Metabolic regulators of inflammation in acute pancreatitis
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Date
27/07/2020Author
Hayes, Alastair John
Metadata
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.