Heme-Regulated eIF2α Kinase (HRI) Inhibition Decreases PKR Activation in HUDEP2 Cells

Introduction: Sickle Cell Disease (SCD) is a group of inherited disorders caused by mutations in the β-globin gene which encodes the hemoglobin subunit β in erythrocytes [1]. Hemoglobin containing the mutant β-globin polymerizes and causes sickling of erythrocytes, which subsequently leads to vaso-occlusion, hemolysis, and activation of the immune system by release of free heme [2]. Heme-regulated eIF2α kinase, also known as heme-regulated inhibitor (HRI/EIF2AK1), the dsRNA-activated kinase Protein Kinase R (PKR/EIF2AK2), PKR-like endoplasmic reticulum kinase (PERK/EIF2AK3) and General Control Non-Depressible 2 (GCN2/EIF2AK4) are four kinases belonging to the eIF2α kinases family and play key functions in the Integrated Stress Response-ATF4 pathway, which is critical for translational control in response to various stress conditions [3]. These kinases are activated respectively by heme depletion, viral infection, endoplasmic reticulum stress, and amino acid starvation, and they phosphorylate eukaryotic initiation factor-2α (eIF2α). Recently, it has been shown that HRI inhibition induces fetal hemoglobin in HUDEP-2 cells and CD34+ hematopoietic progenitor stem cells and prevents sickling, suggesting HRI as a potential therapeutic target for SCD [4, 5]. Moreover, it has been well documented that kinase inhibition can activate compensatory loops (bypass signaling) to circumvent the inhibited target, in particular by overexpression and activation of kinases having the same substrate [6, 7]. Thus, in this study, we investigated if the inhibition of HRI in HUDEP-2 cells lead to compensatory mechanisms by modulation of the expression and activation of the other eIF2α kinases.