Opsin 5 Study: Ultraviolet Raised Melanin in Cultured Cells Only With Added Light-Catching Molecules

A cell and skin-sample study: in cultured cells, ultraviolet light raised pigment above untreated cells only with an added light-catching molecule. Not proof of how people tan.

In a lab dish, human skin pigment cells get one dose of ultraviolet (UV) light. A 2021 cell and skin-sample study says a light sensor called opsin 5 drives their pigment response. Its own figures and methods do not line up.[1]

What this means: This is a cell and skin-sample study with internal inconsistencies, not proof of how people tan. In cultured cells, pigment rose above untreated cells only when a light-catching molecule was added. The key test of whether UV needs opsin 5 rests on one blot that does not clearly show it.[1]

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What was tested

Evidence: lab study (in vitro / ex vivo): human cells and skin explants; no animals, no people. SUBJECT PAPER (2021) · Lan et al., British Journal of Dermatology 2021

The paper asks how melanocytes (skin cells that make the pigment melanin) sense UV light.[1]

• Samples: fresh pigment cells from children's foreskin (HEMs, human epidermal melanocytes); PIG1, an immortalised (altered to keep dividing) melanocyte line; and foreskin explants.[1]
• Explants: small skin pieces kept alive in a dish (ex vivo: outside the body). Donor and explant numbers are not reported.[1]
• UV: one broadband dose, 280-400 nm (nanometres) wavelength, stated as 1.5 J/cm2 (joules of energy per square centimetre). It covers UVA plus UVB (ultraviolet A and B, the two UV bands); the paper writes UVR (ultraviolet radiation).[1]
• Retinal: all-trans retinal, the opsin light-catching molecule, added to some dishes. OPN5 (opsin 5) is a UV- and violet-sensitive light-receptor protein.[1]
• Pigment proteins: TYR (tyrosinase), TRP1 and TRP2 (tyrosinase-related proteins); MITF (microphthalmia-associated transcription factor), the pigment master switch, and its switched-on form p-MITF (phosphorylated).
• Tools and measures: siRNA (small interfering RNA) lowers OPN5; a lentivirus adds extra OPN5 (H-OPN5); NC is the negative control. mRNA (messenger RNA) is a gene's copy; a Western blot ("blot") photographs proteins.

How to read the numbers

• By eye: every value was read by eye from the graphs and is approximate; the paper does not report them as numbers. Untreated = 1.0 unless stated.[1]
• n = 3: means of 3 independent experiments. Tests: one-way ANOVA (analysis of variance) then Tukey's test, comparing group averages pair by pair.[1]

A p-value (written P or p) is roughly how often a difference at least this big would turn up by chance if the treatment did nothing.

Below 0.05 is the usual bar; the smaller it is, the harder the result is to explain by chance alone.

• Stars: ** means P < 0.01 and *** means P < 0.001: by chance fewer than 1 time in 100, or 1 in 1,000. No exact P values, effect sizes or CIs (confidence intervals) are given.[1]
• One blot: each protein photo is one representative blot, with no measurement or statistics.[1] A darker band means more protein.

1. In cells, UV raised pigment only with added retinal

Evidence: lab study, human cells and explants; not people.[1] · Lan et al., British Journal of Dermatology 2021

Fresh cells got nothing, UV, retinal, or retinal + UV. Melanin and tyrosinase activity were read 24 h (hours) and 48 h later.[1]

• Melanin: at 24 h, UV about 1.0 vs retinal + UV about 1.24, starred vs all three groups. At 48 h, about 1.17 vs 1.47, starred vs untreated only.[1]
• Tyrosinase: at 24 h, UV about 0.94 vs retinal + UV about 1.30 (** vs UV). At 48 h, about 1.21 vs 1.75, not marked vs UV.[1]
• Genes and protein, 48 h: only retinal + UV bars are starred. On one blot, the strongest bands are retinal + UV.[1]
• Explants (no retinal added): at 48 h, UV raised melanin staining, about 0.03 vs 0.11 relative units (***).[1]

In the cell graphs, UV alone was never significantly different from untreated cells. For melanin and tyrosinase, retinal + UV beat UV alone at 24 h only, not at 48 h.[1]

For these graphs, see the paper's Supplementary Figure S2e-f (https://pmc.ncbi.nlm.nih.gov/articles/PMC8453816/).[1]

Text vs figure: the text says pigment proteins and genes "in the UVR irradiation group increased significantly". The graphs show that only for retinal + UV.[1]

Limits:[1]

• The explant n may be microscope fields, not explants; one photo shows two pieces per dish. OPN5 was not changed in explants, and the cell melanin stain (Figure S2a) is not quantified.
• Cells grew without keratinocytes (the main outer-skin cells). The claim that "all cells remained viable" rests on a metabolic readout, not a live-cell count.

