title: Mechanisms underlying somatostatin receptor 2 down-regulation of vascular endothelial growth factor expression in response to hypoxia in mouse retinal explants.
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.
authors.affiliation: Department of Biology, University of Pisa, 56127 Pisa, Italy.
authors.forename: Sara
authors.initials: S
authors.lastname: Mei
authors.forename: Maurizio
authors.initials: M
authors.lastname: Cammalleri
authors.forename: Danilo
authors.initials: D
authors.lastname: Azara
authors.forename: Giovanni
authors.initials: G
authors.lastname: Casini
authors.forename: Paola
authors.initials: P
authors.lastname: Bagnoli
authors.forename: Massimo
authors.initials: M
authors.lastname: Dal Monte
mesh_terms: D000818:Animals; D015687:Cell Hypoxia / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D015536:Down-Regulation; D020536:Enzyme Activators / Q000494:pharmacology; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000037:antagonists & inhibitors; D007191:Indazoles / Q000494:pharmacology; D051379:Mice; D008810:Mice, Inbred C57BL; D015282:Octreotide / Q000494:pharmacology; D010766:Phosphorylation / Q000502:physiology; D053824:Protein Tyrosine Phosphatase, Non-Receptor Type 6 / Q000378:metabolism; D034741:RNA, Small Interfering / Q000494:pharmacology; D017481:Receptors, Somatostatin / Q000819:agonists / Q000378:metabolism*; D012160:Retina / Q000378:metabolism*; D050796:STAT3 Transcription Factor / Q000037:antagonists & inhibitors; D014162:Transfection; D015854:Up-Regulation; D042461:Vascular Endothelial Growth Factor A / Q000037:antagonists & inhibitors / Q000378:metabolism*; D040301:Vascular Endothelial Growth Factor Receptor-2 / Q000378:metabolism
chemical_list: D020536:Enzyme Activators; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D007191:Indazoles; D034741:RNA, Small Interfering; D017481:Receptors, Somatostatin; D050796:STAT3 Transcription Factor; D042461:Vascular Endothelial Growth Factor A; C090937:3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole; C093733:somatostatin receptor 2; D040301:Vascular Endothelial Growth Factor Receptor-2; D053824:Protein Tyrosine Phosphatase, Non-Receptor Type 6; C509248:Ptpn6 protein, mouse; D015282:Octreotide