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Background: In the breast, the pleiotropic epigenetic regulator HDAC7 can influence stemness. Materials & Methods: The authors used MCF10 cells knocked-out for HDAC7 to explore the contribution of HDAC7 to IGF1 signaling. Results: HDAC7 buffers H3K27ac levels at the IGFBP6 and IGFBP7 genomic loci and influences their expression. In this manner, HDAC7 can tune IGF1 signaling to sustain stemness. In HDAC7 knocked-out cells, RXRA promotes the upregulation of IGFBP6/7 mRNAs. By contrast, HDAC7 increases FABP5 expression, possibly through repression of miR-218. High levels of FABP5 can reduce the delivery of all-trans-retinoic acid to RXRA. Accordingly, the silencing of FABP5 increases IGFBP6 and IGFBP7 expression and reduces mammosphere generation. Conclusion: The authors propose that HDAC7 controls the uptake of all-trans-retinoic acid, thus influencing RXRA activity and IGF1 signaling.

Epigenetic mechanisms have a main impact on cancer regulation and on microenvironment control. In the context of leiomyosarcoma (LMS), a highly aggressive and immunologically cold cancer, class IIa histone deacetylases (HDACs) and myocyte enhancer factor (MEF2) form a repressive complex that leads to the reorganization of epigenetic landscape. We have recently described the effect of some HDAC inhibitors (HDACi) on chromatin organization and in particular the increase of H3K27Ac genome occupancy near TSS and promoter regions, caused by NKL-54, a HDAC1/2/3 selective inhibitor, and the re-expression of some chemokines can be observed after the treatment with TMP-195, a specific class IIa HDACi. The hypothesis of this project is that the HDAC-MEF2 complex represses the transcription of some immune genes that may have a major role in antitumor response through an epigenetic control of chromatin. To verify this proposal, we knocked-down (KD) MEF2A in SK-UT-1, a high grade LMS cell line, or we treated the same cells with NKL-54 and TMP-195. We compared the impact of MEF2A KD and HDACs inhibition on chemokines and cytokines release by RNA-seq and Luminex profiling, while H3K27ac levels obtained by ChIP-seq were used to quantify enhancers and super-enhancers activation. To evaluate the impact of MEF2A on LMS cells proliferation we performed a clonogenic assay. To test the effectiveness of class IIa HDACs inhibitors in vivo, we aim to generate a syngeneic model of LMS by transforming uterine smooth muscle cells with Large T and Small T antigen, p53175H and by knocking out (KO) ATRX and PTEN. The preliminary data show that KD of MEF2A leads to an increased expression and release of some chemokines and cytokines such as CXCL5, CXCL2 and CXCL8 in SK-UT-1 cells. Class IIa HDACs inhibition further enhances this effect. Furthermore, the absence of MEF2A causes a reduction of colonies formation compared with WT cells, pointing out the fundamental role of MEF2A in LMS cell proliferation. Finally, ChIP-seq analysis of the genomic location of CXCL2 and CXCL5 in HDAC4 and HDAC9 KO shows an increased H3K27Ac signal at the level of the regulative elements of the two genes, indicating the epigenetic reprogramming acting on these loci and achieved by HDACs impairment. High grade LMS are characterized by the formation of HDAC IIa and MEF2 repressive complexes at some specific genomic loci required to sustain LMS malignancy and possibly to weaken LMS immunogenicity. The ablation or the specific inhibition of class IIa HDACs lead to the release of MEF2A from this repressive complex and the subsequent establishment of a transcriptional open chromatin environment that would promote tumor immune infiltration and clearance. These preliminary data, together with previous evidence, highlight the role of HDAC-MEF2 in the context of LMS microenvironment organization, suggesting that the targeting of these partners can boost the immune system activity, and this can be exploited for the development of LMS therapies. Citation Format: Martina Minisini, Emiliano Dalla, Vanessa Tolotto, Claudio Brancolini. The role of HDAC-MEF2 axis in the epigenetic control of immune tumoral microenvironment. [abstract]. In: Proceedings of the AACR Special Conference: Cancer Epigenomics; 2022 Oct 6-8; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2022;82(23 Suppl_2):Abstract nr B014.