Epigenetic regulation of tissue factor inducibility in endothelial cell senescence.

Cellular senescence, a programmed state induced by multiple deleterious triggers, is characterised by permanent cell-cycle exit and altered gene expression and cell morphology. In humans it is considered a tumor suppressor mechanism, mediating removal of damaged or mutated cells from the cell-cycle pool, and may also contribute to the ageing process. In this study, we show that senescent human umbilical vein endothelial cells lose their ability to induce tissue factor (TF), a transmembrane protein with important roles in hemostasis and cancer progression, in response to thrombin or - independently of cell-surface receptors - phorbol-12-myristate-13-acetate. This phenomenon could not be explained by senescence-related alterations in the downstream signal transduction cascade or by accelerated TF mRNA degradation. Rather, using chromatin immuno-precipitation we could show that loss of TF gene inducibility during senescence occurs following chromatin remodelling of the TF promoter resulting from hypo-acetylation of histone H3. These findings were reversible after transduction of presenescent cultures with telomerase reverse transcriptase, enabling late-passage cultures to escape senescence. These results extend the involvement of heterochromatic gene silencing in senescence beyond cell cycle-related genes and suggest a novel anti-cancer mechanism of senescence through inhibition of TF inducibility.