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Home , Allysine, Elastin, Macrophage elastase

Thorax: first published as 10.1136/thx.41.8.577 on 1 August 1986. Downloaded from

Thorax 1986;41:577-585 Review article and the

It is essential that the framework of all multicellular most prominently displayed in newly synthesised or organisms should include some materials with high juvenile elastin, is a glycoprotein that stains with ura- tensile strength and rigidity, such as bone and col- nyl acetate and leads, which appears as small lagen, to maintain shape and mechanical rigidity. In 10-12 nm in diameter concentrated around the addition, there is a requirement for a component with periphery of the amorphous elastin. The intrinsic elasticity that can stretch and undergo elastic are chemically and morphologically quite distinct recoil when required. This property is supplied by an from the amorphous elastin. There is some evidence unusual fibrous , which over 150 years ago was to suggest that the microfibrils are secreted into the given the name elastin. before elastin synthesis and func- Elastin fibres are present in virtually all tion as a nucleation site for future elastin deposition. tissues, although it is only within a few, such as ar- For more details on every aspect of the microfibrils, teries, some , and the lung, that elastin com- readers are referred to other articles.24 prises an appreciable percentage of the total protein. The purification of elastin depends predominantly The ligamentum nuchae of grazing animals and the on its remarkable insolubility, even under harsh, of most contain over 50% elastin on denaturing conditions. The protein has been defined a dry weight basis. The elastin content of on the operationally as the insoluble residue remaining after other hand is quite variable, ranging from as low as a tissue is autoclaved repeatedly or boiled in 01 N 2% in rodents up to 28% in the cow and in man. For NaOH for up to one hour. This procedure was satis- this reason most of the chemical and biological stud- factory for tissues comprised mostly of elastin and ies that have been conducted on elastin have used . Isolation of a relatively intact and pure elas- either the bovine neck or the ascending aorta tin is impossible using this method, however, in tis- of several species. Although the primary aim of this sues that have a low elastin content or are particularly article is to review the role and metabolism of elastin rich in glycoproteins or complex . In http://thorax.bmj.com/ in the lung, most of the background information must recent years milder yet considerably more complex come from elastin derived from other sources. The methods have been used. These methods have been findings may, however, be extrapolated to the lung, compared by Soskels and it is now fairly well since within a given species all appear to be accepted that, even in such complex tissue as the lung, chemically the same, regardless of the tissue of origin. elastin can be purified in the absence of harsh alkali We may safely assume that the essentials of elastin extraction. "Pure" elastin, however, still remains as metabolism are the same in all tissues, although an operationally defined protein residue that has an

apparently much influenced by other components in composition typical of elastin for that on October 2, 2021 by guest. Protected copyright. the extracellular matrix of that particular organ. particular species. This has created some confusion since with slight contamination this protein may Identification and purification of elastin appear as a different elastin. There have been reports of two distinct types of elastin within cartilage6 and The first century of elastin research was primarily synthesised by chick aorta in culture.7 Advanced directed towards the morphological and histological techniques analysing elastin messenger RNA do not features of the elastin fibre. This early history of elas- support this theory.89 Other studies have compared tin research has been reviewed in detail by Hass.' The the amino compositions of elastins derived from vari- elastin fibre is composed of two distinct components. ous organs within the same species. These studies The elastin, or amorphous component, is the major indicate that, once the glycoproteins and other pro- fraction, comprising some 90% of a mature fibre. It teins in close association with elastin have been com- derives its name from the absence of any repeating pletely removed, the compositions are the same structure or banding pattern, when reviewed under an within a species regardless of the tissue of origin.10l electron microscope. The microfibrillar component, Structure of elastin Address for reprint requests: Professor BC Starcher, Department of , University of Texas, Health Center at Tyler, PO Box The amino acid analysis of elastin is consistent with 2003, Tyler TX 75710, USA. its unique physical properties. Almost one third ofthe 577 Thorax: first published as 10.1136/thx.41.8.577 on 1 August 1986. Downloaded from

