Mammalian cells adapted to growth at pH 6.7 have elevated HSP27 levels and are resistant to cisplatin

Tumour response to hypoxia: understanding the hypoxic tumour microenvironment to improve treatment outcome in solid tumours

Hypoxia is a common feature of solid tumours affecting their biology and response to therapy. One of the main transcription factors activated by hypoxia is hypoxia-inducible factor (HIF), which regulates the expression of genes involved in various aspects of tumourigenesis including proliferative capacity, angiogenesis, immune evasion, metabolic reprogramming, extracellular matrix (ECM) remodelling, and cell migration. This can negatively impact patient outcomes by inducing therapeutic resistance. The importance of hypoxia is clearly demonstrated by continued research into finding clinically relevant hypoxia biomarkers, and hypoxia-targeting therapies. One of the problems is the lack of clinically applicable methods of hypoxia detection, and lack of standardisation. Additionally, a lot of the methods of detecting hypoxia do not take into consideration the complexity of the hypoxic tumour microenvironment (TME). Therefore, this needs further elucidation as approximately 50% of solid tumours are hypoxic. The ECM is important component of the hypoxic TME, and is developed by both cancer associated fibroblasts (CAFs) and tumour cells. However, it is important to distinguish the different roles to develop both biomarkers and novel compounds. Fibronectin (FN), collagen (COL) and hyaluronic acid (HA) are important components of the ECM that create ECM fibres. These fibres are crosslinked by specific enzymes including lysyl oxidase (LOX) which regulates the stiffness of tumours and induces fibrosis. This is partially regulated by HIFs. The review highlights the importance of understanding the role of matrix stiffness in different solid tumours as current data shows contradictory results on the impact on therapeutic resistance. The review also indicates that further research is needed into identifying different CAF subtypes and their exact roles; with some showing pro-tumorigenic capacity and others having anti-tumorigenic roles. This has made it difficult to fully elucidate the role of CAFs within the TME. However, it is clear that this is an important area of research that requires unravelling as current strategies to target CAFs have resulted in worsened prognosis. The role of immune cells within the tumour microenvironment is also discussed as hypoxia has been associated with modulating immune cells to create an anti-tumorigenic environment. Which has led to the development of immunotherapies including PD-L1. These hypoxia-induced changes can confer resistance to conventional therapies, such as chemotherapy, radiotherapy, and immunotherapy. This review summarizes the current knowledge on the impact of hypoxia on the TME and its implications for therapy resistance. It also discusses the potential of hypoxia biomarkers as prognostic and predictive indictors of treatment response, as well as the challenges and opportunities of targeting hypoxia in clinical trials.

Microfluidic-enabled self-organized tumor model for in vitro cytotoxicity assessment of doxorubicin

The advent of microfluidic technologies has enabled a better recapitulation of in vitro tumor model with higher biological relevance over conventional monolayer assays. This work built upon a microfluidic system that supported the spontaneous aggregate formation of tumoral cells under flow-induced dynamic physical forces in a confined microchamber without additional matrix materials. Our findings indicated that fluidic streams significantly modulated the biological and architectural features of human breast adenocarcinoma cell (MCF-7), human hepatocarcinoma cell (HepG2), and human cervix adenocarcinoma cell (HeLa) with cell-type-dependent variation. The microfluidic platform was further integrated with a fluorescence detection and imaging system, allowing for non-invasive monitoring of cellular accumulation and spatial distribution of a chemotherapeutic agent, doxorubicin (DOX). The cytotoxic effects of DOX of various concentrations were determined and compared in MCF-7 cells in conventional two-dimensional (2D) static and microfluidic culture conditions. Dose-dependent response to DOX was noticed in both cultures, whereas tumor micronodules grown in microfluidic devices demonstrated significantly lower sensitivity to DOX at increased concentration. Our platform owns promising potentials as a universal modality for bridging traditional 2D cell cultures and in vivo experimentation for preclinical anticancer drug screening.

