Hemin exerts multiple protective mechanisms and attenuates dextran sulfate sodium-induced colitis

Inflammatory bowel disease (IBD) is characterized by recurrent and severe gastrointestinal inflammation. Activation of inflammatory cells, such as TH17 lymphocytes, and/or deficiency of regulatory T cells (Treg) are responsible for the pathogenesis of IBD. As an acute phase reactant, heme oxygenase-1 (HO-1) has been shown to play an anti-inflammatory and immunomodulatory role in many disease processes. In this study, we used a dextran sulfate sodium (DSS)-induced murine colitis model to investigate the effect of upregulating HO-1 by hemin on the development of colonic inflammation.

The mice were enterically challenged with 4% DSS. In addition, some mice were intraperitoneally administered with hemin or Sn-protoporphyrin (SnPP) on days 0, 1, and 6 after DSS treatment. The severity of colitis was evaluated by daily monitoring of weight change and diarrhea. At the end of the experiment, the colon, spleen, and mesenteric lymph nodes were harvested for histology and various immunological assays.

Compared to control groups, DSS challenge markedly induced HO-1 expression in the colon epithelium. Upregulation of HO-1 by hemin was further correlated with attenuation of DSS-induced colitis. In contrast, inhibition of endogenous HO-1 by SnPP aggravated the colitis. To further assess the anti-inflammatory mechanisms, we examined whether hemin enhanced the proliferation of Treg cells and suppressed the production of interleukin (IL)-17. Flow cytometry analysis revealed that hemin markedly expanded the CD4 + CD25 + Foxp3+ Treg population. Moreover, hemin attenuated IL-17 and TH17-related cytokines. This inhibition coincided with the attenuation of DSS-induced colitis. Finally, terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick end-labeling assay showed that hemin treatment markedly reduced programmed cell death of colonic epithelium, indicating that hemin exerts a modulatory effect on the induction of Treg, IL-17, and apoptosis.

These results demonstrate that upregulation of HO-1 by hemin ameliorated experimental colitis. Moreover, our study suggests a broader protective mechanism of hemin.