Suppression of multidrug resistance via inhibition of heat shock factor by quercetin in MDR cells

Mode of Action of Heat Shock Protein (HSP) Inhibitors against Viruses through Host HSP and Virus Interactions

Misfolded proteins after stress-induced denaturation can regain their functions through correct re-folding with the aid of molecular chaperones. As a molecular chaperone, heat shock proteins (HSPs) can help client proteins fold correctly. During viral infection, HSPs are involved with replication, movement, assembly, disassembly, subcellular localization, and transport of the virus via the formation of macromolecular protein complexes, such as the viral replicase complex. Recent studies have indicated that HSP inhibitors can inhibit viral replication by interfering with the interaction of the virus with the HSP. In this review, we describe the function and classification of HSPs, the transcriptional mechanism of HSPs promoted by heat shock factors (HSFs), discuss the interaction between HSPs and viruses, and the mode of action of HSP inhibitors at two aspects of inhibiting the expression of HSPs and targeting the HSPs, and elaborate their potential use as antiviral agents.

Phytol and Heptacosane Are Possible Tools to Overcome Multidrug Resistance in an In Vitro Model of Acute Myeloid Leukemia

Drug resistance is the ability of cancer cells to gain resistance to both conventional and novel chemotherapy agents, and remains a major problem in cancer therapy. Resistance mechanisms are multifactorial and involve more strictly pharmacological factors, such as P-glycoprotein (P-gp) and biological factors such as inhibitor of apoptosis proteins (IAPs) and the nuclear factor-kappa B (NF-κB) pathway. Possible therapeutic strategies for the treatment of acute myeloid leukemia (AML) have increased in recent years; however, drug resistance remains a problem for most pa-tients. Phytol and heptacosane are the major compounds of Euphorbia intisy essential oil (EO) which were demonstrated to inhibit P-gp in a multidrug resistant in vitro model of AML. This study investigated the mechanism by which phytol and heptacosane improve P-gp-mediated drug transport. Phytol suppresses the P-gp expression via NF-κB inhibition and does not seem to act on the efflux system. Heptacosane acts as a substrate and potent P-gp inhibitor, demonstrating the ability to retain the substrate doxorubicin inside the cell and enhancing its cytotoxic effects. Our results suggest that these compounds act as non-toxic modulators of P-gp through different mechanisms and are able to revert P-gp-mediated drug resistance in tumor cells.

Heat Shock Proteins and HSF1 in Cancer

Fitness of cells is dependent on protein homeostasis which is maintained by cooperative activities of protein chaperones and proteolytic machinery. Upon encountering protein-damaging conditions, cells activate the heat-shock response (HSR) which involves HSF1-mediated transcriptional upregulation of a group of chaperones - the heat shock proteins (HSPs). Cancer cells experience high levels of proteotoxic stress due to the production of mutated proteins, aneuploidy-induced excess of components of multiprotein complexes, increased translation rates, and dysregulated metabolism. To cope with this chronic state of proteotoxic stress, cancers almost invariably upregulate major components of HSR, including HSF1 and individual HSPs. Some oncogenic programs show dependence or coupling with a particular HSR factor (such as frequent coamplification of HSF1 and MYC genes). Elevated levels of HSPs and HSF1 are typically associated with drug resistance and poor clinical outcomes in various malignancies. The non-oncogene dependence ("addiction") on protein quality controls represents a pancancer target in treating human malignancies, offering a potential to enhance efficacy of standard and targeted chemotherapy and immune checkpoint inhibitors. In cancers with specific dependencies, HSR components can serve as alternative targets to poorly druggable oncogenic drivers.

Antigiardial Activity of Acetylsalicylic Acid Is Associated with Overexpression of HSP70 and Membrane Transporters

Giardia lamblia is a flagellated protozoan responsible for giardiasis, a worldwide diarrheal disease. The adverse effects of the pharmacological treatments and the appearance of drug resistance have increased the rate of therapeutic failures. In the search for alternative therapeutics, drug repositioning has become a popular strategy. Acetylsalicylic acid (ASA) exhibits diverse biological activities through multiple mechanisms. However, the full spectrum of its activities is incompletely understood. In this study we show that ASA displayed direct antigiardial activity and affected the adhesion and growth of trophozoites in a time-dose-dependent manner. Electron microscopy images revealed remarkable morphological alterations in the membrane, ventral disk, and caudal region. Using mass spectrometry and real-time quantitative reverse transcription (qRT-PCR), we identified that ASA induced the overexpression of heat shock protein 70 (HSP70). ASA also showed a significant increase of five ATP-binding cassette (ABC) transporters (giABC, giABCP, giMDRP, giMRPL and giMDRAP1). Additionally, we found low toxicity on Caco-2 cells. Taken together, these results suggest an important role of HSPs and ABC drug transporters in contributing to stress tolerance and protecting cells from ASA-induced stress.

Chondroitin sulfate-based nanoparticles for enhanced chemo-photodynamic therapy overcoming multidrug resistance and lung metastasis of breast cancer

As a major therapeutic approach for cancer treatment, the effectiveness of chemotherapy is challenged by multidrug resistance (MDR). Herein, we fabricated novel redox-responsive, chondroitin sulfate-based nanoparticles that could simultaneously deliver quercetin (chemosensitizer), chlorin e6 (photosensitizer) and paclitaxel (chemotherapeutic agent) to exert enhanced chemo-photodynamic therapy for overcoming MDR and lung metastasis of breast cancer. In vitro cell study showed that nanoparticles down-regulated the expression of P-glycolprotein (P-gp) on MCF-7/ADR cells and thereby improved the anticancer efficacy of PTX against MCF-7/ADR cells. Moreover, NIR laser irradiation could induce nanoparticles to generate cellular reactive oxygen species (ROS), leading to mitochondrial membrane potential loss, and meanwhile facilitating lysosomal escape of drugs. Importantly, the novel nanoplatform exhibited effective in vivo MDR inhibition and anti-metastasis efficacy through enhanced chemo-photodynamic therapy. Thus, the study suggested that the multifunctional nanoplatform had good application prospect for effective breast cancer therapy.

