Long-term studies on chemically induced liver enlargement in the rat. II. Transient induction of microsomal enzymes leading to liver damage and nodular hyperplasia produced by safrole and Ponceau MX

Pregnane X Receptor Regulates Liver Size and Liver Cell Fate by Yes-Associated Protein Activation in Mice

Activation of pregnane X receptor (PXR), a nuclear receptor that controls xenobiotic and endobiotic metabolism, is known to induce liver enlargement, but the molecular signals and cell types responding to PXR-induced hepatomegaly remain unknown. In this study, the effect of PXR activation on liver enlargement and cell change was evaluated in several strains of genetically modified mice and animal models. Lineage labeling using AAV-Tbg-Cre-treated Rosa26^EYFP mice or Sox9-Cre^ERT , Rosa26^EYFP mice was performed and Pxr-null mice or AAV Yap short hairpin RNA (shRNA)-treated mice were used to confirm the role of PXR or yes-associated protein (YAP). Treatment with selective PXR activators induced liver enlargement and accelerated regeneration in wild-type (WT) and PXR-humanized mice, but not in Pxr-null mice, by increase of cell size, induction of a regenerative hybrid hepatocyte (HybHP) reprogramming, and promotion of hepatocyte and HybHP proliferation. Mechanistically, PXR interacted with YAP and PXR activation induced nuclear translocation of YAP. Blockade of YAP abolished PXR-induced liver enlargement in mice. Conclusion: These findings revealed a function of PXR in enlarging liver size and changing liver cell fate by activation of the YAP signaling pathway. These results have implications for understanding the physiological functions of PXR and suggest the potential for manipulation of liver size and liver cell fate.

Thresholds in Acute and Long Term Animal Studies

The possibilities to define thresholds in general pharmacology and toxicology depend upon the proper knowledge and determination of the most sensitive parameters and relevant targets leading to clinical effects or toxicity. We are generally confronted with the statistical summation of very numerous individual responses and effects at the enzymological, sub-cellular, cellular, organ, or systemic level. In consequence, biological actions of chemicals reveal typical dose-response characteristics described by simple mathematics, and more or less sharp thresholds. Thresholds and targets can change after single and repeated (cumulative) doses by means of altered kinetics and metabolism, by adaptation and modified sensitivity of targets and/or receptors. Most sensitive targets and parameters do not always determine the real thresholds for clinical toxicity. Threshold and specificity of carcinogens can also be modified by similar conditions mentioned. Nutritional factors play an important role in experimental car-dnogenesis, and also for the cancer incidence in man. Frequently, secondary mechanisms such as overloading, induction of cellular growth, mechanical and chemical irritation and damage, and other conditions have simulated real carcinogenic properties of chemical molecules. The important question on the existence and validity of thresholds in carc/nogenes/s must be connected with a solid evaluation of the mode of action of the individual compounds. Otherwise, mathematical calculations of safety limits, definitions of acceptable risks and no-effect levels lack any biological foundation. Finally, the individual perception and social acceptance of risks reveal certain thresholds.

Liver hypertrophy: a review of adaptive (adverse and non-adverse) changes--conclusions from the 3rd International ESTP Expert Workshop

Preclinical toxicity studies have demonstrated that exposure of laboratory animals to liver enzyme inducers during preclinical safety assessment results in a signature of toxicological changes characterized by an increase in liver weight, hepatocellular hypertrophy, cell proliferation, and, frequently in long-term (life-time) studies, hepatocarcinogenesis. Recent advances over the last decade have revealed that for many xenobiotics, these changes may be induced through a common mechanism of action involving activation of the nuclear hormone receptors CAR, PXR, or PPARα. The generation of genetically engineered mice that express altered versions of these nuclear hormone receptors, together with other avenues of investigation, have now demonstrated that sensitivity to many of these effects is rodent-specific. These data are consistent with the available epidemiological and empirical human evidence and lend support to the scientific opinion that these changes have little relevance to man. The ESTP therefore convened an international panel of experts to debate the evidence in order to more clearly define for toxicologic pathologists what is considered adverse in the context of hepatocellular hypertrophy. The results of this workshop concluded that hepatomegaly as a consequence of hepatocellular hypertrophy without histologic or clinical pathology alterations indicative of liver toxicity was considered an adaptive and a non-adverse reaction. This conclusion should normally be reached by an integrative weight of evidence approach.

