S-Adenosylhomocysteine Inhibits NF-κB-Mediated Gene Expression in Hepatocytes and Confers Sensitivity to TNF Cytotoxicity
Chronic alcohol exposure results in liver injury that is driven in part by inflammatory cytokines such as tumor necrosis factor-α (TNF). Hepatocytes are normally resistant to the cytotoxic effects of TNF, but they become sensitized to TNF by chronic alcohol exposure. Recently, we reported that the decrease in the ratio of S-adenosylmethionine (SAM) to S-adenosylhomocysteine (SAH) that occurs with alcoholic liver injury renders hepatocytes sensitive to TNF cytotoxicity. The purpose of this study was to determine whether inhibition of the transcription factor nuclear factor-kappaB (NF-κB) contributed to TNF-induced cell death in hepatocytes with high levels of SAH.
Primary human hepatocytes or HepG2 cells were pre-incubated with a combination of adenosine plus homocysteine to increase SAH levels. Following exposure to TNF, viability was determined by the MTT assay, and activation of the NF-κB pathway was assessed by measuring degradation of cytosolic IκB-α, phosphorylation and translocation of NF-κB to the nucleus, and expression of NF-κB-dependent genes. TNF-induced apoptotic signaling pathways were assessed by monitoring levels of the anti-apoptotic protein, A20, and cleavage products of the caspase-8 substrate, RIP1.
NF-κB-mediated gene expression was inhibited in cells with high SAH, despite the fact that TNF-induced degradation of the cytoplasmic inhibitor IκB-α and accumulation of NF-κB in the nucleus persisted for much longer. In contrast to control cells, the NF-κB that accumulated in the nucleus of cells with high SAH levels was not phosphorylated at serine 536, a modification associated with activation of the transactivation potential of this transcription factor. The inhibition of transactivation by NF-κB resulted in lower mRNA and protein levels of the anti-apoptotic protein A20 and increased cleavage of RIP1.
High SAH levels inhibited NF-κB-mediated gene expression and sensitized primary hepatocytes and HepG2 cells to the cytotoxic effects of TNF. It is likely that crosstalk with other transcription factors is perturbed under these conditions, resulting in still other changes in gene expression.