"Salvage" Pathways to Thymidine Triphosphate Synthesis: Possible Implications for Cancer Chemotherapy1

Home , De novo synthesis, Thymidine monophosphate, Thymidine triphosphate

[CANCER RESEARCH 29, 2398-2403, December 1969] Alternative de Novo and "Salvage" Pathways to Thymidine Triphosphate Synthesis: Possible Implications for Cancer Chemotherapy1

Thomas W. Sneider and Van R. Potter McArdle Memorial Laboratory, The Medical School, University of Wisconsin, Madison, Wisconsin 53706

INTRODUCTION hand before rational attempts at sequential blocking or concurrent blocking can be undertaken." A rather large The development of antitumor agents involves a number of number of similar quotations could easily be added to this problems, some of which have been touched upon in this example. With few exceptions, however, an extremely conference, e.g., host toxicity of the agent, effect of the agent important corollary of the alternative pathway concept has upon natural host defense mechanisms, initial efficacy of the been ignored. In 1950 (24) Potter and Heidelberger stated: agent with respect to its lethal and/or suppressive effects on "The chief conclusion that emerges from a consideration of tumor cells, and the development of resistance by the tumor the multiple pathways open to the individual compounds is cells to the effects of the agent. The question of initial and that the fate of a great many metabolites is not fixed, but may continued effectiveness of the agent is intimately bound up vary widely in different tissues in the same organism and in the with the existence of alternative metabolic pathways in normal same tissue under different circumstances." (italics added). It and cancer tissues. The role of alternative metabolic pathways is now well established that the fate of a given metabolite may in the production of "essential metabolites" was emphasized be either utilization as a building block or further degradation some time ago for both bacterial and mammalian systems (24). towards ultimate breakdown products. It is also widely Since many antitumor agents have been designed to inhibit the understood that many essential metabolites can be produced production of one or more metabolites essential for tumor cell either by de novo or salvage (i.e., preformed) pathways. But in viability or proliferation, the existence of alternative pathways addition, the scientist confronted with baffling results in the that can circumvent the block imposed by a given agent (Chart field of cancer chemotherapy must consider alternative de 1) must be taken into account. That this problem has been novo pathways for the synthesis of essential metabolites. It is not clear whether previous statements about alternative ALTERNATIVE DE NOVO PATHWAYS pathways included the latter probability, and it is our MAY DEFEAT CHEMOTHERAPEUTIC AGENTS intention to make the distinction explicit. ENZ3 ENZ, ->ESSENTIAL METABOLITE- â€¢ESSENTIALPRODUCT POSSIBLE ROUTES TO THYMIDINE TRIPHOSPHATE (e.g. dTTP) (eg ONA) SYNTHESIS

(SALVAGE) ÃŽ The purpose of this paper is to draw attention to the role of ESSENTIAL NUTRIENT Â«- (eg TdR) alternative de novo as well as salvage (preformed) pathways SALVAGE OR PREFORMED leading to the synthesis of thymidine triphosphateâ€”a primary [(PREFORMED) PATHWAYS MAY DEFEAT target of many current chemotherapeutic efforts-especially in CHEMOTHERAPEUTIC AGENTS ENVIRONMENT relation to their existence in a given tissue or various tissues in the same organism. Chart 1. Relationships of salvage (preformed) and alternative de novo pathways for the synthesis of an "essential metabolite" and their role in chemotherapeutic blockade. For abbreviations see Chart 2. Alternative de Novo versus Salvage Pathways

recognized is illustrated by a quotation from Skipper (31 ) who A number of hypothetical routes to the synthesis of maintained that ". . . detailed knowledge of a series of thymidine monophosphate and thymidine triphosphate have biochemical events (as well as alternative pathways) must be at been demonstrated in various systems (Chart 2). The relative contribution of de novo versus salvage (preformed) pathways to the synthesis of thymidine triphosphate depends upon the organism studied. It is known that thymidine (or thymine) is an essential nutrient for certain bacterial mutants. For This study was supported in part by Grant No. P-448 from the example, Escherichia coli 15T"(555-7) displays an absolute American Cancer Society and by Grant No. CA-07125 from the National Cancer Institute, USPHS. need for thymine in the growth medium for the synthesis of

