CONTROL OF PIGMENT PRODUCTION IN MOUSE MELANOMA CELLS IN VITRO Evocation and Maintenance

A clonally derived amelanotic melanoma cell line repeatedly has been forced to produce pigment by the inhibitor of DNA synthesis, I-/3-D-arabinofuranosylcytosine (ara-C) at sublethal levels . One ara-C-derived melanotic line has been cloned, and has continued to produce pigment for 2 years on normal medium . The inhibitor is most effective when administered to synchronized cells in four pulses on successive days at 1 .8 X 10-5 M during the S phase of the cell cycle . Colcemid at a sublethal concentration, and growth on medium solidified with agar also evoked pigment production in this line, but a large number of other inhibitors of biosynthetic processes did not, under the conditions tested . The melanotic lines are active producers of tyrosinase (DOPA oxidase), whereas the amelanotic line produces an inhibitor of tyrosinase activity . Both enzyme and inhibitor are labile at 4 ° C and -20 ° C, and decay of the inhibitor in homogenates of amelanotic cells reveals a low level of residual DOPA oxidase activity. The mean population doubling time of a cloned melanotic line is 23 hr, and that of a cloned amelanotic line 16 .5 hr . A similar decrease in rate of growth is found in other melanotic lines and is believed to be a significant factor in maintaining this differentiated function . Rapid growth may be related to the production of an inhibitor by the amelanotic cells .