578 residues are , about 12% are , and over sues that have been examined, including the lung, the 40% of the remaining amino acids contain hydro- data indicate that elastin synthesis is regulated by the phobic side chains, making this one of nature's most and availability of mRNA .27 non-polar . A survey of the amino acid com- Translation of the mRNAE produces a tropoelastin positions of elastin, from representative species of all molecule containing a short (24-26 residue) signal vertebrate classes and some invertebrate phyla, indi- peptide.8 As the chain elongation proceeds, select cates that elastin is present in all vertebrates except prolyl residues are hydroxylated by prolyl- Agnatha. " hydroxylase.8 '4 The of prolyl residues Renewed interest in elastin biochemistry began in in tropoelastin, unlike collagen, does not affect either the late 1950s and early 1960s with a series of inter- synthesis or stability of the molecule.28 29 Some esting nutritional observations on the effects of cop- investigators have even considered this a serendip- per deficiency in young growing animals, and the itous event that occurs simply because the machinery simultaneous studies on the chemistry of elastin and is present and the molecule resembles collagen in its crosslinks by the Cambridge group headed by many aspects of primary structure. The entire intra- Partridge.'2 13 The most dramatic signs of copper cellular elastin synthetic process requires about 20 deficiency are the fragmented appearance and consid- minutes.2' erable decrease in the amorphous elastin content of Tropoelastin is secreted from the cell into the major blood vessels, leading to aortic and extracellular matrix as a protein with a molecular subsequent death of the animal.'4 It soon became weight of 72000. As tropoelastin molecules become apparent that , a copper metalloenzyme, aligned, perhaps in association with the microfibrils, was required for the formation of crosslinkages that hydrophobic interactions occur, presumably leading stabilise the elastin fibre.'5 In the absence of cross- to coacervation. Lysyl oxidase subsequently converts linking, a soluble, non-functional elastin molecule the epsilon (E) aminos of all but five or six of the total accumulates in the tissues in sufficient quantities for it 37 residues in tropoelastin to aldehydes. These to be isolated. This rather fortunate event (for the very reactive residues () spontaneously con- biochemist) has led to the characterisation of tro- dense to form various Schiff base and aldol conden- poelastin, the soluble monomer form of elastin, which sation crosslinks. Within a few days most of the cross- has now been isolated from several tissues, including links have isomerised into the stable quaternary ligaments, , and lungs.16 Tropoelastin has been pyridinium ring structures of and iso- essential for the sequencing of this otherwise highly desmosine. Several excellent reviews have discussed http://thorax.bmj.com/ crosslinked and very difficult protein. Characteristic the crosslinking process in great detail.30 31 repeating sequences are found throughout the tro- Lysyl oxidase has been isolated from several poelastin molecule. The sequences ala-ala-ala-lys and sources and, although shown to be present in the ala-ala-lys each repeat six times and are believed to be lung, it has not been isolated from that tissue. Copper the sites for future oxidation by lysyl oxidase and sub- is required for enzymatic activity, which explains the sequent crosslink formation.17 Several hydrophobic defects observed in copper deficient repeat sequences are also found in elastin, including animals. Additionally, lathrogens like BAPN inhibit the pentapeptide pro-gly-val-gly-val and a hexa- the , producing the connective tissue defects peptide pro-gly-cal-gly-val-ala. Some have proposed that are seen in . 4 Genetic disorders on October 2, 2021 by guest. Protected copyright. that the pentapeptide repeats form a /B spiral structure have also been described that are characterised by low consisting of / turns while the crosslinked regions lysyl oxidase activity and severe connective tissue form a more rigid a helical conformation. Other defects. 