Effect of 5'-methylthioadenosine on nucleolar morphology and RNA metabolism in cultured Xenopus laevis cells

Inhibition of methylation of DNA and tRNA by adenosine in an adenosine-sensitive mutant of the baby hamster kidney cell line

An adenosine-sensitive (Ados) mutant of baby hamster kidney (BHK) cells, ara-S10d, when treated with a toxic concentration of adenosine (Ado), displayed a substantial elevation of S-adenosylhomocysteine (SAH), S-adenosylmethionine (SAM), and methylthioadenosine (MTA). Wild-type BHK cells treated with the same concentration of Ado (not toxic to these parental cells) produced an elevation of SAH 1.5 times higher than that of ara-S10d cells without a concurrent elevation of SAM or MTA. Inhibition of methylation of DNA and tRNA is greater in ara-S10d cells treated with Ado than that of similarly treated wild-type cells. This inhibition was correlated with the enhanced Ado toxicity, suggesting inhibition of methylation as a possible causal factor for the great increase in Ado sensitivity. Inhibition of methylation may be due to the elevated level of MTA and not solely to the elevation of SAH, a well-known potent inhibitor of numerous methyltransferases.

Effects of sinefungin on rRNA production and methylation in the yeast Saccharomyces cerevisiae

The antifungal agent, Sinefungin (SF), has been shown to be an inhibitor of transmethylation reactions. We report here the effects of SF on the production and methylation of rRNA in the yeast, Saccharomyces cerevisiae. Under conditions of SF treatment which have been shown to affect the regulation of cell proliferation in this yeast, pulse-chase labeling experiments using [methyl-3H]methionine and [3H]uracil indicated that methyl incorporation into rRNA during a short labeling period was inhibited, and stable 18 S rRNA production was differentially decreased. Other experiments quantitating modified nucleotides in newly produced rRNA showed that stable molecules were methylated. Taken together, these results suggest that SF slows methylation of rRNA, and is associated with differential loss of undermethylated 18 S rRNA species.

Multiple mechanisms of adenosine toxicity in an adenosine sensitive mutant of baby hamster kidney (BHK) cells

A class of arabinosyladenine-resistant baby hamster kidney (BHK) cell mutants, isolated in our laboratory, shows cross-resistance to deoxyadenosine, alteration of adenosine kinase, elevation of spontaneous mutation rate, and extreme sensitivity to adenosine. One of these adenosine sensitive mutants, ara-s10d, was isolated spontaneously and studies with Ador revertants suggest the involvement of a single pleiotropic mutation. The enhanced adenosine toxicity in ara-s10d cells can be attributed to pyrimidine nucleotide starvation and to at least one other mechanism, which is associated with a 200-fold elevation of IMP, 3-5 fold elevation of ATP, GTP, S-adenosylmethionine (AdoMet) and methylthioadenosine (MeSAdo).

Effects of 5'deoxy-5'-methylthioadenosine on the metabolism of S-adenosyl methionine

The treatment of transformed rat cells with micromolar amounts of 5'deoxy 5'methyl thioadenosine induces rapid effects on the rate of methylation of DNA concomitantly with alterations of intracellular pools of S-adenosyl methionine and S-adenosyl homocysteine. Pulse chase labelling experiments indicate that 5'deoxy 5'methylthioadenosine does not inhibit the degradation of S-adenosyl homocysteine but inhibits the consumption of S-adenosyl methionine. In vitro transmethylation assays performed with heterologous DNA show that low doses of the thioethernucleoside do not significantly affect the DNA methyltransferase activity of cellular extracts. The biological role of 5'deoxy 5'methylthioadenosine, a natural molecule formed during the synthesis of polyamines is discussed.

Inhibition of the synthesis of polyamines and macromolecules by 5'-methylthioadenosine and 5'-alkylthiotubercidins in BHK21 cells

5'-Methylthioadenosine and four 5'-alkylthiotubercidins were tested for their ability to inhibit polyamine synthesis in vitro and to decrease polyamine concentration and prevent growth of baby-hamster-kidney (BHK21) cells. 5'-Methylthioadenosine and 5'-methylthiotubercidin decreased the activity of spermidine synthase from brain to roughly the same extent, whereas brain spermine synthase was much more strongly inhibited by 5'-methylthioadenosine compared with 5'-methylthiotubercidin. These nucleoside derivatives also inhibited the growth of BHK21 cells and increased the concentration of putrescine. 5'-Methylthioadenosine decreased cellular spermine concentration, whereas 5'-methylthiotubercidin lowered the concentration of spermidine. The activities of ornithine decarboxylase and S-adenosylmethionine decarboxylase were enhanced in cells grown in the presence of 5'-methylthiotubercidin. The growth inhibition produced by these nucleoside derivatives was not reversed by exogenous spermidine or spermine. 5'-Ethylthiotubercidin, 5'-propylthiotubercidin and 5'-isopropylthiotubercidin did not appreciably inhibit spermidine or spermine synthase in vitro or decrease the cellular polyamine content, but effectively prevented the growth of BHK21 cells. All nucleoside derivatives at concentrations of 0.2-1 mm caused a rapid inhibition of protein synthesis. It is concluded that the growth inhibition produced by 5'-methylthioadenosine and 5'-alkylthiotubercidins was not primarily due to polyamine depletion but other target sites, for instance the cellular nucleotide pool, cell membranes etc. must be considered.