Cloning of cDNA for argininosuccinate synthetase mRNA and study of enzyme overproduction in a human cell line

Previous studies of the human cell line RPMI-2650 (wild type) and its canavanine-resistant variants have demonstrated differences in argininosuccinate synthetase activity as follows: canavanine-resistant much greater than wild type grown in citrulline greater than wild type grown in arginine (Su, T.-S., Beaudet, A. L., and O'Brien, W. E. (1981) Biochemistry 20, 2956-2960). A recombinant plasmid containing a 1.55-kilobase insert complementary to the mRNA for human argininosuccinate synthetase was isolated by the combined use of differential colony hybridization and immunoprecipitation of the products of plasmid-selected mRNA translation. Both blot and dot hybridization analysis of polyadenylated RNA indicated a major mRNA species of 1.67 kilobase in all cells, and the levels of mRNA correlated well with the levels of enzyme activity: canavanine-resistant, 180; wild type grown in citrulline, 7; and wild type grown in arginine, 1. One major mRNA species of 1.67 kilobase and one minor species of 2.68 kilobase were observed in wild type and canavanine-resistant cell lines. Reassociation kinetics of pAS1 with genomic DNA from human liver, canavanine-resistant cells, and wild type cells were not significantly different. Blot hybridization of genomic DNA revealed no detectable differences between wild type cells, canavanine-resistant cells, and human leukocytes. The data demonstrated that there were multiple copies, perhaps 10 or more, of argininosuccinate synthetase-like sequences in human DNA and that the canavanine-resistant phenotype was not due to gene amplification.     

Arginine-mediated regulation of an argininosuccinate synthetase minigene in normal and canavanine-resistant human cells.

Regulation of argininosuccinate synthetase (AS) was studied by using minigenes containing 3 kilobases of DNA upstream from the TATAA box and 9 kilobases downstream (including the first four exons of the AS gene) ligated to either the cDNA for AS or to the chloramphenicol acetyltransferase (CAT) gene. Unlike the endogenous AS gene, expression of the CAT minigene was not elevated in Canr1 cells, which overproduce AS compared with parental RPMI-2650 cells. Expression of the CAT minigene in both stable and transient analyses was four- to five-fold higher in RPMI-2650 cells grown in citrulline medium than in cells grown in arginine medium. Although endogenous AS activity is not subject to metabolite regulation in Canr1 cells and expression of the CAT minigene in Canr1 cells was not increased when cells were grown in citrulline medium, expression of the CAT minigene was 10- to 22-fold greater when intracellular arginine pools were depleted by transient starvation for arginine and citrulline.     

Relative argininosuccinate synthetase mRNA levels and gene copy number in canavanine-resistant lymphoblasts

Mutants resistant to the arginine analogue, canavanine, have been isolated from two normal lymphoblast lines, MGL8B2 and MGL33. These mutants constitutively express up to 200-fold higher amounts of structurally normal argininosuccinate synthetase, the urea cycle enzyme that converts citrulline to argininosuccinate. Relative levels of argininosuccinate synthetase mRNA were compared among normal and canavanine-resistant lines using in vitro translation of poly(adenylic acid) RNA and blot hybridization of total cytoplasmic RNA to an argininosuccinate synthetase cDNA. Both of these approaches indicated that the canavanine-resistant lines contain increased steady-state levels of synthetase-specifc mRNA relative to their sensitive parents and that these were roughly correlated with levels of enzyme activity. Blot hybridization of Eco RI-digested genomic DNA preparations revealed no detectable differences in argininosuccinate synthetase structural gene copy number between normal and canavanine-resistant lymphoblasts, demonstrating that the canavanine-resistant phenotype is not caused by gene amplification.     

Sequence for human argininosuccinate synthetase cDNA.

The nucleotide sequence for human argininosuccinate synthetase cDNA was determined by analysis of six clones isolated from a single experiment. The sequence covered 1623 nucleotides including 76 bases of poly(A) and contained a 1236 nucleotide open reading frame encoding a protein of 46,434 daltons. In one cDNA isolate, a cloning artifact or perhaps RNA polymerase error involving addition of an A in a region of six A's within the coding sequence was documented. Single base variations in the 3' untranslated region were examined in detail since detection of DNA polymorphisms in the cDNAs could imply over-expression of both alleles at the active locus in canavanine-resistant cells, i.e. a trans-acting mechanism for enzyme overproduction. However, the sequence from five cDNAs suggested some single base artifacts, and DNA polymorphism remains uncertain. The occurrence of three tandem arginine codons in the 5' untranslated region of the cDNA suggested the possibility of an interaction of arginyl-tRNA with mRNA to regulate RNA processing or half-life as a mechanism for arginine-mediated repression.     

