Glycine N-methyltransferase is an example of functional diversity. Role as a polycyclic aromatic hydrocarbon-binding receptor

Genetic Predisposition to Hepatocellular Carcinoma

Liver preneoplastic and neoplastic lesions of the genetically susceptible F344 and resistant BN rats cluster, respectively, with human HCC with better (HCCB) and poorer prognosis (HCCP); therefore, they represent a valid model to study the molecular alterations determining the genetic predisposition to HCC and the response to therapy. The ubiquitin-mediated proteolysis of ERK-inhibitor DUSP1, which characterizes HCC progression, favors the unrestrained ERK activity. DUSP1 represents a valuable prognostic marker, and ERK, CKS1, or SKP2 are potential therapeutic targets for human HCC. In DN (dysplastic nodule) and HCC of F344 rats and human HCCP, DUSP1 downregulation and ERK1/2 overexpression sustain SKP2-CKS1 activity through FOXM1, the expression of which is associated with a susceptible phenotype. SAM-methyl-transferase reactions and SAM/SAH ratio are regulated by GNMT. In addition, GNMT binds to CYP1A, PARP1, and NFKB and PREX2 gene promoters. MYBL2 upregulation deregulates cell cycle and induces the progression of premalignant and malignant liver. During HCC progression, the MYBL2 transcription factor positively correlates with cells proliferation and microvessel density, while it is negatively correlated to apoptosis. Hierarchical supervised analysis, regarding 6132 genes common to human and rat liver, showed a gene expression pattern common to normal liver of both strains and BN nodules, and a second pattern is observed in F344 nodules and HCC of both strains. Comparative genetics studies showed that DNs of BN rats cluster with human HCCB, while F344 DNs and HCCs cluster with HCCP.

Reversal of High-Fat Diet-Induced Non-Alcoholic Fatty Liver Disease by Metformin Combined with PGG, an Inducer of Glycine N-Methyltransferase

Nonalcoholic fatty liver disease (NAFLD) is a major cause of liver-related morbidities and mortality, and no effective drug treatment currently exists. We aimed to develop a novel treatment strategy to induce the expression of glycine N-methyltransferase (GNMT), which is an important enzyme regulating S-adenosylmethionine metabolism whose expression is downregulated in patients with NAFLD. Because 1,2,3,4,6-pentagalloyl glucose (PGG) is a GNMT inducer, and metformin was shown to upregulate liver mitochondrial GNMT protein expression, the effect of PGG and metformin was evaluated. Biochemical analysis, histopathological examination, immunohistochemical staining, reverse transcription-quantitative PCR (RT-qPCR), Western blotting (WB), proteomic analysis and Seahorse XF Cell Mito Stress Test were performed. The high-fat diet (HFD)-induced NAFLD mice were treated with PGG and metformin. Combination of PGG and metformin nearly completely reversed weight gain, elevation of serum aminotransferases, and hepatic steatosis and steatohepatitis. In addition, the downregulated GNMT expression in liver tissues of HFD-induced NAFLD mice was restored. The GNMT expression was further confirmed by RT-qPCR and WB analysis using both in vitro and in vivo systems. In addition, PGG treatment was shown to increase oxygen consumption rate (OCR) maximum capacity in a dose-dependent manner, and was capable of rescuing the suppression of mitochondrial OCR induced by metformin. Proteomic analysis identified increased expression of glutathione S-transferase mu 4 (GSTM4), heat shock protein 72 (HSP72), pyruvate carboxylase (PYC) and 40S ribosomal protein S28 (RS28) in the metformin plus PGG treatment group. Our findings show that GNMT expression plays an important role in the pathogenesis of NAFLD, and combination of an inducer of GNMT and metformin can be of therapeutic potential for patients with NAFLD.

ZNF165 Is Involved in the Regulation of Immune Microenvironment and Promoting the Proliferation and Migration of Hepatocellular Carcinoma by AhR/CYP1A1

The strong tumorigenic capacity and treatment resistance made hepatocellular carcinoma (HCC) a huge threat to public health. ZNF165, the kruppel family of zinc-finger-containing transcription factors, is expressed in HCC; however, its specific role in HCC and the molecular mechanism are yet to be elucidated. In this study, we observed that ZNF165 was overexpressed in liver cancer tissues and the immune microenvironment; higher ZNF165 expression was correlated with lower overall survival in liver cancer patients. The ZNF165 knockdown in Bel7402 cells revealed the impairment of the tryptophan/kynurenine/AhR/CYP1A1 axis. Moreover, the knockdown of CYP1A1 significantly inhibited the proliferation and migration of HCC cells, and ZNF165 promoted the transcriptional activity of AhR by facilitating the nuclear translocation of CYP1A1. In conclusion, the present study argued that ZNF165 was highly expressed in liver tissues and the immune microenvironment. ZNF165 promoted the proliferation and migration of HCC cells by activating the tryptophan/kynurenine/AhR/CYP1A1 axis and promoting the expression of CYP1A1.

Nuclear localization dictates hepatocarcinogenesis suppression by glycine N-methyltransferase

BACKGROUND

GNMT (glycine N-methyltransferase) is a tumor suppressor gene, but the mechanisms mediating its suppressive activity are not entirely known.

METHODS

We investigated the oncosuppressive mechanisms of GNMT in human hepatocellular carcinoma (HCC). GNMT mRNA and protein levels were evaluated by quantitative RT-PCR and immunoblotting. GNMT effect in HCC cell lines was modulated through GNMT cDNA induced overexpression or anti-GNMT siRNA transfection.

RESULTS

GNMT was expressed at low level in human HCCs with a better prognosis (HCCB) while it was almost absent in fast-growing tumors (HCCP). In HCCB, the nuclear localization of the GNMT protein was much more pronounced than in HCCP. In Huh7 and HepG2 cell lines, GNMT forced expression inhibited the proliferation and promoted apoptosis. At the molecular level, GNMT overexpression inhibited the expression of CYP1A (Cytochrome p450, aromatic compound-inducible), PREX2 (Phosphatidylinositol-3,4,5-trisphosphate-dependent Rac exchange factor 2), PARP1 [Poly (ADP-ribose) polymerase 1], and NFKB (nuclear factor-kB) genes. By chromatin immunoprecipitation, we found GNMT binding to the promoters of CYP1A1, PREX2, PARP1, and NFKB genes resulting in their strong inhibition. These genes are implicated in hepatocarcinogenesis, and are involved in the GNMT oncosuppressive action.

