Phenylalanine 4-monooxygenase and the S-oxidation of S-carboxymethyl-L-cysteine

The identity of the enzyme responsible for the S-oxidation of the mucolytic S-substituted L-cysteine drug, S-carboxymethyl-L-cysteine (SCMC), has been actively investigated for the last 10 years. A genetic polymorphism exists in the oxidation of the thioether moiety that has been identified as a disease susceptibility factor in a number of degenerative diseases. This polymorphism has also been implicated in the wide variation in clinical response to SCMC therapy in man. To date little is known about the molecular enzymology of this reaction but a previous investigation revealed that rat activated phenylalanine 4-monooxygenase (PAH) could S-oxidise both Met- and S-methyl-L-cysteine (SMC) to their S-oxide metabolites. We have investigated the hypothesis that SCMC was also a substrate for activated PAH in the cytosolic faction of the Wistar rat. 1. Substrate and inhibitor investigation revealed that SCMC was a substrate for activated PAH activity in vitro. 2. The large aromatic amino acid hydroxylase monoclonal antibody and the Fe3+ chelator, deferoxamine, completely inhibited both Phe and SCMC oxidation to their respective metabolites. 3. Analysis of the Dixon plots revealed that both Phe and SCMC competitively inhibited each other's oxidation. 4. Correlation studies showed that the rate of production of Tyr was positively correlated to the production of both SCMC and SMC S-oxides in 20 female Wistar rat hepatic cytosolic fractions. These results strongly support the hypothesis that PAH is the enzyme responsible for SCMC S-oxidation in the rat.     

Pharmacogenetics of the S-oxidation of S-carboxymethyl-L-cysteine

The pharmacogenetics of S-carboxymethyl-L-cysteine (SCMC) have been studied in detail. When results from administration of SCMC to 200 volunteers were analysed, there was seen to be a wide interindividual variation in the percentage of sulphoxide metabolites excreted. Computer assisted analysis suggested that the population distribution observed could be most economically represented as two overlapping Gaussian distributions with the smaller mode representing poor sulphoxidisers. This phenomenon appears to be largely genetic in origin and to behave as though controlled by one autosomal recessive gene, but environmental factors may also be important. Poor sulphoxidisers seem to be overrepresented in certain patient populations with chronic diseases. These findings are discussed in terms of oxidative metabolism of sulphur-containing compounds.     

The S-oxidation of S-carboxymethyl-L-cysteine in hepatic cytosolic fractions from BTBR and phenylketonuria enu1 and enu2 mice

1. Mice that were heterozygous dominant for the enu1 and enu2 mutation in phenylalanine monooxygenase/phenylalanine hydroxylase (PAH) resulted in hepatic PAH assays for S-carboxymethyl-L-cysteine (SCMC) that had significantly increased calculated K_m (wild type (wt)/enu1, 1.84–2.12 fold increase and wt/enu2 a 2.75 fold increase in PAH assays). The heterozygous dominant phenotypes showed a significantly reduced catalytic turnover of SCMC (wt/enu1, 6.11 fold decrease and wt/enu2 an 11.25 fold decrease in calculated V_max). Finally, these phenotypes also had a significantly reduced clearance, C_LE (wt/enu1, 13.02 fold and wt/enu2, a 30.80–30.94 fold decrease) The homozygous recessive phenotype (enu1/enu1) was also found to have significantly increased calculated K_m (2.16 fold increase), a significantly reduced calculated V_max (11.35–12.33 fold decrease) and C_LE (24.75–25.00 fold decrease). The enu2/enu2, homozygous recessive phenotype had no detectable PAH activity using SCMC as substrate.

2. The identity of the enzyme responsible for the C-oxidation of L-phenylalanine (L-Phe) and the S-oxidation of SCMC in wt/wt (BTBR) mice was identified using monoclonal antibody and selective chemical inhibitors and was found to be PAH.

3. This in vitro mouse hepatic cytosolic fraction metabolism investigation provides further evidence to support the hypothesis that an individual possessing one variant allele for PAH will result in a poor metaboliser phenotype that is unable to produce significant amounts of S-oxide metabolites of SCMC.

     

Lack of congruence between cysteine dioxygenase activity and S-carboxymethyl-L-cysteine S-oxidation activity in rat cytosol

The identity of the enzyme(s) responsible for the S-oxidation of the mucoactive drug S-carboxymethyl-L-cysteine (SCMC) is unknown but the protein(s) are a susceptibility factor for a number of chronic degenerative diseases. The structural similarities between the amino acid L-cysteine and SCMC have raised the possibility that cysteine dioxygenase (CDO) may be responsible for this biotransformation reaction. Both CDO and SCMC S-oxygenase were found to require Fe2+ for enzymatic activity, and both enzyme activities were inhibited by Fe2+ and Fe3+ chelators. However, sulphydryl group modification of the enzymes resulted in the activation of the S-oxidation of SCMC but inhibition of the S-oxidation of L-cysteine. When the two enzyme activities were quantified in 20 female hepatic cytosolic fractions no linear correlation in the production of their respective metabolites was seen. The results of this investigation indicate that CDO is not responsible for the S-oxidation of SCMC in the rat.     

