Uridine transport and phosphorylation in mouse cells in culture: effect of growth-promoting factors, cell cycle transit and oncogenic transformation

Key role of uridine kinase and uridine phosphorylase in the homeostatic regulation of purine and pyrimidine salvage in brain

Uridine, the major circulating pyrimidine nucleoside, participating in the regulation of a number of physiological processes, is readily uptaken into mammalian cells. The balance between anabolism and catabolism of intracellular uridine is maintained by uridine kinase, catalyzing the first step of UTP and CTP salvage synthesis, and uridine phosphorylase, catalyzing the first step of uridine degradation to beta-alanine in liver. In the present study we report that the two enzymes have an additional role in the homeostatic regulation of purine and pyrimidine metabolism in brain, which relies on the salvage synthesis of nucleotides from preformed nucleosides and nucleobases, rather than on the de novo synthesis from simple precursors. The experiments were performed in rat brain extracts and cultured human astrocytoma cells. The rationale of the reciprocal regulation of purine and pyrimidine salvage synthesis in brain stands (i) on the inhibition exerted by UTP and CTP, the final products of the pyrimidine salvage pathway, on uridine kinase and (ii) on the widely accepted idea that pyrimidine salvage occurs at the nucleoside level (mostly uridine), while purine salvage is a 5-phosphoribosyl-1-pyrophosphate (PRPP)-mediated process, occurring at the nucleobase level. Thus, at relatively low UTP and CTP level, uptaken uridine is mainly anabolized to uridine nucleotides. On the contrary, at relatively high UTP and CTP levels the inhibition of uridine kinase channels uridine towards phosphorolysis. The ribose-1-phosphate is then transformed into PRPP, which is used for purine salvage synthesis.

Pasteurella multocida toxin facilitates inositol phosphate formation by bombesin through tyrosine phosphorylation of G alpha q

The intracellularly acting Pasteurella multocida toxin (PMT) is a potent mitogen that stimulates Gq-dependent formation of inositol trisphosphate. We show that PMT, a nontoxic mutant of PMT (PMTC1165S), and bombesin each stimulate time-dependent phosphorylation of G alpha q at tyrosine 349. Although PMT and PMTC1165S each cause phosphorylation of G alpha q, only the wild-type toxin activates Gq. Pretreatment of cells with wild-type or mutant PMT potentiated the formation of inositol phosphates stimulated by bombesin equally. These data show that PMT potentiates bombesin receptor signaling through tyrosine phosphorylation of Gq and distinguishes between the two proposed models of Gq activation, showing that tyrosine phosphorylation is not linked to receptor uncoupling.

Nucleolar Arf tumor suppressor inhibits ribosomal RNA processing

The p19(Arf) tumor suppressor, a nucleolar protein, binds to Mdm2 to induce p53-dependent cell cycle arrest. Arf also prevents the proliferation of cells lacking Mdm2 and p53, albeit less efficiently. We show that p19(Arf) inhibits production of ribosomal RNA, retarding processing of 47/45S and 32S precursors. These effects correlate with but do not strictly depend upon inhibition of rRNA biosynthesis or cell cycle arrest, are not mimicked by p53, and require neither p53 nor Mdm2. Arf mutants lacking conserved amino acid residues 2-14 do not block rRNA synthesis and processing or inhibit cell proliferation. Evolution may have linked a primordial nucleolar Arf function to Mdm2 and p53, creating a more efficient checkpoint-signaling pathway for coordinating ribosomal biogenesis and cell cycle progression.

Inosine-5'-monophosphate dehydrogenase is required for mitogenic competence of transformed pancreatic beta cells

