Thioredoxin-Interacting Protein (TXNIP) Suppresses Expression of Glutamine Synthetase by Inducing Oxidative Stress in Retinal Muller Glia Under Diabetic Conditions.

BACKGROUND Diabetic retinopathy (DR) is a progressive neurodegenerative disease with early-stage symptoms such as dysfunction of Muller cells, which leads to ganglion cell death. Its pathogenesis is probably associated with oxidative stress and a recently discovered protein, thioredoxin-interacting protein (TXNIP). MATERIAL AND METHODS To explore the role of TXNIP in DR, we cultured Muller cells under diabetic conditions, and then used immunohistochemistry, Western blot, and RT-PCR to detect the expression level of TXNIP under diabetic conditions. We demonstrated the expression level of glutamine synthetase (GS) when TXNIP was inhibited. To explore the potential pathway of TXNIP-induced cell damage in DR, we confirmed the role of IL-1ß under diabetic conditions. RESULTS Diabetes induces TXNIP expressions at mRNA levels, but shows the opposite effect on GS. IL-1ß plays an important role in this pathway. Azaserine effectively increased the expression of GS via attenuating the expression of TXNIP. CONCLUSIONS This study demonstrates the role of TXNIP and its mechanism in DR, provides a possible treatment for DR, and lays a new theoretical foundation for the clinical treatment of DR and other diabetic microvascular changes.

Identification and characterization of genes related to the production of organic acids in yeast.

Organic acids contribute to the flavor of many foods and drinks including alcoholic beverages. To study the cellular processes affecting organic acid production, here we screened collections of Saccharomyces cerevisiae deletion mutants and identified 36 yeast mutants forming a yellow halo on YPD plates containing bromocresol purple, indicating that the pH of the medium had been lowered. The disrupted genes encoded TCA cycle enzymes, transcription factors, signal transducers, and ubiquitin-related proteins. Acetate, pyruvate, and succinate are produced by yeast fermentation in rich medium, and their production was affected by mutations of the genes GTR1, GTR2, LIP5, LSM1, PHO85, PLM2, RTG1, RTG2 and UBP3, and also succinate dehydrogenase-related genes including EMI5, SDH1, SDH2, SDH4, TCM62 and YDR379C-A. Among the genes identified, overexpression of only LIP5 affected the production of acetate in S. cerevisiae. However, overexpression of EMI5, LIP5, RTG2 and UBP3 had a significant effect on the production of acetate, citrate, lactate, and succinate in the bottom-fermenting yeast Saccharomyces pastorianus. Furthermore, phenotypic analysis of the S. cerevisiae disruptants involved in organic acid production showed that azaserine, citrate, ethionine, and sulfite are useful compounds by which mutants with altered organic acid production might be selected. Taken together, these results suggest that the regulation of many organic acids might be simultaneously achieved by activation or inactivation of a single gene.

Crystal structures of Escherichia coli gamma-glutamyltranspeptidase in complex with azaserine and acivicin: novel mechanistic implication for inhibition by glutamine antagonists.

gamma-Glutamyltranspeptidase (GGT) catalyzes the cleavage of such gamma-glutamyl compounds as glutathione, and the transfer of their gamma-glutamyl group to water or to other amino acids and peptides. GGT is involved in a number of biological phenomena such as drug resistance and metastasis of cancer cells by detoxification of xenobiotics. Azaserine and acivicin are classical and irreversible inhibitors of GGT, but their binding sites and the inhibition mechanisms remain to be defined. We have determined the crystal structures of GGT from Escherichia coli in complex with azaserine and acivicin at 1.65 A resolution. Both inhibitors are bound to GGT at its substrate-binding pocket in a manner similar to that observed previously with the gamma-glutamyl-enzyme intermediate. They form a covalent bond with the O(gamma) atom of Thr391, the catalytic residue of GGT. Their alpha-carboxy and alpha-amino groups are recognized by extensive hydrogen bonding and charge interactions with the residues that are conserved among GGT orthologs. The two amido nitrogen atoms of Gly483 and Gly484, which form the oxyanion hole, interact with the inhibitors directly or via a water molecule. Notably, in the azaserine complex the carbon atom that forms a covalent bond with Thr391 is sp(3)-hybridized, suggesting that the carbonyl of azaserine is attacked by Thr391 to form a tetrahedral intermediate, which is stabilized by the oxyanion hole. Furthermore, when acivicin is bound to GGT, a migration of the single and double bonds occurs in its dihydroisoxazole ring. The structural characteristics presented here imply that the unprecedented binding modes of azaserine and acivicin are conserved in all GGTs from bacteria to mammals and give a new insight into the inhibition mechanism of glutamine amidotransferases by these glutamine antagonists.

