The response to injury, in liver and in epithelium generally, is fibrosis. Chronic or recurrent injury leads to cirrhosis, with both intra and extra hepatic complications which include impaired hepatocellular function and portal hypertension. The key pathogenic event in hepatic fibrosis appears to be transition of resident perisinusoidal cells termed hepatic lipocytes (Ito or stellate cells) from quiescence to an """"""""activated"""""""" state. This process is characterized by production of increased amounts of extracellular matrix and de novo expression of smooth muscle alpha actin, the latter characteristic consistent with their transformation to myofibroblasts. We have recently demonstrated that contractility is a further prominent feature of the activated phenotype, elicited in particular by the endothelins, a group of 21 amino acid peptides which are among the most potent vasoactive substances known. Moreover, we have shown that in injured liver preproendothelin-1 mRNA is elevated. These findings suggest that in fibrosing injury, activated lipocytes (i.e., myofibroblasts) contract fibrous bands and/or may cause perisinusoidal constriction, each contributing to altered sinusoidal blood flow patterns and portal hypertension. In addition to its potent contractile effects on lipocytes, we have obtained evidence that endothelin-1 directly stimulates lipocyte activation. While lipocyte contractility and activation have been viewed as separate events, the current information suggests the two may be linked. Collectively, the data indicate that increased production of endothelin in liver plays a central role in liver injury, contributing to both lipocyte activation and contractility. This model, in which endothelin is a key element, may have relevance to other forms of wound healing. The overall aim of this proposal is to elucidate the regulation of endothelins in liver fibrosis in vivo. Toward this goal we will (1) examine the relative expression of endothelin-1 and endothelin-3 as well as endothelin converting enzyme-1 in normal and injured liver (2) characterize endothelin receptors on lipocytes, investigating the possibility that lipocytes possess a novel receptor subtype (3) examine the effect of exogenous endothelin on the liver by producing a transgenic mouse which will exhibit liver specific expression of endothelin-1 and (4) in experimental liver injury, determine whether endothelin antagonists inhibit on lipocyte activation, hepatic fibrosis or sinusoidal blood flow abnormalities. These studies have direct relevance to human liver disease and are likely to lead to new approaches for the treatment of portal hypertension, hepatic fibrosis and possibly other forms of fibrosing injury.
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