The relation of inosine-5'-monophosphate dehydrogenase (IMPDH; the rate-limiting enzyme in GTP synthesis) to mitogenesis was studied by enzymatic assay, immunoblots, and RT-PCR in several dissimilar transformed pancreatic ss-cell lines, using intact cells. Both of the two isoforms of IMPDH (constitutive type 1 and inducible type 2) were identified using RT-PCR in transformed beta cells or in intact islets. IMPDH 2 messenger RNA (mRNA) and IMPDH protein were both regulated reciprocally by changes in levels of their end-products. Flux through IMPDH was greatest in rapidly growing cells, due mostly to increased uptake of precursor. Glucose (but not 3-0-methylglucose, L-glucose, or fructose) further augmented substrate uptake and also increased IMPDH enzymatic activity after either 4 or 21 h of stimulation. Serum or ketoisocaproate also increased IMPDH activity (but not uptake). Two selective IMPDH inhibitors (mycophenolic acid and mizoribine) reduced IMPDH activity in all cell lines, and, with virtually identical concentration-response curves, inhibited DNA synthesis (assessed as bromodeoxyuridine incorporation) in response to glucose, serum, or ketoisocaproate. Inhibition of DNA synthesis was reversible, completely prevented by repletion of cellular guanine (but not adenine) nucleotides, and could not be attributed to toxic effects. Despite the fact that modulation of IMPDH expression by guanine nucleotides was readily detectable, glucose and/or serum failed to alter IMPDH mRNA or protein, indicating that their effects on IMPDH activity were largely at the enzyme level. Precursors of guanine nucleotides failed, by themselves, to induce mitogenesis. Thus, adequate IMPDH activity (and thereby, availability of GTP) is a critical requirement for beta-cell proliferation. Although it is unlikely that further increases in GTP can, by themselves, initiate DNA synthesis, such increments may be needed to sustain mitogenesis.

CSF-1 stimulates nucleoside transport in S1 macrophages

We have examined nucleoside transport (NT) in a cell line derived from primary day 7 murine bone marrow macrophages (S1 macrophages) in response to the macrophage growth factor, colony-stimulating factor 1 (CSF-1). Adenosine and uridine transport in quiescent S1 macrophages occurred primarily by two facilitated diffusional routes, one that was sensitive and one that was relatively resistant to the inhibitor nitrobenzylthioinosine (NBMPR). Addition of CSF-1 to quiescent cultures resulted in increased adenosine and uridine transport with biphasic kinetics with respect to the cell cycle. Basal NT activity was elevated (about twofold) within 15 min of CSF-1 addition, returned to near basal levels by 1 h, and then increased again (three- to fourfold) 8-12 h later, returning again to basal levels by 48 h post CSF-1 stimulation. We propose that the large increase in NT activity at 8-12 h corresponded with the time when cultures synchronously began to enter the S phase of the cell cycle. In addition to these changes in the absolute rates, the proportions of NBMPR-sensitive and NBMPR-insensitive transport also change after CSF-1 addition. Quiescent cultures exhibited primarily NBMPR-insensitive transport while logrithmically growing cultures exhibited primarily NBMPR-sensitive nucleoside transport activity. The increase in the NBMPR-sensitive component of the transport process paralleled a similar increase in the number of high-affinity NBMPR binding sites, suggesting that the mechanism for upregulating NBMPR-sensitive NT activity involves increases in the number of NBMPR-sensitive transporter sites. Interestingly, we were unable to detect Na(+)-dependent concentrative uptake of adenosine, uridine, or formycin-B either in the S1 macrophage cell line or in primary (day 7) murine macrophages. Thus these bone marrow derived macrophages did not display the characteristically large Na(+)-dependent transport systems observed by others in peritoneal macrophages, implying that these two populations of macrophages are, indeed, functionally distinct.

Effects of transformation by v-fps on nucleoside transport in Rat-2 fibroblasts

Important cellular nutrients, including nucleosides and hexose sugars, are rapidly taken up by cells, largely through mediated carrier systems. The present study examined nucleoside and hexose transport activity in normal Rat-2 fibroblasts and clonal derivatives that expressed either the wild-type (C10) or a temperature-sensitive mutant (NA9) form of v-fps, a transforming protein-tyrosine kinase. Initial uptake rates (transport) of adenosine, thymidine, 3-O-methylglucose and 2-deoxyglucose were greater in v-fps-transformed cells than in normal cells. Elevated transport rates were seen in cells that expressed the temperature-sensitive mutant v-fps only after growth at a temperature that was permissive for protein-tyrosine kinase activity. Nucleoside transport rates declined with increasing cell density in both normal and v-fps transformed cells. Analysis of the sensitivity of adenosine transport to inhibition by nitrobenzylthioinosine (NBMPR) indicated that Rat-2 fibroblasts, like many other rat cell types, possess at least two nucleoside transport systems, which can be distinguished by differences in sensitivity to NBMPR. Although both transport activities were elevated in v-fps-transformed cells, a greater increase was seen in the NBMPR-sensitive component than in the NBMPR-insensitive component. Mass law analysis of the binding of [3H]NBMPR indicated that transformed cells had either the same number (NA9) or a smaller number (C10) of NBMPR-binding sites than normal cells, and photolabelling of membrane proteins with [3H]NBMPR identified polypeptides with similar electrophoretic mobilities (55-75 kDa) in both normal and transformed cells. Thus transformation by v-fps resulted in an increase in NBMPR-sensitive transport activity which was not related to either the number of NBMPR-binding sites or the apparent molecular mass of NBMPR-binding polypeptides.

