Expression of Human Hemojuvelin (HJV) Is Tightly Regulated by Two Upstream Open Reading Frames in HJV mRNA That Respond to Iron Overload in Hepatic Cells

Abstract

The gene encoding human hemojuvelin (HJV) is one of the genes that, when mutated, can cause juvenile hemochromatosis, an early-onset inherited disorder associated with iron overload. The 5= untranslated region of the human HJV mRNA has two upstream open reading frames (uORFs), with 28 and 19 codons formed by two upstream AUGs (uAUGs) sharing the same in-frame stop codon. Here we show that these uORFs decrease the translational efficiency of the downstream main ORF in HeLa and HepG2 cells. Indeed, ribosomal access to the main AUG is conditioned by the strong uAUG context, which results in the first uORF being translated most frequently. The reach of the main ORF is then achieved by ribosomes that resume scanning after uORF translation. Furthermore, the amino acid sequences of the uORF-encoded peptides also reinforce the translational repression of the main ORF. Interestingly, when iron levels increase, translational repression is relieved specifically in hepatic cells. The upregulation of protein levels occurs along with phosphorylation of the eukaryotic initiation factor 2␣. Nevertheless, our results support a model in which the increasing recognition of the main AUG is mediated by a tissue-specific factor that promotes uORF bypass. These results support a tight HJV translational regulation involved in iron homeostasis.

Alternative transcription start site selection in Mr-OPY2 controls lifestyle transitions in the fungus Metarhizium robertsii

“…During pathogenesis, two Mr-OPY2 transcripts were produced; the abundant long transcript was inefficiently translated because its 5′UTR has two small uORFs that were proven by two independent experiments to reduce translation efficiency of the downstream major ORF. As described in other studies 29 – 31 , small uORFs can modulate ribosomal access to the AUG start codon of the major ORF, decreasing its translation efficiency. Although the short transcript of Mr-OPY2 has a lower copy number than the long one, it is more efficiently translated.…”

“…3a ). As uORFs suppress translation efficiency in many genes 29 – 31 , we investigated the effect of the uORFs on translation of the main ORF in Mr-OPY2-L . To do this, we cloned the genomic DNA fragment ( gMr-OPY2 ) that contains the two exons, the single intron and the termination region (200 bp), and mutated the AUGs to GUGs in the 5′ UTR L to generate gMr-OPY2 ΔAUGs (Fig.…”

Translation of ABCE1 Is Tightly Regulated by Upstream Open Reading Frames in Human Colorectal Cells

“…Of note, as these uORFs are overlapped in the native configuration, they seem to function in a fail-safe mechanism. A similar mechanism was demonstrated for the two AUG uORFs of the human hemojuvelin mRNA: when uORF1, the strongest inhibitory uORF, is bypassed, it seems that the scanning ribosomes initiate translation at uORF2, maintaining main ORF expression at reduced levels [ 70 ].…”

Translational effects and coding potential of an upstream open reading frame associated with DOPA Responsive Dystonia

“…The impact of uORFs on translation is nucleotide sequence dependent (Geballe and Morris, 1994; Lovett and Rogers, 1996). Mutations falling at any point along the uORF can modulate the degree of repression on the downstream physiological translation (Baek et al, 2009; Cazzola and Skoda, 2000; Kondo et al, 1998; Onofre et al, 2015; Ye et al, 2015). Indeed, interrupting the uATG surrounding sequence by inserting a myc tag at the N terminal of the uORF sequence conferred enhanced GCH1 translation.…”