HDAC6 controls innate immune and autophagy responses to TLR-mediated signalling by the intracellular bacteria Listeria monocytogenes

The role of HDAC6 in enhancing macrophage autophagy via the autophagolysosomal pathway to alleviate legionella pneumophila-induced pneumonia

Legionella pneumophila (L. pneumophila) is a prevalent pathogenic bacterium responsible for significant global health concerns. Nonetheless, the precise pathogenic mechanisms of L. pneumophila have still remained elusive. Autophagy, a direct cellular response to L. pneumophila infection and other pathogens, involves the recognition and degradation of these invaders in lysosomes. Histone deacetylase 6 (HDAC6), a distinctive member of the histone deacetylase family, plays a multifaceted role in autophagy regulation. This study aimed to investigate the role of HDAC6 in macrophage autophagy via the autophagolysosomal pathway, leading to alleviate L. pneumophila-induced pneumonia. The results revealed a substantial upregulation of HDAC6 expression level in murine lung tissues infected by L. pneumophila. Notably, mice lacking HDAC6 exhibited a protective response against L. pneumophila-induced pulmonary tissue inflammation, which was characterized by the reduced bacterial load and diminished release of pro-inflammatory cytokines. Transcriptomic analysis has shed light on the regulatory role of HDAC6 in L. pneumophila infection in mice, particularly through the autophagy pathway of macrophages. Validation using L. pneumophila-induced macrophages from mice with HDAC6 gene knockout demonstrated a decrease in cellular bacterial load, activation of the autophagolysosomal pathway, and enhancement of cellular autophagic flux. In summary, the findings indicated that HDAC6 knockout could lead to the upregulation of p-ULK1 expression level, promoting the autophagy-lysosomal pathway, increasing autophagic flux, and ultimately strengthening the bactericidal capacity of macrophages. This contributes to the alleviation of L. pneumophila-induced pneumonia.

Inflammasomes Are Influenced by Epigenetic and Autophagy Mechanisms in Colorectal Cancer Signaling

Inflammasomes contribute to colorectal cancer signaling by primarily inducing inflammation in the surrounding tumor microenvironment. Its role in inflammation is receiving increasing attention, as inflammation has a protumor effect in addition to inducing tissue damage. The inflammasome's function is complex and controlled by several layers of regulation. Epigenetic processes impact the functioning or manifestation of genes that are involved in the control of inflammasomes or the subsequent signaling cascades. Researchers have intensively studied the significance of epigenetic mechanisms in regulation, as they encompass several potential therapeutic targets. The regulatory interactions between the inflammasome and autophagy are intricate, exhibiting both advantageous and harmful consequences. The regulatory aspects between the two entities also encompass several therapeutic targets. The relationship between the activation of the inflammasome, autophagy, and epigenetic alterations in CRC is complex and involves several interrelated pathways. This article provides a brief summary of the newest studies on how epigenetics and autophagy control the inflammasome, with a special focus on their role in colorectal cancer. Based on the latest findings, we also provide an overview of the latest therapeutic ideas for this complex network.

HDAC6 inhibitor promotes reactive oxygen species-meditated clearance of Staphylococcus aureus in macrophage

Staphylococcus aureus (S. aureus) pneumonia has become an increasingly important public health problem. Recent evidence suggests that epigenetic modifications are critical in the host immune defence against pathogen infection. In this study, we found that S. aureus infection induces the expression of histone deacetylase 6 (HDAC6) in a dose-dependent manner. Furthermore, by using a S. aureus pneumonia mouse model, we showed that the HDAC6 inhibitor, tubastatin A, demonstrates a protective effect in S. aureus pneumonia, decreasing the mortality and destruction of lung architecture, reducing the bacterial burden in the lungs and inhibiting inflammatory responses. Mechanistic studies in primary bone marrow-derived macrophages demonstrated that the HDAC6 inhibitors, tubastatin A and tubacin, reduced the intracellular bacterial load by promoting bacterial clearance rather than regulating phagocytosis. Finally, N-acetyl-L- cysteine, a widely used reactive oxygen species (ROS) scavenger, antagonized ROS production and significantly inhibited tubastatin A-induced S. aureus clearance. These findings demonstrate that HDAC6 inhibitors promote the bactericidal activity of macrophages by inducing ROS, an important host factor for S. aureus clearance and production. Our study identified HDAC6 as a suitable epigenetic modification target for preventing S. aureus infection, and tubastatin A as a useful compound in treating S. aureus pneumonia.

Targeting HDAC6 improves anti-CD47 immunotherapy

BACKGROUND

Cancer cells can overexpress CD47, an innate immune checkpoint that prevents phagocytosis upon interaction with signal regulatory protein alpha (SIRPα) expressed in macrophages and other myeloid cells. Several clinical trials have reported that CD47 blockade reduces tumor growth in hematological malignancies. However, CD47 blockade has shown modest results in solid tumors, including melanoma. Our group has demonstrated that histone deacetylase 6 inhibitors (HDAC6is) have immunomodulatory properties, such as controlling macrophage phenotype and inflammatory properties. However, the molecular and cellular mechanisms controlling these processes are not fully understood. In this study, we evaluated the role of HDAC6 in regulating the CD47/SIRPα axis and phagocytosis in macrophages.

METHODS

We tested the role of HDAC6is, especially Nexturastat A, in regulating macrophage phenotype and phagocytic function using bone marrow-derived macrophages and macrophage cell lines. The modulation of the CD47/SIRPα axis and phagocytosis by HDAC6is was investigated using murine and human melanoma cell lines and macrophages. Phagocytosis was evaluated via coculture assays of macrophages and melanoma cells by flow cytometry and immunofluorescence. Lastly, to evaluate the antitumor activity of Nexturastat A in combination with anti-CD47 or anti-SIRPα antibodies, we performed in vivo studies using the SM1 and/or B16F10 melanoma mouse models.

