Epidermal Melanin Content Research Articles

YTHDF2 Regulates Advanced Glycation End Products-Induced Melanogenesis through Inhibiting A20 Expression in Human Dermal Fibroblasts.

Fibroblast A20 suppresses advanced glycation end products (AGEs)-induced melanogenesis by inhibiting NLRP3 inflammasome activation. AGEs repress A20 expression and significantly m6A-methylate A20 mRNA in fibroblasts. YTHDF2 is the most studied m6A reader protein and can accelerate degradation of m6A-modified mRNA. Whether YTHDF2 regulates AGEs-induced A20 expression and pigmentation is unknown. In this study, we confirmed that YTHDF2 inversely regulated AGEs-BSA-inhibited A20 expression but facilitated AGEs-BSA-activated NF-κB signaling and NLRP3 inflammasome in fibroblasts via YTHDF2 knockdown and overexpression experiments. Mechanistically, YTHDF2 bound to m6A-modified A20 mRNA induced by AGEs-BSA and increased its degradation. Moreover, fibroblast YTHDF2 robustly promoted AGEs-BSA-induced IL-18 level in coculture supernatants and melanin content, tyrosinase activity, and expression of microphthalmia-associated transcription factor and tyrosinase in melanocytes, which were significantly blocked by IL-18 binding protein. Further, fibroblast YTHDF2 markedly increased AGEs-BSA-induced epidermal melanin level in cocultured ex vivo skin and MAPKs activation in melanocytes. Importantly, upregulated dermal YTHDF2 expression was negatively correlated with dermal A20 level and positively associated with both epidermal melanin and dermal AGEs content in sun-exposed skin and lesions of melasma and solar lentigo. These findings suggest that fibroblast YTHDF2 positively regulates AGEs-induced melanogenesis mainly via A20/ NF-κB /NLRP3 inflammasome/ IL-18 /MAPKs axis in an m6A-dependent manner and functions in photoaging-induced hyperpigmentation skin disorders.

Pilot Study to Demonstrate Improvement in Skin Tone and Texture by Treatment with a 1064 nm Q-Switched Neodymium-Doped Yttrium Aluminum Garnet Laser.

The 1064 nm Q-switched neodymium-doped yttrium aluminum garnet (QS Nd:YAG) laser was developed to treat unwanted pigmentation in the skin such as lentigines caused by photoaging, and tattoos from dye/ink insertion. This laser has also been used for non-ablative epidermal rejuvenation (skin toning). To evaluate changes in skin tone, skin texture and overall improvement after a series of treatments with the QS Nd:YAG laser. Participants received seven full-face treatments with M22 or Stellar M22, a 1064 nm QS Nd:YAG laser, at 2-week intervals. The investigators and participants evaluated the improvement in skin tone and texture at 1, 3 and 6 months after the last treatment. Patient satisfaction, patient discomfort, downtime and adverse events were recorded. Histological changes in the treated area were also evaluated. Thirteen women with a median age of 45 years (range, 34-61 years) were included in the study. The majority of the participants (53.9%) had skin type VI. One month after the last treatment session, 38% of participants reported good to very good improvement. This value increased to 100% participant improvement at both the 3-month and 6-month follow-up visits. The reduction in melanin index and the histological analysis demonstrated that the laser procedure contributed to a reduction in epidermal melanin content. Treatments were not associated with high levels of pain or discomfort. The most common immediate post-treatment response was erythema and edema. Most participants were satisfied with the resulting treatment outcome. Skin treatment with the 1064 nm QS Nd:YAG laser module on the M22 and Stellar M22 devices, using a large spot size, low fluence, moderately high repetition rate, improves skin tone and texture in patients with skin types II-VI.

Phantoms for evaluating the impact of skin pigmentation on photoacoustic imaging and oximetry performance.

