Abstract 3087: CD24 regulates tumor cell behaviour in a c-Src dependent fashion

CD24 is a glycosyl phosphatidylinositol (GPI)-anchored membrane protein that is frequently over-expressed in a variety of human carcinomas and is correlated with poor prognosis. In cancer cell lines, changes of CD24 expression can alter several cellular properties like invasion, proliferation and adhesion in vitro and tumor growth in vivo. However, little is known about the mechanisms by which CD24 mediates these effects. Here we have analyzed functional consequences of CD24 knock-down or over-expression in human cancer cell lines of different tumor entities (ovarian, lung, glioma). We show that presence or absence of CD24 was crucial to regulate the amount and activity of c-Src in lipid rafts. CD24-mediated effects on tyrosine-phosphorylated c-Src had impact on regulation of target genes affecting tumor cell invasion, proliferation and apoptosis. Invasive potential of tumor cells was controlled by the expression of the tissue factor pathway inhibitor-2 (TFPI-2), a potent inhibitor of extracellular matrix degradation that can block tumor cell invasion and metastasis. Silencing of CD24 or c-Src enhanced TFPI-2 expression and diminished invasion of SKOV3ip and A549 cells into matrigelTM. Conversely, over-expression of CD24 reduced TFPI-2 expression and enhanced invasion. Furthermore, an inverse correlation between expression of CD24 and TFPI-2 was observed by immunohistochemical analysis of primary breast cancers (N = 1174). TFPI-2 expression was highest in CD24 negative samples and lowered with increasing CD24 expression. Patients with a CD24 low/TFPI-2 high phenotype showed significantly better survival compared to CD24 high/TFPI-2 low patients (p=0.001). Additionally, STAT3 expression and phosphorylation were reduced by CD24 silencing. Diminished STAT3 activity was confirmed by specific reporter assays. Src depletion abrogated levels of p-STAT3 (Y705). In line with that, expression of classical STAT3 target genes like MCL-1, Cyclin D1 and Survivin was reduced after CD24 knock-down. Suppressed cell proliferation and enhanced apoptosis were observed in CD24 knock-down cells, suggesting a pivotal role of regulated target genes. An antibody to CD24 was effective in reducing tumor growth of lung- and pancreatic cancer xenografts in mice. Antibody treatment affected the level of Src-activity in the tumor and altered expression of STAT3 target genes. Our results provide evidence that CD24 regulates TFPI-2 and STAT3 activity via the c-Src non receptor tyrosin kin