Abstract

I will present our work investigating the mechanisms that underpin proliferation-dormancy state changes. Most cells in our body are not actively undergoing cell division cycles and are dormant. The ability for cells to exit dormancy and re-enter the cell cycle is crucial for tissue renewal and immune responses. These mechanisms are highly regulated and are frequently altered in cancer to fuel uncontrolled proliferation. Cellular senescence is another dormant state, but unlike quiescence, these cells are prevented from re-entering the cell cycle and thereby potentially contribute to neoplastic disease. It is crucial that we understand the mechanisms that control proliferation-dormancy state switches. To elucidate these mechanisms, we have studied the exemplar case of Cdk4/6 inhibitors and cultured human epithelial cells. Short term CDK4 /6 inhibitor treatment results in quiescence and reversible cell cycle re-entry. Prolonged CDK4 /6 inhibitor treatment results in cellular senescence. I will show that prolonged treatment results in excessive increases in cell size, a phenotype we call cell overgrowth. Cell overgrowth promotes senescence via biphasic p21 induction. During proliferative arrest, overgrowth is accompanied by imbalanced proteome scaling and osmotic stress, resulting in increased p21 in a p38- and p53-dependent manner. Aberrantly large cells that proliferate following release from CDK4 /6i experience a second phase of p21 induction from replication stress and p21/p53-dependent cell cycle exit. The dormancy fate choice is determined by accumulated p21, integrating past stresses experienced during CDK4 /6i arrest and release. These mechanisms help explain why aberrantly large cells are sensitized to senescence induction, and by extension, why cell hypertrophy is a feature frequently associated with senescence. Why does excessive cell size promote senescence? We hypothesize that allometric scaling of the proteome with excessive cell size challenges cellular homeostasis to trigger senescence. I will present unpublished work to characterize remodeling of the cellular proteome in human fibroblast and epithelial cells in response to various senescence inducers, including oncogenes, acute DNA damage, and chemotherapies, comparing these to quiescent and asynchronous controls. In addition to protein abundance, we have measured protein turnover, subcellular localization and secreted proteins to develop a multidimensional map of stress-induced senescence.