We previously reported that mice deficient in two Se-dependent
glutathione peroxidases, GPx1 and GPx2, have spontaneous
ileocolitis. Disease severity depends on mouse genetic background. Whereas C57BL/6J (B6) GPx1/2-double-knockout (DKO) mice have moderate
ileitis and mild
colitis, 129S1Svlm/J (129) DKO mice have severe
ileocolitis. Because GPx's are
antioxidant enzymes, we hypothesized that elevated
reactive oxygen species trigger
inflammation in these DKO mice. To test whether
NADPH oxidase 1 (Nox1) contributes to
colitis, we generated B6 triple-KO (TKO) mice to study their phenotype. Because the Nox1 gene is X-linked, we analyzed the effects of Nox1 on male B6 TKO mice and female B6 DKO mice with the Nox1(+/-) (het-TKO) genotype. We found that the male TKO and female het-TKO mice are virtually disease-free when monitored from 8 through 50 days of age. Male TKO and female het-TKO mice have nearly no signs of disease (e.g.,
lethargy and perianal
alopecia) that are often exhibited in the DKO mice; further, the slower growth rate of DKO mice is almost completely eliminated in male TKO and female het-TKO mice. Male TKO and female het-TKO mice no longer have the shortened small intestine present in the DKO mice. Finally, the pathological characteristics of the DKO ileum, including the high level of crypt apoptosis (analyzed by apoptotic figures, TUNEL, and cleaved
caspase-3 immunohistochemical staining), high numbers of Ki-67-positive crypt epithelium cells, and elevated levels of monocytes expressing
myeloperoxidase, are all significantly decreased in male TKO mice. The attenuated ileal and colonic pathology is also evident in female het-DKO mice. Furthermore, the male DKO ileum has eightfold higher TNF
cytokine levels than TKO ileum. Nox1
mRNA is highly elevated in both B6 and 129 DKO ileum compared to wild-type mouse ileum. Taking these results together, we propose that
ileocolitis in the DKO mice is caused by Nox1, which is induced by TNF. The milder disease in female het-TKO intestine is probably due to random or imprinted X-chromosome inactivation, which produces mosaic Nox1 expression.