Proton pump inhibitors display anti‐tumour potential in glioma

Abstract Objectives Glioma is one of the most aggressive brain tumours with poor overall survival despite advanced technology in surgical resection, chemotherapy and radiation. Progression and recurrence are the hinge causes of low survival. Our aim is to explain the concrete mechanism in the proliferation and progression of tumours based on tumour microenvironment (TME). The main purpose is to illustrate the mechanism of proton pump inhibitors (PPIs) in affecting acidity, hypoxia, oxidative stress, inflammatory response and autophagy based on the TME to induce apoptosis and enhance the sensitivity of chemoradiotherapy. Findings TME is the main medium for tumour growth and progression. Acidity, hypoxia, inflammatory response, autophagy, angiogenesis and so on are the main causes of tumour progress. PPIs, as a common clinical drug to inhibit gastric acid secretion, have the advantages of fast onset, long action time and small adverse reactions. Nowadays, several kinds of literature highlight the potential of PPIs in inhibiting tumour progression. However, long‐term use of PPIs alone also has obvious side effects. Therefore, till now, how to apply PPIs to promote the effect of radio‐chemotherapy and find the concrete dose and concentration of combined use are novel challenges. Conclusions PPIs display the potential in enhancing the sensitivity of chemoradiotherapy to defend against glioma based on TME. In the clinic, it is also necessary to explore specific concentrations and dosages in synthetic applications.

adverse reactions. 10 The first generation includes omeprazole, lansoprazole and pantoprazole; the second generation includes iprazole, rabeprazole and esomeprazole. Compared with the first-generation PPIs, in terms of drug properties, the second-generation PPIs have the advantages of higher bioavailability, less affected by food and antiacid drugs, slow plasma clearance, less first-pass effect after oral administration, higher stability, less adverse reactions and longer halflife. In clinical, the second-generation PPIs have better effects in inhibiting gastric acid secretion and H. pylori. Besides, they are more effective for relieving pain and the treatment of ulcers, especially, the coalescence of duodenal bulb ulcers. While combined with other medicines, the second-generation PPIs revealed higher security and effectiveness. [11][12][13] Progression and invasion are the main causes of death in malignant tumour patients. 14 The tumour microenvironment (TME) is a multi-complex environment regulated by various cytokines, transcription factors and pathways. 15 One major cause of the progression and invasion is set off by the intimate relationship of glioma cells with the microenvironment. Recent therapeutic anti-tumour approaches focus on such critical components of TME. 16,17 Thus, we searched Pubmed and Cnki conducting for this review.
Our aim is to explain the specific relationship between TME and the progression of glioma. PPIs, the main force for treating the stomach, were found the proficiency to promote the sensitivity of glioma to chemoradiotherapy. Thus, associating with the mechanism of PPIs, another crucial purpose is to make clear the mechanism of PPIs in hypersensitization to inhibit proliferation and progression of glioma based on TME. 18

| EPIDEMIOLOGY OF GLIOMA
Glioma is the most common and malignant primary brain tumour in the CNS caused by canceration of glial cells in the brain and spinal cord. Up to the present, glioma is the most invasive and incurable cancer. 19,20 In the worldwide, about 100,000 cases of diffuse glioma are recorded every year. The annual incidence rate is about 3-8 cases per 10,000 persons. 21 However, the incidence of glioma is increasing gradually.
The peak incidence of primary glioma is between the ages of 55 and 60 years and the incidence rate is higher in men than women. Secondary GBMs tend to affect younger individuals about age 40 years old. 22,23 The main median survival is 15 months under treatment and 5 months without treatment, respectively. 24,25 However, the patients affected by low-grade gliomas may survive for more than 20 years. 26 Up to date, the mainstay of treatment methods contains surgical excision, chemotherapy and radiotherapy. 24 According to the fifth edition of the WHO Classification of Tumours of the CNS (WHO CNS5), the standard of care and prognosis of glioma, circumscribed gliomas are usually benign and recommended for early complete resection, associated with chemotherapy if necessary. Diffuse gliomas and other high-grade gliomas according to their molecule subtype are slightly intractable, with the necessity of chemotherapy. However, for glioblastoma, feasible resection followed by radiotherapy and temozolomide chemotherapy is contained in the current standard of care. 27,28 Glioma stem cells with the characteristics of stem cells and heterogeneous resident nerve cell spheres can promote the recurrence and progression of glioma. 29 In terms of progression, there are two main methods of recurrence after surgical resection. The first is to convert the apoptosis-related factor ligand (FasL) in astrocytes into the paracrine death signal pathway of cancer cells, or effect by inhibiting the axon Pathfinder L1 cell adhesion molecule (L1CAM) to promote the growth of glioma. Serine protease inhibitors (serpins) in glioma progression can inhibit the production of fibrinolytic enzymes and ensure the survival of cancer cells by protecting cancer cells from the biological process of death and promoting the growth of vessels. 30 Moreover, with the extension of treatment time, chemoresistance occurs frequently. Also, it is inevitable for normal tissues adjacent to cancer to be damaged. 31 Due to the resistance to chemoradiotherapy and the aggravation of glioma, local recurrence and distant progression are prone to occur. 32 Moreover, because the glioma is prone to pass across the blood-brain barrier (BBB) adhering to the surface of capillaries and growing around capillaries, the delivery and penetration of therapeutic drugs passed into the brain are restricted resulting in the weakened therapeutic effects. 33 As a result, the clinical therapeutic effect of glioma is deficient.