Extracellular ATP stimulates increases in Na+/K+ pump activity, intracellular pH and uridine uptake in cultures of mammalian cells

Using 3T3 and 3T6 mouse fibroblasts and A431 epidermoid carcinoma cells, we previously observed that extracellular ATP and ADP were mitogens and they synergized with other growth factors (Huang, N., Wang, D. and Heppel, L. A. (1989) Proc. Natl. Acad. Sci. USA 86, 7904-7908). We now report that ATP and ADP stimulated Na+ entry, intracellular alkalinization and Na+/K+ pump activity, which are early events that had been proposed to play a central role in DNA synthesis. In addition, ATP, ADP and AMPPNP stimulated uridine uptake by a pathway involving arachidonic acid metabolism. In A431 cells, activation of protein kinase C also contributed to ATP-dependent stimulation of uridine uptake. Concentrations of indomethacin and pertussis toxin which inhibited uridine uptake also blocked arachidonic acid metabolism and DNA synthesis. ATP acted as a competence factor. Interestingly, ATP did not have to be continuously present to stimulate uridine uptake. It was equally effective even when it was washed away after brief treatment of cells.

Stimulation by xanthine oxidase of 3T3 Swiss fibroblasts and human lymphocytes

Xanthine oxidase stimulates [3H]thymidine incorporation by 3T3 cells even in the absence of any added xanthine, but in the presence of, and synergistically with, insulin or various other growth-stimulating factors. Optimal stimulation was obtained with 2-3 mU enzyme/ml and higher concentrations were toxic. Xanthine oxidase also stimulated human peripheral blood lymphocytes in the presence of phytohemagglutinin and xanthine.

Fibroblast growth factor stimulates protein kinase C in quiescent 3T3 cells without Ca2+ mobilization or inositol phosphate accumulation

To elucidate the transmembrane signalling processes initiated by fibroblast growth factor (FGF), we have studied the effect of recombinant basic FGF (bFGF) on various early events associated with mitogenesis in Swiss 3T3 fibroblasts. bFGF, at mitogenic concentrations, neither induced Ca2+ mobilization from intracellular stores nor increased the accumulation of inositol phosphates. In contrast, bFGF stimulated the phosphorylation of the Mr 80,000 (80K) cellular protein which is a major substrate of protein kinase C. This effect was potentiated by the diacylglycerol kinase inhibitor R59022. Two-dimensional polyacrylamide gel electrophoresis and phosphopeptide mapping showed that the 80K phosphoproteins generated in response to bFGF, bombesin, and phorbol 12,13-dibutyrate were indistinguishable. Down-regulation of protein kinase C prevented bFGF stimulation of 80K phosphorylation. Other protein kinase C-dependent early events such as transmodulation of the epidermal growth factor receptor, cytoplasmic alkalinization, inhibition of vasopressin induced increase in cytosolic [Ca2+], and enhancement of cAMP accumulation in response to forskolin were also induced by bFGF. Similar results were obtained when bFGF was added to quiescent cultures of tertiary mouse embryo fibroblasts. We conclude that bFGF stimulates protein kinase C through a signal transduction pathway distinct from inositol phospholipid turnover and Ca2+ mobilization.

