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Histone deacetylases meet microRNA-associated MMP-9 expression regulation in glucocorticoid-sensitive and -resistant cell lines.

Glucocorticoids are largely used in the treatment of inflammatory pathologies and/or hematological malignancies and regulate the expression of a variety of genes involved in inflammation or metastasis such as matrix metalloproteinases (MMP). Long-term exposure to glucocorticoids can result in failure of responsiveness, which is often associated with an unwanted gene expression. Epigenetic mechanisms are involved in gene expression modulated after development of glucocorticoid resistance but how these mechanisms take place must be further studied. The effects of HDAC inhibitors (HDACi) in a context of glucocorticoid resistance are still not well understood and need to be further investigated. We hypothesized that acquired glucocorticoid resistance associated to HDACi could disturbs epigenetic landscape, especially miR expression, leading to a modulation of MMP-9 gene expression and/or protein secretion, described as largely involved in bone remodeling and tumor invasion in multiple myeloma. To this aim, we used sensitive RPMI-8226 cell line and its dexamethasone- and methylprednisolone-resistant derivatives. The resistant cell lines displayed an 'open chromatin' and an MMP-9 overexpression comparatively to the sensitive cell line. HDACi treatment with MS-275 increased even more MMP-9 overexpression not only at an mRNA level but also at the protein level. We showed that MMP-9 expression regulation was not directly linked with HAT/HDAC balance alterations but rather with the deregulation of MMP-9-targeting miRs. Then, we first demonstrated that miR‑149 downregulation was directly involved in the MMP-9 overexpression following a chronic glucocorticoid exposure and that MS-275 could amplify this overexpression by inhibition of miR‑149 expression and miR‑520c overexpression. Taken together, these results indicate that the use of HDACi in a context of acquired glucocorticoid resistance could modify the epigenetic landscape, highlighting the importance of taking the glucocorticoid response status into consideration in treatment with HDACi.