Zinc

There have been numerous reviews focusing on the impor-tance of vitamin C, vitamin E and selenium for respiratorydiseases such as asthma, but limited studies are available onthe role of dietary zinc (Zn). This paper will attempt toreview the current state of knowledge, while also proposingthe possible importance of Zn in the context of the respira-tory system.

Zinc is a group IIb dietary metal required for the healthyfunctioning of the body. The Australian recommended dietaryintake of Zn is approximately 12 mg/day and this is obtain-able from protein-rich foods, such as red meats, seafood,fresh fruit and vegetables, and dairy products. Over the past30 years, many researchers have demonstrated the criticalrole of Zn in a variety of physiological processes, includinggrowth and development, maintenance and priming of theimmune system and tissue repair and regeneration.1

As Zn is the most widely used biometal in biology, it canbe found in all organs, secretions, fluids and tissues of thebody and is transported via albumin in the circulation.1 Zincpossesses two main properties, which make it an ideal par-ticipator in biological systems. First, Zn is virtually non-toxicas the homeostatic mechanism by which it is regulated is soefficient that no chronic disorders are known to be associatedwith excessive accumulation.2 Second, its physical and chem-ical properties enable it to interact with a variety of enzymesand other proteins that participate in cellular metabolism aswell as in the control of gene transcription.3

In the body Zn exists in two main states: (i) a bound formthat is held on to tightly by metalloproteins and Zn fingerproteins; and (ii) a more loosely bound labile form that participates in intracellular Zn fluxes and is readily depletedin Zn deficiency. Although all organs contain labile intra-cellular Zn, the following tissues are particularly labile Znrich: hippocampus, testis and secretory cells (e.g. pancreaticβ cells and mast cells).4

Respiratory epithelium

The respiratory epithelium is a complex and highly-regulatedinert barrier separating the human airway from the externalenvironment and is therefore constantly exposed to a varietyof exogenous agents (e.g. allergens, inhaled pollutants andviruses) capable of initiating an inflammatory reaction.Although this epithelium is fundamentally a protectivebarrier, one of its important roles is to produce cytokines,growth factors, nitric oxide, matrix metalloproteinases andmany pro- and anti-inflammatory substances.5 Hence thepotential for enhanced oxidative stress, due to the involve-ment of the epithelium in airway inflammation and continualexposure to exogenous environmental agents, is greatlyenhanced in this tissue.6

Airway epithelial damage has been well reported in aller-gic diseases, such as asthma, where tissue injury leads toepithelial desquamation and shedding. This is due to therecruitment and activation of inflammatory cells, such as theeosinophils, mast cells and neutrophils, which release tissue-damaging proteases, chemotactic cytokines and toxic reactiveoxygen species thereby exacerbating the allergic response.5

Why zinc may be beneficial for the respiratory tract

Zinc has been shown to be vital as an anti-oxidant, micro-tubule stabilizer, anti-apoptotic agent, growth cofactor and

Immunology and Cell Biology (2001) 79, 170–177

Special Feature

New insights into the role of zinc in the respiratory epithelium

AI Q TRUONG-TRAN, JOANNE CARTER, RICHARD RUFFIN and PETER D ZALEWSKI

Department of Medicine, University of Adelaide, The Queen Elizabeth Hospital, Woodville, South Australia,Australia

Summary Over the past 30 years, many researchers have demonstrated the critical role of zinc (Zn), a group IIbmetal, in diverse physiological processes, such as growth and development, maintenance and priming of theimmune system, and tissue repair. This review will discuss aspects of Zn physiology and its possible beneficial rolein the respiratory epithelium. Here we have detailed the mechanisms by which Zn diversely acts as: (i) an anti-oxidant; (ii) an organelle stabilizer; (iii) an anti-apopototic agent; (iv) an important cofactor for DNA synthesis;(v) a vital component for wound healing; and (vi) an anti-inflammatory agent. This paper will also review studiesfrom the authors’ laboratory concerning the first attempts to map Zn in the respiratory epithelium and to elucidateits role in regulating caspase-3 activated apoptosis. We propose that Zn, being a major dietary anti-oxidant has aprotective role for the airway epithelium against oxyradicals and other noxious agents. Zn may therefore haveimportant implications for asthma and other inflammatory diseases where the physical barrier is vulnerable andcompromised.

