Graduation Date

Fall 12-14-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Programs

Pathology & Microbiology

First Advisor

Jixin Dong

Abstract

The Hippo pathway is conserved regulator of organ size through control of proliferation, apoptosis, and stem-cell self-renewal. In addition to this important function, many of the canonical signaling members have also been shown to be regulated during mitosis. Importantly, Hippo pathway components are frequently dysregulated in cancers and have attracted attention as possible targets for improved cancer therapeutics. Further exploration of Hippo-YAP (yes-associated protein) signaling has revealed new regulators and effectors outside the canonical signaling network and has revealed a larger non-canonical network of signaling proteins in which canonical Hippo pathway components crosstalk with important cellular homeostasis and apoptosis signaling pathways.

KIBRA is a regulator of the Hippo-YAP pathway, which plays a critical role in tumorigenesis. In the current study, we show that KIBRA is a positive regulator in prostate cancer cell proliferation and motility. We found that KIBRA is tran­scriptionally upregulated in androgen-insensitive LNCaPC4-2 and LNCaP-C81 cells when compared to the parental androgen-sensitive LNCaP cells. Ectopic expression of KIBRA enhances cell proliferation, migration and invasion in both immortalized and cancerous prostate epithelial cells. Accordingly, knockdown of KIBRA reduces migration, invasion, and anchorage-independent growth in LNCaP-C4-2/C81 cells. Moreover, KIBRA expres­sion is induced by androgen signaling and KIBRA is partially required for androgen recep­tor (AR) signaling activation in prostate cancer cells. In line with these findings, we further show that KIBRA is overexpressed in human prostate tumors. Our studies uncover unex­pected results and identify KIBRA as a tumor promoter in prostate cancer.

PDZ-binding kinase (PBK) has been shown to be a target gene of YAP and plays a major role in proliferation and in safeguarding mitotic fidelity in cancer cells. Frequently upregulated in many cancers, PBK drives tumor­igenesis and metastasis. PBK has been shown to be phosphorylated in mitosis by cyclin-dependent kinase 1 (CDK1)/cyclin B, however, no studies have been done examining PBK mitotic phosphorylation in oncogenesis. Additionally to the previously identified Thre­onine-9 phosphorylation, we found that Threonine-24, Serine-32, and Serine-59 of PBK are also phosphorylated. PBK is phosphorylated in vitro and in cells by CDK1 during anti­mitotic drug-induced mitotic arrest and in normal mitosis. We demonstrated that mitotic phosphorylation of Threonine-9 is involved in cytokinesis. The non-phosphorylatable mu­tant PBK-T9A augments tumorigenesis to a greater extent than wild type PBK in breast cancer cells, suggesting that PBK mitotic phosphorylation inhibits its tumor promoting ac­tivity. The PBK-T9A mutant also transforms and increases the proliferation of immortalized breast epithelial cells. Collectively, this study reveals that CDK1-mediated mitotic phos­phorylation of PBK is involved in cytokinesis and inhibits its oncogenic activity.

AMP-activated protein kinase (AMPK) is a heterotrimeric serine/threonine kinase consisting of a catalytic α subunit and two regulatory β and γ subunits. AMPK regulates cellular energy homeostasis and harmonizes proliferation with energy availability. AMPK can shortcut the canonical Hippo pathway to phosphorylate and inhibit YAP directly to constrain proliferation under conditions of cellular stress. Additionally, AMPK has recently been found to sit in the center of a signaling network involving bona-fide tumor suppressors and is associated with cell cycle checkpoints, as AMPK-null cells have mitotic defects. Despite AMPK’s emerging association with the cell-cycle, it still has not been fully delineated how AMPK is regulated by upstream signaling pathways during mitosis. We report for the first time, direct CDK1 phosphorylation of both the catalytic α1 and α2 subunits as well as the β1 regulatory subunit of AMPK in mitosis. We found that AMPK knockout U2OS cells have reduced mitotic indexes and that CDK1 phosphorylation-null AMPK is unable to rescue, demonstrating a role for CDK1 regulation of mitotic entry through AMPK. Our results also denote a vital role for AMPK in promoting proper chromosomal alignment, as loss of AMPK activity leads to misaligned chromosomes and concomitant metaphase delay. Importantly, AMPK expression and activity was found to be critical for paclitaxel chemosensitivity in breast cancer cells and significantly positively correlated with relapse-free survival in systemically treated breast cancer patients.

Available for download on Wednesday, May 15, 2019

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