β1B integrin interferes with matrix assembly but not with confluent monolayer polarity, and alters some morphogenetic properties of FRT epithelial cells

beta 1B is a beta 1 integrin splice variant that differs from the ubiquitous beta 1A in the terminal portion of the cytosolic tail. The expression of this variant in CHO cells results in reduced fibroblast adhesion and motlity (Balzac, F. et al., J. Cell Biol. 127, 557-565 (1994)). We have evaluated the phenotypic changes induced by the expression of beta 1B in the FRT epithelial cell line. Stable transfectants of FRT cells expressing beta 1B or beta 1A human integrins were obtained. The transfected integrins associated with the endogenous alpha subunits and were delivered to the plasma membrane. beta 1B expressing cells attached less efficiently and spread less on fibronectin, laminin or type IV collagen coated dishes. A great reduction of fibronectin fibrils associated to the basal membrane of non-confluent beta 1B transfected cells was observed. This was paralleled by the disappearance of microfilament bundles and loss of basally located focal adhesions. On the contrary, upon beta 1A transfection, a higher amount of fibronectin fibrils, together with microfilament bundles and focal adhesions, was observed. Expression of beta 1B did not significantly modify the ability to manifest the polarized phenotype when cells were grown to confluence on filters in two-chamber-systems. beta 1B-transfected cells showed reduced motile properties when embedded as aggregates in type I collagen gels. Moreover, formation of polarized cysts in suspension culture was impaired. The results show that beta 1B, by interfering with focal adhesion organization, microfilament and fibronectin assembly, cell spreading and migration, affects some morphogenetic properties of FRT epithelial cells.