severe liver damage, including death of changes. liver cells (i.e., hepatocytes). Two mechanisms— The nucleus apoptosis and necrosis—can contribute to hepatocyte begins to death. In apoptosis, the affected cell actively participates break apart, The blebs fuse and the DNA in the cell death process, whereas in necrosis the cell and become breaks into death occurs in response to adverse conditions in the larger; no small pieces. organelles cell’s environment. Numerous factors that may con- The organelles are located in are also tribute to the initiation of hepatocyte apoptosis are the blebs. located in affected by alcohol consumption. These factors include the blebs. the enzyme cytochrome P450 2E1 (i.e., CYP2E1), small The cell The cell molecules (i.e., cytokines) involved in cell communi- breaks into membrane several cation, oxidative stress, and changes in iron ruptures and apoptotic releases the metabolism. Similarly, alcohol consumption can in- bodies; the cell’s content; fluence several factors believed to be involved in hepa- organelles the organelles are still tocyte necrosis, including depletion of the are not functional. energy-storing molecule adenosine-triphosphate, re- functional. duced oxygen levels (i.e., hypoxia) in the liver, oxida- Necrosis Apoptosis tive stress, and bacterial molecules called endotoxins. KEY WORDS: alcoholic liver disorder; necrosis; cytolysis; Structural changes of cells undergoing necrosis or apoptosis. hepatocyte; cytochrome P450; oxidation-reduction; iron; metabolic disorder; ATP (adenosine triphos- of the internal structure of the liver and, subsequently, in phate); hypoxia; endotoxins; biochemical mechanism; severe functional impairment and secondary failure of pathogenesis; literature review other organs, such as the kidney. These multiple compli- cations can lead to the patient’s death. By investigating the mechanisms underlying alcohol’s deleterious effects any people who drink heavily over extended on the liver, researchers hope to ultimately develop new periods of time (i.e., several years) develop diagnostic and therapeutic approaches to prevent these Mincreasingly severe liver damage, including fatty often fatal consequences of alcohol consumption. liver, alcoholic hepatitis, and alcoholic cirrhosis. Fatty Much recent research has focused on the mechanisms liver is caused by the accumulation of fat in the liver. that contribute to hepatocyte death at the cellular level. Alcoholic hepatitis is characterized by extensive inflam- Two processes play a role in hepatocyte destruction— mation of the liver and the destruction of liver cells (i.e., apoptosis and necrosis. This article briefly reviews the hepatocytes). Moreover, scar tissue begins to form, re- differences between these two processes and speculates placing healthy liver tissue. In alcoholic cirrhosis, scar- on some of their underlying mechanisms. The article also ring and cell death progress further, resulting in distortion discusses how heavy alcohol consumption may be associ- ated with the mechanisms that promote these processes. AMIN A. NANJI, M.D., is co-director of the Center for Clinical and Laboratory Investigation, Department of Pathology, Beth Israel Deaconess Medical Center, and DIFFERENCES BETWEEN APOPTOSIS AND NECROSIS associate professor of pathology at Harvard Medical School, Boston, Massachusetts. Although the ultimate results of apoptosis and necrosis are the same (i.e., death of the affected cells), the two pro- SUSANNE HILLER-STURMHÖFEL, PH.D., is a science editor of cesses differ significantly. In apoptosis, the affected cells Alcohol Health & Research World. actively participate by activating a cascade of biochemical VOL. 21, NO. 4, 1997 325 RESEARCH UPDATE reactions that result in cell death. Accordingly, apoptosis Factors That May Contribute to Hepatocyte Apoptosis has been called cell suicide (e.g., Rosser and Gores and Necrosis in Alcoholic Liver Disease 1995).1 In necrosis, however, cell death occurs because of adverse conditions or changes in the cell’s environment. Factor Research Findings Regarding Thus, necrosis can be viewed as the consequence of a Alcohol’s Effects on These Factors “biological accident” that leads to the death of an “inno- Apoptosis cent victim” (Rosser and Gores 1995). Cytochrome • Alcohol metabolism by CYP2E1 Characteristic differences also exist in both the struc- P450 2E1 results in the generation of oxygen ture and the metabolic processes of cells that undergo (CYP2E1) radicals. apoptosis or necrosis (see figure, p. 325) (Rosser and Gores Cytokines • Patients with alcoholic hepatitis 1995). When a cell undergoes apoptosis, the entire cell, and