Studies on cell communication with enucleated human fibroblasts.

Metabolic cooperation, the correction of the mutant phenotype in cells deficient in hypoxanthine phosphoribosyltransferase (HPRT-) by intimate contact with normal cells (HPRT+), represents a form of cell communication that is easily studied with radioautography. In the present study it was found that the formation of cell junctions needed for communication does not require protein synthesis nor is it under the immediate control of the cell nucleus. Enucleated normal cells efficiently communicate with HPRT- mutant cells. The effectiveness of enucleated cells as donors in metabolic cooperation provides evidence that it is the transfer of small molecules, nucleotide, or nucleotide derivatives that is responsible for correction of the mutant phenotype. Karyoplasts (nuclei with small amounts of cytoplasm surrounded by a plasma membrane) are unable to efficiently communicate with intact cells. The utilization of [3H]hypoxanthine by communicating mixtures of HPRT+ and HPRT- human cells is not significantly different than in the normal cells alone. Metabolic cooperation, as studied involves a redistribution of purine-containing compounds among communicating cells.

Urinary metabolites in congenital hyperuricosuria.

The excretion of oxypurine metabolites in urine of patients with congenital hyperuricosuria exceeds, on a creatinine basis, that observed in any previously recognized metabolic anomaly. The ratio of hypoxanthine to xanthine is from 2:1 to 3:1 and results from increased hypoxanthine excretion, in contrast to other hyperuricosuric conditions where ratios of less than one have been reported. Administration of allopurinol (a xanthine-oxidase inhibitor) reduces the excretion of uric acid but results in an equivalent increase in xanthine and hypoxanthine. These features appear to be unique to congenital hyper-uricosuria.

Study of purine metabolism in a xanthinuric female.

A case of xanthinuria is briefly described, and the results of in vivo studies with (14)C-labeled oxypurines are discussed. The data demonstrate that the rate of the turnover of uric acid is normal, despite an extremely small uric acid pool. Xanthine and hypoxanthine pools were measured and their metabolism evaluated. The bulk of the daily pool of 276 mg of xanthine, but only 6% of the 960 mg of hypoxanthine, is excreted. Thus, xanthine appears to be a metabolic end product, whereas hypoxanthine is an active intermediate. Biochemical implications of this finding are discussed.

Disparate enzyme activity in erythocytes and leukocytes. A variant of hypoxanthine phosphoribosyl-transferase deficiency with an unstable enzyme.

A family is reported in which each of two sisters has a son with no detectable hypoxanthine phosphoribosyltransferase (HPRT) (EC 2. 4. 2. 8) in his erythrocytes, a finding considered pathognomonic of Lesch-Nyhan disease. However, neither has the stigmata of the disease. One boy is neurologically normal, and the other is moderately retarded. There was only a slight increase in urinary uric acid, but the amounts of hypoxanthine and xanthine, and their ratios, were similar to those found in Lesch-Nyhan disease, strongly indicating that excesses of these last two oxypurines are not responsible for the symptomatology in that disease. In contrast to the nondetectable HPRT activity in the red blood cells, leukocyte lysates from the two boys have 10-15% of normal activity, possibly reflecting continuing synthesis of an unstable enzyme. This hypothesis is supported by the demonstration that at 4 degrees C HPRT activity was rapidly lost in the propositus while the activity increased in control subjects. The mother's cells were intermediate between the two. The intact and disrupted leukocytes of the hemizygote, in the absence of added phosphoribosyl converted as much hypoxanthine to inosinate as the normal cell, and appropriate tests indicated that under these circumstances enzyme concentration is not rate limiting whereas the concentration of the cosubstrate, phosphoribosyl pyrophosphate, is. The capacity for normal function in the intact mutant cell is more representative of in vivo conditions than the lysate, which may explain the important modification of clinical symptomatology, the relatively mild hyperuricosuria, and the presence of mosaicism in the circulating blood cells of the heterozygotes. A similar explanation may apply to other genetic diseases in which incomplete but severe enzyme deficiencies are found in clinically normal individuals. An associated deficiency in glucose-6-phosphate dehydrogenase in this family permitted confirmation of previous observations on linkage with hypoxanthine phosphoribosyltransferase.

