Gene expression and cell proliferation in rat liver after 2,3,7,8-tetrachlorodibenzo-p-dioxin exposure.

Recently 4 genes (plasminogen activator inhibitor 2, interleukin 1 beta, clone 1, and clone 141) that are transcriptionally or posttranscriptionally responsive to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) have been cloned from a human skin keratinocyte cell line. We determined whether these genes were expressed in the livers of Sprague-Dawley rats following exposure to TCDD and whether there was a relationship between expression and hepatic cell proliferation. TCDD was administered by using a dose loading/maintenance regimen to achieve rapid quasi-steady-state TCDD liver concentrations of 0.03, 30, or 150 ng/g of liver. Gene expression was determined by Northern analysis using polyadenylated mRNA isolated from liver tissue of male and female animals exposed to TCDD for 1 or 14 days and hybridized with the human complementary DNA clone corresponding to one of the four human TCDD-responsive genes. Under low-stringency hybridization conditions, only the expression of clone 1 could be detected. A dose- and time-dependent expression of this gene was observed in the liver of both male and female rats. Expression of clone 1 was not detected in rats subjected to either a two-thirds partial hepatectomy or exposure to a single administration of the hepatic tumor promoters Wy-14643, carbon tetrachloride, or phenobarbital at doses that induce hepatic cell proliferation. Liver:body weight ratios were elevated in rats exposed to the middle and high TCDD doses. Histopathological observation and analysis of serum enzyme levels indicated no evidence of TCDD-induced liver necrosis. Cell proliferation was evaluated immunohistochemically after 7-day 5-bromo-2'-deoxyuridine administration. No increase in total hepatic labeling index was observed for any of the TCDD-exposed treatment groups compared to controls at week 1 or week 2. An increase in the periportal hepatocyte proliferation labeling pattern was observed in TCDD-treated animals. While these results demonstrate that a human TCDD-responsive gene is expressed in the liver of TCDD-treated male and female Sprague-Dawley rats, the expression of this gene is not linked to hepatic cell proliferation or the sex-specific tumor-promoting activity of TCDD.

Brca1 and Brca2 expression patterns in mitotic and meiotic cells of mice.

The mouse homologues of the breast cancer susceptibility genes, Brca1 and Brca2, are expressed in a cell cycle-dependent fashion in vitro and appear to be regulated by similar or overlapping pathways. Therefore, we compared the non isotopic in situ hybridization expression patterns of Brca1 and Brca2 mRNA in vivo in mitotic and meiotic cells during mouse embryogenesis, mammary gland development, and in adult tissues including testes, ovaries, and hormonally altered ovaries. Brca1 and Brca2 are expressed concordantly in proliferating cells of embryos, and the mammary gland undergoing morphogenesis and in most adult tissues. The expression pattern of Brca1 and Brca2 correlates with the localization of proliferating cell nuclear antigen, an indicator of proliferative activity. In the ovary, Brca1 and Brca2 exhibited a comparable hormone-independent pattern of expression in oocytes, granulosa cells and thecal cells of developing follicles. In the testes, Brca1 and Brca2 were expressed in mitotic spermatogonia and early meiotic prophase spermatocytes. Northern analyses of prepubertal mouse testes revealed that the time course of Brca2 expression was delayed in spermatogonia relative to Brca1. Thus, while Brca1 and Brca2 share concordant cell-specific patterns of expression in most proliferating tissues, these observations suggest that they may have distinct roles during meiosis.

Expression of myc, fos and Ha-ras associated with chemically induced cell proliferation in the rat liver.

Events secondary to induced cell proliferation may play a role in the carcinogenic process. These studies investigated the expression of genes associated with growth control in response to two types of cell proliferation stimuli in the livers of male F344 rats. Regenerative hepatocyte proliferation after partial hepatectomy or a single dose of carbon tetrachloride, and mitogenic liver hyperplasia induced by a single dose of phenobarbital or WY-14,643 were assessed by thymidine incorporation and quantitative autoradiography. The expression of myc, fos, and Ha-ras was evaluated by Northern blot analysis of liver derived poly(A)+ mRNA from these same animals. After each treatment, the level of hepatocyte proliferation (labelling index 4-32%) was observed to peak between 24 and 48 h and return to control values by 8 days. In every case, a peak in myc expression was seen between 0.5 and 18 h depending on the proliferative stimulus treatment. A large peak in fos expression was seen at 0.5-2 h but only with the cytotoxic and regenerative proliferative treatments partial hepatectomy or carbon tetrachloride. A broad peak in Ha-ras expression was observed 12 to 36 h after each treatment. These data demonstrate transient expression of these genes following the synchronous induction of hepatocyte proliferation. The increased expression of fos upon treatment with cytotoxicants, but not mitogens, suggests different modes of growth regulation that may be important in understanding the induction of cell proliferation by these two types of agents.

