Role of cytokines in non-genotoxic hepatocarcinogenesis: cause or effect?

Key Characteristics of Human Hepatotoxicants as a Basis for Identification and Characterization of the Causes of Liver Toxicity

Hazard identification regarding adverse effects on the liver is a critical step in safety evaluations of drugs and other chemicals. Current testing paradigms for hepatotoxicity rely heavily on preclinical studies in animals and human data (epidemiology and clinical trials). Mechanistic understanding of the molecular and cellular pathways that may cause or exacerbate hepatotoxicity is well advanced and holds promise for identification of hepatotoxicants. One of the challenges in translating mechanistic evidence into robust decisions about potential hepatotoxicity is the lack of a systematic approach to integrate these data to help identify liver toxicity hazards. Recently, marked improvements were achieved in the practice of hazard identification of carcinogens, female and male reproductive toxicants, and endocrine disrupting chemicals using the key characteristics approach. Here, we describe the methods by which key characteristics of human hepatotoxicants were identified and provide examples for how they could be used to systematically identify, organize, and use mechanistic data when identifying hepatotoxicants.

A role of connexin 43 on the drug-induced liver, kidney, and gastrointestinal tract toxicity with associated signaling pathways

Drug-induced organ toxicity/injury, especially in the liver, kidney, and gastrointestinal tract, is a systematic disorder that causes oxidative stress formation and inflammation resulting in cell death and organ failure. Current therapies target reactive oxygen species (ROS) scavenging and inhibit inflammatory factors in organ injury to restore the functions and temporary relief. Organ cell function and tissue homeostasis are maintained through gap junction intercellular communication, regulating connexin hemichannels. Mis-regulation of such connexin, especially connexin (Cx) 43, affects a comprehensive process, including cell differentiation, inflammation, and cell death. Aim to describe knowledge about the importance of connexin role and insights therapeutic targeting. Cx43 misregulation has been implicated in recent decades in various diseases. Moreover, in recent years there is increasing evidence that Cx43 is involved in the toxicity process, including hepatic, renal, and gastrointestinal disorders. Cx43 has the potential to initiate the immune system to cause cell death, which has been activated in the acceleration of apoptosis, necroptosis, and autophagy signaling pathway. So far, therapies targeting Cx43 have been under inspection and are subjected to clinical trial phases. This review elucidates the role of Cx43 in drug-induced vital organ injury, and recent reports compromise its function in the major signaling pathways.

Connexin-based signaling and drug-induced hepatotoxicity

Being critical mediators of liver homeostasis, connexins and their channels are frequently involved in liver toxicity. In the current paper, specific attention is paid to actions of hepatotoxic drugs on these communicative structures. In a first part, an overview is provided on the structural, regulatory and functional properties of connexin-based channels in the liver. In the second part, documented effects of acetaminophen, hypolipidemic drugs, phenobarbital and methapyriline on connexin signaling are discussed. Furthermore, the relevance of this subject for the fields of clinical and in vitro toxicology is demonstrated.

Relevance for patients: The role of connexin signaling in drug-induced hepatotoxicity may be of high clinical relevance, as it offers perspectives for the therapeutic treatment of such insults by interfering with connexin channel opening.

Oxidative pathways of chemical toxicity and oxidative stress biomarkers in marine organisms

The antioxidant system of marine organisms consists of low molecular weight scavengers and antioxidant enzymes which interact in a sophisticated network. Environmental pollutants can unbalance this system through closely related mechanisms, indirect relationships and cascade effects acting from pre-transcriptional to catalytic levels. Chemically-mediated pathways have the potential to greatly enhance intracellular formation of reactive oxygen species (ROS); at the same time, excessive levels of oxyradicals down-regulate xenobiotics metabolism, with important environmental implications for organisms exposed to chemical mixtures. Interactions between different classes of chemicals, generation of ROS and onset of oxidative stress conditions are partly modulated by changes in levels and functions of redox-sensitive signaling proteins and transcription factors. The Nrf2-Keap1 pathway still remains largely unexplored in marine organisms, despite the elevated degree of identity and similarity with homolog transcripts and proteins from different species. Recent evidences on transcriptional up-regulation of this system are consistent with the capability to provide a prolonged expression of ARE-regulated cytoprotective genes, and to efficiently switch off this mechanism when oxidative pressure decreases. Although gene expression and catalytic activities of antioxidants are often measured as alternative biomarkers in monitoring biological effects of contaminants, conflicting results between molecular and biochemical responses are quite frequent. The links between effects occurring at various intracellular levels can be masked by non-genomic processes affecting mRNA stability and protein turnover, different timing for transcriptional and translational mechanisms, metabolic capability of tissues, post-transcriptional modifications of proteins, bi-phasic responses of antioxidant enzymes and interactions occurring in chemical mixtures. In this respect, caution should be taken in monitoring studies where mRNA levels of antioxidants could represent a snapshot of cell activity at a given time, not an effective endpoint of environmental pollutants.

