Selective inhibition of ion transport mechanisms regulating intracellular pH reduces proliferation and induces apoptosis in cholangiocarcinoma cells

Clinical review of alkalization therapy in cancer treatment

One of the most unique characteristics of cancer metabolism is activated aerobic glycolysis, which is called the “Warburg effect”, and is a hallmark of cancer. An acidic tumor microenvironment (TME) resulting from activated anaerobic glycolysis is associated with cancer progression, multi-drug resistance, and immune escape. Several in vitro and in vivo studies reported that neutralization of the acidic TME by alkalizing agents, such as bicarbonate, resulted in the suppression of cancer progression and a potential benefit for anti-cancer drug responses. In clinical settings, alkalizing effects were achieved not only by alkalizing agents, but also by a following a particular diet. An epidemiological study demonstrated that more fruits and vegetables and less meat and dairy products are associated with an increase in urine pH, which may reflect the alkalizing effect on the body. However, it remains unclear whether alkaline dietary intervention improves the effects of cancer treatment. Moreover, there are few clinical reports to date regarding cancer treatments being performed on patients together with alkalization therapy. In this review, we investigated whether alkalization therapy, which includes an alkaline diet and/or alkalizing agents, improves cancer treatment.

The positive feedback loop of NHE1-ERK phosphorylation mediated by BRAFV600E mutation contributes to tumorigenesis and development of glioblastoma

The v-raf murine sarcoma viral oncogene homolog B1 (BRAF) activating mutation V600E (BRAFV600E) is involved in glioblastoma multiforme (GBM). Na/H exchanger 1 (NHE1), a main pH regulator affecting cell microenvironment, is hyper-expressed in GBM. However, the relationship between BRAFV600E signal pathway and NHE1 in GMB cells remains unclear. This study found that NHE1 was a downstream target of BRAFV600E and an upstream factor of extracellular signal-regulated kinase (ERK). In addition, there was a positive feedback loop between NHE1-ERK phosphorylation under regulation of BRAFV600E mutation contributing to the proliferation and invasion of GBM cells. Moreover, the proliferation and invasion abilities of BRAFV600E-mutant and BRAF wild type GBM cells were all suppressed by the NHE1 inhibitor, BRAFV600E inhibitor and combination of them. The inhibitory effect of combination of the two inhibitors was better than each single drug both in vitro and in vivo. Combination of BRAFV600E and NHE1 inhibitors could be considered as a new therapeutic regimen for GBM, especially for GBM with BRAFV600E.

Alkaline intracellular pH (pHi) activates AMPK-mTORC2 signaling to promote cell survival during growth factor limitation

The mechanistic target of rapamycin (mTOR) complex 2 (mTORC2) signaling controls cell metabolism, promotes cell survival, and contributes to tumorigenesis, yet its upstream regulation remains poorly defined. Although considerable evidence supports the prevailing view that amino acids activate mTOR complex 1 but not mTORC2, several studies reported paradoxical activation of mTORC2 signaling by amino acids. We noted that after amino acid starvation of cells in culture, addition of an amino acid solution increased mTORC2 signaling. Interestingly, we found the pH of the amino acid solution to be alkaline, ∼pH 10. These observations led us to discover and demonstrate here that alkaline intracellular pH (pHi) represents a previously unknown activator of mTORC2. Using a fluorescent pH-sensitive dye (cSNARF1-AM) coupled with live-cell imaging, we demonstrate that culturing cells in media at an alkaline pH induces a rapid rise in the pHi, which increases mTORC2 catalytic activity and downstream signaling to the pro-growth and pro-survival kinase Akt. Alkaline pHi also activates AMPK, a canonical sensor of energetic stress. Functionally, alkaline pHi activates AMPK-mTOR signaling, which attenuates apoptosis caused by growth factor withdrawal. Collectively, these findings reveal that alkaline pHi increases mTORC2- and AMPK-mediated signaling to promote cell survival during conditions of growth factor limitation, analogous to the demonstrated ability of energetic stress to activate AMPK–mTORC2 and promote cell survival. As an elevated pHi represents an underappreciated hallmark of cancer cells, we propose that the alkaline pHi stress sensing by AMPK–mTORC2 may contribute to tumorigenesis by enabling cancer cells at the core of a growing tumor to evade apoptosis and survive.

Disruption of pH Dynamics Suppresses Proliferation and Potentiates Doxorubicin Cytotoxicity in Breast Cancer Cells

The reverse pH gradient is a major feature associated with cancer cell reprogrammed metabolism. This phenotype is supported by increased activity of pH regulators like ATPases, carbonic anhydrases (CAs), monocarboxylate transporters (MCTs) and sodium-proton exchangers (NHEs) that induce an acidic tumor microenvironment, responsible for the cancer acid-resistant phenotype. In this work, we analyzed the expression of these pH regulators and explored their inhibition in breast cancer cells as a strategy to enhance the sensitivity to chemotherapy. Expression of the different pH regulators was evaluated by immunofluorescence and Western blot in two breast cancer cell lines (MDA-MB-231 and MCF-7) and by immunohistochemistry in human breast cancer tissues. Cell viability, migration and invasion were evaluated upon exposure to the pH regulator inhibitors (PRIs) concanamycin-A, cariporide, acetazolamide and cyano-4-hydroxycinnamate. Additionally, PRIs were combined with doxorubicin to analyze the effect of cell pH dynamic disruption on doxorubicin sensitivity. Both cancer cell lines expressed all pH regulators, except for MCT1 and CAXII, only expressed in MCF-7 cells. There was higher plasma membrane expression of the pH regulators in human breast cancer tissues than in normal breast epithelium. Additionally, pH regulator expression was significantly associated with different molecular subtypes of breast cancer. pH regulator inhibition decreased cancer cell aggressiveness, with a higher effect in MDA-MB-231. A synergistic inhibitory effect was observed when PRIs were combined with doxorubicin in the breast cancer cell line viability. Our results support proton dynamic disruption as a breast cancer antitumor strategy and the use of PRIs to boost the activity of conventional therapy.

