| Summary: | The lining of the gastrointestinal tract, a single layer of epithelial cells, forms a dynamic barrier that protects the organism from harmful substances present in the lumen. Maintenance of a healthy gastrointestinal tract requires a delicate balance of proliferation, migration, and apoptosis. In response to damage of the intestinal epithelium, cells immediately surrounding the wounded area rapidly migrate into the denuded area to close the wound and restore the integrity of the epithelium in a process called restitution. A number of growth factors found in the gastrointestinal tract accelerate restitution in in vitro models of intestinal injury. Several of these factors are currently being tested in clinical trials as therapeutic treatments for inflammatory bowel diseases, of which the most promising so far is epidermal growth factor (EGF). However, the molecular mechanisms by which these factors activate intestinal epithelial cell migration are poorly defined. Using an in vitro model of intestinal epithelial cell wound closure developed in our laboratory, we have previously shown that EGF treatment increases intestinal epithelial cell migration to accelerate wound closure. In this dissertation work I have explored the molecular mechanism by which EGF stimulates intestinal epithelial cell migration. The experimental data presented here demonstrate that the small GTPase Rac is required for EGF stimulated cell migration. Treatment of intestinal epithelial cells with EGF rapidly activates Rac and enhances lamellipodia formation at the wound margin. EGF receptor (EGFR) kinase activity is required for increased cell migration and Rac activation in response to EGF. Activation of Src family kinases and phosphoinositide 3-kinase by EGF are required for Rac activation, demonstrating a novel signaling pathway downstream of EGFR that contributes to intestinal epithelial cell migration. These data contribute to our knowledge of the complex molecular mechanism that control intestinal epithelial repair with implications for understanding the role of growth factor regulation of this small GTPase in intestinal cellular migration and differentiation programs in vivo.
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