14 31 sharply contrasting views, derived from physical mea- Once the tropoelastin molecules are crosslinked surements, suggest a highly anisotropic or random they form a three dimensional fibrous network that arrangement for the polypeptide chains.'8 21 often appears branched and fused into a very complex matrix. The elastin fibre can be organised in various Synthesis of elastin configurations in different tissues. In ligaments they extend as long fibres parallel to the direction of stress. Elastin synthesis has been documented in The fibres in the aorta appear to have a lamellar ,22 cells,23 chondrocytes,24 arrangement, forming concentric sheets. In the lung and endothelial cells.25-26 The process is initiated by parenchyma the elastin fibres establish lamellar sheets nuclear transcription from the elastin . The gene surrounding the alveoli. It is apparent, in whatever is quite extensive, containing many large intervening tissue elastin is found, that its orientation takes sequences, suggesting that the final elastin messenger advantage of the protein's main function of being RNA (mRNAE) molecule has undergone major pro- able to stretch to large extensions with little force and cessing from the initial primary transcript.8 In the tis- then return to its original dimensions. The essence of Thorax: first published as 10.1136/thx.41.8.577 on 1 August 1986. Downloaded from

579 the physical properties of elastin has been recently tase that is stored in the azurophil granules of matu- reviewed.2" ring neutrophils at levels of up to 3 jg/106 cells. The current theory, however, relegates neutrophil Degradation of elastin more to pathological processes than to any phys- iological turnover. The enzyme is released into tissues Turnover studies have shown that, once the elastin when the neutrophil dies or encounters molecules to fibre is formed, normal remodelling processes are be phagocytosed. The enzyme has the potential to extremely slow. Studies in man, using sensitive immu- destroy not only elastin but a wide variety of other nological techniques to measure elastin peptides in extracellular matrix components as well as clotting the blood or excreted in the urine,8 sug- factors and complement proteins. gest that less than 1% of the total body elastin pool Macrophages contain low levels of a metal- turns over in a year. By means of the more direct loprotease, which is secreted into the extracellular method of in vivo radiolabelling of elastin, these matrix and has the ability to degrade elastin.36 This observations have been confirmed with several animal elastase has the additional property of not being models, including mice, rats, chicken, and quail.8 32 inhibited by a, antiprotease and can actually digest The mechanisms for elastin degradation in vivo this important elastase inhibitor.37 These cells are of may be divided into two categories-proteolytic particular importance to the lung connective tissue by that degrade uncrosslinked tropoelastin, and the nature oftheir presence in the lung for purposes of enzymes that degrade the fully mature crosslinked defence. and insoluble elastin fibre. Very little is known about the physiological role of Tropoelastin is probably susceptible to a wide produced by other cells, such as fibroblasts range of proteolytic enzymes, yet in only a few or smooth muscle cells. Although the concentration instances has it been systematically studied as a sub- of elastase is very low in these cells, it may be quite strate. On the basis of these experiments and the important in normal elastin metabolism since these results obtained from sequencing data the known are also the cells known to synthesise and secrete elas- tropoelastinases are elastase, trypsin, chymotrypsin, tin on to the extracellular matrix. There are several pronase, , cathepsin G, thrombin, and excellent reviews devoted to the pathological capacity kallikrein. of elastases in the lung.38-42

Elastases inhibitors http://thorax.bmj.com/ Fully crosslinked elastin, on the other hand, is sus- Protease inhibitors are a major weapon in the body's ceptible to attack by a select group of enzymes defence arsenal to protect against excessive pro- classified as elastases. These enzymes are derived from teolytic damage to the lung, and in particular to the several sources, including the pancreas, neutrophils, elastin fibre. Of the many inhibitors circulating in the macrophages, monocytes, platelets, smooth muscle serum, al antiprotease has the greatest impact on cells, and fibroblasts. Although they are designated as elastin metabolism. a, Antiprotease is a glycoprotein these are not in sense of molecular weight 54000 that is synthesised in the elastases, enzymes specific any on October 2, 2021 by guest. Protected copyright. and have been shown to cleave core proteins of pro- liver and is present in the serum at concentrations teoglycan molecules, types III and IV collagen, ranging from 1-8 to 2-0 mg/l. The