Key words: anti-oxidant, epithelium, inflammation, respiratory, zinc.

Correspondence: AQ Truong-Tran, Department of Medicine, University of Adelaide, The Queen Elizabeth Hospital, Woodville,South Australia 5011, Australia. Email: ai.truong-tran@student.adelaide.edu.au

Received 10 October 2000; accepted 10 October 2000.

anti-inflammatory agent in a variety of tissues. We proposethat Zn is important in respiratory tract tissue as a cytopro-tectant against toxins and inflammatory mediators in a similarway to that reported for the endothelium.7,8 Figure 1 describesthe potential roles of Zn in the respiratory epithelium.

Zinc as an anti-oxidant

Zinc has been shown to be an important anti-oxidant as excel-lently reviewed by Powell,9 and this is best demonstrated inanimal studies where there is increased susceptibilty oforgans, such as the lung,10 liver11 and testes,12 to oxidativeinjury in Zn deficient animals. Recent studies in a rat modelof Zn deficiency reported enhanced epithelial lesions in thegastrointestinal epithelium, which were blocked by a nitricoxide synthase inhibitor.13 Similar studies need to be per-formed in the conducting airways to determine if Zn is indeedessential for maintaining the integrity of the airway epithe-lium and for providing protection in oxidative stress. This isparticularly important as oxidative stress is greatly increasedin the respiratory epithelium as a consequence of inflamma-tion, exogenous exposure and leakage from actively respiringmitochondria.14

Zinc can act as an anti-oxidant by a number of mecha-nisms, which may also be important in the respiratorysystem. First, Zn ions can directly act as an anti-oxidant bystabilizing and protecting sulfhydryl-containing proteins thatare important in lung function (e.g. ciliary tubulin, alanytransfer RNA (tRNA) synthetase and Zn finger transcriptionfactors). Zinc may protect these proteins from thiol oxidationand disulphide formation. Zinc can stabilize sulfhydrylgroups by: (i) binding directly to the sulfhydryl groups; (ii) binding to another protein site in close proximity to thesulfhydryl groups and producing a steric hindrance tooxyradicals; or (iii) binding to another site on the proteinresulting in a conformational change that results in a reduc-tion in sulfhydryl reactivity.9 Second, Zn can displace Fe andCu from cell membranes and proteins,15 which can otherwisecause lipid peroxidation and destruction of membrane proteinlipid organization due to their ability to promote the genera-tion of hydroxyl ion (.OH) from H

2O

2and superoxide via the

Fenton reaction.16 This is important as Zn has only one oxi-dation state (II) and therefore cannot undergo these redoxreactions. In addition, Zn can accept a spare pair of electronsfrom oxidants, hence neutralizing their reactivity.17 Third, Znacts indirectly by inducing production of the anti-oxidant

Zinc and the respiratory epithelium 171

Figure 1 Zinc in the respiratoryepithelium. a) Potential target sitesfor the beneficial effects of Zn inthe respiratory epithelium. Thisimage outlines some of the possi-ble beneficial roles of Zn for therespiratory epithelial cells. Zinc isprotective by virtue of being ananti-oxidant, organelle stabilizerand possessing anti-apoptoticactivity (as an inhibitor of caspase-3 activation). Other importantfunctions include stimulation ofDNA synthesis and cell pro-liferation, tissue regeneration andacting as an anti-inflammatoryagent. Finally, the demonstrationof Zn in cilia may indicate a hith-erto unexpected role for this metalion in cilial function. b) Visualiza-tion of Zn in ciliated airwayepithelial cells. Left panel shows abright field UV laser confocalimage of two sheep tracheal cili-ated cells and the right panelshows the corresponding Zinquinfluorescence demonstrating theabundance of Zn in the apicalcytoplasm and cilia. BB, basalbodies.

metallothionein.18 Metallothionein can also release Zn underhigh oxidative stress, which it can contribute to the anti-oxidant defense system.19 Finally, Zn is an important compo-nent of the major anti-oxidant enzyme Cu–Zn superoxidedismutase (Cu–Zn SOD), which is found in the cytoplasm ofairway and alveolar epithelial cells. Cu–Zn SOD acts byremoving superoxide anions. Larsen and colleagues20

recently demonstrated the important protective effect ofCu–Zn SOD in airway inflammation as transgenic mice withelevated levels of Cu–Zn SOD in the lungs were found to bemore resistant to allergen-induced hyperresponsiveness thantheir wild-type counterparts.

