Mitochondrial function and gastrointestinal diseases

Mitochondria are dynamic organelles that function in cellular energy metabolism, intracellular and extracellular signalling, cellular fate and stress responses. Mitochondria of the intestinal epithelium, the cellular interface between self and enteric microbiota, have emerged as crucial in intestinal health. Mitochondrial dysfunction occurs in gastrointestinal diseases, including inflammatory bowel diseases and colorectal cancer. In this Review, we provide an overview of the current understanding of intestinal epithelial cell mitochondrial metabolism, function and signalling to affect tissue homeostasis, including gut microbiota composition. We also discuss mitochondrial-targeted therapeutics for inflammatory bowel diseases and colorectal cancer and the evolving concept of mitochondrial impairment as a consequence versus initiator of the disease.

Epithelial heme oxygenase-1 enhances colonic tumorigenesis by inhibiting ferroptosis

Colorectal cancer has been linked to chronic colitis and red meat consumption, which can increase colonic iron and heme. Heme oxygenase-1 ( Hmox1 ) metabolizes heme and releases ferrous iron, but its role in colonic tumorigenesis is not well-described. Recent studies suggest that ferroptosis, the iron-dependent form of cell death, protects against colonic tumorigenesis. Ferroptosis culminates in excessive lipid peroxidation that is constrained by the antioxidative glutathione pathway. We observed increased mucosal markers of ferroptosis and glutathione metabolism in the setting of murine and human colitis, as well as murine colonic neoplasia. We obtained similar results in murine and human colonic epithelial organoids exposed to heme and the ferroptosis activator erastin, especially induction of Hmox1 . RNA sequencing of colonic organoids from mice with deletion of intestinal epithelial Hmox1 (Hmox1 ΔIEC ) revealed increased ferroptosis and activated glutathione metabolism after heme exposure. In a colitis-associated cancer model we observed significantly fewer and smaller tumors in Hmox1 ΔIEC mice compared to littermate controls. Transcriptional profiling of Hmox1 ΔIEC tumors and tumor organoids revealed increased ferroptosis and oxidative stress markers in tumor epithelial cells. In total, our findings reveal ferroptosis as an important colitis-associated cancer signature pathway, and Hmox1 as a key regulator in the tumor microenvironment.

Interplay of gut microbiota and host epithelial mitochondrial dysfunction is necessary for the development of spontaneous intestinal inflammation in mice

BACKGROUND

Intestinal epithelial cell (IEC) mitochondrial dysfunction involvement in inflammatory bowel diseases (IBD), including Crohn's disease affecting the small intestine, is emerging in recent studies. As the interface between the self and the gut microbiota, IECs serve as hubs of bidirectional cross-talk between host and luminal microbiota. However, the role of mitochondrial-microbiota interaction in the ileum is largely unexplored. Prohibitin 1 (PHB1), a chaperone protein of the inner mitochondrial membrane required for optimal electron transport chain function, is decreased during IBD. We previously demonstrated that mice deficient in PHB1 specifically in IECs (Phb1i∆IEC) exhibited mitochondrial impairment, Paneth cell defects, gut microbiota dysbiosis, and spontaneous inflammation in the ileum (ileitis). Mice deficient in PHB1 in Paneth cells (epithelial secretory cells of the small intestine; Phb1∆PC) also exhibited mitochondrial impairment, Paneth cell defects, and spontaneous ileitis. Here, we determined whether this phenotype is driven by Phb1 deficiency-associated ileal microbiota alterations or direct effects of loss of PHB1 in host IECs.

RESULTS

Depletion of gut microbiota by broad-spectrum antibiotic treatment in Phb1∆PC or Phb1i∆IEC mice revealed a necessary role of microbiota to cause ileitis. Using germ-free mice colonized with ileal microbiota from Phb1-deficient mice, we show that this microbiota could not independently induce ileitis without host mitochondrial dysfunction. The luminal microbiota phenotype of Phb1i∆IEC mice included a loss of the short-chain fatty acid butyrate. Supplementation of butyrate in Phb1-deficient mice ameliorated Paneth cell abnormalities and ileitis. Phb1-deficient ileal enteroid models suggest deleterious epithelial-intrinsic responses to ileal microbiota that were protected by butyrate.

CONCLUSIONS

These results suggest a mutual and essential reinforcing interplay of gut microbiota and host IEC, including Paneth cell, mitochondrial health in influencing ileitis. Restoration of butyrate is a potential therapeutic option in Crohn's disease patients harboring epithelial cell mitochondrial dysfunction. Video Abstract.

Autophagy in Crohn's Disease: Converging on Dysfunctional Innate Immunity

Crohn's disease (CD) is a chronic inflammatory bowel disease marked by relapsing, transmural intestinal inflammation driven by innate and adaptive immune responses. Autophagy is a multi-step process that plays a critical role in maintaining cellular homeostasis by degrading intracellular components, such as damaged organelles and invading bacteria. Dysregulation of autophagy in CD is revealed by the identification of several susceptibility genes, including ATG16L1, IRGM, NOD2, LRRK2, ULK1, ATG4, and TCF4, that are involved in autophagy. In this review, the role of altered autophagy in the mucosal innate immune response in the context of CD is discussed, with a specific focus on dendritic cells, macrophages, Paneth cells, and goblet cells. Selective autophagy, such as xenophagy, ERphagy, and mitophagy, that play crucial roles in maintaining intestinal homeostasis in these innate immune cells, are discussed. As our understanding of autophagy in CD pathogenesis evolves, the development of autophagy-targeted therapeutics may benefit subsets of patients harboring impaired autophagy.

Pak2-mediated phosphorylation promotes RORγt ubiquitination and inhibits colonic inflammation

Dysregulated interleukin-17 (IL-17) expression and its downstream signaling is strongly linked to inflammatory bowel diseases (IBDs). However, the molecular mechanisms by which the function of RORγt, the transcription factor of IL-17, is regulated remains elusive. By a mass spectrometry-based approach, we identify that Pak2, a serine (S)/threonine (T) kinase, directly associates with RORγt. Pak2 recognizes a conserved KRLS motif within RORγt and phosphorylates the S-316 within this motif. Genetic deletion of Pak2 in Th17 cells reduces RORγt phosphorylation, increases IL-17 expression, and induces severe colitis upon adoptive transfer to Rag1^−/− mice. Similarly, reconstitution of RORγt-S316A mutant in Rorc^−/− Th17 cells enhances IL-17 expression and colitis severity. Mechanistically, we demonstrate that Pak2-mediated phosphorylation causes a conformational change resulting in exposure of the ubiquitin ligase Itch interacting PPLY motif and degradation of RORγt. Thus, we have uncovered a mechanism by which the activity of RORγt is regulated that can be exploited therapeutically.

