title: MYC regulates the unfolded protein response and glucose and glutamine uptake in endocrine resistant breast cancer.
abstract: BACKGROUND
  About 70% of all breast cancers are estrogen receptor alpha positive (ER+) and are treated with antiestrogens. However, 50% of ER + tumors develop resistance to these drugs (endocrine resistance). In endocrine resistant cells, an adaptive pathway called the unfolded protein response (UPR) is elevated that allows cells to tolerate stress more efficiently than in sensitive cells. While the precise mechanism remains unclear, the UPR can trigger both pro-survival and pro-death outcomes that depend on the nature and magnitude of the stress. In this study, we identified MYC, an oncoprotein that is upregulated in endocrine resistant breast cancer, as a regulator of the UPR in glucose-deprived conditions.
  
  
  METHODS
  ER+ human breast cancer cell lines (LCC1, LCC1, LY2 and LCC9) and rat mammary tumors were used to confirm upregulation of MYC in endocrine resistance. To evaluate functional relevance of proteins, siRNA-mediated inhibition or small molecule inhibitors were used. Cell density/number was evaluated with crystal violet assay; cell cycle and apoptosis were measured by flow cytometry. Relative quantification of glutamine metabolites were determined by mass spectrometry. Signaling molecules of the UPR, apoptosis or autophagy pathways were investigated by western blotting.
  
  
  RESULTS
  Increased MYC function in resistant cells correlated with increased dependency on glutamine and glucose for survival. Inhibition of MYC reduced cell growth and uptake of both glucose and glutamine in resistant cells. Interestingly, in glucose-deprived conditions, glutamine induced apoptosis and necrosis, arrested autophagy, and triggered the unfolded protein response (UPR) though GRP78-IRE1α with two possible outcomes: (i) inhibition of cell growth by JNK activation in most cells and, (ii) promotion of cell growth by spliced XBP1 in the minority of cells. These disparate effects are regulated, at different signaling junctions, by MYC more robustly in resistant cells.
  
  
  CONCLUSIONS
  Endocrine resistant cells overexpress MYC and are better adapted to withstand periods of glucose deprivation and can use glutamine in the short term to maintain adequate metabolism to support cell survival. Our findings reveal a unique role for MYC in regulating cell fate through the UPR, and suggest that targeting glutamine metabolism may be a novel strategy in endocrine resistant breast cancer.
authors.affiliation: Lombardi Comprehensive Cancer Center and Department of Oncology, Georgetown University School of Medicine, 3970 Reservoir Road NW, Washington, DC 20057, USA. ans33@georgetown.edu.
authors.forename: Ayesha N
authors.initials: AN
authors.lastname: Shajahan-Haq
authors.forename: Katherine L
authors.initials: KL
authors.lastname: Cook
authors.forename: Jessica L
authors.initials: JL
authors.lastname: Schwartz-Roberts
authors.forename: Ahreej E
authors.initials: AE
authors.lastname: Eltayeb
authors.forename: Diane M
authors.initials: DM
authors.lastname: Demas
authors.forename: Anni M
authors.initials: AM
authors.lastname: Warri
authors.forename: Caroline O B
authors.initials: CO
authors.lastname: Facey
authors.forename: Leena A
authors.initials: LA
authors.lastname: Hilakivi-Clarke
authors.forename: Robert
authors.initials: R
authors.lastname: Clarke
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects; D001343:Autophagy / Q000187:drug effects; D001943:Breast Neoplasms / Q000378:metabolism* / Q000473:pathology*; D002453:Cell Cycle / Q000187:drug effects; D045744:Cell Line, Tumor; D019070:Cell Lineage / Q000187:drug effects; D049109:Cell Proliferation / Q000187:drug effects; D002470:Cell Survival / Q000187:drug effects; D019008:Drug Resistance, Neoplasm / Q000187:drug effects; D000091342:Endoplasmic Reticulum Chaperone BiP; D020847:Estrogen Receptor Modulators / Q000494:pharmacology; D004967:Estrogens / Q000494:pharmacology*; D005260:Female; D005947:Glucose / Q000378:metabolism*; D005973:Glutamine / Q000378:metabolism*; D006801:Humans; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism*; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction / Q000187:drug effects; D056811:Unfolded Protein Response / Q000187:drug effects*; D015854:Up-Regulation / Q000187:drug effects
chemical_list: D000091342:Endoplasmic Reticulum Chaperone BiP; D020847:Estrogen Receptor Modulators; D004967:Estrogens; C000717247:HSPA5 protein, human; D016271:Proto-Oncogene Proteins c-myc; D005973:Glutamine; D005947:Glucose