Methionine metabolism in BHK cells: preliminary characterization of the physiological effects of cycloleucine, an inhibitor of s-adenosylmethionine biosynthesis

Triple-Methyl Blockade With Recombinant Methioninase, Cycloleucine, and Azacitidine Arrests a Pancreatic Cancer Patient-Derived Orthotopic Xenograft Model

OBJECTIVES

Methionine addiction is a fundamental and general hallmark of cancer caused by enhanced methyl flux. In the present study, we effected a novel methionine-methylation blockade to target a patient-derived orthotopic xenograft model of pancreatic cancer.

METHODS

The pancreatic cancer patient-derived orthotopic xenograft mouse models were randomized into 6 groups of 8 mice each and treated for 2 weeks: untreated control; azacitidine; oral recombinant methioninase (o-rMETase); o-rMETase plus cycloleucine; o-rMETase plus cycloleucine plus azacitidine (triple-methyl blockade therapy); and gemcitabine (positive control).

RESULTS

Triple-methyl blockade therapy arrested tumor growth (mean relative tumor volume, 1.03 [standard deviation, 0.36]) and was significantly more effective compared with azacitidine (P = 0.0001); o-rMETase (P = 0.007); or o-rMETase plus cycloleucine (P = 0.04). Gemcitabine alone also inhibited but did not arrest tumor growth (mean relative tumor volume, 1.50 [standard deviation, 0.30]). The percentage of cancer cells that were negative for 5-methylcytosine staining in immunohistochemistry, indicating reduction of DNA methylation, increased with triple-methyl blockade therapy (37.5%), compared with gemcitabine (1.8%); o-rMETase (2.8%); azacitidine (9.0%); or o-rMETase plus cycloleucine (10.6%).

CONCLUSIONS

This new concept of triple-methyl blockade therapy has clinical potential for pancreatic cancer, which is currently a recalcitrant disease.

The interferon stimulated gene viperin, restricts Shigella. flexneri in vitro

The role of interferon and interferon stimulated genes (ISG) in limiting bacterial infection is controversial, and the role of individual ISGs in the control of the bacterial life-cycle is limited. Viperin, is a broad acting anti-viral ISGs, which restricts multiple viral pathogens with diverse mechanisms. Viperin is upregulated early in some bacterial infections, and using the intracellular bacterial pathogen, S. flexneri, we have shown for the first time that viperin inhibits the intracellular bacterial life cycle. S. flexneri replication in cultured cells induced a predominantly type I interferon response, with an early increase in viperin expression. Ectopic expression of viperin limited S. flexneri cellular numbers by as much as 80% at 5hrs post invasion, with similar results also obtained for the intracellular pathogen, Listeria monocytogenes. Analysis of viperins functional domains required for anti-bacterial activity revealed the importance of both viperin's N-terminal, and its radical SAM enzymatic function. Live imaging of S. flexneri revealed impeded entry into viperin expressing cells, which corresponded to a loss of cellular cholesterol. This data further defines viperin's multi-functional role, to include the ability to limit intracellular bacteria; and highlights the role of ISGs and the type I IFN response in the control of bacterial pathogens.

Viperin interacts with the kinase IRAK1 and the E3 ubiquitin ligase TRAF6, coupling innate immune signaling to antiviral ribonucleotide synthesis

Virus-inhibitory protein, endoplasmic reticulum-associated, interferon-inducible (viperin) is a radical SAM enzyme that plays a multifaceted role in the cellular antiviral response. Viperin has recently been shown to catalyze the SAM-dependent formation of 3'-deoxy-3',4'-didehydro-CTP (ddhCTP), which inhibits some viral RNA polymerases. Viperin is also implicated in regulating Lys-63-linked polyubiquitination of interleukin-1 receptor-associated kinase-1 (IRAK1) by the E3 ubiquitin ligase tumor necrosis factor receptor-associated factor 6 (TRAF6) as part of the Toll-like receptor-7 and -9 (TLR7/9) innate immune signaling pathways. In these pathways, the poly-ubiquitination of IRAK1 by TRAF6 is necessary to activate IRAK1, which then phosphorylates downstream targets and ultimately leads to the production of type I interferons. That viperin is a component of these pathways suggested that its enzymatic activity might be regulated by interactions with partner proteins. To test this idea, we have reconstituted the interactions between viperin, IRAK1, and TRAF6 by transiently expressing these enzymes in HEK 293T cells. We show that IRAK1 and TRAF6 increase viperin activity ∼10-fold to efficiently catalyze the radical-mediated dehydration of CTP to ddhCTP. Furthermore, we found that TRAF6-mediated ubiquitination of IRAK1 requires the association of viperin with both IRAK1 and TRAF6. Ubiquitination appears to depend on structural changes in viperin induced by SAM binding, but, significantly, does not require catalytically active viperin. We conclude that the synergistic activation of viperin and IRAK1 provides a mechanism that couples innate immune signaling with the production of the antiviral nucleotide ddhCTP.

