GROWTH SUPPRESSION BY P16(INK4) REQUIRES FUNCTIONAL RETINOBLASTOMA PROTEIN

p16(ink4) has been implicated as a tumor suppressor that is lost from a variety of human tumors and human cell lines. p16(ink4) specifically binds and inhibits the cyclin-dependent kinases 4 and 6, In vitro, these kinases can phosphorylate the product of the retinoblastoma tumor suppressor gene. Thus, p16(ink4) could exert its function as tumor suppressor through inhibition of phosphorylation and functional inactivation of the retinoblastoma protein, Here we show that overexpression of p16(ink4) in certain cell types will lead to an arrest in the G(1) phase of the cell cycle, In addition, we show that p16(ink4) Can only suppress the growth of human cells that contain functional pRB. Moreover, we have compared the effect of p16(ink4) expression on embryo fibroblasts from wild-type and RB homozygous mutant mice. Wild-type embryo fibroblasts are inhibited by p16(ink4), whereas the RB nullizygous fibroblasts are not. These data not only show that the presence of pRB is crucial for growth suppression by p16(ink4) but also indicate that the pRB is the critical target acted upon by cyclin D-dependent kinases in the G(1) phase of the cell cycle.