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Maintaining the integrity of the gastrointestinal tract despite the continual presence of microbial flora and injurious agents is essential. Epithelial repair requires restitution and regeneration. During restitution, epithelial cells spread and migrate across the basement membrane to re-establish surface-cell continuity, a process that is independent of cell proliferation. Epithelial continuity depends on a family of small abundant secreted proteins, the trefoil factors (TFFs). The trefoil factor (TFF) family comprises the gastric peptides pS2/TFF1 and spasmolytic peptide (SP)/TFF2, and the intestinal trefoil factor (ITF)/TFF3. Their fundamental action is to promote epithelial-cell restitution within the gastrointestinal tract. TFFs are abundantly secreted onto the mucosal surface by mucus-secreting cells. Their expression is rapidly and coordinately upregulated at the margins of mucosal injury. Secreted TFF acts on adjacent mucosal cell populations either extracellularly (augmenting barrier function) or intracellularly (transcriptional and signaling events). TFF response elements in TFF gene promoters allow increases in TFF expression through auto-induction and cross-induction of other TFFs, in addition to mucin expression and possibly tumor suppression. Cell migration is the result of integrated disruption of cell-cell and cell-substratum adhesion and prevention of apoptosis through cell detachment. Epithelial movement therefore requires integration of motogenic and cell-survival signals. This is achieved by activation of several intracellular signaling pathways that converge on ERK/MAPK and possibly NF-B activation. Serine phosphorylation of the extracellular signal-regulated kinases (ERKs)/mitogen-activated protein kinases (MAPKs) 1 and 2 is central to trefoil factor-mediated signaling, lying downstream of EGFR activation and possibly FAK activation (through recruitment of GRB2 and SOS). Cell migration might result from cooperation between ERK/MAPKs and Rho proteins, FAK activation, beta-integrin clustering and beta-catenin activation. Abrogation of cell death has been shown to require both PI3K activation and ERK/MAPK activation; the former operates through serine/threonine phosphorylation of AKT/protein kinase B, serine phosphorylation of BAD (BCL-2 agonist of cell death) and inhibition of mitochondrial cytochrome c release and formation of the apoptosome (APAF1, caspase-9 (CASP9) and cytochrome c (CYT-c). Translocation of phosphorylated ERK/MAPK to the nucleus leads to amplification and de-restriction of TFF expression to ensure sustained action. (This definition may be outdated - see the DesignNote.)
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