The role of necrosis in hepatocellular proliferation and liver tumors

The safety evaluation of food flavouring substances: the role of metabolic studies

The safety assessment of a flavour substance examines several factors, including metabolic and physiological disposition data. The present article provides an overview of the metabolism and disposition of flavour substances by identifying general applicable principles of metabolism to illustrate how information on metabolic fate is taken into account in their safety evaluation. The metabolism of the majority of flavour substances involves a series both of enzymatic and non-enzymatic biotransformation that often results in products that are more hydrophilic and more readily excretable than their precursors. Flavours can undergo metabolic reactions, such as oxidation, reduction, or hydrolysis that alter a functional group relative to the parent compound. The altered functional group may serve as a reaction site for a subsequent metabolic transformation. Metabolic intermediates undergo conjugation with an endogenous agent such as glucuronic acid, sulphate, glutathione, amino acids, or acetate. Such conjugates are typically readily excreted through the kidneys and liver. This paper summarizes the types of metabolic reactions that have been documented for flavour substances that are added to the human food chain, the methodologies available for metabolic studies, and the factors that affect the metabolic fate of a flavour substance.

Reversing hepatocellular carcinoma progression by using networked biological therapies

The liver is distinguished from other tissues by (a) its detoxifying function, (b) its resistance to apoptosis, and (c) its regenerative response to damage. Hepatocellular carcinoma arises when chronic insults, such as hepatitis or iron overload, constitutively activate this regenerative program. Here, we propose that the proliferative response of the liver to damage underlies the resistance of hepatocellular carcinoma to cytotoxic therapy, and that hepatocellular carcinoma growth should therefore be more readily controlled by using a networked combination of noncytotoxic interventions to interrupt the damage-inducible regenerative pathway. To this end, hepatocellular carcinoma boasts a wealth of potential drug targets, including viral replication, the antiapoptotic immunosuppressant alpha-fetoprotein, hepatic iron overload, inflammatory signaling, extracellular proteases, and growth factors. By blocking these positive feedback loops in parallel, and so returning the host environment to a more normal state, epigenetic repression of tumor-suppressor gene function may be reversed and tumor dormancy restored. Noncytotoxic maneuvers that short circuit damage resistance loops may thus represent an indirect form of gene therapy meriting incorporation into hepatocellular carcinoma clinical trials.

Comparison of the effects of dietary selenium, zinc, and selenium and zinc supplementation on growth and immune response between chick groups that were inoculated with Salmonella and aflatoxin or Salmonella

The effects of four diets (basal diet, Se, Zn, and Se- and Zn-enriched diets) fed to chicks that were administered one of three treatments [Salmonella and aflatoxin inoculation (T1), Salmonella inoculation (T2), or uninoculated (T3)] were investigated for growth and immune responses. We found a significant improvement in growth performance represented by relative body gain (RBG) and feed efficiency (FE), for the Zn- and Se + Zn-enriched diets fed to the T1 and T2 groups. The antibody immune response was significantly improved for the Se enrichment diet in the T1 and T2 groups. The weight of the bursa and thymus, which relate to the level of the immune response, showed significant decreases, whereas the spleen had a significantly increased relative weight (RW) in the T1 group. The variable dietary trace elements supplement increased the thymic RW in the T2 group.

The problem of latency in the development of tumors following exposure to nickel compounds

Six previously published animal studies of tumor production have been reviewed, in order to relate time interval between exposure to nickel and development of tumor formation. Biopsies at intervals before final tumor formation, in some of these experiments, were reviewed to define interim changes between exposure and tumor diagnosis. Correlation between rat and human life span was used to suggest a latency of human tumor expectancy following exposure to nickel.

Risk assessment for aflatoxin: an evaluation based on the multistage model

Lifetime cancer potency of aflatoxin was assessed based on the Yeh et al. study from China in which both aflatoxin exposure and hepatitis B prevalence were measured. This study provides the best available information for estimating the carcinogenic risk posed by aflatoxin to the U.S. population. Cancer potency of aflatoxin was estimated using a biologically motivated risk assessment model. The best estimate of aflatoxin potency was 9 (mg/kg/day)-1 for individuals negative for hepatitis B and 230 (mg/kg/day)-1 for individuals positive for hepatitis B.

The influence of fenarimol on DNA synthesis and mitotic activity in rat liver

Fenarimol administered in one single oral dose of 125 or 62.5 mg kg-1 body wt. day-1, respectively, stimulated rat liver enlargement at a dose-independent rate. Three single doses of fenarimol produced dose-dependent liver growth, whereas five single doses caused no further increase in liver weight. This increase was accompanied by an increase in hepatic DNA synthesis and mitotic activity, with a peak on the first day after the beginning of the experiments. The increase in binuclear hepatocytes and signs of necrosis suggested that the hepatomitogenic effect reflected a compensatory hyperplasia. After both three and five single doses the hepatomitogenic effect was suppressed, as a result of tolerance development.

