Expression of the p16^INK4a tumor suppressor versus other INK4 family members during mouse development and aging

Abstract

Four INK4 proteins can prevent cell proliferation by specifically inhibiting cyclin D-dependent kinases. Both p18^INK4c and p19^INK4d were widely expressed during mouse embryogenesis, but p16^INK4a and p15^INK4b were not readily detected prenatally. Although p15^INK4b, p18^INK4c and p19^INK4d were demonstrated in many tissues by 4 weeks after 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 postnatally. Expression of p16^INK4a and p15^INK4b was induced when mouse embryos were disrupted and cultured as mouse embryo `fibroblasts' (MEFs). The levels of p16^INK4a and p18^INK4c, but not p15^INK4b or p19^INK4d, further increased as MEFs approached senescence. Following crisis and establishment, three of four independently-derived cell lines became polyploid and expressed higher levels of functional p16^INK4a. In contrast, one MEF line that sustained bi-allelic deletions of INK4a initially remained diploid. Therefore, loss of p16^INK4a and other events predisposing to polyploidy may represent alternative processes contributing to cell immortalization. Whereas p18^INK4c and p19^INK4d may regulate pre- and postnatal development, p16^INK4a more likely plays a checkpoint function during cell senescence that underscores its selective role as a tumor suppressor.