The emerging role of KDM5A in human cancer

Histone methylation is a key posttranslational modification of chromatin, and its dysregulation affects a wide array of nuclear activities including the maintenance of genome integrity, transcriptional regulation, and epigenetic inheritance. Variations in the pattern of histone methylation influence both physiological and pathological events. Lysine-specific demethylase 5A (KDM5A, also known as JARID1A or RBP2) is a KDM5 Jumonji histone demethylase subfamily member that erases di- and tri-methyl groups from lysine 4 of histone H3. Emerging studies indicate that KDM5A is responsible for driving multiple human diseases, particularly cancers. In this review, we summarize the roles of KDM5A in human cancers, survey the field of KDM5A inhibitors including their anticancer activity and modes of action, and the current challenges and potential opportunities of this field.


Roles of KDM5A in homeostasis and disease
KDM5A mediates a range of physiological and pathological events, such as cell motility, stemness, and epithelial-mesenchymal transition (EMT), via activating or repressing transcription in demethylase-dependent or independent manners in both homeostasis and disease [11,65,66].

The roles of KDM5A in non-cancer disease
KDM5A is associated with many non-cancer diseases, such as congenital heart disease (CHD) [82] and bacterial, viral, or parasitic infection [79,83,84]. It also mediates renal failure in lipopolysaccharide-induced sepsis of mice [85]. Upregulation of KDM5A inhibits the commitment of marrow-derived mesenchymal stem cells lineage into osteoblasts by decreasing H3K4me3 levels in the promoter region of Runt-related transcription factor 2 (Runx2) (Fig. 3) [86].

KDM5A in human cancer
KDM5A is associated with cancer growth, differentiation, multi-drug resistance, invasion, and metastasis in various cancers ( Table 2 and Fig. 3).

Acute myeloid leukemia (AML)
AML is a cancer with a high lethality that occurs more frequently in older populations [87]. Meanwhile, acute megakaryoblastic leukemia (AMKL), a subtype of AML with similar cell morphology to abnormal megakaryoblasts, accounts for a significant fraction of pediatric AML cases [16,88]. KDM5A is highly expressed in pediatric AMKL with a cytogenetically cryptic fusion NUP98/NSD1 (t(5; 11)(q35; p15)) [89]. The fusion gene NUP98/KDM5A is formed by the fusion of the C-terminal PHD finger of KDM5A to NUP98 [51]. This fusion is required for leukemogenic transformation, by increasing progenitor cell self-renewal and blocking myeloblast differentiation [51,90]. The transplantation of bone marrow cells transduced with NUP98-KDM5A into mice resulted in the development of AML. Mechanistically, marked overexpression of Hoxa cluster genes, most notably Hoxa-5, -7, -9, and -10, is linked with leukemic characteristics [89]. NUP98/JARID1A is involved with promoting expression of the Hoxa gene cluster via specifically Table 1 The roles of KDM5A in homeostasis

Model (cells/tissues/species) Mechanism Functions References
Preadipocytes C/EBPβ blocks Wnt/β-catenin activity in a demethylasedependent fashion and represses Wnt6 transcription Promoting preadipocyte differentiation [75] Reprogramming-resistant fibroblast KDM5A transcriptionally inhibits OCT4 expression Inhibiting reprogramming efficiency [71] Mouse embryonic stem cells KDM5A transcriptionally inhibits cell cycle genes Repressing cell differentiation [74][75][76] Heart KDM5A interacts with CLOCK-BMAL1 to bind to the Per2 promoter, increasing histone acetylation and enhancing transcription in a demethylase-independent fashion Activating CLOCK-BMAL1 and affecting the circadian clock [77] Natural killer cells KDM5A mediates NK cell activation through interacting with p50 to inhibit SOCS1 Activating NK cells [79] IMR-90 cells KDM5A contributes to retinoblastoma-mediated gene silencing Mediating cellular senescence [78] Drosophila KDM5 regulates immune control and gut microbiota maintenance Regulating social behavior [81] binding to their promoters, deregulating the "reader" of histone marks, resulting in the inhibition of the epigenetic program necessary for regular cell differentiation [14]. Furthermore, KDM5A mediates drug resistance to Wee1 inhibition in acute leukemia [91].

