HDAC6, composed of 1,215 amino acids, stands as the largest HDAC. Primarily located in the cytoplasm, this positioning is influenced by the SE14 motif (a tetra-decapeptide repeat domain containing Glu-Ser) and two conserved nuclear export signals (NES1 and NES2) in the N-terminal region (23). The C-terminal ZnF-UBP is essential for binding polyubiquitinated proteins, aiding their degradation through the aggresome-autophagy pathway. This domain enables HDAC6 to interact with ubiquitinated misfolded proteins, directing them to aggresomes for autophagic degradation, thus maintaining cellular protein quality control (24). The two catalytic domains of HDAC6, CD1 and CD2, although similar in structure, have distinct functional roles. CD1 has a broader active site but more limited substrate specificity, preferring peptide substrates with C-terminal acetyllysine residues. On the other hand, CD2 has a greater impact on the total catalytic function of HDAC6, particularly in the deacetylation of tubulin and tau proteins (25). This functional differentiation is crucial for the enzyme's involvement in microtubule dynamics and cell motility.

The structure of HDAC6 is composed of several domains that confer its multifunctional nature. The enzyme's N-terminal region includes a dynein motor-binding domain that assists in the transportation of misfolded proteins to aggresomes along microtubules by HDAC6 (26). This domain interacts with the dynein motor complex, which is essential for retrograde transport within the cell. The linker region connecting the catalytic domains CD1 and CD2 can impact the spatial orientation and availability of substrates. The C-terminal ZnF-UBP domain binds ubiquitinated proteins, linking HDAC6 to the ubiquitin-proteasome system (27).

HDAC6 plays a multifaceted role in cellular homeostasis, extending beyond its traditional function of histone deacetylation. Non-histone proteins, such as α-tubulin, HSP90 and cortactin, are the main targets of this enzyme, playing crucial roles in cell movement, protein breakdown and response to stress.

HDAC6 deacetylates α-tubulin, a key component of the microtubule network. This deacetylation reduces the stability of microtubules, promoting increased cell motility and invasiveness, characteristics often exploited by cancer cells (28). Inhibition of HDAC6 leads to hyperacetylation of α-tubulin, resulting in stabilized microtubules and decreased cell motility (11). This function plays a vital role in activities such as cell movement, axonal transport and intracellular substance movement. Additionally, HDAC6 interacts with cortactin, a protein that binds to actin and controls the movement of the actin cytoskeleton. Cortactin is involved in the formation of lamellipodia and invadopodia, structures essential for cell movement and invasion (29). HDAC6-mediated deacetylation of cortactin enhances its ability to bind to F-actin, thereby promoting actin polymerization and cell motility.

HDAC6 interacts with HSP90, a molecular chaperone essential for the stability and function of numerous client proteins involved in cell growth and survival (30). By deacetylating HSP90, HDAC6 enhances its chaperone activity, which is crucial for the proper folding of nascent proteins and the degradation of misfolded proteins (30). This interaction is particularly important in cells under stress, where the accumulation of misfolded proteins can lead to cellular dysfunction and disease (30). One of the key roles of HDAC6 is in the aggresome-autophagy pathway (31). When the ubiquitin-proteasome system is overwhelmed or impaired, misfolded and aggregated proteins are transported to aggresomes, which are perinuclear inclusion bodies (31). HDAC6 facilitates the transport of these ubiquitinated proteins along microtubules to aggresomes by binding to both the ubiquitinated cargo and the dynein motor complex. Once sequestered in aggresomes, these proteins can be degraded by autophagy, a lysosome-dependent degradation pathway (31). This process is essential for maintaining protein homeostasis and preventing the accumulation of toxic protein aggregates.

HDAC6 facilitates the formation of aggresomes, cellular structures that sequester misfolded proteins (32). By binding to ubiquitinated proteins through its ZnF-UBP domain, HDAC6 transports these proteins to the microtubule-organizing center, where they are sequestered into aggresomes and subsequently degraded by autophagy (27). This function is vital for cell survival under conditions of proteotoxic stress. The aggresome pathway is particularly important in neurons, which are highly susceptible to the accumulation of misfolded proteins due to their long lifespan and complex structure. Inhibition of HDAC6 has been shown to impair aggresome formation and autophagy, leading to increased sensitivity to proteotoxic stress and neuronal degeneration (33).

HDAC6 influences the cellular response to oxidative stress through its impact on mitochondrial activity. It has been shown to affect mitochondrial dynamics and biogenesis through interactions with proteins involved in mitochondrial fusion and fission (34). Blocking HDAC6 activity may result in elevated generation of reactive oxygen species (ROS) and impairment of mitochondria, underscoring its importance in maintaining cellular redox balance (35). Mitochondria play a crucial role in generating ROS within cells, with the equilibrium between mitochondrial fusion and fission being essential for preserving mitochondrial function and preventing oxidative damage (36). HDAC6 interacts with dynamin-related protein 1, a crucial regulator of mitochondrial division, enhancing its deacetylation and activation. This interaction facilitates the division of damaged mitochondria and their removal by mitophagy, a selective form of autophagy that targets dysfunctional mitochondria (37).

HDAC6 is involved in the cellular stress response by regulating the acetylation status of HSPs. HSPs act as molecular chaperones, aiding in the folding of proteins and shielding cells from damage caused by stress. HDAC6 deacetylates HSP90, enhancing its chaperone activity and promoting the stabilization and refolding of stress-damaged proteins (38). This function is particularly important under conditions of oxidative stress, where the accumulation of damaged proteins can lead to cell death. In addition to HSP90, HDAC6 also deacetylates other stress-responsive proteins, such as HSP70 and HSP27 (39). These proteins play crucial roles in protecting cells from heat shock, oxidative stress and other environmental insults. By regulating the acetylation status of these proteins, HDAC6 modulates their chaperone activity and enhances the cellular capacity to cope with stress.

