Summary GDF15 is an established biomarker of cellular stress. The fact that it signals via a specific hindbrain receptor, GFRAL, and that mice lacking GDF15 manifest diet-induced obesity suggest that GDF15 may play a physiological role in energy balance. We performed experiments in humans, mice, and cells to determine if and how nutritional perturbations modify GDF15 expression. Circulating GDF15 levels manifest very modest changes in response to moderate caloric surpluses or deficits in mice or humans, differentiating it from classical intestinally derived satiety hormones and leptin. However, GDF15 levels do increase following sustained high-fat feeding or dietary amino acid imbalance in mice. We demonstrate that GDF15 expression is regulated by the integrated stress response and is induced in selected tissues in mice in these settings. Finally, we show that pharmacological GDF15 administration to mice can trigger conditioned taste aversion, suggesting that GDF15 may induce an aversive response to nutritional stress.

Glycogen is the storage form of carbohydrates in mammals. In humans the majority of glycogen is stored in skeletal muscles (∼500 g) and the liver (∼100 g). Food is supplied in larger meals, but the blood glucose concentration has to be kept within narrow limits to survive and stay healthy. Therefore, the body has to cope with periods of excess carbohydrates and periods without supplementation. Healthy persons remove blood glucose rapidly when glucose is in excess, but insulin-stimulated glucose disposal is reduced in insulin resistant and type 2 diabetic subjects. During a hyperinsulinemic euglycemic clamp, 70–90% of glucose disposal will be stored as muscle glycogen in healthy subjects. The glycogen stores in skeletal muscles are limited because an efficient feedback-mediated inhibition of glycogen synthase prevents accumulation. De novo lipid synthesis can contribute to glucose disposal when glycogen stores are filled. Exercise physiologists normally consider glycogen’s main function as energy substrate. Glycogen is the main energy substrate during exercise intensity above 70% of maximal oxygen uptake (Vo2max⁡) and fatigue develops when the glycogen stores are depleted in the active muscles. After exercise, the rate of glycogen synthesis is increased to replete glycogen stores, and blood glucose is the substrate. Indeed insulin-stimulated glucose uptake and glycogen synthesis is elevated after exercise, which, from an evolutional point of view, will favor glycogen repletion and preparation for new “fight or flight” events. In the modern society, the reduced glycogen stores in skeletal muscles after exercise allows carbohydrates to be stored as muscle glycogen and prevents that glucose is channeled to de novo lipid synthesis, which over time will causes ectopic fat accumulation and insulin resistance. The reduction of skeletal muscle glycogen after exercise allows a healthy storage of carbohydrates after meals and prevents development of type 2 diabetes.

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Context Aggressive pituitary tumours (APTs) are characterised by unusually rapid growth and lack of response to standard treatment. About 1-2% develop metastases being classified as pituitary carcinomas (PCs). For unknown reasons, the corticotroph tumours are overrepresented amongst APTs and PCs. Mutations in the ATRX gene, regulating chromatin remodelling and telomere maintenance, have been implicated in the development of several cancer types, including neuroendocrine tumours. Objective To study ATRX protein expression and mutational status of the ATRX gene in APTs and PCs. Design We investigated ATRX protein expression by using immunohistochemistry in 30 APTs and 18 PCs, mostly of Pit-1 and T-Pit cell lineage. In tumours lacking ATRX immunolabeling, mutational status of the ATRX gene was explored. Results Nine of the 48 tumours (19%) demonstrated lack of ATRX immunolabelling with a higher proportion in patients with PCs (5/18 - 28%) than in those with APTs (4/30 – 13%). Lack of ATRX was most common in the corticotroph tumours, 7/22 (32%), vs 2/24 (8%) in the tumours of the Pit-1 lineage. Loss-of-function ATRX mutations were found in all the nine ATRX immuno-negative cases: nonsense mutations (n=4), frameshift deletions (n=4) and large deletions affecting 22-28 of the 36 exons (n=3). More than one ATRX gene defect was identified in two PCs. Conclusion ATRX mutations occur in a subset of aggressive pituitary tumours and are more common in corticotroph tumours. The findings provide a rationale for performing ATRX immunohistochemistry to identify patients at risk of developing aggressive and potentially metastatic pituitary tumours.