Hepatic HRI eIF2α kinase‐mediated translational suppression activates Nuclear Factor Kappa B in response to heme‐deficiency stress

Abstract

Nuclear factor kappa B (NF‐κB), a key proinflammatory factor involved in inflammation, cancer, apoptosis and immunity, is activated in response to diverse cellular/environmental stresses including ER stress and UV irradiation that activate PERK and GCN2 eIF2α kinases, respectively. We have found that inhibition of heme synthesis in cultured mouse hepatocytes results in acute heme depletion and consequent autoactivation of heme‐regulated inhibitor HRI eIF2α kinase, an exquisite heme‐sensor. The 2‐fold increased eIF2α phosphorylation results in global translational arrest of de novo synthesis of hepatic proteins including IκBα (>5‐fold) that was significantly reversed by heme repletion, but not the proteasome inhibitor MG132. This translational suppression of hepatic IκBα resulted in the nuclear activation of NF‐κB (p65/p50 heterodimer) by >7 fold, as detected by Electrophoretic Mobility Shift Assay (EMSA). To determine the precise role of HRI in IκBα translational suppression, we used HepG2 cells overexpressing HA‐tagged HRI protein. Upon acute heme depletion, this HRI protein was activated with consequently enhanced eIF2α phosphorylation as well as significantly decreased IκBα levels relative to controls. Collectively, our data suggest a plausible role for hepatic HRI in the activation of hepatic NF‐κB inflammatory pathway under heme‐deficiency stress. Supported by NIH grants DK26506, DK26743.