MLL2 regulates glucocorticoid receptor-mediated transcription of ENACα in human retinal pigment epithelial cells

The histone methyltransferase KMT2D maintains cellular glucocorticoid responsiveness by shielding the glucocorticoid receptor from degradation

Because of their ability to induce lymphocyte apoptosis, glucocorticoids (GC) are widely used to treat hematological malignancies such as lymphomas and multiple myeloma. Their effectiveness is often limited, however, due to the development of glucocorticoid resistance by a variety of molecular mechanisms. Here we performed an unbiased genome-wide CRISPR screen with the human T-cell leukemia cell line Jurkat to find previously unidentified genes required for GC-induced apoptosis. One such gene was KMT2D (also known as MLL2 or MLL4), which encodes a histone lysine methyltransferase whose mutations are associated with a variety of cancers, blood malignancies in particular, and are considered markers of poor prognosis. Knockout of KMT2D by CRISPR/Cas9 gene editing in Jurkat and several multiple myeloma cell lines downregulated GR protein expression. Surprisingly, this was not due to a reduction in GR transcripts, but rather to a decrease in the protein's half-life, primarily due to proteasomal degradation. Reconstitution of KMT2D expression restored GR levels. In contrast to the known ability of KMT2D to control gene transcription through covalent histone methylation, KMT2D-mediated upregulation of GR levels did not require its methyltransferase activity. Co-immunoprecipitation and proximity ligation assays found constitutive binding of KMT2D to the GR, which was enhanced in the presence of GC. These observations reveal KMT2D to be essential for the stabilization of cellular GR levels, and suggest a possible mechanism by which KMT2D mutations may lead to GC resistance in some malignancies.

The SCF-FBXW7 E3 ubiquitin ligase triggers degradation of histone 3 lysine 4 methyltransferase complex component WDR5 to prevent mitotic slippage

During prolonged mitotic arrest induced by antimicrotubule drugs, cell fate decision is determined by two alternative pathways, one leading to cell death and the other inducing premature escape from mitosis by mitotic slippage. FBWX7, a member of the F-box family of proteins and substrate-targeting subunit of the SKP1-CUL1-F-Box E3 ubiquitin ligase complex, promotes mitotic cell death and prevents mitotic slippage, but molecular details underlying these roles for FBWX7 are unclear. In this study, we report that WDR5 (WD-repeat containing protein 5), a component of the mixed lineage leukemia complex of histone 3 lysine 4 methyltransferases, is a substrate of FBXW7. We determined by coimmunoprecipitation experiments and in vitro binding assays that WDR5 interacts with FBXW7 in vivo and in vitro. SKP1-CUL1-F-Box-FBXW7 mediates ubiquitination of WDR5 and targets it for proteasomal degradation. Furthermore, we find that WDR5 depletion counteracts FBXW7 loss of function by reducing mitotic slippage and polyploidization. In conclusion, our data elucidate a new mechanism in mitotic cell fate regulation, which might contribute to prevent chemotherapy resistance in patients after antimicrotubule drug treatment.

Histone methyltransferase KMT2D contributes to the protection of myocardial ischemic injury

Histone H3 lysine 4 (H3K4) methyltransferase 2D (KMT2D) plays an important role in cell development in early life. However, the function of KMT2D in adult cells such as cardiomyocytes or neurons has not been reported. In this study, cardiomyocyte-specific KMT2D knockout (KMT2D-cKO) and control (KMT2D-Ctl) mice were exposed to sham or myocardial ischemia (MI) surgery. Depletion of KMT2D aggravated the ischemic area, led to the increased mortality (26.5% in KMT2D-cKO vs 12.5% in KMT2D-Ctl) of the mice, and weakened the left ventricular systolic function. RNA-seq analysis in cardiac tissues identified genes whose expression was changed by MI and KMT2D deletion. Combined with the genome-wide association study (GWAS) analysis, cardiac disease-associated genes Rasd1, Thsd7a, Ednra, and Tns1 were identified. The expression of the Rasd1 was significantly decreased by MI or the loss of KMT2D in vivo. Meanwhile, ChIP assays demonstrated that either MI or loss of KMT2D attenuated monomethylated H3K4 (H3K4me1) enrichment on the enhancer of Rasd1. By generating a KMT2D knockout (H9C2-KO) H9C2 monoclone, we verified that the expression of Rasd1 was controlled by KMT2D, and the expression of Rasd1 was decreased by serum starvation but not low-(O₂) treatment in H9C2 cells. KMT2D has a protective effect on ischemic myocardium by regulating cardiac disease-associated genes including Rasd1. KMT2D is required for the H3K4me1 deposition on the enhancer of Rasd1. Our data for the first time suggest that KMT2D-mediated Rasd1 expression may play an important protective effect on adult cells during nutritional deficiency caused by ischemic injury.

FLII and MLL1 Cooperatively Regulate Aryl Hydrocarbon Receptor-Mediated Transcription in ARPE-19 Cells

Aryl hydrocarbon receptors (AHRs), a class of ligand-dependent nuclear receptors that regulate cellular responses by inducing the expression of various target genes in response to external signals, are implicated in maintaining retinal tissue homeostasis. Previous studies have shown that the regulation of AHR-induced gene expression requires transcriptional co-regulators. However, it is not yet clear how chromatin remodelers, histone methyltransferases and coactivators interact during AHR-mediated gene expression in human retinal cells. In this study, we reveal that the histone methyltransferase MLL1 and the coactivator FLII are involved in AHR-mediated gene expression in retinal pigment epithelial cells. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) significantly increased the expression of CYP1A1, CYP1B1 and AHRR in ARPE-19 cells, whereas FLII or MLL1 depletion significantly reduced the expression of these genes induced by TCDD. Mechanistically, FLII binds to AHR in a ligand-dependent manner in ARPE-19 cells. In particular, the binding of FLII to MLL1 occurs through the GelB domain of FLII. In addition, MLL1 binds to AHR in a ligand-independent manner. FLII is involved in the recruitment of the BRG1 chromatin remodeler and MLL1 histone methyltransferase to the AHR-regulated CYP1A1 gene region in ARPE-19 cells and consequently, plays an important role in RNA polymerase II binding and transcriptional activity by modulating chromatin accessibility. Our results identify the functions and mechanisms of action of FLII and MLL1 in AHR-induced gene expression in human retinal pigment epithelial cells.