Dysfunction of the Myc-induced apoptosis mechanism accompanies c-myc activation in the tumorigenic L929 cell line.

Activation of the c-myc protooncogene, resulting in deregulated, over-expression of the c-Myc protein, can induce both cell proliferation and programmed cell death (apoptosis) in nontransformed cells. Yet, c-myc activation is commonly tolerated in many tumors. This apparent paradox can be resolved if activation of c-myc in transformed cells is associated with loss of Myc-induced apoptosis. To examine this hypothesis, we characterized both the mechanisms of c-myc activation and programmed cell death in the tumorigenic L929 cell line. We showed that activation of c-myc in the L929 cell line involves several distinct mechanisms, including dysfunction of the Myc autosuppression pathway and alteration of c-Myc protein expression. In addition, we demonstrated that L929 cells do not undergo Myc-induced apoptosis. Analysis of somatic cell hybrids revealed that this abrogation of programmed cell death can be partially restored and is likely due to one or more genetic lesions. Our results support the hypothesis that the dysfunction of the Myc-induced apoptosis mechanism can accompany c-myc activation and provide an in vivo example illustrating two cooperative events which can contribute to tumor progression.