Effect of S-oxidation on the anticoagulant effects of 4-hydroxycoumarins, 4-hydroxy-2-pyrones and 1,3-indanediones

S-Oxidation and Anticoagulant Activity in 4-Hydroxycoumarines, 4-Hydroxy-2-pyrones and 1,3-Indanediones Methylthio derivatives of the title compounds as well as their S-oxides and S-dioxides were synthesized and tested for their anticoagulant activities. After oral administration of a single dose to rats, all oxides led to a prothrombin level of less than 25% within twelve hours. Compounds 3a – c and the oxirane 5 are unable to form enols, but nevertheless show strong anticoagulant activity.     

山奈酚对HepG2细胞增殖的影响及诱导其凋亡的机制研究

目的研究山奈酚对人肝癌细胞(HepG2细胞)增殖与凋亡的影响及其机制。方法将HepG2细胞分为空白组和实验组,实验组以CCK-8法检测10个不同浓度的(10,20,30,40,50,60,70,80,90,100μmol·L^-1)山奈酚处理HepG2细胞24 h后的存活率。最终以3个浓度(20,40,50μmol·L^-1)山奈酚作为低、中、高3个浓度实验组。以免疫印迹法检测核苷酸结合寡聚化域样受体蛋白3(NLRP3)和死骨片-1(P62)蛋白表达水平(灰度值)。结果山奈酚处理HepG2细胞24 h后,空白组与低、中、高3个浓度实验组HepG2细胞的存活率分别为(100.00±0.00)%,(87.92±3.13)%,(77.92±4.40)%和(70.53±4.19)%;上述这4组的NLRP3表达水平分别为0.27±0.05,0.50±0.03,0.71±0.08和0.93±0.10;上述这4组的P62表达水平分别为0.54±0.06,0.76±0.05, 0.87±0.04和1.09±0.10。上述指标:3个浓度实验组与空白组比较,差异均有统计学意义(P<0.05,P<0.01,P<0.001)。结论山奈酚可有效抑制HepG2细胞增殖、抑制其自噬并诱导其凋亡,其机制可能为有效促进P62的沉积,同时能诱导凋亡经典途径中NLRP3的合成,从而抑制HepG2细胞自噬并促进其凋亡。     

翻白草油对肝癌HepG2细胞CDK4蛋白的影响

目的:检测翻白草油对肝癌HepG2细胞CDK4蛋白表达的影响,探讨翻白草油抗肝癌的机制.方法:翻白草油作用肝癌HepG2细胞后,用噻唑蓝(MTT)比色法检测HepG2细胞增殖活性和IC50;western blot法检测CDK4蛋白的表达.结果:翻白草油能抑制HepG2细胞增殖,且呈剂量依赖性,IC50值为2.03mg...     

Curcumin-induced genotoxicity and antigenotoxicity in HepG2 cells

Curcumin, a polyphenolic yellow pigment found in turmeric, is commonly used as a coloring agent in foods, drugs, and cosmetics. In our previous study, we found that low levels of curcumin did not increase the reactive oxygen species (ROS) formation and caused no damage to DNA in human hepatoma G2 (HepG2) cells, but at high doses, curcumin imposed oxidative stress and damaged DNA. In the present study, we are determined to investigate the genotoxic and antigenotoxic effects of curcumin using HepG2 cell line, a relevant in vitro model to detect the cytoprotective, antigenotoxic, and cogenotoxic agents. The results of micronucleus (MN) assays showed that, on one hand, curcumin at the high tested concentrations (8 and 16 μg/ml) displayed a small but significant increase in the frequency of MN, and on the other hand, it was observed that the low tested concentration (2 μg/ml) significantly reduced the MN formation induced by the chemotherapeutic agent cyclophosphamide. The present results indicate that curcumin shows both genotoxicity and antigenotoxicity depending on its concentration.     