Zinc as a membrane and cytoskeletal stabilizer

In addition to its role as an anti-oxidant, Zn has other prop-erties advantageous in cytoprotection as it can protect pro-teins and nucleic acids from degradation, while stabilizingthe microtubular cytoskeleton and cellular membranes.21

Similarly, Hennig et al.7 demonstrated the importance of Znin maintaining endothelial cell integrity and vascular barrierfunction. Zinc may play a similar role in the respiratoryepithelium, which also acts as a physical barrier. Numerousstudies have reported that Zn deficiency alters the lipid composition, enzyme activity and protein composition of theplasma membrane skeleton thereby increasing membranepermeability.21 Zinc is also important for tubulin assembly asZn deficiency causes disruption of microtubules.22 Therefore,Zn may also be vital for maintaining the integrity and func-tion of cilia in respiratory epithelial cells.

Zinc as an anti-apoptotic agent

Numerous in vivo studies have demonstrated the importantrole of Zn in the regulation of apoptosis, noting an increasein populations of apoptotic cells in a variety of tissues,including the intestinal epithelium, skin, thymus, testis, retinaand pancreas, in Zn deficient animals.23 Similar increases inapoptosis arose within the neuroepithelium of fetal ratswithin 4 days of maternal Zn deficiency. This interfered withneural closure and was associated with the presence of con-genital abnormalities.24 Whether this applies in humans is notknown; however, one relevant study is that by Mori et al.25

who reported an increase in the apoptosis of keratinocytesaround vesicular lesions of patients with Zn deficiency.Despite the extensive knowledge available there have been nostudies to date reporting on the susceptibility of the respira-tory system to Zn deficiency induced apoptosis in vivo.

In vitro studies have advanced our understanding of thebiochemical mechanisms by which Zn deficiency triggersapoptosis (Fig. 2). One of the earliest studies documentingthe involvement of Zn in suppressing apoptosis was per-formed by Cohen and Duke26 who found that Zn inhibited theactivity of Ca/Mg-dependent endonucleases responsible fornuclear DNA fragmentation in thymocytes. Studies per-formed later, during the 1990s, found that micromolar con-centrations of Zn were able to suppress the activation ofcaspase-3 (see review by Truong-Tran et al.27). This is nowthought to be the principle mechanism by which Zn acts as ananti-apoptotic agent. A possible mechanism by which Zn suppresses caspase-3 activation includes the inhibition of

caspase-6. Caspase-6 not only cleaves the lamin proteins inthe nuclear membrane, inducing nuclear collapse, a distin-guishing characteristic of apoptosis,28 but is also known tocleave pro-caspase-3 into its activated form.29 Yet anotherpossible mechanism by which Zn suppresses apoptosis is byincreasing the B cell lymphoma-2 (Bcl-2)/Bax ratio, therebyincreasing the resistance of cells to apoptosis.30 Althoughrates of apoptosis in different regions of the respiratory tracthave yet to be studied, it has recently been shown that totalcaspase-3 protein by immunocytochemistry is much higher in the bronchial epithelium compared to alveolar pneumo-cytes.31 Figure 2 is a schematic diagram summarizing the bio-chemical pathway of apoptosis and highlights the steps atwhich Zn has been shown to have a suppressive effect.

Zinc is essential for DNA synthesis and cellular growth

Cell proliferation is important in the respiratory epithelium asthere is rapid cell turn over. Zinc is essential for DNA syn-thesis and cell growth and is therefore likely to be one of theregulatory factors in airway cell homeostasis. One of themost telling signs of Zn deficiency is growth retardation asthis metal is vital for pituitary growth hormone secretion andfunction, and it is needed for hepatic insulin like growthfactor-1 (IGF-1). These requirements may be related to theanorexia, decreased food intake and decreased growth,observed in Zn deficient rats.32 Furthermore, MacDonaldet al.33 demonstrated that Zn is essential for IGF-1 mediatedstimulation of cell division. Thymidine uptake was greatlyenhanced when Swiss 3T3 cells were incubated with Zn incombination with IGF-1, as compared to IGF-1 alone.

