Protein acetylation is the process of covalently binding acyl-CoA class A compound to protein-specific amino acid sites under the action of acetyltransferases. Vincent Allfrey and his colleagues discovered histone lysine acetylation modification in 1964 (7). In subsequent studies, they gradually discovered the mechanism of acetylation modification, the discovery and identification of HAT and HDAC, and the discovery and identification of reader domains, which laid the foundation for protein acetylation. With the development of mass spectrometry and proteomics, non-histone acetylation was discovered and the regulatory process of non-histone acetylation was revealed (7). More and more studies have proved that histone acetylation and non-histone acetylation have the same importance in the regulation of biological processes in organisms (7). After the discovery of non-histone acetylation, histone acetyltransferases were also renamed lysine acetyltransferases and histone deacetylases were renamed lysine deacetylases (7). Histone acetylation occurs in the nucleus and is a type of epigenetic regulation that regulates chromatin structure to regulate transcription and DNA repair. Histone hyperacetylation by histone acetyltransferase is associated with transcriptional activation, while histone deacetylation by histone deacetylase is associated with transcriptional repression. Histone acetylation promotes transcription by remodeling higher-level chromatin structure, attenuating histone-DNA interactions, and providing binding sites for transcription activation complexes (12). Histone deacetylation inhibits transcription, and histone deacetylation and acetylation maintain homeostasis by opposing mechanisms, including the assembly of higher-order chromatin structures and the exclusion of bromo domain-containing transcriptional activation complexes (12). Histone acetylation and tumorigenesis are also closely related, and histone acetylation promotes the expression of certain genes that can lead to tumors (13). For example, P300 is a histone lysine acetyltransferase that catalyzes the attachment of acetyl groups to lysine residues, which leads to the activation of several genes, including several oncogenes. Study finds elevated expression of p300 in breast cancer (13).

Non-histone acetylation is involved in most biological processes in organisms and occurs with very high frequency. Non-histone acetylation is involved in key cellular processes related to organism physiology and tumors, such as gene transcription, DNA damage repair, cell division, protein folding, autophagy, cell signaling, and metabolism. For example, HDAC6 acts not only on histones, but also on non-histone substrates to maintain the balance of non-histone acetylation (14). α-Tubulin, the first non-histone substrate of HDAC6, reversibly modulates its homeostasis and in turn affects MT stability and function (15). The α-tubulin acetylation affects intracellular trafficking events through the protein encoded by the cylindromatosis gene, thereby participating in mitosis and affecting the development of the cell cycle (14). Non-histone acetylation modifies protein expression through various mechanisms and affects protein function. For example, regulating protein stability, regulating protease activity, affecting subcellular localization, and regulating protein-protein interactions, etc. Protein acetylation can be classified into three types (N-acetylation, O-acetylation, and K-acetylation) according to acetylation site in a protein amino acid sequence.

Acetylation in eukaryotic cells is in a dynamic equilibrium, which is jointly participated by writer-acetyltransferase, eraser-deacetylase, acetyl coenzyme A, and reader (4) ( Figure 1 ).