Recent reports have raised concerns of potential racial disparities in performance of optical oximetry technologies. To investigate how variable epidermal melanin content affects performance of photoacoustic imaging (PAI) devices, we developed plastisol phantoms combining swappable skin-mimicking layers with a breast phantom containing either India ink or blood adjusted to 50-100% SO2 using sodium dithionite. Increasing skin pigmentation decreased maximum imaging depth by up to 25%, enhanced image clutter, and increased root-mean-square error in SO2 from 8.0 to 17.6% due to signal attenuation and spectral coloring effects. This phantom tool can aid in evaluating PAI device robustness to ensure high performance in all patients.

Concentration-dependent stimulation of melanin production as well as melanocyte and keratinocyte proliferation by melatonin in human eyelid epidermis.

It remains unclear how the multifunctional indoleamine neurohormone, melatonin, alters melanin production and melanocytes within intact human epidermis under physiologically relevant conditions. In the current pilot study, we aimed to clarify this in long-term organ-cultured, full-thickness human eyelid skin, selected for its clinically recognized sensitivity to pigmentation-modulatory hormones. Warthin-Starry histochemistry showed that 100 μM melatonin significantly increased epidermal melanin content and melanocyte dendricity after 6 days of organ culture, even though tyrosinase activity in situ was inhibited, as assessed by quantitative immunohistomorphometry. While the higher melatonin dose tested here (200 μM) did not change epidermal melanization, but again inhibited tyrosinase activity, it increased the number and proliferation of both gp100+ epidermal melanocytes and keratinocytes as well as protein expression of the premelanosomal marker, gp100, ex vivo. Contrary to most previous studies, these eyelid skin organ culture results suggest that long-term melatonin application exerts overall stimulatory, dose-dependent effects on the epidermal pigmentary unit within intact human skin, which appear surprisingly tyrosinase-independent. While these provocative preliminary findings require further work-up and independent confirmation, they encourage one to systematically explore whether prolonged melatonin therapy can (re-)stimulate melanogenesis and increase the pool/activity of epidermal melanocytes in hypopigmented skin lesions.

527 Long-term melatonin treatment stimulates melanin production as well as melanocyte dendricity and proliferation in human epidermis ex vivo

The pigmentary activities of the neurohormone, melatonin, have long fascinated dermatologists. Yet, cell culture reports are contradictory, and it remains unclear how melatonin alters melanocyte physiology in human epidermis. Thus, we have asked in healthy, organ-cultured, full-thickness human eyelid skin whether melatonin stimulates/inhibits melanin production by epidermal melanocytes that operate within their natural skin habitat. By assessing melanin content, tyrosinase activity in situ, melanocyte number, proliferation, and dendricity (gp100/Ki-67 IF), we show here that, after 3 days of organ culture, 200ug/mL melatonin initially reduces the epidermal melanin content, but without affecting tyrosinase activity or melanocyte proliferation/number (no significant changes seen for 100ug/mL).

Skin type specific photobiological response to visible light is mediated by constitutional melanin.

Visible light (VL) induces varying photobiological responses between skin types, likely influenced by inherent melanization. Individual typology angle (ITA) objectively measures skin types. We hypothesize that epidermal melanin content and distribution determine VL response. This study describes clinical and histologic responses to VL and examines the potential role of melanin in the underlying mechanistic pathways. We grouped enrolled participants by ITA (Light=5, Intermediate=4, Dark=7) per colorimetry (CR-400, Konica Minolta). Photoprotected sites were exposed daily to 480 J/cm2 of VL (Fiber-Lite High Intensity Illuminator, Series 180, Dolan Jenner Industries Inc.) for 4 days (total=1920 J/cm2 ), as tolerated. Treated and control sites were biopsied 96h after first exposure. We used hematoxylin and eosin and Fontana-Mason to assess histological changes and melanin deposition, respectively. p53 and Ki67 immunohistochemical stains were done to assess DNA damage and proliferation. Matrix metalloproteinase (MMP)-1 expression was detected by immunohistochemical staining and immunofluorescence microscopy. Darker skin did not tolerate the full VL regimen with blistering occurring in most subjects at doses of 220-880 J/cm2 . Intermediate and Dark skin showed tanning. Light skin developed erythema. p53 counts were highest in Intermediate, followed by Light skin, although this was not statistically significant. VL treatment led to MMP-1 expression and nuclear localization in keratinocytes in Dark and Intermediate but not in Light skin, however differences between groups were not statistically significant. Skin types demonstrate unique biological responses to VL. The role of melanin in photoprotection is well-defined. However, given the pro-apoptotic function of nuclear MMPs, we suggest a potential mechanism by which melanin may mediate VL-induced phototoxicity.