S-allylmercaptocysteine reduces carbon tetrachloride-induced hepatic oxidative stress and necroinflammation via nuclear factor kappa B-dependent pathways in mice

Purpose

To study the protective effects and underlying molecular mechanisms of SAMC on carbon tetrachloride (CCl₄)-induced acute hepatotoxicity in the mouse model.

Methods

Mice were intraperitoneally injected with CCl₄ (50 μl/kg; single dose) to induce acute hepatotoxicity with or without a 2-h pre-treatment of SAMC intraperitoneal injection (200 mg/kg; single dose). After 8 h, the blood serum and liver samples of mice were collected and subjected to measurements of histological and molecular parameters of hepatotoxicity.

Results

SAMC reduced CCl₄-triggered cellular necrosis and inflammation in the liver under histological analysis. Since co-treatment of SAMC and CCl₄ enhanced the expressions of antioxidant enzymes, reduced the nitric oxide (NO)-dependent oxidative stress, and inhibited lipid peroxidation induced by CCl₄. SAMC played an essential antioxidative role during CCl₄-induced hepatotoxicity. Administration of SAMC also ameliorated hepatic inflammation induced by CCl₄ via inhibiting the activity of NF-κB subunits p50 and p65, thus reducing the expressions of pro-inflammatory cytokines, mediators, and chemokines, as well as promoting pro-regenerative factors at both transcriptional and translational levels.

Conclusions

Our results indicate that SAMC mitigates cellular damage, oxidative stress, and inflammation in CCl₄-induced acute hepatotoxicity mouse model through regulation of NF-κB. Garlic or garlic derivatives may therefore be a potential food supplement in the prevention of liver damage.

Origins of injection-site sarcomas in cats: the possible role of chronic inflammation-a review

The etiology of feline injection-site sarcomas remains obscure. Sarcomas and other tumors are known to be associated with viral infections in humans and other animals, including cats. However, the available evidence suggests that this is not the case with feline injection-site sarcomas. These tumors have more in common with sarcomas noted in experimental studies with laboratory animals where foreign materials such as glass, plastics, and metal are the causal agent. Tumors arising with these agents are associated with chronic inflammation at the injection or implantation sites. Similar tumors have been observed, albeit infrequently, at microchip implantation sites, and these also are associated with chronic inflammation. It is suggested that injection-site sarcomas in cats may arise at the administration site as a result of chronic inflammation, possibly provoked by adjuvant materials, with subsequent DNA damage, cellular transformation, and clonal expansion. However, more fundamental research is required to elucidate the mechanisms involved.

Effects of dahuangzhechong pills on cytokines and mitogen activated protein kinase activation in rats with hepatic fibrosis

RELEVANCE TO ETHNOPHARMACOLOGY: Dahuangzhechong pill (DHZCP), a well-known and canonical Chinese medicine formula from "The Synopsis of Prescriptions of the Golden Chamber", is officially approved and recommended by Chinese association of integrative medicine for the prevention and treatment of hepatic fibrosis in China.

AIM OF THE STUDY

To test the hypothesis that therapeutic effects of DHZCP on hepatic fibrosis are conferred by regulating cytokine profile through a mitogen activated protein kinase (MAPK) pathway.

MATERIALS AND METHODS

Hepatic fibrosis is inducted by carbon tetrachloride (CCl(4)) in rats which then were randomly divided into six groups: hepatic fibrosis model group, high dose DHZCP group, low dose DHZCP group, Fufang Biejia Ruangan Pian (FBRP) group, Colchicine group and control group. Pathological, immunohistochemical, multiplex immunoassay and protein expression studies (Western blotting) are performed.

