Abstract 2042 The Glycerol-3-P-ChREBP-FGF21 Axis: How Hepatocyte Nuclei Talk to the Brain About Reductive Stress

Citrin Deficiency (CD) is among the most common rare diseases in the world. CD is caused by biallelic inactivating mutations in SLC25A13, a component of the malate-aspartate shuttle that is critical for moving high energy electrons from cytosolic NADH to the mitochondrial matrix in hepatocytes. People with CD are carbohydrate intolerant and circulate high levels of lactate and urea cycle intermediates. Starting from knowledge that people with CD do not like sweets, we developed a hypothesis that the reductive stress in CD causes metabolite imbalances that drive production of the hepatokine FGF21, which is known to be capable of depressing intake of sweets and ethanol in mice and people. We further suggested that the carbohydrate response element binding protein (ChREBP) may be the key transcription factor driving FGF21 expression in CD but questioned whether any of the nominated activators of ChREBP, which include glucose, specific hexose phosphates, hexose bisphosphates and xylulose-5-phosphate, truly mediate ChREBP activation. Specifically, we hypothesized that if glycerol-3-phosphate is the key activator of ChREBP and ChREBP is the key driver of FGF21 expression, we would be able to explain why fasting, ketogenic diet, exercise and weight loss result in FGF21 circulation (via formation of glycerol and glycerol-3-phosphate) while mitochondrial mutations, carbohydrate overload, ethanol and potentially CD also result in elevated FGF21 circulation (via NADH-dependent conversion of dihydroxyacetone phosphate to glycerol-3-phosphate). Here we show that slc25a13 mutations and glycerol administration individually and additively increase FGF21 circulation in mice. Consistent with our proposed mechanism of action, slc25a13 deletion and glycerol clearly activate ChREBP as shown by induction of ChREBPbeta, liver pyruvate kinase and FGF21 transcripts in liver. To address whether our proposed activator, glycerol-3-phosphate, directly binds ChREBP, we purified the glucose response activation conserved element (GRACE) domain of mouse ChREBP and subjected it to differential scanning fluorimetry. Remarkably, no tested hexose or pentose phosphate metabolite stabilized GRACE versus thermal denaturation by a value greater than the error of the assay, whereas glycerol-3-phosphate stabilized GRACE by 5.9 oC. These data provide the first evidence for a glycerol-3-P –ChREBP-FGF21 axis that operates in human disease, nominate a previously unconsidered activator of ChREBP, and provide a basis for unification of the literature on FGF21 expression, which is now considered to be deeply paradoxical. Moreover, our model suggests that glycerol-3-phosphate accumulation in hepatocytes, by virtue of activating the glycerol-3-P –ChREBP-FGF21 axis, communicates reductive stress from the liver to the brain in a manner that can alter food choices.