Regulation of T cell antitumor immune response by tumor induced metabolic stress

Adaptive T cell immune response is essential for tumor growth control. The efficacy of immune checkpoint inhibitors is regulated by intratumoral immune response. The tumor microenvironment has a major role in adaptive immune response tuning. Tumor cells generate a particular metabolic environment in comparison to other tissues. Tumors are characterized by glycolysis, hypoxia, acidosis, amino acid depletion and fatty acid metabolism modification. Such metabolic changes promote tumor growth, impair immune response and lead to resistance to therapies. This review will detail how these modifications strongly affect CD8 and CD4 T cell functions and impact immunotherapy efficacy.

Effects of extracellular acidity on resistance to chemotherapy treatment: a systematic review

Metabolic alterations in the tumor microenvironment have a complex effect on cancer progression. Extracellular acidity is a consequence of metabolic switch in cancer and results in cell phenotypes with higher resistance to chemotherapeutics. However, mechanisms underlying the relationship between the extracellular acidity and chemoresistance are not clearly understood. This systematic review was carried out by searching the databases PubMed and EMBASE using the keywords "cancer" and "acidosis" or "acidic" and "chemoresistance" or "drug resistance." In vitro and in vivo studies that evaluated the effects of acidification of the tumor microenvironment on chemotherapeutic treatments were included. Literature reviews, letters to the editor, and articles that were not published in English were excluded. The search resulted in a total of 352 articles. After discarding 75 duplicate references, 277 articles were analyzed by sequentially reading through their titles, abstracts, and finally full-text. A total of 14 articles was selected. Acidification of the tumor microenvironment can trigger resistance through different mechanisms, such as increase in drug efflux transporters, inhibition of proton pumps, induction of the unfolded protein response (UPR), and cellular autophagy.

Chemoresistance of Cancer Cells: Requirements of Tumor Microenvironment-mimicking In Vitro Models in Anti-Cancer Drug Development

For decades, scientists have been using two-dimensional cell culture platforms for high-throughput drug screening of anticancer drugs. Growing evidence indicates that the results of anti-cancer drug screening vary with the cell culture microenvironment, and this variation has been proposed as a reason for the high failure rate of clinical trials. Since the culture condition-dependent drug sensitivity of anti-cancer drugs may negatively impact the identification of clinically effective drug candidates, more reliable in vitro cancer platforms are urgently needed. In this review article, we provide an overview of how cell culture conditions can alter drug efficacy and highlight the importance of developing more reliable cancer drug testing platforms for use in the drug discovery process. The environmental factors that can alter drug delivery and efficacy are reviewed. Based on these observations of chemoresistant tumor physiology, we summarize the recent advances in the fabrication of in vitro cancer models and the model-dependent cytotoxicity of anti-cancer drugs, with a particular focus on engineered environmental factors in these platforms. It is believed that more physiologically relevant cancer models can revolutionize the drug discovery process.

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.

Extracellular acidity enhances tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis via DR5 in gastric cancer cells

The tumor microenvironment greatly influences cancer cell characteristics, and acidic extracellular pH has been implicated as an essential factor in tumor malignancy and the induction of drug resistance. Here, we examined the characteristics of gastric carcinoma (GC) cells under conditions of extracellular acidity and attempted to identify a means of enhancing treatment efficacy. Acidic conditions caused several changes in GC cells adversely affecting chemotherapeutic treatment. Extracellular acidity did inhibit GC cell growth by inducing cell cycle arrest, but did not induce cell death at pH values down to 6.2, which was consistent with down-regulated cyclin D1 and up-regulated p21 mRNA expression. Additionally, an acidic environment altered the expression of atg5, HSPA1B, collagen XIII, collagen XXAI, slug, snail, and zeb1 genes which are related to regulation of cell resistance to cytotoxicity and malignancy, and as expected, resulted in increased resistance of cells to multiple chemotherapeutic drugs including etoposide, doxorubicin, daunorubicin, cisplatin, oxaliplatin and 5-FU. Interestingly, however, acidic environment dramatically sensitized GC cells to apoptosis induced by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Consistently, the acidity at pH 6.5 increased mRNA levels of DR4 and DR5 genes, and also elevated protein expression of both death receptors as detected by immunoblotting. Gene silencing analysis showed that of these two receptors, the major role in this effect was played by DR5. Therefore, these results suggest that extracellular acidity can sensitize TRAIL-mediated apoptosis at least partially via DR5 in GCs while it confers resistance to various type of chemotherapeutic drugs.