rats with alcohol-induced liver including the nucleus, separates into numerous fragments injury show elevated levels of tumor (i.e., apoptotic bodies). Simultaneously, the genetic material necrosis factor alpha (TNF-α). (i.e., DNA) of apoptotic cells breaks into a characteristic • Chronic alcohol consumption increases the levels of TNF-α pattern of pieces of varying sizes. During the breakup of receptors on the hepatocytes. the cell, the cell continues to produce proteins and adeno- • Chronic alcohol consumption induces sine triphosphate (ATP), a molecule that is required for the production of transforming most of the cell’s energy-consuming metabolic processes β1 growth factor-beta 1(TGF- ). and which is essential for cell functioning. As a result, Iron metabolism • Chronic alcohol consumption can each apoptotic body, which is surrounded by a piece of increase iron levels in the body and cell membrane, contains intact, functional cell compo- levels of free iron in the liver. nents (i.e., organelles2). Oxidative stress • Alcohol metabolism by CYP2E1 and Necrotic cell death, in contrast, is characterized by the increased levels of free iron increase loss of metabolic functions and of the integrity of the cell the levels of oxygen radicals. membrane. Thus, cells undergoing necrosis cease their Necrosis production of proteins and ATP. Structurally, the cells’ ATP depletion • Patients with alcoholic hepatitis have organelles swell and become nonfunctional during the reduced levels of ATP in their cells. initial stages of necrosis. In addition, the cell membrane forms bubblelike projections (i.e., blebs). These blebs, which Cytokines • Patients with alcoholic hepatitis and rats with alcohol-induced liver injury contain no organelles, fuse and increase in size. Eventually, show elevated levels of TNF-α. the cell membrane ruptures, resulting in the release of the • Chronic alcohol consumption cell’s components into the surrounding tissue. This process increases the levels of TNF-α of cell dissolution is called cytolysis. The released cellular receptors on the hepatocytes. content subsequently induces an inflammatory response in • Chronic alcohol consumption induces the production of TGF-β1. the effected tissue (e.g., the liver). This response is medi- • Other inflammatory cytokines, such ated by three components: (1) certain cells of the immune as interleukin-8, are increased in system that are attracted to the liver; (2) small molecules alcoholic hepatitis. called cytokines that are involved in cell communication; Hypoxia • Alcohol metabolism results in and (3) reactive oxygen species (i.e., oxygen radicals). increased oxygen consumption by This subsequent inflammatory response, which often is the liver. considered an integral part of necrosis, further damages Oxidative stress • Alcohol metabolism by CYP2E1 and the liver tissue. increased levels of free iron increase the levels of oxygen radicals. • Rats receiving an alcohol-containing diet have reduced glutathione levels CAUSES OF APOPTOSIS AND NECROSIS in their mitochondria. • Patients with alcoholic liver disease Numerous factors and mechanisms can induce apoptotic have reduced levels of vitamin E. and necrotic hepatocyte death. Some of these factors and Endotoxin • In alcoholics, the number of mechanisms contribute to both apoptosis and necrosis, endotoxin-producing bacteria in the whereas others play a role in only one of these processes intestine is elevated. In addition, endotoxin can enter the bloodstream 1 more easily, and Kupffer cells have It is important to recognize that apoptosis is not always a deleterious a reduced capacity to detoxify process. Rather, it generally serves to eliminate damaged or malfunc- endotoxin. tioning cells from the body. Thus, apoptosis plays a vital role in ensur- ing the proper functioning of many organs (e.g., the immune system). NOTE: For definitions of terms, see glossary, pp. 330. 2For a definition of this and other technical terms used in this article, see glossary, p. 330. 326 ALCOHOL HEALTH & RESEARCH WORLD RESEARCH UPDATE (see table, p. 326). Chronic alcohol consumption may system. (For more information on cytokines and their role induce or exacerbate many of these mechanisms. in alcoholic liver disease, see the article by McClain et al., pp. 317–320.) Two cytokines that can induce apoptosis α β 3 Contributing Factors to Apoptosis are TNF- and TGF- 1. In the liver, TNF-α is produced and secreted by a certain Among the numerous factors that can contribute to hepa- type of immune cell called Kupffer cells. Several studies tocyte apoptosis, the following
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