RESULTS

DHZCP significantly decreases the levels of alanine aminotransferase, aspartate aminotransferase, hyaluronic acid, laminin, type IV collagen and procollagen III, and reverses hepatic fibrosis in rat model. DHZCP also could reduce the expression of α-smooth muscle actin, and lower the serum level of tumor necrosis factor alpha (TNF-α) and interleukin 13 (IL-13). The expressions of phosphorylated p38 MAPK and extracellular signal-regulated kinase (ERK) are down-regulated, while no significant changes are found in phosphorylation of c-Jun N-terminal kinase (JNK).

CONCLUSIONS

DHZCP can alleviate hepatic fibrosis induced by CCl(4). The anti-fibrotic effects of DHZCP are conferred by decreasing the secretion of TNF-α and IL-13 through down-regulating p38 and ERK phosphorylation.

Mechanisms of resistance of hepatocyte retinoid X receptor alpha-null mice to WY-14,643-induced hepatocyte proliferation and cholestasis

Peroxisome proliferators, such as the lipid-lowering fibrates that function as agonists for peroxisome proliferator-activated receptor alpha (PPARalpha), induce liver tumors in rodents and may produce cholestasis in humans. Considerable attention has focused on peroxisome proliferator-induced hepatocellular carcinoma, a phenomenon not noted in man, whereas limited studies examine fibrates and other therapeutic drugs that induce cholestasis, a common finding in humans. Moreover, the mechanisms by which fibrates induce hepatocyte proliferation and cholestasis are still not fully understood. We have examined the role of hepatocyte retinoid X receptor alpha (RXRalpha), an essential partner of PPARalpha, in modulating WY-14,643-induced hepatocyte proliferation and cholestasis. WY-14,643 treatment induced hepatomegaly in wild type (WT) mice that was also accompanied by induction of the expression of cyclins D1, D3, A2, and B1 and Cdc2 as well as inhibition of Wee 1. Such changes were either absent or greatly reduced in hepatocyte RXRalpha-null mice. Furthermore, neither WY-14,643 treatment nor RXRalpha deficiency affected apoptosis, indicating the importance of PPARalpha/RXRalpha in regulating Wee 1-mediated Cdc2/cyclin B1 expression for cells to enter into mitosis. WY-14,643 treatment also induced cholestasis and liver injury, which is evidenced by induction of alanine aminotransferase, alkaline phosphatase, and hepatic bile acid levels in WT mice. Hepatocyte RXRalpha deficiency protected the mice from WY-14,643-induced liver injury. WY-14,643-mediated induction of the small heterodimer partner, Mrp3, and Cyp3a11 levels was greater in hepatocyte RXRalpha-null than in WT mouse livers suggesting enhanced repression of bile acid synthesis and increased efflux of bile acids into blood for renal excretion as well as hydroxylation of bile acids because of hepatocyte RXRalpha deficiency. These data establish a crucial role of hepatocyte RXRalpha in regulating WY-14,643-mediated cell cycle progression as well as bile acid homeostasis.

Time course investigation of PPARalpha- and Kupffer cell-dependent effects of WY-14,643 in mouse liver using microarray gene expression

Administration of peroxisome proliferators to rodents causes proliferation of peroxisomes, induction of beta-oxidation enzymes, hepatocellular hypertrophy and hyperplasia, with chronic exposure ultimately leading to hepatocellular carcinomas. Many responses associated with peroxisome proliferators are nuclear receptor-mediated events involving peroxisome proliferators-activated receptor alpha (PPARalpha). A role for nuclear receptor-independent events has also been shown, with evidence of Kupffer cell-mediated free radical production, presumably through NAPDH oxidase, induction of redox-sensitive transcription factors involved in cytokine production and cytokine-mediated cell replication following acute treatment with peroxisome proliferators in rodents. Recent studies have demonstrated, by using p47(phox)-null mice which are deficient in NADPH oxidase, that this enzyme is not related to the phenotypic events caused by prolonged administration of peroxisome proliferators. In an effort to determine the timing of the transition from Kupffer cell-to PPARalpha-dependent modulation of peroxisome proliferator effects, gene expression was assessed in liver from Pparalpha-null, p47(phox)-null and corresponding wild-type mice following treatment with 4-chloro-6-(2,3-xylidino)-pyrimidynylthioacetic acid (WY-14,643) for 8 h, 24 h, 72 h, 1 week or 4 weeks. WY-14,643-induced gene expression in p47(phox)-null mouse liver differed substantially from wild-type mice at acute doses and striking differences in baseline expression of immune related genes were evident. Pathway mapping of genes that respond to WY-14,643 in a time- and dose-dependent manner demonstrates suppression of immune response, cell death and signal transduction and promotion of lipid metabolism, cell cycle and DNA repair. Furthermore, these pathways were largely dependent on PPARalpha, not NADPH oxidase demonstrating a temporal shift in response to peroxisome proliferators. Overall, this study shows that NADPH oxidase-dependent events, while detectable following acute treatment, are transient. To the contrary, a strong PPARalpha-specific gene signature was evident in mice that were continually exposed to WY-14,643.

