Mitogenic signalling and the p16INK4a-Rb pathway cooperate to enforce irreversible cellular senescence

The p16(INK4a) cyclin-dependent kinase inhibitor has a key role in establishing stable G1 cell-cycle arrest through activating the retinoblastoma (Rb) tumour suppressor protein pRb(1-5) in cellular senescence. Here, we show that the p16(INK4a)/Rb-pathway also cooperates with mitogenic signals to induce elevated intracellular levels of reactive oxygen species (ROS), thereby activating protein kinase C delta (PKC delta) in human senescent cells. Importantly, once activated by ROS, PKC delta promotes further generation of ROS, thus establishing a positive feedback loop to sustain ROS-PKC delta signalling(6-8). Sustained activation of ROS-PKC delta signalling irreversibly blocks cytokinesis, at least partly through reducing the level of WARTS (also known as LATS1), a mitotic exit network (MEN) kinase required for cytokinesis(9--11), in human senescent cells. This irreversible cytokinetic block is likely to act as a second barrier to cellular immortalization ensuring stable cell-cycle arrest in human senescent cells. These results uncover an unexpected role for the p16(INK4a)-Rb pathway and provide a new insight into how senescent cell-cycle arrest is enforced in human cells.