Protective effect of human epidermal growth factor against the experimental gastric mucosal lesions in rats

The protective effect of human epidermal growth factor (hEGF) on the gastric mucosal lesions in rats was examined in relation to the immunoreactive concentration of plasma. Human EGF (30 micrograms/kg) was administered intravenously, intraperitoneally or subcutaneously. At different times following the administration of hEGF, rats received acidified ethanol solution to induce an experimental gastric mucosal lesion. Sum of lesion length in the gastric mucosa was used as a lesion index. Human EGF administered parenterally markedly decreased the gastric mucosal lesions in 10 min after administration of necrotizing solution, and 10 to 30 min delay was observed in the development of maximal protective action. Profiles of protective potency against the hEGF dose administered intraperitoneally or subcutaneously 30 min before administration of necrotizing solution revealed that the effective dose of hEGF (ED50) was about 5.2 and 2.6 micrograms/kg, for intraperitoneal and subcutaneous administrations, respectively.

Pasteurella multocida toxin: potent mitogen for cultured fibroblasts

Native Pasteurella multocida toxin (PMT) is shown to be an extremely potent mitogen for Swiss 3T3 fibroblasts. Half-maximal stimulation of DNA synthesis was obtained at concentrations of 1 and 2 pM for recombinant PMT (rPMT) and PMT, respectively. The degree of rPMT-induced DNA synthesis was comparable to that elicited by 10% fetal bovine serum and, moreover, was observed in the complete absence of other factors. Cell proliferation was also enhanced by rPMT. The toxin was also a potent mitogen for BALB/c and NIH 3T3 cells, 3T6 cells, and tertiary mouse embryo or human fibroblasts. The mitogenic activity of rPMT was heat-labile. A polyclonal antiserum to PMT inhibited DNA synthesis when added early, but not late, during treatment of the Swiss 3T3 cells with rPMT. A similar time-dependent action of methylamine was also observed. Furthermore, transient exposure of the cells to rPMT at 37 degrees C, but not at 4 degrees C, resulted in a stimulation of DNA synthesis. Thus, toxin action may require cell entry and processing via an acidic compartment. The toxin, at mitogenic concentrations, caused a large increase in the production of inositol phosphates. In contrast, rPMT did not increase the intracellular concentration of cyclic AMP in Swiss 3T3 cells. The basis of rPMT action may afford a unique insight into molecular signaling events involved in the control of cell proliferation.

Epidermal growth factor receptor-mediated selective cytotoxicity of antitumor agents toward human xenografts and murine syngeneic solid tumors

Severe toxic side effects of antiproliferative agents limit their clinical usefulness as antitumor drugs. Recently we observed that the antitumor efficacy of various antitumor agents (5-fluorouracil, tegafur, adriamycin, mitomycin C, cyclophosphamide, and cisplatin) against experimental solid tumors was enhanced by prior or simultaneous administration of human epidermal growth factor (EGF). However, coadministration of EGF did not enhance the toxicity of antitumor agents as measured by LD50 and body weight loss. The above selective potentiation of efficacy of the antitumor agents by human EGF can be characterized as follows. In a dose-dependent manner, human EGF enhanced the efficacy of an antitumor agent (5-FU) treatment against human epidermoid carcinoma A431 transplanted sc in athymic nude mice [ED50 = 2.9 (0.2-49.7, 95% confidence interval) microgram/kg, sc]. Various degrees of enhancement were also observed against other experimental tumors transplanted sc. The degrees of enhancement were directly proportional to the numbers of human EGF binding sites present on tumor cell plasma membrane (threshold of binding site density = 1.5 X 10(3) sites/cell) using 5-FU or cisplatin as an antitumor agent, thus suggesting that the binding of EGF to the receptors on tumor cells is an essential process in enhancing the susceptibility of tumor cells to antitumor agents. Normal cells including intestinal epithelial and bone marrow cells are endowed with fewer EGF binding sites (less than 10(3) sites/cell). This may explain partially the absence of EGF-enhanced cytotoxicity by antitumor agents toward normal cells.

