MDM2 drives resistance to Osimertinib by contextually disrupting FBW7-mediated destruction of MCL-1 protein in EGFR mutant NSCLC.

Background: Overcoming resistance to Osimertinib in epidermal growth factor receptor (EGFR) mutant non-small cell lung cancer (NSCLC) is clinically challenging because the underlying mechanisms are not fully understood. The murine double minute 2 (MDM2) has been extensively described as a tumor promotor in various malignancies, mainly through a negative regulatory machinery on the p53 tumor suppressor. However, the significance of MDM2 on the sensitivity to Osimertinib has not been described.

Methods: Osimertinib resistant cells were generated by standard dose escalation strategy and individual resistant clones were isolated for MDM2 testing. The MDM2 and its mutant constructs (ΔPBD, ΔRING, C464A) were introduced into PC-9, HCC827 and H1975 cells and evaluated for the sensitivity to Osimertinib by MTT assay, colony formation, EdU assay and TUNEL assay. MDM2 expression in resistant cells was manipulated by pharmacological and molecular approaches, respectively. Proteins that were implicated in PI3K/Akt, MAPK/Erk and apoptosis signaling were measured by Western blot analysis. Candidate proteins that interacted with MDM2 were captured by immunoprecipitation and probed with indicated antibodies.

Results: In comparison with parental PC-9 cells, the PC-9 OR resistant cells expressed high level of MDM2. Ectopic expression of MDM2 in PC-9, HCC827 and H1975 sensitive cells generated an Osimertinib resistant phenotype, regardless of p53 status. MDM2 promoted resistance to Osimertinib through a PI3K/Akt and MAPK/Erk-independent machinery, in contrast, MDM2 selectively stabilized MCL-1 protein to arrest Osimertinib-induced cancer cell apoptosis. Mechanistically, MDM2 acted as a E3 ligase to ubiquitinate FBW7, a well-established E3 ligase for MCL-1, at Lys412 residue, which resulted in FBW7 destruction and MCL-1 stabilization. Targeting MDM2 to augment MCL-1 protein breakdown overcame resistance to Osimertinib in vitro and in vivo. Finally, the clinical relevance of MDM2-FBW7-MCL-1 regulatory axis was validated in mouse xenograft tumor model and in NSCLC specimen.

Conclusion: Overexpression of MDM2 is a novel resistant mechanism to Osimertinib in EGFR mutant NSCLC. MDM2 utilizes its E3 ligase activity to provoke FBW7 destruction and sequentially leads to MCL-1 stabilization. Cancer cells with aberrant MDM2 state are refractory to apoptosis induction and elicit a resistant phenotype to Osimertinib. Therefore, targeting MDM2 would be a feasible approach to overcome resistance to Osimertinib in EGFR mutant NSCLC.