How Does ACTH Work? Mechanism of Action, Per the Research

Adrenocorticotropic hormone (ACTH) is a 39-amino-acid peptide derived from the precursor protein pro-opiomelanocortin (POMC) in pituitary corticotroph cells. It belongs to the melanocortin peptide family, alongside the melanocyte-stimulating hormones, and the shared structural core of these peptides explains much of the receptor pharmacology described below. This page describes mechanism only — what receptors the peptide engages and what signalling follows — and separates the parts of that picture that are textbook-established from the parts still described as proposed. Background on the peptide itself is collected on the ACTH learn page.

This page is for educational purposes only and is not medical advice; consult a licensed physician about any medical question or any decision involving a prescription hormone.

The established mechanism: MC2R on the adrenal cortex

The melanocortin system comprises five G-protein-coupled receptors, MC1R through MC5R, and a review of melanocortin signalling mechanisms described these receptors as coupling principally to Gs, activating adenylyl cyclase and raising intracellular cyclic AMP (cAMP), with downstream protein kinase A (PKA) activity and additional reported signalling through ERK pathways (PMID 21222257). Among these receptors, MC2R is distinctive: the same review reported that MC2R responds to ACTH and not to the shorter melanocyte-stimulating hormone peptides, and that its surface expression and function depend on the melanocortin receptor accessory proteins (MRAPs) (PMID 21222257).

That receptor is concentrated on steroidogenic cells of the adrenal cortex. In the classical sequence, ACTH binding raises cAMP, PKA phosphorylates downstream targets, cholesterol delivery to the mitochondria increases and glucocorticoid synthesis follows. The central role of this cascade is reinforced from an unexpected direction — human genetics. A review of gene mutations in Cushing's syndrome reported that defects in components of the cAMP/PKA pathway, including the regulatory and catalytic subunits of protein kinase A, underlie several forms of adrenal cortisol excess (PMID 36660032). In other words, when the pathway that ACTH normally switches on is switched on by mutation instead, cortisol production rises without the hormonal signal — a natural experiment supporting the pathway's position downstream of the receptor.

Why MRAP matters mechanistically

MC2R differs from the other melanocortin receptors in that it is largely retained inside the cell unless an accessory protein escorts it to the membrane; the melanocortin signalling review described MRAP as required for MC2R trafficking and ACTH responsiveness (PMID 21222257). This is why adrenal tissue can be exquisitely ACTH-sensitive while most other tissues are not, even where melanocortin receptors are present.

Step two: cortisol and the glucocorticoid receptor

Most of what is conventionally called "the effect of ACTH" is really the effect of the glucocorticoid it releases. A review of glucocorticoid receptor (GR) biology reported that the receptor exists as multiple isoforms generated by alternative splicing and alternative translation initiation, that it acts through both genomic mechanisms — transactivation and transrepression of target genes — and non-genomic mechanisms, and that tissue-specific isoform patterns help explain variable glucocorticoid sensitivity between individuals and tissues (PMID 24084075). Researchers studying GR signalling in lymphangioleiomyomatosis applied this framework to a specific disease tissue, examining how glucocorticoid receptor signalling behaves in that setting (PMID 42376017).

The practical mechanistic consequence is that the ACTH signal is two receptors deep: MC2R on the adrenal, then GR in target tissue. Variation at either step — receptor expression, accessory proteins, GR isoform mix — changes what the same quantity of circulating peptide does.

Proposed mechanism: direct melanocortin signalling outside the adrenal

A second body of work asks whether ACTH does anything that is not mediated by cortisol. A review of melanocortin regulation of inflammation described melanocortin receptors on immune and resident tissue cells and reported anti-inflammatory signalling attributed particularly to MC1R and MC3R, including effects on cytokine production and leukocyte behaviour (PMID 31649620). A separate review framed ACTH as a "forgotten therapy" and reported that its immunomodulatory actions have been attributed both to adrenal steroidogenesis and to steroid-independent melanocortin receptor signalling on immune cells (PMID 25726511).

This is labelled proposed rather than established for a specific reason: separating a steroid-independent action from the steroid action is methodologically hard in intact animals and people, because ACTH raises cortisol in the same experiment. Much of the supporting work cited in these reviews is cell-based or animal-based (PMID 31649620), and the reviews themselves present mechanism rather than clinical outcome data (PMID 25726511).

Trophic effects on the adrenal cortex: evidence from mouse work

Beyond acute steroid release, the adrenal cortex is a dynamically maintained tissue whose zones renew and convert into one another. In a mouse study, researchers reported that loss of SUMO-specific protease 2 (SENP2) caused isolated glucocorticoid deficiency by blocking adrenal cortex zonal transdifferentiation — that is, by preventing zona glomerulosa cells from converting into cortisol-producing zona fasciculata cells (PMID 36543805). The study is animal work and its findings should be read as such, but it illustrates a mechanistic point relevant to ACTH biology: cortisol output depends not only on acute receptor signalling but on the cellular composition of the cortex that the signal acts upon (PMID 36543805).