Prostate cancer (PCa)
KDM5A is upregulated in PCa tissue compared to normal prostate tissue [95]. KDM5A is also critical for the generation of drug tolerant PCa cells during chronic drug exposure [96]. In addition, KDM5A mediates reduction in methylated H3K4 and thus decreases the levels of two tumor suppression and differentiation genes KLF4 and E-cadherin, which lead to the malignancy of PCa [97]. KDM5A is also capable of promoting PCa progression  Glioblastoma KDM5A promotes cell proliferation, self-renewal, and drug resistance of GBM by regulating Hoxa-9 and -10 and transcription factor FOXM; it promotes migration and invasion of glioma cells via downregulating ZEB1 Promoting proliferation, self-renewal, and drug resistance of GBM and inhibiting the migration and invasion of glioma cells [4,99,100] Lung cancer KDM5A inhibiting the expression of p27 and TFPI2, and upregulating cyclin D1, and ITGB1; it promotes angiogenesis and EMT via Akt signaling Facilitating proliferation, motility, migration, invasion, metastasis, and drug resistance of lung cancer [13,25,101,102] Gastric cancer KDM5A represses its target genes CDKIs (p16, p21, and p27); it transactivates VEGF and promotes gastric tumorigenesis Promoting proliferation metastasis, and angiogenesis [7,8,103,104] Hepatocellular carcinoma KDM5A is negatively regulated by miR-21, and repressed cyclin-dependent kinase inhibitors (CDKIs) Promoting proliferation and inducing senescence [105,106] Renal cell carcinoma KDM5A induces stem-like cancer cells and promote RCC in demethylasedependent manner Facilitating proliferation, metastasis and inducing stemness of cancer cells [47,107] Pancreatic cancer KDM5A transcriptionally inhibits IGF2BP2 expression and MPC-1 Promoting aerobic glycolysis and cell proliferation [108,109] Melanoma KDM5A work as a tumor suppressor gene Improving the response of melanoma to PD-L1 antibody [110][111][112] Ovarian cancer KDM5A promotes cancer progression in demethylase-dependent manner Promoting proliferation EMT, and drug resistance [6,113] via the KDM5A/miRNA-495/YTHDF2/m6A-MOB3B axis [98].

Glioblastoma (GBM)
GBM is a highly lethal cancer due to its ability to infiltrate healthy brain tissue [99]. KDM5A is overexpressed in GBM compared to normal brain tissue and plays multifaced roles in GBM progression depending on the type of GBM [100]. It promotes cell proliferation, self-renewal, and drug resistance of GBM by regulating Hoxa-9 and -10 and the transcription factor FOXM1 in the temozolomide-resistant cell line A172 [99,100]. It is also documented to inhibit migration and invasion of glioma cells via downregulating ZEB1 in A172 and LN-229 cells [4].

Lung cancer
KDM5A is overexpressed in lung cancer tissues and facilitates cell proliferation, invasion, and metastasis of lung cancer via inhibiting the expression of p27 and upregulating cyclin D1, and ITGB1 [10,13,25,101,102]. KDM5A directly binds to the promoters of these three genes and transcriptionally modulated their transcripts [10]. In gefitinib-tolerant human small-cell lung cancer PC9 cells, KDM5A specifically inhibits the proliferation drug-tolerant cells without affecting their parent cells via suppressing the expression of tissue factor pathway inhibitor 2 (TFPI2) [101]. In non-small cell lung cancer cells, KDM5A promotes HIF-1α-VEGF-induced angiogenesis through Akt and induces the epithelial-mesenchymal transition via down-regulating E-cadherin expression and up-regulating N-cadherin and snail expression through activating Akt [25,102]. KDM5A also promotes tumorigenesis of small cell lung cancer suppressing target genes NOTCH1 and NOTCH2 [13].