Membrane transport and the antineoplastic action of nucleoside analogues

This article summarizes recent studies characterizing nucleoside transport in mammalian cells and discusses evidence for a role of membrane transport in the pharmacologic action of nucleoside analogues. Some of these studies have also addressed the controversy concerning the multiplicity in transport routes. It seems clear that erythrocytes and, perhaps, some other mammalian cells possess a single, broadly specific system for transporting nucleosides. However, substantial evidence from valid studies discriminating between transport and intracellular metabolism suggests that at least some mammalian cells, including some tumor cells, possess more than a single system. Evidence now exists for a determining role of membrane transport of nucleoside analogues in their cytotoxicity and, in the case of one pyrimidine nucleoside (AraC), in therapeutic responsiveness in leukemic patients. There are also numerous examples of transport-related resistance to nucleoside analogues. Included in this article are the results of studies from the authors' laboratory pertaining to the therapeutic activity of the purine nucleoside, FAraA, in murine tumor models. These studies provide evidence for a determining role of both membrane transport and intracellular phosphorylation in the selective antitumor action of this agent against murine leukemia. Substantially increased transport inward of FAraA occurs at pharmacologically achievable concentrations of this agent in tumor cells as compared to drug-limiting, normal proliferative epithelium of the small intestine. The basis for this differential appears to be the kinetic duality of FAraA and adenosine transport inward found in tumor cells, but not in proliferative intestinal epithelial cells. Tumor cells have highly saturable (low influx Km) and poorly saturable (high influx Km) systems for adenosine transport, both of which are shared by FAraA. In contrast, proliferative epithelial cells have only a poorly saturable system for these substrates. If a similar kinetic duality of nucleoside transport is found in other tumor cells certain implications arise concerning the significance of the duality to neoplastic transformation.

Uridine kinase: altered subunit size or enzyme expression as a function of cell type, growth stimulation, or mutagenesis

Using antibody prepared against pure uridine kinase from Ehrlich ascites cells, we have measured the expression of enzyme protein by the Western blot technique. Variations were observed in the Mr of the enzyme subunit for uridine kinase from different species: 32,000 (mouse Ehrlich ascites cells), 30,000 (normal human lymphocytes), 28,000 (mouse tissues), 27,500 (rat tissues). For different normal tissues from the same species, there was no significant variation in the subunit size. Transformed human and mouse cell lines, selected for a deficiency of uridine kinase activity in the presence of inhibitors activated by this enzyme, expressed two cross-reacting proteins, one with a normal (30,000) and one with a smaller (21,000) subunit molecular weight than was found in the parental cell line (human lymphoma), or only a smaller protein of Mr 25,000 (mouse lymphoma). Our results show that selection protocols using metabolite inhibitors do not always repress the expression of the enzyme but instead may lead to selection of those cells that have a mutation in the uridine kinase gene, resulting in the expression of an inactive enzyme. The expression of uridine kinase protein changes when cells are stimulated to divide. For both mouse fibroblasts and human lymphocytes, expression of uridine kinase protein as well as activity clearly increased after cells were stimulated to grow. In fibroblasts, increases are seen by 3 hr after stimulation, and plateau after 9 hr at a sevenfold increase. In lymphocytes, no change is seen until 12 hr after stimulation, and a plateau is not reached until 72 hr, with a total increase of approximately 50-fold. There has been considerable interest in the possibility of uridine kinase isozymes. Except for cells that have been mutagenized, the present results show that, as judged by subunit molecular weight, there appears to be only one enzyme form in normal and neoplastic cells or in cells in which uridine kinase activity is induced.

Early stimulation by EGF plus insulin of rRNA, c-fos, and actin mRNA expression: inhibition by cytochalasin D

Stimulation of membrane ruffling is one of the first events induced by addition of growth factors to quiescent cultures. In order to assess the importance of intact cytoskeleton in induction, by EGF + insulin, of early events such as stimulation of rRNA, c-fos, and actin mRNA expression, we studied the effect of cytochalasin D (CD) on these metabolisms. We observed that CD slightly increased rRNA synthesis in nonstimulated cells; conversely, it decreased rRNA synthesis in cells stimulated by EGF + insulin. The maximal inhibition observed was 60%. The c-fos mRNA was not expressed in control cells and was accumulated in cells stimulated by the mixture of EGF + insulin; this accumulation was inhibited by CD. Actin mRNA was expressed in control cells and its expression was stimulated by EGF + insulin. Addition of CD decreased actin mRNA accumulation in stimulated cells but increased this accumulation in unstimulated cells. Our results, taken together, show that CD specifically affected the stimulation of rRNA and mRNA expression induced by growth factors and suggest that intact cytoskeleton and possibly membrane ruffling favored this stimulation.

