Regulation of protein synthesis by the heme-regulated eIF2alpha kinase: relevance to anemias

Abstract

During erythroid differentiation and maturation, it is critical that the 3 components of hemoglobin, alpha-globin, beta-globin, and heme, are made in proper stoichiometry to form stable hemoglobin. Heme-regulated translation mediated by the heme-regulated inhibitor kinase (HRI) provides one major mechanism that ensures balanced synthesis of globins and heme. HRI phosphorylates the alpha-subunit of eukaryotic translational initiation factor 2 (eLF2alpha) in heme deficiency, thereby inhibiting protein synthesis globally. In this manner, HRI serves as a feedback inhibitor of globin synthesis by sensing the intracellular concentration of heme through its heme-binding domains. HRI is essential not only for the translational regulation of globins, but also for the survival of erythroid precursors in iron deficiency. Recently, the protective function of HRI has also been demonstrated in murine models of erythropoietic protoporphyria and beta-thalassemia. In these 3 anemias, HRI is essential in determining red blood cell size, number, and hemoglobin content per cell. Translational regulation by HRI is critical to reduce excess synthesis of globin proteins or heme under nonoptimal disease states, and thus reduces the severity of these diseases. The protective role of HRI may be more common among red cell disorders.

Figure 1

HRI balances heme and globin synthesis by sensing intracellular heme concentrations. During the synthesis of hemoglobin, one molecule of heme is incorporated into each globin chain. When heme concentration is high, heme binds to the second heme-binding domain of HRI and keeps HRI in inactive state, thereby permitting globin protein synthesis and the formation of stable hemoglobin. In heme deficiency, HRI is activated by autophosphorylation. Activated HRI phosphorylates eIF2α and inhibits globin protein synthesis by the mechanism illustrated in Figure 2. HRI, therefore, acts as a feedback inhibitor of globin synthesis to ensure no globin is translated in excess of the heme available for assembly of stable hemoglobin.

Figure 2

Inhibition of the recycling of eIF2 by HRI during translational initiation. eIF2 is a heterotrimeric protein with GTP/GDP-binding site on the γ-subunit. In the initiation of translation, active eIF2-GTP binds initiating methionyl-tRNA and 40S ribosomal subunit to form 43S preinitiation complex. Upon the joining of the 60S ribosomal subunit, GTP bound to eIF2 is hydrolyzed to GDP as an energy source for the joining reaction to form 80S initiating ribosome. To recycle eIF2 for another round of initiation, it is necessary to exchange the bound GDP for GTP. This exchange reaction is catalyzed by another initiation factor eIF2B, which is present in a limiting amount. When eIF2 is phosphorylated by HRI on the Ser51 of the α-subunit, it binds to eIF2B tightly and inactivates eIF2B activity. Depletion of functional eIF2B, therefore, keeps eIF-2 strained in the inactive GDP state and inhibits protein synthesis.

Figure 3

Protein structure of HRI. HRI is divided into 5 domains as indicated. The amino acid sequence of mouse HRI is used here. Heme molecules are marked in red; S denotes the stable heme-binding site, while R denotes the reversible heme-binding site. *Histidine residues that coordinate the heme molecule.

Figure 4

HRI is responsible for the adaptation of microcytic hypochromic anemia in iron deficiency. In iron deficiency, heme concentration declines, which leads to HRI activation and inhibition of globin synthesis as illustrated in Figure 1. This results in decreased hemoglobin and total protein content in Hri^+/+ RBCs. In the absence of HRI (Hri^−/−), protein synthesis continues in the face of heme deficiency, resulting in excess globins. These heme-free globins are unstable and precipitate as inclusions (marked in green) in RBCs and their precursors, causing destruction of these cells. Hri^−/− RBCs are of normal cell size and slightly hyperchromic. However, Hri^−/− mice have decreased RBC number, reticulocytosis, and splenomegaly.

Figure 5

Proposed roles of HRI signaling pathway in stress erythropoiesis. Upon stress in erythroid cells, HRI is activated first to inhibit protein synthesis in order to reduce toxic globin precipitates. We propose that HRI also participates in the second arm of defense by increasing expression of specific genes such as Chop and GADD34, necessary for the adaptation to stress conditions. However, when the stress is insurmountable, this signaling pathway can also lead to apoptosis. In the absence of HRI, excess globins form inclusions and lead to hemolysis as well as apoptosis of erythroid precursors. Consequently, ineffective erythropoiesis occurs.