Regulation of Alpha Hemoglobin Stabilizing Protein (AHSP) mRNA Stability by a 3′ UTR Iron Response Element.

Abstract

AHSP is an erythroid protein that binds the alpha globin subunit of hemoglobin (Hb) to maintain its structure and limit its oxidative activity. Biochemical and genetic studies demonstrate that AHSP is essential for normal Hb homeostasis and may act via at least two mechanisms: First, as a molecular chaperone to promote the folding and stability of alpha globin prior to its incorporation into HbA (alpha2beta2). Second, to bind and detoxify excess alpha globin that accumulates in normal erythroid precursors, and to a larger extent, in beta thalassemic ones. The existence of a potential iron responsive element (IRE) in the 3′-UTR of the AHSP mRNA raises the interesting possibility that iron homeostasis impacts on alpha globin stability via AHSP. IREs form stem-loop structures that bind cytosolic iron-sensing proteins (IRPs). Typically, IRP binding to the 3′ UTR stabilizes mRNA. Association with iron releases the IRP, enhancing mRNA degradation. Computational algorithms identified IRE-like stem-loop structures in AHSP mRNA of multiple species, yet the primary sequences deviate significantly from canonical IRE consensus sequences determined by studies of classical IREs, such as Transferrin receptor and Ferritin. Several lines of evidence now show that the AHSP IRE binds IRPs to regulate mRNA stability in an iron-dependent fashion:1) in vitro gelshift competition assays demonstrate that human and mouse AHSP IREs bind tightly to IRPs.2) AHSP mRNA co-immunoprecipitates with IRPs in erythroid cells in a fashion that depends on an intact IRE.3) AHSP mRNA is destabilized by iron in both erythroid and heterologous cells; disruption of the IRE renders the mRNA constitutively unstable.To study how iron regulates AHSP expression in vivo, we treated mice with iron dextran for 10 days and then examined AHSP mRNA in Ter119+ erythroid progenitors by RT-PCR. We found that short-term iron overload reduced AHSP mRNA levels by about 50% (p > 0.05). Our findings indicate that AHSP mRNA stability is regulated by iron via an atypical 3′-UTR IRE. These findings extend the potential repertoire for functional IREs that do not conform to previously defined canonical consensus sequences. In addition, they provide a potential mechanism by which erythroid cells can regulate globin stability according to iron status. For example, induction of AHSP during iron deficiency might stabilize apo- alpha globin that could accumulate from lack of heme. Conversely, iron overload could destabilize alpha globin by reducing AHSP levels. As such, iron overload, which occurs in patients with beta thalassemia, might aggravate the disease by further elevating the levels of toxic free alpha globin.