Cholangiocytes are key target cells in diseases such as primary biliary cirrhosis and primary sclerosing cholangitis collectively known as cholangiopathies. Cholangiocytes are epithelial cells lining the bile ducts and are responsible for the modification and release of bile into the duodenum. These cells are hormone-responsive and behave in an autocrine fashion (secreting a number of neuroendocrine factors) regulating their own growth and/or apoptosis. During the course of cholangiopathies, cholangiocytes proliferate or are lost (ductopenia) to compensate for the changes in intrahepatic biliary mass. The overall goal of this proposal is to examine the role of biliary histidine decarboxylase (HDC, the enzyme, also expressed by cholangiocytes in the liver, regulating histamine synthesis), histamine synthesis and histamine receptor interaction on cholangiocyte proliferation and/or damage. In support of our proposed objectives, we have previously shown that: (i) histamine (HA) regulates, via its four G-protein coupled receptors (H1-H4), biliary growth/damage;and (ii) the enzyme responsible for histidine to histamine conversion, HDC, is overexpressed in cholangiocytes following extrahepatic bile duct obstruction (BDL). We have also shown that: (i) cholangiocytes express all four subtypes of histamine receptors (HRs);and (ii) H1HR and H2HR stimulate cholangiocyte growth, whereas H3HR or H4HR inhibit biliary growth during cholestasis. In addition to cholangiocytes, histamine is synthesized mainly from mast cells in response to many pathologies and inflammatory diseases. After release histamine is quickly stored or degraded. On the basis of the preliminary and published data, we propose the novel hypothesis that autocrine cholangiocyte release of histamine regulates cholangiocyte response in models of proliferation/damage via HDC and specific histamine receptors. The following specific aims are proposed:
Specific aim 1 : To evaluate the (i) hepatic distribution/expression of HDC in liver sections and purified liver cells including cholangiocytes, hepatocytes and vascular endothelial cells;and (ii) in vivo effects of the modulation of HDC expression on biliary growth/damage in models of a. cholangiocyte proliferation induced by extrahepatic bile duct ligation (BDL);and b. biliary damage (by apoptosis) induced by treatment with the hepatotoxin, carbon tetrachloride (CCl4). The overall goal of this aim is to demonstrate that in models of proliferation (BDL) or damage (CCl4), HDC levels are increased/decreased, respectively, and that alteration of HDC and histamine synthesis will directly affect cholangiocyte proliferative responses.
Specific Aim 2 - To evaluate the specific contribution of histamine receptors during cholangiocyte proliferation/damage and the downstream signaling pathways. To achieve this, we will use pharmacological and molecular approaches to block the specific HRs and to evaluate receptor- specific effects on biliary functions in vitro. The overall goals of this aim are to demonstrate that: (i) our in vivo effects are specific to cholangiocytes;and (ii) to pinpoint the specific histamine receptor responsible for the activation of secondary signaling pathways and effects on cholangiocyte proliferation. These studies will demonstrate that HDC/histamine-mediated effects are regulated by the activation of stimulatory (H1 and H2HRs) and inhibitory (H3 and H4HRs) histamine receptors. At the completion of this project, we expect to have determined the extent and the signaling mechanisms involved through which the proliferation/damage of the biliary epithelium is regulated by the autocrine synthesis of histamine by HDC. The information gained upon the successful completion of these studies are expected to provide important insights into the factors controlling cholangiocyte proliferation/damage, which will ultimately help in the identification of important signaling pathways that can be targeted for the development of therapeutic interventions for chronic cholestatic liver diseases. The study introduces the novel concept that HDC may be an important therapeutic target for the management of biliary disorders. While down-regulation of HDC and histamine synthesis may be important in the inhibition of biliary hyperplasia, overexpression of HDC (leading to enhanced histamine release) may be key for stimulating biliary proliferation in ductopenic conditions associated with biliary damage
The research program proposed herein is significantly relevant to the VA mission and its successful completion will likely benefit Veteran patients. The risk of liver diseases due to alcohol and toxin abuse in US Veterans is increasingly high and is one of the most common reasons for hospitalization, morbidity and mortality in US Veterans. Scientific interest in cholangiocytes (cells lining the biliary tract) is due to the fact that these cells are the target ells in cholangiopathies or diseases like primary biliary cirrhosis (PBC). Management of cholangiopathies (characterized by cholangiocyte proliferation or damage) represents one of the major challenges for the VA. Targeting key players like histamine, that is increased during inflammatory events and may contribute to liver disease progression, during the progression of these diseases will likely lead to new therapies for patients suffering from liver diseases.