Prediction of non-genotoxic carcinogenesis in rats using changes in gene expression following acute dosing

Non-genotoxic carcinogenicity of chemicals is currently routinely evaluated in 2-year rodent bioassays. Therefore, the development of early biomarkers for non-genotoxic carcinogenesis would result in substantial savings in time and expense. The current study investigates whether early changes in gene expression may be developed as markers for cancer. Animals were treated for 1 or 5 days with either non-genotoxic carcinogens (NGTCs) or non-carcinogens and gene expression was analyzed by quantitative PCR (qPCR). We tested two gene signatures previously reported to detect non-genotoxic carcinogens. Using one gene signature it was confirmed that 3/3 non-genotoxic carcinogens and 2/2 non-carcinogens are correctly identified with data from 1 or 5 days of dosing. In contrast an alternative signature correctly identified 0/3 and 2/3 non-genotoxic carcinogens at 1 and 5 days of treatment, respectively and 2/2 non-carcinogens at both time-points. Additionally, we evaluated a novel panel of putative biomarker genes, from the literature, many of which have roles in cell growth and division, including myc, cdc2 and mcm6. These genes were significantly induced by non-genotoxic carcinogens and not by non-carcinogens. Using the average fold-induction across this panel, 2/3 non-genotoxic carcinogens were detected at both 1 and 5 days. These data support the idea that acute changes in gene expression may provide biomarkers for non-genotoxic carcinogenesis but also highlight interesting differences in the sensitivities of distinct gene signatures.     

Determination of the genotoxic status of a chemical

Before a non-genotoxic mechanism of action is proposed for a rodent carcinogen, or predicted for an untested chemical, adequate knowledge of the genotoxic status of the chemical must exist. The current absence of absolute agreement on the set of assays to use when evaluating genotoxicity suggests that a practical approach should be adopted, as outlined, for example, by Arni et al. (1988). In that approach, the chemical is evaluated for gene mutagenicity to Salmonella, for its ability to induce chromosomal aberrations in cultured mammalian cells, and if appropriate, gene mutations in cultured mammalian cells. Consideration of the chemical structure and the expected metabolism of the test chemical also contribute to the classification of an agent as a putative non-genotoxin. Judgement is required when interpreting isolated positive test responses generated in peripheral short-term tests. When attempting to study the mode of action of an established rodent carcinogen that is thought to operate by a non-genotoxic mechanism of action, genotoxicity data generated in vivo, if possible in the target tissue, are of great value. Compounds defined as non-genotoxic by these methods then become candidates for the study of those non-genotoxic toxicities that may predict or explain non-genotoxic rodent carcinogenesis.     

Integrating differentiation and cancer: the Nkx3.1 homeobox gene in prostate organogenesis and carcinogenesis

Abstract Several tissue-specific regulatory genes have been found to play essential roles in both organogenesis and carcinogenesis. In the prostate, the Nkx3.1 homeobox gene plays an important role in normal differentiation of the prostatic epithelium while its loss of function is an initiating event in prostate carcinogenesis in both mouse models and human patients. Thus, the Nkx3.1 homeobox gene provides a paradigm for understanding the relationship between normal differentiation and cancer, as well as studying the roles of homeobox genes in these processes. Here, we review recent findings concerning the roles of Nkx3.1 in development and discuss how its normal function is disrupted in processes of early prostate carcinogenesis.     

Insights into the Role of the Peroxisomal Ubiquitination Machinery in Pex13p Degradation in the Yeast Hansenula polymorpha

The import of matrix proteins into peroxisomes in yeast requires the action of the ubiquitin-conjugating enzyme Pex4p and a complex consisting of the ubiquitin E3 ligases Pex2p, Pex10p and Pex12p. Together, this peroxisomal ubiquitination machinery is thought to ubiquitinate the cycling receptor protein Pex5p and members of the Pex20p family of co-receptors, a modification that is required for receptor recycling. However, recent reports have demonstrated that this machinery plays a role in additional peroxisome-associated processes. Hence, our understanding of the function of these proteins in peroxisome biology is still incomplete. Here, we identify a role for the peroxisomal ubiquitination machinery in the degradation of the peroxisomal membrane protein Pex13p. Our data demonstrate that Pex13p levels build up in cells lacking members of this machinery and also establish that Pex13p undergoes rapid degradation in wild-type cells. Furthermore, we show that Pex13p is ubiquitinated in wild-type cells and also establish that Pex13p ubiquitination is reduced in cells lacking a functional peroxisomal E3 ligase complex. Finally, deletion of PEX2 causes Pex13p to build up at the peroxisomal membrane. Taken together, our data provide further evidence that the role of the peroxisomal ubiquitination machinery in peroxisome biology goes much deeper than receptor recycling alone.     

