Regulation of FasL by NF-kappa B and AP-1 in Fas-dependent thymineless death of human colon carcinoma cells

Cell death due to thymine (dThd) deficiency, associated with the cytotoxic action of 5-fluorouracil in colon cancer, is regulated in thymidylate synth ase-deficient (TS-) human colon carcinoma cells via the Fas (CD95, APO-1) d eath receptor. This was demonstrated by inhibiting the loss in clonogenicit y of TS- cells by anti-Fast and in enhanced survival of TS- clones selected for resistance to Fas-mediated apoptosis, following dThd deprivation, Duri ng thymineless stress in TS- cells, Fas ligand (FasL) is expressed, and its promoter (hFasLPr) is activated. Transactivation of hFasLPr, dependent upo n dThd deficiency, was inhibited following mutation of the binding sites fo r NF-kappa B or AP-1 and by preventing NF-kappa B or AP-1 activation, which inhibited expression of Fast and enhanced clonogenic survival in stable tr ansformants expressing I kappa B alpha M Or DN-MEKK, respectively. These re sults demonstrate the crucial roles for NF-kappa B and AP-1 in the regulati on of Fast in Fas-mediated thymineless death of colon carcinoma cells.