diffusion of this fibronectin, and many of the plasma proteins.33 protein into the lung accounts for over 90% of the The role of these enzymes in normal elastin metab- elastase inhibitory capacity of the alveolar epithelial olism is difficult to assess. It is assumed that elastin fluid. Very stable, inactive complexes are formed with does have a normal turnover, albeit with a very long all elastases except . Individuals half life, and that the turnover observed is not just a with a heritable deficiency in ax, antiprotease (Pi ZZ) reflection of some pathological degradation process; often have circulating inhibitor concentrations only nevertheless no one has yet shown a requirement for 10-20% of normal. This defect is often associated elastase in normal turnover or restructuring,of elastin with early onset of panlobular emphysema and has in tissues as they develop. been instrumental in the development of the hypothe- is secreted into the small intes- sis of protease-antiprotease imbalance in the patho- tine and it is unlikely to have any role in normal elas- genesis of emphysema.38 tin turnover. There have been reports, however, of its detection in the blood,34 and during severe pan- Configuration and function of elastin fibres in the lung creatitis it can damage the lung, inducing vascular injury.35 During the past two decades perhaps no single organ Circulating neutrophil leucocytes produce an elas- in the body has generated more interest in the con- Thorax: first published as 10.1136/thx.41.8.577 on 1 August 1986. Downloaded from

580 nective tissue studies than the lung. This is fitting in fibres. some respects, since before this biochemists seemed to have a particular reluctance in applying their tech- Elastin and early development of the lung niques to studies of this complex tissue. Moreover, during this period there evolved a realisation that no The chronological appearance of elastin in mam- other organ in the body depended so heavily on the malian lung is similar to that seen in studies in other proper architecture and stability of connective tissues tissues.45 Generally, it has been observed that around for proper function, or could be so influenced by envi- the third trimester of pregnancy microfibrillar com- ronmental factors. ponents appear, followed within a few days or weeks, The elastin fibres of the lung are probably the most depending on the species, by deposition of amor- important determinant of lung elasticity under phys- phous elastin, which steadily increases in amount iological pressures. They can stretch to 140% of their until parturition. During embryogenesis elastin syn- resting length before breaking, whereas collagen, thesis in the lung is associated with specific devel- which contributes tensile strength to the lung, can opmental periods. Throughout the canular stage of stretch only about 2%. fetal lung development the elastin concentration is Elastic fibres are ubiquitous in the lung, and as with very low. During the period of alveolarisation elastin other organs they are closely associated with collagen synthesis is dramatically increased and the concen- and proteoglycans. Elastin and collagen fibres from tration in the lung rises. After parturition rapid elas- alveoli, bronchi, interlobular septae, and the pleura tin accumulation continues through the perinatal all appear to have connections with fibres of pul- period, which in man may extend up to seven years. monary . This gives a continuum of fibres Elastin and lung development has been reviewed in throughout the lung such that any force exerted on more detail by Rucker and Dubick.32 the parenchyma is distributed throughout the organ. In the larger pulmonary arteries and airway elastin Experimental disruption of elastin in the lung fibres are present both as circular and as longitudinal bundles. In the vessels they are more prominent in the There is general agreement that the mechanical prop- media but are also evident in the adventitia and erties of the normal lung are much influenced by elas- intima. The fibres are seen in the submucosal areas in tin and other connective tissue components. The het- the airways. Elastin fibres in respiratory bronchioles erogeneity and close association of these components, become more distinct distally, where the alveoli are however, make it very difficult to assess the con- http://thorax.bmj.com/ also more developed. As the fibres continue distally, tribution that each makes to lung function. The most they take on a circular or helical arrangement. This successful approach to defining the mechanical prop- observation was reported early by Pierce and Ebert.43 erties of the lungs in terms of its individual connective They observed that collagen and elastin encircled tissue components is found in studies aimed at pro- respiratory bronchioles and alveolar ducts in a coiled ducing lesions directed towards specific lung proteins. or helical fashion. They proposed that the lung tissue Numerous studies have been reported using vari- unfolds to accommodate its increased volume on ous means to disrupt or degrade lung elastin. When

