Tumor necrosis factor-α (TNF) induces apoptosis in confluent LLCPK₁ epithelial cells, but also activates NF-ϰB, a negative regulator of apoptosis. The presence of increased TNF-induced apoptosis causes a transient increase in epithelial permeability, but the epithelial barrier function recovers, as assessed by measuring the transepithelial electrical resistance, the paracellular flux of mannitol and by the electron microscopic evaluation of the penetration of the electron-dense dye ruthenium red across the tight junctions. The integrity of the epithelial cell layer is maintained by rearrangement of non-apoptotic cells in the monolayer and by the phagocytosis of apoptotic fragments. To study the role of NF-ϰB in an epithelium exposed to TNF, NF-ϰB was inhibited in LLC-PK₁ epithelial cells with either the dietary compound, curcumin, or by transfection with a dominant negative mutant inhibitor IϰBα. Replacement of serine 32 and 36 by alanine has been shown to prevent its phosphorylation and degradation, blocking NF-ϰB activation. Inhibition of NF-ϰB altered the morphology of TNF-induced apoptotic cells, which showed lack of fragmentation and membrane blebbings, and absence of phagocytosis by neighboring cells. TNF treatment of NF-ϰB-inhibited cells also caused altered distribution of the tight junction-associated protein ZO-1, increased epithelial leakiness, and impaired the recovery of the epithelial barrier function, which normally occurs 6 hours after TNF treatment of LLC-PK₁ cells. These data demonstrate that NF-ϰB activation is reqnired for the maintenance of the barrier function of an epithelium undergoing TNF-induced apoptosis.