Glucagon or cyclic AMP-stimulated synthesis of 5-phosphoribosyl 1-pyrophosphate in isolated hepatocytes and inhibition by antimicrotubular drugs

Uric acid inhibits liver phosphorylase a activity under simulated in vivo conditions

We have reported that glycogen synthesis and degradation can occur in vivo without a significant change in the amount of phosphorylase a present. These data suggest the presence of a regulatable mechanism for inhibiting phosphorylase a activity in vivo. Several effectors have been described. AMP stimulates, whereas ADP, ATP, and glucose inhibit activity. Of these effectors, only the glucose concentration changes under normal conditions; thus it could regulate phosphorylase a activity in vivo. We previously have reported that, when all of these effectors were present at physiological concentrations, the net effect was no change in phosphorylase a activity. Addition of caffeine, an independent inhibitor of activity, to the above effectors not only resulted in inhibition but also restored a glucose concentration-dependent inhibition. Because uric acid is an endogenous xanthine derivative, we decided to determine whether it had an effect on phosphorylase a activity. Independently, uric acid did not affect activity; however, when added at a presumed physiological concentration in combination with AMP, ADP, ATP, and glucose, it inhibited activity. A modest but not statistically significant glucose concentration-dependent inhibition was also present. Thus uric acid may play an important role in regulating phosphorylase a activity in vivo.

Role of cellular ribose-5-phosphate content in the regulation of 5-phosphoribosyl-1-pyrophosphate and de novo purine synthesis in a human hepatoma cell line

5-Phosphoribosyl-1-pyrophosphate (PRPP) is an important regulator of de novo purine synthesis. However, the role of ribose-5-phosphate (R5P), the precursor for PRPP, in the regulation of PRPP and de novo purine synthesis has not yet been clarified conclusively. This study was designed to clarify interrelationships between R5P content, PRPP availability, and the rate of de novo purine synthesis in the cultured human hepatoma cell line (HepG2), a plausible model for normal human hepatocytes. Increasing glucose concentration in the culture media from 0 to 10 mmol/L resulted in a 2.9-fold elevation of cellular R5P content (from 107 +/- 31 to 311 +/- 57 nmol/g protein), associated with a correlated increase of 7.14-fold in cellular PRPP availability (from 4.76 +/- 3.4 to 34 +/- 8.4 pmol/mg protein/min) and of 149-fold in the rate of de novo purine synthesis (from 55 to 8,204 dpm/mg protein/h). Plotting the rate of de novo purine synthesis versus R5P content indicates that at a wide range of R5P content, including that prevailing in hepatocytes under physiological conditions, the rate of purine synthesis depends on R5P content. A similar dependence was also demonstrated for PRPP availability. The rate of de novo purine synthesis exhibited a sigmoidal dependence on PRPP availability. The demonstration in human hepatocytes of dependence of the rate of purine synthesis on R5P content has implications concerning the pathogenesis of purine overproduction associated with several inborn and acquired conditions in man.

Promoter regions of the human X-linked housekeeping genes PRPS1 and PRPS2 encoding phosphoribosylpyrophosphate synthetase subunit I and II isoforms

The 5' regions of the human phosphoribosylpyrophosphate synthetase subunit I and II genes (PRPS1 and PRPS2, respectively) were isolated and sequenced. A comparison of the nucleotide sequences between human and rat PRPS1 genes revealed that the sequences around the transcription initiation sites were conserved over 56 nucleotides, and that a TATA-like sequence, a CCAAT box and three putative Sp1 binding sites were present at almost the same positions in the GC-rich sequences. Two major transcription initiation sites were localized in the human PRPS1, one of the two was located 27 nucleotides downstream from the TATA-like sequence, while the upstream initiation site was in the TATA-like sequence. The promoter region of the human PRPS2 gene was also GC-rich and contained a TATA-like sequence, four Sp1 binding sites and a homopyrimidine stretch. The initiation sites were localized at 90 nucleotides upstream from the ATG initiation codon. Chloramphenicol acetyltransferase (CAT)/promoter fusion assays showed that a 2.0 kb region (human PRPS1) and a 1.1 kb region (human PRPS2) possessed the promoter activities in four cell lines. The CAT activities in the three human cell lines tended to correlate with the steady-state mRNA levels of the PRPS1 and PRPS2 genes. These results suggest that the 5' flanking regions cloned contribute to the cell-differential expression of these two genes.