Impact of Polyallylamine Hydrochloride on Gene Expression and Karyotypic Stability of Multidrug Resistant Transformed Cells

The synthetic polymer, polyallylamine hydrochloride (PAA), is found in a variety of applications in biotechnology and medicine. It is used in gene and siRNA transfer, to form microcapsules for targeted drug delivery to damaged and tumor cells. Conventional chemotherapy often does not kill all cancer cells and leads to multidrug resistance (MDR). Until recently, studies of the effects of PAA on cells have mainly focused on their morphological and genetic characteristics immediately or several hours after exposure to the polymer. The properties of the cell progeny which survived the sublethal effects of PAA and resumed their proliferation, were not monitored. The present study demonstrated that treatment of immortalized Chinese hamster cells CHLV-79 RJK sensitive (RJK) and resistant (RJKEB) to ethidium bromide (EB) with cytotoxic doses of PAA, selected cells with increased karyotypic instability, were accompanied by changes in the expression of p53 genes c-fos, topo2-α, hsp90, hsc70. These changes did not contribute to the progression of MDR, accompanied by the increased sensitivity of these cells to the toxic effects of doxorubicin (DOX). Our results showed that PAA does not increase the oncogenic potential of immortalized cells and confirmed that it can be used for intracellular drug delivery for anticancer therapy.

Decreased expression of FBXW7 by ERK1/2 activation in drug-resistant cancer cells confers transcriptional activation of MDR1 by suppression of ubiquitin degradation of HSF1

The acquisition of MDR1-mediated chemoresistance poses a major obstacle to the success of conventional chemotherapeutic agents. HSF1 is also involved in chemoresistance, and several studies have demonstrated the relationship between HSF1 and MDR1 but without any consistent results. Paclitaxel- and doxorubicin-resistant cancer cells showed higher expression of MDR1 and HSF1. Depletion of HSF1 decreased mdr1 expression at mRNA level, and HSF1 directly interacted with the promoter site of mdr1, suggesting its role as a transcriptional regulator of MDR1. Phosphorylation of Ser303/307, which was involved in protein stability of HSF1 by FBXW7-mediated degradation, was found to be important for transcriptional activation of mdr1. Drug-resistant cells showed decreased expression of FBXW7, which was mediated by the activation of ERK1/2, thus indicating that over-activation of ERK1/2 in drug-resistant cells decreased FBXW7 protein stability, which finally inhibited protein degradation of pHSF1 at Ser303/307. There was a positive correlation between immunofluorescence data of pHSF1 at Ser303/307 and MDR1 in carcinogen-induced rat mammary tumors and human lung cancers. These findings identified the post-translational mechanisms of HSF1 transcription in MDR1 regulation of drug resistance development.

Physicochemical characterization of dual action liposomal formulations: anticancer and antimicrobial

Background

Cancer till date remains one of the world's most life threatening disease accompanied by risk of secondary infections. Therefore formulations carrying anticancer drugs which can also decrease the risk of secondary infection are inevitable. Chemotherapeutic drug doxorubicin along with flavonoids quercetin and epigallocatechin gallate (EGCG) is simultaneously loaded on liposomal formulation exploiting the amphiphilic property of the liposomes.

Results

Atomic force microscope imaging reveal the size of liposomal formulation loaded with doxorubicin, quercetin and EGCG to be greater than void liposome confirming the presence of drugs. Liposomal stability is improved by PEGylation; adding to the drug release time in vitro. The charge of phosphatidylcholine is rendered positive by coating the formulation with histone. The average size of the formulation is 342 nm. The encapsulation efficiency of doxorubicin, quercetin and EGCG is found to be 65.8%, 96.8% and 98% respectively. The above formulation demonstrated both anticancer and antimicrobial activity.

Conclusion

The formulation will provide dual anticancer and antimicrobial therapy thereby evading secondary infection in cancer patients along with chemotherapy.

Reversal Effect of Oxypeucedanin on P-glycoprotein-mediated Drug Transport

P-glycoprotein affects the transport of numerous drugs including chemotherapeutic drugs vincristine sulfate (VCR) and docetaxel (DTX), and is one of the main causes for multidrug resistance. Our previous studies have shown that oxypeucedanin (OPD) can enhance the intestinal transit of puerarin and VCR. However, the underlying mechanism is unclear. This study investigated the potential mechanism by which OPD improves P-gp-mediated drug transport. Molecular docking was performed to predict the binding force between OPD and P-gp and the contribution of OPD on P-gp activity. We observed the effect of OPD on the transport of VCR in MDCK-MDR1 cell monolayer and also measured the plasma pharmacokinetic parameters of DTX in the presence and absence of OPD by LC-MS/MS. Moreover, we further investigated the reversal mechanism of OPD on P-gp-mediated drug transport by determining the intracellular accumulation of Rhodamine-123 (Rh123) and P-gp ATPase activity as well as protein expression and mRNA level of P-gp. Our molecular docking results revealed that the binding force between OPD and P-gp was much lower than that between P-gp and verapamil (a P-gp substrate). The transport study in vitro indicated that OPD increased the flux of VCR across MDCK-MDR1 cell monolayer. The in vivo pharmacokinetic parameters data showed OPD increased the absorption of DTX. OPD activated P-gp ATPase activity and enhanced intracellular accumulation of Rh123 in MDCK-MDR1 cells. Western blotting and qRT-PCR outcomes indicated that OPD suppressed P-gp protein expression as well as downregulated P-gp mRNA level. Thus, OPD reverse P-gp-mediated drug transport via inhibition of P-gp activity and P-gp protein expression as well as downregulation of P-gp mRNA level. Our results suggest that OPD could reverse P-gp-mediated drug resistance in tumor cells.

Anthracycline Drugs on Modified Surface of Quercetin-Loaded Polymer Nanoparticles: A Dual Drug Delivery Model for Cancer Treatment

Polymer nanoparticles are vehicles used for delivery of hydrophobic anti-cancer drugs, like doxorubicin, paclitaxel or chemopreventors like quercetin (Q). The present study deals with the synthesis and characterisation of nano formulations (NFs) from Q loaded PLGA (poly lactic-co-glycolic acid) nano particles (NPs) by surface modification. The surface of Q-loaded (NPs) is modified by coating with biopolymers like bovine serum albumin (BSA) or histones (His). Conventional chemotherapeutic drugs adriamycin (ADR) and mitoxantrone (MTX) are bound to BSA and His respectively before being coated on Q-loaded NPs to nano formulate NF1 and NF2 respectively. The sizes of these NFs are in the range 400–500 nm as ascertained by SEM and DLS measurements. Encapsulation of Q in polymer NPs is confirmed from shifts in FT-IR, TGA and DSC traces of Q-loaded NPs compared to native PLGA and Q. Surface modification in NFs is evidenced by three distinct regions in their TEM images; the core, polymer capsule and the coated surface. Negative zeta potential of Q-loaded NPs shifted to positive potential on surface modification in NF1 and NF2. In vitro release of Q from the NFs lasted up to twenty days with an early burst release. NF2 is better formulation than NF1 as loading of MTX is 85% compared to 23% loading of ADR. Such NFs are expected to overcome multi-drug resistance (MDR) by reaching and treating the target cancerous cells by virtue of size, charge and retention.

