S腺苷甲硫氨酸对肝癌细胞急性缺血/缺氧过程中的生物学作用

目的：研究S腺苷甲硫氨酸（SAMe）在急性缺血/缺氧过程中对肝癌细胞HepG2的生物学作用,并通过检测自噬的变化探讨其可能机制。方法：real time-PCR检测自噬特异性基因（Beclin 1）表达的变化;吖啶橙染色后,采用荧光显微镜定性观察自噬;CCK-8法检测HepG2细胞的成活率;用AnnexinⅤ/PI流式细胞仪检测细胞凋亡的变化。结果：SAMe可诱导HepG2细胞的自噬,在单纯缺血和缺血/缺氧处理因素下,HepG2自噬在荧光强度和Beclin 1基因水平分别比空白对照组增强约3.2倍和3.5倍。细胞成活率分别比空白对照组增加了约20%和30%。SAMe对HepG2细胞具有显著的抑制作用,这种抑制作用在缺血/缺氧环境中更加明显,SAMe单独处理组和SAMe预处理结合缺血/缺氧组,其细胞成活率与空白对照组相比分别下降了约30%和70%。随着细胞成活率下降,细胞凋亡比例相应增加,与空白对照组相比,细胞经SAMe处理后凋亡比例增加约18%;细胞经SAMe预处理后再予以缺血/缺氧,凋亡比例增加约30%。结论：在SAMe抑制HepG2细胞生长过程中,自噬起着重要作用。     

腺苷蛋氨酸对酒精性肝损伤大鼠HHcy和TNF-α的影响

目的 观察S-腺苷蛋氨酸（S-adenosylmethionine,SAM）对酒精性肝损伤大鼠的防治作用及机制。方法 48只SD大鼠随机分为4组（每组12只）：对照组、模型组、腺苷蛋氨酸低、高剂量组。除对照组外,其余三组给予酒精、鱼油灌胃配合高脂饮食构建酒精性肝损伤大鼠模型,造模4周后腹腔注射SAM,第8周处死全部大鼠。测定血浆总同型半胱氨酸（total plasma homocysteine,tH-cy）、血清丙氨酸转移酶（ALT）、肝匀浆丙二醛（MDA）和还原型谷胱甘肽（GSH）含量,并应用半定量逆转录聚合酶链反应（RT-PCR）法检测大鼠肝组织TNF-α mRNA水平。结果 与对照组比较,模型组大鼠tHcy、ALT、MDA含量均明显升高（P＜0．01）;GSH水平降低（P〈O．01）;大鼠肝组织TNF-α mRNA表达明显增强（P〈O．01）。与模型组比较,SAM治疗组ALT、MDA明显降低（P〈O．05）;GSH含量升高（P〈O．01）;肝组织内TNF-α-mRNA水平明显降低（P〈O．01）;但血浆tHcy水平无显著变化（P〉O．05）。结论 SAM可明显改善大鼠酒精性肝损伤,其机制可能与其降低肝组织脂质过氧化、抑制肝内TNF-α表达有关,SAM对血浆tHcy水平无显著影响。     

产腺苷蛋氨酸酵母菌株的选育

目的筛选产腺苷蛋氨酸(SAM)的酵母菌并进行诱变育种。方法以选择性培养基筛选酵母菌株并用高效液相色谱检测SAM。利用紫外线和γ-射线处理对菌株进行诱变。结果从34份土样中筛选到1株产SAM的酵母菌Q95菌株,经过5轮紫外线诱变和1轮γ-射线诱变,筛选到1株SAM产量显著提高的正突变株Q6-13。摇瓶发酵27 h后,其SAM产量达到1860μg/mL,与Q95相比提高了86.9%。在15 L外循环气升式生物反应器中补料发酵22 h后,Q6-13的SAM产量达到2540μg/mL。结论通过筛选和诱变,得到了1株高产SAM的酵母菌突变株,为SAM的微生物发酵法规模化生产奠定了基础。     

Distinct cellular expressions of creatine synthetic enzyme GAMT and creatine kinases uCK-Mi and CK-B suggest a novel neuron-glial relationship for brain energy homeostasis

