Extracellular ATP is a potent signaling factor that modulates a variety of cellular functions through the activation of P 2 purinergic receptors in the plasma membrane. These receptors are widely distributed among different liver cell types, including hepatocytes, cholangiocytes, macrophages, and endothelial cells, but the physiologic roles have not been fully defined. Cells release ATP in response to both osmotic and mechanical stimuli, and one mechanism may involve opening of a channel-like pathway (1, 2). In respiratory epithelia, ATP release stimulated by cytosolic cAMP activates outwardly rectified Cl Ϫ channels coupled to P 2U receptors and enhances Cl Ϫ secretion (3).Recent studies in a model liver cell line support an alternative pathway where increases in cell volume induce conductive ATP efflux. In these cells, removal of extracellular ATP or P 2 receptor blockade prevents both Cl Ϫ channel activation and volume recovery (1). These findings suggest functional interactions between ATP release, P 2 receptor stimulation, and Cl Ϫ channel opening in epithelial secretion and volume regulation.Members of the ATP-binding cassette (ABC) 1 protein family are likely to be relevant to this volume regulatory pathway for two reasons. First, while the molecular basis for the transmembrane ATP conductance has not been established, heterologous expression or up-regulation of ABC family members in some cell models is associated with enhanced electrodiffusional ATP release. In cystic fibrosis respiratory epithelia, cAMP fails to stimulate channel-mediated ATP efflux, a response that is present in native epithelia; CFTR gene transfer restores the ATP conductance (3, 4). In other cell lines, ATP release is proportional to the expression of mammalian and Drosophila Mdr1 P-glycoproteins (5, 6). Second, in some but not all cell types, Mdr1 P-glycoproteins regulate swelling-activated Cl Ϫ currents (I Cl-swell ). Effects include enhancement of I Cl-swell and endowment of Cl Ϫ channel sensitivity to protein kinase C (mdr1 gene transfer) and increase in I Cl-swell for a given hypotonic stress (P-glycoprotein overexpression) (7,8). The cellular mechanisms involved in these responses and the implications for other cell types have yet to be clarified.In hepatocytes, P-glycoproteins transport both amphipathic compounds and phospholipids across canalicular membranes into bile (9, 10). However, the functions of multiple other ABC members present in liver cells are unknown. In light of the putative association of certain ABC proteins with channelmediated ATP and Cl Ϫ transport, we sought to investigate the role of hepatocellular ABC proteins in these processes. Findings in rat HTC hepatoma cells were compared with those in a selected population of HTC cells (HTC-R) that overexpress both endogenous and novel Mdr proteins (11). These studies demonstrate that inhibition of P-glycoprotein transport prevents recovery from swelling and that overexpression of Mdr proteins is associated with enhanced ATP release, volume recovery, and cell surv...
Secretion of anionic endo-and xenobiotics is essential for the survival of animal and plant cells; however, the underlying molecular mechanisms remain uncertain. To better understand one such model system-i.e., secretion of bile acids by the liver-we utilized a strategy analogous to that employed to identify the multidrug resistance (mdr) genes. We synthesized the methyl ester of glycocholic acid (GCE), which readily enters cells, where it is hydrolyzed to yield glycocholic acid, a naturally occurring bile acid. The rat hepatomaderived HTC cell line gradually acquired resistance to GCE concentrations 20-fold higher than those which inhibited growth of naive cells, yet intracellular accumulation of radio-
transporting ATPase: genomic organization, alternative splicing, and of the stimulated cell. Thus, mechanical stimulation of structure/function predictions. Hum Mol Genet 1994;3:1647-1656 isolated hepatocytes, including by microinjection, can 3. Wu J, Forbes JR, Chen HS, Cox DW. The LEC rat has a deletion in the evoke [Ca 2/ ] i signals in the stimulated cell as well as copper transporting ATPase gene homologous to the Wilson disease gene. Nat Genet 1994;7:541-545. in the neighboring noncontacting hepatocytes and bile 4. Sternlieb I, Quintana, N, Volenberg I, Schilsky ML. An array of mitochonduct epithelia. This signaling is mediated by release of drial alterations in the hepatocytes of Long-Evans Cinnamon rats. HEPA-ATP or other nucleotides into the extracellular space. TOLOGY 1995;22:1782-1787 cytes that were not in direct contact with the mechanically to increase formation of ductular bile. This concept of coupling between canalicular and ductular bile formation stimulated cell (thus eliminating the possibility of gap junction communication). Similar increases in [Ca 2/ ] i were seen through ATP merits further investigation. This is an important technical consideration given theParacrine signaling by ATP may be potent, but is it ecoin cholangiocytes that were cocultured with hepatocytes that were being mechanically stimulated. Increases in [Ca 2/ ] i in nomical? This last question is somewhat rhetorical. On one hand, ATP represents a source of cellular energy. On the neighboring cells could be blocked by the purinergic receptor antagonist, suramin; by receptor desensitization with ATP other hand, it may be more metabolically costly to synthesize other signaling molecules. Moreover, efficient mechanisms pretreatment; and by apyrase, which depletes extracellular nucleotides through hydrolysis. The investigators also for retrieval of ATP breakdown products from bile exist in the form of apical ATPases and nucleoside transporters. 12 showed that mechanical stimulation did not cause cell injury, as measured by propidium iodide uptake. Taken together, Although intercellular communication in liver may not be cheap, it probably is not that expensive either. these findings suggest a model in which mechanical stimulation of hepatocytes produces release of ATP, which in turn acts on neighboring hepatocytes or cholangiocytes through STEVEN LIDOFSKY, M.D., Ph.D. activation of purinergic receptors. The potential implicationsGastrointestinal Unit of this model are profound.University of California Although hepatocytes are not generally subjected to direct San Francisco, CA mechanical stimulation (unless one counts liver biopsies), ATP release likely occurs under physiological conditions. He-REFERENCES patocytes are constantly subjected to osmotic stress as a con- released from the canaliculus into bile could act on purinergic 12. Che M, Ortiz DF, Arias IM. Primary structure and functional expression receptors on cholangiocytes. The consequent stimulation of of a cDNA encoding the bile canalicular purine-specific Na / -n...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.