Direct examination of histone acetylation on Myc target genes using chromatin immunoprecipitation

Overexpression of c-Myc can lead to altered transcriptional regulation of c ellular genes and to neoplastic transformation. Although DNA binding is cle arly required, the mechanism by which recruitment of c-Myc to target promot ers results in transcriptional activation is highly debated. Much of this c ontroversy 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 ha ve suggested that Myc mediates transcriptional activation by causing an inc rease in the levels of acetylated histones on target promoters. To directly test this model, we employed a chromatin immunoprecipitation assay to exam ine the levels of acetylated histones on the cad promoter. Although Myc was bound to the cad promoter in S phase but not in G(0) phase, we found high levels of acetylated histones on the promoter in both stages. We also exami ned acetylated histones on the cad promoter before and after differentiatio n of U937 cells. Although the levels of c-Myc bound to the cad promoter wer e greatly reduced after differentiation, we saw high levels of acetylated h istones 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 promo ter did not result in a concomitant change in histone acetylation. Thus, re cruitment of a Myc family member to a target promoter does not necessarily influence the amount of acetylated histones at that promoter. Further inves tigations are in progress to define the role of Myc in transcriptional acti vation.