Normal Human Melanocyte Homeostasis as a Paradigm for Understanding Melanoma

Melanocytes, after cell division, separate and migrate along the basement membrane; they extend their dendrites and establish multiple contacts with keratinocytes. Once adhesion is established, keratinocytes control melanocyte growth and expression of cell surface receptors. Most melanomas arise within the epidermis (melanoma in situ) and then invade across the basement membrane. These melanoma cells escape from control by keratinocytes through five major mechanisms: (1) downregulation of receptors important for communication with keratinocytes such as E-cadherin, P-cadherin, and desmoglein, which is achieved through growth factors such as hepatocyte growth factor, platelet-derived growth factor, and endothelin-1 produced by fibroblasts or keratinocytes; (2) upregulation of receptors and signaling molecules important for melanoma cell-melanoma cell and melanoma cell-fibroblast interactions such as N-cadherin, Mel-CAM, and zonula occludens protein-1; (3) deregulation of morphogens such as Notch receptors and their ligands; (4) loss of anchorage to the basement membrane because of an altered expression of cell-matrix adhesion molecules; (5) increased elaboration of metalloproteinases. Thus, investigating normal melanocyte homeostasis helps us to better define how melanoma cells escape the microenvironment created by epidermal keratinocytes and how they develop new cellular partners in fibroblasts and endothelial cells, which support their growth and invasion.