26 2 SUMMARY 27 Drug resistance is a significant hindrance to effective cancer treatment. Although 28 resistance mechanisms of epidermal growth factor receptor (EGFR)-mutant cancer cells to 29 lethal EGFR tyrosine kinase inhibitors (TKI) treatment have been investigated intensively, 30 how cancer cells orchestrate adaptive response under sublethal drug challenge remains 31 largely unknown. Here we find that 2-hour sublethal TKI treatment elicits a transient 32 drug-tolerant state in EGFR-mutant lung cancer cells. Continuous sublethal treatment 33 reinforces this tolerance and eventually establishes long-term TKI resistance. This adaptive 34 process involves H3K9 demethylation-mediated epigenetic upregulation of branched-chain 35 amino acid aminotransferase 1 (BCAT1) and subsequent metabolic reprogramming, which 36 promotes TKI resistance through attenuating reactive oxygen species (ROS) accumulation. 37 Combinational treatment with TKI and ROS-inducing reagents overcomes this drug 38 resistance in preclinical mouse models. Clinical information analyses support the 39 correlation of BCAT1 expression with EGFR TKI response. Collectively, our findings reveal 40 the importance of epigenetically regulated BCAT1-engaged metabolism reprogramming in 41 TKI resistance in lung cancer. 42 HIGHLIGHTS 43 Sublethal EGFR TKI treatment induces transient drug-tolerant state and long-term 44 resistance in EGFR-mutant lung cancer cells 45 Epigenetically regulated BCAT1-mediated metabolic reprogramming orchestrates EGFR 46 TKI-induced drug resistance 47 Combinational treatment with TKI and ROS-inducing agents overcomes the drug 48 resistance induced by EGFR TKI treatment 49 50 51 Cancer cells are notorious for its strong plasticity in response to cytotoxic stress. They 52 may modulate adaptive programs to survive during therapy (Holohan et al., 2013). Upon 53 lethal EGFR TKI exposure, drug-sensitive cancer cells initially undergo drastic apoptotic 54 cell death resulting in notable tumor regression; however, the remaining drug-tolerant cells 55 can follow distinct evolutionary paths and eventually acquire resistance through mutation or 56 non-mutation mechanisms (Hata et al., 2016). However, little is known about how cancer 57 cells orchestrate their adaptive response under sublethal TKI challenge. 58 Epigenetic alterations are one of the major mechanisms underlying the adaptation of 59 cancer cells to drug exposure (Easwaran et al., 2014). Epigenetic changes such as H3K4 60 demethylation or H3K9 and H3K27 methylation has been linked to lethal EGFR TKI 61 induced resistance (Guler et al., 2017; Sharma et al., 2010). 62 Under stressful conditions, cancer cells can reprogram cellular metabolism to support 63 their malignant phenotypes including proliferation, invasion and resistance to drug therapy 64 (Vander Heiden and DeBerardinis, 2017). Recently, Aberrant branched-chain amino acids 65 (BCAAs) metabolism has recently been implicated in various human malignancies 66 including leukemia, liver cancer, pancreatic cancer and non-small c...