APOPTOSIS AND NECROSIS

1 VOL. 21, NO. 4, 1997325 RESEARCH UPDATEAPOPTOSIS AND NECROSISTwo Types of Cell Death in Alcoholic Liver DiseaseAmin A. Nanji, , and Susanne Hiller-Sturmh fel, alcohol consumption over long periods of timecan result in severe liver damage, including death ofliver cells ( , hepatocytes). Two mechanisms APOPTOSIS and NECROSIS can contribute to hepatocytedeath. In APOPTOSIS , the affected cell actively participatesin the cell death process, whereas in NECROSIS the celldeath occurs in response to adverse conditions in thecell s environment. Numerous factors that may con-tribute to the initiation of hepatocyte APOPTOSIS areaffected by alcohol consumption. These factors includethe enzyme cytochrome P450 2E1 ( , CYP2E1), smallmolecules ( , cytokines) involved in cell communi-cation, oxidativestress, and changes in ironmetabolism. Similarly, alcohol consumption can in-fluence several factors believed to be involved in hepa-tocyte NECROSIS , includingdepletion of theenergy-storing molecule adenosine-triphosphate, re-duced oxygen levels ( , hypoxia) in the liver, oxida-tivestress, and bacterial molecules called WORDS: alcoholic liver disorder; NECROSIS ; cytolysis;hepatocyte; cytochrome P450; oxidation-reduction;iron; metabolic disorder; ATP (adenosine triphos-phate); hypoxia; endotoxins; biochemical mechanism;pathogenesis; literature reviewMany people who drink heavily over extendedperiods of time ( , several years) developincreasingly severe liver damage, including fattyliver, alcoholic hepatitis, and alcoholic cirrhosis.

2 Fattyliver is caused by the accumulation of fat in the hepatitis is characterized by extensive inflam-mation of the liver and the destruction of liver cells ( ,hepatocytes). Moreover, scar tissue begins to form, re-placing healthy liver tissue. In alcoholic cirrhosis, scar-ring and cell death progress further, resulting in distortionof the internal structure of the liver and, subsequently, insevere functional impairment and secondary failure ofother organs, such as the kidney. These multiple compli-cations can lead to the patient s death. By investigatingthe mechanisms underlying alcohol s deleterious effectson the liver, researchers hope to ultimately develop newdiagnostic and therapeutic approaches to prevent theseoften fatal consequences of alcohol recent research has focused on the mechanismsthat contribute to hepatocyte death at the cellular processes play a role in hepatocyte destruction APOPTOSIS and NECROSIS . This article briefly reviews thedifferences between these two processes and speculateson some of their underlying mechanisms.

3 The article alsodiscusses how heavy alcohol consumption may be associ-ated with the mechanisms that promote these ANDNECROSISA lthough the ultimate results of APOPTOSIS and necrosisare the same ( , death of the affected cells), the two pro-cesses differ significantly. In APOPTOSIS , the affected cellsactively participate by activating a cascade of biochemicalAMINA. NANJI, , is co-director of the Center forClinical and Laboratory Investigation, Department ofPathology, Beth Israel Deaconess Medical Center, andassociate professor of pathology at Harvard MedicalSchool, Boston, FEL, , is a science editor ofAlcohol Health & Research cellbreaks intoseveralapoptoticbodies; theorganellesare cellNecrosisApoptosisSmall blebsform; thestructure ofthe nucleusbegins to break apart,and the DNAbreaks intosmall pieces. The organelles are alsolocated in the blebs fuseand becomelarger; noorganelles are located inthe cellmembraneruptures andreleases the cell s content;the organellesare changes of cells undergoing NECROSIS or that result in cell death.

