Beta-catenin (Ctnnb1) gene mutations in diethylnitrosamine (DEN)-induced liver tumors in male F344 rats

A critical review of advances in tumor metabolism abnormalities induced by nitrosamine disinfection by-products in drinking water

Intensified sanitation practices amid the recent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) outbreak might result in the increased release of chloramine disinfectants into surface water, significantly promoting the formation of nitrosamine disinfection by-products (DBPs) in drinking water. Unfortunately, these nitrosamine DBPs exhibit significant genotoxic, carcinogenic, and mutagenic properties, whereas chlorinating disinfectants remain in global practice. The current review provides valuable insights into the occurrence, identification, contamination status, exposure limits, and toxicity of the new unregulated disinfection by-products (nitrosamine DBPs) in drinking water. As a result, concentrations of nitrosamine DBPs far exceed allowable limits in drinking water, and prolonged exposure has the potential to cause metabolic disorders, a critical step in tumor initiation and progression. Importantly, based on recent research, we have concluded the role of nitrosamines DBPs in different metabolic pathways. Remarkably, nitrosamine DBPs can induce chronic inflammation and initiate tumors by activating sphingolipid and polyunsaturated fatty acid metabolism. Regarding amino acid and nucleotide metabolism, nitrosamine DBPs can inhibit tryptophan metabolism and de novo nucleotide synthesis. Moreover, inhibition of de novo nucleotide synthesis fails to repair DNA damage induced by nitrosamines. Additionally, the accumulation of lactate induced by nitrosamine DBPs may act as a pivotal signaling molecule in communication within the tumor microenvironment. However, with the advancement of tumor metabolomics, understanding the role of nitrosamine DBPs in causing cancer by inducing metabolic abnormalities significantly lags behind, and specific mechanisms of toxic effects are not clearly defined. Urgently, further studies exploring this promising area are needed.

AFP deletion leads to anti-tumorigenic but pro-metastatic roles in liver cancers with concomitant CTNNB1 mutations

HCC remains one of the most prevalent and deadliest cancers. Serum AFP level is a biomarker for clinical diagnosis of HCC, instead the contribution of AFP to HCC development is clearly highly complex. Here, we discussed the effect of AFP deletion in the tumorigenesis and progression of HCC. AFP deletion in HepG2 cells inhibited the cell proliferation by inactivating PI3K/AKT signaling. Surprisingly, AFP KO HepG2 cells appeared the increasing metastatic capacity and EMT phenotype, which was attributed to the activation of WNT5A/β-catenin signal. Further studies revealed that the activating mutations of CTNNB1 was closely related with the unconventional pro-metastatic roles of AFP deletion. Consistently, the results of DEN/CCl4-induced HCC mouse model also suggested that AFP knockout suppressed the growth of HCC primary tumors, but promoted lung metastasis. Despite the discordant effect of AFP deletion in HCC progression, a drug candidate named OA showed the potent suppression of HCC tumor growth by interrupting AFP-PTEN interaction and, importantly, reduced the lung metastasis of HCC via angiogenesis suppression. Thus, this study demonstrates an unconventional effect of AFP in HCC progression, and suggests a potent candidate strategy for HCC therapy.

Glutamine synthetase limits β-catenin-mutated liver cancer growth by maintaining nitrogen homeostasis and suppressing mTORC1

Glutamine synthetase (GS) catalyzes de novo synthesis of glutamine that facilitates cancer cell growth. In the liver, GS functions next to the urea cycle to remove ammonia waste. As a dysregulated urea cycle is implicated in cancer development, the impact of GS's ammonia clearance function has not been explored in cancer. Here, we show that oncogenic activation of β-catenin (encoded by CTNNB1) led to a decreased urea cycle and elevated ammonia waste burden. While β-catenin induced the expression of GS, which is thought to be cancer promoting, surprisingly, genetic ablation of hepatic GS accelerated the onset of liver tumors in several mouse models that involved β-catenin activation. Mechanistically, GS ablation exacerbated hyperammonemia and facilitated the production of glutamate-derived nonessential amino acids, which subsequently stimulated mechanistic target of rapamycin complex 1 (mTORC1). Pharmacological and genetic inhibition of mTORC1 and glutamic transaminases suppressed tumorigenesis facilitated by GS ablation. While patients with hepatocellular carcinoma, especially those with CTNNB1 mutations, have an overall defective urea cycle and increased expression of GS, there exists a subset of patients with low GS expression that is associated with mTORC1 hyperactivation. Therefore, GS-mediated ammonia clearance serves as a tumor-suppressing mechanism in livers that harbor β-catenin activation mutations and a compromised urea cycle.