Acceleration of purine synthesis in mouse liver by glycogenolytic hormones

Administration (ip) into fed mice of glucagon, epinephrine, vasopressin, oxytocin, angiotensin II, and dibutyryl cyclic AMP (dbcAMP) resulted in a rapid (within 2.5 to 15 min) elevation of PRPP content (two- to threefold) and in acceleration of the rate of de novo purine synthesis (twofold). Inhibition of the epinephrine-stimulated glycogenolysis by 2,5-anhydromannitol diminished markedly the acceleration effect of the hormone on the rate of purine synthesis. Administration of the hormones caused a rapid rise in the liver content of glucose 6-phosphate (G6P) by 15-70% but did not increase the ribose 5-phosphate (R5P) content. Liver ATP content was not affected. The hormones did not cause direct activation of PRPP synthetase, as gauged by the specific activity of the enzyme, its Km for substrates R5P and ATP, and its sensitivity to inhibition by ADP and GDP. The hormones did not increase the liver content of the enzyme activators Pi and Mg2+. The results suggest that the glycogenolytic hormones accelerate purine synthesis by a metabolic mechanism associated with the enhancement of glycogenolysis. PRPP synthesis is probably enhanced by the glycogenolysis-induced alterations in the cellular content of some metabolites other than R5P.

Early mitogenic stimulation of metabolic flux through phosphoribosyl pyrophosphate into nucleotides in Swiss 3T3 cells and requirement of external magnesium for the response

5-Phosphoribosyl 1-pyrophosphate (PRPP) is a common precursor for the synthesis of all nucleotides and also serves as a critical regulator for the synthesis. In spite of a number of studies in vitro on mammalian PRPP synthetase, our understanding of the regulation of PRPP synthesis in situ is very limited. Various mitogens are known to activate purine and pyrimidine de novo biosynthesis and purine base phosphoribosylation as an early response in quiescent mouse fibroblasts. We aimed at elucidation of the underlying mechanism for the possible increase in PRPP synthesis in mitogen-stimulated mouse fibroblasts in culture. In order to quantitatively follow metabolic flux through PRPP into nucleotides, [ribosyl-14C]inosine was enzymatically prepared and used as a tracer to preferentially label intracellular ribose phosphate. The radioactivity incorporation into cellular nucleotides was measured. Evidence supported the validity of the method. Prior exposure of quiescent Swiss 3T3 cells in culture to epidermal growth factor (EGF) plus insulin for 45-60 min enhanced approximately 2-fold the radioactivity incorporation from [ribosyl-14C]inosine into nucleotides, without increasing the specific radioactivity of intracellular free ribose 5-phosphate. [14C]Uracil incorporation into nucleotides, a measure for PRPP-independent ribose phosphate utilization for nucleotide synthesis, was not increased. These and other results indicate that EGF plus insulin stimulates the metabolic flux through PRPP. A similar stimulation was induced by bombesin and melittin in combination with insulin and by fibroblast growth factor alone. Quiescent Swiss 3T6 cells and human fetal fibroblasts showed a similar stimulation of nucleotide synthesis in response to exposure to serum. For characterization of intracellular signaling pathways, we examined effects of several inhibitors and agents on the stimulation. The divalent cation ionophore A23187 mimicked the response to EGF and insulin in Swiss 3T3 cells, thereby suggesting involvement of divalent cation mobilization in this increase. The effect of the ionophore was not additive to that of the growth factors. Omission of Ca2+ from the incubation medium did not affect the response to EGF and insulin, whereas the omission of Mg2+ did abolish the response. Furosemide, an inhibitor of Mg2+ influx, partially inhibited the stimulated synthesis of nucleotides. Thus, the entry of external Mg2+ into the cells may play a critical role in this signal transduction. These results provided an important access to elucidation of the intracellular mechanisms for the mitogen-induced increase in PRPP and nucleotide syntheses.

Increase of urate formation by stimulation of sympathetic hepatic nerves, circulating noradrenaline and glucagon in the perfused rat liver

In the isolated rat liver perfused in situ stimulation of the nerve bundles around the portal vein and the hepatic artery caused an increase of urate formation that was inhibited by the alpha 1-blocker prazosine and the xanthine oxidase inhibitor allopurinol. Moreover, nerve stimulation increased glucose and lactate output and decreased perfusion flow. Infusion of noradrenaline had similar effects. Compared to nerve stimulation infusion of glucagon led to a less pronounced increase of urate formation and a twice as large increase in glucose output but a decrease in lactate release without affecting the flow rate. Insulin had no effect on any of the parameters studied.

