1 Introduction
Tumorigenesis is a complex multi-step process, which is related to the interaction between transcription factors and targets and the formation of tumor microenvironment. Most solid tumors proliferate too fast, and the structure and function of the internal vascular system are immature. When the tumor volume exceeds a certain size, it will lead to the formation of a microenvironment with insufficient blood supply and rapid hypoxia. Hypoxic tumor microenvironment not only does not promote the senescence and apoptosis of tumor cells, but also makes tumor cells more invasive and metastatic, and resists radiotherapy and chemotherapy
[1,2]. In order to adapt to the hypoxic state, tumor cells will increase the rate of glycolysis and form a multi-vascular system. Hypoxia inducible factor (HIF) mediated signal transduction is the most important biological effect of tumor cells in hypoxic microenvironment. Tumor cells maintain cell metabolism and proliferation, promote angiogenesis and nutrient uptake by regulating the expression of HIF protein, and HIF-mediated hypoxic response regulatory pathway plays an important role in tumor invasion and metastasis, tumor drug resistance, immunosuppression and maintenance of cancer stem cells
[3].
The HIF family is mainly composed of HIF-1, HIF-2 and HIF-3 subtypes, and its structure is a heterodimer composed of oxygen sensing regulated α subunits (HIF-1α, HIF-2α and HIF-3α) and structurally expressed HIF-1 β subunits (also known as aryl hydrocarbon receptor nuclear transporter, ARNT)
[4,5]. Among them, HIF-1 has a very broad spectrum of target genes, including nearly 100 target genes related to hypoxia adaptation, inflammation development and tumor growth, so it has become an important target for biological research and drug development
[6⇓⇓⇓⇓~11]. HIF-1α is the main active subunit of HIF-1, which is widely expressed in all tissues. It mediates the response of cells to oxygen levels in vivo by regulating oxygen uptake and transport, and is activated in the case of oxygen reduction or hypoxia. HIF-1 β exists in an oxygen-insensitive form. Both HIF-1α and HIF-1β belong to the bHLH-PAS protein family, and their structures are shown in Fig. 1
[12,13]. Among them, the PAS domain is essential for the formation of heterodimers of HIF-1α and HIF-1β subunits, while the bHLH domain is involved in the binding of heterodimers to hypoxia response element (HRE) -DNA sequences on target genes. In addition to the bHLH and PAS domains, two other retroactivation domains, (TAD):NH
2- terminal (N-TAD) and COOH-terminal (C-TAD), are responsible for regulating the transcription of HIF-1α target genes and preventing the degradation of HIF-1α by the histone acetyltransferases CBP and P300, respectively. In addition, N-TAD is located in the oxygen-sensitive and oxygen-dependent degradation domain (ODD), which is recognized by the ubiquitin ligase pVHL (von Hippel-Lindau) only under normoxic conditions, then undergoes ubiquitination, and triggers rapid degradation by the proteasome pathway
[14].
2 Tumor hypoxia-inducible factor signaling pathway
HIF-1 is involved in multiple signaling pathways in response to hypoxia. Among them, the most important one is the regulation of HIF-1α protein stability and protein expression level, including protein synthesis and transcriptional activity, by oxygen-dependent ubiquitin ligase pVHL.
2.1 Oxygen-dependent regulatory pathway of HIF-1
The heterodimeric HIF-1 building block, composed of HIF-1α and HIF-1β, binds to the hypoxia-responsive element HRE by activating the transcription factor acyltransferase P300, which regulates hundreds of downstream genes related to hypoxia adaptation, inflammation development and tumor growth
[15⇓⇓~18]. The activity of HIF-1α is regulated by Proline hydroxylase domain proteins (PHD) and aspartate hydroxylase Factor-inhibiting HIF (FIH). These hydroxylases are dioxygenases with oxygen and α-ketoglutarate as substrates and ferrous ions and ascorbic acid as co-acting factors. PHD includes three subtypes: PHD1, PHD2 and PHD3. PHD1 is mainly expressed in the nucleus, PHD2 is mainly expressed in the cytoplasm, and PHD3 is expressed both in the nucleus and in the cells
[19].
