Involvement of CCAAT/enhancer binding protein-beta (C/EBPbeta) in epigenetic regulation of mouse methionine adenosyltransferase 1A gene expression

Trichosanthin elicits antitumor activity via MICU3 mediated mitochondria calcium influx

INTRODUCTION

Trichosanthin (TK) is a glycoprotein extracted from the Chinese medicinal herb Trichosanthes kirilowi, which has anti-virus and anti-tumor activity. However, the target and detailed mechanism of TK remains elusive.

OBJECTIVES

We aimed to identify novel antitumor targets of TK in lung adenocarcinoma and study its anti-tumor mechanism.

METHODS

We utilized a Lewis lung carcinoma mouse model to evaluate the inhibition of TK on tumor growth. CCK8 assay was utilized to calculate IC50 of trichosanthin on A549 and H1299. In-vitro cellular assays and in-vivo xenograft mice studies were used to investigate MICU3 overexpression and TK treatment on tumor growth. Fluo-4 dye and JC-1 staining was used to measure the mitochondrial calcium levels and membrane potential. H&E and immunohistochemistry staining were applied the asses the effect of TK on tumor and microenvironment. RNA sequencing was applied to analyze transcriptome changes in TK-treated and MICU3-overexpressed tumor cells. The influence of trichosanthin on DNMT3B expression and MICU3 methylation were detected by qPCR and Western blotting. Transcriptional activity of the MICU3 gene was measured by ChIP-PCR and luciferase assays.

RESULTS

Trichosanthin ihibited the tumor growth in vivo, resulting cancer cell growth inhibition and cell death, with almost no effect on normal cells. IC50 of trichosanthin in A549 and H1299 cells were 62.8 μg/ml and 39.7 μg/ml, respectively. Mitochondrial Calcium Uptake Family complex MICU3 was shown to associated with favorable prognosis and was upregulated upon trichosanthin treatment, along with reduces tumor cell growth and migration, and increased cell death both in vitro and in vivo. Increased mitochondrial calcium level was observed in MICU3 overexpression cells. Pathway analysis of RNA-seq data revealed that cytokine and receptor pathways were enriched in MICU3-overexpressing cells. Trichosanthin decreased DNMT3B expression and altered MICU3 methylation while increased FOSL2 expression and reduced methylation that correlated with increased transcription of the MICU3 gene.

CONCLUSION

Trichosanthin elicits antitumor activity in lung adenocarcinoma via repressing DNMT3B and increasing FOSL2, which in turn induces MICU3-mediated mitochondrial calcium influx and tumor cell death.

Formaldehyde regulates S-adenosylmethionine biosynthesis and one-carbon metabolism

One-carbon metabolism is an essential branch of cellular metabolism that intersects with epigenetic regulation. In this work, we show how formaldehyde (FA), a one-carbon unit derived from both endogenous sources and environmental exposure, regulates one-carbon metabolism by inhibiting the biosynthesis of S-adenosylmethionine (SAM), the major methyl donor in cells. FA reacts with privileged, hyperreactive cysteine sites in the proteome, including Cys120 in S-adenosylmethionine synthase isoform type-1 (MAT1A). FA exposure inhibited MAT1A activity and decreased SAM production with MAT-isoform specificity. A genetic mouse model of chronic FA overload showed a decrease n SAM and in methylation on selected histones and genes. Epigenetic and transcriptional regulation of Mat1a and related genes function as compensatory mechanisms for FA-dependent SAM depletion, revealing a biochemical feedback cycle between FA and SAM one-carbon units.

C/EBPβ Promotes LPS-Induced IL-1β Transcription and Secretion in Alveolar Macrophages via NOD2 Signaling

Objective

C/EBPβ, a crucial transcription factor, regulates innate immunity and inflammatory responses. However, the role played by C/EBPβ in alveolar macrophage (AM) inflammatory responses remains unknown. This study aimed to investigate the role and mechanism of C/EBPβ in alveolar macrophages (AMs) from the transcriptional level and to search for natural compounds targeting C/EBPβ.

