Serum markers of glucagon sensitivity in calorie restricted humans

Calorie restriction (CR), without malnutrition, is the most robust non-genetic intervention to promote longevity through metabolic improvements that delay aging in mice, rats, and primates. Most research aimed at understanding the mechanism by which CR slows aging has focused on insulin and its downstream signaling cascades. We have shown that glucagon, the counter-regulatory hormone to insulin, is essential for CR induced improvements in lifespan, improved metabolic function, and the activation of healthspan and lifespan associated nutrient sensing proteins. This suggests that glucagon is essential for CR to extend both healthspan and lifespan. Although the role of glucagon receptor signaling in aging has not been heavily investigated, the rigor of this work is supported by known effects of glucagon on downstream messengers that extend healthspan. Glucagon increases liver AMP Kinase (AMPK) activity, cyclic AMP (cAMP), and fibroblast growth factor 21 (FGF21), which promote healthy aging. Glucagon limits the protein kinase mechanistic target of rapamycin (mTOR) and IGF-1 bioavailability, which accelerate aging. Highlighting the significance of this proposal, Glucagon receptor agonists are entering the market for the treatment of diabetes and obesity and could be repurposed to promote healthy aging. To understand the potential impact of these agonists, we propose 2 aims using samples collected from CALERIE™ trial participants, to establish a key role of glucagon in CR improved aging. The studies proposed in this R21 create a unique opportunity to study glucagon physiology under long-term calorie restriction in healthy non-obese humans. Aim 1: Using samples collected from participants of the CALERIE™ trial at baseline, 12, and 24 months, we will measure fasted serum glucagon, GLP-1, and FGF21 concentrations, hormones that modulate aging and healthspan. We will assess glucagon sensitivity by calculating the glucagon-alanine index, an established biochemical marker of glucagon sensitivity in humans. Hypotheses: Serum glucagon and the glucagon-alanine index will be decreased, while serum FGF21 will be increased in calorie-restricted people, indicative of improved hepatic glucagon sensitivity. Serum GLP-1 will not be affected by calorie restriction. Aim 2: Assess the relationships of glucagon, the glucagon-alanine index, GLP-1, and FGF21 with markers of lipid homeostasis, glucose homeostasis, IGF-1 and the IGF binding proteins, and measures of whole-body energy homeostasis in CALERIE™ trial participants. Hypothesis: A decrease in fasted serum glucagon, the glucagon-alanine index, and increased FGF21 will be correlated with improved lipid homeostasis (decreased serum cholesterol and triglycerides), improved insulin sensitivity, decreased IGF-1: IGFBP1 ratio (indicative of decreased IGF-1 bioavailability), decreased body lipid, and increased whole-body energy expenditure and lipid oxidation, independent of serum GLP-1 levels.