Variation in expression of genes used for normalization of Northern blots after induction of cell proliferation.

Quantitative knowledge of gene expression can provide valuable information for understanding the action of chemicals that alter cell proliferation and cancer. Accurate quantification of mRNA levels requires the normalization of the gene of interest to a gene with transcriptional levels that do not vary through the cell cycle or with a particular treatment. Changes in expression were examined in proliferating or non-proliferating rat liver for three constitutively expressed 'housekeeping' genes commonly used to normalize mRNA levels from Northern blots. In addition, a direct method of quantifying poly(A)+ mRNA by hybridization with a radiolabelled polythymidylate--poly(T)--probe was compared with traditional methods. Hepatocyte cytolethality and a subsequent wave of hepatocyte proliferation were induced in male Fischer-344 rats by treatment with a single gavage dose of carbon tetrachloride. Induced cell proliferation peaked at 48 h after treatment. Expression of the housekeeping genes actin, glyceraldehyde-3-phosphate-dehydrogenase (GAPDH) and albumin, as well as the proto-oncogene H-ras, was determined by Northern blot analysis at times from 0.5 h to 4 days after treatment. Time-dependent changes were observed in the expression of these genes relative to the levels observed in the untreated control animals. Actin expression peaked at 3.4-fold over control and GAPDH expression was increased by 1.9-fold over control. Albumin mRNA levels varied the least, 1.4-fold over control, indicating that this gene is more appropriate than actin or GAPDH for normalization of proto-oncogene expression under experimental conditions that induce cell proliferation in rat liver. The direct quantification of poly(A)+ mRNA using a poly(T) probe was not influenced by the induction of cell proliferation. This method may be useful when the expression of housekeeping genes is affected by treatment.

Identification of four CCCH zinc finger proteins in Xenopus, including a novel vertebrate protein with four zinc fingers and severely restricted expression.

Tristetraprolin (TTP), the prototype of a class of CCCH zinc finger proteins, is a phosphoprotein that is rapidly and transiently induced by growth factors and serum in fibroblasts. Recent evidence suggests that a physiological function of TTP is to inhibit tumor necrosis factor alpha secretion from macrophages by binding to and destabilizing its mRNA (Carballo, E., Lai, W.S., Blackshear, P.J., 1998. Science, 281, 1001-1005). To investigate possible functions of CCCH proteins in early development of Xenopus, we isolated four Xenopus cDNAs encoding members of this class. Based on 49% overall amino acid identity and 84% amino acid identity within the double zinc finger domain, one of the Xenopus proteins (XC3H-1) appears to be the homologue of TTP. By similar analyses, XC3H-2 and XC3H-3 are homologues of ERF-1 (cMG1, TIS11B) and ERF-2 (TIS11D). A fourth protein, XC3H-4, is a previously unidentified member of the CCCH class of vertebrate zinc finger proteins; it contains four Cx8Cx5Cx3H repeats, two of which are YKTEL Cx8Cx5Cx3H repeats that are closely related to sequences found in the other CCCH proteins. Whereas XC3H-1, XC3H-2, and XC3H-3 were widely expressed in adult tissues, XC3H-4 mRNA was not detected in any of the adult tissues studied except for the ovary. Its expression appeared to be limited to the ovary, oocyte, egg and the early embryonic stages leading up to the mid-blastula transition. Its mRNA was highly expressed in oocytes of all ages, and was enriched in the animal pole cytosol of mature oocytes. Maternal expression was also seen with the other three messages, suggesting the possibility that these proteins are involved in regulating mRNA stability in oocyte maturation and/or early embryogenesis.

Levels of myc, fos, Ha-ras, met and hepatocyte growth factor mRNA during regenerative cell proliferation in female mouse liver and male rat kidney after a cytotoxic dose of chloroform.

