Alternative ferritin mRNA translation via internal initiation

Abstract

Ferritin stores and detoxifies an excess of intracellular iron, and thereby plays an important role in the metabolism of this metal. As unshielded iron promotes oxidative stress, ferritin is crucial in maintaining cellular redox balance and may also modulate cell growth, survival, and apoptosis. The expression of ferritin is controlled by transcriptional and post-transcriptional mechanisms. In light of the well-established transcriptional induction of ferritin by inflammatory signals, we examined how ferritin mRNA translation responds to stress conditions. We first used HT1080 fibrosarcoma cells engineered for coumermycin-inducible expression of PKR, a stress kinase that inhibits protein synthesis in virus-infected cells by phosphorylating eIF2a. In this setting, iron triggered partial ferritin mRNA translation despite a PKR-induced global shutdown in protein synthesis. Moreover, ironmediated ferritin synthesis was evident in cells infected with an attenuated strain of poliovirus; when eIF4GI was cleaved, eIF2a was phosphorylated, and host protein synthesis was inhibited. Under global inhibition of protein synthesis or specific inhibition of ferritin mRNA translation in cells overexpressing PKR or IRP1, respectively, we demonstrate association of ferritin mRNA with heavy polysomes. Further experiments revealed that the 59 untranslated region (59 UTR) of ferritin mRNA contains a bona fide internal ribosomal entry site (IRES). Our data are consistent with the existence of an alternative, noncanonical mechanism for ferritin mRNA translation, which may primarily operate under stress conditions to protect cells from oxidative stress.

Fe 2+ binds iron responsive element-RNA, selectively changing protein-binding affinities and regulating mRNA repression and activation

“…Increasing cellular concentrations of Fe 2þ sufficiently to bind IRE-RNA will lower RNA/IRP affinity and increase eIF4F binding, ribosome assembly, and mRNA translation. During revision, complementary, qualitative data appeared suggesting possible noncanonical translation of ferritin mRNA (39). The recent extensions of the IRE-RNA family to include mRNA encoding a cell cycle protein (40), α-hemoglobin chaperone (41) and amyloid precursor protein (11), in addition to the iron metabolic proteins (e.g., ferritin, ferroportin, and mt-aconitase), indicates that IRE-mRNA control of protein synthesis affects a number of metabolic processes in animal cells.…”

Abnormal body iron distribution and erythropoiesis in a novel mouse model with inducible gain of iron regulatory protein (IRP)-1 function

“…Translational repression of 5′IRE targets is normally achieved by IRP-mediated inhibition of the cap structure-dependent recruitment of the small ribosomal subunit to the mRNA [25]. Interestingly, lack of FTL repression in a context of high IRP1 activity has been observed before [26–29] and could possibly be explained by cap-independent translation via the internal ribosomal entry site recently found in the 5′UTR of FTL mRNA [30]. …”

Mammalian iron metabolism and its control by iron regulatory proteins

“…Increased conversion of c-acon to IRP1 by H 2 O 2 may enhance iron uptake while impairing storage and export, leading to iron overload. However a recent report suggests a mechanism to partially evade IRP action under these conditions [78]. Perhaps the physiological role of ROS involves the constant conversion of a small fraction of c-acon into IRP1.…”

“…A recent study reported that the ferritin IRE, but not the m-acon IRE directly binds ferrous iron, leading to a weakened interaction of IRP1 [109], suggesting an additional mechanism that contributes to the hierarchical regulation of IRE-containing mRNAs. There are also mechanisms to evade IRE action, such as mRNA isoforms that lack the IRE or those that contain internal ribosome entry sites [78,115]. …”