Imbalance of total cellular nucleotide pools and mechanism of the colchicine-induced cell activation

Treatment with colchicine or vinblastine, both inhibitors of microtubule assembly, renders quiescent 3T3 cells in an "activated state" as evidenced by induction of DNA synthesis and other criteria. Microtubule disassembly caused by colchicine or vinblastine brings about a dramatic expansion of total cellular UTP pools with a concomitant diminution in total cellular ATP pools, thus resulting in a marked imbalance in total cellular nucleotide pools. Colchicine and vinblastine also stimulate total cellular RNA synthesis without enhancing uridine phosphorylation, suggesting that these drugs affect the G1 phase of the cell cycle at a point beyond the enhancement of uridine phosphorylation that usually accompanies mitogenic stimulation of quiescent mammalian cells. The markedly expanded cellular UTP pools appear to be necessary for initiation of the colchicine-stimulated DNA synthesis because decreasing cellular UTP pools by addition of D-glucosamine results in a selective inhibition of DNA synthesis in the colchicine-stimulated, but not control, cells. Furthermore, D-glucosamine exerts its inhibitory effect only when it is present in the cultures within the first 14 hr after colchicine treatment. When added at 21 hr, D-glucosamine still decreases cellular UTP pools, but it is no longer inhibitory for DNA synthesis, which commences 14-16 hr after colchicine stimulation. Taxol, an antitumor drug, prevents microtubule disassembly and also blocks such events as expansion of total cellular UTP pools and stimulation of RNA and DNA synthesis, indicating that microtubule depolymerization acts as a primary event initiating the process of cell activation induced by colchicine.

Uridine uptake inhibition as a cytotoxicity test: correlations with the Draize test

The effects of 25 xenobiotic chemicals on the uptake of [3H]-uridine by Balb/c 3T3 cells were assessed. The test compounds, which included alcohols, ethers, esters, ketones, amides, acids and a detergent, inhibited uridine uptake at concentrations lower than those required to kill the cells; thus uridine uptake inhibition is a more sensitive indicator of toxic action than is cell lethality. The concentration of agent required to induce a 50% inhibition in uridine uptake rates after 4 h of treatment was determined for each agent, and this value (UI-50) was used to rank the potency of the test agents. This ranking correlated well with published data on the chemicals' capacity to induce ocular irritation in rabbits (the Draize test). Combinations of agents with differing functional groups produced additive uridine uptake inhibitory effects, suggesting the utility of this approach for the analysis of mixtures. Cells treated with levels of agents that reduced uridine uptake by 60-80% were able to recover most of their uridine uptake capacity after refeeding, indicating that the test shares with in vivo tests the ability to demonstrate recovery from toxic insult. This uridine uptake assay system provides a quantitative and rapid method for assessing toxicity that correlates well with Draize test results.

Cell cycle study on the effect of interferon on synchronized mouse embryo fibroblasts

Mouse embryo fibroblasts synchronized by controlling cultural conditions were used to examine the effects of interferon (IFN) while undergoing a single synchronous cycle of division at the tertiary stage. IFN was added early in G1 and at the G1-S boundary and the duration of specific phases of the cycle were investigated together with biochemical events related to cell cycle progression. Assessment of population distribution by fluorimetric quantitation of DNA content showed that IFN extended G1 and G2 but had no effect on the duration of S phase. Assessment of transport and uptake of exogenous TdR and measurements of specific kinase activity under conditions where DNA synthesis and S phase were not altered showed that IFN had no effect on TdR transport but could markedly reduce TdR uptake, and delay the S phase associated increase of TdR-kinase activity.