2398 CANCER RESEARCH VOL. 29

5-ME-CDP strated. The use of autoradiographic technics that employ labeled thymidine (26) is based on the assumption that DNA fr 5-ME-dCMPâ€”fr 5-ME-dCTP synthesis will always involve thymidine uptake and there is CDP-<-'4â€”UDP \ TDP l! little doubt that many cell types can use thymidine or 77 thymidine precursors from their environment. It is also clear 4 that the normal "replacement" of cells (e.g., intestinal -dCMP frdUMP frdTMP -fr -fr dTTP fr DNA-THYMINE epithelium, vascular polymorphonuclear leukocytes, dermal cells), the "turnover" of tumor cells (35), and dead or dying CdR fr UdR*. Â»l inflammatory and tumor cells in necrotic tumors might yield

?? both local and peripheral pools of salvaged thymidine and deoxycytidine in tumor-bearing animals. Reutilization of these salvaged (preformed) pyrimidines by tumor cells might well render futile chemotherapeutic blockade of de novo dTTP synthesis (Charts 1, 2). Many examples of the "rescue" by exogenous thymidine of amethopterin- or FUdR-blocked Chart 2. Schematic representation of possible alternative de novo and mammalian10"s molar cells thymidine in tissue (or culture IO"4 can molar be cited. 5-methyldeoxy For instance, salvage (preformed) pathways for the synthesis of thymidine mono- or triphosphate. Complex and interlocking regulatory controls within and cytidine) in the culture medium will reverse the inhibition of affecting this scheme (37) are not considered here or in the text. growth of Novikoff hepatoma cells by amethopterin (20). Abbreviations: CDP, UDP, TOP, and 5-ME-CDP, the 5'-diphosphates of That the salvage(preformed) pathway might be important in cytidine, uridine, ribothymidine, and 5-methylcytidine; dCMP, dUMP, in vivo situations is indicated by the data of Feinendegen et al. dTMP, and 5-ME-dCMP, the 5'-monophosphates of deoxycytidine, (8). These authors report that in bone marrow ".. . the deoxyuridine, deoxythymidine, and 5-methyldeoxycytidine; CdR, UdR, TdR, and 5-ME-CdR, 2'-deoxycytidine, 2;-deoxyuridine, magnitude of the reutilization pathway is estimated to involve 2'-deoxythymidine, and 2'-deoxy-5-methylcytidine; U and T, uracil and 35â€”40%ofthe thymidine which is released from DNA by thymine; dTTP and 5-ME-dCTP, the S'-triphosphates of deoxy catabolism." Based on experiments in the rat, Steel and Lamerton (36) reported that "... in bone marrow a degree of thymidine and 5-methyldeoxycytidine; DNA-Thymine, thymine in polymeric DNA; 3'-MDAB, S'-methyW-dimethylaminoazobenzene. local reutilization of DNA (sic) ... is highly probable." Bryant has stated (3) that 5-10% of thymidine in regenerating mouse dTMP2 (16). Other organisms, e.g., the ciliated protozoan liver came from "leukocyte" DNA. Similar conclusions about utilization of the salvage pathway by regenerating mouse liver Tetrahymena pyriformis, lack one or more steps in the early were reached by Robinson and Brecher (30). The "turnover" synthesis of the pyrimidine ring and require the presence of a "totipotent" pyrimidine source in the medium in order to of mouse liver DNA noted by Devik and Halvorsen (7) may be supply derivatives of uracil, cytosine, and thymine for growth a manifestation of the utilization of catabolically formed (40). Although mutant types occur, it is evident that the labeled thymidine. process of selection will move in the direction of wild type In addition to circumvention of a chemotherapeutic block on organisms that can make their own thymidine-5'-mono- or the de novo pathway, the utilization of preformed thymidine (or thymine) from the environment has the added effect of triphosphate. possibly "turning off de novo synthesis. The work of It is not clear if there are any mammalian cells for which thymidine is an essential nutrient in vivo, but the possibility Reichard et al. (29) and of Moore and her coworkers (18,21) cannot be ignored. In the case of the "regenerating" thymus with ribonucleoside diphosphate reducÃasehas shown that after 400 R X-irradiation, Sugino et al. (38) showed a 20-fold dTTP can strongly inhibit this enzyme with respect to increase in the labeling of DNA with TdR-3H when the pyrimidine deoxyribonucleotide synthesis. Moreover, Maley thymus was depleted of nearly all but reticulum cells (9), and and Maley (17) and Ceraci et al. (11) have shown that it showed about a 20-fold dilution of the specific activity of deoxycytidylate deaminase is inhibited by dTTP. That thymidine by de novo pathways in the normal thymus. One exogenous TdR can inhibit de novo synthesis of dTMP in interpretation of these data is that the reticulum cell popula tissue culture material was shown by the work of Morris and tion either lacks or possesses a very weak de novo pathway for Fischer (19) using neoplastic rat cell lines, by Gentry et al. synthesis of dTMP or dTTP. Regardless of whether the salvage (10) with Novikoff hepatoma cells, and by Whittle (39) using (preformed) pathway might, in some instances, be the sole rat thymocytes. source of thymidine triphosphate, the ability of organisms or cell types to utilize thymidine (or pyrimidines convertible to Alternative de Novo Pathways thymidine) from their environment can easily be demon- As illustrated in Chart 2, the main entry point for de novo synthesis of pyrimidine deoxyribonucleotides in mammalian cells is via reduction of the ribose moiety of uridine and Abbreviations used: ADP and GDP, adenosine- and guanosine- cytidine diphosphates (18, 21, 29). The studies of Moore and 5'-diphosphate; AdR, 2 -deoxyadenosine; FUdR, 5-fluoro-2 -deoxy her colleagues (18, 21) on partially purified ribonucleoside uridine; ANIT, a-naphthylisothiocyanate. Also see the legend to Chart reducÃasefrom Novikoff hepatomas as well as whole rat 2. embryos indicate that a single enzyme catalyzes the reduction

