Vitamin D deficiency associated with Crohn's disease and ulcerative colitis: a meta-analysis of 55 observational studies

Novel targets for mucosal healing in inflammatory bowel disease therapy

Inflammatory bowel disease (IBD) is a chronic condition affecting the gastrointestinal tract, primarily manifesting as ulcerative colitis (UC) or Crohn's disease (CD). Both inflammation and disruption of the intestinal epithelial barrier are key factors in IBD pathogenesis. Substantial evidence has revealed a significant association between aberrant immune responses and impairment of the intestinal epithelial barrier in IBD pathogenesis. The components of the intestinal epithelium, particularly goblet cells and Paneth cells, are crucial to gut homeostasis, as they secrete mucin, antimicrobial peptides (AMPs), and cytokines. Furthermore, impairment of epithelial integrity, which is regulated by tight junctions, is a hallmark of IBD pathology. While common treatments for IBD, such as anti-inflammatory drugs, target various signaling pathways with varying efficacies, therapeutic approaches focused on mucosal and epithelial barrier healing have been largely neglected. Moreover, high costs, side effects, and insufficient or inconsistent therapeutic outcomes remain major drawbacks of conventional anti-IBD drugs. Recent studies on epithelial barrier regeneration and permeability reduction have introduced promising therapeutic targets, including farnesoid X receptor (FXR), urokinase-type plasminogen activator (uPA)-urokinase-type plasminogen activator receptor (uPAR) interaction, fecal microbiota transplantation (FMT), and insulin receptor (INSR). Notably, the simultaneous targeting of intestinal inflammation and promotion of epithelial barrier healing shows promise for efficient IBD treatment. Future research should explore targeted therapies and combination treatments, including natural remedies, microbiota colonization, stem cell approaches, and computer-aided drug design. It is also crucial to focus on accurate prognosis and developing a thorough understanding of IBD development mechanisms.

The Immunomodulatory Role of Vitamin D in Regulating the Th17/Treg Balance and Epithelial-Mesenchymal Transition: A Hypothesis for Gallbladder Cancer

The etiology of gallbladder cancer (GBC) is multifactorial, with chronic inflammation resulting from infections, autoimmune diseases, and lifestyle factors playing a pivotal role. Vitamin D deficiency (VDD) has been implicated in the pathogenesis of autoimmune disorders and various malignancies, including GBC. Research on autoimmune diseases highlights the anti-inflammatory properties of vitamin D, suggesting its potential to mitigate disease progression. In oncology, VDD has similarly been linked to increased inflammation, which may contribute to both the initiation and progression of cancer. A critical component in carcinogenesis, as well as in the immunomodulatory effects of vitamin D in autoimmune conditions, is the balance between T-helper 17 (Th17) cells and regulatory T (Treg) cells. We hypothesize that vitamin D may inhibit epithelial-mesenchymal transition (EMT) in GBC by modulating the spatial distribution of tumor-infiltrating T cells, particularly through the regulation of the Th17/Treg balance at the tumor margins. This Th17/Treg imbalance may act as a mechanistic link between VDD and the progression of GBC carcinogenesis. Investigating the role of an Th17/Treg imbalance as a mediator in VDD-induced EMT in GBC not only provides deeper insights into the pathogenesis of GBC but also sheds light on broader mechanisms relevant to the development of other solid organ cancers, given the expanding recognition of the roles of VDD and Th17/Treg cells in cancer biology.

Influence of Vitamin D Receptor Signalling and Vitamin D on Colonic Epithelial Cell Fate Decisions in Ulcerative Colitis

BACKGROUND AND AIMS

Epidemiological studies have shown that subnormal levels of vitamin D (25[OH]D) are associated with a more aggravated clinical course of ulcerative colitis [UC]. Despite an increased focus on the therapeutic importance of vitamin D and vitamin D receptor [VDR] signalling, the mechanisms underlying the effects of the vitamin D-VDR axis on UC remain elusive. Therefore, we aimed to investigate whether exposure to active vitamin D (1,25[OH]2D3/VDR) signalling in human organoids could influence the maintenance of the colonic epithelium.

