BRD4 regulates N6-methyladenosine modification of ESPL1 mRNA via interaction with RNA demethylase ALKBH5 to modulate breast cancer progression

N6-methyladenosine (m6A) is important for RNA metabolism including stability and translation of messenger RNA (mRNA). Recent studies indicate that m6A-methylated RNA can modulate dynamic chromatin accessibility and gene transcription, while the potential link of chromatin to mRNA m6A remains largely unknown. We found that targeted inhibition of bromodomain-containing protein 4 (BRD4) by siRNA or a small compound inhibitor (JQ1) significantly decreases mRNA m6A levels and suppresses the malignancy of breast cancer (BC) cells via increased expression of ALKBH5, a demethylase for mRNA methylation. Mechanistically, inhibition of BRD4 increases the mRNA stability of ALKBH5 via enhanced binding between ALKBH5 3’UTR with RNA-binding protein RALY. Further, BRD4 serves as a scaffold for ubiquitin enzymes TRIM21 and ALKBH5, resulting in the ubiquitination and degradation of ALKBH5 protein. JQ1-increased ALKBH5 then demethylates mRNA of ESPL1, a critical oncogenic driver for breast cancer, and reduces binding between ESPL1 mRNA and m6A reader IGF2BP3, leading to decay of ESPL1 mRNA. Animal and clinical studies confirm a critical role of the BRD4/ALKBH5/ESPL1 pathway in BC progression. Overall, our study sheds light on another layer of gene regulation involving crosstalks between histone modification and RNA methylation. Expression profiling by high throughput sequencing in human MDA-MB-231 cancer cells treated with（n=2） or without JQ1（n=2)