Tumor-targeted docetaxel-loaded hyaluronic acid-quercetin polymeric micelles with p-gp inhibitory property for hepatic cancer therapy

Herein, a novel targeted drug delivery nanosystem based on hyaluronic acid (HA) and quercetin (QU) was designed to improve the in vivo therapeutic efficacy of DTX on HC through HA-CD44 mediated targeting and QU-based p-gp efflux inhibition.

Clusterin inhibition using OGX-011 synergistically enhances zoledronic acid activity in osteosarcoma

Purpose

Despite recent improvements in therapeutic management of osteosarcoma, ongoing challenges in improving the response to chemotherapy warrants new strategies still needed to improve overall patient survival. Among new therapeutic approaches, zoledronic acid (ZOL) represents a promising adjuvant molecule to chemotherapy to limit the osteolytic component of bone tumors. However, ZOL triggers the elevation of heat shock proteins (Hsp), including Hsp27 and clusterin (CLU), which could enhance tumor cell survival and treatment resistance. We hypothesized that targeting CLU using siRNA or the antisense drug, OGX-011, will suppress treatment-induced CLU induction and enhance ZOL-induced cell death in osteosarcoma (OS) cells.

Methods

The combined effects of OGX-011 and ZOL were investigated in vitro on cell growth, viability, apoptosis and cell cycle repartition of ZOL-sensitive or -resistant human OS cell lines (SaOS2, U2OS, MG63 and MNNG/HOS).

Results

In OS cell lines, ZOL increased levels of HSPs, especially CLU, in a dose- and time-dependent manner by mechanism including increased HSF1 transcription activity. The OS resistant cells to ZOL exhibited higher CLU expression level than the sensitive cells. Moreover, CLU overexpression protects OS sensitive cells to ZOL-induced cell death by modulating the MDR1 and farnesyl diphosphate synthase expression. OGX-011 suppressed treatment-induced increases in CLU and synergistically enhanced the activity of ZOL on cell growth and apoptosis. These biologic events were accompanied by decreased expression of HSPs, MDR1 and HSF1 transcriptional activity. In vivo, OGX-011, administered 3 times a week (IP, 20mg/kg), potentiated the effect of ZOL (s.c; 50μg/kg), significantly inhibiting tumor growth by 50% and prolonging survival in MNNG/HOS xenograft model compared to ZOL alone.

Conclusion

These results indicate that ZOL-mediated induction of CLU can be attenuated by OGX-011, with synergistic effects on delaying progression of osteosarcoma.

Contrasting cellular stress responses of Baikalian and Palearctic amphipods upon exposure to humic substances: environmental implications

The species-rich, endemic amphipod fauna of Lake Baikal does not overlap with the common Palearctic fauna; however, the underlying mechanisms for this are poorly understood. Considering that Palearctic lakes have a higher relative input of natural organic compounds with a dominance of humic substances (HSs) than Lake Baikal, we addressed the question whether HSs are candidate factors that affect the different species compositions in these water bodies. We hypothesized that interspecies differences in stress defense might reveal that Baikalian amphipods are inferior to Palearctic amphipods in dealing with HS-mediated stress. In this study, two key mechanisms of general stress response were examined: heat-shock protein 70 (HSP70) and multixenobiotic resistance-associated transporters (ABCB1). The results of quantitative polymerase chain reaction (qPCR) showed that the basal levels (in 3-day acclimated animals) of hsp70 and abcb1 transcripts were lower in Baikalian species (Eulimnogammarus cyaneus, Eulimnogammarus verrucosus, Eulimnogammarus vittatus-the most typical littoral species) than in the Palearctic amphipod (Gammarus lacustris-the only Palearctic species distributed in the Baikalian region). In the amphipods, the stress response was induced using HSs at 10 mg L(-1) dissolved organic carbon, which was higher than in sampling sites of the studied species, but well within the range (3-10 mg L(-1)) in the surrounding water bodies populated by G. lacustris. The results of qPCR and western blotting (n = 5) showed that HS exposure led to increased hsp70/abcb1 transcripts and HSP70 protein levels in G. lacustris, whereas these transcript levels remained constant or decreased in the Baikalian species. The decreased level of stress transcripts is probably not able to confer an effective tolerance to Baikalian species against further environmental stressors in conditions with elevated HS levels. Thus, our results suggest a greater robustness of Palearctic amphipods and a higher sensitivity of Baikalian amphipods to HS challenge, which might prevent most endemic species from migrating to habitats outside Lake Baikal.

Modulation of P-glycoprotein efflux pump: induction and activation as a therapeutic strategy

P-glycoprotein (P-gp) is an ATP-dependent efflux pump encoded by the MDR1 gene in humans, known to mediate multidrug resistance of neoplastic cells to cancer therapy. For several decades, P-gp inhibition has drawn many significant research efforts in an attempt to overcome this phenomenon. However, P-gp is also constitutively expressed in normal human epithelial tissues and, due to its broad substrate specificity, to its cellular polarized expression in many excretory and barrier tissues, and to its great efflux capacity, it can play a crucial role in limiting the absorption and distribution of harmful xenobiotics, by decreasing their intracellular accumulation. Such a defense mechanism can be of particular relevance at the intestinal level, by significantly reducing the intestinal absorption of the xenobiotic and, consequently, avoiding its access to the target organs. In this review, the current knowledge on this important efflux pump is summarized, and a new focus is brought on the therapeutic interest of inducing and/or activating P-gp for limiting the toxicity caused by its substrates. Several in vivo and in vitro studies validating the use of such a therapeutic strategy are discussed. An extensive literature search for reported P-gp inducers/activators and for the experimental models used in their characterization was conducted. Those studies demonstrate that effective antidotal pathways can be achieved by efficiently promoting the P-gp-mediated efflux of deleterious xenobiotics, resulting in a significant reduction in their intracellular levels and, consequently, in a significant reduction of their toxicity.

