Lysophospholipid Mediators in Ovarian Cancer

Statement of Research Plans The goals of the original submission of this award were 1. To define the enzymes, pathways and cellular processes involved in the synthesis and release of LPA by ovarian cancer cells. 2. To define the enzymes and pathways involved in metabolic inactivation of LPA by ovarian cancer cells. 3. To determine the effect of manipulating pathways involved in LPA synthesis and metabolism on ovarian cancer cell growth and tumorigenic potential. This project is in its final year. As such, the majority of the aims have been accomplished as originally proposed and some of our findings have been published. Our work on aims 2 and 3 of the proposal has essentially been completed. We examined the expression of integral membrane lipid phosphatases (LPPs)- enzymes that dephosphorylate and inactivate LPA- in ovarian cancer cells and explored their roles as regulators of ovarian cancer cell responses to LPA. We established that overexpression of a particular LPP isoform, LPP3, in several ovarian cancer cell lines results in decreased growth and increased susceptibility to stimulation of apoptosis by chemotherapy agents. Further work suggested that changes in decreased expression of specific LPP isoforms in ovarian cancer cells compared to normal ovary surface epithelial cells might underlie the increased responsiveness of these cancer cells to LPA. We have also collaborated with a Japanese group to show that gonadotropin releasing hormone which opposes the signaling actions of LPA in ovarian cancer cells, produces a dramatic translocation of LPP3 to the plasma membrane of ovarian cancer cells resulting in increased metabolism of extracellular LPA and decreased ovarian cancer cell responsiveness to LPA. Gonadotropin releasing hormone inhibits ovarian cancer cell growth in some clinical settings to these results are exciting because they provide a potential mechanism for this effect. Aim one of the proposal is the focus of our remaining research efforts. Our plans and aims are essentially as described in the original submission with one exception which is detailed below. We have generated SK-OV-3 derived cell lines expressing the LPA-generating phospholipase autotaxin. Overexpression of autotaxin results in a modest increase in extracellular generation of LPA suggesting that this enzyme participates in the pathway of LPA production by these cells. Our most interesting set of observations relate to the discovery that several ovarian cancer cell lines and primary ovarian cancer cells release small membrane vesicles extracellularly. Our studies indicate that these vesicles are released by exocytosis of the contents of cytoplasmic multivesicular bodies and that their lipid component is a source of substrates for the extracellular production of LPA. These are potentially exciting observations because exosomes have been shown by others to be released by several types of primary neoplasic cells and cell lines. We have used sophisticated mass spectrometry approaches to determine the phospholipid composition of ovarian cancer cell-derived exosomes. The primary goal of our remaining work on this aim of the proposal which represents a minor departure from the aims stated in the original submission is to establish the role of exosomes in the extracellular production of LPA by ovarian cancer cells.