The peroxisome in oxidative stress

Peroxisomes are one of the main sites in the cell where oxygen free radicals are both generated and scavenged. The balance between these two processes is believed to be of great importance for proper functioning of cells and has been implicated in aging and carcinogenesis. We will give an overview of the peroxisomal processes involved in the oxygen radical homeostasis and its implications for the cell.     

Differentiation, cancer, and anticancer activity

Carcinogenesis is a multistep process that results from the development of a variety of defects in the control of differentiation and proliferation. To investigate this concept further, 3T3 T mesenchymal stems cells were employed to establish that a distinct sequence of biological processes is involved in the control of differentiation and proliferation, and that these processes are integrally regulated. Specific defects in these regulatory processes were next established as being involved in carcinogenesis. These defects, however, were found not to be absolute; rather, they appear to involve changes in the stringency by which differentiation and proliferation are integrally regulated. Finally, it was established that when normal or transformed stem cells are induced to undergo nonterminal differentiation (which is one step in the integrated control of proliferation and differentiation), they can be made resistant to carcinogenesis or to revert to a nontransformed state. These data provide strong evidence that a critically important requirement for normal homeostasis is maintenance of intact cellular mechanisms to integrally regulate differentiation and proliferation.     

Nickel(II) acetate-treated Chinese hamster ovary cells differentially express Vimentin, hSNF2H homologue, and H ferritin.

In probing the possible non-genotoxic molecular mechanism(s) of nickel(II)-induced carcinogenesis, we performed a non-radioactive mRNA differential display analysis for nickel(II) acetate-treated Chinese hamster ovary cells (CHO-K1-BH4). Three out of thirty differentially expressed cDNAs had sequences highly similar to known genes. Down-regulation of vimentin and a hSNF2H homologue and up-regulation of ferritin heavy chain were confirmed by Northern blot analysis. The expression of these mRNAs was time- and nickel(II) concentration-dependent. For vimentin, the decrease in mRNA level was concurrent with a decrease in the protein level. For ferritin, the increase in mRNA had no effect on the protein level. Dysregulation of these gene products signifies their involvement in the epigenetic effects of carcinogenic nickel(II) compounds.     

Maturation of the liver-specific peroxisome versus laminin, collagen IV and integrin expression

The interaction of cells with extracellular matrix components contributes to their specific differentiation. We studied hepatic peroxisomes and their changing features during embryonic development, and we immunolocalized in the same tissue the extracellular matrix components laminin and collagen IV as well as the integrin receptor subunits alpha 1, alpha 2, beta 1, and beta 4. Rat and human embryonic liver peroxisome expression were studied at the light- and electron-microscopic level by means of localizing catalase-, D-amino acid oxidase- and polyamine oxidase activities and by means of the immunocytochemistry of six peroxisomal proteins. The successive import of catalase and the peroxisomal beta-oxidation enzymes, the late appearance of the other enzymes, and the gradual increase of peroxisomal size and number to adult values occurred as asynchronous events. Although still immature, peroxisomes were recognized at every stage examined and coexisted with laminin and collagen IV in both species. The beta 1 integrin subunit was immunodetected as early as at 12.5 days in rat. It was concluded that these extracellular matrix factors may be important for the differentiation of liver parenchyma from the liverbud stage onwards. However, the stepwise maturation of the liver-specific peroxisome suggests the involvement of many other regulating factors.     

The glomerulosclerosis gene Mpv17 encodes a peroxisomal protein producing reactive oxygen species.

The mutant mouse strain Mpv17 carries a retroviral insert in its genome which inactivates the Mpv17 gene. At a young age these mice develop glomerulosclerosis and nephrotic syndrome which resembles human disease. We show here that the Mpv17 gene product is highly conserved and encodes a peroxisomal protein. Loss of the Mpv17 protein does not impair peroxisome biogenesis but instead leads to a reduced ability to produce reactive oxygen species (ROS). In turn, overproduction of the Mpv17 gene in transfected cells results in dramatically enhanced levels of intracellular ROS indicating a direct involvement of Mpv17 in ROS production. These data reveal a role for the Mpv17 protein in peroxisomal reactive oxygen metabolism and establish a novel link between peroxisomal ROS production and glomerulosclerosis.     