The LIV-I/LS system as a determinant of azaserine sensitivity of Escherichia coli K-12.

The growth of Escherichia coli is inhibited by an antibiotic compound, azaserine (O-diazoacetyl-L-serine). Previous studies revealed the biochemical properties of azaserine, which involves inhibition of various enzymatic reactions as well as introduction of DNA breakage. However, genetically, nothing has been elucidated except that all the azaserine-resistant strains isolated so far carry lesions in the aroP gene as a primary determinant. Here, we demonstrate that, in addition to AroP, the LIV-I/LS system, an ATP-binding cassette type transporter, is involved in azaserine sensitivity of E. coli, by genetic analysis and transport studies, in which Ki value for azaserine was determined to be approximately 10(-3) M.

Molecular and biochemical elucidation of a cellular phenotype characterized by adenine analogue resistance in the presence of high levels of adenine phosphoribosyltransferase activity.

A mouse embryonal carcinoma cell line isolated for resistance to the adenine analogue 2,6-diaminopurine (DAP) was found to have near-wild-type levels of adenine phosphoribosyltransferase (APRT) activity in a cell-free assay. This DAP-resistant (DAPr) cell line, termed H29D1, also exhibited near-wild-type levels of adenine accumulation and the ability to grow in medium containing azaserine and adenine. Growth in this medium requires high levels of intracellular APRT activity. Using the polymerase chain reaction (PCR) and the dideoxy chain termination sequencing technique, an A-->G transition was discovered in exon 3 of the aprt gene in H29D1. This mutation resulted in an Arg-to-Gln change at amino acid 87 of the APRT protein that, in turn, resulted in a decreased affinity for adenine. An increased sensitivity of APRT to inhibition by AMP was observed when comparing H29D1 to P19, the parental cell line. Using a transgene containing the A-->G mutation, we demonstrated that this mutation is responsible for the biochemical and cellular phenotypes observed for the H29D1 cell line. The approach used in this study provides a definitive method for linking a mutation to a specific cellular phenotype.

Purine utilisation, de novo synthesis and degradation in mouse preimplantation embryos.

The importance of de novo purine synthesis as opposed to the reutilisation of metabolites by salvage pathways, and the nature of the excretory product(s) of purine degradation, have been examined in cultured preimplantation mouse embryos. In the presence of azaserine and mycophenolic acid, which inhibit de novo purine synthesis, embryo cleavage was blocked prior to compaction, the precise stages at which this occurred depended on whether the cultures were established on day 1 or day 2 after fertilisation, and indicated that salvage pathways were insufficient to fulfil the demand for nucleotides during early preimplantation development. The end-product of purine degradation appeared to be xanthine, which was excreted in very small amounts on days 1, 2 and 3, with a pronounced rise from the early to late blastocyst. Uric acid formation or excretion could not be detected. Exogenous hypoxanthine and adenine, which partially inhibited development, were taken up by the embryos and converted to xanthine, most probably by salvage pathways, since the enzyme xanthine oxidase, which converts hypoxanthine directly to xanthine and then to uric acid, could not be detected. Exogenous guanine had little effect on development and was also converted to xanthine, but in this case, the conversion was probably in a single step, via the enzyme guanase.

Photoinhibition of 2-amino-2-carboxybicyclo[2,2,1]heptane transport by O-diazoacetyl-L-serine. An initial step in identifying the L-system amino acid transporter.

