Overexpression of c-Myc can lead to altered transcriptional regulation of cellular genes and to neoplastic transformation. Although DNA binding is clearly required, the mechanism by which recruitment of c-Myc to target promoters results in transcriptional activation is highly debated. Much of this controversy comes from the difficulty in clearly defining a true Myc target gene. We have previously determined that cad is a bona fide Myc target gene and thus now use the cad promoter as a model to study Myc function. Others have shown that Myc can interact indirectly with histone acetylases and have suggested that Myc mediates transcriptional activation by causing an increase in the levels of acetylated histones on target promoters. To directly test this model, we employed a chromatin immunoprecipitation assay to examine the levels of acetylated histones on the cad promoter. Although Myc was bound to the cad promoter in S phase but not in G0 phase, we found high levels of acetylated histones on the promoter in both stages. We also examined acetylated histones on the cad promoter before and after differentiation of U937 cells. Although the levels of c-Myc bound to the cad promoter were greatly reduced after differentiation, we saw high levels of acetylated histones on the cad promoter both before and after differentiation. Finally, we found that a 30-fold change in binding of N-Myc to the telomerase promoter did not result in a concomitant change in histone acetylation. Thus, recruitment of a Myc family member to a target promoter does not necessarily influence the amount of acetylated histones at that promoter. Further investigations are in progress to define the role of Myc in transcriptional activation.
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