Identification of Myc-mediated Death Response Pathways by Microarray Analysis

To understand the mechanisms of Myc-mediated apoptosis induced by DNA damage, we have characterized the death kinetics of three Rat-1 fibroblast cell lines that either overexpress Myc or lack Myc and their parental wild-type cells following exposure to the DNA-damaging agent VP-16, and we monitored the changes in gene expression using microarray. We have identified three groups of genes whose expressions are distinctly regulated during this process. One cluster (Cluster A) revealed a VP-16-dependent but Myc-independent induction of a set of genes that is not linked to the apoptotic response. Two other gene clusters, however, were associated with VP-16-induced apoptosis. Cluster B, which includes p53-responsive genes, was associated with the temporal onset of apoptosis but accounted for only the basal apoptosis. However, Cluster C, which includes c-jun, was highly regulated by Myc and appeared to be critical to mounting the maximal apoptotic response in Myc-expressing cells. Furthermore, the Myc level dropped sharply following VP-16 exposure, which varied inversely with the induction of Cluster C genes, suggesting Myc normally represses their transcription. Thus, we have proposed that removal of Myc-mediated repression of apoptotic signals, combined with Myc-associated acceleration of the p53 responsive pathway, results in complete and rapid cell death following DNA damage.