Reduction of anion exchanger 2 expression induces apoptosis of human hepatocellular carcinoma cells

Antisense oligonucleotide is a promising intervention for liver diseases

As the body's critical metabolic organ, the liver plays an essential role in maintaining proper body homeostasis. However, as people's living standards have improved and the number of unhealthy lifestyles has increased, the liver has become overburdened. These have made liver disease one of the leading causes of death worldwide. Under the influence of adverse factors, liver disease progresses from simple steatosis to hepatitis, to liver fibrosis, and finally to cirrhosis and cancer, followed by increased mortality. Until now, there has been a lack of accepted effective treatments for liver disease. Based on current research, antisense oligonucleotide (ASO), as an alternative intervention for liver diseases, is expected to be an effective treatment due to its high efficiency, low toxicity, low dosage, strong specificity, and additional positive characteristics. In this review, we will first introduce the design, modification, delivery, and the mechanisms of ASO, and then summarize the application of ASO in liver disease treatment, including in non-alcoholic fatty liver disease (NAFLD), hepatitis, liver fibrosis, and liver cancer. Finally, we discuss challenges and perspectives on the transfer of ASO drugs into clinical use. This review provides a current and comprehensive understanding of the integrative and systematic functions of ASO for its use in liver disease.

Roles of Sodium Hydrogen Exchanger (NHE1) and Anion Exchanger (AE2) across Chondrocytes Plasma Membrane during Longitudinal Bone Growth

Mammalian long bone growth occurs through endochondral ossification, majorly regulated by the controlled enlargement of chondrocytes at the growth plate (GP). This study aimed to investigate the roles of Na+/H+ (sodium hydrogen exchanger (NHE1)) and HCO3− (anion exchanger [AE2]) during longitudinal bone growth in mammals. Bones from P10 SpragueDawley rat pups were cultured exvivo in the presence or absence of NHE1 and AE2 inhibitors to determine their effect on long bone growth. Gross morphometry, histomorphometry, and immunohistochemistry were used to assess the bone growth. The results revealed that the culture of the bones in the presence of NHE1 and AE2 inhibitors reduces bone growth significantly (p < 0.05) by approximately 11%. The inhibitor significantly (p < 0.05) reduces bone growth velocity and the length of the hypertrophic chondrocyte zone without any effect on the total GP length. The total GP chondrocyte density was significantly (p < 0.05) reduced, but hypertrophic chondrocyte densities remained constant. NHE1 fluorescence signaling across the GP length was higher than AE2, and their localization was significantly (p < 0.05) inhibited at the hypertrophic chondrocytes zone. The GP lengthening was majorly driven by an increase in the overall GP chondrocyte and hypertrophic chondrocyte densities apart from the regulatory volume phenomenon. This may suggest that NHE1 and AE2 could have a regulatory role in long bone growth.

Chloride Channels and Transporters: Roles beyond Classical Cellular Homeostatic pH or Ion Balance in Cancers

Simple Summary

Roles of chloride-associated transporters have been raised in various cancers. Although complicated ion movements, crosstalk among channels/transporters through homeostatic electric regulation, difficulties with experimental implementation such as activity measurement of intracellular location were disturbed to verify the precise modulation of channels/transporters, recently defined cancerous function and communication with tumor microenvironment of chloride channels/transporters should be highlighted beyond classical homeostatic ion balance. Chloride-associated transporters as membrane-associated components of chloride movement, regulations of transmembrane member 16A, calcium-activated chloride channel regulators, transmembrane member 206, chloride intracellular channels, voltage-gated chloride channels, cystic fibrosis transmembrane conductance regulator, voltage-dependent anion channel, volume-regulated anion channel, and chloride-bicarbonate exchangers are discussed.

Abstract

The canonical roles of chloride channels and chloride-associated transporters have been physiologically determined; these roles include the maintenance of membrane potential, pH balance, and volume regulation and subsequent cellular functions such as autophagy and cellular proliferative processes. However, chloride channels/transporters also play other roles, beyond these classical function, in cancerous tissues and under specific conditions. Here, we focused on the chloride channel-associated cancers and present recent advances in understanding the environments of various types of cancer caused by the participation of many chloride channel or transporters families and discuss the challenges and potential targets for cancer treatment. The modulation of chloride channels/transporters might promote new aspect of cancer treatment strategies.

