Both ET(A) and ET(B) receptors are involved in mitogen-activated protein kinase activation and DNA synthesis of astrocytes: study using ET(B) receptor-deficient rats (aganglionosis rats).
Endothelin (ET) is known to be a potent mitogen in astrocytes. However, the contribution and signalling pathway of ET(A) and/or ET(B) receptor to the proliferation of astrocytes remain unclear. We investigated ET-induced DNA synthesis in astrocytes using ET(B) receptor-deficient mutant rats (aganglionosis rats: sl/sl). Western blotting with anti-ET receptor subtype-specific antibodies and Scatchard analysis of binding revealed that ET(B) receptor expression in astrocytes depended on gene dosage (+/+: sl/+: sl/sl=2: 1:0), whereas ET(A) receptor expression was unchanged among the three genotypes. ET-1 (10 nM) stimulated [3H]thymidine incorporation and mitogen-activated protein kinase (MAP kinase) activity not only in +/+ via both ET(A) and ET(B) receptors, but also in sl/sl astrocytes via ET(A) receptor with about half the extent of those observed in +/+ astrocytes. Treatment with pertussis toxin (PTX) suppressed the ET-1-induced increases in the incorporation and MAP kinase activity in +/+, but not sl/sl astrocytes, indicating that the ET(B) receptor-, but not the ET(A) receptor-, mediated pathway to DNA synthesis involves PTX-sensitive G proteins, e.g. Gi and/or Go (Gi/o). In +/+ astrocytes, ET-1 (1 nM) stimulated cAMP accumulation, and the ET(B) receptor-selective agonist IRL 1620 (1 nM) suppressed 10 microM forskolin-induced cAMP accumulation, suggesting Gs coupling to the ET(A) receptor and Gi/o coupling to the ET(B) receptor. On the other hand, ET-1 did not increase cAMP accumulation in sl/sl astrocytes, although ET-1 (1 nM) suppressed the forskolin-induced response, suggesting Gi/o coupling to the ET(A) receptor. Our results suggest the possibility that the selectivity of G protein for ET(A) receptor is changed from Gs to Gi/o in ET(B) receptor-deficient astrocytes.
The International Research Laboratories, Ciba-Geigy Japan, Takarazuka.
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Sasaki
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Hori
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Oda
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Okada
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Takimoto
D000595:Amino Acid Sequence; D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000201:enzymology* / Q000378:metabolism*; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism*; D002478:Cells, Cultured; D004247:DNA / Q000096:biosynthesis*; D004261:DNA Replication / Q000235:genetics; D004195:Disease Models, Animal; D004789:Enzyme Activation; D005260:Female; D006627:Hirschsprung Disease / Q000235:genetics / Q000378:metabolism*; D008297:Male; D008969:Molecular Sequence Data; D051381:Rats; D011922:Rats, Mutant Strains; D044022:Receptor, Endothelin A; D044023:Receptor, Endothelin B; D017466:Receptors, Endothelin / Q000172:deficiency* / Q000235:genetics / Q000502:physiology*
D044022:Receptor, Endothelin A; D044023:Receptor, Endothelin B; D017466:Receptors, Endothelin; D004247:DNA; D017871:Calcium-Calmodulin-Dependent Protein Kinases