Kathania et al. show that Pak2, a Ser/Thr kinase, associates with RORγt and phosphorylates Ser-316 of RORγt. Deletion of Pak2 in Th17 cells enhances IL-17 expression and colitis severity. Pak2-mediated phosphorylation causes a conformational change resulting in increased ubiquitination of RORγt by the E3 ubiquitin ligase Itch.

RORγt protein modifications and IL-17-mediated inflammation

RORγt, the master transcription factor for cytokine interleukin (IL)-17, is expressed explicitly in Th17 cells, γδT cells, and type 3 innate lymphoid cells in mice and humans. Since dysregulated IL-17 expression is strongly linked to several human inflammatory diseases, the RORγt-IL-17 axis has been the focus of intense research. Recently, several studies have shown that RORγt is modified by multiple post-translational mechanisms, including ubiquitination, acetylation, SUMOylation, and phosphorylation. This review discusses how post-translational modifications modulate RORγt function and its turnover to regulate IL-17-driven inflammation. Broad knowledge of these pathways is crucial for a clear understanding of the pathogenic role of RORγt+IL-17+ cells and for the development of putative therapeutic strategies to target IL-17-driven diseases such as multiple sclerosis, psoriasis, rheumatoid arthritis, systemic lupus erythematosus, and inflammatory bowel disease.

Mitochondria and Inflammatory Bowel Diseases: Toward a Stratified Therapeutic Intervention

Mitochondria serve numerous critical cellular functions, rapidly responding to extracellular stimuli and cellular demands while dynamically communicating with other organelles. Mitochondrial function in the gastrointestinal epithelium plays a critical role in maintaining intestinal health. Emerging studies implicate the involvement of mitochondrial dysfunction in inflammatory bowel disease (IBD). This review presents mitochondrial metabolism, function, and quality control that converge in intestinal epithelial stemness, differentiation programs, barrier integrity, and innate immunity to influence intestinal inflammation. Intestinal and disease characteristics that set the stage for mitochondrial dysfunction being a key factor in IBD, and in turn, pathogenic mitochondrial mechanisms influencing and potentiating the development of IBD, are discussed. These findings establish the basis for potential mitochondrial-targeted interventions for IBD therapy. Expected final online publication date for the Annual Review of Physiology, Volume 84 is February 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Deubiquitination of NLRP6 by Cyld critically regulates colonic inflammation

The inflammasome NLRP6 plays a crucial role in regulating colonic inflammation and cancer. However, the molecular mechanisms by which NLRP6 function is inhibited to prevent excessive inflammation remain unclear. Here, we demonstrate that the deubiquitinase Cyld prevents excessive interleukin 18 (IL-18) production in the colonic mucosa by deubiquitinating NLRP6. We show that deubiquitination inhibited the NLRP6-ASC inflammasome complex and regulated the maturation of IL-18. Cyld deficiency in mice resulted in elevated levels of active IL-18 and severe colonic inflammation and increased colon cancer. Further, in patients with ulcerative colitis, the concentration of active IL-18 was inversely correlated with CYLD expression. Thus, we have identified a novel regulatory mechanism that inhibits the NLRP6-IL-18 pathway in colonic inflammation. (Nat Immunol. 2020. 21(6):626–635. PMID: 32424362)

NLRP6 in host defense and intestinal inflammation

NLRP6 is a member of the NLR (nucleotide-oligomerization domain-like receptor) family of proteins that recognize pathogen-derived factors and damage-associated molecular patterns in the cytosol. The function of NLRP6 has been attributed to the maintenance of epithelial integrity and host defense against microbial infections. Under some physiological conditions, NLRP6 forms a complex with ASC and caspase-1 or caspase-11 to form an inflammasome complex cleaving pro-interleukin-1β (IL-1β) and IL-18 into their biologically active forms. Here, we summarize recent advances in the understanding of the mechanisms of activation of the NLRP6 inflammasome and discuss its relevance to human disease.

Cutting Edge: Hypoxia-Induced Ubc9 Promoter Hypermethylation Regulates IL-17 Expression in Ulcerative Colitis

Dysregulated IL-17 expression is central to the pathogenesis of several inflammatory disorders, including ulcerative colitis. We have shown earlier that SUMOylation of ROR-γt, the transcription factor for IL-17, regulates colonic inflammation. In this study, we show that the expression of Ubc9, the E2 enzyme that targets ROR-γt for SUMOylation, is significantly reduced in the colonic mucosa of ulcerative colitis patients. Mechanistically, we demonstrate that hypoxia-inducible factor 1α (HIF-1α) binds to a CpG island within the Ubc9 gene promoter, resulting in its hypermethylation and reduced Ubc9 expression. CRISPR-Cas9-mediated inhibition of HIF-1α normalized Ubc9 and attenuated IL-17 expression in Th17 cells and reduced diseases severity in Rag1 ^-/- mice upon adoptive transfer. Collectively, our study reveals a novel epigenetic mechanism of regulation of ROR-γt that could be exploited in inflammatory diseases.

Deletion of Cbl-b inhibits CD8+ T-cell exhaustion and promotes CAR T-cell function

Background

Chimeric antigen receptor (CAR) T-cell therapy is an emerging option for cancer treatment, but its efficacy is limited, especially in solid tumors. This is partly because the CAR T cells become dysfunctional and exhausted in the tumor microenvironment. However, the key pathways responsible for impaired function of exhausted cells remain unclear, which is essential to overcome CAR T-cell exhaustion.

