The biochemistry of preneoplastic liver: a common metabolic pattern in hepatocyte nodules

Regulation of expression of drug-metabolizing enzymes by oncogenic signaling pathways in liver tumors: a review

Mutations in genes encoding key players in oncogenic signaling pathways trigger specific downstream gene expression profiles in the respective tumor cell populations. While regulation of genes related to cell growth, survival, and death has been extensively studied, much less is known on the regulation of drug-metabolizing enzymes (DMEs) by oncogenic signaling. Here, a comprehensive review of the available literature is presented summarizing the impact of the most relevant genetic alterations in human and rodent liver tumors on the expression of DMEs with a focus on phases I and II of xenobiotic metabolism. Comparably few data are available with respect to DME regulation by p53-dependent signaling, telomerase expression or altered chromatin remodeling. By contrast, DME regulation by constitutive activation of oncogenic signaling via the RAS/RAF/mitogen-activated protein kinase (MAPK) cascade or via the canonical WNT/β-catenin signaling pathway has been analyzed in greater depth, demonstrating mostly positive-regulatory effects of WNT/β-catenin signaling and negative-regulatory effects of MAPK signaling. Mechanistic studies have revealed molecular interactions between oncogenic signaling and nuclear xeno-sensing receptors which underlie the observed alterations in DME expression in liver tumors. Observations of altered DME expression and inducibility in liver tumors with a specific gene expression profile may impact pharmacological treatment options.

Modulation of the expression of the pi class of glutathione S-transferase by Andrographis paniculata extracts and andrographolide

Andrographis paniculata (Ap) is a commonly used herb for traditional medicine in many Southeast Asian countries. In the present study, we investigated the effect of Ap on the expression of the pi class of glutathione S-transferase (GSTP) in rat primary hepatocytes. Hepatocytes were treated with 25 or 50 microg/mL of ethanol or ethyl acetate extracts of Ap (ApEE or ApEAE) or 10 or 20 microM andrographolide, which is the major active diterpene lactone of Ap, for 48 h. ApEE, ApEAE, and andrographolide dose-dependently induced GSTP protein and mRNA expression. In a GST activity assay, GST activity was significantly higher in cells treated with the maximum concentrations of ApEE, ApEAE, and andrographolide than in control cells (P<0.05). The pTA-2713 luciferase reporter construct containing rat GSTP enhancer 1 (GPE1) was transiently transfected into Clone 9 liver cells. Cells treated with ApEE, ApEAE, and andrographolide showed a dose-dependent increase in luciferase activity. GPE1 deletion abolished the induction efficiency of Ap. Also, the induction of GSTP expression by Ap was inhibited by wortmannin, which is an inhibitor of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. These results indicate that ApEE, ApEAE, and andrographolide induce GSTP expression. This induction is likely related to the PI3K/Akt pathway, and GPE1, an enhancer element in GSTP promoter, is essential for the induction.

CCAAT enhancer-binding protein alpha suppresses the rat placental glutathione S-transferase gene in normal liver

The rat placental glutathione S-transferase (GST-P), an isozyme of glutathione S-transferase, is not expressed in normal liver but is highly induced at an early stage of chemical hepatocarcinogenesis and in hepatomas. Recently, we reported that the NF-E2 p45-related factor 2 (Nrf2)/MafK heterodimer binds to GST-P enhancer 1 (GPE1), a strong enhancer of the GST-P gene, and activates this gene in preneoplastic lesions and hepatomas. In addition to the positive regulation during hepatocarcinogenesis, negative regulatory mechanisms might work to repress GST-P in normal liver, but this remains to be clarified. In this work, we identify the CCAAT enhancer-binding protein alpha (C/EBPalpha) as a negative regulator that binds to GPE1 and suppresses GST-P expression in normal liver. C/EBPalpha binds to part of the GPE1 sequence, and the binding of Nrf2/MafK and C/EBPalpha to GPE1 is mutually exclusive. In a transient-transfection analysis, C/EBPalpha activated GPE1 in F9 embryonal carcinoma cells but strongly inhibited GPE1 activity in hepatoma cells. The expression of C/EBPalpha was specifically suppressed in GST-P-positive preneoplastic foci in the livers of carcinogentreated rats. A chromatin immunoprecipitation analysis showed that C/EBPalpha bound to GPE1 in the normal liver in vivo but did not bind in preneoplastic hepatocytes. Introduction of the C/EBPalpha gene fused with the estrogen receptor ligand-binding domain into hepatoma cells, and subsequent activation by beta-estradiol led to the suppression of endogenous GST-P expression. These results indicate that C/EBPalpha is a negative regulator of GST-P gene expression in normal liver.

Keap1-dependent proteasomal degradation of transcription factor Nrf2 contributes to the negative regulation of antioxidant response element-driven gene expression

Keap1 is a negative regulator of Nrf2, a bZIP transcription factor that mediates adaptation to oxidative stress. Previous studies suggested this negative regulation is a consequence of Keap1 controlling the subcellular distribution of Nrf2. We now report that Keap1 also controls the total cellular level of Nrf2 protein. In the RL34 non-transformed rat liver cell line, Nrf2 was found to accumulate rapidly in response to oxidative stress caused by treatment with sulforaphane, and the accumulation resulted from inhibition of proteasomal-mediated degradation of the bZIP protein. By heterologously expressing in COS1 cells epitope-tagged Nrf2 and an Nrf2DeltaETGE mutant lacking the Keap1-binding site, in both the presence and absence of Keap1 we demonstrate that Nrf2 is subject to ubiquitination and proteasomal degradation independently of both Keap1 and the redox environment of the cell. In oxidatively stressed cells, this is the sole mechanism responsible for Nrf2 degradation. However, under homeostatic conditions Nrf2 is subject to a substantially more rapid mode of proteasomal degradation than it is in oxidatively stressed cells, and this rapid turnover of Nrf2 requires it to interact with Keap1. Within Nrf2, the N-terminal Neh2 domain is identified as the redox-sensitive degron. These data suggest that Keap1 negatively regulates Nrf2 by both enhancing its rate of proteasomal degradation and altering its subcellular distribution.

Activation of mouse Pi-class glutathione S-transferase gene by Nrf2(NF-E2-related factor 2) and androgen

The Pi-class glutathione S-transferases (GSTs) play pivotal roles in the detoxification of xenobiotics, carcinogenesis and drug resistance. The mechanisms of regulation of these genes during drug induction and carcinogenesis are yet to be elucidated. Recently, Nrf2 (NF-E2-related factor 2; a bZip-type transcription factor) knockout mice were shown to display impaired induction of Pi-class GST genes by drugs. It is known that the mouse Pi-class GST gene GST-P1 is expressed predominantly in the male liver, and is regulated by androgen. To determine whether Nrf2 and the androgen receptor regulate GST-P1 directly, we analysed the molecular mechanism of activation of this gene by these factors. The promoter of the GST-P1 gene was activated markedly by Nrf2 in transient transfection analyses. Gel mobility shift assay and footprinting analyses revealed three Nrf2 binding sites: one at the proximal and two at distal elements, located at positions -59, -915 and -937 from the cap site. The fifth intron of the GST-P1 gene contains the androgen-responsive region. Multiple androgen receptor binding sites are clustered within a 500 bp region of this intron. The whole fragment contains a minimum of seven androgen receptor binding sites, which collectively display strong androgen-dependent enhancer activity. However, on division into small fragments containing two or three elements each, individual enhancer activities were dramatically decreased. This suggests that multiple elements work synergistically as a strong androgen-responsive enhancer. Our findings indicate that Nrf2 and the androgen receptor directly bind to and activate the mouse GST-P1 gene.

