Decreases in rat brain aquaporin-4 expression following intracerebroventricular administration of an endothelin ET B receptor agonist.

Aquaporins (AQPs) comprise a family of water channel proteins, some of which are expressed in brain. Expressions of brain AQPs are altered after brain insults, such as ischemia and head trauma. However, little is known about the regulation of brain AQP expression. Endothelins (ETs), vasoconstrictor peptides, regulate several pathophysiological responses of damaged nerve tissues via ET(B) receptors. To show possible roles of ET(B) receptors in the regulation of brain AQP expression, the effects of intracerebroventricular administration of an ET(B) agonist were examined in rat brain. In the cerebrum, the copy numbers of AQP4 mRNAs were highest among AQP1, 3, 4, 5 and 9. Continuous administration of 500 pmol/day Ala(1,3,11,15)-ET-1, an ET(B) selective agonist, into rat brain for 7 days decreased the level of AQP4 mRNA in the cerebrum, but had no effect on AQP1, 3, 5 and 9 mRNA levels. The level of AQP4 protein in the cerebrum decreased by the administration of Ala(1,3,11,15)-ET-1. Immunohistochemical observations of Ala(1,3,11,15)-ET-1-infused rats showed that GFAP-positive astrocytes, but not neurons, activated microglia or brain capillary endothelial cells, had immunoreactivity for AQP4. These findings indicate that activation of brain ET(B) receptors causes a decrease in AQP4 expression, suggesting that ET down-regulates brain AQP4 via ET(B) receptors.

Koyama Y ，Tanaka K 《-》

Endothelins decrease the expression of aquaporins and plasma membrane water permeability in cultured rat astrocytes.

Some of the aquaporins (AQPs), a family of water channel proteins, are expressed in astrocytes. The expression of astrocytic AQPs is altered after brain insults, such as ischemia and head trauma. However, little is known about the regulation of AQP expressions. Endothelins (ETs), which are vasoconstrictor peptides, regulate several pathophysiological responses of astrocytes. In this study, the effects of ETs on AQP expressions and plasma membrane water permeability were examined in cultured rat astrocytes. Determination of AQP mRNA copy numbers revealed that AQP1 and AQP4 were expressed prominently in cultured astrocytes. ET-1 (100 nM) and Ala¹,³,¹¹,¹⁵-ET-1 (an ETB receptor agonist, 50 nM) decreased the AQP4 and AQP9 mRNA levels in cultured astrocytes, but the AQP1, -3, -5, and -8 mRNA levels remained unchanged. BQ788, an ETB receptor antagonist, reduced the effects of ET-1 on astrocytic AQP mRNAs, whereas FR139317, an ETA antagonist, had no effect. Immunoblot analyses revealed that treatment with ET-1 decreased the protein contents of AQP4 and AQP9 in both total cell lysates and plasma membrane fractions of cultured astrocytes. ET-1 and Ala¹,³,¹¹,¹⁵-ET-1 decreased hypoosmolarity-induced water influxes into cultured astrocytes. In the presence of 30 μM Hg²+, which inhibits water movement through AQP1 and AQP9, the hypoosmolarity-induced water influxes were reduced. Phloretin, an inhibitor of AQP9, did not greatly affect the water influxes. The ET-induced decreases in water influxes were obtained in the presence of Hg²+ and phloretin. These results suggest that ET is a factor that regulates AQP expressions and water permeability in astrocytes.

Tanaka K ，Koyama Y 《-》

I.c.v administration of an endothelin ET(B) receptor agonist stimulates vascular endothelial growth factor-A production and activates vascular endothelial growth factor receptors in rat brain.

Vascular endothelial growth factors (VEGFs), a family of angiogenic factors, are upregulated by nerve injuries. To clarify the extracellular signals involved in VEGF production in the brain, the effects of endothelins (ETs), a family of vasoconstricting peptides, were examined. I.c.v. administration of 500 pmol/d Ala(1,3,11,15)-ET-1, an ET(B) receptor agonist, increased the level of VEGF-A mRNA in the rat cerebrum, whereas those of VEGF-B, placental growth factor (PLGF), angiopoietin (ANG)-1, and ANG-2 mRNAs were not largely affected by Ala(1,3,11,15)-ET. The ET-induced increases in cerebrum VEGF-A mRNA were reduced by coadministration of 1 nmol/d BQ788, an ET(B) antagonist. Ala(1,3,11,15)-ET-1 also stimulated the production of VEGF-A proteins in the cerebrum. Immunohistochemical observations in the cerebrum of Ala(1,3,11,15)-ET-1-infused rats showed that glial fibrillary acidic protein (GFAP)-positive astrocytes had VEGF-A immunoreactivity. Neurons, microglia, and brain capillary endothelial cells in the Ala(1,3,11,15)-ET-1-infused rats did not show VEGF-A reactivity. The i.c.v. administration of Ala(1,3,11,15)-ET-1 stimulated tyrosine phosphorylations of VEGF-R1 and R2 receptors in the rat cerebrum, whereas expression levels of total VEGF-R1 and R2 proteins were not largely changed. Immunoreactivity of tyrosine-phosphorylated VEGF-R1 was selectively shown in GFAP-positive astrocytes in the cerebrum of Ala(1,3,11,15)-ET-1-infused rats. Tyrosine-phosphorylated VEGF-R2 proteins were present in astrocytes and brain capillary endothelial cells. These findings indicate that activation of brain ET(B) receptors increases production of VEGF-A and stimulates VEGF receptor signaling in the brain.

Koyama Y ，Nagae R ，Tokuyama S ，Tanaka K ... -  《-》

Intracerebroventricular administration of an endothelin ETB receptor agonist increases expression of tissue inhibitor of matrix metalloproteinase-1 and -3 in rat brain.

Koyama Y ，Baba A ，Matsuda T 《NEUROSCIENCE》

Production of monocyte chemoattractant protein-1 and cytokine-induced neutrophil chemoattractant-1 in rat brain is stimulated by intracerebroventricular administration of an endothelin ETB receptor agonist.

Koyama Y ，Baba A ，Matsuda T 《NEUROREPORT》