Nowadays, it is an urgent problem to explore novel drugs with better curative effects to overcome the resistance and the physiological barriers.

| THE CLINICAL APPLICATION OF PPIS
PPIs are kinds of stomach medicine with minimal side effects, generally. 34 Clinically, they are used to remedy peptic ulcers, gastroesophageal reflux disease, zoai syndrome and upper gastrointestinal bleeding having become the first-line drugs for abnormal gastric acid secretion and related diseases. [35][36][37][38] However, different PPIs have different clinical application tactics and blood-brain barrier penetration (Table 1). [39][40][41][42] PPIs are mostly derivatives of benzimidazole compounds. 43 They are mostly weakly basic drugs with low original drug activity. After being absorbed into the blood, it is transported to the gastric mucosal parietal cells and finally reaches the acidic cavity of the secretory tube where the pH < 4. 44 The technical drug is easy to be ionized and positively charged in this environment to play a preferable role. 45 Due to the low membrane penetration, the drug is continuously aggregated and converted into the form of bioactive hyposulfonic acid and hyposulfonamide under the catalysis of acid, and then mixed with the sulfhydryl (sh) of vacuolar ATPase (V-ATPase) dehydrating and coupling to produce an irreversible covalent disulfide bond to inhibit the H + transport mechanism of the enzyme and acid secretion. 46 PPIs are also H + -K + ATPase inhibitors which may act on the acidsecreting tubules on parietal cells. The acid-secreting tubules secrete acid in the way of H + and K + exchanging through H + -K + ATPase on the membrane to pump H + out of the cells. PPIs cannot transfer the hydrogen of parietal cells to the gastric cavity and inhibit the formation of gastric acid. 44 Omeprazole has the ability to inhibit V-ATPase to change the acidic microenvironment and enhance the sensitivity of drug-resistant cells to taxol. Besides, omeprazole combined with paclitaxel can significantly reduce the tumour volume in animal models with epithelial ovarian cancer. 50 V-ATPase is overexpressed in epithelial ovarian cancer compared with normal ovarian epithelial cells. YAP (Yes-associated protein) is one of the major transcription activation factors playing an important role in regulating cell proliferation and organ development.
YAP is overexpressed and is connected with the progression and multi-drug resistance of the tumour. V-ATPase D1 also known as ATP6V0D1, is the D subunit of the V0 domain. YAP is associated with V-ATPase D1 to promote progression and MDA of the tumour.
Esomeprazole could inhibit the expression of YAP and V-ATPase D1, also the combination of YAP and V-ATPase D1 to promote the sensitivity of tumour cells to conventional anticancer drugs such as PTX. 51 In terms of breast cancer, as the effective chemotherapeutic drugs for breast cancer, raloxifene and doxorubicin combined with omeprazole, lansoprazole and pantoprazole, the viability of breast cancer cells is decreased and the apoptosis is enhanced obviously, compared with raloxifene and doxorubicin solely. 52,53

| TUMOUR MICROENVIRONMENT IS THE MAIN MEDIUM FOR THE PROLIFERATION AND PROGRESSION OF TUMOUR
Progression is the most common phenomenon for patients with glioma which may lead to death. Progression occurs when tumour cells spread from the site of origin to another part of the brain. Progression is a multifactorial process that depends on metabolic changes, gene mutations and TME. Tumour cells, cancer stem cells, cancerassociated fibroblasts (CAFs) and cytokines secreted by these cells, extracellular matrix proteins, blood vessels and various extracellular substances form a complex environment consisting of TME. 56,57 TME is a recognized significant key element affecting tumour occurrence, growth and progression considered as a unit so as to generate a dynamic communication with tumour cells. 58,59 Tumour cells can change and maintain their own survival and development conditions through autocrine and paracrine, so as to promote the growth and development of tumours. 60 TME not only includes the structure, function and metabolism of tumour tissues but is also related to the internal (nuclear and cytoplasmic) and external environment of tumour cells. 61 The main characteristics of the tumour microenvironment are acidification and hypoxia because of the imbalanced steady-state. 62 Therefore, tumours have intracellular alkaline pH and lower extracellular pH ranging from 7.2 to 7.4 and 6.5 to 7.1, respectively. 63,64 The acidic microenvironment strongly contributes to tumour progression by stimulating invasion and progression, inhibiting the immune surveillance of cancers and conferring chemoresistance. 