Methods

Rat AMs were infected with Lv-sh-C/EBPβ and treated with LPS, and the expression levels of iNOS, TNF-α, IL-6, and IL-1β were measured by RT-qPCR, Western blotting, and ELISA. Mechanistically, transcriptome sequencing (RNA-seq) revealed changes in gene expression patterns in AMs after LPS stimulation and C/EBPβ knockdown. Functional enrichment analyses and rescue experiments identified and validated inflammation-associated cell signaling pathways regulated by C/EBPβ. Furthermore, virtual screening was used to search for natural compounds that inhibit C/EBPβ with the structure of helenalin as a reference.

Results

Following stimulation with LPS, AMs exhibited an increased expression of C/EBPβ. C/EBPβ knockdown significantly decreased the expression levels of inflammatory mediators. A total of 374 differentially expressed genes (DEGs) were identified between LPS-stimulated C/EBPβ knockdown and negative control cells. The NOD-like receptor signaling may be a key target for C/EBPβ, according to functional enrichment analyses of the DEGs. Further experiments showed that the muramyl dipeptide (MDP, NOD2 agonist) reversed the downregulation of inflammatory mediators and the NF-κB pathway caused by the C/EBPβ knockdown. The virtual screening revealed that N-caffeoyltryptophan, orilotimod, and petasiphenone have comparable pharmacological properties to helenalin (a known C/EBPβ inhibitor) and demonstrate a great binding capacity to C/EBPβ.

Conclusion

Ablation of C/EBPβ may attenuate LPS-induced inflammatory damage in AMs by inhibiting the NOD2 receptor signaling pathway. Three natural compounds, N-caffeoyltryptophan, orilotimod, and petasiphenone, may be potential C/EBPβ inhibitors.

Keep a watchful eye on methionine adenosyltransferases, novel therapeutic opportunities for hepatobiliary and pancreatic tumours

Methionine adenosyltransferases (MATs) synthesize S-adenosylmethionine (SAM) from methionine, which provides methyl groups for DNA, RNA, protein, and lipid methylation. MATs play a critical role in cellular processes, including growth, proliferation, and differentiation, and have been implicated in tumour development and progression. The expression of MATs is altered in hepatobiliary and pancreatic (HBP) cancers, which serves as a rare biomarker for early diagnosis and prognosis prediction of HBP cancers. Independent of SAM depletion in cells, MATs are often dysregulated at the transcriptional, post-transcriptional, and post-translational levels. Dysregulation of MATs is involved in carcinogenesis, chemotherapy resistance, T cell exhaustion, activation of tumour-associated macrophages, cancer stemness, and activation of tumourigenic pathways. Targeting MATs both directly and indirectly is a potential therapeutic strategy. This review summarizes the dysregulations of MATs, their proposed mechanism, diagnostic and prognostic roles, and potential therapeutic effects in context of HBP cancers.

Mammalian Sulfur Amino Acid Metabolism: A Nexus Between Redox Regulation, Nutrition, Epigenetics, and Detoxification

SIGNIFICANCE

Transsulfuration allows conversion of methionine into cysteine using homocysteine (Hcy) as an intermediate. This pathway produces S-adenosylmethionine (AdoMet), a key metabolite for cell function, and provides 50% of the cysteine needed for hepatic glutathione synthesis. The route requires the intake of essential nutrients (e.g., methionine and vitamins) and is regulated by their availability. Transsulfuration presents multiple interconnections with epigenetics, adenosine triphosphate (ATP), and glutathione synthesis, polyol and pentose phosphate pathways, and detoxification that rely mostly in the exchange of substrates or products. Major hepatic diseases, rare diseases, and sensorineural disorders, among others that concur with oxidative stress, present impaired transsulfuration. Recent Advances: In contrast to the classical view, a nuclear branch of the pathway, potentiated under oxidative stress, is emerging. Several transsulfuration proteins regulate gene expression, suggesting moonlighting activities. In addition, abnormalities in Hcy metabolism link nutrition and hearing loss.