RESULTS

We observed that HDAC6is enhanced the phenotype of antitumoral M1 macrophages while decreasing the protumoral M2 phenotype. In addition, HDAC6 inhibition diminished the expression of SIRPα, increased the expression of other pro-phagocytic signals in macrophages, and downregulated CD47 expression in mouse and human melanoma cells. This regulatory role on the CD47/SIRPα axis translated into enhanced antitumoral phagocytic capacity of macrophages treated with Nexturastat A and anti-CD47. We also observed that the systemic administration of HDAC6i enhanced the in vivo antitumor activity of anti-CD47 blockade in melanoma by modulating macrophage and natural killer cells in the tumor microenvironment. However, Nexturastat A did not enhance the antitumor activity of anti-SIRPα despite its modulation of macrophage populations in the SM1 tumor microenvironment.

CONCLUSIONS

Our results demonstrate the critical regulatory role of HDAC6 in phagocytosis and innate immunity for the first time, further underscoring the use of these inhibitors to potentiate CD47 immune checkpoint blockade therapeutic strategies.

Design, synthesis, insilco study and biological evaluation of new isatin-sulfonamide derivatives by using mono amide linker as possible as histone deacetylase inhibitors

OBJECTIVE

Aim: To evaluate the cytotoxic activity of newly synthesized a series of novel HDAC inhibitors comprising sulfonamide as zinc binding group and Isatin derivatives as cap group joined by mono amide linker as required to act as HDAC inhibitors.

PATIENTS AND METHODS

Materials and Methods: The utilization of sulfonamide as zinc binding group joined by N-alkylation reaction with ethyl-bromo hexanoate as linker group that joined by amide reaction with Isatin derivatives as cap groups which known to possess antitumor activity in the designed of new histone deacetylase inhibitors and using the docking and MTT assay to evaluate the compounds.

RESULTS

Results: Four compounds have been synthesized and characterized successfully by ART-FTIR, NMR and ESI-Ms. the compounds were synthesized and characterized by successfully by ART-FTIR, NMR and ESI- Ms. Assessed for their cytotoxic activity against human colon adenocarcinoma MCF-7 (IC50, I=105.15, II=60.00, III=54.11, IV=56.57, vorinostat=28.41) and hepatoblastoma HepG2 (IC50, I=63.91, II=135.18, III=118.85, IV=51.46, vorinostat=37.50). Most of them exhibited potent HDAC inhibitory activity and significant cytotoxicity.

CONCLUSION

Conclusions: The synthesized compounds (I, II, III and IV) showed cytotoxicity toward MCF-7 and HepG2 cancer cell lines and their docking analysis provided a preliminary indication that they are viable [HDAC6] candidates.

Role of Histone Deacetylase 6 and Histone Deacetylase 6 Inhibition in Colorectal Cancer

Histone deacetylase 6 (HDAC6), by deacetylation of multiple substrates and association with interacting proteins, regulates many physiological processes that are involved in cancer development and invasiveness such as cell proliferation, apoptosis, motility, epithelial to mesenchymal transition, and angiogenesis. Due to its ability to remove misfolded proteins, induce autophagy, and regulate unfolded protein response, HDAC6 plays a protective role in responses to stress and enables tumor cell survival. The scope of this review is to discuss the roles of HDCA6 and its implications for the therapy of colorectal cancer (CRC). As HDAC6 is overexpressed in CRC, correlates with poor disease prognosis, and is not essential for normal mammalian development, it represents a good therapeutic target. Selective inhibition of HDAC6 impairs growth and progression without inducing major adverse events in experimental animals. In CRC, HDAC6 inhibitors have shown the potential to reduce tumor progression and enhance the therapeutic effect of other drugs. As HDAC6 is involved in the regulation of immune responses, HDAC6 inhibitors have shown the potential to improve antitumor immunity by increasing the immunogenicity of tumor cells, augmenting immune cell activity, and alleviating immunosuppression in the tumor microenvironment. Therefore, HDAC6 inhibitors may represent promising candidates to improve the effect of and overcome resistance to immunotherapy.

Nafamostat mesilate prevented caerulein-induced pancreatic injury by targeting HDAC6-mediated NLRP3 inflammasome activation

OBJECTIVE

Nafamostat mesilate (NM), a synthetic broad-spectrum serine protease inhibitor, has been commonly used for treating acute pancreatitis (AP) and other inflammatory-associated diseases in some East Asia countries. Although the potent inhibitory activity against inflammation-related proteases (such as thrombin, trypsin, kallikrein, plasmin, coagulation factors, and complement factors) is generally believed to be responsible for the anti-inflammatory effects of NM, the precise target and molecular mechanism underlying its anti-inflammatory activity in AP treatment remain largely unknown.

METHODS

The protection of NM against pancreatic injury and inhibitory effect on the NOD-like receptor protein 3 (NLRP3) inflammasome activation were investigated in an experimental mouse model of AP. To decipher the molecular mechanism of NM, the effects of NM on nuclear factor kappa B (NF-κB) activity and NF-κB mediated NLRP3 inflammasome priming were examined in lipopolysaccharide (LPS)-primed THP-1 cells. Additionally, the potential of NM to block the activity of histone deacetylase 6 (HDAC6) and disrupt the association between HDAC6 and NLRP3 was also evaluated.

RESULTS

NM significantly suppressed NLRP3 inflammasome activation in the pancreas, leading to a reduction in pancreatic inflammation and prevention of pancreatic injury during AP. NM was found to interact with HDAC6 and effectively inhibit its function. This property allowed NM to influence HDAC6-dependent NF-κB transcriptional activity, thereby blocking NF-κB-driven transcriptional priming of the NLRP3 inflammasome. Furthermore, NM exhibited the potential to interfere the association between HDAC6 and NLRP3, impeding HDAC6-mediated intracellular transport of NLRP3 and ultimately preventing NLRP3 inflammasome activation.

CONCLUSIONS

Our current work has provided valuable insight into the molecular mechanism underlying the immunomodulatory effect of NM in the treatment of AP, highlighting its promising application in the prevention of NLRP3 inflammasome-associated inflammatory pathological damage.

