Endogenous and Dietary Sphingolipids as Modulators in Inflammatory Bowel Disease

炎症性肠病（IBD）是一种由先天免疫和适应性免疫功能紊乱引起的慢性疾病

Inflammatory bowel disease (IBD) is a chronic condition caused by disruption of innate and adaptive immune mechanisms that normally maintain gut homeostasis. IBD also predisposes to the development of colitis- associated colon cancer (CAC). Elucidating mechanisms underlying the development and persistence of IBD could lead to new medical strategies to treat IBD and prevent CAC. Metabolism of sphingolipids is a major activity of gut epithelium that becomes dysregulated in inflamed tissues and is implicated in the pathogenesis of both IBD and CAC. Nonetheless, how sphingolipids mechanistically contribute to IBD development is poorly understood. Within enterocytes, sphingosine kinase 1 (SK1) can metabolize endogenous sphingolipids as well as mammalian dietary sphingolipids, leading to the formation of the bioactive molecule sphingosine-1- phosphate (S1P). S1P regulates lymphocyte trafficking and promotes inflammation and carcinogenesis by signaling through its receptors (S1PR1-5) and by activating STAT3 and NFκB. S1P lyase (SPL), an essential enzyme that is highly expressed in healthy enterocytes, irreversibly degrades S1P, keeping gut S1P levels low. However, SK1 is upregulated during inflammation, and SPL activity is hampered by oxidant stress. These changes result in accumulation of S1P. We generated tissue-specific SPLGutKO mice lacking SPL only in enterocytes. SPLGutKO mice have high gut S1P levels and provide a model for investigating S1P's role in colitis. Using both chemical and infectious models of colitis, we found that SPL inactivation in gut epithelium promotes colitis/CAC. We provide additional evidence that SPLGutKO mice exhibit alterations in immune cell trafficking to the gut, breach of the gut epithelial barrier, and profound changes in the metabolic profiles of gut tissues in the absence of an inflammatory stimulus. Specifically, we observed high levels of platelet activating factor (PAF) and depletion of glutathione (GSH) in SPLGutKO mouse intestines. PAF promotes leukocyte recruitment, activation and reactive oxygen species (ROS) formation through activation of the PAF receptor (PAFR). GSH is the main intracellular antioxidant needed to protect gut epithelium against ROS-mediated injury. Thus, the two key metabolic changes we observed in SPLGutKO mice could enhance oxidant stress while rendering the gut defenseless against that stress. Based on our findings, we hypothesize that sphingolipids influence the development of colitis by perturbing PAF and GSH metabolism, thereby altering immune cell trafficking and epithelial barrier integrity. To test this central hypothesis, we propose three Specific Aims: 1) Establish how sphingolipids perturb the gut metabolome; 2) Determine how sphingolipids facilitate immune cell trafficking to the gut; 3) Elucidate how sphingolipids compromise gut epithelial barrier integrity. By determining how sphingolipids influence gut metabolism of PAF and GSH, and testing causal relationships between sphingolipids, PAF, GSH, gut immune cell trafficking, and gut epithelial barrier functions, we will clarify how sphingolipid metabolism promotes colitis and, alternatively, how this pathway can be targeted to treat IBD.