Eur J Neurosci. 2015 Sep;42(6):2356-70. doi: 10.1111/ejn.13020.
Improvement of cold injury-induced mouse brain edema by endothelin ETB antagonists is accompanied by decreases in matrixmetalloproteinase 9 and vascular endothelial growth factor-A.
- 1Laboratory of Pharmacology, Faculty of Pharmacy, Osaka Ohtani University, 3-11-1 Nishikiori-Kita, Tonda-bayashi, Osaka, 584-8540, Japan.
Brain edema is a potentially fatal pathological state that often occurs after brain injuries such as ischemia and trauma. However, therapeutic agents that fundamentally treat brain edema have not yet been established. We previously found that endothelin ETB receptor antagonists attenuate the formation and maintenance of vasogenic brain edema after cold injury in mice. In this study, the effects of ETB antagonists on matrixmetalloproteinase (MMP)9 and vascular endothelial growth factor (VEGF)-A expression were examined in the cold injury model. Cold injury was performed in the left brain of male ddY mice (5-6 weeks old) for the induction of vasogenic edema. Expression of MMP9 and VEGF-A mRNA in the mouse cerebrum was increased by cold injury. Immunohistochemical observations showed that the MMP9 and VEGF-A were mainly produced in reactive astrocytes in the damaged cerebrum. Intracerebroventricular administration of BQ788 (10 μg) or IRL-2500 (10 μg) (selective ETB antagonists) attenuated brain edema and disruption of the blood-brain barrier after cold injury. BQ788 and IRL-2500 reversed the cold injury-induced increases in MMP9 and VEGF-A expression. The induction of reactive astrocytes producing MMP9 and VEGF-A in the damaged cerebrum was attenuated by BQ788 and IRL-2500. These results suggest that attenuations of astrocytic MMP9 and VEGF-A expression by ETB antagonists may be involved in the amelioration of vasogenic brain edema.
KEYWORDS: astrocyte; blood-brain barrier; brain injury; vascular permeability factors
Brain edema is a pathological state often induced in acute phase of stroke and head trauma, brain tumor, infection and liver failure. Because excess accumulation of water in brain tissue impairs nerve functions and often leads patients to death, management of brain edema formation is clinically important. So, development of novel therapeutic strategies for improvement of brain edema has been required. Many basic investigations were made to clarify mechanisms underlying brain edema formation. From these studies, an important role of astrocytes, a major glial cell in the brain, has been shown. Astrocytes surround most outer surface of brain microvessels by their processes called end-feet, and keep integrity of vascular endothelial cells (Figure). This structure contributes to a large part of blood-brain barrier (BBB). On brain disorders, astrocytes turn their phenotype to “reactive astrpocytes” . Reactive astrocytes produce several soluble factors modulating permeability of brain microvessels, which include VEGF and MMPs. Excess productions of these factors by reactive astrocytes cause disruption of BBB and vasogenic brain edema. We showed that activation of endothelin ETB receptors promote astrocytic activation. Administration of an ETB agonist into rat brain increased number of reactive astrocytes [2,3], while BQ788, an ETB antagonist, reduced induction of reactive astrocytes after brain injury . Moreover, productions of various factors modulating vascular permeability were increased by activation of astrocytic ETB receptors [5,6,7,8,9]. These findings suggest that blockage of astrocytic ETB receptors on brain disorders has beneficial actions to prevent disruption of BBB and brain edema. To verify this strategy, we examined effects of ETB antagonists on disruption of BBB and vasogenic edema formation in a rat model of brain injury. I.c.v. administration of ETB antagonists (BQ788 and IRL2500) improved disruption of BBB and brain edema in rat brain , suggesting beneficial actions of ETB antagonists as an anti-edema drug. Following these findings, the present study was performed to clarify mechanisms underlying anti-brain edema actions of ETB antagonists. MMP9 and VEGF-A are known to be astrocytic factors inducing vasogenic edema. Many studies showed that inhibitors of MMP9 and VEGF-A in brain disorders reduced vasogenic brain edema. However, brain edema is brought from synergistic actions of many edema-inducing factors. So, inhibition of a single factor is thought to be insufficient to prevent brain edema effectively. In contrast, blockage of astrocytic ETB receptors can attenuate actions of multiple edema-inducing factors extensively (Figure). This may be advantage of ETB antagonists as anti-brain edema drugs.
Figure Activation of ETB receptors regulates productions of astrocytic factors inducing vasigenic brain edema. At gliovascular complex, permeability of vascular endothelial cells is regulated under a balance of astrocyte-derived factors. Accompanied with induction of reactive astrocyte, activation of astrocytic ETB receptors (ETB-R) stimulates productions of VEGF-A, MMPs, MCP-1 and CINC-1. Increases in these factors cause disruption of BBB by degradation of basal lamina and tight junctions. In contrast, expressions of angiopoietin-1 (ANG-1) and aquaporin-4 (AQP4) are reduced by ET-1. The altered productions of the astrocytic factors through ETB receptors result in induction of vasogenic edema.
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Yutaka Koyama Ph.D.
Laboratory of Pharmacology,
Faculty of Pharmacy, Osaka Ohtani University,
3-11-1 Nishikiori-Kita, Tonda-bayashi, Osaka 584-8540, Japan