The Fas/FasL Signaling Pathway
Maria Eugenia Guicciardi, Gregory J. Gores
11
11.1
Introduction
Fas (also called CD95 or APO-1) belongs to the death receptor family, a subgroup of the tumor necrosis factor/nerve growth factor (TNF/NGF) receptor superfamily. These cell surface cytokine receptors are able to initiate an apoptotic signaling cascade after binding a group of structurally related ligands or specific antibodies [3, 40]. The members of this family are type I transmembrane proteins with a C- terminal intracellular tail, a membrane-spanning region, and an extracellular N-terminal domain.
Through interaction with the N-terminal domain, the receptors bind their cognate ligands (called death ligands), the majority of which are type II transmembrane proteins belonging to the TNF fam- ily of proteins, and comprised of an intracellular N-terminal domain, a transmembrane region, and a C-terminal extracellular tail. The signature features of the death receptors are represented by a highly homologous region in their extracellular domains containing one to five cysteine-rich repeats, and a 60- to 80-amino acid cytoplasmic sequence known as death domain (DD), which is required to initiate the death signal.
Engagement of death receptors results in initia- tion of the so-called extrinsic pathway of apoptosis, one of the two main signaling pathways leading to apoptotic cell death. The second one is generated by a mitochondrial dysfunction, and is referred to as the intrinsic pathway [23] (Fig. 11.1). Although both signaling pathways are sufficient to trigger apopto- sis, the two are not mutually exclusive and can be simultaneously activated in the same cell through crosstalk between pathways, especially in hepato- cytes.
Apoptosis is essential to preserve liver func- tion and health, as it ensures the efficient removal of unwanted cells (i.e., aged or virus-infected cells) in a highly controlled manner. Apoptotic cells are ultimately fragmented into membrane-bound, or- ganelle-containing corpses (apoptotic bodies),
which are readily engulfed by neighboring phago- cytes, mainly Kuppfer cells; this engulfment proc- ess may, under pathologic conditions, promote liver inflammation and damage by amplifying Fas-me- diated hepatocyte apoptosis through FasL produc- tion by the Kuppfer cells themselves (Fig. 11.2) [9].
Fig. 11.1. Apoptotic pathways. Apoptosis can be induced via the activation of the extrinsic or the intrinsic pathway. Death receptor engagement mediates the extrinsic pathway through the recruitment of adaptor proteins and the formation of the DISC. Initiator caspases (caspase-8 and -10) activated at the DISC directly cleave and activate the effector caspases (caspase-3, - 6, and -7) responsible for the degradation of cellular substrates.
Different stimuli, including growth factor deprivation, DNA damage, and UV light exposure, trigger the intrinsic pathway via the activation of proapoptotic members of the Bcl-2 family of protein (i.e., Bax, Bak), which cause mitochondrial dysfunction.
The action of the proapoptotic members of the Bcl-2 family can be antagonized by anti-apoptotic members of the same family (Bcl-2, Bcl-XL). Following mitochondrial dysfunction, caspase-9, another initiator caspase, is activated and, in turn, activates the effector caspases. See text for details
Although apoptosis in the liver can occur through activation of both the extrinsic and the intrinsic pathway, the extrinsic pathway seems to be by far the most relevant, likely due to the high level of ex- pression of death receptors in hepatic cells. In par- ticular, Fas is constitutively expressed by every cell type in the liver [15], rendering all liver cells sensi- tive to Fas-mediated apoptosis in vivo. Indeed, Fas- induced apoptosis plays a fundamental role in liver physiology by contributing to the elimination of se- nescent cells and maintaining liver homeostasis [1], as well as in pathologic conditions, by ensuring the removal of virus-infected or damaged cells [33, 41].
However, excessive or defective Fas-mediated apop- tosis leads to disease pathogenesis, such as liver fail- ure, fibrosis, and carcinogenesis. In this chapter, we review the molecular mechanisms and regulation of Fas signaling, and the role of the Fas/FasL system in the pathophysiology of the liver.