Apoptosis refers to the orderly death of cells controlled by genes, which is a normal programmed death in order to maintain the stability of the internal environment. In the process of cell apoptosis, it can be divided into the initiation stage, which receives apoptosis signals, interacts with apoptosis regulators, and then activates proteolytic enzymes, resulting in apoptosis (141). However, tumors have the characteristics of avoiding apoptosis, and abnormal apoptosis leads to abnormal tumor growth (142). The abnormal expressions of acetyltransferase and deacetylase can affect the normal apoptosis of cells. For example, PDCD5, a protein associated with apoptosis in human cells, binds to Tip60 and enhances the stability of Tip60 protein under stress-free conditions (143). PDCD5 increases Tip60-dependent K120 acetylation of p53 and is involved in p53-dependent expressions of apoptosis-related genes such as Bax (143). The combination of PDCD5 and Tip60 accelerates DNA damage-induced apoptosis, whereas knockdown of PDCD5 or Tip60 inhibits apoptosis-accelerating activity (143). HDAC1 and HDAC2 double knockout cells show significant activation of apoptosis (144). HDAC6 negatively regulates pro-apoptotic acetylation of p53 at K120 in mesenchymal stem cells (MSCs) (145). Studies have shown that targeting histone acetyltransferases and histone deacetylases can regulate tumor cell apoptosis, thereby affecting tumor growth and development (146). For example, histone deacetylase inhibitors induce apoptosis and autophagy in human neuroblastoma cells (147). Valproic acid induces cell cycle arrest and apoptosis via Hsp70 acetylation and inhibits proliferation of HER2-expressing breast cancer cells (148). When rRNA transcription was inhibited, nucleolar RNA content was reduced. The nucleolar protein Myb-binding protein 1A (MYBBP1A) translocates to the nucleoplasm and increases p53 acetylation as the level of nucleolar RNA content decreases (149). Acetylated p53 enhances p21 and BAX expression and induces apoptosis (149). Targeting protein acetylation to regulate tumor apoptotic activity can provide new therapeutic ideas for the clinical treatment of malignant tumors.

Autophagy is a special substance degradation pathway in cells, which depends on lysosomes for its action (10). The degradation substrates of autophagy include proteins and organelles. The probability of autophagy occurring in normal cells is low, and it mainly occurs in cells under abnormal conditions, such as starvation, hypoxia or organelle damage (150). There are three main types of autophagy. (i) The first type is microautophagy, in which lysosomes wrap a part of the cytoplasm into lysosomes and degrade them. (ii) The second is macroautophagy, which first generates an autophagosome (151). The double-membrane structure of the phagosome, the fruiting body contains the substances that need to be degraded in the cytoplasm, the autophagosome and the lysosome are combined to generate the autophagolysosome, and the acidic substances in the lysosome are used to degrade the autophagosome (151). Substances are degraded. (iii) The third is chaperone-mediated autophagy. Molecular chaperone-mediated autophagy uses heat shock protein 70 to bind to substrates with specific amino acid sequences and transport the substrates to lysosomes for further development (152). In 2004, Shao et al. found that HDAC inhibitor suberoylanilide hydroxamic acid β-D-glucur onide could induce autophagic death of cancer cells, and researchers gradually began to pay attention to the relationship between protein acetylation and autophagy (153). There is a close relationship between histone acetylation and cell autophagy. Histone acetylation can induce the occurrence of cell autophagy in the face of long-term stress, starvation and other harsh environments (154). The most widely studied is the relationship between H4K16ac and H3K56ac and autophagy. In eukaryotic cells, H4K16ac affects chromatin condensation and promotes gene transcription (155).

There is also a close link between non-histone acetylation and cell autophagy. Non-histone protein associated with autophagy that can be acetylated include transcription factors, autophagy-related proteins, and cytoskeletal proteins (7). The Fox O protein family is a transcriptional activator in eukaryotic cells, and acetylation can affect its biological activity. K on Fox O protein can be acetylated by HAT, and the activity of Fox O protein after acetylation is reduced, inhibiting DNA and its interaction, binding to regulate transcription (156). SIRT1 can also affect autophagy by regulating Fox O activity (157). TFEB protein is also a transcription factor that regulates the transcription of autophagy-related genes, such as LC3, and plays an important role in biological processes such as lysosomal biosynthesis and autophagy activation (158). The biological activity of the TFEB protein family is also affected by acetylation modification, and TFEB deacetylation can significantly enhance the autophagy and lysosomal function of cells (159). The TFEB-specific lysine acetylase is GCN5, which can acetylate the K276 and K279 sites of TFEB, affect the formation of TFEB dimers, interfere with the binding of TFEB and its targets, and inhibit autophagy happened (160). Acetylations affect subcellular localization, thereby affecting autophagy (161). In general, BmP300-mediated acetylation sequesters components of the BmAtg8-PE ubiquitin-like system in the nucleus, leading to inhibition of autophagy. Conversely, BmHDAC1-mediated deacetylation leads to nuclear-to-cytoplasmic transfer of components of the BmAtg8-PE ubiquitin-like system, promoting autophagy (161).

