Loss of p62 Binding Allows TIF-IA Accumulation in Senescence, Which Promotes Phenotypic Changes to Nucleoli and the Senescence Associated Secretory Phenotype.

Abstract

A key characteristic of senescent and ageing cells is a reduction in number and increase in size of nucleoli. Although a number of pathways have been suggested, the mechanisms underlying this altered nucleolar phenotype, and the downstream consequences, remain poorly understood. The PolI complex component, TIF-IA, has previously been implicated in regulating this characteristic nucleolar phenotype in response to stress. Here we explored the role of TIF-IA in senescence and ageing. We show that TIF-IA accumulation, particularly in the nucleus and nucleolus, is an early response to oncogene- and therapy-induced senescence (OIS and TIS) in vitro. Using multiple mouse models, we also demonstrate accumulation of TIF-IA in response to senescence induction and ageing in vivo. We demonstrate that TIF-IA accumulation is not required for cell cycle arrest but that in OIS and TIS, it is essential for phenotypic changes to nucleoli, the senescence-associated secretory phenotype (SASP) and establishment of stable senescence. We demonstrate that in proliferating cells, TIF-IA binds the cargo receptor, p62 (SQSTM1), and that accumulation in senescence occurs as a consequence of ATM activation, which disrupts this interaction. Finally, we show that TIF-IA accumulation causes an increase in reactive oxygen species (ROS) levels. Together, these results establish TIF-IA accumulation as a key regulator of the nucleolar phenotype and the SASP in senescence and uncover a novel, p62-dependent mechanism driving this process. These findings offer significant new insights into nucleolar size regulation in senescence and ageing, and suggest a potential relationship with the inflammatory phenotype.