Hepatic GAPDH prevents excessive fasting-induced steatosis via serine-dependent inhibition of diacylglycerol synthesis

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Abstract
Fasting-induced metabolic remodeling is a fundamental process maintaining systemic homeostasis, with profound implications for metabolic disease interventions. During fasting, liver establishes a regulatory network centered on gluconeogenesis that integrates major fuels to keep physiological energy homeostasis. Here, we report that GAPDH, a key enzyme in both glycolysis and gluconeogenesis, senses fasting stress in the liver. Compared to wildtype controls, hepatocyte Gapdh deficiency (GapAlb) mice show comparable blood glucose, decreased amino acid levels and aggravated lipid accumulation in the liver after overnight fasting. Mechanistically, Gapdh depletion decreases serine levels, and serine supplementation inhibits PA-DAG (phosphatidic acid-diacylglycerols) axis by downregulating Lipin1 (a phosphohydrolase in DAG synthesis) upon overnight fasting. Furthermore, knockdown of Lipin1 rescued the effects observed in fasted GapAlb mice. Similarly, GapAlb mice showed enhanced hepatic lipid accumulation under ketogenic diets, and serine supplementation abolished these effects. Together, we find hepatic GAPDH as a central hub in glucose-amino acid-fat metabolism during nutritional limitation.

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