In the presence of sufficient oxygen, HIF-1α has a very short half-life and is degraded intracellularly via an oxygen-dependent degradation (O
2/PHDs/pVHL) pathway (as shown in Figure Synopsis A). There are two proline residues, Pro402 and Pro564, in the ODD of HIF-lα, which can be recognized and hydroxylated by PHD. The hydroxylated HIF-1α binds to the ubiquitin ligase pVHL, which in turn activates the E3-ubiquitin ligase resulting in degradation of HIF-1α
[20,21]. Meanwhile, FIH can hydroxylate the asparagine residue Asn803 in C-TAD, which can inhibit the complex binding of coactivator P300 and HIF, thus inhibiting the transcriptional activity of HIF-1 and blocking its downstream pathway
[22]. Proline hydroxylase (PHD) is the key enzyme in this pathway. In mammals, PHD2 plays a dominant role in regulating HIF-1 and is the key rate-limiting enzyme in the degradation of HIF-1α. Therefore, the design of HIF-1α inhibitors targeting PHD2 has attracted much attention
[23].
Under hypoxic conditions, the hydroxylation ability of PHD and HIF inhibitory factor FIH is significantly reduced, so the prolyl hydroxylation of HIF-1α is reduced, and the degradation pathway is blocked to achieve reversible stabilization of HIF-1α, which makes it possible for HIF-1 α and HIF-1 β (also known as ARNT) to form HIF-1 dimer units (as shown in Figure B)
[24]. Within the nucleus, the bHLH domain of the HIF-1 complex binds to the Hypoxia response element (HRE) in the promoter region of Hypoxia-responsive genes, and C-TAD recruits the coactivator CBP/P300.They jointly activate HIF-1 target genes, initiate and induce the transcription of related downstream effector genes (such as VEGF gene, GLUT gene, etc.), and ultimately trigger a variety of oxygen-tolerant adaptive responses of tissues and cells in a variety of physiological environments, such as angiogenesis, erythropoiesis and apoptosis reduction
[25]. This gene regulation process is essential for embryonic development, normal physiology and the development of many diseases, such as cancer, inflammation and tissue ischemia.
2.2 Oxygen-independent regulatory pathway of HIF-1
2.2.1 Rowth factor signal pathway
HIF-1α expression is also closely associated with two oxygen-independent signal-regulated pathways: the PI3K/Akt/mTOR pathway, the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) and mammalian target of rapamycin (mTOR) pathways, and the mitogen-activated protein kinase/extracellular regulated protein kinase (MAPK/ERK) pathway, both of which can upregulate HIF-1α protein (Figure 2). Phosphorylated eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1) activates ribosomal protein S6 (rpS6) to enhance the translation of HIF-1α mRNA through the activated PI3K/Akt/mTOR pathway
[26,27]. In the MAPK/ERK signaling pathway, ERK1 and ERK2 are phosphorylated, which ultimately enhances the translation of HIF-1α mRNA. Extracellular regulated protein kinase (ERK) is involved not only in the synthesis of HIF-1α but also in its transcriptional activity. It has been found that ERK increases the expression of HIF-1α/P300 complex by phosphorylating the coactivator CBP/P300, thereby stimulating its transcriptional activation function
[28⇓⇓~31].
2.2.2 Heat shock protein 90 signal pathway
Heat shock protein 90 (Hsp90) is also involved in the regulation of HIF-1α. Hsp90 can directly bind to HIF-1α, induce its conformational change, and bind to HIF-1β to initiate its transactivation function
[32]. In addition, Hsp90 can inhibit the degradation of HIF-1α through VHL pathway by stabilizing HIF-1α
[33]. In vitro studies of colorectal cancer, researchers have found that the expression of Hsp90 and HIF-1α is positively correlated and statistically significant, indicating that they may play a synergistic role in the occurrence, development, invasion and metastasis of colorectal cancer
[34]. In addition, Hsp90 was found to bind to the bHLH-PAS domain of HIF-1α under hypoxic conditions, activate the expression of HIF-1α and promote the growth of cancer cells in liver, pancreatic and breast cancer
[35].
2.2.3 Mdm2-p53-mediated ubiquitination and proteasomal degradation pathway
Mdm2 (Mouse double minute 2 homolog) is a ubiquitin protein ligase and p53 is a tumor suppressor gene. Under normoxic conditions, HIF-1α can bind to p53 and allow Mdm2 to mediate ubiquitination and proteasomal degradation of HIF-1α, regulating the level of HIF-1
[36,37]. In hypoxic tumors, the tumor suppressor gene p53 is mutated, which blocks Mdm2-mediated degradation of HIF-1α, resulting in accumulation of HIF-1α protein and enhanced hypoxia tolerance of tumor cells.