Contribution of the glutathione S-transferases to the mechanisms of resistance to aflatoxin B1

The harmful effects of Aflatoxin B1 (AFB1) are a consequence of it being metabolized to AFB1-8,9-epoxide, a compound that serves as an alkylating agent and mutagen. The toxicity of AFB1 towards different cells varies substantially; sensitivity can change significantly during development, can be modulated by treatment with xenobiotics and is decreased markedly in preneoplastic lesions as well as in tumors. Three types of resistance, namely intrinsic, inducible and acquired, can be identified. The potential resistance mechanisms include low capacity to form AFB1-8,9-epoxide, high detoxification activity, increase in AFB1 efflux from cells and high DNA repair capacity. Circumstantial evidence exists that amongst these mechanisms the glutathione S-transferases, through their ability to detoxify AFB1-8,9-epoxide, play a major role in determining the sensitivity of cells to AFB1.

A single dose-response effect of aflatoxin B1 on rapid liver cancer induction in two strains of rats

To investigate rapid liver cancer induction in rats by aflatoxin B1 (AFB1), different single oral doses of AFB1 were given to 3 groups of 1-year-old Buffalo and Wistar rats. The animals were treated once and all survivors were killed 6 weeks later. Control animals received an equal volume of solvent (DMSO), and both groups of animals were maintained under identical conditions throughout the period of experiment. The survival rates were 40% with low and medium doses in AFB1-treated Buffalo and Wistar rats, and 0% in the high-dose Buffalo rats. Slight ante-mortem elevations in serum concentrations of glutamic pyruvic transaminase (SGOT) and glutamic oxaloacetic transaminase (SGPT) were indicative of the persistent damage effect of AFB1 at week 6. Total protein and albumin concentrations were not altered. The percent incidence of altered cell foci (areas) and neoplastic nodules was higher in Wistar than in Buffalo rats given a similar low dose. Various stages of well differentiated hepatocellular carcinomas (0.1-0.2 cm in diameter) developed in 3 of 8 Wistar rats. It thus appears that Wistar rats are more susceptible to hepatocarcinogenesis following a single oral dose of AFB1 than Buffalo rats.

The relationship between cellular ion deregulation and acute and chronic toxicity

Cell injury proceeds through a predictable series of stages as it progresses from reversible to irreversible injury (or "point of no return") and ends eventually in cell death. Ion deregulation is strongly implicated in this process and, in particular, the deregulation of cytosolic Ca2+ ([Ca2+]i) which is thought by most to be a critical step in the transition from reversible to irreversible injury. [Ca2+]i is normally maintained at approximately 100 microM, a level 10,000 times lower than for extracellular Ca2+ [( Ca2+]e). Deregulation may affect any of three Ca2+ buffering systems: the plasma membrane, the mitochondria, and the endoplasmic reticulum. Perturbation of [Ca2+]i is intimately related to perturbation of other ions, including, H+, Na+, and K+. In normal cells, [Ca2+]i elevation is also linked to activation of oncogenes as well as cell division, initiation, wound repair, differentiation, and possibly tumor promotion. In all models of acute injury for which we have measured [Ca2+]i, including ischemia, HgCl2 and calcium inophores, [Ca2+]i always became elevated. This elevation results from influx of [Ca2+]e (ionomycin), redistribution from intracellular stores (NEM, KCN), or from both sources (HgCl2). The degree of [Ca2+]i elevation is correlated with the degree of injury (as determined by blebbing and morphological changes) and cell killing. More recently, much work has been focused on the role of [Ca2+]i in neoplasia. Many stimuli, including the promoter TPA and transforming growth factor beta have been shown to affect normal and transformed cells differently. Both cause differentiation in normal human bronchial epithelial cells but stimulate growth in transformed cells. We propose that deregulation of ions, especially [Ca2+]i, plays an important role, if not a key role, in the initiation of acute and chronic cell injury, including neoplasia. Increases in [Ca2+]i appear to accelerate degradative processes and, unless regulated, lead to cell death.

Nutritional and dietary influences on liver tumorigenesis in mice and rats

There is evidence that liver tumor development in mice can be increased by diets high in calories or fat and by diets low in lipotropes. Data in rats suggest that dietary sucrose or ethanol may have an influence independent of caloric contribution, but the reported results are not entirely consistent. The lipotropic effect on tumorigenesis has been studied extensively in rats and may be the result of several factors, including increased cell proliferation, alteration of macromolecular methylation, changes in cell membrane composition and in xenobiotic metabolism. Comparative studies in mouse liver tumor models may be useful in further investigations.