Introduction Vascular adhesion protein (VAP)-1 is an adhesion molecule and potent amine-oxidase. Activation on hepatic sinusoidal endothelial cells (HSEC) leads to H2O2 release, NFκB activation and expression of gut-homing receptor MAdCAM-1, which promotes homing of gut-tropic lymphocytes to the liver. Given the proposed role of this pathway in hepatic disorders complicating inflammatory bowel disease (IBD); we quantified serum (sVAP-1) titre and intrahepatic/colonic enzyme activity in primary sclerosing cholangitis (PSC)/IBD, as well as investigated consequences of activation with variant amine substrates.
Method sVAP-1 was quantified by ELISA in PSC (n = 105); PBC (90); AIH (99); IBD-alone (50) and healthy controls (21). Correlation with clinical outcome was assessed using Cox proportional hazards assumption/KM-estimates. VAP-1 activity was determined (Amplex red assay) in protein lysates extracted from (a) explanted liver (PSC=9; PBC=10, AIH=5, normal donor, n = 10) and (b) colonic resections (n = 7). Putative VAP-1 substrates were selected based on inclusion in the human metabolome database, and induced kinetic rates measured (Michaelis-Menton analysis). Induction of MAdCAM-1 on HSEC was evaluated quantitatively (cell-ELISA) and functionally (flow-adhesion assays).
Results PSC patients had higher sVAP-1 concentration (median 517 ng/mL) than those with AIH (475), PBC (472), IBD-alone (413) and healthy controls (425) (p < 0.001). sVAP-1 tires >530 ng/ml in PSC were associated with significantly worse transplant-free survival (unadj. HR:2.94, p = 0.008), and retained independent predictive value when controlling for other significant risk factors (cirrhosis, ascending cholangitis and liver biochemistry; adj. HR:3.85, p = 0.003). Intrahepatic VAP-1 enzyme activity was significantly greater in PSC (227 pmol H2O2/min/mg protein) compared with PBC (124), AIH (128) and donor liver (109) (p < 0.001), yet comparable to activity in colonic tissue (220; p=n.s.). The substrate associated with highest VAP-1 enzymatic efficiency was cysteamine (kcat/Km:5.4 × 107); an amine secreted by Escherichia. which induces colitis in mice. Cysteamine+TNFa provision resulted in greater MAdCAM-1 expression by HSEC ELISA than that with other substrates or TNFa alone (p < 0.01), with increased a4b7-dependent adhesion under flow (p < 0.01).
Conclusion Elevated levels of circulating sVAP-1 exist in PSC and predictive of poor outcome. Intrahepatic VAP-1 enzyme activity is also increased in PSC and akin to that seen in the colon. The ability of VAP-1 to catabolise amine substrates secreted by gut commensals/enteric pathogens, provides a theoretical link between altered colonic microbiota and mucosal immunity in the pathogenesis of PSC.
Disclosure of interest None Declared.
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