LPA-dependent Regulation of VCAM-1 in the Ovarian Cancer Metastatic Microenvironment and its Impact on Mesothelial Invasion

Ovarian cancer is the second most common gynecological cancer and is the leading cause of all reproductive cancer deaths in the United States. The poor prognosis is attributed to 75% of patients being diagnosed with advanced stage disease which is accompanied by an overall five-year survival rate of approximately 20-25%. Advanced staged disease is characterized by peritoneal and distant metastasis. Peritoneal metastasis involves the ovarian cancer cells disseminating within the peritoneal cavity where they will implant on surfaces of organs including the omentum, diaphragm, and large and small bowel serosa. The surfaces of these organs are covered by mesothelium, a monolayer of mesothelial cells. Mesothelial cells actively participate in promoting invasion by expressing adhesion molecules including vascular cell adhesion molecule 1 (VCAM-1). Previously, VCAM-1 was demonstrated as an important regulator of mesothelial invasion within the ovarian cancer tumor microenvironment; however, it is not known what regulates mesothelial VCAM-1 expression in ovarian cancer. Lysophosphatidic acid (LPA), a bioactive lipid found at elevated levels in the malignant ascites, regulates several cellular functions critical for tumorigenesis and metastasis such as proliferation, angiogenesis, migration and invasion. Therefore, this we investigated whether LPA targets the ovarian cancer microenvironment, specifically the mesothelium, to promote ovarian cancer metastasis. Using mice that lack expression of the LPA phosphatase, LPP1 (LPP1 KO), and have elevated circulating concentrations of LPA, this thesis reports LPP1 KO mice showed increased peritoneal ovarian cancer growth and a significantly higher incidence of invasive tumors as compared to wild type. Additionally, LPP1 KO had increased mesothelial VCAM-1 expression. From these data, we further studied LPA regulation of mesothelial VCAM-1 to promote mesothelial invasion. Utilizing a co-culture assay system, we observed a significantly reduced ovarian cancer cell invasion of the mesothelium after inhibition of LPA1. Moreover, blocking or inhibiting LPA1 in human mesothelial cells derived from ovarian cancer patients’ ascitic fluid decreased VCAM-1 expression and accumulation both at the transcript and protein level. Furthermore, pulse labeling with S35-Met/Cys indicated a significant decrease in VCAM-1 protein synthesis in cells lacking LPA1 expression compared to control. Importantly, re-expression of exogenous VCAM-1 after LPA1 knockdown in mesothelial cells restores trans-mesothelial invasion of ovarian cancer cells. Taken together, these data demonstrate LPA promotes mesothelial invasion through the regulation of mesothelial VCAM-1 expression. Furthermore, the work in this thesis provides a further understanding of the role LPA plays in ovarian cancer metastatic progression, and these findings provide a potential model for other conditions associated with elevated LPA and chronic VCAM-1 expression.