Early hypomethylation of 2''-O-ribose moieties in hepatocyte cytoplasmic ribosomal RNA underlies the protein synthetic defect produced by CCl4

Carbon tetrachloride (CCl4) treatment of rats produces an early defect in methylation of hepatocyte ribosomal RNA, which occurs concurrently with a defect in the protein synthetic capacity of isolated ribosomes. The CCl4-induced methylation defect is specific for the 2'-O-ribose position, and a corresponding proportional increase in m7G base methylation occurs in vivo. Undermethylated ribosomal subunits (rRNA) from CCl4-treated preparations can be methylated in vitro to a much greater extent than those from control preparations, and in vitro methylation restores their functional capacity. In vitro methylation of treated ribosomal subunits (which restores functional capacity) occurs at 2'-O-ribose positions (largely G residues). In contrast, in vitro methylation of control ribosomal subunits (which does not affect functional activity) represents base methylation as m7G, sites which are apparently methylated in treated preparations in vivo. Methylation/demethylation of 2'-O-ribose sites in rRNA exposed on the surface of cytoplasmic ribosomal subunits may represent an important cellular mechanism for controlling protein synthesis in quiescent hepatocytes, and it appears that CCl4 disrupts protein synthesis by inhibiting this 2'-O-ribose methylation.     

Stable knockdown of PASG enhances DNA demethylation but does not accelerate cellular senescence in TIG-7 human fibroblasts

Demethylation of 5-methylcytosine in genomic DNA is believed to be one of the mechanisms underlying replicative life-span of mammalian cells. Both proliferation associated SNF2-like gene (PASG, also termed Lsh) and DNA methyltransferase 3B (Dnmt3b) knockout mice result in embryonic genomic hypomethylation and a replicative senescent phenotype. However, it is unclear whether gradual demethylation of DNA during somatic cell division is directly involved in senescence. In this study, we retrovirally transduced TIG-7 human fibroblasts with a shRNA against PASG and compared the rate of change in DNA methylation as well as the replicative life-span to control cells under low (3%) and ambient (20%) oxygen. Expression of PASG protein was decreased by approximately 80% compared to control cells following transduction of PASG shRNA gene. The rate of cell growth was the same in both control and PASG-suppressed cells. The rate of demethylation of DNA was significantly increased in PASG-suppressed cells as compared control cells. However, decreased PASG expression did not shorten the replicative life-span of TIG-7 cells. Culture under low oxygen extended the life-span of TIG-7 cells but did not alter the rate of DNA demethylation. While knockout of PASG during development results in genomic hypomethylation and premature senescence, our results show that while down-regulation of PASG expression in a somatic cell also leads to DNA hypomethylation, there is no associated senescent phenotype. These results suggest differences in cellular consequences of hypomethylation mediated by PASG during development compared to that in somatic cells.     

Genome-wide hypomethylation in cancer may be a passive consequence of transformation

Epigenetics describes the study of stable, reversible alterations to the genome that affect gene expression and genome function, the most studied mechanisms are DNA methylation and histone modifications. Over recent years there has been rapid progress to elucidate the nature and role of the mechanisms involved in promoter hypermethylation during carcinogenesis, however, the mechanism behind one of the earliest epigenetic observations in cancer, genome-wide hypomethylation, remains unclear. Current evidence is divided between the hypotheses that hypomethylation is either an important early cancer-causing aberration or that it is a passive inconsequential side effect of carcinogenesis. With recent discoveries of gene–body methylation, fast cyclic methylation of hormone dependent genes and candidate proteins involved in DNA demethylation elucidation of the role of hypomethylation and the mechanism behind it appears ever closer. With the burgeoning use of DNA methyltransferase inhibitors as a cancer therapy there is an increased need to understand the mechanisms and importance of genome-wide hypomethylation in cancer. This review will discuss the timing and potential causes of genomic hypomethylation during carcinogenesis and will propose a way forward to understand the underlying mechanisms.     

A new mutation affecting ribosomal RNA synthesis in Escherichia coli.

A temperature-sensitive mutant of Escherichia coli is described. At the nonpermissive temperature there is a 12-fold reduction in the rate of rRNA synthesis, while tRNA and mRNA syntheses are affected to only a slight extent. Both protein and DNA syntheses also continue at nearly the normal rate. The mutation appears to affect the synthesis of 16S and 23S rRNA equally and has no detectable affect on rRNA maturation. The temperature-sensitive lesion appears to be caused by a single point mutation lying between minutes 21 and 27. It is suggested that this mutation seems to define a new genetic locus involved in the regulation of rRNA synthesis.     

