PLoS One. 2013 Oct 24;8(10):e78495. doi: 10.1371/journal.pone.0078495.

Differences in visceral fat and fat bacterial colonization between ulcerative colitis and Crohn’s disease. An in vivo and in vitro study.

Zulian A, Cancello R, Ruocco C, Gentilini D, Di Blasio AM, Danelli P, Micheletto G, Cesana E, Invitti C.

Diabetes Research Laboratory, Istituto Auxologico Italiano, Milan, Italy.

 

ABSTRACT:

Crohn’s disease (CD) is notably characterized by the expansion of visceral fat with small adipocytes expressing a high proportion of anti-inflammatory genes. Conversely, visceral fat depots in ulcerative colitis (UC) patients have never been characterized. Our study aims were a) to compare adipocyte morphology and gene expression profile and bacterial translocation in omental (OM) and mesenteric (MES) adipose tissue of patients with UC and CD, and b) to investigate the effect of bacterial infection on adipocyte proliferation in vitro. Specimens of OM and MES were collected from 11 UC and 11 CD patients, processed and examined by light microscopy. Gene expression profiles were evaluated in adipocytes isolated from visceral adipose tissue using microarray and RTqPCR validations. Bacteria within adipose tissue were immuno-detected by confocal scanning laser microscopy. Adipocytes were incubated with Enterococcus faecalis and cells counted after 24 h. Morphology and molecular profile of OM and MES revealed that UC adipose tissue is less inflamed than CD adipose tissue. Genes linked to inflammation, bacterial response, chemotaxis and angiogenesis were down-regulated in adipocytes from UC compared to CD, whereas genes related to metallothioneins, apoptosis pathways and growth factor binding were up-regulated. A dense perinuclear positivity for Enterococcus faecalis was detected in visceral adipocytes from CD, whereas positivity was weak in UC. In vitro bacterial infection was associated with a five-fold increase in the proliferation rate of OM preadipocytes. Compared to UC, visceral adipose tissue from CD is more inflamed and more colonized by intestinal bacteria, which increase adipocyte proliferation. The influence of bacteria stored within adipocytes on the clinical course of IBD warrants further investigations.

PMID: 24205244

 

SUPPLEMENT:

In 1932 in the original description of Crohn’s disease (CD), the authors described a hypertrophic adipose tissue surrounding the inflamed intestinal areas. This fat wrapping around the intestinal wall (creeping fat) is detectable early at the onset of the disease and is used as anatomical marker of active lesions by surgeons. Surgeons however describe that all visceral adipose tissue (VAT) depots of CD patients, including the omentum which is anatomically distant from the intestinal lesions, appear hypertrophic similarly to what occurring in obese patients. This observation prompted us to investigate whether there are morphological and molecular differences between VAT of CD and obese patients, and what pathogenic agent might induce a generalized VAT expansion in normal-weight patients as those with CD. Using morphological and microarray analyses, we demonstrated that in patients with active CD, omental fat shows the same inflammatory characteristics of creeping fat, and compared to obese patients, both VAT depots are composed by definitely smaller adipocytes expressing a higher proportion of anti-inflammatory genes (2). These findings led us to hypothesize that in CD, VAT expansion might be a protective phenomenon aimed at controlling the inflammatory response and preventing dissemination of intestinal bacteria. Therefore, we decided to assess whether intestinal bacteria are detectable in VAT of CD. Furthermore, given that VAT of patients with the other main inflammatory bowel disease (IBD), the ulcerative colitis (UC) is not macroscopically hypertrophic as in CD, we compared VAT characteristics of UC and CD patients.

At morphological and molecular level, omental and mesenteric fat of UC appeared composed by small adipocytes and inflamed, although to a lesser degree, than in CD. In particular, visceral fat depots of UC had a lower expression of inflammatory genes, and specifically those related to bacterial response. Prompted by this observation, we examined whether intestinal bacteria were detectable in VAT of the two IBD. We demonstrated that intestinal bacteria are detectable in vivo in mesenteric and omental adipose tissue with a greater perinuclear positivity in CD than in UC adipocytes. Interestingly, the in vitro bacterial infection with Enterococcus faecalis induced an increase in the proliferation rate of omental preadipocytes and adipocytes both, suggesting that in conditions such as bacterial load, adipocytes may change their phenotype and reconvert into stromal-vascular cells. The greater bacterial translocation occurring in CD than in UC is in agreement with the characteristics of the intestinal lesions which are transmural in CD but not in UC, and might explain the exclusive presence of VAT expansion in CD.

In our working hypothesis, bacteria seem to be able to cross an altered intestinal barrier, colonize first the adjacent mesenteric fat and then, through local blood circulation, reach omentum and probably other intra-abdominal organs. As a consequence, inflammatory cells (i.e. macrophages, T lymphocytes) are recruited in VAT depots of IBD patients inducing a further release of pro and anti-inflammatory cytokines by adipocytes and altering their physiology.

Is our initial hypothesis of a hypertrophic “protective” VAT in CD supported by these data? Now we know that intestinal bacteria may reach VAT and localize around and/or within adipocytes. VAT is therefore a reservoir of bacteria, however it is undefined whether this is a protective phenomenon. The small size of adipocytes composing VAT of IBD identifies a peculiar inflammatory condition markedly different from that of obese patients, where inflammation is associated to adipocyte hypertrophy, and likely reflects beneficial changes.

These results pave the way to the development of novel therapeutic strategies for all conditions associated with altered gut permeability (intestinal inflammation, dysbiosis or mucus alteration) that allow translocation of bacteria into the abdominal cavity.

Reference

1. Zulian A, Cancello R, Micheletto G, Gentilini D, Gilardini L, Danelli P, Invitti C. Visceral adipocytes: old actors in obesity and new protagonists in Crohn’s disease? Gut. 2012;61(1):86-94.

 

Contact:

Cecilia Invitti
Department of Medical Sciences and Rehabilitation, Istituto Auxologico Italiano
Via Ariosto 13, 20145 Milan, Italy.
Telephone: +3902619112535 Fax: +3902619112541
e-mail: invitti@auxologico.it
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