Control of the Replicative Life Span of Human Fibroblasts by p16 and the Polycomb Protein Bmi-1

Abstract

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 O₂ 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.

ACKNOWLEDGMENTS

This work was supported by grants from the National Institutes of Health (AG09909 to J.C. and AG16851 to G.P.D.) and startup funds from the Cancer Center, New England Medical Center, Boston, Mass. (G.P.D.).

We thank Miguel Rubio for providing near-senescent 82-6 cells.