The role of hypoxia in the tumor microenvironment and development of cancer stem cell: a novel approach to developing treatment

Hypoxia is a common feature of solid tumors, and develops because of the rapid growth of the tumor that outstrips the oxygen supply, and impaired blood flow due to the formation of abnormal blood vessels supplying the tumor. It has been reported that tumor hypoxia can: activate angiogenesis, thereby enhancing invasiveness and risk of metastasis; increase survival of tumor, as well as suppress anti-tumor immunity and hamper the therapeutic response. Hypoxia mediates these effects by several potential mechanisms: altering gene expression, the activation of oncogenes, inactivation of suppressor genes, reducing genomic stability and clonal selection. We have reviewed the effects of hypoxia on tumor biology and the possible strategiesto manage the hypoxic tumor microenvironment (TME), highlighting the potential use of cancer stem cells in tumor treatment.

Fluorescein Derivatives as Fluorescent Probes for pH Monitoring along Recent Biological Applications

Potential of hydrogen (pH) is one of the most relevant parameters characterizing aqueous solutions. In biology, pH is intrinsically linked to cellular life since all metabolic pathways are implicated into ionic flows. In that way, determination of local pH offers a unique and major opportunity to increase our understanding of biological systems. Whereas the most common technique to obtain these data in analytical chemistry is to directly measure potential between two electrodes, in biological systems, this information has to be recovered in-situ without any physical interaction. Based on their non-invasive optical properties, fluorescent pH-sensitive probe are pertinent tools to develop. One of the most notorious pH-sensitive probes is fluorescein. In addition to excellent photophysical properties, this fluorophore presents a pH-sensitivity around neutral and physiologic domains. This review intends to shed new light on the recent use of fluorescein as pH-sensitive probes for biological applications, including targeted probes for specific imaging, flexible monitoring of bacterial growth, and biomedical applications.

Targeting the pH Paradigm at the Bedside: A Practical Approach

The inversion of the pH gradient in malignant tumors, known as the pH paradigm, is increasingly becoming accepted by the scientific community as a hallmark of cancer. Accumulated evidence shows that this is not simply a metabolic consequence of a dysregulated behavior, but rather an essential process in the physiopathology of accelerated proliferation and invasion. From the over-simplification of increased lactate production as the cause of the paradigm, as initially proposed, basic science researchers have arrived at highly complex and far-reaching knowledge, that substantially modified that initial belief. These new developments show that the paradigm entails a different regulation of membrane transporters, electrolyte exchangers, cellular and membrane enzymes, water trafficking, specialized membrane structures, transcription factors, and metabolic changes that go far beyond fermentative glycolysis. This complex world of dysregulations is still shuttered behind the walls of experimental laboratories and has not yet reached bedside medicine. However, there are many known pharmaceuticals and nutraceuticals that are capable of targeting the pH paradigm. Most of these products are well known, have low toxicity, and are also inexpensive. They need to be repurposed, and this would entail shorter clinical studies and enormous cost savings if we compare them with the time and expense required for the development of a new molecule. Will targeting the pH paradigm solve the "cancer problem"? Absolutely not. However, reversing the pH inversion would strongly enhance standard treatments, rendering them more efficient, and in some cases permitting lower doses of toxic drugs. This article's goal is to describe how to reverse the pH gradient inversion with existing drugs and nutraceuticals that can easily be used in bedside medicine, without adding toxicity to established treatments. It also aims at increasing awareness among practicing physicians that targeting the pH paradigm would be able to improve the results of standard therapies. Some clinical cases will be presented as well, showing how the pH gradient inversion can be treated at the bedside in a simple manner with repurposed drugs.

Towards an Integral Therapeutic Protocol for Breast Cancer Based upon the New H+-Centered Anticancer Paradigm of the Late Post-Warburg Era

A brand new approach to the understanding of breast cancer (BC) is urgently needed. In this contribution, the etiology, pathogenesis, and treatment of this disease is approached from the new pH-centric anticancer paradigm. Only this unitarian perspective, based upon the hydrogen ion (H+) dynamics of cancer, allows for the understanding and integration of the many dualisms, confusions, and paradoxes of the disease. The new H+-related, wide-ranging model can embrace, from a unique perspective, the many aspects of the disease and, at the same time, therapeutically interfere with most, if not all, of the hallmarks of cancer known to date. The pH-related armamentarium available for the treatment of BC reviewed here may be beneficial for all types and stages of the disease. In this vein, we have attempted a megasynthesis of traditional and new knowledge in the different areas of breast cancer research and treatment based upon the wide-ranging approach afforded by the hydrogen ion dynamics of cancer. The concerted utilization of the pH-related drugs that are available nowadays for the treatment of breast cancer is advanced.

The impact of tumour pH on cancer progression: strategies for clinical intervention

Dysregulation of cellular pH is frequent in solid tumours and provides potential opportunities for therapeutic intervention. The acidic microenvironment within a tumour can promote migration, invasion and metastasis of cancer cells through a variety of mechanisms. Pathways associated with the control of intracellular pH that are under consideration for intervention include carbonic anhydrase IX, the monocarboxylate transporters (MCT, MCT1 and MCT4), the vacuolar-type H⁺-ATPase proton pump, and the sodium-hydrogen exchanger 1. This review will describe progress in the development of inhibitors to these targets.

Downstream Effectors of ILK in Cisplatin-Resistant Ovarian Cancer

Despite good responses to first-line treatment with platinum-based combination chemotherapy, most ovarian cancer patients will relapse and eventually develop platinum-resistant disease with poor prognosis. Although reports suggest that integrin-linked kinase (ILK) is a potential target for ovarian cancer treatment, identification of ILK downstream effectors has not been fully explored. The purpose of this study was to investigate the molecular and biological effects of targeting ILK in cisplatin-resistant ovarian cancer. Western blot analysis showed that phosphorylation levels of ILK were higher in cisplatin-resistant compared with cisplatin-sensitive ovarian cancer cells. Further immunohistochemical analysis of ovarian cancer patient samples showed a significant increase in phosphorylated ILK levels in the tumor tissue when compared to normal ovarian epithelium. Targeting ILK by small-interfering RNA (siRNA) treatment reduced cisplatin-resistant cell growth and invasion ability, and increased apoptosis. Differential gene expression analysis by RNA sequencing (RNA-Seq) upon ILK-siRNA transfection followed by Ingenuity Pathway Analysis (IPA) and survival analysis using the Kaplan-Meier plotter database identified multiple target genes involved in cell growth, apoptosis, invasion, and metastasis, including several non-coding RNAs. Taken together, results from this study support ILK as an attractive target for ovarian cancer and provide potential ILK downstream effectors with prognostic and therapeutic value.