Hypoxanthine phosphoribosyltransferase activity in intact fibroblasts from patients with X-linked hyperuricemia.

Discordance between clinical phenotype and the level of a mutant enzyme activity may reflect differences between enzyme function in vivo and that measured by the customary enzyme assays on cell extracts. In the present study, the conversion of hypoxanthine to phosphorylated products was measured in intact skin fibroblasts and in cell extracts from seven patients with mutant hypoxanthine-guanine phosphoribosyltransferase (HPRT) and six control subjects. The patient's phenotypes ranged from asymptomatic hyperuricemia to the Lesch-Nyhan syndrome. Although there was a general correlation between the HPRT activity in cell extracts assayed by the usual methods and the function of the purine salvage pathway in patients, as reflected by urinary oxypurine excretion, there were notable exceptions. A more accurate appraisal of the functioning of the pathway at the cellular level is achieved by measuring the conversion of substrate to product in the intact cell at physiological concentrations of substrates, activators, and product and metabolite inhibitors, and in a physiological ionic environment. In one of the seven patients, the standard enzyme assay indicated normal function, whereas measurements in the intact cell exposed severe dysfunction of the salvage system. In another, the standard assay suggested a severe deficiency not evident in the intact cell or in the patient.

Changes in ornithine decarboxylase activity in rat intestines during aging.

The ornithine decarboxylase (ODC) activity of rat intestines, liver, and brain was found to vary dramatically as animals develop and age. Unusually high activity was present in the small intestines of adult animals. The ODC activity of the small intestines approximated that of fetal tissues and of regenerating rat liver. Putrescine, spermine, and spermidine levels of fetal and adult animal tissues were determined. In all tissues but the stomach mucosa, high putrescine levels correlated with high IDC activity. However, the total polyamine concentrations of the stomach and colon could not be correlated with ODC levels, and no simple relationship between polyamine levels of ODC levels and cellular proliferation in the gut was found.

Biochemical changes in premalignant intestines.

The immunological properties of thymidine kinase from a variety of human tumors suggest that the form of the tumor enzyme resembles that found in the placenta and in the nondividing colonic flat mucosa. To examine the placenta-like characteristics of tumor thymidine kinase, the jejunum and colon from rats ranging in age from fetal to old and from animals treated with dimethylhydrazine (DMH), an intestinal carcinogen, have been studied. In normal jejunum, thymidine kinase activity decreased rapidly with age. Both the activity and the response to phospholipase C and to mercaptans in DMH-induced tumors resembled that of fetal gut, while those in abnormal appearing DMH-treated jejunum were intermediate between normal control of the same age and tumor. Similar but less pronounced changes were seen in the colon. In the jejunum, the level of another enzyme normally associated with rapid cell division, ornithine decarboxylase, was found to be over 100 times that of the liver, colon, and stomach. Treatment of the animals with acetylaminofluorene and with DMH resulted in elevated levels of the enzyme in liver and in colon, respectively, but had little effect on this enzyme in other tissues. The data presented indicate that there were premalignant changes in the levels of both of these enzymes in target tissues of animals treated with carcinogens.

Biochemical changes in preneoplastic rodent intestines.

Two enzymes were examined as potential indicators of early precancerous changes. Ornithine decarboxylase, an enzyme normally associated with rapid cell division, is low in the rapidly dividing, cancer-susceptible colon. The level of this enzyme was also very high in the nondividing cells of the small intestines. Administration of an intestinal carcinogen, dimethylhydrazine, led to a large increase in colonic ornithine decarboxylase but did not affect the enzyme in liver. A liver carcinogen, acetylaminofluorene, induced manyfold increases in ornithine decarboxylase of the liver but not of the colon. Studies of thymidine kinase of the gut showed that this enzyme changed quantitatively and qualitatively throughout the life of the animal, from fetal rat to newborn and adult. The tumor enzyme has many fetal-like properties. Long-term treatment with dimethylhydrazine led to changes in thymidine kinase reminiscent of the fetal enzyme. Short-term treatment caused sharp increases in the thymidine kinase of nondividing cells of the jejunum and the proximal end of the colon; similar changes in the distal end of the colon were slower in appearing and less pronounced.