Synthesis and biological properties of radiohalogenated α,α-disubstituted amino acids for PET and SPECT imaging of amino acid transporters (AATs)

Fluorine-18 and iodine-123 labeled nonnatural alicyclic and methyl branched disubstituted α,α-amino acids are a diverse and useful class of tumor imaging agents suitable for positron emission tomography and single photon emission computed tomography. These tracers target the increased expression of the cell membrane amino acid transporter systems L, ASC, and A exhibited by many human tumor cells. The most established clinical use for these radiolabeled amino acids is imaging primary and recurrent gliomas and primary, recurrent, and metastatic prostate cancer. This review focuses on the synthesis, radiolabeling, and amino acid transport mechanism of a series of nonnatural fluorine-18 and iodine-123 labeled analogs of 1-aminocyclobutane-1-carboxylic acid, 1-aminocyclopentane-1-carboxylic acid, α-aminoisobutyric acid, and α-methylaminoisobutyric acid.

Inhibition of lipopolysaccharide-stimulated TNF-alpha promoter activity by S-adenosylmethionine and 5'-methylthioadenosine

S-adenosylmethionine (SAM) is the principal biological methyl donor and precursor for polyamines. SAM is known to be hepatoprotective in many liver disease models in which TNF-alpha is implicated. The present study investigated whether and how SAM inhibited LPS-stimulated TNF-alpha expression in Kupffer cells (hepatic macrophages). SAM downregulated TNF-alpha expression in LPS-stimulated Kupffer cells at the transcriptional level as suggested by a transfection experiment with a TNF-alpha promoter-reporter gene. This inhibition was not mediated through decreased NF-kappaB binding to four putative kappaB binding elements located within the promoter. The inhibited promoter activity was neither prevented by overexpression of p65 and/or its coactivator p300 nor enhanced by overexpression of coactivator-associated arginine methyltransferase-1, an enzyme that methylates p300 and inhibits a p65-p300 interaction. SAM did not lead to DNA methylation at the most common CpG target sites in the TNF-alpha promoter. Moreover, 5'-methylthioadenosine (MTA), which is derived from SAM but does not serve as a methyl donor, recapitulated SAM's effect with more potency. These data demonstrate that SAM inhibits TNF-alpha expression at the level downstream of NF-kappaB binding and at the level of the promoter activity via mechanisms that do not appear to involve the limited availability of p65 or p300. Furthermore, our study is the first to demonstrate a potent inhibitory effect on NF-kappaB promoter activity and TNF-alpha expression by a SAM's metabolite, MTA.

Methionine depletion induces transcription of the mRNA (N6-adenosine)methyltransferase

This study examines the genetic expression of the S-adenosyl-L-methionine binding subunit of the mRNA (N6-adenosine)methyltransferase (MT-A70) in cultured cells under conditions known to affect transmethylation reactions. Methionine dependence, disrupted methionine metabolism, and increased transmethylation reactions are all phenotypes characteristic of cancer cells. The results show that both methionine depletion and inhibition of S-adenosyl-L-methionine formation can induce up to a four-fold increase in transcription of this S-adenosyl-L-methionine binding subunit. The two splice-variant mRNAs produced from the MT-A70 gene are transcribed at different rates depending on the level of S-adenosyl-L-methionine inhibition. This result may reflect differing Km values toward the substrate for the different enzyme isoforms. 3-Deazaadenosine, an inhibitor known to block certain mRNA transmethylations, was shown to have no effect on MT-A70 gene expression. This result indicates that the control of MT-A70 gene expression is directly related to methionine availability and the subsequent synthesis of S-adenosyl-L-methionine.

