The polycomb protein Bmi-1 represses the INK4a locus, which encodes the tumor 
suppressors p16 and p14(ARF). Here we report that Bmi-1 is downregulated when 
WI-38 human fibroblasts undergo replicative senescence, but not quiescence, and 
extends replicative life span when overexpressed. Life span extension by Bmi-1 
required the pRb, but not p53, tumor suppressor protein. Deletion analysis 
showed that the RING finger and helix-turn-helix domains of Bmi-1 were required 
for life span extension and suppression of p16. Furthermore, a RING finger 
deletion mutant exhibited dominant negative activity, inducing p16 and premature 
senescence. Interestingly, presenescent cultures of some, but not all, human 
fibroblasts contained growth-arrested cells expressing high levels of p16 and 
apparently arrested by a p53- and telomere-independent mechanism. Bmi-1 
selectively extended the life span of these cultures. Low O(2) concentrations 
had no effect on p16 levels or life span extension by Bmi-1 but reduced 
expression of the p53 target, p21. We propose that some human fibroblast strains 
are more sensitive to stress-induced senescence and have both p16-dependent and 
p53/telomere-dependent pathways of senescence. Our data suggest that Bmi-1 
extends the replicative life span of human fibroblasts by suppressing the 
p16-dependent senescence pathway.