METHODS

Intestinal VDR expression was studied by immunohistochemistry, RNA expression arrays, and single-cell RNA sequencing of colonic biopsy specimens obtained from patients with UC and healthy individuals. To characterise the functional and transcriptional effects of 1,25[OH]2D3, we used patient-derived colonic organoids. The dependency of VDR was assessed by knocking out the receptor with CRISPR/Cas9.

RESULTS

Our results suggest that 1,25[OH]2D3/VDR stimulation supports differentiation of the colonic epithelium and that impaired 1,25[OH]2D3/VDR signalling thereby may compromise the structure of the intestinal epithelial barrier, leading to flares of UC. Furthermore, a transcriptional response to VDR activity was observed primarily in fully differentiated cells at the top of the colonic crypt, and this response was reduced during flares of UC.

CONCLUSIONS

We identified an important role of vitamin D signalling in supporting differentiated cell states in the human colonic epithelium, and thereby maintenance of the intestinal barrier integrity. This makes the vitamin D-VDR signalling axis an interesting target for therapeutic efforts to achieve and maintain remission in patients with UC.

Effects of Milk and Dairy on the Risk and Course of Inflammatory Bowel Disease versus Patients' Dietary Beliefs and Practices: A Systematic Review

BACKGROUND

The role of the environment in the pathogenesis of inflammatory bowel disease (IBD) is undisputed, especially in light of numerous epidemiological data showing the increasing prevalence of IBD worldwide. Although no specific environmental factors have been identified, the diet has received the most attention as a potential modifier of the onset and course of IBD and as a therapeutic intervention. The Westernization of the diet is repeatedly cited as a crucial aspect of the change in IBD prevalence, but data on the impact of diet on the course of IBD are still limited and the effectiveness of dietary interventions remains uncertain. Milk remains one of the most discussed dietary agents in IBD.

MATERIALS AND METHODS

We performed a systematic review of the literature published between January 2010 and March 2024 on three databases, Pubmed, Web of Knowledge, and Embase, to assess the impact of milk and dairy products on the risk and course of IBD, as well as patients' dietary beliefs and practices.

RESULTS

We included 37 original studies in our review.

CONCLUSIONS

There is no clear evidence that milk and dairy products influence the incidence and course of IBD. The studies that assess this issue are characterized by great heterogeneity. Milk and dairy are among the most commonly excluded foods by patients with IBD, which may have clinical implications.

Accumulated LPS induced by colitis altered the activities of vitamin D-metabolizing hydroxylases and decreased the generation of 25-hydroxyvitamin D

It is generally accepted that low vitamin D (VD) levels are associated with a high prevalence factor for Inflammatory bowel disease (IBD). IBD patients have observed higher levels of lipopolysaccharide (LPS), ALT, and AST than healthy people. Gut-derived LPS causes inflammatory injury in the liver and kidney. The VD-metabolizing mechanism is involved in the liver and kidney, which means IBD might impact VD metabolism. However, whether IBD affects VD metabolism has not been studied. In vitro LPS resulted in decreased CYP2R1 in liver cells as well as decreased CYP27B1 and increased CYP24A1 in kidney cells, revealing that LPS changed the activities of several hydroxylases. Mice with acute colitis had an increased LPS in serum and liver with mild hepatic injuries, while mice with chronic colitis had a significant elevation of LPS in serum, liver, and kidney with hepatorenal injuries. Thus, the liver hydroxylase for VD metabolism would be the first to be affected in IBD. Consequently, serum 25-hydroxyvitamin D declined dramatically with a significant elevation of 24,25-dihydroxyvitamin D and 1,24,25-trihydroxyvitamin D. Unchanged serum levels of 1,25-dihydroxyvitamin D might be the result of other factors in vivo. In acute colitis, a small dosage (4 IU/day) of cholecalciferol could protect the colon, decrease the serum level of LPS, and finally increase serum 25-hydroxyvitamin D. However, this improvement of cholecalciferol was fading in chronic colitis. These results suggested that VD supplementations for preventing and curing IBD in the clinic should consider hepatorenal hydroxylases and be employed as soon as possible for a better outcome.

Correlation between Treatment Outcomes and Serum Vitamin D Levels As Well As Infliximab Trough Concentration among Chinese Patients with Crohn's Disease

Background

The relationship between vitamin D (vit-D) levels and the effectiveness of infliximab (IFX) in patients with Crohn's disease (CD) remains controversial.