CONCLUSION

Overall, the present data indicate that GNMT exerts a multifaceted suppressive action by interacting with various cancer-related genes and inhibiting their expression.

Knockout of cytochrome P450 1A1 enhances lipopolysaccharide-induced acute lung injury in mice by targeting NF-κB activation

Acute lung injury (ALI) is accompanied by overactivation of multiple pro-inflammatory factors. Cytochrome P450 1A1 (CYP1A1) has been shown to aggravate lung injury in response to hyperoxia. However, the relationship between CYP1A1 and lipopolysaccharide (LPS)-induced ALI is unknown. In this study, CYP1A1 was shown to be upregulated in mouse lung in response to LPS. Using CYP1A1-deficient (CYP1A1-/-) mice, we found that CYP1A1 knockout enhanced LPS-induced ALI, as evidenced by increased TNF-α, IL-1β, IL-6, and nitric oxide in lung; these effects were mediated by overactivation of NF-κB and iNOS. Furthermore, we found that aspartate aminotransferase, lactate dehydrogenase, creatine kinase, and creatinine levels were elevated in serum of LPS-induced CYP1A1-/- mice. Altogether, these data provide novel insights into the involvement of CYP1A1 in LPS-induced lung injury.

Cytochrome P450 1A1 enhances inflammatory responses and impedes phagocytosis of bacteria in macrophages during sepsis

Abstract

The hydroxylase cytochrome P450 1A1 (CYP1A1) is regulated by the inflammation-limiting aryl hydrocarbon receptor (AhR), but CYP1A1 immune functions remain unclear. We observed CYP1A1 overexpression in peritoneal macrophages (PMs) isolated from mice following LPS or heat-killed Escherichia. coli (E. coli) challenge. CYP1A1 overexpression augmented TNF-α and IL-6 production in RAW264.7 cells (RAW) by enhancing JNK/AP-1 signalling. CYP1A1 overexpression also promoted 12S-hydroxy-5Z,8Z,10E,14Z-eicosatetraenoic acid (12(S)-HETE) production in activated RAW, while a 12(S)-HETE antibody attenuated and 12(S)-HETE alone induced inflammatory responses. Macrophages harbouring hydroxylase-deficient CYP1A1 demonstrated reduced 12(S)-HETE generation and LPS-induced TNF-α/IL-6 secretion. CYP1A1 overexpression also impaired phagocytosis of bacteria via decreasing the expression of scavenger receptor A (SR-A) in PMs. Mice injected with CYP1A1-overexpressing PMs were more susceptible to CLP- or E. coli-induced mortality and bacteria invading, while Rhapontigenin, a selective CYP1A1 inhibitor, improved survival and bacteria clearance of mice in sepsis. CYP1A1 and 12(S)-HETE were also elevated in monocytes and plasma of septic patients and positively correlated with SOFA scores. Macrophage CYP1A1 disruption could be a promising strategy for treating sepsis.

Alterations of methionine metabolism in hepatocarcinogenesis: the emergent role of glycine N-methyltransferase in liver injury

The methionine and folate cycles play a fundamental role in cell physiology and their alteration is involved in liver injury and hepatocarcinogenesis. Glycine N-methyltransferase is implicated in methyl group supply, DNA methylation, and nucleotide biosynthesis. It regulates the cellular S-adenosylmethionine/S-adenosylhomocysteine ratio and S-adenosylmethionine-dependent methyl transfer reactions. Glycine N-methyltransferase is absent in fast-growing hepatocellular carcinomas and present at a low level in slower growing HCC ones. The mechanism of tumor suppression by glycine N-methyltransferase is not completely known. Glycine N-methyltransferase inhibits hepatocellular carcinoma growth through interaction with Dep domain-containing mechanistic target of rapamycin (mTor)-interacting protein, a binding protein overexpressed in hepatocellular carcinoma. The interaction of the phosphatase and tensin homolog inhibitor, phosphatidylinositol 3,4,5-trisphosphate-dependent rac exchanger, with glycine N-methyltransferase enhances proteasomal degradation of this exchanger by the E3 ubiquitin ligase HectH. Glycine N-methyltransferase also regulates genes related to detoxification and antioxidation pathways. It supports pyrimidine and purine syntheses and minimizes uracil incorporation into DNA as consequence of folate depletion. However, recent evidence indicates that glycine N-methyltransferase targeted into nucleus still exerts strong anti-proliferative effects independent of its catalytic activity, while its restriction to cytoplasm prevents these effects. Our current knowledge suggest that glycine N-methyltransferase plays a fundamental, even if not yet completely known, role in cellular physiology and highlights the need to further investigate this role in normal and cancer cells.

Role of glycine N-methyltransferase in experimental ulcerative colitis

BACKGROUND AND AIM

Inflammatory bowel diseases (IBDs) are chronic inflammatory disorders with unclear etiology and mechanism(s). Glycine N-methyltransferase (GNMT) plays a central role in inflammatory diseases such as hepatitis and atherosclerosis. However, little is known about the impact of GNMT and the involved mechanism in the pathogenesis of IBD. In the current study, we investigated the role of GNMT in the mouse model of dextran sulfate sodium (DSS)-induced colitis.

METHODS

Protein expression was determined by Western blotting or immunohistochemistry. Histopathology was examined by hematoxylin and eosin staining. Levels of pro-inflammatory cytokines were evaluated by ELISA kits.

RESULTS

GNMT was expressed in the epithelium of the colon under normal conditions, and with DSS treatment, its expression was predominant in infiltrated leukocytes of lesions. Mice with genetic deletion of GNMT (GNMT(-/-) ) showed increased susceptibility to DSS induction of colitis, as revealed by the progression of colitis. Additionally, severe colonic inflammation, including increased crypt loss, leukocyte infiltration, and hemorrhage, was greater with DSS treatment in GNMT(-/-) than wild-type mice. Furthermore, the expression of adhesion molecule and inflammatory mediators in the colon was significantly higher with DSS treatment in GNMT(-/-) than wild-type mice. Moreover, loss of GNMT decreased cell apoptosis in colitis lesions with DSS treatment.

CONCLUSIONS

Collectively, our findings suggest that GNMT may be a crucial molecule in the pathogenesis of DSS-induced colitis. This finding may provide new information for a potential therapeutic target in treating IBD.

