Corticotropin (ACTH)

Overview

Adrenocorticotropic hormone (ACTH), also known as corticotropin, is a 39-amino-acid polypeptide hormone secreted by corticotroph cells of the anterior pituitary gland. It is the principal regulator of glucocorticoid production from the adrenal cortex and serves as the central hormonal effector of the hypothalamic-pituitary-adrenal (HPA) axis — the neuroendocrine system governing the body's response to physiological and psychological stress.

ACTH is cleaved from a much larger precursor protein, proopiomelanocortin (POMC), a 241-amino-acid polypeptide that also gives rise to alpha-melanocyte-stimulating hormone (alpha-MSH), beta-endorphin, beta-lipotropin, and several other bioactive peptides. The tissue-specific processing of POMC determines which peptides are produced: in anterior pituitary corticotrophs, POMC is primarily processed to ACTH and beta-lipotropin, while in the intermediate lobe (in species where it is prominent) and in hypothalamic neurons, ACTH is further cleaved to yield alpha-MSH and corticotropin-like intermediate lobe peptide (CLIP).

The clinical significance of ACTH spans diagnostic endocrinology (ACTH stimulation tests for adrenal insufficiency), therapeutic applications (ACTH gel for infantile spasms and nephrotic syndrome), and its role as the parent molecule from which the research peptide Semax is derived. ACTH's N-terminal fragment ACTH(4-10), also known as the heptapeptide core of Semax, has been studied extensively for nootropic and neuroprotective properties, representing an important intersection between endocrine physiology and peptide neuroscience research.

Structure

ACTH is a single-chain 39-amino-acid polypeptide:

Sequence: Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-Gly-Lys-Lys-Arg-Arg-Pro-Val-Lys-Val-Tyr-Pro-Asn-Gly-Ala-Glu-Asp-Glu-Ser-Ala-Glu-Ala-Phe-Pro-Leu-Glu-Phe

• Molecular formula: C₂₀₇H₃₀₈N₅₆O₅₈S₁
• Molecular weight: ~4,541 Da
• CAS Number: 9002-60-2
• Precursor: Proopiomelanocortin (POMC)

Key structural features:

• Residues 1-13 correspond to alpha-MSH (with N-terminal acetylation and C-terminal amidation in the mature alpha-MSH form)
• Residues 1-24 constitute the biologically active core sufficient for full adrenocortical stimulation (cosyntropin/tetracosactide)
• Residues 25-39 contribute to species specificity and binding affinity but are not essential for receptor activation
• Residues 4-10 (Met-Glu-His-Phe-Arg-Trp-Gly) form the basis of the nootropic peptide Semax (ACTH(4-10) with a C-terminal Pro-Gly-Pro extension)
• Residues 11-13 (Lys-Pro-Val) constitute the anti-inflammatory tripeptide KPV

The overlap of ACTH's N-terminal sequence with alpha-MSH explains ACTH's melanocortin receptor activity, including its capacity to stimulate melanogenesis at high concentrations — a clinically relevant phenomenon observed in conditions of ACTH excess such as Addison's disease, where elevated ACTH produces characteristic hyperpigmentation.

Mechanism of Action

MC2R Signaling at the Adrenal Cortex

ACTH exerts its primary endocrine function through the melanocortin 2 receptor (MC2R), a G-protein-coupled receptor exclusively expressed on adrenocortical cells. MC2R is unique among melanocortin receptors in that it requires a specific accessory protein, MRAP (melanocortin-2 receptor accessory protein), for proper trafficking to the cell surface and functional signaling.

The signaling cascade proceeds as follows:

1. ACTH binding — ACTH binds MC2R on zona fasciculata cells of the adrenal cortex
2. Gs protein activation — receptor coupling to stimulatory G-proteins activates adenylyl cyclase
3. cAMP elevation — increased intracellular cAMP activates protein kinase A (PKA)
4. StAR protein induction — PKA phosphorylates and activates steroidogenic acute regulatory protein (StAR), the rate-limiting step in steroidogenesis
5. Cholesterol mobilization — StAR facilitates cholesterol transfer from the outer to inner mitochondrial membrane
6. Cortisol synthesis — cholesterol is converted through the steroidogenic pathway (cholesterol to pregnenolone to cortisol) via CYP11A1, CYP17A1, and CYP11B1 enzymes

