PERSPECTIVE
Airway Epithelial Integrins: Why So Many?

Dean Sheppard

Department of Medicine, Lung Biology Center and Cardiovascular Research Institute, University of California, San Francisco, San Francisco, California

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Integrins are a large family of heterodimeric transmembrane receptors (1) that can recognize as ligands components of the extracellular matrix and cell-surface counter-receptors of the immunoglobulin and cadherin families (2). The first members of this family were identified as the long-sought "adhesion receptors" that were responsible for attachment of migratory cells to specific components of the extracellular matrix (3). Although this adhesive function of integrins is clearly an important one, it has become increasingly clear over the past few years that integrins are much more than simple examples of cellular glue. Ligation of specific members of the integrin family has now been shown, in various cell types, to be critical for cell survival (4, 5), progression through the cell cycle (6), cellular differentiation (7, 8), establishment of cell polarity, and regulation of expression of a wide variety of genes (8). Integrin ligation has now been shown to induce or inhibit expression of genes encoding matrix metalloproteinases (10), proinflammatory and anti-inflammatory cytokines (11), inhibitors of cell cycle progression (12), and modulators of programmed cell death (5). It is thus clear that, in addition to filling important roles in mechanical processes such as cell adhesion and control of cell shape, integrins function as classic signaling receptors (reviewed by Clark and Brugge [13]).

With the recognition that integrins play important roles in signaling, several laboratories began a concerted effort to identify the biochemical bases of integrin-induced signals. It is now clear that integrins are central components of large, multimeric protein complexes that are assembled at sites of close cellular contact with multivalent integrin ligands (e.g., at sites of contact with the extracellular matrix) (13). Within these complexes, integrins themselves serve as the detectors. In most cells, these complexes also contain docking proteins (e.g., paxillin, cytoskeletal associated substrate of src [p130Cas], and a receptor for activated C kinase [RACK 1] [14]) that can localize effector proteins to these sites and a number of effector proteins that are known to initiate signals mediated by other transmembrane signaling receptor families. The complexity of these signaling machines is underscored by the fact that many of the effectors (e.g., the focal adhesion kinase and src family kinases) can themselves serve as docking proteins for other effectors. The picture that is emerging is quite similar to our current understanding of the biochemical bases of signaling through growth factor receptors and antigen receptors that also organize large multimeric complexes. With respect to all of these families of signaling receptors, current understanding of the general pathways that can be activated greatly exceeds understanding of the bases for signaling specificity in response to a specific receptor in a specific cell type.

In this issue, Jabbour and coworkers show that at least some of the integrins expressed on cultured airway epithelial cells are redistributed on the cell surface in response to the common air pollutant ozone (15). To understand the biologic significance of this finding, it will be important to understand which members of the integrin family are expressed on airway epithelial cells, and what they are really there for. Figure 1 shows the 23 known members of the integrin family, organized by shared  or  subunits. Of these, at least eight can be simultaneously expressed on airway epithelial cells (16) (Steve Nishimura, personal communication). The known ligands of each of these eight integrins are shown in Table 1. One somewhat surprising feature that emerges from this information is that, of the integrins these cells express, only two31 and 64 recognize as their principal ligand a protein that is normally in contact with these cells (the basement membrane protein laminin-5). The results of gene knockout experiments inactivating the genes for the 6, 3, and 4 subunits confirm that these integrins are critical for interactions with epithelial basement membranes in vivo. Both 6 and 4 knockout mice die at birth because of widespread separation of epithelia from their basement membranes (21). The basement membranes underlying epithelial cells from 3 knockout mice are highly disorganized (22), demonstrating a role for this integrin in the structural organization of normal epithelial basement membrane. But why do airway epithelial cells put so much effort into expressing receptors for type I collagen, fibronectin, vitronectin, tenascin, and osteopontin when, under normal circumstances, these proteins are not present in the immediate microenvironment of these stationary cells? Furthermore, why go to the trouble of simultaneously expressing multiple receptors for the same absent ligands?

