JunD is the primary factor mediating inhibition of melanoma tumorigenicity during expression of ATF250–100.(a) Expression of TAM67 attenuates the inhibition of melanoma tumorigenicity after expression of ATF250–100. SW1 cells that stably express control or ATF250–100 were infected with TAM67, and clones expressing TAM67 clones were selected on the basis of resistance to puromycin. Cells expressing TAM67 in combination with control or ATF250–100 were injected (104) s.c. into C3H mice, and tumor size was monitored for 25 d. Data represent mean values (P < 0.003; t test) based on analysis of eight mice per experimental group. (b) Expression of pRS-JunD but not pRS-c-Jun abolishes ATF250–100 ability to inhibit tumorigenicity of SW1 melanoma. The experiment was carried out as indicated in a, except that SW1 cells were infected with indicated pRS-constructs in vitro followed by their s.c. injection into C3H mice. The analysis was performed in groups of eight mice per experimental condition (P < 0.001; t test). (Right) Pictures provide representative data on tumor size at the end of the experiment. (c) Proposed model. The expression of the ATF250–100 results in cytoplasmic accumulation of ATF2 with concomitant inhibition of ATF2 transcription. Consequently, the Jun2 promoter sequences are occupied by c-Jun/JunD, which sensitizes melanoma to apoptosis after treatment (Left). Consistent with this model is the finding that expression of a 10-aa ATF2 peptide, which no longer inhibits ATF2 or Jun2-dependent transcription, suffices to sensitize melanoma cells to basal apoptosis by up-regulating c-Jun/JunD (unpublished data). ATF250–100 association with JNK results in increased activity of c-Jun, which sensitizes melanoma cells to spontaneous (basal) apoptosis, and suffices to reduce tumorigenicity of this otherwise aggressive tumor (Right). ERK, extracellular response kinase.