Protein degradation and iron homeostasis

Regulation of both systemic and cellular iron homeostasis requires the capacity to sense iron levels and appropriately modify the expression of iron metabolism genes. These responses are coordinated through the efforts of several key regulatory factors including F-box and Leucine-rich Repeat Protein 5 (FBXL5), Iron Regulatory Proteins (IRPs), Hypoxia Inducible Factor (HIF), and ferroportin. Notably, the stability of each of these proteins is regulated in response to iron. Recent discoveries have greatly advanced our understanding of the molecular mechanisms governing iron-sensing and protein degradation within these pathways. It has become clear that iron's privileged roles in both enzyme catalysis and protein structure contribute to its regulation of protein stability. Moreover, these multiple pathways intersect with one another in larger regulatory networks to maintain iron homeostasis. This article is part of a Special Issue entitled: Cell Biology of Metals.     

Monothiol CGFS Glutaredoxins and BolA-like Proteins: [2Fe-2S] Binding Partners in Iron Homeostasis

Monothiol glutaredoxins (Grxs) with a signature CGFS active site and BolA-like proteins have recently emerged as novel players in iron homeostasis. Elegant genetic and biochemical studies examining the functional and physical interactions of CGFS Grxs in the fungi Saccharomyces cerevisiae and Schizosaccharomyces pombe have unveiled their essential roles in intracellular iron signaling, iron trafficking, and the maturation of Fe-S cluster proteins. Biophysical and biochemical analyses of the [2Fe-2S] bridging interaction between CGFS Grxs and a BolA-like protein in S. cerevisiae provided the first molecular-level understanding of the iron regulation mechanism in this model eukaryote and established the ubiquitous CGFS Grxs and BolA-like proteins as novel Fe-S cluster-binding regulatory partners. Parallel studies focused on Escherichia coli and human homologues for CGFS Grxs and BolA-like proteins have supported the studies in yeast and provided additional clues about their involvement in cellular iron metabolism. Herein, we review recent progress in uncovering the cellular and molecular mechanisms by which CGFS Grxs and BolA-like proteins help regulate iron metabolism in both eukaryotic and prokaryotic organisms.     

Iron metabolism

The understanding of iron metabolism at the molecular level has been enormously expanded in recent years by new findings about the functioning of transferrin, the transferrin receptor and ferritin. Other recent developments include the discovery of the hemochromatosis gene HFE, identification of previously unknown proteins involved in iron transport, divalent metal transporter 1 and stimulator of Fe transport, and expanded insights into the regulation and expression of proteins involved in iron metabolism. Interactions among principal participants in iron transport have been uncovered, although the complexity of such interactions is still incompletely understood. Correlated efforts involving techniques and concepts of crystallography, spectroscopy and molecular biology applied to cellular processes have been, and should continue to be, particularly revealing.     

Toxoplasma gondii: the model apicomplexan

Toxoplasma gondii is an obligate intracellular protozoan parasite which is a significant human and veterinary pathogen. Other members of the phylum Apicomplexa are also important pathogens including Plasmodium species (i.e. malaria), Eimeria species, Neospora, Babesia, Theileria and Cryptosporidium. Unlike most of these organisms, T. gondii is readily amenable to genetic manipulation in the laboratory. Cell biology studies are more readily performed in T. gondii due to the high efficiency of transient and stable transfection, the availability of many cell markers, and the relative ease with which the parasite can be studied using advanced microscopic techniques. Thus, for many experimental questions, T. gondii remains the best model system to study the biology of the Apicomplexa. Our understanding of the mechanisms of drug resistance, the biology of the apicoplast, and the process of host cell invasion has been advanced by studies in T. gondii. Heterologous expression of apicomplexan proteins in T. gondii has frequently facilitated further characterisation of proteins that could not be easily studied. Recent studies of Apicomplexa have been complemented by genome sequencing projects that have facilitated discovery of surprising differences in cell biology and metabolism between Apicomplexa. While results in T. gondii will not always be applicable to other Apicomplexa, T. gondii remains an important model system for understanding the biology of apicomplexan parasites.     