Regulation of purine uptake in normal and neoplastic cells

Purine bases and purine nucleosides pass the cell membrane by facilitated diffusion. For purine bases two different carrier proteins seem to exist. Purine bases are trapped intracellularly immediately after passage of the cell membrane by the action of purine phosphoribosyltransferases (PRTs). Comparison of kinetic data of transport and intracellular enzyme reactions shows that intracellular metabolism is rate limiting for the whole uptake process. Since phosphate stimulates the uptake of bases, limited availability of phosphoribosylpyrophosphate (PRPP) might play a regulatory role. Purine nucleosides apparently enter cells via a common carrier. Of the nucleosides under investigation, only adenosine was taken up in significant amounts. Uptake of adenosine is mainly determined by the ratio of adenosine deaminase (ADA) and adenosine kinase (AK) activities. For uptake of purine nucleotides sequential action of ecto-5'-nucleotidase (ecto-5'-NT), nucleoside carrier and intracellular metabolism is necessary. Cells without ecto-5'-NT activity did not accumulate radioactivity from nucleotides. Proliferating neoplastic cells (K 562 and HL 60 cells) showed enhanced uptake of purine bases and nucleosides, when compared to quiescent cells (erythrocytes and granulocytes). From initial rates of uptake and intracellular enzyme activities it could be concluded that this enhanced uptake was due to alterations of enzyme pattern in the neoplastic cells.

On the functional symmetry of nucleoside transport in mammalian cells

The transport of uridine and thymidine has been examined in HeLa cells, in Novikoff rat hepatoma cells and in human erythrocytes, with the purpose of comparing influx, efflux and isotopic exchange at chemical equilibrium. The results support the following conclusions: (i) In all three cell types influx and efflux are comparable; (ii) HeLa and Novikoff cells show no trans-effect, while erythrocytes show a 5-fold trans-stimulation; (iii) a single kinetic entity accounts for nucleoside transport in HeLa and Novikoff cells - no parallel routes of permeation with Km less than 40 microM were detected. For the cultured cells, the flux data conform to the kinetic model of a single, carrier-mediated transport system symmetrical with respect to direction, and with equal mobilities of substrate-loaded and empty carrier.

A monoclonal antibody to the human epidermal growth factor receptor

A monoclonal antibody of the IgG class, EGFR1, has been isolated using cells of the epidermoid carcinoma line A431 as immunogen. The A431 antigen recognized by EGFR1 has an apparent molecular weight of approximately 175,000, is a cell-surface molecule which can be specifically cross-linked to EGF, exhibits an EGF-stimulated protein kinase activity, binds to EGFR1 in a number of human cell lines to a degree which parallels EGF binding, and shows EGF-dependent internalization in A431 cells and human fibroblasts. We therefore conclude that EGFR1 is directed against an antigenic site on the human EGF receptor. EGFR1 is not mitogenic for human fibroblasts and does not inhibit EGF binding under a variety of assay conditions. The characterization of EGFR1 has allowed the unambiguous assignment of the structural gene for the human EGF receptor to chromosome 7. Preliminary results suggest that a convenient method for isolating a range of anti-EGF receptor monoclonal antibodies can be developed, based on a hybridoma supernatant screening assay in which positive supernatants bind selectively to a human-mouse cell hybrid containing human chromosome 7.

Evaluation of Mg2+ as an intracellular regulator of uridine uptake

The rate of uptake of the nucleoside uridine increases within minutes after adding a growth stimulus to quiescent 3T3 cells. We have previously shown this uptake rate to be highly sensitive to changes in the intracellular concentration of Mg2+. In the present paper, the alteration of uptake by Mg2+ is shown to occur at the phosphorylation step - the same point at which serum acts to modulate uridine uptake. The serum stimulation of uridine uptake can be mimicked by Mg2+ alone or blocked by partially depleting cells of their Mg2+. Work with cell-free extracts shows that the uridine kinase enzyme responds to Mg2+ in a manner similar to that exhibited by whole cells whose concentrations of Mg2+ have been raised. In addition, the enzyme's inhibition by ATP is relieved by raising the Mg2+ concentration. Thymidine uptake, a reaction which does not respond quickly to mitogenic stimulation, is unaffected by alterations in Mg2+ concentration. These results are discussed in terms of a possible role for Mg2+ as an intracellular regulator of uridine uptake and other reactions of the coordinate response of cells to external effectors.

