AIM: To investigate serum adipokine levels in inflammatory bowel disease (IBD) patients before treatment and after achieving clinical remission.

METHODS: Serum concentrations of six adipokines (tissue growth factor-β1, adiponectin, leptin, chemerin, resistin, and visfatin) were studied in 40 subjects with active IBD [24 subjects with Crohn’s disease (CD) and in 16 subjects with ulcerative colitis (UC)] before and after three months of therapy with corticosteroids and/or azathioprine. Clinical diagnoses were based on ileocolonoscopy, computed tomography or magnetic resonance enterography and histological examination of mucosal biopsies sampled during endoscopy. Serum levels of adipokines were assessed by an indirect enzyme-linked immunosorbent assay. The control group was comprised of 16 age- and sex-matched healthy volunteers.

RESULTS: Baseline leptin concentrations were significantly decreased in both types of IBD compared to controls (8.0 ± 9.1 in CD and 8.6 ± 6.3 in UC vs 16.5 ± 10.1 ng/mL in controls; P < 0.05), and significantly increased after treatment only in subjects with CD (14.9 ± 15.1 ng/mL; P < 0.05). Baseline serum resistin concentrations were significantly higher in CD (19.3 ± 12.5 ng/mL; P < 0.05) and UC subjects (23.2 ± 11.0 ng/mL; P < 0.05) than in healthy controls (10.7 ± 1.1 ng/mL). Treatment induced a decrease in the serum resistin concentration only in UC subjects (14.5 ± 4.0 ng/mL; P < 0.05). Baseline serum concentrations of visfatin were significantly higher in subjects with CD (23.2 ± 3.2 ng/mL; P < 0.05) and UC (18.8 ± 5.3 ng/mL; P < 0.05) than in healthy controls (14.1 ± 5.3 ng/mL). Treatment induced a decrease in the serum visfatin concentrations only in CD subjects (20.4 ± 4.8 ng/mL; P < 0.05). Serum levels of adiponectin, chemerin and tissue growth factor-β1 did not differ between CD and UC subjects compared to healthy controls and also were not altered by anti-inflammatory therapy. Clinical indices of IBD activity did not correlate with adipokine levels.

CONCLUSION: IBD modulates serum adipokine levels by increasing resistin and visfatin release and suppressing leptin production.

Inflammatory bowel diseases (IBDs) such as Crohn’s disease (CD) and ulcerative colitis (UC) are severe and difficult to treat diseases of the digestive tract. Despite considerable progress in the field, the pathophysiology of IBD remains unclear, while its prevalence continues to rise. The most important determinant of the clinical course of IBD is the balance, levels, and ratios of proinflammatory, anti-inflammatory, and immunomodulating factors.

Inflammatory reactions localized in the bowel wall may penetrate the surrounding visceral adipose tissue. Imaging methods provide evidence for hypertrophy of the mesenteric adipose tissue in patients with CD[1-3]. Moreover, submucosal fat deposition (fat halo sign) in the bowel is observed in both CD and UC patients[4]. The anatomic proximity of the bowel and visceral fat favors the activation of adipocytes. Visceral adipose tissue is not only an energy storage site, but is also an active endocrine organ. Activated visceral adipocytes secrete many mediators, cytokines and adipokines, such as leptin, adiponectin, resistin, chemerin, and visfatin. Adipokines influence the immunologic system of the gastrointestinal tract, in some cases worsening IBD by amplifying inflammation via the secretion of proinflammatory interleukins, tumor necrosis factor alpha (TNF-α) and adhesion factors. Visfatin is an example of an adipokine that increases the epithelial expression of TNF-α, interleukin (IL)-l, IL-6, and adhesion molecules[5,6]. Furthermore, the adipokines visfatin, chemerin, and resistin have increased tissue expression in many acute and chronic inflammatory diseases[6-9].

The significance of adipokines for IBD pathophysiology is the subject of intensive research, as on one hand the adipokine levels might serve as an index of inflammatory activity and on the other hand, inhibition of specific adipokines could expand the spectrum of therapeutic interventions for this disease. The aim of the present study was to determine the serum concentrations of tumor growth factor (TGF)-β1, adiponectin, leptin, chemerin, resistin, and visfatin in patients with active CD and UC in the pre-treatment stage and after three months of anti-inflammatory and/or immunosuppressive therapy.

All subjects completed the study. The serum concentrations of adiponectin, TGF-β1, and chemerin were not different between subjects with active IBD and healthy controls, and there was no effect of treatment (Table 2). Baseline serum leptin concentrations were significantly lower in subjects with CD and UC compared with healthy controls (P < 0.05), and significantly increased in CD subjects after three months of treatment (P < 0.05). Baseline serum resistin concentrations were significantly higher in CD and UC subjects compared with healthy controls (P < 0.05), and treatment induced a decrease in levels only in UC subjects (P < 0.05). Baseline serum concentrations of visfatin were also significantly higher in subjects with CD and UC compared to healthy controls (P < 0.05). Treatment induced a decrease in the serum visfatin concentration only in CD subjects (P < 0.05), but the post-therapy level of this adipokine remained significantly higher than that in healthy controls (P < 0.05) (Table 2). There were no correlations detected between any of the adipokines examined or TGF-β1 and the clinical severity of IBD, time from initial diagnosis, age, waist or hip circumference, or BMI (Table 3).