65 The acidic extracellular environment is conducive to tissue damage and the activation of destructive enzymes in the extracellular matrix (ECM) increasing the potential for tumour progression and acquiring cell phenotype of multidrug resistance (MDR). 66 In order to maintain intracellular pH (pHi), the tumour cells have evolved powerful mechanisms to counteract cytoplasmic acidification and expel accumulated protons from cells, including Na + /H + exchanger (NHE), carbonic anhydrase, monocarboxylic acid transporter (MCT) and proton pumps. 67 Pump proton could keep the stability of H + in and outside the microenvironment, and evade the change of intracellular acidity caused by bioenergy conversion. 68 Proton pumps such as V-ATPase, NHE and the carbonic anhydrase are upregulated in cancer cells. 69 In tumours, V-ATPase can pump protons out of cells, alkalize the intracellular environment and acidify the extracellular environment, so as to resist apoptosis and promote tumour aggressiveness. 70 Increased activity of these pH regulators protects cells from changes in cell phenotype caused by pHi fluctuations by squeezing H + in the extracellular space. 71 The metastatic potential of tumour cells is affected by the relationship between tumour cells and ECM. Low pH activates and T A B L E 1 Molecular weight, dose in clinic, blood-brain barrier (BBB) permeability of pump proton inhibitors (PPIs) triggers the secretion of proteolytic enzymes including matrix metalloproteinase-2 (MMP-2), MMP-9, a tissue serine protease, adamalysin-related membrane protease, cysteine protease, cathepsin and gelatinase leading to the degradation and remodelling of ECM, so as to promote tumour invasion and progression. 72 It is reported that the acidic environment after the metabolism of tumours is not conducive to normal cells and results in the immune escape of tumour cells. 73 PPIs can directly change the quantity of T-cell receptors (TCR) or major histocompatibility complex (MHC) so as to improve the recognition and elimination of tumour cells by T cells. 74 Alkalizing the acidic environment of tumour cells can also improve the activity of other effector cells such as natural killer cells (NK) or natural killer T cells (NKT). 75 Under the acidic environment, tumour cells can use the nutrition provided by autophagy of normal cells to meet the needs of tumour cell growth. 76 PPIs can also inhibit autophagy resulting in the lack of nutritional supply for tumour cell proliferation and the death of the tumour. 77 Compared with normal cells, the tumour cells could be more adaptable to the imbalanced pH environment. 78 Because the V-ATPase complex is mainly located at the edge of tumour cells which may induce the acidification of TME and furtherly promote the growth of cancer. 79 In addition, the acidic TME provides extremely suitable living conditions for the proteolytic activity of cathepsins especially the lysosomal cathepsins, since the activity of most of the cathepsins could be enhanced in an acidic condition. 80  EMT is pivotal for wound healing, processes and its occurrence in cancer is known to aggravate invasion, migration and drug resistance in tumours. The occurrence of EMT causes the loss of epithelial characters of tumour cells and the transformation into mesenchymal-like cells and thus enhancing tumour cell proliferation and motility and decreasing cell apoptosis. 84 Transforming growth factor-β (TGF-β) has played a key role in deciding EMT. 85 V-ATPase may promote EMT induced by TGF-β. What is more, low pH is more conducive for tumour cells to produce more TGF-β and promote EMT which implies the growth of cancer. 86 Hypoxia is another major feature of TME associated with malignant progression, therapy resistance and poor prognosis of glioblastomas ( Figure 1). Hypoxia is a pathophysiological condition that generally arises as a consequence of the rapid proliferation of cancer cells as they outgrow their blood supply, therefore, depleting the cells of nutrients and available oxygen. Hypoxia always happens in latestage tumours. Hypoxic tumours are found to be highly aggressive and resistant to chemoradiotherapy because hypoxic tumours require triple the normal radiation dose to achieve the desirable cell death effect as a normal irradiating dose which means that hypoxia is also a key factor in determining the growth of tumour-induced by MDR. 87,88 Previous studies have revealed that oxygenation in glioma is 10 mmHg compared with normal brain tissue which is 40 mmHg and which may arise from radiation resistance.