The creatine/phosphocreatine shuttle system, as catalysed reversibly by creatine kinases, is thought to be essential for the storing and buffering of high phosphate-bound energy in tissues with high energy demand. In the present study, we aimed to clarify the cellular system of creatine biosynthesis and its energy metabolism in the mouse brain by immunohistochemistry for creatine biosynthetic enzyme S-adenosylmethionine:guanidinoacetate N-methyltransferase (GAMT), ubiquitous mitochondrial creatine kinase (uCK-Mi) and brain-type cytoplasmic creatine kinase (CK-B). GAMT was expressed highly in oligodendrocytes and olfactory ensheathing glia and moderately in astrocytes, whereas GAMT was very low in neurons and microglia. By contrast, uCK-Mi was expressed selectively in neurons and localized in their mitochondria in dendrites, cell bodies, axons and terminals. The distinct and almost complementary distribution of GAMT and uCK-Mi suggests that the creatine in neuronal mitochondria is derived not only from the circulation, but also from local glial cells associated with these neuronal elements. By contrast, CK-B was selective to astrocytes among glial populations, and was exclusive to inhibitory neurons among neuronal populations. Interestingly, these cells with high CK-B immunoreactivity are known to be highly resistant to acute energy loss, such as hypoxia and hypoglycemia. Considering that phosphocreatine generates ATP much faster than the processes of glycolysis and oxidative phosphorylation, the highly regulated cellular expressions of creatine biosynthetic and metabolic enzymes suggest that the creatine/phosphocreatine shuttle system plays a role in brain energy homeostasis through a novel neuron-glial relationship.     

Enzymatic methyl esterification of a deamidated form of mouse epidermal growth factor

The enzyme S-adenosylmethionine:protein carboxyl-O-methyl-transferase, type II (EC2.1.1.77; PCMT) from eukaryotes methyl esterifies peptides containing isoAsp residues, which can arise from spontaneous deamidation of labile Asn residues. We report here a study on in vitro methyl esterification of mouse EGF by bovine brain PCMT. This peptide contains two Asn in the sequences Asn¹-Ser² and Asn¹⁶-Gly¹⁷. It is known from the literature that the presence of a small residue on the carboxyl side of asparaginyl makes this residue susceptible to deamidation through the spontaneous formation of a succinimide intermediate. Therefore EGF was incubated under deamidating conditions (pH9.0, 37° for 48 h) and the extent of deamidation monitored by enzymatically measuring the NH₃ produced during the alkali treatment: a release of 0.80 mol NH₃/mol EGF was calculated. The alkali-treated EGF, analyzed by anion-exchange chromatography, shows two major components identified as native EGF (nEGF) and its deamidated form (dEGF). When incubated in the presence of purified PCMT neither nEGF nor dEGF showed any methyl accepting capability. Since it is known that the three-dimensional structure of a protein may hinder the methyl esterification of a potential ethyl accepting site, dEGF was unfolded by reducing and alkylating the intrachain disulfide bridges. Only a slight increase in the methyl accepting capability could be observed. Conversely, when EGF was deamidated after its unfolding, the resulting protein was stoichiometrically methylated by PCMT, presumably at level of isoAsp¹⁶. Our findings strongly suggest that the three-dimensional structure of a protein is a major specificity determinant for both deamidation and methyl esterification processes.     

Early increases in transglutaminase activity and polyamine levels in a Mallory-Denk body mouse model

Rodents treated with 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC) are a model of two hepatic toxic manifestations: porphyria and the appearance of hepatic cytoplasmic protein aggregates (Mallory-Denk Bodies, MDBs). MDBs are induced after long-term DDC feeding, consist primarily of keratins 8 and 18, and contain glutamine-lysine cross-links generated by transglutaminases (TGs). TGs are Ca²⁺-dependent enzymes which catalyze the formation of covalent bonds between proteins and between proteins and polyamines. The aim of the current study was to investigate the time-course of TG hepatic activity in CF1 male mice either acutely or chronically treated with DDC and to correlate this activity with polyamine and porphyrin levels. On day 3 of the treatment, statistically significant increases in TG activity (75%), porphyrin content (6740%) and spermidine levels (73%) were observed. Although not statistically significant, at this time point putrescine levels showed an increase of 52%. The highest TG activity was observed on day 30 (522%), while porphyrin levels were still gradually increasing by day 45 (37,000%). From day 7 of the treatment and until the end of the experiment, putrescine levels remained increased (781%). Spermine levels were not affected by the treatment. The DDC-induced increases in putrescine and spermidine levels herein reported seem to be an early event contributing to the stimulation of liver TG activity, and thus to the promotion of cross-linking reactions between keratin proteins. This in turn would contribute to the formation of protein aggregates, which would lead to the appearance of MDBs. Due to the pro-oxidant and antioxidant properties of polyamines, it is possible to speculate that putrescine and spermidine may also participate at several levels in the oxidative stress processes associated with MDB formation.     