1 alpha,25-Dihydroxyvitamin D(3) inhibits rat liver ultrastructural changes in diethylnitrosamine-initiated and phenobarbital promoted rat hepatocarcinogenesis

The active metabolite of vitamin D, 1 alpha,25-dihydroxyvitamin D(3)[1,25(OH)(2)D(3)] has been receiving increasing attention and has come to the forefront of cancer chemoprevention research as being a regulator of cellular growth, differentiation and death. In the present study, attempts have been made to investigate the in vivo chemopreventive effect of 1,25(OH)(2)D(3) in two-stage rat liver carcinogenesis. Hepatocarcinogenesis was initiated with a single intraperitoneal injection of diethylnitrosamine [DEN] (200 mg/kg b. wt.) at week 4. After a brief recovery period of 2 weeks, all the DEN-treated rats were given phenobarbital (0.05%) in the basal diet and continued thereafter till the completion of the experiment. The results of our experiment showed that the rats which received 1,25(OH)(2)D(3) for 14 weeks (0.3 microg/100 microL propylene glycol, per os, twice a week), starting the treatment 4 weeks prior to DEN injection, exhibited maximum protective effect in maintaining the normal cellular architecture of the hepatocytes than the group of rats which received this micronutrient for only 9 weeks. Moreover, continuous supplementation of 1,25(OH)(2)D(3) maintains the concentration of hepatic microsomal cytochrome P-450 like that of normal vehicle control. Thus, long-term supplementation of 1,25(OH)(2)D(3) significantly (P < 0.001) inhibits hepatic cytosolic lipid peroxidation, thereby protecting the cell membranes from free-radical mediated damage. These results suggest that 1,25(OH)(2)D(3) is useful in the inhibition of rat liver carcinogenesis.

Risk assessment for possible carcinogens: a critical look

Our overall understanding of mechanisms of toxicology in relation to human disease, with prevention of disease as a major objective, depends in part on the development of an adequate number of ways to assess risks, both short term and long term. Despite the cost, the long duration of the test, and some pitfalls, the long-term animal tests remain, to date, the only reliable assay for possible carcinogens. Recent work has concentrated increasingly on the development of short-term tests to replace the long-term tests. Such a development would be most welcome from several points of view. To date, a variety of approaches have been or are being used. These include (1) activation to an alkylating agent with DNA as the most important target, generating possible mutations in DNA and DNA damage with or without repair, (2) induction of cell proliferation, at least a few cycles, with DNA synthesis as the major target, again favoring mutations, and (3) decrease in cell-to-cell communication (gap-junctional intercellular communication) as a supposed test for promotion. None of these proposed assays are reliable indices for possible carcinogenic effects of chemicals or other agents; the scientific basis for this negative conclusion is discussed.

NAD(P)H:Quinone oxidoreductase activity is the principal determinant of beta-lapachone cytotoxicity

beta-Lapachone activates a novel apoptotic response in a number of cell lines. We demonstrate that the enzyme NAD(P)H:quinone oxidoreductase (NQO1) substantially enhances the toxicity of beta-lapachone. NQO1 expression directly correlated with sensitivity to a 4-h pulse of beta-lapachone in a panel of breast cancer cell lines, and the NQO1 inhibitor, dicoumarol, significantly protected NQO1-expressing cells from all aspects of beta-lapachone toxicity. Stable transfection of the NQO1-deficient cell line, MDA-MB-468, with an NQO1 expression plasmid increased apoptotic responses and lethality after beta-lapachone exposure. Dicoumarol blocked both the apoptotic responses and lethality. Biochemical studies suggest that reduction of beta-lapachone by NQO1 leads to a futile cycling between the quinone and hydroquinone forms, with a concomitant loss of reduced NAD(P)H. In addition, the activation of a cysteine protease, which has characteristics consistent with the neutral calcium-dependent protease, calpain, is observed after beta-lapachone treatment. This is the first definitive elucidation of an intracellular target for beta-lapachone in tumor cells. NQO1 could be exploited for gene therapy, radiotherapy, and/or chemopreventive interventions, since the enzyme is elevated in a number of tumor types (i.e. breast and lung) and during neoplastic transformation.

Genetic properties for the suppression of development of putative preneoplastic glutathione S-transferase placental form-positive foci in the liver of carcinogen-resistant DRH strain rats

The post-initiation stage of hepatocarcinogenesis was investigated in carcinogen-resistant inbred DRH rats and the parental strain, carcinogen-sensitive Donryu rats. Male rats at 5 weeks of age from both strains were treated with N-nitrosodiethylamine (200 mg/kg i.p.) followed by feeding with a diet containing 3'-methyl-4-dimethylaminoazobenzene (3'-Me-DAB) from 2 weeks later and were then subjected to partial hepatectomy at 1 week later. At 8 weeks after the start of treatment, the mean area occupied by glutathione S-transferase placental form (GST-P)-positive lesions was about 30% in Donryu rats but less than 4% in DRH rats despite the presence of comparable numbers of foci in the livers of both strains. These observations suggested that clonal expansion of GST-P-positive foci in DRH rat liver was significantly suppressed under these conditions. Furthermore, this genetic property was dominantly inherited in the F1 rats by crosses of DRH and carcinogen-sensitive inbred F344 rats; that is, the induction of GST-P mRNA in the livers of F344 x DRH F1 rats was dominantly suppressed after administration of 3'-Me-DAB for 8 weeks as compared with parental F344 rats under the same conditions. We compared the intrinsic properties related to growth potential of liver cells between adult DRH and Donryu rats. DRH rat liver showed retarded and/or reduced DNA synthesis after partial hepatectomy or a single i.v. injection of lead nitrate and lower activity of telomerase induced by 3'-Me-DAB administration for 1 week, as compared with the Donryu rat liver. The intrinsic properties observed in this study may be related, at least in part, to the low incidence of liver tumors induced by hepatocarcinogens in DRH rats.

Inhibition of early-phase exogenous and endogenous liver carcinogenesis in transgenic rats harboring a rat glutathione S-transferase placental form gene

Hepatocarcinogenesis initiated with N-nitrosodiethylamine (DEN) and that initiated by feeding of a choline-deficient, L-amino acid-defined (CDAA) diet were compared in transgenic male Wistar rats harboring a rat glutathione S-transferase placental form (GST-P) gene (GST-P-Tg rats) and non-transgenic (N-Tg) rats. Eight-week-old GST-P-Tg and N-Tg rats were administered DEN intraperitoneally at 100 mg/kg body weight, subjected to a selection procedure with 2-acetylaminofluorene and CCl4, and killed at the end of weeks 5 and 12. Other groups were fed the CDAA diet for 12 weeks and killed. Five weeks after the DEN treatment, numbers and sizes of gamma-glutamyltransferase (GGT)- or GST-P-positive lesions and 8-hydroxyguanine (8-OHG) levels in the livers were significantly less in GST-P-Tg rats than in N-Tg rats. The lesion numbers were unchanged between the ends of weeks 5 and 12 in GST-P-Tg rats, but decreased in N-Tg rats. The lesion sizes were increased in GST-P-Tg rats, but unchanged in N-Tg rats. While the proliferating cell nuclear antigen labeling indices (PCNA L.I.) in and surrounding the lesions were decreased, more prominently in GST-P-Tg rats than in N-Tg rats, the 8-OHG levels were also decreased but similarly in both cases. After 12 weeks on the CDAA diet, the lesion incidences, numbers and sizes, 8-OHG levels, PCNA L.I. in and surrounding the lesions, and liver injury were significantly less in GST-P-Tg rats than in N-Tg rats. These results indicate that insertion of a rat GST-P transgene alters the early phase of exogenous and endogenous rat hepatocarcinogenesis, presumably due to enhanced detoxification by GST-P expressed both transiently during the initiation and chronically in the altered hepatocyte populations.

