Replacement of 5-methylcytosine by cytosine: a possible mechanism for transient DNA demethylation during differentiation.

In an earlier study it was discovered that when Friend erythroleukemia cells (FELC) were exposed to a variety of chemical agents capable of inducing differentiation, their DNA underwent genome-wide transient demethylation. In an attempt to elucidate the biochemical mechanism responsible for this phenomenon we have induced FELC with 5 mM hexamethylenebisacetamide and labeled the DNA in vivo with a density label, 5-bromodeoxyuridine, and a radioactive label, deoxy[5-3H]cytidine. Newly replicated DNA (heavy-light) was separated from parental DNA (light-light) by isopycnic centrifugation. Incorporation of deoxy[5-3H]cytidine into light-light duplex DNA has been observed only in induced cells concomitantly with the demethylation of the DNA, whereas, in parallel experiments, deoxy[G-3H]adenosine was not incorporated into light-light DNA. It was also found that the labeling of light-light DNA with deoxy[5-3H]cytidine is transient since the 3H label was removed from the DNA during the period of de novo DNA methylation that follows the demethylation. These results, taken together, strongly suggest that the demethylation of the DNA during differentiation is achieved by an enzymatic mechanism whereby 5-methylcytosine is replaced by cytosine

Methionine metabolism in an animal model of sepsis

Background: Sepsis is a disease with high incidence and lethality and is accompanied by profound metabolic disturbances. In mammalian methionine metabolism, S-adenosylmethionine (SAM) is produced, which is important in the synthesis of neurotransmitters and glutathione and as an anti-inflammatory agent. The degradation product and antagonist of SAM is S-adenosylhomocysteine (SAH). In this study, we investigated changes in methionine metabolism in a rodent model of sepsis. Methods: Sepsis was induced in male Wistar rats (n=21) by intraperitoneal injection of bacterial lipopolysaccharide (10 mg/kg). Controls (n=18) received vehicle only. Blood was collected by cardiac puncture 24 h later. Puncture of the suboccipital fossa was performed to collect cerebrospinal fluid (CSF). Methionine metabolites were measured using stable isotope dilution tandem mass spectrometry. Plasma total homocysteine and cysteine were measured by HPLC using fluorescence detection. Glutathione was assayed using a modified enzymatic microtiter plate assay. Results: We observed significantly higher plasma levels of SAM (p<0.001) and SAM/SAH ratio (p=0.004) in septic animals. In CSF, there was also a trend for higher levels of SAM in septic animals (p=0.067). Oxidative stress was reflected by an increase in the ratio of oxidized/reduced glutathione in septic animals (p=0.001). Conclusions: Sepsis is associated with an increase in SAM/SAH ratio in plasma and CSF in rodents. This indicates an altered methylation potential during sepsis, which may be relevant for sepsis-associated impairment of transmethylation reactions, circulation and defense against oxidative stress. If verified in humans, such findings could lead to novel strategies for supportive treatment of sepsis, as methionine metabolism can easily be manipulated by dietary strategies