Abstract. The mouse cell line GD25, which lacks expression of the 131 family of integrin heterodimers due to disruption of the 131 integrin subunit gene, was used for expression of full-length cDNA coding for splice variant A of the mouse 131 integrin subunit. In a stably transformed clone (GD25-131A), the expressed protein was found to form functional heterodimeric receptors together with the subunits et3, ~5, and et6. Both GD25 and GD25-131A attached to fibronectin and formed focal contacts which contained otv133, but no detectable ot5131A. The presence of GRGDS peptide allowed tx5131A to locate to focal contacts of GD25-131A cultured on fibronectin, while the 131-null GD25 cells were unable to attach under these conditions. Affinity chromatography revealed that ct5131A and otv133 could bind to a large cell-binding fragment of fibronectin, a5131A strongly promoted polymerization of fibronectin into a fibrillar network on top of the cells. Whereas little etv133 was colocalized with the fibronectin fibrils in GD25-131A cells, this integrin was able to support fibronectin fibril polymerization in GD25 cells. However, the etv133-induced polymerization was less efficient and occurred mainly in dense cultures of the GD25 cells. Thus, while both a5131A and ave3 are able to support adhesion to fibronectin, etv133 dominates in the formation of focal contacts, and a5131A has a prime function in fibronectin matrix assembly. This is the first report on fibronectin matrix assembly in the absence of 131 integrins.

alpha11beta1 integrin constitutes a recent addition to the integrin family. Here, we present the first in vivo analysis of alpha11 protein and mRNA distribution during human embryonic development. alpha11 protein and mRNA were present in various mesenchymal cells around the cartilage anlage in the developing skeleton in a pattern similar to that described for the transcription factor scleraxis. alpha11 was also expressed by mesenchymal cells in intervertebral discs and in keratocytes in cornea, two sites with highly organized collagen networks. Neither alpha11 mRNA nor alpha11 protein could be detected in myogenic cells in human embryos. The described expression pattern is compatible with alpha11beta1 functioning as a receptor for interstitial collagens in vivo. To test this hypothesis in vitro, full-length human alpha11 cDNA was stably transfected into the mouse satellite cell line C2C12, lacking endogenous collagen receptors. alpha11beta1 mediated cell adhesion to collagens I and IV (with a preference for collagen I) and formed focal contacts on collagens. In addition, alpha11beta1 mediated contraction of fibrillar collagen gels in a manner similar to alpha2beta1, and supported migration on collagen I in response to chemotactic stimuli. Our data support a role for alpha11beta1 as a receptor for interstitial collagens on mesenchymally derived cells and suggest a multifunctional role of alpha11beta1 in the recognition and organization of interstitial collagen matrices during development.

There is currently a great interest in identifying laminin isoforms expressed in developing and regenerating skeletal muscle. Laminin ␣1 has been reported to localize to human fetal muscle and to be induced in muscular dystrophies based on immunohistochemistry with the monoclonal antibody 4C7, suggested to recognize the human laminin ␣1 chain. Nevertheless, there seems to be no expression of laminin ␣1 protein or mRNA in developing or dystrophic mouse skeletal muscle fibers. To address the discrepancy between the results obtained in developing and dystrophic human and mouse muscle we expressed the E3 domain of human laminin ␣1 chain as a recombinant protein and made antibodies specific for human laminin ␣1 chain (anti-hLN-␣1G4/G5). We also made antibodies to the human laminin ␣5 chain purified from placenta. In the present report we show that hLN-␣1G4/G5 antibodies react with a 400-kDa laminin ␣1 chain and that 4C7 reacts with a 380-kDa laminin ␣5 chain. Immunohistochemistry with the hLN-␣1G4/G5 antibody and 4C7 revealed that the two antibodies stained human kidney, developing and dystrophic muscle in distinct patterns. Our data indicate that the previously reported expression patterns in developing, adult, and dystrophic human muscle tissues with 4C7 should be re-interpreted as an expression of laminin ␣5 chain. Our data are also consistent with earlier work in mouse, indicating that laminin ␣1 is largely an epithelial laminin chain not present in developing or dystrophic muscle fibers.

