mc5r

MC5R (melanocortin 5 receptor)

The least clinically advanced member of the melanocortin receptor family. MC5R is a Class A GPCR that couples primarily to Gαs/cAMP and, via a PI3K-regulated Akt-independent pathway, to ERK1/2. Its best-documented role is in exocrine gland secretion — sebaceous gland sebum production, lacrimal protein secretion, Harderian gland porphyrin synthesis, and pheromonal gland output in rodents. Secondary roles in immune cell biology (macrophage immunosuppression, CD8+ T cell regulation) are emerging. Aging relevance centers on the understudied question of whether MC5R decline contributes to age-related exocrine dysfunction (dry eye, xerosis).

Identity

• UniProt: P33032 (MC5R_HUMAN) · reviewed Swiss-Prot entry
• NCBI Gene: 4161
• HGNC symbol: MC5R
• Ensembl: ENSG00000176136
• Chromosomal location: 18p11.21
• Length: 325 amino acids
• Mouse ortholog: Mc5r (used extensively in KO studies)

Receptor biology

MC5R is a seven-transmembrane (7TM) GPCR. On agonist binding it activates adenylate cyclase via Gαs, elevating intracellular cAMP. A second signaling arm activates ERK1/2 through a PI3K-regulated, Akt-independent mechanism, not linked to cAMP or PKA ¹. Both pathways are modulated by the accessory protein MRAP (melanocortin-2 receptor accessory protein), which in humans inhibits MC5R trafficking to the plasma membrane and reduces pharmacological efficacy for NDP-MSH ².

Endogenous ligands (affinity order: α-MSH > ACTH = β-MSH >> γ-MSH) ³:

• α-MSH (alpha-melanocyte-stimulating hormone; alpha-msh) — primary agonist; highest affinity
• ACTH (adrenocorticotropic hormone) — equal affinity to β-MSH
• β-MSH — equal affinity to ACTH
• γ-MSH — substantially lower affinity than the above
• AgRP (agouti-related peptide) — endogenous antagonist; also acts at MC5R (alongside ASIP)

Key structural features:

• 4 predicted N-linked glycosylation sites (extracellular loops)
• Ca²⁺ binding (cofactor role)
• Serine-rich N-terminal motifs required for trafficking to the cell surface; mutations in these cause ER/Golgi retention ⁴
• Transmembrane residues D115, D119 (TM3), F195 (TM5), F254 (TM6) form the ligand-binding pocket ⁵

Tissue expression and function

Exocrine glands

MC5R’s defining physiological role was established by the MC5R-knockout mouse (Mc5r-null, backcrossed 7–9 generations onto C57BL/6J; porphyrin experiments used 129/SvEvTac background): KO animals show markedly impaired sebaceous lipid production (15–20% reduction in total acetone-extractable hair lipids; >50% reduction in sterol esters specifically), impaired fur water repulsion (KO mice absorbed ~twice as much water by body mass vs. WT during swimming — ~5% vs ~2.5% body weight), defective lacrimal protein secretion (α-MSH/ACTH increased lacrimal protein secretion ~80% in WT but not KO), and nearly absent Harderian gland porphyrin synthesis (in 129/SvEv background). Thermoregulation defects in KO mice are attributable to the sebaceous and Harderian lipid deficit — the paper does not implicate sweat/eccrine glands ⁶.

In humans, MC5R mRNA is detected in sebaceous glands and hair follicles by RT-PCR, and MC5R protein localizes to the epidermis, hair follicles, sebaceous glands, and eccrine glands by immunohistochemistry ⁷. MC5R protein is expressed in rat preputial cells (used as a sebaceous gland model); the Thiboutot 2000 paper studied human sebaceous glands and rat preputial cells — not human preputial cells ⁷. In sebocyte culture, MC5R protein is expressed in the cytoplasm of differentiated, lipid-laden sebocytes rather than undifferentiated basal cells — making it a differentiation marker for the sebocyte lineage ⁸.