HDAC6 is essential for controlling various signal transduction pathways, such as NF-κB and MAPK, which are crucial for functions like inflammation, cell viability and programmed cell death (40,41). By deacetylating key components of these pathways, HDAC6 modulates their activity and impacts cellular responses to external stimuli. In the NF-κB pathway, HDAC6 deacetylates and stabilizes the p65 subunit, facilitating its nuclear translocation and transcriptional activity (42). This enhances the activation of NF-κB target genes related to inflammation and immune system function. Studies have shown that inhibiting HDAC6 can reduce NF-κB activity, thereby diminishing inflammatory responses in various cell types. In the MAPK pathway, HDAC6 interacts with and deacetylates crucial signaling molecules like ERK1/2 and JNK (43,44). These interactions impact the initiation and subsequent signaling of the MAPK pathway, which in turn affect cellular functions like growth, differentiation and programmed cell death. HDAC6 inhibition has been observed to alter MAPK signaling, thereby modulating cellular responses to stress and growth factors.

In summary, HDAC6 is integral to processes such as microtubule dynamics, protein degradation, aggresome formation, oxidative stress response, cellular stress response and signal transduction pathways, highlighting its crucial role in cell biology. Understanding the structure and function of HDAC6 provides important insights into its roles in health and disease, emphasizing its potential as a therapeutic target for various conditions. Continued investigation into how HDAC6 regulates these functions, as well as the development of specific HDAC6 inhibitors, holds promise for innovative treatment approaches. These strategies aim to modulate HDAC6 activity to treat diseases characterized by dysregulated protein homeostasis and stress responses.

Cognitive impairments in individuals with obstructive sleep apnea (OSA) are associated with neuroinflammation and oxidative stress caused by intermittent hypoxia (45). New research has highlighted the involvement of TAR DNA-binding protein 43 (TDP-43), HDAC6 and peroxiredoxin 1 (Prdx1) in cognitive decline linked to various degenerative conditions. During this research, individuals with a diagnosis of OSA confirmed by polysomnography were evaluated for cognitive function using the Montreal Cognitive Assessment and blood samples were collected (46). TDP-43 and HDAC6 levels increased in HMC3 cells treated with lipopolysaccharide (LPS), whereas Prdx1 levels decreased. TDP-43 regulated Prdx1 expression by modulating HDAC6, with changes in inflammation and oxidative stress correlating with TDP-43 levels. Administering a specific HDAC6 inhibitor reduced inflammation and oxidative stress caused by LPS through the elevation of Prdx1 levels (46). The research findings suggest that TDP-43 plays a role in affecting Prdx1, leading to increased neuroinflammation and oxidative stress through the regulation of HDAC6 expression.

Parkinson's disease (PD) is a long-term degenerative condition marked by the death of dopamine-producing cells in the substantia nigra pars compacta and striatum, resulting in movement problems (47). Multiple pathogenic mechanisms contribute to PD, and currently, there is no cure. In the pathogenesis of PD, the inflammatory response is crucial, as postmortem examinations of PD patients' brains show increased levels of inflammatory cytokines, such as interleukin (IL)-1β and IL-18 (48). Interestingly, anti-inflammatory medications have demonstrated effectiveness in treating PD. Researchers investigated the impact of tubastatin A (TBA) on protein 3 containing NACHT, LRR and PYD structural domains (NLRP3) activation, SH-SY5Y cell damage and inflammatory response in a study utilizing a 6-hydroxydopamine (6-OHDA)-induced PD model. They also explored its effects on NLRP3 activation and dopaminergic damage in the nigrostriatal system of mice, assessing peroxiredoxin 2 (Prx2) acetylation levels and oxidative stress. The findings showed that TBA suppressed the activation of NLRP3 induced by 6-OHDA, leading to lower levels of NLRP3, mature caspase-1 and IL-1β, as well as reduced gliosis and degeneration of dopaminergic neurons (49). Significantly, TBA reinstated levels of Prx2 acetylation and decreased oxidative stress. Pharmacologically inhibiting HDAC6 using TBA appears to reduce NLRP3-induced inflammation and safeguard dopaminergic neurons, possibly by altering Prx2 acetylation (49). The research indicates that focusing on the deacetylase catalytic region of HDAC6 may offer a promising treatment approach for PD (Fig. 1).

The process of oxidation negatively impacts the ability to heal following a hemorrhage within the brain, and HDAC6 plays a crucial role in initiating this stress (50). Research involving HDAC6 knockout mice showed resistance to oxidative stress following intracerebral hemorrhage (ICH). Lack of HDAC6 resulted in decreased neuronal cell death and reduced levels of proteins associated with cell death. Studies on the mechanisms revealed that HDAC6 interacts with malate dehydrogenase 1 (MDH1) and removes acetyl groups from lysine residues 121 and 298. Exposure of HT22 cells to stressors related to ICH, such as hemoglobin and thrombin, resulted in the suppression of MDH1 acetylation, which was reversed upon HDAC6 inhibition, indicating a connection between HDAC6 and MDH1 (51). Acetylation-mimicking MDH1 mutants, unlike acetylation-resistant ones, protected neurons from oxidative damage. In addition, inhibiting HDAC6 did not decrease brain injury following ICH in the absence of MDH1 (51). Therefore, HDAC6 inhibition appears to mitigate brain damage during ICH by enhancing MDH1 acetylation (Fig. 1).