Effects of traditional artiri La Li formulation on experimental models of vitiligo and mechanisms of melanogenesis

Ethnopharmacological relevanceArtiri La Li Honey Pill (ALLHP) is a traditional medicinal formula that is widely used in Xinjiang, China, for the treatment of vitiligo. Since the cause of vitiligo has not been determined, no satisfactory treatment is available. Clinical interventions include pharmacological treatment with psoralen, usually in conjunction with ultraviolet A (UVA) radiation, but toxic side effects limit this application. Studies on the activity and mechanisms of ALLHP are scarce. Aim of the studyTo verify the therapeutic effect of ALLHP on vitiligo and determine its effectiveness as a theoretical and experimental basis for the development of innovative drugs with independent intellectual property rights and the effective use of local resources. Materials and methodsThe experimental animal model of vitiligo was established by chemical decoloring. Rats were treated with gradient doses of ALLHP. The therapeutic effect was judged by gross observation. The contents of TYR, MAO, AchE and MDA in serum and skin tissue, the number of hair follicles containing melanin in skin tissue, the distribution of epidermal melanin, and the weight index of immune organs were detected, and the therapeutic effect of ALLHP on vitiligo was evaluated. In addition, certain monomer components in ALLHP were used to intervene in the zebrafish juvenile melanin suppression model, and the melanin-activating activities of some monomer components in ALLHP were screened by counting the melanin area ratio. ResultsALLHP increased the number of melanin-containing hair follicles and the epidermal melanin content in the skin of experimental vitiligo animals, repaired the skin cell morphology to a certain extent, increased the content of TYR in serum and skin, and reduced the content of MDA, AchE and MAO. Carvone, Luteolin, Psoralen and Psoraleae phenol and Bakuchiol could increase the melanin area of experimental melanin inhibition in zebrafish. ConclusionAccording to the results of this study, ALLHP can increase the number of melanin-containing hair follicles and the epidermal melanin content in the skin of vitiligo animals and restore skin cell morphology to a certain extent by reducing oxidative stress in epidermal tissue. A wide range of active ingredients may promote melanogenesis with ALLHP.

In vivo imaging with a fast large-area multiphoton exoscope (FLAME) captures the melanin distribution heterogeneity in human skin

Melanin plays a significant role in the regulation of epidermal homeostasis and photoprotection of human skin. The assessment of its epidermal distribution and overall content is of great interest due to its involvement in a wide range of physiological and pathological skin processes. Among several spectroscopic and optical imaging methods that have been reported for non-invasive quantification of melanin in human skin, the approach based on the detection of two-photon excited fluorescence lifetime distinguishes itself by enabling selective detection of melanin with sub-cellular resolution, thus facilitating its quantification while also resolving its depth-profile. A key limitation of prior studies on the melanin assessment based on this approach is their inability to account for the skin heterogeneity due to the reduced field of view of the images, which results in high dispersion of the measurement values. Pigmentation in both normal and pathological human skin is highly heterogeneous and its macroscopic quantification is critical for reliable measurements of the epidermal melanin distribution and for capturing melanin-related sensitive dynamic changes as a response to treatment. In this work, we employ a fast large-area multiphoton exoscope (FLAME), recently developed by our group for clinical skin imaging, that has the ability to evaluate the 3D distribution of epidermal melanin content in vivo macroscopically (millimeter scale) with microscopic resolution (sub-micron) and rapid acquisition rates (minutes). We demonstrate significant enhancement in the reliability of the melanin density and distribution measurements across Fitzpatrick skin types I to V by capturing the intra-subject pigmentation heterogeneity enabled by the large volumetric sampling. We also demonstrate the potential of this approach to provide consistent measurement results when imaging the same skin area at different times. These advances are critical for clinical and research applications related to monitoring pigment modulation as a response to therapies against pigmentary skin disorders, skin aging, as well as skin cancers.