Natural product based leads to fight against leishmaniasis

The growing incidence of parasitic resistance against generic pentavalent antimonials, specifically for visceral disease in Indian subcontinent, is a serious issue in Leishmania control. Notwithstanding the two treatment alternatives, that is amphotericin B and miltefosine are being effectively used but their high cost and therapeutic complications limit their use in endemic areas. In the absence of a vaccine candidate, identification, and characterization of novel drugs and targets is a major requirement of leishmanial research. This review describes current drug regimens, putative drug targets, numerous natural products that have shown promising antileishmanial activity alongwith some key issues and strategies for future research to control leishmaniasis worldwide.

Inhibition of P-Glycoprotein Function and Expression by Kaempferol and Quercetin

The 170 kDa plasma membrane P-glycoprotein (Pgp) causes the efflux of chemotherapeutic drugs from cells and is believed to be an important mechanism in multidrug resistance (MDR) in human cancer. This study demonstrates that some putative flavonoids, i.e., flavonols (quercetin and kaempferol) and isoflavones (genistein and daidzein) markedly increase the sensitivity of the multidrug-resistant human cervical carcinoma KB-V1 cells (high Pgp expression) to vinblastine and paclitaxel dose-dependently, and also decrease the relative resistance of these anti-cancer-drugs in KB-V1 cells. None of the flavonoids had a significant effect on vinblastine and paclitaxel cytotoxicity in wildtype drug-sensitive KB-3-1 cells (lacking Pgp). These flavonoids also caused an increase in intracellular accumulation, and reduced the efflux of Rh123 and 3[H]vinblastine in KB-V1 cells, but not in KB-3-1 cells. The flavonols increased the inhibitory effectiveness of Pgp activity in MDR KB-V1 cells more than isoflavones. Only treatment with flavonols up to 48 h was able to significantly decrease the Pgp expression in a dose-dependent manner in KB-V1 cells. These findings provide evidence that flavonols reduced Pgp expression and function resulting in the inhibition of Pgp activity, but isoflavones modulated intracellular drug levels by inhibiting Pgp function with no effect on Pgp expression. Among the flavonoids tested, flavonols, particularly kaempferol, exhibit the most potent MDR reversing property in KB-V1 cells.

PLGA nanoparticles loaded with etoposide and quercetin dihydrate individually: in vitro cell line study to ensure advantage of combination therapy

PLGA nanoparticles, separately loaded with etoposide (ETN) and quercetin dihydrate (QDN), were prepared by adapting the solvent diffusion (nanoprecipitation) technique. The effect of formulation variables such as amount of polymer, theoretical drug loading, surfactant concentration, and aqueous and organic phase volumes on particle size and entrapment efficiency, were systematically studied. The optimal formulations obtained were of submicron size (153.4 ± 4.2 nm for ETN and 148.6 ± 1.6 nm for QDN) and with low polydispersity indices (0.058 ± 0.02 for ETN and 0.088 ± 0.03 for QDN). The entrapment efficiencies were found as 63.88 ± 1.5 % and 41.36 ± 3.4 % for ETN and QDN, respectively. The characterization of ETN and QDN was done by measuring the zeta potential, TEM, and DSC analysis. The comparison was made in respect of in vitro cytotoxicity assay using cancer cell line A549 (human lung adenocarcinoma epithelial cell line). The results revealed significant increase in cytotoxicity in nanoparticle formulations than their respective free drug. The comparison was also made with respect to cytotoxic activity of individual drug and combination of drugs in the form of free drugs as well as nanoparticles. The combination treatment in the form of nanoparticles is found to produce best results among the treatments used in cytotoxicity studies.

Inducible hsp70 in the regulation of cancer cell survival: analysis of chaperone induction, expression and activity

Understanding the mechanisms that control stress is central to realize how cells respond to environmental and physiological insults. All the more important is to reveal how tumour cells withstand their harsher growth conditions and cope with drug-induced apoptosis, since resistance to chemotherapy is the foremost complication when curing cancer. Intensive research on tumour biology over the past number of years has provided significant insights into the molecular events that occur during oncogenesis, and resistance to anti-cancer drugs has been shown to often rely on stress response and expression of inducible heat shock proteins (HSPs). However, with respect to the mechanisms guarding cancer cells against proteotoxic stresses and the modulatory effects that allow their survival, much remains to be defined. Heat shock proteins are molecules responsible for folding newly synthesized polypeptides under physiological conditions and misfolded proteins under stress, but their role in maintaining the transformed phenotype often goes beyond their conventional chaperone activity. Expression of inducible HSPs is known to correlate with limited sensitivity to apoptosis induced by diverse cytotoxic agents and dismal prognosis of several tumour types, however whether cancer cells survive because of the constitutive expression of heat shock proteins or the ability to induce them when adapting to the hostile microenvironment remains to be elucidated. Clear is that tumours appear nowadays more "addicted" to heat shock proteins than previously envisaged, and targeting HSPs represents a powerful approach and a future challenge for sensitizing tumours to therapy. This review will focus on the anti-apoptotic role of heat shock 70kDa protein (Hsp70), and how regulatory factors that control inducible Hsp70 synthesis, expression and activity may be relevant for response to stress and survival of cancer cells.

Enhancement of carboplatin- and quercetin-induced cell death by roscovitine is Akt dependent and p53 independent in hepatoma cells

PURPOSE

Hepatocellular carcinoma (HCC) is a common malignancy worldwide and has an annual occurrence of one million new cases. Novel therapeutic strategies of increased efficacy in the treatment of HCC-bearing patients would certainly be helpful. Hence, the authors explored the effect of combination treatment of roscovitine with chemotherapeutic drugs or quercetin (Qctn) in hepatoma cells, HepG2 and Hep3B.

METHODS

Cell viability was assessed by MTT assay, cell growth assay, and nuclear morphological changes by DAPI staining. The altered expression of signaling proteins and apoptotic molecules was established by Western blotting.

RESULTS

Roscovitine pretreatment considerably enhanced the drugs and Qctn-induced cell death in HepG2 and Hep3B cells. The exploratory studies revealed that augmented cell killing in HepG2 and Hep3B was mediated via Akt pathway and was independent of p53. pAkt was found to be significantly downregulated in combination treatment of roscovitine with carboplatin or Qctn. Corresponding to reduced expression of pAkt, the downstream molecules Bcl-2 and proactive forms of caspase 9 and caspase 3 were also downregulated indicating apoptosis.

CONCLUSIONS

The present study reports for the first time, in hepatoma cells, the potentiation of carboplatin- and Qctn-induced cell death by the cell cycle inhibitor roscovitine. Roscovitine can thus be considered as a potential therapeutic target in combination with chemotherapeutic drugs or Qctn for treatment of HCC.