Ion Channels, Transporters, and Sensors Interact with the Acidic Tumor Microenvironment to Modify Cancer Progression

Solid tumors, including breast carcinomas, are heterogeneous but typically characterized by elevated cellular turnover and metabolism, diffusion limitations based on the complex tumor architecture, and abnormal intra- and extracellular ion compositions particularly as regards acid-base equivalents. Carcinogenesis-related alterations in expression and function of ion channels and transporters, cellular energy levels, and organellar H⁺ sequestration further modify the acid-base composition within tumors and influence cancer cell functions, including cell proliferation, migration, and survival. Cancer cells defend their cytosolic pH and HCO₃^- concentrations better than normal cells when challenged with the marked deviations in extracellular H⁺, HCO₃^-, and lactate concentrations typical of the tumor microenvironment. Ionic gradients determine the driving forces for ion transporters and channels and influence the membrane potential. Cancer and stromal cells also sense abnormal ion concentrations via intra- and extracellular receptors that modify cancer progression and prognosis. With emphasis on breast cancer, the current review first addresses the altered ion composition and the changes in expression and functional activity of ion channels and transporters in solid cancer tissue. It then discusses how ion channels, transporters, and cellular sensors under influence of the acidic tumor microenvironment shape cancer development and progression and affect the potential of cancer therapies.

The Golgi as a "Proton Sink" in Cancer

Cancer cells exhibit increased glycolytic flux and adenosine triphosphate (ATP) hydrolysis. These processes increase the acidic burden on the cells through the production of lactate and protons. Nonetheless, cancer cells can maintain an alkaline intracellular pH (pHi) relative to untransformed cells, which sets the stage for optimal functioning of glycolytic enzymes, evasion of cell death, and increased proliferation and motility. Upregulation of plasma membrane transporters allows for H+ and lactate efflux; however, recent evidence suggests that the acidification of organelles can contribute to maintenance of an alkaline cytosol in cancer cells by siphoning off protons, thereby supporting tumor growth. The Golgi is such an acidic organelle, with resting pH ranging from 6.0 to 6.7. Here, we posit that the Golgi represents a "proton sink" in cancer and delineate the proton channels involved in Golgi acidification and the ion channels that influence this process. Furthermore, we discuss ion channel regulators that can affect Golgi pH and Golgi-dependent processes that may contribute to pHi homeostasis in cancer.

Drug Repurposing as an Antitumor Agent: Disulfiram-Mediated Carbonic Anhydrase 12 and Anion Exchanger 2 Modulation to Inhibit Cancer Cell Migration

Disulfiram has been used in the treatment of alcoholism and exhibits an anti-tumor effect. However, the intracellular mechanism of anti-tumor activity of Disulfiram remains unclear. In this study, we focused on the modulatory role of Disulfiram via oncogenic factor carbonic anhydrase CA12 and its associated transporter anion exchanger AE2 in lung cancer cell line A549. The surface expression of CA12 and AE2 were decreased by Disulfiram treatment with a time-dependent manner. Disulfiram treatment did not alter the expression of Na⁺-bicarbonate cotransporters, nor did it affect autophagy regulation. The chloride bicarbonate exchanger activity of A549 cells was reduced by Disulfiram treatment in a time-dependent manner without change in the resting pH level. The expression and activity of AE2 and the expression of CA12 were also reduced by Disulfiram treatment in the breast cancer cell line. An invasion assay and cell migration assay revealed that Disulfiram attenuated the invasion and migration of A549 cells. In conclusion, the attenuation of AE2 and its supportive enzyme CA12, and the inhibitory effect on cell migration by Disulfiram treatment in cancer cells provided the molecular evidence supporting the potential of Disulfiram as an anticancer agent.

Anion exchanger 2 suppresses cellular movement and has prognostic significance in esophageal squamous cell carcinoma

Background

Recent studies have reported essential roles for various intracellular pH regulators in epithelial carcinogenesis and tumor progression. The aims of the present study were to investigate the role of anion exchanger 2 (AE2) in the regulation of tumor progression-related genes and the prognostic value of its expression in esophageal squamous cell carcinoma (ESCC).