In vivo characterization of the threshold of laser-induced optical breakdown (LIOB) of a fractional 1064nm Nd:YAG picosecond laser by optical coherence tomography: A step forward to precision laser therapy.

Clinical endpoints serve as an important guide to physicians during laser treatment. However, there are no reliable clinical endpoints for the fractional picosecond laser-induced optical breakdown (LIOB) whose threshold is dependent on the irradiance of the laser and epidermal melanin content of the skin treated. To determine the LIOB threshold in vivo of a 1064nm fractional picosecond laser for different skin colors. A cellular-level resolution optical coherence tomography (OCT) has successfully demonstrated that the in vivo morphological change of LIOB in the epidermis by a fractional picosecond laser. By measuring the melanin content in the area of the skin to treat using a skin imaging device, the physician would therefore be able to determine the threshold of LIOB visually under OCT for the skin of that particular melanin content. The pilot study has demonstrated that OCT may serve as a non-invasive modality to determine the thresholds of LIOB by a fractional picosecond laser. And the established correlation between melanin content and LIOB threshold in this pilot study may further guide physicians, that is, physicians may be able to predict the threshold of LIOB, without having to see the clinical endpoints, by simple measurement of melanin content of the skin to treat preoperatively.

Wavelength optimization for the laser treatment of port wine stains

Based on the well-known principle of selective photothermolysis, laser has been a promising way for the treatment of port wine stains (PWSs). The laser wavelengths used for PWS's clinical treatment include but are not limited to pulsed dye laser (PDL) in 585-600nm, long-pulse 755-nm alexandrite, and 1064-nm Nd:YAG lasers. The objective of this study was to investigate the optimal wavelength for PWS's laser treatment. A two-scale mathematic model was constructed to simultaneously quantify macroscale laser energy attenuation in two-layered bulk skin and microscale local energy absorption on target blood vessels within Krogh unit. The effects of morphological parameters, including epidermal melanin content, epidermal thickness, dermal blood content, blood vessel depth, and diameter on laser energy deposition within target blood vessels, were investigated from the visible to near-infrared bands (500-1100nm). The energy deposition ratio of target blood vessel to epidermal surface was proposed to determine the optimal laser wavelength for PWS with different skin morphological parameters. The bioheat transfer modeling and animal experiment are also conducted to prove our wavelength optimization. The optimal wavelengths for lightly pigmented skin with small and shallow target blood vessels are 580-610nm in the visible band. This wavelength coincides with commercially used PDL. The optimal wavelength shifts to 940nm as the epidermal pigmentation increases or the size and blood vessel depth increases. The optimal wavelength changes to 1005nm as the epidermal pigmentation or the size and burying depth of target blood vessel further increases. Nine hundred forty nanometers can be selected as a general wavelength in PWS treatment to meet the need in most widely morphological structure. Lasers with wavelengths in the 580-610, 940, and 1005nm regions are effective for treating PWS because of their high optical selectivity in blood over the epidermis.

Sun Exposure and Black Skin.

Black skin, compared with white skin, is generally assumed to be more resistant to the consequences of sun exposure due to its epidermal melanin content. However, recent scientific evidence shows that black skin is not completely impervious to sunlight. Both clinical and -experimental data have reported sun-induced effects on black skin. Black skin can warm, burn and peel when -exposed to the sun. Exposure to the sun can also cause hyperpigmentation and photoageing manifestations. The current misconceptions expressed by most black individuals about their resistance to sun-induced effects prevent their engagement in sun safe behaviours. Therefore, the demystification of the harmful effects of the sun needs to be supported by more research studies showing the effectiveness of sun protection strategies for the black population.

