The significance of intestinal apoptosis

Apoptosis occurs at a low level spontaneously in the small intestine (SI). The levels can be raised by a variety of cytotoxic agents including radiation. The apoptosis induced by radiation, and some drugs and the spontaneous apoptosis, show some specificity for the stem cells in the small intestinal crypt. In the colon, these agents target transit cells in the mid crypt. p53 expression is elevated at the same time as apoptosis in the SI but not in the cells undergoing apoptosis. The expression of bcl-2, a survival gene, is largely absent in the SI, but is expressed, albeit weakly, in the stem cells in the colon. Spontaneous apoptosis is observed in p53 null mice which also develop normally suggesting that spontaneous and developmental apoptosis are p53 independent and that spontaneous apoptosis is part of the homeostatic mechanisms maintaining stem cell numbers. Radiation induced apoptosis is completely absent at these early times post-irradiation in p53 nulls. In bel-2 null mice, the levels of spontaneous and radiation induced apoptosis are elevated in the colon. Bax, a death gene, is expressed on the villus and inter-crypt table in the colon suggesting that cells at the end of their lifespan initiate apoptosis. It has been suggested that apoptosis in the SI is a protective mechanism against carcinogenesis in the stem cells of the SI which rarely develops cancer. Cells that possess genetic damage detected. In the large bowel, this mechanism is not effective due to the action of bcl-2. Thus stem cells may persist in this tissue with genetic damage resulting in a higher cancer risk. Furthermore, the lack of spontaneous apoptosis in the colon may result in a gradual increase of the stem cells with time resulting in more ells at risk. (author)