Chemical carcinogenesis: a current biological perspective

Participation of liver stem cells in cholangiocarcinogenesis after aflatoxin B1 exposure of glutathione S-transferase A3 knockout mice

Aflatoxin B1, arguably the most potent human carcinogen, induces liver cancer in humans, rats, trout, ducks, and so on, but adult mice are totally resistant. This resistance is because of a detoxifying enzyme, mouse glutathione S-transferase A3, which binds to and inactivates aflatoxin B1 epoxide, preventing the epoxide from binding to DNA and causing mutations. Glutathione S-transferase A3 or its analog has not been detected in any of the sensitive species, including humans. The generation of a glutathione S-transferase A3 knockout (represented as KO or -/-) mice has allowed us to study the induction of liver cancer in mice by aflatoxin B1. In contrast to the induction of hepatocellular carcinomas in other species, aflatoxin B1 induces cholangiocarcinomas in GSTA3-/- mice. In other species and in knockout mice, the induction of liver cancer is preceded by extensive proliferation of small oval cells, providing additional evidence that oval cells are bipolar stem cells and may give rise to either hepatocellular carcinoma or cholangiocarcinoma depending on the nature of the hepatocarcinogen and the species of animal. The recent development of mouse oval cell lines in our laboratory from aflatoxin B1-treated GSTA3-/- mice should provide a new venue for study of the properties and potential of putative mouse liver stem cells.

Theories of carcinogenesis: an emerging perspective

Four decades ago Leslie Foulds remarked that "Experimental analysis has produced an alarming mass of empirical facts without providing an adequate language for their communication or effective concepts for their synthesis". Examining the relevance of the data avalanche we all generate and are subjected to in the context of the premises and predictions of the current cancer theories may help resolve this paradox. This goal is becoming increasingly relevant given the looming attempts to rigorously model and parameterize crucial events in carcinogenesis (microenvironmental conditions, cellular proliferation and motility), which will require the adoption of reliable premises on which to base those efforts. This choice must be made a priori, as premises are not testable, and data are not free of the theoretical frame used to gather them. In this review we provide a critical analysis of the two main currents in cancer research, one centered at the cellular level of biological organization, the somatic mutation theory, which conceptualizes carcinogenesis as a problem of cell proliferation control, and the other centered at the tissue level, the tissue organization filed theory, which considers carcinogenesis a process akin to organogenesis gone awry.

Oral administration of paclitaxel affects the distribution and metabolism of 2-aminofluorene in various tissues of Sprague-Dawley rats

To evaluate the question of whether or not paclitaxel affects the distribution and metabolism of chemical carcinogens such as 2-aminofluorene (AF) on Sprague-Dawley rats were examined. The AF, acetylated AF and AF metabolites were determined and examined by using high performance liquid chromatography. After having received AF only, AF with paclitaxel at the same time and paclitaxel pretreated for 24 h then treated with AF for 24 h, urine, stool and tissues such as liver, kidneys, stomach, colon, bladder and blood were collected and assayed for AF and its metabolites. Compared to the control group, paclitaxel caused an increase of the metabolites excreted in urine and stool. The major metabolite excreted in urine and stool was 9-OH-AAF. The liver is the major metabolism center and the major residual metabolite of AF in the liver was also 9-OH-AAF.

Short- and intermediate-term carcinogenicity testing--a review. Part 1: the prototypes mouse skin tumour assay and rat liver focus assay

Carcinogenicity testing is by far the most expensive and time-consuming study type of toxicology. For many years, the lifetime exposure with the maximum tolerated dose in two rodent species has been the gold standard of carcinogenicity testing of pharmaceuticals. Major change was introduced by the Fourth International Conference on Harmonization in July 1997; a chronic rodent bioassay in one species and a short-term carcinogenicity assay are regarded as sufficient for registration. Such requirements provide the opportunity to redirect the vast resources previously spent on the lifetime study in the second species. Numerous experimental protocols for short- and intermediate-term carcinogenicity testing in many target tissues have been available for years. The first part of this review describes the basic principles of short- and intermediate-term carcinogenicity testing using the examples of the widely used mouse skin tumour assay and the rat liver foci assay. In the context of these experimental models, the discrimination and quantification of initiating and promoting activity and the use of preneoplastic lesions as endpoints in carcinogenicity testing are described. The review includes the limitations of the models with regard to the extrapolation from effects observed in animal experiments to a potential exposure of humans.