expansion, much as in the expansion of an old fash- the injury is considerable and affects the lower air- on October 2, 2021 by guest. Protected copyright. ioned door spring. In other words, the tissue can ways and alveoli, the lungs usually progress to some expand with no substantial increase in the length of form of emphysema or . These models have the elastin fibres. This is an important concept since been reviewed at great length in several excellent elastin is so intimately associated with collagen that articles.38 -42 the fibres may be unable to undergo axial elongation owing to the inelasticity of collagen. The Setnikar- INTRATRACHEAL INSTILLATION OF ELASTASE Mead model,44 on the other hand, proposes that col- Perhaps the most dramatic model has been the intra- lagen and elastin operate in parallel, and indepen- tracheal administration of pancreatic or neutrophil dently of each other. At low lung volumes the elastin elastase to experimental animals, which results in a fibres are readily extendable, giving the steep part of rapid loss of as much as 40% of lung elastin. Within the volume-pressure curve. As the lung volume minutes of elastase administration surfactant activity increases, the coiled collagen fibres strengthen, pro- is altered, and the elastase has begun to enter the type ducing a decrease in compliance of the combined col- I alveolar epithelium. On entering the lung inter- lagen and elastin networks and increasing the stiffness stitium, the elastase spreads rapidly and degradation of the lungs at maximal lung volumes. In vitro stud- of elastin fibres is initiated. A major portion of the ies, using or elastase for selective dis- enzyme is cleared from the lung either by entering the ruption of connective tissue components, support the blood stream and combining with inhibitors or by theory of parallel functioning of elastin and collagen being engulfed by macrophages. The remaining elas- Thorax: first published as 10.1136/thx.41.8.577 on 1 August 1986. Downloaded from

581 tase continues to attack the elastin fibre for at least internal surface area. The age of the animal during several days. When the degradative process tapers off, exposure appears to be an important factor there is a rapid increase in connective tissue synthesis influencing the extent of lung injury, as nursing ham- as the cells attempt to repair the damaged tissue. sters are more susceptible to permanent lung injury Within a month total elastin content has been than three week old animals or adults.42 54 replaced but, of course, any severed alveolar walls ENDOTOXIN cannot be rebuilt, which results in permanently Mild lung destruction has been observed with enlarged air spaces. It is not clear whether the newly repeated intravenous injections of endotoxin. If rats synthesised elastin actually repairs areas where a rup- are treated with D-galactosamine to lower a, anti- ture would otherwise be imminent. Clearly if the proteinase concentrations just before the adminis- repair process is impaired by inhibition of elastin tration ofendotoxin, there is a considerable reduction crosslinking with BAPN or by cigarette smoke, the in lung elastin with an increase in mean linear inter- resulting emphysema is exacerbated.4b47 Further- cept and lung compliance.55 more, young, rapidly growing animals appear to have less severe emphysema than older animals after elas- INTRATRACHEAL CADMIUM tase administration, which may also reflect the capa- Intratracheal administration of cadmium has been bility of young animals for accelerated elastin syn- shown to cause enlargement of air spaces and, in thesis and repair.48 some instances, fibrosis.42 The mechanism of injury is The severity of emphysema increases substantially not clear and may be modulated to a large degree by between three weeks and several months after elastase peripheral factors, such as the mode of adminis- administration. Studies with tritiated elastase show tration or agents that block the elastin repair process. that only 1% of the initial dose remains in the lung Aerosol administration produces a lesion resembling after four days.49 50 Radioactivity, however, is still human centrilobular emphysema. Instillation of cad- detected in the lung 144 days after administration. It mium in saline, however, causes functional and mor- was suggested that a