Studies on the Regulatory Mechanisms and Molecular Diversity of Integrinmediated Signal Transduction

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Molecular mechanisms underlying the cell-basement membrane interaction were investigated with an emphasis on the integrin-mediated signal transduction. The major achievements are summarized below.1. The major integrin isoforms responsible for cell adhesion onto basement membranes, i.e., integrin α3β1 and α6β1, were purified to homogeneity under non-denaturing conditions. The ligand binding specificities of these integrins were determined by solid-phase binding assays towards a panel of laminin isoforms differing in their α subunit composition. Our results clearly showed that laminin-10 that contains the α5 chain is the most preferred ligand for both laminin-binding integrins.2. Cell adhesion onto laminin-10 through these integrins was found to selectively activate Rac, a member of Rho family GTPases, thereby strongly promoting formation of lamellipodia and cell motility. Rac activation on laminin-10 was associated with Selective tyrosine-phosphorylation of p130Cas, but not FAK, followe … More d by a ternary complex formation among p130Cas, CrkII, and DOCK180, the latter of which is a guanine nucleotide exchange factor for Rac.3. Cell adhesion onto laminin-10 was also found to activate the PI3K-Akt pathway, leading to a strong activation of Akt, Consistent with the role of Akt in cell survival, cells on laminin-10 were much more resistant than those on fibronectin against apoptosis induced. By, serum deprivation. Interestingly, cell survival on fibronectin was dominantly dependent on the ERK pathway, but not the PI3K-Akt pathway.4. Laminin-8, the major laminin isoform in the endothelial basement membrane, was purified to homogeneity from the conditioned medium of human glioma cells. Cell adhesion onto laminin-8 was strictly dependent on integrin α6β1 and preferentially activated Rac, as was the case with cell adhesion onto laminin-10.5. A 30 kD protein was found to copurify with integrin α3β1. This 30 kD protein was identified as CD151, a member of tetraspanin family proteins that span the plasma membrane four times. The CD151/α3β1 integrin complex was found to localize at basolaterai surfaces of epithelial cells, where it was involved in cell-cell adhesion through promoting reorganisation of actin cytoskeleton Less