The multi-functional roles of GNMT in toxicology and cancer

Although glycine N-methyltransferase (GNMT) has been discovered for five decades, its function was not elucidated until recently. In this review, we discuss the multiple roles of GNMT in toxicology and cancer. Besides catalyzing the production of methylglycine (sarcosine) in one carbon metabolism pathway, GNMT was found to be able to bind a number of polycyclic aromatic hydrocarbons and inhibit DNA adducts formation. Moreover, GNMT exerts protective effects against the cytotoxicity and carcinogenicity of benzo(a)pyrene and aflatoxin B(1) in vitro and in vivo. Occupational study showed that workers who had genotypes with higher GNMT promoter activity may have lower content of oxidative damaged DNA products in their urine. In terms of cancer, recent studies using GNMT knockout mouse models demonstrated that GNMT deficiency has high penetrance in inducing the development of steatohepatitis and hepatocellular carcinoma. In terms of the mechanism, besides dysregulation of epigenetic modification, insights have been provided by recent identification of two novel proteins interacting with GNMT-DEPTOR and NPC2. These studies suggest that GNMT not only is involved in mTOR signaling pathway, but also plays an important role in the intracellular trafficking of cholesterol. The implication of these findings to the preventive medicine and translational research will be discussed.

Deficiency of glycine N-methyltransferase aggravates atherosclerosis in apolipoprotein E-null mice

The mechanism underlying the dysregulation of cholesterol metabolism and inflammation in atherogenesis is not understood fully. Glycine N-methyltransferase (GNMT) has been implicated in hepatic lipid metabolism and the pathogenesis of liver diseases. However, little is known about the significance of GNMT in atherosclerosis. We showed the predominant expression of GNMT in foamy macrophages of mouse atherosclerotic aortas. Genetic deletion of GNMT exacerbated the hyperlipidemia, inflammation and development of atherosclerosis in apolipoprotein E-deficient mice. In addition, ablation of GNMT in macrophages aggravated oxidized low-density lipoprotein-mediated cholesterol accumulation in macrophage foam cells by downregulating the expression of reverse cholesterol transporters including ATP-binding cassette transporters-A1 and G1 and scavenger receptor BI. Furthermore, tumor necrosis factor-α-induced inflammatory response was promoted in GNMT-null macrophages. Collectively, our data suggest that GNMT is a crucial regulator in cholesterol metabolism and in inflammation, and contributes to the pathogenesis of atherosclerosis. This finding may reveal a potential therapeutic target for atherosclerosis.

Functional characterization of glycine N-methyltransferase and its interactive protein DEPDC6/DEPTOR in hepatocellular carcinoma

Glycine N-methyltransferase (GNMT) is a tumor suppressor for hepatocellular carcinoma (HCC). High rates of Gnmt knockout mice developed HCC. Epigenetic alteration and dysregulation of several pathways including wingless-type MMTV integration site (Wnt), mitogen-activated protein kinase (MAPK) and Janus kinase and signal transducer and activator of transcription (JAK-STAT) are associated with HCC development in Gnmt knockout mice. We hypothesized that GNMT may regulate signal transduction through interacting with other proteins directly. In this report, we identified a mammalian target of rapamycin (mTOR) inhibitor (DEP domain containing MTOR-interacting protein [DEPDC6/DEPTOR]) as a GNMT-binding protein by using yeast two-hybrid screening. Fluorescence resonance energy transfer assay demonstrated that the C-terminal half of GNMT interact with the PSD-95/Dlg1/ZO-1 (PDZ) domain of DEPDC6/DEPTOR. Immunohistochemical staining showed that 27.5% (14/51) of HCC patients had higher expression levels of DEPDC6/DEPTOR in the tumorous tissues than in tumor-adjacent tissues, especially among HCC patients with hepatitis B viral infection (odds ratio 10.3, 95% confidence interval [CI] 1.05-11.3) or patients with poor prognosis (death hazard ratio 4.51, 95% CI 1.60-12.7). In terms of molecular mechanism, knockdown of DEPDC6/DEPTOR expression in HuH-7 cells caused S6K and 4E-BP activation, but suppressed Akt. Overexpression of DEPDC6/DEPTOR activated Akt and increased survival of HCC cells. Overexpression of GNMT caused activation of mTOR/raptor downstream signaling and delayed G2/M cell cycle progression, which altogether resulted in cellular senescence. Furthermore, GNMT reduced proliferation of HuH-7 cells and sensitized them to rapamycin treatment both in vitro and in vivo. In conclusion, GNMT regulates HCC growth in part through interacting with DEPDC6/DEPTOR and modulating mTOR/raptor signaling pathway. Both GNMT and DEPDC6/DEPTOR are potential targets for developing therapeutics for HCC.

Transcriptional regulation of xenobiotic detoxification in Drosophila

Living organisms, from bacteria to humans, display a coordinated transcriptional response to xenobiotic exposure, inducing enzymes and transporters that facilitate detoxification. Several transcription factors have been identified in vertebrates that contribute to this regulatory response. In contrast, little is known about this pathway in insects. Here we show that the Drosophila Nrf2 (NF-E2-related factor 2) ortholog CncC (cap 'n' collar isoform-C) is a central regulator of xenobiotic detoxification responses. A binding site for CncC and its heterodimer partner Maf (muscle aponeurosis fibromatosis) is sufficient and necessary for robust transcriptional responses to three xenobiotic compounds: phenobarbital (PB), chlorpromazine, and caffeine. Genetic manipulations that alter the levels of CncC or its negative regulator, Keap1 (Kelch-like ECH-associated protein 1), lead to predictable changes in xenobiotic-inducible gene expression. Transcriptional profiling studies reveal that more than half of the genes regulated by PB are also controlled by CncC. Consistent with these effects on detoxification gene expression, activation of the CncC/Keap1 pathway in Drosophila is sufficient to confer resistance to the lethal effects of the pesticide malathion. These studies establish a molecular mechanism for the regulation of xenobiotic detoxification in Drosophila and have implications for controlling insect populations and the spread of insect-borne human diseases.