Although the etiology of IBD remains unclear, the most prevalent hypothesis is that the disease develops due to a genetic predisposition and a hyperactive gut-associated immune response to food allergens and bacteria. Individuals with IBD exhibit hypertrophy of the mesenteric adipose tissue lying in direct proximity to the inflamed bowel as well as alterations in local or serum adipokine concentrations[1].

Adipokines have both pro- and anti-inflammatory effects in IBD patients. Adiponectin is one of the most richly represented adipokines in the blood, exerting many metabolic effects[12-14]. It also has anti-inflammatory effects by increasing the synthesis of an interleukin receptor antagonist and decreasing the dendritic cell release of interferon gamma. Moreover, adiponectin enhances phagocytosis by human macrophages[15,16]. In the present study, no statistically significant increase in serum adiponectin concentration was found in either type of IBD, indicating strong adaptive mechanisms of adiponectin homeostasis.

Leptin, one of the most well studied adipokines, exerts many metabolic, endocrine, and immunologic effects. This adipokine stimulates the proliferation of naïve T-helper lymphocytes, but inhibits the proliferation of T-memory lymphocytes and subsequent production of cytokines[17]. In addition, leptin activates phagocytosis by macrophages[15] and regulates proinflammatory immune responses[18]. The adipose tissue mass is the most important determinant of serum leptin concentration. In our study, despite similar BMI in the three groups of subjects, serum leptin levels were decreased in subjects with both types of IBD, compared with healthy controls. Low leptin concentrations in CD and UC subjects, which are consistent with previous studies[19], may be the result of TNF-α hyperactivity. TNF-α stimulates the temporary release of substantial amounts of leptin in response to inflammation eventually leading to a decrease in leptin-mediated chronic inflammation. Moreover, serum leptin levels increase in CD subjects treated with the TNF-α antagonist infliximab confirming the role of TNF-α in the regulation of leptin release by adipocytes[20]. Similarly, our study showed that a 3-month treatment period with corticosteroids alone or with azathioprine leads to increased blood leptin concentrations, which is particularly noticeable in CD subjects. Interestingly, while some studies report increased expression of leptin mRNA in the mesenteric adipose tissue or increased concentrations of leptin in large bowel lavage fluid from patients with mild to severe CD or UC[3,21], leptin is not detected in normal colonic epithelium. However, it is detected in the subapical region of the epithelial cells during conditions of inflammation[21]. This finding explains the differences between leptin concentrations measured in the serum, tissue, or gut lumen.

Recent studies suggest that TGF-β1 decreases symptoms of IBD by suppressing the immunologic system in the gut and the bowel epithelium through inhibition of immature dendritic cells[22,23]. In our study, TGF-β1 serum concentrations were not significantly different in IBD subjects versus healthy controls, and anti-inflammatory treatment had no effect on TGF-β1 serum levels. These data indicate that any potential role of TGF-β1 in IBD is limited to the mucosa and potential changes in the local concentration of this cytokine cannot be monitored in the systemic circulation.

Chemerin acts as either an activator (initiating stage) or inhibitor (later stages) of inflammation[24,25]. Weigert et al[8] reported elevated serum concentrations of chemerin in IBD patients, especially those with CD. Surprisingly, the highest levels of this adipokine were observed in patients with the non-active form of CD. However, results from this study failed to show significant differences in the baseline chemerin concentrations, though serum levels tended to be higher in IBD patients before treatment.

Resistin is a proinflammatory agent, as it stimulates the synthesis and secretion of TNF-α, IL-12, and adhesive factors upon nuclear factor kappa B activation[26]. Expression of resistin in the mesenteric adipose tissue is elevated in patients with severe CD compared to those with colon cancer[27]. Adipose tissue is not a unique source of resistin as it is also found in mononuclear blood cells, macrophages, and stem cells[28]. The production of resistin by mononuclear cells may be increased by lipopolysaccharides, TNF-α, IL-1, and IL-6[29]. Karmiris et al[19] reported elevated serum levels of resistin in IBD patients, irrespective of disease type. Consistent with these findings, results of this study found significantly higher serum resistin concentrations in both CD and UC subjects compared to healthy controls. Resistin levels returned to baseline levels following three months of treatment in UC, but not CD, subjects.

Adipose tissue produces large amounts of visfatin. Apart from its role in the regulation of insulin receptor sensitivity[30], visfatin also inhibits apoptosis in a neutrophilic response to inflammatory factors[31], and induces the synthesis of interleukins by CD14+ monocytes[5]. Increased serum concentrations of visfatin have been detected during active IBD symptoms[9] and correlate with indices of IBD activity[32]. The findings of the present study confirm this role of visfatin in IBD, as levels were significantly higher in CD and UC subjects compared with healthy controls. Moreover, three months of treatment significantly lowered visfatin levels in CD subjects, indicating that azathioprine-mediated immunosuppression affects the visfatin level.

In summary, bowel inflammation is responsible for elevation of serum levels of some adipokines (resistin, visfatin), while others are not significantly altered (adiponectin, chemerin) or are even suppressed (leptin). In general, the treatment of IBD led to a normalization of adipokine serum levels, though it is unknown if the improvement in leptin and visfatin levels observed only in CD patients is related to disease specificity or the use of azathioprine. Overall, the findings suggest that adipokines are involved in the pathogenesis of IBD. However, the lack of a direct correlation between serum levels and IBD activity implies that adipokines are modulators rather than determinants of IBD severity.