Mitochondrial ROS drive resistance to chemotherapy and immune killing in hypoxic non-small cell lung cancer
Authors: Salaroglio Iris C. , Belisario Dimas Carolina , Akman Muhlis , Vecchia Sofia La , Godel Martina , Anobile Dario Pasquale , Ortone Giacomo , Digiovanni Sabrina , Fontana Simona , Costamagna Costanzo , Rubinstein Menachem , Kopecka Joanna , Riganti Chiara 
Published: May 2022

Abstract
Background.
Solid tumors subjected to intermittent hypoxia are characterized by resistance to chemotherapy and immune-killing by effector T-lymphocytes, particularly tumor-infiltrating Vγ9Vδ2 T-lymphocytes. The molecular circuitries determining this double resistance are not known.
Methods.
We analyzed a panel of 28 human non-small cell lung cancer (NSCLC) lines, using an in vitro system simulating continuous and intermittent hypoxia. Chemosensitivity to cisplatin and docetaxel was evaluated by chemiluminescence, ex vivo Vγ9Vδ2 T-lymphocyte expansion and immune-killing by flow cytometry. Targeted transcriptomic identified efflux transporters and nuclear factors involved in this chemo-immuno-resistant phenotype. The molecular mechanism linking Hypoxia-inducible factor-1α (HIF-1α), CCAAT/Enhancer Binding Protein-β (C/EBP-β) isoforms LAP and LIP, ABCB1, ABCC1 and ABCA1 transporters were evaluated by immunoblotting, RT-PCR, RNA-IP, ChIP. Oxidative phosphorylation, mitochondrial ATP, ROS and depolarization were measured spectrofluorimetrically. The role of ROS/HIF-1α/LAP axis was validated by HIF-1α/LAP knocked-out or overexpressing cells, and in humanized (Hu-CD34+NSG) mice bearing LAP-overexpressing tumors. The clinical meaning of LAP was assessed in 60 NSCLC patients prospectively enrolled, treated with chemotherapy.
Results.
By up-regulating ABCB1 and ABCC1, and down-regulating ABCA1, intermittent hypoxia induced a stronger chemo-immuno-resistance than continuous hypoxia in NSCLC cells. Intermittent hypoxia impaired the electron transport chain, increasing mitochondrial ROS that favor the stabilization of C/EBP-β mRNA mediated by HIF-1α. HIF-1α/C/EBP-β mRNA binding increases the splicing of C/EBP-β toward the production of LAP isoform that transcriptionally induces ABCB1 and ABCC1, promoting the efflux of cisplatin and docetaxel. LAP also decreases ABCA1, limiting the efflux of isopentenyl pyrophosphate, i.e. the endogenous activator of Vγ9Vδ2 T-cells, and reducing the immune-killing. In NSCLC patients subjected to cisplatin-based chemotherapy, C/EBP-β LAP was abundant in hypoxic tumors and was associated with lower response to treatment and survival., in Hu- LAP-overexpressing tumors implanted in CD34+NSG mice recapitulated the chemo-immuno-resistant phenotype of patients. Interestingly, the ROS scavenger mitoquinol induced chemo-immuno-sensitization in immuno-xenografts, by disrupting the ROS/HIF-1α/LAP cascade.
Conclusions.
The impairment of mitochondrial metabolism induced by intermittent hypoxia increases the ROS-dependent stabilization of HIF-1α/LAP complex in NSCLC, producing chemo-immuno-resistance. Clinically used mitochondrial ROS scavengers may counteract such double resistance. Moreover, we suggest C/EBP-β LAP as a new predictive and prognostic factor in NSCLC patients.