Transfection of HepG2 cells with hGSTA4 provides protection against 4-hydroxynonenal-mediated oxidative injury

4-Hydroxynonenal (4-HNE) is a mutagenic ,β-unsaturated aldehyde produced during oxidative injury that is conjugated by several glutathione S-transferase (GST) isoforms. The alpha class human GSTA4-4 enzyme (hGSTA4-4) has a particularly high catalytic efficiency toward 4-HNE conjugation. However, hGST4-4 expression is low in most human cells and there are other aldehyde metabolizing enzymes that detoxify 4-HNE. In the current study, we determined the effect of over-expression of hGSTA4 mRNA on the sensitivity of HepG2 cells to 4-HNE injury. HepG2 cells transfected with an hGSTA4 vector construct exhibited high steady-state hGSTA4 mRNA, high GST–4-HNE catalytic activities, but lower basal glutathione (GSH) concentrations relative to insert-free vector (control) cells. Exposure to 4-HNE elicited an increase in GSH concentrations in the control and hGSTA4 cells, although the dose-response of GSH induction differed among the two cell types. Specifically, hGSTA4 cells had significantly higher GSH concentrations when exposed to 5–15 μM 4-HNE, but not at 20 μM 4-HNE, suggesting extensive GSH utilization at high concentrations of 4-HNE. The hGSTA4 cells exhibited a significant growth advantage relative to control cells in the absence of 4-HNE, and a trend towards increased growth at low dose exposures to 4-HNE. However, the hGSTA4 cells did not exhibit a growth advantage relative to control cells at higher 4-HNE exposures associated with increased GSH utilization. As expected, the hGSTA4 cells showed resistance to 4-HNE stimulated lipid peroxidation at all 4-HNE doses. In summary, our data indicates that over-expression of hGSTA4 at levels conferring high GST–4-HNE conjugating activity confers a partial growth advantage to HepG2 cells and protects against 4-HNE oxidative injury. However, the loss of proliferative capacity of hGSTA4 cells challenged with levels of 4-HNE associated with severe oxidative stress indicates a role of other aldehyde metabolizing enzymes, and/or GSH–electrophile transporter proteins, in providing full cellular protection against 4-HNE toxicity.     

The role of CYP3A4 in amiodarone-associated toxicity on HepG2 cells

Amiodarone is a class III antiarrhythmic drug with potentially life-threatening hepatotoxicity. Recent in vitro investigations suggested that the mono-N-desethyl (MDEA) and di-N-desethyl (DDEA) metabolites may cause amiodarone's hepatotoxicity. Since cytochrome P450 (CYP) 3A4 is responsible for amiodarone N-deethylation, CYP3A4 induction may represent a risk factor. Our aim was therefore to investigate the role of CYP3A4 in amiodarone-associated hepatotoxicity. First, we showed that 50 μM amiodarone is more toxic to primary human hepatocytes after CYP induction with rifampicin. Second, we overexpressed human CYP3A4 in HepG2 cells (HepG2 cells/CYP3A4) for studying the interaction between CYP3A4 and amiodarone in more detail. We also used HepG2 wild type cells (HepG2 cells/wt) co-incubated with human CYP3A4 supersomes for amiodarone activation (HepG2 cells/CYP3A4 supersomes). Amiodarone (10–50 μM) was cytotoxic for HepG2 cells/CYP3A4 or HepG2 cells/CYP3A4 supersomes, but not for HepG2 cells/wt or less toxic for HepG2 cells/wt incubated with control supersomes without CYP3A4. Co-incubation with ketoconazole, attenuated cytotoxicity of amiodarone incubated with HepG2 cells/CYP3A4 or HepG2 cells/CYP3A4 supersomes. MDEA and DDEA were formed only in incubations containing HepG2 cells/CYP3A4 or HepG2 cells/CYP3A4 supersomes but not by HepG2 cells/wt or HepG2 cells/wt with control supersomes. Metabolized amiodarone triggered the production of reactive oxygen species, induced mitochondrial damage and cytochrome c release, and promoted apoptosis/necrosis in HepG2 cells/CYP3A4, but not HepG2 cells/wt. This study supports the hypothesis that a high CYP3A4 activity is a risk factor for amiodarone's hepatotoxicity. Since CYP3A4 inducers are used frequently and amiodarone-associated hepatotoxicity can be fatal, our observations may be clinically relevant.     

索拉非尼在HepG2与LO2细胞的摄取动力学

目的　探讨索拉非尼在肝癌细胞(HepG2)与正常肝细胞(LO2)的摄取动力学特性。方法取正常培养的处于对数期的HepG2与LO2细胞株,加入含索拉非尼系列浓度的培养液,孵育后处理细胞,检测索拉非尼及细胞蛋白浓度,分析摄取动力学参数。结果 HepG2 与LO2细胞对索拉非尼的摄取随着浓度的增加而趋于饱和,但与LO2细胞相比较,HepG2细胞对索拉非尼的摄取量明显增加。当加入10 mol·L-~1的索拉非尼,HepG2与LO2细胞对索拉非尼的摄取随着时间的增加而增加,均在20 min时趋于饱和。HepG2细胞的摄取动力参数V_(max)为(684.14±78.19)pmol·mg protein-~1·min-~1,K_m为(93.3±17.56)μmol·L-~1;LO2细胞的摄取动力参数V_(max)为(335.61±69.73)pmol·mg protein-~1·min-~1,K_m为(135.68±29.34)μmol·L-~1;结果显示HepG2细胞对索拉非尼的摄取速率明显高于LO2细胞。结论索拉非尼在肝癌细胞中摄取量及摄取速率明显高于正常肝细胞,对肝癌细胞具有一定的靶向性。     