Rethinking the Combination of Proton Exchanger Inhibitors in Cancer Therapy

Microenvironmental acidity is becoming a key target for the new age of cancer treatment. In fact, while cancer is characterized by genetic heterogeneity, extracellular acidity is a common phenotype of almost all cancers. To survive and proliferate under acidic conditions, tumor cells up-regulate proton exchangers and transporters (mainly V-ATPase, Na⁺/H⁺ exchanger (NHE), monocarboxylate transporters (MCTs), and carbonic anhydrases (CAs)), that actively extrude excess protons, avoiding intracellular accumulation of toxic molecules, thus becoming a sort of survival option with many similarities compared with unicellular microorganisms. These systems are also involved in the unresponsiveness or resistance to chemotherapy, leading to the protection of cancer cells from the vast majority of drugs, that when protonated in the acidic tumor microenvironment, do not enter into cancer cells. Indeed, as usually occurs in the progression versus malignancy, resistant tumor clones emerge and proliferate, following a transient initial response to a therapy, thus giving rise to more malignant behavior and rapid tumor progression. Recent studies are supporting the use of a cocktail of proton exchanger inhibitors as a new strategy against cancer.

The acidic microenvironment as a possible niche of dormant tumor cells

Although surgical excision, chemo-, and radio-therapy are clearly advanced, tumors may relapse due to cells of the so-called "minimal residual disease". Indeed, small clusters of tumor cells persist in host tissues after treatment of the primary tumor elaborating strategies to survive and escape from immunological attacks before their relapse: this variable period of remission is known as "cancer dormancy". Therefore, it is crucial to understand and consider the major concepts addressing dormancy, to identify new targets and disclose potential clinical strategies. Here, we have particularly focused the relationships between tumor microenvironment and cancer dormancy, looking at a re-appreciated aspect of this compartment that is the low extracellular pH. Accumulating evidences indicate that acidity of tumor microenvironment is associated with a poor prognosis of tumor-bearing patients, stimulates a chemo- and radio-therapy resistant phenotype, and suppresses the tumoricidal activity of cytotoxic lymphocytes and natural killer cells, and all these aspects are useful for dormancy. Therefore, this review discusses the possibility that acidity of tumor microenvironment may provide a new, not previously suggested, adequate milieu for "dormancy" of tumor cells.

Proton channels and exchangers in cancer

Although cancer is characterized by an intratumoral genetic heterogeneity, a totally deranged pH control is a common feature of most cancer histotypes. Major determinants of aberrant pH gradient in cancer are proton exchangers and transporters, including V-ATPase, Na+/H+ exchanger (NHE), monocarboxylate transporters (MCTs) and carbonic anhydrases (CAs). Thanks to the activity of these proton transporters and exchangers, cancer becomes isolated and/or protected not only from the body reaction against the growing tumor, but also from the vast majority of drugs that when protonated into the acidic tumor microenvironment do not enter into cancer cells. Proton transporters and exchangers represent a key feature tumor cells use to survive in the very hostile microenvironmental conditions that they create and maintain. Detoxifying mechanisms may thus represent both a key survival option and a selection outcome for cells that behave as unicellular microorganisms rather than belonging to an organ, compartment or body. It is, in fact, typical of malignant tumors that, after a clinically measurable yet transient initial response to a therapy, resistant tumor clones emerge and proliferate, thus bursting a more malignant behavior and rapid tumor progression. This review critically presents the background of a novel and efficient approach that aims to fight cancer through blocking or inhibiting well characterized proton exchangers and transporters active in human cancer cells. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Theoretical predictions of lactate and hydrogen ion distributions in tumours