As one of the most important tumor suppressors in cells, the interaction between p53 and HIF-1α and their synergistic mechanism in tumor suppressor pathways have attracted more and more attention.
3 Anti-tumor small molecule inhibitors targeting HIF-1
Antitumor small molecule inhibitors acting directly or indirectly on HIF-1 can be roughly divided into the following six categories according to their mechanisms of action: HIF-1α mRNA expression, HIF-1α protein translation, HIF-1 α protein degradation, dimerization of HIF-I α and HIF-I β, HIF -1α binding to DNA, and HIF-1 transcriptional regulation. Some of these compounds have been shown to inhibit tumor growth, metastasis and angiogenesis in vitro and in vivo. The trials in which these small molecules were used alone or in combination in clinical studies of related tumors are shown in Table 1.
3.1 Reduce the expression level of HIF-1α mRNA
3.1.1 EZN-2968
EZN-2968, an RNA antagonist (fig. 3), consists of a 16-Mer locked nucleic acid (LNA) antisense oligonucleotide that specifically targets binding and hinders the expression of HIF-1α mRNA. In normoxic or hypoxic in vitro experiments, EZN-2968 showed selective and durable antagonism (IC
50=1~5 nmol/L) on HIF-1 mRNA and protein expression in prostate cancer cells (15PC3, PC3, and DU145) and glioblastoma cells (U373), resulting in effective inhibition of tumor cell growth
[38]. In vivo, administration of EZN-2968 to normal mice resulted in a significant downregulation of hepatic endogenous HIF-1 and vascular endothelial growth factor (VEGF) in a specific and dose-dependent manner. Tumors in DU145 tumor-bearing nude mice were also effectively inhibited by administration of EZN-2968. Santaris Pharmaceuticals conducted preclinical studies in colorectal cancer, kidney cancer and multiple myeloma. Roche has carried out phase I clinical studies for hepatocellular carcinoma, lymphoma and advanced solid tumors with liver metastasis, and the latest research progress has not been reported.
3.2 Reduce the expression level of HIF-1α protein
3.2.1 2-methoxyestradiol
2-Methoxyestradiol (2ME2, Panzem) is a natural product that targets microtubule formation (Figure 4A), which can directly inhibit the expression of HIF-1α and HIF-2α, induce the down-regulation of HIF-1 expression in tumor cells, and then inhibit tumor growth and angiogenesis
[39,40]. These processes are achieved by inhibiting the translation of HIF-1α. According to reports, 2ME2 has shown good inhibitory effect on prostate cancer, breast cancer and brain cancer, endometriosis and head and neck squamous cell carcinoma in vivo animal experiments
[41,42]. In addition, 2ME2 may also be used in combination with sorafenib for the treatment of liver cancer
[43]. However, clinical trials have shown that although 2ME2 alone or in combination has shown good safety, the anti-tumor efficacy in clinical trials is not ideal
[44⇓⇓~47]. Therefore, the anti-angiogenic and anti-tumor activity of 2ME2 against HIF-1 induction remains to be further studied.
3.2.2 PX-478
PX-478 is the first HIF-1α inhibitor developed by Oncothyreon to enter clinical trials (NCT 00522652) for advanced solid tumors and lymphomas (fig. 4B) by inhibiting the protein expression level of HIF-1α and the transcriptional activity of HIF-1 without affecting the p53 pathway
[48]. The mechanism of PX-478 reducing HIF-1α protein expression is mainly through reducing the level of HIF-1α mRNA expression and the translation of HIF-1 α, and secondly through inhibiting the deubiquitination of HIF-1 α
[49]. In the in vivo antitumor activity test, PX-478 showed good inhibitory activity against human non-small cell lung cancer (A549), small cell lung cancer (SHP-77), breast cancer (MCF7), colon cancer (HT29), prostate cancer (DU145, PC3) and ovarian cancer (OvCar-3) with high expression of HIF-1α
[50]. At the same time, PX-478 can also improve the radiosensitivity of tumor cells, induce cell cycle arrest in G2 phase, promote apoptosis, reduce the expression of COX-2 and PD-L1, and improve the anti-tumor effect of other drugs by combined medication
[51,52].
