Ebook: Zinc in Human Health

A little more than ten years ago, I travelled from Pittsburgh to the Cayman Islands to attend a “Zinc Signals” conference organized by Chris Frederickson. Chris had invited me, then a relative newcomer to the field, to present the work Ian Reynolds and I had recently published on the role of intracellular zinc release in neuronal cell death, to a gathering of approximately thirty or forty scientists that worked primarily on the biology of zinc. That meeting was a true eye-opener for me. For nearly a week, I was surrounded by chemists, immunologists, yeast biologists, and countless other specialists, all brought together by their devotion to the biology of zinc – all wonderfully willing to share their insights, reagents, and a good time snorkeling in the reef known as Stingray City. I made many new friends there, and, importantly, established several new collaborations that, to this day, continue to evolve as well as enrich my scientific life in countless ways. It was in Cayman I became an official zinc scientist.

In 2008, with Glen Andrews leading the way, we established the International Society for Zinc Biology, over which I have the honor of currently presiding. After a highly successful gathering that year of approximately 140 zinc scientists in Banff, Canada, and again in Jerusalem, Israel nearly two years later, we are now in the midst of preparing for our third official conference as a society, to be held in Melbourne, Australia in January 2012.

And now this very timely book! My colleague Lothar Rink has assembled a rich collection of chapters that represent the very essence of the field, a snapshot of the brilliant convergence of medicine and basic science in the study of zinc in human health and, importantly, disease. Leaders of their respective fields have made insightful and thorough contributions to this book, covering topics that range from human nutrition to Alzheimer's disease, from pregnancy to aging, from the digestive system to the brain. To the “zincophile”, the book will offer an intellectually rewarding, scholarly collection of the most important components that constitute the field of zinc biology as it currently stands. To the newcomer, this book represents an opportunity to learn what the field has been up to since Ananda Prasad firmly established zinc as an essential nutrient in humans more than fifty years ago. However, be forewarned, this is a fast-moving field that has finally reached a critical mass of able and productive scientists. The future of zinc research is now here. Enjoy the ride, and as always, think zinc!

Elias Aizenman, Ph.D.

President, International Society for Zinc Biology

Pittsburgh, PA, U.S.A. March 2011

Zinc is an essential trace element. The human body contains 2-3 g of zinc. Since there are no storing systems for zinc in the body, zinc has to be ingested daily and the homeostasis has to be regulated accurately. The recommended daily intake varies between 7-12mg of zinc. The chemical properties of zinc made it to one of the favorite metal ions in biological processes. This results in hundreds of zinc-dependent enzymes and signaling events dependent on zinc. Due to this importance a deficiency in zinc is related to multiple organ dysfunctions. Since zinc deficiency has been described in humans it became more and more obvious that severe zinc deficiency is one of the leading health problems in developing countries and marginal zinc deficiency a common problem in the industrial world. Therefore a detailed knowledge about zinc is necessary to understand dysfunctions in zinc homeostasis and to develop therapies using the biological capacity of this trace element.

Essentiality of zinc for humans and its deficiency was recognized in 1963. During the past 50 years, it has become apparent that deficiency of zinc in humans is widely prevalent. Nutritional deficiency of zinc may affect nearly 2 billion subjects in the developing world. Consumption of cereal proteins high in phytate decreases the bio-availability of zinc. Conditioned deficiency of zinc is also very common. Growth retardation, hypogonadism in males, rough skin, impaired immunity, neuro-sensory disorder and cognitive impairment are some of the major clinical manifestations of zinc deficiency. Zinc is involved in many biochemical functions and nearly 2000 transcription factors require zinc for gene expression. Zinc is also an effective antioxidant and anti-inflammatory agent. In therapeutic dosages, zinc has been used for the treatment of acute diarrhea decreasing mortality in millions of affected infants and children, common cold, Wilson's disease, sickle cell disease and for the prevention of blindness in patients with age related macular degeneration.