Gastric cancer
KDM5A is overexpressed in gastric cancer and increases cell proliferation and metastasis via repressing cyclindependent kinase inhibitors (CDKIs: p16, p21, and p27) [7,8,103,104]. It also facilitates gastric cancer malignancy the through TGF-β1-(p-Smad3)-RBP2-Ecadherin-Smad3 feedback circuit [7]. The CagA-PI3K/ AKT-Sp1-RBP2-Cyclin D1 axis may also act as a possible mechanism for gastric epithelial cell malignant transformation [103]. The enhancement of gastric tumorigenesis by KDM5A was linked with transactivation of vascular endothelial growth factor (VEGF) expression and increased angiogenesis, suggesting that KDM5A overexpression and VEGF activation might play key functions in the progression of human gastric cancer [104].

Hepatocellular carcinoma (HCC)
KDM5A is a prognostic factor for disease-free survival and overall survival of HCC patients [105]. In HCC, KDM5A is negatively regulated by miR-221, and abrogating KDM5A significantly lowered cell proliferation and induced senescence of HCC cells via significantly upregulated CDKIs [106].

Renal cell carcinoma
Renal cell carcinoma (RCC) is a leading cause of death among urological cancers. KDM5A facilitates cell proliferation and metastasis via reducing methylated H3K4 [107]. Silencing KDM5A leads to the induction of apoptosis and cell cycle arrest [107]. KDM5A is also a prognostic indicator for RCC, and it promotes EMT to induce stemness in tumor cells [47].

Melanoma
In melanoma, enhancing KDM5A activity improves the response of melanoma to immune checkpoint blockade programmed cell death protein 1 antibody [110]. In addition, retinoblastoma-binding protein 2-homolog 1 (RBP2-H1), a homolog protein with 54% identity with RBP2 (KDM5A), is downregulated in malignant melanoma [111,112]. RBP2-H1 directly interacts with pRb and regulates the cell cycle and transcriptional. Loss or down-regulation of RBP2-H1 leads to uncontrolled growth and transformed malignant melanomas.

Ovarian cancer
KDM5A is also involved the progression of ovarian cancer. It promotes the proliferation and EMT of ovarian cancer and is correlated with paclitaxel resistance in SKOV3 cells [6]. MiR-421 suppresses proliferation, migration, and invasion of SKOV-3 and OVCAR-8 cells via targeting KDM5A [113].

Pharmacological Targeting of KDM5A in Cancer Therapy
KDM5A is a 2OG-and Fe 2+ -dependent demethylase [37,38]. Due to the conserved binding sites for to 2OG and Fe 2+ in JmjC demethylases [1], several 2OG competitive inhibitors and Fe 2+ chelators have exhibited inhibitory activity against KDM5A [1]. However, despite their potent anti-cancer activity in cellulo and in vivo, none of them have been approved in preclinical or clinical trials due to poor selectivity and organ toxicity [24]. The following sections discuss the main categories of KDM5A inhibitors and their pharmaceutical properties.

2OG analogs
N-Oxalylglycine (NOG, 1, Fig. 4) is a pan-inhibitor of 2OG oxygenase that acts through binding to the 2OG site and chelation of Fe 2+ with its C-1 carboxylate and amido groups [114][115][116]. NOG inhibits KDM5A inhibitor with an IC 50 value of 250 μM in vitro [5]. Compounds 2-4, containing a 2OG substrate-mimicking hydroxamic acid moiety, are also broad-spectrum JmjC-KDM inhibitors with strong in vitro activity. However, their poor selectivity and low in cellulo potency limits their further applications.