Summary of exocrine phenotypes from MC5R-KO mice (Chen 1997; C57BL/6J congenic unless noted):

Gland	Phenotype in MC5R-KO
Sebaceous	15–20% reduction in acetone-extractable hair lipids; >50% reduction in sterol esters; impaired fur waterproofing
Lacrimal	α-MSH/ACTH-stimulated protein secretion abolished; EC₅₀ for ACTH stimulation ~4 nM in WT
Harderian	Nearly complete porphyrin deficiency (129/SvEv strain); porphyrin synthesis and stress-induced release require MC5R
Preputial	Reduced pheromone/odorant output; high MC5R expression confirmed (preputial mRNA ~30× skin level)

Skeletal muscle and adipose

MC5R is the predominant melanocortin receptor subtype in skeletal muscle and white adipose tissue. In these contexts it promotes lipid mobilization and glucose uptake, representing a potential metabolic modulatory role ².

Immune cells

MC5R is expressed on macrophages, where α-MSH/MC5R signaling suppresses phagolysosome activation and promotes recovery of ocular immune privilege following uveitis ⁹. no-fulltext-access — Ng 2021 PDF download failed (no OA URL available in a local paper archive); this claim is attributed to that paper but not independently verified against the full text. Combined deficiency of MC5R and adenosine 2A receptor (A2Ar) in double-knockout (DKO) mice results in significantly fewer CD8+ T cells in spleen and thymus compared to WT and single-KO mice, and confers resistance to experimental autoimmune uveitis (EAU); susceptibility is restored by transfer of WT CD8+ T cells into DKO mice, demonstrating a CD8+ T cell-dependent mechanism ¹⁰.

Xu 2020 notes that MC5R is expressed by B and T lymphocytes ³, but the specific claim of MC5R involvement in B-cell IgG class switching and NK-cell function has not been traced to a verified primary source in this pass. unsourced — B-cell IgG class-switching and NK-cell claims require primary-source DOI confirmation before inclusion as factual claims.

Role in aging

There is no primary study directly linking MC5R to aging mechanisms in mammals as of the literature search date (2026-05-09). The aging relevance is inferential:

1. Exocrine gland decline with age — sebaceous gland activity, lacrimal secretion, and eccrine sweating all decline with aging, contributing to xerosis (dry skin), dry-eye syndrome (xerophthalmia), and impaired thermoregulation — known clinical-aging concerns. Whether MC5R signaling drives this decline (loss of ligand, receptor, or downstream coupling) is not established.

2. Immunosenescence intersections — MC5R’s immunosuppressive role on macrophages is relevant to chronic-inflammation and altered-intercellular-communication, but no study has specifically interrogated MC5R in the context of inflammaging.

3. Skeletal muscle metabolic role — as the predominant melanocortin receptor in skeletal muscle, MC5R may participate in the metabolic derangements of sarcopenic or metabolically aging muscle. This is entirely speculative. needs-human-replication no-mechanism

Dimension	Status	Notes
Pathway conserved in humans?	yes	GPCR/cAMP signaling conserved; MC5R protein confirmed in human sebaceous tissue
Phenotype conserved in humans?	partial	Exocrine decline with aging is a human phenomenon; MC5R’s causal role is undemonstrated
Replicated in humans?	no	No aging-focused human MC5R study; primary evidence is MC5R-KO mouse

needs-human-replication — All functional data is from MC5R-KO rodents. The aging-context claims are indirect and require targeted study.

Pharmacology and druggability

Druggability tier: 3 (predicted druggable; no aging-validated or FDA-approved drug).

Selective MC5R agonists are an active area of discovery but face a fundamental development bottleneck: achieving selectivity over the closely related MC1R and MC4R. MC4R shares high sequence homology, and MC4R agonists/antagonists have independent clinical relevance (obesity: setmelanotide for MC4R pathway). MC1R is the melanoma/pigmentation receptor. Off-target activity at either limits MC5R-selective compound development.

Research probes:

• PL8177 — a cyclic peptide MC1R/MC5R agonist with anti-inflammatory properties; no aging indication; early clinical exploration for inflammatory diseases
• Competitive antagonists (SHU9119, HS024, JKC-363) — pharmacological tools; research use only
• No clinical-stage compound with MC5R as the primary target and an aging indication exists

Therapeutic hypotheses (speculative, not validated):

• MC5R agonists as treatments for dry-eye syndrome or xerosis by restoring exocrine secretory function
• MC5R modulation in inflammatory skin or ocular disease (acne, seborrhea, uveitis)
• Metabolic applications (adipose/muscle MC5R agonism) — overlaps with MC4R obesity pathway concerns

Pathway membership

• melanocortin-system — primary pathway; MC5R is one of five melanocortin receptors (MC1R–MC5R) activated by POMC-derived peptides