View larger version (16K): [in this window] [in a new window]   Figure 1.   Organization of the integrin family. Connecting lines identify all currently known integrin heterodimers.

The short answer to these questions is that no one really knows. However, some hypotheses about functions for airway epithelial integrins can be generated on the basis of the known expression pattern of their ligands. Thus, fibronectin, tenascin, and osteopontin are all greatly enriched in epithelia during development, in the settings of injury and inflammation, and in the stroma surrounding epithelial neoplasms. It is thus conceivable that many of the integrins expressed on airway epithelial cells are present to allow these cells to detect rapidly and respond to dynamic alterations in the matrix that occur during development, in response to injury, and in association with primary and/or metastatic tumors. Preliminary evidence, using colonic epithelial cells (SW480 cells) transfected to express the integrin 6 subunit, suggests that one such effect could be enhancement of epithelial cell proliferation, especially when cells are forced to grow in a three-dimensional culture environment (23). In that experiment, heterologous expression of the 6 subunit (and hence of the integrin v6) also enhanced the ability of these cells to form tumors in nude mice, suggesting that this enhanced ability to grow is directly relevant to epithelial malignancies.

To test the hypothesis that airway epithelial integrins regulate responses to injury or inflammation in vivo, I and others have begun to examine the effects of inactivation of specific integrin subunits using mice expressing integrin null mutations. Thus far, knockouts have been successfully generated for four of the six airway epithelial integrins whose ligands are not normally present in the basement membrane. One limitation to this approach is the requirement of generating live mice. Knockout of the integrin 5 subunit results in embryonic lethality (24), and knockout of the 9 subunit causes death within the first 10 d after birth as a result of bilateral chylothorax (D. Sheppard, unpublished observation). Any inferences about roles for the integrins 51 and 91 in normal adult airways will thus need to await the development of conditional knockouts. Inactivation of the 5 subunit gene (and thus of the v5 integrin) results in normal mice, and these animals have normal responses to a variety of insults to the lungs and other organs (D. Sheppard, unpublished observations). Thus, if v5 plays any role in normal airway epithelial biology, the same role must be played by some other protein or proteins that are unaffected by inactivation of the 5 gene.

For at least one airway epithelial integrin, v6, gene inactivation has provided important clues to in vivo function. Inactivation of the 6 subunit gene (and thus of the integrin v6) in mice results in inflammatory baldness and in dramatic lung and airway inflammation associated with airway hyperresponsiveness to acetylcholine (25). The lung inflammation includes increased numbers of macrophages, lymphocytes, and, in some strains, eosinophils, and marked activation of both macrophages and lymphocytes (26). All of these morphologic effects in the lung are prevented by overexpression of the human 6 subunit as a transgene in a minority of lung epithelial cells (26). These findings demonstrate that one important role for the v6 integrin in epithelial cells is downregulating local inflammatory responses. The results also suggest that abnormalities in epithelial cells could, by themselves, initiate many of the features characteristic of inflammatory airway diseases, such as asthma.

From this brief review, it should be clear that much remains to be learned about the roles of integrins in airway epithelial cells. Of the eight integrins known to be expressed by these cells, only two (31 and 64) are likely to function principally as "adhesion receptors" in healthy adult airways. Based on the fact that the known ligands for the other six integrins expressed are absent from healthy airways but induced or enriched in inflamed and injured airways, it is likely that these receptors influence or direct the epithelial cell's responses to injury. In the case of v6, one role appears to be turning off local inflammatory responses, though the molecular details of how this is accomplished remain to be determined. An explanation of why airway epithelial cells (and many other epithelia) bother to express five other members of the integrin family awaits the results of additional experiments.

	Footnotes

Address correspondence to: Dean Sheppard, M.D., Lung Biology Center, UCSF Box 0854, San Francisco, CA 94143. E-mail: deans{at}itsa.ucsf.edu

(Received in original form March 10, 1998).

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