New Tricks of Our Old Friend the Glucocorticoid Receptor

Ulcerative colitis is the most common form of inflammatory bowel disease (IBD) and a major risk factor for the development of colorectal cancer.¹ Synthetic glucocorticoids are essential drugs in combatting inflammatory diseases and mediate their anti-inflammatory actions by binding the glucocorticoid receptor (GR). Despite its clinical importance, prolonged glucocorticoid treatment is hampered by the emergence of glucocorticoid resistance and detrimental side effects.² Therefore, it is crucial to optimize glucocorticoid use in IBD, which requires an improved understanding of the glucocorticoid action in IBD.

Because GR is ubiquitously expressed, glucocorticoids can affect different cell types in the colon, including immune cells and intestinal epithelial cells (IECs). Using a dextran sulfate sodium (DSS)-induced colitis model, GR deletion in myeloid cells was shown to inhibit the resolution (but not the acute) phase of the disease, which was accompanied by increased infiltration of macrophages and proinflammatory cytokine expression in the colon.³ Yet, for ulcerative colitis the role of GR in IECs has remained largely unresolved.

Earlier work showed that GR deletion in IECs coincided with transient upregulation of proinflammatory chemokines and cytokines in basal conditions,⁴ but an inflammatory or tumorigenic context was not considered. In this issue of Cellular and Molecular Gastroenterology and Hepatology, the group of Reichardt took a more direct approach and used both a DSS-induced colitis model and a colitis-associated colorectal cancer model, in which the intestinal epithelial GR was inducibly deleted (GR^villin mice), to investigate GR action in IECs.⁵ These authors showed that DSS-induced colitis was clearly exacerbated in GR^villin mice compared with GR^flox mice that still express wild-type GR. More specifically, GR^villin mice displayed decreased body weight, worse clinical disease score, reduced colon length, and increased serum interleukin-6 levels compared with GR^flox mice. This is further supported by their findings that compared with GR^flox mice, GR^villin mice showed both aggravated DSS-induced tissue damage, and an even further compromised epithelial barrier integrity.

Diving deeper into the mechanistic basis of the aggravated colitis in GR^villin mice, Reichardt and colleagues further studied the gene expression profile of IEC isolated from colon.⁵ Several chemokines (Cxcl1, Cxcl5, Ccl5) that are involved in the pathogenesis of colitis and attract immune cells, such as neutrophils and monocytes, were only upregulated in IEC of DSS-treated GR^flox mice, suggesting that GR triggers chemokine induction in IEC during inflammation, a proinflammatory action that has been reported before.⁶ In contrast, genes important for the control of the epithelial barrier integrity (Tnfr2 and Mlck) or pathogen sensing (Tlr4) were overexpressed in IECs of GR-devoid GR^villin mice, but not in functional GR-expressing GR^flox mice, indicating GR is required for the repression of these genes. Together this suggests that endogenous glucocorticoids safeguard epithelial permeability and block pattern recognition receptor expression in IECs via binding to an IEC-expressed GR, hereby limiting proinflammatory nuclear factor-κB signaling.

Because chemokine expression was reduced in IEC of DSS-treated GR^villin mice, a logical next step was to investigate how this affected myeloid cell infiltration. In line with the gene expression results, a reduced number of neutrophils, total and inflammatory macrophages, were found in the lamina propria of DSS-treated GR^villin mice compared with GR^flox mice.⁵ This important finding indicates that GR expression in IEC influences recruitment of myeloid cells in the inflamed colon. To link this with the observed aggravated colitis in GR^villin mice, a proinflammatory gene expression analysis was performed in lamina propria cells. Remarkably, chemokine and cytokine expression levels were all strongly upregulated in lamina propria cells of GR^villin mice compared with GR^flox mice, indicative of hyperactivated lamina propria cells.⁵

Because DSS-induced colitis was exacerbated in GR^villin mice, the team of Reichardt further showed that this also influenced tumorigenesis by using the carcinogen azoxymethane.⁵ In a DSS/azoxymethane model, both the number and size of the developed tumors was strongly increased in GR^villin mice compared with GR^flox mice, although not linked to changes in leukocyte infiltration.⁵ In an elegant experiment where colitis and tumorigenesis were induced before GR was deleted in IEC, the severity of colitis and the number and size of the developed tumors showed to be comparable in both genotypes.⁵ This clearly confirmed that the aggravated colitis, and not the deleted GR in IECs, caused the enhanced tumorigenesis. The role of GR in tumorigenesis remains clinically relevant, not least because of the long-term glucocorticoid use to relieve chemotherapy-associated symptoms and perioperative to cancer surgery,⁷ but also because of the ongoing debate concerning the role of glucocorticoids in various solid cancers.^8,9

This study greatly advances the understanding of GR functioning in the colon and highlights that GR’s transcriptional activation and repression mechanisms¹⁰ are involved to control gene expression in IECs. Further questions remain to be resolved. For instance, how would the inducible expression of GR (dimerization) mutants steer these alterations against a GRwt null background? Does GR expression change over time in IECs of patient samples and does this GR remain functional? Notwithstanding these open questions, Reichardt and colleagues clarified that GR action not only keeps the inflammatory response in colitis under control, but may also directly impact the progression toward colorectal cancer. In extension, optimizing glucocorticoid treatment and delivery strategies in patients may even decrease the risk of developing colitis-associated colorectal cancer, highlighting once more the importance of glucocorticoids and maintaining a functional GR in the treatment of IBD.