Changes in tRNA methylase activity of rat kidney following administration of the carcinogen, dimethylnitrosamine

Effects of cortisol on tRNA methylase activities in rat mammary carcinoma

Mammary carcinomas induced in rats by DMBA were divided into three types: I, hard proliferating tumors; II, tumors presenting from an early stage the first signs of cystic degeneration; III, lactating tumors. In all three types, cortisol reduced the protein content by 26%-30%. The already high tRNA methyltransferase activity in type I increased by 200% after cortisol treatment. Hormonal treatment of type II increased the previously reduced control methyltransferases by 37%. In the type III lactating tumors, the total tRNA methyltransferases were inhibited by 35% after cortisol treatment. The methyltransferases of types I and II were separated chromatographically into seven analogous peaks, while the enzymes from type III presented a modified pattern. In each case, cortisol treatment affected the activities of several methyltransferases simultaneously without obvious specificity.

Comparison of urinary modified nucleosides and bases in rats with hepatomas and nephroblastomas

Hepatomas were induced in rats with aflatoxin B1, and nephroblastomas with dimethylnitrosamine. Microscopic examination of livers of aflatoxin-treated rats revealed multinodular hepatocyte hyperplasia at 8 months, and by 13 months all rats had hepatomas. Nephroblastomas were observed by 4 months and by 8 months all rats had developed them. The urinary excretion of several modified nucleosides and bases by normal rats is dependent on body weight and reflects, to a certain extent, their concentrations in tissue tRNA. Increased levels of several modified nucleosides and bases were found in all rats that had cancer. Rats with hepatomas excreted essentially the same modified nucleosides and bases as did those with nephroblastomas; the quantitative patterns of excretion were different, however, suggesting that the urinary modified nucleosides and bases may be used to differentiate between neoplasms. Although the increase in urinary modified nucleosides and bases by tumor-bearing animals results primarily from more rapid turnover of neoplastic tRNAs, the data indicate that increased turnover of mRNA and possibly rRNA may occur in neoplastic tissue. Preliminary data suggest that increases in urinary modified nucleosides and bases may occur during a precancerous stage. The urinary pattern of modified nucleosides and bases by rats with hepatomas is altered if another primary tumor is present. The results obtained from these studies support the use of modified nucleosides and bases in urine as biochemical markers of cancer.

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.