Regulation of purine de novo synthesis in cultured human fibroblasts: the role of P-ribose-PP

Environmental purines decrease Pseudomonas aeruginosa biofilm formation by disrupting c-di-GMP metabolism

Cyclic di-guanosine monophosphate (c-di-GMP) is a bacterial second messenger that governs the lifestyle switch between planktonic and biofilm states. While substantial investigation has focused on the proteins that produce and degrade c-di-GMP, less attention has been paid to the potential for metabolic control of c-di-GMP signaling. Here, we show that micromolar levels of specific environmental purines unexpectedly decrease c-di-GMP and biofilm formation in Pseudomonas aeruginosa. Using a fluorescent genetic reporter, we show that adenosine and inosine decrease c-di-GMP even when competing purines are present. We confirm genetically that purine salvage is required for c-di-GMP decrease. Furthermore, we find that (p)ppGpp prevents xanthosine and guanosine from producing an opposing c-di-GMP increase, reinforcing a salvage hierarchy that favors c-di-GMP decrease even at the expense of growth. We propose that purines can act as a cue for bacteria to shift their lifestyle away from the recalcitrant biofilm state via upstream metabolic control of c-di-GMP signaling.

Small elevations of glucose concentration redirect and amplify the synthesis of guanosine 5'-triphosphate in rat islets

Recent studies suggest a permissive requirement for guanosine 5'-triphosphate (GTP) in insulin release, based on the use of GTP synthesis inhibitors (such as myocophenolic acid) acting at inosine monophosphate (IMP) dehydrogenase; herein, we examine the glucose dependency of GTP synthesis. Mycophenolic acid inhibited insulin secretion equally well after islet culture at 7.8 or 11.1 mM glucose (51% inhibition) but its effect was dramatically attenuated when provided at < or = 6.4 mM glucose (13% inhibition; P < 0.001). These observations were explicable by a stimulation of islet GTP synthesis derived from IMP since, at high glucose: (a) total GTP content was augmented; (b) a greater decrement in GTP (1.75 vs. 1.05 pmol/islet) was induced by mycophenolic acid; and (c) a smaller "pool" of residual GTP persisted after drug treatment. Glucose also accelerated GTP synthesis from exogenous guanine ("salvage" pathway) and increased content of a pyrimidine, uridine 5'-triphosphate (UTP), suggesting that glucose augments production of a common regulatory intermediate (probably 5-phosphoribosyl-1-pyrophosphate). Pathway-specific radiolabeling studies confirmed that glucose tripled both salvage and de novo synthesis of nucleotides. We conclude that steep changes in the biosynthesis of cytosolic pools of GTP occur at modest changes in glucose concentrations, a finding which may have relevance to the adaptive (patho) physiologic responses of islets to changes in ambient glucose levels.

Fibroblast growth factor-dependent metabolism of hypoxanthine via the salvage pathway for purine synthesis in porcine aortic endothelial cells

In this study we examined the metabolism of hypoxanthine in fibroblast growth factor (FGF)-stimulated porcine aortic endothelial cells (PAEC). Our previous report indicated that hypoxanthine in fetal bovine serum (FBS) was an essential component for both basal and FGF-dependent growth of PAEC (Hayashi et al., Exp Cell Res 185: 217-228, 1989). Besides hypoxanthine, the addition of various purine bases and purine nucleosides, but not xanthine, xanthosine or any pyrimidine metabolites, restored the limited growth of PAEC cultured in medium containing 10% dialyzed FBS in the presence or absence of FGF. The metabolism of [14C]hypoxanthine was compared in PAEC treated with and without FGF. Treatment of PAEC with FGF for 24 hr enhanced the radioactivity incorporation from [14C]hypoxanthine into both the acid-soluble and -insoluble fractions approximately 2-fold. Upon chromatographic analyses of hypoxanthine metabolites in the acid-soluble nucleotide fraction, it was found that in control PAEC hypoxanthine was largely metabolized to IMP, adenine nucleotides and uric acid, whereas in FGF-treated cells it was converted to ATP, ADP, GTP, xanthine and uric acid. The radioactivity of IMP was lowered in FGF-stimulated cells. The addition of FGF to PAEC increased phosphoribosyl pyrophosphate (PRPP) synthetase activity by approximately 8-fold and the PRPP content by approximately 2-fold, but it did not increase hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity or hypoxanthine transport. On the other hand, methotrexate, an inhibitor of de novo synthesis of purine, did not affect the growth of PAEC. Analyses of the rate of [14C]formate incorporation into total purine compounds showed that PAEC had a low capacity to synthesize purines de novo, which was not stimulated by FGF. These data indicate that FGF stimulates the synthesis of PRPP necessary for the salvage synthesis of purine nucleotides in conjunction with purine bases, e.g. hypoxanthine.