CRITICAL ISSUES

Knowledge about the crossregulation between pathways is mostly limited to the hepatic availability/removal of substrates and inhibitors. However, advances regarding protein-protein interactions involving oncogenes, identification of several post-translational modifications (PTMs), and putative moonlighting activities expand the potential impact of transsulfuration beyond methylations and Hcy.

FUTURE DIRECTIONS

Increasing the knowledge on transsulfuration outside the liver, understanding the protein-protein interaction networks involving these enzymes, the functional role of their PTMs, or the mechanisms controlling their nucleocytoplasmic shuttling may provide further insights into the pathophysiological implications of this pathway, allowing design of new therapeutic interventions. Antioxid. Redox Signal. 29, 408-452.

Methionine adenosyltransferases in liver health and diseases

Methionine adenosyltransferases (MATs) are essential for cell survival because they catalyze the biosynthesis of the biological methyl donor S-adenosylmethionine (SAMe) from methionine and adenosine triphosphate (ATP). Mammalian cells express two genes, MAT1A and MAT2A, which encode two MAT catalytic subunits, α1 and α2, respectively. The α1 subunit organizes into dimers (MATIII) or tetramers (MATI). The α2 subunit is found in the MATII isoform. A third gene MAT2B, encodes a regulatory subunit β, that regulates the activity of MATII by lowering the inhibition constant (K_i) for SAMe and the Michaelis constant (K_m) for methionine. MAT1A expressed mainly in hepatocytes maintains the differentiated state of these cells whereas MAT2A and MAT2B are expressed in non-parenchymal cells of the liver (hepatic stellate cells [HSCs] and Kupffer cells) and extrahepatic tissues. A switch from the liver-specific MAT1A to MAT2A has been observed during conditions of active liver growth and de-differentiation. Liver injury, fibrosis, and cancer are associated with MAT1A silencing and MAT2A/MAT2B induction. Even though both MAT1A and MAT2A are involved in SAMe biosynthesis, they exhibit distinct molecular interactions in liver cells. This review provides an update on MAT genes and their roles in liver pathologies.

Impaired mitochondrial energy metabolism in Alzheimer's disease: Impact on pathogenesis via disturbed epigenetic regulation of chromatin landscape

The amyloid cascade hypothesis for the pathogenesis of Alzheimer's disease (AD) was proposed over twenty years ago. However, the mechanisms of neurodegeneration and synaptic loss have remained elusive delaying the effective drug discovery. Recent studies have revealed that amyloid-β peptides as well as phosphorylated and fragmented tau proteins accumulate within mitochondria. This process triggers mitochondrial fission (fragmentation) and disturbs Krebs cycle function e.g. by inhibiting the activity of 2-oxoglutarate dehydrogenase. Oxidative stress, hypoxia and calcium imbalance also disrupt the function of Krebs cycle in AD brains. Recent studies on epigenetic regulation have revealed that Krebs cycle intermediates control DNA and histone methylation as well as histone acetylation and thus they have fundamental roles in gene expression. DNA demethylases (TET1-3) and histone lysine demethylases (KDM2-7) are included in the family of 2-oxoglutarate-dependent oxygenases (2-OGDO). Interestingly, 2-oxoglutarate is the obligatory substrate of 2-OGDO enzymes, whereas succinate and fumarate are the inhibitors of these enzymes. Moreover, citrate can stimulate histone acetylation via acetyl-CoA production. Epigenetic studies have revealed that AD is associated with changes in DNA methylation and histone acetylation patterns. However, the epigenetic results of different studies are inconsistent but one possibility is that they represent both coordinated adaptive responses and uncontrolled stochastic changes, which provoke pathogenesis in affected neurons. Here, we will review the changes observed in mitochondrial dynamics and Krebs cycle function associated with AD, and then clarify the mechanisms through which mitochondrial metabolites can control the epigenetic landscape of chromatin and induce pathological changes in AD.