Some of the reasons why Zn is important for cell prolif-eration relate to its presence in the cell nucleus, nucleolus andchromosomes, where it both stabilizes the structure of DNAand RNA and acts as a vital cofactor of many enzymesrequired for DNA and RNA synthesis (e.g. DNA polymerase,RNA polymerase and reverse transcriptase). Zinc is alsoimportant for Zn finger proteins, such as transcription factorIIIA.34 Zinc finger proteins possess a folded domain in whichZn binds to appropriately spaced cysteines and histidinesthereby facilitating the interaction between the transcriptionfactor and DNA specific sequences in the promoter/enhancerregions of certain genes and promoting gene transcription.

Wound healing

Tissue damage to the respiratory epithelium can occur fre-quently as it is often exposed to toxic endogenous and exo-genous substances resulting in varying degrees of damage,from a slight enhancement of the epithelium’s permeability,to a more drastic epithelial cell shedding or denuding of thebasement membrane. After an insult, the respiratory epithe-lium initiates a tissue-healing process that involves the rapidre-epithelialization of the denuded area. Restoration of theintegrity of the epithelium is via the dedifferentiation, spread,and rapid migration of the remaining viable epithelial cellsfound at the edge of the wound over the denuded basementmembrane.35

The epithelium requires Zn and is sensitive to fluxes inplasma Zn. Zinc deficient patients suffering poor healing ofskin lesions and wounds can be treated by increasing dietary

AQ Truong-Tran et al.172

Zn intake or by the use of Zn-impregnated bandages. Theapplication of topical Zn to wounds and lesions producesaccelerated healing via enhanced re-epithelialization.1

Another study supporting the role of Zn in wound healing isthat of Cario et al.36 who found that physiological amounts ofexogenous Zn improved epithelial repair by promotingintestinal epithelial wound healing at the initial step ofepithelial cell restitution.

The wound healing process in the respiratory epitheliumand other tissues is thought to be controlled by the upregula-tion of Zn dependent metalloproteinases (e.g. MMP-9 andMMP-3).35 Metalloproteinases are responsible for the degra-dation of the extracellular matrix (i.e. collagenases,stromelysins and gelatinases). These enzymes possess a cat-alytic mechanism that requires an active site Zn cation thatbinds to a conserved histidine-containing domain.37

Anti-inflammatory effects of Zinc

Labile Zn plays a major role in the control of inflammationvia a number of mechanisms. First, many inflammatory diseases, such as arthritis and asthma, are associated with an increase in the inducible form of nitric oxide (NO) synthase resulting in enhanced NO formation. Studies by

Abou-Mohamed et al.38 demonstrated that Zn is able toinhibit lipopolysaccharide and interleukin-1β-induced NOformation. Second, Zn is anti-inflammatory as it is also ableto inhibit the activation of NF-κβ, a transcription factorimplicated in the expression of many pro-inflammatorygenes. Zinc inhibits NF-κβ activation by blocking the phos-phorylation and degradation of the inhibitory proteins ΙκΒand its multisubunit ΙκΒ kinase, essential reactions requiredfor the activation of NF-κβ.39

A switch from an initial cellular (Th1

predominance) to amore humoral and more pro-inflammatory (Th

2 mediated)

immune response is a feature of many inflammatory diseases,such as asthma, rheumatoid arthritis and food allergies.40 Thesame switch favouring the Th

2 subset also occurs in Zn defi-

ciency and can be reversed when patients are treated with Znsupplements.41 This will be discussed later in the presentreview in the context of a relationship between Zn deficiencyand asthma. Other mechanisms by which Zn may be anti-inflammatory in the respiratory tract include: (i) blocking thebinding of leucocytes to endothelial cells via the interactionbetween leucocyte associated antigen 1 and intercellularadhesion molecule-1 (ICAM-1);42 (ii) blocking the dockingof human rhinovirus on ICAM-1 of somatic cells, therebypreventing viral infections in the respiratory tract;42 and

Zinc and the respiratory epithelium 173

Figure 2 Inhibitory effects ofZn in apoptosis. There are manyinput pathways that trigger cells to die by apoptosis and these con-verge onto a central pathway thatis governed by the mitochondriaand associated proteins, such as theanti-apoptotic B cell lymphoma-2(Bcl-2) and the pro-apoptotic Bax.The activation of caspases, a cys-teine protease family of proteins,results in the biochemical andmorphological characteristics ofapoptosis. Zinc is thought to act atmultiple sites by: (1) inhibitingendonucleases responsible forDNA fragmentation; (2) inhibitingthe activation of caspase-3 and 6,the major executioner caspases;and (3) increasing the Bcl-2 toBax ratio. CAD, calcium activatedDNase; P21, p21waf1/cip1 protein; Psflip, phosphatidyl-serine flip.