Protein acetylation is an important process regulating autophagy and plays an important role in the development of malignant diseases. The phosphorylation of ATG5 (T101) in the lesion tissue of glioblastoma patients is positively regulated by the acetylation modification of the hypoxia-induced autophagy regulator PAK1, which plays an important role in hypoxia-induced autophagy and promotes the occurrence and development of tumors (162). Targeting protein acetylation modification to regulate autophagy activity can provide new therapeutic ideas for clinical treatment of malignant tumors.

A major feature of tumors is uncontrolled proliferation, fueled by corresponding metabolic dysregulation (2). Tumors undergo metabolic reprogramming to promote tumor cell growth, division, invasion and migration. An abnormal response of tumor cell energy metabolism is called the Warburg effect (89). In the presence of oxygen, tumor cells reprogram glucose metabolism by limiting energy metabolism mainly to glycolysis, thereby generating energy for tumor growth (163). Lysine acetylation is a ubiquitous modification in enzymes that catalyze intermediate metabolism. Almost every enzyme in glycolysis, gluconeogenesis, tricarboxylic acid (TCA) cycle, urea cycle, fatty acid metabolism and glycogen metabolism is found to be acetylated in human liver tissue (10). All seven enzymes in the TCA cycle are acetylated (10). Acetylation occurs in most intermediate metabolic enzymes, and acetylation can directly affect the activity or stability of the enzyme (10). The bioenergetic preference of cancer cells promotes tumor acidosis, which in turn results in a marked reduction in glycolysis and glucose-derived acetyl-CoA (164). Protein acetylation affects tumor metabolism by affecting the TCA cycle. CBP acetylates STAT3 to undergo mitochondrial translocation, and STAT3 associates with pyruvate dehydrogenase complex E1, which in turn accelerates the conversion of pyruvate to acetyl-CoA, increases mitochondrial membrane potential and promotes ATP synthesis (165). SIRT5 removes the STAT3 acetyl group, thereby inhibiting its function in mitochondrial pyruvate metabolism (165). The protein also affects lipid metabolism in tumor cells and thus affects tumor development (166). Dynamic regulation of ME1 phosphorylation and acetylation affects lipid metabolism and colorectal tumorigenesis (166). The manner in which SIRT6 deacetylase antagonizes ACAT1 function involves mutually exclusive ME1 S336 phosphorylation and K337 acetylation (166). ACAT1 acetylates GNPAT at K128, which inhibits TRIM21-mediated GNPAT ubiquitination and degradation (167). GNPAT deacetylation by SIRT4 antagonizes the function of ACAT1. GNPAT inhibits TRIM21-mediated degradation of FASN and promotes lipid metabolism. promote the occurrence of liver cancer (167). Studies have shown that lysine acetylation controls metabolic activity by directly blocking the active site of the enzyme (168).

Protein acetylation is closely related to gene transcription. Hyperacetylation promotes gene transcription and expression, while hypoacetylation inhibits gene transcription and expression (12). A large number of proteins involved in chromatin remodeling and cell cycle are acetylated (169). The cell cycle of tumor cells is greatly shortened and disordered. Studies have found that acetylation of tumor cells is also closely related to cell cycle progression (170). Protein acetylation affects tumor cell cycle progression by affecting chromatin remodeling, SIRT2 regulates H4K20me1 deposition through deacetylation of H4K16Ac (acetylation of H4K16), regulates chromatin localization, and affects cell cycle progression (169). Protein acetylation also has effects through the regulation of various factors in the cell cycle. For example, CDC2, a major cyclin-dependent kinase and regulator of S-phase progression and mitosis, is acetylated at residues K6 and K33 in CDC2 (25). SIRT1 interacts with CHK2 and is deacetylated at residure lysine 520, which inhibits CHK2 phosphorylation, dimerization, and thus activation (171). SIRT1 depletion induces CHK2 hyperactivation-mediated cell cycle arrest and subsequent cell death (171). Transcription factor Sp1 is a target of acetylation and is closely associated with cell cycle arrest in colon cancer cell lines (172). Simultaneous regulation of Api5 acetylation and deacetylation is an important factor in cell cycle progression (97).