Treatment of Post-Inflammatory Hyperpigmentation in Patients With Darker Skin Types Using a Low Energy 1,927 nm Non-Ablative Fractional Laser: A Retrospective Photographic Review Analysis.

Post inflammatory hyperpigmentation (PIH) can be difficult to treat especially in patients with darker skin types as darker skin carries increased epidermal melanin content. Various treatments available to improve the appearance of PIH may incite further pigmentation thus making treatment extremely difficult and frustrating. The purpose of this study was to perform a retrospective chart and photographic review to evaluate the efficacy and safety profile of a low energy low density non-ablative fractional1927 nm wavelength laser treatment for PIH in patients with Fitzpatrick skin types IV-VI. A retrospective evaluation of 61 patients with PIH treated with a 1927 nm laser was conducted at a single center. Inclusion criteria required at least 2 treatment sessions so that before and after treatment photographs would be available for comparison and study purposes. Two blinded physician-evaluators using a visual analog scale for percentage of pigmentary clearance in standard photographs assessed treatment efficacy. The mean percent improvement after treatment, evaluated by two dermatologists was 43.24%. The correlation between raters was statistically significant (Pearson's correlation coefficienct of r = 0.59, p < 0.0001). No side effects were observed in the patients treated with the 1927 nm laser. The low energy low density non-ablative fractional 1927 nm wavelength laser is a safe and effective modality for improving post inflammatory hyperpigmentation in patients with darker skin types. Lasers Surg. Med. © 2019 Wiley Periodicals, Inc.

A review of melanin sensor devices.

Knowing how readily the skin produces melanin is invaluable in reducing photochemical and phototherapy overtreatment in dermatology and also in reducing the risk of actinic skin damage and skin cancer from excessive radiant light exposure. The commonly used Fitzpatrick skin type (FST) classification scale is often used to subjectively assess ultraviolet light sensitivity and susceptibility to sunburn following significant sunlight exposure. However, the FST scale falls short in the assessment of nonwhite skin types. Alternatively, commercially available melanin sensor devices, called melanometers, can be used to objectively quantify useful skin parameters such as the epidermal melanin concentration (EMC). This study reviews commercially available melanometers and their use in quantifying epidermal melanin concentration (EMC) and the individual maximum safe radiant exposure (IMSRE) for an individual in clinical, workplace and community settings.

760 Melanin may mediate visible light induced phototoxicity in darker skin types

Visible light (VL), a major component of solar radiation, differentially induces pigmentation in skin of color, an effect likely related to inherent melanization. A chromophore, melanin absorbs light within the VL wavelength. We hypothesize that clinical and histologic responses to VL are determined by epidermal melanin content and distribution. Nuclear matrix metalloproteinase (MMP-1), a proapoptotic mediator of photodamage may play a central role in skin type specific responses to VL. Enrolled participants were grouped by skin type (light=6,dark=6). Photoprotected sites were exposed daily x4days to 480J/cm2 of VL at 200mW(FiberLite High Intensity Illuminator Series 180,Dolan Jenner Industries) for a total dose of 1920J/cm2as tolerated. Treated and control sites were biopsied 24h after the last exposure and stained with Masson’s Trichrome and Fontana-Masson (FM) to assess collagen and melanin deposition, respectively. MMP-1 deposition was detected by indirect immunofluorescence (IF). Dark skin types did not tolerate the full VL regimen with blistering occurring in all subjects at doses of 220-880J/cm2. We found epidermal MMP-1 expression and nuclear localization in keratinocytes in dark but not in light skin. Nuclear localization correlated with areas of highest melanin density per FM stain. While there was no significant difference in MMP expression of treated light skin from control, collagen stains showed decreased stain intensity and thickened, wavy collagen bundles in both light (mean fold change[FC]=1.46,SEM+0.11), and dark skin (FC=1.19,+0.05). Herein, we demonstrate the acute effects of high power VL exposure in light and dark skin. VL can cause degenerative changes in collagen similar to acute UV irradiation or thermal injury, occurring independent of skin melanization. While the role of melanin in photoprotection is well defined, given the pro-apoptotic function of nuclear MMPs, our findings suggest a potential mechanism by which melanin may mediate VL induced phototoxicity. These observations show a “light” and “dark” side to melanin that warrants further investigation.

