Effects of DL-ethionine on mouse liver tRNA base composition

Small-molecule modulators of TRMT2A decrease PolyQ aggregation and PolyQ-induced cell death

Polyglutamine (polyQ) diseases are characterized by an expansion of cytosine-adenine-guanine (CAG) trinucleotide repeats encoding for an uninterrupted prolonged polyQ tract. We previously identified TRMT2A as a strong modifier of polyQ-induced toxicity in an unbiased large-scale screen in Drosophila melanogaster. This work aimed at identifying and validating pharmacological TRMT2A inhibitors as treatment opportunities for polyQ diseases in humans. Computer-aided drug discovery was implemented to identify human TRMT2A inhibitors. Additionally, the crystal structure of one protein domain, the RNA recognition motif (RRM), was determined, and Biacore experiments with the RRM were performed. The identified molecules were validated for their potency to reduce polyQ aggregation and polyQ-induced cell death in human HEK293T cells and patient derived fibroblasts. Our work provides a first step towards pharmacological inhibition of this enzyme and indicates TRMT2A as a viable drug target for polyQ diseases.

Adenosine analogs inhibit the guanine-7-methylation of mRNA cap structures

The adenosine analogs neplanocin A and deazaneplanocin A were observed to inhibit the in vivo guanine-7-methylation of mRNA cap structure using a new assay for hypomethylated RNA. Treatment of cultured mammalian cells with these adenosine analogs resulted in the same extent of hypomethylation of cap structure as did ethionine injection in mice. Neplanocin A and its non-metabolizable analog 3-deazaneplanocin A show the same maximal level of inhibition of methylation suggesting that these adenosine analogs exert their effects by elevating S-adenosylhomocysteine levels rather than by conversion to other inhibitory compounds.

Specific effects of 5-fluoropyrimidines and 5-azapyrimidines on modification of the 5 position of pyrimidines, in particular the synthesis of 5-methyluracil and 5-methylcytosine in nucleic acids

5-Fluoropyrimidines and 5-azapyrimidines were found in our laboratory to be specific inhibitors of modification reactions taking place at the 5 position of pyrimidines in nucleic acids. Thus, 5-fluorouracil and 5-fluorouridine specifically inhibit the formation of 5-methyluracil, pseudouridine, and 5,6-dihydrouracil in tRNA. 5-Fluorocytidine, which is partially biotransformed to 5-fluorouracil derivatives in mammalian cells, inhibits the formation of 5-methyluracil, pseudouridine, 5,6-dihydrouracil, and 5-methylcytosine, and 5-azacytidine is a specific inhibitor of the formation of 5-methylcytosine in tRNA and DNA. Inhibitory effects on tRNA modifications require RNA synthesis, as shown by the observation that various inhibitors of RNA synthesis block the drug effects. An inhibitory low-molecular-weight (4-7S) RNA, consisting mainly of tRNA and pre-tRNA, was isolated from livers of mice after treatment with 5-azacytidine. This RNA, when added to an in vitro tRNA methyltransferase assay, specifically interfered with the formation of 5-methylcytosine in substrate tRNA. Similarly, a DNA inhibiting the synthesis of 5-methylcytosine in an in vitro DNA methylation assay was isolated from L1210 leukemic cells treated with a high dose of 5-azacytidine for a short time. Our data are consistent with the hypothesis that incorporation of 5-azacytosine into positions that are normally occupied by C residues destined to become methylated is required for the inhibition to occur, and a similar situation probably applies to the 5-fluoropyrimidine analogs. Analog base moieties occupying such sites are likely to bind strongly, perhaps irreversibly, to the active sites of the particular modifying enzymes. All our observations with the 5-fluoro- and 5-azapyrimidines are in accord with this hypothesis. It was also observed that administration of 5-azacytidine to mice led to strong inhibition of tRNA cytosine-5-methyltransferase, while at the same time the activities and capacities of purine-specific tRNA methyltransferases became strongly elevated after an initial lag period. We speculate that such increases may represent a response of the cell to the methylation defect induced by the drug. Undermodified tRNAs present in neoplastic cells may also trigger an increased synthesis of modifying enzymes. A scheme has been presented which explains increased tRNA turnover and increased activities of modifying enzymes in neoplastic cells as a consequence of a primary defect in tRNA modification.

Ethionine alters the urinary excretion of N1-methylnicotinamide

Nicotinamide is methylated via S-adenosylmethionine (AdoMet) and excreted in the urine as N1-methylnicotinamide. Since S-adenosylethionine (AdoEth) inhibits nicotinamide methyltransferase, ethionine (0.75 mg/g body wt) was given to rats and its effect on the urinary excretion of N1-methylnicotinamide was examined. Nicotinamide (0.5 mg/g body wt) gave a 13.6-fold increase in the 24-h urine concentration of N1-methylnicotinamide. However, ethionine given 4 h prior to nicotinamide completely prevented this increase by nicotinamide. It appears at ethionine via AdoEth inhibits nicotinamide methyltransferase in vivo and lowers the urine level of N1-methylnicotinamide.

