Abstract
Inflammatory bowel disease (IBD) are chronic inflammatory diseases including Crohn’s disease (CD) and ulcerative colitis (UC). More than 200 genomic regions have been identified and validated (association values〈 5x10-8) to be associated with CD, UC or IBD. These regions may contain multiple genes and the current challenge lies in identifying the causal gene in each of these. To address this problem, we performed a functional genomic screen of 145 genes from validated IBD loci, in a relevant intestinal epithelial cell model (HT-29). The results of this transcriptome-based screening revealed that the candidate IBD gene DUSP16 (a dual specificity phosphatase targeting MAP kinases (MAPKs) phosphorylation) as well as the known IBD gene KSR1 (a scaffold protein regulating the spatiotemporal activation of the ERK) regulate the expression of genes involved in intestinal differentiation and homeostasis. They induce, among others, the expression of the PIGR gene that encodes the polymeric immunoglobulin receptor. PIGR plays a role in transporting dimeric IgA molecules from the basolateral membrane of epithelial cells to the intestinal lumen, via transcytosis, where they play an essential role in protecting the epithelium against intestinal pathogens. Our hypothesis is that DUSP16 and KSR1 modulate the activity of MAPKs in intestinal epithelial cells to induce PIGR expression, thus participating in the maintenance of homeostasis of the intestinal barrier.
To better understand how DUSP16 modulates the expression of PIGR, we used an approach of over- expression (cDNA) and knockdown (shRNA) of DUSP16 in HT-29 cells. Our results confirmed that DUSP16 induction increases the expression of PIGR, whereas a knockdown of DUSP16 reduces the basal level of PIGR. Next we confirmed by Western Blot that the induction of DUSP16 was accompanied by a decrease in MAPK phosphorylation. The involvement of MAPKs was also confirmed through the use of chemical inhibitors specific for each MAPK, with inhibition of ERK and p38 showing the strongest induction of PIGR expression. We are currently analyzing known functional mutants of DUSP16 and KSR1 to determine their impact on MAPK activity and on PIGR expression. This work supports a role for PIGR in disease pathogenesis, adding to two recent studies that documented that patients suffering from UC accumulated somatic mutations in a group of genes regulating the expression of PIGR by Interleukin 17. The mutated genes, including PIGR, were positively selected in inflamed tissues, indicating the importance of the biological function occupied by this gene in the maintenance of homeostasis.
In conclusion, our study successfully identified functional links between two genes from independent IBD loci, and suggests that these DUSP16 and KSR1 play a role in the process of epithelial transcytosis and the development of IBD.
MAP kinases contribute to IL-8 secretion by intestinal epithelial cells via a posttranscriptional mechanism
