Histone Deacetylases Function Research Articles

Targeting histone deacetylase activity in rheumatoid arthritis and asthma as prototypes of inflammatory disease: should we keep our HATs on?

Cellular activation, proliferation and survival in chronic inflammatory diseases is regulated not only by engagement of signal trans-duction pathways that modulate transcription factors required for these processes, but also by epigenetic regulation of transcription factor access to gene promoter regions. Histone acetyl trans-ferases coordinate the recruitment and activation of transcription factors with conformational changes in histones that allow gene promoter exposure. Histone deacetylases (HDACs) counteract histone acetyl transferase activity through the targeting of both histones as well as nonhistone signal transduction proteins important in inflammation. Numerous studies have indicated that depressed HDAC activity in patients with inflammatory airway diseases may contribute to local proinflammatory cytokine production and diminish patient responses to corticosteroid treatment. Recent observations that HDAC activity is depressed in rheumatoid arthritis patient synovial tissue have predicted that strategies restoring HDAC function may be therapeutic in this disease as well. Pharmacological inhibitors of HDAC activity, however, have demonstrated potent therapeutic effects in animal models of arthritis and other chronic inflammatory diseases. In the present review we assess and reconcile these outwardly paradoxical study results to provide a working model for how alterations in HDAC activity may contribute to pathology in rheumatoid arthritis, and highlight key questions to be answered in the preclinical evaluation of compounds modulating these enzymes.

Enhancement of the Antigen-Presenting Cell Function of Mantle Cell Lymphomas (MCL) by Novel Molecularly Based Immunotherapeutic Strategies.

MCL is an aggressive and incurable B-cell malignancy with an intrinsic characteristic to relapse after an initial good response to treatment. Manipulation of the immune system to unleash its well-known specificity and long-lasting protective effect might provide a unique opportunity to induce more durable responses in MCL. In previous studies in an A20 B-cell lymphoma murine model we have demonstrated that augmentation of the antigen-presenting cell function of the malignant B-cell is required for elicitation of an effective anti-lymphoma immunity1. Inhibition of Stat3 signaling, a negative regulator of inflammatory responses, and modulation of histone deacetylases function were identified as two novel targets to augment the immunogenicity of the malignant B-cell. In this study we determined therefore whether manipulation of these intracellular pathways in murine and human MCL cells could result in priming of antigen-specific T-cells and/or restoration of the responsiveness of tolerant T-cells. First, in vitro treatment of FC-muMCL1 cells - cell line derived from a MCL tumor that arise following pristine injection into Em-cyclin D1 transgenic mice- with increasing concentrations of the Stat3 inhibitors, Cucurbitacin I (CuI) or CPA-7 resulted in an enhanced presentation of OVA-peptide to naive CD4+ T-cells specific for a MHC class II restricted epitope of Ovalbumin (OT-II cells). Indeed, these antigen-specific T-cells produce higher levels of IL-2 and IFN-gamma compared to anti-OVA T cells that encountered cognate antigen in FC-muMCL1 cells treated with LPS alone. Similarly, we found that culture of the human MCL cells JEKO or Z138 with allogeneic human peripheral blood mononuclear cells in the presence of increasing concentrations of the Stat3 inhibitors also resulted in increased IL-2 production by T-cells. In the next set of experiments, we determined whether treatment of FC-muMCL1 cells with the hydroxamic acid analogue pan-HDAC inhibitor LAQ824 could influence their antigen-presenting capabilities and their ability to activate T-cell responses. Unlike, MCL cells treated with LPS alone, FC-muMCL1 cells treated with LPS and LAQ824 effectively prime antigen-specific CD4+ T-cells as determined by their production of both IL-2 and IFN-gamma in response to cognate peptide. Furthermore, in vitro treatment of Z138 MCL cells with LAQ824 also led to enhanced IFN-gamma production by allogeneic human PBMCs. Taken together, our findings points to inhibition of Stat3 signaling and inhibition of histone deacetylases as appealing molecularly based immunotherapeutic strategies to augment the immunogenicity of MCL cells.

