Non-small cell lung cancer (NSCLC) with wild-type epidermal growth factor receptor (EGFR) is intrinsic resistance to EGFR-tyrosine kinase inhibitors (TKIs), such as afatinib. Celastrol, a natural compound with antitumor activity, was reported to induce paraptosis in cancer cells. In this study, intrinsic EGFR-TKI-resistant NSCLC cell lines H23 (EGFR wild-type and KRAS mutation) and H292 (EGFR wild-type and overexpression) were used to test whether celastrol could overcome primary afatinib resistance through paraptosis induction. The synergistic effect of celastrol and afatinib on survival inhibition of the NSCLC cells was evaluated by CCK-8 assay and isobologram analysis. The paraptosis and its modulation were assessed by light and electron microscopy, Western blot analysis, and immunofluorescence. Xenografts models were established to investigate the inhibitory effect of celastrol plus afatinib on the growth of the NSCLC tumors in vivo. Results showed that celastrol acted synergistically with afatinib to suppress the survival of H23 and H292 cells by inducing paraptosis characterized by extensive cytoplasmic vacuolation. This process was independent of apoptosis and not associated with autophagy induction. Afatinib plus celastrol-induced cytoplasmic vacuolation was preceded by endoplasmic reticulum stress and unfolded protein response. Accumulation of intracellular reactive oxygen species and mitochondrial Ca 2+ overload may be initiating factors of celastrol/afatinib-induced paraptosis and subsequent cell death. Furthermore, Celastrol and afatinib synergistically suppressed the growth of H23 cell xenograft tumors in vivo. The data indicate that a combination of afatinib and celastrol may be a promising therapeutic strategy to surmount intrinsic afatinib resistance in NSCLC cells. K E Y W O R D S afatinib, celastrol, epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitor (TKI), non-small cell lung cancer, paraptosis, resistance 1 | INTRODUCTION Activating mutations of epidermal growth factor receptor (EGFR) play an important role in the development, invasion, metastasis, and drug resistance of non-small cell lung cancer (NSCLC; Kobayashi et al., 2015; Tsiambas et al., 2016). Treatment with EGFR-tyrosine kinase inhibitors (TKIs), such as first-or second-generation EGFR-TKIs, gefitinib, and erlotinib or afatinib, have become the standard of care for first-line therapy for advanced NSCLC with EGFR activating
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