Mortality and cancer incidence among secondary lead smelter workers

OBJECTIVES

To examine the mortality pattern and the cancer incidence in a cohort of long term smelter workers exposed to lead.

METHODS

The cohort consists of 664 male lead battery workers, employed for at least three months in 1942-87. From 1969 the values of all blood lead samples repeatedly obtained from these workers every two to three months, have been collected in a database. The expected mortality and morbidity 1969-89 was estimated from the county rates, specified for cause, sex, five-year age groups, and calendar year. Individual exposure matrices have been calculated and used for dose-response analyses.

RESULTS

The total cohort showed an increased overall mortality (standardised mortality ratio (SMR) 1.44; 95% confidence interval (95% CI) 1.16-1.79), an increased mortality from ischaemic heart diseases (SMR 1.72; 95% CI 1.20-2.42) and all malignant neoplasms (SMR 1.65; 95% CI 1.09-2.44). These risk estimates were unaffected or slightly decreased when applying a latency period of 15 years, and no dose-response pattern was shown. The non-significantly raised cancer incidence in the gastrointestinal tract (11 malignancies) in the total cohort, increased to a barely significant level in the quartile with the highest cumulative lead exposure (standardised incidence ratio (SIR) 2.34, 95% CI 1.07-4.45). No clear dose response pattern was evident when further subdividing the data into those first employed up to 1969 v those first employed after 1969 when the blood lead monitoring programme started. The risk estimate for malignancies in the gastrointestinal tract was not related to latency time. The cancer incidence was not increased at other sites.

CONCLUSIONS

A slightly increased incidence of gastrointestinal cancers was found in workers exposed to lead and employed before 1970. The lead cohort also showed an increased mortality from ischaemic heart diseases. These risk estimates did not show a dose-response pattern and were not associated with latency time. The results must also be interpreted with caution because of limited numbers, and lack of dietary and smoking data.

Modulation of P-450 IIC7 and IIIA1,2 mRNA in pre-neoplastic liver. Effect of promotion by phenobarbital

P-450 IIC7 and IIIA2 mRNAs are constitutively expressed in the hepatic tissue under developmental control. Both forms--as well as IIIA1, 90% homologous to IIIA2 mRNA--display positive modulation by phenobarbital a prototype inducer of the liver monooxygenases and a strong promoter of experimental chemical hepatocarcinogenesis. In the present work the variations in the concentration of these P-450 mRNA were studied in rats submitted to the hepatocarcinogenic protocol of Solt and Farber. We demonstrate that a decrease in the relative concentrations of P-450 IIC7 and IIIA1, 2 mRNA is set up along the tumor promotion stage. Animals--starting the experimental carcinogenic protocol at pubertal age--show a partial inhibition of the physiological expression of P-450 IIIA1,2 mRNA associated to male sex maturation. Administration of phenobarbital results in an acceleration of the pre-neoplastic process which is concomitant with an induction of P-450 IIC7 as well as IIIA1,2 at the earlier promotion stages. P-450 mRNA concentration markedly decreases as the preneoplastic process develops. While an impaired P-450 IIIA1,2 mRNA relative abundance is observed, an inversion of the modulation of P-450 IIC7 as well as of the male phenotype marker alpha-2u-globulin mRNA arises as the tumor promotion stage progresses, both mRNA becoming repressed in response to phenobarbital.