Carbon Dots for Intracellular pH Sensing with Fluorescence Lifetime Imaging Microscopy

The monitoring of intracellular pH is of great importance for understanding intracellular trafficking and functions. It has various limitations for biosensing based on the fluorescence intensity or spectra study. In this research, pH-sensitive carbon dots (CDs) were employed for intracellular pH sensing with fluorescence lifetime imaging microscopy (FLIM) for the first time. FLIM is a highly sensitive method that is used to detect a microenvironment and it can overcome the limitations of biosensing methods based on fluorescence intensity. The different groups on the CDs surfaces changing with pH environments led to different fluorescence lifetime values. The CDs aqueous solution had a gradual change from 1.6 ns to 3.7 ns in the fluorescence lifetime with a pH range of 2.6-8.6. Similar fluorescence lifetime changes were found in pH buffer-treated living cells. The detection of lysosomes, cytoplasm, and nuclei in living cells was achieved by measuring the fluorescence lifetime of CDs. In particular, a phasor FLIM analysis was used to improve the pH imaging. Moreover, the effects of the coenzymes, amino acids, and proteins on the fluorescence lifetime of CDs were examined in order to mimic the complex microenvironment inside the cells.

A New and Integral Approach to the Etiopathogenesis and Treatment of Breast Cancer Based upon Its Hydrogen Ion Dynamics

Despite all efforts, the treatment of breast cancer (BC) cannot be considered to be a success story. The advances in surgery, chemotherapy and radiotherapy have not been sufficient at all. Indeed, the accumulated experience clearly indicates that new perspectives and non-main stream approaches are needed to better characterize the etiopathogenesis and treatment of this disease. This contribution deals with how the new pH-centric anticancer paradigm plays a fundamental role in reaching a more integral understanding of the etiology, pathogenesis, and treatment of this multifactorial disease. For the first time, the armamentarium available for the treatment of the different types and phases of BC is approached here from a Unitarian perspective-based upon the hydrogen ion dynamics of cancer. The wide-ranged pH-related molecular, biochemical and metabolic model is able to embrace most of the fields and subfields of breast cancer etiopathogenesis and treatment. This single and integrated approach allows advancing towards a unidirectional, concerted and synergistic program of treatment. Further efforts in this line are likely to first improve the therapeutics of each subtype of this tumor and every individual patient in every phase of the disease.

(Patho-)Physiology of Na+/H+ Exchangers (NHEs) in the Digestive System

Na⁺/H⁺ exchangers (NHEs) are expressed in virtually all human tissues and organs. Two major tasks of those NHE isoforms that are located in plasma membranes are cell volume control by Na⁺-uptake and cellular pH regulation by H⁺-extrusion. Several NHEs, particularly NHE 1–4 and 8, are involved in the pathogenesis of diseases of the digestive system such as inflammatory bowel disease (ulcerative colitis, Crohn’s disease) and gastric and colorectal tumorigenesis. In the present review, we describe the physiological purposes, possible malfunctions and pathophysiological effects of the different NHE isoforms along the alimentary canal from esophagus to colon, including pancreas, liver and gallbladder. Particular attention is paid to the functions of NHEs in injury repair and to the role of NHE1 in Barrett’s esophagus. The impact of NHEs on gut microbiota and intestinal mucosal integrity is also dealt with. As the hitherto existing findings are not always consistent, sometimes even controversial, they are compared and critically discussed.

Targeting Tumor Microenvironment by Small-Molecule Inhibitors

The tumor microenvironment (TME) is a hypoxic, acidic, and immune/inflammatory cell–enriched milieu that plays crucial roles in tumor development, growth, progression, and therapy resistance. Targeting TME is an attractive strategy for the treatment of solid tumors. Conventional cancer chemotherapies are mostly designed to directly kill cancer cells, and the effectiveness is always compromised by their penetration and accessibility to cancer cells. Small-molecule inhibitors, which exhibit good penetration and accessibility, are widely studied, and many of them have been successfully applied in clinics for cancer treatment. As TME is more penetrable and accessible than tumor cells, a lot of efforts have recently been made to generate small-molecule inhibitors that specifically target TME or the components of TME or develop special drug-delivery systems that release the cytotoxic drugs specifically in TME. In this review, we briefly summarize the recent advances of small-molecule inhibitors that target TME for the tumor treatment.

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Tumor microenvironment (TME) is an indispensable part of tumor and is an important therapeutic target.

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TME is more penetrable and accessible than tumor cell area.

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Small-molecule inhibitors that target TME are very promising.

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The target efficiency can be improved by specific deliver and release systems.

Hydrogen Ion Dynamics of Cancer and a New Molecular, Biochemical and Metabolic Approach to the Etiopathogenesis and Treatment of Brain Malignancies

The treatment of cancer has been slowly but steadily progressing during the last fifty years. Some tumors with a high mortality in the past are curable nowadays. However, there is one striking exception: glioblastoma multiforme. No real breakthrough has been hitherto achieved with this tumor with ominous prognosis and very short survival. Glioblastomas, being highly glycolytic malignancies are strongly pH-dependent and driven by the sodium hydrogen exchanger 1 (NHE1) and other proton (H⁺) transporters. Therefore, this is one of those pathologies where the lessons recently learnt from the new pH-centered anticancer paradigm may soon bring a promising change to treatment. This contribution will discuss how the pH-centric molecular, biochemical and metabolic perspective may introduce some urgently needed and integral novel treatments. Such a prospective therapeutic approach for malignant brain tumors is developed here, either to be used alone or in combination with more standard therapies.