4 Accordingly, apoptosishas been called cell suicide ( , Rosser and Gores1995).1In NECROSIS , however, cell death occurs because ofadverse conditions or changes in the cell s , NECROSIS can be viewed as the consequence of a biological accident that leads to the death of an inno-cent victim (Rosser and Gores 1995).Characteristic differences also exist in both the struc-ture and the metabolic processes of cells that undergoapoptosis or NECROSIS (see figure, p. 325) (Rosser and Gores1995). When a cell undergoes APOPTOSIS , the entire cell,including the nucleus, separates into numerous fragments( , apoptotic bodies). Simultaneously, the genetic material( , DNA) of apoptotic cells breaks into a characteristicpattern of pieces of varying sizes. During the breakup ofthe cell, the cell continues to produce proteins and adeno-sine triphosphate (ATP), a molecule that is required formost of the cell s energy-consuming metabolic processesand which is essential for cell functioning. As a result,each apoptotic body, which is surrounded by a piece ofcell membrane, contains intact, functional cell compo-nents ( , organelles2).

5 Necrotic cell death, in contrast, is characterized by theloss of metabolic functions and of the integrity of the cellmembrane. Thus, cells undergoing NECROSIS cease theirproduction of proteins and ATP. Structurally, the cells organelles swell and become nonfunctional during theinitial stages of NECROSIS . In addition, the cell membraneforms bubblelike projections ( , blebs). These blebs, whichcontain no organelles, fuse and increase in size. Eventually,the cell membrane ruptures, resulting in the release of thecell s components into the surrounding tissue. This processof cell dissolution is called cytolysis. The released cellularcontent subsequently induces an inflammatory response inthe effected tissue ( , the liver). This response is medi-ated by three components: (1) certain cells of the immunesystem that are attracted to the liver; (2) small moleculescalled cytokines that are involved in cell communication;and (3) reactive oxygen species ( , oxygen radicals).This subsequent inflammatory response, which often isconsidered an integral part of NECROSIS , further damagesthe liver OFAPOPTOSIS ANDNECROSISN umerous factors and mechanisms can induce apoptoticand necrotic hepatocyte death.

6 Some of these factors andmechanisms contribute to both APOPTOSIS and NECROSIS ,whereas others play a role in only one of these processes326 ALCOHOLHEALTH& RESEARCHWORLDRESEARCH UPDATEF actors That May Contribute to Hepatocyte APOPTOSIS and NECROSIS in Alcoholic Liver DiseaseFactorResearch Findings Regarding Alcohol s Effects on These FactorsApoptosisCytochrome Alcohol metabolism by CYP2E1P450 2E1results in the generation of oxygen (CYP2E1) Patients with alcoholic hepatitisand rats with alcohol-induced liverinjury show elevated levels of tumornecrosis factor alpha (TNF- ). Chronic alcohol consumption increases the levels of TNF- receptors on the hepatocytes. Chronic alcohol consumption inducesthe production of transforming growth factor-beta 1(TGF- 1).Iron metabolism Chronic alcohol consumption canincrease iron levels in the body andlevels of free iron in the stress Alcohol metabolism by CYP2E1 andincreased levels of free iron increasethe levels of oxygen depletion Patients with alcoholic hepatitis havereduced levels of ATP in their Patients with alcoholic hepatitis andrats with alcohol-induced liver injuryshow elevated levels of TNF.

7 Chronic alcohol consumption increases the levels of TNF- receptors on the hepatocytes. Chronic alcohol consumption inducesthe production of TGF- 1. Other inflammatory cytokines, suchas interleukin-8, are increased inalcoholic Alcohol metabolism results inincreased oxygen consumption bythe stress Alcohol metabolism by CYP2E1 andincreased levels of free iron increasethe levels of oxygen radicals. Rats receiving an alcohol-containingdiet have reduced glutathione levelsin their mitochondria. Patients with alcoholic liver diseasehave reduced levels of vitamin In alcoholics, the number of endotoxin-producing bacteria in theintestine is elevated. In addition, endotoxin can enter the bloodstreammore easily, and Kupffer cells have a reduced capacity to detoxify : For definitions of terms, see glossary, pp. is important to recognize that APOPTOSIS is not always a deleteriousprocess. Rather, it generally serves to eliminate damaged or malfunc-tioning cells from the body. Thus, APOPTOSIS plays a vital role in ensur-ing the proper functioning of many organs ( , the immune system).

