Mcl-1 confers protection of Her2-positive breast cancer cells to hypoxia: Therapeutic implications

Muhammad Hasan Bashari, Fengjuan Fan, Sonia Vallet, Martin Sattler, Melissa Arn, Claudia Luckner-Minden, Henning Schulze-Bergkamen, Inka Zörnig, Frederik Marme, Andreas Schneeweiss, Michael H. Cardone, Joseph T. Opferman, Dirk Jäger, Klaus Podar*

*Corresponding author for this work

Research output: Journal article (peer-reviewed)Journal article

27 Citations (Scopus)

Abstract

Background: Molecular mechanisms leading to the adaptation of breast cancer (BC) cells to hypoxia are largely unknown. The anti-apoptotic Bcl-2 family member myeloid cell leukemia-1 (Mcl-1) is frequently amplified in BC; and elevated Mcl-1 levels have been correlated with poor prognosis. Here we investigated the pathophysiologic role of Mcl-1 in Her2-positive BC cells under hypoxic conditions. Methods: RNA interference and a novel small molecule inhibitor, EU-5346, were used to examine the role of Mcl-1 in Her2-positive BC cell lines and primary BC cells (sensitive or intrinsically resistant to Her2 inhibitors) under hypoxic conditions (using a hypoxic incubation chamber). Mechanisms-of-action were investigated by RT-PCR, mitochondrial isolation, as well as immunoprecipitation/blotting analysis, and microscopy. The specificity against Mcl-1 of the novel small molecule inhibitor EU5346 was verified in Mcl-1∆/null versus Mcl-1wt/wt Murine Embryonic Fibroblasts (MEFs). Proliferation, survival, and spheroid formation were assessed in response to Mcl-1 and Her2 inhibition. Results: We demonstrate for a strong correlation between high Mcl-1 protein levels and hypoxia, predominantly in Her2-positive BC cells. Surprisingly, genetic depletion of Mcl-1 decreased Her2 and Hif-1α levels followed by inhibition of BC cell survival. In contrast, Mcl-1 protein levels were not downregulated after genetic depletion of Her2 indicating a regulatory role of Mcl-1 upstream of Her2. Indeed, Mcl-1 and Her2 co-localize within the mitochondrial fraction and form a Mcl-1/Her2- protein complex. Similar to genetically targeting Mcl-1 the novel small molecule Mcl-1 inhibitor EU-5346 induced cell death and decreased spheroid formation in Her2-positive BC cells. Of interest, EU-5346 induced ubiquitination of Mcl-1- bound Her2 demonstrating a previously unknown role for Mcl-1 to stabilize Her2 protein levels. Importantly, targeting Mcl-1 was also active in Her2-positive BC cells resistant to Her2 inhibitors, including a brain-primed Her2-positive cell line. Conclusion: Our data demonstrate a critical role of Mcl-1 in Her2-positive BC cell survival under hypoxic conditions and provide the preclinical framework for the therapeutic use of novel Mcl-1- targeting agents to improve patient outcome in BC.

Original languageEnglish
Article number26
JournalBreast Cancer Research
Volume18
Issue number1
DOIs
Publication statusPublished - 26 Feb 2016
Externally publishedYes

Keywords

  • Apoptosis
  • Breast cancer
  • Hypoxia
  • Myeloid cell leukemia-1

ASJC Scopus subject areas

  • Oncology
  • Cancer Research

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