Causes, consequences, and therapy of tumors acidosis

While cancer is commonly described as "a disease of the genes," it is also associated with massive metabolic reprogramming that is now accepted as a disease "Hallmark." This programming is complex and often involves metabolic cooperativity between cancer cells and their surrounding stroma. Indeed, there is emerging clinical evidence that interrupting a cancer's metabolic program can improve patients' outcomes. The most commonly observed and well-studied metabolic adaptation in cancers is the fermentation of glucose to lactic acid, even in the presence of oxygen, also known as "aerobic glycolysis" or the "Warburg Effect." Much has been written about the mechanisms of the Warburg effect, and this remains a topic of great debate. However, herein, we will focus on an important sequela of this metabolic program: the acidification of the tumor microenvironment. Rather than being an epiphenomenon, it is now appreciated that this acidosis is a key player in cancer somatic evolution and progression to malignancy. Adaptation to acidosis induces and selects for malignant behaviors, such as increased invasion and metastasis, chemoresistance, and inhibition of immune surveillance. However, the metabolic reprogramming that occurs during adaptation to acidosis also introduces therapeutic vulnerabilities. Thus, tumor acidosis is a relevant therapeutic target, and we describe herein four approaches to accomplish this: (1) neutralizing acid directly with buffers, (2) targeting metabolic vulnerabilities revealed by acidosis, (3) developing acid-activatable drugs and nanomedicines, and (4) inhibiting metabolic processes responsible for generating acids in the first place.

Rising horizon in circumventing multidrug resistance in chemotherapy with nanotechnology

Multidrug resistance (MDR) is one of the key barriers in chemotherapy, leading to the generation of insensitive cancer cells towards administered therapy. Genetic and epigenetic alterations of the cells are the consequences of MDR, resulted in drug resistivity, which reflects in impaired delivery of cytotoxic agents to the cancer site. Nanotechnology-based nanocarriers have shown immense shreds of evidence in overcoming these problems, where these promising tools handle desired dosage load of hydrophobic chemotherapeutics to facilitate designing of safe, controlled and effective delivery to specifically at tumor microenvironment. Therefore, encapsulating drugs within the nano-architecture have shown to enhance solubility, bioavailability, drug targeting, where co-administered P-gp inhibitors have additionally combat against developed MDR. Moreover, recent advancement in the stimuli-sensitive delivery of nanocarriers facilitates a tumor-targeted release of the chemotherapeutics to reduce the associated toxicities of chemotherapeutic agents in normal cells. The present article is focused on MDR development strategies in the cancer cell and different nanocarrier-based approaches in circumventing this hurdle to establish an effective therapy against deadliest cancer disease.

Targeting tumor-associated acidity in cancer immunotherapy

Checkpoint inhibitors, such as cytotoxic T-lymphocyte-associated protein-4 (CTLA-4) and programmed cell death-1 (PD-1) monoclonal antibodies have changed profoundly the treatment of melanoma, renal cell carcinoma, non-small cell lung cancer, Hodgkin lymphoma, and bladder cancer. Currently, they are tested in various tumor entities as monotherapy or in combination with chemotherapies or targeted therapies. However, only a subgroup of patients benefit from checkpoint blockade (combinations). This raises the question, which all mechanisms inhibit T cell function in the tumor environment, restricting the efficacy of these immunotherapeutic approaches. Serum activity of lactate dehydrogenase, likely reflecting the glycolytic activity of the tumor cells and thus acidity within the tumor microenvironment, turned out to be one of the strongest markers predicting response to checkpoint inhibition. In this review, we discuss the impact of tumor-associated acidity on the efficacy of T cell-mediated cancer immunotherapy and possible approaches to break this barrier.

Role of sodium-hydrogen exchanger isoform 1 in regulating hepatocyte apoptosis induced by hyperammonaemia

BACKGROUND

The "secondary injury" theory of liver failure indicated that hyperammonaemia due to liver failure causes further deterioration of hepatocytes. Our previous studies have demonstrated that high blood ammonia levels may lead to hepatocyte apoptosis, as NH₄Cl loading caused metabolic acidosis and an increase in sodium-hydrogen exchanger isoform 1 (NHE1). In this study, we established a hyperammonia hepatocyte model to determine the role of NHE1 in the regulation of hepatocyte apoptosis induced by NH₄Cl.

MATERIALS AND METHODS

In current studies, intracellular pH (pHi) and NHE1 activity were analyzed using the pHi-sensitive dye BCECF-AM. The results showed that intracellular pH dropped and NHE1 activity increased in hepatocytes under NH₄Cl treatment. As expected, decreased pHi induced by NH₄Cl was associated with increased apoptosis, low cell proliferation and ATP depletion, which was exacerbated by exposure to the NHE1 inhibitor cariporide. We also found that NH₄Cl treatment stimulated PI3K and Akt phosphorylation and this effect was considerably reduced by NHE1 inhibition.

CONCLUSION

This study highlighted the significant role of NHE1 in the regulation of cell apoptosis induced by hyperammonaemia.

Inhibition of pH regulation as a therapeutic strategy in hypoxic human breast cancer cells

Hypoxic cancer cells exhibit resistance to many therapies. This study compared the therapeutic effect of targeting the pH regulatory proteins (CAIX, NHE1 and V-ATPase) that permit cancer cells to adapt to hypoxic conditions, using both 2D and 3D culture models. Drugs targeting CAIX, NHE1 and V-ATPase exhibited anti-proliferative effects in MCF-7, MDA-MB-231 and HBL-100 breast cancer cell lines in 2D. Protein and gene expression analysis in 2D showed that CAIX was the most hypoxia-inducible protein of the 3 targets. However, the expression of CAIX differed between the 3 cell lines. This difference in CAIX expression in hypoxia was consistent with a varying activity of FIH-1 between the cell lines. 3D expression analysis demonstrated that both CAIX and NHE1 were up-regulated in the hypoxic areas of multicellular tumor spheroids. However, the induction of CAIX expression in hypoxia was again cell line dependent. 3D invasion assays conducted with spheroids showed that CAIX inhibition significantly reduced the invasion of cells. Finally, the capability of both NHE1 and CAIX inhibitors to combine effectively with irradiation was exhibited in clonogenic assays. Proteomic-mass-spectrometric analysis indicated that CAIX inhibition might be combining with irradiation through stimulating apoptotic cell death. Of the three proteins, CAIX represents the target with the most promise for the treatment of breast cancer.

