Abstract P5-03-06: Overcoming MEK inhibitor resistance in triple-negative breast cancer by targeting myeloid cell leukemia-1 (MCL1), an anti-apoptotic protein
Authors: Gagliardi M , Chauhan G , Pitner MK , Iles L , Qi Y , Pusztai L , Tripathy D , Bartholomeusz G , Bartholomeusz C 
Published: May 2019

Abstract
Background: Triple-negative breast cancer (TNBC), which affects over 170 000 women worldwide every year, is considered the most arduous to treat subtype of breast cancer. With no targeted therapy, high rates of drug resistance and rapid metastasis, TNBC carries a poor prognosis. The MEK-ERK-MAPK signaling cascade is known to play a role in numerous cancers. Despite the lack of activating Ras/MAPK mutations in breast cancer, transcriptional signatures of this pathway are prevalent in TNBC. Our previous work showed that TNBC patients with tumors overexpressing ERK2 had a lower overall survival rate than did patients with low-ERK2-expressing tumors. MEK inhibitors selumetinib (AZD6244) and pimasertib (AS703026) are active in preclinical models, but not as single agents in the clinic. Using a synthetic lethal siRNA screen, we identified myeloid cell leukemia-1 (MCL1) as a potential contributor to selumetinib resistance. Mcl-1 is an anti-apoptotic protein that is highly amplified in numerous human cancers. It is associated with cell immortalization, transformation, and chemoresistance. Patients with TNBC tumors expressing high levels of Mcl-1 have lower overall survival and distant-metastasis-free survival rates. We hypothesized that Mcl-1 promotes MEK inhibitor resistance in TNBC.
Methods/Results: To model MEK inhibitor resistance, we established selumetinib- and pimasertib-resistant clones of SUM-149 and MDA-MB-231 TNBC cells by continuous exposure to increasing concentrations of inhibitors over a six month period. We confirmed the onset of MEK resistance by demonstrating that resistant cells, in comparison to the parental cells, exhibited no change in cell proliferation upon treatment with the MEK inhibitors. Resistant cells also displayed more effective cell migration and mammosphere formation than parental cells, suggesting a higher fraction of tumor-initiating cells.
We found Mcl-1 to be highly expressed in 83% (15 of 18) of TNBC cell lines but only 30% (3 of 10) of other breast cancer cell lines. Resistant cells had higher levels of Mcl-1 than did parental cells. To determine whether Mcl-1 is required for MEK sensitivity, we treated parental and resistant cells with either selumetinib or pimasertib together with S63845, a highly specific Mcl-1 inhibitor. The Mcl-1 inhibitor restored MEK sensitivity in both resistant cell lines. After treatment with the Mcl-1 inhibitor, the resistant SUM-149 and MDA-MB-231 cells had similar cell proliferation rates to those of their parental counterparts. Similar studies were done using an siRNA against Mcl-1.
Conclusion: Our data demonstrate that Mcl-1 may promote TNBC resistance to MEK inhibitors and that Mcl-1 is a promising target for combination therapy. We will continue to explore the mechanisms of MEK inhibitor resistance by screening for additional genes/pathways involved. Our long-term goal is to design rational combination approaches to counteract the emergence of resistance by using novel molecularly targeted therapeutics.
Citation Format: Gagliardi M, Chauhan G, Pitner MK, Iles L, Qi Y, Pusztai L, Tripathy D, Bartholomeusz G, Bartholomeusz C. Overcoming MEK inhibitor resistance in triple-negative breast cancer by targeting myeloid cell leukemia-1 (MCL1), an anti-apoptotic protein [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P5-03-06.