Carcinogenicity of polyhalogenated biphenyls: PCBs and PBBs

Polychlorinated biphenyls (PCBs) and polybrominated biphenyls (PBBs) are compounds whose physical/chemical properties led to their widespread commercial use. Although their production has been banned or severely limited in most countries since the 1970s, the persistence and stability of these compounds have resulted in a worldwide distribution, especially of PCBs. PBB contamination is limited principally to the state of Michigan, where a series of tragic errors eventually resulted in the accumulation of residues in livestock and the general human population. Long-term exposure to PCBs and PBBs in animals has been associated with the induction of neoplastic nodules in the liver and in some cases hepatocellular carcinoma. This review discusses the role of PCBs and PBBs in the process of carcinogenesis. The mutagenicity/genotoxicity of these compounds, as well as their initiation/promotion potential is discussed. The epidemiology of PCB and PBB exposure is reported along with an estimation of the risk of cancer to humans. Finally, possible molecular mechanisms of action are suggested for polyhalogenated biphenyls in cancer development.

Preneoplastic rat liver cells in vitro: slow progression without promoters, hormones, or growth factors

In order to determine early changes in liver cells during carcinogenesis and to compare them with normal or neoplastic hepatocytes, an experimental model was established which allowed enrichment of this population at early stages of carcinogenesis and provided sufficient viable material for biochemical and cytogenetic analysis. This paper describes a method that allows in vitro selection and propagation of hepatocytes after in vivo initiation by alkylating agents, without the use of hormones, growth factors, or promoters which might affect their progression. From 6 different rat livers (5 initiated by continuous diethylnitrosamine feeding, 1 by a single exposure to N-methyl-N-nitrosourea) we established slow-growing lines, each of which had its own typical characteristics of growth behavior, morphology, and chromosome number. One of these lines (CL 38) transformed spontaneously after 8 weeks in primary culture, with an abrupt change to typical tumor cell behavior such as focal growth, anchorage independence, cloning ability in soft agar, and tumorigenicity in nude mice and newborn rats. In none of the other lines (now in culture for 11-15 months) has a similar abrupt change yet been observed, but all of them show a steady, albeit slow progression towards the properties of neoplastic liver cells, together with a reduction in chromosome number.

Immunocytochemical localization of benzo(a)pyrene-DNA adducts in human tissue

Specimens of human lung, uterine cervix, ovary, and placenta were studied for the presence of benzo(a)pyrene 7,8-diol 9,10 epoxide (BPDE)-DNA adducts by using rabbit anti-BPDE-DNA antibody and light microscopic immunocytochemistry. BPDE-DNA antigenicity was detected in the bronchial epithelial cells, cervical epithelium, oocytes, luteal cells, corpora albicans, and hyalinized media of arteries within the ovaries and trophoblastic cells of the placental villi. In conjunction with immunoassay detection of BPDE-DNA adducts in human peripheral blood lymphocytes, this study demonstrates that a variety of human tissues can metabolize and bind the ubiquitous carcinogen benzo(a)pyrene. The identification and localization of this carcinogen-DNA antigenicity in various tissues and cells may not only help in monitoring exposed persons but also give insight to organ site carcinogenesis, transplacental carcinogenesis, and teratogenesis.

Benzo[a]pyrene metabolism and induction of enzyme-altered foci in regenerating rat liver

