Control of melanin synthesis and secretion by B16/C3 melanoma cells.

In culture, B16/C3 murine melanoma cells grown in the presence of serum undergo melanogenesis at a specific time after plating. At this time, melanin is synthesized intracellularly and then secreted into the extracellular culture fluid. We have found that melanin secretion is dependent on the presence of serum in the growth medium. When confluent cultures are deprived of serum, that is, refed with serum-free medium, cells remain viable but do not undergo melanogenesis. Addition of serum-free medium supplemented with either melanocyte-stimulating hormone (MSH) or dibutyryl cAMP induced melanogenesis in these cells but did not result in melanin secretion. Furthermore, when B16/C3 cells are grown in serum-free, hormone-supplemented medium, they also undergo melanogenesis but fail to release melanin. The addition of serum, however, to B16/C3 cells induced to undergo melanogenesis with MSH, dibutyryl cAMP, or hormone-supplemented medium promotes melanin secretion. Fractionation studies hence revealed that serum contains specific factors capable of inducing melanin secretion. These results demonstrate that factors that regulate melanin synthesis are distinct from those that induce cells to release melanin into their extracellular environment. Furthermore, the ability to induce melanogenesis with single factors will permit us to study the precise sequence of events leading to differentiation in B16/C3 cells under chemically defined conditions.