HPA Axis Regulation

ACTH secretion is regulated through a classical neuroendocrine feedback loop:

• Stimulatory: Corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) from hypothalamic paraventricular nucleus neurons stimulate ACTH release
• Inhibitory: Cortisol exerts negative feedback at both the hypothalamic and pituitary levels, suppressing CRH and ACTH production
• Circadian rhythm: ACTH secretion follows a diurnal pattern with peak levels in the early morning (6-8 AM) and nadir in late evening
• Stress response: Physical injury, psychological stress, hypoglycemia, and inflammation override the circadian pattern, triggering acute ACTH surges

Melanocortin Receptor Cross-Reactivity

Due to its N-terminal sequence identity with alpha-MSH, ACTH binds and activates other melanocortin receptors (MC1R, MC3R, MC4R, MC5R) in addition to MC2R. This cross-reactivity is physiologically relevant — ACTH-mediated MC1R activation on melanocytes explains the hyperpigmentation seen in ACTH-excess states, and MC4R activation may contribute to appetite and metabolic effects during chronic stress.

Research Summary

Applications

Therapeutic Uses

ACTH Gel (H.P. Acthar Gel): Repository corticotropin injection is a purified ACTH preparation in a gelatin matrix providing prolonged release. FDA-approved indications include:

• Infantile spasms (West syndrome) — first-line treatment
• Nephrotic syndrome — particularly steroid-resistant forms
• Multiple sclerosis acute exacerbations
• Rheumatic disorders
• Severe allergic states

Cosyntropin (Synacthen, Cortrosyn): A synthetic ACTH(1-24) fragment (tetracosactide) used diagnostically:

• Standard ACTH stimulation test (250 mcg IV/IM) for diagnosing primary adrenal insufficiency
• Low-dose ACTH test (1 mcg) for secondary adrenal insufficiency

Peptide Research Derivatives

ACTH has served as the parent molecule for several research peptides:

• Semax — ACTH(4-10) with C-terminal Pro-Gly-Pro extension; studied as a nootropic and neuroprotective agent, primarily in Russian clinical research
• KPV — the C-terminal tripeptide (residues 11-13) of the alpha-MSH sequence within ACTH; studied as an anti-inflammatory agent targeting the NF-kB pathway
• ACTH(4-10) — the heptapeptide core fragment with demonstrated effects on attention, memory, and neural plasticity in preclinical models
• Ebiratide — an ACTH(4-9) analog studied for dementia in Japan

Diagnostic Endocrinology

ACTH measurement (via immunoassay) is a cornerstone of differential diagnosis in:

• Cushing's syndrome (ACTH-dependent vs. ACTH-independent)
• Adrenal insufficiency (primary vs. secondary/tertiary)
• Ectopic ACTH syndrome (paraneoplastic cortisol excess)
• Congenital adrenal hyperplasia

Dosing Protocols

The following dosing information reflects FDA-approved clinical guidelines for repository corticotropin (H.P. Acthar Gel). Always consult a qualified healthcare professional.

Important notes: H.P. Acthar Gel is a repository (long-acting) formulation of ACTH purified from porcine pituitary. It stimulates endogenous cortisol production from the adrenal glands. Not interchangeable with synthetic ACTH (cosyntropin/Cortrosyn), which is used only for diagnostic adrenal function testing at a dose of 250 mcg IV/IM. Chronic use carries risks consistent with glucocorticoid excess.

Related Compounds

• Alpha-MSH — the 13-amino-acid melanocortin peptide corresponding to ACTH(1-13) with post-translational modifications; regulates pigmentation, appetite, and inflammation
• KPV — the C-terminal tripeptide of alpha-MSH (ACTH residues 11-13); an anti-inflammatory peptide acting through NF-kB inhibition
• Semax — an ACTH(4-10) derivative studied for cognitive enhancement and neuroprotection
• CRH (Corticotropin-Releasing Hormone) — the hypothalamic peptide that drives ACTH secretion
• Cosyntropin — synthetic ACTH(1-24) used in adrenal function diagnostics
• Beta-endorphin — an opioid peptide co-derived from the POMC precursor alongside ACTH, acting through the opioid receptor system