Enzyme targets of antiglutamine agents in cancer chemotherapy

Inhibition of Glycolysis and Glutaminolysis: An Emerging Drug Discovery Approach to Combat Cancer

Cancer cells have a very different metabolism from that of normal cells from which they are derived. Their metabolism is elevated, which allows them to sustain higher proliferative rate and resist some cell death signals. This phenomenon, known as the "Warburg effect", has become the focus of intensive efforts in the discovery of new therapeutic targets and new cancer drugs. Both glycolysis and glutaminolysis pathways are enhanced in cancer cells. While glycolysis is enhanced to satisfy the increasing energy demand of cancer cells, glutaminolysis is enhanced to provide biosynthetic precursors for cancer cells. It was recently discovered that there is a tyrosine phosphorylation of a specific isoform of pyruvate kinase, the M2 isoform, that is preferentially expressed in all cancer cells, which results in the generation of pyruvate through a unique enzymatic mechanism that is uncoupled from ATP production. Pyruvate produced through this unique enzymatic mechanism is converted primarily into lactic acid, rather than acetyl-CoA for the synthesis of citrate, which would normally then enter the citric acid cycle. Inhibition of key enzymes in glycolysis and glutaminolysis pathways with small molecules has provided a novel but emerging area of cancer research and has been proven effective in slowing the proliferation of cancer cells, with several inhibitors being in clinical trials. This review paper will cover recent advances in the development of chemotherapeutic agents against several metabolic targets for cancer therapy, including glucose transporters, hexokinase, pyruvate kinase M2, glutaminase, and isocitrate dehydrogenase.

Pupylation : A Signal for Proteasomal Degradation in Mycobacterium tuberculosis

This chapter describes the identification of the first prokaryotic ubiquitin-like protein modifier, Pup, which covalently attaches to proteins to target them for destruction by a bacterial proteasome in a manner akin to ubiquitin in eukaryotes. Despite using a proteasome as the end point for proteolysis, Pup and ubiquitin differ in sequence, structure and method of activation and conjugation to protein substrates. Pup is so far the only known posttranslational protein modifier in prokaryotes and its discovery opens the door to the possibility that others are present not only for proteolysis, but also to regulate protein function or localization. Here, we discuss the putative mechanism of activation and conjugation of Pup (termed "pupylation") to target proteins. In addition, because it is unclear whether or not Pup, like ubiquitin, is recycled or degraded during substrate targeting to the proteasome, we propose methods that may identify Pup deconjugation enzymes ("depupylases"). Finally, we outline future directions for Pup research and anti-tuberculosis drug discovery.

Reviving Lonidamine and 6-Diazo-5-oxo-L-norleucine to Be Used in Combination for Metabolic Cancer Therapy

Abnormal metabolism is another cancer hallmark. The two most characterized altered metabolic pathways are high rates of glycolysis and glutaminolysis, which are natural targets for cancer therapy. Currently, a number of newer compounds to block glycolysis and glutaminolysis are being developed; nevertheless, lonidamine and 6-diazo-5-oxo-L-norleucine (DON) are two old drugs well characterized as inhibitors of glycolysis and glutaminolysis, respectively, whose clinical development was abandoned years ago when the importance of cancer metabolism was not fully appreciated and clinical trial methodology was less developed. In this review, a PubMed search using the words lonidamine and 6-diazo-5-oxo-L-norleucine (DON) was undertaken to analyse existing information on the preclinical and clinical studies of these drugs for cancer treatment. Data show that they exhibit antitumor effects; besides there is also the suggestion that they are synergistic. We conclude that lonidamine and DON are safe and potentially effective drugs that need to be reevaluated in combination as metabolic therapy of cancer.

Glutamic acid and its derivatives: candidates for rational design of anticancer drugs

Throughout the history of human civilizations, cancer has been a major health problem. Its treatment has been interesting but challenging to scientists. Glutamic acid and its derivative glutamine are known to play interesting roles in cancer genesis, hence, it was realized that structurally variant glutamic acid derivatives may be designed and developed and, might be having antagonistic effects on cancer. The present article describes the state-of-art of glutamic acid and its derivatives as anticancer agents. Attempts have been made to explore the effectivity of drug-delivery systems based on glutamic acid for the delivery of anticancer drugs. Moreover, efforts have also been made to discuss the mechanism of action of glutamic acid derivatives as anticancer agents, clinical applications of glutamic acid derivatives, as well as recent developments and future perspectives of glutamic acid drug development have also been discussed.

