Abstract 92: CHIP/STUB1 ubiquitin ligase targets MZF1 and loss of its expression in breast cancer unleashes a MZF1-cathepsin pro-oncogenic program
Authors: Mohapatra Bhopal C. , Luan Haitao , Bielecki Timothy A. , Mushtaq Insha , Mirza Sameer , Bailey Tameka A. , Clubb Robert , An Wei , Ahmed Dena , Ansari Rokaya El , Storck Matthew D. , Guda Chittibabu , Sheinin Yuri , Meza Jane L. , Raja Srikumar , Rakha Emad , Band Vimla , Band Hamid 
Published: August 2018

Triple-negative and ErbB2+ breast cancer subtypes carry inherently poorer prognosis. The overall therapeutic response and survival are markedly lower in metastatic disease. This identification of novel pathways that collide with driver oncogenes to promote more aggressive disease and metastasis could help identify biomarkers for selection of patients for more intensive therapy and provide avenues of targeted therapy for new combinatorial therapies. CHIP/STUB1 is a U-box containing ubiquitin ligase that has been established as a negative co-chaperone for HSP90/HSC70. Recent studies have shown its expression to be reduced or lost in several cancers, including breast cancer. However, the relationship of loss of CHIP with breast cancer subtypes and mechanisms by which loss of CHIP promotes oncogenesis in breast cancer are incompletely understood.

IHC analysis of an extensive breast cancer tissue collection revealed that loss of nuclear but not cytoplasmic CHIP predicted aggressive tumorigenesis and shorter survival, with loss of nuclear CHIP in two-thirds of TNBC/ErbB2+ and one-third of ER+ breast cancers. Reduced CHIP expression was confirmed in a panel of breast cancer patient-derived xenograft tumors and in ErbB2+ and TNBC cell lines. Ectopic CHIP expression in TNBC and ErbB2+ breast cancer cell lines suppressed in vitro oncogenic traits and in vivo xenograft tumor growth in nude mice. These analyses established CHIP as a barrier to tumorigenic progression and loss of CHIP as a key event in aggressive tumorigenesis in breast cancer.

To gain mechanistic insights into the linkage of the loss of nuclear CHIP expression with breast cancer aggressiveness, we carried out an unbiased screen for CHIP-regulated nuclear transcription factor-binding to their cognate DNA elements using an array methodology. This analysis identified many candidate transcription factors whose DNA-binding activity was up- or down-regulated upon re-expression of CHIP in CHIP-low ErbB2+ or TNBC cell line models. Western blotting of selected candidates confirmed the CHIP dependence of their expression levels. We focused our further efforts on the Myeloid Zinc Finger 1 (MZF1) protein as a CHIP target since it was recently identified as a positive regulator of cathepsin B/L (CTSB/L)-mediated tumor cell invasion in a model of oncogenic mutant ErbB2 overexpression in MCF-7 cells. We show that CHIP negatively regulates the expression of CTSB/L in TNBC and other breast cancer cell lines. Chemical inhibition of CTSB abrogated breast cancer cell invasion and matrix degradation in vitro and retarded xenograft tumor growth. We suggest that loss of CHIP remodels the cellular transcriptome to unleash critical pro-oncogenic pathways that can provide new therapeutic opportunities in breast cancer.

Citation Format: Bhopal C. Mohapatra, Haitao Luan, Timothy A. Bielecki, Insha Mushtaq, Sameer Mirza, Tameka A. Bailey, Robert Clubb, Wei An, Dena Ahmed, Rokaya El Ansari, Matthew D. Storck, Chittibabu Guda, Yuri Sheinin, Jane L. Meza, Srikumar Raja, Emad Rakha, Vimla Band, Hamid Band. CHIP/STUB1 ubiquitin ligase targets MZF1 and loss of its expression in breast cancer unleashes a MZF1-cathepsin pro-oncogenic program [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 92.