Strain-dependent dysregulation of one-carbon metabolism in male mice is associated with choline- and folate-deficient diet-induced liver injury

Dysregulation of one-carbon metabolism-related metabolic processes is a major contributor to the pathogenesis of nonalcoholic fatty liver disease (NAFLD). It is well established that genetic and gender-specific variations in one-carbon metabolism contribute to the vulnerability to NAFLD in humans. To examine the role of one-carbon metabolism dysregulation in the pathogenesis and individual susceptibility to NAFLD, we used a "population-based" mouse model where male mice from 7 inbred were fed a choline- and folate-deficient (CFD) diet for 12 wk. Strain-dependent down-regulation of several key one-carbon metabolism genes, including methionine adenosyltransferase 1α (Mat1a), cystathionine-β-synthase (Cbs), methylenetetrahydrofolate reductase (Mthfr), adenosyl-homocysteinase (Ahcy), and methylenetetrahydrofolate dehydrogenase 1 (Mthfd1), was observed. These changes were strongly associated with interstrain variability in liver injury (steatosis, necrosis, inflammation, and activation of fibrogenesis) and hyperhomocysteinemia. Mechanistically, the decreased expression of Mat1a, Ahcy, and Mthfd1 was linked to a reduced level and promoter binding of transcription factor CCAAT/enhancer binding protein β (CEBPβ), which directly regulates their transcription. The strain specificity of diet-induced dysregulation of one-carbon metabolism suggests that interstrain variation in the regulation of one-carbon metabolism may contribute to the differential vulnerability to NFLD and that correcting the imbalance may be considered as preventive and treatment strategies for NAFLD.

S-adenosylmethionine in liver health, injury, and cancer

S-adenosylmethionine (AdoMet, also known as SAM and SAMe) is the principal biological methyl donor synthesized in all mammalian cells but most abundantly in the liver. Biosynthesis of AdoMet requires the enzyme methionine adenosyltransferase (MAT). In mammals, two genes, MAT1A that is largely expressed by normal liver and MAT2A that is expressed by all extrahepatic tissues, encode MAT. Patients with chronic liver disease have reduced MAT activity and AdoMet levels. Mice lacking Mat1a have reduced hepatic AdoMet levels and develop oxidative stress, steatohepatitis, and hepatocellular carcinoma (HCC). In these mice, several signaling pathways are abnormal that can contribute to HCC formation. However, injury and HCC also occur if hepatic AdoMet level is excessive chronically. This can result from inactive mutation of the enzyme glycine N-methyltransferase (GNMT). Children with GNMT mutation have elevated liver transaminases, and Gnmt knockout mice develop liver injury, fibrosis, and HCC. Thus a normal hepatic AdoMet level is necessary to maintain liver health and prevent injury and HCC. AdoMet is effective in cholestasis of pregnancy, and its role in other human liver diseases remains to be better defined. In experimental models, it is effective as a chemopreventive agent in HCC and perhaps other forms of cancer as well.

Effects of histone modifications on increased expression of polyamine biosynthetic genes in suicide

Altered polyamine metabolism has been consistently observed as underlying the suicide process. We recently performed a global analysis of polyamine gene expression across the brains of suicide completers, and identified up-regulation of four genes, arginase II (ARG2), S-adenosylmethionine decarboxylase (AMD1), and antizymes 1 and 2 (OAZ1 and OAZ2), which play essential roles in polyamine biosynthesis. To determine if a shared epigenetic mechanism is involved in their overexpression in the prefrontal cortex, we measured promoter levels of tri-methyl modified histone-3-lysine-4 (H3K4me3), a marker of open chromatin, and assessed its association with suicide and gene expression. We identified increased H3K4me3 in the promoter region of OAZ1 in suicide, and found that H3K4me3 was correlated with the expression of OAZ1 and ARG2. Overall, our findings indicate that the H3K4me3 modification plays an important role in the regulation of polyamine biosynthesis, and that this mechanism may be involved in the neurobiology of suicide.