Results

AE2 was strongly expressed in KYSE170 and TE13 cells. The depletion of AE2 in these cells increased cell migration and inhibited the induction of apoptosis. The results of the microarray analysis revealed that various matrix metalloproteinase (MMP) signaling pathway-related genes, such as MMP1, MMP12, and TIMP4, were up- or down-regulated in AE2-depleted KYSE170 cells. Immunohistochemical staining showed that AE2 was primarily located in the cell membranes or cytoplasm of carcinoma cells, and its expression pattern at the invasive front of the tumor was related to the pT category. Prognostic analyses revealed that the low-grade expression of AE2 at the invasive front was associated with shorter postoperative survival.

Conclusions

The results of the present study suggest that reductions in AE2 in ESCC enhance cellular movement by activating MMP signaling pathways and are related to a poor prognosis in patients with ESCC.

Methods

In human ESCC cell lines, knockdown experiments were conducted using AE2 siRNA, and the effects on cellular movement and survival were analyzed. The gene expression profiles of cells were examined using a microarray analysis. An immunohistochemical analysis was performed on 61 primary tumor samples obtained from ESCC patients who underwent esophagectomy.

Knockdown of anion exchanger 2 suppressed the growth of ovarian cancer cells via mTOR/p70S6K1 signaling

Anion exchanger 2 (AE2, encoded by SLC4A2) is a sodium-independent chloride/bicarbonate transporter and implicated in the regulation of intracellular pH and membrane potential. Previous studies have linked AE2 to the tumorigenesis of various cancers. Here, AE2 was identified as an up-regulated protein in ovarian cancer tissues compared to adjacent non-tumor lesions based on quantitative proteomics analysis. AE2 mRNA was also overexpressed in human ovarian cancer samples, and that AE2 overexpression correlated with the shortened survival time of ovarian cancer patients. Short-hairpin RNA-mediated knockdown of AE2 in A2780 and SK-OV-R3 cells inhibited cell growth and induced cell cycle G1 phase arrest. In nude mice, its stable knockdown inhibited the tumorigenicity of A2780 cells. Gene set enrichment analysis on The Cancer Genome Atlas dataset identified that the cell cycle process and mTOR pathway were correlatively with the AE2 expression. Expression of key regulators of G1/S transition (Cyclin D1 and CDK4), and phosphorylation levels of p70S6K were notably reduced in AE2 knockdown cells. Moreover, experiments with mTOR inhibitor suggested that AE2 may promote cell cycle progression through mTOR/p70S6K1 pathway. Together, our results suggest up-regulated AE2 promotes ovarian cancer tumorigenesis by activating mTOR/p70S6K1 pathway and implicate the potential application of AE2 in cancer therapy.

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.

Governing effect of regulatory proteins for Cl(-)/HCO3(-) exchanger 2 activity

Anion exchanger 2 (AE2) has a critical role in epithelial cells and is involved in the ionic homeostasis such as Cl(-) uptake and HCO3(-) secretion. However, little is known about the regulatory mechanism of AE2. The main goal of the present study was to investigate potential regulators, such as spinophilin (SPL), inositol-1,4,5-trisphosphate [IP3] receptors binding protein released with IP3 (IRBIT), STE20/SPS1-related proline/alanine-rich kinase (SPAK) kinase, and carbonic anhydrase XII (CA XII). We found that SPL binds to AE2 and markedly increased the Cl(-)/HCO3(-) exchange activity of AE2. Especially SPL 1-480 domain is required for enhancing AE2 activity. For other regulatory components that affect the fidelity of fluid and HCO3(-) secretion, IRBIT and SPAK had no effect on the activity of AE2 and no protein-protein interaction with AE2. It has been proposed that CA activity is closely associated with AE activity. In this study, we provide evidence that the basolateral membrane-associated CA isoform CA XII significantly increased the activity of AE2 and co-localized with AE2 to the plasma membrane. Collectively, SPL and CA XII enhanced the Cl(-)/HCO3(-) exchange activity of AE2. The modulating action of these regulatory proteins could serve as potential therapeutic targets for secretory diseases mediated by AE2.