Circumvention of multi-drug resistance of cancer cells by Chinese herbal medicines

Multi-drug resistance (MDR) of cancer cells severely limits therapeutic outcomes. A proposed mechanism for MDR involves the efflux of anti-cancer drugs from cancer cells, primarily mediated by ATP-binding cassette (ABC) membrane transporters including P-glycoprotein. This article reviews the recent progress of using active ingredients, extracts and formulae from Chinese medicine (CM) in circumventing ABC transporters-mediated MDR. Among the ABC transporters, Pgp is the most extensively studied for its role in MDR reversal effects. While other MDR reversal mechanisms remain unclear, Pgp inhibition is a criterion for further mechanistic study. More mechanistic studies are needed to fully establish the pharmacological effects of potential MDR reversing agents.

Quercetin: a potential drug to reverse multidrug resistance

This review centers on recent findings with respect to modulating cancer multidrug resistance (MDR) with the well-known flavonoid quercetin. After a short introduction of quercetin, major in vitro and in vivo findings are summarized showing that quercetin is a MDR modulator and thus a potential chemosensitizer. Finally, we contemplate future prospects of modulating MDR in the clinic.

A new dawn for the use of traditional Chinese medicine in cancer therapy

Although traditional Chinese medicine has benefitted one fifth of the world's population in treating a plethora of diseases, its acceptance as a real therapeutic option by the West is only now emerging. In light of a new wave of recognition being given to traditional Chinese medicine by health professionals and regulatory bodies in the West, an understanding of their molecular basis and highlighting potential future applications of a proven group of traditional Chinese medicine in the treatment of a variety of cancers is crucial – this is where their calling holds much hope and promise in both animal and human trials. Furthermore, the rationale for combining conventional agents and modern biotechnological approaches to the delivery of traditional Chinese medicine is an avenue set to revolutionize the future practice of cancer medicine – and this may well bring on a new dawn of therapeutic strategies where East truly meets West.

Effect of bortezomib on human neuroblastoma: analysis of molecular mechanisms involved in cytotoxicity

Background

Bortezomib, a specific and selective inhibitor of the 26S proteasome with antitumor activity against a wide range of malignancies, has been approved for the treatment of relapsed or refractory multiple myeloma and other cancers. Recently, bortezomib has been identified as an effective inhibitor of neuroblastoma cell growth and angiogenesis.

Results

In the present study, we demonstrate that some neuroblastoma cell lines are actually resistant to bortezomib. We have sought to characterize the main pathway by which proteasome inhibition leads to apoptosis, and to define the mechanism responsible for resistance to bortezomib in neuroblastoma cells. Our results show that SB202190, an inhibitor of mitogen-activated protein kinase (MAPK) p38, enhances the ability of bortezomib to induce apoptosis by preventing the phosphorylation of the heat shock protein (HSP) 27.

Conclusion

This study opens the way to further clinical investigations and suggests a potential benefit of using a combination of bortezomib with an inhibitor of p38 MAPK for the treatment of neuroblastoma relapse.

The synergistic reversal effect of multidrug resistance by quercetin and hyperthermia in doxorubicin-resistant human myelogenous leukemia cells

PURPOSE

This study aimed to evaluate the multidrug resistance (MDR) reversal activity of quercetin (Que) in combination with hyperthermia (HT) in human myelogenous leukemia cells K562/A.

METHODS

The cytotoxicity of Que alone and the effect of Que and HT to doxorubicin (Dox) cytotoxicity were determined using MTT assay in K562 and K562/A cells. K562/A cells was heated with or without Que pretreatment, and the protein and mRNA levels of heat shock protein 70 (HSP70) and P-glycoprotein (P-gp) were determined by flow cytometry (FCM) and RT-PCR, respectively. Intracellular accumulation of Dox, cell cycle and apoptosis were monitored with FCM.

RESULTS

Que alone inhibited cell growth in a dose-dependent manner in K562 and K562/A cells. Either Que or HT alone had a weak reversal effect on Dox resistance, however, combination HT and Que showed a much more significant reversal effect on Dox resistance (reverse fold 9.49). The elevated protein expression and mRNA level of HSP70 and P-gp in response to HT were inhibited by Que. Pretreatment with Que caused the cells to accumulate Dox 8.3-fold higher than in control cells. In addition, Que induced apoptosis and G2/M arrest in a dose-dependent manner, and the combination of Que and HT was found to have a synergistic effect on apoptosis.

CONCLUSIONS

Que pretreatment could significantly enhance the MDR reversal activity of HT in resistant cell line, by sensitizing the cell to reversing MDR activity of HT.

Possible drug-metabolism interactions of medicinal herbs with antiretroviral agents

Herbal medicines are widely used by HIV patients. Several herbal medicines have been shown to interact with antiretroviral drugs, which might lead to drug failure. We have aimed to provide an overview of the modulating effects of Western and African herbal medicines on antiretroviral drug-metabolizing and transporting enzymes, focusing on potential herb-antiretroviral drug interactions. Echinacea, garlic, ginkgo, milk thistle, and St. John's wort have the potential to cause significant interactions. In vitro and in vivo animal studies also indicated other herbs with a potential for interactions; however, most evidence is based on in vitro studies. Further pharmacokinetic studies to unveil potential Western and especially African herb-antiretroviral drug interactions are urgently required, and the clinical significance of these interactions should be assessed.

Heat shock-independent induction of multidrug resistance by heat shock factor 1

The screening of two different retroviral cDNA expression libraries to select genes that confer constitutive doxorubicin resistance has in both cases resulted in the isolation of the heat shock factor 1 (HSF1) transcription factor. We show that HSF1 induces a multidrug resistance phenotype that occurs in the absence of heat shock or cellular stress and is mediated at least in part through the constitutive activation of the multidrug resistance gene 1 (MDR-1). This drug resistance phenotype does not correlate with an increased expression of heat shock-responsive genes (heat shock protein genes, or HSPs). In addition, HSF1 mutants lacking HSP gene activation are also capable of conferring multidrug resistance, and only hypophosphorylated HSF1 complexes accumulate in transduced cells. Our results indicate that HSF1 can activate MDR-1 expression in a stress-independent manner that differs from the canonical heat shock-activated mechanism involved in HSP induction. We further provide evidence that the induction of MDR-1 expression occurs at a posttranscriptional level, revealing a novel undocumented role for hypophosphorylated HSF1 in posttranscriptional gene regulation.

