The epithelial lining of the intestine serves as a barrier to lumenal bacteria and can be compromised by pathologic Fas-mediated epithelial apoptosis.
Phosphatidylinositol (PI)3-kinase signaling has been described to limit apoptosis in other systems. We hypothesized that PI3-kinase-dependent pathways regulate Fas-mediated apoptosis and barrier function in intestiynal epithelial cells (IEC). IEC lines (HT-29 and T84) were exposed to agonist anti-Fas antibody in the presence or absence of chemical inhibitors of
PI3-kinase (
LY294002 and
wortmannin). Apoptosis, barrier function, changes in short circuit current (DeltaI(sc)), and expression of adhesion molecules were assessed. Inhibition of
PI3-kinase strongly sensitized IEC to Fas-mediated apoptosis. Expression of constitutively active Akt, a principal downstream effector of the
PI3-kinase pathway, protected against Fas-mediated apoptosis to an extent that was comparable with expression of a genetic
caspase inhibitor, p35.
PI3-kinase inhibition sensitized to apoptosis by increasing and accelerating Fas-mediated
caspase activation. Inhibition of
PI3-kinase combined with cross-linking Fas was associated with increased permeability to molecules that were <400 Da but not those that were >3,000 Da. Inhibition of
PI3-kinase resulted in
chloride secretion that was augmented by cross-linking Fas. Confocal analyses revealed polymerization of actin and maintenance of
epithelial cell adhesion molecule-mediated interactions in monolayers exposed to anti-Fas antibody in the context of
PI3-kinase inhibition. PI3-kinase-dependent pathways, especially Akt, protect IEC against Fas-mediated apoptosis. Inhibition of
PI3-kinase in the context of Fas signaling results in increased
chloride secretion and barrier dysfunction. These findings suggest that agonists of
PI3-kinase such as
growth factors may have a dual effect on intestinal
inflammation by protecting epithelial cells against immune-mediated apoptosis and limiting
chloride secretory
diarrhea.