Endothelin Receptors in Normal and Cirrhotic Human Liver

Lowell Ling1, Rhoda Kuc1, Janet Maguire1, Neil Davie2, Paul Gibbs3, Graeme Alexander4, Anthony Davenport1. 1University of Cambridge, Clinical Pharmacology, Department of Medicine, CB2 0QQ, United Kingdom, 2Pfizer Ltd, Pulmonary vascular disease specialty care, Europe, KT20 7NS, United Kingdom, 3University of Cambridge, Department of Surgery, CB2 2QQ, United Kingdom, 4University of Cambridge, Department of Medicine, CB2 0QQ, United Kingdom.

Medical treatment of portal hypertension (PH) is currently limited with current drug interventions such as prophylactic propranolol which is only effective in 60% of patients.1 PH is often a consequence of cirrhosis, and acute variceal bleed contributes to the high mortality. Endothelin (ET) axis dysfunction has been implicated in the pathogenesis of cirrhosis and PH with elevated plasma ET reported in patients with the disease.2, 3 Animal studies show that treatment with selective ETA or mixed antagonists can reduce portal vein pressure in normal and CCl4 induced cirrhosis.4 These results suggest significant translational potential as clinical meta-analysis showed that reduction of hepatic vein pressure gradient results in reduced rates of primary and recurrent variceal bleeds.5 ET antagonists are currently used for the treatment of pulmonary hypertension, therefore these drugs have the advantage that they could be quickly repurposed to treat PH. The challenge is to confirm and translate the therapeutic benefits seen in animal studies to humans.

Our aim was to identify the distribution of the ET receptor subtypes in normal or cirrhotic hepatic artery and portal vein by immunohistochemistry and to characterise ET pharmacology in the liver parenchyma using autoradiography and radioligand binding assays. Human tissue was obtained with informed consent and ethical approval. Using confocal microscopy, both ETA-like and ETB-like immunoreactivity localised to the endothelium, smooth muscle and adventitia of normal and diseased hepatic artery (n = 3) and portal vein (n = 5). Using [125I]ET-1, competition binding experiments with BQ788 (ETB selective) and sitaxentan (ETA selective) revealed that ETB is the predominant receptor subtype expressed in both normal and cirrhotic liver parenchyma. Expression of receptor subtypes determined by autoradiography showed that ETB is the main receptor subtype in liver lobules in normal and cirrhotic tissue. Our results are consistent with animal studies and quantify the relative expression of ETA and ETB in normal and cirrhotic liver and vessels.

Table 1. Percentage of ETA:ETB receptors in liver parenchyma determined using subtype selective antagonists.

These results show that normal human liver contains one of the highest densities of ETB receptors when compared with other organs, with a further increase in the ETB:ETA ratio in cirrhosis. Our data provides evidence that blockade of beneficial ETB receptors may account for the hepatic toxicity of mixed ET antagonists. Localization of ET receptors within the human liver and in vitro pharmacology of hepatic artery and portal vein are needed to translate and validate ET receptor antagonists as a potential drug therapy for PH.

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2. Kamath PS, Carpenter HA, Lloyd RV, McKusick MA, Steers JL, Nagorney DM, et al. (2000). Hepatic localization of endothelin-1 in patients with idiopathic portal hypertension and cirrhosis of the liver. Liver Transpl., 6(5), 596-602.

3. Hasegawa T, Kimura T, Sasaki T, Okada A (2001). Plasma endothelin-1 level as a marker reflecting the severity of portal hypertension in biliary atresia. J. Pediatr. Surg. 36(11), 1609-1612.

4. Feng HQ, Weymouth ND, Rockey DC (2009). Endothelin antagonism in portal hypertensive mice: implications for endothelin receptor-specific signaling in liver disease. Am J Physiol Gastrointest. Liver Physiol., 297(1), G27-33.

5. D’Amico, Gennaro, Juan Carlos Garcia-Pagan, Angelo Luca, and Jaime Bosch (2006). Hepatic vein pressure gradient reduction and prevention of variceal bleeding in cirrhosis: a systematic review. Gastroenterology, 131(5), 1611-24.