Dietary Tryptophan Supplementation Attenuates Lipopolysaccharide-Induced Acute Lung Injury in a Murine Model of Colitis

Objectives: Inflammatory bowel disease (IBD) is associated with extraintestinal comorbidities, and lung diseases are widespread manifestations. Respiratory bacterial insult is a common illness that results in acute lung injury (ALI) in critical patients. IBD concurrence with respiratory infection may further exacerbate lung injury. Tryptophan (Try), an essential amino acid, is processed by gut microbiota and produces aryl hydrocarbon receptor (AhR) ligands. These ligands can activate the AhR pathway that exerts anti-inflammatory properties and provides protection against mucosal barrier injury. This study investigated the effects of dietary Try on lipopolysaccharide (LPS)-stimulated ALI in mice with colitis induced by dextran sodium sulfate (DSS). Methods: Mice with colitis were allocated to four groups: (1) ND-Sal: normal diet + DSS + intratracheal saline injection; (2) ND-LPS: normal diet + DSS + intratracheal LPS injection; (3) TD-Sal: Try diet + DSS + intratracheal saline injection; (4) TD-LPS: Try diet + DSS + intratracheal LPS injection. Mice were sacrificed 24 h after the intratracheal injection. Results: Results showed that colitis resulted in a high disease activity index. Following induction of ALI in colitis mice, neutrophil populations and inflammatory cytokine levels in bronchoalveolar lavage fluid increased. Gene expression levels associated with toll-like receptor (TLR)4/nuclear factor (NF)-κB signaling were upregulated, and tight junction proteins decreased in the lungs. Dietary Try supplementation decreased circulating LPS levels, suppressed pulmonary TLR4/NF-κB signaling, upregulated AhR/interleukin-22 expression, attenuated oxidative stress and improved the capillary–epithelial barrier integrity in DSS-treated mice. Conclusions: These findings imply that Try may have potential therapeutic significance in bacterial-induced ALI in a colitis condition.