Histone deacetylase 6's function in viral infection, innate immunity, and disease: latest advances

In the family of histone-deacetylases, histone deacetylase 6 (HDAC6) stands out. The cytoplasmic class IIb histone deacetylase (HDAC) family is essential for many cellular functions. It plays a crucial and debatable regulatory role in innate antiviral immunity. This review summarises the current state of our understanding of HDAC6's structure and function in light of the three mechanisms by which it controls DNA and RNA virus infection: cytoskeleton regulation, host innate immune response, and autophagy degradation of host or viral proteins. In addition, we summed up how HDAC6 inhibitors are used to treat a wide range of diseases, and how its upstream signaling plays a role in the antiviral mechanism. Together, the findings of this review highlight HDAC6's importance as a new therapeutic target in antiviral immunity, innate immune response, and some diseases, all of which offer promising new avenues for the development of drugs targeting the immune response.

Transcriptome analysis preliminary reveals the immune response mechanism of golden cuttlefish (Sepia esculenta) larvae exposed to Cd

As a well-known marine metal element, Cd can significantly affect bivalve mollusk life processes such as growth and development. However, the effects of Cd on the molecular mechanisms of the economically important cephalopod species Sepia esculenta remain unclear. In this study, S. esculenta larval immunity exposed to Cd is explored based on RNA-Seq. The analyses of GO, KEGG, and protein-protein interaction (PPI) network of 1,471 differentially expressed genes (DEGs) reveal that multiple immune processes are affected by exposure such as inflammatory reaction and cell adhesion. Comprehensive analyses of KEGG signaling pathways and the PPI network are first used to explore Cd-exposed S. esculenta larval immunity, revealing the presence of 16 immune-related key and hub genes involved in exposure response. Results of gene and pathway functional analyses increase our understanding of Cd-exposed S. esculenta larval immunity and improve our overall understanding of mollusk immune functions.

Acetylation licenses Th1 cell polarization to constrain Listeria monocytogenes infection

T helper 1 (Th1) immunity is typically viewed as a critical adaptation by vertebrates against intracellular pathogens. Identifying novel targets to enhance Th1 cell differentiation and function is increasingly important for anti-infection immunity. Here, through small-molecule screening focusing on epigenetic modifiers during the in vitro Th1 cell differentiation process, we identified that the selective histone deacetylase 6 (HDAC6) inhibitors ricolinostat and nexturastat A (Nex A) promoted Th1 cell differentiation. HDAC6-depleted mice exhibit elevation of Th1 cell differentiation, and decreased severity of Listeria monocytogenes infection. Mechanistically, HDAC6 directly deacetylated CBP-catalyzed acetylation of signal transducer and activator of transcription 4 (STAT4)-lysine (K) 667 via its enzymatic activity. Acetylation of STAT4-K667 is required for JAK2-mediated phosphorylation and activation of STAT4. Stat4K667R mutant mice lost the ability to normally differentiate into Th1 cells and developed severe Listeria infection. Our study identifies acetylation of STAT4-K667 as an essential signaling event for Th1 cell differentiation and defense against intracellular pathogen infections, and highlights the therapeutic potential of HDAC6 inhibitors for controlling intracellular pathogen infections.

Improving cancer immunotherapy by rationally combining oncolytic virus with modulators targeting key signaling pathways

Oncolytic viruses (OVs) represent a new class of multi-modal immunotherapies for cancer, with OV-elicited antitumor immunity being key to their overall therapeutic efficacy. Currently, the clinical effectiveness of OV as monotherapy remains limited, and thus investigators have been exploring various combinations with other anti-cancer agents and demonstrated improved therapeutic efficacy. As cancer cells have evolved to alter key signaling pathways for enhanced cell proliferation, cancer progression and metastasis, these cellular and molecular changes offer promising targets for rational cancer therapy design. In this regard, key molecules in relevant signaling pathways for cancer cells or/and immune cells, such as EGFR-KRAS (e.g., KRAS^G12C), PI3K-AKT-mTOR, ERK-MEK, JAK-STAT, p53, PD-1-PD-L1, and epigenetic, or immune pathways (e.g., histone deacetylases, cGAS-STING) are currently under investigation and have the potential to synergize with OV to modulate the immune milieu of the tumor microenvironment (TME), thereby improving and sustaining antitumor immunity. As many small molecule modulators of these signaling pathways have been developed and have shown strong therapeutic potential, here we review key findings related to both OV-mediated immunotherapy and the utility of small molecule modulators of signaling pathways in immuno-oncology. Then, we focus on discussion of the rationales and potential strategies for combining OV with selected modulators targeting key cellular signaling pathways in cancer or/and immune cells to modulate the TME and enhance antitumor immunity and therapeutic efficacy. Finally, we provide perspectives and viewpoints on the application of novel experimental systems and technologies that can propel this exciting branch of medicine into a bright future.

Extracellular vesicles from Listeria monocytogenes-infected dendritic cells alert the innate immune response

Communication through cell-cell contacts and extracellular vesicles (EVs) enables immune cells to coordinate their responses against diverse types of pathogens. The function exerted by EVs in this context depends on the proteins and nucleic acids loaded into EVs, which elicit specific responses involved in the resolution of infection. Several mechanisms control protein and nucleic acid loading into EVs; in this regard, acetylation has been described as a mechanism of cellular retention during protein sorting to exosomes. HDAC6 is a deacetylase involved in the control of cytoskeleton trafficking, organelle polarity and cell migration, defense against Listeria monocytogenes (Lm) infection and other immune related functions. Here, we show that the protein content of dendritic cells (DCs) and their secreted EVs (DEVs) vary during Lm infection, is enriched in proteins related to antiviral functions compared to non-infected cells and depends on HDAC6 expression. Analyses of the post-translational modifications revealed an alteration of the acetylation and ubiquitination profiles upon Lm infection both in DC lysates and DEVs. Functionally, EVs derived from infected DCs upregulate anti-pathogenic genes (e.g. inflammatory cytokines) in recipient immature DCs, which translated into protection from subsequent infection with vaccinia virus. Interestingly, absence of Listeriolysin O in Lm prevents DEVs from inducing this anti-viral state. In summary, these data underscore a new mechanism of communication between bacteria-infected DC during infection as they alert neighboring, uninfected DCs to promote antiviral responses.

