title: c-Myc dependent expression of pro-apoptotic Bim renders HER2-overexpressing breast cancer cells dependent on anti-apoptotic Mcl-1.
abstract: BACKGROUND
  Anti-apoptotic signals induced downstream of HER2 are known to contribute to the resistance to current treatments of breast cancer cells that overexpress this member of the EGFR family. Whether or not some of these signals are also involved in tumor maintenance by counteracting constitutive death signals is much less understood. To address this, we investigated what role anti- and pro-apoptotic Bcl-2 family members, key regulators of cancer cell survival, might play in the viability of HER2 overexpressing breast cancer cells.
  
  
  METHODS
  We used cell lines as an in vitro model of HER2-overexpressing cells in order to evaluate how anti-apoptotic Bcl-2, Bcl-xL and Mcl-1, and pro-apoptotic Puma and Bim impact on their survival, and to investigate how the constitutive expression of these proteins is regulated. Expression of the proteins of interest was confirmed using lysates from HER2-overexpressing tumors and through analysis of publicly available RNA expression data.
  
  
  RESULTS
  We show that the depletion of Mcl-1 is sufficient to induce apoptosis in HER2-overexpressing breast cancer cells. This Mcl-1 dependence is due to Bim expression and it directly results from oncogenic signaling, as depletion of the oncoprotein c-Myc, which occupies regions of the Bim promoter as evaluated in ChIP assays, decreases Bim levels and mitigates Mcl-1 dependence. Consistently, a reduction of c-Myc expression by inhibition of mTORC1 activity abrogates occupancy of the Bim promoter by c-Myc, decreases Bim expression and promotes tolerance to Mcl-1 depletion. Western blot analysis confirms that naïve HER2-overexpressing tumors constitutively express detectable levels of Mcl-1 and Bim, while expression data hint on enrichment for Mcl-1 transcripts in these tumors.
  
  
  CONCLUSIONS
  This work establishes that, in HER2-overexpressing tumors, it is necessary, and maybe sufficient, to therapeutically impact on the Mcl-1/Bim balance for efficient induction of cancer cell death.
authors: Centre de Recherche en Cancérologie Nantes-Angers - UMR 892 - INSERM/Université de Nantes, Institut de Recherche Thérapeutique de l'Université de Nantes 8 Quai Moncousu BP 7072144007 Nantes Cedex 1 France.
authors: Mario
authors: M
authors: Campone
authors: Bélinda
authors: B
authors: Noël
authors: Cécile
authors: C
authors: Couriaud
authors: Morgan
authors: M
authors: Grau
authors: Yannis
authors: Y
authors: Guillemin
authors: Fabien
authors: F
authors: Gautier
authors: Wilfried
authors: W
authors: Gouraud
authors: Catherine
authors: C
authors: Charbonnel
authors: Loïc
authors: L
authors: Campion
authors: Pascal
authors: P
authors: Jézéquel
authors: Frédérique
authors: F
authors: Braun
authors: Benjamin
authors: B
authors: Barré
authors: Olivier
authors: O
authors: Coqueret
authors: Sophie
authors: S
authors: Barillé-Nion
authors: Philippe
authors: P
authors: Juin
mesh_terms: D017209:Apoptosis; D051017:Apoptosis Regulatory Proteins / Q000235:genetics / Q000378:metabolism*; D000072224:Bcl-2-Like Protein 11; D001943:Breast Neoplasms; D002449:Cell Aggregation; D045744:Cell Line, Tumor; D002470:Cell Survival; D000068338:Everolimus; D005260:Female; D015870:Gene Expression; D055785:Gene Knockdown Techniques; D006801:Humans; D000076222:Mechanistic Target of Rapamycin Complex 1; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D046912:Multiprotein Complexes; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D011401:Promoter Regions, Genetic; D011506:Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D019253:Proto-Oncogene Proteins c-bcl-2 / Q000235:genetics / Q000378:metabolism*; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism*; D034622:RNA Interference; D018719:Erb-b2 Receptor Tyrosine Kinases / Q000378:metabolism*; D015398:Signal Transduction; D020123:Sirolimus / Q000031:analogs & derivatives / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D051020:bcl-X Protein / Q000235:genetics / Q000378:metabolism
chemical_list: D051017:Apoptosis Regulatory Proteins; D000072224:Bcl-2-Like Protein 11; D000068338:Everolimus; D000076222:Mechanistic Target of Rapamycin Complex 1; D008565:Membrane Proteins; D046912:Multiprotein Complexes; D064549:Myeloid Cell Leukemia Sequence 1 Protein; D011506:Proteins; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; D016271:Proto-Oncogene Proteins c-myc; D018719:Erb-b2 Receptor Tyrosine Kinases; D020123:Sirolimus; D058570:TOR Serine-Threonine Kinases; D051020:bcl-X Protein; C494811:BCL2L1 protein, human; C000606775:BCL2L11 protein, human; C489427:MYC protein, human; C498702:PMAIP1 protein, human; C508053:ERBB2 protein, human