11.2
Fas (CD95/APO-1)
and Fas Ligand (FasL/CD95L)
11.2.1
Fas (CD95/APO-1)
Fas is a glycosylated cell-surface protein, ubiqui- tously expressed in various tissues, in particular in thymus, liver, heart, kidney, and in activated mature lymphocytes or virus-transformed lymphocytes.
Although soluble forms of the receptor also exist, whose functions are still largely unknown, the mem- brane-bound form is largely predominant and high- ly biologically active [10]. In order to avoid unneces- sary activation of the apoptotic pathway, Fas expres- sion and localization are tightly regulated through a variety of mechanisms. First of all, only a minimal amount of Fas is expressed on the plasma mem- brane in unstimulated cells, whereas the majority of the receptor localizes in the cytosol, in particular, in the Golgi complex and the trans-Golgi network [6, 66]. After a proapoptotic stimulus, Fas-containing vesicles translocate to the cell surface, increasing Fas expression on the plasma membrane and initiat- ing the apoptotic signal. This mechanism provides an effective tool to regulate the plasma membrane density of the death receptor, and avoid its sponta- neous activation [19, 66]. Fas can also be modulated at a post-translational level, by glycosylation of the receptor [60], as well as at the transcriptional level, by direct regulation of Fas expression. Indeed, a composite binding site for the transcription factor NF- κB is located in the Fas gene promoter [11], and a p53-responsive element has been identified within the first intron of the Fas gene, which cooperates with three sequences in the promoter to upregulate Fas receptor expression [20, 50, 51].
11.2.2
Fas Ligand (FasL/CD95L)
FasL (CD95L) is a type II transmembrane protein with homotrimeric structure, mainly expressed on the cell surface of activated T cells [70]. In the liver, the interaction between FasL-positive cytotoxic T lymphocytes and target cells, such as virus-infected cells or cancer cells, which usually overexpress Fas, represents a powerful tool to eliminate potentially toxic cells [7, 33, 41]. Kupffer cells, liver-specific phagocytes, can also express FasL and induce hepa- tocyte apoptosis [9, 52, 72]. Moreover, hepatocytes themselves overexpress FasL in certain pathological
Fig. 11.2. A vicious cycle of apoptosis. Schematic representa- tion of the amplification of Fas/CD95-mediated hepatocyte apoptosis by Kupffer cell-generated FasL/CD95L. Apoptotic hepatocytes fragment into apoptotic bodies. Engulfment of apoptotic bodies by Kupffer cells induces their activation and the production of FasL/CD95L, which, in turn, exacerbates the apoptotic damage
conditions, such as alcoholic hepatitis and Wilson's disease; this can bind Fas constitutively expressed on the same cell, and induce apoptosis via fratri- cide [21]. FasL also exists in a soluble, trimeric form generated after cleavage by a metalloprotease, but, similarly to the soluble form of Fas, its biological ac- tivity remains controversial [63]. Indeed, although the ability of soluble FasL to induce apoptosis has been documented, high serum levels of FasL often reported in hepatitis, AIDS and several types of tu- mor do not seem to correlate with increased apop- totic damage [74]. The explanation for this apparent contradiction has been provided by recent studies demonstrating that the apoptotic-inducing capac- ity of the soluble form is reduced by over 1,000-fold compared to the membrane-bound FasL [63, 65, 71].
Therefore, even elevated serum levels of soluble FasL might not be sufficient to cause significant apopto- sis. However, the binding of soluble FasL to some matrix proteins (i.e., fibronectin) has been shown to induce trimerization of the ligand, therefore effec- tively mimicking the membrane-bound form [2].
