RESUMEN
The myc proto-oncogenes encode transcriptional regulators whose inappropriate expression is correlated with a wide array of human malignancies. Up-regulation of Myc enforces growth, antagonizes cell cycle withdrawal and differentiation, and in some situations promotes apoptosis. How these phenotypes are elicited is not well understood, largely because we lack a clear picture of the biologically relevant downstream effectors. We created a new biological system for the optimal profiling of Myc target genes based on a set of isogenic c-myc knockout and conditional cell lines. The ability to modulate Myc activity from essentially null to supraphysiological resulted in a significantly increased and reproducible yield of targets and revealed a large subset of genes that respond optimally to Myc in its physiological range of expression. The total extent of transcriptional changes that can be triggered by Myc is remarkable and involves thousands of genes. Although the majority of these effects are not direct, many of the indirect targets are likely to have important roles in mediating the elicited cellular phenotypes. Myc-activated functions are indicative of a physiological state geared toward the rapid utilization of carbon sources, the biosynthesis of precursors for macromolecular synthesis, and the accumulation of cellular mass. In contrast, the majority of Myc-repressed genes are involved in the interaction and communication of cells with their external environment, and several are known to possess antiproliferative or antimetastatic properties.