Date of Degree

2-2018

Document Type

Dissertation

Degree Name

Ph.D.

Program

Biochemistry

Advisor

David A Foster

Committee Members

Jill Bargonetti

James Fagin

Diego Loayza

Yuan-Shan Zhu

Subject Categories

Biochemistry | Cancer Biology | Cell Biology | Molecular Biology

Keywords

rapamycin, TGF-beta, prostate cancer, androgen receptor, E2F, phospholipase D

Abstract

PART I

Prostate cells are hormonally driven to grow and divide. Typical treatments for prostate cancer involve blocking the hormone androgen from activating the androgen receptor (AR) and thus inhibit growth and proliferation of the cancer. Androgen deprivation therapy (ADT) can lead to the selection of cancer cells that grow and divide independently of androgen receptor activation. Prostate cancer cells that are insensitive to androgens commonly display metastatic phenotypes and reduced long-term survival of patients. In this study, we provide evidence that androgen-insensitive prostate cancer cells have elevated phospholipase D (PLD) activity relative to the androgen-sensitive prostate cancer cells. PLD activity has been linked with promoting survival in many human cancer cell lines; and consistent with the previous studies, suppression of PLD activity in the prostate cancer cells resulted in apoptotic cell death. Of significance, suppressing the elevated PLD activity in the androgen-insensitive prostate cancer lines also blocked the ability of these cells to migrate and invade MatrigelTM. Since survival signals are generally an early event in tumorigenesis, the apparent coupling of survival and metastatic phenotypes implies that metastasis could be an earlier event in malignant prostate cancer than generally thought. Resistance to ADT appears to involve an elevation in PLD activity providing a survival program that is coupled to migration and invasion. Interruption of this pathway could provide a therapeutic strategy for treating androgen-insensitive prostate cancer.

PART II

Inhibiting the mammalian Target of Rapamycin Complex 1 (mTORC1) with rapamycin while suppressing Transforming Growth Factor-β (TGF-β) signaling induces apoptosis in many cancer cells. Some cancer cells, though, are resistant to the apoptotic effects of rapamycin treatment in the absence of TGF-β signaling. Both mTORC1 and TGF-β are upstream effectors of Retinoblastoma protein (Rb), a key regulatory protein involved in cell cycle progression from G1 to S-phase. We found that rapamycin-resistant cell lines had a nonfunctional Rb protein and a deleted CDKN2A gene. When Rb function was restored in the rapamycin resistant cells, apoptosis was induced upon rapamycin treatment. When Rb was knocked down in cells with deleted CDKN2A, the cells gained the rapamycin resistant phenotype. The common downstream target of Rb and CDKN2A is E2F1 and inhibition of E2F1 sensitizes the cell to the apoptotic effects of rapamycin. The data suggest that Rb and CDKN2A may be part of compensatory pathways and that the interruption of both pathways is necessary to confer resistance to mTORC1 inhibition. Resistance to mTORC1 inhibition by rapamycin, a downstream target of PLD, appears to involve E2F1 and implicates the involvement of CDKN2A and Rb at the G1-S phase boundary of the cell cycle as a point of therapeutic intervention.

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