Most neurodegeneration data derive from acute toxin (e.g., 6-OHDA) or transgenic overexpression models that capture only single mechanisms of Parkinson's or Alzheimer's pathology (52,53). These paradigms fail to mimic the slow, multifactorial neuronal loss or the age- and sex-dependent penetrance observed in patients (52). HDAC6-knockout mice also show developmental compensation in microtubule dynamics, potentially masking late-life toxicities (54). Human data remain restricted to small post-mortem series; comorbidities and pharmacotherapy exposure confound HDAC6 expression estimates (55). Longitudinal biomarker-driven cohorts and patient-derived induced pluripotent stem cell models will be required to confirm disease stage-specific benefits or risks of HDAC6 inhibition (56,57).

Heart failure involves gene expression regulated by HDACs, and inhibiting these enzymes has shown promise in treating this condition (17). In neonatal rat ventricular myocytes, researchers discovered that pretreatment with seleno-suberoylanilide hydroxamic acid (Se-SAHA) reduced cardiac hypertrophy and fibrosis induced by isoproterenol (ISO). In vitro, Se-SAHA markedly decreased the generation of ROS induced by ISO and restored the levels of superoxide dismutase (SOD)2 and heme oxygenase (HO)-1 expression. Furthermore, Se-SAHA pretreatment inhibited autophagosome buildup and reversed the upregulation of HDAC1 and HDAC6 caused by ISO exposure (58). In a mouse model induced by ISO, Se-SAHA alleviated ventricular systolic dysfunction, hypertrophy and fibrosis, while also suppressing the excessive expression of HDAC1 and HDAC6. Se-SAHA significantly boosted SOD2 activity, enhancing its ability to scavenge free radicals. In addition, Se-SAHA suppressed the increased amounts of microtubule-associated protein 1 light chain 3-II and Beclin-1 in mice with heart failure (58). In conclusion, Se-SAHA exerts cardioprotective effects on ISO-induced heart failure by reducing oxidative stress and inhibiting autophagy (Fig. 2).

Heart failure is often linked to telomere shortening in cardiomyocytes, with arterial hypertension being a primary risk factor (59). Both conditions involve dysregulation of the neurohormonal axis (60). Telomere length in cardiomyocytes was measured in a mouse model of hypertensive atrial fibrillation caused by increased neurohormonal activity [through angiotensin II (AngII) infusion, a high-salt diet and no nephrectomy], as well as in AngII-stimulated cardiomyocytes and endomyocardial biopsy tissues from patients with hypertensive atrial fibrillation. The findings showed that telomere shortening occurred in both laboratory settings and living organisms, associated with left ventricular enlargement and decreased left ventricular function. Telomere shortening was linked to higher levels of superoxide production, increased expression of NADPH oxidase 2 (NOX2), elevated activity of HDAC6, decreased levels of the telomere-specific antioxidant PRDX1 and increased oxidative DNA damage. Inhibition of NOX2 prevented the reduction of PRDX1, DNA damage and telomere shortening, emphasizing NOX2 as a critical producer of ROS (61). Combining the NOX inhibitor apocynin with other treatments reduced the effects of hypertensive hyperlipidemia and telomere shortening. Similarly, the HDAC6 inhibitor TubA, which increases PRDX1 availability, halted telomere shortening in mature heart muscle cells. Thus, the reduction in fractional blood loss caused by neurohormonal hyperactivation in patients with heart failure leads to telomere shortening and oxidative damage in cardiomyocytes, which is reliant on superoxide derived from NOX2 (61). These findings suggest potential targeted treatments for heart failure involving the inhibition of NOX2 and HDAC6 to mitigate oxidative stress and telomere shortening (Fig. 2).

According to the heart failure-gut theory, damage to the intestinal mucosa causes a rise in microbial translocation, resulting in changes to circulating metabolites that contribute to heart failure (62). In this research, human AC16 cells were exposed to doxorubicin to create a heart failure model in a laboratory setting. Scientists studied the impact of indole-3-propionic acid (IPA) on cell survival, programmed cell death, inflammatory response and oxidative damage. The findings indicated that IPA significantly reduced cell death, inflammation and oxidative damage in the treated cells. Structural analysis revealed that IPA binds to HDAC6, leading to reduced HDAC6 levels. However, overexpression of HDAC6 nullified IPA's beneficial effects, indicating the involvement of HDAC6/NOX2 signaling in IPA's action (63). These findings highlight that IPA can mitigate oxidative stress, inflammation and apoptosis in cardiomyocytes by inhibiting HDAC6/NOX2 signaling, demonstrating the potential therapeutic benefits of gut microbiota metabolites like IPA in heart failure treatment (Fig. 2).

Uric acid (UA) accumulation can lead to endothelial dysfunction, oxidative stress and inflammation (64,65). HDACs play a vital role in controlling the progression of pathological conditions in various illnesses. In a study, human umbilical vein endothelial cells were exposed to TSA or had HDAC6 levels reduced, resulting in a reduction of vascular endothelial cell injury caused by UA and an increase in fibroblast growth factor (FGF)21 expression, as well as the activation of AKT, endothelial nitric oxide synthase (eNOS) and forkhead box (Fox)O3a. The protective effects were negated by FGF21 knockdown. In live animals, both TSA and TubA decreased inflammation and tissue damage while enhancing FGF21 production and the activation of AKT, eNOS and FoxO3a in the aortic and kidney tissues of mice with high levels of UA (66). These findings indicate that HDACs, particularly HDAC6 inhibitors, mitigate UA-induced endothelial dysfunction by upregulating FGF21, which in turn activates the PI3K/AKT pathway. Therefore, focusing on HDAC6 shows promise as a treatment approach for UA-induced endothelial dysfunction.

