Mitogenic signalling and the p16^INK4a-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 Cδ (PKCδ) in human senescent cells. Importantly, once activated by ROS, PKCδ promotes further generation of ROS, thus establishing a positive feedback loop to sustain ROS-PKCδ signalling^6-8. Sustained activation of ROS-PKCδ 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.