Inhibiting induction of heat shock proteins as a strategy to enhance cancer therapy

Cancer treatments that incorporate thermal therapy and some systemic therapies induce the production of heat shock or stress proteins. The induced heat shock proteins could lessen the effect of the therapy by inhibiting apoptotic signaling and by acting as molecular chaperones to prevent irreversible cellular damage. Strategies that prevent the induction of heat shock proteins would result in more apoptosis and necrosis, improving the cancer therapy. This paper briefly reviews cancer therapies that induce the stress response, and proposes strategies to reduce the stress response.

Arsenite Regulates Cystic Fibrosis Transmem­brane Conductance Regulator and P-glycoprotein: Evidence of Pathway Independence

In the past, people have argued for and against the theory of reciprocal regulation of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) and P-glycoprotein (Pgp). Data have indicated that this may occur in vitro during drug-induced selection of cells, and in vivo during development. Much of this debate has been caused by a severe lack of mechanistic details involved in such regulation. Our past data indicate that certain Pgp modulators can affect CFTR expression and function. The goal of this study was to investigate the effects of trivalent arsenic (arsenite), a known transcriptional activator of Pgp, on CFTR expression. In vitro analyses in T-84 cells that express basal levels of Pgp and CFTR were conducted using a variety of molecular techniques. Expressions of both genes were altered following treatment with arsenite in a dose- and time-dependent fashion. CFTR expression was suppressed almost three-fold by arsenite, along with a concomitant increase in P-glycoprotein expression. We also report that a member of the MAPK-family, the ERK-mediated signaling cascade is implicated in suppression of CFTR expression following treatment with arsenite. However, this particular pathway is not involved in regulation of P-glycoprotein expression in T-84 cells following treatment with arsenite. Thus, the regulatory pathways that control functional expression of CFTR and P-glycoprotein following arsenite treatment in T-84 cells are distinct and independent.

ABC transporters as multidrug resistance mechanisms and the development of chemosensitizers for their reversal

One of the major problems related with anticancer chemotherapy is resistance against anticancer drugs. The ATP-binding cassette (ABC) transporters are a family of transporter proteins that are responsible for drug resistance and a low bioavailability of drugs by pumping a variety of drugs out cells at the expense of ATP hydrolysis. One strategy for reversal of the resistance of tumor cells expressing ABC transporters is combined use of anticancer drugs with chemosensitizers. In this review, the physiological functions and structures of ABC transporters, and the development of chemosensitizers are described focusing on well-known proteins including P-glycoprotein, multidrug resistance associated protein, and breast cancer resistance protein.

Heat shock proteins and acute leukemias

Heat shock proteins (HSPs) acts as molecular chaperones by helping in the refolding of misfolded proteins and assisting in their elimination if they become irreversibly damaged. HSPs induced by stress treatment have a role in the modulation of apoptosis. The reduction in protein expression levels was correlated with an increased susceptibility to drug-induced apoptosis. HSPs have also been implicated in the resistance of leukemia cells to potential therapeutic agents. The mechanisms of cellular protection used by HSPs have yet to be fully defined. HSPs were shown highly expressed by acute myeloid leukemia (AML) cells as well as by acute lymphoblastic leukemia (ALL) cells. HSP expressions were correlated with that of differentiation antigens and that of drug-resistance and apoptosis proteins. Complete remission (CR) rates were higher in patients with lower expression of HSPs. Overall survival (OS) was significantly longer in patients with lower expression of HSPs. Therapeutically, inhibition of inducible HSP expression or activity should not cause any undesired side effects. HSPs emerge as novel therapeutic targets in anticancer protocols. Early results of phase I studies indicate that 17-allylamino-17-demethoxygeldamycin (17-AAG), capable of binding and disrupting the function of HSP90, results in an acceptable toxicity profile while achieving in vivo disruption of multiple oncogenic client proteins.

Expression of heat-shock proteins is associated with major adverse prognostic factors in acute myeloid leukemia

To identify prognostic factors alternative or additional to drug-resistance and apoptosis proteins, we studied the impact of the expression of heat-shock proteins (HSPs) in 98 newly diagnosed acute myeloid leukemia (AML). HSP27 was expressed by 39%, HSP60 by 26%, HSP70 by 58%, HSP90 by 41%, and HSP110 by 30% of cases. HSP expressions were correlated with that of differentiation antigens (CD34, CD14, CD15, CD33) and that of drug-resistance (MRP, MRK) and apoptosis (Bcl-2) proteins. HSP90 and HSP110 were correlated with FAB subtype and karyotypic grouping. Complete remission (CR) was obtained in 68 cases (69%). Median disease-free survival (DFS) of the 68 remitters was 18.1 months with a 3-year DFS rate of 41%. CR rates were higher in patients with lower expression of HSPs. Overall survival (OS) was significantly longer in patients with lower expression of HSPs. Cytogenetics, CD34 positive expression, MRK positive expression, and HSP110 positive expression remained as pejorative prognostic factors for OS in the multivariate analysis. When considering patients with intermediate risk cytogenetics, HSP110 and MRP positive expressions and CD33 negative expression were of poor outcome, while HSP27 and HSP60 positive expressions appeared of pejorative prognostic value in patients with unfavorable karyotypes.

Expression of mdr49 and mdr65 multidrug resistance genes in larval tissues of Drosophila melanogaster under normal and stress conditions

In situ expression of 2 multidrug resistance genes, mdr49 and mdr65, of Drosophila melanogaster was examined in wild-type third instar larval tissues under physiological conditions and after heat shock or colchicine feeding. Expression of these 2 genes was also examined in tumorous tissues of lethal (2) giant larvae I(2)gl4 mutant larvae. These 2 mdr genes show similar constitutive expression in different larval tissues under physiological conditions. However, they are induced differentially by endogenous (tumorous growth) and exogenous stresses (colchcine feeding or heat shock): whereas heat shock and colchicine feeding induce mdr49, tumorous condition is accompanied by enhanced expression of mdr49 and mdr65 genes.