The ISO-induced mouse heart-failure model reproduces sympathetic over-drive but lacks the metabolic and haemodynamic complexity of human heart failure with preserved ejection fraction. Similarly, telomere-shortening studies rely on young mice with short lifespans and rodent-specific telomerase kinetics, limiting translational value. Larger, multi-ethnic cohorts with pharmacokinetic/pharmacodynamic (PK/PD) sampling and off-target cardiac safety monitoring are still lacking.

The social, health and economic burdens caused by retinal or macular degeneration-induced blindness are significant (67). New therapeutic targets and interventions are urgently needed for the more common atrophic ('dry') age-related macular degeneration (AMD), despite the availability of treatments for neovascular ('wet') AMD (68). Similarly, most inherited retinal diseases lack effective treatments. Although macular and retinal degeneration have genetic and clinical distinctions, they both exhibit similar pathological characteristics, including photoreceptor degeneration, retinal pigment epithelial atrophy, oxidative stress, hypoxia and autophagy deficiencies (69). A study discovered that zebrafish lacking atp6v0e1 and treated with tertiary statin A, an HDAC6 inhibitor, experienced notable enhancements in both photoreceptor outer segment size and visual acuity (70). Furthermore, retinal samples from rd10/rd10 mice that received tertiary statin A showed a significant increase in the quantity of cone photoreceptors located in the outer segments. In vitro studies indicated that ATP6V0E1 influenced hypoxia-inducible factor (HIF)-1α activity, although HDAC6 inhibitors did not significantly impact HIF-1α in the retina (70). Proteomic analysis revealed that the vision restoration mediated by HDAC6 inhibitors was associated with altered expression in ubiquitin-proteasome, phototransduction, metabolism and phagosomal pathways.

Researchers found increased HDAC6 levels and activity in the retinas of human diabetic postmortem donors, streptozotocin (STZ) rats and HuREC cells exposed to high glucose levels in a study using STZ rats as a model for type 1 diabetic retinopathy. Administering TubA, a specific HDAC6 inhibitor, successfully halted the rise in retinal microvascular hyperpermeability and inflammatory markers (71). Additionally, TubA treatment in STZ-induced diabetic rats reduced aging markers, restored the levels and function of sirtuin 1 and decreased the concentrations of ROS and oxidative stress markers such as 4-hydroxynonenal and nitrotyrosine. TubA's inhibition of HDAC6 was linked to the preservation of nuclear factor erythroid 2-related factor 2 (Nrf2)-dependent gene expression and the enhancement of thioredoxin-1 activity, ultimately strengthening its antioxidant properties. The antioxidant benefits of TubA inhibiting HDAC6 in diabetic retinas were confirmed by in vitro research conducted on HuREC cells exposed to glucose stress, which aligned with the in vivo outcomes (71). Therefore, the activation of HDAC6 is crucial in the development of oxidative and nitrosative stress caused by high blood sugar levels in the eye, leading to damage in small blood vessels and possibly diabetic retinopathy.

The impact of HDAC6 upregulation on ARPE-19 cells was investigated by researchers in both normal glucose and high glucose (HG) environments. In HG-stimulated cells, it was discovered that CAY10603 (Cay) treatment led to a significant reduction in intracellular levels of ROS (72,73). The therapy resulted in lower levels of malondialdehyde and myeloperoxidase, as well as increased activity of antioxidant enzymes like superoxide dismutase and catalase. Furthermore, Cay reduced the concentrations of inflammatory cytokines such as TNF-α, IL-1β, IL-6 and monocyte chemoattractant protein-1 in the cellular fluid (72). The therapy notably decreased apoptosis in ARPE-19 cells, as demonstrated by elevated Bcl-2 levels and reduced cleaved caspase-3 and cleaved caspase-9 levels. In addition, Cay decreased the levels of phospho-NF-κB p65, phospho-inhibitor of NF-κBα, NLRP3, cleaved caspase-1 and apoptosis-associated speck-like protein containing a CARD, while simultaneously raising the levels of cytoplasmic NF-κB p65 (72). The findings indicate that Cay may have potential as a treatment for diabetic retinopathy by reducing oxidative stress, inflammation and apoptosis in ARPE-19 cells through its effects on the NF-κB and NLRP3 inflammasome pathways induced by HG.

Although promising results have been reported using STZ-induced diabetic rats and ARPE-19 cell lines, these models only partially replicate the microvascular and neuro- inflammatory complexities of human diabetic retinopathy. Furthermore, the glucose-induced stress in vitro may not fully mimic chronic diabetic conditions in vivo. In addition, interspecies differences in retinal structure and immune response may limit the applicability of these findings to human patients. Future studies using retinal organoids or patient-derived cells may provide more clinically relevant insights.