The metabolism of benzo[a]pyrene (BP) in regenerating rat liver and the induction of enzyme-altered foci (EAF) in the liver of partially hepatectomized rats, treated with BP and promoted with 2-acetylaminofluorene (2-AAF)/CCl4 was investigated. The aim was to examine factors that might be of importance for the tumorigenicity of BP in the regenerating rat liver, such as cytochrome P-450 activity and glutathione levels. In regenerating rat liver, obtained 18 h after partial hepatectomy (PH), the amount of microsomal cytochrome P-450 was reduced by 20% whereas the level of glutathione was elevated by 15% and the cytosolic glutathione transferase activity towards chlorodinitrobenzene and (+/-)-7 beta,8 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydro-BP (BPDE) was unaffected. Microsomes from these animals had a reduced capacity to activate (-)-trans-7,8-dihydroxy-7,8-dihydro-BP (BPD) to DNA-binding products but the pattern of BP metabolites was similar to that observed with control rat liver microsomes. Treatment of rats with 3-methylcholanthrene (MC, 50 mg/kg body wt.) increased cytochrome P-450 levels and glutathione transferase activity towards both substrates. Regenerating livers from these animals retained their cytochrome P-450 level and enzymatic activity towards BP and BPD. Regenerating rat liver microsomes from MC-treated animals were about 35 times more efficient in activating BPD than microsomes from uninduced, partially hepatectomized animals. Intraperitoneal administration of BP (50 mg/kg body wt.) 18 h after PH induced EAF in rats subsequently promoted with 2-AAF/CCl4. Pretreatment of rats with MC 66 h before PH and 84 h before BP administration, increased the number of EAF. In accordance with results by Tsuda et al. (Cancer Res., 40 (1980) 1157-1164), these studies demonstrate that BP is tumorigenic in regenerating rat liver, despite a reduced ability of the liver to activate this compound. Furthermore, MC, an inducer of certain cytochrome P-450 species ("aryl hydrocarbon hydroxylase"), potentiates the effect of BP.

Immunohistochemical demonstration of decreased L-pyruvate kinase in enzyme altered rat liver lesions produced by different carcinogens

Preneoplastic liver lesions were produced in female Wistar rats by application of 25 mg/kg N-nitrosomorpholine (NNM), 14 mg/kg diethylnitrosamine (DENA), 0.075 mg/kg aflatoxin B1 (AFB1) or 160 mg/kg safrole. These carcinogens were administered in two equal doses 12 and 24 h after partial hepatectomy. The animals then received sodium phenobarbital (0.1% in tap water) for up to 410 days. Numerous altered hepatic foci (AHF) and hyperplastic nodules (HN) were detected enzyme histochemically by their negative ATPase reaction after application of AFB1, DENA and NNM; some AHF and HN were also caused by the weak carcinogen safrole. Immunohistochemically these lesions were also L-pyruvate kinase (L-PK)-negative with a high coincidence with regard to their number and area. These results confirm the role of L-PK, an enzyme affecting the pentose phosphate pathway, as a negative marker of preneoplastic liver lesions.

Morphometric and cytophotometric nuclear analysis of altered hepatocyte foci induced by N-nitrosomorpholine (NNM) and aflatoxin B1 (AFB1) in liver of Wistar rats

The progressive morphological changes in the liver during neoplastic transformation have been studied by histological, cytophotometric and morphometric methods in male Wistar rats treated with two carcinogens: N-nitrosomorpholine (NNM) and aflatoxin B1 (AFB1). Cytophotometric and morphometric analysis of hepatocyte nuclei using Feulgen-stained tissue sections were performed in morphologically normal hepatic parenchyma and in early preneoplastic foci composed of altered hepatocytes. Foci of clear cells, mixed cells and large basophilic cells possessed a ploidy distribution similar to the surrounding non-transformed parenchyma, while the small hyperbasophilic cell foci were predominantly diploid. These findings confirm that the foci composed of PAS-negative, small hyperbasophilic cells with an unique diploid content may represent one of the earliest stages in the neoplastic transformation.

Biochemical and molecular epidemiology of human cancer: indicators of carcinogen exposure, DNA damage, and genetic predisposition

