Regulation of Metastasis and Angiogenesis by Autotaxin and its Products

Stracke, Mary

Abstract

Tumor cell migration, invasion, and angiogenesis are important determinants of tumor aggressiveness, and these traits have been associated with the motility-stimulating protein autotaxin (ATX). This protein is a member of the ectonucleotide pyrophosphatase and phosphodiesterase family of enzymes, but unlike other members of this family, ATX also possesses lysophospholipase D activity. This enzymatic activity hydrolyzes lysophosphatidylcholine to lysophosphatidic acid (LPA) and sphingosylphosphorylcholine to sphingosine-1-phosphate (S1P). LPA and S1P are bioactive phospholipids associated with cell growth, motility, and survival, as well as with blood vessel formation and maturation. In a recent study, we have shown a link between ATX expression, LPA, and vascular endothelial growth factor (VEGF) signaling in ovarian cancer cell lines. Exogenous addition of VEGF-A to cultured cells induces ATX expression and secretion, resulting in increased extracellular LPA production. This elevated LPA, acting through LPA4, modulates VEGF responsiveness by inducing VEGF-receptor (VEGFR)-2 expression. Down-regulation of ATX secretion in SKOV3 cells using antisense morpholino oligomers significantly attenuates cell motility responses to VEGF, LPA, ATX, and lysophosphatidylcholine. These effects are accompanied by decreased LPA4 and VEGFR2 expression as well as by increased release of soluble VEGFR1. Because LPA was previously shown to increase VEGF expression in ovarian cancer, our data suggest a positive feedback loop involving VEGF, ATX, and its product LPA that could affect tumor progression in ovarian cancer cells. Since cell motility is a critical component of metastasis formation, we have also focused on the mechanisms by which ATX and its products regulate this process. In a recent study, utilizing a Transwell system to isolate the pseudopodia at the migrating front of fibroblasts, we confirmed previous reports that LPA stimulates vigorous formation of pseudopodia that are rich in RhoA. Removal of LPA results in a slow retraction characterized by gradual loss of vinculin-rich adhesion processes. Although RhoA is generally activated during this process, Rho, mDia, and ROCK are not required. In contrast, S1P added to the lower well of the chamber results in a rapid retraction characterized by a quick spike of RhoA and coalescence of adhesion processes. This S1P-induced retraction requires both RhoA and ROCK, but is only delayed by inhibition of mDIA. These data indicate an important role for pseudopodia in sensing and integrating signals initiated by bioactive lipids that can affect both cellular polarity and pseudopodial function in motility.