Background: Epidemiological and cytological studies have found a potential protective function of Vitamin D3 in colitis-associated colorectal cancer (CAC). In this study, we aim to decipher this effect in an azoxymethane (AOM) and dextran sulfate sodium (DSS) induced CAC mouse model and explore the underlying mechanism primarily focusing on the β-catenin signaling pathway.
Methods: In vitro, colon cancer cell line SW480 were treated with 1,25(OH)2D3 at different concentrations (1, 10, 50 and 100nmol/L) for 24, 48 and 72h and then quantified for proliferation. Transcriptional activity and the protein level of β-catenin were determined in SW480 cells both nontreated (control) and treated with 100nmol/L 1,25(OH)2D3 for 48h. Same assay was applied to SW480 cells with FOXM1 and nVDR siRNA knockdowns separately. In animal experiment, C57BL/6 mice were divided into 6 groups. Group 1 was the naïve control given only saline. Group 2 to 6 were given AOM (12.5mg/kg) by intraperitoneal injection and 2.5% DSS by gastric lavage sequentially to establish CAC model. Group 2 was the model control; Group 3 to 5 received Vitamin D3 at 15, 30, 60IU/g/w before AOM/DSS treatment, while Group 6 (post-dose group) received 60IU/g/w VitD3 5 days after DSS intake. Mice were sacrificed after 14 weeks. Macroscopic and pathologic evaluations were carried out to assess the severity of CAC. The mRNA and protein levels of β-catenin were also detected.
Results: The proliferation of SW480 cells was significantly inhibited by 1,25(OH)2D3 both dose-dependently and time-dependently (p<0.05). 1,25(OH)2D3 significantly decreased the transcriptional activity of β-catenin in SW480 cells by promoting the nuclear export of β-catenin (p<0.05) without affecting the mRNA and protein levels of β-catenin, which could be recovered by knocking down either FOXM1 or nVDR. In mice, Vitamin D intervention groups (G4,5,6) showed a lower tumor number and smaller tumor load than model control (G2) (p<0.05). However, there was no significant difference between the high dose group (G5) and post-dose group (G6). In addition, Vitamin D intervention groups presented lower β-catenin mRNA level than model control (p<0.05). The amount of nuclear β-catenin decreased as the VitD3 dose increased. Finally, the protein expression level of β-catenin was much less in the high dose group (G5) than model control (p<0.05).
Conclusions: 1,25(OH)2D3 downregulates the transcriptional activity of β-catenin by expelling β-catenin out of nucleus and therefore severely dampens the proliferation of SW480 human colon cancer cells. In mice model, VitD3 treatment successfully inhibits the progression from AOM/DSS-induced colitis to colorectal cancer potentially by downregulating the activity of β-catenin signaling pathway.