The primary goal of biochemical and molecular epidemiology is to identify individuals at high cancer risk by obtaining evidence of high exposure to carcinogens, leading to pathobiological lesions in target cells, and/or increased oncogenic susceptibility due to either inherited or acquired host factors. This emerging and multidisciplinary area of cancer research combines epidemiological and laboratory approaches. Because DNA is considered to be an important target for modification by mutagens and carcinogens, damage to DNA can be used as an internal, molecular dosimeter of carcinogen exposure. The reactive species of these carcinogens may directly bind to DNA to form adducts and may indirectly cause secondary DNA lesions, e.g., via induction of free radicals and aldehydes. Highly sensitive and specific methods have been developed to measure the minute amounts of DNA lesions and DNA repair products found in biological specimens from humans exposed to carcinogens. For example, DNA adducts have been measured in cells and tissues from people occupationally exposed to carcinogenic polycyclic aromatic hydrocarbons. Antibodies recognizing carcinogen-DNA adducts have also been detected in human sera. Inherited predisposition to cancer has been revealed by recent advances in molecular genetics, including restriction-fragment-length polymorphism. For example, the hypothesis that rare alleles of the Ha-ras proto-oncogene are associated with an increased risk of lung cancer is currently being tested. These approaches afford the potential of biochemical and molecular epidemiology to predict disease risk for individual persons, instead of for populations, and before the onset of clinically evident disease.

Decrease in glucokinase and glucose-6-phosphatase and increase in hexokinase in putative preneoplastic lesions of rat liver

Preneoplastic liver lesions were produced in female Wistar rats by oral administration of 2-acetylaminofluorene for 165 days succeeded by a carcinogen-free standard diet up to 420 days. During the treatment numerous altered hepatic foci (AHF) and hyperplastic nodules (HN) were detected histochemically by a focal decrease or lack of adenosine-5-triphosphatase and glucose-6-phosphatase (G-6-Pase) activities. In addition, the immunohistochemically demonstrable amount of L-type pyruvate kinase was clearly reduced. The histochemically demonstrated decrease of G-6-Pase was substantiated by microbiochemical determination of the enzyme activity in microdissected material. Moreover, during the experimental period a continuous decrease in glucokinase and an increase in hexokinase was detected microbiochemically within AHF and HN. These alterations indicate a shift in the carbohydrate metabolism from gluconeogenesis to glucose utilization and pentose-phosphate-pathway for biosynthesis of nucleic acids. Beside other oncofetal markers, HK may be used as indicator of the early stages of liver carcinogenesis.

On the clonal origin of tumors: a review of experimental models

A number of current models of carcinogenesis postulate that rare events are critical in the formation of malignant neoplasms. The most fundamental prediction of the hypothesis that these events are rare is the clonality of neoplasms. Evidence from spontaneous neoplasms supports the contention that such neoplasms are clonal. However, there are a number of reasons to believe that spontaneous neoplasms are derived from large numbers of cells. The issue can be resolved with experimental cancer models. In order to determine whether experimentally induced neoplasms are derived from the clonal expansion of single cells, a variety of benign and malignant tumors can be induced in mosaic rodents. These animals comprise 2 genetically distinguishable cell lineages. If the neoplastic tissues obtained from mosaic animals are composed entirely of progeny of one or the other of the 2 cell lineages, it may be concluded that they are clonal. If, on the other hand, the neoplasms began from the proliferation of many cells, then neoplastic masses would be expected to contain cells of both lineages. The results from a number of these experiments have led to the conclusion that chemically induced neoplasms are clonal. Furthermore, malignant neoplasms are generally believed to develop in a stepwise manner. If they were derived from a single cell, then each of the stages leading to the formation of the cancer should be clonal. A variety of stages thought to be necessary precursors of cancer have been analyzed in mosaic rodents. These preneoplastic lesions have been determined to be clonal in origin. Thus, theories of carcinogenesis must account for the rarity of the events critical to the formation of cancer.

Histochemical and immunohistochemical detection of putative preneoplastic liver foci in women after long-term use of oral contraceptives

Localized areas with altered enzyme patterns were observed in liver tissue surrounding focal nodular hyperplasia in women after long-term use of oral contraceptives. These localized lesions were of three different types. Type I lesions were characterized by glycogen storage, a reduction in ATPase and an increase in gamma-glutamyltranspeptidase (gamma-GT) and UDP-glucuronyltransferase (UDP-GT) detected immunohistochemically. Type II lesions, which were morphologically very similar to small hyperplastic nodules, showed only a decreased ATPase reaction. Type III lesions showed an increase in gamma-GT (detected histochemically) and a slight reduction in ATPase. The results indicated that in human liver from patients given oral contraceptives long-term, localized lesions with altered enzyme patterns may occur which are very similar to those observed in animal models during experimental hepatic carcinogenesis.

