Hypoxia Enhances Lysophosphatidic Acid Responsiveness in Ovarian Cancer Cells and Lysophosphatidic Acid Induces Ovarian Tumor Metastasis In vivo

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Abstract
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 1α (HIF1α). LPA-induced cell migration required cytosolic phospholipase A2 (cPLA2) and LPA stimulates cPLA2 phosphorylation in a HIF1α-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 HIFα 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. HIF1α plays a critical role in enhancing and/or sensitizing the role of LPA on cell migration and invasion under hypoxic conditions, where cPLA2 is required for LPA-induced cell migration. (Cancer Res 2006; 66(16): 7983-90)

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