Zinc is most bioavailable from animal flesh, and red meat is the richest common source. In contrast, foods prepared from cereals, legumes, other seeds, and other plant parts may be rich in indigestible zinc-binding ligands, such as phytate and certain dietary fibers, that suppress zinc bioavailability. Zinc absorption from zinc supplements is modulated over time so that the amount of zinc absorbed does not exceed need. Supplements of calcium, iron and folate are reported to suppress zinc absorption under some circumstances. Treatment of zinc deficiency with zinc is most efficacious when other micronutrient are also available in adequate amounts.

Zinc is nutritionally essential and indispensable to growth, development, and maintenance of human functions. There is hardly any cellular process that does not depend on zinc in some way. As a constituent of at least 2800 human proteins and with cellular concentrations of a few hundred micromolar, zinc has unparalleled significance in protein structure, enzymatic catalysis, and cellular regulation. The largest group of zinc metalloenzymes are proteinases. Zinc has a major role in the structural organization of protein domains that interact with DNA/RNA, other proteins, and lipids. Several dozen proteins control cellular and subcellular zinc homeostasis and re-distribution. The control is a prerequisite for regulatory functions of free zinc (II) ions. Fluctuations of cellular free zinc ion concentrations modulate the biological activity of a yet unknown number of additional proteins, suggesting roles of zinc in information transfer.

Life has evolved around Zn and so apparently has the regulation of cell division and cell death. An increase in cytosolic Zn during mid-G1 is essential for the induction of cyclins and other gene products required for the cell to prepare for replication of its DNA. A second requirement for Zn precedes mitotic division of the cell. A role for the action of specific Zn transporters in mitosis is rapidly emerging. The relationship of Zn to cell death is intriguing as this metal ion can both suppress and promote apoptosis, depending upon factors which are still poorly understood. Zn protects many types of cell from oxidant-induced apoptotic cell death by, amongst other things, inhibiting events leading to activation of the caspase executioner proteins. Recent studies showing a role for Zn in macrophage phagocytosis suggests impaired efferocytosis as an alternative mechanism by which apoptotic cells can accumulate in the tissues in Zn deficiency. Under some circumstances, Zn acts as a potent inducer of apoptosis, such as in neurons dying during seizures and brain injury where it contributes to pathogenesis. Other emerging mechanisms of cell death, including autophagy and pyroptosis, are also influenced by Zn and Zn deficiency. In addition to the above roles of Zn as either a suppressor or promoter of cell death, a general feature of apoptosis and autophagic cell death is a substantial increase in fluorophore-detectable, labile Zn within the dying cell's cytoplasm or vacuoles, respectively; the significance of this Zn release is not known. The dual roles of Zn in regulating cell proliferation and cell death point to a pivotal role for this metal ion in tissue homeostasis with important implications for diseases in which the delicate balance between cell birth and cell death goes amiss.

The majority of cellular zinc is tightly bound to proteins, where it has either catalytic or structural functions. Nevertheless, a regulatory role for zinc also exists, which is mediated by the small fraction of free or loosely bound zinc. This chapter discusses the role of free zinc in signal transduction. Different types of zinc signals were observed in eukaryotic cells, ranging from fast changes in less than a minute to signals that accompany cellular differentiation and last for several days. A range of molecular targets for these zinc signals has been identified, including a regulation of the activity of several kinases, phosphatases, phosphodiesterases, caspases, and transcription factors.

Divalent ions like zinc, in its chelatable form, are found ubiquitously in all tissues in the mammalian body. Its amount varies and in some tissues, such as certain areas of the brain and the gastrointestinal tract, it may reach hundreds of micromolar in a specific compartment. Physiological as well as pathological conditions can lead to large changes in both the extra- and intracellular concentration of this ion. Zinc modulates, enhances or inhibits many channels in the course of physiological activity as in the hippocampus or during pathological incidents such as during ischemic episodes. In this chapter we have tried to outline the effect on some of the channels modulated by zinc. Channel modulation is achieved by the slow permeation of zinc through the channel or, like in most cases, due to binding of zinc to specific sites on the channel proteins. We concentrated on major mono and divalent ligand or voltage gated cationic channels since space was not allowed to discuss all the channels, including anionic and non ionic channels modulated by zinc.