Methods

Analysis of RNA-sequencing data from CD8⁺ tumor-infiltrating lymphocytes (TILs) led to identification of Cbl-b as a potential target. The sequencing data were validated using a syngeneic MC38 colon cancer model. To analyze the in vivo role of Cbl-b in T-cell exhaustion, tumor growth, % PD1⁺Tim3⁺ cells, and expression of effector cytokines were analyzed in cbl-b^+/+ and cbl-b^–/– mice. To evaluate the therapeutic potential of Cbl-b depletion, we generated a new CAR construct, hCEAscFv-CD28-CD3ζ.GFP, that recognizes human carcinoembryonic antigen (CEA). cbl-b^+/+ and cbl-b^–/– CEA-CAR T cells were generated by retroviral transduction. Rag^–/– mice bearing MC38-CEA cells were injected with cbl-b^+/+ and cbl-b^–/–; CEA-CAR T cells, tumor growth, % PD1⁺Tim3⁺ cells and expression of effector cytokines were analyzed.

Results

Our results show that the E3 ubiquitin ligase Cbl-b is upregulated in exhausted (PD1⁺Tim3⁺) CD8⁺ TILs. CRISPR-Cas9-mediated inhibition of Cbl-b restores the effector function of exhausted CD8⁺ TILs. Importantly, the reduced growth of syngeneic MC38 tumors in cbl-b^–/– mice was associated with a marked reduction of PD1⁺Tim3⁺ CD8⁺ TILs. Depletion of Cbl-b inhibited CAR T-cell exhaustion, resulting in reduced MC38-CEA tumor growth, reduced PD1⁺Tim3⁺ cells and increased expression of interferon gamma, tumor necrosis factor alpha, and increased tumor cell killing.

Conclusion

Our studies demonstrate that deficiency of Cbl-b overcomes endogenous CD8⁺ T-cell exhaustion, and deletion of Cbl-b in CAR T cells renders them resistant to exhaustion. Our results could facilitate the development of efficient CAR T-cell therapy for solid tumors by targeting Cbl-b.

Nuclear partitioning of Prohibitin 1 inhibits Wnt/β-catenin-dependent intestinal tumorigenesis

The Wnt/β-catenin signaling pathway is aberrantly activated in the majority of colorectal cancer cases due to somatic mutations in the adenomatous polyposis coli (APC) gene. Prohibitin 1 (PHB1) serves pleiotropic cellular functions with dynamic subcellular trafficking facilitating signaling crosstalk between organelles. Nuclear-localized PHB1 is an important regulator of gene transcription. Using mice with inducible intestinal epithelial cell (IEC)-specific deletion of Phb1 (Phb1^iΔIEC) and mice with IEC-specific overexpression of Phb1 (Phb1Tg), we demonstrate that IEC-specific PHB1 combats intestinal tumorigenesis in the Apc^Min/+ mouse model by inhibiting Wnt/β-catenin signaling. Forced nuclear accumulation of PHB1 in human RKO or SW48 CRC cell lines increased AXIN1 expression and decreased cell viability. PHB1 deficiency in CRC cells decreased AXIN1 expression and increased β-catenin activation that was abolished by XAV939, a pharmacological AXIN stabilizer. These results define a role of PHB1 in inhibiting the Wnt/β-catenin pathway to influence the development of intestinal tumorigenesis. Induction of nuclear PHB1 trafficking provides a novel therapeutic option to influence AXIN1 expression and the β-catenin destruction complex in Wnt-driven intestinal tumorigenesis.

Mitochondrial dysfunction during loss of prohibitin 1 triggers Paneth cell defects and ileitis

OBJECTIVE

Although perturbations in mitochondrial function and structure have been described in the intestinal epithelium of Crohn's disease and ulcerative colitis patients, the role of epithelial mitochondrial stress in the pathophysiology of inflammatory bowel diseases (IBD) is not well elucidated. Prohibitin 1 (PHB1), a major component protein of the inner mitochondrial membrane crucial for optimal respiratory chain assembly and function, is decreased during IBD.

DESIGN

Male and female mice with inducible intestinal epithelial cell deletion of Phb1 (Phb1^iΔIEC ) or Paneth cell-specific deletion of Phb1 (Phb1^ΔPC ) and Phb1^fl/fl control mice were housed up to 20 weeks to characterise the impact of PHB1 deletion on intestinal homeostasis. To suppress mitochondrial reactive oxygen species, a mitochondrial-targeted antioxidant, Mito-Tempo, was administered. To examine epithelial cell-intrinsic responses, intestinal enteroids were generated from crypts of Phb1^iΔIEC or Phb1^ΔPC mice.

RESULTS

Phb1^iΔIEC mice exhibited spontaneous ileal inflammation that was preceded by mitochondrial dysfunction in all IECs and early abnormalities in Paneth cells. Mito-Tempo ameliorated mitochondrial dysfunction, Paneth cell abnormalities and ileitis in Phb1^iΔIEC ileum. Deletion of Phb1 specifically in Paneth cells (Phb1^ΔPC ) was sufficient to cause ileitis. Intestinal enteroids generated from crypts of Phb1^iΔIEC or Phb1^ΔPC mice exhibited decreased viability and Paneth cell defects that were improved by Mito-Tempo.

CONCLUSION

Our results identify Paneth cells as highly susceptible to mitochondrial dysfunction and central to the pathogenesis of ileitis, with translational implications for the subset of Crohn's disease patients exhibiting Paneth cell defects.

The Synthetic Small Molecule FL3 Combats Intestinal Tumorigenesis via Axin1-Mediated Inhibition of Wnt/β-Catenin Signaling