In Vitro Interaction of the HIV Protease Inhibitor Ritonavir with Herbal Constituents: Changes in P-gp and CYP3A4 Activity

The purpose of this study was to evaluate in vitro interactions of commercially obtained pure herbal constituents with p-glycoprotein P-gp and cytochrome P-450 3A4 (CYP3A4) activities, which can further modulate the transcellular transport and metabolism kinetics of orally administered drugs. Caco-2 cells grown in the presence of 0.25 micromol/L 1alpha,25-dihydroxy vitamin D3 and multidrug-resistant 1 (MDR1) transfected MDCK cells were used as models to evaluate the effect of purified herbal constituents (quercetin, hypericin, hyperforin from St. John's wort, kaempferol from ginseng, silibinin from milk thistle, and allicin from garlic) on P-gp-mediated efflux of the human immunodeficiency virus (HIV) protease inhibitor ritonavir. In addition, the inhibitory effect of these constituents on CYP3A4-mediated metabolism was determined by using cortisol as a model compound. Silibinin and hyperforin did not significantly alter cellular uptake of H-ritonavir in Caco-2 cells. A similar result was also observed for silibinin when tested in MDR1-MDCK cells. Quercetin, hypericin, and kaempferol exhibited a remarkable inhibition of P-gp-mediated efflux of ritonavir by increasing its cellular uptake in these models. These values were also comparable with the inhibitory effect of quinidine in Caco-2 cells, a well-known inhibitor of P-gp, on ritonavir efflux from Caco-2 cells. Allicin exhibited a concentration-dependent inhibition of ritonavir efflux when tested on MDR1-MDCK cells. There was a significant decrease in the Apical to Basal/Basal to Apical (AP-BL/BL-AP) transport ratio of ritonavir in presence of hypericin, kaempferol, and quercetin. These herbal constituents inhibited the CYP3A4 activity when tested with the Vivid CYP3A4 assay kit, whereas silibinin did not alter cortisol metabolism. Hypericin showed a significant inhibition in reduced nicotinamide adenine dinucleotide phosphate (NADPH)-dependent metabolism of cortisol with 64.6% of intact drug at the end of a 1-hour study. Similarly, kaempferol and quercetin also caused substantial inhibition of cortisol metabolism with 89.7% and 90.1% of intact cortisol, respectively, compared with 45.9% in the control. Prolonged exposure of quercetin resulted in significant increase of mRNA expression of both MDR1 and CYP3A4 levels in Caco-2 cells. However, hyperforin caused upregulation of CYP3A4 and downregulation of MDR1, whereas the effect of silibinin and kaempferol remained inconclusive on these gene expressions. Hypericin, kaempferol, quercetin, and allicin inhibit the efflux and CYP3A4-mediated metabolism of xenobiotics in vitro. Hence, this study warns against the use of herbal constituents along with prescribed HIV protease inhibitors that are substrates for P-gp and/or CYP3A4.

Effects of continuous exposure to digoxin on MDR1 function and expression in Caco-2 cells

The Caco-2 cell line has been used widely for studying intestinal permeability and several transport functions, and express the multidrug resistance transporter MDR1/P-glycoprotein. Previously, the transient exposure to digoxin for 24 h was found to induce MDR1 mRNA in Caco-2 cells. Here, a digoxin-tolerant Caco-2 subline (Caco/DX) was newly established by the continuous exposure of Caco-2 cells to digoxin, and the effects of continuous exposure to digoxin on MDR1 were examined. The 50% growth inhibitory concentration (IC(50)) values for digoxin in Caco-2 and Caco/DX cells were 17.2 and 81.4 nM, respectively. The IC(50) values for paclitaxel, an MDR1 substrate, were 1.0 and 547 nM, respectively, whereas the cytotoxicity of 5-fluorouracil was comparable in both cells. The uptake and efflux of Rhodamine123, an MDR1 substrate, in Caco/DX cells were significantly less and greater, respectively, than those in Caco-2 cells, and these transports were affected by the addition of ciclosporin. The expression of MDR1 mRNA in Caco/DX cells was approximately 2- and 1.7-fold compared with Caco-2 cells and Caco-2 cells treated with 100 nM digoxin for 24 h, respectively. On the other hand, MRP1 mRNA in Caco/DX cells was unchanged. These observations confirmed that the continuous exposure to digoxin, as well as the transient exposure, induced MDR1 in Caco-2 cells.

Quercetin, a dietary-derived flavonoid, possesses antiangiogenic potential

Quercetin, a dietary-derived flavonoid, suppresses tumor growth in vitro and in vivo, and inhibits the activity of tyrosine kinase. The effects of quercetin on the angiogenic process were examined in this study. Quercetin was found to inhibit several important steps of angiogenesis including proliferation, migration, and tube formation of human microvascular dermal endothelial cells in a dose-dependent manner. Additionally, the effect of quercetin on endothelial cell proliferation was confirmed using human umbilical vein endothelial cells. The activity of quercetin on the proliferation of endothelial cells was stronger than that on A549, BEL-7402, MKN-45 tumor cells and NIH-3T3 fibroblast cells. The chicken chorioallantoic membrane assay revealed that addition of quercetin displayed an antiangiogenic effect in vivo. After exposure to quercetin, a decrease in the expression and activity of matrix metalloproteinase-2, which is involved in the angiogenic process of migration, invasion, and tube formation, was observed by reverse transcription-polymerase chain reaction (RT-PCR) and gelatin zymography. These findings suggest that quercetin has antiangiogenic potential and that this effect may be related to an influence on the expression and activity of matrix metalloproteinase-2.

Recent advances in the discovery of flavonoids and analogs with high-affinity binding to P-glycoprotein responsible for cancer cell multidrug resistance

P-glycoprotein (P-gp) is a plasma membrane glycoprotein that confers multidrug resistance on cells by virtue of its ability to exclude cytotoxic drugs in an ATP-dependent manner. The most commonly considered hypothesis is that P-gp acts as an ATP-driven drug-export pump, the mechanism of which is not understood in detail. Therefore, a tremendous effort is being made to find out modulator molecules to inhibit P-gp. We have been developing flavonoid derivatives as a new class of promising modulators using a new in vitro rational-screening assay based on measurements of the binding-affinity toward the C-terminal nucleotide-binding domain (NBD2) of P-gp. This review is focused on our results obtained with a variety of flavonoids. Structure-activity relationships of flavonoids as potential MDR modulators are reported.