Zinc transporters maintain zinc homeostasis in integrative systems through controlling absorption and excretion of this essential micronutrient as well as by regulating inter-organ and intracellular zinc for functional needs. Studies of the 24 zinc transporters derived from two families have relevance to both clinical science and basic science. These studies are necessary to address the questions of what transported zinc does and why multiple zinc transporters have evolved.

Numerous methods have proven useful in the study of zinc in biological systems. This chapter briefly reviews several methods for determining and imaging zinc, particularly in aqueous solutions, cells, and tissues. The sections are written for bioscientists who are potentially interested in a method but may have little experience with it. The methods include fluorescent stains for zinc, fluorescence-based biosensors, X-ray fluorescence, determination of zinc binding affinity, and fiber optic biosensors. For each method the principle and background are described, followed by a brief summary of the procedure and typical results, and concluding with the advantages and disadvantages of each method.

The essentiality of the trace element zinc is anchored in its role as a cofactor in a number of enzymes and in its function as a signaling molecule. Among other cell systems, the immune system requires zinc for adequate functionality. Not only zinc deprivation but also exceedingly high zinc levels result in immune dysfunction, depicting the significance of a regulated zinc homeostasis for the immune response. Disturbances of zinc homeostasis affect multiple aspects of the immune system, including hematopoiesis, innate immunity, adaptive immune response and processes involved in immune regulation. Zinc deficiency impairs the development of lymphocyte progenitors and modifies the differentiation of cells of the innate immune system. Furthermore, immune cell function is adversely affected during zinc deprivation, resulting in compromised lymphocyte immune reaction combined with dysregulated innate immune responses. Consequently, the development and progression of various infections and diseases is influenced by alterations in the zinc homeostasis. For several decades, the immunobiology of zinc has been studied. This chapter aims to discuss the overall impact of zinc on the immune system.

Zinc is an essential micronutrient critical to the maintenance of a healthy immune system. Zinc deficiency is often linked with an increased risk for infectious diseases especially among at-risk populations such as young children and pregnant women. Supplementation with zinc has been shown to prevent and treat diarrhea among children under 5 years of age decreasing both diarrhea morbidity and mortality. Zinc deficiency is also correlated with risk for respiratory infections, but the benefit of supplementation appears to be limited to more severe episodes and populations with high rates of zinc deficiency. While there is evidence suggesting a correlation between zinc deficiency and the prevalence of malaria, measles, HIV, and tuberculosis, few studies have shown a benefit of supplementation for either prevention or treatment of these infections. The World Health Organization currently recommends zinc supplementation for the treatment of diarrhea among children under 5 years of age.

Zinc availability affects the host response to major trauma and infection and is associated with the classic acute phase response. During the acute phase response plasma zinc levels precipitously decline as a result of mobilization of zinc into the intracellular compartment to assist with vital metabolic functions that enable immune defense, tissue repair and recovery. Despite knowing some but not all of the essential roles that zinc plays in host defense, little is known regarding metabolic zinc requirements at the onset of and during the acute phase response in the context of critical illness. Recent evidence suggests that zinc supplementation may have a beneficial role in the critically ill and that zinc deficiency, prior to host insult, may be disadvantageous. In this review, new data on zinc metabolism, its implication in the pathogenesis of critical illness with a focus on sepsis, and its therapeutic effects are summarized.

Zinc (Zn) is essential for normal cell structure and physiology. Its deficiency causes growth retardation, immunodeficiency, and neuronal degeneration. Zn homeostasis is tightly controlled through Zn transporters and metallothioneins, which regulate Zn concentration, its distribution in individual cells, and contribute to Zn signaling in cells. Zn intracellular signaling regulates immune reactions as well as hard and connective tissue development. Although many molecules involved in these processes have Zn-binding motifs, the molecular mechanisms of Zn's role have not been clarified. Recently, we and other groups demonstrated that Zn signaling plays diverse and specific roles in vivo and in vitro, in studies on the genetic knockout of Zn transporter functions. In this section, we discuss the impact of Zn signaling on the mast cell-mediated allergy response, T cell-mediated immune response, and development of hard and connective tissues. We also describe Zn signaling dysregulation as a leading health problem in allergy and immune response, and in skeletal and connective tissues' development.