Colorectal cancer exhibits aberrant activation of Wnt/β-catenin signaling. Many inhibitors of the Wnt/β-catenin pathway have been tested for Wnt-dependent cancers including colorectal cancer, but are unsuccessful due to severe adverse reactions. FL3 is a synthetic derivative of natural products called flavaglines, which exhibit anti-inflammatory and cytoprotective properties in intestinal epithelial cells, but has not been previously tested in cell or preclinical models of intestinal tumorigenesis. In vitro studies suggest that flavaglines target prohibitin 1 (PHB1) as a ligand, but this has not been established in the intestine. PHB1 is a highly conserved protein with diverse functions that depend on its posttranslational modifications and subcellular localization. Here, we demonstrate that FL3 combats intestinal tumorigenesis in the azoxymethane-dextran sodium sulfate and Apc^Min/+ mouse models and in human colorectal cancer tumor organoids (tumoroids) by inhibiting Wnt/β-catenin signaling via induction of Axin1 expression. FL3 exhibited no change in cell viability in normal intestinal epithelial cells or human matched-normal colonoids. FL3 response was diminished in colorectal cancer cell lines and human colorectal cancer tumoroids harboring a mutation at S45 of β-catenin. PHB1 deficiency in mice or in human colorectal cancer tumoroids abolished FL3-induced expression of Axin1 and drove tumoroid death. In colorectal cancer cells, FL3 treatment blocked phosphorylation of PHB1 at Thr258, resulting in its nuclear translocation and binding to the Axin1 promoter. These results suggest that FL3 inhibits Wnt/β-catenin signaling via PHB1-dependent activation of Axin1. FL3, therefore, represents a novel compound that combats Wnt pathway-dependent cancers, such as colorectal cancer. SIGNIFICANCE: Targeting of PHB1 by FL3 provides a novel mechanism to combat Wnt-driven cancers, with limited intestinal toxicity. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/17/3519/F1.large.jpg.

The influence of mitochondrial-directed regulation of Wnt signaling on tumorigenesis

Mitochondria are dynamic organelles that play a key role in integrating cellular signaling. Mitochondrial alterations are evident in all stages of tumorigenesis and targeting mitochondrial pathways has emerged as an anticancer therapeutic strategy. The Wnt-signaling pathway regulates many fundamental cellular functions such as proliferation, survival, migration, stem-cell maintenance, and mitochondrial metabolism and dynamics. Emerging evidence demonstrates that mitochondrial-induced regulation of Wnt signaling provides an additional mechanism to influence cell-fate decisions. Crosstalk between mitochondria and Wnt signaling presents a feedforward loop in which Wnt activation regulates mitochondrial function that, in turn, drives Wnt signaling. In this mini-review, we will discuss the recent evidence revealing the mitochondrial control of Wnt signaling and its implications for tumorigenesis and anticancer therapeutic targeting.

Deubiquitination of NLRP6 inflammasome by Cyld critically regulates intestinal inflammation

The inflammasome NLRP6 plays a crucial role in regulating inflammation and host defense against microorganisms in the intestine. However, the molecular mechanisms by which NLRP6 function is inhibited to prevent excessive inflammation remain unclear. Here, we demonstrate that the deubiquitinase Cyld prevents excessive interleukin 18 (IL-18) production in the colonic mucosa by deubiquitinating NLRP6. We show that deubiquitination inhibited the NLRP6-ASC inflammasome complex and regulated the maturation of IL-18. Cyld deficiency in mice resulted in elevated levels of active IL-18 and severe colonic inflammation following Citrobacter rodentium infection. Further, in patients with ulcerative colitis, the concentration of active IL-18 was inversely correlated with CYLD expression. Thus, we have identified a novel regulatory mechanism that inhibits the NLRP6-IL-18 pathway in intestinal inflammation.

Gut bacteria signaling to mitochondria in intestinal inflammation and cancer

The gastrointestinal microbiome plays a pivotal role in physiological homeostasis of the intestine as well as in the pathophysiology of diseases including inflammatory bowel diseases (IBD) and colorectal cancer (CRC). Emerging evidence suggests that gut microbiota signal to the mitochondria of mucosal cells, including epithelial cells and immune cells. Gut microbiota signaling to mitochondria has been shown to alter mitochondrial metabolism, activate immune cells, induce inflammasome signaling, and alter epithelial barrier function. Both dysbiosis of the gut microbiota and mitochondrial dysfunction are associated with chronic intestinal inflammation and CRC. This review discusses mitochondrial metabolism of gut mucosal cells, mitochondrial dysfunction, and known gut microbiota-mediated mitochondrial alterations during IBD and CRC.

Gut Microbiota Contributes to Spontaneous Colitis in E3 Ligase Itch-Deficient Mice

Inflammatory bowel diseases are associated with complex shifts in microbiota composition. However, it remains unclear whether specific subsets of commensal bacteria induce inflammatory bowel diseases in genetically susceptible hosts. In this study, we found that deficiency of the E3 ligase Itch, which leads to spontaneous colitis and rectal prolapse, is associated with alteration of the gut microbiota. 16S rRNA sequencing showed expansion of colitogenic Bacteroides sp. in Itch^-/- mice. Treatment with broad-spectrum antibiotics substantially reduced colonic inflammation in Itch^-/- mice. Microbiota of Itch^-/- mice failed to induce spontaneous colitis upon transfer to Itch^+/+ mice but aggravated chemically induced colitis. Furthermore, we found that Bacteroides vulgatus, which is expanded in Itch^-/- mice, was sufficient to induce colon inflammation in Itch^-/- mice.

IL-17-driven intestinal fibrosis is inhibited by Itch-mediated ubiquitination of HIC-5

Intestinal fibrosis is a major complication in inflammatory bowel diseases, but the regulatory mechanism that inhibits fibrosis remains unclear. Here we demonstrate that Itch^-/-myofibroblasts express increased amounts of profibrotic collagen type I and α-SMA in response to IL-17. Mechanistically, we demonstrate that Itch directly binds to HIC-5 and targets it for K63-linked ubiquitination to inhibit IL-17-driven intestinal fibrosis. Reconstitution of Itch^-/- myofibroblasts with wild-type Itch but not the Itch-C830A mutant normalized the expression of profibrotic genes. Similarly, shRNA-mediated inhibition of HIC-5 normalized the expression of profibrotic gene expression. Thus, we have uncovered a novel mechanism by which Itch negatively regulates intestinal fibrosis.