Manipulating extracellular tumour pH: an effective target for cancer therapy

The pH in tumour cells and the tumour microenvironment has played important roles in cancer development and treatment.

The Role of pH Regulation in Cancer Progression

Frequently observed phenotypes of tumours include high metabolic activity, hypoxia and poor perfusion; these act to produce an acidic microenvironment. Cellular function depends on pH homoeostasis, and thus, tumours become dependent on pH regulatory mechanisms. Many of the proteins involved in pH regulation are highly expressed in tumours, and their expression is often of prognostic significance. The more acidic tumour microenvironment also has important implications with regard to chemotherapeutic and radiotherapeutic interventions. In addition, we review pH-sensing mechanisms, the role of pH regulation in tumour phenotype and the use of pH regulatory mechanisms as therapeutic targets.

Cellular acidification as a new approach to cancer treatment and to the understanding and therapeutics of neurodegenerative diseases

During the last few years, the understanding of the dysregulated hydrogen ion dynamics and reversed proton gradient of cancer cells has resulted in a new and integral pH-centric paradigm in oncology, a translational model embracing from cancer etiopathogenesis to treatment. The abnormalities of intracellular alkalinization along with extracellular acidification of all types of solid tumors and leukemic cells have never been described in any other disease and now appear to be a specific hallmark of malignancy. As a consequence of this intracellular acid-base homeostatic failure, the attempt to induce cellular acidification using proton transport inhibitors and other intracellular acidifiers of different origins is becoming a new therapeutic concept and selective target of cancer treatment, both as a metabolic mediator of apoptosis and in the overcoming of multiple drug resistance (MDR). Importantly, there is increasing data showing that different ion channels contribute to mediate significant aspects of cancer pH regulation and etiopathogenesis. Finally, we discuss the extension of this new pH-centric oncological paradigm into the opposite metabolic and homeostatic acid-base situation found in human neurodegenerative diseases (HNDDs), which opens novel concepts in the prevention and treatment of HNDDs through the utilization of a cohort of neural and non-neural derived hormones and human growth factors.

Value of pH regulators in the diagnosis, prognosis and treatment of cancer

Altered metabolism, associated with acidification of the extracellular milieu, is one of the major features of cancer. As pH regulation is crucial for the maintenance of all biological functions, cancer cells rely on the activity of lactate exporters and proton transporters to regulate their intracellular pH. The major players in cancer pH regulation are proton pump ATPases, sodium-proton exchangers (NHEs), monocarboxylate transporters (MCTs), carbonic anhydrases (CAs) and anion exchangers (AEs), which have been shown to be upregulated in several human malignancies. Thanks to the activity of the proton pumps and transporters, tumours acidify their microenvironment, becoming more aggressive and resistant to therapy. Thus, targeting tumour pH may contribute to more effective anticancer strategies for controlling tumour progression and therapeutic resistance. In the present study, we review the role of the main pH regulators expressed in human cancer cells, including their diagnostic and prognostic value, as well as their usefulness as therapeutic targets.

Cancer: fundamentals behind pH targeting and the double-edged approach

The highly regulated pH of cells and the less-regulated pH of the surrounding extracellular matrix (ECM) is the result of a delicate balance between metabolic processes and proton production, proton transportation, chemical buffering, and vascular removal of waste products. Malignant cells show a pronounced increase in metabolic processes where the 10- to 15-fold rise in glucose consumption is only the tip of the iceberg. Aerobic glycolysis (Warburg effect) is one of the hallmarks of cancer metabolism that implies excessive production of protons, which if stayed inside the cells would result in fatal intracellular acidosis (maintaining a strict acid-base balance is essential for the survival of eukaryotic cells). Malignant cells solve this problem by increasing mechanisms of proton transportation which expel the excess acidity. This allows cancer cells to keep a normal intracellular pH, or even overshooting this mechanism permits a slightly alkaline intracellular tendency. The proton excess expelled from malignant cells accumulates in the ECM, where chronic hypoxia and relative lack of enough blood vessels impede adequate proton clearance, thus creating an acidic microenvironment. This microenvironment is quite heterogeneous due to the tumor's metabolic heterogeneity and variable degrees of hypoxia inside the tumor mass. The acidic environment (plus other necessary cellular modifications) stimulates migration and invasion and finally intravasation of malignant cells which eventually may result in metastasis. Targeting tumor pH may go in two directions: 1) increasing extracellular pH which should result in less migration, invasion, and metastasis; and 2) decreasing intracellular pH which may result in acidic stress and apoptosis. Both objectives seem achievable at the present state of the art with repurposed drugs. This hypothesis analyzes the altered pH of tumors and its implications for progression and metastasis and also possible repurposed drug combinations targeting this vulnerable side of cancer development. It also analyzes the double-edged approach, which consists in pharmacologically increasing intracellular proton production and simultaneously decreasing proton extrusion creating intracellular acidity, acid stress, and eventual apoptosis.

Poly(styrenesulfonate)-Modified Ni-Ti Layered Double Hydroxide Film: A Smart Drug-Eluting Platform

Drug-eluting stents (DESs) are widely used in the palliative treatment of many kinds of cancers. However, the covered polymers used in DESs are usually associated with stent migration and acute cholecystitis. Therefore, developing noncovered drug-loading layers on metal stents is of great importance. In this work, Ni-Ti layered double hydroxide (Ni-Ti LDH) films were prepared on the surface of nitinol via hydrothermal treatment, and the LDH films were further modified by poly(styrenesulfonate) (PSS). The anticancer drug doxorubicin could be effectively loaded onto the modified films, and drug release could be smartly controlled by the pH. Besides, the drug absorption amounts of cancer cells cultured on the films could be effectively improved. These results indicate that the PSS-modified LDH film may become a promising drug-loading platform that can be used in the design of DESs.