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.

The Polyploid State Plays a Tumor-Suppressive Role in the Liver

Most cells in the liver are polyploid, but the functional role of polyploidy is unknown. Polyploidization occurs through cytokinesis failure and endoreduplication around the time of weaning. To interrogate polyploidy while avoiding irreversible manipulations of essential cell-cycle genes, we developed orthogonal mouse models to transiently and potently alter liver ploidy. Premature weaning, as well as knockdown of E2f8 or Anln, allowed us to toggle between diploid and polyploid states. While there was no detectable impact of ploidy alterations on liver function, metabolism, or regeneration, mice with more polyploid hepatocytes suppressed tumorigenesis and mice with more diploid hepatocytes accelerated tumorigenesis in mutagen- and high-fat-induced models. Mechanistically, the diploid state was more susceptible to Cas9-mediated tumor-suppressor loss but was similarly susceptible to MYC oncogene activation, indicating that polyploidy differentially protected the liver from distinct genomic aberrations. This suggests that polyploidy evolved in part to prevent malignant outcomes of liver injury.

Molecular genetics of hepatocellular neoplasia

Hepatocellular carcinoma (HCC) is the sixth most common malignancy and the third leading cause of cancer deaths worldwide. Proper classification and early identification of HCC and precursor lesions is essential to the successful treatment and survival of HCC patients. Recent molecular genetic, pathologic, and clinical data have led to the stratification of hepatic adenomas into three subgroups: those with mutant TCF1/HNF1 alpha gene, those with mutant beta-catenin, and those without mutations in either of these loci. Hepatic adenomas with alpha-catenin mutations have a significantly greater risk for malignant transformation in comparison with the other two subgroups. Telangiectatic focal nodular hyperplasia has now been reclassified as telangiectatic adenoma due to the presence of non-random methylation patterns, consistent with the monoclonal origin which is similar to hepatic adenoma and HCC. HCC precursor lesions demonstrate unique molecular alterations of HSP70, CAP2, glypican 3, and glutamine synthetase that have proven useful in the histologic diagnosis of early HCC. Though specific genetic alterations depend on HCC etiology, the main proteins affected include cell membrane receptors (in particular tyrosine kinase receptors) as well as proteins involved in cell signaling (specifically Wnt/beta-catenin, Ras/Raf/MEK/ERK and PI3K/Akt/mTOR pathways), cell cycle regulation (i.e. p53, p16/INK4, cyclin/cdk complex), invasiveness (EMT, TGF-beta) and DNA metabolism. Advances in gene expression profiling have provided new insights into the molecular genetics of HCC. HCCs can now be stratified into two clinically relevant groups: Class A, the low survival subclass (overall survival time 30.3+/- 8.02 months), shows strong expression signatures of cell proliferation and antiapoptosis genes (such as PNCA and cell cycle regulators CDK4, CCNB1, CCNA2, and CKS2) as well as genes involving ubiquitination and sumoylation; Class B, the high survival subclass (overall survival time 83.7 +/-10.3 months), does not have the above expression signature. In fact, insights into HCC-specific alterations of signal transduction pathways and protein expression patterns have led to the development of new therapeutic agents with molecular targets such as EGFR, VEGF, or other multi-kinase inhibitors. In the future, these specific molecular alterations in HCC can potentially serve as diagnostic tools, prognostic markers, and/or therapeutic targets with the potential to alter clinical outcomes.