(iii) inhibiting the release of preformed mediators from mastcells and basophils (e.g. histamine)43 and eosinophils (e.g.eosinophil cationic protein).44

Localization of zinc in the respiratory epithelium

Studies by our laboratory have reported the first attempts tolocalize Zn in the cells and tissues of the respiratory systemand to study the biochemical role of Zn in regulating apop-tosis.45 Levels and distribution of intracellular labile Zn, weredetermined using a novel UV-excitable Zn-specific fluo-rophore Zinquin, which has previously enabled the imagingof distinct pools of labile Zn in a range of cell types andtissues.46–49 By using Zinquin we have reported45 that: (i) themalignant bronchial epithelial cell line NCI-H292 had twicethe Zinquin fluorescence of the malignant alveolar cell lineA549 (Table 1); (ii) airway epithelial cells were relatively richin labile Zn when compared with alveolar epithelial cells incryostat sections; (iii) labile Zn lines the apical and lumenalside of the entire length of the conducting airways; and (iv)this Zn is especially concentrated in the mitochondrial-rich,apical cytoplasmic region, immediately below the cilia ofprimary tracheobronchial epithelial cells (Fig. 1b).

There are several reasons why airway epithelial cells havehigher basal levels of intracellular Zn compared to alveolarepithelial cells. First, upper airway epithelial cells are morelikely to be challenged or damaged by foreign pollutants dueto their positioning. Hence these cells may require higher Zn

levels for protection and cellular repair. Second, upperairway epithelial cells possess mucin-secreting granules thatare rich in carboxylated glycosaminoglycans that are largelyacidic in nature and can trap more positively charged labileZn within granules. Finally, these cells have a more rapid turnover rate and thus will require higher intracellular Zn levelsfor Zn-dependent DNA synthesis.

Zinc suppresses caspase activation and apoptosis inrespiratory epithelial cells

In order to determine the importance of labile intracellular Znin the survival of respiratory epithelial cells we have previ-ously investigated the role of this Zn in regulating oxyradical-induced apoptosis.45 It is interesting to note that our observeddistribution of labile Zn in these cells closely matches that ofthe inactive form of the major executioner enzyme in apop-tosis, caspase-3, as detected by immunocytochemistry inhuman tracheobronchial epithelium.31

The zinc status of malignant NCI-H292 and A549 andprimary ciliated airway epithelial cells from sheep wasmanipulated using a membrane permeable Zn chelator TPEN(N,N,N′,N′-tetrakis-{2-pyridylmethyl}-ethylenediamine),which binds tightly to labile pools of Zn making it function-ally Zn deficient, and the Zn ionophore sodium pyrithione,which has a supplementary effect by transporting exogenousZn into cells. We have found that Zn is clearly an importantfactor for the survival of these cells as chelation of labile Zn

AQ Truong-Tran et al.174

Table 1 Comparison of Zinc-dependent Zinquin fluorescence in A549 and NCI-H292 malignant cell lines

Zinc Status A549 cells NCI-H292 cellsZinquin fluorescence n Zinquin fluorescence n

(pixels)* (pixels)*

Basal Zn levels 11.78 ± 0.28a 192 23.14 ± 2.18b 29Zn supplementation 59.86 ± 1.61c 307 95.10 ± 3.32d 58Zn depletion 7.22 ± 0.26e 187 9.70 ± 0.74a 28

*Zinquin fluorescence is expressed as average pixels ± SEM. Cells were loaded with exogenous Zn using sodium pyrithione, a Zn ionophore,and depleted of Zn using TPEN, a membrane permeable Zn chelator.46

a–eValues that share the same alphabetical superscript are not significantly different at the 5% level of confidence.