Molecular structure of the human argininosuccinate synthetase gene: occurrence of alternative mRNA splicing.

The human genome contains one expressed argininosuccinate synthetase gene and ca. 14 pseudogenes that are dispersed to at least 11 human chromosomes. Eleven clones isolated from a human genomic DNA library were characterized extensively by restriction mapping, Southern blotting, and nucleotide sequencing. These 11 clones represent the entire expressed argininosuccinate synthetase gene that spans 63 kilobases and contains at least 13 exons. The expressed gene codes for two mRNAs that differ in their 5' untranslated sequences and arise by alternative splicing involving the inclusion or deletion of an entire exon. In normal human liver and cultured fibroblasts, the predominant mature argininosuccinate synthetase mRNA lacks sequences encoded by exon 2 in the expressed gene. In contrast, the predominant argininosuccinate synthetase mRNA in baboon liver contains exon 2 sequences. A transformed canavanine-resistant human cell line in which argininosuccinate synthetase activity is 180-fold higher than that in wild-type cells contains abundant amounts of both forms of the argininosuccinate synthetase mRNA. The mRNA lacking exon 2 sequences is the more abundant mRNA species in the canavanine-resistant cells. These observations show that splicing of the argininosuccinate synthetase mRNA is species specific in primates and varies among different human cell types.     

Citrulline metabolism in normal and citrullinemic human lymphocyte lines

Citrullinemia is one of the five aminoacidurias associated with the Krebs-Henseleit urea cycle. A long-term lymphocyte line (UM-21) derived from a patient with this disease and nine of ten clones of this line were found to have no activity for the enzyme argininosuccinate synthetase (AS), as demonstrated by their inability to grow in medium in which citrulline had been substituted for arginine, by their inability to incorporate arginine-C14 derived from citrulline-C14 into cellular protein, and by direct enzyme assay. One clone had normal or nearly normal argininosuccinate synthetase activity, as demonstrated by the same criteria. Nutritional "variants" able to grow logarithmically in medium containing citrulline were isolated from UM-21 and three clones. The apparent Kms of AS for citrulline in UM-21, the ten clones, the variant lines, and a normal line were measured and fell into three groups: AS in UM-21 and nine clones had no measurable apparent Km for citrulline; AS in the variant cells had apparent Kms for citrulline of approximately 20 mM; and AS in the normal cell line and one clone had apparent Kms for citrulline of 0.2 mM. The data suggest that the defect in the citrullinemic cell lines is due to a mutation in the structural gene coding for argininosuccinate synthetase.     

Control of argininosuccinate synthetase by arginine in human lymphoblasts

Human lymphoblasts in long-term culture have the enzyme activities necessary to convert citrulline to arginine: argininosuccinate synthetase and argininosuccinate lyase. Upon transfer from arginine-supplemented to citrulline-supplemented medium, lymphoblasts exhibit a lag period before resuming exponential growth. During this lag the specific activity of argininosuccinate synthetase increases an average of 60-fold. Argininosuccinate lyase activity remains unchanged. If normal lymphoblasts are starved in arginine-deficient medium without citrulline or if argininosuccinate lyase--deficient lymphoblasts are transferred to citrulline-containing medium, argininosuccinate synthetase activity increases linearly for several days and reaches even higher levels. Cycloheximide blocks the increase in enzyme activity. Cells grown in citrulline medium and pulse labeled with 35S-methionine incorporate more 35S-methionine into argininosuccinate synthetase protein than cells grown in arginine; the rate of disappearance of radioactively labeled enzyme is the same in citrulline- and arginine-grown cells. Arginine or a closely related metabolite thus appears to repress the synthesis of argininosuccinate synthetase of human lymphoblasts in culture.     

Induction of astrocyte argininosuccinate synthetase and argininosuccinate lyase by dibutyryl cyclic AMP and dexamethasone

Arginine is an intermediate in the elimination of excess nitrogen and is the substrate for nitric oxide synthesis. Arginine synthesis has been reported in brain tissue. We have studied the activity of the arginine biosynthetic enzymes argininosuccinate synthetase and argininosuccinate lyase in dexamethasone and/or dibutyryl cyclic AMP treated rat astrocyte cultures. Argininosuccinate lyase activity was stimulated by treatment with either effector and an additive effect was obtained when both agents were added simultaneously. Argininosuccinate synthetase was also increased in dexamethasone treated astrocytes. The effect of dibutyryl cyclic AMP on argininosuccinate synthetase was variable, suggesting a role for additional factors in its regulation as compared to argininosuccinate lyase. Regulation of arginine synthesis in astrocytes may be important to insure that arginine is not limiting for nitric oxide synthesis in neural tissue.     

