Differential induction and regulation of peroxisomal enzymes: predictive value of peroxisome proliferation in identifying certain nonmutagenic carcinogens

Hypolipidemic drugs and certain plasticizers markedly increase the number of peroxisomes in liver parenchymal cells. Continued exposure to peroxisome proliferators has been shown to produce essentially similar pleiotropic responses leading eventually to the development of liver tumors in rats and mice. These agents are not mutagenic in short-term test systems and do not appear to interact with or damage DNA. Accordingly, the events leading to or associated with the induction of peroxisome proliferation have been postulated to play a role in the development of liver tumors. Recent evidence indicates that persistent peroxisome proliferation leads to the formation of 8-hydroxyguanosine in rat liver DNA, which supports the role for oxidative stress. The mRNAs of the three peroxisomal beta-oxidation genes are induced over 20-fold in the livers of rats treated with nafenopin, Wy-14643, BR-931, and other structurally diverse peroxisome proliferators. This increase in beta-oxidation mRNAs is evident within 30 min to 1 hr and was maximal 8 to 16 hr after the administration of a single dose of these agents by gavage. The peroxisomal catalase and urate oxidase mRNAs increase about 2-fold in the livers of rats treated chronically with peroxisome proliferators. These results indicate that peroxisome proliferators differentially regulate different peroxisomal enzymes. The tissue specificity of peroxisomal beta-oxidation gene regulation by xenobiotics supports the contention that the development of liver tumors following exposure to peroxisome proliferators correlates well with the inducibility of peroxisome proliferation and the beta-oxidation genes. Although these agents are known to exert mitogenic response in liver, it is unlikely that stimulation of DNA synthesis alone is responsible for tumor development. Cell proliferation may, however, play a secondary role. The morphological phenomenon of peroxisome proliferation should serve as a simple, sensitive, and valuable biological indicator for the identification of nongenotoxic or nonmutagenic chemicals that may be carcinogenic. An understanding of the cellular and molecular basis of peroxisome proliferation is a prerequisite for the evaluation of toxicological implications of this phenomenon.

Effect of nitroxazepine on bone marrow cells of mice

The antidepressant drug, nitroxazepine, was tested in mice for mutagenicity in bone marrow cells by a micronucleus test. The drug was administered orally at a dose of 4.5, 9, and 13.5 mg per mouse. The results show that the drug is found to be mutagenic in mouse bone marrow cells.

Almost total conversion of pancreas to liver in the adult rat: a reliable model to study transdifferentiation

Study of transdifferentiation provides an excellent opportunity to investigate various factors and mechanisms involved in repression of activated genes and derepression of inactivated genes. Here we describe a highly reproducible in vivo model, in which hepatocytes are induced in the pancreas of adult rats that were maintained on copper-deficient diet containing a relatively non-toxic copper-chelating agent, triethylenetetramine tetrahydrochloride (0.6% w/w) for 7-9 weeks and then returned to normal rat chow. This dietary manipulation resulted in almost complete loss of pancreatic acinar cells at the end of copper-depletion regimen, and in the development of multiple foci of hepatocytes during recovery phase. In some animals, liver cells occupied more than 60% of pancreatic volume within 6-8 weeks of recovery. Northern blot analysis of total RNA obtained from the pancreas of these rats revealed the expression of albumin mRNA. Albumin was demonstrated in these pancreatic hepatocytes by immunofluorescence. The advantages of this model over the previously described models are: a) low mortality (10%), b) depletion of acinar cells, and c) development of multiple foci of hepatocytes in 100% of rats.

Development of hepatocytes in the pancreas of hamsters treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Transdifferentiation is a process in which one differentiated cell type is converted to another. A unique example of transdifferentiation is the development of hepatocytes from pancreatic cells in adult hamsters and rats. In this communication we report the induction of pancreatic hepatocytes in hamsters that were given 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Two or 6 intraperitoneal (ip) injections of TCDD at a dose of 100 micrograms/kg body weight at 4-wk intervals induced pancreatic hepatocytes in 75% and 89% of the animals respectively. In animals given only two doses of TCDD each pancreas contained one to two hepatic foci, whereas when six injections were administered multiple hepatic foci were observed. By hematoxylin and eosin stain and by periodic acid Schiff stain, the pancreatic hepatocytes were morphologically identical to those in normal liver. Although the exact mechanism by which TCDD induces the transformation is not clear, it is conceivable that TCDD acting through receptor-mediated mechanisms is activating the repressed liver-specific genes in the pancreas.