5-N-Substituted-2-(substituted benzenesulphonyl) glutamines as antitumor agents. Part II: Synthesis, biological activity and QSAR study

Cancer is a major killer disease throughout human history. Thus, cancer becomes a major point of interest in life science. It was proved that cancer is a nitrogen trap and tumor cells are avid glutamine consumers. The non-essential amino acid glutamine, which is a glutamic acid derivative, supplies its amide nitrogen to tumor cells in the biosynthesis of purine and pyrimidine bases of nucleic acids as well as takes part in protein synthesis. Based on these and in continuation of our composite programme of development of new potential anticancer agents through rational drug design, 17 new 5-N-Substituted-2-(substituted benzenesulphonyl) glutamines were selected for synthesis. These compounds as well as 36 earlier synthesized glutamine analogues were screened for antitumor activity using percentage inhibition of tumor cell count as the activity parameter. QSAR study was performed with 53 compounds in order to design leads with increased effectiveness for antitumor activity using both physicochemical and topological parameters. QSAR study showed that steric effect on the aromatic ring is conducive to the activity. n-butyl substitution on aliphatic side chain and atom no 12 is important for antitumor activity of glutamine analogues.

Isolated liver perfusion permits administration of high doses of chemotherapeutic agents. Comparison with hepatic artery infusion

Tumor cells are dependent on glutamine metabolism and acivicin, which is a selective glutamine antagonist, has been shown to effectively retard tumor growth in several malignancies. However, systemic treatment with acivicin is associated with significant side effects. The purpose of the present study was to examine whether use of an in vivo isolated liver perfusion model may allow administration of lethal doses of acivicin and compare it to regional infusion of acivicin in the hepatic artery. Five days after tumor inoculation, acivicin was administered by an isolated liver perfusion model or by regional infusion via the hepatic artery. It was found that regional infusion of acivicin (5 and 10 mg/kg) via the hepatic artery caused systemic illness and diarrhea, and all animals in this group died within 3 days. In contrast, we observed no signs of systemic illness, diarrhea or hepatocellular injury in rats receiving isolated liver perfusion with or without acivicin (10 mg/kg) administration. Noteworthy, we found that isolated perfusion with acivicin reduced the glutamine content in liver tumors by 39% compared to perfusion with control medium. In line with this, it was found that isolated perfusion with acivicin (10 mg/kg) inhibited tumor growth in the liver. Taken together, this study suggests that application of the isolated liver perfusion model avoids the toxic and lethal effects of high doses of chemotherapy, herein acivicin, and may provide a useful approach to treat liver tumors in vivo.

Combined, functional genomic-biochemical approach to intermediary metabolism: interaction of acivicin, a glutamine amidotransferase inhibitor, with Escherichia coli K-12

Acivicin, a modified amino acid natural product, is a glutamine analog. Thus, it might interfere with metabolism by hindering glutamine transport, formation, or usage in processes such as transamidation and translation. This molecule prevented the growth of Escherichia coli in minimal medium unless the medium was supplemented with a purine or histidine, suggesting that the HisHF enzyme, a glutamine amidotransferase, was the target of acivicin action. This enzyme, purified from E. coli, was inhibited by low concentrations of acivicin. Acivicin inhibition was overcome by the presence of three distinct genetic regions when harbored on multicopy plasmids. Comprehensive transcript profiling using DNA microarrays indicated that histidine biosynthesis was the predominant process blocked by acivicin. The response to acivicin, however, was quite complex, suggesting that acivicin inhibition resonated through more than a single cellular process.