Functional analysis of a histone deacetylase-like protein of Thermus caldophilus GK24 in mammalian cell

The function of eukaryotic histone deacetylase (HDAC) has been extensively studied for its critical role in transcriptional regulation and carcinogenesis. However that of the prokaryotic counterpart remains largely unknown. Recently, we cloned HDAC-like protein in Thermus caldophilus GK24 ( Tca HDAC) from a genomic library of the microorganism based on homology analysis with human HDAC1. To explore the function of Tca HDAC in mammalian cells, Tca HDAC gene expressing vector was transfected into a human fibrosarcoma cell line, HT1080. Tca HDAC was mainly localized in nuclei of the mammalian cells as a human HDAC1 was, due to an N-terminal HDAC association domain. We further generated histidine-substituted Tca HDAC mutants and investigated their role in biochemical and cellular activity of the enzyme. Tca HDAC mutants exhibited dramatic loss of enzymatic activity and conditioned media (CM) from HT1080 cells transfected with mutant Tca HDAC was unable to stimulate angiogenic phenotypes of endothelial cells in vitro whereas that of wild Tca HDAC did. Collectively, these results demonstrate that a prokaryotic histone deacetylase from T. caldophilus GK24 is functionally active in mammalian cells and its function in gene expression is conserved from prokaryotes to eukaryotes.

Chemistry of acetyl transfer by histone modifying enzymes: structure, mechanism and implications for effector design

The post-translational modification of histones plays an important role in chromatin regulation, a process that insures the fidelity of gene expression and other DNA transactions. Of the enzymes that mediate post-translation modification, the histone acetyltransferase (HAT) and histone deacetylase (HDAC) proteins that add and remove acetyl groups to and from target lysine residues within histones, respectively, have been the most extensively studied at both the functional and structural levels. Not surprisingly, the aberrant activity of several of these enzymes have been implicated in human diseases such as cancer and metabolic disorders, thus making them important drug targets. Significant mechanistic insights into the function of HATs and HDACs have come from the X-ray crystal structures of these enzymes both alone and in liganded complexes, along with associated enzymatic and biochemical studies. In this review, we will discuss what we have learned from the structures and related biochemistry of HATs and HDACs and the implications of these findings for the design of protein effectors to regulate gene expression and treat disease.

Class IIa histone deacetylases: regulating the regulators

In the last decade, the identification of enzymes that regulate acetylation of histones and nonhistone proteins has revealed the key role of dynamic acetylation and deacetylation in various cellular processes. Mammalian histone deacetylases (HDACs), which catalyse the removal of acetyl groups from lysine residues, are grouped into three classes, on the basis of similarity to yeast counterparts. An abundance of experimental evidence has established class IIa HDACs as crucial transcriptional regulators of various developmental and differentiation processes. In the past 5 years, a tremendous effort has been dedicated to characterizing the regulation of these enzymes. In this review, we summarize the latest discoveries in the field and discuss the molecular and structural determinants of class IIa HDACs regulation. Finally, we emphasize that comprehension of the mechanisms underlying class IIa HDAC functions is essential for potential therapeutic applications.

Functional and Physical Interactions between Yeast 14-3-3 Proteins, Acetyltransferases, and Deacetylases in Response to DNA Replication Perturbations

The highly conserved 14-3-3 proteins participate in many biological processes in different eukaryotes. The BMH1 and BMH2 genes encode the two functionally redundant Saccharomyces cerevisiae 14-3-3 isoforms. In this work we provide evidence that defective 14-3-3 functions not only impair the ability of yeast cells to sustain DNA replication in the presence of sublethal concentrations of methyl methanesulfonate (MMS) or hydroxyurea (HU) but also cause S-phase checkpoint hyperactivation. Inactivation of the catalytic subunit of the histone acetyltransferase NuA4 or of its interactor Yng2, besides leading to S-phase defects and persistent checkpoint activation in the presence of genotoxic agents, is lethal for bmh mutants. Conversely, the lack of the histone deacetylase subunit Rpd3 or Sin3 partially suppresses the hypersensitivity to HU of bmh mutants and restores their ability to complete DNA replication in the presence of MMS or HU. These data strongly suggest that reduced acetyltransferase functionality might account for the S-phase defects of bmh mutants in the presence of genotoxic agents. Consistent with a role of 14-3-3 proteins in acetyltransferase and deacetylase regulation, we find that acetylation of H3 and H4 histone tails is reduced in temperature-sensitive bmh mutants shifted to the restrictive temperature. Moreover, Bmh proteins physically interact, directly or indirectly, with the Esa1 acetyltransferase throughout the cell cycle and with the Rpd3 deacetylase specifically during unperturbed S phase and after HU treatment. Taken together, our results highlight a novel role for 14-3-3 proteins in the regulation of histone acetyltransferase and deacetylase functions in the response to replicative stress.