Increased skin tumorigenesis in mice lacking pi class glutathione S-transferases

The activity of chemical carcinogens is a complex balance between metabolic activation by cytochrome P450 monooxygenases and detoxification by enzymes such as glutathione S-transferase (GST). Regulation of these proteins may have profound effects on carcinogenic activity, although it has proved impossible to ascribe the observed effects to the activity of a single protein. GstP appears to play a very important role in carcinogenesis, although the precise nature of its involvement is unclear. We have deleted the murine GstP gene cluster and established the effects on skin tumorigenesis induced by the polycyclic aromatic hydrocarbon 7, 12-dimethylbenz anthracene and the tumor promoting agent 12-O-tetradecanoylphorbol-13-acetate. After 20 weeks, a highly significant increase in the number of papillomas was found in the GstP1/P2 null mice [GstP1/P2(-/-) mice, 179 papillomas, mean 9.94 per animal vs. GstP1/P2(+/+) mice, 55 papillomas, mean 2.89 per animal, (P < 0.001)]. This difference in tumor incidence provides direct evidence that a single gene involved in drug metabolism can have a profound effect on tumorigenicity, and demonstrates that GstP may be an important determinant in cancer susceptibility, particularly in diseases where exposure to polycyclic aromatic hydrocarbons is involved, for instance in cigarette smoke-induced lung cancer.

Changes in drug-metabolizing enzymes of rats in ciprofibrate-induced hepatic nodules

1. Premalignant rat liver nodules produced in the resistant hepatocyte model, by exposure to carcinogenic chemicals (diethyl nitrosamine and 2-acetamidofluorene), and partial hepatectomy, exhibit decreased xenobiotic hydroxylase activities and increased conjugase activities, which are considered responsible for increased resistance to xenobiotic toxicity. 2. However, premalignant rat liver nodules generated by feeding the hypolipidaemic, peroxisomal proliferating drug, ciprofibrate, in a hypolipidaemic model, exhibit decreased hydroxylase activities but decreased conjugase activities also. 3. It is considered that reactive oxygen species (ROS) are generated in both the resistant hepatocyte model and in the hypolipidaemic model, resulting in lipid peroxidation, loss of haem, cytochromes and hydroxylase activities. 4. However, whereas there is a rebounding compensation of conjugase enzymes in the resistant hepatocyte model, this does not occur with the hypolipidaemic model, as peroxidation is probably persistent and the conjugases are continuously destroyed.

Transcriptional and post-transcriptional mechanisms can regulate cell-specific expression of the human Pi-class glutathione S-transferase gene

Previous studies from this laboratory have identified transcriptional mechanisms that are utilized to increase expression of the human glutathione S-transferase gene GSTP1 in a multidrug-resistant derivative (VCREMS) of the human mammary carcinoma cell line MCF7 [Moffat, McLaren and Wolf (1994) J. Biol. Chem. 269, 16397-16402]. The data presented here provide strong evidence that post-transcriptional mechanisms can also play an important role in determining cell-specific expression of the GSTP1 gene. GSTP1 mRNA levels were shown to be elevated 3.1-fold in the human bladder carcinoma cell line EJ compared with VCREMS cells. Despite this observation, transient transfection assays revealed a decreased rate of GSTP1 promoter activity in EJ cells. Indeed, GSTP1 transcriptional repressor activity, mediated by a region located between nucleotides -105 and -86 (as we have previously described in MCF7 cells), was observed in EJ cells. However, in contrast with our results in MCF7 cells, the EJ repressor activity did not displace the essential nuclear complex bound to the C1 promoter element (-73 to -54) in vitro. In addition, competition experiments indicated that an AP-1-like protein is an integral component of the C1-bound complex in EJ cells. Interestingly, experiments utilizing actinomycin D to inhibit transcription demonstrated significantly greater stability of GSTP1 mRNA in EJ cells than in VCREMS cells. These findings suggest that cell-specific differences in the rates of GSTP1 mRNA decay provide the predominant mechanism responsible for elevated expression of the GSTP1 gene in EJ cells.

Aryl sulfotransferase IV deficiency in rat liver carcinogenesis initiated with diethylnitrosamine and promoted with N-2-fluorenylacetamide or its C-9-oxidized metabolites

Down regulation of aryl sulfotransferase IV (AST IV) in promotion/progression of liver carcinogenesis by N-2-fluorenylacetamide (2-FAA) has been established. This study examined whether the C-9 oxidized metabolites of 2-FAA, which have recently been shown to promote diethylnitrosamine (DEN)-initiated liver carcinogenesis in male Sprague-Dawley rats, effect the above change. Hence, in DEN-initiated rats, the effects of promoting regimens of 9-OH-2-FAA or 9-oxo-2-FAA, 15 oral doses at 50 and 100 mumol/kg of body weight, were compared to those of 2-FAA at 50 mumol/kg of body weight and of the vehicle on the activity of N-hydroxy(OH)-2-FAA sulfotransferase (ST), an isozyme of AST IV and AST IV expression and distribution. Relative to the vehicle, treatment with the fluorenyl compounds led to decreased levels in hepatic N-OH-2-FAA ST activity and development of hepatic nodules and tumors which had still lower levels of the ST activity than the respective remnant livers. At approximately 8 months after treatment with the C-9-oxidized compounds at doses twice that of 2-FAA, the extents of decreases in the hepatic N-OH-2-FAA ST activity and cytosolic AST IV protein in tumors were comparable to those with 2-FAA. Immunocytochemical analysis showed close association of AST IV deficiency with neoplastic liver lesions. In comparison to N-OH-2-FAA, 9-OH-2-FAA had only low and 9-oxo-2-FAA lacked sulfate acceptor activity in the presence of male rat liver cytosol or AST IV. At 3.3-fold greater concentration than N-OH-2-FAA, 9-oxo-2-FAA inhibited (27%) the sulfate acceptor activity of N-OH-2-FAA in the presence of AST IV, which suggested interference by 9-oxo-2-FAA at the active site. Although the C-9-oxidized compounds do not appear to be substrates for N-OH-2-FAA ST, their ability to cause a decrease in N-OH-2-FAA ST activity and protein similar to that of 2-FAA supports their role in hepatocarcinogenesis. Whereas 9-OH-2-FAA had a 3.9-fold greater sulfate acceptor activity in the presence of female than male rat liver cytosol and inhibited dehydroepiandrosterone ST activity of female rat liver, N-OH-2-FAA and 9-oxo-2-FAA inhibited estrone ST activity of male rat liver, suggesting that the C-9-oxidized compounds as well as N-OH-2-FAA are substrates for STs other than AST IV.

