The precise pathophysiological mechanisms causing BE is still unclear, but the combination of gastric acid and bile acids from the gastroduodenal reflux is commonly acknowledged as the key factor in the development of BE [26
]. At low pH, bile acids are thought to cause esophageal mucosal injury, which has been substantiated both in vitro
and in animal model systems [27
The NR1I family of orphan nuclear receptors are known to prevent toxic accumulations of xenobiotics within cells by regulating a broad range of cellular transporters [17
]. The nuclear receptor PXR is a member of this family and functions in the enterohepatic organs as a detoxifier and regulator of bile acid homeostasis [12
]. It can bind a variety of bile acids [32
] and subsequently regulate the expression of a multitude proteins that transport bile acids across cell membranes [34
]. These include the multidrug resistance (MDR)1 gene [37
], which encodes the efflux protein P-glycoprotein that removes xenobiotics from cells [39
]. Other bile acid transporters that are induced by PXR include the multidrug resistance associated protein (MRP)2 and 3 [40
] and the organic anion transporting polypeptide (OATP)1 and 2 [42
]. From studies in mice it was concluded that the function of PXR is of particular importance when bile acid concentrations reach pathophysiologic levels [33
PXR expression is known in healthy liver and intestinal tract, but in cancer it has yet to be explored. Therefore, in this study we investigated the expression and significance of PXR in esophageal pathology. We did not detect PXR in normal squamous epithelium or in the squamous epithelium of RE patients. PXR did however express at both mRNA and protein level in columnar epithelium, and was significantly lower in adjacent squamous esophageal epithelium of the same patient. In samples from adenocarcinoma patients PXR was clearly observed the nucleus. PXR mRNA levels between BE and EAC do not differ, but nuclear PXR protein expression does increase in EAC. Perhaps, this is an effect of difference posttranscriptional modifications between the stages. It could also indicate a translocation from the cytoplasm to the nucleus occuring during progression from BE to EAC, as our studies showed translocation from cytoplasm to the nuclei of adenocarcinoma cells in vitro after stimulation with rifampicine or litholic acid. These processes and their significance to PXR function need to be further explored, and a first step in this could be Western blot analysis on subcellular fractions of BE and EAC cells.
Previous studies have suggested that PXR expression in cancer cells can interfere with the metabolism and responsiveness to chemotherapeutics, such as irinotecon and tamoxifen [46
]. They suggest this drug resistance involves the metabolizing enzyme CYP3A4, one of the key target genes of PXR [15
]. These effects on the metabolism of anticancer agents are especially important considering that PXR ligands include endogenous steroids and bile acids, as well as numerous environmental chemicals and dietary constituents. It has yet to be investigated whether higher levels of PXR in the esophagus also affects responsiveness to chemotherapy.
Given the relatively low rare allele frequency for SNP 8055C/T, our population size may have been insufficient to detect a statistically significant association. Validation of our findings will require a well-characterized population from a multicenter study. Recent studies associate PXR polymorphisms with other pathogenic conditions of the gastrointestinal tract, such as inflammatory bowel disease [20
] and primary sclerosing cholangitis [21
]. Since associations with the two PXR SNPs in this study are in line with previous findings in IBD [20
], this draws attention on a possible link of the functional effect of these SNPs with chronic inflammation. It is well known that inflammation, through the activation of NF-κB pathway leads to a decrease of CAR, PXR and RXR-alpha expression and the expression of their target genes. In addition, it has recently been shown that the mutual repression between PXR and NF-κB signalling pathways provides a molecular mechanism linking xenobiotic metabolism and inflammation [48
Although it cannot be ruled out that the observed link between BE and PXR levels is not the cause but only the consequence of the metaplasia from squamous to intestinal-type mucosa, the link with PXR-activity associated SNPs suggest a active role of PXR in BE pathophysiology. Further research should focus on the biologic function of PXR in BE and EAC, especially because PXR protein expression was observed in only few nuclei in Barrett's epithelium whereas EAC tissue was abundant with PXR positive nuclei. Here, we chose LCA to study nuclear translocation as it is the endogenous ligand with the highest binding affinity for PXR. As supraphysiological levels of LCA were used to stimulate esophageal cells, further research will be required using extensive stimulation assays that mimick the in vivo
situation by long-term repetitive stimulations with a mix of bile acids in physiologic concentrations as recently performed [49
]. For a complex disease such as BE, development and validation of representative animal models will be of great value to investigate whether PXR plays a protective role in the development of BE or if it has a detrimental effect on neoplastic progression.