persistent low level of elastase phological abnormalities characteristic of pulmonary might remain for months, giving rise to the pro- fibrosis. When BAPN is administered simultaneously gressive lesions in these animals. Other studies, with cadmium instillation the animals develop however, suggest that functional elastase activity in changes typical of bullous emphysema, with no indi- the lung lasts only a few days after administration.5" cation of fibrosis.56 This again illustrates the The outcome of studies with chloromethyl ketone importance of elastin and perhaps collagen repair http://thorax.bmj.com/ inhibitors also argues against prolonged enzymatic processes not only in the degree of severity but also in activity.52 Another suggestion is that the stress of the nature of the lesion. The role played by elastase in breathing has an effect on the injured connective this model is not clear. Neutrophils are pulled into the tissue framework, leading to further damage of alveo- lungs of hamsters after cadmium installation and lar walls. have been shown to lose azurophilic granules and Light and electron microscopic examination shows elastase.57 If, however, the animals are depleted of lesions resembling human panlobular emphysema, neutrophils before being given cadmium and BAPN, the destruction of elastin fibres with including the lesions are just as severe as those produced in on October 2, 2021 by guest. Protected copyright. decreased numbers of enlarged and distorted alveoli. hamsters with normal neutrophil levels.58 Quantitative histological techniques reflect these changes; the mean linear intercept is increased and the COPPER DEFICIENCY internal surface area is decreased. Physiological stud- Copper deficiency prevents normal crosslink for- ies show an increase in lung compliance, total lung mation in both elastin and collagen. When weanling capacity, residual volume, and functional residual rats from copper deficient dams were continued on a capacity. Apparently there is also some destruction of copper deficient diet for six to 10 weeks, their lungs lung collagen, but this lesion has not been localised. showed a significant reduction in elastin content along with increased mean linear intercepts.1442 The NITROUS OXIDE emphysematous changes were not reversible with Another model ofemphysema results from prolonged copper supplementation once the lesions were exposure of experimental animals to nitrous oxide, formed. Similar observations have been made with which may result in small airway lesions, an increase copper deficient pigs and hamsters.42 Yet, inter- in neutrophil and macrophage populations in lavage estingly, the lungs from these animals were not fluid, and a loss of lung elastin.53 Total elastin returns different from controls in total elastin content. It was to normal with the cessation of exposure to nitrous postulated that low ceruloplasmin concentrations oxide. Mean linear intercept and lung volumes resulted in oxidative cleavage of the elastin fibres, increase and there is a corresponding decrease in the producing emphysematous changes without appre- Thorax: first published as 10.1136/thx.41.8.577 on 1 August 1986. Downloaded from

582 ciable removal of elastin. smoke causes an accumulation in the respiratory The mottled mouse has a genetic defect affecting bronchi of alveolar macrophages, which appear to be copper metabolism, which results in low lysyl oxidase filled with pigments and are metabolically and mor- activity and lack of normal connective tissue cross- phologically activated. The activated macrophage has linking.40 These animals develop progressive pan- the ability to secrete chemoattractants and secre- lobular emphysema with increased compliance and tagogues for neutrophils, as well as secrete a metal- decreased elastin recoil. Mean linear intercepts are loprotease capable of digesting elastin and oi anti- increased and internal surface area is decreased. protease. The end result is a clustering of large These animals develop more severe emphysema when numbers of neutrophils and macrophages, poised to exposed to nitrous oxide59 and could provide a sensi- release considerable amounts of elastolytic enzymes tive model system for testing compounds that might at the site where the earliest signs of centrilobular potentiate lung injury. emphysema are detected. In addition to this, the alve- olar macrophages, as well as cigarette smoke, are rich Repair of lung damage sources of oxidising agents. One potential action of these oxidants would be to oxidise the res- Although there are some differences between the ani- idue found at the of cx1 proteinase inhibitor. mal models, the importance of the integrity of the This has been shown by selective