The role of glutamine and glucose analogues in metabolic inhibition of human myeloid leukaemia in vitro

1. Glutamine analogues L-[alpha S,5S]-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (acivicin) and 6-diazo-5-oxo-L-norleucine (DON) have been shown to possess cytotoxic activity against a wide variety of animal and human xenografted solid tumours, however their potential in man has been limited by toxicity. 2. We have analysed the effects of acivicin and DON on glutamine utilization to determine whether the reason for the disappointing therapeutic profile is solely due to the inefficient inhibition of glutamine metabolism. 3. Human myeloid leukaemic cells treated with acivicin inhibited ribonucleotide biosynthesis but not energy production via glutaminolysis and had little effect on viability, whereas treatment with DON inhibited both ribonucleotide biosynthesis and glutamine oxidation and resulted in reduced viability. 4. Treatment of the myeloid leukaemic cells with the glucose analogue 2-deoxy-D-glucose in addition to DON potentiated the inhibition of de novo nucleotide biosynthesis, glutaminolysis and glycolysis, and caused a further reduction in cell viability. 5. These results provide further support for the essential role of glutamine in cellular metabolism, and indicate that use of the glutamine analogue DON in the treatment of acute myeloid leukaemia may be more clinically effective if used in combination with 2-deoxy-D-glucose.

Transport and membrane binding of the glutamine analogue 6-diazo-5-oxo-L-norleucine (DON) in Xenopus laevis oocytes

We have examined transport and membrane binding of 6-diazo-5-oxo-L-norleucine (DON, a photoactive diazo-analogue of glutamine) and their relationships to glutamine transport in Xenopus laevis oocytes. DON uptake was stereospecific and saturable (Vmax of 0.44 pmol/oocyte.min and a Km of 0.065 mM). DON uptake was largely Na+ dependent (80% at 50 microM DON) and inhibited (greater than 75%) by glutamine and arginine (substrates of the System B0,+ transporter) at 1 mM. Glutamine and DON show mutual competitive inhibition of Na(+)-dependent transport. Preincubation of oocytes in medium containing 0.1 mM DON for 24 or 48 hr depressed the Vmax for System B0,+ transport (as measured by Na(+)-dependent glutamine uptake), this effect was highly specific (neither D-DON nor the System B0,+ substrates glutamine and D-alanine showed any independent effect) and required Na+ ions. Glutamine (1 mM in preincubation medium) protected transport from inhibition by DON. The possibility that specific inactivation of System B0,+ by DON reflects attachment of DON to the transporter was tested by examining the binding of [14C]DON to Xenopus oocyte membranes. Oocytes incubated in 100 mM NaCl in the presence of [14C]DON for up to 48 hr showed 2.4-fold higher 14C-binding to membranes than oocytes incubated in choline chloride. Na(+)-dependent DON binding (31 +/- 11 fmol/micrograms membrane protein) was suppressed by external glutamine, arginine or alanine and was largely confined to a membrane protein fraction of 48-65 kDa (as assessed by SDS-polyacrylamide gel electrophoresis). The present studies indicate that DON and glutamine uptake in oocytes are both mediated by System B0,+ and demonstrate the DON binding to a particular membrane protein fraction is associated with inactivation of the transporter, offering the prospect of using [14C]DON as a covalent label for the transport protein in order to facilitate its isolation and subsequent biochemical characterization.

Substrate-specificity of glutamine transporters in membrane vesicles from rat liver and skeletal muscle investigated using amino acid analogues

We investigated the effects of glutamine and histidine analogues on glutamine transport processes in membrane vesicles prepared from rat liver (sinusoidal membrane) and skeletal muscle (sarcolemma). L-[14C]Glutamine is transported in these membranes predominantly by Systems N/Nm (liver and muscle respectively), and to a lesser extent by Systems A and L (e.g. about 60, 20 and 20% of total flux respectively via Systems N, A and L at 0.05 mM-glutamine in liver membrane vesicles). The glutamine anti-metabolites 6-diazo-5-oxo-L-norleucine and acivicin were relatively poor inhibitors of glutamine uptake into liver membrane vesicles (less than 25% inhibition at 20-fold excess) and appeared primarily to inhibit System A activity (i.e. N-methylaminoisobutyric acid-inhibitable glutamine uptake). In similar experiments azaserine (also a glutamine anti-metabolite) inhibited approx. 50% of glutamine uptake, apparently by inhibition of System A and also of System L (i.e. 2-amino-2-carboxybicyclo[2,2,1]heptane-inhibitable glutamine uptake). Glutamate gamma-hydroxamate, aspartate beta-hydroxamate, histidine and N'-methylhistidine were all strong inhibitors of glutamine uptake into liver membrane vesicles (greater than 65% inhibition at 20-fold excess), but neither homoglutamine nor N'-methylhistidine produced inhibition. L-Glutamate-gamma-hydroxamate was shown to be a competitive inhibitor of glutamine transport via System N (Ki approximately 0.6 mM). Glutamine uptake in sarcolemmal vesicles showed a similar general pattern of inhibition as in liver membrane vesicles. The results highlight limits on the substrate tolerance of System N; we suggest that the presence of both an L-alpha-amino acid group and a nitrogen group with a delocalized lone-pair of electrons (amide or pyrrole type), separated by a specific intramolecular distance (C2-C4 chain equivalent), is important for substrate recognition by this transporter.