In vivo inhibition of the rate of de novo purine synthesis in rat liver by glucagon

The rates of de novo purine and protein synthesis were assessed in vivo in rat liver after bolus administration of glucagon. The specific activity of hepatic purines and the specific activity ratio of hepatic purine/protein were used as an index of the rate of de novo purine synthesis and the rate relative to protein. Glucagon at doses of 0.01 mg to 0.1 mg/200 g body weight (BW), administered as an intravenous bolus, inhibited dose-dependently the rate of de novo purine synthesis and the rate relative to protein although it increased dose-dependently the hepatic concentration of 5-phosphoribosyl 1-pyrophosphate (PRPP). The inhibition of the rate of de novo purine synthesis recovered to control levels during the period between 60 and 90 minutes after glucagon administration. Dibutyryl cyclic AMP (Bt2 cAMP) partially mimicked this effect of glucagon in that it did not increase the rate of de novo purine synthesis in spite of increased concentrations of PRPP. These results suggest that an intravenous bolus of glucagon inhibits the rate of de novo purine synthesis through increasing cAMP concentration. The data are consistent with inhibition of amidophosphoribosyltransferase (ATase) in spite of increased PRPP concentrations.

Beta-adrenergic stimulation inhibits calcium efflux and alters the morphology of cultured arterial muscle cells

Beta-adrenoreceptor stimulation with isoproterenol (IP) rapidly and reversibly rounded and arborized smooth muscle cells (SMC) cultured from rat aorta. The arborized SMC remained firmly attached to the substratum via a network of long, dendritelike processes. Arborization of the SMC correlated closely with increases in cellular cAMP produced by IP and a variety of other compounds. The intracellular calcium antagonist 8-(N,N-diethylamino)-octyl-3,4,5-trimethoxybenzoate (TMB-8) also rounded and arborized the SMC. Antitubulin compounds potently blocked arborized by IP, dibutyryl cAMP, and TMB-8. The release of cell-bound 45Ca2+ was followed in the presence and absence of IP. IP strikingly increased the amount of 45Ca2+ that remained cell bound between 20 and 120 min. Propranolol and colchicine prevented IP from inhibiting the release of cell-bound 45Ca2+. These results suggest that the modulation of Ca2+ transport is involved in the arborization of cultured SMC by cAMP.

Mitogenic enhancement of purine base phosphoribosylation in Swiss mouse 3T3 cells

Increased uptake of guanosine and the purine bases, adenine, hypoxanthine, and guanine, occurs within minutes after addition of fresh serum or purified growth-promoting agents to density-arrested cultures of Swiss 3T3, Swiss 3T6, and tertiary mouse embryo fibroblasts. Enhancement of uptake is maximal by about 50 min, is potentiated by combinations of growth promoters, and involves a process distinguishable from that of the enhanced uridine uptake on the basis of time course, pattern of growth-factor responsiveness and the failure of uridine to inhibit purine uptake. Enhanced purine uptake in 3T3 cells results from stimulation of the phosphoribosylation of purine bases and consequent trapping of nucleotide derivatives rather than from increased rates of membrane transport of the purine compounds. Increased availability of 5-phosphoribosyl 1-pyrophosphate (PRPP), identifiable within 30 min of serum addition, provides a likely explanation for increased purine base phosphoribosylation. Mitogenic stimulation of purine uptake is unaffected by cycloheximide but is markedly reduced in sodium-free medium. Enhanced purine uptake does not clearly depend upon changes in concentrations of effectors of intracellular PRPP synthesis. Nevertheless, when the possibility of direct mitogenic activation of PRPP synthase was studied, no differences were found in activities of this enzyme in extracts from stimulated and unstimulated 3T3 cells and mouse embryo fibroblasts.