A review of the molecular mechanisms of chemically induced neoplasia in rat and mouse models in National Toxicology Program bioassays and their relevance to human cancer

Tumor response in the B6C3F1 mouse, F344 rat, and other animal models following exposure to various compounds provides evidence that people exposed to these or similar compounds may be at risk for developing cancer. Although tumors in rodents and humans are often morphologically similar, underlying mechanisms of tumorigenesis are often unknown and may be different between the species. Therefore, the relevance of an animal tumor response to human health would be better determined if the molecular pathogenesis were understood. The underlying molecular mechanisms leading to carcinogenesis are complex and involve multiple genetic and epigenetic events and other factors. To address the molecular pathogenesis of environmental carcinogens, the authors examine rodent tumors (e.g., lung, colon, mammary gland, skin, brain, mesothelioma) for alterations in cancer genes and epigenetic events that are associated with human cancer. National Toxicology Program (NTP) studies have identified several genetic alterations in chemically induced rodent neoplasms that are important in human cancer. Identification of such alterations in rodent models of chemical carcinogenesis caused by exposure to environmental contaminants, occupational chemicals, and other compounds lends further support that they are of potential human health risk. These studies also emphasize the importance of molecular evaluation of chemically induced rodent tumors for providing greater public health significance for NTP evaluated compounds.

Anti-Cripto Mab inhibit tumour growth and overcome MDR in a human leukaemia MDR cell line by inhibition of Akt and activation of JNK/SAPK and bad death pathways

Doxorubicin (DOX) selection of CCRF-CEM leukaemia cell line resulted in multidrug resistance (MDR) CEM/A7R cell line, which overexpresses MDR, 1 coded P-glycoprotein (Pgp). Here, we report for the first time that oncoprotein Cripto, a founding member of epidermal growth factor-Cripto-FRL, 1-Criptic family is overexpressed in the CEM/A7R cells, and anti-Cripto monoclonal antibodies (Mab) inhibited CEM/A7R cell growth both in vitro and in an established xenograft tumour in severe combined immunodeficiency mice. Cripto Mab synergistically enhanced sensitivity of the MDR cells to Pgp substrates epirubicin (EPI), daunorubicin (DAU) and non-Pgp substrates nucleoside analogue cytosine arabinoside (AraC). In particular, the combination of anti-Cripto Mab at less than 50% of inhibition concentrations with noncytotoxic concentrations of EPI or DAU inhibited more than 90% of CEM/A7R cell growth. Cripto Mab slightly inhibited Pgp expression, and had little effect on Pgp function, indicating that a mechanism independent of Pgp was involved in overcoming MDR. We demonstrated that anti-Cripto Mab-induced CEM/A7R cell apoptosis, which was associated with an enhanced activity of the c-Jun N-terminal kinase/stress-activated protein kinase and inhibition of Akt phosphorylation, resulting in an activation of mitochondrial apoptosis pathway as evidenced by dephosphorylation of Bad at Ser136, Bcl-2 at Ser70 and a cleaved caspase-9.

Possible tumor development from double positive foci for TGF-alpha and GST-P observed in early stages on rat hepatocarcinogenesis

Expression of TGF-alpha during promotion of neoplastic development from GST-P-positive foci in rat chemical hepatocarcinogenesis was investigated. One-hundred male F344 rats were given a single intraperitoneal injection of DEN (200 mg/kg bodyweight) and subjected to two-thirds partial hepatectomy at week 3. Commencing 2 weeks from the start, PB at doses of 0 or 500 p.p.m. was fed to the rats for 46 weeks. Groups of 10 rats were killed at weeks 4, 8, 16, 32, 48 and their livers were immunohistochemically examined for expression of GST-P and TGF-alpha. TGF-alpha-positive foci and single positive cells were observed from week 4, partially overlapping with GST-P-positive foci but being much fewer. Numbers of TGF-alpha-positive lesions did not increase from weeks 4-48, but their areas showed increment at weeks 32 and 48, especially with PB administration. Almost all of the tumors observed at weeks 16, 32 and 48 were positive for TGF-alpha (98%). In addition, epidermal growth factor receptor overexpression was observed in most TGF-alpha-positive lesions (foci and tumors). The proliferating cell nuclear antigen labeling index in double positive foci for GST-P and TGF-alpha was significantly higher than that in TGF-alpha-negative foci. In conclusion, TGF-alpha may be closely related with promotion from altered foci to neoplasms in rat hepatocarcinogenesis. Our data suggest that double positive foci for GST-P and TGF-alpha in the early stages of rat hepatocarcinogenesis may develop into tumors with promotion.