Microenvironmental acidosis in carcinogenesis and metastases: new strategies in prevention and therapy

Much effort is currently devoted to developing patient-specific cancer therapy based on molecular characterization of tumors. In particular, this approach seeks to identify driver mutations that can be blocked through small molecular inhibitors. However, this approach is limited by extensive intratumoral genetic heterogeneity, and, not surprisingly, even dramatic initial responses are typically of limited duration as resistant tumor clones rapidly emerge and proliferate. We propose an alternative approach based on observations that while tumor evolution produces genetic divergence, it is also associated with striking phenotypic convergence that loosely correspond to the well-known cancer “hallmarks”. These convergent properties can be described as driver phenotypes and may be more consistently and robustly expressed than genetic targets. To this purpose, it is necessary to identify strategies that are critical for cancer progression and metastases, and it is likely that these driver phenotypes will be closely related to cancer “hallmarks”. It appears that an antiacidic approach, by targetting a driver phenotype in tumors, may be thought as a future strategy against tumors in either preventing the occurrence of cancer or treating tumor patients with multiple aims, including the improvement of efficacy of existing therapies, possibly reducing their systemic side effects, and controlling tumor growth, progression, and metastasis. This may be achieved with existing molecules such as proton pump inhibitors (PPIs) and buffers such as sodium bicarbonate, citrate, or TRIS.

Selective Tumor Cell Inhibition Effect of Ni-Ti Layered Double Hydroxides Thin Films Driven by the Reversed pH Gradients of Tumor Cells

Nitinol is widely fabricated as stents for the palliation treatment of many kinds of cancers. It is of great importance to develop nitinol stents with selective tumor cell inhibition effects. In this work, a series of pH sensitive films composed of Ni(OH)2 and Ni-Ti layered double hydroxide (Ni-Ti LDH) with different Ni/Ti ratios were prepared on the surface of nitinol via hydrothermal treatment. The films with specific Ni/Ti ratios would release a large amount of nickel ions under acidic environments but were relatively stable in neutral or weak alkaline medium. Cell viability tests showed that the films can effectively inhibit the growth of cancer cells but have little adverse effects to normal cells. Besides, extraordinarily high intracellular nickel content and reactive oxygen species (ROS) level were found in cancer cells, indicating the death of cancer cells may be induced by the excessive intake of nickel ions. Such selective cancer cell inhibition effect of the films is supposed to relate with the reversed pH gradients of tumor cells.

BI-1 enhances Fas-induced cell death through a Na+/H+-associated mechanism

The role of Bax inhibitor-1 (BI-1) in the protective mechanism against apoptotic stimuli has been studied; however, as little is known about its role in death receptor-mediated cell death, this study was designed to investigate the effect of BI-1 on Fas-induced cell death, and the underlying mechanisms. HT1080 adenocarcinoma cells were cultured in high concentration of glucose media and transfected with vector alone (Neo cells) or BI-1-vector (BI-1 cells), and treated with Fas. In cell viability, apoptosis, and caspase-3 analyses, the BI-1 cells showed enhanced sensitivity to Fas. Fas significantly decreased cytosolic pH in BI-1 cells, compared with Neo cells, and this decrease correlated with BI-1 oligomerization, mitochondrial Ca2+ accumulation, and significant inhibition of sodium-hydrogen exchanger (NHE) activity. Compared with Neo cells, a single treatment of BI-1 cells with the NHE inhibitor EIPA or siRNA against NHE significantly increased cell death, which suggests that the viability of BI-1 cells is affected by the maintenance of intracellular pH homeostasis through NHE.

Glycolysis, tumor metabolism, cancer growth and dissemination. A new pH-based etiopathogenic perspective and therapeutic approach to an old cancer question

Cancer cells acquire an unusual glycolytic behavior relative, to a large extent, to their intracellular alkaline pH (pHi). This effect is part of the metabolic alterations found in most, if not all, cancer cells to deal with unfavorable conditions, mainly hypoxia and low nutrient supply, in order to preserve its evolutionary trajectory with the production of lactate after ten steps of glycolysis. Thus, cancer cells reprogram their cellular metabolism in a way that gives them their evolutionary and thermodynamic advantage. Tumors exist within a highly heterogeneous microenvironment and cancer cells survive within any of the different habitats that lie within tumors thanks to the overexpression of different membrane-bound proton transporters. This creates a highly abnormal and selective proton reversal in cancer cells and tissues that is involved in local cancer growth and in the metastatic process. Because of this environmental heterogeneity, cancer cells within one part of the tumor may have a different genotype and phenotype than within another part. This phenomenon has frustrated the potential of single-target therapy of this type of reductionist therapeutic approach over the last decades. Here, we present a detailed biochemical framework on every step of tumor glycolysis and then proposea new paradigm and therapeutic strategy based upon the dynamics of the hydrogen ion in cancer cells and tissues in order to overcome the old paradigm of one enzyme-one target approach to cancer treatment. Finally, a new and integral explanation of the Warburg effect is advanced.

Cariporide and other new and powerful NHE1 inhibitors as potentially selective anticancer drugs--an integral molecular/biochemical/metabolic/clinical approach after one hundred years of cancer research

In recent years an increasing number of publications have emphasized the growing importance of hydrogen ion dynamics in modern cancer research, from etiopathogenesis and treatment. A proton [H⁺]-related mechanism underlying the initiation and progression of the neoplastic process has been recently described by different research groups as a new paradigm in which all cancer cells and tissues, regardless of their origin and genetic background, have a pivotal energetic and homeostatic disturbance of their metabolism that is completely different from all normal tissues: an aberrant regulation of hydrogen ion dynamics leading to a reversal of the pH gradient in cancer cells and tissues (↑pH_i/↓pH_e, or “proton reversal”). Tumor cells survive their hostile microenvironment due to membrane-bound proton pumps and transporters, and their main defensive strategy is to never allow internal acidification because that could lead to their death through apoptosis. In this context, one of the primary and best studied regulators of both pH_i and pH_e in tumors is the Na⁺/H⁺ exchanger isoform 1 (NHE1). An elevated NHE1 activity can be correlated with both an increase in cell pH and a decrease in the extracellular pH of tumors, and such proton reversal is associated with the origin, local growth, activation and further progression of the metastatic process. Consequently, NHE1 pharmaceutical inhibition by new and potent NHE1 inhibitors represents a potential and highly selective target in anticancer therapy. Cariporide, being one of the better studied specific and powerful NHE1 inhibitors, has proven to be well tolerated by humans in the cardiological context, however some side-effects, mainly related to drug accumulation and cerebrovascular complications were reported. Thus, cariporide could become a new, slightly toxic and effective anticancer agent in different human malignancies.