Design, synthesis, and biological evalution of bifunctional inhibitors against Hsp90-HDAC6 interplay

HDAC6 and Hsp90, existing as a cytosolic complex play an important role in maintaining the protein homeostasis. The interplay of HDAC6 and Hsp90 has attracted wide attention due to their important role and promise as therapeutic targets in malignant cancers. Therefore, the discovery of dual inhibitors targeting HDAC6 and Hsp90 is of high importance. In the present study, we describe the design, synthesis, and biological evaluation of bifunctional inhibitors against HDAC6 and Hsp90 interplay. In particular, compound 6e shows a significant inhibitory activity against both HDAC6 and Hsp90 with IC₅₀ values of 106 nM and 61 nM, respectively. Compound 6e promotes the acetylation of HDAC6 substrate proteins such as α-tubulin and Hsp90 via HDAC6 inhibition, and also induces the degradation of Hsp90 clients such as Her2, EGFR, Met, Akt, and HDAC6 via Hsp90 inhibition. Compound 6e consequently furnishes potent antiproliferative effect on gefitinib-resistant H1975 non-small cell lung cancer (NSCLC) with a GI₅₀ value of 1.7 μM. In addition, compound 6e successfully achieved significant tumor growth inhibition in H1975 NSCLC xenograft model without noticeable abnormal behavior, body weight changes, and apparent ocular toxicity. We conclude that compound 6e constitutes an excellent tool as well as a valuable lead for assessment of Hsp90 and HDAC6 dual inhibition with a single molecule.

Sp1 S-Sulfhydration Induced by Hydrogen Sulfide Inhibits Inflammation via HDAC6/MyD88/NF-κB Signaling Pathway in Adjuvant-Induced Arthritis

Histone deacetylase 6 (HDAC6) acts as a regulator of the nuclear factor kappa-B (NF-κB) signaling pathway by deacetylating the non-histone protein myeloid differentiation primary response 88 (MyD88) at lysine residues, which is an adapter protein for the Toll-like receptor (TLR) and interleukin (IL)-1β receptor. Over-activated immune responses, induced by infiltrated immune cells, excessively trigger the NF-κB signaling pathway in other effector cells and contribute to the development of rheumatoid arthritis (RA). It has also been reported that HDAC6 can promote the activation of the NF-κB signaling pathway. In the present study, we showed that HDAC6 protein level was increased in the synovium tissues of adjuvant-induced arthritis rats. In addition, hydrogen sulfide (H₂S) donor S-propargyl-cysteine (SPRC) can inhibit HDAC6 expression and alleviate inflammatory response in vivo. In vitro study revealed that HDAC6 overexpression activated the NF-κB signaling pathway by deacetylating MyD88. Meanwhile, sodium hydrosulfide (NaHS) or HDAC6 inhibitor tubastatin A (tubA) suppressed the pro-inflammatory function of HDAC6. Furthermore, the reduced expression of HDAC6 appeared to result from transcriptional inhibition by S-sulfhydrating specificity protein 1 (Sp1), which is a transcription factor of HDAC6. Our results demonstrate that Sp1 can regulate HDAC6 expression, and S-sulfhydration of Sp1 by antioxidant molecular H₂S ameliorates RA progression via the HDAC6/MyD88/NF-κB signaling pathway.

Structure-based prediction of HDAC6 substrates validated by enzymatic assay reveals determinants of promiscuity and detects new potential substrates

Histone deacetylases play important biological roles well beyond the deacetylation of histone tails. In particular, HDAC6 is involved in multiple cellular processes such as apoptosis, cytoskeleton reorganization, and protein folding, affecting substrates such as ɑ-tubulin, Hsp90 and cortactin proteins. We have applied a biochemical enzymatic assay to measure the activity of HDAC6 on a set of candidate unlabeled peptides. These served for the calibration of a structure-based substrate prediction protocol, Rosetta FlexPepBind, previously used for the successful substrate prediction of HDAC8 and other enzymes. A proteome-wide screen of reported acetylation sites using our calibrated protocol together with the enzymatic assay provide new peptide substrates and avenues to novel potential functional regulatory roles of this promiscuous, multi-faceted enzyme. In particular, we propose novel regulatory roles of HDAC6 in tumorigenesis and cancer cell survival via the regulation of EGFR/Akt pathway activation. The calibration process and comparison of the results between HDAC6 and HDAC8 highlight structural differences that explain the established promiscuity of HDAC6.

HDAC6 inhibitor ACY-1215 improves neuropathic pain and its comorbidities in rats of peripheral nerve injury by regulating neuroinflammation