Comparisons of liver transfer RNA methyltransferase and adenosylmethionine decarboxylase activities of male and female rats.

The activities of liver transfer RNA (tRNA) methyltransferases from control or ovariectomized female rats were found to be higher than those of control or castrated males. Administration of testosterone to ovariectomized females caused activity to decrease to the level of the males. Conversely, administration of estrogen to castrated males resulted in liver enzyme levels similar to those of the females. When the substrates for in vitro methylation were either mixed heterologous tRNA's from Escherichia coli or mixed homologous methyl-deficient tRNA from livers of ethionine-treated rats, the difference in activity between males and females was about 35%. When amino acid-specific tRNA's from E. coli were used as substrates, the ratios of activity of enzymes from females to that of males were: tRNANfMet 1.5; tRNAMetMet 1.1; tRNASer3 1.85; tRNAPhe 1.1; and tRNATyr 1.25, indicating that there are qualitative as well as quantitative differences in the liver tRNA methyltransferases of the two sexes. The adenosylmethionine decarboxylase activity of female rat liver preparations was approximately double that found for males. Testosterone, given to ovariectomized females, lowered the activity of this enzyme to about the same level as that of males. It is not clear whether the observed sex-related differences in activity of several adenosylmethionine-utilizing liver enzymes represent isolated phenomena or are indicative of a sex-related difference in the rate of liver adenosylmethionine turnover.

Ethionine-induced changes in rat liver transfer RNA methylation.

We have confirmed the finding by Rajalakshmi that transfer RNA (tRNA) from livers of ethionine-treated rats can act as a substrate for homologous tRNA-methylating enzymes in vitro. This methyl-deficient tRNA from liver can be methylated in vitro by enzymes from normal or ethionine-treated rats. The in vitro inhibition of tRNA methylation that follows ethionine treatment can be at least partially relieved in vitro. The liver extracts from ethionine-treated animals contained a low-molecular-weight inhibitor of tRNA methylation. Dialysis of enzyme preparations from ethionine-treated, but not control, rats resulted in large increases in tRNA methylase activity, with either Escherichia coli or homologous tRNA's as substrate. Furthermore, the tRNA methylase activity of control rat liver enzyme extracts was greatly depressed by dialysate from liver homogenates of ethionine-treated rats. After 5 days of ethionine administration the liver tRNA methylase activities were significantly higher than those of control preparations despite the continued presence of the dialyzable inhibitor(s). The liver tRNA's from these animals were poorer methyl acceptors than those from 3-day-treated rats, although still better than tRNA's from untreated rats. These observations have been interpreted to indicate that ethionine causes the accumulation in the liver of inhibitors of tRNA methylation. Early in the course of ethionine administration, methyl-deficient tRNA can be isolated. When the period of ethionine treatment is extended, the organism attempts to maintain homeostasis by production of increased amounts of tRNA-methylating enzymes. The increased quantities of these enzymes are able to overcome, at least partially, the effects of the inhibitors and to decrease the extent to which methyl-deficient tRNA is produced.

Time dependence of ethionine-induced changes in rat liver transfer RNA methylation.

Methyl-deficient transfer RNA (tRNA) and subnormal levels of tRNA-methylating enzymes were found in the livers of female rats that had received injections of 250 mg DL-ethionine per kg body weight per day and 120 mg adenine per kg body weight per day for 2 days. Adenine alone had no effect. When the ethionine plus adenine injections were continued for longer periods of time, liver tRNA-methylating enzyme activity measured in vitro gradually increased and exceeded that of the controls. Concurrently, the relative methyl deficiency of liver tRNA decreased. The latter was evident because of the decreased ability of the tRNA to accept methyl groups during in vitro methylation catalyzed by homologous enzymes. Liver tRNA from animals that were treated with ethionine for 7 days could accept only about 40% as many methyl groups as could tRNA from animals that had received ethionine for only 2 days. No further significant change in methyl deficiency of the tRNA was seen when ethionine administration was extended to a total of 14 days. Enzyme preparations from ethionine-treated, but not control, rat livers contained dialyzable substances that inhibited the tRNA methylases and altered the base specificity of these enzymes. Although S-adenosylhomocysteine and S-adenosylethionine were found to be present in the liver preparations, neither of these substances could account for the observed changes in specificity.

