Cell proliferation and tumour promotion by ethinyl estradiol in rat hepatocarcinogenesis

Biochemical analysis of the initial carcinogenic changes that induce preneoplastic and neoplastic cell populations during chemical hepatocarcinogenesis in rats

To analyze the initial carcinogenic changes that induce preneoplastic and neoplastic cell populations in the rat liver, a short-term in vivo promotion assay method was developed. Preneoplastic foci and nodules were quantitated with glutathione S-transferase P-form (GST-P) and γ-glutamyl transpeptidase. Among the four agents tested, benzyl isothiocyanate (BITC) demonstrated the strongest promotor activity, producing very large nodules composed of 2¹⁸ to 2²⁰ cells in the rat liver. In addition, a choline/methionine-deficient (CMD) diet, which strongly inhibits protein synthesis, exhibited lower but distinct promotive activity, giving rise to large nodules composed of 2¹¹ to 2¹³ cells. Based on the collected stereologic and biochemical data as well as the results of DNA microarray analysis, preneoplastic foci and nodules were strongly indicated to grow without cell division. The absence of cell division indicates the absence of mutations in the genetic mechanism, and vice versa; thus, preneoplastic cell induction can be considered nongenetic. Furthermore, the nodules were markedly more susceptible to promoter agents than hepatocytes as to die of necrosis. Based on these experimental findings, neoplastic cell induction was logically deduced to be nongenetic. The present analysis may help improve the knowledge of the "unknowable mechanism of cancer initiation" of rat chemical hepatocarcinogenesis.

Decreased Diethylnitrosamine-induced Liver Preneoplastic Lesions by Estradiol-3-benzoate Treatment

To clarify whether inhibitory effect of estrogen on liver tumor is associated with cell proliferation, we investigated its role in diethylnitrosamine (DEN)-induced rat preneoplastic lesions, with time sequenced manners. F344 male rats (n = 90) were divided into three groups at 5 weeks of age. The mini-osmotic pumps providing a continuous infusion of DEN was implanted into the abdominal cavity of each animal in group 1, 2 and 3 at 6 weeks of age. To see the effect of estrogen, pellet containing 1 or 10 μg of estradiol- 3-benzoate (EB) was implanted subcutaneously in the animals of groups 2 or 3, respectively, one week prior to DEN treatment. Ten animals of each group were euthanized at 10, 14 and 18 weeks after DEN treatment. Liver tissues at each time point were fixed in 10% phosphate-buffered formalin and were processed and embedded in paraffin and 5 μm sections mounted on a silanized slide. Glutathione S-transferase placental form (GST-P) positive foci and 5-bromo-2-deoxyuridine (BrdU) labeling cells were detected at each time point. Area of GST-P positive foci in DEN+EB 1 or 10 μg group was significantly decreased compared to DEN alone at 14 weeks (p < 0.01 or p < 0.05, respectively) an at 18 weeks (p < 0.05 or p < 0.01, respectively). BrdU index in DEN+EB 1 or 10 μg groups was significantly decreased compared to DEN alone at 14 weeks and at 18 weeks (p < 0.01). Taken together, we conclude that EB treatment decrease the DEN-induced liver preneoplastic lesions and this may be associated with decrease of cellular proliferation.

Role of a novel CAR-induced gene, TUBA8, in hepatocellular carcinoma cell lines

Phenobarbital (PB), a nongenotoxic carcinogen, activates the nuclear constitutive active/androstane receptor (CAR), resulting in the transcriptional induction or repression of various hepatic genes. We previously demonstrated that liver tumors developed after chronic PB treatment only when CAR is present. To understand the molecular mechanism of tumor promotion, cDNA microarray analysis was performed. We identified tubulin alpha 8 (TUBA8) as one of the candidate genes that may be involved in liver tumor promotion. Tuba8 mRNA was induced with PB treatment in mouse livers before tumor development as well as in tumor tissues. Because the functions of TUBA8 are unknown in liver, we investigated the effects of TUBA8 gene expression on cell growth, proliferation, and cell migration. Sense or antisense cDNA for Tuba8 was stably transfected into Huh7 and HepG2 cells. Exogenous overexpression of Tuba8 inhibited cell growth and proliferation in Huh7 but not in HepG2 cells, while cell migration was increased in HepG2 cells but not Huh7 cells. These results indicate that TUBA8 can play a role in the regulation of cell growth, proliferation, and cell migration in a cell-specific manner in vitro, suggesting that TUBA8 may contribute to mouse liver tumorigenesis through these functions.

