The aggressive phenotype of breast cancer tumors expressing estrogen receptor activating mutations: the role of chemotherapy resistance and predilection of liver metastases

Endocrine therapy is the mainstay treatment for estrogen receptor-α (ER)-positive metastatic breast cancer (MBC) patients. However, all MBC patients who initially responded to the treatment will relapse and receive chemotherapy. Our lab identified novel activating-mutations, Y537S and D538G, in the ER ligand-binding domain (LBD). These mutated-ERs (mut-ER) confer constitutive transcriptional activity and resistance to endocrine therapies in nearly 40% of the patients. Furthermore, these patients comprise a unique sub-group characterized by increased liver metastasis (LM) and a worse prognosis.

Our major goal is to study the role of the mitochondria in promoting LM formation and conferring chemo-resistance.

We will use MCF-7 cells stably-expressing WT or LBD-mutations. MCherry-labeled cells will be co-cultured with primary human hepatocytes or using fresh liver slices, and conduct proteomics of isolated mitochondria. For chemo-resistance, we will focus on the role of MDR1 and c-jun pathways and assess gene expression, transcriptional activity, and study clinical samples of patients harboring these mutations.

Mass spectrometry revealed differential expression of several proteins that may be relevant for LM, with the mitochondrial-transporter SLC25A15 in the highest-ranking. Indeed, both mRNA and protein levels were higher in the mut-ERs and form metastasis-like structures when co-cultured with liver slices. We showed that mut-ERs confer chemo-resistance and revealed an elevation in MDR1 and c-Jun. Inhibition of the JNK pathway decreased MDR1 expression, decreased viability, and increased apoptosis. This study is likely to reveal unique targets for the development of novel treatment strategies aiming at restricting the development of LM and pave the way for novel therapies.