Metabolism and action of amino acid analog anti-cancer agents

The preclinical pharmacology, antitumor activity and toxicity of seven of the more important amino acid analogs, with antineoplastic activity, is discussed in this review. Three of these compounds are antagonists of L-glutamine: acivicin, DON and azaserine; and two are analogs of L-aspartic acid: PALA and L-alanosine. All five of these antimetabolites interrupt cellular nucleotide synthesis and thereby halt the formation of DNA and/or RNA in the tumor cell. The remaining two compounds, buthionine sulfoximine and difluoromethylornithine, are inhibitors of glutathione and polyamine synthesis, respectively, with limited intrinsic antitumor activity; however, because of their powerful biochemical actions and their low systemic toxicities, they are being evaluated as chemotherapeutic adjuncts to or modulators of other more toxic antineoplastic agents.

Effects of intermediary metabolites and electron transport inhibitors on action of chloroquine on Brugia pahangi and Onchocerca volvulus

We examined the possibility that chloroquine is interfering with aerobic energy-generating processes in the adult filarial parasites, Brugia pahangi and Onchocerca volvulus. Using motility of these parasites as an assay of drug effect, we found that micromolar concentrations of chloroquine caused significant paralysis, but only in alkaline medium (pH 8.4). The addition of 12 mM glutamine or 10 mM albizziin to the medium completely antagonized drug-induced paralysis. In addition, in B. pahangi, all of the tricarboxylic acid cycle intermediates (10 mM) except citrate and pyruvate antagonized the effect of chloroquine on motility; in O. volvulus, oxaloacetate as well as glutamine inhibited the effect of the drug. The effect of chloroquine on both parasites was enhanced when it was used in combination with 10 microM acivicin, a glutamine antimetabolite. Here motility of B. pahangi was reduced significantly within 24-48 hr at acidic (6.8) neutral (7.4) and alkaline (8.4) pH. This effect was partially reversible by glutamine (12 mM). Motility of O. volvulus was reduced to near zero within 4 hr with this drug combination. Antimycin A and rotenone, both electron transport inhibitors, also synergized with chloroquine at any pH to produce paralysis in B. pahangi. The effects of the rotenone and chloroquine combination were reversed in the presence of 10 mM succinate. However, glutamine (12 mM) was unable to antagonize the effects of chloroquine plus antimycin A on the motility of B. pahangi. These findings suggest that chloroquine may be inhibiting aerobic energy metabolism in the filariae, possibly at the level of electron transport. Furthermore, since chloroquine is well-tolerated but only weakly filaricidal in vivo, the data indicate that use of this drug in combination with other inhibitors of aerobic energy metabolism may be a chemotherapeutically useful approach to the treatment of filariases.

Chromatographic analysis of purine precursors in mouse L1210 leukemia

A number of antagonists of nucleotide metabolism with anti-cancer activity affect the de novo purine pathway. To determine the biochemical mechanisms of cytotoxicity of these drugs, assay procedures have been developed for measurement of the levels of intermediates proximal to IMP in the pathway for de novo purine biosynthesis in mouse L1210 leukemia cells. Purine precursors have been synthesized in vitro from [14C]glycine using enzymes from chicken liver. These 14C-labeled intermediates have been used as marker compounds to define retention times for metabolites of leukemia cells separated by HPLC and the chromatographic mobilities of these intermediates after two-dimensional thin-layer chromatography. These new chromatographic procedures have been used in combination to determine the steady-state concentrations for purine precursors in mouse L1210 leukemia cells in the exponential phase of growth: N-formylglycineamide ribotide (16 microM); N-formylglycineamidine ribotide (4.7 microM); 5-aminoimidazole ribotide (4.0 microM); 4-carboxy-5-aminoimidazole ribotide (0.46 microM); N-succino-5-aminoimidazole-4-carboxamide ribotide (11 microM); 5-aminoimidazole-4-carboxamide ribotide (16 microM); 5-formamidoimidazole-4-carboxamide ribotide (2.7 microM); and IMP (57 microM). The metabolic effects of tiazofurin (25 microM) upon mouse L1210 leukemia cells growing in culture define a "metabolic crossover point" at the reaction catalyzed by IMP dehydrogenase (EC 1.1.1.205) which confirms previous reports of inhibition of this enzyme.