High levels of lactate and H(+)-ions play an important role in the invasive and metastatic cascade of some tumours. We develop a mathematical model of cellular pH regulation focusing on the activity of the Na(+)/H(+) exchanger (NHE) and the lactate/H(+) symporter (MCT) to investigate the spatial correlations of extracellular lactate and H(+)-ions. We highlight a crucial role for blood vessel perfusion rates in determining the spatial correlation between these two cations. We also predict critical roles for blood lactate, the activity of the MCTs and NHEs on the direction of the cellular pH gradient in the tumour. We also incorporate experimentally determined heterogeneous distributions of the NHE and MCT transporters. We show that this can give rise to a higher intracellular pH and a lower intracellular lactate but does not affect the direction of the reversed cellular pH gradient or redistribution of protons away from the glycolytic source. On the other hand, including intercellular gap junction communication in our model can give rise to a reversed cellular pH gradient and can influence the levels of pH.

Regulation of tumour intracellular pH: a mathematical model examining the interplay between H+ and lactate

Non-invasive measurements of pH have shown that both tumour and normal cells have intracellular pH (pHi) that lies on the alkaline side of neutrality (7.1-7.2). However, extracellular pH (pHe) is reported to be more acidic in some tumours compared to normal tissues. Many cellular processes and therapeutic agents are known to be tightly pH dependent which makes the study of intracellular pH regulation of paramount importance. We develop a mathematical model that examines the role of various membrane-based ion transporters in tumour pH regulation, in particular, with a focus on the interplay between lactate and H(+) ions and whether the lactate/H(+) symporter activity is sufficient to give rise to the observed reversed pH gradient that is seen is some tumours. Using linear stability analysis and numerical methods, we are able to gain a clear understanding of the relationship between lactate and H(+) ions. We extend this analysis using perturbation techniques to specifically examine a rapid change in H(+)-ion concentrations relative to variations in lactate. We then perform a parameter sensitivity analysis to explore solution robustness to parameter variations. An important result from our study is that a reversed pH gradient is possible in our system but for unrealistic parameter estimates-pointing to the possible involvement of other mechanisms in cellular pH gradient reversal, for example acidic vesicles, lysosomes, golgi and endosomes.

BACPTDP: a water-soluble camptothecin pro-drug with enhanced activity in hypoxic/acidic tumors

Hypoxia is a common feature of solid tumors. Up-regulation of hypoxia-inducing factor-1 (HIF-1) occurs in the majority of primary malignant tumors and in two-thirds of metastases, while most normal tissues are negative. HIF-1 induces the glycolytic phenotype, which creates an acidic extracellular microenvironment and associated pH gradient such that drugs that are weak acids are selectively taken up and retained in acidic tumors. 7-Butyl-10-amino-camptothecin (BACPT) is a prime example of an agent that can exploit the tumor pH gradient for enhanced selectivity.

PURPOSE

This study profiles the antitumor activity of BACPT in vitro and its water-soluble dipeptide ester, BACPTDP, in vivo.

METHODS

Antitumor activity was evaluated by proliferation assays in cancer cell lines and in murine xenograft models for human neuroblastoma (IMR-32), colon (HT29), ovarian (SK-OV-3), pancreatic (Panc-1), glioma (SF-295) and non-small-cell lung (NCI-H460) cancers.

RESULTS

BACPT had superior antiproliferative activity compared to established drugs in monolayer cultures of human neuroblastoma and pancreatic tumor cell lines and in 3-dimensional histocultures of colon and primary ovarian cancer. Antitumor activity of BACPTDP was comparable to irinotecan in IMR-32, HT29, SF-295 and NCI-H460 xenografts, significantly greater in SK-OV-3 and in Panc-1 where complete regressions were observed. Combination of BACPT with gemcitabine produced additive to synergistic interactions in Panc-1 cells that were independent of drug ratio and optimal when gemcitabine was administered 24 h prior to BACPT.