11.3
Fas/FasL Signaling
Engagement of Fas by either agonistic antibodies or FasL leads to the trimerization of the receptor, fol- lowed by recruitment of the adaptor molecule FADD (Fas-associated protein with death domain)/MORT- 1 (mediator of receptor-induced toxicity), a cytosol- ic protein with a C-terminal death domain, and a death effector domain at the N-terminus [78]. FADD
associates with the receptor through interaction of its death domains, while its death effector domain binds to a correspondent death effector domain or a caspase recruitment domain (CARD) in the pro- domain of inactive initiator caspases, such as pro- caspase-8 and procaspase-10. The resulting com- plex is called the death-inducing signaling complex (DISC) (Fig. 11.3). Recruitment and accumulation of procaspase-8 and/or -10 at the DISC results in self- processing, spontaneous activation of the caspase via autoproteolytic cleavage, and initiation of a pro- teolytic cascade. Procaspase-8 and -10 are proteo- lytically processed with similar kinetics, and both can initiate apoptosis independently of each other, as well as participate in the same apoptotic pathway [36, 79]. A family of proteins of viral origin called v-FLIPs (viral FLICE-inhibitory proteins) has been found to exert an anti-apoptotic activity by inhibit- ing the activation of effector caspases at the DISC.
v-FLIPs contain two death effector domains that en- able them to bind to FADD and block caspase-8 ac- tivation [46]. It has not yet been established whether they also inhibit caspase-10 activation. The human cellular homolog, c-FLIP (also called I-FLICE or Casper or Usurpin) [76], exists in a short and a long isoform, as a result of different splice variants. The short form, c-FLIP
S, consists only of two death ef- fector domains, and structurally resembles v-FLIP.
The long form, c-FLIP
Lconsists of two death effec- tor domains and a caspase-like domain, and closely resembles caspase-8, except that it contains an inac- tive enzymatic site, and, therefore, has no cysteine protease activity. Both forms of c-FLIP are recruited and bind to the DISC upon stimulation [62], but while c-FLIP
Smay competitively inhibit procaspase-
Fig. 11.3. The Fas/CD95 death-inducing signaling complex (DISC). Schematic rep- resentation of the DISC formed after the engagement of FasL/CD95L to Fas/CD95.
The death domain (DD) on the adapter protein FADD interacts with the receptor’s death domain, whereas the death effector domain (DED) binds the correspondent death effector domain in the pro-domain of the inactive initiator caspase-8 and/or -10. The short form of the inhibitor of ap- optosis cFLIP (cFLIPS) binds to the DED of FADD and prevents the recruitment of the initiator pro-caspases to the DISC, whereas the long form of cFLIP (cFLIPL) allows the recruitment of procaspase-8 to the DISC, but inhibits its processing into the active form. See text for details
8 recruitment to the DISC, c-FLIP
Lallows recruit- ment and even partial cleavage of procaspase-8.
However, by mechanisms yet to be clarified, c-FLIP
Lprevents further proteolytic processing of caspase-8 to generate the active subunits of the enzyme [37]
(Fig. 11.3).
Downstream of the DISC formation, activation of effector caspases, such as caspase-3, -6 and -7, which are ultimately responsible for the degrada- tion of key cellular components, can occur via two different signaling pathways. Based on the signaling pathway preferentially activated after Fas stimula- tion, cells have been classified into type I and type II [61] (Fig. 11.4). In type I cells, large amounts of caspase-8 are activated at the DISC, which, in turn, directly cleave and activate caspase-3. In these cells, prevention of mitochondrial dysfunction by over- expression of the anti-apoptotic proteins Bcl-2 or Bcl-X
Ldoes not block the activation of caspase-8 or caspase-3, nor does it inhibit apoptosis, suggesting a mitochondria-independent activation of a caspase cascade. In contrast, in type II cells, DISC forma-
tion is strongly reduced, and activation of caspases, including caspase-8, occurs mainly downstream of mitochondria, as both caspase activation and apop- tosis can be prevented by overexpression of Bcl-2 or Bcl-X
L. Notably, Fas induces mitochondrial dysfunc- tion in both type I and type II cells, but only in type II cells are mitochondria essential for the execution of the apoptotic program, whereas in type I cells they likely function solely as amplifiers of the apoptotic signal [61]. Mitochondrial dysfunction during Fas signaling is initiated by caspase-8-mediated cleav- age of Bid, a pro-apoptotic, BH3-only member of the Bcl-2 family of proteins [39, 43]. The generated 15- kDa fragment (tBid) translocates to the mitochon- dria, and contributes to the formation of pores on the outer mitochondrial membrane, resulting in the release of apoptogenic factors, such as cytochrome c [39, 43], AIF (apoptosis-inducing factor) [73], and SMAC (second mitochondria-derived activator of caspases)/Diablo (direct IAP-binding protein with low pI) [14, 77]. In the cytosol, cytochrome c associ- ates with the cofactor Apaf-1 (apoptosis-activating factor 1) and procaspase-9, and forms a complex named apoptosome. Through an energy-requiring reaction, procaspase-9 in the apoptosome is proc- essed into the mature enzyme and, in turn, activates caspase-3, starting a caspase cascade downstream of the mitochondrium.