beta-Catenin-mediated signaling: a novel molecular target for chemoprevention with anti-inflammatory substances

Inflammation is thought to play a role in the pathophysiology of cancer. Accumulating evidence from clinical and laboratory-based studies suggests that substances with anti-inflammatory activities are potential candidates for chemoprevention. Recent advances in cellular and molecular biology of cancer shed light on components of intracellular signaling cascades that can be potential molecular targets of chemoprevention with various anti-inflammatory substances. Although cyclooxygenase-2, a primary enzyme that mediates inflammatory responses, has been well recognized as a molecular target for chemoprevention by both synthetic and natural anti-inflammatory agents, the cellular signaling mechanisms that associate inflammation and cancer are not still clearly illustrated. Recent studies suggest that beta-catenin-mediated signaling, which regulates developmental processes, may act as a potential link between inflammation and cancer. This review aims to focus on beta-catenin-mediated signaling pathways, particularly in relation to its contribution to carcinogenesis, and the modulation of inappropriately activated beta-catenin-mediated signaling by nonsteroidal anti-inflammatory drugs and chemopreventive phytochemicals possessing anti-inflammatory properties.

Molecular mechanisms of hepatocarcinogenesis in transgenic mouse models of liver cancer

Overexpression of c-myc and transforming growth factor-alpha (TGF-alpha) has been frequently observed in human hepatocellular carcinoma (HCC),suggesting a pivotal role played by these protooncogenes in liver oncogenesis. In order to investigate the molecular events underlying human hepatic malignant transformation, we have generated c-myc and c-myc/ TGF-alpha transgenic mice that are prone to liver cancer. These transgenic mice develop HCCs with different incidence, kinetics and histopathological features. Indeed, co-expression of c-myc and TGF-alpha transgenes results in a dramatic synergistic effect on liver tumor development when compared with respective single transgenic lines, including a shorter latency period and a more aggressive phenotype. The more malignant histopathological features characteristic of c-myc/ TGF-alpha HCCs are the result of the increased proliferation and reduced apoptosis in this model of liver cancer when compared with single parental lines. Accordingly, c-myc and c-myc/l TGF-alpha transgenic mice display a different molecular pathogenesis of HCC. Importantly, the genetic and molecular mechanisms that are involved in c-myc and c-myc/ TGF-alpha liver cancer development are major oncogenic events in human hepatocarcinogenesis, indicating that these mouse models represent a useful tool to dissect and elucidate the molecular basis of human HCC.

Inherent growth advantage of (pre)malignant hepatocytes associated with nuclear translocation of pro-transforming growth factor alpha

