PP55α-Directed PP2A in Cellular Senescence

Cellular senescence is a permanent cell cycle arrest that serves as a tumor suppressive mechanism but also contributes to aging and age-related diseases. Senescence is primarily regulated by the p16/RB and p53/p21 pathways in response to genotoxic stress, such as ionizing radiation (IR). While the role of p53 in senescence is well characterized, the mechanisms controlling p16 transcription remain unclear. The CDKN2A locus, which encodes p16, is epigenetically silenced in normal cells by polycomb repressive complexes (PRC1/2) through histone modifications such as H2AK119ub and H3K27me3. Upon IR exposure, this repression is lifted, leading to p16 expression and senescence induction. However, the regulatory mechanisms that control this process remain poorly understood. Our preliminary findings identify PR55α, a regulatory subunit of the PP2A phosphatase, as a key suppressor of p16 expression and IR-induced senescence. PR55α overexpression prevents senescence in normal human pancreatic ductal (HPNE) cells following IR, while PR55α knockdown restores p16 expression and senescence in pancreatic cancer (HPAF/CD18) cells. We hypothesize that PR55α/PP2A dephosphorylates key epigenetic regulators, such as CBX8, enhancing PRC1/2 activity and repressing p16 transcription. To test this, we propose three aims: Aim 1: Determine how PR55α regulates PcG protein function and suppresses p16 transcription. We will investigate whether PR55α dephosphorylates CBX8 and enhances PRC1/2-mediated chromatin repression of CDKN2A using chromatin immunoprecipitation (ChIP) and mass spectrometry. Aim 2: Define the mechanisms by which PR55α regulates IR-induced senescence. We will assess PR55α's effects on DNA damage response (DDR), oncogenic signaling (YAP, ERK, β-catenin, c-Myc), and the Yamanaka factors (KLF4, SOX2, OCT3) in senescence regulation. Aim 3: Assess PR55α’s role in p16 regulation and IR-induced senescence in vivo. Using inducible PR55α overexpression and knockout mouse models, we will evaluate PR55α's impact on p16 expression and senescence following IR exposure. These studies will elucidate a novel phospha-tase-mediated mechanism of p16 regulation and senescence induction. By defining PR55α’s role in these processes, we may uncover new therapeutic targets for cancer and age-related diseases.