Abstract

Expression of full-length p16^INK4a blocks _v3 integrin-dependent cell spreading on vitronectin but not collagen IV. Similarly, G₁-associated cell cycle kinases (CDK) inhibitory (CKI) synthetic peptides derived from p16^INK4a, p18^INK4c and p21^Cip1/Waf1, which can be delivered directly into cells from the tissue culture medium, do not affect non-_v3-dependent spreading on collagen IV, laminin and fibronectin at concentrations that inhibit cell cycle progression in late G₁. The _v3 heterodimer remains intact after CKI peptide treatment but is immediately dissociated from the focal adhesion contacts. Treatment with phorbol 12-myristate 13-acetate (PMA) allows _v3 to locate to the focal adhesion contacts and the cells to spread on vitronectin in the presence of CKI peptides. The cdk6 protein is found to suppress p16^INK4a-mediated inhibition of spreading and is also shown to localize to the ruffling edge of spreading cells, indicating a function for cdk6 in controlling matrix-dependent cell spreading. These results demonstrate a novel G₁ CDK-associated integrin regulatory pathway that acts upstream of _v3-dependent activation of PKC as well as a novel function for the p16^INK4a tumour suppressor protein in regulating matrix-dependent cell migration.