Effects of nitrogen starvation on "stable" RNA synthesis in Rhodotorula gracilis cells

In exponentially growing cells of Rhodotorula gracilis, transferto a nitrogen-free medium causes the acceleration of RNA breakdownprocesses and the arrest of the synthesis of 5 s and 4 s RNA,followed by a progressive fall in the formation of mature rRNAspecies. At the same time, the accumulation of protein graduallydecreases, and the final block corresponds to the block of theaccumulation of rRNA. Labelling experiments suggest that theprogressive arrest of the formation of mature rRNA species depends,at least to a good extent, on the breakdown of the newly-synthesizedrRNA precursor due to hypomethylation of the molecule. The possiblerelationship between the level of amino acids and the synthesisof 5 s and 4 s RNA is discussed. (Received April 22, 1976; )     

DNA hypomethylation and human diseases

Changes in human DNA methylation patterns are an important feature of cancer development and progression and a potential role in other conditions such as atherosclerosis and autoimmune diseases (e.g., multiple sclerosis and lupus) is being recognised. The cancer genome is frequently characterised by hypermethylation of specific genes concurrently with an overall decrease in the level of 5 methyl cytosine. This hypomethylation of the genome largely affects the intergenic and intronic regions of the DNA, particularly repeat sequences and transposable elements, and is believed to result in chromosomal instability and increased mutation events. This review examines our understanding of the patterns of cancer-associated hypomethylation, and how recent advances in understanding of chromatin biology may help elucidate the mechanisms underlying repeat sequence demethylation. It also considers how global demethylation of repeat sequences including transposable elements and the site-specific hypomethylation of certain genes might contribute to the deleterious effects that ultimately result in the initiation and progression of cancer and other diseases. The use of hypomethylation of interspersed repeat sequences and genes as potential biomarkers in the early detection of tumors and their prognostic use in monitoring disease progression are also examined.     

Non-coordinate synthesis and methylation of tRNA following excision of plant tissue

Summary The relationship between the synthesis and methylation of nucleic acids in tissue slices from higher plant storage organs has been investigated. Although the observed nucleic acid synthesis is mainly an expression of rRNA synthesis the highest level of methylation occurs in tRNA. Unlike the synthesis and methylation of rRNA which appears completely coupled, the methylation of tRNA is not tightly coupled to its synthesis. It is suggested that a pool of undermethylated tRNA exists in the tissue prior to incubation and that methylation of this tRNA initially controls the rate of protein synthesis in the tissue slices.     

The methyl donor S-Adenosylmethionine inhibits active demethylation of DNA: a candidate novel mechanism for the pharmacological effects of S-Adenosylmethionine

S-Adenosylmethionine (AdoMet) is the methyl donor of numerous methylation reactions. The current model is that an increased concentration of AdoMet stimulates DNA methyltransferase reactions, triggering hypermethylation and protecting the genome against global hypomethylation, a hallmark of cancer. Using an assay of active demethylation in HEK 293 cells, we show that AdoMet inhibits active demethylation and expression of an ectopically methylated CMV-GFP (green fluorescent protein) plasmid in a dose-dependent manner. The inhibition of GFP expression is specific to methylated GFP; AdoMet does not inhibit an identical but unmethylated CMV-GFP plasmid. S-Adenosylhomocysteine (AdoHcy), the product of methyltransferase reactions utilizing AdoMet does not inhibit demethylation or expression of CMV-GFP. In vitro, AdoMet but not AdoHcy inhibits methylated DNA-binding protein 2/DNA demethylase as well as endogenous demethylase activity extracted from HEK 293, suggesting that AdoMet directly inhibits demethylase activity, and that the methyl residue on AdoMet is required for its interaction with demethylase. Taken together, our data support an alternative mechanism of action for AdoMet as an inhibitor of intracellular demethylase activity, which results in hypermethylation of DNA.     

Influence of pH on the rate of ribosomal ribonucleic acid synthesis during sporulation in Saccharomyces cerevisiae.

The rate of synthesis of ribosomal RNA (rRNA) is much slower during sporulation than during vegetative growth of yeast. If sporulating cells are transferred from normal incubation conditions at pH 8.8 to the same medium adjusted to pH 7.0, the rate of rRNA synthesis increased to approach that observed in vegetative cells. The response to the pH change is quite rapid, occurring within 10 min. THE PH-dependent, rate-limiting step appears to be in the processing of 35S ribosomal precursor RNA to the final 26S and 18S RNA species. A similar pH effect also was found for the rate of protein synthesis. However, no change in respiration was observed when the pH was lowered. These results indicate that the observed differences in rate of rRNA synthesis in vegetative and sporulating cells are a consequence of pH and are not intrinsic to sporulation. The results also support the correlation between rRNA processing and protein synthesis.     

Regulation of synthesis of ribosomal protein in Neurospora crassa

In Neurospora crassa during a nutritional shift-down transition of growth, the synthesis of rRNA is for about 2 h largely inhibited and the rate of protein synthesis is only partially reduced (by about 25 %). During this period the relative rate of synthesis of individual ribosomal proteins, measured irrespectively of their incorporation into ribosomes, is reduced by 70–80%. The ribosomal proteins synthesized during the shift are stable. Thus, the synthesis of ribosomal proteins appears in N. crassa to be coordinately regulated with that of rRNA.