Biochemical basis of hypoxanthine-guanine phosphoribosyltransferase deficiency in nine families

The concentration of hypoxanthine-guanine phosphoribosyltransferase (HPRT) cross-reacting material (CRM) was determined in haemolysates and/or lymphoblast lysates from nine patients with complete or partial deficiency of HPRT activity. Two of the patients had the fully developed Lesch-Nyhan syndrome and although they had undetectable HPRT activity, small amounts of CRM were found. HPRT-specific mRNA was not detected in lymphoblast lysates from one of these patients, while lysates from the other had a much reduced concentration. Samples from three patients with less than 0.1% of normal HPRT activity but with minor or no neurological manifestations were also found to contain small amounts of CRM. The other four patients whose HPRT activities ranged from 3 to 10% of normal were found to have CRM concentrations which varied from 26 to 100% of normal. In one patient with a partial deficiency the Km for 5-phospho-alpha-D-ribosyl-1-pyrophosphate was five times normal.

Human hypoxanthine-guanine phosphoribosyltransferase. Development of a spectrophotometric assay and its use in detection and characterization of mutant forms

A simple and rapid spectrophotometric assay for the estimation of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) activity in human tissues is described. It is based on the increase in absorbance at 257.5 nm which occurs when the substrate guanine is converted to its 5'-mononucleotide, GMP. The assay has been developed to measure HGPRT activity in erythrocyte and lymphocyte lysates and in brain homogenates, and has been used in the screening of patients with hyperuricaemia and/or hyperuricosuria for HGPRT deficiency. It has also been used to determine the steady-state kinetic constants of a mutant form of the enzyme. The spectrophotometric assay is compared with the radioactive assay currently used to measure HGPRT activity.

Purine synthesis de novo in cultured lymphoblast cells derived from patients with gout

Rates of de novo purine synthesis in lymphoblast cell cultures derived from ten patients with gout were compared with those from control individuals. Since the growth rate of the culture, an assay procedure was developed to account for the variation in lymphoblast growth rates and to permit valid quantitative comparison between purine synthesis in each cell line. Clear differences were demonstrated between the rates of purine synthesis in cells from normal control subjects and those from patients with a deficiency of hypoxanthine-guanine phosphoribosyltransferase activity (HPRT-deficient). Lymphoblasts from the gouty patients showed purine synthesis either within the normal range or intermediate between this and the HPRT-deficient cells. In patients having normal renal function, de novo purine synthesis of lymphoblast cells correlated with the degree of urate production as reflected by the urinary excretion of urate over a 24 h period. Three patients, with demonstrable excessive production of urate in vivo, exhibited increased purine synthesis in lymphoblasts. This increased synthesis did not appear to result from any of the enzyme mutations currently recognized as responsible for abnormal purine metabolism.

HPRT-deficiency associated with normal PRPP concentration and APRT activity

Deficiencies of HPRT are usually associated with increased concentrations of PRPP and increased levels of APRT activity in erythrocytes. We report the case of a male with a partial deficiency of HPRT in whom these two parameters were normal. The clinical features of this patient were those associated with severe hyperuricaemia and gout. Studies of intact erythrocytes showed rates of incorporation of [14C]hypoxanthine and of [14C]adenine into purine nucleotides which were almost indistinguishable from normal. However, HPRT activity in erythrocyte lysates was only 9% of normal. In cell extracts of cultured lymphoblasts, the HPRT activity was 20% of control values and the APRT activity was normal. The PRPP concentration and the rate of de novo purine synthesis in cultured lymphoblasts were both intermediate between controls and lymphoblasts from patients with the Lesch-Nyhan syndrome.