Chloroform is a liver carcinogen in mice and a kidney carcinogen in rats. It is thought to act through a non-genotoxic-cytotoxic mode of action. Changes in expression of growth control genes accompanying chloroform-induced cytolethality and regeneration may play a part in the development of chloroform-induced tumors. In this experiment, we examined the levels of the myc, fos, Ha-ras, met and hepatocyte growth factor mRNA in livers of female B6C3F(1) mice and kidneys of male F-344 rats to detect changes in gene expression following a single, cytotoxic gavage dose of chloroform in corn oil. Poly A+ RNA was purified from homogenates of livers of mice treated with 350mg/kg chloroform and kidneys of rats treated with 180 mg/kg chloroform and used for Northern blot analysis. Livers of female mice showed large transient increases in levels of myc and fos mRNA while levels of Ha-ras, met and the hepatocyte growth factor gene mRNA remained near control levels. In the male rat kidney, levels of myc mRNA increased after treatement, while levels of mRNA of all other genes examined remained near control levels. This pattern of gene expression is consistent with that induced by other cytotoxic carcinogens and suggest that alteration of the myc and fos genes could be involved in the regenerative cell proliferation that ultimately could play a role in chloroform-induced tumors.

Cell proliferation rates in common cancer target tissues of B6C3F1 mice and F344 rats: effects of age, gender, and choice of marker.

Increasing emphasis is being placed on mode of action for chemical carcinogens as an important consideration for risk assessment. Many rodent carcinogens appear to act through nongenotoxic mechanisms, such as induced cell proliferation. Information on cell proliferation rates based on species, age, gender, tissue, and choice of marker will provide a foundation for incorporating such measurements into rodent toxicity studies. Cell proliferation was evaluated in liver, kidney, skin, and forestomach of control male and female B6C3F1 mice and F344 rats at 7, 10, 13, and 20 weeks of age. Proliferating cell nuclear antigen (PCNA), an endogenous cell proliferation marker, and bromodeoxyuridine (BrdU) administered by ip injection 2 hr before euthanization were compared as markers of cell proliferation. Only in liver were BrdU and PCNA labeling indices (LIs; S phase only) statistically similar. As expected, the PCNA proliferating index (PI; G1 + S + G2 + M phases) was consistently greater than the S phase LI in all tissues examined. Age-related differences in LI were evident in liver and kidney, whereas LIs in the forestomach and skin were not age- dependent. In all tissues examined, gender- and species-related differences in cell proliferation were detected. Although BrdU and PCNA LIs were often statistically different, they both provided a useful indication of cell proliferation rates in the tissues examined. These results provide potentially useful information for designing rodent toxicity studies and biological models of carcinogenesis.

Brca1 is expressed independently of hormonal stimulation in the mouse ovary.

BRCA1 mutations lead to cancer susceptibility in hormonally dependent tissues such as the ovary and breast. To test the hypothesis that Brca1 expression in the ovary is hormonally regulated and specifically regulated by a functional estrogen receptor, we examined its expression by in situ hybridization in ovaries from virgin, pregnant, and lactating mice, in hypophysectomized mice treated with hormones, and in estrogen-receptor-deficient mice. To determine the relationship between Brca1 expression and cell cycle, serial and adjacent sections of ovary were evaluated for proliferating cell nuclear antigen by immunohistochemistry. Regardless of the model, Brca1 was consistently expressed in granulosa and thecal cells of follicle populations that proliferate independently of hormonal stimulation. Expression was similar in these same follicle populations in the ovaries of estrogen-receptor-deficient mice, in which the lack of this estrogen receptor results in abnormal and incomplete follicular development. Brca1 expression was diminished in the granulosa and thecal cells of hormonally dependent antral follicles. Brca1 expression was also localized to luteal cells of recently formed corpora lutea and corpora lutea associated with pregnancy, but it was greatly diminished in regressing corpora lutea in cycling mice. In all cases, Brca1 expression correlated to S-phase proliferating cell nuclear antigen nuclear staining. Thus, Brca1 expression in the mouse ovary occurs independently of hormonal status and in the absence of a major estrogen receptor-mediated pathway; it is, however, closely correlated with cell cycle in mouse ovarian granulosa, thecal, and luteal cell.

Expression of myc, fos, and Ha-ras in the livers of furan-treated F344 rats and B6C3F1 mice.