Arsenic Inhibits DNA Mismatch Repair by Promoting EGFR Expression and PCNA Phosphorylation

Both genotoxic and non-genotoxic chemicals can act as carcinogens. However, while genotoxic compounds lead directly to mutations that promote unregulated cell growth, the mechanism by which non-genotoxic carcinogens lead to cellular transformation is poorly understood. Using a model non-genotoxic carcinogen, arsenic, we show here that exposure to arsenic inhibits mismatch repair (MMR) in human cells, possibly through its ability to stimulate epidermal growth factor receptor (EGFR)-dependent tyrosine phosphorylation of proliferating cellular nuclear antigen (PCNA). HeLa cells exposed to exogenous arsenic demonstrate a dose- and time-dependent increase in the levels of EGFR and tyrosine 211-phosphorylated PCNA. Cell extracts derived from arsenic-treated HeLa cells are defective in MMR, and unphosphorylated recombinant PCNA restores normal MMR activity to these extracts. These results suggest a model in which arsenic induces expression of EGFR, which in turn phosphorylates PCNA, and phosphorylated PCNA then inhibits MMR, leading to increased susceptibility to carcinogenesis. This study suggests a putative novel mechanism of action for arsenic and other non-genotoxic carcinogens.     

Evidence for and possible mechanisms of non-genotoxic carcinogenesis in the rodent thyroid

Thyroid tumours are a common finding in toxicity tests in rodents. It is known that prolonged administration of antithyroid drugs leads to the development of multiple thyroid tumours, and the role of genotoxic and non-genotoxic mechanisms in this needs definition. The role of drugs with an antithyroid action in thyroid carcinogenesis requires a knowledge of thyroid physiology. This review briefly discusses the anatomy and physiology of the thyroid before concentrating on the cellular pathology of the changes that take place in the transition from a normal to a neoplastic thyroid cell. The malignant cell is characterised by excess growth and invasiveness. The normal thyroid cell does not possess an unlimited growth potential because of a growth-desensitising mechanism (GDM) of the antioncogene type. Spontaneous thyroid carcinogenesis requires three key steps which are presumed to arise by mutation and clonal selection: the loss of the GDM, the acquisition of TSH-independent growth, and the acquisition of invasiveness. The sequence of the cell biological changes involved is not fully understood, but it has been shown that IGF-1 is a necessary co-factor for the growth-stimulating effect of TSH in the normal cell, and that autocrine production of IGF-1 is a feature of spontaneous thyroid adenomas. Another early change that has been shown in both experimental and human thyroid tumours is mutation of one of the ras oncogenes. In carcinogenesis due to the prolonged administration of an agent known to interfere with thyroid hormone metabolism and to induce a high TSH, two rather than three key steps will be required for carcinogenesis, as the development of TSH independent growth will not confer any selective advantage. We have shown that monoclonal lesions induced in this way regress when the goitrogen is withdrawn and therefore retain TSH dependence. The development of the other two key changes--the loss of the GDM and the acquisition of invasiveness--may be due to genotoxic or non-genotoxic mechanisms. They can occur in man in the absence of any known mutagenic agent. In patients with dyshormonogenesis a congenital defect in one of the steps of thyroid hormone synthesis is associated with multiple tumour production. It is reassuring that in these patients, exposed to decades of high TSH levels, benign lesions are common, but malignant thyroid tumours are very rare. The occurrence of thyroid tumours following the use of substances known to interfere with thyroid hormone metabolism does not itself exclude a genotoxic component to the carcinogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)     

DNA methylation of nuclear receptor genes--possible role in malignancy

The members of the nuclear receptor superfamily are known to mediate a wide array of basic biological processes, such as regulation of cell growth and differentiation, and induction of apoptosis. In several human malignancies, this central control function of nuclear receptors is disturbed, which seems to play an important role in tumor development and progression. Many nuclear receptor genes have been reported to be downregulated in malignancies; however, only a few mutations, gene arrangements, deletions or similar genetic changes have been shown to occur in these tumors.During the last decade, increasing attention has been directed towards epigenetic mechanisms of gene regulation such as DNA methylation. Many nuclear receptor genes can be silenced through aberrant methylation in tumors; epigenetic silencing, therefore, represents an additional mechanism that modifies expression of key genes during carcinogenesis.This review will give insights into the role of DNA methylation in the silencing of nuclear receptor genes and its involvement in human malignancies.     

