Inhibition of lipopolysaccharide-stimulated TNF-α promoter activity byS-adenosylmethionine and 5′-methylthioadenosine

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Abstract
S-adenosylmethionine (SAM) is the principal biological methyl donor and precursor for polyamines. SAM is known to be hepatoprotective in many liver disease models in which TNF-α is implicated. The present study investigated whether and how SAM inhibited LPS-stimulated TNF-α expression in Kupffer cells (hepatic macrophages). SAM downregulated TNF-α expression in LPS-stimulated Kupffer cells at the transcriptional level as suggested by a transfection experiment with a TNF-α promoter-reporter gene. This inhibition was not mediated through decreased NF-κB binding to four putative κB binding elements located within the promoter. The inhibited promoter activity was neither prevented by overexpression of p65 and/or its coactivator p300 nor enhanced by overexpression of coactivator-associated arginine methyltransferase-1, an enzyme that methylates p300 and inhibits a p65-p300 interaction. SAM did not lead to DNA methylation at the most common CpG target sites in the TNF-α promoter. Moreover, 5′-methylthioadenosine (MTA), which is derived from SAM but does not serve as a methyl donor, recapitulated SAM's effect with more potency. These data demonstrate that SAM inhibits TNF-α expression at the level downstream of NF-κB binding and at the level of the promoter activity via mechanisms that do not appear to involve the limited availability of p65 or p300. Furthermore, our study is the first to demonstrate a potent inhibitory effect on NF-κB promoter activity and TNF-α expression by a SAM's metabolite, MTA.