Fatty acid oxidation and autophagy promote endoxifen resistance and counter the effect of AKT inhibition in ER-positive breast cancer cells

Abstract

Tamoxifen (TAM) is the first-line endocrine therapy for estrogen receptor-positive (ER+) breast cancer (BC). However, acquired resistance occurs in ∼50% cases. Meanwhile, although the PI3K/AKT/mTOR pathway is a viable target for treatment of endocrine therapy-refractory patients, complex signaling feedback loops exist, which can counter the effectiveness of inhibitors of this pathway. Here, we analyzed signaling pathways and metabolism in ER+ MCF7 BC cell line and their TAM-resistant derivatives that are co-resistant to endoxifen using immunoblotting, quantitative polymerase chain reaction, and the Agilent Seahorse XF Analyzer. We found that activation of AKT and the energy-sensing kinase AMPK was increased in TAM and endoxifen-resistant cells. Furthermore, ERRα/PGC-1β and their target genes MCAD and CPT-1 were increased and regulated by AMPK, which coincided with increased fatty acid oxidation (FAO) and autophagy in TAM-resistant cells. Inhibition of AKT feedback activates AMPK and ERRα/PGC-1β-MCAD/CPT-1 with a consequent increase in FAO and autophagy that counters the therapeutic effect of endoxifen and AKT inhibitors. Therefore, our results indicate increased activation of AKT and AMPK with metabolic reprogramming and increased autophagy in TAM-resistant cells. Simultaneous inhibition of AKT and FAO/autophagy is necessary to fully sensitize resistant cells to endoxifen.

A Free Fatty Acid Receptor Agonist Inducing Autophagy in HT-29 Cells by Downregulating The AKT/mTOR Signaling Pathway in Fibrin Gel Matrices

“…AKT is the main signaling pathway essentially transmitting mitogen and growth factors. Besides, AKT inhibits BAD protein via phosphorylation and represses induced apoptosis [10,14].…”

“…Previous studies demonstrated that stress causes cell death including apoptosis and autophagy and leads to activation of AMPK and suppressor of mTOR [10,11].…”

Energy metabolism pathways in breast cancer progression: The reprogramming, crosstalk, and potential therapeutic targets

“…The AMPK activation promoted activation of AKT, while inhibition of AKT feedback suppressed the expression AMPK. AMPK additionally increased CPT1, which finally lead to the increase in fatty acid oxidation [138] . The molecular mechanism of sorafenib's antitumoral activity in BC is impairment of glucose metabolism by sustained activation of AMPK [147] .…”

“…1 ). In BC tumor and cell lines, the expression of CPT1A increased in ER-positive compared with ER-negative [137] via upregulating expression of ERRα and peroxisome proliferator activated receptor gamma coactivator 1 (PGC-1) β by activation of AMPK [138] , finally countering the effect of AKT inhibition. CPT1A overexpression significantly attenuated the proliferation in MDA-MB231 BC cells when compared with basal expression control.…”

Nuclear Receptor-Mediated Metabolic Reprogramming and the Impact on HR+ Breast Cancer

“…This idea is supported by studies from other solid tumors where CPT1A expression was associated with metastatic ability, EMT and drug resistant phenotype and its inhibition re-sensitized resistant cells to radiation, hormone therapy and/or and chemo-therapy [ 180 , 181 , 241 , 242 , 243 , 244 , 245 , 246 ]. Mechanistically, endocrine-resistant cells have been shown to express high levels of ERRα/PGC-1β and increased expression of a number of target genes, including CPT1A, which has been shown to increase FAO [ 247 ]. Likewise, CD36, which facilitates cellular import of fatty acids from the micro-environment, is upregulated in endocrine-resistant tumors and cell lines.…”