Thromboxane receptor α mediates tumor growth and angiogenesis via induction of vascular endothelial growth factor expression in human lung cancer cells

The role of thromboxane receptor α (TPα) in tumor growth and angiogenesis was investigated in a nude mice model and in cell culture. Stable human lung cancer A549 cells over-expressing TPα (A549-TPα) was generated and used to inoculate athymic nude mice. A549-TPα cells induced greater tumor growth and increased vascularization in tumors than in the control A549 cells. Increased angiogenesis was further verified by studying the induction of vascular endothelial growth factor (VEGF) in A549-TPα cells. I-BOP, an agonist of TP, stimulated the expression of VEGF in this cell line as well as in another human lung cancer H157 cells in a time and dose dependent manner. The expression of VEGF was determined at both the mRNA and protein levels. The signaling pathways that are involved in I-BOP-induced VEGF expression were further examined by the use of inhibitors. Inhibition of the extracellular signal-regulated kinase (ERK) activation blocked the induction almost completely indicating that ERK activation was an essential step in the induction. Each of the three upstream kinases, protein kinase A, EGFR kinase and Src kinase, contributed partially to the overall induction. However, PI 3-kinase and protein kinase C had minimal contribution. These results indicate that activation of the TPα induces the expression of VEGF through multiple signaling pathways.