Lysophosphatidic acid (LPA) is elevated in ascites of ovarian cancer patients and stimulates growth and other activities of ovarian cancer cells in vitro. Tissue hypoxia is a critical factor for tumor aggressiveness and metastasis in cancers. We tested whether the ascites of ovarian cancer is hypoxic and whether hypoxia influences the effects of LPA on ovarian cancer cells. We found that ovarian ascitic fluids were hypoxic in vivo. Enhanced cellular responsiveness to LPA, including migration and/or invasion of ovarian cancer cells, was observed under hypoxic conditions. This enhancement could be completely blocked by geldanamycin or a small interfering RNA targeting hypoxia-inducible factor 1A (HIF1A). LPA-induced cell migration required cytosolic phospholipase A 2 (cPLA 2 ) and LPA stimulates cPLA 2 phosphorylation in a HIF1A-dependent manner under hypoxia conditions. Furthermore, we show for the first time that exogenous LPA enhances tumor metastasis in an orthotopic ovarian cancer model and HIFA expression in tumors. 17-Dimethylaminoethylamino-17-demethoxygeldanamycin (an inhibitor of the heat shock protein 90) effectively blocked LPA-induced tumor metastasis in vivo. Together, our data indicate that hypoxic conditions are likely to be pathologically important for ovarian cancer development. HIF1A plays a critical role in enhancing and/or sensitizing the role of LPA on cell migration and invasion under hypoxic conditions, where cPLA 2 is required for LPAinduced cell migration. (Cancer Res 2006; 66(16): 7983-90)

We have reported previously that levels of lysophosphatidic acid (LPA) are elevated in the blood and ascites from patients with ovarian cancer. LPA stimulates proliferation of ovarian cancer cells and has been proposed as an autocrine growth factor. Here, we show that a novel autocrine loop of LPA promotes the migration of ovarian cancer cells, which is a critical step of tumor metastasis. We report that laminin, but not other extracellular matrix proteins, induces LPA production in ovarian cancer cells. A neutralizing antibody against beta1 integrin and a calcium-independent phospholipase A2-specific inhibitor, HELSS, block both LPA production and the haptotactic activity of laminin. Exogenously added LPA restores the migratory ability of HEY ovarian cancer cells to laminin. These data suggest that laminin-induced cell migration is mediated by LPA. We further show that a specific receptor for LPA, LPA3, is required for mediating the chemotactic activity of LPA. In addition, we show that cytosolic PLA2 is required for cell migration and its activation is phosphatidylinositol-3 kinase-dependent. These findings have revealed a new mechanism of crosstalk between a beta1 integrin receptor and a G protein-coupled receptor.

Calcium-independent phospholipase A 2 (iPLA 2 ) plays a pivotal role in phospholipid remodeling and many other biological processes, including inflammation and cancer development. iPLA 2 can be activated by caspase-3 via a proteolytic process in apoptotic cells. In this study we identify novel signaling and functional loops of iPLA 2 activation leading to migration of non-apoptotic human ovarian cancer cells. The extracellular matrix protein, laminin-10/11, but not collagen I, induces integrin-and caspase-3-dependent cleavage and activation of overexpressed and endogenous iPLA 2 . The truncated iPLA 2 (amino acids 514 -806) generates lysophosphatidic acid and arachidonic acid. Arachidonic acid is important for enhancing cell migration toward laminin-10/11. Lysophosphatidic acid activates Akt that in turn acts in a feedback loop to block the cleavage of poly-(ADP-ribose) polymerase and DNA fragmentation factor as well as prevent apoptosis. By using pharmacological inhibitors, blocking antibodies, and genetic approaches (such as point mutations, dominant negative forms of genes, and siRNAs against specific targets), we show that ␤ 1 , but not ␤ 4 , integrin is involved in iPLA 2 activation and cell migration to laminin-10/ 11. The role of caspase-3 in iPLA 2 activation and cell migration are supported by several lines of evidence. 1) Point mutation of Asp 513 (a cleavage site of caspase-3 in iPLA 2 ) to Ala blocks laminin-10/11-induced cleavage and activation of overexpressed iPLA 2 , whereas mutation of Asp 733 to Ala has no such effect, 2) treatment of inhibitors or a small interfering RNA against caspase-3 results in decreased cell migration toward laminin-10/11, and 3) selective caspase-3 inhibitor blocks cleavage of endogenous iPLA 2 induced by laminin-10/11. Importantly, small interfering RNA-mediated down-regulation of endogenous iPLA 2 expression in ovarian carcinoma HEY cells results in decreased migration toward laminin, suggesting that our findings are pathophysiologically important.

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Ovarian cancer is a highly metastatic disease. Lysophosphatidic acid (LPA) levels are elevated in ascites from ovarian cancer patients, but its potential role in ovarian cancer metastasis has just begun to be revealed. In this work, we show that LPA stimulates invasion of primary ovarian cancer cells, but not ovarian epithelial or borderline ovarian tumor cells, although these benign cells indeed respond to LPA in cell migration. We have found that LPA downregulates tissue inhibitor of metalloproteinases (TIMPs). TIMP2 and TIMP3 play functional role in LPA-induced invasion as negative regulators. G(i) protein, phosphatidylinositol-3 kinase (PI3K), p38 mitogen-activated protein kinase (MAPK), cytosolic phospholipase A(2) and urokinase type plasminogen activator (uPA) are required for LPA-induced cells invasion. TIMP3 may affect two independent downstream targets, vascular endothelial growth factor receptor and p38 MAPK. In vivo, LPA stimulates tumor metastasis in an orthotopic ovarian tumor model, which can be inhibited by a PI3K inhibitor, LY294002. In summary, LPA is likely a key component for promoting ovarian metastasis in vivo. LPA downregulates TIMP3, which may have targets other than metalloproteinases. Our in vivo metastasis mouse model is useful for studying the efficacy of therapeutic regimes of ovarian cancer.