Metastatic breast lesions are difficult to treat because they are resistant to chemotherapy. There are currently very few therapeutics that are available to prevent breast cancer metastasis or reduce drug resistance. The ability of breast cancer cells to metastasize to distant tissues is dependent upon migration, invasion, and intravasation into blood or lymphatic vessels. Neoangiogenesis into the tumor provides oxygen, nutrients, and a physical network of vessels that facilitates metastasis. Phospholipase D1 (PLD1), a lipid-signaling enzyme, has been proposed to be involved in these tumorigenic pathways, making its inhibition a promising target for metastasis. However, while it has been demonstrated through in vitro studies and tumor explant approaches that PLD1 has potential roles in many processes important for cancer progression, the function of PLD1 has not been characterized in breast cancer models in which the cancer is initiated in vivo via genetic drivers. The hypothesis we thus explored was to examine if PLD1 has a role in breast cancer progression and metastasis using a well-established genetic mouse system that better models the human disease. We show here that when PLD1 is ablated in the PyMT transgenic breast cancer mouse model, there is a remarkable decrease in lung metastases and neoangiogenesis. However, no major changes in breast cancer initiation, proliferation, macrophage recruitment, migration, or invasion were observed. We further conducted PCR array analysis of angiogenic genes, which identified the involvement of PLD1 in the potent sphingosine-1-phosphate (S1P) proangiogenic signaling cascade.
Sphingosine-1-phosphate receptor 1 (S1PR1) regulates neoangiogenesis through migration and invasion of endothelial cells and formation of vascular tubes. The S1PR1 ligand, S1P, is produced by the enzyme sphingosine kinase 1 (SK1) in tumor cells, macrophages, and endothelial cells. S1P is a bioactive metabolite that drives tumorigenesis, angiogenesis, and chemotherapy resistance. S1P can bind as both an autocrine and paracrine agent, causing the upregulation of S1PR1 expression and perpetuating a proangiogenic phenotype. Crucial to the phosphorylation of sphingosine to S1P is the activation of the SK1 enzyme by phosphatidic acid (PA), the signaling lipid generated by PLD1 hydrolysis of the membrane phospholipid phosphatidylcholine. We show that PLD1-ablated tumors have reduced SK activity, as determined through assessing S1P production, and S1PR1 expression, despite having normal or potentially increased SK1 expression. This reduction of SK activity suggests that PLD1-generated PA plays an important role in SK1 activation in this setting. In our model, when tumor and endothelial cells lack PLD1, breast tumors have decreased endothelial marker expression, vascular density, and dysregulated tube formation, providing correlative functional support for the importance of the inhibition of the SK-S1P-S1PR1 pathway.
Breast cancer metastasis is a complicated process wherein tumor cells intravasate blood or lymphatic vessels, survive in circulation, extravasate, and proliferate in other tissues. The many pathways that can be targeted for pharmacological inhibition, ultimately fall into two categories, prevention and treatment. Here, we show that PLD1 ablation reduces the number and size of breast metastases to the lung. Dysregulated neoangiognesis could, at least in part, underlie this finding through decreasing the incidence of tumor cell intravasation. Treatment for metastasis includes combinations of drugs used for primary tumor proliferation, to which metastatic lesions develop resistance. Interestingly, the PLD1-ablated mice have markedly smaller breast metastases, and S1P inhibition protects against chemotherapy resistance. PLD1 inhibition might be useful for the treatment of established metastatic foci by decreasing their size and increasing their sensitivity to chemotherapy. PLD1 activity provides a new target for the potential prevention and treatment of breast cancer metastasis.
|Advisor:||Frohman, Michael A.|
|Commitee:||Chan, Chia-Hsin (Lori), Snider, Ashley J., Talmage, David A.|
|School:||State University of New York at Stony Brook|
|Department:||Molecular and Cellular Pharmacology|
|School Location:||United States -- New York|
|Source:||DAI-B 78/06(E), Dissertation Abstracts International|
|Subjects:||Pharmacology, Biochemistry, Oncology|
|Keywords:||Angiogenesis, Breast cancer, Lipid signaling, Metastasis, Oncology, Pharmacology|
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