Metabolic regulators of inflammation in acute pancreatitis

Abstract

Acute pancreatitis (AP) is a common inflammatory disease with multiple aetiological triggers, most commonly gallstones and alcohol consumption, but with no specific treatment. One in four patients with AP develop organ dysfunction, requiring critical care support and have a high risk of death. During systemic inflammation, and specifically during AP, there is increased flux through the kynurenine pathway of tryptophan metabolism due to induction of rate-limiting enzymes by inflammatory mediators, leading to elevated circulating 3- hydroxykynurenine (3HK) levels. In clinical AP in humans, plasma 3HK levels correlate with clinical severity. 3HK is hazardous to many cell types, principally through the generation of reactive oxygen species. Production of 3HK can be blocked by inhibiting kynurenine 3- monooxygenase (KMO), the enzyme which catalyses kynurenine hydroxylation. KMO has a prominent role in regulating the systemic inflammatory response during severe AP, but the mechanisms that link metabolism through KMO and systemic inflammation have eluded discovery, until now. In this thesis, I firstly show that the KMO blockade in genetically-altered mice protects against critical illness and improves recovery in an experimental model of severe AP. This more severe model consisted of 2% sodium taurocholate retrograde ductal infusion along with simultaneous implantation of a moderately invasive telemeter device with chest leads, which monitored heart rate, locomotor activity and body temperature. The telemetry system was required to continuously monitor for clinical signs of recovery and deterioration in this potentially lethal model, and because the pancreatic ductal infusion after the device implantation would not be technically feasible, the device had to be implanted at the same time as pancreatitis induction. I discovered a hepatocyte-restricted role for KMO, where mice generated to lack Kmo solely in hepatocytes (Kmoalb-cre) showed elevated plasma kynurenine and 3HK levels, reduced 13C6-3-hydroxykynurenine tracer clearance, and transcriptomic alterations in key innate immunity pathways in liver tissue, specifically modulating expression of canonical toll-like receptor pathway signalling genes. Although Kmoalb-cre mice with elevated 3HK did not significantly differ in pancreas injury metrics, multiple-organ neutrophilia infiltrate or routine biochemistry at an early timepoint (24-hr) using, these mice succumbed fatally earlier and more readily to experimental AP over 7-days using a less severe recovery telemetered model of AP, thus indicating an impaired recovery and increased susceptibility for critical illness. The less severe, recovery model, also utilised 2% sodium taurocholate infusion, but the telemeter device was smaller, lighter and faster to implant and only measured locomotor activity and temperature. Therapeutically reducing 3HK to undetectable levels through systemic blockade using a highly-specific KMO inhibitor rescued the phenotype, protecting against critical illness and early mortality in the 7-day recovery telemetered model of AP. In vitro, interleukin-1β was found to synergise with 3HK to cause cellular apoptosis, a mode of cell death previously shown to occur in multiple organ failure during experimental severe AP by our research group, thereby demonstrating a cytotoxic effect by 3HK and innate immune mediators. Together, these findings establish the KMO product 3HK as a modulator of innate immunity that exhibits a complex interaction with inflammatory cytokines during critical illness to promote excess morbidity and death from multiple organ failure that may be rescued by systemic KMO blockade.