Raymond E. Boissy

The effect of niacinamide on reducing cutaneous pigmentation and suppression of melanosome transfer

Summary Background Cutaneous hyperpigmentation occurs in multiple conditions. In addition, many Asian women desire a lighter skin colour. Thus, there is a need for the development of skin lightening agents. Niacinamide is a possible candidate.

Melanosome transfer to and translocation in the keratinocyte.

Abstract Complexion coloration in humans is primarily regulated by the amount and type of melanin synthesized by the epidermal melanocyte. However, additional and equally contributing factors consist of (1) efficient transfer of melanin from the melanocytes to the neighboring keratinocytes and (2) distribution and degradation of the transferred melanosomes by the recipient keratinocytes. Once synthesized in the cell body of the epidermal melanocyte, pigmented melanosomes are translocated down the dendrites and captured at the dendritic tips via various cytoskeletal elements. Molecules recently identified that participate in this process consist of Rab27a, myosin‐Va and melanophilin. Eventually, these peripherally localized melanosomes are transferred to keratinocytes by a presently undefined mechanism. The protease‐activated receptor‐2 (PAR‐2) and unidentified surface lectins and glycoproteins facilitate this transfer process. Once incorporated into the keratinocytes, melanosomes are distributed individually or as clusters, aggregated towards the apical pole of the nucleus, and degraded as the keratinocytes undergo terminal differentiation and desquamation. Ultraviolet irradiation (UVR) can modulate the process of melanosome transfer from the melanocytes to the keratinocytes. UVR can upregulate expression of PAR‐2 and lectin‐binding receptors and increase phagocytic activity of cultured keratinocytes. Therefore, many cellular and molecular events that occur after melanogenesis contribute to skin color.

The patterns of melanosome distribution in keratinocytes of human skin as one determining factor of skin colour.

Background  One determining factor of skin colour is the distribution pattern of melanosomes within keratinocytes. Melanosomes in keratinocytes of light skin as in Caucasians are distributed as membrane‐bound clusters, whereas the melanosomes in keratinocytes of dark skin as in African/American individuals tend to be larger and distributed individually. It has been shown that melanin content, melanin composition and the size of melanosomes in the human epidermis vary considerably with both ethnicity and chronic sun exposure.

DeoxyArbutin: a novel reversible tyrosinase inhibitor with effective in vivo skin lightening potency

Abstract:  Modulation of melanogenesis in the melanocytes can be achieved using chemicals that share structural homologies with the substrate tyrosine and as thus competitively inhibit the catalytic function of tyrosinase. We have developed a new tyrosinase inhibitor, deoxyArbutin (dA), based on this premise. DeoxyArbutin demonstrates effective inhibition of mushroom tyrosinase in vitro with a Ki that is 10‐fold lower that hydroquinone (HQ) and 350‐fold lower than arbutin. In a hairless, pigmented guinea pig model, dA demonstrated rapid and sustained skin lightening that was completely reversible within 8 weeks after halt in topical application. In contrast, HQ induced a short but unsustained skin lightening effect whereas kojic acid and arbutin exhibit no skin lightening effect. Results from a panel of safety tests supported the overall establishment of dA as an actionable molecule. In a human clinical trial, topical treatment of dA for 12 weeks resulted in a significant or slight reduction in overall skin lightness and improvement of solar lentigines in a population of light skin or dark skin individuals, respectively. These data demonstrate that dA has potential tyrosinase inhibitory activity that can result in skin lightening and may be used to ameliorate hyperpigmentary lesions.

Mechanisms Regulating Skin Pigmentation: The Rise and Fall of Complexion Coloration

Skin pigmentary abnormalities are seen as aesthetically unfavorable and have led to the development of cosmetic and therapeutic treatment modalities of varying efficacy. Hence, several putative depigmenting agents aimed at modulating skin pigmentation are currently being researched or sold in commercially available products. In this review we will discuss the regulation of processes that control skin complexion coloration. This includes direct inhibition of tyrosinase and related melanogenic enzymes, regulation of melanocyte homeostasis, alteration of constitutive and facultative pigmentation and down-regulation of melanosome transfer to the keratinocytes. These various processes, in the complex mechanism of skin pigmentation, can be regulated individually or concomitantly to alter complexion coloration and thus ameliorate skin complexion diseases.

Melanocortin 1 receptor genotype: An important determinant of the damage response of melanocytes to ultraviolet radiation

The melanocortin 1 receptor gene is a main determinant of human pigmentation, and a melanoma susceptibility gene, because its variants that are strongly associated with red hair color increase melanoma risk. To test experimentally the association between melanocortin 1 receptor genotype and melanoma susceptibility, we compared the responses of primary human melanocyte cultures naturally expressing different melanocortin 1 receptor variants to α‐melanocortin and ultraviolet radiation. We found that expression of 2 red hair variants abolished the response to α‐melanocortin and its photoprotective effects, evidenced by lack of functional coupling of the receptor, and absence of reduction in ultraviolet radiation‐induced hydrogen peroxide generation or enhancement of repair of DNA photoproducts, respectively. These variants had different heterozygous effects on receptor function. Microarray data confirmed the observed differences in responses of melanocytes with functional vs. non‐functional receptor to α‐melanocortin and ultraviolet radiation, and identified DNA repair and antioxidant genes that are modulated by α‐melanocortin. Our findings highlight the molecular mechanisms by which the melanocortin 1 receptor genotype controls genomic stability of and the mutagenic effect of ultraviolet radiation on human melanocytes.—Kadekaro, A. L., Leachman, S., Kavanagh, R. J., Swope, V., Cassidy, P., Supp, D., Sartor, M., Schwemberger, S., Babcock, G., Wakamatsu, K., Ito, S., Koshoffer, A., Boissy, R. E., Manga, P., Sturm, R. A., Abdel‐Malek, Z. A. Melanocortin 1 receptor genotype: an important determinant of the damage response of melanocytes to ultraviolet radiation. FASEB J. 24, 3850–3860 (2010). www.fasebj.org

