Contribution of p16 INK4a to replicative senescence of human fibroblasts

Abstract

In standard conditions of tissue culture, human fibroblasts undergo a limited number of population doublings before entering a state of irreversible growth arrest termed replicative senescence or M1. The arrest is triggered by a combination of telomere dysfunction and the stresses inflicted by culture conditions and is implemented, at least in part, by the cyclin-dependent kinase inhibitors p21 CIP1 and p16 INK4a. To investigate the role of p16 INK4a, we have studied fibroblasts from members of melanoma prone kindreds with mutations in one or both copies of the CDKN2A locus. The mutations affect the function of p16 INK4a but not of the alternative product, p14 ARF. The p16 INK4a-defective fibroblasts have an above average life span, compared to the heterozygous and normal age-matched controls, but they arrest with characteristics typical of senescence. Using agents that are known to bypass M1, such as DNA tumor virus oncoproteins or the Bmi1 transcriptional repressor, we provide evidence that p16 INK4a defective cells arrest at a stage that is operationally between M1 and M2 (crisis). As well as indicating that p16 INK4a contributes to but is not essential for replicative senescence of human fibroblasts, our data reveal considerable heterogeneity in the levels and accumulation of p16 INK4a in different strains.