Endothelin-receptor antagonists are proapoptotic and antiproliferative in human colon cancer cells

Endothelin (ET)-I can act as an autocrine/paracrine growth factor or an antiapoptotic factor in human cancers. To study the role of ET-I in human colon cancer, proliferation and apoptosis of colon carcinoma cells was investigated using human HT-29 and SW480 colon carcinoma cells, ET-I was secreted by these cells. Treatment of cells with bosentan, a dual ETA/B-receptor antagonist, decreased cell number. Inhibition of DNA synthesis by bosentan was observed only in the presence of serum. Exogenously added ET-I did not increase DNA synthesis in serum-deprived cells. SW480 cells were sensitive and HT-29 cells were resistant to FasLinduced apoptosis. Bosentan sensitised resistant HT-29 cells to FasLinduced, caspase-mediated apoptosis, but not to TNF-alpha-induced apoptosis. Bosentan and/or FasLigand (FasL) did not modulate the expression of caspase-8 or FLIP. Bosentan sensitisation to apoptosis was reversed by low concentrations ( 10(-13) 10(-10) m), but not by high concentrations (10(-9) 10(-7) M) of ET-I. These results suggest that the binding of ET-I to high-affinity sites inhibits FasL-induced apoptosis, while the binding of either ET-I or receptor antagonists to low-affinity sites promotes FasL-induced apoptosis. In conclusion, endothelin signalling pathways do not induce human colon cancer cell proliferation, but are survival signals controling resistance to apoptosis. (C) 2003 Cancer Research UK.