Human tyrosinase related protein‐1 (TRP‐1) does not function as a DHICA oxidase activity in contrast to murine TRP‐1

Abstract: Tyrosinase related protein‐1 is a melanocyte specific protein and a member of the tyrosinase gene family which also includes tyrosinase and TRP‐2 (DOPAchrome tautomerase). In murine melanocytes, TRP‐1 functions as a 5,6‐dihydroxyindole‐2‐carboxylic acid [DHICA] oxidase during the biosynthetic conversion of tyrosine to eumelanin and mutations affecting TRP‐1 result in the synthesis of brown rather than black pelage coloration. In this study, we examined the putative DHICA oxidase activity of TRP‐1 in human melanocytes using several approaches. We first utilized a line of cultured melanocytes established from a patient with a form of oculocutaneous albinism completely lacking expression of TRP‐1 (OCA3). This line of melanocytes endogenously exhibited the same amount of DHICA oxidase activity as control melanocytes expressing TRP‐1. In other experiments, cultured human fibroblasts were transfected with a cDNA for TRP‐1, in either the sense or antisense direction, or with the retroviral vector alone. TRP‐1 expression was induced in fibroblasts transfected with the TRP‐1 cDNA in the sense direction only. Although TRP‐1 was expressed by sense‐transfected cells, there was no significant DHICA oxidase activity above controls. These results demonstrate that human TRP‐1 does not use DHICA as a substrate for oxidation.

What's the use of generating melanin?

The CONTROVERSIES featured in the August 1998 issue of Exp Dermatol have discussed the controls of melanogenesis (Schallreuter et al., Exp Dermatol 7: 143-150, 1998). Now, we explore the biological functions of the endproduct of melanogenesis, the various melanin biopolymers. As delineated in the subsequent contributions, melanins are functionally much more complex and fascinating than their evident colour-awarding and UV-light-filtering properties suggest, and dermatologists and pigment biologists alike have yet to discover and define the full range of biologically relevant melanin functions in health and disease.

Effective inhibition of melanosome transfer to keratinocytes by lectins and niacinamide is reversible.

Abstract:  Skin pigmentation results in part from the transfer of melanized melanosomes synthesized by melanocytes to neighboring keratinocytes. Plasma membrane lectins and their glycoconjugates expressed by these epidermal cells are critical molecules involved in this transfer process. In addition, the derivative of vitamin B3, niacinamide, can inhibit melanosome transfer and induce skin lightening. We investigated the effects of these molecules on the viability of melanocytes and keratinocytes and on the reversibility of melanosome‐transfer inhibition induced by these agents using an in vitro melanocyte–keratinocyte coculture model system. While lectins and neoglycoproteins could induce apoptosis in a dose‐dependent manner to melanocytes or keratinocytes in monoculture, similar dosages of the lectins, as opposed to neoglycoproteins, did not induce apoptosis to either cell type when treated in coculture. The dosages of lectins and niacinamide not affecting cell viability produced an inhibitory effect on melanosome transfer, when used either alone or together in cocultures of melanocytes–keratinocytes. Cocultures treated with lectins or niacinamide resumed normal melanosome transfer in 3 days after removal of the inhibitor, while cocultures treated with a combination of lectins and niacinamide demonstrated a lag in this recovery. Subsequently, we assessed the effect of niacinamide on facial hyperpigmented spots using a vehicle‐controlled, split‐faced design human clinical trial. Topical application of niacinamide resulted in a dose‐dependent and reversible reduction in hyperpigmented lesions. These results suggest that lectins and niacinamide at concentrations that do not affect cell viability are reversible inhibitors of melanosome transfer.

Functional analysis of keratinocytes in skin color using a human skin substitute model composed of cells derived from different skin pigmentation types

Skin color is one of the most distinct features in the human race. To assess the mechanisms of skin color variation, human skin substitutes (HSS) were constructed by grafting mixtures of cultured keratinocytes and melanocytes from a combination of donor skin types, together with light skin derived fibroblasts, into chambers inserted onto the back skin of severe combined immunodeficient (SCID) mice. The resulting complexion coloration of the HSS was relatively darker and lighter when dark and light skin derived keratinocytes, respectively, were combined with melanocytes derived from either light or dark skin. The melanin content in the epidermis and the maturation stage of melanosomes in basal keratinocytes were significantly increased in the HSS composed of dark compared to light skin derived keratinocytes. In addition, the ratio of individual/clustered melanosomes in recipient keratinocytes was increased in the former as opposed to the latter HSS. The genetic expression of endothelin‐1, proopiomelanocortin, microphthalmia‐associated transcription factor, tyrosinase, GP100, and MART1 were increased in HSS composed of dark vs. light skin derived keratinocytes. These data suggest that our HSS is a promising melanogenic model that demonstrates the role of the keratinocyte in regulating in part both melanogenesis and distribution of transferred melanosomes.—Yoshida, Y., Hachiya, A., Sriwir‐iyanont, P., Ohuchi, A., Kitahara, T., Takema, Y., Visscher, M. O., Boissy, R. E. Functional analysis of keratinocytes in skin color using a human skin substitute model composed of cells derived from different skin pigmentation types. FASEB J. 21, 2829–2839 (2007)