Molecular and genetic events in schistosomiasis-associated human bladder cancer: role of oncogenes and tumor suppressor genes

Carcinoma of the urinary bladder is the most common malignancy in many tropical and subtropical countries and is mainly due to endemic schistosomal infection. Schistosomiasis-associated bladder cancer defines a characteristic pathology and cellular and molecular biology that differs from urothelial carcinoma of non-schistosomal origin. N-Nitroso compounds are suspected etiologic agents in the process of bladder cancer induction during schistosomiasis. Elevated levels of DNA alkylation damage have been detected in schistosome-infected bladders and are accompanied by an inefficient capacity of DNA repair mechanisms. Consequently, high frequency of G --> A transition mutations were observed in the H-ras gene and at the CpG sequences of the p53 tumor suppressor gene. Genetic changes have also been detected in the c-erbB-1 and c-erbB-2 oncogenes and in the cdkn2 and Rb tumor suppressor genes. The potential application of these mutational patterns in providing a biological marker suitable for the biomonitoring and early detection of this neoplasm could indicate new avenues of approach that might alleviate the problem in the future. It can also assist in elucidating the mechanisms by which schistosomiasis augments human bladder cancers.

Sp1-mediated transcriptional activation of the human Pi class glutathione S-transferase promoter

Previous studies in this laboratory have identified an essential AP-1 recognition sequence (C1 region; -69 to -63) in th human Pi class glutathione s-transferase (GSTP1) promoter and a negatively acting regulatory element (-105 to -86) that acts to suppress GSTP1 transcription in the human mammary carcinoma cell line, MCF7 (1). The data presented here further delineate the functional characteristics of the GSTP1 promoter by examining the significance of two potential binding sites for the transcription factor, Sp1 (-57 to -49 and -47 to -39). The introduction of mutations within these Sp1-like elements and the use of Sp1 antisera in electrophoretic mobility shift assays demonstrated that Sp1 was bound to this region of the GSTP1 promoter in three different cell lines, MCF7, VCREMS, and EJ. Moreover, these in vitro studies indicated that only one of the two putative Sp1 response elements was utilized. Transient transfection assays using GSTP1 promoter constructs that incorporated mutations of the Sp1 elements clearly demonstrated that binding of Sp1 to the GSTP1 promoter was absolutely required for optimal levels of GSTP1 transcription. In particular, disruption of the distal Sp1 recognition motif (-57 to -49) markedly reduced GSTP1 promoter activity in each cell line, thus indicating preferential binding of Sp1 to the distal site. However, insertion of the repressor binding site (-105 to -86) into these constructs suggested that Sp1 was not involved in mediating the suppressive effects of the GSTP1 transcriptional repressor in MCF7 cells, because inhibition of Sp1 binding did not alleviate repressor activity. Therefore, these studies provide strong evidence that Sp1 plays a central role in regulating basal levels of GSTP1 transcription.

Anticarcinogenic biological response of Mikania cordata: reflections in hepatic biotransformation systems

The chemopreventive role of an Indian medicinal plant Mikania cordata (Compositae), which is consumed as vegetable and advocated in folk-medicine, has been evaluated through its effects on Phase 1 and 2 of the hepatic drug-detoxifying enzyme system in rats. Although oral administration of a methanolic extract of this plant root (50, 100 or 150 mg/kg for 4, 8 or 12 weeks) has been found to have very little or no effect on hepatic microsomal cytochrome P-450 and cytochrome b5 contents as well as NADPH cytochrome c reductase activity, it afforded a marked induction of uridine diphosphoglucuronyl transferase activities of liver microsomes. The extract also significantly increased the activities of microsomal uridine diphosphoglucose dehydrogenase, reduced nicotinamide adenine dinucleotide (phosphate): quinine reductase and cytosolic glutathione s-transferases with a concomittant elevation in the contents of reduced glutathione. All these effects were found to be dose-dependent and maintained during 12 weeks of the extract treatment. Results of the study clearly indicate that the intracellular contents of active intermediates of various xenobiotics including chemical carcinogens would be reduced by the specific enhancement of drug-detoxifying enzymes in the liver of rats treated with the plant extract.

Overexpression of a cytochrome P-450 of the 2a family (Cyp2a-5) in chemically induced hepatomas from female mice

Chemical hepatocarcinogenesis in female mice, induced by 5,9-dimethyl(7H)dibenzo[c,g]carbazole, leads to the overexpression of a cytochrome P-450 of the 2a family. This protein was identified as Cyp2a-5, by the use of immunoblots obtained from isoelectric focusing gels. This method allowed the distinction of Cyp2a-5 from Cyp2a-4, another mouse liver cytochrome P-450, by taking advantage of their slightly different pI values. The theoretical pI values, determined from the amino acid sequence, were pI 9.91 for Cyp2a-4 and pI 10.01 for Cyp2a-5. Other structurally related forms were not detected. In hepatomas from female mice, only the Cyp2a-5 form was overexpressed (2-3 fold). Male mice showed a weak expression of Cyp2a-4 and Cyp2a-5 in control liver samples and in hepatomas. The expression of both forms was increased more than fivefold upon castration. Pyrazole induces specifically the Cyp2a-5 form. The Cyp2a-5 overexpression was correlated with enhanced microsomal coumarin-7-hydroxylase and testosterone-15 alpha-hydroxylase activities. An immunohistochemical study showed that Cyp2a-4 and Cyp2a-5 are expressed uniformly in female livers, but centrilobularly in male livers. In hepatomas, this localisation is perturbed; in females we observed a focal cell localisation, and the Cyp2a-containing cells were often hypertrophic and polyploid. In hepatomas from male mice, the Cyp2a-containing cells became dispersed. From a comparison with other studies, the Cyp2a-5 overexpression appears to be a general feature of hepatocarcinogenesis in mice.

Possible mechanisms of alteration in the capacities of carcinogen metabolizing enzymes during schistosomiasis and their role in bladder cancer induction

Carcinoma of the urinary bladder is the most common malignancy in many tropical and subtropical countries. There is a well documented association with chronic urinary schistosomal infection, and bladder cancer associated with schistosomiasis is a major cause of morbidity and mortality in the endemic areas. Many factors have been suggested as possible causative agents in schistosome-associated bladder carcinogenesis but theories concerning the possible role of schistosomal infection in altering host metabolism of chemical carcinogens have received most attention. In experimental schistosomiasis there is a common pattern of changes in the activities of several hepatic Phase I and Phase II enzymes. Phase I enzymes show increased activities in the early stages of infection but these activities are reduced to below their preinfection levels in the intermediate and late chronic stages of the disease. The activities of Phase II enzymes are altered in favour of the deconjugation pathways in the later stages of the disease. The possible basic mechanisms that might be involved in such changes during parasitism and their potential role in the induction of bladder neoplasia are discussed.

