Translational control by iron-responsive elements

Transcriptional Regulation of Gene Expression by Metalloproteins

FNR and SoxR are transcriptional regulators containing an iron–sulfur cluster. The iron–sulfur cluster in FNR acts as an oxygen sensor by reacting with oxygen. The structural change of the iron–sulfur cluster takes place when FNR senses oxygen, which regulates the transcriptional regulator activity of FNR through the change of the quaternary structure. SoxR contains the [2Fe–2S] cluster that regulates the transcriptional activator activity of SoxR. Only the oxidized SoxR containing the [2Fe–2S]2+ cluster is active as the transcriptional activator. CooA is a transcriptional activator containing a protoheme that acts as a CO sensor. CO is a physiological effector of CooA and regulates the transcriptional activator activity of CooA. In this review, the biochemical and biophysical properties of FNR, SoxR, and CooA are described.

Out of balance--systemic iron homeostasis in iron-related disorders

Iron is an essential element in our daily diet. Most iron is required for the de novo synthesis of red blood cells, where it plays a critical role in oxygen binding to hemoglobin. Thus, iron deficiency causes anemia, a major public health burden worldwide. On the other extreme, iron accumulation in critical organs such as liver, heart, and pancreas causes organ dysfunction due to the generation of oxidative stress. Therefore, systemic iron levels must be tightly balanced. Here we focus on the regulatory role of the hepcidin/ferroportin circuitry as the major regulator of systemic iron homeostasis. We discuss how regulatory cues (e.g., iron, inflammation, or hypoxia) affect the hepcidin response and how impairment of the hepcidin/ferroportin regulatory system causes disorders of iron metabolism.

Translational regulation of BACE-1 expression in neuronal and non-neuronal cells

As the main beta-secretase of the central nervous system, BACE-1 is a key protein in the pathogenesis of Alzheimer's disease. Excessive expression of the protein might cause an overproduction of the neurotoxic beta-amyloid peptide. Therefore, a tight regulation of BACE-1 expression is expected in vivo. In addition to a possible transcriptional control, the BACE-1 transcript leader contains features that might constitute mechanisms of translational regulation of protein expression. Moreover, recent work has revealed an increase of BACE-1 protein and beta-secretase activity in some Alzheimer's disease patients, although a corresponding increase of transcript has not been reported. Here we show that BACE-1 translation could be modulated at multiple stages. The presence of several upstream ATGs strongly reduces the translation of the main open reading frame. This inhibition could be overcome with conditions that favour skipping of upstream ATGs. We also report an alternative splicing of the BACE-1 transcript leader that reduces the number of upstream ATGs. Finally, we show that translation driven by the BACE-1 transcript leader is increased in activated astrocytes independently of the splicing event, indicating yet another mechanism of translational control. Our findings might explain why increases in BACE-1 protein or activity are reported in the brain of Alzheimer's disease patients even in the absence of changes in transcript levels.

Translational control of Scamper expression via a cell-specific internal ribosome entry site

The mRNA of Scamper, a putative intracellular calcium channel activated by sphingosylphosphocholine, contains a long 5' transcript leader with several upstream AUGs. In this work we have investigated the role this sequence plays in the translational control of Scamper expression. The cytosolic transcription machinery of a T7 RNA polymerase recombinant vaccinia virus was used to avoid artifacts arising from cryptic promoters or mRNA processing. Based on transient transfection experiments of dicistronic and bi-monocistronic plasmids expressing reporter genes, we present evidence that the 5' transcript leader of Scamper contains a functional internal ribosome entry site (IRES). Our data indicate that Scamper translation in Madin-Darby canine kidney cells is driven by a cap-independent mechanism supported by the IRES activity of its mRNA. Finally, the Scamper IRES appears to be the first IRES with specificity for kidney epithelial cells.

The use of antioxidants in Friedreich's ataxia treatment

Friedreich's ataxia is the most common recessive ataxia associated with life-threatening cardiomyopathy. It results from a loss of function of frataxin that ultimately leads to oxidative insult, particularly to neurons and cardiomyocytes. The disease is progressive, the oxidative insult being presumably subsequent to an abnormal iron/sulfur cluster synthesis that causes mitochondrial respiratory chain disease and impaired signalling of one antioxidant pathway. After a detailed in vitro study, idebenone, a short chain homologue of coenzyme Q(10) with potent antioxidant properties, was given to patients. The antioxidant had a dramatic and rapid effect on the cardiomyopathy in most patients. Although a subset of patients also report various improvements, implying that idebenone could have a broader spectrum of action including some neurological improvements, the antioxidant did not have noticeable effects on the ataxia. Several hypotheses on the mechanisms that could account for the contrasting effects of the antioxidant on clinical symptoms of Friedreich's ataxia are discussed in this review. The considerable difficulties still being encountered in ascertaining the effect of antioxidants on the course of the neurological condition are also considered.

