Skin & Anti-Ageing

GHK (glycyl-L-histidyl-L-lysine) is a naturally occurring human tripeptide with high affinity for copper(II) ions (binding constant ~10^-16 M). Plasma GHK levels decrease with age: approximately 200 ng/mL at age 20 → approximately 80 ng/mL at age 60. Proposed mechanisms of action: stimulation of collagen and glycosaminoglycan synthesis (via activation of fibroblasts and decoration synthesis), promotion of angiogenesis, anti-inflammatory effects (suppression of TNF-α, IL-6), antioxidant activity (copper is a cofactor for superoxide dismutase), attraction of immune cells to wound sites, and stimulation of nerve growth. The copper delivery function is significant — copper is essential for lysyl oxidase (collagen crosslinking), superoxide dismutase (antioxidant defence), and cytochrome c oxidase (mitochondrial energy production). Clinical evidence: published studies show wound healing acceleration in animal models and improved skin quality in small human trials, but large RCTs are lacking.

The term 'cosmeceutical' was coined by Albert Kligman in 1984 to describe products with biologically active ingredients that have both cosmetic and therapeutic effects. Critically, regulatory frameworks do not recognise this category: products are regulated either as cosmetics (requiring only safety substantiation, no efficacy proof) or as drugs/medicines (requiring clinical trials and regulatory approval). In the US, any product claiming to 'treat', 'cure', or 'modify the structure or function of the body' is classified as a drug regardless of marketing positioning. Peptide-containing skincare products exist in this regulatory grey zone — they are marketed as cosmetics (avoiding drug claims) but imply therapeutic benefits through terminology like 'stimulates collagen production' or 'reduces wrinkle depth'. The lack of regulatory efficacy requirements means: formulations may contain peptides at sub-therapeutic concentrations, delivery may be inadequate for dermal penetration, and clinical claims may not be supported by robust evidence.

Dermal peptide categories: (1) Pharmaceutical dermal peptides: cyclosporine ophthalmic (dry eye disease — immunosuppression at ocular surface), bacitracin/gramicidin (topical antibiotics), and investigational wound healing peptides. (2) Cosmeceutical peptides: signal peptides (Matrixyl/palmitoyl pentapeptide-4 — collagen synthesis stimulation; Matrixyl 3000/palmitoyl tripeptide-1 + palmitoyl tetrapeptide-7 — ECM component production), carrier peptides (GHK-Cu/copper peptide — delivering copper for enzymatic collagen synthesis and antioxidant activity), neurotransmitter-inhibiting peptides (acetyl hexapeptide-3/Argireline — claims to reduce wrinkles by inhibiting neuromuscular junction activity, sometimes called 'topical Botox'), and enzyme-inhibiting peptides (soybean-derived peptides inhibiting protease-mediated collagen degradation). Evidence quality varies: pharmaceutical peptides have Phase III RCT data; cosmeceutical peptides typically have small studies, in vitro data, or no published clinical evidence.

Elastin is encoded by a single gene (ELN) and produced primarily during foetal development and early childhood — adult elastin production is minimal (<1% per year turnover). The elastin precursor tropoelastin (approximately 72 kDa) is secreted by fibroblasts, cross-linked extracellularly by lysyl oxidase to form desmosine and isodesmosine crosslinks (unique to elastin), and deposited onto a microfibrillar scaffold of fibrillin-1 (mutations in fibrillin-1 cause Marfan syndrome). Mature elastic fibres can stretch to 150% of their resting length and recoil. Elastin degradation: neutrophil elastase and MMP-12 (macrophage metalloelastase) are the primary elastin-degrading enzymes. UV exposure induces MMP expression, accelerating elastin degradation (solar elastosis — the accumulation of degraded, non-functional elastin is a hallmark of photoaged skin). Because adult elastin replacement is minimal, preventing degradation is more impactful than stimulating synthesis for maintaining skin elasticity.

