Immunopharmacology and inflammationProtective effect of 7-O-succinyl macrolactin A against intestinal inflammation is mediated through PI3-kinase/Akt/mTOR and NF-κB signaling pathways
Graphical abstract
Introduction
Inflammatory bowel disease (IBD) is a chronic inflammatory condition of the gastrointestinal tract, encompassing two major types, Crohn׳s disease (CD) and ulcerative colitis (UC) (Elson and Weaver, 2005, Podolsky, 2002). Although its etiology is largely unknown, it is widely accepted that genetic, environmental, and intestinal immune factors interact (Khor et al., 2011). For the majority of IBD patients, nonspecific immunosuppression with aminosalicylates and corticosteroids is considered standard therapy. In patients with severe flare-up who are refractory to steroid therapy and facing the risk of life-threatening complications, fast working immunosuppressants such as cyclosporine have been considered as a choice of therapy (D׳Haens et al., 2001, Lichtiger et al., 1994, Loftus et al., 2003). However, instead of cyclosporine A, which has severe side effects, recent studies have suggested rapamycin, one of the macrolide compounds which has antifungal and immunosuppressant activities, as an effective agent for treatment of refractory IBD (Farkas et al., 2006, Massey et al., 2008, Ogino et al., 2012). Rapamycin inhibits mammalian target of rapamycin (mTOR) and prevents PI3K and Akt signaling in response to growth factors, hormones, and nutrients (Tokunaga et al., 2004).
During the recurrent inflammatory process, attachment and infiltration of leukocytes into the damaged gut epithelium by pro-inflammatory cytokines such as TNF-α is one of the critical steps of inflammation and tissue injury in IBD-inflicted mucosa (Spiik et al., 2002, van Assche and Rutgeerts, 2002), which was also demonstrated in colonic biopsy samples from patients with Crohn׳s disease (Ellis et al., 1998) (Nikolaus et al., 1998, Pallone and Monteleone, 1996, Van Deventer, 1997). TNF-α induces expression of other pro-inflammatory cytokines and adhesion molecules which are required for leukocyte attachment and infiltration through activation of transcription factor NF-κB (Baldwin, 1996, Barnes and Karin, 1997, Chen and Manning, 1995). Activation of NF-κB by TNF-α stimulus relies on phosphorylation-dependent ubiquitination and degradation of inhibitor of κB (IκB) proteins mediated by IκB kinase (IKK) complex (Ghosh and Karin, 2002). In addition, TNF-α has been reported to stimulate activation of the phosphoinositide-3-kinase (PI3K)/Akt pathway which, in turn, phosphorylates the IKK complex (Ozes et al., 1999).
7-O-succinyl macrolactin A (SMA) is a derivative of macrolactin A (MA). The activity of MA is derived from the presence of a large macrocyclic lactone ring and it is known to have anti-biotic and immunosuppressive activities (Gustafson et al., 1989). In addition to its antibacterial activity against vancomycin-resistant enterococci and methicillin-resistant staphylococcus aureus (Kim et al., 2011), MA protects not only T-lymphoblast cells against human HIV viral replication (Gustafson et al., 1989), but also neuronal cells against glutamate toxicity (Kim et al., 1997). SMA has better antibacterial activity and less cytotoxicity than MA (Romero-Tabarez et al., 2006). However, despite its biological activities, the potential of MA and SMA for treatment of IBD has not been fully investigated. In present study, we attempted to determine whether SMA exerts inhibitory effects on intestinal inflammation in vitro and in vivo, and the signaling pathway of SMA action.
Section snippets
Materials
The HT-29 human colon cancer cell line and U937 human monocytic cell line were purchased from American Type Culture Collection (ATCC, Manassas, VA, U.S.). RPMI-1640, fetal bovine serum (FBS), and penicillin-streptomycin solution were purchased from Thermo Scientific HyClone (Logan, UT, U.S.). M199 and Trizol reagent were purchased from Invitrogen (Carlsbad, California, U.S.). TNF-α was purchased from Biosource-Invitrogen (Carlsbad, California, U.S.).
SMA suppresses colon inflammation in vitro and in vivo
Leukocyte recruitment to the mucosa is a critical step in the initiation and perpetuation of intestinal inflammation, and TNF-α is the major pro-inflammatory mediator involved in the pathogenesis of IBD. In order to determine whether SMA inhibits such inflammatory processes, we examined the effect of SMA on TNF-α-induced adhesion of monocytes to colon epithelium using an in vitro co-culture model. Application of TNF-α resulted in a remarkable increase in adhesion of monocytic U937 cells to
Discussion
In the current study, we demonstrated that SMA, a macrolide, induces potent inhibition of intestinal inflammation not only in an in vitro cell adhesion model of monocytes to colonic epithelial cells but also in a rat model of TNBS-induced colitis. More importantly, we demonstrated that oral administration of SMA is more efficacious than 5-ASA, an active metabolite of sulfasalazine, the most commonly prescribed anti-inflammatory agents for treatment of IBD. In addition, SMA also inhibited
Acknowledgement
This work was supported by the 2011 Yeungnam University Research Grant.
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These authors contributed equally to this work.