Depletion of glutathione (GSH), mitochondrial damage and oxidative stress are key factors in the development of acetaminophen (APAP) hepatotoxicity (74-76). A study has indicated that APAP-induced liver tissues and AML-12 cells show significantly increased HDAC6 expression and decreased malate dehydrogenase 1 (MDH1) levels. The novel HDAC6 inhibitor LT-630 has shown potential as a treatment for APAP-induced liver injury. LT-630 was found to enhance both the expression and acetylation of MDH1. When MDH1 was overexpressed in APAP-stressed AML-12 cells, there was an increase in the NADPH/NADP+ ratio, GSH levels were elevated and apoptosis was reduced (77). Notably, the beneficial effects of LT-630 were reversed when MDH1 was silenced with small inhibitory RNA (77). Thus, LT-630 appears to protect against liver damage by modulating MDH1 and reducing oxidative stress caused by APAP. The hepatoprotective effects of LT-630 and HDAC6 inhibition in acetaminophen-induced liver injury were evaluated primarily in murine hepatocytes and acute liver damage models. However, APAP hepatotoxicity in humans can exhibit substantial interindividual variability due to genetic, metabolic and environmental factors. Furthermore, acute liver failure models may not predict responses in chronic liver disease settings. More diverse and chronic liver models are needed, along with PK profiling of HDAC6 inhibitors in hepatic tissue.

Sepsis, a serious illness caused by the body's overwhelming response to infection, is a leading factor in the high rates of illness and death in intensive care units (78). This syndrome can lead to the failure of various organs, such as the lungs, kidneys and liver, potentially culminating in multiple organ dysfunction syndrome (79,80). Researchers discovered in an experiment utilizing a cecal ligation and puncture (CLP) model to induce sepsis that HDAC6 plays a role in the advancement of sepsis by reducing the levels of prohibitin 1 (PHB1). Blocking HDAC6 activity significantly mitigated the effects of CLP-induced sepsis by preventing mitochondrial dysfunction and lowering oxidant production, which in turn protected rats from oxidative damage. The downregulation of PHB1 by HDAC6 interfered with the mitochondrial respiratory chain, resulting in increased oxidant production and oxidative stress, leading to severe oxidative damage in multiple organs (81). Although the CLP-induced sepsis model is a widely used and clinically relevant murine model, it still differs from human sepsis in terms of immune system complexity, timing of interventions and disease heterogeneity. The role of HDAC6 in human sepsis remains poorly characterized and differences in mitochondrial responses among species may influence outcomes. To improve translational relevance, studies using human immune cells or sepsis patient samples are warranted.

LIRI occurs due to hypoxia followed by blood flow restoration in the lungs (82,83). Research indicates that LIRI increases both the activity and expression of HDAC6. Inhibiting HDAC6 has been shown to provide protection against various types of IRIs and offers protective effects in different lung injury models. Research on MLE-12 cells showed that blocking HDAC6 decreased apoptosis, oxidative stress, inflammation and mitochondrial dysfunction caused by hypoxia/reoxygenation (H/R) (84). The protective impact was facilitated by the stimulation of the Nrf2/heme oxygenase (HO-1) signaling pathway and the deactivation of the ERK/NF-κB signaling pathway. However, these protective effects were partially reversed when MLE-12 cells were pretreated with the Nrf2 inhibitor ML385 or the ERK activator LM22B-10. Therefore, blocking HDAC6 can reduce lung epithelial cell damage caused by H/R by activating the Nrf2/HO-1 pathway and deactivating the ERK/NF-κB pathway (85). The findings from H/R cell models and murine LIRI studies demonstrate the potential of HDAC6 inhibition in mitigating oxidative lung injury. However, these models represent acute injury phases and may not reflect chronic lung injury, co-morbidities or the surgical and ventilatory complexities present in human transplantation contexts. Validation in larger animal models and ex vivo perfused human lungs could provide stronger translational evidence.

Osteoarthritis, a prevalent degenerative joint condition in older individuals, is characterized by cartilage damage, inflammation of the synovial membrane and hardening of the bone just beneath the cartilage (86,87). These pathological changes lead to chronic joint pain and dysfunction, significantly affecting quality of life and increasing healthcare costs (87). Researchers have found elevated levels of HDAC6 in the cartilage of osteoarthritic mice and in tert-butyl hydroperoxide (TBHP)-treated chondrocytes in vitro. Treatment with TubA, an HDAC6 inhibitor, effectively reduced HDAC6 expression, alleviated oxidative stress and lowered levels of apoptotic proteins, thereby promoting chondrocyte survival and preventing extracellular matrix degradation (88). Additionally, TubA-induced HDAC6 inhibition activated autophagy in chondrocytes, while autophagy inhibitors negated TubA's protective effects (88). The results suggest that targeting HDAC6 could be a potential treatment option for osteoarthritis. However, while HDAC6 inhibition reduced oxidative damage and preserved cartilage in mouse models and TBHP-treated chondrocytes, the spontaneous and slowly progressive nature of human osteoarthritis is not fully captured in these systems. Rodent joints (compared with human joints) differ in size, biomechanics and immune response (89,90). In addition, therapeutic timing and long-term outcomes remain underexplored. Studies using human chondrocytes or advanced 3D joint-on-chip models could help address these gaps.

Nanoplastics (NPs), originating from microplastics, present potential health hazards. Polystyrene (PS)-NPs of 100 nm can penetrate cells, including mouse embryonic fibroblasts (MEFs), where their accumulation leads to inflammation and oxidative stress (91,92). PS-NPs are internalized via endocytosis and predominantly gather at juxtamembranous sites, avoiding the nucleus, and are eventually cleared from cells after exposure stops (93,94). Their proximity to the nucleus is likely due to retrograde transport along microtubes. Researchers discovered that treating PS-NP-exposed MEFs with inhibitors of HDAC6, dynamin or microtubule polymerization markedly reduced both intracellular and intranuclear accumulation (95). Furthermore, HDAC6 inhibitors facilitated the rapid clearance of PS-NPs. Blocking retrograde transport also diminished the activation of antioxidant response pathways, inflammation, oxidative stress and ROS production (95). Therefore, inhibiting the reverse transport of non-biodegradable PS-NPs promotes their quick export via exocytosis, thereby reducing cytotoxicity. The study of PS-NPs in murine embryonic fibroblasts provides a useful model for nanotoxicity, but their intracellular behavior, clearance kinetics and interactions with the immune system in humans are still poorly understood. Furthermore, in vitro systems lack the systemic complexity and exposure routes of environmental NP exposure. Bridging in vitro findings with in vivo toxicology and human exposure assessment will be crucial for real-world risk evaluation.