Comparison of constitutive and inducible levels of expression of peroxisomal beta-oxidation and catalase genes in liver and extrahepatic tissues of rat

Previous studies from our laboratories have shown that carcinogenic peroxisome proliferators significantly increase the mRNA levels of peroxisomal beta-oxidation genes in the rat liver by enhancing the transcriptional activity. Because of a good correlation between the inducibility of peroxisome proliferation and carcinogenicity of this class of xenobiotics, we proposed that sustained induction of peroxisomal beta-oxidation system and the resultant oxidative stress form the basis for carcinogenesis. Since this concept implies that tumors should develop only in tissues which display maximal peroxisome proliferation, we have now assessed the degree to which catalase and the three beta-oxidation genes are expressed in liver and 12 extrahepatic tissues of adult rats fed for 2 weeks a diet containing 0.025% ciprofibrate (w/w), a peroxisome proliferator. In the ciprofibrate-treated rats, the levels of catalase mRNA increased to less than 2-fold in liver, kidney, intestine, and heart, but no change was detected in other tissues. The mRNA levels of the three genes of beta-oxidation system in the liver of adult rats treated with ciprofibrate increased greater than 20-fold. In contrast, in the kidney, small intestine, and heart the increases in the mRNA levels of all three beta-oxidation genes were small and varied from 2- to 4-fold following ciprofibrate treatment. Ciprofibrate did not significantly increase the levels of these mRNAs in the other nine tissues. These results correlated well with the levels of peroxisomal beta-oxidation activity, peroxisome volume density, and the immunologically quantified proteins in various tissues. These results provide evidence for the presence of beta-oxidation enzymes in peroxisomes of many tissues of rat and for tissue (cell)-specific differences in the inducibility of mRNAs of these beta-oxidation genes. The marked inducibility of beta-oxidation genes in liver and subsequent development of liver tumors support the hypothesis that tumors develop in tissues that show inducibility of peroxisome proliferation vis a vis beta-oxidation system following exposure to peroxisome proliferators.

Lack of expression of glutathione-S-transferase P, gamma-glutamyl transpeptidase, and alpha-fetoprotein messenger RNAs in liver tumors induced by peroxisome proliferators

Many structurally unrelated nonmutagenic peroxisome proliferators induce altered areas, neoplastic nodules, and hepatocellular carcinomas in rats. Unlike the lesions induced by genotoxic hepatocarcinogens, these lesions do not stain positively for the phenotypic markers gamma-glutamyl transpeptidase (GGT) and glutathione-S-transferase P (GST-P). To ascertain whether the absence of immunocytochemically detectable GST-P and GGT proteins in peroxisome proliferator-induced neoplastic lesions is due to the absence of specific mRNAs, we analyzed the total RNA isolated from hepatocellular carcinomas induced by three different peroxisome proliferators (ciprofibrate, Wy-14643, and BR-931) and the genotoxic carcinogens, 2-acetylaminofluorene and aflatoxin B1 (AFB), for the presence of GST-P, GGT, and alpha-fetoprotein (AFP) mRNAs. Northern and dot blot analysis of total RNA isolated from liver tumors induced by three different peroxisome proliferators revealed no detectable GST-P, GGT, and AFP mRNAs. GST-P mRNA was also not detected in a transplantable hepatocellular carcinoma established from a liver tumor induced by ciprofibrate. In contrast, GST-P mRNA levels were high in primary liver tumors induced by both 2-acetylaminofluorene and AFB and the two transplantable hepatocellular carcinomas established from such tumors. By immunoblot method, GST-P protein was found to be abundant in both primary and transplantable liver tumors induced by genotoxic carcinogens but not in those derived from peroxisome proliferator treatment. The GGT and AFP mRNAs were also not found in all 18 liver tumors induced by peroxisome proliferators that were analyzed and also in the ciprofibrate-derived transplantable liver tumor. The expression of GGT and AFP genes in liver tumors induced by 2-acetylaminofluorene and AFB was variable. These studies with peroxisome proliferators show that the GST-P and GGT gene derepression is not essential for the hepatocarcinogenesis or successful tumor transplantation. Further characterization of the molecular basis for the differential expression, particularly of the GST-P gene in liver tumors, may help identification of the critical event(s) in hepatocarcinogenesis by genotoxic carcinogens and nongenotoxic peroxisome proliferators.

Carcinogenicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in the Syrian golden hamster

The carcinogenic potential of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) was examined in Syrian golden hamsters, at different dose levels. Twenty-one percent of the hamsters that received a total of 600 micrograms/kg body weight of TCDD either by the s.c. or i.p. route developed squamous cell carcinomas of the skin of facial region within 12-13 months from the beginning of the experiment. Neoplasms were not observed in any other organs. These studies suggest that TCDD may be a complete carcinogen in hamsters, the species most resistant to the toxic effect of this compound.

Induction of peroxisome proliferation and hepatic tumours in C57BL/6N mice by ciprofibrate, a hypolipidaemic compound

The hepatic effects of ciprofibrate, a potent peroxisome proliferator, were evaluated in male C57BL/6N mice, a mouse strain with very low incidence of spontaneous liver tumour development. Dietary feeding of ciprofibrate (0.0125% or 0.025% w/w) for 2 weeks resulted in a marked proliferation of peroxisomes (9-fold increase) and several-fold increase (8- to 10-fold) in the activity of peroxisomal beta-oxidation enzymes. Feeding ciprofibrate at 0.025% concentration for 15 months followed by a 0.0125% for 6 months led to the development of hepatic adenomas in 8/14 (57%) and hepatocellular carcinomas (HCC) in 3/14 (21%) mice. In mice given 0.0125% ciprofibrate for 18 months 5 of 8 (62%) and 3 of 8 (37%) developed adenomas and HCC respectively. Similar to the findings observed in rats, both the adenomas and HCC were negative for gamma-glutamyltranspeptidase. These results in C57BL/6N mice of hepatocarcinogenic effect of ciprofibrate, a non-genotoxic chemical, indicate that peroxisome proliferation can be used as a reliable parameter to evaluate the carcinogenicity of hypolipidaemic compounds.

Immunocytochemical localization of liver-specific proteins in pancreatic hepatocytes of rat

Hepatocytes are induced in the pancreas of rats maintained first on a copper-deficient diet for 8 weeks and then on normal rat chow. These cells are morphologically identical to parenchymal cells of the liver. These hepatocytes contain two liver-specific proteins: carbamyl phosphate synthetase I, a mitochondrial matrix protein that participates in the conversion of ammonia to carbamyl phosphate; and urate oxidase, an enzyme that catalyzes the oxidation of uric acid to allantoin. In addition, we also present evidence indicating that dietary administration of ciprofibrate induces peroxisomal beta-oxidation pathway enzymes, while the levels of catalase are unaltered in pancreatic hepatocytes. These observations along with the previously published results further establish the identity of pancreatic hepatocytes to parenchymal cells of liver and clearly indicate that transdifferentiation of pancreatic cells to hepatocytes is associated with activation of several liver-specific genes.

Immunocytochemical localization of urate oxidase, fatty acyl-CoA oxidase, and catalase in bovine kidney peroxisomes

We investigated the localization of urate oxidase, peroxisomal fatty acyl-CoA oxidase, and catalase in bovine kidney by immunoblot analysis and protein A-gold immunocytochemistry, using the respective polyclonal monospecific antibodies raised against the enzymes purified from rat liver. By immunoblot analysis, these three proteins were detected in bovine kidney and bovine liver homogenates. Subcellular localization of these three enzymes in kidney was ascertained by protein A-gold immunocytochemical staining of Lowicryl K4M-embedded tissue. Peroxisomes in bovine kidney cortical epithelium possessed crystalloid cores or nucleoids, which were found to be the exclusive sites of urate oxidase localization. The limiting membrane, the marginal plate, and the matrix of renal peroxisomes were negative for urate oxidase staining. In contrast, catalase and fatty acyl-CoA oxidase were found in the peroxisome matrix. These results demonstrate that, unlike rat kidney peroxisomes which lack urate oxidase, peroxisomes of bovine kidney contain this enzyme as well as peroxisomal fatty acyl-CoA oxidase.