Zinc is essential for cells and intracellular zinc levels are maintained within appropriate boundaries by zinc transporters. However, increasingly, changes in intracellular zinc have been observed in a number of different cancers. Furthermore, these changes in zinc levels are often accompanied by parallel alterations in the expression of different zinc transporters. In this chapter we will detail what is known about zinc and zinc transporter levels in different cancers with particular emphasis on breast cancer, which has been examined most thoroughly. It is hoped that what has been discovered regarding the involvement of zinc and zinc transporters in breast cancer will be applicable to future investigations of other cancers.

The essentiality of zinc is highlighted in pregnancy where many fundamental processes are dependent on Zn. In addition to adequate intake during pregnancy and lactation, it is recognised that a range of pathological outcomes are associated with zinc deficiency including; birth defects, growth retardation, impaired immune function and increased susceptibility to infection, skin disorders and central nervous system dysfunction. Furthermore, our understanding of pregnancy outcomes has been advanced by the better understanding of mechanisms that underlie the altered supply of Zn to the fetus in response to the maternal exposure to toxins, infection and disease. The metal binding protein, metallothionein is important in altering Zn distribution following exposure to alcohol or infection.

Ageing is an inevitable biological process with gradual and spontaneous biochemical and physiological changes and increased susceptibility to diseases. Nutritional zinc may remodel these change with subsequent healthy ageing, because zinc improves the inflammatory/immune response as shown by “in vitro” and “in vivo” studies. However, in ageing, the zinc daily dietary intake is reduced than that one recommended by the RDA. Many causes can be involved: among them, inadequate mastication, psychosocial factors, drugs interactions, altered cellular processes [zinc transporters and Metallothioneins (MT)]. These processes are very relevant because the intracellular zinc homeostasis is regulated by buffering MT and zinc transporters assigning to zinc a role of “second messenger”. Physiological zinc supplementation in elderly improves these functions with however contradictory data. Therefore, the choice of old subjects for zinc supplementation has to be better considered in relation to the specific genetic background of MT and IL-6, because the latter is involved both in MTmRNA and in intracellular zinc homeostasis. The genetic variations of IL-6 -174G/C locus when associated with those ones of MT1A +647A/C locus are useful tools for the choice of old people for zinc supplementation because improving the inflammatory/immune response, suggesting the relevance of zinc-MT gene interaction for healthy ageing and longevity.

Zinc is an essential nutrient that is required for a broad range of biological functions. The potential exists for zinc to impact on chronic disease. The aims of this chapter are twofold: firstly to evaluate the epidemiological and clinical data that report on zinc status and atherosclerosis in humans, and secondly to determine the potential mechanisms of the interaction of zinc with atherosclerosis risk factors. There are conflicting reports of the relationship between atherosclerosis and zinc status, as assessed by dietary intake of zinc and/or the measurement of zinc concentrations in healthy and diseased tissues. The balance of epidemiological studies points to an association between zinc deficiency and atherosclerosis however the studies are hampered by the lack of a decisive biomarker of zinc status. Clinical trials are mainly of zinc supplementation, and these show a decrease in plasma high-density lipoprotein cholesterol concentrations leading to an increased risk of heart disease. Impaired zinc homeostasis has been associated with increased levels of oxidative stress and the induction of widespread genomic and proteomic changes that relate to cardiovascular disease. Potential mechanisms of the influence of zinc on atherogenesis studied in rodent models and in cell culture include its interaction with a wide range of cellular redox and inflammatory processes, such as NF-kB, NO, PPAR, and PKC signalling pathways. In conclusion, zinc is likely to be involved in atherogenesis through its interactions with lipoprotein metabolism, inflammation and oxidative stress. Further progress will be made when improved methods of measuring zinc status are developed.