chemical oxidation elastin fibre in maintaining normal lung function is to yield a relatively ineffective inhibitor that associ- apparent. Equally important is the ability of the lung ates with elastase some 2000 times more slowly than to turn on elastin synthesis and remodel areas where the native protein.38 60 61 There is also the potential lesions have occurred. for direct oxidant damage to lung cells or cellular Often when the lung is injured and there is cellular components such as lipids, cofactors, and nucleic destruction in the parenchyma, reparative processes acids. Recently there have been suggestions that go awry and the lungs become fibrotic. This process is endogenous antioxidant systems within the lung, such usually equated with a considerable increase in col- as ceruloplasmin, vitamin C, or methionine lagen, yet there is an equally important increase in sulphoxide-peptide reductase, may be adversely total cell mass, as well as in elastin and the other com- affected by cigarette smoke, lowering the lung's ponents of the extracellular matrix. Agents that may defence against oxidants.6' The elastin maturation cause interstitial pulmonary fibrosis are diverse, process may itself be impaired by cigarette smoke. including oxygen and oxides of nitrogen and sulphur, Chronic exposure of hamsters to cigarette smoke, http://thorax.bmj.com/ inorganic dusts, infectious agents, and drugs such as after a single intratracheal dose of elastase, reduced bleomycin. Many patients with adult respiratory dis- the rate of desmosine formation in resynthesised elas- tress syndrome who have survived develop what may tin.47 Lung lysyl oxidase activity appeared be considered a form of rapidly progressing inter- significantly lowered in the animals exposed to ciga- stitial pulmonary fibrosis. The mechanism of fibrosis rette smoke. Similar in vitro studies have also shown is unknown. It is apparent that the major pulmonary that the ability of lysyl oxidase to convert the lysine inflammatory and immune effector cells, neutrophils, residues of tropoelastin to aldehydes is blocked by have critical roles in extracts from cigarette smoke.62 macrophages, and lymphocytes on October 2, 2021 by guest. Protected copyright. this process through the release of enzymes, oxidants, chemotactic factors, growth factors, and secre- Effect of infection tagogues. Acting in concert with probably several types of lung cells, such as the septal cells of the alve- Bacterial infection in the lung is another means of olar walls, smooth muscle cells, fibroblasts, and endo- delivering destructive potential to the elastin fibres. thelial and epithelial cells, there is a rapid move to On the one hand, infection can draw substantial num- repair the injury and restore normal lung architecture bers of neutrophils to the lung. Even more damaging and function. An apparent loss of regulatory control, may be an elastase released from organisms such as as well as problems in directing the replacement pro- Pseudomonas aeruginosa. Patients with cystic fibrosis teins to the appropriate sites of injury, may result in a commonly have serious infection with this organism connective tissue build up that may virtually oblit- that proves impossible to eradicate. As the lifespan of erate the alveolar air spaces. these patients has dramatically increased in recent years, the incidence of emphysema has also gone up. Effect of tobacco smoke Increased degradation of elastin has been documen- ted in these patients by increased excretion of The effect of tobacco smoke on lung elastin is desmosine.63 The assumption is made that the extremely complicated, affecting many facets of con- increased desmosines originate from the lung and are nective tissue metabolism. Inhalation of cigarette a reflection of the damage being done to elastin fibres, Thorax: first published as 10.1136/thx.41.8.577 on 1 August 1986. Downloaded from

583 but this has not been documented. the effects of a poor nutritional state sooner than a protein like elastin with its extremely slow turnover. Effect of nutrition Malnutrition in infants, however, particularly during the period of alveolarisation, could have pronounced One area of growing interest is the role of nutrition in and perhaps permanent harmful effects on elastin chronic lung disease. This topic has been recently architecture and lung function. reviewed in detail.60 64 For many years it has been known that excessive weight loss and starvation are Consequences of stability positively correlated with the development of emphysema. Deprivation of essential nutrients, such In many respects elastin is a perfectly designed pro- as trace minerals and vitamins, could foster con- tein for its role in normal lung