Enzymic programs of rat bone marrow and the impact of acivicin and tiazofurin

The in vivo actions of two antimetabolites, acivicin (NSC-163501) and tiazofurin (NSC-286193), were examined on the enzymic programs of rat bone marrow. From the bone marrow of the femurs, 100,000 g supernatant fractions were prepared; enzymic activities were measured by isotopic assays, and cellularity was determined. In the normal bone marrow, the specific activities of pyrimidine de novo synthetic enzymes, CDP reductase, dTMP synthase, CTP synthase, carbamoyl-phosphate synthase II (synthase II), orotidine 5'-phosphate decarboxylase and aspartate carbamoyltransferase, were 1, 2.7, 5, 10, 63 and 601 nmol/hr/mg protein, respectively, whereas those of the salvage enzymes, deoxycytidine, thymidine, cytidine and uridine kinases were 3, 43, 149, and 367 nmol/hr/mg protein, respectively. In purine biosynthesis, the activities of the de novo synthetic enzymes, IMP dehydrogenase, formylglycinamidine ribonucleotide (FGAM) synthase, GMP synthase, amidophosphoribosyl-transferase (AT) and adenylosuccinate synthase were 16, 8, 107, 78 and 124 nmol/hr/mg protein, respectively, and those of the salvage enzymes, adenine, hypoxanthine and guanine phosphoribosyl-transferases, were 340, 407, and 1018 nmol/hr/mg protein, respectively. The sequence of events was elucidated after a single i.p. injection of acivicin (5 mg/kg) or tiazofurin (200 mg/kg). Within 2 hr after acivicin injection, CTP, GMP and FGAM synthases lost 85-90%, while AT and synthase II lost 50 and 80%, respectively, of their activities. The activities rose to near normal range by 72-96 hr. The bone marrow cellularity decreased, reaching a nadir at 24 and 48 hr, and returning to normal range by 72 and 92 hr; thymidine kinase activity followed a similar pattern. Tiazofurin injection depressed IMP dehydrogenase activity to 20% by 2 hr with a rebound to normal range by 48 and 72 hr. The cellularity decreased more slowly, reaching its lowest point at 24 hr and returning to normal range at 72 hr. For acivicin the marked depletion of the activities of the glutamine-utilizing enzymes and for tiazofurin that of IMP dehydrogenase might account, in part at least, for the bone marrow toxicity of these antimetabolites. Because of the presence in the bone marrow of high activities of purine and pyrimidine salvage enzymes, it should be possible to design methods utilizing nucleosides and nucleobases to protect the bone marrow from the action of antimetabolites.

Inactivation by acivicin of rat brain CTP and GMP synthetases and depression of CTP and GTP concentrations

Evidence was provided that injection of acivicin (25 mg/kg, i.p.) into the rat inactivated brain CTP and GMP synthetases. Under the same circumstances, CTP and GTP concentrations in the rat brain decreased following the decline in the activities of CTP and GMP synthetases. The decrease in enzymic activities and nucleotide concentrations progressed with time. The decline in CTP and GMP synthetase activities and CTP and GTP concentrations caused by acivicin occurred more slowly and to a lesser extent than in liver and hepatoma 3924A. The delay in the expression of acivicin action in the rat brain was attributed to a possible slower entrance of acivicin and the lower concentration than might have been attained in the rat brain. These considerations are based on the rapid disappearance of acivicin from rat plasma noted earlier. The decline in CTP concentration in rat brain might interfere with neuronal function. The decline in GTP concentration might be expressed through the depletion of biopterins which are generated from GTP in the brain. The possible relevance to the biochemical basis of paranoid schizophrenia which occurs reversibly after high-dose acivicin or tiazofurin treatment was discussed.