Cancer cells have unlimited replicative potential with continuous proliferative signals (2). Normal cells and tissues release growth signals in an orderly manner, and these growth signals instruct cells to grow, divide and differentiate in an orderly manner, thereby ensuring the stability of cell numbers and the homeostasis of the internal environment, thereby maintaining normal tissue structure and function (2). However, tumor cell proliferation signals are abnormal and can continuously obtain proliferation signals from a variety of different pathways. In the abnormal proliferation of tumor cells, protein acetylation plays an important role. For example, acetylation at the K323 site of PGK1 is an important regulatory mechanism that promotes its enzymatic activity and cancer cell metabolism (173).

Acetyltransferase and deacetylase dynamically regulate the balance of acetylation, affecting the apoptosis and autophagy of tumor cells and other death methods, thereby affecting the proliferation of tumor cells. For example, Api5 is a known anti-apoptotic and nuclear protein responsible for inhibiting cell death under serum starvation conditions (97). The only known post-translational modification of Api5 is acetylation at K251. The K251 acetylation in Api5 is responsible for its stability, whereas the deacetylated form of Api5 is unstable (97). Inhibition of acetylation by p300 results in a decrease in Api5 levels, whereas inhibition of deacetylation by HDAC1 results in an increase in Api5 levels (97). Acetylation also affects the proliferation of tumor cells by affecting the activities of various metabolic enzymes in cells. For example, PKM2 K305 acetylation reduces PKM2 enzymatic activity and promotes its lysosome-dependent degradation through chaperone-mediated autophagy (CMA) (174). Degrade and promote tumor growth through chaperone-mediated autophagy (174). Ribonucleotide reductase (RNR) catalyzes the de novo synthesis of deoxyribonucleoside diphosphates (dNDPs), which provide dNTP precursors for DNA synthesis (175). Acetylation at residue K95 in RRM2 results in a reduction of the dNTP pool, DNA replication fork arrest, and inhibition of tumor cell growth in vitro and in vivo (175). P300 acetylates MAT IIα at K81 and destabilizes MAT IIα by promoting its ubiquitination and subsequent proteasomal degradation, inhibits tumor cell growth, and is reduced in human hepatocellular carcinoma (176). Inactivation of HDAC2 leads to elevated TPD52 acetylation, which impairs the interaction between TPD52 and HSPA8, resulting in impaired CMA function and tumor growth in vivo (177). Acetylation-dependent regulation of CMA oncogenic function in PCa by TPD52 suggests the possibility of targeting the TPD52-mediated CMA pathway to control PCa progression (177). p21 depletion converts KLF4 from a cell cycle inhibitor to a promoter of bladder cancer cell proliferation (178). Furthermore, KLF4 is acetylated in a p21-dependent manner to inhibit bladder cancer cell growth as a tumor suppressor (178). Since tumor cell proliferation is affected by acetylation modifications, drugs targeting acetylation can be used to treat abnormal tumor growth. For example, Rg3 extracted from ginsenosides has antiproliferative activity against melanoma by reducing HDAC3 and increasing p53 acetylation in vitro and in vivo (179). Therefore, Rg3 may serve as a potential therapeutic agent for the treatment of melanoma (179). Therapeutic modalities targeting acetyltransferases and deacetylases are also a potentially effective tumor treatment modality.