CONCLUSIONS

BACPTDP is a water-soluble camptothecin pro-drug that spontaneously generates the lipid-soluble active agent, BACPT. This topoisomerase inhibitor exploits solid tumor physiology for improved selectivity and activity against multiple tumor types with particular promise for use in treating pediatric neuroblastoma and pancreatic carcinoma.

Expression of chemoresistance-related genes and heat shock protein 72 in hyperthermic isolated limb perfusion of malignant melanoma: an experimental study

Hyperthermic isolated limb perfusion (HILP) is considered an established treatment for multiple locoregional intransit metastases in malignant melanoma of the extremities. Various mechanisms such as the expression of chemoresistance genes and heat shock proteins by the tumor may be responsible for varying response rates and locoregional recurrences of the treatment. The aim of the experimental animal study was to investigate the direct impact of HILP on such mechanisms of resistance. Tissue temperature, administration of the cytostatic drug, and duration of perfusion were varied. Expression of the chemoresistance genes mdr1, mrp1, mrp2, and lrp and of heat shock protein 72 (HSP72) in the tumor tissue was analysed using RT-PCR and western blot analysis. The untreated SK-MEL-3 tumor expressed mdr1, mrp1, and lrp, but not mrp2. Neither variation of temperature, administration of the cytostatic drug, nor duration of perfusion changed the expression of this “resistance pattern”. In contrast to the cytostatic drug, hyperthermia causes a persistent induction of HSP72. Both observations could offer a potential explanation for failure of HILP in malignant melanoma.

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.

Proteomics-based strategy to delineate the molecular mechanisms of the metastasis suppressor gene BRMS1

The breast cancer metastasis suppressor 1 (BRMS1) gene has been shown to suppress metastasis without affecting the growth of the primary tumor in mouse models. It has also been shown to suppress the metastasis of tumors derived from breast, melanoma, and, more recently, ovarian carcinoma (see ref 1). However, how BRMS1 exerts its metastasis suppressor function remains unknown. To shed light into its metastatic mechanism of action, the sensitive 2D-DIGE analysis coupled with MS has been used to identify proteins differentially expressed by either overexpressing (Mel-BRMS1) or silencing BRMS1 (sh635) in a melanoma cell line. After comparison of the protein profiles from WT, Mel-BRMS1, and sh635 cells, 79 spots were found to be differentially expressed. Mass spectrometry analysis allowed the unambiguous identification of 55 polypeptides, corresponding to 43 different proteins. Interestingly, more than 75% of the identified proteins were down-regulated in Mel-BRMS1 cells compared to WT. In contrast, all the identified proteins in sh635 cells extracts were up-regulated compared to WT. Most of the deregulated proteins are involved in cell growth/maintenance and signal transduction among other cell processes. Six differentially expressed proteins (Hsp27, Alpha1 protease inhibitor, Cofilin1, Cathepsin D, Bone morphogenetic protein receptor2, and Annexin2) were confirmed by immunoblot and functional assays. Excellent correlation was found between DIGE analysis and immunoblot results, indicating the reliability of the analysis. Available evidence on the reported functions of the identified proteins supports the emerging role of BRMS1 as negative regulator of the metastasis development. This work opens an avenue for the molecular mechanisms' characterization of metastasis suppressor genes with the aim to understand their roles.

Efficacy of chemotherapeutic agents under hypoxic conditions in pulmonary adenocarcinoma multidrug resistant cell line

Hypoxia is often observed in solid tumors. The aim of this study was to investigate the efficacy of seven cytotoxic drugs against the pulmonary adenocarcinoma multidrug-resistant cell line A549/MDR under hypoxia (3% O(2)), and to explore the possible mechanisms for the change of efficacy. The efficacy of cytotoxic drugs under hypoxic conditions was different from that under normoxia. Proliferation of A549/MDR cells was enhanced under hypoxia and no close correlation was found between proliferation and cytotoxic effects. Under hypoxia, the efficacy of rhodamine123 efflux was unchanged; the culture medium became more acidic and the generation of reactive oxygen species (ROS) was decreased. The intracellular fluorescence intensity of daunorubicin was much lower in this acidic microenvironment. These results indicate that susceptibility to drugs was greatly influenced by hypoxia and different intracellular drug concentrations induced by microenvironment acidification which may be the main cause of the change in drug efficacy. In addition, proliferation may change resistance to study drugs under hypoxia for A549/MDR cells. The decreased generation of ROS may be another reason for the resistance of A549/MDR cell line to daunorubicin under hypoxic conditions. Drug exclusion mediated by P-gp may not be the key reason.

