Studies of the ethylation of rat liver transfer ribonucleic acid after administration of L-ethionine

Analysis of time-related metabolic fluctuations induced by ethionine in the rat

The time-course of metabolic events following response to a model hepatotoxin ethionine (800 mg/kg) was investigated over a 7 day period in rats using high-resolution (1)H NMR spectroscopic analysis of urine and multivariate statistics. Complementary information was obtained by multivariate analysis of (1)H MAS NMR spectra of intact liver and by conventional histopathology and clinical chemistry of blood plasma. (1)H MAS NMR spectra of liver showed toxin-induced lipidosis 24 h postdose consistent with the steatosis observed by histopathology, while hypertaurinuria was suggestive of liver injury. Early biochemical changes in urine included elevation of guanidinoacetate, suggesting impaired methylation reactions. Urinary increases in 5-oxoproline and glycine suggested disruption of the gamma-glutamyl cycle. Signs of ATP depletion together with impairment of the energy metabolism were given from the decreased levels in tricarboxylic acid cycle intermediates, the appearance of ketone bodies in urine, the depletion of hepatic glucose and glycogen, and also hypoglycemia. The observed increase in nicotinuric acid in urine could be an indication of an increase in NAD catabolism, a possible consequence of ATP depletion. Effects on the gut microbiota were suggested by the observed urinary reductions in the microbial metabolites 3-/4-hydroxyphenyl propionic acid, dimethylamine, and tryptamine. At later stages of toxicity, there was evidence of kidney damage, as indicated by the tubular damage observed by histopathology, supported by increased urinary excretion of lactic acid, amino acids, and glucose. These studies have given new insights into mechanisms of ethionine-induced toxicity and show the value of multisystem level data integration in the understanding of experimental models of toxicity or disease.

Rapid changes in glucose metabolism following the administration of ethionine: its role in regulating hepatic protein synthesis

The relationship between the changes in portal glucagon, insulin, glucose, and hepatic protein synthesis were investigated during ethionine intoxication. There was a 50% decrease in blood glucose, a seven-fold increase in portal glucagon and a 90% increase in portal insulin, all of which coincided temporally with the inhibition of hepatic protein synthesis. When reversal of ethionine intoxication was initiated with adenine it simultaneously restored blood glucose, insulin, glucagon, and protein synthesis. Protein synthesis could not be adequately restored by glucose, but in this case hepatic ATP levels did not increase. In addition, glucose given by stomach tube prior to ethionine did not prevent the action of ethionine, though it did maintain plasma glucose levels and prevented the decrease in plasma insulin and increase in plasma glucagon. These results show that in vivo regulation of hepatic protein synthesis during ethionine intoxication is not likely to be mediated by portal insulin, glucose, and glucagon.

1,7-Diethylguanosine formation in tRNA chemical ethylation by ethionine and ethylnitrosourea

The mechanism of ethionine carcinogenesis and more generally the relationship between alkylation of nucleic acids by chemical carcinogens and oncogenesis still remain obscure. In the present study the rat liver tRNA ethylation by L-[ethyl-1-3H]ethionine was reinvestigated by examining in particular the highly radioactive 'pyrimidine-nucleotide-like' fraction found earlier in acid hydrolysates of hepatic tRNA from ethionine-treated rats. The following results were obtained: (1) ultraviolet-spectral and chromatographic analyses showed the presence of 1,7-diethylguanosine in this 'pyrimidine-nucleotide-like' fraction; (2) the dialkyl compound was recovered exclusively in the form of imidazole-ring-opened derivatives. When [1-14C]ethylnitrosourea was used as alkylating agent, the in vivo ethylation pattern of tRNA from various organs of rat showed an analogous radioactive 'pyrimidine-nucleotide-like' fraction as main radioactive product. On the contrary, tRNA ethylation pattern after in vitro reaction with [1-14C]ethylnitrosourea exhibited a main radioactivity peak (85% of the total radioactivity recovered) in coincidence of the chromatographic area of 1,7-diethylguanine. The 1,7-diethylguanosine moieties of tRNA were extremely labile both under physiological and alkaline conditions. The 1,7-diethylguanine-associated radioactivity was completely lost from [14C]ethyl-tRNA after only 7 h incubation at 37 degrees C and pH 7.3, while at pH 11.4 this process was preceded by the conversion of the 1,7-diethylguanosine residues into imidazole-ring-opened derivatives.

Characterization of rat liver cells transformed in culture by DL-ethionine

A rat liver-derived epithelial cell line transformed with DL-ethionine and the corresponding control cell line were characterized according to morphological and cytochemical criteria to establish their origin from liver epithelium and to identify cellular changes due to transformation by DL-ethionine. The presence of intermediate junctions confirms the epithelial nature; glycogen accumulation and glucose-6-phosphatase activity confirm the hepatic origin of the cells. Persistent alterations resulting from ethionine transformation were variations in cell shape and size, focal multilayered growth, an increase in the nucleolar:nuclear ratio, and a reduction in the number of cells displaying a primary cilium. Hyperplasia of the inner nuclear membrane, elongation and branching of mitochondria, and a reduction in the length and frequency of cell junctions were also characteristic of the transformed cells.