11.4
Fas/FasL in Liver Diseases
Dysregulation of hepatocyte apoptosis often associ- ates with liver diseases (Fig. 11.5). Downregulation of apoptosis leads to diseases associated with exces- sive cell growth, such as hepatocellular carcinoma.
On the contrary, excessive hepatocyte apoptosis is a feature of viral and autoimmune hepatitis, acute hepatic failure, cholestatic diseases, alcoholic and non-alcoholic hepatitis, chemotherapeutic-induced liver damage, as well as transplantation-associated liver damage, such as ischemia/reperfusion injury and graft rejection. The role of the Fas/FasL system in several human liver diseases associated with dis- ruption of apoptosis is described in greater detail in this section.
Fig. 11.4. Fas/CD95-mediated apoptotic pathways. Schematic representation of Fas-mediated apoptotic pathways in type I and type II cells. The amount of active caspase-8 generated at the DISC determines whether the cell activates a mitochondrial- independent (type I) or mitochondrial-dependent (type II) path- way of caspase activation and apoptosis. See text for details
11.4.1
Pathologic Conditions Associated with Reduced Fas-mediated Apoptosis
Hepatocellular Carcinoma
Hepatocellular carcinoma, the most common pri- mary malignancy of the liver, has multiple etiolo- gies, including environmental, nutritional, and met- abolic factors, as well as chronic viral infections. The role of Fas as an anticarcinogenic agent is suggested by the evidence that Fas-defective animals show increased risk of developing tumors. Consistently, Fas expression is downregulated in several tumors, including hepatocellular carcinoma, which origi- nate from tissue that previously expressed normal levels of Fas [28, 38]. Reductions of Fas expression of various extents have been described in hepatocar- cinomas, which inversely correlate with the severity of the disease. Indeed, the most significant reduc- tions in Fas expression are found in poorly differ- entiated, advanced carcinomas, and negatively cor- relate with patient survival [32, 53, 68]. The loss of Fas represents an advantageous adaptation for the cancer cell, because it allows the cell to survive the attack by FasL-expressing cytotoxic T lymphocytes and NK cells. In addition, tumor cells often express FasL, which enables them to actively kill the im- mune cells and create immune privileged sites [24, 27, 68]. Therefore, therapeutic approaches aimed to restore Fas expression and sensitivity to Fas-medi- ated apoptosis in tumor cells may be effective in the therapy of hepatocellular carcinomas. Indeed, several chemotherapeutic drugs induce tumor cell apoptosis by causing DNA damage and activation of the nuclear phosphoprotein p53, which, in turn, has been found to upregulate Fas and increase sen- sitivity to Fas-mediated apoptosis [50, 51]. However, a study recently performed using an experimental mouse model of chronic hepatitis-induced hepato- cellular carcinoma has demonstrated that inhibi- tion of FasL may actually reduce the development of cancer [54]. FasL has a proinflammatory activity [34, 64], and a status of continuous inflammation leads to massive cells loss and liver regeneration that, in turn, may significantly increase the chance of mutagenic events. Therefore, FasL is not only di- rectly responsible for hepatocyte killing, but is also indirectly involved in chronic liver dysplasia and hepatocellular carcinoma development by inducing an inflammatory response. Thus, early in the dis- ease process, inhibiting FasL and its inflammatory signaling would prevent the milieu necessary for carcinogenesis to occur. These new findings have to
be considered in order to design a better therapeutic approach for hepatocellular carcinomas.
