The p38MAPK-MK2-HSP27 Pathway Regulates the mRNA Stability of the Senescence-Associated Secretory Phenotype

Abstract

2Age is a significant risk factor for the development of cancer. Both age-dependent 24 accumulation of cell autonomous mutations within preneoplastic cells and increases in 25 senescent stromal cells within the tumor microenvironment are thought to collaborate to 26 drive tumorigenesis. Senescent cells express pro-tumorigenic factors termed the 27 senescence-associated secretory phenotype (SASP), subject to a variety of regulatory 28 mechanisms that are not fully elucidated. Previous work demonstrated that p38 29 mitogen-activated protein kinase (p38MAPK)-dependent regulation of AUF1 occupancy 30 on SASP factor mRNAs post-transcriptionally stabilizes many SASP mRNAs and 31 contributes to their increased expression. Here, we address the mechanism by which 32 p38MAPK regulates AUF1's occupancy and activity on SASP factor mRNAs. We found 33 that the p38MAPK-MK2-HSP27 pathway regulates both mRNA stability and AUF1 34 occupancy in cells induced to senesce. Furthermore, the tumor-promoting abilities of 35 senescent stromal cells were lost upon inhibition of MK2, suggesting that this pathway 36 is a viable therapeutic target within the tumor microenvironment. 37 38 senescence is characterized by an altered cell morphology that includes a flattened 46 3 appearance and development of stress fibers, increased expression of senescence-47 associated β-galactosidase (SA-β-gal), and expression of the senescence-associated 48 secretory phenotype or SASP (also known as the senescence messaging secretome, or 49 SMS) [2][3][4]. This secretory phenotype can lead to an immune-mediated clearance of 50 tumor cells, but when it occurs in stromal cells it functions as a potent tumor promoter 51 [2,[5][6][7]. The SASP consists of a large number of pro-inflammatory cytokines and other 52 immune modulators, matrix remodeling proteins, and pro-proliferative factors, among 53 others, that are coordinately upregulated in senescent cells. Senescent fibroblast-54 derived SASP can promote epithelial cell proliferation and epithelial-to-mesenchymal 55 transition (EMT), to allow for tumor extravasation and metastasis. In addition, the SASP 56 can remodel the stromal compartment to create an immunosuppressive environment 57 conducive to tumor proliferation [2, 7-9]. 58 59 Immediately following a senescence-inducing stimulus, cells (referred to here as pre-60 senescent) begin to upregulate numerous SASP factors through the activity of 61 transcription factors like NFκB but do not yet display classic signs of senescence, 62 including a flattened morphology or SA-β-Gal activity [10][11][12].We recently 63 demonstrated that the maintenance of SASP factor upregulation occurs via a transition 64 from a transcriptionally-driven process in pre-senescent cells to a post-transcriptional 65 stabilization mechanism in cells displaying the morphological changes and SA-β-Gal 66 activity characteristic of senescent cells [10]. In spite of their increased transcriptional 67 levels, several SASP factor mRNAs in pre-senescent cells, including IL-6, IL-8, and GM-68 C...