Inhibition of cell proliferation and glutathione S-transferase by ascorbyl esters and interferon in mouse glioma

Mouse glioma-26 (G-26) cell line established in this laboratory was used in the study. The in vitro effect of ascorbyl esters, viz., ascorbyl-palmitate (As-P), -stearate (As-S) and mouse interferon-alpha/beta (MulFN-alpha/beta) on the glioma cell viability, proliferation and glutathione S-transferase (GST) activity was investigated. Cell viability and proliferation were examined by colorimetric MTT assay and [3H]-thymidine incorporation, respectively. Incubation (24h) of G-26 cells with As-S, As-P or MulFN-alpha/beta, resulted in a dose dependent decrease in cell viability (IC50 = 125 microM As-S; 175 microM As-P and 3.6 x 10(4) U/ml MulFN-alpha/beta) and proliferation (IC50 = 157 microM As-S; 185 microM As-P and 3.6 x 10(4) U/ml MulFN-alpha/beta). A combined exposure to 175 microM As-S and 800 U/ml of MulFN-alpha/beta resulted in a greater than an additive effect on cell viability and proliferation. The inhibition of cell proliferation/viability by interferon was species specific and was observed only with homologous MulFN-alpha/beta, but not with human interferon-alpha lymphoblastoid or human interferon-beta. Ascorbyl esters inhibited cytosolic GST activity (1-50 = 15.0 microM As-S and 28.5 microM As-P) towards 1-chloro-2,4-dinitrobenzene in a dose dependent manner. The apparent Ki values for affinity purified GST, deduced from Dixon plots were 0.95 microM and 2.0 microM for As-S and As-P, respectively. Significant inhibition of GST was also observed in the cytosol isolated from G-26 cells exposed to 300 microM As-S or 800 U/ml MulFN-alpha/beta.

Induction of drug-metabolizing enzymes in human pancreatic cancer and chronic pancreatitis

Chronic pancreatitis and pancreatic cancer have both been linked with occupational exposure to organic chemicals. These chemicals are known to be metabolized within the liver by the cytochrome P-450 family of enzymes, and indeed are able to induce levels of these enzymes as evidence of their interaction. The purpose of this study was therefore to see if these enzyme systems were altered in chronic pancreatitis and pancreatic cancer. Immunocytochemistry of four phase I drug-metabolizing enzymes (cytochromes P-450 IIIA1, P-450 IIE, P-450 IA2, and NADPH cytochrome P-450 oxido-reductase) and one phase II enzyme [glutathione S-transferase (GST) 5-5] was therefore performed on pancreas and/or liver biopsy samples from organ donors and compared with patients with chronic pancreatitis or pancreatic cancer. In samples from donor subjects, the types and levels of drug-metabolizing enzymes in hepatocytes were similar to those seen in pancreatic acinar cells. In material from patients with chronic pancreatitis or pancreatic cancer, cytochrome P-450 enzyme levels were greater in both the liver and the pancreas than those seen in the donor group, while GST levels were unchanged. Islets of Langerhans showed high levels of P-450 IA2 in the donor group, with clear induction of P-450 IIIA1 and NADPH cytochrome P-450 oxidoreductase in patients with chronic pancreatitis but not in the pancreatic cancer group. Levels of GST 5-5 were also induced in the islets. The present findings raise the possibility of an aetiological relationship between elevated levels of drug-metabolizing enzymes and the subsequent development of disease.

Inducibility of gamma-glutamyltransferase by dexamethasone in rat liver: relationship with the cytochrome P-450 content

Gamma-glutamyltransferase (GGT) inducibility by dexamethasone (DEX) in rat liver decreased by about 95% within the first 14 days of life, while the liver content of cytochrome P-450 (P-450) increased by about 500%. Cobaltic Protoporphyrin IX (CPP), given on the 9th day of life, caused a temporary depression of the P-450 liver content, with maximal effects 3 and 4 days after the administration of CPP. GGT induction by DEX was significantly higher in CPP-treated rats than in untreated ones, with maximum induction coinciding with the maximal decrease of P-450. These effects were CPP dose-dependent.

Fumonisins: isolation, chemical characterization and biological effects

The fumonisin B mycotoxins (FB1 and FB2) have been purified and characterized from corn cultures of Fusarium moniliforme strain MRC 826. Fumonisin B1 (FB1), the major fumonisin produced in culture, has been shown to be responsible for the major toxicological effects of the fungus in rats, horses and pigs. Recent investigations on the purification of compounds with chromatographic characteristics similar to FB1 have led to the identification of two new fumonisins, FB3 and FB4. Fumonisins A1 and A2, the N-acetyl derivatives of FB1 and FB2 respectively, were also purified and shown to be secondary metabolites of the fungus. Short-term carcinogenesis studies in a rat liver bioassay indicated that over a period of 15 to 20 days, at dietary levels of 0.05-0.1%, FB2 and FB3 closely mimic the toxicological and cancer initiating activity of FB1 and thus could contribute to the toxicological effects of the fungus in animals. In contrast, no biological activity could be detected for FA1 under identical experimental conditions. These studies and others have indicated that the fumonisin B mycotoxins, although lacking mutagenicity in the Salmonella test or genotoxicity in the DNA repair assays in primary hepatocytes, appear to induce resistant hepatocytes similar to many known hepatocarcinogens.

Membrane biochemistry and chemical hepatocarcinogenesis

Biochemical membrane alterations appearing during the process of chemical carcinogenesis are described. Emphasis is put on membrane composition, structure, and biogenesis. In this presentation the knowledge gained from experimental studies of liver and skin in the process of cancer development is acknowledged. Important biochemical changes have been reported in lipid composition, fatty acid saturation, constitutional enzyme expression, receptor turnover and oligomerization. Functional consequences of the altered membrane structure is discussed within the concepts of regulation of cell proliferation, regulation of membrane receptor expression, redox control, signal transduction, drug metabolism, and multidrug resistance. Data from malignant tumours and normal tissue are addressed to evaluate the importance of the alterations for the process and for the eventual malignant transformation.

Ethoxyquin-induced resistance to aflatoxin B1 in the rat is associated with the expression of a novel alpha-class glutathione S-transferase subunit, Yc2, which possesses high catalytic activity for aflatoxin B1-8,9-epoxide

A purification scheme has been devised for two ethoxyquin-inducible Alpha-class glutathione S-transferases (GSTs) which possess at least 25-fold greater activity towards aflatoxin B1 (AFB1)-8,9-epoxide than that exhibited by the GSTs (i.e. F, L, B and AA) that have been described previously. These two enzymes are both heterodimers and both contain a subunit of Mr 25,800. This subunit has been isolated from both of the GST isoenzymes and, after cleavage with CNBr, it has been subjected to automated amino acid sequencing. The primary structure of the Mr 25,800 subunit revealed that it forms part of a subfamily of Alpha-class GSTs which possess closest identity (about 92%) with the Yc subunit of apparent Mr 27,500, which is encoded by the recombinant cDNA clone pGTB42 [Telakowski-Hopkins, Rodkey, Bennett, Lu & Pickett (1985) J. Biol. Chem. 260, 5820-5825]. As these two GSTs possess less than 70% sequence identity with the Ya1 and Ya2 subunits, both of Mr 25,500, the constitutively expressed Yc subunit of Mr 27,500 has been renamed Yc1 and the ethoxyquin-inducible GST of Mr 25,800 has been designated Yc2. Using this nomenclature, the two GSTs with high activity for AFB1-8,9-epoxide are Ya1Yc2 and Yc1Yc2. Although evidence suggests that induction of Yc2 is responsible for the high detoxification capacity of livers from ethoxyquin-treated rats for AFB1-8,9-epoxide, resistance towards AFB1 may be multifactorial in this instance as dietary ethoxyquin also induces the Ya1, Ya2 and Yc1 subunits about 2.2-, 10.9- and 2.7-fold respectively. Besides the induction of GST by ethoxyquin, activity towards AFB1-8,9-epoxide is also elevated in the livers of neonatal rats and in livers that contain preneoplastic nodules. Western blotting experiments show that Yc2 is not present in hepatic cytosol from adult rats fed on normal diets but is expressed in neonatal rat livers and in the livers of adult rats that contain preneoplastic nodules that have arisen as a consequence of consuming diets contaminated with AFB1.

