Glucocorticoid receptor inhibits Müller glial galectin-1 expression via DUSP1-dependent and -independent deactivation of AP-1 signalling.

Galectin-1/LGALS1 is a hypoxia-induced angiogenic factor associated with diabetic retinopathy (DR). Recently, we elucidated a hypoxia-independent pathway to produce galectin-1 in Müller glial cells stimulated by interleukin (IL)-1β. Here we revealed glucocorticoid receptor (GR)-mediated inhibitory mechanisms for Müller glial galectin-1/LGALS1 expression. Activator protein (AP)-1 site in the LGALS1 enhancer region, to which activating transcription factor2, c-Fos and c-Jun bind, was shown to be essential for IL-1β-induced galectin-1/LGALS1 expression in Müller cells. Ligand (dexamethasone or triamcinolone acetonide)-activated GR induced dual specificity phosphatase (DUSP)1 expression via the glucocorticoid response element and attenuated IL-1β-induced galectin-1/LGALS1 expression by reducing phosphorylation of these AP-1 subunits following AKT and extracellular signal-regulated kinase (ERK)1/2 deactivation. Moreover, activated GR also caused DUSP1-independent down-regulation of IL-1β-induced LGALS1 expression via its binding to AP-1. Administration of glucocorticoids to mice attenuated diabetes-induced retinal galectin-1/Lgals1 expression together with AKT/AP-1 and ERK/AP-1 pathways. Supporting these in vitro and in vivo findings, immunofluorescence analyses showed co-localization of galectin-1 with GR and phosphorylated AP-1 in DUSP1-positive glial cells in fibrovascular tissues from patients with DR. Our present data demonstrated the inhibitory effects of glucocorticoids on glial galectin-1 expression via DUSP1-dependent and -independent deactivation of AP-1 signalling (transactivation and transrepression), highlighting therapeutic implications for DR.

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