The fact that neuropathic pain (NP) has no effective therapy and is frequently accompanied by psychiatric comorbidities is well established. Aberrant neuroinflammation plays an important role in the development and maintenance of NP. HDAC6 inhibitors have been demonstrated to ameliorate mechanical allodynia brought on by chemotherapy and peripheral nerve damage. However, its pharmacological mechanisms and its effects on NP-related mental disorders have not been fully elucidated. The present study was dedicated to exploring the effects of ACY-1215 (a specific HDAC6 inhibitor) on neuroinflammation and behavioral abnormalities associated with NP. In this work, spinal nerve ligation (SNL) was performed as an NP model on rats. Mechanical allodynia, cognitive impairment, and depressive-like behavior caused by SNL were attenuated by continuous intraperitoneal injection of ACY-1215. Moreover, ACY-1215 administration suppressed SNL-induced neuroinflammatory responses (including microgliosis, the elevation of pro-inflammatory factors IL-1β and TNF-α) in ligation of the ipsilateral spinal dorsal horn (iSDH), hippocampus (HPC) and prefrontal cortex (PFC). Mechanistically, MyD88-dependent pro-inflammatory pathways (MyD88/NF-κB and MyD88/ERK) were activated in the iSDH following SNL and were inhibited by ACY-1215. Moreover, ACY-1215 enhanced the acetylation modification of MyD88 and inhibited the SNL-induced elevation of MyD88 without affecting its transcription in the iSDH. These findings suggest that pharmacological inhibition of HDAC6 can ameliorate NP and its psychiatric complications through modulating neuroinflammation, in part by blocking the MyD88-mediated pro-inflammatory pathways. The possible mechanism is that ACY-1215 prevents the elevation of MyD88 reactivity by increasing its acetylation level. Notably, neither SNL nor ACY-1215 significantly altered MyD88 expression in HPC and PFC, indicating differentiated pro-inflammatory mechanisms in the supraspinal neural regions.

The Role of HDAC6 in Autophagy and NLRP3 Inflammasome

Autophagy fights against harmful stimuli and degrades cytosolic macromolecules, organelles, and intracellular pathogens. Autophagy dysfunction is associated with many diseases, including infectious and inflammatory diseases. Recent studies have identified the critical role of the NACHT, LRR, and PYD domain-containing protein 3 (NLRP3) inflammasomes activation in the innate immune system, which mediates the secretion of proinflammatory cytokines IL-1β/IL-18 and cleaves Gasdermin D to induce pyroptosis in response to pathogenic and sterile stimuli. Accumulating evidence has highlighted the crosstalk between autophagy and NLRP3 inflammasome in multifaceted ways to influence host defense and inflammation. However, the underlying mechanisms require further clarification. Histone deacetylase 6 (HDAC6) is a class IIb deacetylase among the 18 mammalian HDACs, which mainly localizes in the cytoplasm. It is involved in two functional deacetylase domains and a ubiquitin-binding zinc finger domain (ZnF-BUZ). Due to its unique structure, HDAC6 regulates various physiological processes, including autophagy and NLRP3 inflammasome, and may play a role in the crosstalk between them. In this review, we provide insight into the mechanisms by which HDAC6 regulates autophagy and NLRP3 inflammasome and we explored the possibility and challenges of HDAC6 in the crosstalk between autophagy and NLRP3 inflammasome. Finally, we discuss HDAC6 inhibitors as a potential therapeutic approach targeting either autophagy or NLRP3 inflammasome as an anti-inflammatory strategy, although further clarification is required regarding their crosstalk.

HDAC6 contributes to human resistance against Mycobacterium tuberculosis infection via mediating innate immune responses

Tuberculosis (TB), which is caused by Mycobacterium tuberculosis (Mtb), remains a major cause of morbidity and mortality worldwide. Increasing lines of evidence indicate that certain individuals, which are termed resisters, are naturally resistant to TB infection. The resister phenotype has been linked to host efficient innate immune responses, but the underlying mechanisms and the key immune factors remain unclear. Here, we find that upon Mtb infection, monocyte-derived macrophages (MDMs) from TB resisters exhibited distinctly higher production of TNF-α, IL-1β and IL-6, higher ratio of bacteria in acidic vacuoles, and lower intracellular bacterial loads, as compared to that from the healthy controls, individuals with latent TB infection, and TB patients. Such enhanced anti-Mtb immune capacity of macrophages from resisters largely depends on histone deacetylase 6 (HDAC6), whose expression is specifically maintained in MDMs from TB resisters during Mtb infection. Furthermore, we demonstrate that HDAC6 is required for acidification of Mtb-containing phagosomes in macrophages, thus controlling the intracellular survival of Mtb. Taken together, these findings unravel an indispensable role of HDAC6 in human innate resistance against Mtb infection, suggesting that HDAC6 may serve as a marker for individual TB risk as well as a novel host-directed anti-TB therapeutic target.

HDAC6 and CXCL13 Mediate Atopic Dermatitis by Regulating Cellular Interactions and Expression Levels of miR-9 and SIRT1

Histone deacetylase 6 (HDAC6) has been known to regulate inflammatory diseases. The role of HDAC6 in allergic skin inflammation has not been studied. We studied the role of HDAC6 in atopic dermatitis (AD) and the mechanisms associated with it. The decreased expression or chemical inhibition of HDAC6 suppressed AD by decreasing autophagic flux and cellular features of AD. AD increased expression levels of the Th1 and Th2 cytokines, but decreased expression levels of forkhead box P3 (FoxP3) and interleukin-10 (IL-10) in an HDAC6-dependent manner. CXC chemokine ligand 13 (CXCL13), which was increased in an HDAC6-depenednt manner, mediated AD. MiR-9, negatively regulated by HDAC6, suppressed AD by directly regulating the expression of sirtuin 1 (SIRT1). The downregulation or inhibition of SIRT1 suppressed AD. Experiments employing culture medium and transwell suggested that cellular interactions involving mast cells, keratinocytes, and dermal fibroblast cells could promote AD; HDAC6 and CXCL13 were found to be necessary for these cellular interactions. Mouse recombinant CXCL13 protein increased HDAC6 expression in skin mast cells and dermal fibroblast cells. CXCL13 protein was found to be present in the exosomes of DNCB-treated skin mast cells. Exosomes of DNCB-treated skin mast cells enhanced invasion potentials of keratinocytes and dermal fibroblast cells and increased expression levels of HDAC6, SIRT1 and CXCL13 in keratinocytes and dermal fibroblast cells. These results indicate that HDAC6 and CXCL13 may serve as targets for the developing anti-atopic drugs.

