Regulation of Melanogenesis in Melanocytes

In humans the major stimulus for cutaneous pigmentation is ultraviolet radiation (UVR). Little is known about the mechanism underlying this response, in part because of the complexity of interactions in whole epidermis. Using a recently developed culture system, human melanocytes were exposed daily to a physiologic range of UVR doses from a solar simulator. Responses were determined 24 hours after the last exposure. There was a dose-related increase in melanin content per cell and uptake of 14C-DOPA, accompanied by growth inhibition. Cells from donors of different racial origin gave proportionately similar increases in melanin, although there were approximately tenfold differences in basal values. Light and electron microscopy revealed UVR-stimulated increases in dendricity as well as melanosome number and degree of melanization, analogous to the well-recognized melanocyte changes following sun exposure of intact skin. Similar responses were seen with Cloudman S91 melanoma cells, although this murine cell line required lower UVR dosages and fewer exposures for maximal stimulation. These data establish that UVR is capable of directly stimulating melanogenesis. Because cyclic AMP elevation has been associated in some settings with increased pigment production by cultured melanocytes, preliminary experiments were conducted to see if the effects of UVR were mediated by cAMP. Both alpha-MSH and isobutylmethylxanthine (IBMX), as positive controls, caused a fourfold increase in cAMP level in human melanocytes and/or S91 cells, but following a dose of UVR sufficient to stimulate pigment production there was no change in cAMP level up to 4 hours after exposure. Thus it appears that the UVR-induced melanogenesis is mediated by cAMP-independent mechanisms.

We report the synthesis of a probe that permits the visualization by electron microscopy of acidic organelles in intact cells. This probe, 3-(2,4-dinitroanilino)-3'-amino-N-methyldipropylamine (DAMP), is a basic congener of dinitrophenol that readily diffuses into intact cells. Its primary and tertiary amino groups (apparent pKa, 10.6) allow it to be concentrated in acidic organelles and to be retained there after fixation with aldehydes. The dinitroarene moiety of DAMP can then be localized with mouse monoclonal antibodies directed against dinitrophenol. The antibodies, in turn, can be visualized by light or electron microscopy by reaction with rabbit anti-mouse antibodies coupled to rhodamine or horseradish peroxidase, respectively. We have used these methods to show that DAMP concentrates in a variety of membrane-bound structures in cultured fibroblasts, including classic multivesicular bodies (resembling lysosomes), intermediate-sized vesicles with multiple shapes (resembling endosomes), and an abundant population of very small spherical vesicles. A small fraction of coated vesicles is labeled with DAMP. Labeling with DAMP does not occur when the pH gradient of fibroblasts is disrupted by the ionophore monensin or the weak base chloroquine. DAMP should be a useful probe for exploring the assembly, distribution, and function of acidic organelles by electron microscopy.