Abstract 5449: Defining the transcription regulatory network in differentiating human acute myeloid leukemia cells.
Authors: Roberts Michael , Forrester Jeffrey , Epstein Eric , Saffie Rizwan , Changoor Stuart , Campbell Tracy , Oldach Phoebe , Stanley Natalie , Baum Christina , Collins Eric , Szabat Amelia 
Published: August 2014

The acute myeloid leukemia cell line, HL-60, has been studied extensively as a model system for hematopoietic cell differentiation. Treatment with phorbol esters establishes a signaling network that causes genetic reprogramming resulting in cell cycle arrest, macrophage differentiation and apoptosis. We have used DNA microarray analyses to define changes in genome-wide expression patterns at multiple time points during the 24-hour period post-treatment.

As the cells abandon the leukemia program and transition to a macrophage-like state, approximately 1500 genes undergo a statistically significant change in their expression levels. Here we report on the subset of those genes encoding DNA sequence-specific transcription factors (GO:0003700) and regulators of gene expression (GO:0010468). These genes are hypothesized to mediate genetic reprogramming through the activation or repression of target genes responsible for the physiological changes observed. Cluster analysis of the 78 genes defined as transcription factors reveals a group of rapidly induced "immediate early" genes whose expression is maintained throughout the time course and includes members of the EGR, AP-1(FOS/JUN), and NR4A gene families. A set of "delayed early" transcription factor genes show elevated and sustained expression by 3 hours post treatment and include members of the MAF, GATA and NF-κB families. Transcription factor genes showing sustained down-regulation of expression by 3 hours post treatment are represented by MYC, TSC22D3 and FLI1. Similarly, we report the expression patterns for 90 genes involved in transcription regulation and together describe the network of transcription effectors that likely modulate the expression of many of the remaining ∼1300 genes showing significantly changing expression levels.

To begin to identify the key drivers of genetic reprogramming at the level of transcriptional regulation, the promoters of these 1500 genes have been scanned for enriched sequence motifs, and the resulting combinatorial occurrence pattern used to predict expression cluster membership. We report the results of promoter sequence structure clustering and its correlation with expression clusters.

Preliminary results suggest the transcription factor Egr1 is a driver of transcriptional reprogramming, so we are establishing an Egr1-inducible HL-60 cell line using the "Tet-On" expression system. A comparison of the changes in gene expression resulting from Egr1 up-regulation alone with the phorbol ester induced reprogramming will help dissect the specific role Egr1 plays in transitioning the leukemia cells away from the cancer phenotype and toward the differentiated and ultimately apoptotic phenotypes.

Citation Format: Michael Roberts, Jeffrey Forrester, Eric Epstein, Rizwan Saffie, Stuart Changoor, Tracy Campbell, Phoebe Oldach, Natalie Stanley, Christina Baum, Eric Collins, Amelia Szabat. Defining the transcription regulatory network in differentiating human acute myeloid leukemia cells. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5449. doi:10.1158/1538-7445.AM2013-5449