Date of Graduation

2015

Document Type

Dissertation

Degree Type

PhD

College

School of Medicine

Department

Microbiology, Immunology, and Cell Biology

Committee Chair

Scott A Weed

Committee Co-Chair

Laura F Gibson

Committee Member

Lan Guo

Committee Member

Michael D Schaller

Committee Member

William Tse

Abstract

The zinc-finger pluripotency factor Kruppel-like factor 4 (KLF4) exerts context dependent roles in the maintenance of tissue homeostasis and the pathobiology of many malignancies, including breast cancer. Despite the multitude of studies focused on the role of KLF4 across these contexts, the signaling programs that are regulated by this factor in breast cancer cells remain unclear. In this dissertation we deliniate three KLF4-dependent protumorigenic signaling axes that endow triple-negative breast cancer (TNBC) cells with enhanced capacity to resist cell stress.;In the first study (Chapter 2), we found that KLF4 promotes the expression of two microRNAs (miRs), miR-206 and miR-21 (miR-206/21), to positively regulate RAS-ERK signaling and dependent cell phenotypes in TNBC cells. The collaborative action of miR-206/21 suppressed RAS-inhibitory GTPase activating protein (GAP) activity by repressing the translation of the GAP RASA1 and the Neurofibromatosis-1 (NF-1) GAP associated protein SPRED1. This attenuation of GAP activity resulted in increased levels of wild-type RAS-GTP (WT-RAS-GTP) levels and RAF-ERK activation. Interestingly, KLF4-miR-206/21 promoted RAS-ERK signaling in cells harboring activating RAS-mutations, and this regulation of pathway activity was attributed to the selective action of GAPs on WT-RAS proteins. This study uncovered a role for GAP proteins in regulating RAS-ERK signaling in RAS-mutant and RAS-WT cells alike and is consistent with the emerging paradigm in which the output of RAS signaling in RAS-mutant cells is dependent on the levels of WT-RAS-GTP.;In the second study (Chapter 3), we identified that KLF4 and miR-206 are functional markers of TNBC mammary cancer stem-like cells (MaCSCs) which promote cell survival against cell stresses including matrix detatchment, growth in immunocompromised mice, and against cytotoxic chemotherapy. Interestingly, the modulation of endogenous KLF4 and miR-206 only had subtle effects on MaCSC population. miR-206 repressed the translation of the pro-apoptotic molecules CX43 and the well-established miR-21 target PDCD4. Suppression of CX43 and PDCD4 promoted TNBC cell survival, and reduced levels of both proteins was observed in MaCSCs compared to non stem-like cells. These results suggest that low PDCD4 and CX43 contribute to the enhanced chemo- and radio-resistance of MaCSCs, and implicate KLF4-miR-206 signaling in enforcing tumor cell survival.;Finally, in the third study (Chapter 4), we found that KLF4 mediates TNBC cell resistance toward MEK 1/2 inhibition by promoting the transcription of the receptor tyrosine kinase (RTK), PDGFRbeta. This RTK has been functionally implicated as a critical resistance factor that protects cells from the cytostatic effects of RAS-ERK pathway inhibition. KLF4 antagonized cMYC mediated repression of PDGFRbeta, and either KLF4 or PDGFRbeta was sufficient to promote MEK 1/2 inhibitor resistant TNBC cell proliferation. Surprisingly, the time dependent re-emergence of RAS-ERK signaling upon MEK 1/2 inhibitor treatment was not dependent on KLF4 or PDGFRbeta, and thus our results suggest that alternative pathways may be involved in endowing TNBC cell resistance to MEK 1/2 inhibition.;Collectively, the studies presented in dissertation suggest that KLF4 is a critical mediator of cell survival in TNBC cells. In agreement with this role for KLF4, in a recently published study, we found that KLF4 collaborates with a related KLF member, KLF5, to promote lapatinib resistance in HER2+ breast cancer cells by regulating the expression of the anti-apoptotic proteins MCL1 and BCL-XL. Our results point to the targeting of KLF4 and/or KLF4-dependent effectors in the treatment of TNBC.

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