The clarification of mechanisms that negatively regulate the invasive behavior of human glioma cells is of great importance in order to find new methods of treatment. In this study, we have focused on the negative regulation of lysophosphatidic acid (LPA)-induced migration in glioma cells. Using small interference RNA and dominant-negative gene strategies in addition to pharmacological tools, we found that isoproterenol (ISO) and sphingosine-1-phosphate (S1P) negatively but differently regulate the LPA-induced migration. ISO-induced suppression of the migration of glioma cells occurs via  2 -adrenergic receptor/cAMP/Epac/Rap1B/inhibition of Rac, whereas S1P has been shown to suppress the migration of the cells through S1P 2 receptor/Rho-mediated down-regulation of Rac1. The expression of tumor suppressor phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is required for the inhibitory ISO-induced and Rap1B-mediated actions on the migration, Rac1 activation, and Akt activation in response to LPA. Thus, the PTENmediated down-regulation of phosphatidylinositol 3-kinase activity may be involved in the regulation of Rap1B-dependent inhibition of Rac1 activity. These findings suggest that there are at least two distinct inhibitory pathways, which are mediated by the S1P 2 receptor and  2 -adrenergic receptor, to control the migratory, hence invasive, behavior of glioma cells.
INTRODUCTIONThe malignant astrocytomas, the anaplastic astrocytoma and glioblastoma multiforme, are the most common glial tumors, with an annual incidence of three to four per 100,000 of population (DeAngelis, 2001). Because radical surgical treatment of malignant glial tumors is impossible because of the highly expressed migratory and invasive features of glioma cells, further understanding of the complex regulation and signaling molecules involved in glioma invasion is still needed. Lysophosphatidic acid (LPA) has been shown to play a significant role in human tumorigenesis as a factor to increase the motility and invasiveness of different cell types (Imamura et al., 1993;Stam et al., 1998). In glioma cells as well, extracellularly added LPA induced the proliferation and migration of stimulated cells (Malchinkhuu et al., 2005). A major part of extracellular LPA is thought to be produced from lysophosphatidylcholine by autotaxin/lysophospholipase D (Tokumura et al., 2002;Umezu-Goto et al., 2002). In the CNS as well, LPA production seems to occur in a similar way . Recent evidence suggested that autotaxin is overexpressed in glioblastoma multiforme and contributes to the cell motility of glioblastomas (Kishi et al., 2006). For this reason, revealing new substances or mechanisms that could negatively regulate the LPA-induced invasive behavior is of great importance for devising new and effective treatment modalities to counteract tumor cells.Our previous studies (Malchinkhuu et al., 2005(Malchinkhuu et al., , 2008 and those from other laboratories (Lepley et al., 2005) suggest that sphingosine 1-phosphate (S1P) and its receptors are ...