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Peptides in Dermal Fibroblast and Keratinocyte Models
For laboratory research use only. The compounds discussed here are sold strictly for in vitro research. They are not drugs, supplements or cosmetics and are not for human or animal use. This article summarizes published literature for reference and is not medical or usage advice.
Peptides and skin cells have a long shared history in the literature, mostly because dermal fibroblasts and keratinocytes are easy to culture and respond measurably to short sequences. Three families dominate the published work: matrikines, fragments released when extracellular matrix proteins are broken down, melanocortins acting on pigment cells, and antimicrobial peptides made by the epidermis itself. Below is what the primary papers actually measured in these models, plus the physical constraint that governs everything, which is that the stratum corneum is very good at keeping molecules out.
Key findings
- GHK-Cu stimulated collagen synthesis in cultured fibroblasts from between 10 to the minus 12 and 10 to the minus 11 molar, peaking at 10 to the minus 9 molar, with no change in cell number [1].
- A GHK triplet occurs in the alpha-2 chain of type I collagen, so proteases can liberate it from the matrix itself [1].
- Human epidermal keratinocytes and melanocytes both secrete proopiomelanocortin, and POMC stimulated cyclic AMP at the MC-1R and raised melanogenesis in human pigment cells [2].
- Passive penetration of the stratum corneum generally requires a molecular weight under about 1 kDa plus suitable lipophilicity; unaided macromolecules do not get through [3].
- In a 12 week, double blind, split-face study of 93 women aged 35 to 55, a moisturizer with 3 ppm palmitoyl-KTTKS reduced measured fine lines against the same moisturizer without it [4].
The barrier comes first
Any discussion of peptides applied to skin has to start with the stratum corneum, because it decides what is even possible. The working rule from the delivery literature is a maximum molecular weight around 1 kDa together with high lipophilicity and a certain polarity, and macromolecules without assistance do not penetrate human skin at all [3]. Physical approaches such as electroporation, sonophoresis and iontophoresis, or chemical penetration enhancers, are what change that arithmetic.
Which is why the peptides with the most published cell data are so short. GHK is three residues. KTTKS is five. These sit at the small end on purpose, and the palmitoyl group attached to KTTKS in cosmetic formulations is there to raise lipophilicity, not to do anything at the receptor. That single fact explains most of the formulation chemistry you encounter in this area.
Matrikines: GHK and KTTKS
Maquart and colleagues reported the core GHK result in 1988 [1]. GHK, a tripeptide with affinity for copper(II) that was isolated from human plasma, raised collagen synthesis in fibroblast cultures starting somewhere between 10 to the minus 12 and 10 to the minus 11 molar and peaking at 10 to the minus 9 molar. Importantly, the effect was independent of any change in cell number, so it was not simply more cells making more collagen. The authors also noted that a GHK triplet sits in the alpha-2 chain of type I collagen, which suggests the peptide can be cut out of the matrix by proteases where tissue is being remodelled. That is the defining property of a matrikine: the matrix is both the substrate and the source of the signal.
Pickart’s later review catalogued a broader set of reported actions for GHK and GHK-Cu, including increased synthesis of collagen, elastin, metalloproteinases and anti-proteases, chemoattraction of repair cells, and increased fibroblast proliferation [5]. Read that one with the author’s commercial involvement in mind, which is disclosed in the paper, and treat the 1988 concentration-response as the firmer measurement.
KTTKS is a fragment of the type I collagen propeptide, and the palmitoylated version is the one tested. Robinson and colleagues ran a 12 week, double blind, placebo controlled, split-face randomized study in 93 Caucasian women aged 35 to 55, comparing a moisturizer against the same moisturizer containing 3 ppm pal-KTTKS [4]. Quantitative image analysis and expert grading both showed reduced fine lines versus control, and the material was well tolerated. Three parts per million is a strikingly low concentration, which is consistent with the picomolar to nanomolar potencies seen for matrikines in culture.
Melanocortins in pigment cells
The second family acts on melanocytes. Alpha-MSH binds MC1R, raises cyclic AMP, and increases tyrosinase driven melanin synthesis. What Rousseau and colleagues added is that the epidermis runs this system locally [2]. Keratinocytes and matched melanocytes from five healthy donors both secreted POMC, the precursor. Processing was incomplete, so the precursor itself left the cells. Alpha-MSH was secreted at much lower levels and only by keratinocytes, and neither cell type released ACTH, though the processing enzymes PC1, PC2 and 7B2 were present in both. Hair follicle melanocytes secreted both POMC and alpha-MSH, and corticotrophin-releasing hormone increased that through the CRH receptor 1. In MC-1R transfected cells, POMC stimulated cyclic AMP, less potently than ACTH or alpha-MSH did, and increased melanogenesis and dendricity in human pigment cells.
