p16(INK4A) and p19(ARF) act in overlapping pathways in cellular immortalization

The INK4A locus encodes two independent but overlapping genes, p16(INK4A) a nd p19(ARF), and is frequently inactivated in human cancers. The unusual st ructure of this locus has lead to ambiguity regarding the biological role o f each gene. Here we express, in primary mouse embryonic fibroblasts (MEFs) , antisense RNA constructs directed specifically towards either p16(INK4A) or p19(ARF). Such constructs induce extended lifespan in primary MEFs; this lifespan extension is reversed upon subsequent elimination of the p16(INK4 A) or p19(ARF) antisense constructs. In immortal derivatives of cell lines expressing antisense p16(INK4A) or p19(ARF) RNA, growth arrest induced by r ecovery of p16(INK4A) expression is bypassed by compromising the function o f the retinoblastoma protein (Rb), whereas growth arrest induced by re-expr ession of p19(ARF) is overcome only by simultaneous inactivation of both th e Rb and the p53 pathways. Thus, the physically overlapping p16(INK4A) and p19(ARF) genes act in partly overlapping pathways.