Carcinogenesis is a multistep process whereby normal cells are transformed into malignant cells. The process is characterized by major biological changes shared by most neoplastic cells called hallmarks of cancer. These transformational events relies on multiple alterations at genetic and epigenetic levels leading to abnormal cell growth ^[1][DOI: 10.2174/1568026615666150825141330].

Over the past years protein lysine acetylation has emerged as a key post-translational modification in the coordination of tightly regulated biological functions and alterations of the acetylome profiles are associated with various pathological conditions such as cancer.

The acetylation status of lysine residues within histone and non-histone proteins is finely tuned by the concert action of histone acetyltransferases (HATs) and histone deacetylases (HDACs) catalyzing the addition and removal of the acetyl groups, respectively. Recently, there was a particular focus on HDAC6 coming from its unique properties to control multiple cellular pathways linked to cell growth, survival, and migration. Accordingly, the use of HDAC6 inhibitors alone or in combination with additional chemotherapeutic agents appear as a promising strategy to treat various cancers.

3.3.2. HDAC6 in CML

3.3.2.1.Degradation of BCR-ABL via Deacetylation of HSP90α by HDAC6 in the Cytoplasm

Although little research exists on HDAC6 in the context of CML, this protein has a function that makes it particularly interesting in the context of such pathology. HDAC6 deacetylates heat shock protein (HSP90)α, which is involved in the stabilization of the oncogenic tyrosine kinase breakpoint cluster region-Abelson (BCR-ABL) protein ^[114] [186] protein. In the acetylated form, HSP90α loses its chaperone function, which leads to the degradation of its client proteins by the proteasome (Figure 3A). The importance of the acetylation status of HSP90α in the protein degradation of BCR-ABL makes HDAC6 inhibitors potentially promising molecules for the treatment of CML. Pan-HDAC inhibitors are capable of inducing the inhibition of HDAC6, as well as the downregulation of HDAC6 using si-RNA, which increases the acetylation of HSP90α, and in turn increases the ubiquitination of the BCR-ABL protein, decreasing its expression in K562 cells ^[116][117][188,189].

3.3.3 HDAC6 inhibitors in CLL

HDAC6 is upregulated in CLL patient samples, cell lines, and euTCL1 transgenic mouse models compared to normal controls. Accordingly, this pathology could be an interesting target for selective HDAC6 inhibitors, as genetic silencing of HDAC6 improves the survival of euTCL1 mice. Moreover, the chemical inhibitor ACY738 reduces the proliferation of CLL B cells leading to their apoptosis. Together with ibrutinib, this HDAC6 inhibitor triggers synergistic cell death in vivo ^[120][doi:10.3389/fimmu.2020.590072]. Beyond the direct effect on pathological B cells, HDAC6 inhibition improves CLL-induced immunosuppression of CLL T cells. HDAC6 inhibitors enhances immune checkpoint blockade in CLL so that combination treatment with ACY738 potentializes the in vivo antitumor effect of anti-PD-1 and anti-PD-L1 antibody treatments with increased cytotoxic CD8+ T cells ^[120][doi:10.3389/fimmu.2020.590072].

3.3.4 HDAC6 inhibitors in AML

Several HDAC6 inhibitors were assessed as single agents in AML. The HDAC6 inhibitor ST80 shows potent antileukemic activity in myeloid cell lines and primary AML blasts at low micromolar concentrations, leading to preferential acetylation of  a-tubulin ^[121][doi:10.1016/j.leukres.2012.02.026]. HDAC inhibitors with a central naphthoquinone structure selectively inhibit HDAC6 in the AML cell line MV4-11, further decreasing mutant FLT-3 protein and constitutively active signal transducer and activators of transcription (STAT)5 levels and reducing extracellular regulated kinase (ERK) phosphorylation ^[122][doi:10.1021/cb200134p]. HDAC inhibitors with the 2-(oxazol-2-yl)phenol moiety as a novel zinc-binding group exhibited selective inhibition against HDAC1 and class IIb HDACs (HDAC6 and HDAC10) in the MV-4-11 AML cells ^[123][doi:10.1039/C4MD00401A]. The in vivo potency of the selective and orally-available HDAC6 inhibitor N-Hydroxy-4-(2-methoxy-5-(methyl(2-methylquinazolin-4-yl)amino)phenoxy)butanamide 23bb was better against MV4-11 AML cells compared to SAHA or ACY-1215 ^[124][doi:10.1021/acs.jmedchem.5b01342]. The O-aminobenzamide-based HDAC inhibitor compound 13e down-regulates HDAC6 in MV4-11 cells. 13e induces apoptotic cell death and cycle arrest most likely mediated by a p53-dependent pathway ^[125][doi:10.1021/acs.jmedchem.8b00136]. The parthenolide-SAHA hybrid compound 26 more potently reduces the viability of the resistant HL-60/ADR AML cell line compared to SAHA, triggering intrinsic apoptosis and reducing the protein expression levels of HDAC1, HDAC6 and the multidrug resistance-associated protein 1 (ABCC1) leading to an intracellular accumulation of drugs ^[126][doi:10.1016/j.bioorg.2019.03.056]. The HDAC6-selective inhibitor PTG-0861 induces apoptosis in MV4-11 AML cells with limited cytotoxicity against non-malignant cells ^[127][doi:10.1016/j.ejmech.2020.112411].