Differential regulation of cytochrome(s) P450 2B1/2 by phenobarbital in hepatic hyperplastic nodules induced by aflatoxin B1 or diethylnitrosamine plus 2-acetylaminofluorene in male F344 rats

Phenobarbital (PB) is an effective growth stimulator of hepatic hyperplastic nodules developed with diethylnitrosamine and 2-acetylaminofluorene plus partial hepatectomy (the Solt-Farber model), but it does not apparently stimulate the growth of preneoplastic lesions produced with aflatoxin B1 (AFB). Some studies have suggested a correlation between the induction of specific cytochrome P450 enzymes and the tumor promoting effects produced by repeated treatment with PB. To examine this hypothesis further, hepatic hyperplastic nodules were produced with AFB (10 ip doses of AFB, 150 micrograms/kg/day, followed by partial hepatectomy) or by a modified Solt-Farber protocol (DEN/AAF), and the effects of PB on nodule growth and expression of cytochrome(s) P450 2B1 and/or P450 2B2 (P450 2B1/2) were determined. Both treatment protocols (without PB) produced multiple, large nodules within 10-17 weeks of carcinogen administration. These nodules stained intensely for glutathione S-transferase p (GST-p; GST7-7) and gamma-glutamyl transpeptidase (GGT) and weakly for P450 2B1/2. Pentoxyphenoxazone dealkylation activity was decreased to less than 50% of the surrounding tissue levels in both types of nodules. PB treatment of animals with DEN/AAF-induced nodules greatly increased P450 2B1/2 expression in surrounding tissues, whereas most, but not all, nodules were not inducible. Pentoxyphenoxazone dealkylation was increased 31- to 35-fold in surrounding tissue, but it was increased only 2-fold in pooled nodular tissue, relative to untreated control liver. In contrast to the DEN/AAF group, immunohistochemical staining and pentoxyphenoxazone dealkylation in the AFB group demonstrated that P450 2B1/2 was equally inducible in nodular and surrounding tissues. Short-term treatment (5 days) with PB produced a 2-fold increase in the number and total area of GGT-positive nodules in the DEN/AAF group, but it had no significant effect on the number, size distribution, or total area of GGT-positive nodules in the AFB group. All large GGT-positive nodules in the DEN/AAF group were nonresponsive to induction of P450 2B1/2, whereas all of the GGT-positive nodules which were responsive to P450 2B1/2 induction by PB in this group were relatively small. The size and area of AFB-induced GGT-positive nodules was not affected by PB treatment, and P450 2B1/2 in all of these nodules was inducible by PB. Although a causal, inverse relationship between the responsiveness of nodules to PB induction of P450 2B1/2 and their reaction to PB growth stimulation cannot be firmly established, these data are consistent with such a hypothesis.

Keynote address: multidrug resistance: a pleiotropic response to cytotoxic drugs

Tumor cells exposed in tissue culture to one of several different classes of antineoplastic agents, including anthracyclines, vinca alkaloids, epipodophyllotoxins, and certain antitumor antibiotics, can develop resistance to the selecting agent and cross resistance to the other classes of agents. This phenomena of multidrug resistance is generally associated with decreased drug accumulation and overexpression of a membrane glycoprotein. This membrane protein, referred to as P-glycoprotein, apparently acts as an energy-dependent drug efflux pump. Multidrug resistance in human MCF-7 breast cancer cells selected for resistance to adriamycin (AdrR MCF-7) is associated with amplification and overexpression of the mdr1 gene which encodes P-glycoprotein. A number of other changes are also seen in this resistant cell line including alterations in Phase I and Phase II drug metabolizing enzymes. Similar biochemical changes occur in a rat model for hepatocellular carcinogenesis and are associated in that system with broad spectrum resistance to hepatotoxins. The similar changes in these two models of resistance suggests that these changes might be part of a battery of genes whose expression can be altered in response to cytotoxic stress, thus rendering the cell resistant to a wide variety of cytotoxic agents.

The role of glutathione-dependent enzymes in drug resistance

Glutathione and glutathione-dependent enzymes are ubiquitously distributed through nature. These enzyme systems appear to have evolved to protect cells from toxic and mutagenic environmental chemicals. There is now unequivocal evidence demonstrating that these enzymes play a role in chemical resistance in a variety of phylogeny including, bacteria, plants and insects. There is also increasing circumstantial, as well as genetic evidence which indicates that these enzymes are also a determinant in the sensitivity of tumor cells to anticancer drugs, particularly alkylating agents and those drugs whose toxic effects are mediated by free radicals. In this review some of the experimental data which leads to these conclusions is discussed.

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.

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.

Gamma-glutamyltransferase induction by dexamethasone in cytochrome P-450-depleted rat liver

The effects of the cytochrome P-450 depletion by cobaltic protoporphyrin IX on the postnatal glucocorticoid-inducibility of the membrane-bound enzyme gamma-glutamyltransferase have been assessed in the rat liver. Dexamethasone-induced gamma-glutamyltransferase activity in 14-, 28- and 77-day-old rats was high, weak and absent, respectively, and inversely correlated with the physiological cytochrome P-450 activity. In the liver acinus, the enzyme was reexpressed by the zone 1 and zone 2 hepatocytes in suckling rats, substantially only by the zone 1-hepatocytes in just weaned rats. Following cytochrome P-450 depletion, gamma-glutamyltransferase induction by dexamethasone was more rapid, more intense and more extended in the liver acinus, occurring also in the zone 3 hepatocytes in suckling rats, in the zone 2 and a few zone 3 hepatocytes in just weaned rats. Further, the enzyme induction occurred also in adult rats in the zone 1 and in some zone 2 cells. This shows that cytochrome P-450 modulates the extent of hepatic gamma-glutamyltransferase induction by dexamethasone in postnatal rat-hepatocytes. The phenomenon may be consequent on hormone biotransformation changes caused by the cytochrome P-450 depletion.