Activity-based probes for proteomic profiling of histone deacetylase complexes

Histone deacetylases (HDACs) are key regulators of gene expression that require assembly into larger protein complexes for activity. Efforts to understand how associated proteins modulate the function of HDACs would benefit from new technologies that evaluate HDAC activity in native biological systems. Here, we describe an active site-directed chemical probe for profiling HDACs in native proteomes and live cells. This probe, designated SAHA-BPyne, contains structural elements of the general HDAC inhibitor suberoylanilide hydroxamic acid (SAHA), as well as benzophenone and alkyne moieties to effect covalent modification and enrichment of HDACs, respectively. Both class I and II HDACs were identified as specific targets of SAHA-BPyne in proteomes. Interestingly, multiple HDAC-associated proteins were also enriched by SAHA-BPyne, even after denaturation of probe-labeled proteomes. These data indicate that certain HDAC-associated proteins are directly modified by SAHA-BPyne, placing them in close proximity to HDAC active sites where they would be primed to regulate substrate recognition and activity. We further show that SAHA-BPyne can be used to measure differences in HDAC content and complex assembly in human disease models. This chemical proteomics probe should thus prove valuable for profiling both the activity state of HDACs and the binding proteins that regulate their function.

Abnormal histone acetylase and deacetylase expression and function in lung inflammation

Abnormal histone acetylase and deacetylase expression and function in lung inflammation

Histone Deacetylase-6 (HDAC6) Modulates Akt and STAT3 Activity Via Heat Shock Protein (Hsp) 90 in Human Multiple Myeloma (MM) Cells.

Histone deacetylase 6 (HDAC6) has an essential role to recruit ubiquitinated proteins to transport to aggresomes, which ultimately induces lysosomal protein degradation. We have shown that inhibition of proteasomes with bortezomib and of aggresomes with HDAC6 inhibitor Tubacin demonstrated significant cytotoxicity in MM cell lines and MM patient tumor cells in vitro (Hideshima T et al., PNAS 2005, 102: 8597–8572). In this study, we further examined the biologic significance of HDAC6 inhibition by Tubacin in MM cells. We found that HDAC6 is constitutively associated with heat shock protein (Hsp) 90 in MM cell lines which is enhanced by Tubacin, as evidenced by co-immunoprecipitation. Since Akt and STAT3 have been shown to play important role in proliferation, anti-apoptosis, and drug resistance in MM cells; and all are client proteins of Hsp90, we next further examined whether inhibition of HDAC6 could modulate activities of these proteins via Hsp90. Importantly, Tubacin enhanced phosphorylation of Akt, associated with augmentation of Hsp90 acetylation. Hsp90 inhibitor 17-AAG downregulated Akt phosphorylation associated with enhanced interaction of Hsp90 with Akt, which was partially blocked by Tubacin. On the other hand, 17-AAG did not enhance acetylation of α-tubulin or ubiquitination of proteins, suggesting that Hsp90 does not affect HDAC6 function. Furthermore, we found that STAT3 is also constitutively associated with Hsp90. Importantly, both Tubacin and 17-AAG inhibit phosphorylation of STAT3 in a dose- and time-dependent fashion in MM cells. Taken together, our data indicate that HDAC6 has an important role not only in aggresomal protein degradation, but also in MM cell pathogenesis by modulating Akt and STAT3 signaling cascades via Hsp90 acetylation in MM cells.

Inhibition of Tumor Necrosis Factor-α-Inducible Inflammatory Genes by Interferon-γ Is Associated with Altered Nuclear Factor-κB Transactivation and Enhanced Histone Deacetylase Activity

Airway smooth muscle (ASM) cells can act as effector cells in the initiation and/or perpetuation of airway inflammation in asthma by producing various inflammatory chemokines or cytokines. Previous studies from our laboratory and others showed that the combination of tumor necrosis factor-alpha (TNFalpha) and interferon-gamma (IFNgamma) or endogenous IFNbeta results in a synergistic induction of various pro-inflammatory genes, including CD38 and regulated upon activation normal T-cell expressed and secreted (RANTES), in ASM cells. In contrast to these studies, we found that IFNgamma (1000 U/ml) markedly inhibited TNFalpha-induced expression of interleukin (IL)-6, IL-8, and eotaxin by 66.29+/-3.33, 43.86+/-7.11, and 63.25+/-6.46%, respectively. These genes were also found to be NF-kappaB-dependent in that TNFalpha-induced expression of IL-6, IL-8, and eotaxin was dose-dependently inhibited by the selective IKKbeta inhibitor 4-(2'-aminoethyl)amino-1,8-dimethylimidazo[1,2-a]quinoxaline (BMS-345541) (1-30 microM). Using a luciferase reporter construct containing kappaB sites, we found that IFNgamma (10-1000 U/ml) inhibits NF-kappaB-dependent gene transcription in a dose-dependent manner. Moreover, IFNgamma failed to affect TNFalpha-induced IkappaKbeta phosphorylation or IkappaB degradation as well as nuclear NF-kappaB/DNA interaction. It is noteworthy that IFNgamma decreases TNFalpha-induced histone acetyl transferase (HAT) and increases histone deacetylase (HDAC) activities. Finally, trichostatin A, an HDAC inhibitor, prevents IFNgamma inhibitory action on TNFalpha-induced gene expression. Together, our data indicate that IFNgamma is a potent inhibitor of specific TNFalpha-inducible inflammatory genes by acting on NF-kappaB transactivation via the modulation of HDAC function.

