We previously found that capsaicin induces tight-junction (TJ) opening accompanied with cofilin dephosphorylation/activation in intestinal Caco-2 cells. Here, we examined the role of cofilin in TJ regulation, and analyzed the structural events that lead to TJ opening. We transfected Caco-2 cells with wild-type cofilin [cofilin(wt)] or its constitutively active mutant cofilin(S3A). We found that the decreases in trans-epithelial electrical resistance (TER) was slower in cofilin(wt) transfectants and faster in cofilin(S3A) mutants than in vector controls. Moreover, cofilin clephosphorylation corresponded to the rate of TER decrease. Capsaicin treatment changed the localization of TJ proteins and altered the F-actin structure, but in a manner different from those depend oil myosin light chain kinase (MLCK). These results strongly support the importance of cofilin in TJ opening, suggesting cofilin as a target for TJ permeability regulation in epithelia] cells. (C) 2008 Elsevier Inc. All rights reserved.