Table 2 Relationship between Zinc and asthma

Serum Zn Hair Zn(µg/dL) (µg/gm)

Author Control Asthma Control Asthma

Goldey et al. 198455 83 84 169 160n = 8 n = 8 n = 21 n = 29

Di Toro et al. 198756 NA NA 147 ± 9 99 ± 6n = 19 n = 43

P ≤ 0.05el-Kholy et al. 199057 88.4 ± 11.0 70.3 ± 13.2 194.5 ± 18.6 167.5 ± 23.0

n = 20 n = 22 n = 20 n = 22P ≤ 0.001 P ≤ 0.001

Kadrabova et al. 199658 89 ± 2 80 ± 1 NA NAn = 33 n = 22

P ≤ 0.05

resulted in the rapid activation of caspase-3-like activity anddown stream events of apoptosis. Furthermore, in primarysheep ciliated tracheal epithelial cells, H

2O

2gave an increase

over the control in DEVD-caspase activity of 1.24 ± 0.12units/µg protein/h; TPEN gave an increase of 0.52 ± 0.14units/µg protein/h, while the two in combination gave a syn-ergistic increase of 2.58 ± 0.53 units/µg protein/h (P ≤ 0.05).45

Hence Zn depletion not only results in the rapid activation ofcaspase-3-like activity, but also greatly increases respiratoryepithelial cell susceptibility to oxyradical induced apoptosis.

Alternatively Zn supplementation via 1 µmol/L sodiumpyrithione and 25 µmol/L exogenous ZnSO

4resulted in a

59.3% inhibition of H2O

2-induced DEVD-caspase activation

(P ≤ 0.005) and thus suppression of apoptosis in primarysheep ciliated tracheal epithelial cells.45

Is zinc beneficial or detrimental for respiratorydiseases?

Excessive inhalation of zinc salts, such as zinc oxide, has previously been implicated in causing metal fume fever, aninfluenza-like illness that results from an acute or subacuterespiratory tract inflammation (mild interstitial pneumonia)and is characterized by bronchial hyper-responsiveness,myalgias and fever. If left untreated or undetected fume fevercan also lead to occupational asthma.50

In contrast brief exposures to zinc sulphate aerosols havebeen shown to protect against allergic bronchoconstriction inguinea pigs, possibly by blocking histamine release frommast cells.51 Zinc has also been shown to directly decreasethe incidence of respiratory infections in young children fromdeveloping countries. It has been reported that there was a45% decrease in the incidence and prevalence of acute lowerrespiratory infection in children of 6–35 months of agereceiving 10 mg of supplemental Zn daily.52

Similarly, Zn may be beneficial in reducing the severity ofsymptoms of the common cold. However, there has been conflicting data arguing the pros and cons of Zn lozenges inthe treatment of colds. One recent randomized, doubleblinded, placebo-controlled study by Prasad et al.53 support-ive of Zn, reported a shorter mean overall duration of coldsymptoms (4.5 vs 8.1 days), cough (3.1 vs 6.3 days), andnasal discharge (4.1 vs 5.8 days) when compared with theplacebo group. It has been proposed that Zn acts by prevent-ing viral docking, capsid formation and replication in the respiratory epithelium.42,53 Several factors, such as thedosage, route of administration and the form of Zn saltsgiven, may influence the outcome of whether Zn is good orbad for the respiratory system. Another issue which needs tobe taken into account is whether certain diseases have anunderlying hypozincaemia, for example, in chronic inflam-matory diseases such as rheumatoid arthritis39 and asthma.This may influence the recovery rate when supplementedwith exogenous Zn.

Is there a relationship between asthma and zincdeficiency?

The increase in prevalence of asthma is strongly dependenton environmental factors including diet. Numerous studieshave suggested that significant decreases in the intake of

dietary anti-oxidants may be an important contributing factorto the increasing incidence of asthma over the last threedecades.54

The first consistent study investigating the Zn status ofbronchial asthmatics was that of Goldey et al. in 198455

(Table 2) who reported a reduction in the Zn content of hairin asthmatics, but due to a limited sample size these resultswere not considered statistically significant. However in1987, a larger study by Di Toro and colleagues56 reported asignificant decrease in Zn hair status in allergic and asthmaticchildren, suggesting that asthmatic children were at risk of Zndeficiency. Further adding to these findings, el-Kholy andcolleagues57 and Kadrabova et al.58 reported similar results,but extended the observations to show a significant drop inserum Zn levels. It was proposed that adequate dietary intakeand Zn supplementation may decrease the severity of asth-matic attacks by correcting this underlying hypozincaemia.57