Key interactors / related proteins

• alpha-msh — primary endogenous agonist; POMC-derived
• pomc — precursor gene for α-, β-, γ-MSH and ACTH; all endogenous MC5R ligands are POMC cleavage products
• mc1r — sister receptor; convergent ligand set; selectivity overlap is pharmacological bottleneck
• mc4r — sister receptor; obesity relevance; high-homology risk for off-target effects

Gaps and limitations

• needs-human-replication — No primary aging study exists; all exocrine phenotype data derives from MC5R-KO mice.
• no-mechanism — The mechanism linking MC5R activity levels to age-related exocrine decline is entirely unstudied.
• needs-gtex-query — GTEx aging-correlation not queried; MC5R is tissue-restricted and low-abundance in bulk-tissue assays.
• Ensembl ID corrected from ENSG00000198822 (which is GRM3, a different gene) to ENSG00000176136 (confirmed MC5R via UniProt P33032 cross-reference and Ensembl REST lookup). HGNC:6933 and NCBI Gene 4161 confirmed correct.
• unsourced — B-cell IgG class-switching and NK-cell claims: Xu 2020 confirms B/T lymphocyte MC5R expression broadly, but the specific IgG class-switching and NK-cell functional claims have not been traced to a primary source.
• dose-response-unclear — No human dose-response data for any MC5R-targeted agent in any indication.

Footnotes

Footnotes

1. doi:10.1016/j.mce.2009.01.014 · Rodrigues AR et al. · Mol Cell Endocrinol 2009 · in-vitro (HEK293 cells expressing rat MC5R-GFP) · α-MSH activates ERK1/2 (EC₅₀ ~7.3 nM) through a PI3K-regulated, Akt-independent pathway; cAMP, PKA, PKC, and Akt are not required; abolished by wortmannin and LY294002 ↩

2. doi:10.3390/ijms23158727 · Ji L-Q et al. · Int J Mol Sci 2022 · review · pharmacology + energy-metabolism regulation; covers MRAP interactions and skeletal muscle/adipose expression ↩ ↩²

3. doi:10.1007/s00018-020-03511-0 · Xu Y et al. · Cell Mol Life Sci 2020 · review · comprehensive MC5R signaling pathway overview in health and disease; covers immunomodulation, thermoregulation, sebogenesis ↩ ↩²

4. doi:10.1016/j.bbamcr.2017.04.004 · Rodrigues AR et al. · Biochim Biophys Acta Mol Cell Res 2017 · in-vitro · serine-rich N-terminal motifs required for MC5R cell-surface targeting ↩

5. doi:10.1021/bi3013593 · Yang Y et al. · Biochemistry 2013 · in-vitro (mutagenesis) · ligand-binding pocket residues identified by site-directed mutagenesis ↩

6. doi:10.1016/s0092-8674(00)80467-5 · Chen W et al. · Cell 1997 · in-vivo (mouse, KO) · model: Mc5r-null backcrossed 7–9 generations onto C57BL/6J (most experiments); 129/SvEvTac used for Harderian porphyrin experiments · local PDF available in a local paper archive ↩

7. doi:10.1046/j.1523-1747.2000.00094.x · Thiboutot D et al. · J Invest Dermatol 2000 · in-vitro (human sebaceous gland tissue + rat preputial cells) · MC5R mRNA detected in human sebaceous glands by RT-PCR; MC5R protein confirmed in human epidermis, hair follicles, sebaceous glands, and eccrine glands by IHC; rat preputial cells used as a sebaceous gland model (not human preputial cells) ↩ ↩²

8. doi:10.1016/j.peptides.2005.05.030 · Zhang L et al. · Peptides 2006 · in-vitro (human tissue) · MC5R as differentiation marker of human sebocytes; not_oa in a local paper archive ↩

9. doi:10.1080/09273948.2020.1849735 · Ng TF et al. · Ocul Immunol Inflamm 2021 · in-vivo (mouse) · MC5R on macrophages mediates α-MSH immunosuppression; loss → heightened retinal damage in uveitis ↩

10. doi:10.3389/fimmu.2021.742154 · McDonald T et al. · Front Immunol 2021 · in-vivo (mouse, double-KO) · MC5R/A2Ar combined deficiency reduces CD8+ T cells and confers autoimmune-disease resistance ↩