Isonicotinic acids
2,4-Pyridinedicarboxylic acid (2,4-PDCA, 5) is a pandemethylase inhibitor with an IC 50 value of 4.92 µM against KDM5A as measured using an in vitro amplified luminescence proximity homogeneous assay (AlphaScreen assay) [117]. However, 5 also exhibits poor permeability and low selectivity for KDM5A over other KDM5 demethylases [118]. Many analogues of 5 have been synthesized and their structure-activity relationships have been explored to improve the permeability and KDM5A selectivity of 5. Replacement of the hydrogen at the C5 position of 5 with different groups had a slight effect on KDM5A inhibitory activity (comparing IC 50 values among compounds 5-9, Fig. 5) [119]. Meanwhile, C6 substitutions 5 would greatly alter inhibitory activity against KDM5A (for example, comparing IC 50 values for compounds 9-11 and 13, Fig. 5). Moreover, modification of the C4 side group with ethyl or methyl would greatly affect the activity of compounds (comparing IC 50 values between 11 and 12; between 13 and 14; and among 15, 16, and 17, Fig. 5). These results indicated that having a carboxyl group at the C4 position is important for potency against KDM5A [118]. Apart from in vitro activity, the in cellulo activity of these isonicotinic acid derivatives were also studied. KDM5-C70 (14), an ethyl ester derivative of the most potent hit compound KDM5-C49 (13), is cell-permeable but also retains significant in vitro pan-KDM5 inhibitory activity. 14 showed antiproliferative activity against myeloma cells, with genome-wide increase in H3K4me3 observed [114,115]. , the corresponding amide analogue of 14, is an efficient and selective histone lysine demethylase 5 (KDM5) antagonist with IC 50 values < 72 nM for KDM5A-D [116]. This compound globally raises H3K4 methylation at transcription start sites and reduces the proliferation of multiple myeloma (MM1S) cell proliferation.

Pyrimidinones
Pyrimidine derivatives were also identified as KDM5A inhibitors in vitro and in cellulo. CPI-455 (Fig. 6, 18) is a selective and pan-KDM5 inhibitor with 10 nM potency against KDM5A [120]. In Hela cells, 18 globally increased H3K4me3 levels [120]. Like other demethylase inhibitors, 18 could also antagonize KDM5B activity (IC 50 = 3 nM). In accordance with the roles of KDM5A and KDM5B in drug resistance, 18 reduced drug-tolerance by up to 20-fold in several cancer cell models [26]. In addition, 18 showed relatively long retention time (total C max = 192 μM) and excellent oral activity (100 mg/ kg) in vivo [26]. CPI-4203 (19) is a compound structurally related to 18 but is a less potent KDM5A inhibitor in vitro [26]. Substitution of the C5 position of the o-xylene group of 19 with a chlorine group improved cell potency and pharmacokinetic profile of compound 20 [121]. An optimized compound (21) based on comprehensive structure-activity relationship analysis on the C5-and C6-positions of 18 (Fig. 6) showed slightly lower KDM5A inhibition activity (IC 50 = 15.1 nM) but improved potency against PC9 cells (EC 50 from 5.2 μM to 0.34 μM) compared to the parent compound [121]. To develop a more potent KDM5A inhibitor in cellulo,   Fig. 7), a potent and highly selective KDM5 antagonist with IC 50 values of approx. 25 nM against KDM5A-C. In MKN45 cells, 27 engaged KDM5A and induced accumulation of its substrates H3K4me2 and H3K4Me3, without affecting the levels of H3K4me1, H3K27me2, and H3K9me2/3. A further study also found that 27 has good in cellulo activity, as indicated by its ability to inhibit MKN45 cell proliferation, migration and wound healing and [122]. Liang's group also reported another pyrazole derivative, the racemic 28, with an IC 50 value of 0.260 μM (Fig. 7) from high-throughput screening. The cis-isomer (30, IC 50 = 0.16 μM) of 28 has about fivefold lower IC 50 than the trans-isomer (29, IC 50 = 0.77 μM). However, even the more potent 26 was ineffective against PC9 cells (EC 50 > 30 μM) [123]. After structure-activity relationship (SAR) analysis and optimization, a more active compound (31) was identified with in cellulo (EC 50 = 960 μM) and in vivo activity. Additionally, 31 showed improved potency, reduced lipophilicity (Log D = 1.3 at pH 7.4), and lower molecular weight compared to the lead compound 30. Two compounds (32 and 33) containing one or two pyrazole rings wares documented and patented as highly potent KDM5A inhibitors with activity in the nanomolar range [124].