Neutral amino acid uptake into mammalian cells occurs predominantly through the L, A, and ASC carrier-mediated transport systems. The proteins responsible for transport by these systems have not been isolated, and the three pathways presently are defined by their amino acid specificity and physiologic parameters. We have found that the amino acid derivative, O-diazoacetyl-L-serine (azaserine), is a potentially useful probe for identification of the L-(leucine-favoring) system transporter in human T-lymphocytes. Uptake of azaserine competitively inhibits the uptake of the prototype L-system amino acid, 2-amino-2-carboxybicycloheptane (BCH). Azaserine undergoes photolytic cleavage with 365 nm incident light to yield a highly reactive carbene intermediate and free N2. Following photolysis of [14C]azaserine in a suspension of lymphocytes, the 14C label is detectable within a crude cytoplasmic membrane preparation, and this process is inhibited by a 50-fold excess of unlabeled azaserine or 2-amino-2-carboxybicycloheptane, suggesting that the 14C-product is associated with the membranes at or near the L-system transport site. Furthermore, photolysis of azaserine in the presence of lymphocytes results in specific irreversible inhibition of L-system transport. Thus, photolysis of azaserine provides an initial step toward the identification of the L-system transporter.

Genetic demonstration of bidirectionality in the high affinity purine base transporter of mutant mouse S49 cells.

A mutant cell line was selected from wild type S49 lymphoblasts that expressed a novel high affinity purine base transport system not found in parental cells or any other mammalian cell line (Aronow, B., Toll, D., Patrick, J., Hollingsworth, P., McCartan, K., and Ullman, B. (1986) Mol. Cell. Biol. 6, 2957-2962). In order to determine whether this nucleobase transport system was bidirectional, mutant cell lines possessing this high affinity base transport capability were derived from a nucleoside transport-deficient derivative of an adenylosuccinate synthetase-deficient S49 cell line. The resulting progeny excreted significantly greater amounts of purine into the cell culture medium than parental cells. This purine was identified as hypoxanthine. These results demonstrate genetically that the high affinity purine base transport system can mediate both the influx and efflux of hypoxanthine.

Expression of a novel high-affinity purine nucleobase transport function in mutant mammalian T lymphoblasts.

The single nucleoside transport function of mouse S49 lymphoblasts also transports purine bases (B. Aronow and B. Ullman, J. Biol. Chem. 261:2014-2019, 1986). This transport of purine bases by S49 cells is sensitive to inhibition by dipyridamole (DPA) and 4-nitrobenzylthioinosine, two potent inhibitors of nucleoside transport. Therefore, wild-type S49 cells cannot salvage low hypoxanthine concentrations in the presence of 10 microM DPA and 11 microM azaserine; the latter is a potent inhibitor of purine biosynthesis. Among a mutagenized wild-type population, a cell line, JPA2, was isolated which could proliferate in 50 microM hypoxanthine-11 microM azaserine-10 microM DPA. The basis for the survival of JPA2 cells under these selective conditions was expression of a unique, high-affinity purine nucleobase transport function not present in wild-type cells. JPA2 cells could transport 5 microM concentrations of hypoxanthine, guanine, and adenine 15- to 30-fold more efficiently than parental cells did. Kinetic analyses revealed that the affinity of the JPA2 transporter for all three purine bases was much greater than that of the wild-type nucleobase transport system. Moreover, nucleobase transport in JPA2 cells, unlike that in parental cells, was insensitive to inhibition by DPA, 4-nitrobenzylthioinosine, sulfhydryl reagents, and nucleosides. No alterations in nucleoside transport capability, phosphoribosylpyrophosphate levels, or purine phosphoribosyltransferase enzymes were detected in JPA2 cells. Thus, JPA2 cells express a novel nucleobase transport capability which can be distinguished from the nucleoside transport function by multiple biochemical parameters.

Monoclonal antibodies against tumour-associated antigens: mycoplasma as a major technical obstacle and its possible circumvention by azaserine selection medium.

