Somatostatin protects photoreceptor cells against high glucose–induced apoptosis

This is an open-access article distributed under the terms of the Creative Commons Attribution License.-- et al.[Purpose]: Many cellular and molecular studies in experimental animals and early retinal function tests in patients with diabetic retinopathy (DR) have shown that retinal neurodegeneration is an early event in the pathogenesis of the disease. Somatostatin (SST) is one of the most important neuroprotective factors synthesized by the retina: SST levels are decreased in parallel to retinal neurodegeneration in early stages of DR. In this study, we characterized the induction of apoptosis (programmed cell death) in a 661W photoreceptor-like cell line cultured under high glucose (HG) conditions and the effect of SST. [Methods]: A 661W photoreceptor-like cell line and retinal explants from 10-week-old male C57BL/6 mice were cultured under HG conditions and treated with SST. [Results]: Hyperglycemia significantly reduced the cellular viability by increasing the percentage of apoptotic cells, and this effect was ameliorated by SST (p˂0.05). Activation of caspase-8 by hyperglycemia was found in the 661W cells and retinal explants and decreased in the presence of SST (p˂0.05). Moreover, we detected activation of calpain-2 associated with hyperglycemia-induced cell death, as well as increased protein tyrosine phosphatase 1B (PTP1B) protein levels; both had a pattern of cleavage that was absent in the presence of SST (p˂0.05). Treatment of the 661W cells and retinal explants with SST for 24 h increased the phosphorylation of type 1 insulin-like growth factor receptor (IGF-IR; tyrosine 1165/1166) and protein kinase B (Akt; serine 473), suggesting this survival signaling is activated in the neuroretina by SST (p˂0.05). [Conclusions]: This study has provided new mechanistic insights first into the involvement of calpain-2 and PTP1B in the loss of cell survival and increased caspase-8-dependent apoptosis induced by hyperglycemia in photoreceptor cells and second, on the protective effect of SST against apoptosis by the enhancement of IGF-IR-mediated Akt phosphorylation.This work was supported by grants from European Union: (EUROCONDOR project grant agreement number 278,040) and FP7-People-2012-CIG (grant agreement number 333,594), Spanish Ministry of Economy and Competitivity: SAF2012-35562 and SAF2015-65267R cofunded by the Fondos Europeos de Desarrollo Regional de la Unión Europea [FEDER] and Centro de Investigación Biomédica en Red de Diabetes y Enfermedades Metabólicas Asociadas (CIBERdem).Peer Reviewe