Objective

To evaluate the interaction between vit-D levels and the response to IFX therapy in patients with CD.

Methods

This was a retrospective cohort study. Serum vit-D and IFX trough concentrations (TC) were measured in 84 patients, and statistical analyses were performed.

Results

The total vit-D deficiency rate at enrollment, at week 14 and week 38, was 64.3%, 41.67%, and 37.5%, respectively (P < 0.001). CD activity index (CDAI) (120, range, 93-142.75) and simplified endoscopic activity score for CD (SES-CD) (2, range, 0-4) at week 14 were lower than that of enrollment (CDAI, 136.5, range, 101.25-196; SES-CD 13, range, 5-23) (P < 0.001). The biochemical remission (BR), clinical remission (CR), endoscopic remission (ER), and response (ERe) rates of week 38 were 76.1%, 88.5%, 22.4%, and 67.2%, respectively. vit-D levels at enrollment were positively correlated with CDAI at week 38 (P = 0.024). IFX serum TC was related to BR (P = 0.036), CR (P = 0.032) at week 14, and ERe (P = 0.009) at week 38.

Conclusion

Among Chinese patients with CD, vit-D levels prior to IFX therapy are related to CDAI scores, and IFX serum TC is associated with BR, CR, and ERe.

Nutrition, Nutritional Status, Micronutrients Deficiency, and Disease Course of Inflammatory Bowel Disease

During the disease course, most Inflammatory Bowel Disease patients present a condition of malnutrition, undernutrition, or even overnutrition. These conditions are mainly due to suboptimal nutritional intake, alterations in nutrient requirements and metabolism, malabsorption, and excessive gastrointestinal losses. A suboptimal nutritional status and low micronutrient serum levels can have a negative impact on both induction and maintenance of remission and on the quality of life of Inflammatory Bowel Disease patients. We performed a systematic review including all the studies evaluating the connection between nutrition, nutrition status (including undernutrition and overnutrition), micronutrient deficiency, and both disease course and therapeutic response in Inflammatory Bowel Disease patients. This systematic review was performed using PubMed/MEDLINE and Scopus. Four main clinical settings concerning the effect of nutrition on disease course in adult Inflammatory Bowel Disease patients were analyzed (induction of remission, maintenance of remission, risk of surgery, post-operative recurrence, and surgery-related complications). Four authors independently reviewed abstracts and manuscripts for eligibility. 6077 articles were found; 762 duplicated studies were removed. Out of 412 full texts analyzed, 227 were included in the review. The evidence summarized in this review showed that many nutritional aspects could be potential targets to induce a better control of symptoms, a deeper remission, and overall improve the quality of life of Inflammatory Bowel Disease patients.

Association of Vitamin D Deficiency and Insufficiency with Pathology in Hospitalized Patients

Vitamin D deficiency is one of the most common medical conditions, with approximately one billion people having low vitamin D levels. Vitamin D is associated with a pleiotropic effect (immunomodulatory, anti-inflammatory and antiviral), which can be essential for a better immune response. The aim of this research was to evaluate the prevalence of vitamin D deficiency/insufficiency in hospitalized patients focusing on demographic parameters as well as assessing the possibility of its associations with different comorbidities. Of 11,182 Romanian patients evaluated in the study over 2 years, 28.83% had vitamin D deficiency, 32.11% insufficiency and 39.05% had optimal vitamin D levels. The vitamin D deficiency was associated with cardiovascular disorders, malignancies, dysmetabolic disorders and SARS-CoV2 infection, older age and the male sex. Vitamin D deficiency was prevalent and showed pathology association, while insufficiency of vitamin D (20-30 ng/mL) had lower statistical relevance and represents a grey zone in vitamin D status. Guidelines and recommendations are necessary for homogeneity of the monitoring and management of inadequately vitamin D status in the risk categories.

Transcriptomic analyses of treatment-naïve pediatric ulcerative colitis patients and exploration of underlying disease pathogenesis

BACKGROUND

Ulcerative colitis (UC) is a form of chronic inflammatory bowel disease of nonspecific origin. This study used an RNA-Sequencing (RNA-Seq) approach to evaluate the transcriptomic landscape of a well-stratified treatment-naïve pediatric UC patient population by comparing them with healthy control children. The data were analyzed to evaluate the mechanisms driving UC-related intestinal inflammation and fibrosis.