Heat Shock Protein 27 Protects L929 Cells from Cisplatin-Induced Apoptosis by Enhancing Akt Activation and Abating Suppression of Thioredoxin Reductase Activity

PURPOSE

Heat shock protein 27 (Hsp27) is up-regulated in multiple malignancies and implicated in cisplatin resistance. It is attempted to know how Hsp27 endues cell with cisplatin resistance by interfering with upstream of both apoptosis signal-regulating kinase 1 (ASK1)/p38 mitogen-activated protein kinase-activated apoptotic signaling and serine/threonine kinase Akt-dependent survival signaling.

EXPERIMENTAL DESIGN

The mouse L929 cells stably transfected with human Hsp27 or its dominant-negative mutant and the human cervical cancer HeLa cells transfected with Hsp27 siRNA were used. The cisplatin-induced apoptosis and activation of ASK1, p38, and Akt were compared in control cells, cells overexpressing Hsp27, and cells with their endogenous Hsp27 knocked down.

RESULTS

Hsp27 effectively protected the cells from cisplatin-induced DNA fragmentation. The p38 inhibitors obviously decreased whereas Akt inhibitors markedly increased the apoptotic fraction in cisplatin-treated cells. Overexpression of Hsp27 doubly enhanced the drug-induced Akt activation while substantially depressing the drug-induced activation of ASK1 and p38. Knockdown of the endogenous Hsp27 in HeLa cells resulted in the effects opposite to that observed in the Hsp27-overexpressing cells. Enhancement of Akt activation is associated with complex formation between Akt and Hsp27, whereas depression of ASK1/p38 activation is attributed to a reversion of the drug-induced inhibition of thioredoxin reductase activity and subsequent oxidation of thioredoxin.

CONCLUSIONS

Hsp27 endues cells with cisplatin resistance via depression of the drug-induced ASK1/p38 activation and enhancement of the drug-induced Akt activation. This study revealed the intervention of Hsp27 in upstream of both ASK1/p38 apoptotic signaling and phosphatidylinositol 3-kinase/Akt survival signaling. Therefore, the inhibition of Hsp27 may be a novel strategy of cancer chemotherapy.

Camptothecin analogs with enhanced activity against human breast cancer cells. II. Impact of the tumor pH gradient

Human breast tumors often exist in an acidic and hypoxic microenvironment, which can promote resistance to radiation and chemotherapies. A tumor-selective pH gradient arises in these tumors which favors uptake and retention of drugs like camptothecin that are weak acids. We evaluated the effect of alkyl substitutions at the 7 position in seven CPTs with varying groups at the 10 position on modulation by acidic extracellular pH in three human breast cancer cell lines. Growth inhibition was assessed by propidium iodide staining of nucleic acids in human breast cancer cells cultured at either extracellular pH 6.8 or 7.4 that were (1) hormone-sensitive (MCF-7/wt), (2) hormone insensitive (MDA-MB-231), or (3) alkylator-resistant (MCF-7/4-hc). Over 10-fold pH modulation was observed in 7-halomethyl analogs of methylenedioxy-CPT and in 7-alkyl analogs of 10-amino-CPT. Of 39 analogs tested, the overall pattern of activity across breast tumor cell lines was similar with some notable exceptions. For example, 7-propyl-10-amino-CPT was modulated 16- to 20-fold by acidic extracellular pH in the MCF-7 cell lines, but only 6-fold in MDA-MB-231 cells. One mechanism that can contribute to pH modulation is enhanced cellular drug uptake and retention. In MCF-7/wt cells, uptake of 10-amino-CPT increased 4-fold, while retention increased over 10-fold at acidic extracellular pH. In addition, gene expression analysis of MCF-7/wt cells indicated that expression of a number of genes changed under acidic culture conditions, including down-regulation of the CPT efflux protein pump breast cancer resistance protein (BCRP). Interestingly, expression of topoisomerase I, the molecular target of CPT, was not affected by acidic growth conditions. These results highlight the importance of maintaining key features of tumor physiology in cell culture models used to study cancer biology and to discover and develop new anticancer drugs. While several substitutions at the 7 and 10 positions enhance potency, 7-halomethyl and 10-amino CPT analogs show selective activity at the acidic pH common to the microenvironment of most solid tumors.