Hepatic Enzyme Induction

Hepatic enzyme induction is generally an adaptive response associated with increases in liver weight, induction of gene expression, and morphological changes in hepatocytes. The additive growth and functional demands that initiated the response to hepatic enzyme induction cover a wide range of stimuli including pregnancy and lactation, hormonal fluctuations, dietary constituents, infections associated with acute-phase proteins, as well as responses to exposure to xenobiotics. Common xenobiotic enzyme inducers trigger pathways involving the constitutive androstane receptor (CAR), the peroxisome proliferator-activated receptor (PPAR), the aryl hydrocarbon receptor (AhR), and the pregnane-X-receptor (PXR). Liver enlargement in response to hepatic enzyme induction is typically associated with hepatocellular hypertrophy and often, transient hepatocyte hyperplasia. The hypertrophy may show a lobular distribution, with the pattern of lobular zonation and severity reflecting species, strain, and sex differences in addition to effects from specific xenobiotics. Toxicity and hepatocarcinogenicity may occur when liver responses exceed adaptive changes or induced enzymes generate toxic metabolites. These undesirable consequences are influenced by the type and dose of xenobiotic and show considerable species differences in susceptibility and severity that need to be understood for assessing the potential effects on human health from similar exposures to specific xenobiotics.

Alteration of liver cell function and proliferation: differentiation between adaptation and toxicity

Exposure of experimental animals to biologically effective levels of chemicals, either endogenous or exogenous, the latter of either synthetic or natural origin, elicits a response(s) that reflects the diverse ways in which the various units of organization of an organism deal with chemical perturbation. For some chemicals, an initial response constitutes an adaptive effect that maintains homeostasis. Disruption of this equilibrium at any level of organization leads to an adverse effect, or toxicity. The livers of laboratory animals and humans, like other organs, undergo programmed phases of growth and development, characterized by proliferation followed by differentiation. With organ maturity, the process of differentiation leads to the commitment of differentiated cells to constitutive functions that maintain homeostasis and to specialized functions that serve organismal needs. In the mature livers of all species, proliferation of all cell types subsides to a low level, Thus, the mature liver consists of 2 types of cells: intermediate cells, the hepatocytes, which replicate infrequently, but can respond to signals for replication, and replicating cells, the stem cells, endothelial, Kupffer, and stellate cells (Ito or pericytes), bile duct epithelium, and granular lymphocytes (pit cells). Quantifiable alterations or effects at the molecular level underlie alterations at the organelle level, which in turn lead to alterations at the cellular level, which can ultimately be manifested as a change in the whole organism. Alterations can be quantal (binary), either all or none, as with cell replication, cell necrosis or apoptosis, and cell differentiation, which take place at the cellular level. They can also be graded or continuous (nonbinary), as with enzyme induction, organelle hypertrophy, and extracellular matrix elaboration, occurring either at the intra- or extra (supra) cellular level. Any quantifiable change induced in the function or structure of a cell or tissue constitutes a response or effect. Each of the several types of cell in the liver responds to a given stimulus according to its localization and function. Generally, renewing cells are more vulnerable to chemical injury than intermediate cells, which are largely quiescent. Hepatic adaptive responses usually involve actions of the chemical on cellular regulatory pathways, often receptor mediated, leading to changes in gene expression and ultimately alteration of the metabolome. The response is directed toward maintaining homeostasis through modulation of various cellular and extracellular functions. At all levels of organization, adaptive responses are beneficial in that they enhance the capacity of all units to respond to chemical induced stress, are reversible and preserve viability. Such adaptation at subtoxic exposures is also referred to as hormesis. In contrast, adverse or toxic effects in the liver often involve chemical reaction with cellular macromolecules and produce disruption of homeostasis. Such effects diminish the capacity for response, can be nonreversible at all levels of organization, and can compromise viability. An exposure that elicits an adaptive response can produce toxicity with longer or higher exposures (ie, above a threshold) and the mechanism of action changes with the effective dose. A variety of hepatic adaptive and toxic effects has been identified. Examples of adaptive effects are provided by phenobarbital and ciprofibrate, whereas p-dichlorobenzene and 2-acetylaminofluorene illustrate different toxic effects. The effects of chemicals in the liver are, in general, similar between experimental animals and humans, although exceptions exist. Thus, identification and monitoring of both types of effect are integral in the safety assessment of chemical exposures.

Focal nodular hyperplasia of the liver after intraconazole treatment

A 38-year-old woman developed focal nodular hyperplasia of the liver after she had received a 4-month treatment with intraconazole 200 mg/d for a fungal infection of her fingernails. Because the patient underwent yearly liver ultrasound examinations because of the removal of a breast carcinoma, when the tumor was discovered incidentally, it was clear that it had developed within the past year after she had begun receiving intraconazole. Although various chemical agents and drugs have been considered as possible etiologic factors in the development of focal nodular hyperplasia of the liver, cases occurring after intraconazole therapy have not been reported before. Apart from the theoretical considerations with regard to the pathogenesis of nodular hyperplasia of the liver, this case could gain practical importance, as it shows a new adverse effect of a drug that has been used in more than 34 million patients over the past 10 years. Furthermore, this case should draw attention to the possibility of drug-induced benign hepatic tumors, as they may mimic malignant and metastatic disorders, which might be especially alarming in patients undergoing routine examinations after removal of malignant tumors, such as our patient.