3.2.3 Bay 87-2243
The Bay 87-2243 compound (fig. 4C) found by Bayer in high-throughput screening can effectively inhibit the accumulation of HIF-1α and HIF-2α proteins. Under hypoxic conditions, Bay 87-2243 can effectively reduce the protein level of HIF-1α by inhibiting the activity of mitochondrial complex I and preventing the inactivation of PHD-mediated degradation of HIF-1α by ROS. At the same time, Bay 87-2243 can reduce the expression level of HIF-1α protein and HIF-1 target genes, and inhibit the growth of tumor cells
[53]. In the aspect of drug combination, the combined use of Bay 87-2243 and Vemurafenib can effectively inhibit the tumor growth of a BRAF mutant xenogeneic melanoma transplanted tumor nude mouse model,This may be related to the participation of HIF-1 in the expression of multiple glycolytic genes, and means that Bay 87-2243 will become a new direction of cancer treatment in combination with other therapies that reduce the oxygen level of cancer cells and the utilization rate of extracellular glucose
[53]. In addition, the combination of Bay 87-2243 and histone deacetylase HDAC inhibitor showed better anti-liver cancer and anti-migration effects
[54]. At present, the compound is undergoing phase I clinical studies in patients with advanced malignant tumors (NCT01297530).
3.2.4 ENMD-1198
ENMD-1198, a tubulin-binding agent (Figure 4D), is an analog of 2ME2 with antiproliferative and antiangiogenic effects
[55]. ENMD-1198 can effectively inhibit the phosphorylation of MAPK/ERK, PI3K/Akt and FAK, among which FAK is an important mediator of tumor cell invasion. ENMD-1198 can also significantly reduce the activity of HIF-1α and STAT3, which is an important transcription factor regulating VEGF-A in cancer cells, resulting in the decrease of VEGF mRNA expression. In addition, ENMD-1198 can inhibit signal transduction of multiple pathways and interfere with transcriptional regulation. For example, it inhibits tumor cell migration and cell invasion by interfering with EGF and HGF-mediated signaling
[56]. Molecular mechanism studies have shown that ENMD-1198 can disrupt HIF-1α-mediated hypoxia signal transduction in hepatocellular carcinoma cells, hinder tumor angiogenesis, and achieve the effect of inhibiting tumor proliferation
[56]. ENMD-1198 has been shown to be well tolerated and safe in humans in phase I clinical trials. Based on the pharmacokinetic results, a dose of 425 mg·m
-2·d
-1 is recommended for phase II clinical trials. Current trial data suggest that the stable phase of disease is effectively prolonged in treated patients, so further study of ENMD-1198 is of great value
[57].
3.2.5 Manassantin A and derivatives
The natural product Manassantin A (Fig. 4E) is a potent HIF-1 inhibitor, and LXY 6006, as its structurally simplified derivative (Fig. 4F), also inhibits HIF-1 activity by blocking hypoxia-induced HIF-1α accumulation in the nucleus
[58,59]. In vitro and in vivo, LXY 6006 selectively inhibited the growth of lung cancer, breast cancer and pancreatic cancer cells more effectively than Manassantin A
[59]. Manassantin A is composed of a tetrahydrofuran core region and a side chain region. Kwak et al. considered that the tetrahydrofuran core region played an important role in HIF-1 inhibition by studying the structure-activity relationship (SAR) of the five-membered ring and six-membered ring analogues of tetrahydrofuran in Manassantin A
[60]. In addition, they also reported for the first time that Manassantin A combined with EGFR inhibitor gefitinib can effectively inhibit tumor growth synergistically, with a tumor inhibition rate of 80%, which shows good therapeutic potential of combination therapy compared with the 30% tumor inhibition rate of Manassantin A or gefitinib alone.
LXY6090 (Figure 4G) is a Manassantin a derivative further optimized on the basis of LXY 6006, which is easier to synthesize in technology and has more significant anti-tumor effect in vitro and in vivo, with a IC
50 value of 4.11 nmol/L
[61]. LXY 6090 also plays an anti-tumor role by reducing the expression of HIF-1α protein, inhibiting proteasome degradation and accumulation of HIF-1α protein.