Role of transcriptional and posttranscriptional regulation of methionine adenosyltransferases in liver cancer progression

Down-regulation of the liver-specific MAT1A gene, encoding S-adenosylmethionine (SAM) synthesizing isozymes MATI/III, and up-regulation of widely expressed MAT2A, encoding MATII isozyme, known as MAT1A:MAT2A switch, occurs in hepatocellular carcinoma (HCC). Here we found Mat1A:Mat2A switch and low SAM levels, associated with CpG hypermethylation and histone H4 deacetylation of Mat1A promoter, and prevalent CpG hypomethylation and histone H4 acetylation in Mat2A promoter of fast-growing HCC of F344 rats, genetically susceptible to hepatocarcinogenesis. In HCC of genetically resistant BN rats, very low changes in the Mat1A:Mat2A ratio, CpG methylation, and histone H4 acetylation occurred. The highest MAT1A promoter hypermethylation and MAT2A promoter hypomethylation occurred in human HCC with poorer prognosis. Furthermore, levels of AUF1 protein, which destabilizes MAT1A messenger RNA (mRNA), Mat1A-AUF1 ribonucleoprotein, HuR protein, which stabilizes MAT2A mRNA, and Mat2A-HuR ribonucleoprotein sharply increased in F344 and human HCC, and underwent low/no increase in BN HCC. In human HCC, Mat1A:MAT2A expression and MATI/III:MATII activity ratios correlated negatively with cell proliferation and genomic instability, and positively with apoptosis and DNA methylation. Noticeably, the MATI/III:MATII ratio strongly predicted patient survival length. Forced MAT1A overexpression in HepG2 and HuH7 cells led to a rise in the SAM level, decreased cell proliferation, increased apoptosis, down-regulation of Cyclin D1, E2F1, IKK, NF-κB, and antiapoptotic BCL2 and XIAP genes, and up-regulation of BAX and BAK proapoptotic genes. In conclusion, we found for the first time a post-transcriptional regulation of MAT1A and MAT2A by AUF1 and HuR in HCC. Low MATI/III:MATII ratio is a prognostic marker that contributes to determine a phenotype susceptible to HCC and patients' survival.

CONCLUSION

Interference with cell cycle progression and I-kappa B kinase (IKK)/nuclear factor kappa B (NF-κB) signaling contributes to the antiproliferative and proapoptotic effect of high SAM levels in HCC.

MicroRNA Signature in Alcoholic Liver Disease

Alcoholic liver disease (ALD) is a major global health problem. Chronic alcohol use results in inflammation and fatty liver, and in some cases, it leads to fibrosis and cirrhosis or hepatocellular carcinoma. Increased proinflammatory cytokines, particularly TNF alpha, play a central role in the pathogenesis of ALD. TNF alpha is tightly regulated at transcriptional and posttranscriptional levels. Recently, microRNAs (miRNAs) have been shown to modulate gene functions. The role of miRNAs in ALD is getting attention, and recent studies suggest that alcohol modulates miRNAs. Recently, we showed that alcohol induces miR-155 expression both in vitro (RAW 264.7 macrophage) and in vivo (Kupffer cells, KCs of alcohol-fed mice). Induction of miR-155 contributed to increased TNF alpha production and to the sensitization of KCs to produce more TNF alpha in response to LPS. In this paper, we summarize the current knowledge of miRNAs in ALD and also report increased expression of miR-155 and miR-132 in the total liver as well as in isolated hepatocytes and KCs of alcohol-fed mice. Our novel finding of the alcohol-induced increase of miRNAs in hepatocytes and KCs after alcohol feeding provides further insight into the evolving knowledge regarding the role of miRNAs in ALD.