The development of tumor is divided into multiple stages. In the early stage, the primary lesion proliferates indefinitely, and after the formation of an obvious primary lesion, the function of the organ in which it is located is affected (2). Although the primary tumor is extremely malignant, the cause of death in most patients is the abnormal growth of metastatic tumors in sites other than the primary tumor (180). The reasons for these metastases are also unresolved and need to be discovered and solved urgently. Studies have found that protein acetylation is one of the important factors affecting tumor cell metastasis (6). For example, isocitrate dehydrogenase 1 (IDH1) is hyperacetylated in CRC primary tumors and liver metastases (181), sirtuin-2 is the deacetylase of IDH1, and SIRT2 overexpression significantly inhibits CRC cell proliferation, migration and invasion (181). COL6A1 is dysregulated in several human malignancies, and upregulation of H3K27 acetylation-activated COL6A1 promotes cell migration and invasion by inhibiting the STAT1 pathway in OS cells and promotes osteosarcoma lung metastasis (182). ZMYND8 acetylation of P300 at residues K1007 and K1034 is required for HIF activation and breast cancer progression and metastasis (183). TGF-β-activated kinase 1 (TAK1) stimulates phosphorylation by TGF-β and then induces acetylation of tubulin through αTAT1 activation, which subsequently activates AB cell migration and invasion (184). AFP acetylation promotes its oncogenic effects by blocking binding to the phosphatase PTEN and the pro-apoptotic protein caspase-3, thereby increasing signaling of proliferation, migration and invasion and reducing apoptosis (185). In HCC cells, hepatitis B virus X protein (HBx) and palmitic acid (PA) increased the levels of acetylated AFP by disrupting SIRT1-mediated deacetylation (185). AFP acetylation plays an important role in hepatocellular carcinoma progression (185). miR-15a-5p reduces histone H4 acetylation by inhibiting ACSS2 expression, inhibiting acetyl-CoA activity (186). miR-15a-5p inhibits lipid metabolism by inhibiting ACSS2-mediated acetyl-CoA activity and histone acetylation, thereby inhibiting a novel mechanism of lung cancer cell metastasis (186). In addition to histone acetylation affecting tumor cell invasion and migration, non-histone acetylation also affects tumor metastasis. For example, elevated levels of alpha-tubulin acetylation are sufficient reasons for the metastatic potential of breast cancer (187). Metastatic breast cancer cells exhibit high levels of alpha-tubulin acetylation, extending along microantenna (McTN) protrusions (187). Mutation of acetylation sites on α-tubulin and enzymatic regulation of this post-translational modification had a dramatic effect on McTN frequency and reattachment of suspended tumor cells (187). Reducing alpha-tubulin acetylation significantly inhibited migration but not proliferation (187). Targeting protein acetylation to affect tumor invasion and migration may serve as a potentially effective therapeutic strategy.

HAT is one of the important targets of tumor therapy. HAT inhibitors are inhibitors of protein acetyltransferase, which can inhibit its activity and reduce the level of protein acetylation. Three types of HAT inhibitors have been reported, dual substrate inhibitors, natural compounds and synthetic compounds (201). HAT inhibitors are widely used in tumor treatment. Currently, the main researches are drug inhibitors targeting CBP/P300 and small molecule inhibitors of HAT domain (201) ( Table 3 ).