Moreover, compared with normal cells, the expression of hypoxia-inducible factor-1α (HIF-1α) and vascular endothelial growth factor (VEGF) in adjacent tissues was significantly higher. 89 This results in an enhanced infiltration of cellular and plasma components into the brain, further inducing the reconstruction of TME, as well as maintaining the stem cell phenotype of tumour cells promoting progression and drug resistance. 97,98 In addition, nitric oxide synthases (NOS), widely expressed in gliomas, use L-arginine to produce primary RNS type NO that interacts with O2 À , generating ONOO À . NO is more effective in quenching superoxide and reacts with O 2 to form other nitrogen oxides such as NO 2 , as a free radical, which in turn may react with NO to yield N 2 O 3 reacting with biomolecules (lipids, proteins and DNA), potentially leading to cell death ( Figure 1). 18,99 NO plays a role in the activation of NF-κB which is a major transcription factor in glioma progression. IκB kinase-independent NF-κB activation may involve NO-induced IκB nitration. RNS may disrupt the Keap1-Nrf2 complex which can prolong the activation of Nrf2 and promote the antioxidant state inducing survival of tumour cells. 100 RNS also inhibits wild-type p53 through cysteine (Cys) oxidation and tyrosine (Tyr) nitration contributing to glioma genesis furtherly. 101 Another reason promoting progression and recurrence is MDR. A formidable obstacle for MDR is the blood-tumour barrier (BTB) and blood-brain barrier (BBB), filtering barriers of capillaries. They exclude most compounds except highly lipidized small molecules of less than 400 daltons, rendering potentially powerful anti-cancer drugs impotent for GBM treatment. Thus, breaking the BTB or BBB will significantly impact GBM treatment. 98 Actually, the BBB is composed of non-fenestrated brain endothelial cells (BECs) of the capillary wall, which is surrounded by pericytes, astrocytes, perivascular neurons, a basal membrane and an extracellular matrix, forming the highly organized neurovascular unit. 102 Besides the nitrosourea compounds carmustine (BCNU) and lomustine (CCNU) as well as the platinum agents cisplatin and carboplatin, the most widely used chemotherapeutic drug is temozolomide (TMZ), an imidazotetrazine derivative of dacarbazine. TMZ could penetrate into the CNS and has 96%-100% bioavailability. 103 However, the concentration of TMZ is far away efficient. TMZ is one of the substrates of P-glycoprotein (P-gp), an important efflux pump which locates on the apical membrane side of endothelial cells forming BBB and serves as a maintainer of the integrity and the polarity of BBB. P-gp not only hinders brain entry of a large number of xenobiotics including potentially toxic substances and therapeutic agents but also transports the compounds that have crossed the BBB back into the circulation ( Figure 2). 104 Due to the overexpression of P-gp at the BBB of glioma, only 20% of TMZ regarding a systemic dose is able to enter the cerebral parenchyma. 105 In addition, BBB also accounts for the limited efficacy of other chemotherapeutic agents in GBM, such as etoposide, irinotecan, vincristine and cisplatin. Undeniably, BBB and BTB greatly contribute to the MDR. 106 The mechanisms of MDR in tumour cells which is in the TME include four aspects: (1)  (MDR-1) expression in the cell membrane, which is responsible for expelling various drugs from tumour cells to form multidrug resistance. 109 Weakly alkaline drugs can lead to capturing ions based on being protonated in an acidic extracellular environment, which hinders the effects of anti-cancer chemotherapy drugs. In general, most chemotherapy drugs are weakly alkaline and the characteristics are prone to ionize in an acidic environment and with high polarity, so it is not suitable to pass through the cell membrane. 110,111 Once entering tumour cells, drugs are encapsulated in acidic organelles and the efficacy will be reduced or ineffective. Even several chemotherapy drugs induce the production of V-ATPase in tumours maintaining the acid environment. Lysosomal vesicle or endosomes style structures in the acid environment can further expel drugs out of cells or eliminate drugs by activating relative secretory pathways. And then these structures can be further reused to enhance drug resistance limiting the drug's effects on its molecular targets (mainly DNA). Therefore, the concentration of chemotherapeutic drugs that can play a toxic and anti-cancer role in cancer cells is still very low. 112 In the hypoxia and glucose deficient tumour microenvironment, the expression of mRNA and protein of Topo is decreased and associated with decreased DNA breakage leading to the decreased cytotoxicity of chemotherapy drugs. At the same time, the content of complexes and chemotherapy drugs-TOPO-DNA is decreased resulting in MDR. 113 The immune system's response to the tumour can impact the glioma's survival, proliferation and invasiveness. 114 Tumour-associated macrophages (TAMs) are formed by peripheral blood monocytes infiltrating into solid tumour tissues accounting for a large proportion of tumour stromal cells. TAMs are an important component of TME. It has been reported that macrophages are related to immunosuppression and immune escape. 115 In the early stages, tumour cells release chemokines to attract macrophages and other inflammatory cells reaching the extracellular matrix. Then TAMs can penetrate the basement membrane so that tumour cells escape the bondage of the basement membrane and reach the surrounding normal tissue matrix. 