Date of Graduation
Doctor of Philosophy (PhD)
14-3-3ζ is a ubiquitously expressed family member of proteins that have been implicated to have oncogenic potential through its interactions and involvement in cancer initiation and progression. 14-3-3ζ belongs to the highly conserved 14-3-3ζ protein family and modulates numerous pathways in cancer. Overexpression of 14-3-3ζ is an early event, occurs in more than 40% of human breast cancer cases, and is associated with disease recurrence and poor prognosis. Metabolic reprogramming is a hallmark of cancer. Cancer cells elevate aerobic glycolysis to produce metabolic intermediates and reducing equivalents, thereby facilitating cellular adaptation to the adverse environment and sustaining fast proliferation. Interestingly, new evidence has emerged that metabolic alteration may arise at early stages of breast cancer. However, little is known about what triggers metabolic reprogramming and how it mechanistically contributes to breast cancer initiation and progression. In this dissertation, I have characterized the functional role of 14-3-3ζ in metabolic alteration, cancer initiation and progression. The bioinformatic analyses of gene expression profiling from early-stages breast premalignant lesions showed that the expression of 14-3-3ζ is strongly correlated with the expressions of glycolytic genes, especially lactate dehydrogenase A (LDHA). Interestingly, this positive correlation was also preserved in the advanced stage of breast cancer. Experimentally, my work demonstrated that increasing 14-3-3ζ expression in human non-transformed mammary epithelial cells (hMECs), MCF10A and MCF12A, transcriptionally up-regulated LDHA expression and increased glycolytic activity, which increased colony formation and promoted early transformation of hMECs. Conversely, knockdown of LDHA in these 14-3-3ζ-overexpressing hMECs significantly decreased glycolytic activity and inhibited early transformation. Mechanistically, up-regulation of LDHA in 14-3-3ζ-overexpressing hMECs was directly mediated by the cAMP-response element-binding (CREB) transcription factor through 14-3-3ζ-mediated activation of the MEK-ERK signaling axis. Blocking MEK-ERK pathway in 14-3-3ζ-high expressing hMEC-derived MCF10DCIS.COM tumor lesions, significantly decreasing LDHA expression, reducing tumor cell proliferation, and effectively inhibiting tumor growth. Taken together, my studies demonstrate that 14-3-3ζ has pleiotropic functions on cancer metabolism dysregulation and tumorigenesis. While 14-3-3ζ has been identified as critical mediator in breast cancer initiation and early metabolic transformation, another key finding of this dissertation is that discovery of tumor cells selectively preserve high 14-3-3ζ expression during tumor progression process. 14-3-3ζ may involve in cell fitness mechanism that benefits cell survival and proliferation during cancer progression. Cancer is a sequential process of cell clone selection and competition. During cell competition; a “fit” clone population with better growth advantages outcompetes other subclones and eliminates “unfit” subclones. I found that 14-3-3ζ-low cells are eradicated by 14-3-3ζ-high cells when they grow together (HET tumors); however, 14-3-3ζ-low cells can still survive only when surrounded by cells with similar expression levels of 14-3-3ζ. Mechanistically, 14-3-3ζ-low tumor cells produce a high level of cytokine macrophage inhibitory factor (MIF). Juxtacrine signaling involving MIF, its receptor CXCR2 and downstream production of interleukin-8 (IL-8), augmented cell proliferation and reduced cell apoptosis in 14-3-3ζ-high cells. Disruption of IL-8 or its upstream signaling, MIF or CXCR2, led to diminish cell fitness in 14-3-3ζ-high tumor cells and reduced tumor growth. Moreover, unlike conventional cell fitness, this study has revealed that 14-3-3ζ-high cells out-compete to sequester MIF, thereby causing cell death of 14-3-3ζ-low cells. I show that cancer cells may utilize part of immunity to trigger winner-loser cell interaction that determines the cell fate in solid tumors. Targeting the MIF-CXCR2-IL-8 axis could be an effective strategy to intervene in breast cancer progression. In summary, my work demonstrates that 14-3-3ζ has two distinct roles involving in cancer metabolism and cell competition, which may be developed into novel therapeutic strategies to target human breast cancer.
Breast Cancer, Cancer Metabolism, Cell Competition, 14-3-3