Inhibition of 6-methyladenine formation decreases the translation efficiency of dihydrofolate reductase transcripts

Cycloleucine was used to inhibit the formation of internal N6-methyladenosine residues in the messenger ribonucleic acid transcripts from cultured methotrexate resistant mouse sarcoma cells. Cells cultured in cycloleucine produced transcripts deficient in N6-methyladenosine residues and the 2'-O-methylated nucleosides of the cap structure; however, the formation of the 7-methylguanine nucleoside of the cap was not effected. Cytoplasmic polyadenylated transcripts were isolated from cells which had been pretreated with media containing cycloleucine and translated in an in vitro translation assay. The levels of translated dihydrofolate reductase were then analyzed by polyacrylamide gel electrophoresis. The amount of dihydrofolate reductase protein produced from the transcripts of the cycloleucine treated cells was 20% less than untreated transcripts. Ribonuclease protection assays demonstrated little difference in the cytoplasmic levels of dihydrofolate reductase transcripts between treated and untreated cells suggesting that the decrease in translation efficiency was not caused solely by an alteration in the processing or cytoplasmic transport of the transcripts. Translation of in vitro transcribed transcripts showed the presence of 2'-O-methylated nucleosides in the cap structure had a negative effect on translation efficiency, demonstrating that the results observed from cycloleucine treatment could not be due to the inhibition of 2'-O-methylation in the cap. These experiments therefore suggest that an inhibition of N6-methyladenosine residues in dihydrofolate reductase transcripts significantly alters their rate of translation.

Induction of murine erythroleukemia cell differentiation is associated with methylation and differential stability of poly(A)+ RNA transcripts

Murine erythroleukemia (MEL) cells exposed to DMSO were assessed for their ability to methylate poly(A)+ RNA and accumulate RNA transcripts of globin and nonglobin genes (c-myc, beta-actin and MER5). Cells were pulse-labeled with L-[methyl-3H]methionine, cytoplasmic RNA was isolated, selected for poly(A)+ RNA and analyzed by HPLC chromatography for methylated nucleosides. When MEL cells were exposed to inhibitors of RNA methylation (neplanocin A, 3-deazaneplanocin A and cycloleucine) and assessed for their ability to differentiate by DMSO, accumulate RNA transcripts, produce hemoglobin, methylate poly(A)+ and poly(A)- RNA and synthesize S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH), we observed the following: (a) MEL cells treated with DMSO underwent hypermethylation in poly(A)+ RNA that preferentially occurred at the 5'-cap structures (7-methylguanosine and 2'-O-methylcytidine and 2'-O-methyluridine); (b) inducer-treated MEL cells exhibited a decrease in the intracellular level of SAH that led to a lower ratio of SAH/SAM, an event that favors methylation; and (c) treatment of MEL cells with inhibitors of RNA methylation suppressed methylation of poly(A)- and poly(A)+ RNA, reversed the ratio SAH/SAM seen in differentiated MEL cells and prevented differentiation to occur. Moreover, we observed that treatment of MEL cells with selective inhibitors of RNA methylation caused fragmentation of beta major globin and c-myc mRNAs, two RNA transcripts coded by developmentally regulated genes, while had no detectable effect on the structural integrity of poly(A)+ RNA transcripts transcribed by two housekeeping genes (beta-actin and MER5). These data indicate that induction of erythroid cell differentiation of MEL cells is associated with changes in methylation of poly(A)+ RNA and selective differential stability of RNA transcripts, two events that might be related to each other.

Contribution of 4-methylthio-2-oxobutanoate and its transaminase to the growth of methionine-dependent cells in culture. Effect of transaminase inhibitors

