Abstract 1514: Acquired differentiation and loss of malignancy of pulmonary metastases in breast cancer
Authors: Liu Huiping 
Published: August 2015

Cancer stem cells (CSCs) or tumor initiating cells (TICs) were demonstrated in a leukemia transplant experiment in 1930s with a single mouse leukemic cell as well as in recent studies of human leukemia and solid tumors. Our work demonstrated that breast CSCs are able to mediate spontaneous lung metastasis in patient-derived breast tumor xenografts (PDXs) in vivo (PNAS, 2010). We further characterized the phenotypic and molecular characteristics of spontaneous metastases versus primary breast tumors. We collected the tumor cells from the lungs and conducted orthotopic implantation back into the mouse mammary fat pads in order to make a more metastatic tumor model. Surprisingly, relative to the parental primary breast tumors, the lung metastasis (met)-derived mammary tumors exhibited a slower growth rate and a reduced metastatic potential with a more differentiated epithelial status based on gene array analyses. The up-regulated genes in the met-derived tumors included promoters of cell adhesion and differentiation as well as tumor suppressors and cell cycle inhibitors, such as KRT19 (CK19), CEACAM1 (CD66a), and CDKN1A. Many of the down-regulated genes were promoters of self-renewal, cell motility, and cell cycle, such as EZH2, SMAD2, and CCNB1. Clinically, this signature correlated with breast tumor subtypes. Consistently, the differentiation scores of the lung met-derived tumors were significantly higher than the primary tumors (p = 0.007). We then examined the functions of microRNA candidates in regulating CSC-mediated metastasis and therapy resistance as well as their clinical relevance.

By miRNA analyses, thirty-two miRNAs were differentially expressed with twenty-seven miRNAs being up-regulated in the met-derived tumors, including miR-200 family members which induce epithelial phenotype and inhibits self-renewal, miR-30 family members, miR-138, and others. Further clinical studies demonstrated that miR-30c is independently associated with favorable distant relapse free survival of breast tumor patients (GSE22216 n = 210). Subsequent mechanistic work characterized that miR-30c inhibits both CSC-mediated metastasis and chemotherapy resistance by a direct targeting of cytoskeleton genes twinfilin TWF1 and vimentin (VIM). We also identified miR-138 as a novel regulator of invasion and epithelial-mesenchymal transition in breast cancer cells, acting by directly targeting the polycomb epigenetic regulator EZH2. Notably, based on the UNC breast tumor database analyses (n = 337), the miR-138 activity signature served as a novel independent prognostic marker for patient survival beyond traditional pathologic variables, intrinsic subtypes or a proliferation gene signature. Both miR-30c and miR-138 can be regulated by the transcription factor GATA3. Our results highlight the loss of malignant character in some lung micro-metastatic lesions and the epigenetic regulation of this phenotype.

Note: This abstract was not presented at the meeting.

Citation Format: Huiping Liu. Acquired differentiation and loss of malignancy of pulmonary metastases in breast cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1514. doi:10.1158/1538-7445.AM2015-1514