116 At the same time, TAMs and tumour cells can promote angiogenesis by releasing enzymes to generate angiogenesis, such as MMP2, MMP-7, MMP-9, MMP-12 and cyclooxygenase-2 (COX-2) to improve the invasiveness and motility of cells. 117,118 Neovascularization can provide nutrition and oxygen for tumour growth and provide a path for tumour cell progression. 119 TAMs also could release cytokines and growth factors directly to promote growth, such as VEGF, tumour necrosis factor-α (TNF-α) interleukin-8 (IL-8) and so on. 120 Hypoxia in TME can further induce macrophages to produce HIF-1α and promote angiogenesis. 121 CAFs exist in the stroma of tumour cells which are associated with EMT and angiogenesis. 122 Fibroblasts activate HIF-1α and NF-κB signalling pathways to stimulate oxidative stress, autophagy and glycolysis. These decomposition products create nutrient support for tumour growth. 123 Besides, CAFs produce a variety of cytokines and extracellular matrix proteins including stromal cell-derived factor 1 derived factor 1 (SDF1), hepatocyte growth factor (HGF), VEGF, platelet-derived growth factor (PDGF) and TGF-β to provide a basis for the progress and progression. 124 Mast cells (MC) secret fibroblast growth factor 2 (FGF-2), VEGF and TGF-β to promote the progress and progression. 125 A variety of cytokines produced by tumour cells can induce the proliferation of myeloid-derived suppressor cells (MDSCs), such as COX-2, IL-6 and F I G U R E 2 The mechanism of pump proton inhibitors (PPIs) for defending multidrug resistance granulocyte-macrophage colony-stimulating factor (GM-CSF) and VEGF can further enhance the immunosuppression of myeloid suppressor cells and enhance immune escape together. 126,127 This kind of neuro-inflammatory TME can lead to the loss of BBB integrity. 128 The consequences of a compromised BBB are deleteriously exposing the brain to potentially harmful concentrations of substances from the peripheral circulation, adversely affecting neuronal signalling and abnormal immune cell infiltration. 129 All of these can lead to disruption of brain homeostasis.
Autophagy also displays a key role in the growth of glioma. Autophagy is an important catabolic process of substances in cells. 130 It wraps the wrong proteins or damaged organelles through autophagy vesicles with double-layer membrane structure, fuses with lysosomes and hydrolyzes with lysosomal acid hydrolases to produce biological molecules such as amino acids which are finally reused by cells to realize the circulation of substances in cells. 131 In the early stage of tumour progression, autophagy will inhibit tumour progression. In the middle and late stages of tumour progression, autophagy will protect the tumour from stimulating and anoikis apoptosis so as to promote progression. 132 Anoikis is a special form of programmed cell death induced by the loss of contact between cells and extracellular matrix and other cells. Usually, tumour cells gather together and closely adhere to the extracellular matrix to form a 'home' for self-function and survival. When they break away from the cell adhesion matrix and lose the connection between cells, anoikis apoptosis will generate. 133 Moreover, tumours will produce extensively damaged proteins, organelles and other harmful components after chemotherapy and radiotherapy. At this time, the activity of autophagy is improved to remove harmful substances in time and provide emergency and raw materials for DNA damage repairing which may result in a poor prognosis of tumour treatment. 134 With the long-term administration of chemistry, autophagy could gather the tumour cells to assemble into blocks promoting progression. Moreover, autophagy may induce the movement of damaged proteins, mitochondria and stressors including ROS to maintain the activation of advanced glioma. 135 Chemotherapy could activate the autophagy genes like autophagy-related protein 5 (Atg5), LC3 and others involved in autophagic pathways inducing progression. 136,137 PI3K-Akt-mTOR is a key signalling pathway that is related to autophagy. 137 mTOR acts as the main regulator of autophagy containing two complexes called mammalian target of rapamycin complex 1 (mTORC1) and mammalian target of rapamycin complex 2 (mTORC2). MAPK inhibits mTORC1 activity, which leads to suppression of autophagy activating unc-51-like kinase 1 (ULK1). Under ER stress, Ca 2+ is released into the cytoplasm and triggers autophagy by activating the MAPK-TOR signal pathway. In addition, in the tumour microenvironment, cancer cells experience hypoxia resulting in the exposure of hydrophobic regions of misfolded/ unfolded proteins and accumulation largely in the ER inducing the upregulation of unfolded protein response (UPR) and the expression of autophagy genes inducing invasion and progression. In addition to canonical UPR, proteotoxicity also stimulates the selective, autophagy-dependent, removal of discrete ER domains loaded with misfolded proteins to further alleviate ER stress. These mechanisms can favour the progression and long-term survival of advanced glioma cells. 141,142 It has been evidenced that autophagy could increase the expression of NF-κB to activate MMP inducing invasion and progression. 143 MMP plays a key role in the invasion process by degrading many elements of ECM, including collagens, fibronectin and laminin. 144 It was reported that MMP is localized in vasculature cells and tumour cells of malignant astrocytomas. 145 MMP inhibition significantly decreases invasion, migration and tumour progression in advanced glioma cells. 146 All of the tumour cells would upregulate glycolysis to reduce the energy supplied by mitochondrial. Autophagy provides a source of energy in this process to promote the survival and progression of advanced glioma. 76,147 In terms of the pathological of gliomas, some mutations will indeed cause cells to continuously enter the cell cycle for mitosis, escape apoptosis, contact inhibition and immunosuppression and make cells continuously obtain energy so as to cause metabolic abnormalities and induce angiogenesis, hypoxia and necrosis of brain tumours, such as IDH mutation, H3K27M mutation, TERT mutation, MGMT mutation and so on. These mutations will activate the expression of various signal pathways and form the basis for the occurrence and development of glioma. 148

