mTORC1-Independent Reduction of Retinal Protein Synthesis in Type 1 Diabetes

Abstract

Poorly controlled diabetes has long been known as a catabolic disorder with profound loss of muscle and fat body mass resulting from a simultaneous reduction in protein synthesis and enhanced protein degradation. By contrast, retinal structure is largely maintained during diabetes despite reduced Akt activity and increased rate of cell death. Therefore, we hypothesized that retinal protein turnover is regulated differently than in other insulin-sensitive tissues, such as skeletal muscle. Ins2Akita diabetic mice and streptozotocin-induced diabetic rats exhibited marked reductions in retinal protein synthesis matched by a concomitant reduction in retinal protein degradation associated with preserved retinal mass and protein content. The reduction in protein synthesis depended on both hyperglycemia and insulin deficiency, but protein degradation was only reversed by normalization of hyperglycemia. The reduction in protein synthesis was associated with diminished protein translation efficiency but, surprisingly, not with reduced activity of the mTORC1/S6K1/4E-BP1 pathway. Instead, diabetes induced a specific reduction of mTORC2 complex activity. These findings reveal distinctive responses of diabetes-induced retinal protein turnover compared with muscle and liver that may provide a new means to ameliorate diabetic retinopathy.

Phosphatase control of 4E-BP1 phosphorylation state is central for glycolytic regulation of retinal protein synthesis

“…The present results suggest that Akt (principally Akt1 and Akt3) regulates retinal protein synthesis and that inhibition of glycolysis influences protein synthesis through a mechanism involving Akt but independent of mTORC1. Of note, these results in normal ex vivo retinas compare closely with the reduction of Akt and mTOR activities we observed in retinas from diabetic rats and mice (13,39) and the lack of effect of diabetes on retinal mTORC1 activity. These results also strongly show the relevance of the ex vivo retina method for the investigation of in vivo pathophysiological mechanisms of disease.…”

“…The retina has a high rate of glucose uptake, and 90% of glucose is metabolized to lactate by glycolysis (49,50), providing the majority of retinal ATP. This high rate of glucose uptake is in line with a high anabolic activity, as reflected by the rate of protein synthesis, which is approximately twice that of skeletal muscle (13), and a high basal rate of Akt activity compared with muscle and liver (38). The present results suggest that Akt (principally Akt1 and Akt3) regulates retinal protein synthesis and that inhibition of glycolysis influences protein synthesis through a mechanism involving Akt but independent of mTORC1.…”

“…However, although 4E-BP1 dephosphorylation starts later than ATP depletion, a direct correlation between ATP loss and 4E-BP1 dephosphorylation was not observed (control: r 2 ϭ 0.7, 2-DG: r 2 ϭ 0.75; both nonsignificant). Both 4E-BP1 and S6 showed different susceptibilities to glycolysis and Akt inhibition, so we directly analyzed total (mTORC1 and mTORC2) mTOR kinase activity per the method used in our recently published study (13). Total mTOR activity was significantly reduced by ϳ25% by 2-DG treatment, (Fig.…”

“…As a consequence, diabetes-induced reduction of retinal insulin receptor and Akt activity (37, 39) may lead to reduced biosynthetic capacity essential for retinal physiology and thus compromise cell viability. In addition, altered metabolite profiles in the retina during diabetes may further disturb essential anabolic pathways and limit adaptive mechanisms (1).We (13,14) showed recently that both retinal insulin receptor signaling and excess glucose concentrations regulate retinal cell death and protein synthesis in diabetic rats and mice, thus revealing a clear interplay between growth factor and nutrient regulation of cell viability. In the present study, we undertook a specific assessment of the regulatory function of the Akt/mTOR pathway…”

“…We (13,14) showed recently that both retinal insulin receptor signaling and excess glucose concentrations regulate retinal cell death and protein synthesis in diabetic rats and mice, thus revealing a clear interplay between growth factor and nutrient regulation of cell viability. In the present study, we undertook a specific assessment of the regulatory function of the Akt/mTOR pathway and glycolysis on retinal protein synthesis.…”