Of particular relevance to Westernized countries, Schwartzand Weiss59 conducted a large scale American study(n = 9074) that found a negative relationship between wheez-ing and serum zinc:copper ratio. Furthermore, Soutar et al.54

investigated the relationship between allergic diseases anddietary anti-oxidants and noted that there was an increase inthe presence of atopy, bronchial reactivity and the risk ofallergic-type symptoms in adults with the lowest intake ofdietary Zn.58

Despite these studies, the significance of these correla-tions between the severity of asthmatic symptoms and low Znlevels is not yet fully understood. Hence, future studies arerequired to fully appreciate the importance of varying Znstatus and its effect on the clinical symptoms and pathologi-cal changes noted in asthma.

Possible mechanisms of zinc deficiency in asthma

While acknowledging that dietary changes over the pastseveral decades have resulted in a decreased intake of freshfoods containing anti-oxidants, we propose that severalintrinsic factors may contribute to a low Zn status in asth-matics. First, like other inflammatory diseases, a redistribu-tion in plasma Zn to the liver can occur during excessivestress. This has been attributed to the release of leucocyteendogenous mediator from activated phagocytes, which thenstimulates movement of Zn from plasma to hepatocytes inallergic reactions.57 Second, the immune system is extremelydependent on the availability of Zn for maintaining itshomeostasis. Inflammatory diseases can cause an increase inthe demand for Zn as: (i) Zn is essential for producing thethymic hormone thymulin necessary for regulating T-celldevelopment and activation; and (ii) Zn is crucial for the activation of natural killer cells, phagocytic cells and forgranulocytes, such as mast cells and eosinophils.60 As aresult, greater demand for Zn by the immune system could bea contributing factor to the Zn deficiency noted in inflam-matory diseases. Zinc deficiency itself is detrimental forinflammation as it results in dramatic increases in thenumber, size and activation state of mast cells.60 This furtherexacerbates damage via increasing chemotaxis of eosinophilsand neutrophils, which creates a continuous cycle of oxida-tive damage. Zinc deficiency can also cause a prematureswitch from the Th

1dependent cellular immune response to

Zinc and the respiratory epithelium 175

a Th2

dependent pro-inflammatory humoral response.41 Thisshift in the Th

1/Th

2balance promotes enhanced levels of

IL-4, IL-5, leukotriene B4

(LTB4) and prostaglandin E

2

(PGE2) release, all of which have been implicated in promot-

ing the pathogenesis of allergic diseases such as asthma.60

Third, although reactive oxygen species are formed as anormal component of cellular respiration, in asthma there isa reported imbalance between the flux of oxidants generatedand the presence and/or activation of cellular anti-oxidantdefence mechanisms. This especially relates to Cu-Zn SOD,which is normally required to detoxify superoxide anions.60

At least three studies have demonstrated a significantdecrease in Cu-Zn SOD activity in erythrocytes61 and respir-atory epithelial cells.62,63 One possible explanation for thedecrease in activity of Cu-Zn SOD may be that in order forthe body to compensate for increased oxidative stress, it mustupregulate its anti-oxidant production, hence increasing itsneed for biochemically active Zn. Therefore, if a hypo-zincaemia exists and tissue Zn becomes limiting duringinflammation, the activity of Cu-Zn SOD may be compro-mised. Finally, because the respiratory epithelial cells are richin Zn their loss, through shedding into the airways duringasthmatic episodes, will further deplete Zn reserves.

Conclusions

This review has attempted to integrate the available informa-tion concerning Zn physiology and the structure and functionof the respiratory system, an area which to date has beenpoorly studied. The advent of new technologies, such as thevisualization of labile pools of Zn by Zn fluorophores, willenable new insights into the functionality of Zn in the respir-atory tract, and its relevance to inflammatory diseases such asasthma. We believe that a full understanding into the rele-vance of Zn for the respiratory system can only be achievedonce this information is acquired. Analogies can be drawnwith the gastrointestinal system where Zn has been clearlyshown to have protective effects against ulcers, diarrhoea andmucosal damage.13

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