METHODS

Intestinal mucosal samples from five pediatric UC patients and five healthy controls were analyzed by RNA-Seq, and results were verified by qPCR. A CRISPR/Cas9 approach was used to knock out the expression of HLA-DRB5, and molecular biology techniques were used for additional mechanistic studies.

RESULTS

In these analyses, 2290 genes were found to be differentially expressed between the UC and control samples, of which 1258 and 1032 were upregulated and downregulated, respectively. Gene Ontology analysis showed that these genes were enriched in extracellular matrix (ECM)-related processes and that 7 of 8 differentially expressed genes of interest (PIK3CD, IL1β, IL1α, TIMP1, MMP1, MMP12, COL6A3, and HLADRB5) were upregulated and involved in ECM-receptor interaction and inflammatory bowel disease-related pathways. Increased HLA-DRB5 expression driven by intestinal bacteria was found to promote IL-1α secretion, leading to intestinal inflammation and fibrosis, suggesting a possible target for the treatment of UC.

CONCLUSION

These data suggest that intestinal inflammation is present in pediatric UC patients for extended periods before the onset of symptoms, and intestinal fibrosis begins even during the early stages of UC. Intestinal bacteria were also found to trigger intestinal inflammation and fibrosis, with HLA-DRB5 playing a central role in this process.

Functional Plasmon-Activated Water Increases Akkermansia muciniphila Abundance in Gut Microbiota to Ameliorate Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is associated with dysbiosis and intestinal barrier dysfunction, as indicated by epithelial hyperpermeability and high levels of mucosal-associated bacteria. Changes in gut microbiota may be correlated with IBD pathogenesis. Additionally, microbe-based treatments could mitigate clinical IBD symptoms. Plasmon-activated water (PAW) is known to have an anti-inflammatory potential. In this work, we studied the association between the anti-inflammatory ability of PAW and intestinal microbes, thereby improving IBD treatment. We examined the PAW-induced changes in the colonic immune activity and microbiota of mice by immunohistochemistry and next generation sequencing, determined whether drinking PAW can mitigate IBD induced by 2,4,6-trinitrobenzene sulfonic acid (TNBS) and dysbiosis through mice animal models. The effects of specific probiotic species on mice with TNBS-induced IBD were also investigated. Experimental results indicated that PAW could change the local inflammation in the intestinal microenvironment. Moreover, the abundance of Akkermansia spp. was degraded in the TNBS-treated mice but elevated in the PAW-drinking mice. Daily rectal injection of Akkermansia muciniphila, a potential probiotic species in Akkermansia spp., also improved the health of the mice. Correspondingly, both PAW consumption and increasing the intestinal abundance of Akkermansia muciniphila can mitigate IBD in mice. These findings indicate that increasing the abundance of Akkermansia muciniphila in the gut through PAW consumption or other methods may mitigate IBD in mice with clinically significant IBD.

The Crosstalk between Vitamin D and Pediatric Digestive Disorders

Vitamin D is a cyclopentane polyhydrophenanthrene compound involved mainly in bone health and calcium metabolism but also autophagy, modulation of the gut microbiota, cell proliferation, immune functions and intestinal barrier integrity. The sources of vitamin D include sunlight, diet and vitamin D supplements. Vitamin D3, the most effective vitamin D isoform is produced in the human epidermis as a result of sunlight exposure. Vitamin D undergoes two hydroxylation reactions in the liver and kidney to reach its active form, 1,25-dihydroxyvitamin D. Recent studies highlighted a complex spectrum of roles regarding the wellbeing of the gastrointestinal tract. Based on its antimicrobial effect, it was recently indicated that vitamin D supplementation in addition to standard eradication therapy might enhance H. pylori eradication rates. Moreover, it was suggested that low levels of vitamin D might also be involved in the acquisition of H. pylori infection. In terms of celiac disease, the negative effects of vitamin D deficiency might begin even during intrauterine life in the setting of maternal deficiency. Moreover, vitamin D is strongly related to the integrity of the gut barrier, which represents the core of the pathophysiology of celiac disease onset, in addition to being correlated with the histological findings of disease severity. The relationship between vitamin D and cystic fibrosis is supported by the involvement of this micronutrient in preserving lung function by clearing airway inflammation and preventing pathogen airway colonization. Moreover, this micronutrient might exert anticatabolic effects in CF patients. Inflammatory bowel disease patients also experience major benefits if they have a sufficient level of circulating vitamin D, proving its involvement in both induction and remission in these patients. The findings regarding the relationship between vitamin D, food allergies, diarrhea and constipation remain controversial, but vitamin D levels should be monitored in these patients in order to avoid hypo- and hypervitaminosis. Further studies are required to fill the remaining gaps in term of the complex impact of vitamin D on gastrointestinal homeostasis.

