Cellular senescence promotes skin carcinogenesis through p38MAPK and p44/p42MAPK signaling

Cellular senescence entails an irreversible growth arrest that evolved in part to prevent cancer. Paradoxically, senescent cells secrete pro-inflammatory and growth-stimulatory molecules, termed the senescence-associated secretory phenotype (SASP), which is correlated with cancer cell proliferation in culture and xenograft models. However, at what tumor stage and how senescence and SASP act on endogenous tumor growth in vivo is unknown. To understand the role of senescence in cancer etiology, we subjected p16-3MR transgenic mice, which permit the identification and selective elimination of senescent cells in vivo, to the well-established two-step protocol of squamous cell skin carcinoma (SCC), in which tumorigenesis is initiated by a carcinogen 7,12-dimethylbenz[α]anthracene (DMBA) and then promoted by 12-O-tetradecanoyl-phorbol-13-acetate (TPA). We show that TPA promotes skin carcinogenesis by inducing senescence and SASP. Systemic induction of senescence in non-tumor bearing p16-3MR mice using a chemotherapy followed by the two-step carcinogenesis protocol potentiated the conversion of benign papillomas to carcinomas by elevating p38 MAPK and MAPK/ERK signaling. Ablation of senescent cells reduced p38 MAPK and MAPK/ERK signaling, thereby preventing the progression of benign papillomas to carcinomas. Thus, we show for the first time that senescent cells are tumor promoters, not tumor initiators, and that they stimulate skin carcinogenesis by elevating p38 MAPK and MAPK/ERK signaling. These findings pave the way for developing novel therapeutics against senescence-fueled cancers.