3.2.6 NNC 55-0396
NNC 55-0396 (fig. 4h), is an inhibitor of T-type Ca
2+ channels
[62]. NNC 55-0396 inhibited mitochondrial HIF-1α expression mediated by reactive oxygen species in a dose-dependent manner by inhibiting protein synthesis. NNC 55-0396 also inhibits the phosphorylation of mTOR and p70S6K and regulates the translation and expression of HIF-1α. In addition, NNC 55-0396 can reduce the stability of HIF-1α protein by regulating protein synthesis and degradation pathways, and inhibit tumor angiogenesis in vitro and in vivo. In the glioblastoma-bearing mice model, NNC 55-0396 showed a good anti-tumor effect, and the research showed that the anti-tumor mechanism of NNC 55-0396 was to inhibit the stability of HIF-1α protein through the signal transduction related to T-type Ca
2+ channels
[63]. Regulating Ca
2+ channels may also be a new way to improve the anti-tumor therapeutic effect.
3.2.7 Quinazolin-4-on
Huang et al. reported a high-throughput screening method based on the reporter gene of β-lactamase receptor mediated by HIF-1 in cells, and screened a series of compounds with HIF-1 signaling pathway inhibitory activity from 73000 compounds. Among them, Quinazolin-4-one 1 (Figure 4 I) not only has good HIF-1 inhibitory activity, but also has good druggability
[64][64,65]. Further studies showed that quinazolin-4-ones could induce rapid apoptosis of tumor cells (BJ-TERT/LT/ST/RASV12 cells) by acting on RAS-RAF-MEK-dependent signaling pathway. Oncogene RAS can stimulate the expression of HIF-1α through the RAF/MEK/ERK pathway, so the inhibitory effect of these quinazolin-4-ones on HIF-1α may be carried out through the RAS signaling pathway
[66,67].
3.2.8 Remodelin
Remodelin is a natural product that specifically targets and inhibits the N-acetyltransferase protein NAT 10 (Figure 4j). By inhibiting the activity of the N-acetyltransferase protein NAT 10, Remodelin has been shown to control cancer cell migration and cell invasion
[68⇓~70]. Studies have shown that Remodelin can significantly inhibit hypoxia-induced HIFs expression and inhibit HIFs-related angiogenesis, and the molecule itself has little toxicity, so it is a potential anti-tumor, anti-inflammatory and angiogenesis disease treatment drug. Especially in the treatment of tumors with VHL mutations (such as Renal cell carcinomas, RCC), Remodelin shows a highly effective inhibitory effect on the expression of HIFs, which makes it possible to develop a specific drug targeting VHL mutations
[71].
3.3 Small molecular compounds for promoting the degradation of HIF-1α protein
3.3.1 Indole/benzimidazole derivatives
The anti-platelet aggregating agent YC-1 (fig. 5A) was effective in reducing HIF-1 protein levels by inducing degradation of the C-terminus of HIF-1
[72]. YC-1 can also inhibit the activation of mTOR/PI3K/Akt pathway and NF-κB under hypoxic conditions, and its antitumor and antiangiogenic effects have also been demonstrated in various tumor models in vivo and in vitro
[73][74,75]. In addition to YC-1, recent studies have found that many YC-1 derivatives also show good inhibitory effect on HIF-1. The structure-activity relationship of 1,3-disubstituted indoles, the chemical skeleton of YC-1, was studied, and the furan ring was found to be the key pharmacophore for inhibiting HIF-1
[76].
3.3.2 Apigenin
Apigenin is a natural flavonoid HIF-1α inhibitor (Figure 5B). There are two main mechanisms: (1) It inhibits the phosphorylation of AKT and HDM2 by PI3K/AKT/p70S6K1 and PI3K/AKT/GSK-3 pathways, improves the stability of p53, and leads to the down-regulation of HIF-1α
[77⇓⇓~80]; (2) down-regulate the expression of HIF-1α by interfering with the function of heat shock protein Hsp 90
[81,82]. Apigenin can inhibit the expression of HIF-1α and VEGF in tumor cells under both hypoxic and normoxic conditions, so it is also an effective inhibitor of tumor angiogenesis. In addition, apigenin can eliminate the resistance of tumor to paclitaxel, and the combination of apigenin and other anticancer drugs provides a new therapeutic method for the treatment of drug-resistant tumors
[83]. Currently, preclinical trials on the absorption, metabolism, and excretion of apigenin and apigenin glycosides (NCT 03526081) have been completed.
