Id-1 delays senescence but does not immortalize keratinocytes

Defining the molecular basis responsible for regulating the proliferative p otential of keratinocytes has important implications for normal homeostasis and neoplasia of the skin. Under current culture conditions, neonatal fore skin-derived human keratinocytes possess a relatively short replicative lif espan, Recently it was reported that forced overexpression of the helix-loo p-helix protein Id-1 was capable of immortalizing keratinocytes, secondary to activation of telomerase activity and suppression of p16/Rb-mediated gro wth arrest pathways. To investigate the relationship between Id-1, telomera se activity, telomere length, pie, Rb cell cycle regulators, and senescence , whole populations of keratinocytes were infected with a retrovirus to ind uce overexpression of Id-1. In these unselected cultures, enhanced Id-1 lev els clearly extended the lifespan of keratinocytes, but Id-1 did not preven t the onset of replicative senescence. Under these experimental conditions, Id-1 expression did not trigger induction of telomerase activity, and ther e was progressive shortening of the telomeres that was accompanied by eleva ted pie levels and prevalence of active Rb, The ability of Id-1 to postpone , but not prevent, senescence may be related to partial inhibition of pie e xpression, as the Id-1-overexpressing cultures displayed a decreased capaci ty for 12-O-tetradecanoylphorbol-13-acetate-mediated p16 induction, Thus, w hile no immortalization was observed, Id-1 could delay the onset of replica tive senescence in unselected human keratinocyte populations.