Anacardiic acid, a natural compound extracted from natural plants, is a p300/CBP histone acetyltransferase inhibitor, significantly reduces the viability of PTEN-/- cells not in PTEN+/+ cells by inducing apoptosis (209). Delphinoside induces p53-mediated apoptosis in human prostate cancer LNCaP cells by inhibiting HDAC activity and activating p53 acetylation (211). Therefore, delphinidin may have a role in the prevention of prostate cancer (211). There are also synthetic compounds acting on HAT, targeting HAT as inhibitors to regulate intracellular acetylation homeostasis (210). A-485 competes with acetyl-CoA. A-485 selectively inhibits proliferation of lineage-specific tumor types, including several hematological malignancies and androgen receptor-positive prostate cancer (202). WM-3835 is a potent and highly specific HBO1 (KAT7 or MYST2) inhibitor that directly binds to the acetyl-CoA binding site of HBO1 33 WM-3835 activates apoptosis while inhibiting osteosarcoma (OS) cells proliferation, migration and invasion (216). WM-3835 has antitumor activity and potently inhibits the growth of osteosarcoma xenografts in mice (216). TH1834 dihydrochloride is a specific Tip60 (KAT5) histone acetyltransferase inhibitor (215). TH1834 dihydrochloride induces apoptosis and increases DNA damage in breast cancer cells. TH1834 dihydrochloride does not affect the activity of the related histone acetyltransferase MOF. Anticancer activity (215). Combination therapy of CK1 inhibitor SR3029 and Tip60 inhibitor MG149 had stronger inhibitory effects on β-catenin acetylation, transcription of Wnt target genes, and viability and proliferation of colon cancer cells (214). Transcriptional activity of β-catenin can be regulated through the CK1δ/ϵ-β-catenin-Tip60 axis, which may be a potential therapeutic target for colon cancer (214).

HAT activators are activators that act on protein acetyltransferases and can activate acetyltransferases to increase the level of protein acetylation. For example, CTB can induce acetylation of P53 protein by increasing the expression of P300, thereby inducing significant cell death in MCF-7, but it may be well tolerated in MRC-5 (217). Therefore, CTB can be applied in cancer treatment (217). The research on HAT activators is not very extensive, and most of them are activators targeting the CBP/P300 complex (217) ( Table 4 ).

HDACs are found to be abnormally expressed in malignant tumors (219). The expression of HDACs is closely related to clinical treatment prognosis and tumor occurrence and development. In liver cancer, inhibition of HDAC2 expression can promote histone acetylation in the promoter region of MIR22HG, thereby upregulating the expression of MIR22HG, promoting the production of miR-22-5p, and ultimately increasing the sensitivity to radiotherapy (64). In acute B lymphocytic leukemia, inhibits the activity of HDAC3, which enhances the sensitivity of acute B lymphocytic leukemia cells to drugs by inhibiting the JAK/signal transducer and activator of transcription 3 signaling pathway (220). Inhibition of HDAC8 activity causes cytotoxic effects, cell cycle arrest in human monocytic leukemia followed by apoptosis, and cytostatic effects in p53-deficient human myelocytic leukemia cells (73). SIRT1/2 inhibition results in HSPA5 acetylation and dissociation from EIF2AK3, leading to endoplasmic reticulum stress response, which in turn upregulates ATF4 and dit4, triggering autophagy (86). Sirtuins have become a promising target for a novel class of anti-cancer drugs. HDAC inhibitor can reverse this phenomenon and reactivate the expression of tumor suppressors, and HDAC inhibitor can act on histone acetylation and non-histone acetylation to inhibit tumor growth, invasion and metastasis, and has become a clinically effective anti-tumor drug (221) ( Table 5 ).

Studies have shown that HDAC inhibitor has a significant inhibitory effect on P53, HSP90, NF-κB factors and multiple dephosphorylation enzymes, and a variety of HDAC inhibitors have been developed (59). The FDA has developed and approved several HDAC inhibitors for clinical cancer treatment. HDAC inhibitors are mainly divided into five categories according to different structures, including short-chain fatty acids, amides, hydroxamic acids, cyclic peptides, and chemical substances extracted from plants (265). Among histone deacetylase inhibitors, fatty acids are one of the less commonly used inhibitors. Valproic acid is an anticonvulsant drug that has been used clinically in bipolar disorder (266). The study found that valproic acid can also inhibit histone deacetylase 9, affect Notch cell signaling, and inhibit the activity of human neuroblastoma cells (267). The HDAC inhibitor of the benzamide class is the first inhibitor that selectively targets class I HDACs. There are also a large number of benzamide drugs in clinical trials for tumor treatment (59). The enzyme kinetics study of aminobenzamide-based HDAC inhibitors shows that the aminobenzamide motif has a tight binding mechanism (slow start/slow shutdown) unlike the classical fast-on/fast-off kinetics of hydroxamic acid-based HDAC inhibitors (268).

