Epigenetic abnormalities are common in hematologic malignancies, including multiple myeloma (MM), and their effects can be efficiently counteracted by a class of tumor suppressor microRNAs, named epi-miRNAs. Given the oncogenic role of histone deacetylases (HDACs) in MM, we investigated if their activity could be antagonized by miR-29b, a well-established epi-miRNA. We demonstrated here that miR-29b specifically targets HDAC4 and we highlighted that both molecules are involved in a functional loop. In fact, silencing of HDAC4 by shRNAs inhibited MM cell survival and migration and triggered apoptosis and autophagy, along with induction of miR-29b expression by promoter hyperacetylation, leading to downregulation of pro-survival miR-29b targets (SP1, MCL-1). Moreover, treatment with the pan-HDAC inhibitor SAHA upregulated miR-29b, overcoming the negative control exerted by HDAC4. Importantly, overexpression or inhibition of miR-29b respectively potentiated or antagonized SAHA activity on MM cells, as also shown in vivo by a strong synergism between miR-29b synthetic mimics and SAHA in a murine xenograft model of human MM. Altogether, our results shed light on a novel epigenetic circuitry regulating MM cell growth and survival, and open new avenues for miR-29b-based epi-therapeutic approaches in the treatment of this malignancy.
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