Is there an association between an increase in c-myc RNA steady state levels and c-myc methylation in HL-60 cells treated with 3-deaza-(+/-)-aristeromycin, an indirect inhibitor of methylation?

Effects of 3-deazaadenosine on apoptosis-related gene transcripts in HL-60 cells

The effect of the transmethylation inhibitor 3-deazaadenosine on transcription levels of genes associated with apoptosis was investigated in HL-60 cells. After incubation of HL-60 cells with 100 microM 3-deazaadenosine for 45 min., a schedule known to perturb transmethylation metabolites and initiate apoptosis in these cells, a 50% decrease in c-myc and a 50% increase in bcl-2 RNA steady-state levels compared to control cells were observed. Transcription levels of c-myc continued to decrease after extended exposure to 3-deazaadenosine, while bcl-2 mRNA levels dropped to 25% and 30% below those in control cells after 1.5 hr and 3 hr, respectively. The expression levels of the bcl-2 related bax gene, showed a similar pattern as bcl-2; a 60% increase was initially measured, but after 1.5 and 3 hr, bax transcripts were 80% and 70% respectively, of those found in untreated cells. Another bcl-2 related gene, bcl-x, was previously reported to generate two transcripts in human cells. The long variant bcl-x1 acts as bcl-2, while the short form bcl-xs induces apoptosis. We were unable to detect bcl-xs transcripts in untreated and 3-deazaadenosine treated cells by the highly sensitive reverse transcriptase polymerase chain reaction method. This suggests that this gene product may not be involved in 3-deazaadenosine induced apoptosis in HL-60 cells. Bcl-x1 mRNA levels, however, slowly decreased with about 50% after 1.5 or 3 hr 3-deazaadenosine treatment. It is concluded that 3-deazaadenosine initiated apoptosis affects c-myc, bcl-2, bax and bcl-x1 mRNA levels.

Association of arsenic-induced malignant transformation with DNA hypomethylation and aberrant gene expression

Inorganic arsenic, a human carcinogen, is enzymatically methylated for detoxication, consuming S-adenosyl-methionine (SAM) in the process. The fact that DNA methyltransferases (MeTases) require this same methyl donor suggests a role for methylation in arsenic carcinogenesis. Here we test the hypothesis that arsenic-induced initiation results from DNA hypomethylation caused by continuous methyl depletion. The hypothesis was tested by first inducing transformation in a rat liver epithelial cell line by chronic exposure to low levels of arsenic, as confirmed by the development of highly aggressive, malignant tumors after inoculation of cells into Nude mice. Global DNA hypomethylation occurred concurrently with malignant transformation and in the presence of depressed levels of S-adenosyl-methionine. Arsenic-induced DNA hypomethylation was a function of dose and exposure duration, and remained constant even after withdrawal of arsenic. Hyperexpressibility of the MT gene, a gene for which expression is clearly controlled by DNA methylation, was also detected in transformed cells. Acute arsenic or arsenic at nontransforming levels did not induce global hypomethylation of DNA. Whereas transcription of DNA MeTase was elevated, the MeTase enzymatic activity was reduced with arsenic transformation. Taken together, these results indicate arsenic can act as a carcinogen by inducing DNA hypomethylation, which in turn facilitates aberrant gene expression, and they constitute a tenable theory of mechanism in arsenic carcinogenesis.

Effect of adenosine analogues on the expression of matrix metalloproteinases and their inhibitors from human dermal fibroblasts

The effect of the cytostatic and antiviral adenosine analogues 3-deazaadenosine (c3Ado) and 3-deaza-(+/-)-aristeromycin (c3Ari) on human skin fibroblasts was studied. Variables examined were cell morphology, viability, DNA fragmentation, expression of matrix metalloproteinases (MMPs) and matrix metalloproteinase inhibitors (TIMPs). None of these variables were changed when cells were exposed to c3Ari concentrations ranging from 10(-5) to 10(-3) M or 10(-5) M c3Ado. However, large changes in cell morphology, viability and expression of MMPs and MMP inhibitors occurred when fibroblasts were treated with 10(-4) or 10(-3) M c3Ado. Cells rounded up, shrank in volume, some detached and viability was lost without any detectable fragmentation of DNA. These changes in morphology and viability were associated with a differentiated expression of MMPs and MMP inhibitors. A large increase in collagenase activity occurred, and depending on the concentration of the adenosine analogue and the length of treatment, this change in activity could be shown to be due to one or a combination of the following factors: an increased synthesis of the collagenase protein, a decreased production of TIMP-1 or an increased activity of the collagenase superactivator, stromelysin. In contrast to this, treatment with c3Ado resulted in a decreased gelatinase activity, which in part could be attributed to an increased production of an inhibitor that seemed to affect gelatinase but not collagenase. The cellular changes induced by c3Ado seemed to reflect some of the alteration in the metabolic machinery that appears during a drug-induced or programmed/controlled death of a dermal cell. The different effects exerted by these two adenosine analogues on dermal fibroblasts can at least in part explain why c3Ado have previously been shown to be more toxic than c3Ari in animal models.