Icant growth inhibition in MM cell lines and patient MM cells, without toxicity in PBMCs. In contrast, modest or no growth inhibitory effect of HDAC1 or HDAC2 knockdown was recognized. Consistent with our previous studies using non-selective HDAC inhibitors (ie, SAHA, LAQ824, LBH589) 257, the MM cell growth inhibitory effect induced by either HDAC3 knockdown or BG45 is associated with markedly increased p21WAF1, followed by apoptosis evidenced by cleavage of caspases and PARP. Taken together, these results strongly suggest that classI HDAC inhibitor- or non-selective HDAC inhibitor-induced MM cell growth inhibition is due to HDAC3 inhibition. They further suggest that more selective HDAC3 inhibitor may have a more favorable side effect profile than class-I or non-selective HDAC inhibitors. We have previously shown that both non-selective HDAC inhibitors and HDAC6-selective inhibitors tubacin and ACY-1215 significantly enhance bortezomib-induced cytotoxicity in MM cells, associated with dual proteasome and aggresome blockade 6, 7. Since nonselective HDAC inhibitors can block both class-I (HDAC1, 2, 3 and 8) and class-IIb (HDAC6, 10), we next determined whether the enhanced cytotoxicity of bortezomib combined with non-selective HDAC inhibitors is due solely to HDAC6 inhibition, or also to class-I HDAC blockade. Importantly, MS275, but not Merck60, augments bortezomibinduced cytotoxicity in MM cells. Moreover, both HDAC3 knockdown and BG45 similarly significantly enhance bortezomib-induced cytotoxicity, confirming the pivotal role of HDAC3 blockade in mediating enhanced cytotoxicity in combination with bortezomib.ADC-Related Custom Services Bortezomib with HDAC6 inhibitors achieves dual inhibition of proteasomal and aggresomal protein degradation and accumulation of polyubiquitinated proteins 6, 7, which was not observed by bortezomib and HDAC3 knockdown.WU-04 Therefore differential mechanisms of action of HDAC3 (class-I) versus HDAC6 (class-IIb) inhibition mediate enhanced bortezomib-induced cytotoxicity in MM cells.PMID:34856019 NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author ManuscriptLeukemia. Author manuscript; available in PMC 2014 September 16.Minami et al.PageWe have shown that the BM microenvironment induces MM cell proliferation, survival, drug resistance, and migration 20, 28. The JAK2/STAT3 pathway mediates MM cell survival by regulating anti-apoptotic proteins including Mcl-1, Bcl-xL, and survivin 17, 291; therefore, inhibition of JAK2/STAT3 pathway is a potential therapeutic target. Indeed, we and others have shown that STAT3 inhibition by RNAi or small molecule inhibitors significantly inhibits MM cell growth 15, 17, 32. Importantly, we here found that HDAC3 knockdown markedly decreases both tyrosine (Y705) and serine (S727) phosphorylation of STAT3. Moreover, either HDAC3 knockdown or BG45 inhibit p-STAT3 and MM cell growth, even in the presence of exogenous IL-6 or BMSC culture supernatants. Previous studies have shown that STAT3 acetylation is regulated by HDAC3 in multiple cancers 14, 19, 33, indicating that STAT3 is one of non-histone substrate proteins were hyperacetylated by HDAC3 inhibition. We therefore examined the impact of HDAC3 inhibition on STAT3 acetylation. Consistent with previous studies, we observed that acetylation of STAT3 in MM cells is upregulated by both HDAC3 knockdown and BG45. Since HDAC3 knockdown or inhibition triggers both upregulation of acetylation and downregulation of phosphorylation of STAT3, these results suggest cross.