HDAC6 Substrate Discovery Using Proteomics-Based Substrate Trapping: HDAC6 Deacetylates PRMT5 to Influence Methyltransferase Activity

Histone deacetylase 6 (HDAC6) is upregulated in a variety of tumor cell lines and has been linked to many cellular processes, such as cell signaling, protein degradation, cell survival, and cell motility. HDAC6 is an enzyme that deacetylates the acetyllysine residues of protein substrates, and the discovery of HDAC6 substrates, including tubulin, has revealed many roles of HDAC6 in cell biology. Unfortunately, among the wide variety of acetylated proteins in the cell, only a few are verified as HDAC6 substrates, which limits the full characterization of HDAC6 cellular functions. Substrate trapping mutants were recently established as a tool to discover unanticipated substrates of histone deacetylase 1 (HDAC1). In this study, we applied the trapping approach to identify potential HDAC6 substrates. Among the high confidence protein hits after trapping, protein arginine methyl transferase 5 (PRMT5) was successfully validated as a novel HDAC6 substrate. PRMT5 acetylation enhanced its methyltransferase activity and symmetrical dimethylation of downstream substrates, revealing possible crosstalk between acetylation and methylation. Substrate trapping represents a powerful, systematic, and unbiased approach to discover substrates of HDAC6.

LincRNA-Cox2 regulates IL6/JAK3/STAT3 and NF-κB P65 pathway activation in Listeria monocytogenes-infected RAW264.7 cells

Listeria monocytogenes (Lm) can lead to high mortality rates relative to other foodborne pathogens. Lm-induced inflammation is partly characterized by macrophage activation. Long non-coding RNAs (lncRNAs) have important roles in various biological processes. However, it is unknown how lncRNAs regulate the host response to Lm infection. To identify the role of lncRNA in Lm infection, we used in vitro and in vivo models. We found that lincRNA-Cox2 was highly expressed in Lm-infected RAW264.7 cells. LincRNA-Cox2 knockdown resulted in reduced proinflammatory cytokines, apoptosis, migration ability and enhanced phagocytosis of Lm. LincRNA-Cox2 knockdown also reduced the phosphorylation of Janus kinase 3 (JAK3) and signal transducer and activator of transcription (STAT3) and the nuclear translocation of nuclear factor (NF)-κB P65, which are known to be involved in inflammatory responses. Experimentally inhibiting the protein and phosphorylation levels of STAT3 resulted in reduced proinflammatory cytokines and enhanced phagocytosis of Lm by the RAW264.7 cells. Our research suggests that lincRNA-Cox2 plays important roles in inflammation, the phagocytic function and cell migration ability of RAW264.7 cells by activating interleukin (IL)-6/JAK3/STAT3 signaling, and lincRNA-Cox2 also regulates NF-κB P65 nuclear translocation. Our research provides new insights into the regulatory role of lincRNA-Cox2 in Lm infection.

Oxa Analogues of Nexturastat A Demonstrate Improved HDAC6 Selectivity and Superior Antileukaemia Activity

The acetylome is important for maintaining the homeostasis of cells. Abnormal changes can result in the pathogenesis of immunological or neurological diseases, and degeneration can promote the manifestation of cancer. In particular, pharmacological intervention in the acetylome with pan-histone deacetylase (HDAC) inhibitors is clinically validated. However, these drugs exhibit an undesirable risk-benefit profile due to severe side effects. Selective HDAC inhibitors might promote patient compliance and represent a valuable opportunity in personalised medicine. Therefore, we envisioned the development of HDAC6-selective inhibitors. During our lead structure identification, we demonstrated that an alkoxyurea-based connecting unit proves to be beneficial for HDAC6 selectivity and established the synthesis of alkoxyurea-based hydroxamic acids. Herein, we report highly potent N-alkoxyurea-based hydroxamic acids with improved HDAC6 preference compared to nexturastat A. We further validated the biological activity of these oxa analogues of nexturastat A in a broad subset of leukaemia cell lines and demonstrated their superior anti-proliferative properties compared to nexturastat A.

The isothiocyanate sulforaphane inhibits mTOR in an NRF2-independent manner

BACKGROUND

The isothiocyanate sulforaphane (SFN) has multiple protein targets in mammalian cells, affecting processes of fundamental importance for the maintenance of cellular homeostasis, among which are those regulated by the stress response transcription factor nuclear factor erythroid 2 p45-related factor 2 (NRF2) and the serine/threonine protein kinase mechanistic target of rapamycin (mTOR). Whereas the way by which SFN activates NRF2 is well established, the molecular mechanism(s) of how SFN inhibits mTOR is not understood.

HYPOTHESIS/PURPOSE

The aim of this study was to investigate the mechanism(s) by which SFN inhibits mTOR STUDY DESIGN AND METHODS: We used the human osteosarcoma cell line U2OS and its CRISPR/Cas9-generated NRF2-knockout counterpart to test the requirement for NRF2 and the involvement of mTOR regulators in the SFN-mediated inhibition of mTOR.

RESULTS

SFN inhibits mTOR in a concentration- and time-dependent manner, and this inhibition occurs in the presence or in the absence of NRF2. The phosphatidylinositol 3-kinase (PI3K)-AKT/protein kinase B (PKB) is a positive regulator of mTOR, and treatment with SFN caused an increase in the phosphorylation of AKT at T308 and S473, two phosphorylation sites associated with AKT activation. Interestingly however, the levels of pS552 β-catenin, an AKT phosphorylation site, were decreased, suggesting that the catalytic activity of AKT was inhibited. In addition, SFN inhibited the activity of the cytoplasmic histone deacetylase 6 (HDAC6), the inhibition of which has been reported to promote the acetylation and decreases the kinase activity of AKT.

CONCLUSION

SFN inhibits HDAC6 and decreases the catalytic activity of AKT, and this partially explains the mechanism by which SFN inhibits mTOR.

