Translational control by hepatic eIF2‐alpha kinases: Constitutive activation/autophosphorylation of heme regulated inhibitor (HRI) in wild type (wt) mice and compensation by PERK in HRI‐knockout mice

Abstract

Phosphorylation of the eukaryotic initiation factor eIF2‐α subunit controls protein translation via any of four known eIF2α kinases. We have shown that in cultured rat hepatocytes heme‐depletion activates HRI and suppresses global hepatic protein synthesis. This effect is heme‐reversible. To determine the role of hepatic HRI in regulating protein synthesis, we examined cultured hepatocytes from human, rat, and wt or HRI‐knockout (KO) mice. Immunoblotting (IB) analyses revealed that hepatic HRI exists in all species except KO mice, albeit at lower than erythroid levels. While rat or human hepatic HRI exists largely as a 76kDa species, mouse HRI exists as a constitutively phosphorylated ≈ 92kDa species. Heme‐depletion of wt mouse hepatocytes resulted in further eIF2α phosphorylation, but this was not heme‐reversible, unlike that of rat HRI. To determine whether another eIF2α kinase contributed to this effect we examined the ER‐stress inducible PKR‐like ER kinase, PERK. Indeed, IB analyses revealed that hepatic heme depletion markedly induced both PERK and phosphoPERK content in wt mouse hepatocytes. Genetic deletion of HRI constitutively induced PERK expression and phosphorylation and led to accumulation of ubiquitinated proteins in HRI‐KO hepatocytes, thus indicating basal ER‐stress. These findings reveal that PERK may play a significant compensatory role in hepatic translational control. Supported by DK26506 and DK26743.