3.3.3 Bisphenol A
Bisphenol A (BPA) is an endocrine-disrupting compound (Figure 5C) that is a xenoestrogen. Studies have shown that BPA can destabilize HIF-1α and promote the degradation of HIF-1α protein by disrupting the binding of HIF-1αand Hsp90 in hepatocellular carcinoma cells, thereby inhibiting the hypoxic response of tumor cells
[84]. The two methyl groups in the central position of BPA are critical for its binding to Hsp 90 and are the key sites for selective recognition of BPA structure by Hsp 90. Under both normoxic and hypoxic conditions, BPA can reduce the content of HIF-1α protein in Hep3B hepatoma cells, indicating that the degradation of HIF-1α by BPA is not through the pVHL-proteasome pathway, but the specific pathway is not yet clear. In addition, BPA also inhibits the induction of EPO by cobalt, which stabilizes HIF-1 protein by inhibiting the interaction between HIF-1α and ubiquitin ligase pVHL. Therefore, the degradation of HIF-1α by BPA was also enhanced in the presence of cobalt. BPA also degrades HIF-1α in the presence of proteasome inhibitor MG132, which induces HIF-1α accumulation even under normoxic conditions. These results suggest that BPA degrades HIF-1α in cells through a proteasome-independent pathway.
3.4 Inhibitors of dimerization of HIF-1α and HIF-1β
3.4.1 Acriflavine
Acriflavine (ACF), composed of 3,6-diamino-10-methylchloroacridine (Acriflavine) and 3,6-diaminoacridine (proflavine) (fig. 6a), can directly bind to the PAS-B domain of HIF-1α and HIF-2α, block their heterodimerization with HIF-1 β, and inhibit HIF-1 and HIF-2 and their transcriptional activity
[85]. By co-immunoprecipitation assay, the IC
50 of ACF inhibiting the dimerization of HIF-1 or HIF-2 was about 1 μmol/L. In hypoxic cells, ACF inhibited the binding of HIF-1α to DNA in a dose-dependent manner, and also hindered the binding of HIF-2 to VEGF and PDK1 genes. ACF also blocked hypoxia-induced VEGF and GLUT1 mRNA transcription, but did not affect HIF-1α protein accumulation. ACF inhibits tumor cell growth mainly through inhibition of angiogenesis and inhibition of HIF-1 function, and its anti-tumor effect can also be significantly enhanced when combined with sunitinib. Therefore, the combination of HIF-1α/HIF-1β dimer inhibitors with other anti-cancer drugs is expected to become a new strategy for cancer treatment
[86].
3.5 Inhibitors of HIF-1 binding to DNA
3.5.1 Doxorubicin and Daunorubicin
Doxorubicin (DXR) (fig. 6B) and Daunorubicin (DNR) (fig. 6C) are anthracycline derivatives approved by the US Food and Drug Administration (FDA) for breast cancer, bladder cancer, lymphoma, etc., and belong to the topoisomerase inhibitor family
[87][88]. In hypoxic tumor cells, DXR and DNR hinder the transcriptional activity of HIF-1 by intercalating into DNA and inducing DNA strand breaks mediated by topoisomerase II to disrupt the binding of HIF-1 to DNA. Second, DXR and DNR can also significantly reduce the levels of target genes responsible for tumor angiogenesis and growth, such as VEGF and GLUT1, in a dose-dependent manner
[89].
3.5.2 Echinomycin (NSC-13502, Echinomycin)
NSC-13502, a quinoxaline-substituted cyclic peptide antibiotic isolated from Streptomyces Acanthopanacis (Fig. 6d), is a small molecule known to bind DNA in a sequence-specific manner
[90]. NSC-13502 can inhibit the binding of HIF-1 and endogenous HRE of VEGF promoter, and then inhibit the expression of VEGF mRNA induced by hypoxia
[91]. It has been found that NSC-13502 can selectively eliminate cancer stem cells in allogeneic tumor transplantation models, which has a significant effect on leukemia and lymphoma
[92]. NSC-13502 can effectively target acute myeloid leukemia initiating cells without affecting the normal function of hematopoietic stem cells
[93].
3.6 Inhibitors of HIF-1 transcriptional activity
3.6.1 Bortezomib
Bortezomib (PS-341, Velade) is a new generation of proteasome inhibitor for the treatment of multiple myeloma and mantle cell lymphoma (Figure 7 a), which directly induces apoptosis of tumor cells by inhibiting proteasome-mediated proapoptotic and antiapoptotic protein regulation, and inhibits angiogenesis by reducing VEGF levels
[94⇓~96]. Bortezomib inhibits the transcriptional activity of HIF-1α mainly through the following three mechanisms: (1) it inhibits the transcriptional activity of HIF-1α by enhancing FIH-mediated inhibition of P300, a coactivator that plays an important role in HIF-1 α transcription
[97]; (2) down-regulating hypoxia-induced HIF-1α protein expression by inhibiting Akt/mTOR/p70S6K/S6RP pathway; (3) Blocking the phosphorylation of p44/42 MAPK, reducing the accumulation of HIF-1α in the nucleus, and reducing the transcription level of HIF-1α
[98].