Heterocyclic compounds
Some heterocyclic compounds exhibit good inhibitory activity against KDM5A. Imidazopyridine O4I3 (Fig. 8,  34) promoted the reprogramming efficiency of pioneering human induced pluripotent stem cell via inhibiting KDM5A and thus enriching H3K4me3 at the OCT4 promoter [71]. Ryuvidine (Fig. 8, 35), a KDM5A inhibitor identified by the AlphaScreen method, could prevent generation, and inhibit the growth of gefitinib-tolerant human small-cell lung cancer PC9 cells without affecting parental cells [101]. Compound 37 is an irreversible KDM5A inhibitor, derived from 36 (Fig. 8). 37 contains a (dimethylamine but-2-enamido) phenyl moiety and covalently interacts with Cys481 of KDM5A [125]. PBIT (38) inhibits KDM5A with an IC 50 of 6.01 μM in vitro. Treatment of cancer cells with 38 inhibited removal of H3K4me3, and the inhibition of proliferation was especially effective against cells expressing higher levels of KDM5A [126]. Compound 39 is a potent and specific KDM5A antagonist reported by our group. It displayed significantly greater specificity for KDM5A over KDM4A and other KDM5 family members, and is much more specific than currently described KDM5A antagonists.
In the mechanism, 39 blocked the interaction between Fig. 7 The chemical structures of pyrazoles. SAR structure-activity relationship KDM5A and its substrate H3K4me3, and thus transcriptionally upregulated the levels of p16 and p27 levels via increasing occupations of H3K4me3 on their promoters [12]. Tetrazoylhydrazide (Fig. 8, 40) was initially identified as a KDM4A inhibitor, but it also inhibits KDM5A demethylase activity with an IC 50 value of 10.4 μM [127].
40 competitively binds to KDM5A, using its tetrazole moiety as an isostere of the C5-carboxyl group of 2OG. Replacement of the terminal hydrazide of 40 almost abrogated its inhibitory activity against KDM5A, while the extension of two or three carbon atoms for the alkyl chain led to less than twofold loss in potency. Based on high-throughput screening, Gale et al. screened and identified a 3-thio-1,2,4-triazole derivative YUKA1 (Fig. 8, 41) as a KDM5A inhibitor with an in vitro IC 50 value of 2.66 ± 0.69 µM, this compound reduced gefitinib-and trastuzumab-induced drug tolerance in EGFRmutant lung cancer cells and HER2 + breast cancer cells, respectively [92]. Aminodarone derivatives (42)(43)(44) are a new class of KDM5A inhibitors with potencies in the ~ 40 μM range, which were identified using a HaloTag assay for displacement of histone H3K4me3 from PHD3 [128]. This study demonstrates the feasibility of targeting noncatalytic domains of JmjC-KDMs for the first time and opens a window for the development of allosteric demethylase inhibitors.

