FAS-FAS ligand interaction has been implicated in increased enterocyte apoptosis seen in immune-mediated bowel injury. However, scant information exists on the role of FAS in physiological enterocyte turnover. In the present study, the regulation of enterocyte FAS and FAS ligand expression by cytokines and its functional role in human intestinal epithelial cell apoptosis and proliferation were analyzed with two different models: a nontransformed human intestinal epithelial cell line (HIEC) and normal colonic explant cultures. HIEC constitutively expressed FAS, as analyzed by flow cytometry. However, stimulation with agonistic anti-FAS antibody (1-500 ng/ml) did not induce HIEC apoptosis. In contrast, in the presence of tumor necrosis factor ␣ (TNF␣) and/or interferon ␥ (IFN␥), HIEC became highly susceptible to FAS-induced apoptosis. The sensitizing effect to FAS-induced apoptosis was mediated via TNF␣-and IFN␥-induced upregulation of FAS expression (maximally 348%). Receptor studies showed that the effect of TNF␣ on FAS was mediated via the p55 TNF receptor. In colonic organ cultures, IFN␥ and TNF␣ also enhanced colonocyte FAS expression, resulting in a markedly increased apoptotic response to stimulation of this receptor, as shown by in situ terminal deosyuridine triphosphate nick-end staining. Neither FAS ligand expression nor its induction by cytokines was observed in HIEC or colonic explants. Proliferation studies showed that FAS is not implicated in regulating HIEC growth. These findings suggest that, despite the fact that normal human enterocytes express FAS, costimulatory factors, such as TNF␣ or IFN␥, abundantly secreted under inflammatory conditions, are necessary to sensitize intestinal epithelial cells to FAS-induced apoptosis by upregulating this receptor.