Roles of Src family kinase, Ras, and mTOR signaling in intestinal epithelial homeostasis and tumorigenesis.

The turnover of intestinal epithelial cells (IECs) is relatively rapid (3 to 5 days in mouse and human), and this short existence and other aspects of the homeostasis of IECs are tightly regulated by various signaling pathways including Wnt-β-catenin signaling. Dysregulation of IEC homeostasis likely contributes to the development of intestinal inflammation and intestinal cancer. The roles of receptor protein tyrosine kinases and their downstream signaling molecules such as Src family kinases, Ras, and mammalian target of rapamycin (mTOR) in homeostatic regulation of IEC turnover have recently been evaluated. These signaling pathways have been found to promote not only the proliferation of IECs but also the differentiation of progenitor cells into secretory cell types such as goblet cells. Of note, signaling by SFKs, Ras, and mTOR has been shown to oppose the Wnt-β-catenin signaling pathway and thereby to limit the number of Lgr5+ intestinal stem cells or of Paneth cells. Such cross talk of signaling pathways is important not only for proper regulation of IEC homeostasis but for the development of intestinal tumors and potentially for anticancer therapy.