| PPIs can induce apoptosis by changing the acidic tumour microenvironment
More and more studies have proved that the acidic environment outside the tumour cells plays an important role in the development, infiltration, dissemination and drug resistance of the tumour (Figure 3). 149 PPIs inhibit the activity of V-ATPase in the acidic environment outside the gastric cancer cells to hinder the proton transport, so as to change the pH gradient of gastric cancer cells and cause cell inactivation ( Figure 3). The changed pH affects the structure and activity of almost F I G U R E 3 Mechanism of pump proton inhibitors (PPIs) inducing glioma cell damage every enzyme, then, these active enzymes affect various signals and pathways of cells. 150 For example, V-ATPase is involved in the Wnt/β-Catenin signalling pathway which promotes tumour development and progression. Pantoprazole can inhibit V-ATPase so that this pathway is blocked. 151 The concrete mechanism of V-ATPase is to maintain the balance of abnormal pH gradient inside and outside the cell. 152  The activated caspase-3 cleaves the corresponding cytoplasmic and nuclear substrates, resulting in the apoptosis of the tumour finally. 158 One research report that the rate of immune cell infiltration  162 Lactic acid could also activate the autophagy process of cells to reuse intracellular substances to maintain cell survival. 163,164 At the same time, several studies illustrate that lactic acid also inhibits the apoptosis of tumour cells by maintaining the content of NADPH and high expression of anti-apoptotic protein. 165,166 In a word, lactic acid can be the main reason leading to tumour cells escaping apoptosis. PPIs could harbour acid production to invert the proliferation induced by lactic acid. 167

| PPIs can induce apoptosis by changing the anoxic tumour microenvironment
Oxidative stress is a popular research factor to recover the tumour even the glioma. In general, the damage induced by oxidative stress is mainly due to the imbalance between the antioxidant defence system and the excessive formation of ROS. 168 In the normal functional states, the production and removal of ROS are in a dynamic balance.
However, if the production of ROS exceeds the removal as given an acute stimulation, then, the excessive free radicals will cause irreversible oxidation damage to the body. 169 The main resources of ROS are mitochondria and cytoplasm. 170 173 Studies also show that activated monocytes contacting with tumours in TME could generate a high quantity of ROS producing more impact on DNA damage, metabolic activity, membrane structure and relative protein synthesis. 174 177 It could also promote DNA to produce pyrimidine dimer. 178 In addition, modification of base groups occurs, such as acetylation which further affects protein synthesis, cytoskeleton and DNA damage repair. 179 When taking protein into consideration, ROS can break and crosslink protein or polypeptide chains resulting in protease inactivation and metabolic disorder. 180 Besides, due to the high expression of SOD and catalase enzymes in glioma; there is an accelerated conversion of superoxide to hydrogen peroxide in tumour cells which makes astrocytes particularly sensitive to damage induced by ROS. 181 PPIs have been used to modulate the pHi, disturb the mitochondrial membrane potential and produce excessive ROS leading to apoptosis ( Figure 3). 182 Several studies implied the ROS in the TME may promote more macrophages to inhibit the progression of tumour. 183 In addition, PPIs have been applied to enhance the secretion of gastrin, and then enhance the secretion of insulin by islet cells, further inhibiting glycolysis and strengthening oxidative stress. 183 Studies concluded that P-ATPase plays a crucial role in defending against ROS. 184  The acidic microenvironment can inhibit the production and function of CD4, CD25, factor Forkhead box P3 (FOXp3) and iT-regs through the PI3K/Akt/mTOR signal pathway. 241 IL-10 produced by iT-regs decreased the production of ROS in the vascular wall, the activity of NADPH and improved vascular endothelial dysfunction. 242 However, it has been proven that omeprazole and pantoprazole have the ability to reduce the iT-regs differentiation in an acidic microenvironment so as to suppress the glioma. 241

| PPIs can inhibit tumour progression by inhibiting tumour-related inflammation
