Calprotectin, a heterodimer of S100A8 and S100A9, is a proinflammatory cytokine released from ultraviolet radiation-exposed keratinocytes. Calprotectin binds to Toll-like receptor 4, the receptor for advanced glycation end-products, and extracellular matrix metalloproteinase inducer on target cells to stimulate migration. Melanocytes and melanoma cells produce little if any calprotectin, but they do express receptors for the cytokine. Thus, keratinocyte-derived calprotectin has the potential to activate melanocytes and melanoma cells within the epidermis in a paracrine manner. We examined the ability of calprotectin to stimulate proliferation and migration in normal human melanocytes and melanoma cells in vitro. We first showed, by immunofluorescence and quantitative RT-PCR, that the melanocytic cells employed expressed a calprotectin receptor, the receptor for advanced end-products. We then demonstrated that calprotectin significantly enhanced proliferation, migration, and Matrigel invasion in both normal human melanocytes and melanoma cells. Thus, calprotectin is one of the numerous paracrine factors released by ultraviolet radiation-exposed keratinocytes that may promote melanomagenesis and is a potential target for melanoma prevention or therapy. 1. Introduction In normal human epidermis, each melanocyte associates with approximately 35 keratinocytes to form an “epidermal melanin unit” [1]. The keratinocytes in each unit exert considerable control over the behavior of the associated melanocyte, via interactions of cell-cell adhesion molecules and release of paracrine factors. Ultraviolet radiation (UVR) can substantially alter keratinocyte-melanocyte interactions. UVR exposure enhances keratinocyte production of a wide variety of paracrine factors, including interleukins, growth factors, interferons, and chemokines, that may profoundly affect melanocyte and melanoma cell proliferation, migration, and gene expression [1–4]. Such paracrine modulation of melanocyte behavior may promote melanomagenesis [1, 4]. Within the cell, S100 proteins, in the form of heterodimers or homodimers, control the localization and activity of a variety of target proteins; however, the S100 proteins S100A8 and A9 are also secreted from cells as calprotectin, a heterodimeric proinflammatory cytokine [5]. Calprotectin exerts its effects by binding to a variety of receptors on the surface of target cells, including Toll-like receptor 4 (TLR4), the receptor for advanced glycation end-products (RAGE), and extracellular matrix metalloproteinase inducer (EMMPRIN) [5–7]. S100A8
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