Mitochondrial STAT3 contributes to transformation of Barrett's epithelial cells that express oncogenic Ras in a p53-independent fashion

Metaplastic epithelial cells of Barrett's esophagus transformed by the combination of p53-knockdown and oncogenic Ras expression are known to activate signal transducer and activator of transcription 3 (STAT3). When phosphorylated at tyrosine 705 (Tyr705), STAT3 functions as a nuclear transcription factor that can contribute to oncogenesis. STAT3 phosphorylated at serine 727 (Ser727) localizes in mitochondria, but little is known about mitochondrial STAT3's contribution to carcinogenesis in Barrett's esophagus, which is the focus of this study. We introduced a constitutively active variant of human STAT3 (STAT3CA) into the following: 1) non-neoplastic Barrett's (BAR-T) cells; 2) BAR-T cells with p53 knockdown; and 3) BAR-T cells that express oncogenic H-Ras(G12V). STAT3CA transformed only the H-Ras(G12V)-expressing BAR-T cells (evidenced by loss of contact inhibition, formation of colonies in soft agar, and generation of tumors in immunodeficient mice), and did so in a p53-independent fashion. The transformed cells had elevated levels of both mitochondrial (Ser727) and nuclear (Tyr705) phospho-STAT3. Introduction of a STAT3CA construct with a mutated tyrosine phosphorylation site into H-Ras(G12V)-expressing Barrett's cells resulted in high levels of mitochondrial phospho-STAT3 (Ser727) with little or no nuclear phospho-STAT3 (Tyr705), and the cells still formed tumors in immunodeficient mice. Thus tyrosine phosphorylation of STAT3 is not required for tumor formation in Ras-expressing Barrett's cells. We conclude that mitochondrial STAT3 (Ser727) can contribute to oncogenesis in Barrett's cells that express oncogenic Ras. These findings suggest that agents targeting STAT3 might be useful for chemoprevention in patients with Barrett's esophagus.

Prohibitin 1 modulates mitochondrial function of Stat3

Mitochondrial dysfunction in intestinal epithelial cells (IEC) is thought to precede the onset of inflammatory bowel diseases (IBD). Expression of Prohibitin 1 (PHB), a mitochondrial protein required for optimal electron transport chain (ETC) activity, is decreased in mucosal biopsies during active and inactive IBD. In addition to its activities as a transcription factor, Signal Transducer and Activator of Transcription 3 (Stat3) resides in the mitochondria of cells where phosphorylation at S727 is required for optimal ETC activity and protects against stress-induced mitochondrial dysfunction. Here, we show that PHB overexpression protects against mitochondrial stress and apoptosis of cultured IECs induced by TNFα, which is a pro-inflammatory cytokine involved in IBD pathogenesis. Expression of pS727-Stat3 dominant negative eliminates protection by PHB against TNFα-induced mitochondrial stress and apoptosis. PHB interacts with pS727-Stat3 in the mitochondria of cultured IECs and in colonic epithelium from wild-type mice. Our data suggest a protective role of PHB that is dependent on pS727-Stat3 to prevent mitochondrial dysfunction in IECs. Reduced levels of PHB during IBD may be an underlying factor promoting mitochondrial dysfunction of the intestinal epithelium.

Sphingosine-1-phosphate: Driver of NFκB and STAT3 persistent activation in chronic intestinal inflammation and colitis-associated cancer

Inflammatory bowel disease is associated with an increased risk of colorectal cancer. Colitis-associated cancer is a classical inflammation-driven cancer in which constitutive NFκB and STAT3 activation drive tumorigenesis. Recent findings published by Liang et al. in Cancer Cell demonstrate that sphingosine kinase 1 (SphK1)-mediated upregulation of sphingosine-1-phosphate (S1P) drives a persistent NFκB/IL-6/STAT3/sphingosine-1-phosphate receptor 1 (S1PR1) amplification loop that is critical to the development of chronic colitis and colitis-associated cancer. FTY720, an antagonist of S1PR1, abolished persistent NFκB/IL-6/STAT3 signaling and reduced the development and progression of colitis-associated cancer. Targeting SphK1/S1P/S1PR1 may provide a therapeutic option to prevent the progression of colitis to cancer.

Nrf2 is not required for epithelial prohibitin-dependent attenuation of experimental colitis

Inflammatory bowel disease is associated with increased reactive oxygen species (ROS) and decreased antioxidant response in the intestinal mucosa. Expression of the mitochondrial protein prohibitin (PHB) is also decreased during intestinal inflammation. Our previous study showed that genetic restoration of colonic epithelial PHB expression [villin-PHB transgenic (PHB Tg) mice] attenuated dextran sodium sulfate (DSS)-induced colitis/oxidative stress and sustained expression of colonic nuclear factor erythroid 2-related factor 2 (Nrf2), a cytoprotective transcription factor. This study investigated the role of Nrf2 in mediating PHB-induced protection against colitis and expression of the antioxidant response element (ARE)-regulated antioxidant genes heme oxygenase-1 (HO-1) and NAD(P)H quinone oxidoreductase-1 (NQO-1). PHB-transfected Caco-2-BBE human intestinal epithelial cells maintained increased ARE activation and decreased intracellular ROS levels compared with control vector-transfected cells during Nrf2 knockdown by small interfering RNA. Treatment with the ERK inhibitor PD-98059 decreased PHB-induced ARE activation, suggesting that ERK constitutes a significant portion of PHB-mediated ARE activation in Caco-2-BBE cells. PHB Tg, Nrf2(-/-), and PHB Tg/Nrf2(-/-) mice were treated with DSS or 2,4,6-trinitrobenzene sulfonic acid (TNBS), and inflammation and expression of HO-1 and NQO-1 were assessed. PHB Tg/Nrf2(-/-) mice mimicked PHB Tg mice, with attenuated DSS- or TNBS-induced colitis and induction of colonic HO-1 and NQO-1 expression, despite deletion of Nrf2. PHB Tg/Nrf2(-/-) mice exhibited increased activation of ERK during colitis. Our results suggest that maintaining expression of intestinal epithelial cell PHB, which is decreased during colitis, reduces the severity of inflammation and increases colonic levels of the antioxidants HO-1 and NQO-1 via a mechanism independent of Nrf2.

