Implication of S-adenosylhomocysteine hydrolase in inhibition of TNF-α- and IL-1β-induced expression of inflammatory mediators by AICAR in RPE cells

PURPOSE. AMP-activated protein kinase ( AMPK) has been suggested to be a novel signaling pathway in regulating inflammation. The role of AMPK in retinal pigment epithelial cell inflammatory response is addressed using AMPK activator 5-aminoimidazole-4-carboxamide riboside ( AICAR). METHODS. Protein expression and activation of signaling molecules were detected by immunoblotting. Cytokines were determined by ELISA kits. AMPK alpha expression was knockdown by siRNAs. RESULTS. AICAR inhibited tumor necrosis factor (TNF)-alpha- or interleukin (IL)-1 beta-induced production of IL-6, IL-8, and monocyte chemotactic protein (MCP)-1 and of intercellular adhesion molecule ( ICAM)-1 expression in human RPE cells. The inhibitory effect on cytokine production and ICAM-1 expression persisted in the RPE cells in which AMPK was knocked down by AMPK siRNA. Moreover, an adenosine kinase inhibitor 5'-iodotubercidin, which effectively abolished AMPK activation caused by AICAR, did not reverse the anti-inflammatory effect of AICAR. In comparison, anti-inflammatory effects of AICAR were mimicked by adenosine but not inosine, the metabolites of AICAR. Finally, with the exception of TNF-alpha-induced IL-6 production, adenosine dialdehyde, an inhibitor of S-adenosylhomocysteine hydrolase, was found to block cytokine production and ICAM-1 expression. CONCLUSIONS. Regardless of the ability of AICAR to activate AMPK, the inhibitory effects of AICAR on cytokine production and ICAM-1 expression were not associated with AMPK. The mechanism of AICAR inhibition may be attributed to the interference of adenosylmethionine-dependent methylation.