Methylation of cortical brain proteins from patients with HIV infection

OBJECTIVES: Experimental models of vitamin B12 deficient-neuropathy are characterized by central nervous system protein hypomethylation. The encephalitis/vacuolar myelopathy complicating HIV infection and subacute combined degeneration of the cord due to vitamin B12 deficiency share similar biochemical and pathological abnormalities. Altered central nervous system methylation may be important in the pathogenesis of HIV encephalitis. To test this hypothesis we compared brain protein methylation of HIV-positive, and control, subjects. MATERIALS AND METHODS: Carboxymethyltransferase activity was assayed in postmortem cortical brain samples obtained from 16 control patients (9 males); mean age (59+/-5.1 years, range 21-87 years), 9 HIV-positive patients (7 males, 6 IVDA, 3 homosexual, 4 with HIV encephalitis, mean age 37, range 23-45), and 3 patients with Alzheimer's disease (mean age 78 years). RESULTS: The amount of radiolabelled SAM (S-adenosylmethionine) incorporated into carboxymethyl, and N-methylation sites within brain proteins from cortical white matter in vitro was significantly lower (P<0.05) in the HIV+ group vs controls. Carboxymethyltransferase activity was similar in the HIV-infected brains irrespective of the presence or absence of HIV encephalitis. Mean cortical methyl group incorporation was also lower in the Alzheimer's disease group compared to controls. CONCLUSION: The observation of reduced in vitro methylation of brain proteins from patients with HIV infection and Alzheimer's disease suggests that fewer unmethylated sites exist due to relative protein hypermethylation in vivo. The absence of hypomethylation in the brains of patients with HIV encephalitis suggests that hypomethylation is not necessary for the development of HIV encephalitis.     

S-adenosylmethionine regulates thiopurine methyltransferase activity and decreases 6-mercaptopurine cytotoxicity in MOLT lymphoblasts

Six-mercaptopurine (6-MP) is a pro-drug widely used in treatment of various diseases, including acute lymphoblastic leukaemia (ALL). Side-effects of thiopurine therapy have been correlated with thiopurine methyltransferase (TPMT) activity.We propose a novel TPMT-mediated mechanism of S-adenosylmethionine (SAM)-specific effects on 6-mercaptopurine (6-MP) induced cytotoxicity in a model cell line for acute lymphoblastic leukemia (MOLT). Our results show that exogenous SAM (10-50 μM) rescues cells from the toxic effects of 6-MP (5 μM) by delaying the onset of apoptosis. We prove that the extent of methylthioinosine monophosphate (MeTIMP) induced inhibition of de novo purine synthesis (DNPS) determines the concentrations of intracellular ATP, and consequently SAM, which acts as a positive modulator of TPMT activity. This leads to a greater conversion of 6-MP to inactive 6-methylmercaptopurine, and thus lower availability of thioinosine monophosphate for the biotransformation to cytotoxic thioguanine nucleotides (TGNs) and MeTIMP. We further show that the addition of exogenous SAM to 6-MP treated cells maintains intracellular SAM levels, TPMT activity and protein levels, all of which are diminished in cells incubated with 6-MP. Since TPMT mRNA levels remained unaltered, the effect of SAM appears to be restricted to protein stabilisation rather than an increase of TPMT expression. We thus propose that SAM reverses the extent of 6-MP cytotoxicity, by acting as a TPMT-stabilizing factor.This study provides new insights into the pharmacogenetics of thiopurine drugs. Identification of SAM as critical modulator of TPMT activity and consequently thiopurine toxicity may set novel grounds for the rationalization of thiopurine therapy.     

Alcohol-induced S-adenosylhomocysteine accumulation in the liver sensitizes to TNF hepatotoxicity: possible involvement of mitochondrial S-adenosylmethionine transport

Hepatocytes are resistant to tumor necrosis factor-alpha- (TNF) induced killing/apoptosis under normal circumstances, but primary hepatocytes from rats chronically fed alcohol have increased TNF cytotoxicity. Therefore, there must be mechanism(s) by which alcohol exposure "sensitizes" to TNF hepatotoxicity. Abnormal metabolism of methionine and S-adenosylmethionine (SAM) are well-documented acquired metabolic abnormalities in ALD. S-adenosylhomocysteine (SAH) is the product of SAM in hepatic transmethylation reactions, and SAH hydrolase (SAHH) is the only enzyme to metabolize SAH to homocysteine and adenosine. Our previous studies demonstrated that chronic intracellular accumulation of SAH sensitized hepatocytes to TNF cytotoxicity in vitro. In the current study, we extended our previous observations by further characterizing the effects of chronic alcohol intake on mitochondrial SAM levels in liver and examining its possible involvement in SAH sensitization to TNF hepatotoxicity. Chronic alcohol consumption in mice not only increased cytosolic SAH levels, but also decreased mitochondrial SAM concentration, leading to decreased mitochondrial SAM to SAH ratio. Moreover, accumulation of hepatic SAH induced by administration of 3-deaza-adenosine (DZA-a potent inhibitor of SAHH) enhanced lipopolysaccharide (LPS)/TNF hepatotoxicity in mice in vivo. Inhibition of SAHH by DZA resulted not only in accumulation of cytoplasmic SAH, but also in depletion of the mitochondrial SAM pool. Further studies using mitochondrial SAM transporter inhibitors showed that inhibition of SAM transport into mitochondria sensitized HepG2 cells to TNF cytotoxicity. In conclusion, our results demonstrate that depletion of the mitochondrial SAM pool by SAH, which is elevated during chronic alcohol consumption, plays a critical role in SAH induced sensitization to TNF hepatotoxicity.