In response to environmental cues, enzymes that influence the functions of proteins, through reversible post-translational modifications supervise the coordination of cell behavior like orchestral conductors. Class IIa histone deacetylases (HDACs) belong to this category. Even though in vertebrates these deacetylases have discarded the core enzymatic activity, class IIa HDACs can assemble into multiprotein complexes devoted to transcriptional reprogramming, including but not limited to epigenetic changes. Class IIa HDACs are subjected to variegated and interconnected layers of regulation, which reflect the wide range of biological responses under the scrutiny of this gene family. Here, we discuss about the key mechanisms that fine tune class IIa HDACs activities.

The MEF2-class IIa histone deacetylase (HDAC) axis operates in several differentiation pathways and in numerous adaptive responses. We show here that nuclear active HDAC4 and HDAC7 display transforming capability. HDAC4 oncogenic potential depends on the repression of a limited set of genes, most of which are MEF2 targets. Genes verified as targets of the MEF2-HDAC axis are also under the influence of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway that affects MEF2 protein stability. A signature of MEF2 target genes identified by this study is recurrently repressed in soft tissue sarcomas. Correlation studies depicted two distinct groups of soft tissue sarcomas: one in which MEF2 repression correlates with PTEN downregulation and a second group in which MEF2 repression correlates with HDAC4 levels. Finally, simultaneous pharmacological inhibition of the PI3K/Akt pathway and of MEF2-HDAC interaction shows additive effects on the transcription of MEF2 target genes and on sarcoma cells proliferation. Overall, our work pinpoints an important role of the MEF2-HDAC class IIa axis in tumorigenesis.

Transcriptional networks supervising class IIa HDAC expression are poorly defined. Here we demonstrate that MEF2D is the key factor controlling HDAC9 transcription. This control, which is part of a negative feed-back loop during muscle differentiation, is hijacked in cancer. In leiomyosarcomas the MEF2D/HDAC9 vicious circuit sustains proliferation and cell survival, through the repression of the death receptor FAS. Comprehensive genome-wide studies demonstrate that HDAC4 and HDAC9 control different genetic programs and show both specific and common genomic binding sites. Although the number of MEF2-target genes commonly regulated is similar, only HDAC4 represses many additional genes that are not MEF2D targets. As expected, HDAC4−/− and HDAC9−/− cells increase H3K27ac levels around the TSS of the respective repressed genes. However, these genes rarely show binding of the HDACs at their promoters. Frequently HDAC4 and HDAC9 bind intergenic regions. We demonstrate that these regions, recognized by MEF2D/HDAC4/HDAC9 repressive complexes, show the features of active enhancers. In these regions HDAC4 and HDAC9 can differentially influence H3K27 acetylation. Our studies describe new layers of class IIa HDACs regulation, including a dominant positional effect, and can contribute to explain the pleiotropic actions of MEF2 TFs.

