PERSPECTIVE
Repairing the Cables of the Lung

Charles Kuhn III

Department of Pathology, Memorial Hospital of Rhode Island, Pawtucket, Rhode Island; and Brown University School of Medicine, Providence, Rhode Island

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On May 6, 1996, a truck driver operating under the influence of alcohol rammed his 18-wheeler into one of the supporting pillars of Boston's Tobin Bridge, shutting down its lower deck which serves as the main conduit for traffic into the city from the north, and triggering a mammoth traffic jam. Initially, the authorities feared that the repairs would necessitate tearing down and rebuilding that part of the bridge, crippling traffic flow for months. Fortunately, a contractor was located, touted by the local press as the only one in the country with the necessary technology, who was able to jack up the roadway, reposition the damaged support, and complete the repairs in a matter of days. Analogously, in the current issue of the AJRCMB, investigators from Boston University describe an in vitro system that demonstrates how lung fibroblasts can repair damaged elastic fibers without degrading them and replacing them with newly synthesized elastin (1).

Elastic fibers, accompanied by collagen, form a continuous cable-like network that winds down the alveolar ducts and encircles the mouths of alveoli, exerting tension inward along the ducts to counter the outward tension in the alveolar walls and surface film (2). Elastic fibers are constructed of two major components: microfibrils, which are synthesized first during development; and an amorphous component synthesized subsequently and deposited on the microfibrils. The major constituent of the amorphous material is elastin, an insoluble polymer of tropoelastin molecules covalently bound together by cross-links of such remarkable stability that the elastin remains insoluble in boiling 0.1 N NaOH. The Boston University investigators used [¹⁴C]lysine to pulse label elastic fibers in cultures of lung fibroblasts. When they later treated such cultures with pancreatic elastase, they found that damaged elastin remaining in the matrix of the cultures was ultrastructurally abnormal and had become alkali soluble. After some further weeks in culture in the absence of exogenous label, the radioactivity associated with the damaged elastin had again become alkali insoluble, indicating that the damage had been repaired and, in addition, the ultrastructural abnormalities had disappeared. Because nearly all of the [¹⁴C]lysine in the elastin was post-translationally modified to form cross-links, they concluded that the repair involved salvage of existing cross-linked elastin peptides.

There is no direct evidence that salvage repair occurs following elastolytic damage in vivo. Ultrastructurally, damaged elastic fibers can be recognized for several days in vivo following an intratracheal elastase injection (3). In our own work, when we shifted from assaying elastin in the lung as the hot alkali-insoluble residue (4) to an assay based on desmosine content of the lung (5), the apparent elastin content of the lung 24 h after an elastase injection increased, suggesting that there is indeed a residue of alkali-soluble elastin suitable to serve as a substrate for salvage repair. In vivo, however, the elastase treatment sets in motion a complex inflammatory and biosynthetic response not fully replicated in culture. As part of that response there is increased de novo synthesis of elastin and collagen, increased activity of prolyl hydroxylase and lysyl oxidase, and a change in the composition of glycosaminoglycans in the tissue (4).

What are the implications of this unusual mechanism of dealing with damaged protein? Experiments have shown that the ability of lung tissue to repair or replace elastin damaged in the course of tissue injury profoundly influences the outcome. Injury that is selective for elastin results in emphysema, examples being the injury produced by injection of pancreatic elastase or papain, enzymes relatively specific for elastin. When the repair of the damaged elastin in these models is prevented by inhibitors of cross-linking, the emphysema is markedly more severe (5). If the lung is injured nonspecifically, i.e., with CdCl₂, inhibition of connective tissue cross-linking converts the resultant pathology from pulmonary fibrosis to emphysema (6). In the model of elastase-induced injury it has been suggested that despite the restoration of the elastin content of the lung to normal subsequent to insult, emphysema results because the organization of the elastic fibers is disrupted and the network discontinuous. If this is correct, a salvage-repair mechanism using the existing elastic fibers would preserve the pre-existing organization of elastic fibers better than a process in which damaged fibers are resorbed and new ones synthesized. A major artificiality of the elastase model is that it produces massive injury to elastin all at one time, and repair mechanisms are overwhelmed. With injury produced a little at a time over a more extended period, salvage repair could minimize the disruption to the architecture of the connective tissue fibers, thereby helping to prevent overdistension of air spaces.