Lysosomal involvement in the removal of clofibrate-induced rat liver peroxisomes. A biochemical and morphological analysis

Peroxisomal proliferators induce in rodents hepatic hyperplasia and hypertrophy; the significant increase in the peroxisomal population is accompanied by specific and reversible induction of some peroxisomal enzymes. In suckling rats born from clofibrate-treated mothers, a massive removal of proliferated organelles occurs within 3 days of recovery. In the present paper we examined the early stages of the recovery period in liver of male rats treated with clofibrate for 5 days. The lysosomal involvement in the removal of drug-induced peroxisomes was investigated under physiological conditions, ie in the absence of inhibitors of the autophagic process. Biochemical results indicate that peroxisomal β-oxidation, but not catalase activity, returns to the control values within the examined period. Total acid phosphatase activity is not affected by clofibrate treatment, but following fractionation on a linear density gradient the lysosomal marker enzyme activity is shifted towards lower density values, particularly at day 1 and 2 of recovery. This class of organelles possibly represents lysosomes involved in active autophagic processes. Acid phosphatase cytochemistry shows an increase of lysosome number at day 1 of recovery. Combination of acid phosphatase cytochemistry either with catalase cytochemistry or with catalase immunogold labelling allows to reveal organelles containing both marker enzymes. These results strongly support the involvement of autophagic processes in the removal of proliferated peroxisomes.     

Peroxisomes in dental tissues of the mouse

Patients with mild forms of peroxisomal biogenesis disorders show facial dysmorphism and exhibit dentition problems accompanied by enamel hypoplasia. However, no information is available on the role of peroxisomes in dental and paradontal tissues. Therefore, we studied the distribution of these organelles, their protein composition and the expression of corresponding genes during dental development and in mature decalcified teeth in mice. Perfusion-fixed heads of mice of different developmental stages (E13.5 to adult) were cut in sagittal direction into two halves and embedded in paraffin for serial sectioning and subsequent peroxidase-based immunohistochemistry or double-immunofluorescence preparations. Frozen, unfixed heads of newborn mice were used for cryosectioning and subsequent laser-assisted microdissection of ameloblasts and odontoblasts, RNA isolation and RT-PCR analysis. Our results revealed the presence of peroxisomes already in the bud stage of dental development. An increase in peroxisome abundance was noted during differentiation of ameloblasts and odontoblasts with the highest number of organelles in Tomes’ processes of mature ameloblasts. A strong heterogeneity of peroxisomal enzyme content developed within differentiated dental cell types. A drastic down-regulation of catalase in maturing ameloblasts was noted in contrast to high levels of lipid metabolizing enzymes in peroxisomes of these cells. As known from the literature, differentiated ameloblasts are more prone to oxidative damage which could be explained by the low catalase levels inside of this cell type.     

Autophagy in nuclear receptor PPARγ-deficient mouse prostatic carcinogenesis

Peroxisome proliferator-activated receptor-gamma (PPARγ) is a major modulator of cellular lipid metabolism and organelle differentiation. To understand whether autophagy is involved in the processes of dysregulated fatty acid oxidation and induced oxidative stress accompanying prostatic carcinogenesis, we characterized in vitro and in vivo models of PPARγ- and PPARγ2-deficiency in mouse prostate epithelia. Autophagy accompanied the altered cellular proliferation and de-differentiation that resulted in PPARγ-/γ2-deficient mouse prostatic intraepithelial neoplasia (mPIN). Electron microscopy showed accumulated defective lysosomes and autophagic vacuoles in PPARγ-/γ2-deficient cells, suggestive of autophagy. Gene expression profiling indicated a major dysregulation of cell cycle control and metabolic signaling networks related to peroxisomal, mitochondrial and lysosomal maturation, lipid oxidation and degradation. Further, the putative autophagic phenotypes of PPARγ-null cells could be rescued by re-expression of either the PPARγ1 or -γ2 isoform. Our paper examines the links between autophagy and PPARγ-related subcellular and histopathological changes taking place during murine prostatic carcinogenesis.