Glucocorticoid Metabolism – Cortisol and HPA Axis

What Is Glucocorticoid Metabolism?

Glucocorticoid metabolism encompasses all biochemical processes involved in the production, transport, action, and degradation of glucocorticoids. Glucocorticoids are steroid hormones produced in the adrenal cortex. The most important glucocorticoid in humans is cortisol. These hormones play a central role in energy, carbohydrate, and protein metabolism, as well as in the regulation of the immune system and the body stress response.

Synthesis of Glucocorticoids

The synthesis of glucocorticoids takes place in the zona fasciculata of the adrenal cortex. The precursor molecule is cholesterol, which is converted through several enzymatic steps -- first into pregnenolone and ultimately into cortisol. This process is regulated by the hypothalamic-pituitary-adrenal (HPA) axis:

• The hypothalamus releases corticotropin-releasing hormone (CRH).
• CRH stimulates the pituitary gland to secrete ACTH (adrenocorticotropic hormone).
• ACTH stimulates the adrenal cortex to produce and release cortisol.
• Elevated cortisol levels suppress CRH and ACTH release via negative feedback.

Cortisol secretion follows a distinct circadian rhythm: peak levels occur in the early morning (shortly before waking), while the lowest levels are found in the evening and at night.

Transport in the Blood

In the bloodstream, approximately 90% of cortisol is bound to transport proteins. The primary carrier is corticosteroid-binding globulin (CBG, transcortin), while a smaller fraction binds to albumin. Only the free, unbound fraction of cortisol is biologically active and able to enter target cells to exert its effects.

Mechanism of Action

Glucocorticoids primarily act through intracellular receptors known as glucocorticoid receptors (GR). After entering a target cell, cortisol binds to the cytosolic receptor. The resulting hormone-receptor complex translocates to the cell nucleus, where it influences the gene expression of numerous target genes. This explains the wide-ranging effects of these hormones throughout the body.

Biological Effects

Metabolic Effects

• Gluconeogenesis: Promotion of new glucose production from non-carbohydrate precursors (e.g., amino acids) in the liver, raising blood glucose levels.
• Protein catabolism: Breakdown of proteins in muscle and connective tissue to provide amino acids as an energy source.
• Lipolysis and fat redistribution: Mobilization of fatty acids from peripheral fat depots, while simultaneously promoting fat accumulation in the trunk and face (characteristic of chronically elevated cortisol levels).
• Antagonism of insulin action: Glucocorticoids counteract the effects of insulin and can promote insulin resistance.

Immunological and Anti-Inflammatory Effects

• Inhibition of prostaglandin and leukotriene synthesis through induction of lipocortin (annexin A1).
• Suppression of pro-inflammatory cytokines (e.g., interleukin-1, TNF-alpha).
• Reduction of vascular permeability and edema formation.
• Inhibition of immune cell proliferation and activity (lymphocytes, macrophages).

Additional Effects

• Increase in cardiac output and blood pressure.
• Inhibition of bone metabolism (long-term: increased risk of osteoporosis).
• Influence on the central nervous system: mood, cognition, and sleep.
• Inhibition of collagen synthesis and wound healing with chronically elevated levels.

Breakdown and Excretion

Cortisol is primarily metabolized in the liver, where it is converted into water-soluble, inactive metabolites through reduction and conjugation reactions (mainly glucuronidation). These metabolites are subsequently excreted by the kidneys in the urine. The main metabolites are tetrahydrocortisol and tetrahydrocortisone. A portion of cortisol is converted to the biologically inactive form cortisone by the enzyme 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2), which is particularly important in the kidney to prevent overstimulation of mineralocorticoid receptors.

Clinical Relevance: Disorders of Glucocorticoid Metabolism

Cushing Syndrome

Cushing syndrome results from chronic overproduction of cortisol or from long-term use of glucocorticoid medications. Typical features include central obesity, moon face, muscle weakness, hypertension, osteoporosis, and elevated blood glucose levels.

Addison Disease

Addison disease (primary adrenal insufficiency) is characterized by insufficient cortisol production. Symptoms include chronic fatigue, weight loss, low blood pressure, and a characteristic hyperpigmentation of the skin.

Congenital Adrenal Hyperplasia

In congenital adrenal hyperplasia (CAH), enzymatic defects in cortisol synthesis lead to cortisol deficiency and compensatory overproduction of androgens, which can result in virilization and other hormonal disturbances.

Therapeutic Use of Glucocorticoids

Synthetic glucocorticoids such as prednisolone, dexamethasone, and budesonide are widely used in medicine due to their potent anti-inflammatory and immunosuppressive properties. Indications include bronchial asthma, rheumatoid arthritis, inflammatory bowel disease, allergic reactions, and organ transplantation. However, long-term use carries significant risks, as the pharmacological effects mirror those of chronically elevated endogenous cortisol levels.

References

1. Chrousos, G. P.: Stress and disorders of the stress system. Nature Reviews Endocrinology, 5(7):374-381, 2009. DOI: 10.1038/nrendo.2009.106
2. Nicolaides, N. C., Chrousos, G. P., Charmandari, E.: Adrenal Insufficiency. In: Feingold, K. R. et al. (Hrsg.): Endotext. MDText.com, Inc., South Dartmouth (MA), 2000-2023. Available at: https://www.ncbi.nlm.nih.gov/books/NBK279083/
3. Stewart, P. M., Krone, N. P.: The Adrenal Cortex. In: Melmed, S. et al. (Hrsg.): Williams Textbook of Endocrinology, 13th edition. Elsevier, Philadelphia 2016, pp. 489-555.