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

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-alpha is implicated. The present study investigated whether and how SAM inhibited LPS-stimulated TNF-alpha expression in Kupffer cells (hepatic macrophages). SAM downregulated TNF-alpha expression in LPS-stimulated Kupffer cells at the transcriptional level as suggested by a transfection experiment with a TNF-alpha promoter-reporter gene. This inhibition was not mediated through decreased NF-kappaB binding to four putative kappaB 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-alpha 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-alpha expression at the level downstream of NF-kappaB 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-kappaB promoter activity and TNF-alpha expression by a SAM's metabolite, MTA.