11.4.2
Pathologic Conditions Associated with Excessive Fas-mediated Apoptosis
Viral Hepatitis
Viral hepatitis is mainly caused by infection with hepatitis B (HBV) or C virus (HCV). However, the virus itself has very mild cytopathic effects on the infected host cells, and the extensive tissue damage associated with viral hepatitis is generally the result of host immune response to viral antigen. During viral hepatitis, specific classes of cytotoxic T lym- phocytes (CTL) recognize and kill viral antigen-ex- pressing HBV- or HCV-infected hepatocytes to clear the virus from the liver. This causes the initial liver damage, which is subsequently exacerbated by the influx of antigen-non-specific inflammatory cells.
The killing of viral antigen-positive hepatocytes by CTL occurs via apoptosis, as demonstrated by the presence of apoptotic bodies, once referred to
Fig. 11.5. Role of Fas/CD95-mediated apoptosis in the patho- genesis of liver diseases. Schematic representation of the role of Fas/CD95-mediated apoptosis in liver health and disease. Under physiological conditions, the number of cells killed by apoptosis equals the number of cells produced by cell division. Imbalance between cell production and cell death leads to different liver diseases. See text for details
as Councilman bodies, in the liver of patients with viral hepatitis. In particular, Fas, although not the only apoptotic pathway involved, seems to play a key role in this process. Indeed, Fas expression is increased in the liver of patients with chronic hepa- titis B and C, and directly correlates with disease activity such as periportal and intralobular inflam- mation [21, 31, 42, 47, 49, 80]. It is not clear whether Fas expression is mainly regulated by virus-specific protein expression or by inflammatory cytokines, such as interleukin-1, generated after the first im- mune response. Areas of FasL-positive infiltrating mononuclear cells are also common in the liver of HBV- and HCV-infected patients, confirming the importance of the Fas/FasL system in the removal of infected cells by CTL during viral hepatitis [21, 31, 47, 49, 80]. Nevertheless, the role of the HBV and HCV proteins in Fas-mediated apoptosis remains controversial. The HBV X-gene product (HBx) has been shown to stimulate the apoptotic turnover of hepatocytes [75], as well as to activate NF- κB and c-Jun N-terminal kinase (JNK) pathways and there- fore to protect liver cells from apoptosis [13, 57].
Similarly, HCV proteins inhibit Fas-mediated ap- optosis and death in transgenic mice by preventing the release of cytochrome c from the mitochondria [44]. Therefore, hepatitis virus proteins may either sensitize the hepatocyte to Fas-induced apoptosis or inhibit apoptosis to maintain persistent infection.
Alcoholic Hepatitis
Although the pathogenesis of alcoholic hepatitis and cirrhosis is still poorly understood, apopto- sis certainly plays an important role both in vitro and in vivo. Apoptosis is a characteristic feature of experimental ethanol-induced liver injury [5, 22].
Moreover, hepatocyte apoptosis has recently been identified in liver biopsies of patients with alcoholic hepatitis, with a correlation with the disease sever- ity, being most abundant in patients with high bi- lirubin and AST levels, and grade 4 steatohepatitis [35, 56]. Among the several mechanisms proposed to explain alcohol-induced hepatocyte apoptosis, there is the activation of death receptor pathways, in particular, the Fas/FasL and TNF- α/TNF-R1 signal- ing. Patients with alcoholic hepatitis express higher levels of Fas and FasL in the hepatocytes compared to healthy subjects, which renders the cells more susceptible both to cytotoxic T lymphocyte-medi- ated apoptosis, and to cell death by autocrine and/
or paracrine mechanisms [56]. The increased ex- pression of Fas and FasL may result from TNF- α- induced activation of NF- κB, a transcription factor that can upregulate both these genes [11]. Indeed, TNF- α serum levels are elevated during alcoholic
hepatitis, and are directly involved in hepatocyte apoptosis [45]. Moreover, a recent study suggested that, in addition to a direct cytotoxic effect on the hepatocyte, the TNF- α/TNF-R1 system is required also for Fas-mediated cell death, as demonstrated by the increased resistance of TNF-R1/TNF-R2 double knockout mice to Fas-induced fulminant liver in- jury [12]. Thus, it appears that both Fas and TNF-R1 contribute to ethanol-mediated liver injury through a synergistic action in inducing hepatocyte apopto- sis.
