Analysis of oncogene-induced senescence

Senescence, like apoptosis, functions to remove damaged cells from the cell cycle and thereby prevents the accumulation of multiple genetic changes that drive tumourigenesis. In contrast to apoptosis – also commonly referred to as programmed cell-death – senescence is characterised by an irreversible cell-cycle arrest. A number of stimuli have been reported to induce senescence including oncogenic stimuli, e.g. oncogenic RAS–RAF signalling; telomere shortening; culture stress; oxidative stress and DNA damaging agents. However, many of the senescence signals generated by these stimuli remain poorly defined. Oncogenes are drivers of tumourigenesis and therefore a complete understanding of the senescence signals generated by oncogenes will be important for advancing our knowledge of how tumours evade senescence and for the rational design of prosenescence cancer therapies. Three proposed signals include DNA replicational stress, oxidative DNA damage from an increase in intracellular reactive oxygen species (ROS) and p38 stress-activated protein kinase (SAPK) signalling. In this thesis, data are presented that support that an oncogenic RAF-induced senescent cell-cycle arrest can occur in rat Schwann cells without an increase in ROS or a DNA damage response (DDR) as well as p38 SAPK signalling, and thereby suggest that the arrest is induced by an alternative senescence signal. Furthermore, data are presented that show that low oxygen does not prevent oncogenic RAS-induced senescence in rodent or human cells and thereby question the role of ROS in oncogene-induced senescence. Senescence by definition is an irreversible cell-cycle arrest. However, the stability of the cell-cycle arrest in the absence of a continuous oncogenic signal has been relatively unexplored. This is an important question as the answer is also likely to impact the rational design of pro-senescence cancer therapies. Analyses of rat Schwann cells and human fibroblasts using an inducible RAF protein construct that enables reversible RAF signalling show that an oncogenic RAF-induced senescent arrest is inherently more stable in human cells than rodent cells but that the arrest is ultimately reversible in both species