Pleiotropic Effects of Vitamin D in Patients with Inflammatory Bowel Diseases

The multifaceted activity of vitamin D in patients with inflammatory bowel disease (IBD) presents a challenge for further research in this area. Vitamin D is involved in the regulation of bone mineral metabolism, it participates in the regulation of the immune system, and it is an underlying factor in the pathogenesis of IBD. Additionally, vitamin D affects Th1 and Th2 lymphocytes, influencing the release of cytokines and inhibiting tumor necrosis factor (TNF) expression and the wnt/β-catenin pathway. As far as IBDs are concerned, they are associated with microbiota dysbiosis, abnormal inflammatory response, and micronutrient deficiency, including vitamin D hypovitaminosis. In turn, the biological activity of active vitamin D is regulated by the vitamin D receptor (VDR) which is associated with several processes related to IBD. Therefore, in terms of research on vitamin D supplementation in IBD patients, it is essential to understand the metabolic pathways and genetic determinants of vitamin D, as well as to identify the environmental factors they are subject to, not only in view of osteoporosis prevention and therapy, but primarily concerning modulating the course and supplementation of IBD pharmacotherapy.

Reducing Disease Activity of Inflammatory Bowel Disease by Consumption of Plant-Based Foods and Nutrients

Inflammatory bowel disease is a chronic and recurring inflammatory condition of the gastrointestinal tract encompassing ulcerative colitis and Crohn's disease. Although the pathogenesis of inflammatory bowel disease remains to be fully elucidated, environmental factors such as diet are believed to play a pivotal role in the onset and management of inflammatory bowel disease. Diet is thought to play an essential role in intestinal inflammation due to its regulatory effects on the microbiota, gut immune system, and epithelial barrier function. Although the evidence remains insufficient to draw firm conclusions on the role of specific dietary components in gastrointestinal diseases, studies have suggested that a Western diet with high intakes of total fats, omega-6 fatty acids, and meat have been associated with intestinal inflammation and relapse of inflammatory bowel disease. In contrast to a Western diet, plant-based diets often result in a reduced intake of total fats and meats and an increased intake of plant fibers which may contribute to reduced intestinal inflammation. This review critically examines the influence of plant-based dietary components on the clinical disease course of inflammatory bowel disease. Furthermore, this review discusses the benefits and possible limitations of plant-derived dietary components in the treatment of inflammatory bowel disease while addressing the principal type of disease and the anatomic site of inflammation within the gastrointestinal tract. Finally, this review points out important directions for future research on the role of diet in inflammatory bowel disease. A better understanding of the role of diet and intestinal inflammation may pave the way for novel dietary interventions and specific foods- or food supplements, which can support the treatment of inflammatory bowel disease.

1,25-Dihydroxyvitamin D3 attenuates IL-1β secretion by suppressing NLRP1 inflammasome activation by upregulating the NRF2-HO-1 pathway in epidermal keratinocytes