Control of intracellular heme levels: Heme transporters and heme oxygenases

Heme serves as a co-factor in proteins involved in fundamental biological processes including oxidative metabolism, oxygen storage and transport, signal transduction and drug metabolism. In addition, heme is important for systemic iron homeostasis in mammals. Heme has important regulatory roles in cell biology, yet excessive levels of intracellular heme are toxic; thus, mechanisms have evolved to control the acquisition, synthesis, catabolism and expulsion of cellular heme. Recently, a number of transporters of heme and heme synthesis intermediates have been described. Here we review aspects of heme metabolism and discuss our current understanding of heme transporters, with emphasis on the function of the cell-surface heme exporter, FLVCR. Knockdown of Flvcr in mice leads to both defective erythropoiesis and disturbed systemic iron homeostasis, underscoring the critical role of heme transporters in mammalian physiology.     

Mechanisms of mammalian iron homeostasis

Iron is vital for almost all organisms because of its ability to donate and accept electrons with relative ease. It serves as a cofactor for many proteins and enzymes necessary for oxygen and energy metabolism, as well as for several other essential processes. Mammalian cells utilize multiple mechanisms to acquire iron. Disruption of iron homeostasis is associated with various human diseases: iron deficiency resulting from defects in the acquisition or distribution of the metal causes anemia, whereas iron surfeit resulting from excessive iron absorption or defective utilization causes abnormal tissue iron deposition, leading to oxidative damage. Mammals utilize distinct mechanisms to regulate iron homeostasis at the systemic and cellular levels. These involve the hormone hepcidin and iron regulatory proteins, which collectively ensure iron balance. This review outlines recent advances in iron regulatory pathways as well as in mechanisms underlying intracellular iron trafficking, an important but less studied area of mammalian iron homeostasis.     

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.     

Molecular and cellular bases of iron metabolism in humans

Iron is a microelement with the most completely studied biological functions. Its wide dissemination in nature and involvement in key metabolic pathways determine the great importance of this metal for uniand multicellular organisms. The biological role of iron is characterized by its indispensability in cell respiration and various biochemical processes providing normal functioning of cells and organs of the human body. Iron also plays an important role in the generation of free radicals, which under different conditions can be useful or damaging to biomolecules and cells. In the literature, there are many reviews devoted to iron metabolism and its regulation in proand eukaryotes. Significant progress has been achieved recently in understanding molecular bases of iron metabolism. The purpose of this review is to systematize available data on mechanisms of iron assimilation, distribution, and elimination from the human body, as well as on its biological importance and on the major iron-containing proteins. The review summarizes recent ideas about iron metabolism. Special attention is paid to mechanisms of iron absorption in the small intestine and to interrelationships of cellular and extracellular pools of this metal in the human body.     

The ras oncogene--an important regulatory element in lower eucaryotic organisms.

The ras proto-oncogene in mammalian cells encodes a 21-kilodalton guanosine triphosphate (GTP)-binding protein. This gene is frequently activated in human cancer. As one approach toward understanding the mechanisms of cellular transformation by ras, the function of this gene in lower eucaryotic organisms has been studied. In the yeast Saccharomyces cerevisiae, the RAS gene products serve as essential function by regulating cyclic adenosine monophosphate metabolism. Stimulation of adenylyl cyclase is dependent not only on RAS protein complexed to GTP, but also on the CDC25 and IRA gene products, which appear to control the RAS GTP-guanosine diphosphate cycle. Although analysis of RAS biochemistry in S. cerevisiae has identified mechanisms central to RAS action, RAS regulation of adenylyl cyclase appears to be strictly limited to this particular organism. In Schizosaccharomyces pombe, Dictyostelium discoideum, and Drosophila melanogaster, ras-encoded proteins are not involved with regulation of adenylyl cyclase, similar to what is observed in mammalian cells. However, the ras gene product in these other lower eucaryotes is clearly required for appropriate responses to extracellular signals such as mating factors and chemoattractants and for normal growth and development of the organism. The identification of other GTP-binding proteins in S. cerevisiae with distinct yet essential functions underscores the fundamental importance of G-protein regulatory processes in normal cell physiology.     

Simon Labbé's work on iron and copper homeostasis

Iron and copper have a wealth of functions in biological systems, which makes them essential micronutrients for all living organisms. Defects in iron and copper homeostasis are directly responsible for diseases, and have been linked to impaired development, metabolic syndromes and fungal virulence. Consequently, it is crucial to gain a comprehensive understanding of the molecular bases of iron- and copper-dependent proteins in living systems. Simon Labbé maintains parallel programs on iron and copper homeostasis using the fission yeast Schizosaccharomyces pombe (Schiz. pombe) as a model system. The study of fission yeast transition-metal metabolism has been successful, not only in discerning the genes and pathways functioning in Schiz. pombe, but also the genes and pathways that are active in mammalian systems and for other fungi.     

