Michael A. Peplowski

Serine proteases decrease intestinal epithelial ion permeability by activation of protein kinase Cζ

Epithelial permeability to ions and larger molecules in the gut is essential for fluid balance, and its dysregulation contributes to intestinal pathology. We investigated the effect of digestive serine proteases on epithelial paracellular permeability. Trypsin, chymotrypsin, and elastase elicited sustained increases in transepithelial resistance (R(TE)) in polarized monolayers of three intestinal epithelial cell lines. This effect was reflected by decreases in paracellular conductances of Na+ and Cl- and a concomitant decrease in permeability to 3,000 molecular weight dextran. The enzyme activities of the proteases were required, yet activators of known protease-activated receptors (PARs) did not reproduce the effect of these proteases on R(TE). PKCzeta isoform-specific inhibitor significantly reduced the trypsin-induced increase in R(TE) whereas PKCzeta activity was increased in cells treated with trypsin and chymotrypsin compared with control cells; this activity was reduced to control levels in the presence of PKCzeta-specific inhibitor. Ca2+ chelators and pharmacological inhibitors of cell signaling support the role for PKCzeta in the protease-induced effect. Finally, we showed that treatment with the serine proteases increased occludin immunostaining and zonula occludin-1 coimmunoprecipitation with occludin in the detergent-insoluble fraction of cell lysates, and these increases were ablated by pretreatment with PKCzeta-specific inhibitor. This finding indicates increased insertion of occludin into the cell junctional complex. These data demonstrate a role for serine proteases in the facilitation of epithelial barrier function through a mechanism that is independent of PARs and is mediated by activation of PKCzeta.

Proteinase-activated Receptor 2 (PAR2) Decreases Apoptosis in Colonic Epithelial Cells

Background: Inflammatory bowel disease management lacks therapies that heal the epithelial barrier. Results: PAR2 activation increases activities of MEK1/2 and PI3K in intestinal epithelial cells, which blocks apoptosis. Conclusion: Cytokine-induced apoptosis in colonic epithelial cells is inhibited by PAR2 signaling. Significance: PAR2 is important in maintaining intestinal epithelial homeostasis. Mucosal biopsies from inflamed colon of inflammatory bowel disease patients exhibit elevated epithelial apoptosis compared with those from healthy individuals, disrupting mucosal homeostasis and perpetuating disease. Therapies that decrease intestinal epithelial apoptosis may, therefore, ameliorate inflammatory bowel disease, but treatments that specifically target apoptotic pathways are lacking. Proteinase-activated receptor-2 (PAR2), a G protein-coupled receptor activated by trypsin-like serine proteinases, is expressed on intestinal epithelial cells and stimulates mitogenic pathways upon activation. We sought to determine whether PAR2 activation and signaling could rescue colonic epithelial (HT-29) cells from apoptosis induced by proapoptotic cytokines that are increased during inflammatory bowel disease. The PAR2 agonists 2-furoyl-LIGRLO (2f-LI), SLIGKV and trypsin all significantly reduced cleavage of caspase-3, -8, and -9, poly(ADP-ribose) polymerase, and the externalization of phosphatidylserine after treatment of cells with IFN-γ and TNF-α. Knockdown of PAR2 with siRNA eliminated the anti-apoptotic effect of 2f-LI and increased the sensitivity of HT-29 cells to cytokine-induced apoptosis. Concurrent inhibition of both MEK1/2 and PI3K was necessary to inhibit PAR2-induced survival. 2f-LI was found to increase phosphorylation and inactivation of pro-apoptotic BAD at Ser112 and Ser136 by MEK1/2 and PI3K-dependent signaling, respectively. PAR2 activation also increased the expression of anti-apoptotic MCL-1. Simultaneous knockdown of both BAD and MCL-1 had minimal effects on PAR2-induced survival, whereas single knockdown had no effect. We conclude that PAR2 activation reduces cytokine-induced epithelial apoptosis via concurrent stimulation of MEK1/2 and PI3K but little involvement of MCL-1 and BAD. Our findings represent a novel mechanism whereby serine proteinases facilitate epithelial cell survival and may be important in the context of colonic healing.

The Canadian MD/PhD training program needs reinstated support.

The Canadian Institutes of Health Research (CIHR) recently terminated its MD/PhD training program without clear alternative funding in place. This misguided decision must urgently be reversed, as it has the potential to diminish a unique pool of graduates at the forefront of translational research.

Interferon-γ suppresses intestinal epithelial aquaporin-1 expression via Janus kinase and STAT3 activation.

