Exposure of mice to total body irradiation induces nuclear factor kappaB
(NFkappaB) activation in a tissue-specific manner. In addition to the spleen,
lymph nodes, and bone marrow, the tissues that exhibit NFkappaB activation now
include the newly identified site of the intestinal epithelial cells. NFkappaB
activated by total body irradiation mainly consists of NFkappaB p50/RelA
heterodimers, and genetically targeted disruption of the NFkappaB p50 gene in
mice significantly decreased the activation. By comparing tissue damage and
lethality in wild-type and NFkappaB p50 knockout (p50-/-) mice after they were
exposed to increasing doses of total body irradiation, we additionally examined
the role of NFkappaB activation in total body irradiation-induced tissue damage.
The results show that p50-/- mice are more sensitive to total body
irradiation-induced lethality than wild-type mice (LD50/Day 7: wild-type = 13.12
Gy versus p50-/- = 7.75 Gy and LD50/Day 30: wild-type = 9.31 Gy versus p50-/- =
7.81 Gy). The increased radiosensitivity of p50-/- mice was associated with an
elevated level of apoptosis in intestinal epithelial cells and decreased survival
of the small intestinal crypts compared with wild-type mice (P < 0.01). In
addition, RelA/TNFR1-deficient (RelA/TNFR1-/-) mice also exhibited a significant
increase in intestinal epithelial cell apoptosis after they were exposed to total
body irradiation as compared with TNFR1-deficient (TNFR1-/-) mice (P < 0.01). In
contrast, no significant increase in total body irradiation-induced apoptosis or
tissue injury was observed in bone marrow cells, spleen lymphocytes, and the
liver, heart, lung, and kidney of p50-/- mice in comparison with wild-type mice.
These findings indicate that activation of NFkappaB selectively protects the
small intestine against ionizing radiation-induced damage.