The role of a Schiff base scaffold, N-(2-hydroxy acetophenone) glycinate-in overcoming multidrug resistance in cancer

Drug resistance is a problem that hinders the numerous successes of chemotherapeutic intervention of cancer and continues to be a major obstacle for cures. Till date, several attempts have been made to develop suitable multidrug resistance (MDR) reversing agents. But, throughout the clinical development of MDR reversing agents, patients repeatedly suffer from toxicities. So far, some anticancer activity of Schiff bases which are the condensation products of carbonyl compounds and primary amines and their metal complexes has been described. But, overcoming multidrug resistance, by the use of such small molecules still remain unexplored. Under this backdrop, in search of less toxic and more effective MDR reversing agents our laboratory has developed the different metal chelates of Schiff base N-(2-hydroxy acetophenone)glycinate (NG) which is structurally similar to azatyrosine [L-β-(5-hydroxy-2-pyridyl)-alanine] that inhibits tumor formation by deactivating the c-Raf-1 kinase and c-Ha-ras signalling pathway. A decade-long research proposes possible strategies to overcome MDR by exploiting the chemical nature of such metal chelates. In this review we have catalogued the success of metal chelates of NG to overcome MDR in cancer. The review depict that the problem of MDR can be circumvent by synchronized activation of immunogenic cell death pathways that utilize the components of a host's immune system to kill cancer cells in combination with other conventional strategies. The current wealth of preclinical information promises better understanding of the cellular processes underlying MDR reversing activity of metal derivatives of NG and thus exposes several cellular targets for rational designing of new generation of Schiff base metal chelates as MDR reversing agents.

The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters

The mammalian Slc4 (Solute carrier 4) family of transporters is a functionally diverse group of 10 multi-spanning membrane proteins that includes three Cl-HCO3 exchangers (AE1-3), five Na(+)-coupled HCO3(-) transporters (NCBTs), and two other unusual members (AE4, BTR1). In this review, we mainly focus on the five mammalian NCBTs-NBCe1, NBCe2, NBCn1, NDCBE, and NBCn2. Each plays a specialized role in maintaining intracellular pH and, by contributing to the movement of HCO3(-) across epithelia, in maintaining whole-body pH and otherwise contributing to epithelial transport. Disruptions involving NCBT genes are linked to blindness, deafness, proximal renal tubular acidosis, mental retardation, and epilepsy. We also review AE1-3, AE4, and BTR1, addressing their relevance to the study of NCBTs. This review draws together recent advances in our understanding of the phylogenetic origins and physiological relevance of NCBTs and their progenitors. Underlying these advances is progress in such diverse disciplines as physiology, molecular biology, genetics, immunocytochemistry, proteomics, and structural biology. This review highlights the key similarities and differences between individual NCBTs and the genes that encode them and also clarifies the sometimes confusing NCBT nomenclature.

Saponins isolated from Asparagus induce apoptosis in human hepatoma cell line HepG2 through a mitochondrial-mediated pathway

OBJECTIVE

Many scientific studies have shown that Asparagus officinalis has an antitumour effect and enhances human immunity, but the active components and the antitumour mechanisms are unclear. We investigated the effects of saponins isolated from Asparagus on proliferation and apoptosis in the human hepatoma cell line HepG2.

METHODS

HepG2 cells were treated with varying concentrations of Asparagus saponins at various times. Using mtt and flow cytometry assays, we evaluated the effects of Asparagus saponins on the growth and apoptosis of HepG2 cells. Transmission electron microscopy was used to observe the morphology of cell apoptosis. Confocal laser scanning microscopy was used to analyze intracellular calcium ion concentration, mitochondrial permeability transition pore (mptp), and mitochondrial membrane potential (mmp). Spectrophotometry was applied to quantify the activity of caspase-9 and caspase-3. Flow cytometry was used to investigate the levels of reactive oxygen species (ros) and pH, and the expressions of Bcl2, Bax, CytC, and caspase-3, in HepG2 cells.

RESULTS

Asparagus saponins inhibited the growth of HepG2 cells in a dose-dependent manner. The median inhibitory concentration (IC(50)) was 101.15 mg/L at 72 hours. The apoptosis morphology at 72 hours of treatment was obvious, showing cell protuberance, concentrated cytoplasm, and apoptotic bodies. The apoptotic rates at 72 hours were 30.9%, 51.7%, and 62.1% (for saponin concentrations of 50 mg/L, 100 mg/L, 200 mg/L). Treatment with Asparagus saponins for 24 hours increased the intracellular level of ros and Ca(2+), lowered the pH, activated intracellular mptp, and decreased mmp in a dose-dependent manner. Treatment also increased the activity of caspase-9 and caspase-3, downregulated the expression of Bcl2, upregulated the expression of Bax, and induced release of CytC and activation of caspase-3.

CONCLUSIONS

Asparagus saponins induce apoptosis in HepG2 cells through a mitochondrial-mediated and caspase-dependent pathway, suggesting that they may be a potent agent for the treatment of hepatocellular carcinoma.