In AML, inhibition of HDAC6 was essentially investigated in combination with other pharmacologically active compounds at a pre-clinical level. For instance, a combination of 17-(allylamino)-17-demethoxygeldanamycin (17-AAG), a synthetic derivative of the ansamycin benzoquinone antibiotic geldanamycin, with the HDAC6 inhibitor tubacin reduces the viability of primary AML samples, validating HDAC6 as a HSP90 client protein also in AML and that its hyperacetylation facilitates the anticancer potential of 17-AAG ^[128][doi:10.1182/blood-2008-03-143644]. LBH-589 and PXD101 inhibit HDAC1 and HDAC6 and synergize with cytarabine to induce cell death in pediatric AML, accompanied by DNA damage induction and increased Bim expression levels ^[129][doi:10.1371/journal.pone.0017138]. Similarly, Bim protein induction and inhibition of nuclear factor-kappa B (NF-kB) pathway were identified as a mechanistic basis for the synergistic anti-cancer effects of belinostat in combination with the proteasome inhibitor bortezomib in AML and ALL cells ^[130][doi:10.1111/j.1365-2141.2011.08591.x]. The selective JAK2/HDAC6 dual inhibitor 20a shows excellent in vivo antitumor efficacy in HEL AML mouse xenograft assays and synergizes with the antifungal drug fluconazole ^[131][doi:10.1021/acs.jmedchem.8b00393]. The selective HDAC6 inhibitor MPT0G211 combined with doxorubicin displays anti-cancer effect by inducing a DNA damage response associated with increased Ku70 acetylation and BAX activation in HL-60 and MOLT-4 AML cell lines. Accordingly, ectopic expression of HDAC6 successively reverses the apoptosis triggered by the combined treatment ^[132][doi:10.1186/s13148-018-0595-8].

3.3.5 HDAC6 inhibitors in ALL

The HDAC6 inhibitor tubacin enhances the anti-cancer effects of the Na+/K+-ATPase inhibitor ouabain or the proteasome inhibitor MG-13 against pre-B and T ALL cells in vitro and in vivo. These results suggest that selectively targeting HDAC6 alone or in combination with conventional chemotherapeutic drugs could provide a novel approach for ALL therapy ^[133][doi:10.3109/10428194.2011.570821]. Similarly, belinostat synergizes with the proteasome inhibitor bortezomib to kill ALL cells through Bim up-regulation and NF-kB inhibition. Altogether the perturbation of intracellular microtubular transport network, combined with the interference with protein homeostasis via proteasomal inhibition, could be a general and efficient mechanism explaining the synergistic effect observed ^[130][doi:10.1111/j.1365-2141.2011.08591.x]. MPT0G211 combined with vincristine interrupts ALL mitosis via interference with microtubular dynamics leading to apoptosis. In vivo, MPT0G211 plus doxorubicin or vincristine reduces tumor growth xenograft models ^[132][doi:10.1186/s13148-018-0595-8].

Remarkably, it has been shown that the inhibition of HDAC6 using either the pan-HDAC inhibitor trichostatin, the selective HDAC6 inhibitor tubacin, or a genetic knock-down efficiently reduces Notch3 signaling through a post-translational-mediated protein down-regulation, leading to enhanced apoptosis in T-ALL cells and impairing leukemia growth in mice xenografted with T-ALL cell lines and primary human T-ALL cells. These results highlights the therapeutic potential of HDAC6 targeting in Notch3-addicted tumors ^[134][doi:10.1038/s41388-018-0234-z].

3.3.6 HDAC6 inhibitors in other hematological malignancies

Inhibition of HDAC6 activity increases CD20 levels in B-cell tumor cell lines and malignant patient cells, potentializing the in vivo effect of anti-CD20 monoclonal antibodies like rituximab. Translation of CD20 mRNA is significantly enhanced after HDAC6 inhibition as CD20 mRNA was abundant within the polysomal fraction, indicating a post-transcriptional function of HDAC6. Collectively, these findings suggest HDAC6 inhibition is a rational therapeutic strategy to be implemented in combination therapies with anti-CD20 monoclonal antibodies and open up novel avenues for the clinical use of HDAC6 inhibitors ^[135][doi:10.1002/mc.22983].

The HDAC6 inhibitor A452 combined with the Bruton's tyrosine kinase inhibitor ibrutinib efficiently kills non-Hodgkin lymphoma cells, including follicular lymphoma ^[135][doi:10.1002/mc.22983].

The HDAC6 inhibitor KT-531 displays the highest anti-cancer potency against T-cell prolymphocytic leukemia (T-PLL) cells compared to other hematological neoplasms, together with safe differential toxicity compared to non-transformed cell lines. Accordingly, HDAC6 is overexpressed in primary T-PLL patient samples in which KT-531 exerts a potent anti-cancer activity. Moreover, a combination of KT-531 with various approved drugs including bendamustine, idasanutlin, and venetoclax shows promising synergistic effects against T-PLL patient cells ^[136][doi:10.1021/acs.jmedchem.1c00420].