8 2 For a definition of this and other technical terms used in this article, seeglossary, p. 21, NO. 4, 1997327 RESEARCH UPDATE(see table, p. 326). Chronic alcohol consumption mayinduce or exacerbate many of these Factors to ApoptosisAmong the numerous factors that can contribute to hepa-tocyte APOPTOSIS , the following may be particularly rele-vant in alcoholic liver disease: The enzyme cytochrome P450 2E1 ( , CYP2E1) Cytokines, such as tumor NECROSIS factor-alpha (TNF- ) and transforming growth factor-beta 1 (TGF- 1) Oxidative stress Altered iron The liver is the primary site of alcohol meta-bolism. Several pathways for alcohol metabolism exist in hepatocytes. One of these pathways involves a group ofenzymes collectively known as cytochrome P450, whichprimarily serves to detoxify harmful substances. One ofthese enzymes is CYP2E1. Alcohol metabolism byCYP2E1 results in the formation of highly reactive, oxygen-containing molecules called oxygen radicals. Oxygenradicals are chemically or electrically unstable; therefore,they quickly interact with other molecules in the cells, suchas fat molecules ( , lipids), proteins, and DNA.

9 Forexample, oxygen radicals can interact with the lipids thatcomprise the membranes within and surrounding the process is called lipid peroxidation. As a result of lipidperoxidation, the membranes gradually are degraded. Thisloss of membrane integrity severely impairs cell function,and the cells eventually die through an apoptotic have demonstrated that the CYP2E1-dependent pathway of alcohol-induced lipid peroxidationand subsequent hepatocyte APOPTOSIS occurs in rats receivinga high-fat diet ( , a diet rich in lipid components calledpolyunsaturated fatty acids) plus alcohol (Nanji et ,b). Other investigators have evaluated the relation-ship among CYP2E1, unsaturated fatty acids, lipid perox-idation, and apoptotic cell death by exposing a hepatocytecell line that overproduces CYP2E1 to the unsaturated fattyacid arachidonic acid (Chen et al. 1997). This treatmentresulted in lipid peroxidation and hepatocyte that prevented the formation of oxygen radicalsby CYP2E1 or that eliminated, or scavenged, these radicals( , antioxidants) prevented apoptotic hepatocyte observations further support the hypothesis thatalcohol-induced activity of CYP2E1 may contribute toalcohol-induced liver are small molecules secreted primarilyby cells of the immune system in order to communicatewith other immune cells and cells outside the immunesystem.

10 (For more information on cytokines and their rolein alcoholic liver disease, see the article by mcclain et al.,pp. 317 320.) Two cytokines that can induce apoptosisare TNF- and TGF- the liver, TNF- is produced and secreted by a certaintype of immune cell called Kupffer cells. Several studieshave found that TNF- levels were elevated in patientswith alcoholic hepatitis and in rats with alcohol-inducedliver injury ( mcclain et al. 1993; Nanji et al. 1994c). Toexert its effects on the cell, TNF- must interact with specif-ic docking molecules ( , receptors) on the cell s types of TNF- receptors exist. Hepatocytes carryreceptors that respond even to low TNF- , hepatocytes are naturally sensitive to the alcohol consumption increases the number ofthese receptors on the hepatocytes, thereby enhancing thecells sensitivity to TNF- even further (Deaciuc et al. 1995).In addition, hepatocytes carry a receptor called CD95, orFas, which is similar to the TNF- receptors. This recep-tor interacts with a specific molecule ( , Fas ligand) foundon the surface of certain immune cells ( , cytotoxic Tcells).