Okadaic acid stimulates expression of Fas receptor and Fas ligand by activation of nuclear factor kappa-B in human oral squamous carcinoma cells

In the present study, we used western blot and RT-PCR analysis to examine the expression of proteins and mRNAs of Fas receptor and Fas ligand in human oral squamous carcinoma SCC-25 cells treated with okadaic acid. Treatment with okadaic acid enhanced the expression of proteins and mRNAs of both Fas receptor and Fas ligand in SCC-25 cells. The amount of IκB-α in whole cell lysates decreased, while the level of NF-κB in nucleus increased, in the okadaic acid-treated cells. Okadaic acid-treatment also alters the cellular localization of NF-κB, from cytoplasm to nuclei. To investigate the activation of NF-κB in okadaic acid-treated SCC-25 cells, we performed electrophoretic mobility gel shift assay using nuclear extracts and the consensus oligonucleotide for NF-κB DNA binding site. The binding of nuclear proteins to the oligonucleotide of NF-κB increased when the cells had been treated with 20 nM okadaic acid for 4 h. We transfected the cells with pFLF1, which has the promoter region of Fas receptor gene containing NF-κB binding site. A luciferase reporter gene assay demonstrated that the activity in the cells transfected with pFLF1 and treated with 20 nM okadaic acid increased in a time-dependent manner and that the activity was more than three-fold over that in the control cells. Our results suggest that NF-κB activated at early stages in the okadaic acid-treated SCC-25 cells stimulated the promoter activity of Fas receptor in the cells leading to the apoptotic death of these cells.