p53 restoration in small cell lung cancer identifies a latent cyclophilin-dependent necrosis mechanism (RNA-Seq I)
The p53 tumor suppressor regulates multiple context-dependent tumor  suppressive programs. Although p53 is mutated in ~90% of small cell lung cancer  (SCLC) tumors, how p53 mediates tumor suppression in this context is unknown. Here,  using a mouse model of SCLC in which endogenous p53 expression can be  conditionally and temporally regulated, we show that SCLC tumors maintain a  requirement for p53 inactivation. However, we identified tumor subtype heterogeneity  between SCLC tumors such that p53 reactivation induces a canonical senescence  response in a subset of tumors, while, in others, p53 induces a non-apoptotic form of  cell death that culminates in necrosis. We pinpointed the cyclophilin family of peptidyl  prolyl cis-trans isomerases as critical determinants of a p53-induced transcriptional  program that is specific to SCLC tumors and cell lines that are poised to undergo p53-  mediated necrosis. Importantly, inhibition of cyclophilin isomerase activity, or genetic  ablation of specific cyclophilin genes, suppresses p53-mediated necrosis by limiting p53  transcriptional output without impacting p53 chromatin binding. Our study demonstrates  that a previously unappreciated intertumoral heterogeneity in SCLC can influence the  biological response to p53 restoration, describes a novel mechanism of p53-regulated  cell death, and uncovers putative mechanisms for the treatment of this most-recalcitrant  tumor type. Overall design: RNA-sequencing analysis of Type D and Type V SCLC cells after 72hrs of p53 restoration and/or CsA treatment.