DECEMBER 1969 2399

of ADP, GDP, UDP, and CDP. No information on the direct inhibited to only 85% of control levels. Since the treated reduction of phosphorylated derivatives of ribothymidine culture showed no dead or dying cells (as determined by the (1-0-D-ribofuranosylthymine) to dTMP or its phosphorylated lack of uptake of the vital dye trypan blue), the possibility of derivatives is available. Since ribothymidine occurs both as the TdR having been salvaged from the environment is slight. free nucleoside and its phosphorylated derivatives, and in Hence the use of an alternative de novo path is strongly transfer RNA's in mammalian tissues, it is of some importance implicated. It should be emphasized that selection of liver cells to determine if the ribonucleoside diphosphate reducÃasecan possessing a certain route to dTMP is probably not exclusively utilize TDP as a substrate. Similarly, no information on the involved in these studies. utilization of 5-methylcytidine diphosphate as a substrate for That the "pathway of choice" to dTMP might vary in the this enzyme is available. same cell type under different circumstances is also suggested The "classical" de novo route to dTMP synthesis involves the by recent azo dye feeding studies in the rat. During the feeding of 3'-MDAB to rats, the activities of certain enzymes of dTTP deamination of dCMP to form dUMP, which is subsequently metabolism in liver vary in a marked and characteristic methylated to form dTMP. As will be discussed below, fashion.3 Although histologie changes during 3'-MDAB feeding however, the preferred route to dTMP synthesis in some mammalian tissues is via methylation of dUMP formed by have been described as bile duct cell proliferation (28), the reduction of the 2'-hydroxyl group of UDP. An alternative pattern of changes in "dTTP enzymes" during bile duct cell path to dTMP is via deamination of 5-methyldeoxycytidylate. proliferation induced by a-naphthylisothiocyanate (ANIT) are Nishihara et al. (22) and Ceraci et al. (11) have shown that strikingly different (32). Studies involving separation of hepatocyte from nonhepatocyte nuclei in livers from partially purified dCMP deaminase catalyzes the deamination 3'-MDAB-fed rats suggested that the "bue duct cell' pro of 5-MedCMP as well as dCMP. Karlstrom and Larsson (15) found that as much as 75% of the dTTP formed in E. coli must liferation might be proliferation of morphologically altered have been synthesized via deamination of 5-MedCTP. That this hepatocytes or hepatocyte stem cells.3 These experiments alternative route can occur in other organisms is indicated by are based on the work of Bushnell et al. (5), who found that studies of Bieliavsky and Leong (2) on sea urchin embryos and the nuclei of rat liver were distributed in two classes when on neoplastic mouse cells in tissue culture by Price et al. (27). homogenates were subjected to centrifugation on discon It is apparentâ€”without exhaustive citation of the literatureâ€” tinuous sucrose gradients (1.6 M sucrose layered over 2.3 M that there can be at least three de novo routes to dTMP sucrose). Hepatocyte nuclei (N-I) penetrated both layers of synthesis with the possibility of a fourth via reduction of the sucrose and formed a pellet in the tube while nonhepatocyte ribose moiety of TDP. Consideration of this fact, and of the nuclei (N-II) were unable to penetrate the heavy sucrose several possible paths for utilization of preformed pyrimidine layer and remained in the supernatant. Bushnell et al. (5) deoxyribonucleosides, leads to the crucial question of whether demonstrated that the nuclear RNA in the N-I fraction was a given normal or tumor cell is invariably "fixed" with respect preferentially labeled if orotic acid-14C was the precursor, to its path to dTMP synthesis or if the route used can vary whereas N-II nuclear RNA was more highly labeled when cytidine-14C was employed (Table 1). under the same or different circumstances. Table 1 Unique versus Multiple Paths to dTMP in Specific Tissues Disintegrations per minute in RNA per Â¿igRNA Perhaps the most telling evidence that a given cell type can Radioactive precursor N-I N-II Ratio N-I/N-II utilize different paths to dTMP synthesis is found in the case of hepatocytes. Although it has been maintained (see refer Orotic acid-H 3870 Â±450 421+40 9.19 Cytidine-3H 321 Â±85 173 Â±16 ences in Ref. 4) that dTMP is synthesized in regenerating rat 1.85 liver via the sequence CDP -Â»dCMP -Â»dUMP -Â»dTMP;the Data from Bushnell et al. (5). Adult male rats were injected intiapeiitoneally 1.5 hours after the onset of feeding (60% protein diet) work of Hecht and Potter (12) and Crone and Itzhaki (6) with either orotic acid-3H or cytidine-3H. Thirty minutes after demonstrated that the predominant sequence was UDP -Â» -> injection the rats were killed by decapitation, the livers removed, and dUMP -Â»dTMP. However, the studies of Robinson and nuclear fractions N-I and N-II obtained and analyzed as described in Brecher (30), Bryant (3), and Devik and Halvorsen (7) that that report. Values are means of separate determinations on three rats per isotope used. have already been cited indicate that regenerating mouse liver utilizes the salvage path (TdR ->-dTMP) to a significant The separation of N-I and N-II nuclei from livers of rats that extent. That some alternative pathway (either TdR -Â»dTMP, were fed 3'-MDAB for various periods of time are shown in TDP-Â»-Â»dTMP,or5-ME-dCMP -Â»dTMP)is used in human liver Charts 3â€”5.It is evident that the distribution of DNA between is suggested by the observation (1) that substantial regen hepatocyte (N-I) and nonhepatocyte (N-II) nuclei shifts from eration of the liver of a patient on large doses of FUdR 80% in N-I at zero days of dye feeding to 20% in that fraction occurred following resection of most of the liver because of a after 17 days of feeding the carcinogen (Chart 3). The data in hepatoma. Recent studies (T. Sneider, unpublished data) Chart 4 show that little incorporation of TdR-3 H (2-hour provide more clearcut evidence for use of an alternative de labeling periods) occurs in either N-I or N-II for the first 10 novo path in liver cells. Young adult Buffalo rat liver cells days of feeding even though at that point the distribution of grown in tissue culture in medium lacking added thy nudine, but having 1CTS molar amethopterin as well as UdR-3H or AdR-3H, showed complete inhibition of UdR-3H incorpora T. W. Sneider, D. E. Bushnell, and V. R. Potter. The Distribution and Synthesis of DNA in Two Classes of Rat Liver Nuclei during Azo tion into DNA. The incorporation of AdR-3H into DNA was Dye-induced Hepatocarcinogenesis, submitted for publication.