Biochemistry of iron uptake

Recent information gained from genetic and biochemical studies of iron transport in yeast, coupled with the identification of specific mutations causing iron uptake disorders in mice and man, has provided new clues about the mechanisms involved in iron uptake. This article summarizes these discoveries and discusses their impact on our current understanding of the biochemistry of iron uptake.     

Systems biology of lipid metabolism: From yeast to human

Lipid metabolism is highly relevant as it plays a central role in a number of human diseases. Due to the highly interactive structure of lipid metabolism and its regulation, it is necessary to apply a holistic approach, and systems biology is therefore well suited for integrated analysis of lipid metabolism. In this paper it is demonstrated that the yeast Saccharomyces cerevisiae serves as an excellent model organism for studying the regulation of lipid metabolism in eukaryotes as most of the regulatory structures in this part of the metabolism are conserved between yeast and mammals. Hereby yeast systems biology can assist to improve our understanding of how lipid metabolism is regulated.     

"Sleeping beauty": quiescence in Saccharomyces cerevisiae.

The cells of organisms as diverse as bacteria and humans can enter stable, nonproliferating quiescent states. Quiescent cells of eukaryotic and prokaryotic microorganisms can survive for long periods without nutrients. This alternative state of cells is still poorly understood, yet much benefit is to be gained by understanding it both scientifically and with reference to human health. Here, we review our knowledge of one "model" quiescent cell population, in cultures of yeast grown to stationary phase in rich media. We outline the importance of understanding quiescence, summarize the properties of quiescent yeast cells, and clarify some definitions of the state. We propose that the processes by which a cell enters into, maintains viability in, and exits from quiescence are best viewed as an environmentally triggered cycle: the cell quiescence cycle. We synthesize what is known about the mechanisms by which yeast cells enter into quiescence, including the possible roles of the protein kinase A, TOR, protein kinase C, and Snf1p pathways. We also discuss selected mechanisms by which quiescent cells maintain viability, including metabolism, protein modification, and redox homeostasis. Finally, we outline what is known about the process by which cells exit from quiescence when nutrients again become available.     

Mechanisms of aging and energy metabolism in Caenorhabditis elegans

Aging studies on diverse species ranging from yeast to man have culminated in the delineation of several signaling pathways that influence the process of senescent decline and aging. While understanding these interlinked signal-transduction cascades is becoming even more detailed and comprehensive, the cellular and biochemical processes they impinge upon to modulate the rate of senescent decline and aging have lagged considerably behind. This fundamental question is one of the most important challenges of modern aging research and has been the focus of recent research efforts. Emerging findings provide insight into the facets of cellular metabolism which can be fine-tuned by upstream signaling events to ultimately promote longevity. Here, we survey the mechanisms regulating aging in the simple nematode worm Caenorhabditis elegans, aiming to highlight recent discoveries that shed light into the interface between aging signaling pathways and cellular energy metabolism. Our objective is to review the current understanding of the processes involved and discuss mechanisms that are likely conserved in higher organisms.     

Molecular mechanisms of iron uptake in fungi

Fungi, like all free-living organisms, are in competition for limiting nutrients. In accumulating iron, fungi are faced also with a trace metal whose aqueous and redox chemistry make it both relatively bio-unavailable and strongly cytotoxic. Successful adaptation to this environmental context has provided fungi with an iron uptake strategy that has three features: it relies on redox cycling to enhance iron bio-availability and reduce iron cytotoxicity; it includes both high- and low-affinity pathways that are mechanistically distinct; and it is autoregulating so as to maintain intracellular iron homeostasis. Using Saccharomyces cerevisiae as a paradigm, this review summarizes current knowledge about the four pathways by which this yeast accumulates iron. These four pathways include: siderophore iron accumulation; high affinity iron uptake via an iron permease; and two lower affinity uptake pathways through relatively non-specific divalent metal ion transporters. All of these four pathways are directly or indirectly dependent on the activity of metalloreductase activity expressed extracellularly on the plasma membrane. A variety of experimental and genomics data indicate that this resourcefulness is shared by many, if not most, fungi. On the other hand, while the autoregulation of iron metabolism in Baker's yeast is well-understood, little is known about the apparent homeostatic mechanisms in these other yeasts and fungi. The integration of these multiple uptake mechanisms and their regulation into over-all iron homeostasis in yeast concludes this brief review.