Reduced cytochrome P4501A activity and recovery from oxidative stress during subchronic benzo[a]pyrene and benzo[e]pyrene treatment of rainbow trout

This study assessed the role of aryl hydrocarbon receptor (AHR) affinity, and cytochrome P4501A (CYP1A) protein and activity in polyaromatic hydrocarbon (PAH)-induced oxidative stress. In the 1-100nM concentration range benzo[a]pyrene (BaP) but not benzo[e]pyrene (BeP) competitively displaced 2nM [(3)H]2, 3, 7, 8-tetrachloro-dibenzo-p-dioxin from rainbow trout AHR2α. Based on appearance of fluorescent aromatic compounds in bile over 3, 7, 14, 28 or 50days of feeding 3μg of BaP or BeP/g fish/day, rainbow trout liver readily excreted these polyaromatic hydrocarbons (PAHs) and their metabolites at near steady state rates. CYP1A proteins catalyzed more than 98% of ethoxyresorufin-O-deethylase (EROD) activity in rainbow trout hepatic microsomes. EROD activity of hepatic microsomes initially increased and then decreased to control activities after 50days of feeding both PAHs. Immunohistochemistry of liver confirmed CYP1A protein increased in fish fed both PAHs after 3days and remained elevated for up to 28days. Neither BaP nor BeP increased hepatic DNA adduct concentrations at any time up to 50days of feeding these PAHs. Comet assays of blood cells demonstrated marked DNA damage after 14days of feeding both PAHs that was not significant after 50days. There was a strong positive correlation between hepatic EROD activity and DNA damage in blood cells over time for both PAHs. Neither CYP1A protein nor 3-nitrotyrosine (a biomarker for oxidative stress) immunostaining in trunk kidney were significantly altered by BaP or BeP after 3, 7, 14, or 28days. There was no clear association between AHR2α affinity and BaP and BeP-induced oxidative stress.

Activation of group IVC phospholipase A(2) by polycyclic aromatic hydrocarbons induces apoptosis of human coronary artery endothelial cells

Exposure to environmental pollutants, such as polycyclic aromatic hydrocarbons (PAHs) found in coal tar mixtures and tobacco sources, is considered a significant risk factor for the development of heart disease in humans. The goal of this study was to determine the influence of PAHs present at a Superfund site on human coronary artery endothelial cell (HCAEC) phospholipase A(2) (PLA(2)) activity and apoptosis. Extremely high levels of 12 out of 15 EPA high-priority PAHs were present in both the streambed and floodplain sediments at a site where an urban creek and its adjacent floodplain were extensively contaminated by PAHs and other coal tar compounds. Nine of the 12 compounds and a coal tar mixture (SRM 1597A) activated group IVC PLA(2) in HCAECs, and activation of this enzyme was associated with histone fragmentation and poly (ADP) ribose polymerase (PARP) cleavage. Genetic silencing of group IVC PLA(2) inhibited both (3)H-fatty acid release and histone fragmentation by PAHs and SRM 1597A, indicating that individual PAHs and a coal tar mixture induce apoptosis of HCAECs via a mechanism that involves group IVC PLA(2). Western blot analysis of aortas isolated from feral mice (Peromyscus leucopus) inhabiting the Superfund site showed increased PARP and caspase-3 cleavage when compared to reference mice. These data suggest that PAHs induce apoptosis of HCAECs via activation of group IVC PLA(2).

Benzo(a)pyrene induced glycine N-methyltransferase messenger RNA expression in Fundulus heteroclitus embryos

Glycine N-methyltransferase (GNMT) is a mediator in the methionine and folate cycles, and is responsible for the transfer of a methyl group from S-adenosylmethionine (SAM) to glycine forming S-adenosylhomocysteine (SAH) and sarcosine. All the known DNA methyltransferases use SAM as a methyl donor thus, GNMT is critically involved in regulation of DNA methylation. Altered GNMT activities have been associated with liver pathologies including hepatocellular carcinoma. The homotetramer form of GNMT is enzymatically active, but the homodimeric form has been suggested as the 4S PAH-binding protein which may mediate CYP1A expression. To further understand the role of GNMT in benzo(a)pyrene (BaP)-related toxicity, full length Fundulus heteroclitus GNMT cDNA was cloned from adult liver. The open reading frame (ORF) of GNMT is 888 base pairs long and encodes a deduced protein of 295 amino acids which has 74% identity with human GNMT. Expression of GNMT mRNA was determined by quantitative RT-PCR. In unfertilized, 2days postfertilization (dpf), and 3 dpf embryos GNMT was constitutively higher than in 4, 7, 10 or 14 dpf embryos. Embryos were also exposed to waterborne BaP at 10 and 100 μg L⁻¹, and by 10 dpf the higher BaP dose caused increased expression of GNMT mRNA. These results suggest that PAH exposure may alter expression of an important physiological methylation mediator. Future work will be necessary to determine enzyme level effects of BaP exposure as well.

Glycine N-methyltransferase affects the metabolism of aflatoxin B1 and blocks its carcinogenic effect

Previously, we reported that glycine N-methyltransferase (GNMT) knockout mice develop chronic hepatitis and hepatocellular carcinoma (HCC) spontaneously. For this study we used a phosphoenolpyruvate carboxykinase promoter to establish a GNMT transgenic (TG) mouse model. Animals were intraperitoneally inoculated with aflatoxin B(1) (AFB(1)) and monitored for 11 months, during which neither male nor female GNMT-TG mice developed HCC. In contrast, 4 of 6 (67%) male wild-type mice developed HCC. Immunofluorescent antibody test showed that GNMT was translocated into nuclei after AFB(1) treatment. Competitive enzyme immunoassays indicated that after AFB(1) treatment, the AFB(1)-DNA adducts formed in stable clones expressing GNMT reduced 51.4% compared to the vector control clones. Experiments using recombinant adenoviruses carrying GNMT cDNA (Ad-GNMT) further demonstrated that the GNMT-related inhibition of AFB(1)-DNA adducts formation is dose-dependent. HPLC analysis of the metabolites of AFB(1) in the cultural supernatants of cells exposed to AFB(1) showed that the AFM(1) level in the GNMT group was significantly higher than the control group, indicating the presence of GNMT can enhance the detoxification pathway of AFB(1). Cytotoxicity assay showed that the GNMT group had higher survival rate than the control group after they were treated with AFB(1). Automated docking experiments showed that AFB(1) binds to the S-adenosylmethionine binding domain of GNMT. Affinity sensor assay demonstrated that the dissociation constant for GNMT-AFB(1) interaction is 44.9 microM. Therefore, GNMT is a tumor suppressor for HCC and it exerts protective effects in hepatocytes via direct interaction with AFB(1), resulting in reduced AFB(1)-DNA adducts formation and cell death.