HDAC6 is gaining recognition for its significant role in cancer, particularly through its impact on oxidative stress. HDAC6 regulates the acetylation of various non-histone proteins, affecting cellular redox balance, protein degradation and stress responses. In cancer, HDAC6 is often upregulated, increasing oxidative stress, which in turn promotes tumor progression and resistance to therapy. The enzyme's regulation of ROS production, mitochondrial function and autophagy highlights its critical role in cancer biology. Targeting HDAC6 with specific inhibitors has demonstrated potential in reducing tumor growth and overcoming chemoresistance by mitigating oxidative stress and enhancing cellular homeostasis. This section reviews HDAC6's role in cancer development and progression through its influence on oxidative stress, emphasizing the therapeutic potential of HDAC6 inhibition in oncology.

Resistance to chemotherapy in OSCC presents a major obstacle, frequently leading to cancer recurrence and metastasis (96,97). Cancer stem cell (CSC) subpopulations are notably resistant to treatment due to epigenetic regulation mechanisms (98). HDACs are crucial epigenetic regulators of gene expression, with HDAC6 playing a role in processes such as oxidative stress, autophagy and the DNA damage response (99). Researchers found that HDAC6 accumulates in cisplatin-resistant (CisR) cell lines and CSCs, which showed reduced DNA damage and ROS levels, along with increased expression of PRDX2. Treatment with TubA, a specific HDAC6 inhibitor, heightened oxidative stress and DNA damage while reducing PRDX2 levels (100). Additionally, TubA induced apoptosis and diminished the stemness phenotype in both CisR and CSC cells (100). The results indicate that targeting HDAC6 with drugs such as TubA might be able to address chemoresistance and decrease CSC characteristics in OSCC.

Cells constantly produce ROS, which can lead to oxidative stress if their concentrations become excessive (101). ROS play a crucial role in regulating the Hippo pathway (102). Mps one binder 1 (MOB1), an essential component of the Hippo signaling pathway, is phosphorylated by mammalian STE20-like protein kinase 1/2 (MST1/2) and enhances the activity of large tumor suppressor kinase 1/2 (LATS1/2) (103). It interacts with the acetyltransferase CREB-binding protein (CBP) and gets acetylated at lysine 11 (MOB1-K11) (104). Conversely, HDAC6 deacetylates MOB1. When MOB1 is acetylated at K11, its binding affinity for the E3 ligase Praja2 decreases, leading to its ubiquitination and stabilization. The acetylation of MOB1 increases its phosphorylation, leading to the activation of LATS1. Oxidative stress inhibits the degradation of CBP, leading to the acetylation of MOB1, which is consistent with the activity of MST1/2 kinases and opposes the deacetylation effect of HDAC6. This links oxidative stress to Hippo pathway activation. In vitro, the mutant MOB1-K11R, which lacks acetylation, enhances the proliferation, migration and invasion of lung cancer cells, and in vivo, it accelerates tumor growth compared to wild-type MOB1 (105). The interaction between oxidative stress and CBP controls the acetylation of MOB1-K11, leading to the activation of LATS1 and the Hippo pathway. This activation prevents the nuclear translocation of yes-associated protein/transcriptional co-activator with PDZ-binding motif and hinders tumor progression (Fig. 3).

In non-small cell lung cancer (NSCLC) with KRAS mutations, changes in liver kinase B1 (LKB1) frequently result in a worse prognosis than mutations in TP53 (106). LKB1 plays a vital role as a tumor suppressor, regulating multiple signaling pathways in response to energy deprivation (106). A recent study on blocking HDAC6 using both pharmacological and genetic techniques revealed its impact on the function of various key glycolytic enzymes. Researchers used mouse cell lines with KRAS/LKB1 (KL) and KRAS/TP53 mutations to account for the variability caused by germline and somatic mutations in human models. They examined the metabolic characteristics and responses to HDAC6 blockade, assessing the impact on cancer cell proliferation in vitro and tumor development in vivo. The findings suggested that KL mutant cells possessed intrinsically reduced levels of redox-responsive coenzymes, rendering them more susceptible to the HDAC6 inhibitor ACY-1215 (107). This vulnerability was due to their diminished capacity to ramp up compensatory metabolic processes and manage oxidative stress. Furthermore, the concurrent use of HDAC6 blockers and glutaminase inhibitors resulted in a notable increase in cell mortality and enhanced the body's ability to combat tumors in a KL lung cancer models, both in vitro and in vivo (107). Thus, targeting HDAC6 presents a potential opportunity in KL NSCLC by exploiting the cells' weakened capacity to handle glycolytic dysfunction, offering a promising strategy for managing KRAS-mutant NSCLC with concurrent LKB1 mutations.