Our separate write-up on Melanotan 2 covers the synthetic side of that receptor family in more detail.
Antimicrobial peptides from the epidermis
The third group is made by skin cells as part of barrier function. Alpha-MSH and its C-terminal tripeptide KPV inhibited Staphylococcus aureus colony formation and affected Candida albicans viability and germ tube formation in vitro, with cellular cyclic AMP proposed as the mechanism, and neutrophil killing of both organisms was enhanced rather than reduced [6]. Cathelicidin LL-37 is the better studied example in human epidermis; we cover it in our LL-37 overview.
| Peptide | Length | Model where characterized | Measured endpoint |
|---|---|---|---|
| GHK-Cu | 3 residues | Fibroblast culture [1] | Collagen synthesis, maximal at 1 nM |
| Pal-KTTKS | 5 residues, palmitoylated | 93 subject split-face study [4] | Fine line measurement by image analysis |
| Alpha-MSH / POMC | 13 residues / precursor | Human keratinocytes and melanocytes [2] | cAMP at MC-1R, melanogenesis, dendricity |
| KPV | 3 residues | S. aureus, C. albicans cultures [6] | Colony formation, germ tube formation |
Reading this literature carefully
A few recurring problems are worth naming. Cell culture concentrations and formulation concentrations are not comparable, since what reaches a fibroblast in intact tissue after a topical application is unknown unless someone measured it. Copper complexation is a real variable for GHK, because the peptide and the copper complex are different entities and papers do not always specify which was tested. And split-face cosmetic studies measure what they measure, graded appearance over weeks, which is not the same evidence class as a concentration-response curve in culture.
On the material side, short peptides like these are simple to synthesize and easy to get wrong in characterization. Purity by reversed phase HPLC and identity by mass spectrometry are the minimum, with orthogonal identity testing expected for reference quality material, and counterion content affects how much peptide is actually in a weighed sample [7]. Check the certificate of analysis rather than a number printed on a label.
Frequently asked questions
What is a matrikine?
A peptide fragment released by proteolysis of an extracellular matrix protein that then acts as a signal itself. GHK qualifies because its sequence occurs in the alpha-2 chain of type I collagen [1].
Why are these peptides so short?
Because penetration of the stratum corneum broadly requires molecules under about 1 kDa, and unaided macromolecules do not cross human skin [3].
Does GHK need copper?
The 1988 fibroblast work tested the copper complex GHK-Cu, and GHK has known affinity for copper(II) [1]. Copper-free GHK has been studied separately, so the two should not be treated as interchangeable.
References
- Maquart FX, Pickart L, Laurent M, Gillery P, Monboisse JC, Borel JP. Stimulation of collagen synthesis in fibroblast cultures by the tripeptide-copper complex glycyl-L-histidyl-L-lysine-Cu2+. FEBS Lett. 1988. PMID 3169264
- Rousseau K, Kauser S, Pritchard LE, et al. Proopiomelanocortin (POMC), the ACTH/melanocortin precursor, is secreted by human epidermal keratinocytes and melanocytes and stimulates melanogenesis. FASEB J. 2007. PMID 17317724
- Münch S, Wohlrab J, Neubert RHH. Dermal and transdermal delivery of pharmaceutically relevant macromolecules. Eur J Pharm Biopharm. 2017. PMID 28647443
- Robinson LR, Fitzgerald NC, Doughty DG, Dawes NC, Berge CA, Bissett DL. Topical palmitoyl pentapeptide provides improvement in photoaged human facial skin. Int J Cosmet Sci. 2005. PMID 18492182
- Pickart L. The human tri-peptide GHK and tissue remodeling. J Biomater Sci Polym Ed. 2008. PMID 18644225
- Cutuli M, Cristiani S, Lipton JM, Catania A. Antimicrobial effects of alpha-MSH peptides. J Leukoc Biol. 2000. PMID 10670585
- McCarthy D, Han Y, Carrick K, et al. Reference Standards to Support Quality of Synthetic Peptide Therapeutics. Pharm Res. 2023. PMID 36949371