Evidence for and possible mechanisms of non-genotoxic carcinogenesis in rodent liver

Doses of chemicals which induce hepatocellular necrosis usually induce hepatic tumours if the dosing is frequent and is maintained for long periods. Such necrosis is usually evident within 48 h of the first administration. Similarly, chemicals that lead to marked proliferation of peroxisomes in the liver also usually induce hepatic tumours on pretracked regular dosing. For both of these phenomena failure to produce a certain level of effect, or to maintain it for sufficiently long periods, can result in the observation of a non-carcinogenic response. The exact dose/time requirements for carcinogenicity have not been defined and may be species/strain/sex-specific. Some chemicals induce liver enlargement and mitogenesis in the absence of overt hepatotoxic effects. The early phases of hepatomegaly are associated with mitogenic effects that can be measured as cells in S-phase within the first few days of administration. The later stages of hepatomegaly appear to be associated more with cellular hypertrophy. Both effects appear to be threshold-related. Further, sustained hepatomegaly is associated with proliferation of SER and the induction of a range of liver enzymes. These changes (mitogenesis, hepatomegaly, enzyme induction), in isolation, are less definitive indicators of carcinogenicity, but they occur for a sufficient number of liver-specific carcinogens that their role as early indicators is worthy of confirmed study. The major area of study required for all possible early markers of hepatocarcinogenicity is to establish the dose and time dependence of these changes in relation to the eventual appearance of tumours. Finally, the specificity of all these markers require evaluation by the study of appropriate non-carcinogens.

Liver tumors in rodents: extrapolation to man

Man is a poor model for the prediction of agents that are hepatocarcinogenic for laboratory rodents. Relatively few agents are known to cause any form of primary liver cancer in man. The most important is hepatitis B virus, for which there is possibly a model in the woodchuck but not one in rats or mice. The only other agents known to cause primary liver cancer in man are certain steroid hormones, vinyl chloride, and thorium dioxide. There are animal models for the first two of these and a reasonable expectation that thorium dioxide would produce liver tumors in animals if the appropriate experiments were done. Aflatoxin, a potent hepatocarcinogen in rats and other species but not mice, is strongly suspected of being an important human hepatocarcinogen in certain geographical areas of the world, but the evidence is circumstantial. There is no more than a weak association between the nutritional type of cirrhosis secondary to excessive intake of alcohol and increased primary liver cancer in man, and no evidence at all that ethanol per se causes liver tumors in mice, rats, hamsters, or mastomys. By contrast, a very large number of chemicals to which people in the West have been exposed for many decades have been found to be hepatocarcinogens in laboratory rodents. In most cases the levels of exposure required to produce liver tumors in rodents far exceed those to which man is normally exposed. The problem is to guess whether low-level exposure to such rodent hepatocarcinogens poses any real liver cancer threat to man?The mortality from primary liver cancer is very low in countries such as England and Wales where there is widespread exposure to low doses of both natural and synthetic agents which, in high dosage, cause liver tumors in rodents. This suggests that, if there is any risk, it can only be very small. Death rate data collected in England and Wales by the Registrar General are consistent with there having been a small increase in the incidence of primary liver cancer in England and Wales during the past 20 years, but the apparent increase might well be a consequence of revisions in the International Classification of Diseases system and not real. During the first half of the present century the age-standardized incidence of primary liver cancer in England and Wales was falling.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of phthalic acid esters on the liver and thyroid

The effects, over periods from 3 days to 9 months of administration, of diets containing di-2-ethylhexyl phthalate are very similar to those observed in rats administered diets containing hypolipidemic drugs such as clofibrate. Changes occur in a characteristic order commencing with alterations in the distribution of lipid within the liver, quickly followed by proliferation of hepatic peroxisomes and induction of the specialized P-450 isoenzyme(s) catalyzing omega oxidation of fatty acids. There follows a phase of mild liver damage indicated by induction of glucose-6-phosphatase activity and a loss of glycogen, eventually leading to the formation of enlarged lysosomes through autophagy and the accumulation of lipofuscin. Associated changes are found in the kidney and thyroid. The renal changes are limited to the proximal convoluted tubules and are generally similar to changes found in the liver. The effects on the thyroid are more marked. Although the levels of thyroxine in plasma fail to about half normal values, serum triiodothyronine remains close to normal values while the appearance of the thyroid varies, very marked hyperactivity being noted 7 days after commencement of treatment, this is less marked at 14 days, but even after 9 months treatment there is clear cut evidence for hyperactivity with colloid changes which indicate this has persisted for some time. Straight chain analogs of di-2-ethylhexyl phthalate, di-n-hexyl phthalate and di-n-oxtyl phthalate differ entirely in their short-term effects on the liver and kidney but have similar effects on the thyroid. The short-term in vivo hepatic effects of the three phthalate esters can be reproduced in hepatocytes in tissue culture. All three phthalate esters, as well as clofibrate, have early marked effects on the metabolism of fatty acids in isolated hepatocytes. The nature of these changes is such as to increase storage of lipid in the liver. A hypothesis is presented to explain the progress from these initial metabolic effects to the final formation of liver tumors.