EPP is caused by mutations in the FECH gene (encoding ferrochelatase, the final enzyme in the haem biosynthesis pathway) or rarely the ALAS2 gene (X-linked protoporphyria, XLP). Ferrochelatase deficiency → accumulation of protoporphyrin IX (PPIX) in erythrocytes, plasma, and skin. PPIX is a potent photosensitiser — when it absorbs light (peak absorption at 408nm/Soret band, with additional peaks at 505, 540, 575, 630nm), it generates reactive oxygen species → rapid endothelial cell damage, mast cell degranulation, and complement activation → burning pain, oedema, and erythema within minutes of sun exposure. Afamelanotide mechanism in EPP: SC implant (60-day release) → MC1R activation on melanocytes → increased eumelanin production → eumelanin absorbs the light wavelengths that would otherwise activate PPIX → reduced phototoxic reactions → increased pain-free time outdoors. Clinical benefit: Scenesse Phase III showed significantly increased time in direct sunlight without pain.

Eumelanin is a heterogeneous polymer of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) units. Its dark colour (brown to black) results from extended conjugation within the polymer. Photoprotective properties: eumelanin absorbs UV radiation across a broad spectrum (UVB 290-320nm, UVA 320-400nm, and visible light), dissipating >99.9% of absorbed energy as heat through ultrafast internal conversion (photophysics preventing photochemical damage). Eumelanin also scavenges reactive oxygen species generated by UV exposure. The photoprotective capacity of eumelanin is the biological basis for afamelanotide's therapeutic effect in EPP — increased eumelanin deposition in the epidermis reduces the amount of UV-visible light that penetrates to deeper skin layers where protoporphyrin IX is deposited, thereby reducing phototoxic reactions.

Intrinsic (chronological) ageing affects skin through: reduced cellular proliferation (keratinocyte turnover slows from ~28 days in young skin to >40 days in elderly), decreased collagen synthesis (approximately 1% decline per year after age 30 — fibroblast senescence reduces both collagen production rate and collagen quality), reduced elastin network (degradation without replacement), thinning of dermis and epidermis (reduced ECM and fewer cell layers), decreased vascularity (fewer dermal capillaries), reduced sebaceous gland activity (drier skin), and decline in immune surveillance cells (Langerhans cells). At the molecular level: telomere shortening (limiting cell division capacity), mitochondrial DNA mutations (reducing cellular energy production), accumulation of advanced glycation end-products (AGEs — crosslinking collagen, reducing its flexibility), and altered gene expression (reduced growth factor production). Intrinsic ageing is genetically programmed and cannot be fully prevented, though lifestyle factors (nutrition, sleep, stress management) modulate its rate.

Keratinocytes constitute approximately 90% of epidermal cells and are responsible for producing keratin (the structural protein of skin, hair, and nails) and maintaining the epidermal barrier. Keratinocyte lifecycle: stem cells in the basal layer divide → daughter cells progressively differentiate as they migrate upward through stratum basale → stratum spinosum → stratum granulosum → stratum corneum (where they are terminally differentiated, enucleated corneocytes). This transit takes approximately 28-40 days. During differentiation, keratinocytes produce keratins (K5/K14 in basal layer → K1/K10 in suprabasal layers), lipids (ceramides, cholesterol, fatty acids that form the stratum corneum lipid matrix), and involucrin/loricrin (forming the cornified envelope). Growth factors stimulating keratinocyte proliferation and migration: EGF, KGF (keratinocyte growth factor/FGF-7), TGF-α, and IGF-1. Understanding keratinocyte biology is fundamental to wound healing (re-epithelialisation) and skin barrier research.

Matrikines are peptide fragments released during ECM degradation that act as signalling molecules, triggering cellular responses to repair the damaged matrix. Key concept: the ECM is not merely structural but contains embedded biological information — specific proteolytic fragments activate cell surface receptors (integrins, elastin-binding protein, CD44) to stimulate matrix synthesis, cell migration, and proliferation. Examples: the GFOGER sequence in type I collagen binds integrin α2β1 to promote cell adhesion and differentiation; the VGVAPG sequence in elastin binds the elastin-binding protein to stimulate fibroblast chemotaxis and MMP-1 secretion; and the RGD sequence in fibronectin binds multiple integrins to regulate cell adhesion. Cosmeceutical matrikine-inspired peptides: palmitoyl pentapeptide-4 (KTTKS) is a matrikine derived from type I collagen's C-terminal propeptide — it signals fibroblasts to increase collagen production.

