To improve cancer patient outcome significantly, we must understand the mechanisms regulating stem-like cancer cells, which have been implicated as a cause of metastasis and treatment resistance. The transcription factor C/EBPδ can exhibit pro- and anti-tumorigenic activities, but the mechanisms underlying the complexity of its functions are poorly understood. Here we identify a role for breast cancer cell intrinsic C/EBPδ in promoting phenotypes that have been associated with cancer stem cells (CSCs). While C/EBPδ expression is not abundant in most metastatic breast cancers, our data support a pro-tumorigenic role of C/EBPδ when expressed in subsets of tumor cells and/or through transient activation by the tumor microenvironment or loss of substrate adhesion. Using genetic mouse models and human breast cancer cell lines, we show that deletion or depletion of C/EBPδ reduced expression of stem cell factors and stemnness markers, sphere formation and self-renewal, along with growth of tumors and established experimental metastases in vivo. C/EBPδ is also known as a mediator of the innate immune response, which is enhanced by hypoxia and interleukin-6 (IL-6) signaling, two conditions that also play important roles in cancer progression. Our mechanistic data reveal C/EBPδ as a link that engages two positive feedback loops, in part by directly targeting the IL-6 receptor (IL6RA) gene, and, thus, amplifying IL-6 and HIF-1 signaling. This study provides a molecular mechanism for the synergism of tumor microenvironmental conditions in cancer progression with potential implications for the targeting of CSCs.
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We are thankful for superb support through services provided by Leidos Biomedical Research, Inc., FNLCR (Laboratory Animal Sciences Program, Protein Expression Laboratory, Illustrations, and Graphical Support); and the NCI/CCR cores (Small Animal Imaging, Flow Cytometry, Optical Microscopy, and Analysis Laboratory); and Data Management Services, Inc. for assistance with Statistics. We thank Linda Miller, Suzanne Specht, Rena Mao, and the student interns for their valuable contributions; Heidi Dowst for providing clinical information related to the PDX models; and all the investigators who kindly shared their valuable reagents (see Methods) or provided critical comments on the manuscript. This research was supported by the Intramural Research Program of the NIH, National Cancer Institute, in part with Federal Funds from the Frederick Cancer Institute (NIH) under contract no. HHSN261200800001E. LED and MTL were supported by BCRF Founders Fund Grant CPRIT DP150069 and V-Foundation grant T2014-010. The content of this publication does not necessarily reflect the views or policies of the Department of Health and Human Services, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government.
KB and ES conceived the project and designed the study. KB, DM-V, SS, and ES designed, conducted, and/or analyzed the experiments. LED and MTL provided PDX reagents, data, resources, and advice. GHS and LED provided technical support and supervision for animal model studies. KB, MTL, and ES wrote the manuscript.
Conflict of interest
MTL is a limited partner in StemMed Ltd, and a Manager in StemMed Holdings, its general partner. He also holds an equity stake in Tvardi Therapeutics Inc. LED is a compensated employee of StemMed Ltd. The remaining authors declare that they have no conflict of interests.
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Balamurugan, K., Mendoza-Villanueva, D., Sharan, S. et al. C/EBPδ links IL-6 and HIF-1 signaling to promote breast cancer stem cell-associated phenotypes. Oncogene 38, 3765–3780 (2019). https://doi.org/10.1038/s41388-018-0516-5
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