Activation of the p21 pathway of growth arrest and apoptosis by the β₄ integrin cytoplasmic domain

The integrin α₆β₄, a receptor for members of the laminin family of basement membrane components, contributes to the function of epithelial cells and their oncogenically transformed derivatives. In our efforts to study α₆β₄-mediated functions in more detail and to assess the contribution of the β₄ cytoplasmic domain in such functions, we identified a rectal carcinoma cell line that lacks expression of the β₄ integrin subunit. This cell line, termed RKO, expresses α₆β₁ but not α₆β₄, and it interacts with laminin-1 less avidly than similar cell lines that express α₆β₄. We expressed a full-length β₄ cDNA, as well as a mutant cDNA that lacks the β₄ cytoplasmic domain, in RKO cells and isolated stable subclones of these transfectants. In this study, we report that subclones that expressed the full-length β₄ cDNA in association with endogenous α₆ exhibited partial G₁ arrest and apoptosis, properties that were not evident in RKO cells transfected with either the cytoplasmic domain mutant or the expression vector alone. In an effort to define a mechanism for these observed changes in growth, we observed that expression of the α₆β₄ integrin induced expression of the p21 (WAF1; CiP1) protein, an inhibitor of cyclin-dependent kinases. These data suggest that the β₄ integrin cytoplasmic domain is linked to a signaling pathway involved in cell cycle regulation in the β₄ transfected RKO cells.