High-Fat Diet/Palmitate–Induced ER Stress Promotes Protein O-GlcNAcylation in Retina and Retinal Muller Cells

The incidence of type 2 diabetes, the most common cause of diabetic retinopathy (DR), is rapidly on the rise due to the overconsumption of calorie rich diets. Using an animal model of diet-induced obesity/pre-type 2 diabetes, we evaluated the mechanism responsible for increased O-GlcNAcylation of retinal proteins following consumption of a diet high in saturated fat (HFD), as dysregulated O-GlcNAcylation contributes to the development of DR. In the retina of mice fed a HFD for 4 weeks, increased O-GlcNAcylation was observed concomitant with markers of endoplasmic reticulum (ER) stress. Similarly, in TR-MUL retinal cells in culture, addition of the saturated fatty acid palmitate or the ceramide analog Cer6 to culture medium increased protein O-GlcNAcylation and ER stress. One potential mechanism whereby ER stress increases O-GlcNAcylation is by upregulating flux through the hexosamine biosynthetic pathway (HBP) via increased expression of the rate-limiting enzyme glutamine-fructose-6-phosphate amidotransferase (GFAT). In the retina of mice fed a HFD and in TR-MUL cells exposed to Cer6, mRNA and protein expression of GFAT2, but not GFAT1 were increased. Similarly, in TR-MUL cells treated with the ER stress inducer thapsigargin (TG) increased O-GlcNAcylation was observed concomitant with an increase in GFAT2. Alternatively, ER stress inhibitor prevented the effect. GFAT2 knockdown via siRNA attenuated O-GlcNAcylation induced by either TG or Cer6. The glial cell specific transcription factor OASIS (old astrocyte specifically-induced substance) plays a central role in coordinating the unfolded protein response. Unlike GFAT1, GFAT2 contains a putative binding site for OASIS. Both HFD and Cer6 increased OASIS expression, and OASIS was sufficient to promote GFAT2 expression and increase O-GlcNAcylation. Overall, the results support a model whereby HFD-induced ER stress acts to promote O-GlcNAcylation in retina through an OASIS/GFAT2-dependent mechanism.