Superactivity of human phosphoribosyl pyrophosphate synthetase due to altered regulation by nucleotide inhibitors and inorganic phosphate

Phosphoribosyl pyrophosphate (PPRibP) synthetase activity was studied in cultured fibroblasts and lymphoblasts from a male child (patient 2-A) in whom inherited purine nucleotide and uric acid overproduction are accompanied by neurological deficits. Chromatographed or partially purified preparations of the child's enzyme showed 5-6-fold increased inhibitory constants (I0.5) for the noncompetitive inhibitors GDP and 6-methylthioinosine monophosphate but normal responsiveness to the competitive inhibitors ADP and 2,3-diphosphoglycerate. Activation of the PPRibP synthetase of patient 2-A by Pi was also abnormal with 3-4-fold reduced apparent KD values for Pi. Superactivity of the PPRibP synthetase of this child thus appeared to result from a combination of regulatory defects; selective resistance to noncompetitive inhibitors and increased responsiveness to Pi activation. Selective growth of the patient's fibroblasts in medium containing 6-methylthioinosine confirmed the functional significance of the in vitro inhibitor resistance of the aberrant enzyme. Fibroblasts and lymphoblasts derived from patient 2-A showed increased concentrations and rates of generation of PPRibP as well as increased rates of the pathways of purine base salvage and purine nucleotide synthesis de novo. The magnitudes of these increases in the child's cells exceeded those in cells with catalytically superactive PPRibP synthetases. These alterations as well as the in vitro kinetic abnormalities in the patient 2-A enzyme were expressed to a reduced degree in fibroblasts from the child's affected mother, supporting the proposal that this woman is a heterozygous carrier for X-linked enzyme superactivity.

Intracellular activity of HPRT Cape Town: purine uptake and growth of cultured cells in selective media

The low activity of the human variant HPRT Cape Town is associated with substrate inhibition by hypoxanthine and guanine in vitro. The intracellular activity of this variant was investigated by study the relative uptake of radiolabelled purine nucleotide precursors and the growth in selective media of EBvirus-transformed lymphoblasts prepared from the proband (TK). These cells incorporated less than 10% of the [14C]hypoxanthine and [14C]guanine of the control cells; while their purine de novo synthesis was accelerated 8-fold. In selective media the HPRT Cape Town cells grew in a similar manner to HPRT-ve cells. These results indicate that if substrate inhibition is responsible for the low intracellular activity of HPRT Cape Town, the concentration of either hypoxanthine or guanine in the vicinity of the active site of the enzyme must be greater than the Ki(app) for these substrates, 118 and 28 mumol L(-1) respectively. Evidence is presented that the intracellular concentration of guanine, but not hypoxanthine, is well in excess of the Ki(app) in cultured lymphoblasts.

The concentration of 5-phosphoribosyl 1-pyrophosphate in monolayer tumor cells and the effect of various pyrimidine antimetabolites

5-Phosphoribosyl 1-pyrophosphate (PRPP) was determined in several murine and human cancer cell lines grown in monolayer, and harvested by trypsinization. For all cell lines a large variation in the PRPP concentration (5-1300 pmol/ 10(6) cells was found. A 1-hr incubation in Dullbecco's medium reduced the variation in PRPP concentration. After this incubation the highest concentration was found in the murine B16 melanoma cell line (about 200 pmol/10(6) cells). The human melanoma cell lines IGR3 and M5 and the human colon carcinoma cell line WiDr contained about 100 pmol/10(6) cells. After this preincubation of 1-hr these cell suspensions were used to study the effect of several antimetabolites on PRPP concentration. A 2-hr incubation with 1mM N-(phosphonacetyl)-L-aspartate (PALA) increased the PRPP concentration only in M5 cells, whereas methotrexate caused an increase in all cell lines. When 5-fluorouracil (5FU) was added, no significant decrease was found in any cell line. Addition of 5FU after a 2-hr preincubation with PALA resulted in a lower concentration in B16, M5 and WiDr cells. The prodrug, 5-fluoro-5' deoxyuridine altered the PRPP concentration only in in WiDr cells when it was added after PALA. The activity of the 5FU metabolizing enzyme orotate phosphoribosyl transferase was comparable in B16, M5 and WiDr cells, but much lower in IGR3 cells.

