Regulation of FasL by NF-κB 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 synthase-deficient (TS-) human colon carcinoma cells via the Fas (CD95, APO-1) death receptor. This was demonstrated by inhibiting the loss in clonogenicity of TS- cells by anti-Fast and in enhanced survival of TS- clones selected for resistance to Fas-mediated apoptosis, following dThd deprivation, During thymineless stress in TS- cells, Fas ligand (FasL) is expressed, and its promoter (hFasLPr) is activated. Transactivation of hFasLPr, dependent upon dThd deficiency, was inhibited following mutation of the binding sites for 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 transformants expressing I kappa B alpha M Or DN-MEKK, respectively. These results demonstrate the crucial roles for NF-kappa B and AP-1 in the regulation of Fast in Fas-mediated thymineless death of colon carcinoma cells.