ATP depletion in rat cholangiocytes leads to marked internalization of membrane proteins.

Intrahepatic bile ducts (BD) are a critical target of injury in the postischemic liver. Decreased vascular perfusion causes characteristic changes in the morphology of the ductular epithelia including a loss of secondary membrane structures and a decrease in plasma membrane surface area. Using adenosine triphosphate (ATP) depletion of cultured normal rat cholangiocytes (NRC) to model ischemic ducts, the present studies examined the fate of apical membrane proteins to determine whether membrane recycling might contribute to rapid functional recovery. Apical proteins, including gamma-glutamyl transpeptidase (GGT), Na(+)-glucose cotransporter (SGLT1), and apically biotinylated proteins, were not shed into the luminal space during ATP depletion. Instead, labeling of surface proteins after ATP depletion showed a significant decrease in GGT and SGLT1, consistent with membrane internalization. Similarly, z-axis confocal microscopy of biotinylated apical proteins also showed protein internalization. During ATP recovery, SGLT1 transport activity remained profoundly depressed even after 24 hours of recovery, indicating that the function of the internalized apical proteins is not rapidly recovered. These studies suggest that the membrane internalization in ATP-depleted cholangiocytes is a unidirectional process that contributes to prolonged functional deficits after restoration of normal cellular ATP levels. This sustained decrease in transport capacity may contribute to the development of ductular injury in postischemic livers.

Domain-specific purinergic signaling in polarized rat cholangiocytes.

In cholangiocytes, adenine nucleotides function as autocrine/paracrine signals that modulate ductular ion transport by activation of purinergic receptors. The purpose of these studies was to identify cellular signals that modulate ATP release and nucleotide processing in polarized normal rat cholangiocytes. In Ussing chamber studies, selective exposure of the apical and basolateral membranes to ATP or adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS) stimulated increases in short-circuit current. Apical purinergic receptor agonist preference was consistent with the P2Y(2) subtype. In contrast, basolateral ADP was more potent in stimulating transepithelial currents, consistent with the expression of different basolateral P2 receptor(s). Luminometric analysis revealed that both membranes exhibited constitutive ATP efflux. Hypotonic exposure enhanced ATP release in both compartments, whereas decreases in ATP efflux during hypertonicity were more prominent at the apical membrane. Increases in intracellular cAMP, cGMP, and Ca(2+) also increased ATP permeability, but selective effects on apical and basolateral ATP release differed. Finally, the kinetics of ATP degradation in apical and basolateral compartments were distinct. These findings suggest that there are domain-specific signaling pathways that contribute to purinergic responses in polarized cholangiocytes.

The lipid products of phosphoinositide 3-kinase contribute to regulation of cholangiocyte ATP and chloride transport.

ATP stimulates Cl(-) secretion and bile formation by activation of purinergic receptors in the apical membrane of cholangiocytes. The purpose of these studies was to determine the cellular origin of biliary ATP and to assess the regulatory pathways involved in its release. In Mz-Cha-1 human cholangiocarcinoma cells, increases in cell volume were followed by increases in phophoinositide (PI) 3-kinase activity, ATP release, and membrane Cl(-) permeability. PI 3-kinase signaling appears to play a regulatory role because ATP release was inhibited by wortmannin or LY294002 and because volume-sensitive current activation was inhibited by intracellular dialysis with antibodies to the 110 kDa-subunit of PI 3-kinase. Similarly, in intact normal rat cholangiocyte monolayers, increases in cell volume stimulated luminal Cl(-) secretion through a wortmannin-sensitive pathway. To assess the role of PI 3-kinase more directly, cells were dialyzed with the synthetic lipid products of PI 3-kinase. Intracellular delivery of phosphatidylinositol 3, 4-bisphosphate, and phosphatidylinositol 3,4,5-trisphosphate activated Cl(-) currents analogous to those observed following cell swelling. Taken together, these findings indicate that volume-sensitive activation of PI 3-kinase and the generation of lipid messengers modulate cholangiocyte ATP release, Cl(-) secretion, and, hence, bile formation.

Endogenous ATP release regulates Cl- secretion in cultured human and rat biliary epithelial cells.

P2Y receptor stimulation increases membrane Cl- permeability in biliary epithelial cells, but the source of extracellular nucleotides and physiological relevance of purinergic signaling to biliary secretion are unknown. Our objectives were to determine whether biliary cells release ATP under physiological conditions and whether extracellular ATP contributes to cell volume regulation and transepithelial secretion. With the use of a sensitive bioluminescence assay, constitutive ATP release was detected from human Mz-ChA-1 cholangiocarcinoma cells and polarized normal rat cholangiocyte monolayers. ATP release increased rapidly during cell swelling induced by hypotonic exposure. In Mz-ChA-1 cells, removal of extracellular ATP (apyrase) and P2 receptor blockade (suramin) reversibly inhibited whole cell Cl- current activation and prevented cell volume recovery during hypotonic stress. Moreover, exposure to apyrase induced cell swelling under isotonic conditions. In intact normal rat cholangiocyte monolayers, hypotonic perfusion activated apical Cl- currents, which were inhibited by addition of apyrase and suramin to bathing media. These findings indicate that modulation of ATP release by the cellular hydration state represents a potential signal coordinating cell volume with membrane Cl- permeability and transepithelial Cl- secretion.

Reorganization of cholangiocyte membrane domains represents an early event in rat liver ischemia.

Cholangiocytes contribute significantly to bile formation through the vectorial secretion of water and electrolytes and are a focal site of injury in a number of diseases including liver ischemia and post-transplantation liver failure. Using ischemia in intact liver and adenosine triphosphate (ATP) depletion in cultured cells to model cholangiocyte injury, these studies examined the effects of metabolic inhibition on cholangiocyte viability and structure. During 120 minutes of ischemia or ATP depletion, cell viability and tight junctional integrity in cholangiocytes were maintained. However, both the in vivo and in vitro models displayed striking alterations in the secondary structure of the plasma membrane. After 120 minutes, the basolateral (BL) interdigitations were diminished and the apical (Ap) microvilli were significantly decreased in number. The BL and Ap membrane surface areas decreased by 42 +/- 8% and 63 +/- 2%, respectively. Despite these changes, F-actin remained predominantly localized to the membrane domains. In contrast, in a time course that paralleled the loss of microvilli, the actin-membrane linking protein ezrin progressively dissociated from the cytoskeleton. These studies indicate that cholangiocyte ATP depletion induces characteristic, domain-specific changes in the plasma membrane and implicate alterations in the membrane-cytoskeletal interactions in the initiation of the changes. Pending the re-establishment of the differentiated domains, the loss of specific secondary structures may contribute to impaired vectorial bile duct secretion and postischemic cholestasis.