EXPRESSION OF THE P16(INK4A) TUMOR-SUPPRESSOR VERSUS OTHER INK4 FAMILY MEMBERS DURING MOUSE DEVELOPMENT AND AGING

Four INK4 proteins can prevent cell proliferation by specifically inhi biting cyclin D-dependent kinases. Both p18(INK4c) and p19(INK4d) were widely expressed during mouse embryogenesis, but p16(INK4a) and p15(I NK4b) were not readily detected prenatally. Although p15(INK4b), p18(I NK4c) and p19(INK4d) were demonstrated in many tissues by 4 weeks afte r birth, p16(INK4a) protein expression was restricted to the lung and spleen of older mice, with increased, more widespread mRNA expression during aging. Transcripts encoding the INK4a alternative reading frame product p19(ARF) were not detected before birth but were ubiquitous p ostnatally. Expression of p16(INK4a) and p15(INK4b) was induced when m ouse embryos were disrupted and cultured as mouse embryo 'fibroblasts' (MEFs). The levels of p16(INK4a) and p18(INK4c), but not p15(INK4b) o r p19(INK4d), further increased as MEFs approached senescence. Followi ng crisis and establishment, three of four independently-derived cell lines became polyploid and expressed higher levels of functional p16(I NK4a). I, contrast, one MEF line that sustained bi-allelic deletions o f INK4a initially remained diploid. Therefore, loss of p16(INK4a) and other events predisposing to polyploidy may represent alternative proc esses contributing to cell immortalization. Whereas p18(INK4a) and p19 (INK4d) may regulate pre- and postnatal development, p16(INK4a) more l ikely plays a checkpoint function during cell senescence that undersco res its selective role as a tumor suppressor.