Targeting histone deacetylase SIRT1 selectively eradicates EGFR TKI-resistant cancer stem cells via regulation of mitochondrial oxidative phosphorylation in lung adenocarcinoma

Lung adenocarcinoma (LAD) is a human malignancy successfully treated with the tyrosine kinase inhibitor (TKI) gefitinib; however, the enrichment of therapy resistant cancer stem cells (CSCs) in such patients is assumed to be a source of treatment failure. Evaluation of LAD cell populations treated with the TKI inhibitor gefitinib identified unique aspects of a subpopulation of tumor cells exhibiting stemlike properties and mitochondriaspecific metabolic features along with their reliance on sirtuin 1 (SIRT1) for survival advantage. This addiction to bioenergetic metabolism in LAD treated with EGFRtargeted therapy suggests that mitochondria] targeting should be synthetically lethal using established cytotoxic therapies. Accordingly, loss of the phenotype present in resistant CSC clones either by targeting the energy metabolism with tigecycline, a mitochondria] DNAtranslation inhibitor, or tenovin6 (TV6), a SIRT1 inhibitor, inhibited their dependency on mitochondria] oxidative phosphorylation (mtOXPHOS) and sensitized them for a more pronounced and longlasting TKI therapeutic effect. The results specifically demonstrated that combined therapy with TV6 and gefitinib resulted in tumor regression in xenograft mouse models, whereas administration of a single agent showed no such efficacy. Importantly, combined treatment with TV6 also decreased the effective dose of gefitinib necessary for treatment response. Clinical analysis demonstrated that highprofile SIRT1 and mtOXPHOS proteins were associated with recurrence and poor prognosis in LAD patients. These observations support the CSC hypothesis for cancer relapse and advocate use of mitochondriatargeting inhibitors as part of combinatorial therapy in a variety of clinical settings, as well as for reducing firstline TKI dosage in LAD patients.