Angiotensin-(1-7) Attenuates Protein O-GlcNAcylation in the Retina of Mice Fed a High-Fat Diet

Increased retinal protein O-GlcNAcylation occurs in response to hyperglycemia and contributes to diabetic retinopathy (DR). Alternatively, systemic pharmacological blockade of the Renin-Angiotensin System (RAS) lowers the incidence of DR. The classic RAS that systemically regulates blood pressure is defined by the ACE/AngII/AT1R axis; this is counter-regulated by actions of the ACE2/Angiotensin-(1-7) (Ang1-7)/Mas axis of the RAS. Increased endogenous production of Ang1-7 is responsible for many of the beneficial effects of cardioprotective RAS blockers such as ACE inhibitors; however the mechanism whereby DR outcomes are improved remains unresolved. The hypothesis here is that Ang1-7 acts to reduce retinal O-GlcNAcylation by activating Protein Kinase A (PKA) and thereby inhibiting the rate-limiting enzyme of the hexosamine biosynthetic pathway, glutamine-fructose-6-phosphate amidotransferase (GFAT). Mice fed a 60% high fat diet (HFD) for 11 weeks and treated with the ACE inhibitor captopril (50 mg/L) for 3 weeks exhibited a Mas receptor-dependent attenuation of retinal protein O-GlcNAcylation as compared to vehicle treated controls. To determine the mechanism by which Ang1-7 regulates O-GlcNAcylation, R28 retinal neurons were treated with Ang1-7 and the adenylate cyclase activator forskolin. Both Ang1-7 and forskolin increased cAMP concentrations and attenuated levels of protein O-GlcNAcylation induced by treatment with the O-GlcNAcase inhibitor thiamet G. Classically, cAMP activates PKA, which phosphorylates and inhibits GFAT. Overall, these studies establish the first link between dysregulation of the well-studied physiological RAS and diabetes-induced retinal tissue damage as a consequence of excessive protein O-GlcNAcylation.