Effects of 12 Ca2+ antagonists on multidrug resistance, MDR1-mediated transport and MDR1 mRNA expression

The effects of 12 Ca(2+) antagonists on MDR1 were examined by two independent models: the inhibitory effect on MDR1-mediated transport of [(3)H]digoxin using MDR1-overexpressing LLC-GA5-COL150 cell monolayers and the reversal effect on cytotoxicity of vinblastine or paclitaxel using MDR1-overexpressing Hvr100-6 cells. The inhibitory effects on [(3)H]digoxin transport were assessed as the 50% inhibitory concentration during 4 h exposure, and the values were the lowest for nicardipine (4.54 microM), manidipine (4.65 microM) and benidipine (4.96 microM), followed by bepridil (10.6 microM), barnidipine (12.6 microM), efonidipine (13.0 microM), verapamil (13.2 microM) and nilvadipine (18.0 microM). The reversal effect on cytotoxicity was assessed by the 50% growth inhibitory concentration after 3 days exposure, and the resistance to vinblastine or paclitaxel in Hvr100-6 cells was reversed by manidipine, verapamil, benidipine, barnidipine, and nicardipine, in that order. Bepridil, barnidipine, efonidipine, verapamil and nilvadipine showed similar inhibitory effects on [(3)H]digoxin transport, but barnidipine and verapamil showed a stronger effect in reversal of cytotoxicity. Real-time quantitative RT-PCR assay indicated a decrease in MDR1 mRNA expression by barnidipine and verapamil. It is concluded that Ca(2+) antagonists cannot only be direct inhibitors of MDR1 but that some may at the same time act as inhibitors of expression of MDR1 via down-regulation of MDR1 mRNA.

Heat shock protein expression in human tumours grown in severe combined immunodeficient mice

The constitutional expression of heat shock proteins (HSP) 27, 70 and 90 in human breast, colon and ovarian cancer cells transplanted into severe combined immunodeficient (SCID) mice was evaluated. In addition their induced expression under chemotherapeutic stress was analyzed. The oestrogen receptor positive breast cancer cell lines (MCF-7, T47D) demonstrated an increased level of HSP 27 and 70 expression compared with oestrogen receptor negative cell lines (BT20, HBL100). After 5-fluorouracil application for 4 days, HSP 27 and 70 expression was increased in HT29 colon tumours. Hence, the human/SCID mouse model is well suited to evaluate the constitutional and induced expression of human HSPs under various experimental conditions.

Regulation of multidrug resistance 1 (MDR1)/P-glycoprotein gene expression and activity by heat-shock transcription factor 1 (HSF1)

Infection of HeLa cells with adenovirus-carrying HSF1(+) cDNA, which encodes a mutated form of HSF1 with constitutive transactivation capacity, increased multidrug resistance 1 (MDR1) mRNA level and P-glycoprotein (P-gp) cell surface content and stimulated rhodamine 123 accumulation and vinblastine efflux activity. On the other hand, infection with adenovirus-carrying HSP70 and HSP27 cDNAs did not increase MDR1/P-gp expression. HSF1 regulates MDR1/P-gp expression at the transcriptional level, since HSF1(+) bound the heat-shock consensus elements (HSEs) in the MDR1 gene promoter and also activated the expression of an MDR1 promoter-driven reporter plasmid (pMDR1(-1202)). In addition, heat-shock increased pMDR1(-1202) promoter activity but not the activity of a similar reporter plasmid with point mutations at specific HSEs, and the heat-induced increase was totally inhibited by co-transfection with an expression plasmid carrying HSF1(-), a dominant negative mutant of HSF1. The stress inducers arsenite, butyrate, and etoposide also increased pMDR1(-1202) promoter activity, but the increase was not inhibited (in the case of butyrate) or was only partially inhibited (in the case of arsenite and etoposide) by HSF1(-). These results demonstrate that HSF1 regulates MDR1 expression, and that the HSEs present in the -315 to -285 region mediate the heat-induced activation of the MDR1 promoter. However, other factors may also participate in MDR1 induction by stressing agents.

Heat shock proteins--modulators of apoptosis in tumour cells

Apoptosis is a genetically programmed, physiological method of cell destruction. A variety of genes are now recognised as positive or negative regulators of this process. Expression of inducible heat shock proteins (hsp) is known to correlate with increased resistance to apoptosis induced by a range of diverse cytotoxic agents and has been implicated in chemotherapeutic resistance of tumours and carcinogenesis. Intensive research on apoptosis over the past number of years has provided significant insights into the mechanisms and molecular events that occur during this process. The modulatory effects of hsps on apoptosis are well documented, however, the mechanisms of hsp-mediated protection against apoptosis remain to be fully defined, although several hypotheses have been proposed. Elucidation of these mechanisms should reveal novel targets for manipulating the sensitivity of leukaemic cells to therapy. This review aims to explain the currently understood process of apoptosis and the effects of hsps on this process. Several proposed mechanisms for hsp protection against apoptosis and the therapeutic implications of hsps in leukaemia are also discussed.

A novel association between the human heat shock transcription factor 1 (HSF1) and prostate adenocarcinoma

A search for differentially expressed genes in a pair of nonmetastatic (PC-3) versus metastatic variant (PC-3M) human prostate carcinoma cell lines led to identification of the human heat shock factor (HSF1) as an overexpressed gene product in PC-3M cells. Analysis of primary prostate cancer specimens indicated that HSF1 is generally up-regulated in most of the malignant prostate epithelial cells relative to the normal prostate cells. Among the known effectors of HSF1 action, constitutive levels of HSP70 and HSP90 are not significantly altered by the naturally elevated expression of HSF1 as in PC-3M cells or by transduced overexpression of HSF1 in PC-3 cells. The basal levels of HSP27 in both cases are, however, consistently increased by two- to threefold. With respect to response to heat shock, high basal concentration of HSP90 is not further enhanced in these cells, and HSP70 is up-regulated irrespective of HSF1 level. Heat shock, however, causes an increase in HSP27 when HSF1 is up-regulated, except when the expression of HSF1 is already too high. These results document for the first time that HSF1 is overexpressed in human prostate cancer cells, at least one consequence of which in the prostate cancer cell lines tested is stimulation of both basal and stress-induced expression of HSP27, an important factor in cell growth, differentiation, or apoptosis.

Apoptosis induction by arsenic: mechanisms of action and possible clinical applications for treating therapy-resistant cancers

Arsenic, a known carcinogen, may be useful in cancer treatment. Arsenic may be effective in counteracting drug resistance because it appears to induce apoptosis in tumor cells independently of p53 activation, thereby allowing it to be directed against p53-defective cancers. The role of MAP kinases in arsenic-induced apoptosis in tumor cells is important and may be influenced by reactive oxygen species or glutathione. This review focuses on recent findings from this and other laboratories regarding the mechanism(s) of arsenic-induced apoptosis in tumor cells and considers their relevance in the clinical treatment of therapy-resistant cancers. Copyright 2000 Harcourt Publishers Ltd.