2. Opsin 5 is present in these cells

Evidence: lab study, human cells and explants; not people.[1] · Lan et al., British Journal of Dermatology 2021

• OPN5 was found in fresh and PIG1 cells by mRNA tests, blots and antibody staining; explants by staining.[1]
• Baseline mRNA: about 0.000026 (fresh) and 0.000037 (PIG1), relative to a steady reference gene. By eye, that is very low.[1]
• After UV: OPN5 mRNA rose only with retinal + UV, about 0.013 vs 0.003 (about 4 times; **, n = 3). UV alone did not significantly raise it.[1]
• Protein (one blot): by eye, UV alone is not stronger than untreated; retinal + UV is strongest. For this one blot, the authors call the rise "slightly".[1]

Text vs figure: the text says the OPN5 gene "appeared to increase significantly" after UV; the graph shows that only with retinal.[1]

Limits:[1]

• Untreated OPN5 mRNA is about 0.00003 in Figure 1a but about 0.003 in Figure 1d: about 100-fold apart by eye, same axis label, unexplained.
• By eye, Figures 3a and 4g (one blot each) show clearly stronger OPN5 bands with UV, unlike Figure 1e (retinal status not stated).
• The antibodies are polyclonal (a mix of antibodies), not checked on cells lacking OPN5. By eye, the cell stain fills the cytoplasm (cell interior); the legend says plasma membrane.
• In explants, by eye, the stain covers much of the outer skin, and the antibody listing has an error (Problem 4).
• The legend says OPN5 "colocalized with melanocyte marker MelanA"; the text says stain intensity was higher after UV. Neither is measured.
• So OPN5 in explant melanocytes is reported, not shown.

Context, not this paper: a 2015 study found melanocyte opsin mRNA "does not change in response to physiological UVR doses". It also found "three OPN5 splice isoforms, two of which encode truncated proteins" (shortened versions).[5]

What this means: The receptor is present and can change in these conditions; that does not show it absorbs the UV.[1]

3. Without UV, changing opsin 5 changed pigment

Evidence: lab study, human cells only; not people.[1] · Lan et al., British Journal of Dermatology 2021

Lowering OPN5 in fresh cells, with no UV step described (untreated / NC / OPN5 siRNA):[1]

• Melanin: about 0.048 / 0.046 / 0.024, about half (** vs both). MITF and TYR mRNA also fell (starred vs both).[1]
• One blot: pigment-protein bands are fainter. OPN5 mRNA fell to about 0.09 of control; by eye, the protein fell less.[1]

Raising OPN5 in PIG1 cells: TYR, TRP1, TRP2 and MITF mRNA all rose (starred), for example TYR about 0.065 vs 0.10.[1]

On the one blot shown (Figure 2i), by eye, of the pigment proteins the rise is clear mainly for TYR with extra OPN5. The text says "Overexpression of OPN5 resulted in upregulated expression of TYR, TRP1, TRP2 and MITF".[1]

View panel i of Figure 2 in the open-access paper (figure not reproduced here: its CC BY-NC-ND 4.0 licence does not allow commercial reuse)

Panel i of Figure 2: one blot of PIG1 cells. By eye, of the pigment proteins the rise is clear mainly for TYR with extra OPN5.

Credit: Panel i of Figure 2 from Lan Y, Zeng W, Dong X, Lu H. British Journal of Dermatology 2021;185:391-404, doi:10.1111/bjd.19797. © 2021 The Authors. CC BY-NC-ND 4.0 (https://creativecommons.org/licenses/by-nc-nd/4.0/). Unaltered single-panel crop.

Reading key: PIG1 = a human melanocyte (pigment) cell line, a name not an acronym; Control = cells with the control virus; H-OPN5 = cells given extra OPN5 by a lentivirus (overexpression); OPN5 = opsin 5; TYR = tyrosinase; TRP1, TRP2 = tyrosinase-related proteins 1 and 2; MITF = microphthalmia-associated transcription factor; β-actin = beta-actin, the loading check.

In one line: On this one blot, by eye, extra OPN5 goes with a clearly stronger TYR band; TRP1 rises slightly; TRP2 and MITF look little changed.