Role of Epigenetics in the Regulation of Immune Functions of the Skin

This review is intended to illuminate the emerging understanding of epigenetic modifications that regulate both adaptive and innate immunity in the skin. Host defense of the epidermis and dermis involves the interplay of many cell types to enable homeostasis; tolerance to the external environment; and appropriate response to transient microbial, chemical, and physical insults. To understand this process, the study of cutaneous immunology has focused on immune responses that reflect both adaptive learned and genetically programmed innate defense systems. However, recent advances have begun to reveal that epigenetic modifications of chromatin structure also have a major influence on the skin immune system. This deeper understanding of how enzymatic changes in chromatin structure can modify the skin immune system and may explain how environmental exposures during life, and the microbiome, lead to both short-term and long-term changes in cutaneous allergic and other inflammatory processes. Understanding the mechanisms responsible for alterations in gene and chromatin structure within skin immunocytes could provide key insights into the pathogenesis of inflammatory skin diseases that have thus far evaded understanding by dermatologists.

HDAC6 Mediates Poly (I:C)-Induced TBK1 and Akt Phosphorylation in Macrophages

Macrophages are derived from monocytes in the bone marrow and play an important role in anti-viral innate immune responses. Macrophages produce cytokines such as interferons and IL-10 upon viral infection to modulate anti-viral immune responses. Type I interferons (IFNs) promote anti-viral defense. IL-10 is a suppressor cytokine that down-regulates anti-viral immune responses. HDAC6 is a tubulin deacetylase that can modulate microtubule dynamics and microtubule-mediated cell signaling pathways. In the present study, we investigated the potential role of HDAC6 in macrophage anti-viral responses by examining poly (I:C)-induced IFN-β and IL-10 production in mouse bone marrow-derived macrophages (BMDMs). We also investigated the role of HDAC6 in poly (I:C)-induced anti-viral signaling such as TBK1, GSK-3β, and Akt activation in mouse BMDMs. Our data showed that HDAC6 deletion enhanced poly (I:C)-induced INF-β expression in macrophages by up-regulating TBK1 activity and eliminating the inhibitory regulation of GSK-3β. Furthermore, HDAC6 deletion inhibited poly (I:C)-induced suppressor cytokine IL-10 production in the BMDMs, which was associated with the inhibition of Akt activation. Our results suggest that HDAC6 modulates IFN-β and IL-10 production in macrophages through its regulation of TBK1, GSK-3β, and Akt signaling. HDAC6 could act as a suppressor of anti-viral innate immune responses in macrophages.

HDAC6 inhibitors: Translating genetic and molecular insights into a therapy for axonal CMT

Histone deacetylase 6 (HDAC6) plays a central role in various processes that are key for neuronal survival. In this review, we summarize the current evidence related to disease pathways in the axonal form of Charcot-Marie-Tooth disease (CMT) and highlight the role of HDAC6 in these pathways. We hypothesize that HDAC6 might in fact actively contribute to the pathogenesis of certain forms of axonal CMT. HDAC6 plays a deacetylase activity-dependent, negative role in axonal transport and axonal regeneration, which are both processes implicated in axonal CMT. On the other hand, HDAC6 coordinates a protective response during elimination of toxic misfolded proteins, but this is mostly mediated independent of its deacetylase activity. The current mechanistic insights on these functions of HDAC6 in axonal CMT, along with the selective druggability against its deacetylase activity, make the targeting of HDAC6 particularly attractive. We elaborate on the preclinical studies that demonstrated beneficial effects of HDAC6 inhibitors in axonal CMT models and outline possible modes of action. Overall, this overview ultimately provides a rationale for the use of small-molecule HDAC6 inhibitors as a therapeutic strategy for this devastating disease.

G protein-coupled receptor kinase 2 (GRK2) as a multifunctional signaling hub

Accumulating evidence indicates that G protein-coupled receptor kinase 2 (GRK2) is a versatile protein that acts as a signaling hub by modulating G protein-coupled receptor (GPCR) signaling and also via phosphorylation or scaffolding interactions with an extensive number of non-GPCR cellular partners. GRK2 multifunctionality arises from its multidomain structure and from complex mechanisms of regulation of its expression levels, activity, and localization within the cell, what allows the precise spatio-temporal shaping of GRK2 targets. A better understanding of the GRK2 interactome and its modulation mechanisms is helping to identify the GRK2-interacting proteins and its substrates involved in the participation of this kinase in different cellular processes and pathophysiological contexts.

Autophagy in intracellular bacterial infection

Autophagy is a conserved intracellular degradation process enclosing the bulk of cytosolic components for lysosomal degradation to maintain cellular homeostasis. Accumulating evidences showed that a specialized form of autophagy, known as xenophagy, could serve as an innate immune response to defend against pathogens invading inside the host cells. Correspondingly, infectious pathogens have developed a variety of strategies to disarm xenophagy, leading to a prolonged and persistent intracellular colonization. In this review, we first summarize the current knowledge about the general mechanisms of intracellular bacterial infections and xenophagy. We then focus on the ongoing battle between these two processes.

Histone deacetylase inhibitors impair the host immune response against Mycobacterium tuberculosis infection

Histone deacetylase inhibitors (HDACi), a novel class of anti-cancer drug, have been recently reported to suppress host immunity and increase susceptibility to infection. Tuberculosis, a leading infectious disease killer caused by Mycobacterium tuberculosis (M.tb), is basically the product of the interaction between bacterial virulence and host resistance. However, the effects of HDACi in host immunity against M.tb is largely unknown. In this study, we found that HDACi including Trichostatin A (TSA) and suberoylanilide hydroxamic acid (SAHA) significantly impaired phagocytosis and killing activity of macrophage. In line with these findings, we noted that M.tb induced reactive oxygen species (ROS) production and autophagy are significantly suppressed by TSA. Transcriptome analysis revealed that the suppression of autophagy by TSA might due to its inhibiting autophagy-regulating genes such as CACNA2D3, which regulates intracellular Ca²⁺ levels. Finally, we confirmed that HDACi including TSA and SAHA significantly exacerbated the histopathological damage and M.tb load in the lung of M.tb infected mice. Taken together, our results indicated that HDACi at least TSA and SAHA significantly impaired macrophage immunity against M.tb and therefore increase susceptibility to TB, our findings raised the concern that the potential side effects of HDACi on latent TB reactivation should be considered in clinic.