Small heat shock proteins HSP27 and alphaB-crystallin: cytoprotective and oncogenic functions

Heat shock protein-27 (HSP27) and alphaB-crystallin are ubiquitous small heat shock proteins whose expression is induced in response to a wide variety of physiological and environmental insults. They allow the cells to survive in otherwise lethal conditions. Various mechanisms have been proposed to account for the cytoprotective functions of these small heat shock proteins. First, these proteins are powerful molecular chaperones whose main function is to prevent the aggregation of nascent and stress-accumulated misfolded proteins. Second, they interact directly with various components of the tightly regulated programmed cell death machinery, upstream and downstream of the mitochondrial events. Third, they appear to play a role in the proteasome-mediated degradation of selected proteins. Both HSP27 and alphaB-crystallin were also proposed to participate in the development of neurodegenerative diseases and malignant tumors in which their overexpression could induce drug resistance. Altogether, these properties suggest that these small heat shock proteins are appropriate targets for modulating cell death pathways.

Stress response inhibits the nephrotoxicity of cisplatin

Salt loading and saline hydration are used to protect patients from cisplatin-induced nephrotoxicity. The mechanism by which salt exerts its protective effect is unknown. As part of an ongoing study of cisplatin nephrotoxicity, an in vitro assay system was developed that models the in vivo exposure and response of proximal tubule cells to cisplatin. In this study, it was discovered that the toxicity of cisplatin toward LLC-PK1 cells varied dramatically according to the tissue culture media used for 3-h cisplatin exposure. Further experiments revealed that minor variations in the sodium concentration among standard tissue culture media modulated cisplatin nephrotoxicity. NaCl has been shown to protect against cisplatin-induced nephrotoxicity in vivo but has never before been demonstrated in vitro. NaCl did not alter the cellular accumulation of cisplatin. NaCl altered the osmolarity of the external media, and its effect was replicated by substituting equiosmolar concentrations of impermeant anions or cations. The change in osmolarity triggered a stress response within the cell that modulated sensitivity to cisplatin. These data resolve several long-standing controversies regarding the mechanism by which salt loading protects the kidney from cisplatin-induced nephrotoxicity.

Development of cross-resistance between heat and cisplatin or hydroxyurea treatments in FaDu squamous carcinoma cells

BACKGROUND

Induction of hyperthermia by radiofrequency ablation is gaining popularity in treating a variety of solid tumors. This study examined an impact of sublethal heat treatment interacted with chemotherapeutic drugs on the survival of head and neck squamous carcinoma cells using in vitro model.

MATERIALS AND METHODS

FaDu cells were subjected to heat treatment at 42 degrees C or 45 degrees C for 15 min either before or after exposure to cisplatin or hydroxyurea. The survival of cells was determined by 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide assay. The RNA and protein levels of various heat shock proteins were examined by reverse transcription polymerase chain reaction and Western blot analysis, respectively. Cell cycle progression was analyzed by flow cytometry with propidium iodide staining.