Sphaeropsidin A shows promising activity against drug-resistant cancer cells by targeting regulatory volume increase

Despite the recent advances in the treatment of tumors with intrinsic chemotherapy resistance, such as melanoma and renal cancers, their prognosis remains poor and new chemical agents with promising activity against these cancers are urgently needed. Sphaeropsidin A, a fungal metabolite whose anticancer potential had previously received little attention, was isolated from Diplodia cupressi and found to display specific anticancer activity in vitro against melanoma and kidney cancer subpanels in the National Cancer Institute (NCI) 60-cell line screen. The NCI data revealed a mean LC50 of ca. 10 µM and a cellular sensitivity profile that did not match that of any other agent in the 765,000 compound database. Subsequent mechanistic studies in melanoma and other multidrug-resistant in vitro cancer models showed that sphaeropsidin A can overcome apoptosis as well as multidrug resistance by inducing a marked and rapid cellular shrinkage related to the loss of intracellular Cl(-) and the decreased HCO3 (-) concentration in the culture supernatant. These changes in ion homeostasis and the absence of effects on the plasma membrane potential were attributed to the sphaeropsidin A-induced impairment of regulatory volume increase (RVI). Preliminary results also indicate that depending on the type of cancer, the sphaeropsidin A effects on RVI could be related to Na-K-2Cl electroneutral cotransporter or Cl(-)/HCO3 (-) anion exchanger(s) targeting. This study underscores the modulation of ion-transporter activity as a promising therapeutic strategy to combat drug-resistant cancers and identifies the fungal metabolite, sphaeropsidin A, as a lead to develop anticancer agents targeting RVI in cancer cells.

Ion channels and transporters in metastasis

An elaborate interplay between ion channels and transporters, components of the cytoskeleton, adhesion molecules, and signaling cascades provides the basis for each major step of the metastatic cascade. Ion channels and transporters contribute to cell motility by letting through or transporting ions essential for local Ca2+, pH and--in cooperation with water permeable aquaporins--volume homeostasis. Moreover, in addition to the actual ion transport they, or their auxiliary subunits, can display non-conducting activities. They can exert kinase activity in order to phosphorylate cytoskeletal constituents or their associates. They can become part of signaling processes by permeating Ca2+, by generating local pH-nanodomains or by being final downstream effectors. A number of channels and transporters are found at focal adhesions, interacting directly or indirectly with proteins of the extracellular matrix, with integrins or with components of the cytoskeleton. We also include the role of aquaporins in cell motility. They drive the outgrowth of lamellipodia/invadopodia or control the number of β1 integrins in the plasma membrane. The multitude of interacting ion channels and transporters (called transportome) including the associated signaling events holds great potential as therapeutic target(s) for anticancer agents that are aimed at preventing metastasis. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Pterostilbene simultaneously induced G0/G1-phase arrest and MAPK-mediated mitochondrial-derived apoptosis in human acute myeloid leukemia cell lines

Background

Pterostilbene (PTER) is a dimethylated analog of the phenolic phytoalexin, resveratrol, with higher anticancer activity in various tumors. Herein, the molecular mechanisms by which PTER exerts its anticancer effects against acute myeloid leukemia (AML) cells were investigated.

Methodology and Principal Findings

Results showed that PTER suppressed cell proliferation in various AML cell lines. PTER-induced G0/G1-phase arrest occurred when expressions of cyclin D3 and cyclin-dependent kinase (CDK)2/6 were inhibited. PTER-induced cell apoptosis occurred through activation of caspases-8-9/-3, and a mitochondrial membrane permeabilization (MMP)-dependent pathway. Moreover, treatment of HL-60 cells with PTER induced sustained activation of extracellular signal-regulated kinase (ERK)1/2 and c-Jun N-terminal kinase (JNK)1/2, and inhibition of both MAPKs by their specific inhibitors significantly abolished the PTER-induced activation of caspases-8/-9/-3. Of note, PTER-induced cell growth inhibition was only partially reversed by the caspase-3-specific inhibitor, Z-DEVE-FMK, suggesting that this compound may also act through a caspase-independent pathway. Interestingly, we also found that PTER promoted disruption of lysosomal membrane permeabilization (LMP) and release of activated cathepsin B.