The role of zinc in the brain was a subject of intense interest, based particularly on the observation that a chelatable zinc pool is found at glutamatergic synaptic vesicles and is co-released during synaptic activity. Studies utilizing nutritional zinc deficiency or chelation linked brain zinc to cell death during ischemia and a role in development as well as learning and memory. Numerous studies in recent years led to identification of multiple zinc transporters among them the ZnT3 that is responsible for zinc accumulation in the vesicles. Generation of transgenic animals lacking specific zinc transporters together with the availability of zinc dyes led a dramatic progress in our insight on the role of brain zinc homeostasis. In this chapter we will discuss mechanisms by which zinc induces neuronal signaling or neuronal death, and how these may link zinc to learning and memory, seizure, ischemia and Alzheimer's disease (AD). Finally, we will focus on the emerging importance of zinc signaling in glia and glial-neuronal interaction.

In addition to the milieu of systems this nutritionally essential element contributes to in the human body, the divalent cation zinc also participates in neuronal signalling within the nervous system. Specifically, a subset of zinc within the brain is located in the synaptic vesicles of some glutatmatergic neurons and can be released at the presynaptic terminal in an activity-dependent manner. The location of zinc releasing neurons and pathways, and the co-release of synaptic zinc with neurotransmitters such as glutamate, supports the involvement of synaptic zinc in neuromodulation and, specifically, in cortical plasticity. In fact, chelation of extracellular zinc in vitro has been demonstrated to inhibit LTP, while sensory deprivation or sensory stimulation in intact animals can lead to activity-dependent increases or decreases of synaptic zinc, respectively.

Depression is a chronic recurring illness that is associated with significant disability, morbidity and mortality. Depression is twice as prevalent in women as men and the risk factor for prenatal depression and postpartum depression is similar to the risk factor for depression in general. Attention deficit hyperactivity disorder (ADHD) is a developmental disease, characterized by overactivity, impulsivity and inattentiveness. ADHD affects about 5-9% of children aged between 5 and 14 years and 2-3 times as many boys as girls. The presence of ADHD in childhood is a major risk of the development of neurobehavioral disorder in adults. Despite the intensive research of both diseases, the exact mechanisms involved in the pathophysiology and treatment of depression or ADHD is still unknown.

This paper is focused mostly on depression and ADHD in the context of the role of zinc deficiency in the pathogenesis of these diseases, changes in animal behavior, the role of zinc treatment and zinc supplementation, and possible biological mechanisms involved in these relationships.

The maintenance of normal zinc levels is crucial to a variety of cellular functions, with zinc also shown to be a key mediator of a number of central and peripheral disorders. Within the brain, there is now mounting evidence for a pivotal role for zinc dyshomeostasis in the onset and pathological and symptomatic progression of Alzheimer's disease- the most common form of dementia. In this chapter we will discuss the evidence for a disturbance in zinc levels in both the brain and peripheral compartments in Alzheimer's disease. We will further outline the various cellular targets that may be affected by this and how it may contribute to the pathophysiology of Alzheimer's disease. Finally, we will discuss the notion that zinc dyshomeostasis may represent a therapeutic target for this progressive neurodegenerative disorder.

Over the past three decades, we have learned much about the pathophysiology of brain ischemic injury. Much of this work focused on effects initiated by rapid release of the excitatory neurotransmitter, glutamate, and downstream Ca²⁺ overload. Yet therapeutic interventions based upon these mechanisms have had limited success. More recently, a second divalent cation, Zn²⁺, has garnered considerable attention as a signal ion and mediator of damage in brain ischemia. Zn²⁺ accumulates in damaged neurons after ischemia in many areas of the mammalian forebrain and contributes in diverse ways to the injury process. The past decade has seen rapid advances in our understanding of ways in which Zn²⁺ may contribute to distinct stages of the ischemic neurodegneration cascade. It is hoped that this emerging understanding will suggest new and more effective treatments to decrease morbidity after cerebral ischemia.