The role of Caspase-2 and PIDD1 in lung tumorigenesis and response to genotoxic stress

Platinum-based chemotherapy is the standard-of-care for non-small cell lung cancer. As with other chemotherapies, not all patients respond positively to treatment and almost all patients will develop chemotherapy-resistant disease. Using the KrasG12D-driven mouse model of lung adenocarcinoma, our lab identified upregulation of p53-induced with a death domain isoform 1 (Pidd1) in resistant tumors in response to long-term cisplatin chemotherapy treatment. Remarkably, PIDD1 expression in vitro induces p53-dependent resistance to cisplatin as well as other DNA, damaging chemotherapeutics.;PIDD1 expression leads to assembly of large molecular weight complexes named PIDDosomes. The Caspase-2-PIDDosome is critical for PIDD1-induced chemotherapy resistance. Our lab demonstrated that the Caspase-2-PIDDosome dynamically regulates p53 activity via proteolytic modification of MDM2, the master negative regulator of p53. In unstressed cells, MDM2 binds to and ubiquitinates p53, targeting it for proteasomal degradation. Upon stress, Pidd1 is upregulated, ultimately leading to formation of the Caspase-2-PIDDosome, and subsequent Caspase-2 activation. Activated Caspase-2 cleaves MDM2 into two fragments: p60 and p30. p60 maintains the ability to bind p53, but cannot ubiquitinate p53, resulting in increased p53 protein stability. The role of the Caspase-2-PIDDosome in regulating the p53-MDM2 feedback loop is not well characterized in tumorigenesis or chemotherapy response.;In this study, I utilize the KrasG12D-driven mouse model of lung adenocarcinoma to investigate tumorigenesis and chemotherapy response in Caspase-2-deficient and Pidd1-deficient mice. These data demonstrate that Caspase-2 is a tumor suppressor in lung adenocarcinoma primarily by inhibiting cell proliferation. Caspase -2-deficient tumors respond to chemotherapy; however, rapid tumor cell proliferation following treatment reduces the long-term therapeutic efficacy of chemotherapy. Unexpectedly, Pidd1-deficiency does not impact lung tumorigenesis or chemotherapy response. Mechanistic investigation in vitro revealed that ATM phosphorylation of PIDD1 is critical for Caspase-2-PIDDosome assembly, Caspase-2-mediated MDM2 cleavage, cell cycle arrest, and resistance to DNA damaging agents. This pathway is not modulated by exogenous MDM2 or its cleavage product p60. Further, pharmacological inhibition of the p53-MDM2 negative feedback-loop using nutlin-3 does not alter PIDD1-induced growth arrest or cisplatin resistance. Together these findings demonstrate that Caspase-2 and PIDD1 have distinct functions in vivo and elucidate the Caspase-2-PIDDosome signaling network in p53-dependent response to DNA damage.