MEF2s are pleiotropic transcription factors (TFs) which supervise multiple cellular activities. During the cell cycle, MEF2s are activated at the G 0 /G 1 transition to orchestrate the expression of the immediate early genes in response to growth factor stimulation. Here we show that, in human and murine fibroblasts, MEF2 activities are downregulated during late G 1 . MEF2C and MEF2D interact with the E3 ligase F-box protein SKP2, which mediates their subsequent degradation through the ubiquitinproteasome system. The cyclin-dependent kinase 4 (CDK4)/cyclin D1 complex phosphorylates MEF2D on serine residues 98 and 110, and phosphorylation of these residues is an important determinant for SKP2 binding. Unscheduled MEF2 transcription during the cell cycle reduces cell proliferation, whereas its containment sustains DNA replication. The CDK inhibitor p21/ CDKN1A gene is a MEF2 target gene required to exert this antiproliferative influence. MEF2C and MEF2D bind a region within the first intron of CDKN1A, presenting epigenetic markers of open chromatin. Importantly, H3K27 acetylation within this regulative region depends on the presence of MEF2D. We propose that following the initial engagement in the G 0 /G 1 transition, MEF2C and MEF2D must be polyubiquitylated and degraded during G 1 progression to diminish the transcription of the CDKN1A gene, thus favoring entry into S phase. In vertebrates, the family of MEF2s comprises 4 members-MEF2A, -B, -C, and -D-as well as some splicing variants (1). Common features of all MEF2 members are the MADS box (MCM1, agamous, deficiens, serum response factor) and the adjacent MEF2 domain positioned within the highly conserved amino-terminal region (1). These domains are involved in recognizing the YTA(A/T) 4 TAR DNA motif, in mediating the formation of homo-and heterodimers, and in the interaction with different cofactors (1). The carboxy-terminal half is much less conserved. It encompasses the transactivation domains and the nuclear localization signal (2). The different family members exhibit specific but also overlapping patterns of expression, during either embryogenesis or adult life (1, 3). MEF2s are subjected to intense supervision by environmental signals, in order to couple the gene expression signature to the organism requirements (1). MEF2s oversee the expression of several genes, depending on and in cooperation with other transcription factors (TFs) (3, 4). In addition, MEF2s can also operate as repressors of transcription when in complexes with class IIa histone deacetylases (HDACs) (5,6,7,8).The extent of genes under the influence of MEF2s justifies the pleiotropic activities and the assorted cellular responses attributed to these TFs. During development, in general, expression of MEF2 is linked to the activation of differentiation programs (1). In various scenarios, the onset of MEF2 expression coincides with the withdrawal from the cell cycle (9). Specific ablation of MEF2C in neural/progenitor cells impacts differentiation but not their survival or proliferation (...

The contribution of MEF2 TFs to the tumorigenic process is still mysterious. Here we clarify that MEF2 can support both pro-oncogenic or tumor suppressive activities depending on the interaction with co-activators or co-repressors partners. Through these interactions MEF2 supervise histone modifications associated with gene activation/repression, such as H3K4 methylation and H3K27 acetylation. Critical switches for the generation of a MEF2 repressive environment are class IIa HDACs. In leiomyosarcomas (LMS), this two-faced trait of MEF2 is relevant for tumor aggressiveness. Class IIa HDACs are overexpressed in 22% of LMS, where high levels of MEF2, HDAC4 and HDAC9 inversely correlate with overall survival. The knock out of HDAC9 suppresses the transformed phenotype of LMS cells, by restoring the transcriptional proficiency of some MEF2-target loci. HDAC9 coordinates also the demethylation of H3K4me3 at the promoters of MEF2-target genes. Moreover, we show that class IIa HDACs do not bind all the regulative elements bound by MEF2. Hence, in a cell MEF2-target genes actively transcribed and strongly repressed can coexist. However, these repressed MEF2-targets are poised in terms of chromatin signature. Overall our results candidate class IIa HDACs and HDAC9 in particular, as druggable targets for a therapeutic intervention in LMS.

While MEF2 transcription factors are well known to cooperate in orchestrating cell fate and adaptive responses during development and adult life, additional studies over the last decade have identified a wide spectrum of genetic alterations of MEF2 in different cancers. The consequences of these alterations, including triggering and maintaining the tumorigenic process, are not entirely clear. A deeper knowledge of the molecular pathways that regulate MEF2 expression and function, as well as the nature and consequences of MEF2 mutations are necessary to fully understand the many roles of MEF2 in malignant cells. This review discusses the current knowledge of MEF2 transcription factors in cancer.

No abstract

HDAC7 is a pleiotropic transcriptional coregulator that controls different cellular fates. Here, we demonstrate that in human mammary epithelial cells, HDAC7 sustains cell proliferation and favours a population of stem‐like cells, by maintaining a proficient microenvironment. In particular, HDAC7 represses a repertoire of cytokines and other environmental factors, including elements of the insulin‐like growth factor signalling pathway, IGFBP6 and IGFBP7. This HDAC7‐regulated secretome signature predicts negative prognosis for luminal A breast cancers. ChIP‐seq experiments revealed that HDAC7 binds locally to the genome, more frequently distal from the transcription start site. HDAC7 can colocalize with H3K27‐acetylated domains and its deletion further increases H3K27ac at transcriptionally active regions. HDAC7 levels are increased in RAS‐transformed cells, in which this protein was required not only for proliferation and cancer stem‐like cell growth, but also for invasive features. We show that an important direct target of HDAC7 is IL24 , which is sufficient to suppress the growth of cancer stem‐like cells.