Tissue specificity and species differences in the distribution of urate oxidase in peroxisomes

The localization of urate oxidase in different tissues of rat and in the livers of selected mammalian species was investigated by immunoblot analysis and protein A-gold immunoelectron microscopy. Urate oxidase was purified from rat liver and used as an antigen to generate polyclonal antibodies in the rabbit. The antibodies were found to be monospecific by immunodiffusion and immunoblot analyses. By immunoblot analysis, urate oxidase was detected in the livers of rat, two strains of mice, hamster, dog, cat, and cow, but not in the Cynomolgus monkey and human liver. Urate oxidase was not detected by immunoblot method in rat kidney, jejunal mucosa, adrenal gland, testis, and pancreas. The subcellular localization of urate oxidase was ascertained by the protein A-gold immunocytochemical staining of the Lowicryl K4M embedded tissues. Urate oxidase was localized exclusively in the crystalloid core of the peroxisome in hepatic parenchymal cells of rat, mouse, hamster, dog, cat, and cow. The limiting membrane and the matrix of hepatic peroxisomes in these species were negative for the staining. The marginal plates of feline, canine, and bovine hepatic peroxisomes were also negative for urate oxidase. This enzyme was also not detected within the peroxisomes of human and monkey livers by the immunocytochemical technique. Peroxisomes (microperoxisomes) in extrahepatic rat tissues did not stain positively for urate oxidase by the protein A-gold immunocytochemical method, although they were positive for catalase. Fatty acyl-CoA oxidase was present in peroxisomes of jejunal mucosa, Leydig cells of test-is and pancreas but not in adrenal gland. Administration of a hepatic peroxisome proliferator, ciprofibrate or Wy-14643, failed to induce urate oxidase in rat liver. These results indicate that urate oxidase is a liver specific protein in rat and its localization within the liver peroxisomes of six mammals, excluding man and a nonhuman primate, and that its localization is limited exclusively to the crystalloid core. Unlike fatty acyl-CoA oxidase, urate oxidase does not appear to be inducible significantly by peroxisome proliferator treatment in the rat liver.

Animal models of exocrine pancreatic carcinogenesis

In order to understand the evolution, histogenesis, and biological behaviour of exocrine pancreatic carcinoma, some reproducible experimental models have been developed in certain rodent species. To date, more than 16 chemicals, many of them structurally unrelated, have been shown to induce pancreatic tumors. Although some of these chemicals appear species specific in their effect on the pancreas, others have been shown to be capable of inducing pancreatic tumors in more than one species. In hamsters, the administration of diisopropylnitrosamine or its oxidized metabolites leads to the development of ductal adenocarcinomas that histologically resemble human pancreatic carcinomas. The histogenesis of the ductal type of adenocarcinoma in hamsters is complex, and appears to involve both the duct cells and dedifferentiated acinar cells. All pancreatic tumors in rats develop from acinar cells showing variable degrees of differentiation, regardless of the type of carcinogen used. The type of pancreatic lesions that develop in mice are also of acinar cell origin. In guinea pigs the tumors are adenocarcinomas of the ductal type and are shown to be derived from dedifferentiated acinar cells that have undergone duct-like transformation. Irrespective of the type of tumor that develops in these experimental animals, all of these models can be successfully used to evaluate the various modifying (risk) factors and biological behaviour of these neoplasms.

Absence of gamma-glutamyltranspeptidase activity in lung metastasis in rats with hepatocellular carcinomas induced by ciprofibrate, a peroxisome proliferator

The incidence of lung metastasis in rats with hepatocellular carcinomas (HCC) induced by ciprofibrate, a peroxisome proliferator and the expression of gammaglutamyl transeptidase (GGT) in the metastatic lesions was studied. HCC were induced in 75 male F-334 rats by chronic dietary administration of ciprofibrate (0.025% w/w) for 16-22 months. The incidence of lung metastasis was 25% in rats killed between 14 and 16 months which increased to 56% in rats killed between 20 and 22 months. The lung metastases were multifocal and present in both the blood vessels and parenchyma. All the metastatic foci examined for the expression of GGT by histochemical stain were devoid of this enzyme. The results of this study clearly demonstrate the malignant behavior of ciprofibrate-induced liver tumors and the absence of GGT expression in metastatic lesions a phenotypic property that is peculiar to the primary HCC induced by several peroxisome proliferators.

Cytogenetic effects of amitriptyline hydrochloride in somatic and germ cells of mice

Amitriptyline hydrochloride, an antidepressant drug, was tested in mice for mutagenic effects in the somatic cells by the micronucleus test and in the germ cells by the air drying technique of Evans et al. Mice were treated orally with the drug at a dose of 70, 140 and 210 mg/kg body weight. The results indicate that the drug is capable of inducing mutations both in the mitotic and meiotic cells of mice.

