Vitamin D receptor is a novel transcriptional regulator for Axin1.
Axin1 is a scaffold protein in the β-catenin destruction complex, which, if disrupted, contributes to pathogenesis of various human diseases, including colorectal carcinogenesis and inflammatory bowel diseases (IBD). We have previously demonstrated that Salmonella infection promotes the degradation and plasma sequestration of Axin1, leading to bacterial invasiveness and inflammatory responses. Vitamin D and the vitamin D receptor (VDR) appear to be important regulators of IBD and colon cancer. Although VDR and Axin1 are all involved in intestinal inflammation, it remains unclear whether these processes are related or function independently. In the current study, we hypothesize that VDR is an important regulator for the maintenance of physiological level of Axin1.



Using the intestinal epithelial conditional VDR knockout mouse model (VDRΔIEC) and cultured cell lines, influences of VDR status on the expression of Axin1 was evaluated by Western blots and real-time PCR. Loss- and gain-of-function assays were used to investigate the regulation of VDR on Axin1 at the transcriptional and translational levels. Cells were treated with cycloheximide or actinomycin for molecular mechanistic studies. Candidate genomic VDR binding sites for Axin1 were tested by chromatin immunoprecipitation (ChIP) assay. Physical interactions among VDR, Axin1, and β-catenin were tested by immunoprecipitation. Cellular localization of Axin1 with different VDR status was determined by fractionation and immunohistochemistry.



We found that VDR deletion led to lower protein and mRNA levels of Axin1, whereas knockdown of Axin1 did not change the expression level of VDR protein. Immunoprecipitation data did not support physical interaction between VDR and Axin1. The VDR regulation of Axin1 was through a VDR genomic binding site for Axin1 gene on the regulatory region. Fractionation data showed that cytosolic Axin1 was significantly reduced due to VDR deletion, leaving the nuclear fraction unchanged. In ileum, Axin1 was distributed in the cytosol of apical epithelium and crypts.



VDR is important for the maintenance of physiological level of Axin1. The discovery of Axin1 as a VDR target gene provides novel and fundamental insights into the interactions between the VDR and β-catenin signaling pathways.
Department of Biochemistry, Rush University, Chicago, IL, 60612, USA.
Dapeng
D
Jin
Division of Gastroenterology and Hepatology, Department of Medicine, University of Illinois at Chicago, Chicago, IL, 60612, USA.
Yong-Guo
YG
Zhang
Department of Biochemistry, Rush University, Chicago, IL, 60612, USA.
Shaoping
S
Wu
Division of Gastroenterology and Hepatology, Department of Medicine, University of Illinois at Chicago, Chicago, IL, 60612, USA.
Rong
R
Lu
Division of Gastroenterology and Hepatology, Department of Medicine, University of Illinois at Chicago, Chicago, IL, 60612, USA.
Zhijie
Z
Lin
Department of Biochemistry, Rush University, Chicago, IL, 60612, USA.
Yuanyuan
Y
Zheng
Department of Biochemistry, Rush University, Chicago, IL, 60612, USA.
Honglei
H
Chen
Department of Biochemistry, Rush University, Chicago, IL, 60612, USA.
Gabriella
G
Cs-Szabo
Department of Biochemistry, Rush University, Chicago, IL, 60612, USA; Division of Gastroenterology and Hepatology, Department of Medicine, University of Illinois at Chicago, Chicago, IL, 60612, USA. Electronic address: junsun7@uic.edu.
Jun
J
Sun
D000818:Animals; D060466:Axin Protein / Q000378:metabolism*; D001665:Binding Sites; D002467:Cell Nucleus / Q000378:metabolism; D003110:Colonic Neoplasms / Q000378:metabolism; D003513:Cycloheximide / Q000737:chemistry; D003600:Cytosol / Q000378:metabolism; D003609:Dactinomycin / Q000737:chemistry; D004848:Epithelium / Q000378:metabolism; D005347:Fibroblasts / Q000378:metabolism; D045325:HCT116 Cells; D019073:HT29 Cells; D006801:Humans; D007249:Inflammation; D007413:Intestinal Mucosa / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D011401:Promoter Regions, Genetic; D018167:Receptors, Calcitriol / Q000378:metabolism*; D015398:Signal Transduction; D051176:beta Catenin / Q000378:metabolism
C555451:AXIN1 protein, human; D060466:Axin Protein; C555452:Axin1 protein, mouse; D018167:Receptors, Calcitriol; D051176:beta Catenin; D003609:Dactinomycin; D003513:Cycloheximide
Axin1; Beta-catenin; IBD; Inflammation; Intestine; Transcriptional factor; VDR; VDR genomic binding site; Vitamin D; Wnt