• Where should I look? The TYR row: control (left) vs extra OPN5 (right).
• What am I comparing? Band darkness with and without extra OPN5.
• What does that support? On this one blot, extra OPN5 goes with a clearly stronger TYR band.
• What does it not establish? That TRP1, TRP2 and MITF rose as the text says, or that melanin rose.

Limits: no rescue test or second independent siRNA is shown, so hits on other genes are not ruled out. MITF mRNA fell further than OPN5 mRNA itself.[1]

Melanin was not measured after overexpression. PIG1 is immortalised with two human papillomavirus genes (HPV16 E6/E7), not primary cells. Retinal use is not stated.[1][4]

This supports a light-independent ("inherent") role for OPN5 in pigment production in cultured cells.[1]

4. Key test: does UV need opsin 5?

Evidence: lab study, human cells only; not people.[1] · Lan et al., British Journal of Dermatology 2021

The paper says: with OPN5 knocked down, UV no longer raised TYR, TRP1, TRP2 or MITF; they "did not change significantly".[1]

The blot (Figure 3a): one unmeasured blot, four lanes (untreated, OPN5 siRNA, UV, UV + siRNA).[1]

• By eye, the UV + siRNA lane has stronger TYR, TRP2, MITF and OPN5 bands than the siRNA-only lane.
• By eye, its OPN5 band is close to the UV lane, so the knockdown looks weak there.
• By eye, p-MITF is strong only in the UV lane: only switched-on MITF disappears. "Significantly" has no test shown.

View panel a of Figure 3 in the open-access paper (figure not reproduced here: its CC BY-NC-ND 4.0 licence does not allow commercial reuse)

Panel a of Figure 3: one blot of fresh pigment cells. The paper says UV needs OPN5; by eye, only p-MITF clearly disappears.

Credit: Panel a of Figure 3 from Lan Y, Zeng W, Dong X, Lu H. British Journal of Dermatology 2021;185:391-404, doi:10.1111/bjd.19797. © 2021 The Authors. CC BY-NC-ND 4.0 (https://creativecommons.org/licenses/by-nc-nd/4.0/). Unaltered single-panel crop.

Reading key: HEMs = human epidermal melanocytes (fresh skin pigment cells); UVR = ultraviolet radiation (UV light); si-OPN5 = OPN5 knocked down with small interfering RNA (siRNA); + / − = treatment given / not given; OPN5 = opsin 5, a UV/violet-sensitive light-receptor protein; TYR = tyrosinase, the main melanin-making enzyme; TRP1, TRP2 = tyrosinase-related proteins 1 and 2 (pigment enzymes); MITF = microphthalmia-associated transcription factor (pigment master switch); p-MITF = phosphorylated (switched-on) MITF; β-actin = beta-actin, a steady protein used as the loading check.

In one line: The paper's key knockdown blot: by eye, only switched-on MITF is lost.

• Where should I look? The two right lanes (UV, then UV + siRNA), ending with p-MITF.
• What am I comparing? Band darkness after UV, with and without OPN5 knockdown.
• What does that support? On this one blot, switched-on MITF is lost when OPN5 is knocked down.
• What does it not establish? That TYR, TRP2 or MITF stop rising; by eye, those bands still look stronger.

Other blots (one each):[1]

• In PIG1 (one blot), extra OPN5 plus UV gave the strongest TYR band; the text says "significantly higher", but no statistics are shown. By eye, the PIG1 UV-only lane is not clearly above control for TYR or MITF.
• With the MITF blocker ML329, UV did not raise TYR, TRP1 or TRP2; that blot has no UV-only lane.

Limits:[1]

• Melanin was not measured under knockdown plus UV. So the abstract's "OPN5 activation was necessary for UVR‐induced increase in cellular melanin" is not tested.
• ML329 places MITF downstream of UV, not of OPN5 specifically. By eye, its MITF lanes differ between two same-design blots (Figures 3d and 4i). Retinal use in Figure 3 is not stated.
• Other melanocyte UV sensors, rhodopsin (another opsin) and TRPA1 (transient receptor potential ankyrin 1, an ion channel), were not compared. So "the key sensor" is the authors' conclusion, not shown.[6][7]
• Several abstract sentences closely parallel a 2013 abstract about TRPA1. So do not treat the abstract as an exact summary of this paper's experiments.[6]

What this means: That UV needs OPN5 is the authors' claim, not something the one blot shows.[1]

5. Calcium, signal enzymes and a proposed chain

Evidence: lab study, human cells only; not people.[1] · Lan et al., British Journal of Dermatology 2021

