5-Bromodeoxyuridine-tolerant melanoma cells in vitro and in vivo.

The thymidine analog 5-bromodeoxyuridine (BrUdR) incorporated into DNA has been successfully used as a selective agent (1, 2) or mutagen (3) on various animal cells. Some of its effects are understandable in terms of base substitution in the genetic material; others, however, such as the reversible suppression of cytodifferentiation (4-7), remain unexplained. We have examined the effects of prolonged treatment with high BrUdR concentrations on the in vitro and in vivo characteristics of a murine malignant melanoma (B 16) cell line and have already reported the isolation and altered phenotype of B16 cells tolerant to 45 /Ag BrUdR/ml (8). Moreover, these did not require continued exposure to the analog-containing environment (9). The data presented here review and extend our previous observations and support the hypothesis that B16 cells tolerant to high analog doses (1) are stable amelanotic variants and (2) do not retain their altered phenotype by virtue of residual BrU-DNA. We also demonstrate, by means of cesium chloride (CsCl) equilibrium-centrifugation analysis, that tolerant cells incorporate less analog into DNA than control cells. When cells chronically exposed to BrUdR were tested in vivo, tumorigenicity was reduced but not completely extinguished. On the other hand, cells withdrawn from the BrUdR-containing media eventually become as tumorigenic as control cells.