Enhanced inflammation is a risk factor for many cancers. Immunosuppression caused by inflammation is one of the main reasons for poor prognosis and the short survival of glioma patients. 238 Macrophages are natural immune cells that can be found in most TMEs. 243  Several relative experiments showed that TNF-α and IL-6 declined apparently in the serum of cancer cells in mice after intragastric omeprazole which implied that PPIs inhibit the tumour progression caused by fat metabolism. 254 Besides, PPIs may decrease the quality of IL-4 directly and furtherly stimulate the production of TNF-α. 260 Omeprazole and pantoprazole have been already proved to decrease pro-inflammatory factors such as TNF and IL-6, and increase antiinflammatory factors such as IL-10 implying the potential for anti-tumour. 193 As a result, PPIs may give play to anti-inflammatory and cancer clearance. Furthermore, PPIs could transfer M2 macrophages to M1 macrophages to fight cancer. 261  The mammalian target of rapamycin (mTOR) is a highly conservative serine/threonine-protein kinase that belongs to the PI3K-related protein kinase family. 112 MTOR can phosphorylate Atg13 preventing it from interacting with complexes containing Atg1 and Atg17, and inhibiting autophagy assembly so as to reduce radiation resistance. 265 The activation of mTOR could induce aerobic glycolysis through upre-  270 This process could generate excessive ROS leading to lysosomal chamber instability which will affect mitochondrial membrane potential and mitochondrial energy supplement to suppress hypoxia and autophagy in advanced glioma promoting the anti-glioma. 270 In addition, more ROS will promote the release of pro-apoptotic molecules into the cytoplasm and further promote autophagy. 271 It has been reported that PPIs can induce autophagy by activating ERK and JNK and inhibit mTOR signalling by activating MAPK. 201,272 There is also evidence showing that PPIs mediate the strong upregulation of Beclin-1 by activating NF-κB which is responsible for the ROSinduced autophagy making the foundation for anti-tumour. 273,274 Besides, PPIs could moderate the acidic microenvironment to inhibit the activation of lysosomal cathepsins to suppress MMP increasing sensitivity to TMZ. Finally, the progression of the tumour was inhibited. 275 PPIs may activate MAPK kinase, reduce blood glucose levels and block glycolysis induced by TNF-α so as to invert the Warburg effect which could activate the autophagy promoting progression. 276

| PPIS SENSITIZE CHEMORADIOTHERAPY PROMOTING APOPTOSIS
Chemoradiotherapy resistance is an important element to early recurrence, treatment failure and dismal prognosis of glioblastoma. 277 With growing amounts of evidence that chemoradiotherapy may induce the more normal cells to glioma leading to recurrence and progression. 278,279 Even if the concentration of drugs entering plasma and cerebrospinal fluid is enough to inhibit the progression of the tumour, the drug resistance and progression of glioma also happen from time to time. 280 Thus, hoping to make use of PPIs and chemoradiotherapy together to play a good effect.
The radiation may lead to the mutation of mitochondrion respiratory chain complex I and II and DNA which will lead to more ROS in glioma. 281 Excessive ROS could damage DNA containing mitochondrion DNA leading to respiratory chain dysfunction, even higher levels of free radicals in the cells, and the disbalance of O 2 À in and outside the cells, and, further, block oxygen supplement. 282 In addition, ionizing radiation (IR) will produce more free radicals which combine with oxygen and further lead to permanent DNA damage. 283 Thus, radiotherapy can irreversibly damage DNA through reactive oxygen species. However, the tolerance of normal cells to radiation dose is limited, and the radiation dose cannot be increased continuously. 284 Hypoxic tumour cells continue to exist after radiation and transform into a more invasive phenotype. 19 PPIs such as esomeprazole could generate more ROS persistently to make up for the deficiency of oxidative stress caused by limited radiation. 285 The inhibitory effect of many chemotherapeutic drugs depends on the oxygen partial pressure of the TME. In general, the sensitivity of chemotherapy was the strongest at the peak of oxygen partial pressure. 286,287 Because of the ability to enhance oxygen partial pressure through enhancing oxidative stress, PPIs could increase the sensitivity to chemotherapy. However, we need to explore the dose and concentration of different PPIs with the maximum oxygen partial pressure in TME.
The inverted pH gradient of cell membrane inside and outside tumour is an important mechanism leading to drug resistance. 288 The acidic extracellular microenvironment makes a chemical and physical shelter for the anti-tumour effect of weakly alkaline chemotherapeutic drugs. Once the chemotherapeutic drugs are protonated and alkalized, they cannot pass through the plasma membrane. 241,272 Chemotherapeutic drugs in tumour cells enter lysosomes, acidic organelles, and acidic vesicles through protonation and neutralization, and are further discharged out of cells. 289  PPIs can prevent the occurrence of EMT by interfering with the expression of the TGF-β/Smad signal pathway and NF-κB in the acidic microenvironment. [296][297][298][299] It also has been proved that rabeprazole administration induced cell death and reduced cell migration together with EMT by inhibiting Akt and Gsk-3β phosphorylation, which in turn suppressed the EMT. In addition to molecular aberrations and hypoxia, pump protons may facilitate epithelial to mesenchymal transition by modulating the pH of the TME. We found rabeprazole could play a better role in the unbuffered acidic environment to inhibit EMT. 300 What is more, pantoprazole inhibits breast cancer resistance protein (BCRP) and has a chemo-sensitizing activity, thereby contributing to improved delivery of imatinib (a kind of drug for treating leukaemia) into the CNS. 299 It has been proved that treatment of the cancer cells with the PPIs resulted in order of magnitude reduction in the halfmaximal inhibitory concentration (IC 50 ) values. 301 Esomeprazole enhances radiosensitivity in radiation-resistant squamous cell carcinoma of the head and neck. 285 PPIs could also increase chemotherapeutic drug uptake by the tumour cells. 302 PPIs, as the small molecules, could pass through BBB and BTB (Table 1).