Calcium: in retinal-treated cells, UV raised calcium about 2-3 times: about 21 vs 60 in fresh cells (***), 0.29 vs 0.62 in PIG1 (**).[1]

Signal enzymes (single unquantified blots; retinal use not stated): UV switched on two protein-switching enzymes, CAMKII (calcium/calmodulin-dependent protein kinase II) and PKC (protein kinase C). UV did not switch on MAPKs (mitogen-activated protein kinases).[1]

By eye, OPN5 knockdown weakened both signals. For CAMKII the text says "this is abolished when OPN5 is silenced"; a faint band remains by eye.[1]

By eye, total PKC also looks higher after UV, so a rise per PKC molecule is not shown.[1]

The proposed chain (Figure 5): UV, then OPN5, calcium, CAMKII, PKC, MITF, the pigment enzymes and melanin. It is a proposed model: several links were inferred, not tested directly.[1]

Limits:[1]

• Calcium was never measured with OPN5 knocked down; the OPN5 link rests on p-CAMKII as a stand-in.
• Calcium was measured without outside calcium, so any rise comes from stores inside the cell. Yet the Discussion says OPN5 "activates Ca2+ channels" (calcium channels), untested.
• ML329 did not block UV's p-PKC rise, so PKC is placed upstream of MITF; that blot has no UV-only lane.
• No calcium, G-protein (signal relay) or kinase blockers were used, so the abstract's signalling chain and the CAMKII-to-PKC order are inferred.
• With broadband UV and no wavelength test, OPN5 as the absorber is not shown. The Discussion itself says the retinal-bound pigment "has an absorption maximum in the visible region".

Context, not this paper: a 2025 review reports human OPN5 "preferentially activates Gq-mediated Ca2⁺ signaling"; this paper cites Gi coupling. Gq and Gi are two types of G protein (signal relays).[8]

What this means: The Figure 5 chain is a proposed explanation, not an observation.[1]

Problems in the paper

Evidence: the paper's own Methods and figures; calculations by the team.[1][2]

1. UV dose: "30 W cm–2 for 1–1·5 min" (W: watts) gives 1800-2700 J/cm2, or 1.8-2.7 J/cm2 if read as milliwatts (thousandths), not 1.5. Figure S1d uses 2 J/cm2, a dose not in the Methods list.[1][2]
2. Retinal units: 0.5 µmol (micromoles) per mL (millilitre) vs per litre in the Methods is 1000-fold apart, probably a unit error. From the stated 12 mmol/L (millimoles per litre) ethanol stock, per mL means about 4% ethanol. The 6 µmol/mL top viability dose means about 50% ethanol, implausible for live cells. The paper does not say which value was used.[1][2]
3. Duplicate siRNA: of four listed siRNA sequences, 1 and 4 are identical and 2 overlaps 1, so at most three distinct targets are listed. Figure S3a shows only one siRNA, not the four the Methods describe.[1][2]
4. Antibody mismatch: catalogue A0428, listed as "goat antirabbit", is goat anti-mouse. The paper's own cell-staining paragraph lists it as "goat antimouse". If used as written, the green could be the mouse MelanA (melanocyte-marker) antibody, which the paper does not resolve.[1][3]
5. Figure 3a: the text says UV no longer raised pigment proteins after OPN5 knockdown, but by eye the one blot shows only p-MITF lost.[1]

Who funded it

• Funding (quoted): 'Funding sources Supported by the National Natural Science Foundation of China (no. 81673069, no. 81972920).'[1]
• Conflicts of interest (COI), quoted: 'Conflicts of interest The authors declare they have no conflicts of interest.'[1]

What this doesn't prove

• Not proof in people: human cells and skin explants only. Not proof of how people tan.[1]
• Not that UV needs OPN5: the key evidence is one unquantified blot whose lanes, by eye, do not match the text.[1]
• Not that OPN5 absorbs the UV, or the full chain: no wavelength test, and Figure 5 is a proposed model.[1]
• Statistics: stars only, so no CI-vs-P check is possible; the legends apply ANOVA plus Tukey to two-group comparisons.[1]
• Missing basics: donors, ethics approval, consent and a UV spectrum are not reported.[1]
• Old paper: SUBJECT PAPER (2021), outside the 12-month window, so not current evidence beyond what it reports itself.[1]

Not advice: this is not medical or skincare advice. Nothing here says what anyone should do about sun exposure, tanning or pigment conditions.[1]

In short

Back to the dish: pigment cells made more melanin after UV only with added retinal, beating UV alone only at 24 h. Opsin 5 is present, and changing it changed pigment genes.[1]

