A Role for Fibroblast-Derived SASP Factors in the Activation of Pyroptotic Cell Death in Mammary Epithelial Cells

In normal tissue homeostasis, bidirectional communication between different cell types can shape numerous, biological outcomes. Within the microenvironment, there exist many cell types including epithelial cells, tumor cells, immune cells, and stromal cells. One cell type that is prevalent in tissues surrounding tumors are fibroblasts, which often undergo senescence, transition in myofibroblasts, or carcinoma associated fibroblasts (CAFs) and greatly influence shaping the composition of the microenvironment. Fibroblasts have been shown to be prone to undergo senescence, exemplified by an irreversible cell cycle arrest in response to a variety of stimuli such as bleomycin (BIS) or oncogene induced senescence (OIS). Senescent cells exhibit alterations in gene expression and are characterized by substantial changes in their secretome; termed the senescence associated secretory phenotype (SASP). While the role of fibroblast derived SASP factors on cancer cells has been well studied, the impact of these factors on normal epithelial cells remains poorly understood. The cell type and senescence inducer specific effects can greatly influence the composition of the SASP, often comprised of pro-inflammatory factors. The nature of the secreted factors from senescent cells can dictate whether senescence will function in a tumor suppressive or tumor promoting fashion. Our studies have found that treatment of normal mammary epithelial cells with conditioned media from senescent fibroblasts (SASP CM) results in a robust caspase 1 dependent, pyroptotic cell death in three normal mammary epithelial cell lines. The capacity of SASP CM to cause cell death is maintained across multiple senescence-inducing stimuli, including BIS and OIS. Moreover, the addition of HRas (G12V) or ErbB2 into normal mammary epithelial cells mitigates the ability of SASP CM to induce cell death. Furthermore, MDA-MB-231 and MDAMB-436 breast cancer cells also attenuated the ability of SASP CM to induce celldeath. While the role of senescent fibroblasts in cancer has been well characterized, further studies are needed to elucidate the impact of senescent fibroblasts and their SASP factors on the surrounding cell non-cancerous cell types including endothelial cells, other fibroblasts, or immune cells. Greater characterization of the context, cell type and senescence inducer specific effects of SASP from senescent cells will be important in informing the use of therapeutic avenues such as senolytics, which selectively eliminate senescent cells.