function. The unusual nective tissue abnormalities and predispose the lung amino acid composition and lysine derived crosslinks to injury through several means. Unusually low con- provide the elastin fibre with great distensibility and centrations of copper could affect lysyl oxidase activ- recoil properties. They also lend chemical stability to ity, retarding crosslink formation in elastin and col- the fibre, which is susceptible to few proteolytic lagen. Superoxide dismutase and ceruloplasmin also enzymes and chemical injuries. Complications arise in require copper for activity and both may function as conjunction with this inherent stability. Mature elas- antioxidants. Selenium is required for tin has an extremely low turnover rate. Once the deli- peroxidase activity and iron is required for catalase. cate architecture of the alveolar walls has been con- Vitamins E and C are effective scavengers of free rad- structed and the continuum ofconnective tissue fibres icals and deficiencies of these may also affect the lung is established, the components are meant to remain in cells' defence against oxidant damage. that configuration. After the fetal and early perinatal A more direct effect on lung connective tissue may stages of lung development we apparently have no be seen with a deficiency in vitamin B6. Several stud- contingency programme for initiating a new and ies have indicated that elastin crosslinking is impaired architecturally correct alveolus if the original struc- in B6 deficient animals, implying that pyridoxal phos- ture has been destroyed. So what has man done in his phate is a for lysyl oxidase.64 A recent study, infinite wisdom but devise every means possible to however, presents evidence suggesting that the B6 bring tobacco smoke and other pollutants and drugs effect on elastin crosslinking is due to a block in the into direct contact with this very durable but not in-

conversion of to .65 The destructible connective tissue matrix? Our defence http://thorax.bmj.com/ raised concentrations of homocysteine then block the system is magnificent, as we signal for macrophages conversion of to the desmosines through the and neutrophils to fight this unforeseen menace. As formation of thiazine derivatives with the aldehyde the neutrophils degranulate and release their enzymes functional group. there is disparity between the finely tuned ratio of Supplementing a copper deficient diet with a high elastase to antiprotease. Every injury sustained by content of vitamin C was shown many years ago to alveolar elastin that is not repaired hastens the inevi- worsen the effects of copper deficiency and further table cleavage of the alveolar wall. If the injury is reduce the elastin content of the aorta.66 Recently it perpetuated, as is the case with cigarette smoke, alve- has been observed that smooth muscle cells in culture olar walls are slowly cleaved, leaving greatly enlarged on October 2, 2021 by guest. Protected copyright. synthesise significantly less elastin when the medium air spaces and a lung without elastic recoil. Appar- is supplemented with vitamin C.8 67 One could specu- ently, during some phase of alveolar damage the late that, with today's overindulgence in vitamin sup- resynthesis of elastin and the repair of fibres has an plements, excessive vitamin C concentrations could appreciable effect on the development of the lesion. slow down elastin synthesis in the lung during acute The nutritional state and age at onset of injury may or chronic lung injury and thereby impair the crucial be of paramount importance during this period. elastin replacement process. Starvation in young growing rats produces a gen- Protecting elastin eral loss of connective tissues, which may result in emphysematous like changes in lung structure.42 This What avenues are being considered for preventing or effect was not seen in older animals, where growth at least curtailing the destruction of the elastin fibre in retardation did not occur.68 A direct relationship the lung? One approach is to increase the serum elas- between malnutrition and the elastin fibre component tase inhibitory capacity. Replacement treatment with of the adult human lung will be difficult to show. a, antiprotease is currently being evaluated with lim- Other lung elements, such as surfactant, collagen, and ited numbers of patients with a genetic deficiency in the respiratory muscles, which all play important a, antiprotease.69 Assessment of the efficacy of this parts in normal lung function, would probably feel method will be difficult and it is the topic of a recent Thorax: first published as 10.1136/thx.41.8.577 on 1 August 1986. Downloaded from

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