An Inducible, Large-Intestine-Specific Transgenic Mouse Model for Colitis and Colitis-Induced Colon Cancer Research.

BACKGROUND: Animal models are an important tool to understand intestinal biology. Our laboratory previously generated C57BL/6-Tg(Car1-cre)5Flt transgenic mice (CAC) with large-intestine-specific Cre recombinase (Cre) expression as a model to study colon health. AIM: To expand the utility of the CAC mouse model by determining the impact of chemically induced colitis on CAC transgene expression. METHODS: CAC mice were crossed to Rosa reporter mice (Rosa26R (flox/flox) ) with a lox-STOP-lox signal controlling ß-galactosidase (ßgal) expression and then further crossed with Apc(CKO/CKO) mice in some experiments to delete Apc alleles (Apc (Δ580) ). Initially, 8-week-old CAC(Tg/WT);Rosa26R (flox/WT) ;Apc (Δ580/WT) mice were treated with dextran sulfate sodium (DSS) in drinking water (5 days, 0, 0.65, 1.35, or 2.0 %). Colon tissue damage and ßgal labeling were analyzed 10 day after stopping DSS. Next, 8-week-old CAC(Tg/WT);Rosa26R(flox/flox) mice were treated with 0 or 1.35 % DSS, and colonic ßgal labeling was assessed at 30 day post-DSS treatment. Finally, 10-week-old CAC(Tg/WT);Apc (Δ580/WT) mice were treated with DSS (0 or 2 %) for 5 days and colonic tumors were analyzed at 20 weeks. RESULTS: CAC(Tg/WT);Rosa26R (flox/WT) ;Apc (Δ580/WT) mice had a DSS dose-dependent increase in colon epithelial damage that correlated with increased epithelial ßgal labeling at 10 days (r (2) = 0.9, ß = 0.75). The ßgal labeling in CAC(Tg/WT);Rosa26R(flox/flox) mice colon remained high at 30 days, especially in the crypts of the healed ulcer. DSS also increased colon tumor incidence and multiplicity in CAC(Tg/WT);Apc (Δ580/WT) mice. CONCLUSIONS: DSS-mediated epithelial damage induces a persistent, Cre-mediated recombination of floxed alleles in CAC mice. This enables the examination of gene function in colon epithelium during experimental colitis and colitis-induced colon cancer.

Constitutively active RAS signaling reduces 1,25 dihydroxyvitamin D-mediated gene transcription in intestinal epithelial cells by reducing vitamin D receptor expression.

High vitamin D status is associated with reduced colon cancer risk but these studies ignore the diversity in the molecular etiology of colon cancer. RAS activating mutations are common in colon cancer and they activate pro-proliferative signaling pathways. We examined the impact of RAS activating mutations on 1,25 dihydroxyvitamin D (1,25(OH)2D)-mediated gene expression in cultured colon and intestinal cell lines. Transient transfection of Caco-2 cells with a constitutively active mutant K-RAS (G12 V) significantly reduced 1,25(OH)2D-induced activity of both a human 25-hydroxyvitamin D, 24 hydroxyase (CYP24A1) promoter-luciferase and an artificial 3X vitamin D response element (VDRE) promoter-luciferase reporter gene. Young Adult Mouse Colon (YAMC) and Rat Intestinal Epithelial (RIE) cell lines with stable expression of mutant H-RAS had suppressed 1,25(OH)2D-mediated induction of CYP24A1 mRNA. The RAS effects were associated with lower Vitamin D receptor (VDR) mRNA and protein levels in YAMC and RIE cells and they could be partially reversed by VDR overexpression. RAS-mediated suppression of VDR levels was not due to either reduced VDR mRNA stability or increased VDR gene methylation. However, chromatin accessibility to the VDR gene at the proximal promoter (-300bp), an enhancer region at -6kb, and an enhancer region located in exon 3 was significantly reduced in RAS transformed YAMC cells (YAMC-RAS). These data show that constitutively active RAS signaling suppresses 1,25(OH)2D-mediated gene transcription in colon epithelial cells by reducing VDR gene transcription but the mechanism for this suppression is not yet known. These data suggest that cancers with RAS-activating mutations may be less responsive to vitamin D mediated treatment or chemoprevention.

Vitamin D Receptor-Dependent Signaling Protects Mice From Dextran Sulfate Sodium-Induced Colitis.

Low vitamin D status potentiates experimental colitis, but the vitamin D-responsive cell in colitis has not been defined. We hypothesized that vitamin D has distinct roles in colonic epithelial cells and in nonepithelial cells during colitis. We tested this hypothesis by using mice with vitamin D receptor (VDR) deletion from colon epithelial cells (CEC-VDRKO) or nonintestinal epithelial cells (NEC-VDRKO). Eight-week-old mice were treated with 1.35% dextran sulfate sodium (DSS) for 5 days and then euthanized 2 or 10 days after removal of DSS. DSS induced body weight loss and increased disease activity index and spleen size. This response was increased in NEC-VDRKO mice but not CEC-VDRKO mice. DSS-induced colon epithelial damage and immune cell infiltration scores were increased in both mouse models. Although the epithelium healed between 2 and 10 days after DSS administration in control and CEC-VDRKO mice, epithelial damage remained high in NEC-VDRKO mice 10 days after removal of DSS, indicating delayed epithelial healing. Gene expression levels for the proinflammatory, M1 macrophage (Mɸ) cytokines tumor necrosis factor-α, nitric oxide synthase 2, and interleukin-1ß were significantly elevated in the colon of NEC-VDRKO mice at day 10. In vitro experiments in murine peritoneal Mɸs demonstrated that 1,25 dihydroxyvitamin D directly inhibited M1 polarization, facilitated M2 polarization, and regulated Mɸ phenotype switching toward the M2 and away from the M1 phenotype. Our data revealed unique protective roles for vitamin D signaling during colitis in the colon epithelium as well as nonepithelial cells in the colon microenvironment (i.e., modulation of Mɸ biology).