Repression of ferritin light chain translation by human eIF3

Abstract

16Iron is essential to life, but excess iron is detrimental as it catalyzes reactive hydroxyl radical 17 production. To prevent iron toxicity, intracellular iron homeostasis is regulated by ferritin, a 18 protein complex composed of a mixture of two subunits: ferritin light chain (FTL) and ferritin 19 heavy chain (FTH). Ferritin expression is regulated post-transcriptionally by the iron response 20 proteins (IRPs), which bind an RNA hairpin -the iron responsive element (IRE) -located in 21 the FTL and FTH mRNA 5ʹ-untranslated region (5ʹ-UTR). Here, we show FTL translation is also 22 regulated in an IRP-independent manner by eukaryotic translation initiation factor 3 (eIF3). We 23 find that eIF3 represses FTL translation by interacting with a region of the 5ʹ-UTR immediately 24 downstream of the IRE. Furthermore, we find that loss of eIF3-mediated regulation of FTL 25 translation likely contributes to a subset of clinically-observed hyperferritinemia cases. 26 Introduction 27Iron is essential for a spectrum of metabolic pathways and cellular growth. However, if 28 not properly managed, excess iron catalyzes the production of harmful radicals that lead to lipid 29 peroxidation, protein and DNA damage, and ultimately cell death (Palmer, Roberts, and Bero 30 1994). To safeguard against these damaging effects, cells ubiquitously express ferritin, an iron-31 sequestering protein complex. Ferritin is composed of a dynamic mixture of 24 individual 32 subunits of two types, the ferritin heavy chain (FTH) and the ferritin light chain (FTL), which 33 assemble into a hollow shell that encapsulates and stores up to 4,500 Fe 3+ atoms (Harrison and 34 Arosio 1996) (Anderson and McLaren 2012). Though the two subunits that make up this 35 complex are structurally similar, they are not functionally interchangeable (Harrison and Arosio 36 1996). The FTH subunit catalyzes the oxidation of Fe 2+ to Fe 3+ , while the FTL subunit lacks 37 catalytic activity and instead serves as a nucleation site for mineralization, increases complex 38All rights reserved. No reuse allowed without permission.was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. ribosomal subunit recruitment and consequently inhibits translation. This inhibition prevents the 50 presence of excess ferritin and ensures sufficient levels of accessible iron. In response to an 51 increase in the labile iron pool (LIP) or under high iron conditions, IRP1 is loaded with an iron 52 sulfur cluster produced by mitochondria that prevents IRE binding, and IRP2 is targeted for 53 proteasomal degradation (Fig. 1A,B) (Wilkinson and Pantopoulos 2014). These processes 54 relieve the transcript from IRP/IRE generated steric hindrances, allowing for translation to 55 proceed and for the excess iron to be deposited into newly formed ferritin complexes. IRP-IRE 56 interactions are known to be disrupted in clinically relevant cases of hyperferritinemia, in which 57 single nucleotide mutations throughout the IRE ...

In Vivo RNAi-Mediated eIF3m Knockdown Affects Ribosome Biogenesis and Transcription but Has Limited Impact on mRNA-Specific Translation

“…Recently, it has been shown that eIF3 acts as a repressor of Ftl1 translation in vitro. 33 Our results confirm the link between eIF3 and Ftl1 and, for the first time, provide evidence for the repression of Ftl1 translation by eIF3 in vivo in the liver.…”

eIF3 interacts with histone H4 messenger RNA to regulate its translation

“…eIF3 interacts with a stem-loop structure in the coding sequence of H4 mRNA It was previously established that human translation initiation factor eIF3 can target mRNAs in a transcriptspecific manner and can function as an activator or repressor of translation (19)(20)(21). The majority of the mRNAs identified contain a single eIF3-binding site predominantly located within 5'UTR RNA structural elements (19).…”

“…The translation of several additional mRNAs is repressed by eIF3. This is the case for the cell proliferation regulator BTG1 (19), ferritin light chain (FTL) mRNA (21), and msl-2 mRNA in Drosophila melanogaster (66). How eIF3 binding can contribute to the negative translation regulation of an mRNA and modulation of histone translational regulation in particular is unclear at this stage.…”

“…The initiation factor eIF3 is capable of selectively recruiting and controlling the expression of several cellular mRNAs by binding to specific stem loops (19)(20)(21). This regulation occurs primarily through interactions with 5'UTR structural elements, but the role of eIF3 in regulation is not yet clearly established (19,22), nor is the mechanism by which eIF3 selects its mRNA targets.…”

Eukaryotic initiation factor EIF-3.G augments mRNA translation efficiency to regulate neuronal activity

“…Recent work has begun to reveal that different eIF3 subunits can selectively regulate translation in a manner depending on cell type, mRNA targets, and post-translational modification. Interaction of eIF3 RNA-binding subunits with specific 5′UTR stem-loop structures of mRNAs can trigger a translational switch for cell proliferation in human 293T cells (Lee et al 2015), and can also act as a translational repressor, such as the case for human Ferritin mRNA (Pulos-Holmes et al 2019). Under cellular stress, such as heat shock, the eIF3 complex circumvents cap-dependent protein translation initiation and recruits ribosomes directly to m6A marks within the 5′UTR of mRNAs encoding stress response proteins (Meyer et al 2015).…”