HDAC6 depletion improves cystic fibrosis mouse airway responses to bacterial challenge

The hypothesis of this study was that Hdac6 depletion would restore cystic fibrosis (CF) responses to bacterial challenge to more wild type profiles using a CF mouse model. CF mice harboring the F508del Cftr mutation respond to bacterial challenge with 25,000 CFU Pseudomonas aeruginosa embedded into agarose beads to slow clearance. CF mice respond significantly more aggressively to this challenge compared to WT mice with respect to bacterial clearance, weight loss, neutrophil recruitment, and MIP-2 production. Depletion of Hdac6 expression in the CF mice (CF/Hdac6) significantly improves these responses to more WT levels. Weight loss in response to infection is most severe in CF mice and significantly attenuated in CF/Hdac6 mice. Bacterial levels are reduced at a faster rate in CF/Hdac6 mice compared to CF mice where infection persists. Percent neutrophils in lung lavage fluid post-infection are significantly higher in CF mice, but returned to WT levels with CF/Hdac6 mice. Similarly, CF Mip-2 levels are restored to WT levels in the absence of Hdac6 expression. These data demonstrate that Hdac6 depletion restores CF responses to bacterial challenge to WT-like profiles and offer a potential therapeutic avenue for addressing inflammation and infection in CF airways independently of Cftr correction.

Supramolecular Complexes in Cell Death and Inflammation and Their Regulation by Autophagy

Signaling activation is a tightly regulated process involving myriad posttranslational modifications such as phosphorylation/dephosphorylation, ubiquitylation/deubiquitylation, proteolytical cleavage events as well as translocation of proteins to new compartments within the cell. In addition to each of these events potentially regulating individual proteins, the assembly of very large supramolecular complexes has emerged as a common theme in signal transduction and is now known to regulate many signaling events. This is particularly evident in pathways regulating both inflammation and cell death/survival. Regulation of the assembly and silencing of these complexes plays important roles in immune signaling and inflammation and the fate of cells to either die or survive. Here we will give a summary of some of the better studied supramolecular complexes involved in inflammation and cell death, particularly with a focus on diseases caused by their autoactivation and the role autophagy either plays or may be playing in their regulation.

Targeted inhibition of histone deacetylase 6 in inflammatory diseases

Targeting epigenetic modification of gene expression represents a promising new approach under investigation for the treatment of inflammatory diseases. Accumulating evidence suggests that epigenetic mechanisms, such as histone modification, play a crucial role in a number of inflammatory diseases, including rheumatoid arthritis, asthma, and contact hypersensitivity. Consistent with this role, histone deacetylase (HDAC) inhibitors have shown efficacy in the treatment of inflammatory diseases. In particular, selective inhibitors of HDAC6, a cytoplasmic member of the HDAC family that contains two deacetylase domains, are under investigation as a potential treatment strategy for inflammatory diseases due to their ability to regulate inflammatory cells and cytokines. Here, we review recent findings highlighting the critical roles of HDAC6 in a variety of inflammatory diseases, and discuss the therapeutic potential of HDAC6 inhibitors in these settings.

Epigenetic Regulation of Autophagy: A Path to the Control of Autoimmunity

Autoimmune diseases are a significant cause of debilitation and mortality globally and are in need of cost-effective therapeutics. Autophagy is a cellular pathway that facilitates immune modulation involved in both pathogen control and autoimmunity. Regulation is multifactorial and includes a number of epigenetic pathways which can involve modification of DNA-binding histones to induce autophagy-related mRNA synthesis or microRNA and decapping-associated mRNA degradation which results in autophagy suppression. Appreciation of epigenetic-based pathways involved in autophagy and autoimmunity may facilitate application of a burgeoning group of epigenetic pharmaceuticals to these important diseases.

2,4‑Di‑tert‑butylphenol, a potential HDAC6 inhibitor, induces senescence and mitotic catastrophe in human gastric adenocarcinoma AGS cells

The natural product 2,4‑di‑tert‑butylphenol (DTBP) has a wide spectrum of biological functions, including anticancer activities, although the underlying mechanisms are poorly understood. Here, we found that DTBP induces senescence in human gastric adenocarcinoma AGS cells as evidenced by upregulation of p21 and Rb and increased β‑galactosidase activity. DTBP also induces mitotic catastrophe and generates multinucleated cells, which is accompanied by an increase in the proportion of polymerized tubulin, possibly caused by inhibition of HDAC6 enzyme activity. In silico docking analysis showed that DTBP docked at the entrance of the ligand-binding pocket of the HDAC6 enzyme. Accordingly, DTBP represents a promising lead structure for the development of HDAC6 inhibitors, with an improvement in specificity conferred by modification of the cap group. We propose for the first time that the underlying mechanism of the anticancer activity of DTBP is attributed to inhibition of HDAC6 activity.

Implications of the HDAC6-ERK1 feed-forward loop in immunotherapy

The oncogene HDAC6 controls numerous cell processes that are related to tumorigenesis and metastasis, and has recently arisen as a target to treat malignancies. The ERK cascade is a classic pathway driving oncogenesis, and the components of this pathway are either highly mutated in cancers or are vital in cancer's pathological activity. The interactions between these important components of tumor proliferation have been examined, and our research has demonstrated that they regulate each other as evidenced by different posttranslational modifications. Preclinical evidence also supports clinical trials cotargeting these two pathways, which may provide better efficacy than single treatment. Furthermore, HDAC6 and ERK both participate in the regulation of T cell maturation and may have implications on the functions of immune cells. This leads to the possibility of connecting HDAC6 and ERK to immunotherapy. In this review, we summarize the published studies about the interaction of HDAC6 and ERK cascade and their relationship to cancers. We also include the association of HDAC6 and ERK to immune system and discuss the plausibility of linking these to immunotherapy.