Melanocytes reside in the basal layer of the epidermis at a density of approximately 1,000-2,000 per mm² (relatively constant across ethnicities — skin colour differences result from melanin type, amount, and distribution rather than melanocyte number). Each melanocyte services approximately 36 keratinocytes through dendritic processes — the epidermal melanin unit. Melanin is synthesised in specialised organelles called melanosomes, which are transferred to surrounding keratinocytes via dendritic tips. Melanosome transfer mechanisms: cytocrine secretion (membrane fusion), phagocytosis (keratinocytes engulf melanosome-containing dendritic tips), and exocytosis/endocytosis (melanosomes released extracellularly then taken up). In darker skin, melanosomes are larger, more numerous, and distributed individually throughout keratinocytes; in lighter skin, they are smaller, fewer, and clustered. Afamelanotide stimulates both melanogenesis within melanosomes and melanosome transfer to keratinocytes.

Melanogenesis pathway: α-MSH binds MC1R on melanocytes → Gαs activation → adenylyl cyclase → cAMP → PKA → CREB phosphorylation → MITF (microphthalmia-associated transcription factor) upregulation → increased transcription of melanogenic enzymes: tyrosinase (rate-limiting enzyme converting tyrosine → DOPA → dopaquinone), TRP-1 (tyrosinase-related protein 1), and TRP-2/DCT (dopachrome tautomerase). Dopaquinone branches into eumelanin pathway (brown-black, photoprotective — via DHI and DHICA intermediates) or pheomelanin pathway (red-yellow, less protective — via cysteinyl-DOPA when cysteine is available). Afamelanotide ([Nle4, D-Phe7]-α-MSH) has approximately 100-1000× the potency of native α-MSH at MC1R, preferentially stimulating eumelanin production. The resulting increased eumelanin absorbs UV radiation and dissipates it as heat, providing photoprotection that benefits EPP patients by reducing protoporphyrin-mediated phototoxic reactions.

Pheomelanin is a sulphur-containing polymer formed when cysteine conjugates with dopaquinone (diverting the melanogenic pathway from eumelanin production). Its colour ranges from yellow to red. Pheomelanin is less photoprotective than eumelanin and, problematically, can generate reactive oxygen species upon UV exposure (through photosensitisation mechanisms involving the benzothiazine chromophore). Individuals with MC1R loss-of-function variants (common in Celtic populations) produce predominantly pheomelanin, resulting in red hair, fair skin, and increased UV sensitivity and melanoma risk. The eumelanin:pheomelanin ratio is the primary determinant of constitutive photoprotection. Afamelanotide shifts this ratio toward eumelanin by strongly activating MC1R — even in individuals with partial MC1R loss of function, supraphysiological stimulation can increase eumelanin production.

Photoageing (extrinsic ageing) results from cumulative UV exposure: UVB (290-320nm) causes direct DNA damage (cyclobutane pyrimidine dimers, 6-4 photoproducts), while UVA (320-400nm) generates ROS that cause indirect DNA damage, lipid peroxidation, and protein modification. Chronic UV exposure → persistent MMP induction (MMP-1, -3, -9 degrading collagen and elastin), reduced procollagen synthesis (UV-activated AP-1 transcription factor inhibits type I collagen gene expression), solar elastosis (accumulation of degraded, tangled elastic fibres — the histological hallmark of photoageing), and melanocyte irregularity (lentigines/age spots). Clinically: wrinkles, rough texture, dyspigmentation, telangiectasia, and loss of elasticity. Photoageing can be partially prevented (sun protection) and partially treated (retinoids, which stimulate collagen synthesis and normalise cell turnover). Peptide-based anti-photoageing strategies target: collagen stimulation, MMP inhibition, antioxidant protection, and melanocyte regulation.

