Amino Acid Deprivation Decreases Intracellular Levels of Reactive Oxygen Species in Hepatic Stellate Cells

Abstract

In eukaryotic cells amino acid deprivation triggers a response aimed to ensure cell survival in stress conditions. In the present work we analyzed the effects of amino acid deprivation on intracellular levels of reactive oxygen species (ROS) of hepatic stellate cells (HSC), a key cell type in the development of liver fibrosis. Histidine deprivation caused in the human immortalized HSC cell line LX-2 a fast decrease of intracellular ROS levels that was also observed in HSC incubated either with leucine-free or amino acid-free medium, but not with glucose-free medium. Phosphorylation of GCN2 kinase and its substrate eIF2α was induced by histidine deprivation. Reversion studies and activation of GCN2 by tRNA and the proteasome inhibitor MG-132 showed a correlation between GCN2 phosphorylation and diminished ROS levels. However, a lack of correlation between eIF2α phosphorylation and ROS levels was found using salubrinal, an inhibitor of eIF2α phosphorylation, suggesting a role for GCN2 unrelated to its activity as eIF2α kinase. LX-2 cells treated with histidine-free medium presented reduced SOD activity that could account for the decrease on ROS levels. Histidine deprivation as well as activation of GCN2 by treatment with tRNA, caused an increase in LX-2 cell viability, suggesting amino acid restriction to present a protective effect in HSC which is mediated by GCN2 activation.