Sex-specific metabolic responses to glucagon receptor agonism and modulation of the FGF21-glucagon axis in female mice.

Glucagon receptor agonism, particularly when combined with incretin analogues, is currently being explored as a treatment for obesity to improve cardiometabolic health, given glucagon's key role in regulating energy homoeostasis. However, male-biased preclinical studies limit our understanding of sex-specific responses to glucagon receptor activation, especially regarding fibroblast growth factor 21 (FGF21), a major downstream effector of glucagon signalling. To test whether responses to glucagon receptor agonism are sex dependent and modulated by FGF21, we compared a long-acting glucagon analogue (LA-Gcg) with the GLP-1 analogue semaglutide in diet-induced obese male and female mice. We then used female Fgf21 knockout (KO) mice to probe the role of the FGF21-glucagon axis in the response to glucagon receptor agonism. LA-Gcg induced greater weight loss, reduced food intake and more strongly altered hepatic gene expression in males, whereas semaglutide effects were comparable between sexes. LA-Gcg impaired glucose tolerance more severely in females than in males. This impairment was exacerbated in female Fgf21 KO mice, despite similar reductions in body weight between genotypes. Notably, FGF21 deficiency potentiated diet-induced obesity in females but had minimal impact under chow diet, fasting or voluntary exercise. Collectively, these findings reveal that both sex and FGF21 modulate metabolic responses to glucagon-based therapies, emphasizing the importance of including female models in preclinical metabolic research to better predict therapeutic efficacy. KEY POINTS: Biological sex is known to affect metabolism, yet this variable remains largely underexplored in metabolic research. In males, glucagon's metabolic benefits often involve another hormone, FGF21 (fibroblast growth factor 21), but this relationship is largely unstudied in females. A long-acting glucagon (LA-Gcg) treatment caused less weight loss in obese female mice, failing to reduce their food intake, unlike in males. LA-Gcg also worsened glucose tolerance in females. Female mice lacking the Fgf21 gene were more susceptible to diet-induced obesity; although LA-Gcg treatment still reduced their weight and cleared liver fat, the absence of FGF21 worsened the drug-induced glucose intolerance. Our findings highlight sex-specific differences in metabolic responses to glucagon, emphasizing the need to consider sex as a key variable in the development of glucagon-based therapies.