Increased UDP-glucuronyltransferase and gamma-glutamyltranspeptidase in enzyme-altered rat liver lesions produced by low doses of aflatoxin B1

Preneoplastic liver lesions were produced in female Wistar rats by low doses of aflatoxin B1 (Model 1: administration of 37.5 micrograms/kg 12 and 24 h after partial hepatectomy; Model 2: continuous application of 3.5 micrograms/kg in tap water daily for 28 days with partial hepatectomy after 14 days. The animals then received sodium phenobarbital, 0.1% in tap water, for 180 to 400 days). In both models numerous altered hepatic foci (AHF) and hyperplastic nodules (HN) were detected enzyme histochemically by their negative ATPase and positive gamma-glutamyltranspeptidase reactions. Immunohistochemically these lesions were also UDP-glucuronyltransferase positive. Increased UDP-glucuronyltransferase adds to permanent alterations of a number of drug metabolizing enzymes observed in a variety of different tumor models. These alterations are responsible for the toxin-resistant phenotype (Faber 1984b). Increased gamma-glutamyltranspeptidase was detected both enzyme histochemically and immunohistochemically; whereas gamma-glutamyltranspeptidase activity was present in both AHF/HN and in periportal areas by enzyme histochemistry, the immunohistochemical method selectively stained gamma-glutamyltranspeptidase in AHF and HN. Immunohistochemically detectable UDP-glucuronyltransferase and gamma-glutamyltranspeptidase are markers of putative precancerous liver lesions which may be useful in the analysis of the prestages of liver carcinogenesis.

Development of nitrosourea-induced brain tumours--with a special note on changes occurring during latency

Nitrosourea compounds fulfil the requirements of ideal carcinogens, in that they produce a high incidence of tumours selectively and consistently in the nervous system, they induce neoplasms that have morphological and biological similarities to naturally occurring neural tumours in man and animals and they represent an environmental hazard. N-Ethyl-N-nitrosourea (ENU) has a preferential transplacental and neonatal action, a single intravenous or intraperitoneal injection inducing an almost 100% incidence of neural neoplasms. Transplacental carcinogenesis by ENU provides an ideal experimental model in which the sequential development of brain tumours can be studied. Investigations of the early stages of brain carcinogenesis induced by transplacentally administered ENU have shown that: (1) the earliest histologically detectable changes appear in 8-wk-old animals and are composed of stem cells; (2) these early lesions are frequently multiple and become larger with increasing age; (3) they are not distributed at random, but occur in areas in which tumours develop later; (4) the carcinogens endow the cells, even only 2 days after exposure, with a malignant potential which may subsequently become manifest either in vivo or in culture; (5) half-way through latency, malignant cells are present in the brain; (6) the emergence of the malignant phenotype in cultured brain cells is a stepwise process which culminates in the concomitant appearance of tumorigenicity and invasiveness.

Trends in Scandinavian Cell Toxicology

Over a period of two years the Scandinavian Society for Cell Toxicology* has met three times (on 21 October 1983 and 6–7 June 1984 in Uppsala, Sweden, and on 6–7 September 1985 in Roskilde, Denmark), to present research advances in cell toxicology and to discuss the effects of xenobiotics in isolated and cultured cells. The first part of this report represents a summary of these discussions. In the second part, some of the individual research reports presented by the participating members are summarised. The purpose is to give a review of problems currently dealt with in Scandinavian laboratories associated with the Society for Cell Toxicology.