Cholestatic Liver Disease
Cholestasis is defined as an impairment of bile flow through the liver. As a consequence, high concen- trations of bile acids accumulate within the hepato- cytes, causing tissue damage and liver failure. Sever- al studies demonstrated that one of the mechanisms by which bile acids, especially hydrophobic bile ac- ids, induce liver damage is by triggering hepatocyte apoptosis. Indeed, hydrophobic bile acids, such as deoxycholic and glycodeoxycholic acid, are able to cause hepatocyte apoptosis in vitro [16, 26, 30, 58, 66]. More remarkably, massive hepatocyte apoptosis is clearly detectable in the liver of bile duct-ligated mice, an animal model of extrahepatic cholestasis [48]. Although bile acids have detergent properties and could potentially damage the cell membranes, they actually need to be transported into the cell to trigger apoptosis, as cells lacking a functional bile acid transporter are resistant to bile acid-induced apoptosis [25]. It has been shown that elevated con- centrations of bile acids within the hepatocyte can induce Fas translocation from its intracellular loca- tions to the plasma membrane, where the increased surface density triggers its oligomerization and ini- tiates the apoptotic signal [66]. Indeed, bile acid-in- duced apoptosis largely occurs via a Fas-dependent, FasL-independent mechanism, both in vitro [16], and in vivo [48]. In addition, a recent study dem- onstrated that, in a model of chronic cholestasis, Fas-mediated cytoxicity promotes the development of liver fibrosis, the result of excessive deposition of extracellular matrix during the wound-healing re- sponse that follows a prolonged injury to the liver [8]. In the absence of Fas, long-term, bile duct-ligat- ed mice showed reduced markers of fibrosis, such as expression of α-smooth muscle actin and collagen deposition, as compared to Fas-expressing animals, suggesting that inhibition of Fas-mediated hepato- cyte apoptosis may prevent liver fibrogenesis.
Although Fas plays a major role in executing
bile acid-mediated apoptosis, other pathways are
also likely to be involved [48]. Recent reports have
demonstrated that, in the absence of Fas, bile acids
increase the transcription and oligomerization of another death receptor, TRAIL-R2/DR5, suggesting that bile acid cytotoxicity can also be mediated by TRAIL/TRAIL-R2 signaling pathway [29]. Both Fas and TRAIL-R2 signal apoptosis through activation of caspase-8/-10 and Bid, therefore targeted inhibi- tion of caspases or Bid could have therapeutic rel- evance in the treatment of cholestatic liver diseases.
Indeed, inhibition of Bid by injection of antisense oligonucleotides has already been demonstrated to reduce hepatocyte apoptosis and liver damage in bile duct-ligated mice [30].
Wilson's Disease
Wilson's disease is a genetic disorder caused by excessive copper storage in different organs and tissues, including the liver. Liver sections from pa- tients with Wilson's disease show significant hepa- tocyte apoptosis associated with upregulation of Fas and FasL on the hepatocyte cell membrane [69].