Metal complexes
The rhodium (III) complex 45 ( Fig. 9) is the first metalbased antagonist of KDM5A described [29]. It blocked the KDM5A-H3K4me3 interaction in human TNBC cells and increased the amplification of p27 gene promoters. With an IC 50 of 23.2 ± 1.8 nM for KDM5A, this complex was selective for KDM5A over other KDMs and additionally exhibited anti-proliferative effects towards TNBC in vivo. Interestingly, 45 exhibited more potent inhibitory activity on KDM5A in vitro and in cellulo than its congener 46.
The binding modes of KDM5A inhibitors Comprehensive efforts have been directed towards characterizing the binding modes of KDM5A interactions with diverse inhibitors. Cheng and Yan's group have explored the crystal structures of the JmjC domain of KDM5A in complex with the co-factor α-ketoglutarate (αKG), Mn 2+ , and eight KDM5 inhibitors [129]. They determined the complex structure of KDM5A-αKG-Mn 2 + and found that metal ion can binds to six ligands (side chains of His-483, Glu-485, and His-571, two oxygen atoms of αKG, and a water molecule) in an octahedral coordination sphere. In the catalytic mechanism, the final site would be bound by an O 2 molecule to begin the demethylation process via accepting a hydrogen atom from the substrate. The inhibitor 13 binds to KDM5A JmjC domain in a similar fashion to αKG, with RMS deviation of only 0.3 A˚ across 293 pairs of α-C atoms between the two KDM5A structures. The isonicotinic acid group of 13 engages the αKG binding region, with its terminal carboxyl group interacting via H-bonds with Lys-501, Tyr-409 and water, in a similar manner to αKG. Like αKG, 13 provides two ligands for metal coordination, through its pyridine and aminomethyl nitrogen atoms and generates comparable non-polar interactions with KDM5A. However, rather than engaging the 4th and 5th ligand sites as αKG does, 13 moves to sites 5 and 6, while a water molecule switches from the 6th to the 4th site. The distinct binding interactions between αKG and 13 in the bidentate interaction with the metal species results in a conformational shift of the side chain of Asn-493. In the αKG-bound conformation, Asn-493 forms a bridge between αKG and Gln-557, creating a water-free interface between αKG and Asn-493/ Gln-557. However, in the 13-bound structure, Asn-493 H-bonds with a water molecule in the 4th ligand position, while a second water molecule engages with the terminal carboxylate of the isonicotinic acid group. Superimposition of the two structures indicates that Asn-493 shifts via a 180-degree rotation of the side-chain torsional angle c1, producing an interface between 13 and Asn-493/Gln-557 that contains at least three well-structured water molecules. Apart from Asn-493, there is no significant deviation in the active site between the two complexes. All other seven compounds exhibited nearly the same interaction, underscoring the role of the Asn-493 local substructure in mediating catalysis, and also providing a molecular justification for the finding that alteration of the carboxylate (6-11 and 13) reduces inhibitory activity significantly. Inhibitor-specific interactions with R-73, Q-85, D-412, W-470, Q-535, N-575, or N-585 were recorded. This binding analysis could be employed for the future development of efficient KDM5A ligands in several ways. Firstly, the chemical space near Cys-481, which is a residue unique to the KDM5A, could be exploited. Secondly, functional groups could be added to protrude into an adjacent water-occupied groove containing multiple unique residues of KDM5A. Third, multiple branches could be combined into a single molecule. Finally, functional groups could be modified to optimize interactions with the contacted residues. Fig. 9 The chemical structures of metal complexes

Perspective and predicament for study KDM5A roles in cancers
Mounting evidence supports that KDM5A is crucial in regulating embryonic stem cell differentiation via subtly controlling gene activation and suppression [130,131]. KDM5A has variable expression statuses in different cancer tissues and its overexpression is closely related to multi-drug resistance of many cancer cells [59]. Since KDM5A can act as either as a tumor suppressor [4] or an oncogene in cancer [6,8], its functional profile in different cancer types should be mapped. KDM5A acts by demethylase-dependently regulating mRNA and miRNA transcripts and participates in cancer proliferation, metastasis, and tumorigenesis [4][5][6]132]. However, the underlying mechanisms of KDM5A in cancer stemness, drug resistance and autophagy still require further research. There are already some studies that validate the non-demethylase roles of the lysine-specific demethylases (KDMs), which means these enzymes are not just merely methylation erasers. All KDMs possess binding domains that lack catalytic activity, and their associations with chromatin or nucleosomes could be considered as a type of protein-protein or protein-nucleic acid interactions where the covalent methylation/demethylation events play a part. Thus, targeting the KDM binding domains offers potential for novel therapeutic strategies [133]. However, such an approach requires the premise of establishing a method which can selectively detect KDM activity in cells. KDM5A has been documented to promote tenascin C (TNC) expression and invasion in a demethylase-independent manner [94]. KDM5A is involved in lengthening of DICER1 via maintenance of 3′ UTR length [115]. KDM5A also exhibits its demethylaseindependent functions via formatting complexes with other proteins. For example, it promotes Per2 promoter transcription in a demethylase-independent manner through forming complex with the circadian rhythm-regulatory transcription factors CLOCK and BMAL1 [77]. KDM5A also interacts physically with HDAC complexes and reduce radiosensitivity of the HeLa and MCF-7 [134,135]. All the studies showed that blocking the interaction between KDM5A and its ligands proteins may be an alternative strategy for modulating the non-demethylase activity of KDM5A.