2400 CANCER RESEARCH VOL. 29

Â°0

Chart 3. Distribution of DNA in hepatocyte nuclei (N-I) and nonhepatocyte nuclei (N-II) expressed as a percentage of the total DNA IT 21 28 per liver per 100 gm body weight in livers from rats fed 3'-MDAB. Data DAYS ON V-MDAB DIET from Sneider et al. (see Footnote 3). Adult male rats fed 60% casein diet containing 0.05% 3'-MDAB were killed at the times shown, the Chart 4. Incorporation of TdR-3H into DNA from hepatocyte (N-I) livers removed, and the nuclear fractions N-I and N-II isolated and and nonhepatocyte (N-II) nuclei. Data from Sneider et al. Adult male rats fed 60% casein diet containing 0.05% 3'-MDAB were analyzed as described in that paper. The average values of 10 rats are intraperitoneally injected with TdR-3H (20 fie/100 gm body wt.) two shown at Week 0, 4 at 7 and 10 days of dye feeding, 10 at 14 days, 4 at hours prior to killing on the days shown. Nuclear fractions were 17 days, 24 at 21 days, and 18 at 42 days. The % of total liver DNA prepared and analyzed for TdR-3H incorporation into DNA as found in N-I nuclei is shown by the closed circles (â€¢ â€¢)andin N-II by the crosses (X X). For abbreviations see Chart 2. described in that report. Each point represents average incorporation values for the number of rats noted in the legend of Chart 3. Incorporation into N-I nuclei is shown by closed circles (â€¢ â€¢)and DNA between the two classes of nuclei had shifted to equal amounts. The specific activity of the DNA in N-I nuclei, into N-II by crosses (X X). For abbreviations see Chart 2. expressed in relation to the specific activity of the N-II DNA (Chart 5), shows that the proliferation noted starting at Day

14 of dye feeding was accompanied by a greater rate of DNA a synthesis in the N-I (hepatocyte) fraction. The available r evidence suggests, but does not prove, that the 3'-MDAB"bile * duct" proliferation may actually involve hepatocytes. Since the alterations in "dTTP enzymes" in livers from 3'-MDAB-fed rats also do not resemble changes noted during ANIT feeding (32), during early stages of liver "regeneration" (4), or in hepatocellular carcinomas (34), there is a good possibility that r hepatocytes under these circumstances use a different route to & dTMP synthesis. More detailed studies in these systems with O 7 IO 14 17 21 28 35 42 emphasis on the individual cell types (13, 14) in mixed DAYS ON 3'-MDAB DIET populations seems in order. Chart 5. The specific activities of TdR-3H-labeled DNA in hepatocyte (N-I) nuclei expressed as a ratio to the specific activities of DNA in CONCLUSIONS nonhepatocyte (N-II) nuclei. Data from Sneider et a/.3 Raw data shown in Chart 4 were used in the calculation of these ratios. For It should be clear from the examples cited that alternative de abbreviations see Chart 2. novo and salvage(preformed) pathways to synthesis of dTTP (or any essential metabolite) can defeat chemotherapeutic efforts to block that synthesis. Moreover, a priori predictions given cell type might be "totipotent" with respect to multiple about the route to dTTP synthesis in given normal or cancer paths to a given productâ€”thepathway utilized at any instant tissues cannot be made with certainty. The latter statement is being dependent upon such factors as presence of preformed emphasized because, in addition to selection of cell popula product in the cellular environment, antimetabolite blocks, tions that utilize pathways different from those employed by and so on. It should also be noted that given cell types that are another population of the same cell type, it is possible that a in the process of arriving at their final differentiated state