The pro-peptide of transforming growth factor alpha (proTGFalpha) was recently found in hepatocyte nuclei preparing for DNA replication, which suggests a role of nuclear proTGFalpha for mitogenic signalling. This study investigates whether the nuclear occurrence of the pro-peptide is involved in the altered growth regulation of (pre)malignant hepatocytes. In human hepatocarcinogenesis, the incidence of proTGFalpha-positive and replicating nuclei gradually increased from normal liver, to dysplastic nodules, to hepatocellular carcinoma. ProTGFalpha-positive nuclei almost always were in DNA synthesis. Also, in rat hepatocarcinogenesis, proTGFalpha-positive nuclei occurred in (pre)malignant hepatocytes at significantly higher incidences than in unaltered hepatocytes. For functional studies unaltered (GSTp(-)) and premalignant (GSTp(+)) rat hepatocytes were isolated by collagenase perfusion and cultivated. Again, DNA synthesis occurred almost exclusively in proTGFalpha-positive nuclei. GSTp(+) hepatocytes showed an approximately 3-fold higher frequency of proTGFalpha-positive nuclei and DNA replication than GSTp(-) cells. Treatment of cultures with the mitogen cyproterone acetate (CPA) elevated the incidence of proTGFalpha-positive nuclei and DNA synthesis in parallel. Conversely, transforming growth factor beta1 (TGFbeta1) lowered both. These effects of CPA and TGFbeta1 were significantly more pronounced in GSTp(+) than in GSTp(-) hepatocytes. In conclusion, nuclear translocation of proTGFalpha increases in the course of hepatocarcinogenesis and appears to be involved in the inherent growth advantage of (pre)malignant hepatocytes.

Disruption of beta-catenin pathway or genomic instability define two distinct categories of liver cancer in transgenic mice

BACKGROUND & AIMS

Human liver cancer can be divided into 2 categories that are characterized by activation of beta-catenin and genomic instability. Here we investigate whether similar categories exist among 5 transgenic models of liver cancer, including c-myc, transforming growth factor-alpha, E2F-1, c-myc/transforming growth factor-alpha, and c-myc/E2F-1 mice.

METHODS

The random amplified polymorphic DNA method was used to assess the overall genomic instability, and chromosomal loci affected by genomic alterations were determined by microsatellite analysis. beta-Catenin mutations and deletions were analyzed by polymerase chain reaction and sequencing screening. Cellular localization of beta-catenin and expression of alpha-fetoprotein, a prognostic marker of hepatocellular carcinoma, were investigated by immunohistochemistry.

RESULTS

Liver tumors from the transgenic mice could be divided into 2 broad categories characterized by extensive genomic instability (exemplified by the c-myc/transforming growth factor-alpha mouse) and activation of beta-catenin (exemplified by the c-myc/E2F-1 mouse). The c-myc/transforming growth factor-alpha tumors displayed extensive genomic instability with recurrent loss of heterozygosity at chromosomes 1, 2, 4, 6, 7, 9, 12, 14, and X and a low rate of beta-catenin activation. The genomic instability was evident from the early dysplastic stage and occurred concomitantly with increased expression of alpha-fetoprotein. The c-myc/E2F-1 tumors were characterized by a high frequency of beta-catenin activation in the presence of a relatively stable genome and low alpha-fetoprotein levels.

CONCLUSIONS

We have identified 2 prototype experimental models, i.e., c-myc/transforming growth factor-alpha and c-myc/E2F-1 mice, for the 2 categories of human hepatocellular carcinoma characterized by genomic instability and beta-catenin activation, respectively. These mouse models will assist in the elucidation of the molecular basis of human hepatocellular carcinoma.

No involvement of beta-catenin gene mutation in gastric carcinomas induced by N-methyl-N-nitrosourea in male F344 rats

The agent N-methyl-N-nitrosourea (MNU) is a direct acting carcinogen and induces well-differentiated adenocarcinoma on the rat gastric mucosa. In this study, 27 histopathologically verified gastric carcinomas induced in male F344 rats were analyzed for mutations in the N-terminal phosphorylation sites (codons 1-51) of the beta-catenin gene by using polymerase chain reaction (PCR)-single strand conformation polymorphism (SSCP) assays. In parallel studies, the specific localization of the beta-catenin protein was also examined by immunohistochemical analysis. No mutations in the beta-catenin gene were found in any of 27 gastric carcinomas induced by MNU. Immunohistochemical analysis resulted in the beta-catenin protein to be localized in the plasma membrane but cytoplasmic and/or nuclear accumulation of beta-catenin was not identified in any of these carcinomas. These results suggest that mutations in the beta-catenin gene are less contributory to the development of rat gastric carcinomas induced by MNU. This animal model may provide a system for evaluating the mechanism of human gastric carcinogenesis that is not associated with beta-catenin gene mutations.