Conclusion

Taken together, our results suggest that PTER induced HL-60 cell death via MAPKs-mediated mitochondria apoptosis pathway and loss of LMP might be another cause for cell apoptosis induced by PTER.

Regulation and roles of bicarbonate transporters in cancer

A unifying feature of solid tumors is a markedly altered pH profile compared to normal tissues. This reflects that solid tumors, despite completely different origins, often share several phenotypic properties with implications for intra- and extracellular pH. These include: a metabolic shift in most cancer cells toward more acid-producing pathways, reflecting both oncogenic signaling and the development of hypoxia in poorly perfused regions of the tumors; the poorly perfused and often highly dense tumor microenvironment, reducing the diffusive flux of acid equivalents compared to that in normal tissues; and the markedly altered regulation of the expression and activity of pH-regulatory transport proteins in cancer cells. While some of these properties of tumors have been well described in recent years, the great majority of the research in this clinically important area has focused on proton transport, in particular via the Na⁺/H⁺ exchanger 1 (SLC9A1, NHE1) and various H⁺ ATPases. We have, however, recently demonstrated that at least under some conditions, including in vitro models of HER2 positive breast cancer, and measurements obtained directly in freshly dissected human mammary carcinomas, bicarbonate transporters such as the electroneutral Na⁺, HCO⁻₃ cotransporter (SLC4A7, NBCn1), are upregulated and play central roles in pH regulation. In this review, we summarize and discuss the current knowledge regarding the regulation and roles of bicarbonate transporters in cancer. Furthermore, we present new analyses of publicly available expression data demonstrating widely altered expression levels of SLC4- and SLC26 family transporters in breast-, lung-, and colon cancer patients, and we hypothesize that bicarbonate transporter dysregulation may have both diagnostic and therapeutic potential in cancer treatment.

Hispolon induces apoptosis through JNK1/2-mediated activation of a caspase-8, -9, and -3-dependent pathway in acute myeloid leukemia (AML) cells and inhibits AML xenograft tumor growth in vivo

Hispolon is an active phenolic compound of Phellinus igniarius, a mushroom that was recently shown to have antioxidant and anticancer activities in various solid tumors. Here, the molecular mechanisms by which hispolon exerts anticancer effects in acute myeloid leukemia (AML) cells was investigated. The results showed that hispolon suppressed cell proliferation in the various AML cell lines. Furthermore, hispolon effectively induced apoptosis of HL-60 AML cells through caspases-8, -9, and -3 activations and PARP cleavage. Moreover, treatment of HL-60 cells with hispolon induced sustained activation of JNK1/2, and inhibition of JNK by JNK1/2 inhibitor or JNK1/2-specific siRNA significantly abolished the hispolon-induced activation of the caspase-8/-9/-3. In vivo, hispolon significantly reduced tumor growth in mice with HL-60 tumor xenografts. In hispolon-treated tumors, activation of caspase-3 and a decrease in Ki67-positive cells were observed. Our results indicated that hispolon may have the potential to serve as a therapeutic tool to treat AML.

Suppression of mammalian bone growth by membrane transport inhibitors

Bone lengthening during skeletal growth is driven primarily by the controlled enlargement of growth plate (GP) chondrocytes. The cellular mechanisms are unclear but membrane transporters are probably involved. We investigated the role of the Na(+)/H(+) antiporter (NHE1) and anion exchanger (AE2) in bone lengthening and GP chondrocyte hypertrophy in Sprague-Dawley 7-day-old rat (P7) bone rudiments using the inhibitors EIPA (5-(N-ethyl-N-isopropyl)amiloride) and DIDS (4,4-diidothiocyano-2,2-stilbenedisulphonate), respectively. We have also determined cell-associated levels of these transporters along the GP using fluorescent immunohistochemistry (FIHC). Culture of bones with EIPA or DIDS inhibited rudiment growth (50% at approx. 250 and 25 µM, respectively). Both decreased the size of the hypertrophic zone (P < 0.05) but had no effect on overall length or cell density of the GP. In situ chondrocyte volume in proliferative and hypertrophic zones was decreased (P < 0.01) with EIPA but not DIDS. FIHC labeling of NHE1 was relatively high and constant along the GP but declined steeply in the late hypertrophic zone. In contrast, AE2 labeling was relatively low in proliferative zone cells but increased (P < 0.05) reaching a maximum in the early hypertrophic zone, before falling rapidly in the late hypertrophic zone suggesting AE2 might regulate the transition phase of chondrocytes between proliferative and hypertrophic zones. The inhibition of bone growth by EIPA may be due to a reduction to chondrocyte volume set-point. However the effect of DIDS was unclear but could result from inhibition of AE2 and blocking of the transition phase. These results demonstrate that NHE1 and AE2 are important regulators of bone growth.

