The endothelin system in human glioblastoma

Endothelin-1 (ET-1) is a powerful mitogenic and/or anti apoptotic peptide produced by many cancer cells. To evaluate the potential role of the endothelin system in glioblastoma we first determined the cellular distribution of the mRNA and proteins of the components of the endothelin system, preproendothelin-l (PPET-1), endothelin-converting enzyme-1 (ECE-1), and ETA and ETB receptors in human glioblastoma tissue and glioblastoma cell lines. PPET-1, ECE-I, and ETA receptor were highly expressed in glioblastoma vessels and in some scattered glioblastoma areas whereas ETB receptor was mainly found in cancer cells. This suggests that glioblastoma vessels constitute an important source of ET-1 that acts on cancer cells via the ET receptor. Four human glioblastoma cell lines expressed mRNA for all of the components of the ET-1 pathway. Bosentan, a mixed ETA and ETB receptor antagonist, induced apoptosis in these cell lines in a dose-dependent manner. Apoptosis was potentiated by Fas Ligand (APO-1L, CD95L), a pro-apoptotic peptide, only in LNZ308 cells, corresponding to the known functional Fas expression in these cell lines. LNZ308 cells also expressed the long and short forms of the cellular FLICE/caspase-8 inhibitory protein (FLIP). Bosentan and a protein kinase C inhibitor down-regulated short FLIP in these cells. ET-1 induced transient phosphorylation of extracellular signal-regulated kinase but did not induce long-term thymidine incorporation in LNZ308 glioblastoma cells. These results suggest that, in glioblastoma cells, ET-I, mainly acting via the ETB receptor, is a survival/anti-apoptotic factor produced by tumor vasculature, but not a proliferation factor, involving protein kinase C and extracellular signal-regulated kinase pathways, and stabilization of the short form of FLIP.