In the clinic, PPIs have been proven the ability to overcome the resistance of conventional chemotherapeutic drugs for breast cancer and radiation. The use of PPIs in 6754 patients suffering from breast cancer significantly not only improved the overall survival rate of these patients and reduced the disease recurrence rate but overcome the resistance of conventional chemotherapy drugs and radiation. 303 Besides, based on clinical epidemiological comparative trials, PPIs can assist radiotherapy and chemotherapy to delay the overall survival of 206 patients with rectal cancer. 56 What is more, Omeprazole not only improved the effect of radiotherapy and chemotherapy but also delayed the recurrence and complications of 125 rectal cancer patients. 304 The combined use of PPIs in the treatment of laryngeal cancer can reduce the incidence of pharyngeal reflux and mucositis. 305 Although there has been no relative research studying the synergistic effect of chemoradiotherapy and PPIs in glioma, based on the above-mentioned relevant clinical epidemiological study about the administration of PPIs in other cancers and preclinical pharmacologic or toxicologic metabolism and mechanism analysis. Of course, improper dosage selection of PPIs and chemoradiotherapy will also lead to the aggravation of adverse reactions. A clinical study showed that the inappropriate dose of PPIs led to the further aggravation of radiation pneumonia after radiotherapy for lung cancer. 306 In general, PPIs may be a potential valuable anti-cancer drug to promote chemoradiotherapy sensitivity so as to block the growth of glioma.

| THE ACUTE SIDE EFFECT OF PPIS AND THE NECESSITY OF COMBINING PPIS WITH CHEMORADIOTHERAPY
The basic side effects of PPIs contain nausea, 307 diarrhoea or headache, 308 and even more serious side effects such as subacute cutaneous lupus erythematosus, 309 and interstitial nephritis and pneumonia. 310 After a long period of the epidemiological follow-up survey, patients with long-term use of PPIs may get complications of suppurative liver abscess, pulmonary tuberculosis and inflammatory bowel disease. 311 Inflammatory mediators can cause metabolic disorders as being with PPIs for a long time, such as IL-1, IL-6, TNF-α, INF-γ and prostaglandin E2 (PGE2) which may induce an increase in muscle decomposition and a decrease of muscle synthesis lead to the decrease of muscle mass and muscle strength, even furtherly subside with decreased physical function, polymyositis and rhabdomyolysis. 312 Besides, mitochondrial would be impaired and intestinal microbial would change based on the proximal pH of the intestine increases. 313 It has been proved that PPIs elevate the pH of the stomach and upper gut causing more bacteria, even pathogenic bacteria, to survive in the gastrointestinal tract and enter the gut gradually. 314 Vitamin B12, vitamin C, vitamin D, magnesium, calcium, iron, β-carotene and zinc will be reduced due to the decreased relative quality of food release and body absorption. [315][316][317][318][319] Suppressing magnesium may inhibit the absorption of Vitamin D implying the release of adipocytokine which is involved in the regulation of glucose levels and fatty acid breakdown inducing insulin resistance so that the appearance of obesity and contribute to an increase of the inflammatory state. 315 Based on a reported real case, PPIs may be associated with several mental diseases. 320 Thus, it is urgent to design specific and efficient combined medication or united anti-glioma therapies to treat glioma.

| CONCLUSIONS AND FUTURE DIRECTIONS
The tumour microenvironment is the main medium for proliferation, progression and multidrug resistance of tumours. Acidity and hypoxia, as well as the relative inflammatory response and autophagy, are the main influencing factors for tumour progression and progression in TME. PPIs could promote apoptosis and sensitize chemoradiotherapy through altering the release of inflammatory factors and cytokines of inflammatory cells, autophagy and promoting oxidative stress based on NF-κB, MAPK, Keap1/NRF/ARE, PI3K/Akt signal pathways in TME.
PPIs could be considered as an adjuvant treatment strategy to be combined with medication to fight glioma. In order to better predict the therapeutic effect, COX can be considered a prognostic indicator.
In terms of the practical application scope of PPIs nowadays,

ACKNOWLEDGMENT
The authors acknowledge and appreciate all the collegues for their valuable efforts on this paper.

CONFLICT OF INTEREST
The authors declare no conflicts of interest.

DATA AVAILABILITY STATEMENT
Data sharing is not applicable to this article as no new data were created or analyzed in this study.