Coinduction of endothelial nitric oxide synthase and arginine recycling enzymes in aorta of diabetic rats.

Decreased availability of arginine and impaired production of NO (nitric oxide) have been implicated in the development of endothelial dysfunction. Citrulline formed by the NOS reaction is recycled to arginine by the citrulline-NO cycle, which is composed of NOS, argininosuccinate synthetase (AS), and argininosuccinate lyase. Therefore, we investigated the alterations of these enzymes in the aorta of streptozotocin (STZ)-induced diabetic rats. eNOS and AS mRNAs were increased by three- to fourfold 1-2 weeks after STZ treatment and decreased at 4 weeks. AL mRNA was weakly induced. Induction of eNOS and AS proteins was also observed. Cationic amino acid transporter (CAT)-1 mRNA remained little changed, and CAT-2 mRNA was not detected. The plasma nitrogen oxide levels were increased 1-2 weeks after STZ treatment and decreased at 4 weeks. Transforming growth factor-beta1 (TGF-beta1) mRNA in the aorta was also induced. TGF-beta1 induced eNOS and AS mRNAs in human umbilical vein endothelial cells but inhibited the proliferation of HUVEC. These results indicate that eNOS and AS are coinduced in the aorta in early stages of STZ-induced diabetic rats and that the induction is mediated by TGF-beta1. The results also suggest that TGF-beta1 works antiatherogenically at early stages of diabetes by increasing NO production, whereas prolonged elevation of TGF-beta1 functions atherogenically by inhibiting endothelial cell growth.     

Regulation of nitric oxide production by arginine metabolic enzymes

Nitric oxide (NO) is synthesized from arginine by NO synthase (NOS), and the availability of arginine is one of the rate-limiting factors in cellular NO production. Citrulline, which is formed as a by-product of the NOS reaction, can be recycled to arginine by successive actions of argininosuccinate synthetase (AS) and argininosuccinate lyase (AL), forming the citrulline-NO cycle. AS and sometimes AL have been shown to be coinduced with inducible NOS (iNOS) in various cell types including activated macrophages, vascular smooth muscle cells, glial cells, neuronal PC12 cells, and pancreatic beta-cells. Cationic amino acid transporter (CAT)-2 is induced in activated macrophages but not in PC12 cells. On the other hand, arginase can downregulate NO production by decreasing intracellular arginine concentrations. iNOS and arginase activities are regulated reciprocally in macrophages by cytokines, and this may guarantee the efficient production of NO. In contrast, iNOS and arginase isoforms (type I and II) are coinduced in lipopolysaccharide (LPS)-activated macrophages. These results indicate that NO production is modulated by the uptake, recycling, and degradation of arginine.     

Organization and control in the arginine biosynthetic pathway of Neurospora.

Eight enzymes involved in the conversion of acetylglutamate to arginine in Neurospora crassa were studied. The data indicate that of three enzymes early in the sequence, only the first, acetylglutamate kinase, is a nonorganellar enzyme. The next two, N-acetyl-gamma-glutamyl-phosphate reductase and acetylornithine aminotransferase, are in the mitochondrion, which was previously shown to contain the subsequent enzymes: acetylornithine-glutamate acetyltransferase, ornithine carbamyltransferase, and carbamyl-phosphate synthetase A (arginine specific). The last two enzymes of the pathway, argininosuccinate synthetase and argininosuccinate lyase, were previously shown to be cytosolic. All enzymes but one have low amplitudes or repression. Their levels respond little to arginine excess and are about twofold elevated (threefold for ornithine carbamyltransferase) as a result of arginine limitation in the arg-12-8 strain. No restriction of the incorporation of mitochondrial enzymes into mitochondria could be detected when the levels of these enzymes were elevated. Two enzymes, acetylglutamate kinase and carbamyl-phosphate synthetase A, which initiate the synthesis of the ornithine and guanidino moieties of arginine, respectively, show the lowest specific activities in crude extract. These enzymes display special regulatroy features. Acetylglutamate kinase, which has a typically low amplitude of repression, is subject to feedback inhibition. Carbamyl-phosphate synthetase A is wholly insensitive to arginine or citrulline in vitro or in vivo, but displays a very large amplitude of repression (about 60-fold). It is unique in that it can be almost completely repressed by growth of mycelia in excess arginine. These data suggest that mitochondrial localization may be incompatible with a mechanism of feedback inhibition by a cytosolic effector, arginine. Further, they suggest that the high repressibility of carbamyl-phosphate synthetase A compensates for its feedback insensitivity.     

