Activation of the HPA axis regulates metabolism<i>via</i>GR-dependent and independent pathways in fasting elephant seal pups

Activation of the hypothalamic-pituitary-adrenal (HPA) axis increases the release of glucocorticoids, which regulate metabolism via glucocorticoid receptor (GR) signaling. Although the response to HPA activation is well characterized in many species, little is known about how the HPA axis regulates cellular signaling during extreme physiological conditions such as prolonged fasting, which naturally increases cortisol in elephant seals. We previously showed that daily intramuscular injections of adrenocorticotropin (ACTH) with or without the GR antagonist RU486 (ACTH+RU486) for 4 days increase aldosterone and cortisol at 2 h, with the effects on cortisol but not aldosterone diminishing at 4 days. Similarly, blocking endogenous GR signaling induces a compensatory increase in aldosterone but not cortisol, suggesting that aldosterone plays a role in the response to HPA axis activation in fasting elephant seal pups. Here, we conducted plasma metabolite profiling and RNA-seq analyses in blood and muscle samples collected from fasting elephant seal pups challenged with daily intramuscular injections of ACTH with and without RU486. Integrated analyses of parallel blood gene expression and metabolomics data showed distinct metabolic signatures between samples collected 2h and 4 days after initial ACTH treatment, which were strongly suppressed by RU846, suggesting that GR drives the observed response. Whereas several carbohydrates were downregulated at 2h, pteridines, vitamins, and amino acids were enriched at 4 days, suggesting that repeated stimulation of the HPA axis affects substrate partitioning and cytoprotection. Moreover, enrichment analyses on differentially expressed genes in skeletal muscle showed rapid (2h) upregulation of transcriptional programs involving adipogenesis, TGF-β signaling, and myogenesis, whereas repeated exposure to ACTH upregulated pathways related to heme metabolism, immunity, and inflammation while downregulating notch signaling. Apoptosis, androgen response, oxidative phosphorylation, fatty acid metabolism, and oxidative damage were downregulated, whereas DNA repair, xenobiotic metabolism, hypoxic signaling, estrogen response, and cholesterol homeostasis were upregulated independently of time. GR antagonism elicited differential responses in hypoxic signaling, estrogen response, angiogenesis, fatty acid metabolism, and notch signaling. Our results show that while GR signaling directly regulates several metabolic pathways in fasting elephant seals, the transcriptional response to ACTH also involves other mediators, potentially aldosterone. Research funded by UC Berkeley. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.