Tritiated-thymidine-induced increased DNA content and irreversible differentiation in a human melanoma cell line

When cultured in the presence of 2.0 microCi/ml (methyl-3H)thymidine (3H-TdR) the growth rate of 6 human melanoma lines and 1 subline progressively slowed then stopped, a change that was accompanied by loss of reproductive viability as assessed by colony formation in agar, but unaccompanied by a comparable inhibition of thymidine incorporation. In all cases increases in cell size, nuclear size and DNA content were observed. In 1 cell line only, MM96, these changes were accompanied by a profound increase in morphological differentiation. Despite this MM96 did not show increased differentiation in response to 2 x 10(-7) M alpha melanocyte stimulating-hormone (alpha MSH), which in fact stimulated growth, or in response to 10(-3) M N6,O2'-dibutyryl adenosine 3':5'-monophosphate (db-cAMP), 10(-3) M theophylline or 5 x 10(-4)M guanosine-5'-triphosphate (GTP), all of which retarded growth. With none of the cell lines was differentiation increased in response to 10(-3)M db-cAMP although in each case growth was retarded. These results reinforce the importance of colony assays vs DNA synthesis studies in assessing reproductive viability and show that supra-reproductively lethal levels of 3H-TdR can by-pass defects in the differentiation pathway of at least one, but not all, human melanoma cell lines.