The ␣4 laminin subunit is a component of endothelial cell basement membranes. An antibody (2A3) against the ␣4 laminin G domain stains focal contact-like structures in transformed and primary microvascular endothelial cells (TrHBMECs and HMVECs, respectively), provided the latter cells are activated with growth factors. The 2A3 antibody staining colocalizes with that generated by ␣v and ␤3 integrin antibodies and, consistent with this localization, TrHBMECs and HMVECs adhere to the ␣4 laminin subunit G domain in an ␣v␤3-integrin-dependent manner. The ␣v␤3 integrin/2A3 antibody positively stained focal contacts are recognized by vinculin antibodies as well as by antibodies against plectin. Unusually, vimentin intermediate filaments, in addition to microfilament bundles, interact with many of the ␣v␤3 integrin-positive focal contacts. We have investigated the function of ␣4-laminin and ␣v␤3-integrin, which are at the core of these focal contacts, in cultured endothelial cells. Antibodies against these proteins inhibit branching morphogenesis of TrHBMECs and HMVECs in vitro, as well as their ability to repopulate in vitro wounds. Thus, we have characterized an endothelial cell matrix adhesion, which shows complex cytoskeletal interactions and whose assembly is regulated by growth factors. Our data indicate that this adhesion structure may play a role in angiogenesis.

Laminin-5 (LN5) is a matrix component of epithelial tissue basement membranes and plays an important role in the initiation and maintenance of epithelial cell anchorage to the underlying connective tissue. Here we show that two distinct LN5 function-inhibitory antibodies, both of which bind the globular domain of the ␣3 subunit, inhibit proliferation of epithelial cells. These same antibodies also induce a decrease in mitogenactivated protein kinase activity. Inhibition of proliferation by the function-perturbing LN5 antibodies is reversed upon removal of the antibodies and can be overcome by providing the antibody-treated cells with exogenous LN5 and rat tail collagen. Because epithelial cells use the integrin receptor ␣3␤1 to interact with both LN5 and rat tail collagen, we next investigated the possibility that integrin ␣3␤1 is involved in mediating the proliferative impact of LN5. Proliferation of human epithelial cells is significantly inhibited by a function-perturbing ␣3 integrin antibody. In addition, antibody activation of ␤1 integrin restores the proliferation of epithelial cells treated with LN5 functionperturbing antibodies. These data indicate that a complex comprising LN5 and ␣3␤1 integrin is multifunctional and contributes not only to epithelial cell adhesion but also to the regulation of cell growth via a signaling pathway involving mitogen-activated protein kinase. We discuss our study in light of recent evidence that LN5 expression is up-regulated at the leading tips of tumors, where it may play a role in tumor cell proliferation. INTRODUCTIONCell interaction with elements of the extracellular matrix impacts their adherence, motility, as well as protein and gene expression (for example, see Adams and Watt, 1993;Roskelly et al., 1995). In intact normal tissue epithelial cells bind to extracellular matrix molecules, which are organized into a complex multiprotein structure called the basement membrane. The major components of the latter include type IV collagen, proteoglycans, and laminins. One laminin isoform, laminin-5 (LN5), 1 in particular plays an important role in establishing firm adherence of epithelial cells to the basement membrane, because it is necessary for the assembly and maintenance of stable anchorage devices between epithelial cells and matrix called hemidesmosomes (Baker et al., 1996;Green and Jones, 1996). However, recent reports also indicate that LN5 is expressed at the budding tips of invading tumor cells, i.e., at sites where cancer cells are undergoing cell division but where there are most likely no hemidesmosomes (Pyke et al., 1994(Pyke et al., , 1995. This provides an indication § Corresponding author. E-mail address: j-jones3@nwu.edu. 1 Abbreviations used: BrdU, bromodeoxyuridine; DAPI, 4,6-diamidino-2-phenylindole; EGF, epidermal growth factor; FN, fibronectin; hLN5, human laminin-5; Ig, immunoglobulin; LN, laminin; MAP, mitogen-activated protein; MAPK, MAP kinase; RTC, rat tail collagen; rtLN5, rat laminin-5.© 1999 by The American Society for Cell Biology 259 that L...

The ␣4 laminin subunit is a component of the basement membrane of blood vessels where it codistributes with the integrins ␣v␤3, ␣3␤1, and ␣6␤1. An antibody against the G domain (residues 919-1207; G 919 -1207 ) of the ␣4 laminin subunit inhibits angiogenesis in a mouse-human chimeric model, indicating the functional importance of this domain. Additional support for the latter derives from the ability of recombinant G 919 -1207 to support endothelial cell adhesion. In particular, endothelial cell adhesion to G 919 -1207 is half-maximal at 1.4 nM, whereas residues 919-1018 and 1016 -1207 of the G domain are poor cellular ligands. Function blocking antibodies against integrins ␣v␤3 and ␤1 and a combination of antibodies against ␣3 and ␣6 integrin subunits inhibit endothelial cell attachment to G 919 -1207 . Moreover, both ␣v␤3 and ␣3␤1 integrin bind with high affinity to G 919 -1207 . Together, our studies demonstrate that the G domain of laminin ␣4 chain is a specific, high affinity ligand for the ␣v␤3 and ␣3␤1 integrin heterodimers and that these integrins, together with ␣6␤1, function cooperatively to mediate endothelial cell-␣4 laminin interaction and hence blood vessel development. We propose a model based on these data that reconcile apparent discrepancies in the recent literature with regard to the role of the ␣v␤3 integrin in angiogenesis. matrix ͉ matrix receptor ͉ blood vessels

Mechanical ventilation is a valuable treatment regimen for respiratory failure. However, mechanical ventilation (especially with high tidal volumes) is implicated in the initiation and/or exacerbation of lung injury. Hence, it is important to understand how the cells that line the inner surface of the lung [alveolar epithelial cells (AECs)] sense cyclic stretching. Here, we tested the hypothesis that matrix molecules, via their interaction with surface receptors, transduce mechanical signals in AECs. We first determined that rat AECs secrete an extracellular matrix (ECM) rich in anastamosing fibers composed of the α3 laminin subunit, complexed with β1 and γ1 laminin subunits (i.e. laminin-6), and perlecan by a combination of immunofluorescence microscopy and immunoblotting analyses. The fibrous network exhibits isotropic expansion when exposed to cyclic stretching (30 cycles per minute, 10% strain). Moreover, this same stretching regimen activates mitogen-activated-protein kinase (MAPK) in AECs. Stretch-induced MAPK activation is not inhibited in AECs treated with antagonists to α3 or β1 integrin. However, MAPK activation is significantly reduced in cells treated with function-inhibiting antibodies against the α3 laminin subunit and dystroglycan, and when dystroglycan is knocked down in AECs using short hairpin RNA. In summary, our results support a novel mechanism by which laminin-6, via interaction with dystroglycan, transduces a mechanical signal initiated by stretching that subsequently activates the MAPK pathway in rat AECs. These results are the first to indicate a function for laminin-6. They also provide novel insight into the role of the pericellular environment in dictating the response of epithelial cells to mechanical stimulation and have broad implications for the pathophysiology of lung injury.