Biological photoprotection mechanisms: constitutive (baseline melanin content, SC thickness, urocanic acid in sweat — absorbing UV), inducible (tanning response — UV exposure → DNA damage → p53 activation → POMC/α-MSH production by keratinocytes → MC1R activation on melanocytes → melanogenesis; delayed tanning takes 3-5 days to develop), and enzymatic (DNA repair enzymes — nucleotide excision repair fixing UV-induced pyrimidine dimers). Afamelanotide provides pharmacological photoprotection by: directly stimulating melanogenesis without requiring UV-induced DNA damage (bypassing the need for the UV→p53→α-MSH cascade), achieving higher eumelanin levels than natural tanning in many individuals, and maintaining melanisation during periods of minimal sun exposure (the implant provides continuous MC1R stimulation for ~60 days). Complementary external photoprotection: broad-spectrum sunscreen (SPF ≥30), protective clothing, and behavioural UV avoidance. For EPP patients, afamelanotide supplements but does not replace external photoprotection measures.

Signal peptides in cosmeceuticals are short amino acid sequences designed to mimic ECM degradation fragments (matrikines) that naturally signal fibroblasts to produce new matrix components. Key examples: palmitoyl pentapeptide-4 (KTTKS — a type I collagen fragment, palmitoylated for skin penetration; studies show stimulation of collagen I, III, and fibronectin synthesis in human fibroblast cultures; limited clinical trial data showing modest improvement in wrinkle depth), palmitoyl tripeptide-1 (GHK — growth factor-like peptide stimulating collagen synthesis; the tripeptide core is identical to the GHK copper peptide without the metal), and palmitoyl hexapeptide-12 (stimulating hyaluronic acid and collagen production). Penetration is a major challenge: even with palmitoylation (adding a C-16 fatty acid to improve lipophilicity), these peptides must cross the SC, which limits delivery to the superficial dermis. Clinical evidence for most signal peptides is based on small, often manufacturer-funded studies.

Skin barrier integrity depends on three components: physical barrier (SC bricks-and-mortar structure — corneocytes, lamellar lipid matrix, cornified envelope), chemical/biochemical barrier (acidic pH 4.5-5.5 maintained by fatty acids and amino acids — the acid mantle; natural moisturising factors/NMFs — hygroscopic amino acids, urea, lactate derived from filaggrin degradation — maintaining SC hydration), and immunological barrier (antimicrobial peptides — defensins, cathelicidins/LL-37, dermcidin; toll-like receptors on keratinocytes detecting pathogens). Barrier disruption cycle: SC damage → increased TEWL → dehydration → inflammation → protease activation → further barrier degradation. Restoration: ceramide-dominant lipid replacement, NMF replenishment, pH normalisation, and allowing keratinocyte differentiation to rebuild normal SC. For peptide skincare research, barrier function assessment (TEWL, SC hydration by corneometry, and skin pH) provides objective endpoints for evaluating product effects.

The stratum corneum (SC) follows the 'bricks and mortar' model: corneocytes (flattened, keratin-filled dead cells — the bricks) embedded in a lamellar lipid matrix (ceramides, cholesterol, free fatty acids in a 1:1:1 molar ratio — the mortar). SC thickness varies: approximately 10-15 cell layers (10-20μm) on most body sites, up to 100+ layers on palms and soles. The SC provides: barrier function (preventing transepidermal water loss — normal TEWL is 5-10 g/m²/h; values >25 indicate barrier disruption), UV protection (absorbing/scattering UV radiation), chemical resistance (preventing penetration of most exogenous substances), and microbial defence (acidic pH 4.5-5.5 and antimicrobial peptides create an inhospitable environment for pathogens). The SC is the primary barrier to transdermal peptide delivery — its lipophilic, tightly packed structure excludes large hydrophilic molecules like peptides, necessitating technologies like microneedles or iontophoresis for transdermal peptide delivery.

TEWL is measured using evaporimeters (open-chamber — Tewameter; closed-chamber — VapoMeter; or unventilated-chamber systems) that detect water vapour flux from the skin surface. Normal TEWL values: forearm approximately 5-10 g/m²/h, face approximately 10-15 g/m²/h. Elevated TEWL indicates barrier disruption (atopic dermatitis: 15-25 g/m²/h; severely damaged skin: >25 g/m²/h). TEWL is used as: a diagnostic measure of skin barrier function, an endpoint in barrier repair studies, a quality control metric for skincare product efficacy, and a research tool for evaluating peptide formulation effects on skin integrity. For transdermal peptide delivery research, TEWL measurement confirms whether delivery technologies (microneedles, iontophoresis) have created transient barrier disruption sufficient for peptide penetration without causing excessive or prolonged skin damage.