Specific immunosuppressive effects of constant infusion of 2'-deoxycoformycin.

The effect of continuous infusion into C57BL/6J mice of 2'-deoxycoformycin (DCF), a tight-binding inhibitor of adenosine deaminase, on the biological function of bone marrow stem cells and T- and B-lymphocytes was evaluated. Greater than 85% inhibition of adenosine deaminase in erythrocytes, thymus, and bone marrow was noted after DCF infusion at 0.4 mg per kg body weight per day, while lesser extents of inhibition were characteristic of spleen and lymph nodes. The reconstitution of lethally irradiated C57BL/6J mice with bone marrow cells from DCF- and 0.9% NaCl infused mice of the same strain was compared. The two groups of animals were virtually identical with respect to (a) the number of spleen colony-forming units, (b) the response of splenic lymphocytes to both B- and T-cell mitogens, (c) hematological analysis of peripheral blood elements, and (d) survival time, thus strongly supporting the lack of effect of DCF infusion on the capacity of stem cells to differentiate. In contradistinction, DCF infusion was highly lymphocytotoxic as noted by the severe necrosis in both B- and T-cell regions in lymph nodes and spleen and by the dramatic weight reduction in spleen and thymus. Histopathology of other tissues including bone marrow was normal except for the occurrence of hepatitis. A striking decrease in blastogenesis induced by the mitogens concanavalin A, phytohemagglutinin, and Escherichia coli lipopolysaccharides was also observed after DCF infusion. Consistent with these data, in vitro incubation of bone marrow cells with DCF did not impair the number of spleen colony-forming units produced in lethally irradiated mice. These data suggest a potential use for adenosine deaminase inhibitors in the prevention of graft-versus-host disease in hematopoietic transplantation.

Role of murine tumor models in cancer treatment research.

Two major factors have contributed to a widely held disenchantment with murine tumor models for drug screening in cancer research: (a) the higher costs of these models in comparison to studies performed with tumor cells in vitro; and (b) the perception that these models have failed to demonstrate satisfactory correlation of chemosensitivity with analogous human tumor types; i.e., murine tumors generally have proved to be sensitive to many more agents than are found to be active in the clinic. The perceived failure of the murine models is discussed with particular reference to the difference in criteria used for evaluating drug sensitivity in murine tumor models versus clinical trials, and we conclude that the perception about murine models is not tenable in light of present information. The very important role of murine tumor models in optimizing dosage and administration schedules and, most importantly, in the development of a new drug to its most useful potential in combination chemotherapy is discussed. The value of this in vivo methodology is stressed.

Relationships between non-extractable DNA and the bacterial growth cycle.

Escherichia coli DNA has been fractionated into extractable and non-extractable DNA after deproteinization of detergent-lysed cell preparations with chloroform-isoamyl alcohol. The former was extracted with dilut buffered saline whereas the latter remained in the interphase layer associated with residual cellular debris from which almost 40 percent could be released by incubating with pronase. About 20-25 percent more amino acid residues were bound to the pronase-released DNA than to the extractable DNA, but the relative distribution of the residues in the two DNA samples was virtually identical. The specific activities and the relative amounts of denser (1.709g-cm-3) and lighter (on the surface of CsCl gradients) DNA fractions from E. coli, grown in the presence of labeled thymidine, indicated that these two corresponded to extractable and non-extractable DNA, respectively. The relative amounts of the two fractions varied with the growth phase primarily as a function of the growth rate. Age and metabolic state of cells in the culture or those used as inocula could modify this relative distribution. When growth rate was maximal, the ratio of the two remained at about 1. During lag phase when no appreciable net synthesis of DNA could be detected, there was a rapid and preferential incorporation of labeled thymidine into non-extractable DNA. A disproportionate increase in the fraction of the total DNA which was extractable, was also abserved but only when stationary phase cultures were used as inocula. Complete equilibration of the label in the two DNA fractions was attained only after cultures had reached mid-log phase of growth. Similar results were obtained when prelabeled cells were used. These data have been interpreted as suggesting that the rate of cell growth and DNA synthesis are related to the number or size of sites of attachment of DNA to some cellular structure. Newly synthesized DNA would be attached to different sites on this structure and initiation of DNA replication in lag phase would require reorientation of the two kinds of DNA. Small peptides which are firmly bound to the DNA and which vary quantitatively with the rate of DNA synthesis could perhaps be involved in the attachment to the sites.