Epigenetic regulation of spermidine/spermine N1-acetyltransferase (SAT1) in suicide

We have recently shown that the expression of spermidine/spermine N1-acetyltransferase (SAT1) is downregulated across the brains of suicide completers, and that its expression is influenced by genetic variations in the promoter. Several promoter polymorphisms in SAT1, including rs6526342, have been associated with suicide and other psychiatric disorders, and display haplotype-specific effects on expression. However, these effects cannot explain total variability in SAT1 expression, and other regulatory mechanisms, such as epigenetic factors, may also be at play. In this study, we assessed the involvement of epigenetic factors in controlling SAT1 expression in the prefrontal cortex of suicide completers by mapping CpG methylation across a 1880-bp region of the SAT1 promoter, and measuring levels of tri-methylated histone-3-lysine 27 (H3K27me3) at the promoter in suicide completers and controls. Our results demonstrated that CpG methylation was significantly negatively correlated with SAT1 expression. Although overall or site-specific CpG methylation was not associated with suicide or SAT1 expression, we observed high levels of methylation at the polymorphic CpG site created by rs6526342, indicating a relationship between promoter haplotypes and methylation. There was no association between H3K27me3 and suicide, nor was this modification associated with SAT1 expression. Overall, our results indicate that epigenetic factors in the promoter region of SAT1 influence gene expression levels, and may provide a mechanism for both our previous findings of haplotype-specific effects of promoter variations on SAT1 expression, as well as the widespread downregulation of SAT1 expression observed in the brains of suicide completers.

Difference in expression of hepatic microRNAs miR-29c, miR-34a, miR-155, and miR-200b is associated with strain-specific susceptibility to dietary nonalcoholic steatohepatitis in mice

The importance of dysregulation of microRNA (miRNA) expression in nonalcoholic steatohepatitis (NASH) has been increasingly recognized; however, the association between altered expression of miRNAs and pathophysiological features of NASH and whether there is a connection between susceptibility to NASH and altered expression of miRNAs are largely unknown. In this study, male inbred C57BL/6J and DBA/2J mice were fed a lipogenic methyl-deficient diet that causes liver injury similar to human NASH, and the expression of miRNAs and the level of proteins targeted by these miRNAs in the livers were determined. Administration of the methyl-deficient diet triggered NASH-specific changes in the livers of C57BL/6J and DBA/2J mice, with the magnitude being more severe in DBA/2J mice. This was evidenced by a greater extent of expression of fibrosis-related genes in the livers of methyl-deficient DBA/2J mice. The development of NASH was accompanied by prominent changes in the expression of miRNAs, including miR-29c, miR-34a, miR-155, and miR-200b. Interestingly, changes in the expression of these miRNAs and protein levels of their targets, including Cebp-β, Socs 1, Zeb-1, and E-cadherin, in the livers of DBA/2J mice fed a methyl-deficient diet were more pronounced as compared with those in C57BL/6J mice. These results show that alterations in the expression of miRNAs are a prominent event during development of NASH induced by methyl deficiency and strongly suggest that severity of NASH and susceptibility to NASH may be determined by variations in miRNA expression response. More important, our data provide a mechanistic link between alterations in miRNA expression and pathophysiological and pathomorphological features of NASH.