Constitutive active/androstane receptor promotes hepatocarcinogenesis in a mouse model of non-alcoholic steatohepatitis

The nuclear receptor constitutive active/androstane receptor (CAR) acts as a sensor of toxic byproducts derived from the endogenous metabolism and exogenous chemicals. We previously reported that CAR is responsible for exacerbating hepatic injury and fibrosis in a dietary model of non-alcoholic steatohepatitis (NASH) via upregulation of lipid peroxidation. In this study, we investigated the pathological roles of the CAR in the development of hepatocellular carcinoma in NASH model. CAR+/+ and CAR-/- mice were fed methionine- and choline-deficient (MCD) diet after tumor initiation with a single dose of the genotoxic carcinogen diethylnitrosamine (DEN) at 2 weeks of age. Interestingly, the MCD diet dramatically promoted DEN-induced hepatocarcinogenesis in CAR+/+ mice. However, the deletion of CAR leads to a significantly lower tumor incidence and smaller tumor diameter. Hepatocytes of MCD-treated-CAR+/+ mice showed a significantly higher staining frequency of Ki-67, a marker of cell proliferation, and exhibited a higher expression of c-Myc and FoxM1 transcripts compared with MCD-treated CAR-/- mice. Immunohistochemistry revealed the nuclear translocation of CAR thus suggesting that the activation of CAR signaling increased in the hepatocytes of CAR+/+ mice fed MCD diet. In addition, in vitro experiments using the CAR stably expressed cell line with TCPOBOP have suggested that CAR activation directly leads to cell proliferation. Survival was significantly lower in the CAR+/+ mice fed the MCD diet in comparison with the CAR-/- mice. Taken together, these results suggest that CAR may therefore play a critical role in the hepatocarcinogenesis of the murine NASH model via the upregulation of cell proliferation.

Insights into testicular damage induced by ethinylestradiol in rats

The present study was conducted to clarify the mechanisms of testicular toxicity induced by ethinylestradiol using a rat model maintaining testicular testosterone levels. Twelve-week-old male SD rats were implanted subcutaneously with testosterone (800 mg)-filled tubes on the back 2 days before ethinylestradiol treatment, and subsequently administered orally 10 mg/kg/day ethinylestradiol for 4 consecutive weeks. At termination, measurements of hormone levels in serum and the testis, sperm head counts in the testis, weights of genital organs and histopathological examination were performed. Results show that the supply of testosterone alone induced markedly increased serum testosterone levels, slightly decreased testicular testosterone levels, and atrophic Leydig cells. Treatment of rats with ethinylestradiol alone significantly decreased testosterone levels in serum and the testis, sperm head counts, and weights in the testis, epididymis and prostate. Histological features included atrophy of Leydig cells, decreased number of elongated spermatids, degeneration of germ cells, and tubular atrophy. Co-administration of testosterone almost completely prevented the aforementioned changes brought about by ethinylestradiol, except for Leydig cell atrophy. From these results, we attribute testicular toxicity during ethinylestradiol exposure to the suppression of testicular testosterone levels.

The orphan nuclear receptor constitutive active/androstane receptor is essential for liver tumor promotion by phenobarbital in mice

Hepatocellular carcinoma (HCC) is known to progress through a step often called tumor promotion. Phenobarbital (PB) is the prototype of nongenotoxic cacinogens that promote HCC in rodents. The molecular target of PB to elicit the promotion has been the subject of intense investigations over the last 30 years since it was discovered. The nuclear receptor constitutive active/androstane receptor (CAR) is activated by PB as well as by various other xenobiotics such as therapeutic drugs and environmental pollutants. CAR activation results in the transcriptional induction of numerous hepatic genes including those that encode xenobiotic-metabolizing enzymes such as a set of cytochrome P450s. In addition to PB, many CAR activators are nongenotoxic carcinogens, but the role of CAR in liver tumor promotion remains unexplored. Using Car(-/-) mice, we have here examined tumor promotion by chronic treatment with PB in drinking water after tumor initiation with a single dose of the genotoxic carcinogen diethylnitrosamine. None of the Car(-/-) mice developed either eosinophilic foci or advanced liver tumors, whereas all Car(+/+) mice developed HCC and/or adenoma by 39 weeks. The results indicate that CAR is the molecular target of promotion by PB and that activation of this receptor is an essential requirement for liver tumor development.

Cyclin A2 and c-myc mRNA expression in ethinyl estradiol induced liver proliferation

The time-course of c-myc and cyclin A2 mRNA expression was determined in the liver of male Sprague-Dawley rats during transient liver cell proliferation induced by a single dose of ethinyl estradiol (EE), and was compared to that during liver regeneration following two-thirds hepatectomy (PH). Cell proliferation was assessed in terms of 5'-bromodeoxyuridine (BrdU) labeling. EE administration and PH both increased BrdU labeling between 18 and 48 h, with peak values at 18 and 24 h. An early (2 h) increase in BrdU labeling was observed after EE but not PH. Maximal increases in cyclin A2 mRNA levels and BrdU labeling coincided after both EE and PH, and cyclin A2 mRNA expression was proportional to the intensity of the proliferative response. In contrast, the degree of c-myc mRNA expression was similar after EE administration and PH, but the time course was different: c-myc gene expression rose concomitantly with DNA replication after EE, while after PH it increased during the prereplicative phase. This indicates that the pattern of c-myc gene expression in the liver is strongly related to the type of proliferative response.