Atrazine represses S100A4 gene expression and TPA-induced motility in HepG2 cells

Atrazine (ATZ) is probably the most widely used herbicide in the world. However there are still many controversies regarding its impacts on human health. Our investigations on the role of pesticides in liver dysfunctions have led us to detect an inhibition of FSP1 expression of 70% at 50 μm and around 95% at 500 μM of ATZ (p < 0.01). This gene encodes the protein S100a4 and is a clinical biomarker of epithelial–mesenchymal transition (EMT), a key step in the metastatic process. Here we investigated the possible effect of ATZ on cell migration and noticed that it prevents the EMT and motility of the HepG2 cells induced by the phorbol ester TPA. ATZ decreases Fak pathway activation but has no effect on the Erk1/2 pathway known to be involved in metastasis in this cell line. These results suggest that ATZ could be involved in cell homeostasis perturbation, potentially through a S100a4-dependant mechanism.     

Mechanisms of tolvaptan-induced toxicity in HepG2 cells

Tolvaptan, a vasopressin receptor 2 antagonist used to treat hyponatremia, has recently been reported to be associated with an increased risk of liver injury. In this study, we explored the underlying mechanisms of hepatotoxicity of tolvaptan using human HepG2 cells. Tolvaptan inhibited cell growth and caused cell death in a concentration- and time-dependent manner. Tolvaptan treatment led to delayed cell cycle progression, accompanied by decreased levels of several cyclins and cyclin-dependent kinases. Tolvaptan was found to cause DNA damage, as assessed by alkaline comet assays; this was confirmed by increased levels of 8-oxoguanine and phosphorylation of histone H2AX. Exposure of HepG2 cells to tolvaptan enhanced cytochrome C release and triggered apoptosis by modulating Bcl-2 family members. The activation of p38 contributed to tolvaptan-mediated apoptosis via down-regulation of Bcl-2. Proteasome inhibition altered tolvaptan-induced cell cycle deregulation and enhanced tolvaptan-induced apoptosis and cytotoxicity. Moreover, tolvaptan treatment induced autophagy. Inhibition of autophagy by knocking-down an autophagy-related gene increased tolvaptan-induced apoptosis and cytotoxicity. Taken together, our findings suggest that the cytotoxicity of tolvaptan results from delayed cell cycle progression, the induction of DNA damage, and the execution of apoptosis. In addition, a number of signaling pathways were perturbed by tolvaptan and played an important role in its cytotoxicity.     

阿司匹林联合华法林对HepG2细胞中CYP3A4酶的抑制作用

目的研究阿司匹林联用华法林对肝药物代谢酶CYP3A4活性的影响及其机制。方法不同质量浓度的阿司匹林、华法林及联合用药处理Hep G2细胞48 h,采用MTT法检测细胞存活率;通过荧光素酶报告基因技术检测各组对PXR转录酶活性和酶CYP3A4活性的影响;采用荧光定量PCR法和Western blotting法检测Hep G2细胞的酶CYP3A4的m RNA和蛋白表达水平。结果与对照组比较,阿司匹林组、联合用药组细胞的孕烷X受体(PXR)转录酶活性、酶CYP3A4活性均显著降低(P0.01),CYP3A4 m RNA和蛋白表达水平显著降低(P0.05、0.01),华法林组均无显著差异。结论阿司匹林联用华法林能够抑制药物代谢酶CYP3A4活性,其机制可能是通过抑制PXR受体的m RNA和蛋白表达实现的。     

Intracellular disposition of arabinogalactan and asialofetuin in HepG2 cells

The binding and uptake of arabinogalactan and asialofetuin in HepG2 cells was kinetically characterized using ^I25I-labeled ligands. The number of binding sites (n) and the association constant (K) of arabinogalactan was 1.9 × 10⁵ ± 1.2 × 10⁵ sites/cell and 5.0 × 10⁶ ± 3.9 × 10⁶ M^? 1, respectively, whereas the n and K_a of asialofetuin was 2.7 × 10⁵ ± 1.1 × 10⁵ sites/cell and 1.1 × 10⁷ ± 0.7 × 10⁷ M^? 1, respectively. These results suggest that the binding capacity of HepG2 cells for arabinogalactan is lower than that for asialofetuin. Moreover, the amount of arabinogalactan uptake by HepG2 cells was lower than that of asialofetuin. Thus, asialofetuin was preferentially bound and internalized by hepatoma cells compared to arabinogalactan.