Preferential over-expression of the class alpha rat Ya2 glutathione S-transferase subunit in livers bearing aflatoxin-induced pre-neoplastic nodules. Comparison of the primary structures of Ya1 and Ya2 with cloned class alpha glutathione S-transferase cDNA sequences

Normal rat liver expresses Ya (Mr 25,500), Yc (Mr 27,500) and Yk (Mr 25,000) Class Alpha glutathione S-transferase (GST) subunits. The Ya-type subunit can be resolved into two separate polypeptides, designated Ya1 and Ya2, by reverse-phase h.p.l.c. In rat livers that possess aflatoxin B1-induced pre-neoplastic nodules, a marked increase is observed in the expression of Ya1, Ya2, Yc and Yk; of these subunits, Ya2 exhibited the greatest increase in concentration. The Ya1 and Ya2 subunits isolated from nodule-bearing livers were cleaved with CNBr, and the purified peptides were subjected to automated amino-acid-sequence analysis. Differences in the primary structures of the two Ya GST subunits were found at positions 31, 34, 107 and 117. These data demonstrate that Ya1 and Ya2 are distinct polypeptides and are the products of separate genes. The amino acid sequences obtained from Ya1 and Ya2 were compared with the cloned cDNAs pGTB 38 [Pickett, Telakowski-Hopkins, Ding, Argenbright & Lu (1984) J. Biol. Chem. 259, 4112-4115] and pGTR 261 [Lai, Li, Weiss, Reddy & Tu (1984) J. Biol. Chem. 259, 5182-5188], which encode rat Ya-type subunits. From these comparisons it appears probable that Ya1 represents the GST subunit encoded by pGTR 261, whereas Ya2 represents the subunit encoded by pGTB 38. It is likely that the over-expression of Ya1 and Ya2 in nodule-bearing livers is of major significance in the acquired resistance of nodules to aflatoxin B1, since previous work [Coles, Meyer, Ketterer, Stanton & Garner (1985) Carcinogenesis 6, 693-697] has shown that the Ya-type GST subunit has high activity towards aflatoxin B1 8,9-epoxide.

Expression of human glutathione S-transferases in Saccharomyces cerevisiae confers resistance to the anticancer drugs adriamycin and chlorambucil

Adaptation and resistance to chemicals in the environment is a critical part of the evolutionary process. As a result, a wide variety of defence systems that protect cells against chemical insult have evolved. Such chemical resistance mechanisms appear to play a central role in determining the sensitivity of human tumours to treatment with chemotherapeutic drugs. The glutathione S-transferases (GST) are important detoxification enzymes whose over-expression has been associated with drug-resistance. In order to evaluate this possibility we have expressed the human Alpha-class and Pi-class GST cDNAs that encode GST B1B1 and GST pi in the yeast Saccharomyces cerevisiae. The expression of GST B1B1 or GST pi resulted in a marked reduction in the cytotoxic effects of chlorambucil, a bifunctional alkylating agent, and an anthracycline, adriamycin. These data provide direct evidence that the over-expression of GST in cells can confer resistance to anticancer drugs.

Poly(A+)RNA levels of growth-, differentiation- and transformation-associated genes in the progressive development of hepatocellular carcinoma in the rat

The development of chemically induced hepatocellular carcinoma in the rat proceeds through a series of premalignant changes that may ultimately progress to a primary malignant tumor. Using the selection technique based on diminished binding of preneoplastic hepatocytes to tissue culture plates precoated with asialofetuin, we have isolated poly(A+)RNA from early preneoplastic foci as well as preneoplastic persistent nodules and primary hepatocellular carcinoma induced by the Solt-Farber protocol in the Fischer rat. The steady-state poly(A+)RNA levels of genes traditionally associated with growth, differentiation and/or transformation were then determined to address the question of their temporal expression in the multistep nature of cancer development. Ornithine decarboxylase- and P53-specific transcripts did not significantly change in preneoplastic foci but were increased in later-stage preneoplastic nodules and hepatocellular carcinoma. Albumin-specific transcripts were decreased in all hepatocellular carcinoma but there was no consistent coordinated increase in alpha-fetoprotein-specific transcripts. c-myc and raf transcripts increased at the very early preneoplastic foci stage and continued to increase throughout the neoplastic process. No L-myc or N-myc transcripts could be detected in any RNA sample. c-Ha-ras-specific transcripts were essentially unaltered in all RNA samples whereas no c-Ki-ras or N-ras transcripts could be detected throughout the neoplastic process. In addition, no dominant-acting transforming mutations in the ras gene family were detected by DNA transfection experiments using NIH/3T3 cells.

Effects of non-genotoxic hepatocarcinogens phenobarbital and nafenopin on phenotype and growth of different populations of altered foci in rat liver

Non-genotoxic hepatocarcinogens share the ability to induce liver growth in rodents. Phenobarbital (PB), as one prototype compound, promotes the development of liver tumors; altered cell foci of the clear-eosinophilic phenotype, also identified by gamma-glutamyltransferase expression, appear to be precursor lesions. These foci seem to over-respond to the growth-inducing effect of PB. In contrast, the question as to whether peroxisome inducers are also tumor promoters is still unsettled. We will present evidence which strongly suggests that the peroxisome inducer, nafenopin (Naf), promotes tumor development in rat livers by stimulating selective growth of a hitherto undescribed subtype of altered foci. This subtype is characterized by weak diffuse cytoplasmic basophilia of its hepatocytes. Initiation in rats by aflatoxin B1 followed by promotion with Naf produced numerous adenomas and carcinomas; their morphology resembled that of the weakly basophilic foci. Both clear-eosinophilic and weakly basophilic foci appear "spontaneously" in the liver of aging rats. Promotion of such lesions by PB-type compounds or peroxisome inducers may explain cancer formation by these non-genotoxic agents.

The glutathione S-transferases: an update

Over the last 15 years, we have passed through an initial period in which multiple forms of GST in various organs and different species were identified and characterized. The focus of current research is to define the role of the numerous isozymes in cell function, to ascertain the relationship between structure and function of different isozymes and to determine how the expression of GST is regulated in different tissues. During these studies, it is expected that new roles for the GST will be proposed, and this family of multifunctional proteins will continue to hold the interest of numerous investigators for many years.

Gamma-glutamyltransferase induction by glucocorticoids in rat liver: age-dependence, time-dependence, dose-dependence, and intralobular distribution

Postnatal responsiveness of rat-liver gamma-glutamyltransferase (GGT) to glucocorticoids (GC) has been defined by investigating: age-dependence, time-dependence, hormonal dose-dependence, and lag-time of the enzyme re-expression; half-life of the induced enzyme activity; dynamics of the enzyme reappearance in the liver tissue. Hydrocortisone-acetate (HC) or dexamethasone (DEX) were administered to the animals starting 1, 2, 3, 4, or 5 d before killing, at the doses of 25 micrograms or 1 microgram/(g b. w. x d), respectively. In 14 d old rats, after a lag-time of about 20 h (DEX) or 30 h (HC), GGT activity progressively increased up to 38 and 31 times the control value, respectively, at 5th d; the enzyme re-expression was linearly hormone dose-dependent; half-life of the induced enzyme activity was about 36 h. In 21 d old rats, GGT re-induction behaved as in 14 d old animals, except that the induced activity was about half that of each correspondent treatment. In 28 d old rats, a very low but significant GGT activity was re-expressed only after hormonal treatments longer than 48 h. In 35 and 77 d old rats, significant GGT activity was never re-induced. GGT was re-expressed in liver parenchyma, with a defined space-course. In 14 d old rats, GGT reappeared first in periportal areas, then in acinar zone 1, finally in acinar zone 2. While the animals were ageing, GGT re-expression occurred to lesser and lesser extents in liver tissues, because of a progressive space-restriction from acinar zones 1 and 2 to zone 1 and finally, in 35 d old rats, to periportal areas. In adults, GGT was re-expressed only by rare hepatocytes in periportal spaces. Acinar zone-3 hepatocytes did never re-express GGT, irrespectively of the animal age. Thus, 2 rat hepatocyte populations could be distinguished (1 responsive, the other unresponsive to GC for GGT re-expression), the relative proportion of which changes in favour of the unresponsive one while the animal ages. Hepatic GGT re-induction by GC, occurring after a long lag-time, does not follow the typical model of hormonal induction. Previous permissive cell changes seem to be required. Hepatocyte-GGT re-expression by GC appears to be inversely correlated with the differentiation level and the cytochrome P-450 amount (activity) of the cell as limiting factors for the triggering of the enzyme induction.