Hydroxamic acid HDAC inhibitors are the first class of HDAC inhibitors to be developed (59). Vorinostat is the first HDAC inhibitor on the market. At appropriate concentrations, vorinostat can inhibit HDAC1, 2, 3, 6, inhibit the activity of HDAC, and lead to significant hyperacetylation of H4 at residues lysine 5, 8, 12, 1, and 6 (269). These hyperacetylation are closely related to transcriptional changes, and vorinistat can simultaneously increase or decrease the transcription of specific genes in tumor cells, suggesting that HDAC inhibitor can have completely opposite effects throughout the genome (265). Virinostat is currently approved for the treatment of cutaneous T-cell lymphoma (CTCL). Studies have shown that vorinostat has activity in the treatment of recurrent glioblastoma multiforme (270). Clinically, it can be used in combination with other drugs to treat tumors (270). Vorinostat is clinically used in combination with gefitinib in the treatment of lung cancer to enhance the induction of apoptosis of lung cancer cells (271). Panobinostat is involved in many biological processes, including DNA replication and repair, chromatin remodeling, gene transcription, cell cycle progression, protein degradation and cytoskeleton reorganization (226). For example, in prostate cancer, Panobinostat reverses HepaCAM gene expression and inhibits proliferation by increasing histone acetylation (226). Panobinostat can also be used in combination with other drugs to improve treatment efficiency, such as in acute myeloid leukemia, studies have shown that the combination of panobinostat differentiation and arsenic trioxide apoptosis can significantly improve survival (272). Another HDAC inhibitor is SIRT inhibitor, inhibition of SIRT1 and SIRT2 induces cancer cell apoptosis and plays multiple roles in regulating autophagy (86). Salermide in NSCLC cells, inhibiting SIRT1 and 2 by acetylating HSPA5, and then activating ATF4 and dit4 to inhibit the mTOR signaling pathway, thereby inducing pro-survival autophagy (86). Ginsenoside Rg1 inhibits cell proliferation and induces cellular senescence in acute myeloid leukemia cells CD34+CD38- leukemia stem cells by activating Sirtuin 1 (SIRT1)/tuberous sclerosis complex 2 (TSC2) signaling pathway (273). Capsaicin attenuates cell migration by enhancing corticosteroid and -catenin acetylation in bladder cancer cells through SIRT1 targeting and inhibition (274). Capsaicin-reduced cell migration is associated with downregulation of sirtuin 1 (SIRT1) deacetylase, possibly through proteasome-mediated protein degradation (274). Combination therapy of SIRT1/2 inhibitor and drug autophagy inhibitor is an effective therapeutic strategy (86). Some studies have found that synthetic HDAC inhibitors may have toxic side effects such as atrial fibrillation, researchers turned their attention to natural inhibitors extracted from plants (59). Plant-derived inhibitors also showed good activity in inhibiting tumors. For example, hawthorn polyphenol extract (HPE) can significantly reduce ROS levels, apoptosis and inflammation-related factor expression in cells, and also inhibit AMPK/SIRT1/NF-κB and miR-34a/SIRT1/p53 pathways by regulating acetylation (275). Pathway is involved in hyperglycemia-induced inflammation and apoptosis of human retinal epithelial cells (275). These inhibitors can significantly inhibit tumor proliferation, migration and invasion, and can induce apoptosis and induce autophagy (59). However, the application of these inhibitor drugs in clinical practice requires more in-depth research.