Methylations of adenosine residues (m6A) in pre-mRNA are important for formation of late simian virus 40 mRNAs

Cycloleucine, a competitive inhibitor of methionine transferase was used to generate in vivo partially methylated mRNA in SV40-infected BSC-1 cells. Cycloleucine at 0.5 mg/ml causes more than a 30% decrease in internal m6As of late SV40 mRNA with only minor effect on the dimethyladenosine of the 5' caps m7GpppmAm. After treatment with 2 and 5 mg/ml of cycloleucine, internal m6As were reduced by 10- and 100-fold, respectively. The inhibition of BSC-1 mRNA methylations paralleled that observed for late SV40 mRNAs. In cells exposed to 2 mg/ml cycloleucine production of late SV40 mRNA was inhibited by 80% whereas the amount of SV40 nuclear RNA was only slightly reduced. Size fractionation of SV40 nuclear RNA from cycloleucine-treated cells revealed a loss of SV40 19 S RNA with a corresponding increase of fragmented RNA sedimenting between 11 to 5 S, so that the total amount of SV40 RNA in the nucleus was almost unchanged. Analysis of viral transcription complexes from cells treated with cycloleucine indicated that SV40 transcription was not affected by cycloleucine. SV40-transformed cells, in contrast to BSC-1 cells, were able to process and transport undermethylated RNA. When transformed cells were treated with 2 mg/ml cycloleucine no changes in quantities or size of cytoplasmic and nuclear RNA were detected. The data argues for a role of internal m6A moieties in modulating the processing-linked transport of mRNA from the nucleus to the cytoplasm of nontransformed cells. Transformed cells may escape these controls due to structural alterations in their perinuclear regions.

Searches for ultimate chemical carcinogens and their reactions with cellular macromolecules

Studies on a variety of chemical carcinogens have demonstrated that their ultimate reactive and carcinogenic forms are strong electrophiles. Some carcinogens, such as alkylating agents, are in their ultimate forms as administered, but most require metabolism to these active derivatives. The ultimate carcinogens react, usually non-enzymatically, with nucleophilic constituents in vivo. Of particular interest in regard to their possible importance in carcinogenesis have been the covalent interactions of these electrophilic reactants with cellular informational macromolecules, the DNAs, RNAs, and proteins. Current data are consistent with the idea that the initiation step of chemical carcinogenesis is a mutagenic event and is caused by alteration of DNA by the ultimate carcinogens. The nature of the carcinogen metabolite(s) involved in the promotion phase has not been determined, but there appears to be no requirement that they be electrophilic. The development of the concept of ultimate chemical carcinogens as strong electrophilic reactants is reviewed, especially with respect to the studies carried in the authors' laboratory.

Effect of L-ethionine on macromolecular synthesis in mitogen-stimulated lymphocytes

2--4 mM L-ethionine completely inhibits DNA synthesis in phytohaemagglutinin- or concanavalin A-stimulated lymphocytes even though it does not prevent the morphological changes characteristic of blast formation. Evidence is presented which indicates that complete commitment to DNA synthesis as well as a substantial increase in the rates of RNA and protein synthesis can occur in the presence of ethionine. Ethionine, however, does inhibit methylation of tRNA and prevents mitogen-induced increase in the activity of histone-modifying enzymes. All of these effects of exposure to ethionine are completely reversible. Removal of ethionine after 24 h or more of exposure results in a rapid, synchronous wave of DNA synthesis, an increase in the rate of methylation of RNA and an increase in activity of histone-modifying enzymes.

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.

Effect of ethionine on the rough endoplasmic reticulum from male and female rat liver

Ethionine causes a decrease in the amount of rough endoplasmic reticulum in rat liver, the effect being greater in female than in male rats. Rough endoplasmic reticulum isolated from rat liver 24 hr after ethionine injection and stripped of its ribosomes partially lost its in vitro ribosome binding capacity. However, no differences were detected between the binding affinities of ribosomes, isolated from either untreated animals or intoxicated rats, to stripped rough membranes derived from normal rats. Structural changes occur in the rough endoplasmic reticulum of the ethionine treated rats, while the ribosomes are still bound to the membrane.

Effects of DL-ethionine on mouse liver tRNA base composition

Treatment of mice with DL-ethionine and adenine causes a reduction of all methylated bases of liver tRNA. This effect is dose-dependent and specific for the methylated bases. Individual methylated components are affected to different extents, m22G being most sensitive to inhibition.