GBM is a challenging brain tumor that is both aggressive and drug-resistant, characterized by genetically unstable and invasive cells that pose significant treatment difficulties (108,109). This has driven the need for new therapeutic approaches. A group of scientists have studied a range of hydroxamic acids containing abiraterone as possible dual blockers of cytochrome P450 family 17 subfamily A member 1 (CYP17A1) and HDAC6 to treat GBM. Among these, Compound 12 stood out for its ability to induce apoptosis, suppress genes linked to tumor recurrence, increase oxidative stress and trigger DNA damage responses (110). Molecular modeling studies confirmed that Compound 12 has strong inhibitory effects on both CYP17A1 and HDAC6. Compound 12 was found to efficiently decrease tumor growth in vivo in xenograft and orthotopic mouse models, with no notable adverse effects (110). These findings highlight the potential of dual CYP17A1 and HDAC6 inhibition as a promising strategy to overcome resistance in GBM therapy, offering a new avenue for more effective treatments.

The multifunctional roles of HDAC6 extend far beyond its involvement in neurodegenerative diseases, cardiovascular disorders and metabolic conditions, as its influence is significantly noted across various types of cancers. Previously, it has been shown that HDAC6 has a role in the development and advancement of liver (111), breast (112), colorectal (113,114), gastric (115), ovarian and endometrial cancer (116). These findings emphasized the importance of HDAC6 in regulating responses to oxidative stress, cytoskeletal dynamics, epithelial-mesenchymal transition, protein homeostasis and intercellular communication, all of which play crucial roles in cancer progression and metastasis (117-120). HDAC6 can remove acetyl groups from non-histone proteins like α-tubulin (121), HSP90 (122), cortactin (123) and various transcription factors, enabling it to control essential mechanisms that cancer cells use to survive, grow and spread. The inhibition of HDAC6 has shown promising therapeutic potential in preclinical models by disrupting these oncogenic processes, offering new avenues for cancer treatment (124,125). Building on this foundation, it is crucial to explore the roles of HDAC6 in other cancer types, further elucidating its broad impact on tumorigenesis and its potential as a therapeutic target. HDAC6's influence spans across a diverse array of malignancies, including bladder, pancreatic (126), cervical and gallbladder cancer (127). In these cancers, HDAC6 modulates oxidative stress, enhances cell motility and affects key signaling pathways that are pivotal for cancer cell survival and resistance to therapy (127-130). The enzyme's regulatory functions often intersect with vital cellular mechanisms such as autophagy, apoptosis and immune response modulation, underscoring its central role in maintaining the malignant phenotype (Table I).

A potent HDAC6 inhibitor, TubA, has shown protective effects against multiple oxidative stress-related diseases (136). As an example, HDAC6 can be found constitutively in the mouse retina as well as in the cone-like mouse cell line 661W. HDAC6 can be inhibited by the specific inhibitor TubA, leading to acetylation of α-tubulin, HDAC6's primary substrate (137). When hydrogen peroxide produces oxidative stress, TubA promotes cell survival and activates heat shock transcription factor 1, activating HSP70 and HSP25 (137). In addition, oxidative stress inhibits the peroxiredoxin 1 (Prx1) redox-regulated protein in 661W cells. Prx1's peroxide-reducing activity depends on its acetylation, which HDAC6 inhibits (37). Photoreceptors may be protected from damage by Prx1-retained activity after preincubation with TubA. An analysis of hereditary vision loss in zebrafish (dyeucd6) was carried out to determine whether TubA treatment has a therapeutic effect on visual function. The application of TubA in vivo led to hyperacetylation of alpha-microtubulin, as well as the ability to rescue visual function and retinal morphology (70,137). Therefore, inhibiting HDAC6 and modulating peroxiredoxin activity may play an important role in protecting retinal cells, particularly photoreceptors, which are exposed to high levels of ROS.

Tubacin is a selective HDAC6 inhibitor that has demonstrated protective effects in models of cardiovascular disease. Tubacin treatment in endothelial cells decreased ROS generation by blocking NADPH oxidase function and boosting eNOS activity, resulting in elevated NO levels (138). This led to improved endothelial function and reduced vascular inflammation. Tubacin's ability to stabilize microtubules by inhibiting HDAC6-mediated deacetylation of α-tubulin also contributes to its protective effects on the endothelium, promoting better cellular structure and function. In atherosclerosis models, Tubacin has been shown to reduce plaque formation and stabilize existing plaques by decreasing oxidative stress and inflammation (139). The compound's capacity to regulate the NF-κB and MAPK signaling pathways, crucial in inflammatory reactions, enhances its promise as a treatment for cardiovascular conditions.

It has been determined that ACY-1215 (Ricolinostat) provides the most effective treatment for macrophages activated by LPS (41,140,141). The treatment dose was determined using flow cytometry. In a study using RAW264.7 macrophages, controls, LPS-treated macrophages and ACY-1215-treated macrophages were compared. LPS-treated cells were treated with ACY-1215 (10 µM) 2 h before LPS treatment, and ROS, inflammatory cytokines, mitochondrial ultrastructure, mitochondrial membrane potential, RNA and protein expression were determined after 24 h of LPS treatment. In addition, HDAC6 knockdown was investigated in relation to the inflammatory response of LPS-activated RAW264.7 macrophages. It was demonstrated that inhibition of HDAC6 suppressed the production of ROS and inhibited the expression of pro-inflammatory cytokines in LPS-activated RAW264.7 cells, including TNF-α, IL-1 and IL-6, in the cells (41). Furthermore, ACY-1215 restored the mitochondrial membrane potential and ultrastructure of LPS-activated macrophages to normal levels (41). As well as inhibiting HDAC6 expression, ACY-1215 activated MAPK and NF-κB signaling pathways by normalizing Toll-like receptor 4, Nrf2 and HO-1 protein expression (142).