284 POSTER Lenalidomide and CC-4047 inhibit the proliferation of Namalwa cancer cells while expanding CD34+ progenitor cells. New insights on the combination therapy with HDAC inhibitors for hematological cancers

Histone deacetylase (HDAC) and histone acetyltransferase (HAT) functions are associated with various cancers, and the inhibition of HDAC has been found to arrest disease progression. Here, we have investigated the gene expression profiles of leukaemic cells in response to the HDAC inhibitor trichostatin A (TSA) using oligonucleotide microarrays. Nucleosomal histone acetylation was monitored in parallel and the expression profiles of selected genes were confirmed by quantitative polymerase chain reaction (PCR). A large number of genes (9% of the genome) were found to be similarly regulated in CCRF-CEM and HL-60 cells in response to TSA, and genes showing primary and secondary responses could be distinguished by temporal analysis of gene expression. A small fraction of genes were highly sensitive to histone hyper-acetylation, including XRCC1, HOXB6, CDK10, MYC, MYB, NMI and CBFA2T3 and many were trans-acting factors relevant to cancer. The most rapidly repressed gene was MKRN3, an imprinted gene involved in the Prader–Willi syndrome.

Abnormal histone acetylase and deacetylase expression and function in lung inflammation

Post-translational modifications in DNA and histone proteins are heritable changes that are not coded for in the DNA sequence itself but play an important role in the control of gene expression. These modifications include histone acetylation, methylation, ubiquitination, sumoylation and phosphorylation. These changes are not only critical for generating diversity of cell types during mammalian development, but are also important for maintaining the stability and integrity of the expression profiles of different cell types. Until recently, the study of human disease has focused on genetic mechanisms rather than on non-coding events. However, it is becoming increasingly clear that altered patterns of histone modifications can lead to several major pathologies. This review focuses on histone acetylation and its role in inflammatory gene expression. Interestingly, the expression and activity of enzymes that regulate this modification have been reported to be abnormal in the airways of patients with respiratory disease. Histone modifications, despite being heritable and stably maintained, are also potentially reversible and there is scope for the development of "epigenetic therapies" for disease.

Mechanism of the Long Range Anti-silencing Function of Targeted Histone Acetyltransferases in Yeast

Transcriptionally silent chromatin in Saccharomyces cerevisiae is associated with histone hypoacetylation and is formed through the action of the Sir histone deacetylase complex. A histone acetyltransferase (HAT) targeted near silent chromatin can overcome silencing at a distance by increasing histone acetylation in a sizable region. However, how a tethered HAT acetylates distant nucleosomes has not been resolved. We demonstrate here that targeting the histone H3-specific HAT Gcn5p promotes acetylation of not only histone H3 but also histone H4 in a broad region. We also show that long range anti-silencing and histone acetylation by targeted HATs can be blocked by nucleosome-excluding sequences. These results are consistent with the contention that a tethered HAT promotes stepwise propagation of histone acetylation along the chromatin. Because histone hypoacetylation is key to the formation and maintenance of transcriptionally silent chromatin, it is believed that acetylation promoted by a targeted HAT disrupts silent chromatin thereby overcoming silencing. However, we show that the acetylated and transcriptionally active region created by a tethered HAT retains structural hallmarks of Sir-dependent silent chromatin and remains associated with Sir proteins indicating that tethered HATs overcome silencing without completely dismantling silent chromatin.