RNA interference of argininosuccinate synthetase restores sensitivity to recombinant arginine deiminase (rADI) in resistant cancer cells

Background  

Sensitivity of cancer cells to recombinant arginine deiminase (rADI) depends on expression of argininosuccinate synthetase (AS), a rate-limiting enzyme in synthesis of arginine from citrulline. To understand the efficiency of RNA interfering of AS in sensitizing the resistant cancer cells to rADI, the down regulation of AS transiently and permanently were performed in vitro, respectively.     

Some aspects of the regulation of arginine biosynthesis in soybean cell cultures

The levels of the activities of argininosuccinate synthetase and argininosuccinate lyase were measured in soybean (glycine max L. var. Mandarin) cell suspension cultures grown in the presence or absence of exogenous arginine. In some experiments, actinomycin D or cycloheximide were also added to the cultures, at critical stages of their growth. The results obtained led to the conclusion that activity of argininosuccinate synthetase is subject to significant inhibition by levels of arginine similar to those found to occur within the cells. Argininosuccinate lyase activity appeared to be enhanced, when arginine levels were increased above those occurring physiologically. Both enzymes appeared to be subject to inactivation, possibly via proteolysis.     

Inhibition of Arginyl-Transfer Ribonucleic Acid Synthetase Activity of Escherichia coli by Arginine Biosynthetic Precursors

The arginine biosynthetic precursors, ornithine, citrulline, and argininosuccinate, inhibit arginyl-transfer ribonucleic acid (tRNA) synthetase (EC 6.1.1.13, arginine: soluble RNA ligase, adenosine monophosphate) activity in the in vitro attachment assay system. Ornithine is the most potent, argininosuccinate is next, and citrulline is least effective. The implications of these results are discussed in relation to arginyl-tRNA synthetase activity and the level of the arginine biosynthetic enzymes during conditions of restricted and unrestricted supply of arginine to cells.     

Channeling of urea cycle intermediates in situ in permeabilized hepatocytes

Preferential use of endogenously generated intermediates by the enzymes of the urea cycle was observed using isolated rat hepatocytes made permeable to low molecular weight compounds with alpha-toxin. The permeabilized cells synthesized [14C]urea from added NH4Cl, [14C]HCO3-, ornithine, and aspartate, using succinate as a respiratory substrate; with all substrates saturating, about 4 nmol of urea were formed per min/mg dry weight of cells. Urea usually accounted for about 40-50% of the total (NH3 + ornithine)-dependent counts, arginine for less than 10%, and citrulline for about 30%. Very tight channeling of arginine between argininosuccinate lyase and arginase was shown by the fact that the addition of a 200-fold excess of unlabeled arginine to the incubations did not decrease the percentage of counts found in urea or increase that found in arginine, even though a substantial amount of the added arginine was hydrolyzed inside the cells. The channeling of argininosuccinate between its synthetase and lyase was demonstrated by similar observations; unlabeled argininosuccinate added in 200-fold excess decreased the percentage of counts in urea by only 25%. Channeling of citrulline from its site of synthesis by ornithine transcarbamylase in the mitochondrial matrix to argininosuccinate synthetase in the cytoplasmic space was also shown. These results strongly suggest that the three "soluble" cytoplasmic enzymes of the urea cycle are grouped around the mitochondria and are spatially organized within the cell in such a way that intermediates can be efficiently transferred between them.     

Negative and positive regulation in trans of gene expression from adeno-associated virus vectors in mammalian cells by a viral rep gene product.

We previously described use of the human parvovirus, adeno-associated virus (AAV), as a vector for transient expression in mammalian cells of the gene for chloramphenicol acetyltransferase (CAT). In the AAV vector, pTS1, the CAT gene is expressed under the control of the major AAV promoter p40. This promoter is embedded within the carboxyl-terminal region of an open reading frame (orf-1) which codes for a protein (rep) required for AAV DNA replication. We show here that the rep product has additional trans-acting properties to regulate gene expression. First, deletion or frame-shift mutations in orf-1, which occurred far upstream of p40, increased expression of CAT in human 293 (adenovirus-transformed) cells. This increased CAT expression was abolished when such mutant AAV vectors were transfected into 293 cells together with a second AAV vector which could supply the wild-type AAV rep product in trans. Thus, an AAV rep gene product was a negative regulator, in trans, of expression of CAT in uninfected 293 cells. In adenovirus-infected 293 cells, the function of the AAV rep product was more complex, but in some cases, it appeared to be a trans activator of the expression from p40. In HeLa cells, only trans activation by rep was seen in the absence or presence of adenovirus. Neither activation nor repression by the rep product required replication per se of the AAV vector DNA. Thus, trans-acting negative or positive regulation of gene expression by the AAV rep gene is modulated by factors in the host cell and by the helper adenovirus.