DECEMBER 1969 2401

Downloaded from cancerres.aacrjournals.org on September 25, 2021. © 1969 American Association for Cancer Research. Thomas W. Sneider and Van R. Potter might well use metabolic pathways quite different from those 13. Johnston, I. R., Mathias, A. P., Pennington, F., and Ridge, D. The Fractionation of Nuclei from Mammalian Cells by Zonal Centrif- employed by the fully differentiated cell (33). The importance ugation. Biochem. 1., 109: 127-135, 1968. to chemotherapy of this statement lies in the possibility (23, 25) that cancer cells represent cells that have been "frozen" at 14. Johnston, I. R., Mathias, A. P., Pennington, F., and Ridge, D. Distribution of RNA Polymerase Activity among the Various one or more stages of differentiation. Classes of Liver Nuclei. Nature, 220: 668-675, 1968. The more recent information that has been presented in this 15. Karlstrom, O., and Larsson, A. Significance of Ribonucleotide paper serves to reemphasize, with respect to the dTTP Reduction in the Biosynthesis of Deoxyribonucleotides in pathway, the contention of Potter and Heidelberger (24) that Escherichia coli. European J. Biochem., 3: 164-170, 1967. has already been cited in the introduction concerning the 16. Lark, K. G., Repko, T., and Hoffman, E. The Effect of Amino muliplicity of metabolic pathways "in different tissues in the Acid Deprivation on Subsequent Deoxyribonucleic Acid Replication. Biochim. Biophys. Acta, 76: 9-24, 1963. same organism and in the same tissue under different circumstances." It is hoped that further consideration of these 17. Maley, G. F., and Maley, F. The Purification and Properties of Deoxycytidylate Deaminase from Chick Embryo Extracts. J. Biol. possibilities will be of benefit to the scientist designing Chem.,239: 1168-1176,1964. antimetabolites affecting the dTTP pathway as well as to the 18. Moore, E. C., and Huribert, R. B. Regulation of Mammalian clinician who uses these agents. Deoxyribonucleotide Biosynthesis by Nucleotides as Activators and Inhibitors. J. Biol. Chem., 241: 4802-4809, 1966. ACKNOWLEDGMENTS 19. Morris, N. R., and Fischer, G. A. Studies Concerning Inhibition of the Synthesis of Deoxycytidine by Phosphorylated Derivatives of The authors thank Mrs. Lynn Dean for her expert technical assistance Thymidine. Biochim. Biophys. Acta, 42: 183-184, 1960. in performing some of the studies cited in this paper. 20. Morse, P. A., and Potter, V. R. Pyrimidine Metabolism in Tissue Culture Cells Derived from Rat Hepatomas. I. Suspension Cell Cultures Derived from the Novikoff Hepatoma. Cancer Res., 25: REFERENCES 499-508,1965. 21. Murphree, S., Moore, E. C., and Beali, P. T. Regulation by 1. Barrett, O., and Cohen, A. Liver Regeneration after Major Nucleotides of the Activity of Partially Purified Ribonucleotide Hepatectomy: Effect of Chemotherapy on Growth and Function- Reductase from Rat Embryos. Cancer Res. 28: 860-863, 1968. Case Report. Cancer, 22: 268-273, 1968. 22. Nishihara, M., Chrambach, A., and Aposhian, H. V. The Deoxy 2. Bieliavsky, N., and Leong, G. F. The Effect of FUdR on cytidylate Deaminase Found in Bacillus subtilis Infected with Segmentation and Incorporation of H-Uracil in Amphibian Phage SP8. Biochemistry, 6: 1877-1886,1967. Embryos. Exptl. Cell Res., 47: 261-270, 1967. 23. Potter, V. R. Recent Trends in Cancer Biochemistry: The 3. Bryant, B. J. Reutilization of Leukocyte DNA by Cells of Importance of Studies on Fetal Tissue. Canadian Cancer Con Regenerating Liver. Exptl. Cell Res., 27: 70-79, 1962. ference, 8: 9-30,1969. 