Similarly, hepatocyte apoptosis and Fas expression have been found to be increased in a model of cop- per overload in vitro [69]. As already suggested in alcoholic hepatitis, the simultaneous expression of Fas and FasL on the same cell membrane may pro- mote fratricide killing of neighboring cells. Copper accumulation within the hepatocyte causes oxida- tive stress, which, in turn, may promote Fas activa- tion and apoptosis [4]. The upregulation of Fas like- ly occurs via the activation of the tumor suppressor gene p53, which follows the oxidative stress-induced DNA damage. Indeed, treatment of hepatoma cells with copper results in a transient increase in p53 and Fas expression, the latter being a consequence of p53 transcriptional activity [55, 69]. Inhibition of either FasL or caspases effectively reduces apopto- sis with similar results, suggesting that Fas might be the only apoptotic signal involved in copper-in- duced apoptosis. Therefore therapies aimed to in- hibit either Fas or FasL or caspases might be useful in the treatment of Wilson's disease and could re- duce the need for transplantation in the acute form of this disease.
Non-alcoholic Steatohepatitis
Non-alcoholic steatohepatitis (NASH) is the most severe form of non-alcoholic fatty liver disease (NAFLD), characterized by the presence of mac- rovesicular steatosis along with inflammatory ac- tivity, and sometimes associated with fibrosis. The molecular mechanisms involved in tissue damage during NASH are poorly understood. However, it has recently been demonstrated that Fas expression, activation of caspase-3 and -7 and hepatocyte ap-
optosis are enhanced in the liver of NASH patients, and positively correlated with the biochemical and histopathologic markers of liver injury [17, 18]. Mi- tochondrial function is often impaired in the liver of subjects with NASH [59]. Activation of Fas results in mitochondrial dysfunction as a consequence of the activation of Bid and its translocation to the mi- tochondria. Moreover, mitochondrial dysfunction is associated with generation of reactive oxygen, which is also able to induce apoptosis, further ex- acerbating tissue injury and inflammation. Thus, Fas inhibition may be an effective therapy to reduce liver damage and prevent development of cirrhosis in NASH.
11.5 Conclusions
The Fas/FasL system plays a key role in maintaining liver homeostasis and function through regulation of cell death and survival. Indeed, several liver dis- eases are associated with either Fas overexpression or downregulation, which directly correlates with the onset of the disease and its severity. Hepatocyte apoptosis often represents the early stage of many liver diseases, independent of their etiology, and it occurs mainly through engagement of death recep- tors on the plasma membrane, especially Fas. Un- controlled hepatocyte apoptosis can progress into liver injury if the number of cells dying is signifi- cantly higher than the number of cells replaced by cell division (Fig. 11.5). Moreover, if the amount of apoptotic bodies overwhelms the clearance capac- ity by phagocytes, the apoptotic bodies undergo a process of autolysis resulting in the release of pro- inflammatory factors, which exacerbate the inflam- matory response and tissue damage. In addition, he- patic stellate cells are activated by the phagocytosis of apoptotic bodies, and generate a pro-fibrogenic response by promoting collagen deposition in the liver parenchyma and formation of fibrotic tissue.
In conclusion, therapeutic approaches aimed to
modulate Fas-mediated apoptosis may ultimately
be effective in reducing liver damage in several hu-
man liver diseases. Preliminary studies on animal
models of liver injury have already generated prom-
ising data regarding the feasibility and effectiveness
of genetic inhibition of Fas as a possible therapy to
prevent fulminant liver failure [67, 81].
Acknowledgments
This work was supported by grants from the Na- tional Institute of Health DK 41876 (to GJG) and the Mayo Foundation, Rochester, Minnesota.
Selected Reading
The following readings are suggested to help un- derstand the signal transduction through the death receptors and the fundamental role of Fas in liver physiology and pathophysiology.
Locksley RM, Killeen N, Lenardo MJ. The TNF and TNF recep- tor superfamily: integrating mammalian biology. Cell 2001;104:487–501. [40]
Scaffidi C, Fulda S, Srinivasan A et al. Two CD95 (APO-1/Fas) sign- aling pathways. EMBO J 1998;17:1675–1687. [61]
Faubion WA, Gores GJ. Death receptors in liver biology and pathobiology. Hepatology 1999;29:1–4. [15]
Galle PR, Hofmann WJ, Walczak H et al. Involvement of the APO- 1/Fas (CD95) receptor and ligand in liver damage. J Exp Med 1995;182:1223–1230. [21]
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