Targeting histone deacetylase in cancer therapy

Histone deacetylase (HDAC) is recognized as one of the promising targets for cancer treatment as many HDAC inhibitors have entered clinical trials for both solid and liquid tumors. Nevertheless, the mechanisms underlying the antiproliferative effects of HDAC inhibitors remain elusive. Although they have been shown to regulate the transcription of a defined set of genes through chromatin remodeling, increasing evidence suggests that modifications of the epigenetic histone code may not be the primary mechanism for HDAC inhibitor-mediated growth inhibition and apoptosis in cancer cells. While histones still represent a primary target for the physiological function of HDACs, the antitumor effect of HDAC inhibitors might also be attributed to transcription-independent mechanisms by modulating the acetylation status of a series of nonhistone targets. Also noteworthy is the effect of HDAC inhibitors on Akt downregulation through the alteration of protein phosphatase 1 (PP1) complex formation. To provide an overview of the use of HDAC inhibitors in cancer treatment, this review addresses the following subjects: (1) the physiological relevance of HDAC-mediated acetylation of histone and nonhistone substrates, (2) the chemical biology of HDACs and development of a novel class of HDAC inhibitors, and (3) the protein acetylation-independent effect of HDAC inhibitors on the activation status of signaling kinases.

Inhibition of Histone Deacetylation Blocks Cardiac Hypertrophy Induced by Angiotensin II Infusion and Aortic Banding

A number of distinct stress signaling pathways in myocardium cause cardiac hypertrophy and heart failure. Class II histone deacetylases (HDACs) antagonize several stress-induced pathways and hypertrophy. However, cardiac hypertrophy induced by transgenic overexpression of the homeodomain only protein, HOP, can be prevented by the nonspecific HDAC inhibitors trichostatin A and valproic acid, suggesting that alternate targets that oppose class II HDAC function might exist in myocardium. We tested the effects of several HDAC inhibitors, including a class I HDAC-selective inhibitor, SK-7041, on cardiac hypertrophy induced by angiotensin II (Ang II) treatment or aortic banding (AB). Cardiac hypertrophy was induced by chronic infusion of Ang II or by AB in mice or rats and evaluated by determining the ratio of heart weight to body weight or to tibia length, cross-sectional area, or echocardiogram. Cardiac hypertrophy induced by Ang II or AB for 2 weeks was significantly reduced by simultaneous administration of trichostatin A, valproic acid, or SK-7041. Echocardiogram revealed that exaggerated left ventricular systolic dimensions were relieved by HDAC inhibitors. HDAC inhibitors partially reversed preestablished cardiac hypertrophy and improved survival of AB mice. The expressions of atrial natriuretic factor, alpha-tubulin, beta-myosin heavy chain, and interstitial fibrosis were reduced by HDAC inhibition. These results suggest that the predominant effect of HDAC inhibition, mainly mediated by class I HDACs, is to prevent cardiac hypertrophy in response to a broad range of agonist and stretch stimuli.

The C. elegans histone deacetylase HDA-1 is required for cell migration and axon pathfinding

Histone proteins play integral roles in chromatin structure and function. Histones are subject to several types of posttranslational modifications, including acetylation, which can produce transcriptional activation. The converse, histone deacetylation, is mediated by histone deacetylases (HDACs) and often is associated with transcriptional silencing. We identified a new mutation, cw2, in the Caenorhabditis elegans hda-1 gene, which encodes a histone deacetylase. Previous studies showed that a mutation in hda-1, e1795, or reduction of hda-1 RNA by RNAi causes defective vulval and gonadal development leading to sterility. The hda-1(cw2) mutation causes defective vulval development and reduced fertility, like hda-1(e1795), albeit with reduced severity. Unlike the previously reported hda-1 mutation, hda-1(cw2) mutants are viable as homozygotes, although many die as embryos or larvae, and are severely uncoordinated. Strikingly, in hda-1(cw2) mutants, axon pathfinding is defective; specific axons often appear to wander randomly or migrate in the wrong direction. In addition, the long range migrations of three neuron types and fasciculation of the ventral nerve cord are defective. Together, our studies define a new role for HDA-1 in nervous system development, and provide the first evidence for HDAC function in regulating neuronal axon guidance.