Furan administered by gavage for 2 yr has been reported to induce hepatocellular carcinomas in male and female B6C3F1 mice and in male but not female F344 rats. Chronic exposure studies in our laboratory using bioassay conditions showed extensive hepatocellular toxicity and sustained increases in regenerative cell proliferation after 1, 3, and 6 wk of treatment in male and female rats and male mice. Altered expression of growth-control genes associated with this hyperproliferative state may enhance the susceptibility of these genes to mutation or may provide a selective growth advantage to preneoplastic cells. Quantitative northern blot analysis of mRNA was used to examine the expression of the oncogenes myc, fos, and Ha-ras in the livers of animals treated with furan. In male rats, a single administration of 30 mg/kg furan produced necrosis and a subsequent wave of cell proliferation 48 h after treatment and induced transient peaks in the expression of myc, fos, and Ha-ras 6-24 h after treatment. In male rat liver from our cell proliferation studies, only a slight increase in myc expression was seen at the end of week 1 of treatment. However, beginning at week 3 and increasing at week 6, up to a 15-fold increase over control values was observed in the expression of myc in the treated animals. The only other notable increase in expression observed in any animals from the cell proliferation study was a threefold increase in myc at week 6 in treated female rats. The absence of an increase in Ha-ras expression in the male mouse liver suggests that the unique pattern of Ha-ras mutations previously reported in furan-induced mouse liver tumors is not due to increased mutational susceptibility related to overexpression of this gene. The lack of sustained expression of myc, fos, and Ha-ras in rapidly proliferating liver suggests that continuous expression of these genes is not necessary to maintain increased rates of cell replication. The large increase in myc expression in male but not female rats suggests an adaptive change that may be related to the sex-specific incidence of furan-induced hepatocellular carcinomas in rats.

Effects of fixation on RNA extraction and amplification from laser capture microdissected tissue.

One of the key end points for understanding the molecular basis of carcinogenesis is the quantitation of gene expression in specific cell populations. Microdissection techniques allow extraction of morphologically distinct cells for molecular analysis. A recent advance in microdissection uses the PixCell laser capture microdissection (LCM) system, which allows for precise removal of pure cell populations from morphologically preserved tissue sections. The objective of this study was to determine the optimal fixation protocol for analyzing RNA from tissue samples using LCM. Optimal fixation must provide acceptable morphology, allow proper laser capture of selected cells, and preserve the integrity of mRNA. We evaluated the effects of both cross-linking and precipitive-type fixatives on frozen and paraffin-embedded mouse liver tissue. For assessment of the quality of the mRNA in LCM samples generated from various fixed tissues, reverse transcription-polymerase chain reaction (RT-PCR)-amplified mouse liver beta2-microglobulin mRNA was detected with ethidium bromide. We also examined mouse glyceraldehyde-3-phosphate-dehydrogenase by using the fluorogenic TaqMan system for real-time quantitative detection of RT-PCR products. Frozen tissues yielded more RT-PCR product than did paraffin-embedded tissues. In both frozen and paraffin-embedded tissues, differences were observed between the fixatives. Precipitive fixatives, such as ethanol and acetone, consistently produced more RT-PCR amplification product than did cross-linking fixatives such as formalin. Optimal fixation protocols for LCM analysis will facilitate the examination of gene expression in specific cell populations, accelerating investigations of the molecular differences responsible for the phenotypic changes observed during carcinogenesis.

CYP2J subfamily cytochrome P450s in the gastrointestinal tract: expression, localization, and potential functional significance.

Our laboratory recently described a new human cytochrome P450 arachidonic acid epoxygenase (CYP2J2) and the corresponding rat homologue (CYP2J3), both of which were expressed in extrahepatic tissues. Northern analysis of RNA prepared from the human and rat intestine demonstrated that CYP2J2 and CYP2J3 mRNAs were expressed primarily in the small intestine and colon. In contrast, immunoblotting studies using a polyclonal antibody raised against recombinant CYP2J2 showed that CYP2J proteins were expressed throughout the gastrointestinal tract. Immunohistochemical staining of formalin-fixed, paraffin-embedded intestinal sections using anti-CYP2J2 IgG and avidin-biotin-peroxidase detection revealed that CYP2J proteins were present at high levels in nerve cells of autonomic ganglia, epithelial cells, intestinal smooth muscle cells, and vascular endothelium. The distribution of this immunoreactivity was confirmed by in situ hybridization using a CYP2J2-specific antisense RNA probe. Microsomal fractions prepared from human jejunum catalyzed the NADPH-dependent metabolism of arachidonic acid to epoxyeicosatrienoic acids as the principal reaction products. Direct evidence for the in vivo epoxidation of arachidonic acid by intestinal cytochrome P450 was provided by documenting, for the first time, the presence of epoxyeicosatrienoic acids in human jejunum by gas chromatography/mass spectrometry. We conclude that human and rat intestine contain an arachidonic acid epoxygenase belonging to the CYP2J subfamily that is localized to autonomic ganglion cells, epithelial cells, smooth muscle cells, and vascular endothelium. In addition to the known effects on intestinal vascular tone, we speculate that CYP2J products may be involved in the release of intestinal neuropeptides, control of intestinal motility, and/or modulation of intestinal fluid/electrolyte transport.