RESULTS

FaDu cells preheated to 45 degrees C exhibited an increased resistance to hydroxyurea but not to cisplatin. The heat treatment resulted in induction of HSP70 expression at transcript and protein levels, but there was no change in expression of HSP90beta and HSP27. After heat treatment, cells accumulated in S-phase at 3 h and proceeded to G(2)/M phase at 24 h. When cells pre-exposed to drugs for 24 h, the cisplatin-treated cells exhibited a higher thermotolerance than the hydroxyurea-treated cells at heat treatment of 45 degrees C. Cisplatin and hydroxyurea caused cells to accumulate in S-phase and increased the protein expression of HSP27 but not HSP90beta and HSP70.

CONCLUSION

FaDu cells surviving the heat treatment expressed HSP70 and disrupted cell cycle progression, which resulted in developing a resistance to subsequent hydroxyurea treatment. However, the heat treatment did not have an effect on the sensitivity to cisplatin. In the reversed procedure, pre-exposure to hydroxyurea and cisplatin resulted in developing a thermotolerance.

Immunohistochemical and clinicopathologic analysis of response to neoadjuvant therapy for esophageal squamous cell carcinoma

Recently, neoadjuvant chemotherapy combined with radiation (NAT) has been used in the active treatment of progressive esophageal cancer (T4). However, many patients are resistant to supplemental therapy, and it is necessary to to be aware that side-effects may occur. Accordingly, to minimize adverse reactions and cost, it is important to determine the indications for NAT. We investigated 34 patients with T4 esophageal squamous cell carcinoma and examined the relation between the effects of NAT and immunohistochemical or additional clinicopathologic factors. There was no relation between clinicopathologic factors and immunohistochemical findings (p53 or hsp27 expression), and no clinicopathologic factors showed a relation to a supplemental therapeutic effect. In addition, there was no correlation between p53 staining and therapeutic effects (P=0.734). In contrast, there was a correlation (P=0.0058) between hsp27 staining and therapeutic effect. In conclusion, the usefulness of hsp27 immunostaining in predicting the therapeutic effect of NAT was confirmed in T4 esophageal squamous cell carcinoma.

A low-pH culture condition enhances the radiosensitizing effect of wortmannin

PURPOSE

The radiosensitizing effect of wortmannin on human tumor cells in a low-pH microenvironment was compared with that in a neutral-pH environment.

METHODS AND MATERIALS

A172 human glioblastoma cells, A549 human lung adenocarcinoma cells, and HMV-1 human melanoma cells were treated with 20 microM wortmannin 2 h before irradiation, and cell survival was examined. A low-pH microenvironment was simulated by exposing cells to low-pH culture medium for 24 h before wortmannin treatment. The effects of wortmannin on the repair of DNA double-strand breaks (dsbs) after 50-Gy irradiation in both low- and neutral-pH conditions were measured by pulsed-field gel electrophoresis. Expression of the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) in low-pH conditions was also compared with that in neutral-pH conditions by Western blot analysis.

RESULTS

The radiosensitizing effect of wortmannin was greater in low-pH cultures than in neutral-pH cultures for all cell lines. The fast-rejoining component of DNA dsb repair was inhibited more strongly in low-pH than in neutral-pH conditions, although there was little difference in DNA-PKcs expression between groups.

CONCLUSIONS

The low-pH culture condition, which was designed to mimic the microenvironment of the central tumor mass in actively proliferating solid tumors, enhanced the radiosensitizing effect of wortmannin by inhibiting the fast-rejoining component of DNA dsb repair and by prolonging the retention of nonrejoined DNA dsbs.

pH and drug resistance in tumors

Uptake of weakly ionizing drugs by tumours is greatly influenced by the interstitial and intracellular pH, and the ionization properties of the drug. Extracellular pH in tumors is acidic, while the intracellular pH is in the neutral-to-alkaline range. Tumors of the bladder, kidney and gastrointestinal system in particular are exposed to extremes of pH. Strategies for exhancing and exploiting acid-outside plasmalemmal pH gradients to drive the uptake of weak acid drugs into tumors are discussed, as are techniques for alkalinizing tissues to improve response to weak base drugs. The participation of acidic intracellular vesicles in non-specific drug resistance is explored. Copyright 2000 Harcourt Publishers Ltd.