Anti-metastatic prostacyclins inhibit the adhesion of colon carcinoma to endothelial cells by blocking E-selectin expression

Prostacyclins have long been shown to have anti-metastatic activity. One hypothesis is their modulation of cell adhesion molecule (CAM) expression by target organ endothelial cells. We have postulated that prostacyclin, its analogs, and mechanistic mimics decrease colon carcinoma adhesion to cytokine-stimulated endothelial cells by blocking endothelial expression of the adhesion molecule E-selectin, but not the vascular cell adhesion molecule-1 (VCAM-1). Cultured human microvascular endothelial cells (HDMEC) were pre-incubated with prostacyclin (PGI₂), dibutyrl-CAMP (dbcAMP), forskolin (FOR), and/or iso-methylbutylxanthine (IBMX) for 15 min, then co-incubated with the cytokine tumor necrosis factor (TNF) for 4 h. HDMEC surface expression of E-selectin and VCAM-1 was evaluated by flow cytometry and ELISA. Adherence of ⁵¹Cr-labeled colon carcinoma cells to HDMEC monolayers was then determined. In parallel assays, HDMECs were incubated with anti-E-selectin and anti-VCAM-1 monoclonal antibody (1:100) prior to the addition of tumor cells. Prostacyclins, its analogs, and mimics significantly reduced E-selectin expression by HDMEC, while the reduction of VCAM-1 expression was much less pronounced. Prostacyclins also significantly decreased colon carcinoma adherence to stimulated HDMECs. The inhibition of E-selectin expression, but not VCAM-1 expression, corresponded to the reduction of tumor cell adherence. Prostacyclin's effects on tumor adhesion were nullified by pre-incubation with E-selectin antibody. The inhibition of colon carcinoma adherence to cytokine-stimulated endothelial cells treated with prostacyclin, its analogs, and mimics appears to result from blocking endothelial E-selectin, but not VCAM-1, expression. These data support the hypothesis that prostacyclins may exert their anti-metastatic effect, in part, by inhibiting CAM-mediated adherence of colon carcinoma to endothelial cells in metastatic target organs.