␣-Dystroglycan (␣-DG) represents a highly glycosylated cell surface molecule that is expressed in the epithelial cell-basement membrane (BM) interface and plays an essential role in epithelium development and tissue organization. The ␣-DG-mediated epithelial cell-BM interaction is often impaired in invasive carcinomas, yet roles and underlying mechanisms of such an impaired interaction in tumor progression remain unclear. We report here a suppressor function of laminin-binding glycans on ␣-DG in tumor progression. In aggressive prostate and breast carcinoma cell lines, lamininbinding glycans are dramatically decreased, although the amount of ␣-DG and ␤-dystroglycan is maintained. The decrease of lamininbinding glycans and consequent increased cell migration were associated with the decreased expression of ␤3-N-acetylglucosaminyltransferase-1 (␤3GnT1). Forced expression of ␤3GnT1 in aggressive cancer cells restored the laminin-binding glycans and decreased tumor formation. ␤3GnT1 was found to be required for laminin-binding glycan synthesis through formation of a complex with LARGE, thus regulating the function of LARGE. Interaction of the laminin-binding glycans with laminin and other adhesive molecules in BM attenuates tumor cell migratory potential by antagonizing ERK/AKT phosphorylation induced by the components in the ECM. These results identify a previously undescribed role of carbohydrate-dependent cell-BM interaction in tumor suppression and its control by ␤3GnT1 and LARGE.glycosylation ͉ cell adhesion ͉ basement membrane ͉ carcinoma I nteraction of epithelial cells with basement membrane (BM) is mediated by cell adhesion molecules, which operate at the interface of epithelial cell-ECM and regulate cell growth, motility, and differentiation by integrating signals from ECM or soluble factors (1-3). One of the most important epithelial cell-BM interactions is mediated by ␣-dystroglycan (␣-DG) on epithelial cells (4).␣-DG is a cell surface receptor for several major BM proteins, including laminin, perlecan, and agrin. A laminin G-like domain in all these glycoproteins binds to a unique glycan structure attached to ␣-DG, and this interaction has been shown to be critical in assembling BM (5, 6). This unique glycan structure is referred to as laminin-binding glycans hereafter. ␣-DG is not attached directly to the plasma membrane but is bound to it through attachment to the transmembrane protein ␤-dystroglycan (␤-DG), which binds to the cytoplasmic protein dystrophin, which, in turn, binds to the actin cytoskeleton and many adaptor molecules involved in cellular signaling (4,5).␣-DG is highly glycosylated and contains both N-linked glycans and mucin type O-glycans. The mucin type O-glycans are clustered in a mucin-like domain at the N-terminal of mature ␣-DG, which includes unique O-mannosyl glycans and sialic acid ␣233Gal␤134GlcNAc␤132Man␣13Ser/Thr (7). Defects in glycosylation of the O-mannosyl glycans have been shown to cause muscular dystrophy (8). So far, 7 glycosyltransferases or glycosyltransferase-like ...

Integrin α11β1 is a stromal cell-specific receptor for fibrillar collagens and is overexpressed in carcinoma-associated fibroblasts (CAFs). We have investigated its direct role in cancer progression by generating severe combined immune deficient (SCID) mice deficient in integrin α11 (α11) expression. The growth of A549 lung adenocarcinoma cells and two patient-derived non-small cell lung carcinoma (NSCLC) xenografts in these α11 knockout (α11−/−) mice was significantly impeded, as compared with wild-type (α11+/+) SCID mice. Orthotopic implantation of a spontaneously metastatic NCI-H460SM cell line into the lungs of α11−/− and α11+/+ mice showed significant reduction in the metastatic potential of these cells in the α11−/− mice. We identified that collagen cross-linking is associated with stromal α11 expression, and the loss of tumor stromal α11 expression was correlated with decreased collagen reorganization and stiffness. This study shows the role of integrin α11β1, a receptor for fibrillar collagen in differentiation of fibroblasts into CAFs. Furthermore, our data support an important role for α11 signaling pathway in CAFs, promoting tumor growth and metastatic potential of NSCLC cells and being closely associated with collagen cross-linking and the organization and stiffness of fibrillar collagen matrices.