A Novel Function for the Tumor Suppressor p16(INK4a) : Induction of Anoikis via Upregulation of the α(5)β(1) Fibronectin Receptor

The tumor suppressor gene p16(INK4a) inhibits the kinase activity of the cyclin-dependent kinase 4–6/cyclin D complexes and subsequent phosphorylation of critical substrates necessary for transit through the G1 phase of the cell cycle. Recent studies suggested that control of the G1/S boundary might not be the sole biological function of p16(INK4a). We hypothesized that p16(INK4a) might influence hitherto unknown critical features of a malignant epithelial phenotype, such as anchorage dependence. Here we provide evidence that stable transfection of p16(INK4a) restitutes apoptosis induction upon loss of anchorage (anoikis) in a variety of human cancer cells. Anoikis in p16(INK4a)-transfected cells was evidenced by DNA fragmentation and poly(ADP-ribose) polymerase cleavage upon cultivation on polyhydroxyethylmethacrylate-coated dishes and was associated with suppression of anchorage-independent growth as well as complete loss of tumorigenicity. p16(INK4a)-mediated anoikis was due to selective transcriptional upregulation of the α(5) integrin chain of the α(5)β(1) fibronectin receptor as detected by FACS(®) analysis, immunoprecipitation, Northern blotting, and nuclear run-on assays. Addition of soluble fibronectin and inhibitory α(5) antibodies to nonadherent cells completely abolished p16(INK4a)-mediated anoikis, whereas laminin was ineffective. Furthermore, antisense-induced downregulation of the α(5) integrin chain in p16(INK4a)-transfected cells restored resistance to anoikis. These data suggest a novel functional interference between a cell cycle–regulating tumor suppressor gene and membrane-bound integrins, thus regulating a hallmark feature of an epithelial transformed phenotype: susceptibility to anoikis.