Bromodomain Containing Protein-4: The Epigenetic Origin of Pulmonary Arterial Hypertension

Rationale: Pulmonary arterial hypertension (PAH) is a vasculopathy characterized by enhanced pulmonary artery (PA) smooth muscle cell (PASMC) proliferation and suppressed apoptosis. Decreased expression of microRNA-204 has been associated to this phenotype. By a still elusive mechanism, miR-204 downregulation promotes the expression of oncogenes including NFAT, Bcl-2 and Survivin. In cancer, increased expression of the epigenetic reader Bromodomain-containing protein 4 (BRD4) sustains cell survival and proliferation. Interestingly, BRD4 is a predicted target of miR-204 and has binding sites on the NFAT promoter region. Objective: To investigate the role of BRD4 in PAH pathogenesis. Methods and Results: BRD4 is upregulated in lungs, distal PAs and PASMCs of PAH patients compared to controls. With mechanistic in vitro experiments, we demonstrated that BRD4 expression in PAH is miR-204 dependent. We further studied the molecular downstream targets of BRD4 by inhibiting its activity in PAH-PASMCs using a clinically available inhibitor JQ1. JQ1 treatment in PAH-PASMCs increased p21 expression, thus triggering cell cycle arrest. Furthermore, BRD4 inhibition, by JQ1 or siBRD4, decreased the expression of three major oncogenes, which are overexpressed in PAH: NFATc2, Bcl-2 and Survivin. Blocking this oncogenic signature led to decreased PAH-PASMC proliferation and increased apoptosis in a BRD4-dependant manner. Indeed, pharmacological JQ1 or molecular (siRNA) inhibition of BRD4 reversed this pathological phenotype in addition to restoring mitochondrial membrane potential and to increasing cells spare respiratory capacity. Moreover, BRD4 inhibition in vivo reversed established PAH in the Sugen/hypoxia rat model. Conclusions: BRD4 plays a key role in the pathological phenotype in PAH, which could offer new therapeutic perspectives for PAH patients.