Inflammatory bowel diseases are associated with dysregulated electrolyte and water transport and resultant diarrhea. Aquaporins are transmembrane proteins that function as water channels in intestinal epithelial cells. We investigated the effect of the inflammatory cytokine, interferon-γ, which is a major player in inflammatory bowel diseases, on aquaporin-1 expression in a mouse colonic epithelial cell line, CMT93. CMT93 monolayers were exposed to 10 ng/mL interferon-γ and aquaporin-1 mRNA and protein expressions were measured by real-time PCR and western blot, respectively. In other experiments, CMT93 cells were pretreated with inhibitors or were transfected with siRNA to block the effects of Janus kinases, STATs 1 and 3, or interferon regulatory factor 2, prior to treatment with interferon-γ. Interferon-γ decreased aquaporin-1 expression in mouse intestinal epithelial cells in a manner that did not depend on the classical STAT1/JAK2/IRF-1 pathway, but rather, on an alternate Janus kinase (likely JAK1) as well as on STAT3. The pro-inflammatory cytokine, interferon-γ may contribute to diarrhea associated with intestinal inflammation in part through regulation of the epithelial aquaporin-1 water channel via a non-classical JAK/STAT receptor signalling pathway.

Tumor necrosis factor α decreases aquaporin 3 expression in intestinal epithelial cells through inhibition of constitutive transcription

Inflammatory diseases of the gut are associated with altered electrolyte and water transport, leading to the development of diarrhea. Epithelially expressed aquaporins (AQPs) are downregulated in inflammation, although the mechanisms involved are not known. We hypothesized that AQP3 expression in intestinal epithelial cells is altered in intestinal inflammation and that these changes are driven by tumor necrosis factor (TNF) α. Human colonic adenocarcinoma (HT‐29) cells were treated with TNFα to investigate signaling mechanisms in vitro. AQP3 expression was assessed by real‐time PCR and radiolabeled glycerol uptake, with select inhibitors and a luciferase reporter construct used to further elucidate intracellular signaling. AQP3 expression was downregulated in HT‐29 cells treated with TNFα. Luciferase reporter construct experiments revealed that TNFα downregulated constitutive transcriptional activity of the AQP3 promoter, and inhibition of MEK/ERK and nuclear factor κB (NF‐κB) signaling prevented the decrease in AQP3 mRNA expression. Constitutive AQP3 expression was suppressed by specificity protein (Sp) 3, and knockdown of this transcription factor bound to the AQP3 promoter was able to partially prevent the TNFα‐induced downregulation of AQP3. TNFα signals through MEK/ERK and NF‐κB to enhance the negative transcriptional control of AQP3 expression exerted by Sp3. Similar mechanisms regulate numerous ion channels, suggesting a common mechanism by which both ion and water transport are altered in inflammation.

Interferon gamma decreases intestinal epithelial aquaporin 3 expression through downregulation of constitutive transcription

Aquaporin (AQP) 3 expression is altered in inflammatory bowel diseases, although the exact mechanisms regulating AQP abundance are unclear. Although interferon gamma (IFNγ) is centrally involved in intestinal inflammation, the effect of this cytokine on AQP3 expression remains unknown. HT-29 human colonic epithelial cells were treated with IFNγ to assess AQP3 mRNA expression by real-time RT-PCR and functional protein expression through the uptake of radiolabelled glycerol. Transient knockdown of signal transducer and activator of transcription 1 (STAT1), STAT3, Sp1, and Sp3 were performed to determine the involvement of these transcription factors in the IFNγ-induced signalling cascade. AQP3 promoter regions involved in the response to IFNγ were assessed using a luciferase reporter system. Likewise, enteroids derived from human colonic biopsies were also treated with IFNγ to assess for changes in AQP3 mRNA expression. IFNγ decreased AQP3 mRNA expression in HT-29 cells in a time- and concentration-dependent manner and reduced functional AQP3 protein expression (decreased 3H-labelled glycerol uptake). IFNγ also reduced AQP3 expression in enteroids derived from human colonic biopsies. Knockdown of STAT1 partially prevented the IFNγ-induced downregulation of AQP3 expression, whereas STAT3 and Sp3 knockdowns resulted in increased baseline expression of AQP3 but did not alter IFNγ-induced downregulation. Constitutive transcription of AQP3 is downregulated by IFNγ as demonstrated using the luciferase reporter system, with Sp3 bound to the AQP3 promoter as shown by chromatin immunoprecipitation. AQP3 constitutive transcription in intestinal epithelial cells is downregulated by IFNγ. This response requires STAT1 that is postulated to drive the downregulation of AQP3 expression through increased acetylation or decreased deacetylation the AQP3 promoter, ultimately resulting in decreased constitutive transcription of AQP3.Key messages• IFNγ suppresses the expression of AQP3 in intestinal epithelial cells.• Proximal AQP3 promoter elements are sufficient to drive constitutive expression and mediate the IFNγ-induced downregulation of AQP3 mRNA expression.• IFNγ-induced suppression of AQP3 is dependent upon STAT1 expression, but not STAT3, Sp1, or Sp3.

W1665 Aquaporin 3 Expression Is Downregulated in HT29 Colonic Epithelial Cells Exposed to Tumor Necrosis Factor α and Interferon γ

of the Na+/K+ ATPase was unaltered. CONCLUSION: These studies demonstrate a novel role for the FXR in downregulating colonic epithelial secretory capacity. This antisecretory effect of FXR activation appears to be distinct from that elicited by DCA and is mediated by inhibition of Na+/K+ ATPase activity. By virtue of their ability to downregulate epithelial secretory function, our data suggest that FXR agonists may have therapeutic potential as anti-diarrhoeal agents.