The roles of iron in health and disease

Iron is vital for almost all living organisms by participating in a wide variety of metabolic processes, including oxygen transport, DNA synthesis, and electron transport. However, iron concentrations in body tissues must be tightly regulated because excessive iron leads to tissue damage, as a result of formation of free radicals. Disorders of iron metabolism are among the most common diseases of humans and encompass a broad spectrum of diseases with diverse clinical manifestations, ranging from anemia to iron overload and, possibly, to neurodegenerative diseases. The molecular understanding of iron regulation in the body is critical in identifying the underlying causes for each disease and in providing proper diagnosis and treatments. Recent advances in genetics, molecular biology and biochemistry of iron metabolism have assisted in elucidating the molecular mechanisms of iron homeostasis. The coordinate control of iron uptake and storage is tightly regulated by the feedback system of iron responsive element-containing gene products and iron regulatory proteins that modulate the expression levels of the genes involved in iron metabolism. Recent identification and characterization of the hemochromatosis protein HFE, the iron importer Nramp2, the iron exporter ferroportin1, and the second transferrin-binding and -transport protein transferrin receptor 2, have demonstrated their important roles in maintaining body's iron homeostasis. Functional studies of these gene products have expanded our knowledge at the molecular level about the pathways of iron metabolism and have provided valuable insight into the defects of iron metabolism disorders. In addition, a variety of animal models have implemented the identification of many genetic defects that lead to abnormal iron homeostasis and have provided crucial clinical information about the pathophysiology of iron disorders. In this review, we discuss the latest progress in studies of iron metabolism and our current understanding of the molecular mechanisms of iron absorption, transport, utilization, and storage. Finally, we will discuss the clinical presentations of iron metabolism disorders, including secondary iron disorders that are either associated with or the result of abnormal iron accumulation.

Iron is a regulatory component of human IL-1beta production. Support for regional variability in the lung

The human lung accumulates iron with senescence. Smoking escalates the accumulation of iron, and we have demonstrated regional variability in the accumulation of iron in smokers' lungs. Iron has been reported to influence the production of a number of proinflammatory mediators, including human interleukin (IL)-1beta. We postulated that we could (1) demonstrate regional differences in the release of IL-1beta from human alveolar macrophages and (2) influence the production of IL-1beta in human macrophages by altering intracellular iron concentrations. To test these hypotheses, alveolar macrophages were obtained by independent lavage of the upper and lower lobes of healthy volunteers (both smokers and nonsmokers), after which the ability of each population to secrete IL-1beta was quantified, together with their ability to produce tumor necrosis factor-alpha, IL-6, and IL-8. Additionally, we established an in vitro model of "iron-loaded" cells of the human myelomonocytic cell line THP-1 in order to examine more directly the effect of iron and its chelation on the secretion of IL-1beta. We report here that an intracellular, chelatable pool of iron expands with exogenous iron-loading as well as with lipopolysaccharide (LPS) stimulation and appears to suppress transcription of IL-1beta, whereas shrinkage of this pool by early chelation augments transcription of IL-1beta beyond that induced by LPS alone. And finally, we demonstrate a regional relationship in the lung between excess alveolar iron and the production of human alveolar macrophage-derived IL-1beta, suggesting a partnership between iron and inflammation that may have clinical significance, especially in relation to lung diseases with a regional predominance.

Haem precursor delta-aminolaevulinic acid induces activation of the cytosolic iron regulatory protein 1

Control of cellular iron homoeostasis is performed by iron regulatory protein 1 (IRP1) through post-transcriptional modifications. This protein is sensitive to intracellular iron availability, being activated at low iron levels and inactivated at high iron levels, conditions that signal the increased expression of the transferrin receptor or of ferritin respectively. IRP1 is known to be activated by some oxidants such as H2O2 and NO. delta-Aminolaevulinic acid (ALA), previously found to produce reactive oxygen species and a carbon-centred radical, to release iron from ferritin, and to increase rat liver and brain non-haem iron and ferritin, was investigated for its effects on IRP1 activity in cultured hamster pulmonary fibroblasts. We have found that 1-2 mM ALA produced a 2-3-fold activation of IRP. On incubation with 1-4 mM succinylacetone methyl ester, a potent ALA dehydratase inhibitor, a 3-4-fold activation of the protein was observed, accompanied by a 40% increase in the intracellular ALA concentration. When cells were incubated in the presence of ALA or succinylacetone methyl ester, N-acetylcysteine inhibited IRP1 activation, suggesting that the observed effect is mediated by an oxidative process. We surmise that ALA-induced IRP1 activation might act as a co-sensor of iron homoeostasis.

Location of the internal ribosome entry site in the 5' non-coding region of the immunoglobulin heavy-chain binding protein (BiP) mRNA: evidence for specific RNA-protein interactions

The 220 nucleotide 5'non-coding region (5'NCR) of the human immunoglobulin heavy chain binding protein (BiP) mRNA contains an internal ribosome entry site (IRES) that mediates the translation of the second cistron in a dicistronic mRNA in cultured mammalian cells. In this study, experiments are presented that locate the IRES immediately upstream of the start-site AUG codon in the BiP mRNA. Furthermore, crosslinking of thiouridine-labeled BiP IRES-containing RNA to cellular proteins identified the specific binding of two proteins, p60 and p95, to the 3'half of the BiP 5'NCR. Interestingly, both p60 and p95 bound also specifically to several viral IRES elements. This correlation suggests that p60 and p95 could have roles in internal initiation of cellular and viral IRES elements.