Perspectives for the future development of KDM5A inhibitors
Several KDM5A inhibitors have displayed promising results against cancers [26,29] Thirdly, KDM5A inhibitors are also faced with the common challenges during drug design or optimization such as improving cell permeability and stability. Finally, only inhibiting KDM5A demethylase activity may be insufficient to decrease the expression of their target genes. To address the above challenges, some solutions have already been proposed. Firstly, some non-catalytic domains have been documented to regulate KDM5A demethylase activity and blocking these less highly conserved domains can potentially abrogate the catalytic activity of KDM5A in a selective fashion. Secondly, given the enzymatic properties of KDM5A demethylase, the identification of an allosteric regulatory site and development of allosteric inhibitors is also an alternative strategy for targeting this enzyme. Thirdly, impeding the interaction between KDM5A and its partner proteins may be also a viable strategy to improve the selectivity of KDM5A inhibitors. Fourthly, to improve cell permeability and stability of KDM5A inhibitors, medicinal chemistry principles can be applied to optimize the properties of the lead compounds. Fifthly, plant-derived natural products (PDNPs) have been considered as a unique source of biologically active molecules and scaffolds for drugdiscovery [137]. Xue-fu-Zhu-Yu decoction, a famous Chinese herbal formula, has been documented to protect rats from retinal ischemia via inhibiting KDM5A and PKM2 and thus downregulating levels of HIF-1α and VEGF [137]. Results like these suggest that PDNPs may emerging trend for identifying KDM5A inhibitors for multiple cancer therapy. Finally, combination therapy is also a promising avenue to develop KDM5A inhibitors, which may accelerate inhibitors into clinic development. For example, H3K4me3 tends to be highly enriched at active promoter regions adjacent to transcriptional initiation sites. Meanwhile, histone deacetylase 1 (HDAC1) is reported to negatively regulate mitotic chromatin binding of the notch effector RBP-J in a KDM5A-dependent fashion via coregulating their downstream genes. Thus it may be feasible for an HDAC1 inhibitor to be applied synergistically with KDM5A inhibitors to weaken the viability of cancer cells [138]. Other modulators of such as EZH2, E2F4, and small ubiquitin-like modifier 2 may also be feasible for combination with KDM5A inhibitors for cancer therapy [70,80,139].
In summary, KDM5A plays important roles in mediating differentiation and gene transcription processes. KDM5A demethylase represents a unique anti-cancer target because its inhibition can be effective at impeding tumor progression and multi-drug resistance in KDM5A-amplified cancers. Continued identification of specific inhibitors of KDM5A is needed to enhance our understanding of this enzyme in cancer biology. However, although preclinical studies of KDM5A inhibitors for cancer therapy have been reported, no specific KDM5A inhibitor has been advanced into clinical trials. Potent and selective KDM5A inhibitors with druglike characteristics could potentially be used alone or in combination with other differentiation therapies [140], immunotherapeutics [141], or chromatin-targeting agents [142], hold promise for future anticancer therapy.