Epidermal Fluence Threshold Determination by Real-Time Melanin Measurements.

Epidermal preservation is essential during laser treatment for vascular, hair, and benign pigment dyschromias. Epidermal tolerance is determined by epidermal melanin content, fluence, pulse width, wavelength, skin cooling, and spot size. The authors' objective was to determine the maximum epidermal tolerance for the long-pulse alexandrite 755 nm and the long-pulse neodymium-doped yttrium aluminum garnet (Nd:YAG) 1064-nm lasers for varying epidermal melanin content. Skin melanin measurements were performed at the test sites with a melanin reader, and 0.5 to 1 second of refrigerated air precooled the skin. Then, alexandrite and Nd:YAG laser test spots of 5 to 18 mm were delivered in a series of ascending fluences using 5-, 20-, and 50-ms pulse widths. Skin response at 24 to 48 and 96 hours was scored from 0 to 15 varying from "no reaction" to "severe scabbing." Alexandrite laser, mean threshold fluences increased by a factor of 1.2 increasing from 5 to 20 ms, and by a factor of 1.4 increasing from 5 to 50 ms, among subjects with a melanin index (MI) from 9 to 25 (Fitzpatrick skin phototype I-III). The Nd:YAG fluence to reach epidermal tolerance was 6X the fluence with the alexandrite laser for the same MI in subjects with MI 26 to 35. Epidermal melanin measurements are quantitative and objective, therefore, improving treatment setting determination by decreasing the risk of overtreatment or undertreatment.

High correlation between skin color based on CIELAB color space, epidermal melanocyte ratio, and melanocyte melanin content.

BackgroundTo treat skin color disorders, such as vitiligo or burns, melanocytes are transplanted for tissue regeneration. However, melanocyte distribution in the human body varies with age and location, making it difficult to select the optimal donor skin to achieve a desired color match. Determining the correlations with the desired skin color measurement based on CIELAB color, epidermal melanocyte numbers, and melanin content of individual melanocytes is critical for clinical application.MethodFifteen foreskin samples from Asian young adults were analyzed for skin color, melanocyte ratio (melanocyte proportion in the epidermis), and melanin concentration. Furthermore, an equation was developed based on CIELAB color with melanocyte ratio, melanin concentration, and the product of melanocyte ratio and melanin concentration. The equation was validated by seeding different ratios of keratinocytes and melanocytes in tissue-engineered skin substitutes, and the degree of fitness in expected skin color was confirmed.ResultsLinear regression analysis revealed a significant strong negative correlation (r = − 0.847, R2 = 0.717) between CIELAB L* value and the product of the epidermal melanocyte ratio and cell-based melanin concentration. Furthermore, the results showed that an optimal skin color match was achieved by the formula.DiscussionWe found that L* value was correlated with the value obtained from multiplying the epidermal melanocyte ratio (R) and melanin content (M) and that this correlation was more significant than either L* vs M or L* vs R. This suggests that more accurate prediction of skin color can be achieved by considering both R and M. Therefore, precise skin color match in treating vitiligo or burn patients would be potentially achievable based on extensive collection of skin data from people of Asian descent.

Momentum transfer Monte Carlo for the simulation of laser speckle imaging and its application in the skin.