Absence of gamma-glutamyl transpeptidase activity in neoplastic lesions induced in the liver of male F-344 rats by di-(2-ethylhexyl)phthalate, a peroxisome proliferator

Male F-344 rats were fed a diet containing 2% di-(2-ethylhexyl)phthalate (DEHP) for 95 weeks. Liver nodules and/or hepatocellular carcinomas (HCC) developed in 6/10 rats fed DEHP and none were found in controls (P less than 0.005 by chi 2 test). All the nodules and HCC were negative for gamma-glutamyl transpeptidase. In the non-tumorous portions of liver, the hepatocytes contained an increased number of peroxisomes and extensive accumulation of lipofuscin. By immunocytochemical analysis, the liver peroxisomes in rats treated chronically with DEHP had visually detectable decrease in the H2O2-degrading catalase and increase in H2O2-producing fatty acyl-CoA oxidase. These results show that higher dietary level of DEHP, which causes substantially greater degree of peroxisome proliferation than the 1.2% dietary level used in the National Toxicology Program bioassay (1982, Publication no. NTP-80-37, Tech. Report Series No. 217), can induce liver tumors in male rats.

Coordinate polypeptide expression during hepatocarcinogenesis in male F-344 rats: comparison of the Solt-Farber and Reddy models

The Solt-Farber resistant hepatocyte (RH) and Reddy (dietary peroxisome proliferator) hepatocarcinogenesis protocols were utilized to induce both preneoplastic and neoplastic nodules in male F-344 rats. Total cellular polypeptides from normal liver, ciprofibrate (CP)-induced and RH nodules were analyzed for both qualitative and quantitative changes using computer-assisted, high resolution two-dimensional polyacrylamide gel electrophoresis. Approximately 800-1000 cytosolic and 1000-1200 particulate polypeptides were readily separated and detected using an ultrasensitive silver stain. The two-dimensional polyacrylamide gel electrophoresis patterns were very similar for each tissue with respect to both the number of polypeptides detected and the overall patterns. Three cytosolic polypeptides, E, 6.90/47; F, 6.90/46; and G, 6.50/28 (designated pI/Mr X 10(-3], and two particulate polypeptides, B, 5.90/43; and D, 5.70/21; were detected in CP nodules but not in normal liver. Polypeptides B and D were also detected in RH nodules. No qualitative polypeptide differences were detected among the individual preneoplastic or individual neoplastic CP nodules or between preneoplastic and neoplastic CP nodules. Numerous quantitative changes in both known markers for hepatocarcinogenesis and in as yet unidentified polypeptides were noted. In RH nodules the Ya subunit of glutathione-S-transferase B (GST-B) and the Yb subunit of GST-A were increased 2-4-fold as compared to normal liver or in replicating liver following a 70% partial hepatectomy, while in CP nodules the Yb subunit was unaltered and the Ya subunit increased 4-fold as compared to normal. The Yp subunits of GST-P were increased from almost nondetectable levels in normal liver to one of the most abundant cytosolic polypeptides in RH nodules. In contrast, the Yp subunits were not detected in any of the CP nodules either on the two-dimensional polyacrylamide gel electrophoresis gels themselves or following Western transfer and immunoblot analysis with antibody against GST-P. Two additional polypeptide spots, which may represent Yc charge shift variants, appeared at the same molecular weight as the constitutively expressed Yc subunit of GST-B but shifted one charge unit each toward the acidic region in CP nodules. DT-diaphorase which was increased 2-3-fold in RH nodules was unaltered in CP nodules. In addition to these changes in known markers, 34 (22 cytosolic and 12 particulate) polypeptides were significantly increased while 27 (12 cytosolic and 15 particulate) polypeptides were decreased during CP-induced hepatocarcinogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification, properties, and immunocytochemical localization of human liver peroxisomal enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase

A molecular understanding of genetic disease in which peroxisomal functions are impaired depends on analysis of the structure of normal and mutant enzymes of peroxisomes. We report experiments describing the isolation, characterization, and immunocytochemical localization of enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase bifunctional enzyme (PBE) of the peroxisomal fatty acid beta-oxidation system from normal human liver and compared it with that of rat liver enzyme. The human enzyme, purified approximately equal to 2300-fold by ion-exchange chromatography, is homogeneous as judged by NaDodSO4/PAGE. This PBE is localized exclusively in the matrix of peroxisomes in liver cells by the protein A/gold immunocytochemical method. The human PBE is similar to rat enzyme in size (Mr, approximately equal to 79,000), isoelectric point (pI, 9.8), pH optima, molecular structure as observed by rotary shadowing, and peptide pattern on NaDodSO4/PAGE after proteolytic digestion with Staphylococcus aureus V8 protease. The human and rat enzymes differed in their immunological properties by having partial identity with each other; this is reflected in their slightly dissimilar composition of the amino acids aspartic acid, threonine, glutamic acid, tyrosine, and glycine. COOH-terminal amino acid were similar for both the enzymes: -Gly-Ser-Leu-Ile-COOH. These results suggest that the human and rat liver PBE may be different in their amino acid sequences at their antigenic sites.

