β‐Hydroxybutyrate (βOHB) Activates Gpr109a to Contribute to the Anti‐vascular Aging Effect of Autophagy
Authors: McCarthy Cameron Grant , Chakraborty Saroj , Singh Avinash , Schreckenberger Zachary , Mell Blair , Wenceslau Camilla Ferreira , Joe Bina 
Published: April 2020

The aged phenotype is classically viewed as the accumulation of cellular debris and dysfunctional organelles. Although waste products are an inevitable consequence of normal cellular metabolism, multiple systems are devoted to its repair, clearance, or recycling. Autophagy is the constitutively active catabolic process responsible for cellular degradation. Autophagic activity has also been implicated as a modulator of longevity, as well as inversely related with chronological vascular aging and premature vascular aging associated with hypertension. However, the mechanisms by which autophagy exerts anti‐vascular aging effects are still unknown. Evolutionarily, autophagy functions to mobilize nutrients in times of starvation. Specifically, hepatic autophagy releases free fatty acids from triglycerides, which are oxidized to generate ketone bodies. Therefore, we hypothesized that fasting would increase the systemic expression of ketone body β‐hydroxybutyrate (βOHB) in Dahl salt‐sensitive rats fed a low salt‐diet, and this would be prevented by inhibition of autophagy with chloroquine (CQ). As hypothesized, fasting increased serum βOHB, and treatment with CQ blocked this increase (mmol/L, non‐fasting: 0.38±0.01 vs. fasting: 1.01±0.06* vs. fasting+CQ: 0.45±0.04, *p<0.05) (Figure
). Next we questioned whether βOHB could potentially contribute to the anti‐vascular aging effect of autophagy. Previously, we have observed that βOHB is a potent vasodilator via endothelial potassium channels. However, it is not known if βOHB acts as an agonist or antagonist to any specific receptor to mediate endothelium‐dependent relaxation. To help answer this question we needed to switch to a genetic knockout model, as pharmacological antagonists for the βOHB receptors, Gpr109a and Gpr41, do not exist. βOHB activates Gpr109a, whereas it is generally recognized as an antagonist of Gpr41. Therefore, we hypothesized that vascular relaxation to βOHB would be significantly attenuated in isolated mesenteric resistance arteries from Gpr109a−/− mice, but not Gpr41−/− mice. As hypothesized, vasodilatory effect of βOHB was significantly blunted in Gpr109a−/− mice, but not Gpr41−/− mice [Emax (%relaxation), WT: 75±2 vs. Gpr109a−/−: 53±3* vs. Gpr41−/−: 73±1, *p<0.05] (Figure
). Overall, these data reveal that ketone body βOHB is stimulated by autophagy and that activation of Gpr109a by βOHB can cause vasodilation of isolated resistance arteries. Significantly, our data define a novel mechanism that underlies the anti‐vascular aging effect of autophagy.Support or Funding InformationAmerican Heart Association (18POST34060003) and National Institutes of Health (R00GM118885 and R01HL143082).Fasting (24 h) stimulates serum βOHB expression and this is prevented by autophagy inhibition (chloroquine 50 mg/kg i.p.; CQ). n=6–10. One way ANOVA: *p<0.05.Figure 1βOHB mediates vasodilation partially via Gpr109a, and not Gpr41. Concentration response curves to βOHB. n=2–4. Two‐way ANOVA: *p<0.05. vs. wild type (WT).Figure 2