Perfluorooctanoic acid-induced hepatic toxicity following 21-day oral exposure in mice

Perfluorooctanoic acid (PFOA) is a member of the perfluoroalkyl acids that have wide commercial applications and is a widespread pollutant of toxicological importance that has been detected in environmental matrices. The NOAEL and LOAEL of PFOA in rodent were reported 1 and 10 ppm, respectively. The current study characterizes the hepatic toxicities of PFOA in mice. Male ICR mice were exposed continuously to 0, 2, 10, 50 and 250 ppm of PFOA in drinking water for 21 days. Food and water consumption decreased in mice exposed to 250 ppm of PFOA. Mean body weight gain was reduced in mice exposed to 50 and 250 ppm of PFOA. The size and relative weight of the liver increased dose-dependently in PFOA-treated mice. Serum enzyme activities, alanine aminotransferase and aspartate aminotransferase, increased in mice exposed to PFOA in a dose-dependent manner. In the histopathological evaluation, the liver of PFOA-treated mice showed remarkable hepatocytomegaly and acidophilic cytoplasm. At the high doses of PFOA, diffuse hepatic damage by multifocal coagualation and liquefaction necrosis were noted. In contrast to the remarkable change of liver, the kidney had little change. The size and relative weights of the kidney, biomarkers of kidney damage (blood urea nitrogen, creatinine), and histopathological changes had no differences between PFOA-untreated and PFOA-treated mice. In conclusion, our results demonstrate that PFOA causes a toxic effect on the liver but not to the kidney.

Modes of action and species-specific effects of di-(2-ethylhexyl)phthalate in the liver

The industrial plasticizer di-(2-ethylhexyl)phthalate (DEHP) is used in manufacturing of a wide variety of polyvinyl chloride (PVC)-containing medical and consumer products. DEHP belongs to a class of chemicals known as peroxisome proliferators (PPs). PPs are a structurally diverse group of compounds that share many (but perhaps not all) biological effects and are characterized as non-genotoxic rodent carcinogens. This review focuses on the effect of DEHP in liver, a primary target organ for the pleiotropic effects of DEHP and other PPs. Specifically, liver parenchymal cells, identified herein as hepatocytes, are a major cell type that are responsive to exposure to PPs, including DEHP; however, other cell types in the liver may also play a role. The PP-induced increase in the number and size of peroxisomes in hepatocytes, so called 'peroxisome proliferation' that results in elevation of fatty acid metabolism, is a hallmark response to these compounds in the liver. A link between peroxisome proliferation and tumor formation has been a predominant, albeit questioned, theory to explain the cause of a hepatocarcinogenic effect of PPs. Other molecular events, such as induction of cell proliferation, decreased apoptosis, oxidative DNA damage, and selective clonal expansion of the initiated cells have been also been proposed to be critically involved in PP-induced carcinogenesis in liver. Considerable differences in the metabolism and molecular changes induced by DEHP in the liver, most predominantly the activation of the nuclear receptor peroxisome proliferator-activated receptor (PPAR)alpha, have been identified between species. Both sexes of rats and mice develop adenomas and carcinomas after prolonged feeding with DEHP; however, limited DEHP-specific human data are available, even though exposure to DEHP and other phthalates is common in the general population. This likely constitutes the largest gap in our knowledge on the potential for DEHP to cause liver cancer in humans. Overall, it is believed that the sequence of key events that are relevant to DEHP-induced liver carcinogenesis in rodents involves the following events whereby the combination of the molecular signals and multiple pathways, rather than a single hallmark event (such as induction of PPARalpha and peroxisomal genes, or cell proliferation) contribute to the formation of tumors: (i) rapid metabolism of the parental compound to primary and secondary bioactive metabolites that are readily absorbed and distributed throughout the body; (ii) receptor-independent activation of hepatic macrophages and production of oxidants; (iii) activation of PPARalpha in hepatocytes and sustained increase in expression of peroxisomal and non-peroxisomal metabolism-related genes; (iv) enlargement of many hepatocellular organelles (peroxisomes, mitochondria, etc.); (v) rapid but transient increase in cell proliferation, and a decrease in apoptosis; (vi) sustained hepatomegaly; (vii) chronic low-level oxidative stress and accumulation of DNA damage; (viii) selective clonal expansion of the initiated cells; (ix) appearance of the pre-neoplastic nodules; (x) development of adenomas and carcinomas.