Interfering with pH regulation in tumours as a therapeutic strategy

The high metabolic rate of tumours often leads to acidosis and hypoxia in poorly perfused regions. Tumour cells have thus evolved the ability to function in a more acidic environment than normal cells. Key pH regulators in tumour cells include: isoforms 2, 9 and 12 of carbonic anhydrase, isoforms of anion exchangers, Na+/HCO3- co-transporters, Na+/H+ exchangers, monocarboxylate transporters and the vacuolar ATPase. Both small molecules and antibodies targeting these pH regulators are currently at various stages of clinical development. These antitumour mechanisms are not exploited by the classical cancer drugs and therefore represent a new anticancer drug discovery strategy.

pH control mechanisms of tumor survival and growth

A distinguishing phenotype of solid tumors is the presence of an alkaline cellular feature despite the surrounding acidic microenvironment. This phenotypic characteristic of tumors, originally described by Otto Warburg, arises due to alterations in metabolism of solid tumors. Hypoxic regions of solid tumors develop due to poor vascularization and in turn regulate the expression of numerous genes via the transcription factor HIF-1. Ultimately, the tumor microenvironment directs the development of tumor cells adapted to survive in an acidic surrounding where normal cells perish. The provision of unique pH characteristics in tumor cells provides a defining trait that has led to the pursuit of treatments that target metabolism, hypoxia, and pH-related mechanisms to selectively kill cancer cells. Numerous studies over the past decade involving the cancer-specific carbonic anhydrase IX have re-kindled an interest in pH disruption-based therapies. Although an acidification of the intracellular compartment is established as a means to induce normal cell death, the defining role of acid-base disturbances in tumor physiology and survival remains unclear. The aim of this review is to summarize recent data relating to the specific role of pH regulation in tumor cell survival. We focus on membrane transport and enzyme studies in an attempt to elucidate their respective functions regarding tumor cell pH regulation. These data are discussed in the context of future directions for the field of tumor cell acid-base-related research.

High expression of ErbB2 contributes to cholangiocarcinoma cell invasion and proliferation through AKT/p70S6K

AIM: To compare the impact of ErbB2 on cell invasion and proliferation in cholangiocarcinoma (CCA) cell lines.

METHODS: Level of endogenous ErbB2 expression in three CCA cell lines, namely HuCCA-1, KKU-100 and KKU-M213, was determined by real-time reverse-transcriptase polymerase chain reaction. Two ErbB2 inhibitory methods, a small molecule ErbB2 kinase inhibitor (AG825) and siRNA, were used to disrupt ErbB2 function in the cell lines. CCA cell invasion, motility and proliferation under ErbB2-disrupted conditions were detected using Transwell and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays. In addition, ErbB2 downstream effectors were investigated by Western blotting analysis.

RESULTS: Suppression of ErbB2 activity, using a specific kinase inhibitor (AG825), reduced invasion, motility and proliferation of all three CCA cell lines. The ability of this drug to inhibit neoplastic properties (invasion, motility and proliferation) increased concomitantly with the level of ErbB2 expression. Similarly, knockdown of ErbB2 level by siRNA inhibited cell invasion and proliferation of KKU-M213, a high-ErbB2-expressing cell, better than those of the lower-ErbB2-expressing cells, HuCCA-1 and KKU-100. Thus, both inhibitory methods indicated that there is more ErbB2-dependency for malignancy of the high-ErbB2-expressing cell, KKU-M213, than for that of low-ErbB2-expressing ones. In addition, interrupting ErbB2 activity decreased phosphorylation of AKT and p70S6K, but not extracellular signal-regulated kinase 1/2, in the high-ErbB2-expressing CCA cell line.

CONCLUSION: Our data indicated that high ErbB2 expression enhances CCA invasion, motility and proliferation via the AKT/p70S6K pathway, which suggests the possibility of targeting these molecules for CCA therapy.

Proton pump inhibitor-induced tumour cell death by inhibition of a detoxification mechanism

This review presents a possible new approach against cancer, as represented by inhibition of proton pumps, a mechanism used by tumour cells to avoid intracellular accumulation of toxic substances. Proton pump inhibitors (PPIs) belong to a family of pro-drugs that are currently used in the treatment of peptic diseases needing acidity to be activated. PPIs target the acidic tumour mass, where they are metabolized, thus blocking proton traffic. Proton pump inhibition triggers a rapid cell death as a result of intracellular acidification, caspase activation and early accumulation of reactive oxygen species into tumour cells. As a whole, the devastating effect of PPIs on tumour cells suggest the triggering of a fatal cell toxification. Many human tumours, including melanoma, osteosarcoma, lymphomas and various adenocarcinomas are responsive to PPIs. This appears highly conceivable, in as much as almost all human tumours are acidic and express high levels of proton pumps. Paradoxically, metastatic tumours appear to be more responsive to PPIs being more acidic than the majority of primary tumours. However, two clinical trials test the effectiveness of PPIs in chemosensitizing melanoma and osteosarcoma patients. Indeed, tumour acidity represents a very potent mechanism of chemoresistance. A majority of cytotoxic agents, being weak bases, are quickly protonated outside and do not enter the cells, thus preventing drugs to reach specific cellular targets. Clinical data will provide the proof of concept on the use of PPIs as a new class of antitumour agent with a very low level of systemic toxicity as compared with standard chemotherapeutic agents.

An integral approach to the etiopathogenesis of human neurodegenerative diseases (HNDDs) and cancer. Possible therapeutic consequences within the frame of the trophic factor withdrawal syndrome (TFWS)

A novel and integral approach to the understanding of human neurodegenerative diseases (HNDDs) and cancer based upon the disruption of the intracellular dynamics of the hydrogen ion (H(+)) and its physiopathology, is advanced. From an etiopathological perspective, the activity and/or deficiency of different growth factors (GFs) in these pathologies are studied, and their relationships to intracellular acid-base homeostasis reviewed. Growth and trophic factor withdrawal in HNDDs indicate the need to further investigate the potential utilization of certain GFs in the treatment of Alzheimer disease and other neurodegenerative diseases. Platelet abnormalities and the therapeutic potential of platelet-derived growth factors in these pathologies, either through platelet transfusions or other clinical methods, are considered. Finally, the etiopathogenic mechanisms of apoptosis and antiapoptosis in HNDDs and cancer are viewed as opposite biochemical and biological disorders of cellular acid-base balance and their secondary effects on intracellular signaling pathways and aberrant cell metabolism are considered in the light of the both the seminal and most recent data available. The "trophic factor withdrawal syndrome" is described for the first time in English-speaking medical literature, as well as a Darwinian-like interpretation of cellular behavior related to specific and nonspecific aspects of cell biology.