BOB Ped Surg 2023 - Maria Tecos, AAP - Presentation

Hello and thank you for the opportunity to share our work. I have no disclosures. You're on call and you get consulted on a 14-day ex-31-week male with a distended tender abdomen, skin changes and free air on abdominal X-ray. You suspect necrotizing enterocolitis, take him to the OR and discover a massive amount of threatened bowel. These fragile patients are the driving force for the investigative efforts of our lab. Short gut syndrome has the potential to result in intestinal failure that can progress to metabolic derangements and death. These patients face a 25% mortality within two years, and approximately 25% of survivors go on to have intestinal transplantation, but less than 50% ever regain enteral autonomy. TPN is a significant contributor to liver injury in short gut syndrome, but our murine small bowel resection model doesn't use TPN, and we found that intestinal loss alone is sufficient to mount oxidative stress that results in liver fibrosis. A question then became why and how does this happen? We know that different parts of intestine specialize in different things, so we began to consider the potential role of enterohepatic circulation and bile acid recycling. This led us to question if removing the ileum where bile acids are reabsorbed would have a different liver injury phenotype compared to removing the jejunum. We designed an experiment comparing mice that underwent sham, 50% proximal and 50% distal small bowel resection. All mice were fed liquid diet for 10 weeks after operation. At that time, liver injury, bile acid speciation, and bile acid metabolism were assessed. When we analyzed serum from these mice, we found evidence of liver injury as early as post-operative week two in the proximal resection mice, which persisted to post-operative week 10. Liver injury was only found in the proximal resection mice, giving us early evidence that removing the ileum may be protective. We then evaluated the level of liver fibrosis with Sirius red staining and expression of collagen 1A1, both of which were present at higher levels in proximal resection mice with distal resection again showing liver protection. Next, we examined inflammatory markers of oxidative stress as characterized by tumor necrosis factor alpha, NADPH oxidase and glutathione synthetase. These were all increased in the proximal resection group compared to sham and distal resection. The proximal resection group also had a compensatory increase in an acute phase protein known to increase against oxidative stress called lipocalin 2. To test our theory that the functional differences of the jejunum and ileum in bile acid recycling were driving these inflammatory changes, we verified that removal of the ileum decreased the size of the bile acid pools. This bolstered our notion that the distal resection prevented the recycling of bile acids. Bile acid pool speciation revealed the distal resection to have decreased taurolithocholic acid or TLCA, which is the most toxic, insoluble, hydrophobic bile acid and an increase in the most soluble hydrophilic bile acid tauro ursodeoxycholic cholic acid or Tudka for short. Conversely, the proximal resection bile acid profile was the exact opposite. The recycling and retention of toxic bile acids likely contributes to proximal resection liver fibrosis. We then measured CYP7A1, the rate limiting step of novo bile acid synthesis in the liver and found it to be increased in the distal resection group. This corroborates the idea that removing the ileum interrupts the recycling of toxic bile acids, yielding a healthier overall bile acid pool via new bile acid production, which in turn protects the liver. Of course, this discovery inspired further questions, chiefly if the skewing of the bile acid pool toward a healthier profile protected the liver from fibrosis, then could the administration of healthy bile acids rescue the liver injury phenotype of the proximal resection mice. We repeated the experiment with the supplementation of the healthy bile acid Tudka to the proximal resection mice, which also acts as an endogenous antioxidant and cellular chaperone. The Tudka supplemented group also exhibited significantly less oxidative stress, and the previously observed compensatory increase in lipocalin 2 was absent. Liver fibrosis as measured by Sirius red staining was rescued by Tudka. When we reassess the bile acid speciation after Tudka supplementation, we found that the addition of Tudka did indeed rescue the bile acid profile to a healthier milieu. To sum, the liver injury seen in our proximal small bowel resection model is likely the result of direct liver exposure to oxidative stress and toxic bile acids via enterohepatic circulation. Conversely, removing the ileum where bile acid absorption occurs, shields the liver from inflammation and stimulates healthy new bile acid production, resulting in a unique phenotype that protects the liver. Further, the liver fibrosis observed in proximal small bowel resection is rescued with the supplementation of Tudka. Here, we have shown that manipulation of bile acid recycling can combat the liver injury resulting from small bowel resection. Thank you to my incredible mentor Dr. Warner, my labmates Dr. Steinberger, Courtney and Onifer, Dr. Gauer PhD, our collaborators Drs. Davidson and Rubin, and Dr. Simencovich who generously provided funding for these projects. For further reading, please see your other publications on this topic.