MLL4/KMT2D histone methyltransferase and JUNB cooperate in a feed-forward loop to support AP-1 dependent TGF-β signaling

Transforming growth factor β (TGF-β) signaling is a highly pleiotropic pathway with an important role in development, homeostasis and cancer. Chromatin regulators contribute to the regulation of TGF-β responsive transcription. Requirement of subunits of the MLL3/MLL4 histone methyltransferase complexes for TGF-β responses have been reported. However, their exact roles are not fully understood. To investigate the functions of these complexes, we employed CRISPR/Cas9 genome editing to inactivate the KMT2C/MLL3 or KMT2D/MLL4 genes in human diploid epithelial cells. Time-course RNA-seq experiments revealed the requirement of MLL4 but not of MLL3 for TGF-β transcriptional responses. CUT&RUN experiments showed that MLL4 binding increases after TGF-β treatment and is especially enriched at AP-1 transcription factor binding sites. Interestingly, TGF-β-induced chromatin binding of MLL4 correlates with increases in H3K27ac but not in H3K4me1 modifications. Furthermore, TGF-β treatment sets off SMAD2-induced JUNB expression, which forms a feed-forward loop with MLL4. By inhibiting the activities of AP-1, the BAF chromatin remodeler, or the CBP/p300 histone acetyltransferase, we found that AP-1 binding and these chromatin regulators are all necessary for TGF-β induction of MLL4 binding and transcriptional activation of its genomic targets. Taken together, our study reveals distinctive roles for the MLL3 and MLL4 paralogs in the transcriptional response to TGF-β. In contrast to MLL3, MLL4 forms a feed-forward loop of JUNB, the BAF complex, and CBP/p300 to sustain transcription activation by TGF-β.

Genes & Development