Prohibitin attenuates colitis-associated tumorigenesis in mice by modulating p53 and STAT3 apoptotic responses

Although inflammatory bowel disease is associated with higher risk of colorectal cancer, the precise pathogenic mechanisms underlying this association are not completely understood. Prohibitin 1 (PHB), a protein implicated in the regulation of proliferation, apoptosis, and transcription, is decreased in intestinal inflammation. In this study, we have established a key function for PHB in mediating colitis-associated cancer. Wild-type and transgenic (Tg) mice specifically overexpressing PHB in intestinal epithelial cells were subjected to a classical two-stage protocol of colitis-associated carcinogenesis. In addition, wild-type and p53 null human cell models were used to assess PHB interaction with STAT3 and p53. Wild-type mice exhibited decreased mucosal PHB protein expression during colitis-associated carcinogenesis. Tg mice exhibited decreased susceptibility in a manner associated with increased apoptosis, p53, Bax, and Bad expression plus decreased Bcl-xL and Bcl-2 expression. PHB overexpression in wild-type but not p53 null human cells increased expression of Bax, Bad, and caspase-3 cleavage. In wild-type p53 cells, PHB overexpression decreased basal and interleukin-6-induced STAT3 activation and expression of the STAT3 responsive genes Bcl-xL and Bcl-2. PHB coimmunoprecipitated with phospho-STAT3 in addition to p53 in cultured cell lysates and colon mucosa. This is the first study to show interaction between PHB and STAT3 in vivo. In summary, our findings suggest that PHB protects against colitis-associated cancer by modulating p53- and STAT3-mediated apoptosis. Modulation of PHB expression in intestinal epithelial cells may offer a potential therapeutic approach to prevent colitis-associated carcinogenesis.

Nanoparticle-based therapeutic delivery of prohibitin to the colonic epithelial cells ameliorates acute murine colitis

BACKGROUND

Intestinal epithelial expression of antioxidants and nuclear factor kappa B (NF-κB) contribute to mucosal barrier integrity and epithelial homeostasis, two key events in the pathogenesis of inflammatory bowel disease (IBD). Genetic restoration of intestinal epithelial prohibitin 1 (PHB) levels during experimental colitis reduces the severity of disease through sustained epithelial antioxidant expression and reduced NF-κB activation. To determine the therapeutic potential of restoring epithelial PHB during experimental colitis in mice, we assessed two methods of PHB colonic mucosal delivery: adenovirus-directed administration by enema and poly(lactic acid) nanoparticle (NPs) delivery by gavage.

METHODS

As a proof-of-principle to demonstrate the therapeutic efficacy of PHB, we utilized adenovirus-directed administration by enema. Second, we used NPs-based colonic delivery of biologically active PHB to demonstrate therapeutic use for human IBD. Colitis was induced by oral administration of dextran sodium sulfate (DSS) in water for 6-7 days. Wildtype mice receiving normal tap water served as controls.

RESULTS

Both methods of delivery resulted in increased levels of PHB in the surface epithelial cells of the colon and reduced severity of DSS-induced colitis in mice as measured by body weight loss, clinical score, myeloperoxidase activity, proinflammatory cytokine expression, histological score, and protein carbonyl content.

CONCLUSIONS

This is the first study to show oral delivery of a biologically active protein by NPs encapsulated in hydrogel to the colon. Here we show that therapeutic delivery of PHB to the colon reduces the severity of DSS-induced colitis in mice. PHB may represent a novel therapeutic target in IBD.

The role and therapeutic potential of prohibitin in disease

Prohibitin 1 (PHB1), a pleiotropic protein in the cell, has been implicated in the regulation of proliferation, apoptosis, transcription, mitochondrial protein folding, and as a cell-surface receptor. This diverse array of functions of PHB1 is attributed to the cell type studied and its subcellular localization. This review discusses recent data that indicate a diverse role of PHB1 in disease pathogenesis and suggest that targeting PHB1 may be a potential therapeutic option for treatment of diseases including cancer, inflammatory bowel disease, insulin resistance/type 2 diabetes, and obesity. These diseases are associated with increased oxidative stress and mitochondrial dysfunction and therefore, the role of PHB1 in both responses will also be discussed.

Prohibitin inhibits tumor necrosis factor alpha-induced nuclear factor-kappa B nuclear translocation via the novel mechanism of decreasing importin alpha3 expression

Expression of prohibitin 1 (PHB), a multifunctional protein in the cell, is decreased during inflammatory bowel disease (IBD). Little is known regarding the regulation and role of PHB during intestinal inflammation. We examined the effect of tumor necrosis factor alpha (TNF-alpha), a cytokine that plays a central role in the pathogenesis of IBD, on PHB expression and the effect of sustained PHB expression on TNF-alpha activation of nuclear factor-kappa B (NF-kappaB) and epithelial barrier dysfunction, two hallmarks of intestinal inflammation. We show that TNF-alpha decreased PHB protein and mRNA abundance in intestinal epithelial cells in vitro and in colon mucosa in vivo. Sustained expression of prohibitin in intestinal epithelial cells in vitro and in vivo (prohibitin transgenic mice, PHB TG) resulted in a marked decrease in TNF-alpha-induced nuclear translocation of the NF-kappaB protein p65, NF-kappaB/DNA binding, and NF-kappaB-mediated transcriptional activation despite robust IkappaB-alpha phosphorylation and degradation and increased cytosolic p65. Cells overexpressing PHB were protected from TNF-alpha-induced increased epithelial permeability. Expression of importin alpha3, a protein involved in p50/p65 nuclear import, was decreased in cells overexpressing PHB and in colon mucosa of PHB TG mice. Restoration of importin alpha3 levels sustained NF-kappaB activation by TNF-alpha during PHB transfection. These results suggest that PHB inhibits NF-kappaB nuclear translocation via a novel mechanism involving alteration of importin alpha3 levels. TNF-alpha decreases PHB expression in intestinal epithelial cells and restoration of PHB expression in these cells can protect against the deleterious effects of TNF-alpha and NF-kappaB on barrier function.

Prohibitin is a novel regulator of antioxidant response that attenuates colonic inflammation in mice

BACKGROUND & AIMS

Increased free radicals and/or impaired antioxidant defenses have been shown to play a pathogenetic role in human and animal models of inflammatory bowel disease. Our previous studies showed that prohibitin (PHB) levels are decreased during colitis and that cultured intestinal epithelial cells overexpressing PHB are protected from oxidative stress. This study investigated the effect of intestinal epithelial cell-specific PHB overexpression on oxidative stress associated with experimental colitis and the potential mechanism by which PHB functions as an antioxidant using PHB transgenic mice.

