The formation of bile by the liver results from apical secretion of bile salts and organic solutes from hepatocytes and chloride, bicarbonate and water from cholagniocytes. This vectorial secretion requires the strict polarization and coordinated regulation of distinct transporters in both cell types. The distribution and activity of essential transporter proteins in hepatocytes (e.g. multi-drug resistance protein 2; mrp2) and cholangiocytes (e.g. cystic fibrosis transmembrane conductance regulator; CFTR) is regulated through cAMP. Decreased activity or distribution of transporters represents a putative basis of many cholestatic liver diseases. In other cell types, PDZ domain-expressing proteins tether membrane proteins to the cytoskeleton and moderate their distribution, retention, clustering and activity within membrane microdomains. In epithelial cells, Ezrin-Radixin-Moesin binding phosphoprotein 50 (EBP50) has been implicated in modulating cAMP-dependent apical transport events. Recent studies in our laboratory demonstrate EBP50 is highly concentrated at the apical domain of hepatocytes and cholangiocytes. Disruption of the EBP50-CFTR interaction in cholangiocytes results in the loss of cAMP activation of CFTR and CFTR-dependent cell volume regulation. A mechanism for EBP50 to amplify these effects on CFTR were revealed in studies that showed EBP50 is capable of oligomerizing both in vitro and in vivo. Hypothesizing that EBP50 serves a pivotal role in the capacity and regulation of bile formation, the proposed studies will (1) characterize the regulation and functional implications of EBP50 oligomerization; (2) delineate the physiologic role of EBP50 in modulating CFTR distribution and activity in cholangiocytes; and (3) compare and contrast the EBP50-CFTR functional paradigm developed cholangiocytes to the functional consequences of the EBP50-mrp2 interaction in hepatocytes. This line of investigation holds great promise in providing novel insights into the mechanisms underlying regulated bile formation and cholestatic liver disease.
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