Xenobiotics and xenoestrogens in fish: modulation of cytochrome P450 and carcinogenesis

As is the case with mammals, an ever-increasing number of cytochromes P450 (CYPs) are being characterized from fish. The focus of work on fish CYPs has been primarily on environmental induction of CYP1A by pollutants such as the polycyclic aromatic hydrocarbons, polychlorinated biphenyls, dioxins and dibenzofurans. This response has been the basis for a sensitive biomonitoring tool of ecosystem health for a number of years. Studies have documented a correlation between CYP1A induction, pollutant levels and tumor incidence, especially in bottom-dwelling species. The rainbow trout has been utilized as a tumor model to document the role of CYP1A modulation in the inhibition or promotion of cancer. Fish are also very responsive to the class of chemicals known as xenoestrogens. Recent evidence is presented documenting the modulation of CYPs by xenoestrogens and their potential role as modulators of the tumor response. In this paper, we summarize the current knowledge concerning the occurrence of CYPs in fish and focus on the role of CYP1A induction in environmental monitoring of various genotoxic carcinogens and in the modulation of cancer in the trout model. Finally, the important class of aquatic pollutants known as xenoestrogens have now been shown to modulate CYP levels perhaps leading to alterations in tumor response or other adverse effects.

3,2'-Dimethyl-4-aminobiphenyl-induced gallbladder carcinogenesis and effects of ethinyl estradiol in hamsters

The effects of ethinyl estradiol (EE) on 3,2'-dimethyl-4-aminobiphenyl (DMAB)-induced carcinogenesis were examined in Syrian golden hamsters. DMAB was subcutaneously injected in corn oil at a concentration of 100 mg/kg once a week for 20 weeks and EE was administered in the diet at a dose of 0.75 ppm throughout the experiment. Some animals were killed at week 20 and all surviving ones were killed at week 50. Gallbladder tumors (adenomas and carcinomas) were induced in 6 of 15 hamsters (40%) in the DMAB + EE group and 5 of 14 (36%) in the DMAB alone group in males, and in 6 of 13 (46%) in the DMAB + EE group and 1 of 8 (13%) in the DMAB alone group in females at week 50. A clearer enhancing effect of EE on DMAB gallbladder carcinogenesis was observed for tumor multiplicity (No./animal) for both sexes; from 0.36 to 0.67 in males and from 0.14 to 0.62 in females. Thus, DMAB was demonstrated to be carcinogenic in the gallbladder of hamsters and EE enhanced this DMAB-induced gallbladder tumorigenesis.

Oxidative dehalogenation of 2-fluoro-17 alpha-ethynyloestradiol in vivo. A distal structure-metabolism relationship of 17 alpha-ethynylation

Metabolic activation to catechols and their oxidation products is variously considered to contribute to the genotoxic, cytotoxic, transforming and tumour-promoting activities of exogenous steroidal oestrogens. 2-Fluoro-17 alpha-ethynyloestradiol (2-FEE2) was synthesized as a prototype of pharmacologically active derivatives of 17 beta-oestradiol which are resistant to metabolic activation in vivo. It possessed high affinity for the rat uterine oestrogen receptor and was oestrogenic in rats. Biliary metabolites of [6,7-3H]2-FEE2 (0.73 mumol/kg, 157 micrograms/kg, i.v.) in female rats were characterized: 87% of the radiolabel was excreted, principally as 2-FEE2 glucuronide, over 6 hr. Although 2-fluoro-17 beta-oestradiol is not metabolized to C-2 oxygenated products in vivo, 2-FEE2 underwent rapid and appreciable oxidative defluorination. 2-Hydroxy-17 alpha-ethynyloestradiol and 2-methoxy-17 alpha-ethynyloestradiol represented, respectively, 8% and 13% of the dose. Fluorination nevertheless restricted C-2 oxygenation to ca. 28% of that which 17 alpha-ethynyloestradiol undergoes in female rats. C-4 oxygenation of 2-FEE2, resulting in catechol formation, occurred but to a lesser extent (ca. 12% of dose). None of the major and identified minor biliary metabolites was a product of metabolic activation at the ethynyl function. A mechanistic rationalization of the long range enhancement by 17 alpha-ethynylation of oxidative defluorination at C-2 is presented.