The growth in culture of methionine-dependent transformed cells of human, rat and mouse origin was arrested in the absence of L-methionine (Met) but took place in the presence of 4-methylthio-2-oxobutanoic acid (MTOB), the keto acid of Met. From 24 hr after seeding, cells grew in 0.1 mM MTOB medium at a rate comparable to that in 0.1 mM Met medium. Using [35S]MTOB, it was found that the Met synthesized was used in normal MRC-5 cells and in transformed HeLa cells to the same extent for protein, adenosylmethionine and adenosylhomocysteine syntheses. However, when the free Met content was examined, it was found to be 3-fold greater in HeLa than in MRC-5 cells. To examine the importance of this free Met for the growth of transformed cells, the transaminase responsible for converting MTOB to Met was chosen as a target enzyme for the synthesis of compounds with potential inhibitory activity. Since this is a multisubstrate enzyme, reduced Schiff bases were prepared containing both pyridoxal or other aromatic groups, as one constituent, and L-Met or other amino-acids in the free acid or ester or amide form, as the other constituent. Only esters containing the pyridoxal moiety and Met or certain of its structural analogues exhibited good selective growth inhibitory activity in that there was little (20%) or no effect on the growth of normal MRC-5 and derm cells, respectively, while that of transformed HeLa, HEp-2 and L1210 cells was strongly inhibited (80%). This inhibition was accompanied by a concomitant decrease in the activity of the MTOB transaminase in both HeLa and MRC-5 cells treated with 3c the most potent inhibitor. However, using [35S]MTOB it was found that MTOB itself accumulated 48% in HeLa but only 12% in MRC-5 cells treated with 3c. On the contrary [35S]Met formed from [35S]MTOB increased 3.7-fold in MRC-5 inhibitor-treated cells showing 20% growth inhibition whereas it decreased 38% in HeLa-treated cells showing 80% growth inhibition. This decrease in cellular Met in HeLa is not responsible for growth arrest. Indeed the growth of HeLa cells could not be restored by adding a 10-fold excess of Met. Since MTOB can alleviate Met-dependence, the intracellular homeostasis of this metabolite may play a hitherto unsuspected role in controlling cell growth.

Early acinar cell changes in caerulein-induced interstitial acute pancreatitis in the rat

Early ultrastructural and immunohistochemical changes caused by supramaximal secretory stimulation with caerulein were studied in the rat pancreas. The morphological basis for the earlier reported decrease of pancreatic juice secretion after supramaximal caerulein was the appearance of swollen and irregular zymogen-like material containing structures with short segments of budding bristle-coated membranes in the apical parts of acinar cells. Images of exocytosis of zymogen granules were only few. Later, marked vacuolization and signs of autophagocytosis are seen in the basal cytoplasm. Immunohistochemistry showed that the large zymogen containing structures were intensively labelled for trypsin at the early stages of the experiment (4-30 min). Later (1-2 h), the vacuoles were empty or contained occasional, small-labelled granules only. The pancreozymin-receptor antagonist proglumide as well as cycloleucine that inhibits protein synthesis by inhibiting the synthesis of S-adenosylmethionine, effectively prevented the caerulein induced acinar cell changes. The irregular zymogen containing structures with coated pits on their surface indicate disturbed zymogen granule formation leading to the accumulation of large lakes of zymogen material and finally to marked autophagocytosis in acinar cells. The effects of caerulein are receptor-mediated and depend on the process of methylation in the formation of zymogen granules.

Relative effects of S-adenosylmethionine depletion on nucleic acid methylation and polyamine biosynthesis

Treatment of cultured L1210 cells with 1 mM-L-2-amino-4-methoxy-cis-but-3-enoic acid (L-cisAMB), a methionine-analogue inhibitor of S-adenosylmethionine (AdoMet) synthetase (EC 2.5.1.6), produced a rapid and near-total depletion of AdoMet by 4 h. After this, the pools recovered to 60% of control by 48 h, apparently because of an increase in AdoMet synthetase activity. Both AdoMet depletion and the accompanying increase in synthetase activity were substantially enhanced by lowering methionine concentrations in the media from 100 microM to 30 microM, the minimal concentration that supports cell growth at control values. During a 4 h incubation in media containing 30 microM-methionine, 1-5 mM-L-cisAMB depleted cellular AdoMet to undetectable values, and inhibited nucleic acid methylation by 44-72% and RNA methylation by 60-87%. Under these same treatment conditions, putrescine pools increased by about 3-fold, whereas spermidine pools decreased by only 20% and spermine pools remained the same. Pool changes were accompanied by a 2-4-fold increase in ornithine decarboxylase activities and AdoMet activities. Thus the rapid depletion of AdoMet pools by L-cisAMB results immediately in a decrease in methyl-transfer reactions involving nucleic acids, whereas, by contrast, biosynthesis of higher polyamines appears to be minimally affected, owing to compensatory increases in key enzyme activities.

