Evidence for Enhanced Exosome Production in Aromatase Inhibitor-Resistant Breast Cancer Cells

Aromatase inhibitors (AIs) represent the standard anti-hormonal therapy for post-menopausal estrogen receptor-positive breast cancer, but their efficacy is limited by the emergence of AI resistance (AI(R)). Exosomes act as vehicles to engender cancer progression and drug resistance. The goal of this work was to study exosome contribution in AI(R)mechanisms, using estrogen-dependent MCF-7 breast cancer cells as models and MCF-7 LTED (Long-Term Estrogen Deprived) subline, modeling AI(R). We found that exosome secretion was significantly increased in MCF-7 LTED cells compared to MCF-7 cells. MCF-7 LTED cells also exhibited a higher amount of exosomal RNA and proteins than MCF-7 cells. Proteomic analysis revealed significant alterations in the cellular proteome. Indeed, we showed an enrichment of proteins frequently identified in exosomes in MCF-7 LTED cells. The most up-regulated proteins in MCF-7 LTED cells were represented by Rab GTPases, important vesicle transport-regulators in cancer, that are significantly mapped in small GTPase-mediated signal transduction, protein transport and vesicle-mediated transport Gene Ontology categories. Expression of selected Rab GTPases was validated by immunoblotting. Collectively, we evidence, for the first time, that AI(R)breast cancer cells display an increased capability to release exosomes, which may be associated with an enhanced Rab GTPase expression. These data provide the rationale for further studies directed at clarifying exosome's role on endocrine therapy, with the aim to offer relevant markers and druggable therapeutic targets for the management of hormone-resistant breast cancers.