The liver has a key role in inter-organ communication by secreting most circulating plasma proteins. However, the mechanisms governing hepatic protein secretion remain unclear. Here we show that hepatic protein secretion follows a diurnal rhythm regulated by food intake in humans and mice. Using liver microsomal proteomics, we find that proteins implicated in the early secretory pathway, such as protein glycosylation and folding in the endoplasmic reticulum (ER) and Golgi apparatus, exhibit a rhythmic expression profile, which is abolished in Bmal1-knockout mice. Mechanistically, we show that hepatic glycogenolysis provides substrates for protein N-glycosylation. In mice, perturbing hepatic glycogenolysis with pharmacological or nutritional interventions leads to ER stress and attenuates diurnal protein secretion. We confirm these results in humans, as genetic variants associated with glycogen storage disease and congenital disorders of glycosylation also alter hepatic protein secretion. Overall, our work uncovers hepatic glycogen metabolism as a circadian regulator of protein secretion.
