Resistance mechanisms during endocrine treatment in breast cancer

Abstract

Prolonged endocrine therapy is the mainstay of treatment for ER+ breast cancer patients. However, resistance develops in many patients which leads to more aggressive disease. Understanding the mechanisms of acquired resistance that emerge as a consequence of prolonged endocrine treatment remains critical. This study aimed to use gene expression profiling to discover induced mechanisms shared by a panel of MCF7-derived acquired resistant cells that underpin endocrine resistant growth. The in vitro panel represents resistance to oestrogen deprivation, tamoxifen or fulvestrant and includes long-term (3year) models to better-mimic clinical endocrine exposure. Affymetrix 1.0ST microarrays detected 572 genes induced in all resistant models versus MCF7. Over-represented ontologies, pathways and functional classification for these genes revealed induction of oxidative phosphorylation (OxPhos) and TCA cycle enzymes in the resistant models, a finding further confirmed by mass spectrometry. Increased oxygen consumption, NADH dehydrogenase and/or cytochrome C oxidase activity was detected in resistant cells, and targeting with OxPhos inhibitors Metformin or Antimycin A confirmed growth-dependency on OxPhos. Western blotting for AMPK (energy sensor) activity and its downstream anabolic targets (ACC, mTOR/P70S6K) showed Metformin reduced fatty acid and protein synthesis in growth-sensitive endocrine resistant cells. In silico analysis inferred clinical relevance since many TCA/OxPhos genes associated with earlier relapse in ER+ and/or tamoxifen treated patients. Monitoring basal glycolysis (extracellular lactate) and growth impact of 2DG or glutamine restriction demonstrated glycolysis and glutaminolysis also contribute to endocrine resistance. The microarrays furthermore revealed that metabolic kinases PCK2, ALDH18A1 and PFKFB2, and components of cell response to Zn were commonly-induced which may additionally help endocrine resistant growth. This study has revealed increased OxPhos arises as a consequence of prolonged endocrine treatment and is a key bioenergetic pathway sustaining resistance. Since resistant growth is Metformin-sensitive, such targeting of this energy pathway (alongside further antihormones or glycolysis/glutaminolysis inhibitors) could help treat resistance