Pancreatic acinar cell regeneration following copper deficiency-induced pancreatic necrosis

Rats maintained on a diet deficient in copper for up to 8-10 weeks exhibit marked necrosis and depletion of acinar cells in the pancreas. When these animals are subsequently fed a copper-supplemented diet, foci of hepatocyte differentiation emerge by about 12 weeks. The present study deals with pancreatic changes during copper depletion and determines the extent of regenerative capacity of the pancreas during the early phases of copper supplementation. During the period of copper depletion, the pancreas gradually decreased in size, and by eight weeks of deficiency weighed approx. 30% as much as the control pancreas. Light-microscopic examination showed focal necrosis of acinar cells at 4 weeks, loss of lobular architecture by six weeks and loss of 85-90% of acinar tissue by eight weeks. Regeneration of the pancreas was initiated by feeding copper-supplemented diet at the end of eight weeks of copper deficiency. The wet weight of the pancreas increased gradually, and by 17 days it weighed 50% more than on the first day of regeneration. Mitotic activity was observed mainly in the acinar cells, beginning at 24 h and reaching a maximum of 19 +/- 1/1000 acinar cells at 72 h, and decreasing steadily thereafter. [3H]Thymidine autoradiography showed a labeling index of 51 +/- 1.7/1000 acinar cell nuclei at 24 h, reaching a peak value of 261 +/- 5.5/1000 acinar cells at 96 h. By 17 days, pancreatic regeneration was only partially complete; however, the proliferative activity was still persistent, albeit at a slower pace. Morphometric analysis of the pancreas on the 17th day of regeneration showed that acinar tissue occupied only 37% of the volume, as compared to 84% in the normal pancreas. These studies clearly demonstrate that acinar tissue of the pancreas following copper deficiency is capable of regeneration, but that the recovery of the pancreas is only partial. Additional studies are necessary to establish the role of this early acinar cell regeneration in pancreatic hepatocyte differentiation.

Tannic acid-induced nucleolar changes in hepatocytes transplanted into syngeneic or xenogeneic host and in hepatocytes maintained in primary culture

In this study we have investigated the effect of a single dose of tannic acid, administered s.c., on the nucleolar ultrastructure of hepatocytes transplanted into a syngeneic or xenogeneic host in order to evaluate the validity of our hepatocyte transplantation system as an in vivo alternative to the use of whole animals to test for species and strain differences to the effects of hepatotoxins. Within 4-6 h following tannic acid injection, rat hepatocytes transplanted into the anterior chamber of eye and inguinal fat pads of rat and athymic nude mouse, showed changes of nucleolar components, with separation of ribonucleoprotein containing granules into discrete dark zones. These dark areas were surrounded by light areas consisting of granular and fibrillar components of the nucleolus. These changes were identical to tannic acid-induced nucleolar alterations in the homotopic liver. Hamster and rat hepatocytes xenotransplanted into athymic nude mice also displayed prominent nucleolar alterations in response to tannic acid. The similarity and extent of nucleolar alterations observed in transplanted hepatocytes and the in situ homotopic liver cells attest to the usefulness of the hepatocyte transplantation system for the evaluation of species differences in biological response to toxic/carcinogenic effects of xenobiotics.