Liver tyrosine kinase activation during early stages of chemical hepatocarcinogenesis

Protein phosphorylation at tyrosine residues is believed to be involved in several important cellular processes because tyrosine-specific protein kinase activation is associated with stimulation of cellular proliferation by hormones and growth factors, embryogenesis, and retroviral cell transformation. Because cell proliferation is thought to be an essential component of chemical carcinogenesis, liver tyrosine-specific protein kinase activity was examined during the early stages of the Solt and Farber chemical hepatocarcinogenesis model. Rats were given diethylnitrosamine in one dose (200 mg/kg, IP) followed by 2 weeks of dietary 0.02% 2-acetylamino-fluorene starting at day 14 after diethylnitrosamine, followed by partial hepatectomy on day 21. By day 32 this regimen produces a relatively synchronized population of hyperplastic liver nodules up to 1.5 mm in diameter. Rats were sacrificed on day 32, their livers were perfused with cold normal saline, homogenized, and centrifuged at 1,000g for 10 min. The resulting supernatant was centrifuged at 30,000g for 30 min and the pellet was assayed for tyrosine kinase activity using the synthetic peptide [Val5]angiotensin II as substrate. Rats that received the complete regimen had a 2.6-fold increase in their liver tyrosine kinase activity as compared to sham controls (2.4 pmoles/min/mg protein vs 6.4 pmoles/min/mg protein, P less than .05). In contrast, rats that received a partial regimen (ie, partial hepatectomy, or 2-acetylaminofluorene + partial hepatectomy, or diethylnitrosamine + 2-acetylaminofluorene) did not have elevated tyrosine kinase activity nor did they have hyperplastic nodules. These preliminary data suggest that activation of liver tyrosine kinase is associated with the very early stages of chemical hepatocarcinogenesis.

Trophic effects on the pancreas of trypsin and bile salt deficiency in the small-intestinal lumen

The effect of trypsin inhibitor-containing diets was studied in rats, mice, and hamsters for 30 weeks. In rats, pancreatic weight, DNA, RNA, and protein increased in response to a diet of raw soya flour (containing trypsin inhibitor). In mice, pancreatic weight, DNA, RNA and protein content increased with the same diet. Only rats developed micro- and macro-nodules. In rats, longer treatment with raw soya flour resulted in further growth in the pancreas, with ultimate development of adenomas and carcinomas of the acinar pancreas. The pancreatic changes were reversible up to 6 months of feeding the raw soya diet, but thereafter became irreversible. Pancreatic growth, similar to that produced by trypsin inhibitor, was also produced by cholestyramine, perhaps as a result of its bile salt-binding properties. We conclude that removal of proteases and bile salts from the upper small intestine results in pancreatic growth, which may become neoplastic.

Influences of various xenobiotic inducers on cytocidal toxicity of lasiocarpine and senecionine in primary cultures of rat hepatocytes

The influences of in vivo pretreatment with phenobarbitone (PB), 3-methylcholanthrene (3-MC), 2,2',4,4',5,5'-hexachlorobiphenyl (HCBP), and 3,3',4,4'-tetrachlorobiphenyl (TCBP) on cytocidal hepatotoxicity of two pyrrolizidine alkaloids, lasiocarpine (LC) and senecionine (SC), were compared in short-term primary cultures of rat hepatocytes. Toxicity was measured by release of lactate dehydrogenase (LDH) into culture medium at 24 h. LC was slightly more toxic to control hepatocytes than SC in the graded response range of 10-160 microM. PB and HCBP (a PB-type polychlorobiphenyl inducer) similarly potentiated toxicity of SC, and each diminished the degree to which cell killing by LC and SC was inhibited by SKF-525-A. By comparison, 3-MC and TCBP (a 3-MC-type PCB inducer) each diminished toxicity of SC but had little effect on toxicity of LC. Alpha-naphthoflavone (ANF) potentiated toxicity of both LC and SC in hepatocytes induced by 3-MC or TCBP but had little effect on responses of hepatocytes induced by either PB or HDBP. These results indicate that xenobiotics that induce similar patterns of cytochrome P-450 isozymes have qualitatively similar modulating influences on cytocidal hepatotoxicity of pyrrolizidine alkaloids in primary cultures. However, the observed modulating effects could not be explained solely on the basis of altered activation rates by the cytochrome P-450 species known to be induced by the various xenobiotics.