4. Bucher, N. L. R. Regeneration of Mammalian Liver. Intern. Rev. 24. Potter, V. R., and Heidelberger, C. Alternative Metabolic Pathways. Cytology, 15: 245-300, 1963. Physiol. Rev., 30: 487-512, 1950. 5. Bushnell, D. E., Whittle, E. D., and Potter, V. R. Differential 25. Potter, V. R., and Watanabe, M. Some Biochemical Essentials of Utilization of Pyrimidines for RNA Synthesis in Two Classes of Malignancy: The Challenge of Diversity. In: C. J. D. Zarafonetis Rat Liver Nuclei. Biochim. Biophys. Acta, 179: 497-499, 1969. (ed.), Proceedings of the International Conference on Leukemia- 6. Crone, M., and Itzhaki, S. On the Relative Functioning of the Lymphoma. pp. 36-46. Philadelphia: Lea and Febiger, 1968. Pathways for Formation of Thymidine Nucleotides in the Regen 26. Prescott, D. M., and Bender, M. A. Preparation of Mammalian erating Liver and Spleen of the Rat. Biochim. Biophys. Acta, 95: Metaphase Chromosomes for Autoradiography. Methods Cell 8-13,1965. Physiol., 1: 381-385,1964. 7. Devik, F., and Halvorsen, K. Observations by Biochemical Analysis 27. Price, F. M., Rotherham, J., and Evans, V. J. Pyrimidine and Autoradiography on Labelled [sic] Deoxyribonucleic Acid in Nucleoside Requirements of a Neoplastic C3H Mouse Cell Strain. J. the Normal and Regenerating Liver of Mice. Nature, 797: 148-150, Nati. Cancer Inst., 39: 529-538, 1967. 1963. 28. Price, J. M., Karman, V. W., Miller, E. C., and Miller, J. A. 8. Feinendegen, L. E., Bond, V. P., and Hughes, W. L. Physiological Progressive Microscopic Alterations in the Livers of Rats Fed the Hepatic Carcinogens 3'-Methyl-4-dimethylaminoazobenzene and Thymidine Reutilization in Bone Marrow. Proc. Soc. Exptl. Biol. Med., 722: 448-455, 1966. 4 -Fluoro-4-dimethylaminoazobenzene. Cancer Res., 12: 193-200, 9. Frenkel, E. P., Sugino, Y., Bishop, R. C., and Potter, R. L. Effect 1952. of X-radiation on DNA Metabolism in Various Tissues of the Rat. 29. Reichard, P. The Biosynthesis of Deoxyribose. New York: John VI. Correlative Morphologic and Biochemical Changes during Wiley and Sons, 1967. Regeneration of the Thymus. Radiation Res., 19: 701-716,1963. 30. Robinson, S. H., and Brecher, G. Delayed Incorporation of 10. Gentry, G. A., Morse, P. A., and Potter, V. R. Pyrimidine Tritiated Thymidine into DNA. Science, 142: 392-393, 1963. Metabolism in Tissue Culture Cells Derived from Rat Hepatomas. 31. Skipper, H. E., Thomson, J. R., and Bell, M. Attempts at Dual III. Relationship of Thymidine to the Metabolism of Other Blocking of Biochemical Events in Cancer Chemotherapy. Cancer Pyrimidine Nucleosides in Suspension Cultures Derived from the Res., 14: 503-507, 1954. Novikoff Hepatoma. Cancer Res., 25: 517-525, 1965. 32. Sneider, T. W., Krawitt, E., and Potter, V. R. Selected Enzymes of 11. Ceraci, G., Rossi, M., and Scarano, E. Deoxycytidylate Amino- Primidine Nucleotide Metabolism in Livers from Rats Fed hydrolase. I. Preparation and Properties of the Homogeneous a-Naphthylisothiocyanate. Cancer Res., in press. Enzyme. Biochemistry, 6: 183-191, 1967. 33. Sneider, T. W., and Potter, V. R. Deoxycytidylate Deaminase and 12. Hecht, L. I., and Potter, V. R. Nucleic Acid Metabolism in Related Enzymes of Thymidine Triphosphate Metabolism in Regenerating Rat Liver. III. Intermediates in the Synthesis of DNA Hepatomas and Precancerous Rat Liver. Advan. Enzyme Regul., 7: Pyrimidine Nucleotides. Cancer Res., 16: 999-1004, 1956. 375-394, 1969.