Due to its simplicity and low cost, laser speckle imaging (LSI) has achieved widespread use in biomedical applications. However, interpretation of the blood-flow maps remains ambiguous, as LSI enables only limited visualization of vasculature below scattering layers such as the epidermis and skull. Here, we describe a computational model that enables flexible in-silico study of the impact of these factors on LSI measurements. The model uses Monte Carlo methods to simulate light and momentum transport in a heterogeneous tissue geometry. The virtual detectors of the model track several important characteristics of light. This model enables study of LSI aspects that may be difficult or unwieldy to address in an experimental setting, and enables detailed study of the fundamental origins of speckle contrast modulation in tissue-specific geometries. We applied the model to an in-depth exploration of the spectral dependence of speckle contrast signal in the skin, the effects of epidermal melanin content on LSI, and the depth-dependent origins of our signal. We found that LSI of transmitted light allows for a more homogeneous integration of the signal from the entire bulk of the tissue, whereas epi-illumination measurements of contrast are limited to a fraction of the light penetration depth. We quantified the spectral depth dependence of our contrast signal in the skin, and did not observe a statistically significant effect of epidermal melanin on speckle contrast. Finally, we corroborated these simulated results with experimental LSI measurements of flow beneath a thin absorbing layer. The results of this study suggest the use of LSI in the clinic to monitor perfusion in patients with different skin types, or inhomogeneous epidermal melanin distributions.

Theoretical review of the treatment of pigmented lesions in Asian skin.

Asian skin has a higher epidermal melanin content, making it more likely to develop adverse pigmentary reactions following laser surgery. The nanosecond lasers are the gold standard for the treatment of pigmented lesions, but the risk of complications, such as post-inflammatory hyperpigmentation, is increased in dark-skinned patients. Intense Pulsed Light (IPL) or long-pulsed lasers are available for treating superficial pigmented lesions, and fewer complications are seen when using these devices compared to the nanosecond lasers. Nanosecond lasers are essential in the treatment of dermal melanosis. Recently, picosecond lasers have been investigated. Picosecond lasers will also play an important role in the treatment of pigmented lesions.

Melanosome distribution patterns affecting skin reflectance: Implications for the in vivo estimation of epidermal melanin content.

Several techniques employed in the in vivo estimation of epidermal melanin content rely on the assumption that the effects of different distribution patterns of aggregated melanin (clustered within the melanosomes) on skin spectral responses, particularly across the 600-1350 nm range, can be ignored. Accordingly, for all practical purposes, only the non-aggregated (colloidal) form of melanin is taken into account by these techniques. In this paper, however, we demonstrate through predictive computer simulations that these responses are directly influenced by the occurrence of both forms of melanin. Our in silico findings, in turn, indicate that such an assumption may lead to inaccurate estimations of epidermal melanin content.

The Peripheral Clock Regulates Human Pigmentation

Although the regulation of pigmentation is well characterized, it remains unclear whether cell-autonomous controls regulate the cyclic on-off switching of pigmentation in the hair follicle (HF). As human HFs and epidermal melanocytes express clock genes and proteins, and given that core clock genes (PER1, BMAL1) modulate human HF cycling, we investigated whether peripheral clock activity influences human HF pigmentation. We found that silencing BMAL1 or PER1 in human HFs increased HF melanin content. Furthermore, tyrosinase expression and activity, as well as TYRP1 and TYRP2 mRNA levels, gp100 protein expression, melanocyte dendricity, and the number gp100+ HF melanocytes, were all significantly increased in BMAL1 and/or PER1-silenced HFs. BMAL1 or PER1 silencing also increased epidermal melanin content, gp100 protein expression, and tyrosinase activity in human skin. These effects reflect direct modulation of melanocytes, as BMAL1 and/or PER1 silencing in isolated melanocytes increased tyrosinase activity and TYRP1/2 expression. Mechanistically, BMAL1 knockdown reduces PER1 transcription, and PER1 silencing induces phosphorylation of the master regulator of melanogenesis, microphthalmia-associated transcription factor, thus stimulating human melanogenesis and melanocyte activity in situ and in vitro. Therefore, the molecular clock operates as a cell-autonomous modulator of human pigmentation and may be targeted for future therapeutic strategies.