METHODS

Colitis was induced using 2 established mouse models (Salmonella typhimurium and dextran sodium sulfate) in PHB transgenic mice and wild-type littermates. Oxidative stress was determined by measuring glutathione and protein carbonyl levels in the cecum or colon. Nuclear factor erythroid 2-related factor 2 (Nrf2), a transcriptional regulator of oxidant responses, expression, and activation, was assessed in colon mucosa and cultured intestinal epithelial cells overexpressing PHB.

RESULTS

Cells overexpressing PHB showed sustained Nrf2 nuclear accumulation and DNA binding during oxidant stress. PHB transgenic mice exhibited decreased oxidative stress and colitis and increased Nrf2 messenger RNA expression, nuclear protein translocation, and DNA binding compared with wild-type littermates during colitis.

CONCLUSIONS

These results show that PHB is a regulator of Nrf2 expression in intestinal epithelial cells during oxidative conditions and prevents inflammation-associated oxidative stress and injury through sustained activation of Nrf2. Our data show that PHB is a novel regulator of antioxidants and suggest that restoration of PHB levels represents a potential therapeutic approach in inflammatory bowel disease.

Invasive Escherichia coli are a feature of Crohn's disease

Crohn's disease (CD) and ulcerative colitis (UC) are idiopathic inflammatory conditions of the gut. Our goal was to investigate if invasive Escherichia coli strains were present in patients with inflammatory bowel disease (IBD). Bacterial strains were isolated from biopsy material obtained from normal controls, and patients with a clinical diagnosis of CD and UC. Invasive bacteria were characterized by gentamicin protection assay and biochemical profiling (Api-20E). Strains were characterized by induction of cytokine expression in epithelial and macrophage cell cultures, measurement of epithelial barrier function, and confocal microscopy. Of all invasive bacterial strains in CD 98.9% were identified as E. coli as opposed to 42.1% in UC and 2.1% in normal controls. Epithelial invasion in vitro was significantly higher for CD-associated E. coli (8.4%, +/-5.5 of initial inoculum (I/O)) in comparison to UC (2.5%, +/-0.4 I/O), but highest for strains from inflamed CD tissue (11.3%, +/-4.3 I/O). Both, CD and UC E. coli strains induced high mean TNF-alpha expression in macrophage cell lines (2604.8 pg/10(5) cells, +/-447.4; 2,402.6 pg/10(5) cells, +/-476.3, respectively), but concentrations were significantly higher for isolates from inflamed CD tissue (3071.3 pg/10(5) cells, +/-226.0). Invasive E. coli from IBD tissue induced similar concentrations of interleukin (IL)-8 in epithelial cell cultures, but strains from inflamed CD tissue induced significantly less epithelial IL-8 (674.1 pg/10(5) cells, +/-58.0 vs 920.5 pg/10(5) cells, +/-94.6). IBD-associated E. coli strains significantly decreased transepithelial resistance, induced disorganization of F-actin and displacement of ZO-1, and E-cadherin from the apical junctional complex (AJC). In comparison to normal controls and UC, E. coli are more prevalent in CD, are highly invasive, and do not encode for known effector proteins. E. coli strains from IBD patients regulate cytokine expression and epithelial barrier function, two pathological features of IBD.

Interleukin-6 transcriptionally regulates prohibitin expression in intestinal epithelial cells

Prohibitin (PHB) is a highly conserved protein that has multiple functions in the cell. We recently demonstrated that PHB plays an important role in combating oxidative stress and its expression is down-regulated in human and animal models of inflammatory bowel disease. Little is known regarding the regulation of PHB expression in intestine or other tissues. In this study we examined the regulation of PHB expression in intestinal epithelial cells using the model cell line Caco2-BBE. We successfully cloned the 1192-bp human PHB promoter region and identified the transcription start site 1594 bp upstream from the translation start site due to an intervening intron. We show that the acute phase cytokine interleukin-6 (IL-6) increases PHB protein and mRNA abundance and induces PHB promoter activation. The IL-6 response element site in the PHB promoter is required for maximal basal promoter activity and responsiveness to IL-6. IL-6 also increases binding of nuclear proteins to the IL-6 response element in the PHB promoter that are supershifted by a STAT3 antibody. Both basal promoter activity and IL-6 responsiveness are attenuated by signal transducer and activator of transcription 3 short interference RNA, suggesting that signal transducer and activator of transcription 3 mediates PHB activity by IL-6. Confirming these in vitro results, IL-6(-/-) mice exhibit reduced PHB expression in the colon compared with wild-type mice. These results suggest that IL-6 modulates PHB expression in cultured intestinal epithelial cells and in the intestine in vivo.

Interleukin-6 induces keratin expression in intestinal epithelial cells: potential role of keratin-8 in interleukin-6-induced barrier function alterations

Keratin 8 (K8) and keratin-18 (K18) are the major intermediate filament proteins in the intestinal epithelia. The regulation and function of keratin in the intestinal epithelia is largely unknown. In this study we addressed the role and regulation of K8 and K18 expression by interleukin 6 (IL-6). Caco2-BBE cell line and IL-6 null mice were used to study the effect of IL-6 on keratin expression. Keratin expression was studied by Northern blot, Western blot, and confocal microscopy. Paracellular permeability was assessed by apical-to-basal transport of a fluorescein isothiocyanate dextran probe (FD-4). K8 was silenced using the small interfering RNA approach. IL-6 significantly up-regulated mRNA and protein levels of K8 and K18. Confocal microscopy showed a reticular pattern of intracellular keratin localized to the subapical region after IL-6 treatment. IL-6 also induced serine phosphorylation of K8. IL-6 decreased paracellular flux of FD-4 compared with vehicle-treated monolayers. K8 silencing abolished the decrease in paracellular permeability induced by IL-6. Administration of dextran sodium sulfate (DSS) significantly increased intestinal permeability in IL-6-/- mice compared with wild type mice given DSS. Collectively, our data demonstrate that IL-6 regulates the colonic expression of K8 and K18, and K8/K18 mediates barrier protection by IL-6 under conditions where intestinal barrier is compromised. Thus, our data uncover a novel function of these abundant cytoskeletal proteins, which may have implications in intestinal disorders such as inflammatory bowel disease wherein barrier dysfunction underlies the inflammatory response.