But the one blot behind whether UV needs opsin 5 does not clearly show it, and the paper has dose, unit, sequence and antibody errors. Cell and skin-sample work, not proof of how people tan.[1]

Context: a related patent (idea, not evidence)

Evidence: patent record (idea, not evidence). Not a study; no data assessed.[9]

L'Oréal SA holds French patent FR3067933B1, granted 2020-07-17, on opsin 3 (OPN3), a different skin opsin (light-receptor protein) from OPN5, and skin pigmentation.[9]

In plain terms, the granted main claim covers siRNAs aimed at OPN3's mRNA to modulate skin pigmentation. An siRNA (small interfering RNA) is the gene-lowering tool this paper used on OPN5. Further claims cover pigmentation from visible blue-violet light (400-470 nm).[9]

A patent shows a granted claim, not that anything works. It concerns OPN3 and visible light, not OPN5 and UV: context only, not evidence for or against this paper.[9]

No patent was found that claims OPN5 in melanocytes, but the search was not exhaustive. It covered Google Patents only, top 25 results per query; Lens.org, PATENTSCOPE and Espacenet (other patent databases) were unavailable.[10]

Sources

DOI (digital object identifier): a permanent paper link. PMC (PubMed Central): a free full-text library. PMID: a PubMed record number.

1. SUBJECT PAPER (2021). Opsin 5 is a key regulator of ultraviolet radiation-induced melanogenesis in human epidermal melanocytes. Lan Y, Zeng W, Dong X, Lu H. Br J Dermatol (British Journal of Dermatology). 2021;185(2):391-404. doi:10.1111/bjd.19797. PMID 33400324; PMC8453816. Licence CC BY-NC-ND 4.0 (Creative Commons Attribution-NonCommercial-NoDerivatives 4.0). Peer-reviewed; preprint: no. https://doi.org/10.1111/bjd.19797 | free: https://pmc.ncbi.nlm.nih.gov/articles/PMC8453816/ (with supplementary figures S1-S3).
2. Calculated by this Article's team from values stated in the paper (not reported by the paper itself): dose arithmetic, retinal dilutions, siRNA overlap vs NCBI (National Center for Biotechnology Information) record NM_181744.4 (the OPN5 gene).
3. Beyotime catalogue A0428; Antibody Registry AB_2893435 (goat anti-mouse secondary). Public product records, used only to identify the antibody the paper lists as A0428.
4. Cellosaurus (a cell-line database) CVCL_S410 (PIG1). Public record, used only for how the PIG1 cell line was made.
5. CONTEXT, not this paper: Haltaufderhyde 2015, PMID 25267311. https://pubmed.ncbi.nlm.nih.gov/25267311/
6. CONTEXT, not this paper: Bellono 2013, PNAS (Proceedings of the National Academy of Sciences), PMID 23345429. https://pubmed.ncbi.nlm.nih.gov/23345429/
7. CONTEXT, not this paper: Wicks NL, Chan JW, Najera JA et al. UVA phototransduction drives early melanin synthesis in human melanocytes. Current Biology 2011;21:1906-11. PMID 22055294. https://pubmed.ncbi.nlm.nih.gov/22055294/
8. CONTEXT, not this paper: Ma J, 2025 review, PMID 41327490. https://pubmed.ncbi.nlm.nih.gov/41327490/
9. Patent record (idea, not evidence), CONTEXT, not this paper: L'Oréal SA. Modulateurs de l'opsine 3 dans la modulation de la pigmentation de la peau [Modulators of opsin 3 in the modulation of skin pigmentation]. FR3067933B1 (granted 2020-07-17); FR3067933A1 (published 2018-12-28); application FR1755671A, filed 2017-06-21. Google Patents record. The record is in French; the English comes from Google's machine translation. https://patents.google.com/patent/FR3067933B1/en | A1: https://patents.google.com/patent/FR3067933A1/en
10. Patent search by this Article's team, 2026-10-28: Google Patents only, top 25 results per query; Lens.org, PATENTSCOPE and Espacenet unavailable. Not exhaustive.

Images: panel a of Figure 3 and panel i of Figure 2 from Lan Y, Zeng W, Dong X, Lu H, British Journal of Dermatology 2021;185:391-404, doi:10.1111/bjd.19797, © 2021 The Authors, CC BY-NC-ND 4.0 (https://creativecommons.org/licenses/by-nc-nd/4.0/). Unaltered.

Not medical advice: this summarizes published research for general information only.