Regulation of uridine kinase activity in BALB/C-3T3 cells by serum components

After the stimulation of quiescent density-inhibited BALB/c-3T3 cells with fresh bovine calf serum, uridine kinase activity measured in cellular extracts increased between hours 3 and 6 of incubation and remained elevated through 12 h after stimulation. The addition of either partially purified platelet-derived growth factor (PDGF) or platelet-poor plasma (PPP) also caused increased uridine kinase activity by 6 h, but the increased activity was not maintained and the activity returned to the prestimulated level by 12 h. However, when PDGF and PPP were added in combination an increased level of uridine kinase activity was maintained in a manner similar to that seen after the addition of serum. The components of PPP eluted in the void volume from Sephadex G-50 chromatography did not induce uridine kinase activity when present alone, although they did act synergistically with PDGF to allow the maintenance of elevated levels or uridine kinase activity over the period from 6 to 12 h after stimulation. Thymidine kinase activity was not induced by the addition of either PDGF or PPP alone, although either serum or the combination of PDGF and PPP did produce and induction of thymidine kinase activity in late G1.

Nutrient transport in a bovine lens epithelial cell line

A bovine calf lens epithelial cell line (CLE-1) that synthesizes crystallin has been established in culture and some of its transport properties have been characterized using both cells and membrane vesicles derived from them. The membrane vesicles fractionate with high recovery of plasma membrane markers, showing a 40-fold purification of 5'-AMPase and a 20-fold decrease in the specific activity of the mitochondrial marker enzyme succinic dehydrogenase relative to a cell homogenate. Transport sites demonstrated higher specific activity than has been seen in vesicles from cell lines studied previously. The uptake of alpha-amino isobutyric acid (AIB) (an alanine analog) by CLE-1 cells is stimulated four- to fivefold by Na+ and exhibits a Km of 5.4 mM with a Vmax of 50 pmoles/min.microgram of cell protein. The uptake of leucine was not Na+ stimulatable. The uptake of AIB by the cells was reduced by 43% at confluence. Thus, the cell density dependent behavior of the uptake of the alanine amino acid family in CLE-1 is similar to that of various fibroblast cells. The Na+ caused a threefold stimulation of AIB uptake in the membrane vesicles, while vesicular uptake of leucine was unaffected by Na+. The uptake of adenine, guanine, uridine, and guanosine was also tested in these vesicles. The substrates were rapidly accumulated, came to a steady state distribution within 1-2 minutes, and were recovered as the unaltered compounds after uptake.

The relationship between uracil nucleotide concentrations and glycogen synthesis in hepatocytes from fed and fasted rats

The relationship between glycogen synthesis and uracil nucleotide content was studied in rat hepatocytes. When hepatocytes were incubated in the presence of uridine in the incubation medium there was an increase in the rate of incorporation of U-14C-glucose into glycogen. In hepatocytes incubated in the absence of uridine for 1 hr there were large decreases in the cellular contents of UDPG and UTP, while in the presence of 5 mM uridine the concentrations of these nucleotides increased 2 to 3 fold. In hepatocytes from fasted rats uracil nucleotide contents were lower than in hepatocytes from fed rats and the effect of uridine on glycogen synthesis was greater.

Synergistic stimulation of early events and DNA synthesis by phorbol esters, polypeptide growth factors, and retinoids in cultured fibroblasts

12-O-Tetradecanoyl-phorbol-13-acetate (TPA), in the absence of serum, acts synergistically with a range of polypeptide growth factors to stimulate DNA synthesis in quiescent Swiss 3T3 cells. These growth factors include epidermal growth factor (EGF), insulin, and the peptide produced by BHK cells transformed by SV-40 virus (fibroblast-derived growth factor, FDGF). Retinoids also show mitogenic synergism with TPA or polypeptide growth factors. The spectrum of mitogenic synergisms displayed by TPA are similar to those of vasopressin, a pituitary peptide. However, TPA and vasopressin do not synergistically interact to stimulate DNA synthesis in quiescent 3T3 cells. This suggests that TPA and vasopressin act via an identical biochemical pathway. Several lines of evidence suggest rapid postreceptor convergence of the mitogenic mechanisms of action of the hormone and the tumor promotor. Thus, vasopressin and TPA both inhibit EGF binding to cellular receptors. Furthermore, TPA and vasopressin induce a similar array of early events in quiescent cells--most strikingly, identical stimulation of Rb+ influx. Stimulation of ion flux is suggested as the possible convergence point of the pathway by which TPA and vasopressin act as mitogens.