Pharmacological properties of dibenzo[a,c]cyclooctene derivatives isolated from Fructus Schizandrae chinensis. II. Induction of phenobarbital-like hepatic monooxygenases

Schizandrin (Sin) A, B and C, Schizandrol (Sol) A and B and Schizandrer (Ser) A and B were isolated from Fructus Schizandrae chinensis, a traditional Chinese tonic. These components are the derivatives of dibenzo[a,c]cylooctene. Dimethyl-4,4'-dimethoxy-5,6,5',6'-dimethylenedioxy-biphenyl-2,2'-dicarboxylate (DDB) is an intermediate for synthesizing Sin C. The effect of these compounds on rat liver microsomal monooxygenases and epoxide hydrolase has been studied. Among these compounds, Sin B, Sin C, Sol B and DDB significantly increased rat liver cytochrome P-450 concentration, NADPH-cytochrome c reductase, benzphetamine and aminopyrene demethylase activities. The four compounds also markedly stimulated proliferation of smooth endoplasmic reticulum of liver cells. Metyrapone (1 mM) inhibited to a same extent (about 50%) the activity of aminopyrene demethylase of microsomes from rats treated by Sin B, Sin C, Sol B, DDB and phenobarbital (PB). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of liver microsomal preparations showed that Sin B and Sol B induce a major protein band of P-450 similar to that induced by PB. In addition, the effect of Sin B-, Sol B- and DDB-treated rat liver microsomes on [G-3H]-benzo[a]pyrene (BP) metabolism and covalent binding of reactive metabolites to DNA, in vitro, resembles that of PB. Dual induction of rats by Sol B and BP decreased mutagenicity of BP.

Monooxygenase activity and ultrastructural changes of liver in the course of chronic exposure of rats to vinyl chloride

The activity of microsomal cytochrome P-450 monooxygenase and ultrastructure of the liver have been studied in rats exposed dynamically to 50, 500, and 20,000 ppm of vinyl chloride (VC) over 10 months. After 1 and 3 months of exposure to 500 and 20,000 ppm of VC, the level of cytochrome P-450 was slightly lower than in the control animals and upon continuation of exposure it was restored to the original level accompanied by slight increase of activity of aniline p-hydroxylase. Liver enlargement, developed in the course of the exposure, was accompanied by ultrastructural alterations beginning in the 3rd month of exposure to all concentrations of VC. Development of hepatic alterations (hypertrophy of smooth and rough endoplasmic reticulum, swelling of mitochondria, accumulation of lipid droplets, focal cytoplasmic degradation) is discussed with regard to the activity of microsomal monooxygenase system in metabolizing VC to toxic metabolites.

Liver growth and tumorigenesis in rats

Liver enlargement is frequently reported in studies on the short-term toxicity of chemicals. In many such studies no histological evidence of damage is present but biochemically there is often an increased microsomal enzyme activity (MEA) which is interpreted to represent a type of work hypertrophy. In a few instances, the MEA in the enlarged liver is either normal or less than normal. In such instances histochemical evidence of liver damage (depression of G-6-Pase and autophagy) is found. A compound which produced the latter changes is Ponceau MX. When administered for up to 21 months at a dose-level which produces biochemical and histochemical evidence of liver injury, a series of changes were observed consisting of progerssive diminution of MEA, areas of glycogen accumulation and centrilobular fatty change and these were followed first by nodular hyperplasia and then by frank carcinoma. The protective effect of increased MEA in carcinogenesis was shown by the reduction in tumour incidence on the administration of phenobarbitone simultaneously with acetylaminofluorene, 4-dimethyl aminoazo benzene and diethylnitrosamine. But no such protective effect is seen if the phenobarbitone is administered after treatment with these carcinogens. In fact the number of tumours is enhanced presumably due to preferential stimulation of the growth of malignant cells.