Haplotypes, loss of heterozygosity, and expression levels of glycine N-methyltransferase in prostate cancer

PURPOSE

Glycine N-methyltransferase (GNMT) affects genetic stability by regulating DNA methylation and interacting with environmental carcinogens. In a previous study, we showed that GNMT acts as a susceptibility gene for hepatocellular carcinoma. Here, we report on our efforts to characterize the haplotypes, loss of heterozygosity (LOH), and expression levels of the GNMT in prostate cancer.

EXPERIMENTAL DESIGN

Peripheral blood mononuclear cell DNA collected from 326 prostate cancer patients and 327 age-matched controls was used to determine GNMT haplotypes. Luciferase reporter constructs were used to compare the promoter activity of different GNMT haplotypes. GNMT LOH rates in tumorous specimens were investigated via a comparison with peripheral blood mononuclear cell genotypes. Immunohistochemical staining was used to analyze GNMT expression in tissue specimens collected from 5 normal individuals, 33 benign prostatic hyperplasia patients, and 45 prostate cancer patients.

RESULTS

Three major GNMT haplotypes were identified in 92% of the participants: A, 16GAs/DEL/C (58%); B, 10GAs/INS/C (19.9%); and C, 10GAs/INS/T (14.5%). Haplotype C carriers had significantly lower risk for prostate cancer compared with individuals with haplotype A (odds ratio, 0.68; 95% confidence interval, 0.48-0.95). Results from a phenotypic analysis showed that haplotype C exhibited the highest promoter activity (P < 0.05, ANOVA test). In addition, 36.4% (8 of 22) of the prostatic tumor tissues had LOH of the GNMT gene. Immunohistochemical staining results showed abundant GNMT expression in normal prostatic and benign prostatic hyperplasia tissues, whereas it was diminished in 82.2% (37 of 45) of the prostate cancer tissues.

CONCLUSIONS

Our findings suggest that GNMT is a tumor susceptibility gene for prostate cancer.

Effects of substrate and potassium on the betaine-synthesizing enzyme glycine sarcosine dimethylglycine N-methyltransferase from a halophilic methanoarchaeon Methanohalophilus portucalensis

Methanohalophilus portucalensis FDF1 can synthesize the compatible solute betaine de novo through the methylation of glycine, sarcosine and dimethylglycine with the methyl group from S-adenosylmethionine. After separation by DEAE-Sephacel ion chromatography using a KCl step gradient, glycine, sarcosine and dimethylglycine methytransfer (GSDMT) activities were detected in a single peak. The estimated molecular weight of GSDMT was 240 kDa and 2-D gel analysis indicated it was separated into four subunits (52 kDa) with different pI. The PBE94 chromatofocusing column also separated GSDMT into four protein peaks A, B, C, D. Both peak B and D proteins possessed GSDMT activity, while the peak A protein only exhibited SDMT activity. The multiple methyltransferase activities of the large complex appear to be unique compared to other methyltransferases used in betaine synthesis. Further methyltransferase assays in response to different concentrations of KCl indicated that the peak D protein exhibited low GSDMT activity only when K(+) < or = 0.4 M. The peak B protein exhibited a higher GSDMT activity at 0.4 M K(+), while the peak A protein exhibited SDMT activity only at higher K(+) (0.8 M). These results suggest that the internal K(+) concentration regulates GSDMT activities and affects the net betaine accumulation in the cells.

Cross-species comparative toxicogenomics as an aid to safety assessment

Cross-species comparative toxicogenomics has the potential for improving the understanding of the different responses of animal models to toxicants at a molecular level. This understanding could then lead to a more accurate extrapolation of the risk posed by these toxicants to humans. Cross-species comparative studies have been carried out at the genomic sequence level and using microarrays to examine changes in global mRNA profiles. However, these studies face considerable bioinformatic challenges in terms of identifying which genes are truly orthologous across species. The resources to analyse such studies, in the context of such orthologues, beg improvement. Finally, the experimental design of such studies needs to be carefully considered to make their results fully interpretable. These issues are discussed, along with the current state-of-the-art cross-species comparative toxicogenomics in this review.

The DHR96 nuclear receptor regulates xenobiotic responses in Drosophila

Exposure to xenobiotics such as plant toxins, pollutants, or prescription drugs triggers a defense response, inducing genes that encode key detoxification enzymes. Although xenobiotic responses have been studied in vertebrates, little effort has been made to exploit a simple genetic system for characterizing the molecular basis of this coordinated transcriptional response. We show here that approximately 1000 transcripts are significantly affected by phenobarbital treatment in Drosophila. We also demonstrate that the Drosophila ortholog of the human SXR and CAR xenobiotic receptors, DHR96, plays a role in this response. A DHR96 null mutant displays increased sensitivity to the sedative effects of phenobarbital and the pesticide DDT as well as defects in the expression of many phenobarbital-regulated genes. Metabolic and stress-response genes are also controlled by DHR96, implicating its role in coordinating multiple response pathways. This work establishes a new model system for defining the genetic control of xenobiotic stress responses.

Evidence of spatially extensive resistance to PCBs in an anadromous fish of the Hudson River

Populations of organisms that are chronically exposed to high levels of chemical contaminants may not suffer the same sublethal or lethal effects as naive populations, a phenomenon called resistance. Atlantic tomcod (Microgadus tomcod) from the Hudson River, New York, are exposed to high concentrations of polycyclic aromatic hydrocarbons (PAHs) and bioaccumulate polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs). They have developed resistance to PCBs and PCDDs but not to PAHs. Resistance is largely heritable and manifests at early-life-stage toxic end points and in inducibility of cytochrome P4501A (CYP1A) mRNA expression. Because CYP1A induction is activated by the aryl hydrocarbon receptor (AHR) pathway, as are most toxic responses to these compounds, we sought to determine the geographic extent of resistance to CYP1A mRNA induction by PCBs in the Hudson River tomcod population. Samples of young-of-the-year tomcod were collected from seven locales in the Hudson River, extending from the Battery at river mile 1 (RM 1) to RM 90, and from the Miramichi River, New Brunswick, Canada. Laboratory-reared offspring of tomcod adults from Newark Bay, in the western portion of the Hudson River estuary, were also used in this study. Fish were partially depurated in clean water and intraperitoneally injected with 10 ppm coplanar PCB-77, 10 ppm benzo[a]pyrene (BaP), or corn oil vehicle, and levels of CYP1A mRNA were determined. CYP1A was significantly inducible by treatment with BaP in tomcod from the Miramichi River, from laboratory-spawned offspring of Newark Bay origin, and from all Hudson River sites spanning 90 miles of river. In contrast, only tomcod from the Miramichi River displayed significantly induced CYP1A mRNA expression when treated with PCB-77. Our results suggest that the population of tomcod from throughout the Hudson River estuary has developed resistance to CYP1A inducibility and probably other toxicities mediated by the AHR pathway. Tomcod from the Hudson River may represent the most geographically expansive population of vertebrates with resistance to chemical pollutants that has been characterized.