Vorinostat, also called SAHA, is a broad-spectrum HDAC inhibitor that has been authorized for treating cutaneous T-cell lymphoma (143). Although not selective for HDAC6, Vorinostat has shown potential to modulate oxidative stress and improve the efficacy of anticancer therapies. During preclinical research, Vorinostat therapy increased levels of ROS in malignant cells, resulting in increased cell death and decreased tumor cell proliferation (144). The potential of Vorinostat as a therapeutic agent for various cancers is supported by its ability to inhibit multiple HDACs and regulate different cellular pathways related to oxidative stress and protein homeostasis (145). The compound's broad-spectrum activity makes it a valuable addition to the arsenal of anticancer therapies, particularly in cases where selective HDAC6 inhibitors may not be sufficient.

HDAC6 inhibitors such as TubA, ACY-1215 (ricolinostat) and tubacin have shown therapeutic efficacy in preclinical models of oxidative stress-related diseases. However, a thorough understanding of their PK and toxicity is critical for clinical translation.

TubA exhibits high selectivity for HDAC6 with nanomolar potency. In murine models, it shows moderate bioavailability when administered intraperitoneally and a relatively short half-life (~1-2 h) due to rapid hepatic metabolism (146). Limited oral bioavailability has been reported, necessitating formulation optimization for systemic administration. Toxicological studies in rodents indicate that TubA is generally well tolerated at therapeutic doses, with minimal off-target effects (147). However, long-term toxicity and potential immunosuppressive effects require further investigation (147,148).

ACY-1215 (ricolinostat) is a clinically advanced HDAC6-selective inhibitor with favorable PK. Phase I clinical trials in patients with multiple myeloma and lymphoma revealed an elimination half-life of 3-5 h and good oral bioavailability (149,150). Ricolinostat was generally well tolerated, with the most common adverse effects being fatigue, diarrhea and transient cytopenias. These findings support its use in combination therapies. However, reversible inhibition of immune function and hepatic enzyme elevation have been observed in certain patients, underscoring the need for careful monitoring (151).

Tubacin, while effective in vitro, has drawbacks of poor metabolic stability and low systemic exposure in vivo, which limits its development as a clinical candidate. Its use has primarily been restricted to proof-of-concept studies due to its unfavorable PK profile (152,153).

Despite promising early-phase safety data, most HDAC6 inhibitors have not yet undergone large-scale toxicity studies in humans. The long-term effects on normal tissue homeostasis, off-target deacetylation events and potential reproductive toxicity remain largely unexplored. Further PK modeling and toxicity profiling in diverse species are essential to advance these compounds toward regulatory approval (153).

Although numerous animal models, including rodent models of neurodegeneration, cancer and cardiovascular diseases, have been used to assess the efficacy of HDAC6 inhibitors, these models often fail to fully recapitulate the complexity of human diseases (154,155). For instance, rodent models typically do not mimic the slow, multifactorial progression of diseases like Parkinson's or Alzheimer's (139). Similarly, the short lifespan and lack of sex- and age-related variability in animal models limit their ability to predict long-term therapeutic outcomes in humans (156). Furthermore, these models often do not account for the impact of comorbidities and multifactorial conditions that are common in human patients, complicating the translation of preclinical findings to human treatments (156).

Most current research on HDAC6 inhibitors is based on short-term studies and long-term toxicity data remain scarce (157). Chronic inhibition of HDAC6, particularly in neuronal cells, may impair autophagic processes and protein degradation, leading to potential neurotoxicity (158). The long-term effects of HDAC6 inhibition on other organ systems, such as the heart, liver and kidneys, are largely unknown. Studies investigating off-target effects and cumulative toxicity are essential for assessing the safety of these compounds in prolonged use (137).

The PK profiles of HDAC6 inhibitors, particularly in humans, are still not fully understood. Variability in oral bioavailability, half-life and hepatic metabolism can affect both the efficacy and safety of these drugs. For instance, certain inhibitors, such as TubA, show promising preclinical efficacy but have been associated with high systemic exposure and off-target effects at therapeutic doses, raising concerns about their safe application in humans. Furthermore, drug-drug interactions remain a significant issue, particularly in the context of patients taking multiple medications for complex conditions like cancer or cardiovascular disease (159).

Clinical trials (NCT01997840, NCT02189343, NCT02091063, NCT03176472) of HDAC6 inhibitors, such as Ricolinostat (ACY-1215), have shown encouraging results in terms of target engagement and initial safety, but large-scale, randomized controlled trials are still lacking (137,149). The absence of longitudinal studies means that the efficacy of these inhibitors in real-world clinical settings remains uncertain. Furthermore, phase II and III trials focusing on hard clinical endpoints (e.g., survival, disease progression) are essential to confirm the long-term benefits of HDAC6 inhibitors in treating neurodegenerative diseases, cancer and cardiovascular disorders.

A significant challenge in HDAC6 inhibitor development is the lack of reliable biomarkers that can measure drug efficacy and predict patient response. For instance, biomarkers like acetyl-α-tubulin have been suggested, but their use in clinical trials is still under investigation. The absence of predictive biomarkers complicates patient stratification and limits the ability to tailor treatment to specific genetic profiles or disease stages (153,160,161).

Another limitation is the lack of data on the safety of HDAC6 inhibitors in vulnerable populations, including elderly patients, children and those with comorbidities. The effect of HDAC6 inhibition on reproductive health, immune function and neurological development is still under-researched, and these concerns will need to be addressed through dedicated clinical trials.