Evidence that the methylation inhibitor cycloleucine causes accumulation of a discrete ribosomal RNA precursor in hamster mitochondria

A novel RNA fraction, 'Cy RNA,' that accumulates in mitochondria when hamster cells are treated with the methylation inhibitor cycloleucine, has been characterized by high resolution acrylamide gel electrophoresis and DNA-RNA hybridization. Cy RNA ran in gels as a discrete band, with an apparent chain length of 2 600. It hybridized specifically to restriction fragments containing genes for the mitochondrial ribosomal RNAs. We infer that Cy RNA is a discrete polycistronic ribosomal RNA precursor transcript, whose processing is dependent on normal methylation.

Synergistic growth inhibiting effect of nitrous oxide and cycloleucine in experimental rat leukaemia

Nitrous oxide (N2O) inactivates the vitamin B12-dependent enzyme methionine synthetase with subsequent impairment of folate metabolism and a reduction of cellular proliferation. Indications exist that this effect is antagonized by S-adenosylmethionine (SAM), and it was investigated whether combination with an inhibitor of SAM synthesis, cycloleucine, would result in increased inhibition of growth in rat leukaemia model (BNML). Leukaemic growth was compared in untreated rats, in rats treated with either nitrous oxide/oxygen (1:1) or cycloleucine (50 mg kg-1 i.p.), and in rats receiving both agents. Combined treatment resulted in the strongest reduction of leukaemic infiltration in spleen and liver, and this reduction often was more than the added effects of single treatments. Peripheral leukocyte counts were also lowest after combined treatment. The deoxyuridine suppression test, measuring folate-dependent de novo synthesis of thymidine, was more severely disturbed with combined treatment. Levels of vitamin B12 in plasma were reduced in rats receiving N2O, but an increase in plasma folate occurred in all treated rats. These results indicate that a reduction of SAM synthesis by cycloleucine can increase the disturbance of folate metabolism that is caused by nitrous oxide, with a potentiation of the effects on leukaemic growth.

Evidence that cycloleucine affects the high-affinity systems of amino acid uptake in cultured human fibroblasts

The influence of cycloleucine on kinetic parameters of uptake of L-alanine, L-proline and L-leucine into cultured human fibroblasts was examined under initial-rate conditions with substrate concentrations of 0.05-10 mM and 5 mM-cycloleucine. Kinetic data obtained by computer analysis showed that, in the absence of cycloleucine, cell uptake was heterogeneous for each amino acid. L-Alanine and L-leucine entered by two transport systems with different affinities; L-proline was taken up by one saturable transport system plus a diffusion-like process. This heterogeneity disappeared in the presence of cycloleucine, since the high-affinity systems were no longer detectable. The remaining process had the same kinetic constants as the low-affinity system for alanine and leucine and a KD similar to the diffusion constant for proline. The influence of cycloleucine on the amino acid uptake was not specific either to the amino acid concerned or to a particular transport system, since the three neutral amino acid-transport systems, A, ASC and L, were involved in these experiments. This influence was shown to be unaffected by the absence of Na+ (for leucine uptake). ATP content of the cells was identical in the presence or in the absence of cycloleucine.

Effect of methionine deprivation on methylation and synthesis of macromolecules

The growth of 4 tumour-cell lines (Walker rat mammary carcinoma (W-256), a mouse lymphoma (TLX5), a mouse bladder carcinoma (MB) and a human bladder carcinoma (EJ) was much reduced when methionine in the culture medium was substituted by homocysteine. In contrast, a human embryonic fibroblast line grew equally well under such conditions. Although homocysteine alone was unable to support growth of W-256 it stimulated growth at low methionine concentrations. When W-256 was cultured for 24 h in medium containing homocysteine only, the extent of methylation of nucleic acids and the acid-soluble pool of methionine were decreased. However, under such conditions there was an increased methylase activity towards both endogenous substrate and E. coli tRNA. The effect of methionine removal was to cause a large increase in the Vmax value for methylation of tRNA, without any change in the Km value towards S-adenosyl-L-methionine (SAM). For both W-256 and TLX5, methionine deprivation caused a rapid inhibition of RNA biosynthesis, followed by inhibition of DNA synthesis, while protein synthesis tended to increase. This suggests that the inability of W-256 and TLX5 to survive and grow in methionine-deficient, homocysteine-supplemented medium is not due to insufficient methionine for protein biosynthesis, but may be related to an enhanced methylating activity of some tumour-cell lines.