Okadaic acid-induced, naringin-sensitive phosphorylation of glycine N-methyltransferase in isolated rat hepatocytes

Glycine N-methyltransferase (GNMT) is an abundant cytosolic enzyme that catalyses the methylation of glycine into sarcosine, coupled with conversion of the methyl donor, S -adenosylmethionine (AdoMet), into S -adenosylhomocysteine (AdoHcy). GNMT is believed to play a role in monitoring the AdoMet/AdoHcy ratio, and hence the cellular methylation capacity, but regulation of the enzyme itself is not well understood. In the present study, treatment of isolated rat hepatocytes with the protein phosphatase inhibitor okadaic acid, was found to induce an overphosphorylation of GNMT, as shown by proteomic analysis. The analysis comprised two-dimensional gel electrophoretic separation of (32)P-labelled phosphoproteins and identification of individual protein spots by matrix-assisted laser-desorption ionization-time-of-flight mass spectrometry. The identity of GNMT was verified by N-terminal Edman sequencing of tryptic peptides. Chromatographic separation of proteolytic peptides and (32)P-labelled amino acids suggested that GNMT was phosphorylated within a limited region, and only at serine residues. GNMT phosphorylation could be suppressed by naringin, an okadaic acid-antagonistic flavonoid. To assess the possible functional role of GNMT phosphorylation, the effect of okadaic acid on hepatocytic AdoMet and AdoHcy levels was examined, using HPLC separation for metabolite analysis. Surprisingly, okadaic acid was found to have no effect on the basal levels of AdoMet or AdoHcy. An accelerated AdoMet-AdoHcy flux, induced by the addition of methionine (1 mM), was likewise unaffected by okadaic acid. 5-Aminoimidazole-4-carboxamide riboside, an activator of the hepatocytic AMP-activated protein kinase, similarly induced GNMT phosphorylation without affecting AdoMet and AdoHcy levels. Activation of cAMP-dependent protein kinase by dibutyryl-cAMP, reported to cause GNMT phosphorylation under cell-free conditions, also had little effect on hepatocytic AdoMet and AdoHcy levels. Phosphorylation of GNMT would thus seem to play no role in regulation of the intracellular AdoMet/AdoHcy ratio, but could be involved in other GNMT functions, such as the binding of folates or aromatic hydrocarbons.

Transient induction of increased aldehyde dehydrogenase 3A1 levels in cultured human breast (adeno)carcinoma cell lines via 5'-upstream xenobiotic, and electrophile, responsive elements is, respectively, estrogen receptor-dependent and -independent

Transient up-regulation of ALDH3A1, CYP1A1 and CYP1B1 transcription by transient exposure to aryl hydrocarbon receptor (AhR) ligands, e.g. 3-methylcholanthrene, is via transient transactivation of xenobiotic responsive elements (XRE) present in the 5'-upstream regions of these genes. Others have shown that AhR ligand-mediated induction of increased CYP1A1 levels in cultured human breast (adeno)carcinoma cell lines is apparently estrogen receptor (ER)-dependent, i.e. it was observed in ER(+) cell lines but not in ER(-) cell lines, whereas AhR ligand-mediated induction of increased CYP1B1 levels is ER-independent, i.e. it was observed in both ER(+) and ER(-) cell lines. The present investigation established that transient, AhR ligand/XRE-mediated induction of increased ALDH3A1 levels in human breast (adeno)carcinoma cell lines was, like that of CYP1A1 and unlike that of CYP1B1, apparently ER-dependent. Thus, transient exposure to 3-methylcholanthrene induced increased levels of ALDH3A1 in five cultured human breast (adeno)carcinoma cell lines that were documented as being ER(+), viz., MCF-7/0, MCF-7/OAP, T-47D, ZR-75-1 and MDA-MB-468, but failed to induce increased levels of this enzyme in four cultured human breast (adeno)carcinoma cell lines that have been historically viewed as being ER(-), viz., MDA-MB-231, SK-BR-3, HS-578-T and MDA-MB-435. Somewhat at odds with the foregoing, transient exposure to 3-methylcholanthrene also induced increased levels of ALDH3A1 and CYP1A1 in cultured, essentially ER(-), human breast epithelial MCF-10A cells. These cells, like cultured human breast (adeno)carcinoma cells, are immortal, but unlike the latter, are not tumorigenic. Transient induction of increased ALDH3A1 levels can also be effected by agents that are not AhR ligands, viz., electrophiles such as catechol, and thus, cannot up-regulate ALDH3A1 transcription via transactivation of a 5'-upsteam region XRE. Rather, they are thought to up-regulate ALDH3A1 transcription via transient transactivation of an electrophile responsive element (EpRE) that is putatively also present in the 5'-upstream region of this gene. Electrophile-initiated/EpRE-mediated induction of increased ALDH3A1 levels was found to be ER-independent. Thus, catechol transiently induced increased levels of ALDH3A1 in the five ER(+) human breast (adeno)carcinoma cell lines, the four ER(-) human breast (adeno)carcinoma cell lines, and the ER(-), immortal but not tumorigenic, human breast epithelial cell line.

