Evidence that nuclear factor-kappa B activation is critical in oxidant-induced disruption of the microtubule cytoskeleton and barrier integrity and that its inactivation is essential in epidermal growth factor-mediated protection of the monolayers of intestinal epithelia.

Abstract

Using monolayers of intestinal (Caco-2) cells, we showed that oxidants disrupt the microtubule cytoskeleton and barrier integrity; epidermal growth factor (EGF) was protective via stabilization of the microtubules. Because proinflammatory conditions activate nuclear factor-kappaB (NF-kappaB), we hypothesized that oxidants disrupt barrier integrity through activation of NF-kappaB and that EGF protects by suppressing NF-kappaB. Parental cells were pretreated with EGF or NF-kappaB or inhibitory kappaBalpha (I-kappaBalpha) modulators. Other cells were stably transfected with varying levels of a dominant negative mutant for the NF-kappaB inhibitor I-kappaBalpha. Both types of cells were grown as monolayers and then exposed to oxidant (H2O2). We then monitored monolayer barrier integrity (permeability), stability of the microtubule cytoskeleton (confocal microscopy, immunoblotting), intracellular levels of the I-kappaBalpha (immunoblotting), translocation, and activity of NF-kappaB (immunoblotting, sensitive enzyme-linked immunosorbent assay). Monolayers were also fractionated and processed to assess alterations in 1) polymerized tubulin (S2; an index of cytoskeletal integrity) and 2) monomeric tubulin (S1; an index of disassembly) (polyacrylamide gel electrophoresis fractionation and immunoblotting). We found the following: 1) Oxidants caused I-kappaBalpha degradation, NF-kappaB translocation, NF-kappaB (p50 and p65 subunits) activation, tubulin disassembly ( upward arrow S1, downward arrow S2), microtubule architectural instability, and barrier disruption. I-kappaBalpha stabilizers and NF-kappaB inhibitors [e.g., carbobenzyloxy-leuleu-leucinol (MG-132), lactacystin] suppressed oxidants injurious effects. 2) EGF (10 ng/ml) stabilized I-kappaBalpha and prevented both NF-kappaB translocation and activation while protecting monolayers against oxidants. 3) In stably transfected cells, transfection-induced stabilization of I-kappaBalpha by itself led to EGF-like protective effects. In these mutant cells, protection was not potentiated by EGF (10 ng/ml). Conclusions are 1) oxidants induce disruption of the cytoskeleton and intestinal barrier integrity, in part, through I-kappaBalpha degradation and subsequent NF-kappaB activation, 2) I-kappaBalpha stabilization is by itself protective, mimicking EGF, and 3) EGF protects cell monolayers through I-kappaBalpha stabilization and NF-kappaB inactivation. To our knowledge, this is the first report that NF-kappaB can affect the dynamics of cytoskeletal assembly and intestinal barrier integrity.