Enhancement of intracellular 5-phosphoribosyl 1-pyrophosphate levels as a major factor in the 6-azauridine-induced stimulation of carbamoyl phosphate synthesis in mouse spleen slices

Brief exposure to 6-azauridine stimulates the production of carbamoyl phosphate for de novo pyrimidine biosynthesis in vitro in slices of haematopoietic spleen from anaemic mice (preceding paper). In studies of the underlying mechanism for this response we turned our attention to changes in the level of substrates and effectors for carbamoyl-phosphate synthetase II. Intermediates of the orotic acid pathway and 6-azauridine had little effect on the synthetase activity in vitro. 6-Azauridine 5'-monophosphate (6-AzaUMP) stimulated synthetase II, possibly in an allosteric manner. However, in view of the potency as an activator and the tissue levels, 6-azaUMP may be only partially responsible for the stimulation. Adenine nucleotide levels in the tissue showed only minor changes after brief exposure (15 min) to 6-azauridine. The level of UTP and UDP, potent inhibitors for synthetase II, showed no significant change. The level of 5-phosphoribosyl 1-pyrophosphate (PPRibP), a potent positive effector for the synthetase II, showed a more than 1.5-fold increase after 15 min. The relative importance of these factors was evaluated by assay of the synthetase, partially purified from mouse spleen, under simulated conditions in vitro. The results indicated that the enhanced level of PPRibP played a major role in increasing the production of carbamoyl phosphate. In Ehrlich ascites cells in vitro, where 6-azauridine did not increase carbamoyl phosphate production, the basal PPRibP level was high (range over 0.1 mM) and the changes in the level, brought about by the analogue, were relatively small.

Regulation of phosphoribosylpyrophosphate synthetase by endogenous purine and pyrimidine compounds and synthetic analogs in normal and leukemic white blood cells

Phosphoribosylpyrophosphate (PRPP) is essential for the formation of both purine and pyrimidine nucleotides as well as for the active nucleotide form of some chemotherapeutic agents. The formation of PRPP is catalyzed by by enzyme PRPP synthetase, and many different compounds are known to affect the activity of this enzyme. This report examines the effects of endogenous purine and pyrimidine nucleotides, nucleosides, and several analogs of these compounds on the activity of PRPP synthetase from different types of normal and leukemic white blood cells (i.e. normal lymphocytes, normal granulocytes, phytohemagglutinin-stimulated lymphocytes, and acute and chronic leukemic cells). Our results show that the effect varied with each individual compound, and the magnitude of the effect was dependent on the source of the enzyme. Since it appears possible to differentially affect PRPP synthetase activity from the different types of leukemic cells, this enzyme may be a potential target site in the chemotherapy of leukemia.

Alterations in glomerular RNA in diabetic rats: roles of glucagon and insulin

Incorporation in vivo of labeled orotate into RNA and total nucleotides was measured in isolated glomeruli and whole renal cortex. In 2-day diabetic animals, glomerular RNA was increased, and there was greater incorporation of orotate into total nucleotides and RNA as compared with controls. Insulin reversed the exaggerated incorporation at infusion rates that corrected hyperglucagonemia without reducing plasma glucose and with only minimal changes in insulin concentrations. The addition of glucagon to insulin infusions reproduced the increased incorporation observed in untreated diabetics. Similar changes occurred in renal cortex, where differences in orotate incorporation into nucleotide precursors seemed to be the main cause for alterations in RNA labeling. Isotope incorporation in glomeruli correlated positively with plasma glucagon, but not with insulin or glucose concentrations. Although in 7-month diabetic animals orotate incorporation into RNA was less than in controls, probably as a consequence of renal disease, 24-hour insulin infusion decreased it further. Our results confirm that in the diabetic kidney, abnormal uracil nucleotide metabolism and increased cellular content of RNA are demonstrable in glomeruli as in the renal cortex. These changes appear to be related directly to hyperglucagonemia.

Phosphoribosylpyrophosphate bioavailability in diabetic rat renal cortex in vivo

Experimental diabetes induces increased content of RNA and UTP in the renal cortex. Studies were designed to assess the bioavailability of 5-phosphoribosyl-1-pyrophosphate (PRPP) in the diabetic renal cortex because PRPP is an important determinant of the de novo synthesis of nucleotides. The tissue bioavailability of PRPP determines the effects of orotate or adenine administration on UTP, ATP, and GTP content and on the incorporation of labeled precursors into UTP and ATP. Diabetic and control rats with chronic intravenous cannulas were infused over 2.5-24 h with orotate or adenine. Orotate administration induced greater decreases in ATP and GTP and in labeled adenine incorporation into ATP concomitant with smaller increases in UTP in controls than in diabetic animals. Adenine administration induced a greater decrease of labeled orotate incorporation into UTP and a smaller increase in ATP in controls than in diabetic animals. Prolonging the adenine infusion resulted in disappearance of these differences. The results are compatible with greater initial bioavailability of PRPP in the diabetic renal cortex than in controls but with a rate of maximal PRPP generation that is the same in both tissues.