In experiments aimed at generating monoclonal antibodies against tumour associated antigens, mice are usually immunized with cancer cells (or membrane fractions prepared from it) obtained from surgery or cultured in vitro. In both situations there is the danger of introducing mycoplasmas into the hybridoma cultures even if the myeloma cell line and the mice used for immunization or feeder cell preparation are not infected. Mycoplasmas kill hybridoma cells during HAT selection because they excessively degrade thymidine. The use of azaserine instead of aminopterin in hybrid selection circumvents the necessity for thymidine and therefore may allow survival and growth of hybridomas in the presence of mycoplasmas.

Effect of pyridoxal deficiency on pancreatic DNA damage and nodule induction by azaserine.

The effects of pyridoxal deficiency on the genotoxicity and nodule inducing ability of azaserine in rat pancreas were examined. Azaserine at a dose of 10 mg/kg body weight which causes substantial DNA damage in normal rat pancreas, failed to induce DNA damage detectable by alkaline elution in the pancreas of pyridoxal-deficient rats. Studies of the distribution of [14C]azaserine in rat tissues revealed that uptake of azaserine in pancreas of pyridoxal deficient rats was not significantly different from that of normal rats. The ability of a structurally unrelated amino acid carcinogen N delta-(N-methyl-N- nitrosocarbamoyl )-L-ornithine to damage rat pancreatic DNA was not affected by pyridoxal deficiency. In another study, the pyridoxal antagonist 4'-deoxypyridoxine was administered i.p. to rats prior to and during azaserine treatment. Four months later, quantitative sterological analysis of atypical acinar cell nodules revealed that there was a significant reduction in the number but not size of nodules in the pancreases of 4'-deoxypyridoxine-treated rats. These results confirm the relationship of the induction of DNA damage by azaserine to its ability to induce pancreatic tumors, and support previous studies of azaserine metabolism, strongly suggesting that the in vivo activation of this carcinogen is pyridoxal dependent.

Enhancing activity of rat tissue extracts for induction of lambda prophage by L-azaserine.

We studied the effect of rat tissue extracts on induction of lambda prophage in Escherichia coli (lambda) by L-azaserine. Hepatic and pancreatic extracts, primarily the cytosolic fraction, markedly increased the rate of induction. Hepatic extracts from lipotrope-deficient rats were somewhat more active than extracts from normal rats. The enhancing activity in normal rat hepatic cytosol was partially characterized. It reduced by about one-half the dose of azaserine required for a given purpose. The enhancement was increased by preincubating the bacterial cells with cytosol; cells retained the effect after cytosol was removed. Enhancing activity was inhibited strongly by the amino acids phenylalanine, tryptophan, and tyrosine; to lesser extents by leucine, methionine, and serine; and not at all by proline or glutamine. It was eliminated by dialysis of the cytosol and reduced by omission of nicotinamide adenine dinucleotide phosphate (NADP) from the reaction mixture. Heating the cytosol to 60 degrees C or 80 degrees C or varying the pH of the reaction mixture from 6 to 8 had no significant effect. Treating the cytosol with trypsin appeared to release an inhibitor of the activity. Glutathione, cysteine, and beta-mercaptoethanol also enhanced lambda induction by azaserine, but the cytosolic activity was not affected by the thiol-inactivating compound diethylmaleate (DEM). The results suggest that factors in cytosol interact with bacterial cells to facilitate transport of azaserine into the cells, primarily through the aromatic amino acid transport system. A small molecule, not a free thiol compound, appears to be involved. It may serve to establish reducing conditions protective for azaserine, the probable mechanism of action of sulfhydryl compounds.

Binding of [14C] azaserine to DNA and protein in the rat and hamster.

The binding of [14C] azaserine or its metabolites to DNA and protein in the organs of rats and hamsters was determined at various time after treatment with [14C] azaserine. The specific activity of 14C labelling of DNA and protein was determined. Rat liver DNA and protein were most extensively labelled at 90 min post-injection, but by 24 h the specific activity decreased to the levels found in pancreas and kidney. Thymus contained negligible amounts of radioactivity at all time-points. DNA and protein from hamster pancreas contained more label than did DNA and protein from rat pancreas. The results suggest that factors other than DNA binding play a role in determining the species and organ specificity of azaserine.