As a scaffold protein, BET can read epigenetic code, recognize histone acetylation or non-histone acetylation, and regulate gene expression, and play an important role in cell function (115). However, abnormal expression of BET leads to abnormal gene expression, resulting in abnormal cell function, which is related to the development of many malignant diseases. The study found that the abnormal expression of BRD4 is related to glioma, and the expression in glioma is significantly higher than that in normal tissue (130); BRD4 inhibitors effectively penetrated the blood-brain barrier and targeted glioma tumor tissue, but had little effect on normal brain tissue (130). Therefore, BRD4 is a target for the treatment of glioma (130). Targeting BET protein therapy is a very promising tumor treatment strategy. The BET-bromodomain-specific inhibitors JQ1, I-BET and I-BET151 represent initial successes in the development of BET inhibitors (276). The small molecule BET inhibitor drug, JQ1, is a potent growth inhibitor for many cancers and holds promise for cancer therapy (276). However, studies have found that JQ1 can activate other oncogenic pathways and may affect epithelial-to-mesenchymal transition (EMT) (276). That is to say, JQ1 has an unexpected role in promoting prostate cancer invasion (276). In the application of tumor treatment, attention should be paid to the possible toxic and side effects of JQ1. BET inhibitor treatment in HCC cell lines reduces cell migration by downregulating SMARCA4 (277). GS-5829 inhibits CLL cell proliferation and induces leukemia cell apoptosis by deregulating key signaling pathways such as BLK, AKT, ERK1/2, and MYC (278). BRD2 supports borderline activity and raises the possibility that pharmacological BET inhibitors may partially affect gene expression by interfering with regional borderline function (279). Disruption of negative autoregulation by BET inhibitor (BETi) leads to a marked increase in BCL6 transcription, which further activates the mTOR signaling pathway by inhibiting tumor suppressor death-associated protein kinase 2 (123).

The effectiveness of BET-specific targeted inhibitors is often affected by tumor drug resistance (280). There is also an urgent need to address the issue of BET inhibitor resistance. Prostate cancer-associated SPOP mutations confer resistance to BET inhibitors by stabilizing BRD4 (281). Tumor-suppressive effects of SPOP in prostate cancer, where it acts as a negative regulator of BET protein stability, and also provides a molecular mechanism for resistance to BET inhibitors in individuals with prostate cancer carrying SPOP mutations (281). Prostate cancer-associated SPOP mutants display impaired binding to BET proteins, leading to reduced proteasomal degradation and accumulation of the protein in prostate cancer cell lines and patient specimens, and causing resistance to BET inhibitors (282). Transcriptomic and BRD4 enzymatic analysis revealed enhanced expression of GTPase RAC1 and cholesterol biosynthesis-related genes, and activation of AKT-mTORC1 signaling due to BRD4 stabilization (282). Resistance to BET inhibitors in SPOP-mutant prostate cancer can be overcome by combination with AKT inhibitors and further supports the evaluation of SPOP mutations as biomarkers to guide BET inhibitor-directed therapy in prostate cancer patients (282).

Although research on BET inhibitors is still a research focus, the combination use of BET inhibitors with other drugs is also being explored. BET inhibitors can be used in combination with other types of inhibitors in order to promote the therapeutic effect or reduce adverse reactions (283). For example, the combination of BET inhibitor I-BET762 and PARP inhibitor Talazoparib Synergy is used in the treatment of SCLC and has a synergistic effect (283). At the same time, a strategy of combined application of HDAC inhibitor and JQ1 inhibitor has shown good efficacy in the treatment of AML (284). Based on the combination drug strategy, dual-target inhibitors of HDAC and BET are also being developed, and have shown more significant efficacy than single-target inhibitors in the treatment of pancreatic cancer (285). This multi-targeted drug can ensure the efficacy and durability of the anti-cancer effect, and this combination approach also reduces the possibility of tumor resistance (285). This provides a new scope of research for BET inhibitors in the treatment of tumors. BET and HDAC inhibitors are synergistic at reduced doses, suggesting a potential approach to avoid overlapping toxicities of the two drug classes (280). The combination of CPI-0610 with a PRAP inhibitor has been found to better address PRAP inhibitor resistance in ovarian cancer patients (286). It also proposes new therapeutic strategies to address PARP inhibitor resistance using drugs already approved or in clinical development that have the potential to rapidly transform and benefit a broad range of ovarian cancer patients (286) ( Table 6 ).