The nucleotide-binding oligomerization domain-like receptor family pyrin domain-containing protein (NLRP) inflammasome is a key inflammatory signaling pathway activated via a two-step signaling process consisting of priming and activation steps. Several studies have shown that 1,25-dihydroxyvitamin D3 (1,25(OH)2VD3) inhibits the priming step required for NLRP3 inflammasome activation in immune cells. However, as activating the NLRP1 inflammasome in keratinocytes does not necessarily require a priming step, whether 1,25(OH)2VD3 inhibits NLRP1 activation in unprimed keratinocytes is currently unknown. In this study, we showed that 1,25(OH)2VD3 inhibits nigericin-induced NLRP1 inflammasome activation in unprimed keratinocytes. 1,25(OH)2VD3 suppressed nigericin-induced interleukin-1β (IL-1β) secretion and caspase-1 activation in human primary keratinocytes. In addition, 1,25(OH)2VD3 significantly inhibited the formation of apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) oligomers and specks, but not caspase-1 enzymatic activity, suggesting that 1,25(OH)2VD3 prevents NLRP1-ASC complex assembly in keratinocytes. Vitamin D receptor (VDR)-knockdown abolished the inhibitory effects of 1,25(OH)2VD3 on nigericin-induced ASC oligomerization and IL-1β secretion, suggesting that 1,25(OH)2VD3 suppresses inflammasome activation via VDR signaling. Furthermore, nigericin induced K+ efflux and cellular reactive oxygen species (ROS) production, and 1,25(OH)2VD3 pretreatment suppressed nigericin-induced ROS production. 1,25(OH)2VD3 increased the expression of both nuclear factor erythroid 2-related factor 2 (NRF2) and heme oxygenase-1 (HO-1), whereas HO-1 inhibition or NRF2 and HO-1 knockdown abrogated the inhibitory effects of 1,25(OH)2VD3 on IL-1β secretion. Our results indicate that 1,25(OH)2VD3 inhibits nigericin-induced activation step of NLRP1 inflammasome activation in unprimed keratinocytes. Our findings reveal the mechanism underlying the inhibitory effect of 1,25(OH)2VD3, which involves NRF2-HO-1 pathway activation through the VDR, providing further insight into the potential function of 1,25(OH)2VD3 as a therapeutic agent for inflammasome-related skin diseases.

Present and Future Therapeutic Approaches to Barrier Dysfunction

There is converging and increasing evidence, but also uncertainty, for the role of abnormal intestinal epithelial barrier function in the origin and development of a growing number of human gastrointestinal and extraintestinal inflammatory disorders, and their related complaints. Despite a vast literature addressing factors and mechanisms underlying changes in intestinal permeability in humans, and its connection to the appearance and severity of clinical symptoms, the ultimate link remains to be established in many cases. Accordingly, there are no directives or clinical guidelines related to the therapeutic management of intestinal permeability disorders that allow health professionals involved in the management of these patients to carry out a consensus treatment based on clinical evidence. Instead, there are multiple pseudoscientific approaches and commercial propaganda scattered on the internet that confuse those affected and health professionals and that often lack scientific rigor. Therefore, in this review we aim to shed light on the different therapeutic options, which include, among others, dietary management, nutraceuticals and medical devices, microbiota and drugs, and epigenetic and exosomes-manipulation, through an objective evaluation of the scientific publications in this field. Advances in the knowledge and management of intestinal permeability will sure enable better options of dealing with this group of common disorders to enhance quality of life of those affected.

The 25(OH)D3, but Not 1,25(OH)2D3 Levels Are Elevated in IBD Patients Regardless of Vitamin D Supplementation and Do Not Associate with Pain Severity or Frequency

Due to its immunomodulatory effect, vitamin D has been associated with clinical parameters and outcomes in inflammatory bowel diseases (IBDs) which are chronic conditions of the gastrointestinal tract. Upon synthesis or digestion, vitamin D is metabolized in the liver to form 25(OH)D3, the major circulating metabolite. Further renal hydroxylation generates 1,25(OH)2D3, the most potent metabolite. Our aim was to examine the association between vitamin D levels, and its supplementation and pain intensity in 39 IBD patients and 33 healthy individuals. 25(OH)D3 and 1,25(OH)2D3 serum levels were measured. Each subject filled out visual analog scale (VAS) and Laitinen’s pain assessment scales. Laboratory results were obtained, and disease activity was assessed. Linear regression was employed to investigate the correlation between 25(OH)D3, 1,25(OH)2D3 and pain intensity, clinical activity parameters, C-reactive protein, disease duration, and dietary habits. In IBD patients, 25(OH)D3 was increased, whereas 1,25(OH)2D3 was not. Vitamin D3 supplementation did not influence their levels. No correlation was found between pain scores, disease activity, inflammatory status, disease duration or dietary habits and both forms of vitamin D. Elevated 25(OH)D3 and normal 1,25(OH)D3 were found in IBD patients as compared to the controls. We discovered no effect from supplementation and no association between pain severity and vitamin D.