Atypical acinar cell lesions of the pancreas in mice induced by 4-hydroxyaminoquinoline-1-oxide

The carcinogenic effect of 4-hydroxyaminoquinoline-1-oxide (4-HAQO) on exocrine pancreas of mice after a single intravenous (i.v.) injection was investigated. At a dose of 24 mg/kg body weight, 100% of the mice killed at the end of 40 weeks had developed atypical acinar cell foci (AAF) in the pancreas. Each pancreas contained an average of 18.3 +/- 2.7 AAF. Cells in AAF were arranged as acini and their cytoplasm contained eosinophilic zymogen granules. The nuclei were large, round to oval, basally located and contained 1-2 prominent nucleoli. Cells in AAF showed a markedly increased mitotic activity with an index of 4 +/- 0.7/1000 cells as compared to a value of 0.25 +/- 0.25 in the acinar cell population of control pancreas. Autoradiographic studies showed a labeling index of 23 +/- 3 in AAF as compared to only 1.5 +/- 0.29 in the pancreatic acinar cells of controls. These findings indicate that 4-HAQO induces pancreatic lesions in mice which are quite similar to the acidophilic foci observed in rat pancreas and may serve as another useful model for studies of pancreatic carcinogenesis.

Peroxisome proliferator-induced hepatocarcinogenesis: levels of activating and detoxifying enzymes in hepatocellular carcinomas induced by ciprofibrate

It is generally held that altered areas, neoplastic nodules and hepatocellular carcinomas (HCC) induced by mutagenic chemical carcinogens are resistant to the effects of hepatotoxins. This characteristic is attributed to the marked decrease in activating (phase I) enzymes and a several-fold increase in detoxifying (phase II) enzymes. In previous studies, we have shown that hepatic neoplastic lesions induced by non-mutagenic peroxisome proliferators differed from mutagenic carcinogen-induced lesions by lacking gamma-glutamyl transpeptidase and the placental form of glutathione S-transferase. In this study we have examined ciprofibrate-induced HCC for phase I and phase II enzymes. These tumors showed a marked decrease in cytochrome P-450 (53%), cytochrome b5 (79%) and aryl hydrocarbon hydroxylase (55%) activities compared to normal livers. Interestingly, activities of phase II enzymes in these tumors, such as UDP-glucuronyltransferases and sulfotransferases were decreased or remained the same as in the normal livers. In addition, the activity of epoxide hydrolase was also decreased markedly in all peroxisome proliferator-induced HCC. The decrease in the activity of various enzymes appears not to be due to the direct effect of ciprofibrate, since no inhibitory effect was observed after adding this compound in vitro. These findings further amplify the differences between the hepatic lesions induced by mutagenic hepatocarcinogens and non-mutagenic peroxisome proliferators suggesting a divergence in the mechanism by which peroxisome proliferators induce liver tumors.

Xenobiotic-induced peroxisome proliferation: role of tissue specificity and species differences in response in the evaluation of the implications for human health

Significant increases in the number of peroxisomes and in the activity of H2O2 generating peroxisomal fatty acid beta-oxidation enzyme system occur in liver parenchymal cells of mice, rats and certain other species exposed to several structurally dissimilar hypolipidemic drugs and certain phthalate-ester plasticizers. These agents, referred to as peroxisome proliferators, are considered as novel hepatocarcinogenic agents in view of their non-mutagenic and non-genotoxic nature. The lack of mutagenicity of these agents combined with consistent coupling of proliferation of H2O2 generating peroxisomes led to the hypothesis that persistent proliferation of peroxisomes serves as an endogenous initiator of neoplastic transformation by inducing oxidative stress. The mechanism by which oxidative stress leads to neoplastic transformation is, however, not clear. This postulated link between peroxisome proliferation and carcinogenicity implies that tumors should develop only in organs displaying profound peroxisome proliferation. Current evidence indicates that maximal peroxisome proliferation is a tissue-specific phenomenon, restricted largely to the hepatocyte. This tissue-specific biological response suggests that interaction of these structurally dissimilar xenobiotics with a receptor(s) might be the mechanism responsible for peroxisome proliferation and the selective increase in the rate of transcription of peroxisomal fatty acid beta-oxidation enzyme genes without significantly affecting the transcriptional rate of peroxisomal marker enzyme catalase gene. Identification and molecular characterization of peroxisome proliferator specific receptor(s) and elucidation of the mechanisms responsible for the differences in interspecies responses will be necessary for the evaluation of the implications for human health, since the carcinogenicity is not directly attributable to the chemical but to the adaptive responses of the host.

Effect of primidone in somatic and germ cells of mice

Primidone, an anti-convulsant drug, was tested in mice for mutagenicity in somatic cells by the micronucleus test and in germ cells by the sperm-head abnormality assay. Mice were treated orally with the drug at doses of 4.37, 8.75 and 13.11 mg/mouse. The results indicate that the drug is capable of inducing mutations both in somatic and germ cells of mice.