Prohibitin protects against oxidative stress in intestinal epithelial cells

Prohibitin (PHB) is an evolutionarily conserved and ubiquitously expressed protein whose expression or function in intestinal diseases is not known. In this study, we examined the expression and role of PHB in oxidative stress associated with inflammatory bowel disease. Our results show that PHB primarily localizes to the mitochondria in intestinal epithelial cells. Its expression is down-regulated during active human Crohn's disease, experimental colitis in vivo, and oxidative stress in vitro. PHB overexpression increases the expression of glutathione-S-transferase pi and protects from oxidant-induced depletion of glutathione. Finally, PHB overexpression decreases accumulation of reactive oxygen metabolites, as well as increased permeability induced by oxidative stress in intestinal epithelial cells. Together, these results suggest that PHB constitutes a previously unrecognized cellular defense against oxidant injury. Thus, strategies to modulate PHB levels may constitute a novel therapeutic approach for intestinal inflammatory diseases, wherein oxidative stress plays a critical role in tissue injury and inflammation.

Tumor necrosis factor (TNF) alpha increases collagen accumulation and proliferation in intestinal myofibroblasts via TNF receptor 2

Intestinal fibrosis is an incurable complication of Crohn's disease involving increased numbers of collagen-producing myofibroblasts. Tumor necrosis factor (TNF) alpha has defined proinflammatory roles in Crohn's disease but its role in fibrosis is unclear. We tested the hypothesis that TNFalpha increases collagen accumulation and proliferation in intestinal myofibroblasts and has additive effects in combination with insulin-like growth factor (IGF) I. The mechanisms, TNF receptor isoform, and downstream signaling pathways were examined. Intestinal myofibroblasts from wild-type (WT) mice or mice homozygous for disruption of genes encoding TNFR1 (TNFR1-/-), TNFR2 (TNFR2-/-), or both (TNFR1/2-/-), were treated with TNFalpha, IGF-I, or both. In WT cells, TNFalpha and IGF-I stimulated type I collagen accumulation and DNA synthesis in an additive manner. IGF-I, but not TNFalpha, stimulated type I collagen gene activation. TNFalpha, but not IGF-I, induced tissue inhibitor of metalloproteinase-1 (TIMP-1) expression and reduced matrix metalloproteinases-2 activity and collagen degradation. TNFalpha also activated ERK1/2. These responses to TNFalpha were absent in TNFR2-/- and TNFR1/2-/- myofibroblasts, whereas TNFR1-/- cells showed similar responses to WT. Inhibition of ERK1/2 diminished TNFalpha induced DNA synthesis in WT and TNFR1-/- cells. Differences in TNFalpha-induced STAT3/DNA binding activity and not NFkappaB and AP-1 transcriptional activation correlated with impaired collagen accumulation/TIMP-1 induction in TNFR2(-/-) cells. Constitutively active STAT3 rescued TIMP-1 expression in TNFR2-/- cells. We conclude that TNFalpha and IGF-I may additively contribute to fibrosis during intestinal inflammation. TNFR2 is a primary mediator of fibrogenic actions of TNFalpha acting through ERK1/2 to stimulate proliferation and through STAT3 to stimulate TIMP-1 and inhibit collagen degradation.

Growth hormone reduces the severity of fibrosis associated with chronic intestinal inflammation

BACKGROUND & AIMS

Growth hormone (GH) is used to treat growth delay in children with Crohn's disease and in patients with short-bowel syndrome. GH can increase collagen accumulation in intestinal mesenchymal cells, raising concern that GH therapy could exacerbate fibrosis in patients with Crohn's disease. We tested if GH treatment altered inflammation or fibrosis during chronic, experimental granulomatous enterocolitis.

METHODS

Ileum and cecum of Lewis rats were subserosally injected with peptidoglycan-polysaccharide (PG-APS) or control human serum albumin. At the onset of chronic PG-APS-induced inflammation, rats were administered recombinant human GH or vehicle for 14 days. Fibrosis and inflammation were quantified by gross gut disease scoring, histologic scoring, type I collagen, and cytokine expression in cecum. Abundance and localization of suppressor of cytokine signaling-3 (SOCS-3) messenger RNA and/or protein were determined in cecum. Effect of GH, cytokines, or PG-APS on SOCS-3 synthesis was measured in intestinal myofibroblasts. Myofibroblasts overexpressing SOCS-3 were used to test whether SOCS-3 inhibits collagen accumulation.

RESULTS

In PG-APS-injected rats, GH modestly reduced gross adhesions and mesenteric contractions, cecal fibrosis score, and collagen expression, but had no effect on intestinal inflammation. GH increased SOCS-3 messenger RNA and protein abundance in PG-APS rats and SOCS-3 messenger RNA was localized to the periphery of granulomas. GH in combination with cytokines or PG-APS, but not alone, induced SOCS-3 synthesis in intestinal myofibroblasts. Myofibroblasts overexpressing SOCS-3 showed reduced cytokine-induced collagen accumulation.

CONCLUSIONS

GH modestly reduces intestinal fibrosis associated with chronic experimental enterocolitis and stimulates expression of antifibrogenic SOCS-3, suggesting that GH therapy in inflammatory bowel disease should not exacerbate fibrosis.

Growth factors in inflammatory bowel disease: the actions and interactions of growth hormone and insulin-like growth factor-I

Multiple growth hormones (GHs) and factors are relevant to inflammatory bowel disease (IBD) due to their trophic effects on epithelial cells to promote mucosal integrity, renewal, and repair, on mesenchymal cells to promote wound healing, and on intestinal immune cells to modulate inflammation. The anabolic effects of GHs and factors outside the intestine are relevant to minimizing nutritional insufficiency, catabolic state, and the inability to maintain body weight due to inflammation-induced malabsorption. GHs and factors can, however, have a dual role, whereby trophic effects can be beneficial but, if excessive, can promote complications including the increased risk of intestinal tumors/adenocarcinoma and fibrosis. This review focuses on GH and insulin-like growth factor (IGF-I), for which evidence suggests such a dual role may exist. The actions of GH and IGF-I on the healthy intestine are compared with effects during intestinal inflammation or associated complications. Interactions between these growth factors and others relevant to IBD are considered. The role of the newly discovered suppressors of cytokine signaling proteins, which may dictate the balance between beneficial and excessive actions of GH and IGF-I, is also addressed.