Over-expressed estrogen receptor-alpha up-regulates hTNF-alpha gene expression and down-regulates beta-catenin signaling activity to induce the apoptosis and inhibit proliferation of LoVo colon cancer cells

Epidemiologic studies reported that the prevalence of hereditary non-polyposis colon cancer (HNPCC) in male is about 1.5-fold higher than that in female. Decreases in circulatory estrogen (E(2)) have been reported to downregulate the expression of E(2) receptor (ER) and significantly increase the risk of colorectal cancer. Patients that received E(2) replacement therapy were found to have a reduction in the incidence of colon adenoma and carcinoma. Furthermore, significant decreases in the expression of ER have been found in colorectal cancer specimens. Evidences strongly suggest the protective roles of E(2) and ER against colorectal cancer. However, the mechanisms of ERalpha effects on colorectal cancer cells remained un-clear. LoVo cells were transient transfected to overexpress ERalpha, DNA fragmentation and the activated caspases measurements were performed to evaluate apoptotic effects. Western blotting was used to evaluate protein levels, and luciferase activity assay to measure the Htnf-a promoter activity. The results clearly demonstrated that overexpressed ERalpha with or without E(2) (10(-8) M) treatment could activate caspase -8, -9, and 3 and induce DNA fragmentation in LoVo cell. At the same time, overexpressed ERalpha plus E(2) significantly increases the expression and promoter activity of hTNF-alpha, and the DNA fragmentation effect induced by E(2) plus ERalpha were reduced by the addition of hTNF antibody (0.1 ng(ml). In addition, E(2) plus ERalpha significantly upregulated p21 and p27 levels and downregulated the beta-catenin and its target genes, cyclin D1 and Rb, which regulate the cell cycle and cell proliferation. The results indicate that E(2) plus overexpressed ERalpha induce LoVo cell apoptosis might mediate through the increase of hTNF-alpha gene expression, which in turn activate caspase-8, -9 and caspase-3 and lead to the DNA fragmentation and apoptosis. E(2) plus ERalpha also showed the downregulation of beta-catenin signalings implicating the suppression of proliferation and metastasis of colorectal cells. Efforts aiming at enhancing ERalpha expression and(or activity may be proved to be an alternative therapy against colorectal cancer.

Hepatotoxicity and mechanism of action of haloalkanes: carbon tetrachloride as a toxicological model

