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Mechanisms underlying somatostatin receptor 2 down‐regulation of vascular endothelial growth factor expression in response to hypoxia in mouse retinal explants
Author(s) -
Mei Sara,
Cammalleri Maurizio,
Azara Danilo,
Casini Giovanni,
Bagnoli Paola,
Dal Monte Massimo
Publication year - 2012
Publication title -
the journal of pathology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.964
H-Index - 184
eISSN - 1096-9896
pISSN - 0022-3417
DOI - 10.1002/path.3006
Subject(s) - stat3 , vascular endothelial growth factor , hypoxia (environmental) , somatostatin , endocrinology , neovascularization , angiogenesis , octreotide , medicine , biology , chemistry , cancer research , microbiology and biotechnology , signal transduction , organic chemistry , oxygen , vegf receptors
Abstract Hypoxia is a trigger of VEGF expression, the primary cause of retinal pathologies characterized by neovascularization. During hypoxia, transcription factors such as STAT3 and HIF‐1 promote the increase in VEGF expression. Octreotide, a somatostatin receptor 2 (sst 2 )‐preferring agonist, reduces retinal VEGF expression and neovascularization. To investigate the intracellular pathways linking sst 2 activation to the inhibition of hypoxia‐induced VEGF up‐regulation, we used pharmacological approaches and siRNA in mouse retinal explants cultured in normoxia or hypoxia. In hypoxic explants in which STAT3 or HIF‐1 was inhibited, we observed the existence of reciprocal interactions between STAT3 and HIF‐1, which synergistically induced VEGF expression. Octreotide prevented hypoxia‐induced activation of STAT3 and HIF‐1, and the downstream increase in VEGF expression, as evaluated in hypoxic explants treated with pharmacological inhibitors of STAT3 or HIF‐1 and in normoxic explants in which pharmacological activators of STAT3 or HIF‐1 were used to mimic a hypoxia‐like response. The effect of octreotide on STAT3 activation is in part indirect, through the blockade of VEGFR‐2 phosphorylation. The effect of octreotide on STAT3, HIF‐1, VEGFR‐2, and VEGF required Src homology region 2 domain‐containing phosphatase 1 (SHP‐1). In hypoxic extracts, octreotide induced SHP‐1 phosphorylation and activation, and inhibiting SHP‐1 abolished the octreotide effect on STAT3, HIF‐1, VEGFR‐2, and VEGF. The central role of SHP‐1 in the modulation of STAT3 and HIF‐1 was confirmed in normoxic explants in which pharmacologically activated SHP‐1 prevented the effect of STAT3 or HIF‐1 activation. Immunohistochemical studies showed that under hypoxia sst 2 and VEGF are expressed by retinal vessels, thus indicating a possible direct effect of octreotide on VEGF‐containing endothelial cells. These data clarify the mechanism by which octreotide prevents hypoxia‐induced VEGF up‐regulation and support the effectiveness of octreotide in treatment of oxygen‐induced retinopathies. These results may have implications in designing therapies targeting STAT3 and/or HIF‐1 aimed at preventing retinal neovascularization. Copyright © 2012 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.