Abstract P1-13-18: EGFR signaling contributes to acquired resistance to CDK4/6 inhibitors in ER+ breast cancer cells in vitro and in vivo
Authors: Lypova Nadiia , Lanceta Lilibeth , Daugherty Susan , Chesney Jason , Imbert-Fernandez Yoannis 
Published: March 2023

Abstract
Breast cancer driven by different hormone receptors, including estrogen receptor (ER+), is responsible for approximately 70-80% of cases among women. While endocrine therapy (ET) of ER+ primary tumors with antiestrogens or aromatase inhibitors is an effective first-line therapy, its success is limited by intrinsic and acquired resistance. Response to ET can be limited due to overexpression of cyclin D1, which promotes the activation of cyclin-dependent kinases 4 and 6 (CDK4/6). Selective inhibition of CDK4/6 and ER signaling is now standard-of-care therapy for ER+ metastatic breast cancer. CDK4/6 inhibitors (CDK4/6i), including palbociclib (PD), ribociclib, and abemaciclib, used in combination with ET, have shown improvement in progression-free survival compared to ET alone in the metastatic setting. However, the inevitable development of acquired resistance significantly limits the efficacy of this targeted therapy. Rewired signaling driven by different oncogenes, including the epidermal growth factor receptor (EGFR), overcomes the targeted inhibition of CDK4/6, which, in turn, allows cell cycle progression contributing to acquired resistance. We evaluated the expression and activity of EGFR in the panel of different matching pairs of ER- and ER+ CDK4/6i-sensitive (pS) and CDK4/6i-resistant (pR) breast cancer cell lines in vitro. Increased EGFR expression in ER+ MCF7/pR cells correlated with elevated ER phosphorylation, suggesting a direct cross-talk between EGFR and ER signaling in PD-resistant cells. Stimulation with EGF promoted ER activation, whereas estrogen stimulation promoted EGFR activation in ER+ pS and pR cell lines, indicating a direct cross-regulation between these molecular targets. Treatment of MCF7/pR cells with CDK4/6i resulted in elevated EGFR mRNA expression, confirming the dependency of resistant cells on EGFR signaling. Moreover, exposure to the EGFR inhibitors dramatically inhibited the proliferation of MCF7/pR cells compared to the parental cells, suggesting that EGFR could be an essential bypass signaling regulator contributing to resistance to CDK4/6i. We found that combined treatment with EGFRi and CDK4/6i significantly inhibited Rb phosphorylation in pR cells improving the effect of CDK4/6i on cell cycle arrest. We evaluated EGFR expression in MCF7/pS and pR mouse xenograft models in vivo. We observed a significant elevation in EGFR expression and phosphorylation in response to PD treatment, confirming the importance of EGFR signaling in maintaining the acquired resistance to CDK4/6i in ER+ breast cancer cells. Taken together, our findings suggest that EGFR signaling plays an important role in acquired resistance to CDK4/6i in ER+ breast cancer cells in vitro and in vivo and targeting EGFR with small-molecule inhibitors (like erlotinib or gefitinib) or chimeric monoclonal antibodies (i.e. cetuximab) can be a promising approach to improve CDK4/6i efficacy in breast cancer patients.
Citation Format: Nadiia Lypova, Lilibeth Lanceta, Susan Daugherty, Jason Chesney, Yoannis Imbert-Fernandez. EGFR signaling contributes to acquired resistance to CDK4/6 inhibitors in ER+ breast cancer cells in vitro and in vivo. [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P1-13-18.