Purine metabolism and urate biosynthesis in isolated chicken hepatocytes

The metabolism of some purine compounds to urate and their effects on de novo urate synthesis in chicken hepatocytes were investigated. The purines, listed in descending order of rates of catabolism to urate, were hypoxanthine, xanthine, inosine, guanosine, guanine, IMP, GMP, adenosine, AMP, and adenine. During a 1-h incubation period, conversion to urate accounted for more than 80% of the total quantities of guanine, guanosine, and inosine metabolized, but only 42% of the adenosine and 23% of the adenine metabolism. Adenine, adenosine, and AMP inhibited de novo urate synthesis [( 14C]formate incorporation into urate), whereas the other purines, especially guanine, guanosine, and GMP, stimulated de novo urate synthesis. When hepatocytes were incubated with glutamine and adenosine, AMP, guanine, guanosine, or GMP, the rates of de novo urate synthesis were lower than the additive effects of glutamine and the purine in separate incubations. Increasing phosphate concentrations had no effect on urate synthesis in the absence of added purines but, in combination with adenosine, AMP, guanosine, or GMP, increased urate synthesis. These results indicate that the ratio of adenine to guanine nucleotides and the interaction between substrates and purine nucleotides are involved in the regulation of urate biosynthesis in chicken liver.

Purines as 'hyper-repressors' of glucose transport. A role for phosphoribosyl diphosphate

Under selected conditions the rate of glucose transport and the intracellular phosphoribosyl diphosphate (PPRibP) concentrations of chick-embryo fibroblasts are inversely correlated. This relationship holds when cells are incubated with mannose, fructose, xylose or various concentrations of glucose. The metabolic inhibitors 2,4-dinitrophenol, rotenone and Methylene Blue increased glucose transport and decreased PPRibP. The addition of any pyrimidine or purine base or ribonucleoside dramatically depleted PPRibP pools, regardless of the carbon source. Addition of guanine (10 microM) or hypoxanthine (100 microM) decreased transport in glucose-grown chick cells to barely detectable values, but did not affect increases observed in cells depressed by substitution of xylose for glucose. Guanosine, inosine and the purine analogues 6-thioguanine, 6-thioguanosine, 8-azaguanine and 6-methylmercaptopurine riboside sharply decreased transport in glucose-grown cells and blocked the increase in transport resulting from the replacement of glucose by fructose or xylose in the culture medium.

Purine oversecretion in cultured murine lymphoma cells deficient in adenylosuccinate synthetase: genetic model for inherited hyperuricemia and gout

Alterations in several specific enzymes have been associated with increased rates of purine synthesis de novo in human and other mammalian cells. However, these recognized abnormalities in humans account for only a few percent of the clinical cases of hyperuricemia and gout. We have examined in detail the rates of purine production de novo and purine excretion by normal and by mutant (AU-100) murine lymphoma T cells (S49) 80% deficient in adenylosuccinate synthetase [IMP:L-aspartate ligase (GDP-forming), EC 6.3.4.4]. The intracellular ATP concentration of the mutant cells is slightly diminished, but their GTP is increased 50% and their IMP, four-fold. Compared to wild-type cells, the AU-100 cells excrete into the culture medium 30- to 50-fold greater amounts of purine metabolites consisting mainly of inosine. Moreover, the AU-100 cell line overproduces total purines. In an AU-100-derived cell line, AU-TG50B, deficient in adenylosuccinate synthetase and hypoxanthine/guanine phosphoribosyltransferase (IMP:pyrophosphate phosphoribosyltransferase, EC 2.4.2.8), purine nucleoside excretion is increased 50- to 100-fold, and de novo synthesis is even greater than that for AU-100 cells. The overexcretion of purine metabolites by the AU-100 cells seems to be due to the primary genetic deficiency of adenylosuccinate synthetase, a deficiency that requires the cell to increase intracellular IMP in an attempt to maintain ATP levels. As a consequence of elevated IMP pools, large amounts of inosine are secreted into the culture medium. We propose that a similar primary genetic defect may account for the excessive purine excretion in some patients with dominantly inherited hyperuricemia and gout.

