An eRNA Transcription Checkpoint For Diverse Signal-dependent Enhancer Activation Programs
While many or most signal and ligand-dependent pathways function by directing binding of specific transcription factors to large cohorts of regulatory enhancers, the possible “universal” strategies underling activation of these lorge enhancer programs by diverse signals remains poorly understood. Here, we report that ligands, exemplified by estrogen-17-b (E2), cause the recruitment of DNA-PKcs-phosphorylated KAP1 to estrogen receptor a (ERa)-bound enhancers, inhibiting its E3 SUMO ligase activity for the CDK9 subunit of P-TEFb, which permits formation of active P-TEFb required for eRNA elongation overcoming this checkpoint and permitting enhancer activation. We find that most, or all, signal and ligand enhancer activation programs similarly require the loss of SUMOligase function by phoshorylation of KAP1 to prevent Pol II pausing at regulated enhancers. This is based on activation of P-TEFb consequent to deSUMOylation of CDK9, permitting eRNA synthesis. Overcoming this checkpoint appears to be a widely, if not universally, used strategy for signal-dependent enhancer activation because similar events occur on AR and NF-kB-regulated enhancers and even following depolarization acutely-activated neuronal enhancers. This study reveals an unexpected strategy required to overcome a checkpoint common to most or all signal/ligand-regulated enhancers critical to activate broad programs of biologically-important regulatory enhancers that implement the instructions of the endocrine system. Overall design: For hormone treatments, cells were incubated at 37°C and 5% CO2 for at least 3 days in phenol red-free DMEM (GIBCO/Invitrogen) supplemented with 5% charcoal dextran-stripped FBS (GIBCO/Invitrogen). 17-ß-Estradiol (E2; Steraloids, Inc.) was added to a final concentration of 100 nM. For knock-down of KAP1 genes, the shRNA were employed.