2402 CANCER RESEARCH VOL. 29

34. Sneider, T. W., Potter, V. R., and Morris, H. P. Enzymes of 38. Sugino, Y., Frenkel, E. P. and Potter, R. L. Effect of X-radiation Thymidine Triphosphate Synthesis in Selected Morris Hepatomas. on DNA Metabolism in Various Tissues of the Rat. V. DNA Cancer Res., 29: 40-54,1969. Metabolism in Regenerating Thymus. Radiation Res., 19: 35. Steel, G. G., Adams, K., and Barrett, J. C. Analysis of the Cell 682-700, 1963. Population Kinetics of Transplanted Tumors of Widely Differing Growth Rate. Brit. J. Cancer, 20: 784-800, 1966. 39. Whittle, E. D. Effect of Thymidine on Deoxyribonucleic Acid Synthesis and Cytidine Metabolism in Rat-Thymus Cells. Biochim. 36. Steel, G. G., and Lamerton, L. F. The Turnover of Tritium from Biophys. Acta, 114: 44-60,1966. Thymidine in Tissues of the Rat. Exptl. Cell Res., 37: 117-131, 1965. 40. Wykes, J. R., and Prescott, D. M. The Metabolism of Pyrimidine 37. Sugino, Y. Metabolism of Deoxyribonucleotides. Ann. Report Inst. Compounds by Tetrahymena pyriformis. J. Cell. Physiol, 72: Virus Res. (Kyoto University), 9: 1-29, 1966. 173-184,1968.

DECEMBER 1969 2403

Downloaded from cancerres.aacrjournals.org on September 25, 2021. © 1969 American Association for Cancer Research. Alternative de Novo and ''Salvage'' Pathways to Thymidine Triphosphate Synthesis: Possible Implications for Cancer Chemotherapy

Thomas W. Sneider and Van R. Potter

Cancer Res 1969;29:2398-2403.

Updated version Access the most recent version of this article at: http://cancerres.aacrjournals.org/content/29/12/2398.citation

E-mail alerts Sign up to receive free email-alerts related to this article or journal.

Reprints and To order reprints of this article or to subscribe to the journal, contact the AACR Publications Subscriptions Department at [email protected].

Permissions To request permission to re-use all or part of this article, use this link http://cancerres.aacrjournals.org/content/29/12/2398.citation. Click on "Request Permissions" which will take you to the Copyright Clearance Center's (CCC) Rightslink site.