The activation of retinal HCA2 receptors by systemic beta-hydroxybutyrate inhibits diabetic retinal damage through reduction of endoplasmic reticulum stress and the NLRP3 inflammasome

Abstract

ObjectiveThe role of the hydroxycarboxylic acid receptor 2 (HCA2) in the retinal damage induced by diabetes has never been explored. In this context, the present study highlights an upregulation of retinal HCA2 receptors in diabetic C57BL6J mice. Moreover, we illustrate that HCA2 receptors exert an anti-inflammatory effect on the retinal damage induced by diabetes when activated by the endogenous ligand β-hydroxybutyrate,MethodologySeven-to-10-week-old C57BL6J mice were rendered diabetic by a single intraperitoneal injection of streptozotocin (75 mg/kg of body weight) and monitored intermittently over a 10-week period extending from the initial diabetes assessment. Mice with a fasting blood glucose level higher than 250 mg/dl for 2 consecutive weeks after streptozotocin injection were treated twice a week with intraperitoneal injections of 25-50-100 mg/kg β-hydroxybutyrate.ResultsInterestingly, while the retinal endoplasmic reticulum stress markers (pPERK, pIRE1, ATF-6α) were elevated in diabetic C57BL6J mice, their levels were significantly reduced by the systemic intraperitoneal treatment with 50 mg/kg and 100 mg/kg β-hydroxybutyrate. These mice also exhibited high NLRP3 inflammasome activity and proinflammatory cytokine levels. In fact, the elevated levels of retinal NLRP3 inflammasome activation markers (NLRP3, ASC, caspase-1) and of the relative proinflammatory cytokines (IL-1β, IL-18) were significantly reduced by 50 mg/kg and 100 mg/kg β-hydroxybutyrate treatment. These doses also reduced the high apoptotic cell number exhibited by the diabetic mice in the retinal outer nuclear layer (ONL) and increased the ONL low connexin 43 expression, leading to an improvement in retinal permeability and homeostasis.ConclusionsThese data suggest that the systemic treatment of diabetic C57BL6J mice with BHB activates retinal HCA2 and inhibits local damage.

β-Hydroxybutyrate Oxidation Promotes the Accumulation of Immunometabolites in Activated Microglia Cells

“…The role of BHB in microglia and neuroinflammation has been previously studied in various disease models [ 26 , 36 , 38 , 53 , 54 , 55 , 56 , 57 ]. However, the metabolic effects of BHB on microglia activation and the underlying metabolic reprogramming remain unknown.…”

“…It can act as a signalling molecule by direct binding to the G-protein-coupled receptor GPR109A or as a histone deacetylase (HDAC) inhibitor or indirectly, via oxidative metabolism with the subsequent production of acetyl-CoA and NADH [ 35 ]. Whereas the direct signalling effects of BHB in brain and microglia have been extensively studied in different systems [ 33 , 36 , 37 , 38 , 39 , 40 , 41 ], the metabolism of BHB in microglia has not been previously characterised. This lack of knowledge represents a barrier towards our full understanding of the effects of this nutrient in microglia and in the wider context of brain-related diseases.…”

Interleukin-1β is a potential therapeutic target for periodontitis: a narrative review

“…It has an anti-inflammatory effect on retinal damage. 145 BHB prevents NLRP3 inflammasome activation in both mouse and human neutrophils and reduces urate-crystal-induced inflammation in individuals with gout. 144 Compared with the currently used IL-1β or IL-1 receptor antagonists, MCC950 and BHB are less expensive to produce.…”

Resolvin D1 reduces mitochondrial damage to photoreceptors of primary retinal cells exposed to high glucose

“…This was indicated by a decreased expression of the gap junction integrity marker Cnx43 on the surface of mitochondria. By favoring gap junction integrity, Cnx43 maintains cell survival (Li & Roy, ; Li, Sato, Haimovici, Okamoto, & Roy, ), whereas decrement of retinal Cnx43 provokes derangement of retinal cells in diabetics (Bobbie et al, ; Maisto et al, ; Trotta et al, ). Therefore, a strategic increase in RvD1 bioavailability and the appropriate activation of FPR2 counteracts the primary retinal cell damage.…”