Hepatic glycine N-methyltransferase is up-regulated by excess dietary methionine in rats

Glycine N-methyltransferase (GNMT) regulates S-adenosylmethionine (SAM) levels and the ratio of SAM:S-adenosylhomocysteine (SAH). In liver, methionine availability, both from the diet and via the folate-dependent one-carbon pool, modulates GNMT activity to maintain an optimal SAM:SAH ratio. The regulation of GNMT activity is accomplished via posttranslational and allosteric mechanisms. We more closely examined GNMT regulation in various tissues as a function of excess dietary methyl groups. Sprague Dawley rats were fed either a control diet (10% casein plus 0.3% L-methionine) or the control diet supplemented with graded levels (0.5-2%) of L-methionine. Pair-fed control groups of rats were included due to the toxicity associated with high methionine consumption. As expected, the hepatic activity of GNMT was significantly elevated in a dose-dependent fashion after 10 d of feeding the diets containing excess methionine. Moreover, the abundance of hepatic GNMT protein was similarly increased. The kidney had a significant increase in GNMT as a function of dietary methionine, but to a much lesser extent than in the liver. For pancreatic tissue, neither the activity of GNMT nor the abundance of the protein was responsive to excess dietary methionine. These data suggest that additional mechanisms contribute to regulation of GNMT such that synthesis of the protein is greater than its degradation. In addition, methionine-induced regulation of GNMT is dose dependent and appears to be tissue specific, the latter suggesting that the role it plays in the kidney and pancreas may in part differ from its hepatic function.

Getting the adrenaline going: crystal structure of the adrenaline-synthesizing enzyme PNMT

BACKGROUND

Adrenaline is localized to specific regions of the central nervous system (CNS), but its role therein is unclear because of a lack of suitable pharmacologic agents. Ideally, a chemical is required that crosses the blood-brain barrier, potently inhibits the adrenaline-synthesizing enzyme PNMT, and does not affect other catecholamine processes. Currently available PNMT inhibitors do not meet these criteria. We aim to produce potent, selective, and CNS-active PNMT inhibitors by structure-based design methods. The first step is the structure determination of PNMT.

RESULTS

We have solved the crystal structure of human PNMT complexed with a cofactor product and a submicromolar inhibitor at a resolution of 2.4 A. The structure reveals a highly decorated methyltransferase fold, with an active site protected from solvent by an extensive cover formed from several discrete structural motifs. The structure of PNMT shows that the inhibitor interacts with the enzyme in a different mode from the (modeled) substrate noradrenaline. Specifically, the position and orientation of the amines is not equivalent.

CONCLUSIONS

An unexpected finding is that the structure of PNMT provides independent evidence of both backward evolution and fold recruitment in the evolution of a complex enzyme from a simple fold. The proposed evolutionary pathway implies that adrenaline, the product of PNMT catalysis, is a relative newcomer in the catecholamine family. The PNMT structure reported here enables the design of potent and selective inhibitors with which to characterize the role of adrenaline in the CNS. Such chemical probes could potentially be useful as novel therapeutics.

Inhibition of liver methionine adenosyltransferase gene expression by 3-methylcolanthrene: protective effect of S-adenosylmethionine

Methionine adenosyltransferase (MAT) is an essential enzyme that catalyzes the synthesis of S-adenosylmethionine (AdoMet), the most important biological methyl donor. Liver MAT I/III is the product of the MAT1A gene. Hepatic MAT I/III activity and MAT1A expression are compromised under pathological conditions such as alcoholic liver disease and hepatic cirrhosis, and this gene is silenced upon neoplastic transformation of the liver. In the present work, we evaluated whether MAT1A expression could be targeted by the polycyclic arylhydrocarbon (PAH) 3-methylcholanthrene (3-MC) in rat liver and cultured hepatocytes. MAT1A mRNA levels were reduced by 50% following in vivo administration of 3-MC to adult male rats (100 mg/kg, p.o., 4 days' treatment). This effect was reproduced in a time- and dose-dependent fashion in cultured rat hepatocytes, and was accompanied by the induction of cytochrome P450 1A1 gene expression. This action of 3-MC was mimicked by other PAHs such as benzo[a]pyrene and benzo[e]pyrene, but not by the model arylhydrocarbon receptor (AhR) activator 2,3,7,8-tetrachlorodibenzo-p-dioxin. 3-MC inhibited transcription driven by a MAT1A promoter-reporter construct transfected into rat hepatocytes, but MAT1A mRNA stability was not affected. We recently showed that liver MAT1A expression is induced by AdoMet in cultured hepatocytes. Here, we observed that exogenously added AdoMet prevented the negative effects of 3-MC on MAT1A expression. Taken together, our data demonstrate that liver MAT1A gene expression is targeted by PAHs, independently of AhR activation. The effect of AdoMet may be part of the protective action of this molecule in liver damage.

Metabolism-based polycyclic aromatic acetylene inhibition of CYP1B1 in 10T1/2 cells potentiates aryl hydrocarbon receptor activity

We have used polycyclic aromatic hydrocarbon (PAH) alkyne metabolism-based inhibitors to test whether CYP1B1 metabolism is linked to aryl hydrocarbon receptor (AhR) activation in mouse embryo fibroblasts (MEF). 1-ethynylpyrene (1EP) selectively inactivated CYP1B1 dimethylbenzanthracene (DMBA) metabolism in C3H10T1/2 MEFs; whereas 1-(1-propynyl)pyrene (1PP) preferentially inhibited CYP1A1 activity in Hepa-1c1c7 mouse hepatoma cells (Hepa). In each cell type >90% inhibition of DMBA metabolism after 1 h treatment with each inhibitor (0.1 microM) was progressively reversed and then increased to levels seen with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induction (fourfold stimulation). It was found that 0.1 microM 1EP and 1PP maximally induce CYP1B1 and CYP1A1 mRNA levels in10T1/2 and Hepa cells, respectively, after 6 h. 1-Ethylpyrene (EtP), which lacks the activatable acetylene moiety, was far less effective as an inhibitor and as an inducer. AhR activation is essential for 1EP induction as evidenced by the use of AhR antagonists and AhR-deficient MEFs and absence of induction following inhibition of DMBA metabolism with carbon monoxide (CO). Inhibition of CYP1B1 was linked to enhanced AhR activation even at early stages prior to significant ligand depletion. 1EP and EtP were similarly effective in stimulating AhR nuclear translocation, though 5-10 times slower compared with TCDD, and produced no significant down-regulation of the AhR. TCDD activated AhR/Arnt complex formation with an oligonucleotide xenobiotic response element far more extensively than 1EP or EtP, even at concentrations of 1EP that increased CYP1B1 mRNA to similar levels. CO did not influence these responses to EtP, event hough CO treatment potentiated EtP induction of CYP1B1 mRNA. These differences suggest a fundamental difference between PAH/AhR and TCDD/AhR complexes where CYP1B1 metabolic activity regulates the potency, rather than the formation of the AhR/Arnt complex.