The methylation of tRNA

The methylation of tRNA is a post-transcriptional modification which is achieved by specific enzymes, the tRNA methylases, with S adenosylmethionine as a methyl donor. The level and pattern of methylation are characteristic of the tRNA species and origin. Abnormally methylated tRNAs have been obtained, in vivo and in vitro, by a variety of methods, and their properties have been studied. The tRNA methylases are found in all cells and tissues. Their activity varies with the differentiation state of the cells, and under the influence of many internal and external factors ; it is especially elevated in embryonic and cancerous tissues. These enzymes are very unstable, and none of them has been purified to homogeneity. We present here their known properties and we propose a theory concerning their specificity. Finally, after reviewing the few available experimental data, we discuss the current hypotheses and speculations about the roles and functions of tRNA methylation.

The effect of ethionine on ribonucleic acid synthesis in rat liver

1. By 1h after administration of ethionine to the female rat the appearance of newly synthesized 18SrRNA in the cytoplasm is completely inhibited. This is not caused by inhibition of RNA synthesis, for the synthesis of the large ribosomal precursor RNA (45S) and of tRNA continues. Cleavage of 45S RNA to 32S RNA also occurs, but there was no evidence for the accumulation of mature or immature rRNA in the nucleus. 2. The effect of ethionine on the maturation of rRNA was not mimicked by an inhibitor of protein synthesis (cycloheximide) or an inhibitor of polyamine synthesis [methylglyoxal bis(guanylhydrazone)]. 3. Unlike the ethionine-induced inhibition of protein synthesis, this effect was not prevented by concurrent administration of inosine. A similar effect could be induced in HeLa cells by incubation for 1h in a medium lacking methionine. The ATP concentration in these cells was normal. From these two observations it was concluded that the effect of etionine on rRNA maturation is not caused by an ethionine-induced lack of ATP. It is suggested that ethionine, by lowering the hepatic concentration of S-adenosylmethionine, prevents methylation of the ribosomal precursor. The methylation is essential for the correct maturation of the molecule; without methylation complete degradation occurs.

Further investigation of the increased transfer ribonucleic acid methylase activity in tumours of the mouse colon

1. Extracts prepared from tumours of the mouse colon induced by 1,2-dimethylhydrazine were considerably more active in catalysing the methylation of tRNA than were extracts from normal colon. The enhanced activity was observed when both unfractionated ;methyl-deficient' tRNA and purified tRNA preparations from yeast and bacteria were used as substrates for methylation. 2. The methylated bases produced in these reactions were identified. There were no differences between the products of the reaction catalysed by extracts of tumour and normal colon. 3. The increased activity of tRNA methylases was not due to the presence in the extracts of stimulatory or inhibitory molecules of low molecular weight such as polyamines or S-adenosylhomocysteine. 4. Other enzymes concerned with tRNA metabolism (RNA polymerase, ATP-tRNA adenylyltransferase, aminoacyl-tRNA ligases) were also increased in activity in the tumour tissue. 5. The extent of methylation of a limiting amount of tRNA was greater when tumour extracts were compared with controls, but in no case was it possible to achieve a stoicheiometric methylation of the purified tRNA preparations used as substrates, and the tumour extracts were not able to methylate tRNA obtained from normal mouse colon. We conclude that the tumours contained greater activities of tRNA methylases but that there was no evidence for changes in the specificity of these enzymes during neoplastic growth.

Methyldeficient mammalian 4s RNA: evidence for L-ethionine-induced inhibition of N6-dimethyladenosine synthesis in rat liver tRNA

The nucleotide composition of 4s RNA from livers of rats fed with a diet containing 0.3% D-ethionine was found to be identical with that from untreated animals. In contrast, one single modified nucleotide was absent in 4s RNA from livers of rats fed with a 0.3% L-ethionine diet. The minor nucleo=tide was also absent in liver 4s RNA from rats fed with a 0.3% L-ethionine diet followed by ten days of normal food. It was identified after dephosphorylation by ultraviolet absorption spectra, cochromatography with authentic material and mass spectra as N(6)-dimethyladenosine. It is concluded that S-adenosylethionine, the primary product of L-ethionine in the liver, causes strong and selective inhibition of the specific RNA-methylase responsible for adenosine to N(6)-dimethyl=adenosine methylation in rat liver 4s RNA. Compared to the strong inhibition of N(6)-dimethyladenosine formation described here, L-ethionine-dependent ethylation of liver 4s RNA is far less efficient. The quantitation of l-methyladenosine, ribothymidine and 3'-terminal adenosine in this 4s RNA as well as its aminoacid acceptor activity is typical for tRNA; hence it may be concluded that N(6)-dimethyladenosine is a component of rat liver tRNA. This may demonstrate the first evidence for the existence of specifically methyl-deficient mammalian tRNA. A possible correlation between the activity of L-ethionine as a liver carcinogen and its ability to induce the formation of methyl-deficient tRNA by selectively inhibiting the synthesis of N(6)-dimethyladenosine on the tRNA level in the same organ is discussed.