Selective blockade of mGlu5 metabotropic glutamate receptors protects rat hepatocytes against hypoxic damage

Abstract

Western blot analysis of protein extracts from rat liver revealed the presence of the mGlu5 receptor, one of the G-protein-coupled receptors activated by glutamate (named ''metabotropic glutamate receptors'' or mGlu receptors). mGlu5 expression was particularly high in extracts from isolated hepatocytes, where levels were comparable with those seen in the rat cerebral cortex. The presence of mGlu5 receptors in hepatocytes was confirmed by reversetranscription polymerase chain reaction (RT-PCR) analysis, immunohistochemistry in neonate or adult rat liver, as well as by immunocytochemical analysis in HepG2 hepatoma cells, where the receptor appeared to be preferentially distributed in cell membranes. Interestingly, mGlu1 receptors (which are structurally and functionally homologous to mGlu5 receptors) were never found in rat liver or hepatocytes. In hepatocytes exposed to anoxic conditions for 90 minutes, glutamate, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and quisqualate, which all activate mGlu5 receptors, accelerated the onset and increased the extent of cell damage, while 4-carboxy-3-hydroxyphenylglycine (4C3HPG), an agonist of mGlu2/3 receptors, was inactive. 2-Methyl-6-(2-phenyl-1-ethynyl)-pyridine (MPEP), a novel, noncompetitive, highly selective mGlu5 receptor antagonist, not only abolished the toxic effect of 1S,3R-ACPD, but, unexpectedly, was protective by itself against anoxic damage. This suggests that hepatocytes express mGlu5 receptors and that activation of these receptors by endogenous glutamate facilitates the development of anoxic damage in hepatocytes. (HEPATOLOGY 2000; 31:649-655.)Glutamate is the major excitatory neurotransmitter in the central nervous system (CNS), and evidence for peripheral glutamatergic fibers in mammals is still lacking. However, glutamate receptors have been identified in peripheral organs, including taste buds, 1 myenteric plexus, 2 and pancreatic islet cells. 3 Glutamate receptors may either form membrane ion channels (ionotropic glutamate receptors) or couple to guanosine triphosphate-binding proteins (''metabotropic'' glutamate [mGlu] receptors). mGlu receptors form a family of 8 subtypes, which are subdivided into 3 groups on the basis of their structural homology, pharmacological profile, and transduction pathways. Group I includes mGlu1 and -5 receptors, which are coupled to polyphosphoinositide (PI) hydrolysis and activated by 3,5-dihydroxyphenylglycine (DHPG) and quisqualate; group II (mGlu2 and -3) and -III (mGlu4, -6, -7, and -8) mGlu receptors are instead negatively coupled to adenylyl cyclase activity. 4,5 1S,3R-ACPD behaves as a non-subtype-selective agonist, with a preferential activity on group I and -II mGlu receptors. 4 We were intrigued by the finding that 1S,3R-ACPD and quisqualate stimulate PI hydrolysis in cultured hepatocytes, and this effect is reduced by the mixed mGlu receptor antagonist, ␣-methyl-4-carboxyphenylglycine (MCPG). 6 This suggests that either mGlu1 or -5 (or a related unknown receptor subtype) are expressed and...

Expression of the metabotropic glutamate receptor 5 (mGluR5) induces melanoma in transgenic mice

“…In bone cells, glutamate or NMDA application increases NMDA receptor currents (7). Similarly, mGluR5 expression has been observed in many types of cells other than neurons, including astrocytes, hepatocytes, melanocytes, osteoblast cells (8)(9)(10)(11), fibroblast cells (12), and more recently, stem cells (13,14). In astrocytes, mGluR5 triggers intracellular Ca 2+ release that is potentiated by adenosine receptor activation (8).…”

Metabotropic Glutamate Receptors in Peripheral Tissues: Implications for Toxicology

“…In subsequent studies, expression of mGluR5, but not mGluR1, and mGluR3 in rat liver was demonstrated (Do et al, 2007;Storto et al, 2000a). Moreover, mGluR3 were shown to be up-regulated in response to persistent hypoxic status such as fibrotic/cirrhotic conditions in rat liver macrophages, exerting a role in functional metabolism and viability in this tissue (Do et al, 2007), although it has been shown that an agonist of mGluR2/3 had no effect on rat hepatocyte death induced by anoxia (Storto et al, 2000a). On the contrary, endogenous mGluR5 activation is associated with liver damage induced by lipopolysaccharide and d-galactosamine (Jesse et al, 2009) or by acetaminophen in mice (Storto et al, 2003).…”

mGlu receptors in endocrine organs

“…40 mGlu3 receptors were demonstrated to be upregulated in response to persistent hypoxic status such as fibrotic/cirrhotic conditions in the rat liver, exerting protective effects and improving hepatic function, 41 although an agonist of mGlu2/3 receptors had no effect on rat hepatocyte death induced by anoxia. 42 On the contrary, endogenous mGlu5 receptor activation is associated with liver damage induced by lipopolysaccharide and d-galactosamine 43 or by acetaminophen 44 in mice. Storto et al (2000) 42 suggested that mGlu5 receptor activity is induced by glutamate released from rat hepatocytes exposed to anoxic conditions.…”

“…42 On the contrary, endogenous mGlu5 receptor activation is associated with liver damage induced by lipopolysaccharide and d-galactosamine 43 or by acetaminophen 44 in mice. Storto et al (2000) 42 suggested that mGlu5 receptor activity is induced by glutamate released from rat hepatocytes exposed to anoxic conditions. In turn, selective blockade of mGlu5 receptor protects against hepatocyte death induced by hypoxia 45 and oxidative stress 44 in rodents.…”