5-Azacytidine induction of mouse endogenous type C virus and suppression of DNA methylation.

Abstract

5-Azacytidine was found to induce the expression of BALB:virus-1 and BALB:virus-2 from K-BALB cells and ecotropic endogenous virus from AKR2B cells. Efficiency of the induction was high and comparable to that by 5-bromodeoxyuridine. The level of methylcytosine in newly synthesized DNA was drastically decreased when K-BALB cells were treated with 5-azacytidine. There was an inverse relationship between the level of DNA modification and the frequency of virus expression.Mammalian DNA contains about 2-7% of its cytosine methylated in the 5 position (1). Methylation of DNA was proposed to play a regulatory role in gene expression of eukaryotic cells (2), although the precise mechanism involved is unknown. 5-Azacytidine (5azaCyd), a cytidine analogue with nitrogen in the 5 position, has recently been shown to induce myotube formation in mouse embryonic fibroblast culture (3) and at the same time inhibit methylation of DNA (4).All strains of laboratory mice carry DNA sequences of endogeneous type C viruses in an unexpressed form (5). Expression of the endogenous viruses can be induced by treatment of cells with various agents. These agents include halogenated pyrimidines such as 5-bromodeoxyuridine (BrdUrd) and 5-iododeoxyuridine (IdUrd) (6, 7), inhibitors of protein synthesis such as cycloheximide and puromycin (8), an amino acid analogue (9), and DNA-damaging agents (10-12). In this communication, we report that 5-azaCyd induces expression of endogenous virus as efficiently as does BrdUrd, one ofthe most potent inducers so far studied. Our data also indicate that DNA modification might be involved in the repression of endogenous virus genome in mammalian cells. MATERIALS AND METHODSCell Culture. All cultures were grown in Eagle's minimal essential medium supplemented with 10% heat-inactivated calf serum. K-BALB cells (13) X 10"l becquerels) for ['4C]thymidine, 2 pug/ml for 5-azaCyd, 30 ,ug/ml for BrdUrd, or 20 pg/ml for cycloheximide. The dish receiving ['4C]thymidine was incubated for 10 min and the cells were treated with trypsin. Appropriate numbers of the cells were filtered onto glass-fiber filters. The filters were washed with 5% trichloroacetic acid and with ethanol. Radioactivity incorporated was used as a measure ofDNA synthesis. The dishes were incubated for 5 hr with various inducers and assayed for virus production by the procedure described above.Base Analysis. Methylcytosine content in the newly synthesized DNA strand was assayed as described by others (4) with minor modifications. K-BALB cells were seeded onto 60-mm dishes at 5 X 105 cells per dish. After overnight incubation, cultures were treated with 5azaCyd in medium. containing [methyl-3H]methionine at 4 ,uCi/ml and [2-'4C]thymidine at 0.005 ,uCi/ml. DNA was collected and hydrolyzed with 60% perchloric acid as described (4). After neutralization with 6 M KOH, 100 ,ul ofthe hydrolysate was spotted onto cellulose thinlayer glass plates together with 5 ,ug each ofmethylcytosine and thymine. The plates were then developed with H2O in dimens...

Proliferation and Differentiation Requirements for the Induction of Two Retroviral Loci during B-Cell Activation

“…It has been suggested that alterations in DNA methylation might provide a means of controlling gene expression (Razin & Riggs, 1980). Consistent with this idea, an inverse relationship has been observed between the level of proviral DNA methylation and virus expression (Niwa & Sugahara, 1981 ;Groudine et al, 1981 ;Conklin et al, 1982). It should be noted, however, that studies of the oestrogen responsive chicken gene vitellogenin have suggested that oestrogen treatment results in a specific demethylation even in a tissue which does not express the vitellogenin gene (Wilks et al, 1982).…”

“…To examine the link between BXV amplification and p.f.c, inhibition further, we asked whether 5-azacytidine, a virus inducer in fibroblast cultures (Niwa & Sugahara, 1981 ;Groudine et al, 1981), was also active on lymphocytes and if so, what, if any, effect might this drug have on B-cell differentiation. The data presented in Fig.…”

Azacytidine-induced reactivation of a DNA repair gene in Chinese hamster ovary cells.

“…AC is also capable of reactivating silent genes on the inactive X chromosome (13,29), which is known to be hypermethylated at the glucose 6-phosphate dehydrogenase and hypoxanthine guanine phosphoribosyl transferase loci (45,48,50). It also activates latent retroviruses, and these events have also been shown to be associated with the loss of 5-methylcytosine (5mC) (5,14,31). All these results support the hypothesis that hypermethylation of DNA sequences can shut off transcription and that AC reactivates genes by demethylating these sequences.…”

Changes in structure and methylation pattern in a cluster of thymidine kinase genes.

“…Studies with inhibitors of DNA methylation suggest that the inhibition of DNA methylation increases the frequency of expression of methylated, inactive genes in vivo (2,4,9,20), and it has been demonstrated that methylated DNA is expressed more poorly than unmethylated DNA after transfer into mammalian cells (18,25,33,36).…”