Studies on the mechanism of inhibition of tRNA methylation by 3,4-dihydroxyphenylethylamine.

A Lineweaver-Burk analysis of a kinetic study of tRNA methylation by a 30-50% (NH4)2SO4 fraction from a weanling rat liver extract showed competitive inhibition with a Km for S-adenosylmethionine = 0.66 - 10(-6) M and a Ki for 3,4-dihydroxyphenylethylamine (dopamine) = 4 - 10(-5) M. The dopamine-inhibited methylation of tRNA appears to be linear with time. Rapid-flow dialysis studies indicated a S-adenosylmethionine binding constant of 0.65 - 10(-6) M. Dopamine appeared to interfere with the binding of S-adenosylmethionine to the weanling rat liver protein preparation but did not affect the binding of S-adenosylmethionine to protein in several systems in which dopamine did not inhibit tRNA methylase activity.

Increased activity of rat liver N2-guanine tRNA methyltransferase II in response to liver damage.

Alterations in rat liver transfer RNA (tRNA) methyltransferase activities have been observed after liver damage by various chemicals or by partial hepatectomy. The qualitative and quantitative nature of these activity changes and the time course for their induction have been studied. Since homologous tRNAs are essentially fully modified in vivo, E. coli tRNAs were used as in vitro substrates for the rat liver enzymes in these studies. Each of the liver-damaging agents tested rapidly caused increases in activities of the enzyme(s) catalyzing methyl group transfer to tRNAs that have an unmodified guanine at position 26 from the 5' end of the molecule. This group of tRNAs includes E. coli tRNANfmet, tRNAAla1, tRNALeu1, or Leu2, and tRNASer3 (Group 1). In each case N2-methylguanine and N2,N2-dimethylguanine represented 90% or more of the products of these in vitro methylations. The product and substrate specificity observed are characteristic of N2-guanine methyltransferase II (S-adenosyl-L-methionine : tRNA (guanine-2)-methyltransferase, EC 2.1.1.32). In crude and partially purified preparations derived from livers of both control and treated animals this enzyme activity was not diminished significantly by exposure to 50 degrees C for min. The same liver-damaging agents induced little or no change in the activities of enzymes that catalyze methyl group transfer to various other E. coli tRNAs that do not have guanine at position 26 (Group 2). The results of mixing experiments appear to rule out the likelihood that the observed enzyme activity changes are due to stimulatory or inhibitory materials present in the enzyme preparations from control or treated animals. Thus, our experiments indicate that liver damage by each of several different methods, including surgery or administration of chemicals that are strong carcinogens, hepatotoxins, or cancer-promoting substances, all produce changes in liver tRNA methyltransferase activity that represent a selective increase in activity of N2-guanine tRNA methyltransferase II. It is proposed that the specificity of this change is not fortuitous, but is the manifestation of an as yet unidentified regulatory process.

Effects of amines on macromolecular methylation.

Histone methylation by extracts of rat brain or liver was inhibited and tRNA methylation stimulated by the addition of a number of naturally occurring polyamines. The effect was age independent although the methylase activities are highly age-related. Spermine and/or histamine stimulated methylation of cytosine and adenine to a far grease activity, were more sensitive to inhibition by adenosine than were liver extracts. Adenosine inhibited the methylation of guanine to a greater extent than of cytosine or adenine. Methylation of both tRNA and histone by liver enzyme was inhibited by L-dopa, dopamine and epinephrine. Methylation by brain enzyme was also blocked, but less extensively. The response of liver extracts to these catecholamines was highly age-related. The phenolic amines, octopamine, synephrine, serotonin and tyramine, stimulated tRNA methylation slightly while inhibiting histone methylation by both liver and brain extracts and these effects showed no age dependency. Analysis of the data suggests that most of these compounds do not act by competing for the available S-adenosylmethionine.