Thyroid hormone receptor-mediated regulation of the methionine adenosyltransferase 1 gene is associated with cell invasion in hepatoma cell lines

The thyroid hormone T(3) regulates differentiation, growth, and development. We demonstrated that methionine adenosyltransferase 1A (MAT1A) was positively regulated by T(3) identified by cDNA microarray previously. The expression of the MAT1A was upregulated by T(3) in hepatoma cell lines overexpressing thyroid hormone receptors (TRs). Additionally, these findings indicate that MAT1A may be regulated by CCAAT/enhancer binding protein (C/EBP). The critical role of the C/EBP binding sites was confirmed by the reporter or chromatin immuno-precipitation (ChIP) assay. In addition, C/EBP was upregulated in hepatoma cells after T(3) treatment and ectopic expression of MAT1A inhibited cell migration and invasion in J7 hepatoma cells. Conversely, knockdown of MAT1A expression increased cell migration. Together, these findings suggest that the expression of the MAT1A gene is mediated by C/EBP and is indirectly upregulated by T(3). Finally, TR was downregulated in a small subset of hepatocellular carcinoma cells concomitantly reduced the expression of C/EBPalpha and MAT1A.

Transcriptional regulation of mouse mast cell protease-2 by interleukin-15

Mast cells (MCs) play a critical role in innate and adaptive immunity through the release of cytokines, chemokines, lipid mediators, biogenic amines, and proteases. We recently showed that the activities of MC proteases are transcriptionally regulated by intracellularly retained interleukin-15 (IL-15), and we provided evidence that this cytokine acts as a specific regulator of mouse mast cell protease-2 (mMCP-2). Here, we show that in wild-type bone marrow-derived mast cells (BMMCs) IL-15 inhibits mMCP-2 transcription indirectly by inducing differential expression and mMCP-2 promoter binding of the bifunctional transcription factors C/EBPbeta and YY1. In wild-type BMMCs, C/EBPbeta expression predominates over YY1 expression, and thus C/EBPbeta preferentially binds to the mMCP-2 promoter. In IL-15-deficient BMMCs, the opposite is found: YY1 expression predominates and binds to the mMCP-2 promoter at the expense of C/EBPbeta. Hypertranscription of the mMCP-2 gene in IL-15-deficient BMMCs is associated with histone acetylation and, intriguingly, with methylation of non-CpG dinucleotides within the MCP-2 promoter. This suggests a novel model of cytokine-controlled protease transcription: non-CpG methylation maintains a chromosomal domain in an "open" configuration that is permissive for gene expression.

Genetic and epigenetic control of molecular alterations in hepatocellular carcinoma

Comparative analysis of hepatocellular carcinoma (HCC) in rat strains that are either susceptible or resistant to the induction of HCC has allowed the mapping of genes responsible for inherited predisposition to HCC. These studies show that the activity of several low penetrance genes and a predominant susceptibility gene regulate the development of hepatocarcinogenesis in rodents. These studies shed light on the epidemiology of human HCC. The identified genes regulate resistance to hepatocarcinogenesis by affecting the capacity of the initiated cells to grow autonomously and to progress to HCC. Analysis of the molecular alterations showed highest iNos cross-talk with IKK/NF-kB and RAS/ERK pathways in most aggressive liver lesions represented by HCC in the susceptible F344 rats. Unrestrained extracellular signal-regulated kinase (Erk) activity linked to proteasomal degradation of dual-specificity phosphatase 1 (Dusp1), a specific ERK inhibitor, by the CKS1-SKP2 ubiquitin ligase complex was highest in more aggressive HCC of genetically susceptible rats. Furthermore, deregulation of G1 and S phases of the cell cycle occurs in HCC of susceptible F344 rats, leading to pRb hyperphosphorylation and elevated DNA synthesis, whereas a block to G1-S transition is present in the HCC of resistant BN rats. Importantly, similar alterations in the signaling pathways that regulate cell cycle progression were found in human HCC with poorer prognosis (as defend by patients' survival length), whereas human HCC with better prognosis had molecular characteristics similar to the lesions in the HCC of resistant rat strains. This review discusses the role of molecular alterations involved in the acquisition of resistance or susceptibility to HCC and the importance of genetically susceptible and resistant rat models for the identification of prognostic markers, and chemopreventive or therapeutic targets for the biological network therapy of human disease.