The use of many halogenated alkanes such as carbon tetrachloride (CCl4), chloroform (CHCl3) or iodoform (CHI3), has been banned or severely restricted because of their distinct toxicity. Yet CCl4 continues to provide an important service today as a model substance to elucidate the mechanisms of action of hepatotoxic effects such as fatty degeneration, fibrosis, hepatocellular death, and carcinogenicity. In a matter of dose,exposure time, presence of potentiating agents, or age of the affected organism, regeneration can take place and lead to full recovery from liver damage. CCl4 is activated by cytochrome (CYP)2E1, CYP2B1 or CYP2B2, and possibly CYP3A, to form the trichloromethyl radical, CCl3*. This radical can bind to cellular molecules (nucleic acid, protein, lipid), impairing crucial cellular processes such as lipid metabolism, with the potential outcome of fatty degeneration (steatosis). Adduct formation between CCl3* and DNA is thought to function as initiator of hepatic cancer. This radical can also react with oxygen to form the trichloromethylperoxy radical CCl3OO*, a highly reactive species. CCl3OO* initiates the chain reaction of lipid peroxidation, which attacks and destroys polyunsaturated fatty acids, in particular those associated with phospholipids. This affects the permeabilities of mitochondrial, endoplasmic reticulum, and plasma membranes, resulting in the loss of cellular calcium sequestration and homeostasis, which can contribute heavily to subsequent cell damage. Among the degradation products of fatty acids are reactive aldehydes, especially 4-hydroxynonenal, which bind easily to functional groups of proteins and inhibit important enzyme activities. CCl4 intoxication also leads to hypomethylation of cellular components; in the case of RNA the outcome is thought to be inhibition of protein synthesis, in the case of phospholipids it plays a role in the inhibition of lipoprotein secretion. None of these processes per se is considered the ultimate cause of CCl4-induced cell death; it is by cooperation that they achieve a fatal outcome, provided the toxicant acts in a high single dose, or over longer periods of time at low doses. At the molecular level CCl4 activates tumor necrosis factor (TNF)alpha, nitric oxide (NO), and transforming growth factors (TGF)-alpha and -beta in the cell, processes that appear to direct the cell primarily toward (self-)destruction or fibrosis. TNFalpha pushes toward apoptosis, whereas the TGFs appear to direct toward fibrosis. Interleukin (IL)-6, although induced by TNFalpha, has a clearly antiapoptotic effect, and IL-10 also counteracts TNFalpha action. Thus, both interleukins have the potential to initiate recovery of the CCl4-damaged hepatocyte. Several of the above-mentioned toxication processes can be specifically interrupted with the use of antioxidants and mitogens, respectively, by restoring cellular methylation, or by preserving calcium sequestration. Chemicals that induce cytochromes that metabolize CCl4, or delay tissue regeneration when co-administered with CCl4 will potentiate its toxicity thoroughly, while appropriate CYP450 inhibitors will alleviate much of the toxicity. Oxygen partial pressure can also direct the course of CCl4 hepatotoxicity. Pressures between 5 and 35 mmHg favor lipid peroxidation, whereas absence of oxygen, as well as a partial pressure above 100 mmHg, both prevent lipid peroxidation entirely. Consequently, the location of CCl4-induced damage mirrors the oxygen gradient across the liver lobule. Mixed halogenated methanes and ethanes, found as so-called disinfection byproducts at low concentration in drinking water, elicit symptoms of toxicity very similar to carbon tetrachloride, including carcinogenicity.

The peroxisome proliferator-activated receptor alpha (PPARalpha): role in hepatocarcinogenesis

The peroxisome proliferator-activated receptor alpha (PPARalpha) is a member of the nuclear receptor superfamily and mediates most of the known biological effects of peroxisome proliferators. The latter represents a large group of chemicals that include the fibrate hyperlipidemic drugs, the pthalate plasticizers, various solvents and degreasing agents, and endogenous hormones and fatty acids. Peroxisome proliferators are classical members of the nongenotoxic group of chemical carcinogens that do not require metabolic activation to electrophiles in order to exert their harmful effects. These chemicals are of particular concern to regulatory agencies since they can only be detected by long-term carcinogen bioassays using rodents. The mechanism of the carcinogenic action of peroxisome proliferators is beginning to emerge. PPARalpha-null mice are resistant to hepatocarcinogenesis indicating that this receptor is necessary for cancer. However, recent studies indicate that Kupffer cells, in a PPARalpha independent manor, are required for the major effects of peroxisome proliferators on cell proliferation. An interaction between PPARalpha and estrogen carcinogenesis has also been elucidated.

Receptor-mediated hepatocarcinogenesis: role of hepatocyte proliferation and apoptosis

The rodent liver is a target organ for the action of several non-genotoxic carcinogens. These include dioxins, polychlorinated biphenyls, phenobarbital, peroxisome proliferators and organochlorine pesticides. These chemicals disrupt the homeostasis of the liver by perturbing hepatocyte cell death and proliferation, causing hyperplasia leading to tumour formation. Significant progress has been made towards elucidating the mechanisms of action of these toxicants since the discovery of receptors that bind specific classes of xenobiotics. Dioxins and polychlorinated biphenyls bind to the aryl hydrocarbon receptor, phenobarbital binds to the constitutive androstane receptor and peroxisome proliferators act via the their activated receptor alpha. These three receptors have ligand-dependent transcription activities and therefore mediate changes in gene expression in response to toxicant exposure. The development of transgenic mouse strains where the genes for these receptors are disrupted has demonstrated that receptor activity is essential for the toxicity of these carcinogens. This implies that changes in the expression of key target genes control proliferation and apoptosis in the xenobiotic-induced hepatocyte phenotype.