Mucosal vitamin D signaling in inflammatory bowel disease

Epidemiological studies have identified vitamin D (25(OH)D) deficiency to be highly prevalent among patients with inflammatory bowel disease (IBD), and low serum levels correlate with a higher disease activity and a more complicated disease course. The link to IBD pathogenesis has been subject of investigations, primarily due to the distinct immunological functions of vitamin D signaling, including anti-inflammatory and anti-fibrotic actions. Vitamin D is a pleiotropic hormone that executes its actions on cells through the vitamin D receptor (VDR). A leaky gut, i.e. an insufficient intestinal epithelial barrier, is thought to be central for the pathogenesis of IBD, and emerging data support the concept that vitamin D/VDR signaling in intestinal epithelial cells (IECs) has an important role in controlling barrier integrity. Here we review the latest evidence on how vitamin D promotes the interplay between IECs, the gut microbiome, and immune cells and thereby regulate the intestinal immune response. On the cellular level, vitamin D signaling regulates tight junctional complexes, apoptosis, and autophagy, leading to increased epithelial barrier integrity, and promotes expression of antimicrobial peptides as part of its immunomodulating functions. Further, intestinal VDR expression is inversely correlated with the severity of inflammation in patients with IBD, which might compromise the positive effects of vitamin D signaling in patients with flaring disease. Efforts to reveal the role of vitamin D in the pathophysiology of IBD will pave the road for the invention of more rational treatment strategies of this debilitating disease in the future.

Vitamin D and intestinal homeostasis: Barrier, microbiota, and immune modulation

Vitamin D plays a pivotal role in intestinal homeostasis. Vitamin D can impact the function of virtually every cell in the gut by binding to its intracellular receptor (VDR) and subsequently transcribing relevant genes. In the lumen, the mucus layer and the underlying epithelium serve to keep resident microbiota at bay. Vitamin D ensures an appropriate level of antimicrobial peptides in the mucus and maintains epithelial integrity by reinforcing intercellular junctions. Should bacteria penetrate the epithelial layer and enter the interstitium, immune sentinel cells (e.g. macrophages, dendritic cells, and innate lymphoid cells) elicit inflammation and trigger the adaptive immune response by activating Th1/Th17 cells. Vitamin D/VDR signaling in these cells ensures clearance of the bacteria. Subsequently, vitamin D also quiets the adaptive immune system by suppressing the Th1/Th17 cells and favoring Treg cells. The importance of vitamin D/VDR signaling in intestinal homeostasis is evidenced by the development of a chronic inflammatory state (e.g. IBD) when this signaling system is disrupted.

Diminished Vitamin D Receptor Protein Levels in Crohn's Disease Fibroblasts: Effects of Vitamin D

Vitamin D (VD) deficiency has been associated to Crohn’s disease (CD) pathogenesis, and the exogenous administration of VD improves the course of the disease, but the mechanistic basis of these observations remains unknown. Vitamin D receptor (VDR) mediates most of the biological functions of this hormone, and we aim to analyze here the expression of VDR in intestinal tissue, epithelial cells, and fibroblasts from CD patients. The effects of VD on a fibroblast wound healing assay and murine intestinal fibrosis are also analyzed. Our data show diminished VDR protein levels in surgical resections and epithelial cells from CD patients. In intestinal fibroblasts isolated from damaged tissue of CD patients, we detected enhanced migration and decreased VDR expression compared with both fibroblasts from non-damaged tissue of the same CD patient or control fibroblasts. Treatment with VD increased VDR protein levels, avoided the accelerated migration in CD fibroblasts, and prevented murine intestinal fibrosis induced by the heterotopic transplant model. In conclusion, our study demonstrates diminished VDR protein levels associated with enhanced migration in intestinal fibroblasts from damaged tissue of CD patients. In these cells, VD accumulates VDR and normalizes migration, which supports that CD patients would benefit from the VD anti-fibrotic therapeutic value that we demonstrate in a murine experimental model.