The mechanism of salvage repair remains unknown. Stone and co-workers presented suggestive evidence that it requires the participation of the cross-linking enzyme lysyl oxidase and requires close association between cells and the damaged elastic fibers, but that it does not require the production of new soluble tropoelastin (1). Normal elastic fiber assembly is a relatively complex process in which the tropoelastin is deposited on a scaffolding of microfibrils and linked to a specific microfibrillar protein by cysteine residues near its carboxyl end (7). Selected lysines are oxidatively deaminated by lysyl oxidase to the aldehyde allysine and brought into register so that specific allysines can undergo aldol condensation or react with unmodified lysine via Schiff base formation. The process is relatively rapid and soluble tropoelastin can become incorporated into alkali-insoluble elastin with 24 h (8). The formation of desmosine and isodesmosine cross-links by secondary reactions is nonenzymatic and is a much longer process. Salvage repair is slower and was far from complete 1 wk after the elastolytic injury had occurred. If, as proposed, lysyl oxidase is required, the process may be slow because kinetically unfavored lysyl residues are the ones left to be oxidized during repair, because the proteolyzed elastin is less basic than tropoelastin and hence has less affinity for lysyl oxidase, or for other reasons. For example, the state of the microfibrils in the proteolyzed matrix was not investigated.

Newly formed allysines in elastin undergoing salvage repair could form Schiff bases with abnormal amino termini generated by peptide bond hydrolysis during the proteolytic injury. Thus salvage repair would result in the formation of abnormal cross-links rather than restoration of the normal peptide bonds (9). Newly generated COOH groups might diminish the hydrophobicity of the elastin, which is important for its mechanical properties. Thus the elastic behavior of repaired elastin might be abnormal.

Elastin laid down in childhood ordinarily lasts a lifetime (10). During this time, it may undergo peptide chain cleavage by proteases and oxidants leaking from nearby cells, direct effects of inhaled air pollutants (11), etc. If this damage undergoes salvage repair by a mechanism like that just described, structural abnormalities may accumulate, which could explain the gradual loss of elasticity in the lung that is a universal accompaniment of aging (12).

The precedent for repairing macromolecules instead of replacing them is DNA. Even the most dedicated aficionados of the extracellular matrix would not have the hubris to claim that our fibers are as important to nature as our genes. Nonetheless, the observation that evolution has provided a mechanism for patching up elastin when it is damaged emphasizes the importance of maintaining the integrity of the mechanical stress-resistive elements of the tissue.

	Footnotes

Address correspondence to: Dr. Charles Kuhn, Department of Pathology, Memorial Hospital of Rhode Island, 111 Brewster Street, Pawtucket, RI 02860.

(Received in original form April 16, 1997).

	References

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2. Wilson, T. A., and H. Bachofen. 1982. A model for mechanical structure of the alveolar duct. J. Appl. Physiol. 52: 1064-1070 [Abstract/Free Full Text].

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5. Kuhn, C., and B. C. Starcher. 1980. The effect of lathyrogens on elastase induced emphysema. Am. Rev. Respir Dis. 122: 453-460 [Medline].

6. Niewoehner, D. E., and J. R. Hoidal. 1982. Lung fibrosis and emphysema: divergent responses to a common injury? Science 217: 356-360 .

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8. Faris, B., L. E. Jackson, B. M. Schreiber, B. M. Martin, H. V. Jones, and C. A. Franzblau. 1991. A controlled precursor add-back model of elastogenesis in smooth muscle cell cultures. Matrix 11: 367-372 [Medline].

9. Stone, P. J., S. M. Morris, B. M. Martin, M. P. McMahon, B. Faris, and C. Franzblau. 1988. Repair of protease-damaged elastin in neonatal rat aortic smooth muscle cell cultures. J. Clin. Invest. 82: 1644-1654 .

10. Winters, R. S., B. A. Burnette-Vick, and D. A. Johnson. 1994. Ozone, but not nitrogen dioxide, fragments elastin and increases its susceptibility to proteolysis. Am. J. Respir. Crit. Care Med. 150: 1026-1031 [Abstract].

11. Shapiro, S. D., S. K. Endicott, M. A. Province, J. A. Pierce, and E. J. Campbell. 1991. Marked longevity of human parenchymal elastic fibers deduced from prevalence of D-aspartate and weapons-related radiocarbon. J. Clin. Invest. 87: 1828-1834 .

12. Kuhn, C. 1997. Morphology of the aging lung. In The Lung: Scientific Foundations. R. G. Crystal, J. B. West, P. J. Barnes, and E. R. Weibel, editors. Lippincott-Raven, Philadelphia. 2187-2192.