Lack of an absolute requirement for the native aryl hydrocarbon receptor (AhR) and AhR nuclear translocator transactivation domains in protein kinase C-mediated modulation of the AhR pathway

Protein kinase C (PKC)-mediated modulation of the aryl hydrocarbon receptor (AhR) pathway was examined in CHOK1-derived L10.I cells stably transfected with the pGUDLUC6.1 reporter; pGUDLUC6.1 is solely controlled by four dioxin-responsive enhancer elements. Co treatment of L10.I cells with 10 nM 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) and 81 nM phorbol 12-myristate 13-acetate (PMA), an activator of sn-1,2-diacylglyerol binding PKCs, enhanced transactivation of the reporter construct several-fold relative to cells treated with a saturating 10 nM TCDD dose alone; this effect was dubbed the "PMA effect." A domain swapping and deletional analysis of the native AhR and AhR nuclear translocator (ARNT) protein transactivation domains (TADs) was performed to determine if these domains are absolutely required for the AhR x ARNT dimer-mediated PMA effect in the L10.I model system; controls demonstrate the suitability of the L10.I model for these analyses and that endogenous AhR and ARNT levels are extremely low in this model. Transient coexpression of the AhR and ARNT-474-FLAG, an ARNT protein lacking the native ARNT TAD, in L10.I cells reveals the native ARNT TAD is not absolutely required for the AhR x ARNT-474-FLAG dimer to mediate the PMA effect. Transient coexpression of AhRDeltaCVP, a chimeric AhR protein in which the native AhR TAD has been replaced with the VP16 (herpes simplex virus protein 16) TAD (which control experiments demonstrate is unaffected by PMA), and ARNT in L10.I cells indicates that the native AhR TAD is not absolutely required for this AhRDeltaCVP x ARNT dimer to mediate the PMA effect. These observations strongly suggest that PKC-mediated modulation of the AhR pathway is not absolutely dependent on coactivators recruited to the AhR. ARNT dimer by the native TADs of the AhR and its heterodimerization partner ARNT.

The existence of the 4S polycyclic aromatic hydrocarbon-protein binding in 14-day-old chick embryo liver

Cytochrome P-450IA1, the isozyme most closely associated with aryl hydrocarbon hydroxylase (AHH), is regulated by two high-affinity binding proteins, the 4S polycyclic aromatic hydrocarbon (PAH)-binding protein which primarily binds PAHs and the 8S Ah (dioxin) receptor which binds 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and like congeners. The present study was conducted to determine whether the 4S protein existed in 14-day-old chick embryo liver when AHH activity is maximal to determine if they are linked as is the 8S Ah receptor and to confirm the existence of the dioxin receptor by investigating their ligand binding characteristics in the presence and absence of sodium molybdate, an agent that stabilizes steroid hormone receptors and partially stabilizes the dioxin receptor. Competitive ligand binding studies were performed with liver cytosol from livers of male 14-day-old chick embryos using [3H]-benzo[a]pyrene (B[a]P) or [3H]-TCDD in the presence and absence of a 200-fold excess of B[a]P, benzo[e]pyrene (B[e]P), 3-methylcholanthrene (3-MC), and tetrachlorodibenzofuran (TCDBF). Specific PAH-binding activity was assayed using sucrose gradient analysis. In the absence of molybdate, the 4S PAH-binding protein had high affinity for B[a]P, B[e]P, 3-MC, but very low affinity for TCDBF; the Ah receptor exhibited high affinity for TCDBF. In the presence of sodium molybdate, the Ah receptor was stabilized while the 4S PAH-binding protein was relatively unaffected. These results affirm the existence of two distinct PAH-binding proteins in 14-day-old chick embryo liver cytosol and suggest a linkage of the 4S protein to AHH.

Dexamethasone regulation of the rat 3alpha-hydroxysteroid/dihydrodiol dehydrogenase gene

Rat liver 3alpha-hydroxysteroid/dihydrodiol dehydrogenase (3alpha-HSD/DD), a member of the aldo-keto reductase superfamily, inactivates circulating steroid hormones and may contribute to the carcinogenicity of polycyclic aromatic hydrocarbons (PAHs) by oxidizing trans-dihydrodiols to reactive o-quinones with the concomitant generation of reactive oxygen species. The 3alpha-HSD/DD gene has been cloned, and its 5'-flanking region contains a negative response element (NRE; -797 to -498 bp) that may repress constitutive expression by binding to Oct transcription factors. Upstream from the NRE are three distal imperfect glucocorticoid response elements (GRE1, GRE2, and GRE3); in addition, a proximal imperfect GRE (GRE4) is adjacent to an Oct binding site in the NRE. When rat hepatocytes were cultured on Matrigel and exposed to dexamethasone (Dex), steady state levels of 3alpha-HSD/DD mRNA were increased 4-fold in a dose-dependent manner, yielding an EC50 value of 10 nM. Time to maximal response was 24 hr, and the effect was blocked with the anti-glucocorticoid RU486. Measurement of the half-life of 3alpha-HSD/DD mRNA, with and without Dex treatment, indicated that the increase in steady state mRNA levels was not due to increased mRNA stability. By contrast, nuclear run-off experiments using nuclei obtained from Dex-stimulated hepatocytes indicated that Dex increased transcription of the rat 3alpha-HSD/DD gene. Tandem repeats of the imperfect GRE1, GRE2, GRE3, and GRE4 were inserted into thymidine kinase-chloramphenicol acetyl-transferase vectors and cotransfected with the human glucocorticoid receptor into human hepatoma cells. On treatment with Dex, maximal trans-activation of the chloramphenicol acetyl-transferase reporter gene activity was mediated via the proximal GRE (GRE4). These data imply that GRE4 is a functional cis-element and that binding of the occupied glucocorticoid receptor to this element increases 3alpha-HSD/DD gene transcription. A model is proposed for the positive and negative regulation of the rat 3alpha-HSD/DD gene by the glucocorticoid receptor and Oct transcription factors, respectively.