Identification of 7-carboxymethylguanine in DNA from pancreatic acinar cells exposed to azaserine.

Studies were undertaken to determine the identity of an azaserine:DNA adduct. The most probable adduct, 7-carboxymethylguanine, was synthesized. DNA isolated from pancreatic acinar cells treated in culture with [14C]azaserine was hydrolyzed under neutral conditions to liberate N-alkylated purines. The neutral hydrolysate was subjected to high-performance liquid chromatography along with the synthetic standard. One of the radioactive peaks from the treated DNA was found to cochromatograph with 7-carboxymethylguanine in three systems: reverse phase; anion exchange; and ion pair reverse phase. These results suggest that azaserine metabolism in acinar cells results in carboxymethylation of DNA, supporting previously proposed models of azaserine degradation.

Studies of DNA damage in rat pancreas and liver by 6-diazo-5-oxo-L-norleucine, ethyl diazoacetate and azaserine.

One hour following intraperitoneal injection of the pancreatic and liver carcinogen azaserine, 10 mg/kg (0.06 mmol/kg), DNA damage is present in both pancreas and liver of Wistar/Lewis rats as determined with alkaline sucrose gradients. Single injections (0.06 mmol/kg) of either of 2 structural analogues of azaserine, 6-diazo-5-oxo-L-norleucine (DON) and ethyl diazoacetate (EDA), do not damage pancreatic or liver DNA. EDA administered at 0.5 mmol/kg damages liver DNA, but not pancreatic DNA. Total radioactivity in pancreas and liver of Wistar rats 1 h after intravenous injection of [14C]azaserine, (10 microCi/kg), is 2.8 and 1.3 times respectively, the level of 14C-activity in pancreas and liver following injection [14C]EDA. Three to four times as much [14C]EDA localizes in the liver of Wistar rats as in the pancreas. Azaserine (0.06 mmol/kg weekly for 6 weeks) induces atypical acinar cell nodules (AACN) in pancreas of Wistar/Lewis rats. DON (0.06 mmol/kg weekly for 6 weeks) induces an elevated incidence, but low number of AACN in pancreas. EDA (0.10 mmol/kg weekly for 6 weeks) does not induce pancreatic AACN.

Studies of pancreatic nodule induction and DNA damage by D-azaserine.

The ability of the D-isomer of azaserine to induce atypical acinar cell nodules (AACN) in pancreas and to cause DNA damage in pancreas and liver was evaluated. Rats were injected with equivalent doses of D- or L-azaserine and numbers of AACN were counted after 4 months. DNA damage in pancreas and liver of rats treated in vivo, and in pancreatic acinar cells treated in vitro with D- or L-azaserine was determined by alkaline elution. Results show that D-azaserine does not significantly induce AACN in pancreas, nor does it cause extensive DNA damage in comparison with L-azaserine, suggesting that the differential effect of the 2 isomers is related to stereospecificity in either transport or metabolism.

[De novo purine biosynthesis. In vitro measurement in hyperuricemia (author's transl)]. / Mesure de la biosynthèse de novo des bases puriques dans les hyperuricémies.

De novo purine biosynthesis has been investigated in circulating blood lymphocytes in vitro. N-formyl-glycinamide ribonucleotide (FGAR) has been mesured using 14C-formate incorporation in the presence of azaserine, a metabolic inhibitor blocking the metabolical pathway at the level of FGAR synthesis. Such a synthesis was measured in 20 healthy controls, 24 patients with primary gout (11 on allopurinol therapy) and 26 patients with chronic renal failure and secondary hyperuricemia (8 on allopurinol therapy). Among gouty patients without allopurinol therapy, FGAR synthesis was normal in 5 and increased in the others. FGAR synthesis was decreased in patients with renal failure whatever the therapy. However, FGAR synthesis remained increased in patients with a primary gout complicated with renal insufficiency. The test we propose for de novo purine biosynthesis measurement is simple and of value to analyse the patho-physiology of hyperuricemia and its therapy. The test allows an acurate discrimination between primary and secondary hyperuricemia in the presence of renal insufficiency.