Microsomal cytochrome P-450-linked monooxygenase systems and lipid composition of human hepatocellular carcinoma

The tissues of hepatocellular carcinoma were operatively resected from six patients. All four components of the systems of microsomal cytochrome P-450-linked monooxygenase of the tissues were investigated and compared to those of normal liver tissue. The concentrations of cytochromes P-450, P-420 and b5 were measured optically and the concentrations of all components except cytochrome P-450 were measured by the Western blotting method followed by immunochemical staining. In microsomes of hepatocellular carcinoma tissues, there was as much cytochrome P-450 and other redox components as in the normal liver tissues, but cytochrome P-450 in liver cancer tissues was unstable and easily converted to cytochrome P-420. The specific activities of NADPH- and NADH-ferricyanide and cytochrome c reductase of each sample were also measured. In the microsomes of the cancer tissues, the specific activities were remarkably reduced compared with those of normal liver tissues. The lipid compositions of the microsomes and the phospholipid/cholesterol ratios (w/w) were 13.1 +/- 3.13 in the cancer tissues and 43.0 +/- 6.74 in normal liver tissues. This difference of the lipid composition elucidates the instability of cytochrome P-450 molecules and the inefficiency of the electron transport of cytochrome P-450-linked monooxygenase systems.

Decreased cytosolic levels of the heme binding Z protein in rat hepatocyte nodules and hepatocellular carcinomas

Hepatocyte nodules and hepatocellular carcinomas were induced in male Fischer rats using the resistant hepatocyte model. The immunoreactive cytosolic levels of the heme-binding Z protein (HBP) were reduced by 56% (P less than 0.001; 2-tailed t-test) in early hepatocyte nodules (25 weeks) and hepatocellular carcinomas (10-12 months). This finding is in accordance with the previously reported reduced heme content of hepatocyte nodules and is consistent with the postulated role for HBP in intracellular heme transport and distribution. The immunoreactive levels of the glutathione S-transferase isozymes (GST) which like HBP bind heme, were elevated 2-fold (P less than 0.01) in early and late hepatocyte nodules and were unchanged in hepatocellular carcinomas.

Amplification and increased expression of alpha class glutathione S-transferase-encoding genes associated with resistance to nitrogen mustards

Glutathione-dependent enzymes play a central role in the protection of cells from cytotoxic chemicals and have been implicated in the intrinsic and acquired resistance of tumors to cytotoxic drugs. We have generated a Chinese hamster ovary line resistant to bifunctional nitrogen mustards and in this report have characterized and isolated the protein that represents the major observable phenotypic difference between the drug-sensitive and drug-resistant cell lines. This purified protein is shown to be an alpha class glutathione S-transferase comprising YcYc subunits and possessing a pI value of approximately 8.0. The intracellular level of the Yc subunit is elevated greater than 40-fold in the drug-resistant cell line, which could account for the increase in glutathione S-transferase (RX:glutathione R-transferase; EC 2.5.1.18) activity toward both 1-chloro-2,4-dinitrobenzene and cumene hydroperoxide. Other glutathione S-transferase subunits within this gene family are also elevated. These changes are accompanied by a significant elevation in alpha class mRNA levels. Southern analysis indicates that the genes coding for these proteins are amplified 4- to 8-fold in the drug-resistant cell line. In addition, gamma-glutamyl transpeptidase [(5-glutamyl)-peptide:amino acid 5-glutamyltransferase; EC 2.3.2.2] activity is increased 3.6-fold in the drug-resistant Chinese hamster ovary cell line, which may explain the increase in cellular glutathione level. In this case no gene amplification was seen. These data indicate that gene amplification may be important in drug resistance toward alkylating agents and also that other enzymes in glutathione homeostasis are involved.

Glutathione and related enzyme activity in human lung cancer cell lines

Glutathione levels were measured in 30 human lung cancer lines. Lower levels were detected in cell lines derived from small cell lung cancer specimens compared to non-small cell lines (mean 42 vs. 130 nmol mg-1 protein, P = 0.005). However, no difference were detected between cell lines derived from previously untreated patients, compared to those derived from patients who had received chemotherapy. Non-small cell lines were found to have increased activity of 4 detoxification enzymes compared to small cell lines, although these differences did not reach statistical significance: glutathione transferase activity (69 vs. 36 units, P = 0.137), glutathione reductase (139 vs. 82 units, P = 0.05), gamma-glutamyl transpeptidase (9.39 vs. 3.03 units, P = 0.072) and superoxide dismutase (20 vs. 13.6 units, P = 0.137). As the cell lines exhibit a similar chemosensitivity pattern to that observed in clinical practice, these differences in glutathione and detoxification enzyme levels may prove to be important indicators of intrinsic drug resistance often seen in patients with non-small cell lung cancer.

Morphometric evaluation of hepatocellular proliferative lesions in the rat liver

Classification of rat hepatocellular proliferative lesions can vary between pathologists as the many qualitative histologic criteria have not been satisfactorily evaluated and ranked for prognostic value. Computer-assisted morphometry offers an objective method to evaluate certain cellular features. The Solt-Farber resistant hepatocyte model was used in this study to produce populations of rats with a full range of hepatocellular proliferative lesions. Cellular features within the lesions were then measured morphometrically and the data were analyzed by animal age and by subjective lesion diagnosis. The nuclear/cytoplasmic ratio followed by the cell area and nuclear area were found to be the most important parameters for separating microscopic foci and islands of cellular alteration, an early hyperplastic lesion, from other hepatocellular proliferative lesions. The coefficient of variation, as a relative measure of heterogeneity, increased in a linear manner for cell, nuclear and nucleolar areas as the animals aged and was significantly higher for cell and nuclear area in hepatocellular carcinoma compared to other hepatocellular proliferative lesions. Hepatocyte nodules (representing primarily late hyperplastic lesions) and persistent hepatocyte nodules (lesions with similarities to hepatocellular adenoma) could not be satisfactorily separated within the limits of this study. As these borderline lesions show a continuum of cytologic change, other features, such as architectural change, are necessary to satisfactorily classify them on a subjective basis. An alternative approach is to use discriminant functions derived from morphometric studies.

Decreased expression of cytochrome P-452 in the resistance phenotype characteristic of putative preneoplastic hepatocyte nodules during hepatocarcinogenesis

Hepatocyte nodules, a characteristic early step in the development of liver cancer in rats, has a distinctive resistance phenotype including a large decrease in total cytochromes P-450 and in two isozymes induced by phenobarbital and two by 3-methylcholanthrene. In this study, it has been observed that the nodules show a large decrease in an additional cytochrome P-450, cytochrome P-452, which is very active in the hydroxylation of lauric acid at C-11 and C-12. The decrease in activity of this microsomal cytochrome P-452 is of the same order of magnitude as the decreases in the other cytochrome P-450 components. These observations are consistent with the hypothesis that there is some more basic alteration in the synthesis or availability of heme and that the changes in the activities of the cytochromes P-450 are secondary.

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.