TARGETED DELETION OF BETA-1-INTEGRINS IN F9-EMBRYONAL CARCINOMA-CELLS AFFECTS MORPHOLOGICAL-DIFFERENTIATION BUT NOT TISSUE-SPECIFIC GENE-EXPRESSION

The integrin superfamily of heterodimeric transmembrane adhesion receptors mediates many cell-cell and cell-matrix interactions whose functions are believed to be critical for normal morphogenesis and differentiation. By eliminating the beta1 integrin gene through homologous recombination, we have assessed the role of the beta1 integrin family in the F9 embryonal carcinoma model for endodermal differentiation. F9 cells were unexpectedly found to maintain three copies of the beta1 gene and complete elimination required three sequential rounds of targeting to generate triple knockout lines (beta1 TKO). Elimination of the beta1 integrin family of adhesion receptors from F9 cells resulted in reduced adhesion to fibronectin, laminin and collagen, but strongly enhanced adhesion to vitronectin. The absence of beta1 integrins did not promote significant compensatory up-regulation of either beta3 or beta5 subunits, both of which are known to act as vitronectin receptors when associated with alphav. The loss of beta1 integrins severely affected morphological differentiation when the beta1-deficient cells were induced to differentiate to either parietal or visceral endoderm. Parietal endoderm derived from beta1-deficient cells retained a rounded morphology and migrated poorly on both fibronectin and vitronectin. Visceral endoderm derived from beta1-deficient cells were also unable to form a normal, confluent epithelial monolayer; instead, a non-contiguous layer containing clumps of disorganized cells was observed. However, loss of beta1 integrins did not interfere with induction by differentiating agents of tissue-specific gene products for either visceral or parietal endoderm. These results suggest that beta1 integrins mediate morphological differentiation (migration and epithelial formation) but not tissue-specific gene expression in induced F9 cells, and that these two processes are not necessarily linked in this system.