What corticotroph tumour biology reveals about the upstream switch

ACTH secretion itself is regulated, and disease states where that regulation fails have been informative. A review of the genetics of Cushing's disease reported that somatic mutations in the USP8 gene are found in a substantial share of corticotroph adenomas and that these mutations act by impairing degradation of epidermal growth factor receptor, sustaining signalling that drives POMC transcription and ACTH secretion (PMID 28850717). A complementary review of gene mutations across Cushing's syndrome described how pituitary-driven and adrenal-driven disease arise through different molecular routes, including cAMP/PKA pathway defects at the adrenal level (PMID 36660032).

Pharmacology aimed at this axis maps neatly onto the mechanism. A review of new and emerging drug therapies for Cushing's disease described agents grouped by where they act — pituitary-directed compounds targeting corticotroph receptors, adrenal steroidogenesis inhibitors blocking cortisol synthesis, and glucocorticoid receptor blockade at the tissue level (PMID 30048162). A more recent review of new trends in treating Cushing's disease reported the same three-tier framework alongside updated agents and surgical context (PMID 39526050). Each tier corresponds to one node of the ACTH pathway described above.

Preclinical model work

Newer mechanistic work uses patient-derived tissue models. In a study using patient-derived organoids from pituitary neuroendocrine tumours, researchers reported that ceritinib inhibited both tumour cell growth and ACTH production (PMID 41083089). That work is laboratory-based and organoid-based, not a clinical trial, and the study was framed as an exploration of tumour biology rather than as evidence of patient outcomes (PMID 41083089).

ACTH and the mineralocorticoid zone

ACTH is usually discussed as a glucocorticoid signal, but the adrenal cortex also makes aldosterone. A review of the pathogenesis of primary aldosteronism described the mechanisms by which aldosterone production becomes autonomous, including somatic mutations affecting ion channels and transporters in adrenocortical cells and the resulting dysregulation of the zona glomerulosa (PMID 41638802). Read alongside the mouse zonation work, which reported that blocking zona glomerulosa-to-fasciculata conversion produced isolated glucocorticoid deficiency (PMID 36543805), it underlines that the adrenal cortex is a layered organ in which ACTH signalling is one of several overlapping regulatory inputs.

Established versus proposed, side by side

Mechanism-Linked Safety Signals: What Studies Report

Because the pathway terminates in glucocorticoid action, the recognised harms described in this literature are the harms of cortisol excess. Reviews of Cushing's disease therapy described sustained hypercortisolism as the clinical problem those drugs are designed to reverse, with treatment strategies divided between pituitary-directed agents, steroidogenesis inhibitors and glucocorticoid receptor blockade (PMID 39526050). The earlier review of emerging therapies similarly reported that drug selection in this setting involves balancing efficacy against adrenal insufficiency and other class-specific tolerability concerns (PMID 30048162). At the opposite end of the spectrum, the mouse genetics work reported that disrupting adrenal zonation produced isolated glucocorticoid deficiency (PMID 36543805) — a reminder that both excess and insufficiency are failure modes of this axis. None of these papers constitutes safety data for any non-clinical use of the peptide.

Limits of the mechanism evidence

• Review-heavy. Several of the key sources are narrative reviews of receptor pharmacology rather than primary experiments (PMID 21222257).
• Species and system gaps. The zonation finding came from genetically modified mice (PMID 36543805), and the tumour growth finding came from patient-derived organoids rather than treated patients (PMID 41083089).
• Confounded pathways. Steroid-dependent and steroid-independent melanocortin effects are difficult to disentangle experimentally, as the immunology review noted (PMID 25726511).
• Disease-derived inference. Much of what is known about the upstream switch comes from tumour genetics rather than normal physiology (PMID 28850717).

Understanding a mechanism is not the same as predicting an outcome. The papers summarised here describe receptors, second messengers and transcriptional machinery; they do not establish what any given exposure would do in a healthy person, and none of this material should be read as guidance.

References

• Melanocortin signalling mechanisms (Advances in Experimental Medicine and Biology, 2010)
• The biology of the glucocorticoid receptor: new signaling mechanisms in health and disease (Journal of Allergy and Clinical Immunology, 2013)
• ACTH: The forgotten therapy (Seminars in Immunology, 2015)
• Genetics of Cushing's disease (Clinical Endocrinology, 2018)
• New and emerging drug therapies for Cushing's disease (Expert Opinion on Pharmacotherapy, 2018)
• Melanocortin Regulation of Inflammation (Frontiers in Endocrinology, 2019)
• Loss of SUMO-specific protease 2 causes isolated glucocorticoid deficiency by blocking adrenal cortex zonal transdifferentiation in mice (Nature Communications, 2022)
• Gene Mutations in Cushing's Syndrome (Molecular Syndromology, 2023)
• New Trends in Treating Cushing's Disease (touchREVIEWS in Endocrinology, 2024)
• Ceritinib inhibits growth and ACTH production of PitNETs: Insights from patient-derived organoids (Pharmacological Research, 2025)
• Pathogenesis of primary aldosteronism (Vitamins and Hormones, 2026)
• Exploring glucocorticoid receptor signalling in lymphangioleiomyomatosis (ERJ Open Research, 2026)