Aminoacylation of undermethylated mammalian transfer RNA

To study the role of 5-methylcytidine in the aminoacylation of mammalian tRNA, bulk tRNA specifically deficient in 5-methylcytidine was isolated from the livers of mice treated with 5-azacytidine (18 mg/kg) for 4 days. For comparison, more extensively altered tRNA was isolated from the livers of mice treated with DL-ethionine (100 mg/kg) plus adenine (48 mg/kg) for 3 days. The amino acid acceptor capacity of these tRNAs was determined by measuring the incorporation of one of eight different 14C-labeled amino acids or a mixture of 14C-labeled amino acids in homologous assays using a crude synthetase preparation isolated from untreated mice. The 5-methylcytidine-deficient tRNA incorporated each amino acid to the same extent as fully methylated tRNA. The tRNA from DL-ethionine-treated livers showed an overall decreased amino-acylation capacity for all amino acids tested. The 5-methylcytidine-deficient tRNA from DL-ethionine-treated mice were further characterized as substrates in homologous rate assays designed to determine the Km and V of the aminoacylation reaction using four individual 14C-labeled amino acids and a mixture of 14C-labeled amino acids. The Km and V of the reactions for all amino acids tested using 5-methylcytidine-deficient tRNA as substrate were essentially the same as for fully methylated tRNA. However, the Km and V were increased when liver tRNA from mice treated with DL-ethionine plus adenine was used as substrate in the rate reaction with [14C]lysine as label. Our results suggest that although extensively altered tRNA is a poorer substrate than control tRNA in both extent and rate of aminoacylation, 5-methylcytidine in mammalian tRNA is not involved in the recognition of the tRNA by the synthetase as measured by aminoacylation activity.

Inhibition of tRNA methylation in vitro and in whole cells by an oncostatic S-adenosyl-homocysteine (SAH) analogue: 5'-deoxy 5'-S-isobutyl adenosine (SIBA)

A high increase in the amount of methylated tRNA bases was found in vivo in Rous sarcoma virus infected and transformed chick embryo fibroblasts in comparison with normal cells, tRNA methylases extracted from transformed cells showed also higher activity in vitro with a heterologous substrate. 5'-deoxy-5'-S-isobutyl adenosine, (a structural analogue of S-adenosyl-L homocysteine), which inhibits virus-induced cell transformation, inhibits also the increase of incorporation of labelled methyl groups into tRNA in infected and transformed cells. When normal cells are grown in the presence of this inhibitor, undermethylated tRNAs are obtained. The effect of the drug is different in normal, infected and transformed cells. The methylation of the different bases is inhibited in vitro and in vivo to various extent. The effect of this substance on tRNA methylation may be the cause of its inhibitory effect on cell transformation.

Base composition studies on transfer RNA from normal and regenerating rat liver

The base composition of bulk tRNA isolated from regenerating rat liver, 12, 18, 24 and 30 h after partial hepatectomy, was determined by a 3H derivative method. Only a few minor statistically significant changes (2--11%), as compared to sham-operated liver, were found at 18, 24 and 30 h after hepatectomy. These included a reduction in the amounts of adenosine and 3-(3-amino-3-carboxypropyl)-uridine, and an increase in the amounts of 1-methyl-adenosine, 1-methylguanosine, 3-methylcytidine and pseudouridine. Similarly, when the base composition of tRNA fractions from control and 24-h regenerating rat liver, partially purified by one-dimensional polyacrylamide gel electrophoresis, was determined, no gross differences were observed. These results suggest that the process of liver regeneration is not accompanied by a gross alteration of the modification pattern of tRNA.

Selective inhibition of uracil tRNA methylases of E. coli by ethionine

L-ethionine has been found to inhibit uracil tRNA methylating enzymes in vitro under conditions where methylation of other tRNA bases is unaffected. No selective inhibitor for uracil tRNA methylases has been identified previously. 15 mM L-ethionine or 30 mM D,L-ethionine caused about 40% inhibition of tRNA methylation catalyzed by enzyme extracts from E. coli B or E. coli M3S (mixtures of methylases for uracil, guanine, cytosine, and adenine) but did not inhibit the activity of preparations from an E. coli mutant that lacks uracil tRNA methylase. Analysis of the 14CH3 bases in methyl-deficient E. coli tRNA after its in vitro methylation with E. coli B3 enzymes in the presence or absence of ethionine showed that ethionine inhibited 14CH3 transfer to uracil in tRNA, but did not diminish significantly the 14CH3 transfer to other tRNA bases. Under similar conditions 0.6 mM S-adenosylethionine and 0.2 mM ethylthioadenosine inhibited the overall tRNA base methylating activity of E. coli B preparations about 50% but neither of these ethionine metabolites preferentially inhibited uracil methylation. Ethionine was not competitive with S-adenosyl methionine. Uracil methylation was not inhibited by alanine, valine, or ethionine sulfoxide. It is suggested that the thymine deficiency that we found earlier in tRNA from ethionine-treated E. coli B cells, resulted from base specific inhibition by the amino acid, ethionine, of uracil tRNA methylation in vivo.

Lysine transfer RNA2 is the major target for L-ethionine in the rat

Ethionine, a hepatocarcinogen, ethylates macromolecules in vivo especially tRNA of rat liver. When rats were injected with L-[ethyl-3H]ethionine, the tRNA fraction of the liver was found to be labeled. One tRNA with the highest specific activity was purified and identified as lysine-tRNA2.

Alterations of tRNA modification in mammalian systems: the effect of ethionine

The relationship between the modification of tRNA and its ability to act as a substrate for homologous tRNA modification enzymes in vitro was studied. The tRNA extracted from the livers of rats was active as a substrate for in vitro methylation with extracts from normal rat liver 19 h after treatment with L-ethionine (35 mg/100 g/24 h). After 4 weeks of feeding a diet containing o.25% DL-ethionine, the tRNA was a poor substrate for methylation in vitro, even though it was deficient in methylated nucleosides. Only 18% and 7% of the available sites could be methylated after 67 h and 4 weeks, respectively, of ethionine treatment. 3-(3-amino-3-carboxypropyl)uridine, a nucleoside that is also synthesized from S-adenosylmethionine, was assayed in individual tRNAs by their reactivity with the N-hydroxysuccinimide ester of phenoxyacetic acid. The reactivity of tRNAIle, tRNAAsn, and tRNAThr was decreased by treatment with ethionine at 67 h as well as at 2 and 4 weeks, although no difference could be detected at 19 h.