Hepatic NADH reductive stress underlies common variation in metabolic traits

The cellular NADH/NAD(+) ratio is fundamental to biochemistry but the extent to which it reflects versus drives metabolic physiology in vivo is poorly understood. Here, we report the in vivo application of LbNOX(1), a bacterial water-forming NADH oxidase, to assess the metabolic consequences of directly lowering the hepatic cytosolic NADH/NAD(+) ratio in mice. By combining this genetic tool with metabolomics, we identify circulating α-hydroxybutyrate (αHB) as a robust marker of elevated hepatic cytosolic NADH/NAD(+) ratio, also known as reductive stress. In humans, elevations in circulating αHB levels have previously been associated with impaired glucose tolerance(2), insulin resistance(3), and mitochondrial disease(4), and are associated with a common genetic variant in GCKR(5), which has previously been associated with many seemingly disparate metabolic traits. Using LbNOX, we demonstrate NADH reductive stress mediates the effects of GCKR variation on many metabolic traits including circulating triglycerides, glucose tolerance, and FGF21 levels. Our work identifies elevated hepatic NADH/NAD(+) as a latent metabolic parameter that is shaped by human genetic variation and contributes causally to key metabolic traits and diseases. Moreover, it undescores the utility of genetic tools such as LbNOX to empower studies of “causal metabolism.”