3.6.2 Epidithioketopiperazine(±)-3
Block et al. reported a small molecule inhibitor, Epidithioketopiperazine (±) -3 (ETP (±) -3), that inhibits the transcriptional activation function of HIF-1α by specifically binding to the P300 CH1 domain to prevent HIF-1α from binding to the transcriptional coactivator P300/CBP (Fig. 7B)
[99]. The inhibitor is a mixture of diastereoisomers of the ETP molecule comprising two isoforms that are nearly identical in affinity and activity, capable of down-regulating transcription of cellular endogenous HIF-1α target genes. In hypoxic MCF7 cells, ETP (±) -3 inhibited the expression of LOX, a HIF-1α target gene, in a dose-dependent manner (IC
50 of approximately 300 nmol/L)
[99]. Therefore, designing small molecule inhibitors that can reduce hypoxia-induced gene expression by targeting transcriptional coactivators is also a potential and efficient way to inhibit tumor growth.
3.6.3 Indenopyrazole 2l
Minegishi et al. studied the inhibitory effect of a series of indolopyrazole compounds on the transcriptional activity of HIF-1α under hypoxic conditions by using dual-luciferase reporter gene technology
[100]. In this study, an alternative inhibitor for HIF-1 related biological studies was proposed to detect the inhibition of HIF-1 transcriptional activity in HeLa cells under hypoxic conditions, and to detect its anti-proliferative activity against human colon cancer cell line HCT 116, human liver cancer cell line HepG 2, human prostate cancer cell line PC3 and human cervical cancer cells HeLa by MTT assay. In addition, Indenopyrazole 2l (fig. 7c) was found to have the strongest inhibitory effect on hypoxia-induced HIF-1 transcriptional activity, with a IC
50 of 14 nmol/L. Indenopyrazole 2l inhibited the expression of VEGF induced by hypoxia, but had no effect on the expression of HIF-1 mRNA and the accumulation of HIF-1 protein, and had no effect on the heterodimerization of HIF-1α/HIF-1β in the nucleus.
4 Prospect
HIF-1 interacts with a variety of upstream and downstream proteins, participates in different signaling pathways, mediates hypoxic signal transduction, regulates a series of compensatory responses to hypoxic environment, such as cell metabolism, angiogenesis, tumor growth and metastasis, and plays an important role in the growth and development of the body and physiological and pathological processes. In 2019, HIF-related research achievements won the Nobel Prize in Physiology or Medicine, and hypoxia-induced targeted anti-tumor has increasingly become a research hotspot in the field of biomedicine.
In recent years, with the in-depth study of HIF-1 by researchers, more and more small molecule inhibitors have been developed, and these compounds have shown good anti-tumor effects in vivo and in vitro anti-tumor models, and some of them have successfully entered the clinical development stage. In addition, Doxorubicin and Daunorubicin, which have been marketed, have also been confirmed to have good HIF-1α inhibitory activity. Therefore, the development of small molecule inhibitors targeting HIF-1 or HIF-1α is considered to be an effective strategy for anti-tumor research.
Because the HIF-1 signaling pathway broadly intersects with many other cellular pathways, the use of HIF-1 inhibitors in combination with other anti-cancer drugs has also been demonstrated to be a highly effective therapeutic modality in anti-cancer research trials. In addition, clinical studies have found that the resistance to radiotherapy and chemotherapy is closely related to the high-level expression of HIF-1α protein. Therefore, the combination of HIF-1α inhibitors and other anti-cancer drugs can play a good role in the process of radiotherapy and chemotherapy, and effectively improve the problem of drug resistance. However, due to the complexity of signal transduction involved in HIF-1, it is still a great challenge for medicinal chemistry to develop precise and efficient HIF-1 inhibitors to achieve good therapeutic effect in the anti-tumor process while maintaining the molecular safety and druggability of drugs.
the National Natural Science Foundation of China(21772144)
the National Key R&D Program of China(2020YFA0907903)
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