The Nucleation and Maintenance of Heterochromatin by a Histone Deacetylase in Fission Yeast

Posttranslational modifications of histones play an essential role in heterochromatin assembly. Whereas the role of Clr4/Suv39h-mediated methylation of histone H3 at lysine 9 (H3K9) in heterochromatin assembly is well studied, the exact function of histone deacetylases (HDACs) in this process is unclear. We show that Clr3, a fission yeast homolog of mammalian class II HDACs, acts in a distinct pathway parallel to RNAi-directed heterochromatin nucleation to recruit Clr4 and mediate H3K9 methylation at the silent mating-type region and centromeres. At the mat locus, Clr3 is recruited at a specific site through a mechanism involving ATF/CREB family proteins. Once recruited, Clr3 spreads across the 20 kb silenced domain that requires its own HDAC activity and heterochromatin proteins including Swi6/HP1. We also demonstrate that Clr3 contributes to heterochromatin maintenance by stabilizing H3K9 trimethylation and by preventing histone modifications associated with active transcription, and that it limits RNA polymerase II accessibility to naturally silenced repeats at heterochromatin domains.

Individual histone deacetylases in Drosophila modulate transcription of distinct genes

Lysine residues on the N-terminal tails of histones in chromatin are the primary targets of histone acetyltransferases (HATs) and histone deacetylases (HDACs) in eukaryotes. Regulation of histone acetylation by these two classes of enzymes plays significant roles in controlling transcriptional activity in cells. Eukaryotic organisms have several different HDACs, but the biological roles of each HDAC are still not clear. To understand the physiological functions of HDACs, we characterized six different Drosophila HDACs, including Rpd3, HDAC3, HDAC4, HDAC6-S, HDAC6-L, and Sir2, by developmental expression pattern, transcriptional profiles of target genes, and sensitivity to HDAC inhibitors. We found that each HDAC has a distinct temporal expression pattern and regulates transcription of a unique set of genes. Furthermore, we demonstrated differential sensitivity of HDACs to inhibitors. These results show that each individual HDAC plays different roles in regulating genes involved in various biological processes.

Genomewide analysis of nucleosome density histone acetylation and HDAC function in fission yeast

We have conducted a genomewide investigation into the enzymatic specificity, expression profiles, and binding locations of four histone deacetylases (HDACs), representing the three different phylogenetic classes in fission yeast (Schizosaccharomyces pombe). By directly comparing nucleosome density, histone acetylation patterns and HDAC binding in both intergenic and coding regions with gene expression profiles, we found that Sir2 (class III) and Hos2 (class I) have a role in preventing histone loss; Clr6 (class I) is the principal enzyme in promoter-localized repression. Hos2 has an unexpected role in promoting high expression of growth-related genes by deacetylating H4K16Ac in their open reading frames. Clr3 (class II) acts cooperatively with Sir2 throughout the genome, including the silent regions: rDNA, centromeres, mat2/3 and telomeres. The most significant acetylation sites are H3K14Ac for Clr3 and H3K9Ac for Sir2 at their genomic targets. Clr3 also affects subtelomeric regions which contain clustered stress- and meiosis-induced genes. Thus, this combined genomic approach has uncovered different roles for fission yeast HDACs at the silent regions in repression and activation of gene expression.

The Synergistic Combination of the Farnesyl Transferase Inhibitor Lonafarnib and Paclitaxel Enhances Tubulin Acetylation and Requires a Functional Tubulin Deacetylase

Farnesyl transferase (FT) inhibitors (FTI) are anticancer agents developed to target oncogenic Ras proteins by inhibiting Ras farnesylation. FTIs potently synergize with paclitaxel and other microtubule-stabilizing drugs; however, the mechanistic basis underlying this synergistic interaction remains elusive. Here we show that the FTI lonafarnib affects the microtubule cytoskeleton resulting in microtubule bundle formation, increased microtubule stabilization and acetylation, and suppression of microtubule dynamics. Notably, treatment with the combination of low doses of lonafarnib with paclitaxel markedly enhanced tubulin acetylation (a marker of microtubule stability) as compared with either drug alone. This synergistic effect correlated with FT inhibition and was accompanied by a synergistic increase in mitotic arrest and cell death. Mechanistically, we show that the combination of lonafarnib and paclitaxel inhibits the in vitro deacetylating activity of the only known tubulin deacetylase, histone deacetylase 6 (HDAC6). In addition, the lonafarnib/taxane combination is synergistic only in cells lines expressing the wild-type HDAC6, but not a catalytic-mutant HDAC6, revealing that functional HDAC6 is required for the synergy of lonafarnib with taxanes. Furthermore, tubacin, a specific HDAC6 inhibitor, synergistically enhanced tubulin acetylation in combination with paclitaxel, similar to the combination of lonafarnib and paclitaxel. Taken together, these data suggest a relationship between FT inhibition, HDAC6 function, and cell death, providing insight into the putative molecular basis of the lonafarnib/taxane synergistic antiproliferative combination.