EGR1 is critical for gastrin-dependent upregulation of anion exchanger 2 in gastric cancer cells

The essential anion exchanger (AE) involved in bicarbonate secretion is AE2/SLC4A2, a membrane protein recognized to be relevant for the regulation of the intracellular pH in several cell types. Here we report that gastrin, a major gastrointestinal hormone, upregulates the expression of AE2 mRNA and protein in a cholecystokinin B receptor dependent manner in gastric cancer cells. The upregulated species of AE2 mRNA originates from the classical upstream promoter of the AE2 gene (here referred to as AE2a1) which provides the binding site for transcription factors early growth response 1 (EGR1) and SP1. EGR1 upregulated the AE2 expression that can be competitively inhibited by SP1 in co-transfection experiments. This competitive inhibition was avoided in cells because the SP1 expression was time-staggered to EGR1 in response to gastrin. Overexpression or knockdown of EGR1 consistently increased or decreased the expression of AE2. Our data linked a novel signal pathway involved in gastrin-stimulated AE2 expression.

Gastrin inhibits a novel, pathological colon cancer signaling pathway involving EGR1, AE2, and P-ERK

Human anion exchanger 2 (AE2) is a plasma membrane protein that regulates intracellular pH and cell volume. AE2 contributes to transepithelial transport of chloride and bicarbonate in normal colon and other epithelial tissues. We now report that AE2 overexpression in colon cancer cells is correlated with expression of the nuclear proliferation marker, Ki67. Survival analysis of 24 patients with colon cancer in early stage or 33 patients with tubular adenocarcinoma demonstrated that expression of AE2 is correlated with poor prognosis. Cellular and molecular experiments indicated that AE2 expression promoted proliferation of colon cancer cells. In addition, we found that transcription factor EGR1 underlies AE2 upregulation and the AE2 sequester p16INK4a (P16) in the cytoplasm of colon cancer cells. Cytoplasmic P16 enhanced ERK phosphorylation and promoted proliferation of colon cancer cells. Gastrin inhibited proliferation of colon cancer cells by suppressing expression of EGR1 and AE2 and by blocking ERK phosphorylation. Taken together, our data describe a novel EGR1/AE2/P16/P-ERK signaling pathway in colon carcinogenesis, with implications for pathologic prognosis and for novel therapeutic approaches.

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.

Role of the HBx oncoprotein in carbonic anhydrase 9 induction

Carbonic anhydrase 9 (CA9), as one of the most hypoxia-responsive genes, has been associated almost exclusively with hypoxic tumors. Its principal role is in pH regulation which helps tumor cells overcome intracellular acidosis and survive extended periods of time with low oxygen. Hypoxia-inducible factor 1 (HIF-1) is the main transcriptional activator of CA9. Hepatitis B virus X protein (HBx) has been shown to increase the transcriptional activity of HIF-1. HBx is often expressed from the gene integrated in the hepatocytes infected persistently and contributes significantly to alterations in host gene expression that can lead to the development of hepatocellular carcinoma (HCC) associated with Hepatitis B virus (HBV). The aim of this study was to determine the effect of HBx on expression of CA9. Transient transfection of HBx led to an increase in the expression of CA9 as assessed by RT-PCR and Western blotting. HBx was able to increase CA9 promoter activity significantly in several cell lines. The effect was mediated via HIF-1 and a functional HRE element located -10/-3 bp upstream of the CA9 transcription initiation site. These data suggest that CA9 may be involved in the development of HCC by contributing to the survival of hepatocytes infected with HBV in liver tissue with fibrosis.