Modulation of purine synthesis and phosphoribosylpyrophosphate content in human fibroblasts at different population doublings

Human diploid fibroblasts of embryonic origin which display the limited lifespan phenomenon, respond to purine base deprivation with a large accumulation of phosphoribosylpyrophosphate when studied at early population doubling levels. This suggests that the purine salvage enzymes are the main consumers of phosphoribosylpyrophosphate in such cells. There is a strong positive correlation between the rate of de novo purine synthesis and the phosphoribosylpyrophosphate content, consistent with its role in the first and rate-limiting reaction of de novo purine synthesis. With increasing population doublings, the cells become less responsive to purine base deprivation; the phosphoribosylpyrophosphate does not increase significantly and a lower rate of de novo purine synthesis is observed. As the rate of cell replication decreases with serial passages, it becomes apparent that such cells appropriately adjust the concentration of phosphoribosylpyrophosphate, thereby controlling the rate of purine synthesis according to their needs for cellular growth.

Effect of L-glutamine antagonists on 5-phosphoribosyl 1-pyrophosphate levels in P388 leukemia and in murine colon adenocarcinomas in vivo

The intratumoral content of 5-phosphoribosyl 1-pyrophosphate (PRPP) and the activity of the enzymes anabolizing and catabolizing the sugar phosphate were determined following i.p. administration of an LD10 dose of an L-glutamine antagonist or saline to tumor-bearing animals. Elevation of PRPP pool size following administration of L-[alpha S,5S]-alpha-amino-3-chloro-4,5-dihydro-5-isopazoleacetic acid (NSC-163501) (AT-125) was maximal at 8 hr and returned to pretreatment levels by 24 hr. In P388 leukemia, dose for dose, at 4 hr, 6-diazo-5-oxo-L-norleucine (NSC-7365) (DON) was the most potent of the L-glutamine antagonists in elevating basal PRPP pool size (50% above control) followed by AT-125 and azaserine, 300 and 100% above control respectively. Moreover, such augmentation in PRPP pool size preferentially affected P388 tumor rather than the small intestine. Following i.p. administration of LD10 doses of AT-125, DON and azaserine, the specific activities of PRPP anabolizing and catabolizing enzymes were determined. A significant inhibition of PRPP amidotransferase was demonstrated with DON and AT-125 (P less than 0.05), and no inhibition with azaserine. A similar modulation of PRPP pool size demonstrated in vivo following administration of 250 mg/kg of ART-125 in mice bearing colonic adenocarcinoma lines. It was suggested that a significant increase of PRPP pool size might cause the possible synergism of a selected L-glutamine antagonist and 5-fluorouracil as reported after the appropriately scheduled administration of methotrexate and 5-fluorouracil.

Elucidation of aberrant purine metabolism: application to hypoxanthine-guanine phosphoribosylstransferase- and adenosine kinase-deficient mutants, and IMP dehydrogenase- and adenosine deaminase-inhibited human lymphoblasts

We propose that the ratio of [14C]formate-labelled purine nucleosides and bases (both intra and extracellular) to nucleic acid purines provides, in exponentially growing cultures, a sensitive index for comparative studies of purine metabolism. This ratio was 4-fold greater for an HGPRT- mutant than for the parental HGPRT+ human lymphoblast line. The major components of the labelled nucleoside and base fraction were hypoxanthine and inosine. By blocking adenosine deaminase activity with coformycin we found that approx. 90% of inosine was formed directly from IMP rather than the route IMP leads to AMP leads to adenosine leads to inosine. The ratio of labelled base + nucleosides to nucleic acids was essentially unchagned for an AK- lymphoblast line and 2-fold greater than control for an HGPRT(-)-KAK- line, demonstrating that a deficiency of adenosine kinase alone has little effect on the accumulation of purine nucleosides and bases. Although adenosine was a minor component of the nucleoside and base fraction, the adenosine fraction increased from 3 to 13% with the addition of coformycin to the HGPRT(-)-AK- line. In the parental and HGPRT- lines, adenosine was shown to be primarily phosphorylated rather than deaminated at concentrations less than 5 microM. Inhibition of IMP dehydrogenase activity by mycophenolic acid caused a 12- and 3-fold increase in the rate of production of labelled base and nucleoside in the parent and HGPRT- cells respectively. These results suggest that a mutationally induced partial deficiency in the activities converting IMP to guanine nucleotides may result in an increased catabolism of IMP.