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Melasma Pathogenesis: a Review of the Latest Research, Pathological Findings, and Investigational Therapies Volume 25 Number 10| October 2019| Dermatology Online Journal || Review 25(10):1 Melasma pathogenesis: a review of the latest research, pathological findings, and investigational therapies Susruthi Rajanala BA, Mayra B de Castro Maymone MD DSc, Neelam A Vashi MD Affiliations: Department of Dermatology, Boston University School of Medicine, Boston, Massachusetts, USA Corresponding Author: Neelam A. Vashi MD, Assistant Professor of Dermatology, Director, Boston University Center of Ethnic Skin, Director, Cosmetic and Laser Center, Boston University School of Medicine, 609 Albany Street, J108, Boston, MA 02118, Tel: 617-638-5524, Email: [email protected] pathogenesis involving an interplay of keratinocytes, Abstract mast cells, gene regulation abnormalities, increased Melasma is an acquired hyperpigmentation disorder vascularization, and basement membrane disruption most commonly affecting females with darker skin [3]. It is essential that clinicians are familiar with the types. It is triggered by several factors including sun evolving pathogenesis of melasma, as this may aid in exposure, genetic influences, and female sex successful combination treatment options for this hormones. The pathology of melasma extends notoriously difficult and relapsing condition. This beyond melanocytes and recent literature points to review aims to provide a summary of the more novel interactions between keratinocytes, mast cells, gene regulation abnormalities, neovascularization, and pathological findings and latest investigational disruption of basement membrane. This complex therapies. pathogenesis makes melasma difficult to target and likely to recur post treatment. A better understanding of the latest pathological findings is key to Discussion developing novel and successful treatment options. Publications describing melasma pathogenesis were This review aims to provide a summary of the more primarily found through a PubMed literature search. novel pathological findings and latest investigational Keywords included: melasma, pathology, therapies. melanogenesis, melanocytes, keratinocytes, mast cells, ultraviolet (UV) radiation, basement membrane, vascularization, prostaglandins, Keywords: melasma, hyperpigmentation, melasma estrogen, progesterone, therapeutics, and pathogenesis, melasma treatment tranexamic acid. Several articles were reviewed for relevancy from 1981 to 2018 and references of the Introduction selected articles were also searched for relevant Melasma is an acquired hyperpigmentation that is articles. A total of 35 references were included. characterized by bilateral irregular brown macules Literature included was of a variety of types and patches on sun exposed areas of the face and including basic science research, randomized less frequently, the forearms [1]. It is most commonly controlled trials, commentaries, and reviews. found in darker skinned females — typically Asian or Melanocytes Hispanic women — with Fitzpatrick skin types III-IV. The increase in the amount of melanin production in It is triggered by a variety of factors including sun melasma is well accepted. However, whether or not exposure, genetics, and female sex hormones [2]. this increase is accompanied by a quantitative The pathology of melasma is complex although it increase in melanocytes is debated [4, 5]. The specific was initially thought to involve only melanocytes. mechanisms behind increased melanin deposition Evolving research points to a more heterogeneous are still being elucidated. It has been shown that UV - 1 - Volume 25 Number 10| October 2019| Dermatology Online Journal || Review 25(10):1 radiation leads to an upregulation of melanocyte- factor controls the expression of tyrosinase, an stimulating hormone (MSH) receptors — also known enzyme responsible for several steps in as melanocortin-1 receptors (MC1-R) — on melanogenesis [8]. UV radiation is also shown to melanocytes, allowing for greater binding of the endogenously generate 1,2-diacylglycerols (DAGs), a hormones and therefore more melanin production type of second messenger, from plasma membrane [6]. Proopiomelanocortin (POMC) is cleaved to phospholipids of melanocytes via phospholipase C produce the peptides -melanocyte stimulating and D (PLC and PLD) pathways. These DAGs go on to hormone (-MSH) and adrenocorticotropic activate tyrosine and therefore increase hormone (ACTH) in response to UV [7]. When these melanogenesis (Figure 1), [9]. peptides bind to and activate MC1-R, they increase The tumor suppressor protein p53 may also play a levels of protein kinase A (PKA), which role in UV-induced melanogenesis. This protein phosphorylates the cAMP response element (CREB), upregulates POMC production in keratinocytes post (Figure 1). UVB damage, which leads to increased melanin cAMP response element is a transcription factor for production. In addition, the protein also increases microphthalmia associated transcription factor transcription of hepatocyte nuclear transcription (MITF), a key regulator in the melanin synthesis factor-1alpha (HNF-1alpha), which induces pathway. Microphthalmia associated transcription tyrosinase downstream to increase melanin Figure 1. Flow diagram showing the total number of mobile apps found from the Apple App Store and the number of apps excluded from analysis (including reason for removal). - 2 - Volume 25 Number 10| October 2019| Dermatology Online Journal || Review 25(10):1 production even in the absence of keratinocytes Granzyme B, released directly by mast cells, further (Figure 1), [8]. Although the above pathways induce damages the extracellular matrix (ECM). Tryptase melanogenesis in all skin, the response to UV light is may also contribute to solar elastosis by triggering exaggerated and expression of -MSH is sustained in the production of elastin [3]. melasma lesions, augmenting the production of Finally, mast cells induce hypervascularization, melanin [7]. another prominent clinical finding in melasma, by Solar Elastosis and Photoaging secreting proteins such as vascular endothelial Solar elastosis refers to the accumulation of growth factor (VEGF), fibroblast growth factor-2 abnormal elastic tissue in the dermis resulting from (FGF-2), and transforming growth factor-B (TGF-B). chronic sun exposure or photoaging. Melasma These angiogenic factors increase the size, density, patients have been found to have high levels of solar and dilatation of vessels in affected skin, and present elastosis in affected skin. In addition, histological another therapeutic target when treating melasma analysis shows that melasma skin tends to have (Figure 1), [2, 3]. thicker and more curled and fragmented elastic fibers when compared to normal skin [3]. Basement Membrane Damage Basement membrane abnormalities play a key role in Ultraviolet B exposure can stimulate keratinocytes to melasma pathology. As described above, UV increase melanocyte and therefore, melanin damage activates MMP2 and MMP9 to degrade type production by secreting various growth factors, IV and VI collagen in the basement membrane [3]. cytokines, and hormones including inducible nitric Cadherin 11, an adhesion molecule that is oxide synthase or iNOS [10]. UVB irradiation also upregulated in melasma skin, can then mediate increases plasmin production by keratinocytes. This interaction between fibroblasts and melanocytes enzyme leads to higher levels of arachidonic acid and promote melanogenesis [15]. Cadherin 11 is also and alpha-MSH and therefore stimulates the melanin responsible for upregulating MMP1 and MMP2 synthesis pathway [11]. These factors all lead to expression, leading to further collagen degradation hyperpigmentation of the affected skin. and accumulation of elastotic material in melasma Furthermore, it has been suggested that even visible skin. These effects may even be independent of UV light may play a role in the pathogenesis of melasma, irradiation [15]. Basement membrane damage also especially in darker skin types (Fitzpatrick types IV- allows the movement of melanocytes and melanin VI), by interacting with the opsin 3 sensor (Figure 1), granules down into the dermis, which contributes to [12]. the persistent and recurring nature of melasma. Mast Cells and Neovascularization Therefore, trauma induced by lasers or any therapies Numbers of mast cells are higher in melasma skin that further aggravate the basement membrane may than in unaffected skin [8]. UV exposure triggers the worsen the disease. Similarly, restoration of the release of histamine from these mast cells, leading to basement membrane may limit recurrence (Figure downstream effects [13]. Histamine binding at the 1), [3]. H2 receptor activates the tyrosinase pathway and induces melanogenesis. This finding may help Dermal Inflammation elucidate the link between the inflammatory process Prolonged UV radiation causes dermal inflammation in UV radiation and the hyperpigmentation that and activates fibroblasts. These cells then secrete follows [14]. stem cell factor (SCF), which may diffuse into and induce melanogenesis in the overlying epidermis In addition, UV radiation also increases the [16]. Likewise, levels of stem cell growth factor production of mast cell tryptase, which activates receptor, also known as c-kit, are also upregulated in matrix metalloproteinase (MMP) precursors. These melasma lesions. When c-kit binds to SCF, it activates active enzymes then go on to degrade type IV the tyrosine kinase pathway responsible for collagen and damage the basement membrane. melanogenesis [17]. Dermal inflammation is also
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