Access Type

Open Access Dissertation

Date of Award

January 2014

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Pathology

First Advisor

Hyeong-Reh C. Kim

Abstract

The PDGF family consists of four members; while PDGF A and B are secreted as active dimers, PDGF C and D are secreted as latent dimers that undergo serine protease-mediated extracellular proteolytic activation. Gene expression analysis of breast cancer cell lines showed that PDGF C expression is associated with Basal B subtype breast cancer cells which have cancer stem cell-like characteristics. Furthermore, PDGF C expression is associated with triple-negative (estrogen receptor-, progesterone receptor- and HER2/neu-negative) breast cancer cells, a challenging type of breast cancer to treat. During the course of our study, we discovered a splice variant of PDGF C encoding the truncated PDGF C protein (t-PDGF C). Specific aims of this dissertation are to determine the role of full-length PDGF C (FL-PDGF C) and t-PDGF C in breast cancer and to characterize their subcellular localizations. This study found that although t-PDGF C presumably lacks the signal peptide, it is secreted as a heterodimer with FL-PDGF C which can undergo extracellular proteolytic activation. Furthermore, PDGF C was found in the nuclear fraction of breast cancer cells, suggesting an uncharacterized function in breast cancer. A putative nuclear localization sequence in PDGF C showed little effect on its nuclear localization as determined by the point mutagenesis assay. Interestingly, we found that the serine protease cleavage site in the hinge region plays a critical role for both extracellular proteolytic processing and nuclear accumulation of PDGF C, suggesting that the biochemical processing and the subcellular localization are co-regulated. For the functional study, we established in vitro cell models engineered to overexpress PDGF C isoforms or inhibit its expression. This study found that PDGF C expression correlates with cell proliferation, invasive phenotype and anchorage-independent growth in vitro. Importantly, t-PDGF C expression further promoted PDGF C-induced phenotypic transformation. PDGF C downregulation decreased tumor growth and metastatic potential in vivo. Taken together, this study identified PDGF C and its splice variant as key signaling molecules in breast cancer. In addition, once thought of as primarily an extracellular signaling molecule, nuclear localization marks a potentially important paradigm shift in PDGF C biology.

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