Effects of uridine diphosphoglucose (UDPG) infusion on 5-phosphoribosyl pyrophosphate (PRPP) levels of mouse tissues.

Uridine diphosphoglucose (UDPG) has been shown to have tissue-specific effects that have proved to be of clinical value in the treatment of some liver ailments. In an effort to determine something about the mechanism of action, we investigated the effect of UDPG on the levels of 5-phosphoribosyl pyrophosphate (PRPP) and PRPP synthetase in mouse liver, spleen and transplanted tumors. Three strains of mice were studied with and without tumors under various experimental conditions. Balb/c mice were infused with UDPG intraperitoneally at levels of 0.16 g/kg/day (0.28 mmole) to 1.6 g/kg/day (2.8 mmoles) for 5 days. At the low dose rate the PRPP level in the liver was found to increase 3-fold. A slight increase was noted in the activity of PRPP synthetase. However, when the UDPG was infused at a level of 2.8 mmoles/kg/day, the increases in both the synthetase and PRPP were inhibited. Both CRF1 and CD8 mice were less sensitive to the effects of UDPG per se. However, the high level of PRPP in the tumors they carried was greatly affected by the UDPG infusion. The tumor-specific inhibition of PRPP suggests that this action might prove to be useful combination therapy with inhibitors of purine and pyrimidine nucleotide synthesis in various rescue regimens. UDPG was found to enter cells intact before it was cleaved into glucose phosphate and UMP. The fact that UDPG was also found in the membrane fraction suggests that either there is a specific transport mechanism or UDPG exerts its action via interaction with the cell membrane.

Inhibition by methotrexate of the stable incorporation of 5-fluorouracil into murine bone marrow DNA.

As a consequence of the inhibition of de novo purine synthesis by methotrexate (MTX) there is an increase in 5-phosphoribosyl-1-pyrophosphate (PRPP) concentration. In cells where 5-fluorouracil (FUra) is activated via orotate phosphoribosyltransferase (OPRtase), increased PRPP results in greater conversion of FUra to nucleotides. In the murine CD8F1 breast tumor system, MTX markedly enhances the antitumor activity of FUra, increasing both the activation of FUra to FUMP and the incorporation of FUTP into RNA. However, in contrast to reported tumor tissue culture studies, MTX pretreatment in vivo prevents the stable incorporation of FUra into CD8F1 bone marrow DNA. Pretreatment with MTX (300 mg/kg) 2.5 hr prior to [3H]FUra (100 mg/kg), with a 2-hr labeling, reduced the level of FUra in DNA from 921 pmol to 66 pmol/mg of DNA. Without MTX pretreatment, 59% of the total incorporation of FUra into nucleic acids was into DNA when FUra was administered. After MTX the percentage of incorporation into DNA was reduced to 9%. Additionally, the ratio of [3H]FUra to 32P in DNA when both were given simultaneously was reduced by greater than 90%, suggesting that MTX must be specifically blocking the incorporation of FUra rather than nonspecifically preventing its incorporation by inhibiting DNA synthesis. In contrast, MTX failed to reduce the formation of DNA containing fluorouracil residues from FdUrd. To test whether MTX prevents the initial incorporation of FUra into DNA, or acts to enhance removal by the DNA glycosylase repair system, DNA was prelabeled in vivo with [3H]FUra, and MTX or MTX plus dThd was then administered. The level of FUra in bone marrow DNA was not reduced by subsequent treatment with MTX, or MTX plus dThd, indicating that MTX does not enhance the removal of FUra from DNA. The level of total free fluorodeoxynucleotides formed from FUra was reduced by two-thirds following MTX pretreatment, suggesting that the action of MTX in preventing the stable incorporation of FUra into DNA was to reduce the availability of FdUTP.