Interaction of selected alkoxy naringenin oximes with model and bacterial membranes.

Naringenin is a flavonoid found in many fruits and herbs, most notably in grapefruits. In recent years, this compound and its derivatives have been of great interest due to their high biological activity, including fungicidal and bactericidal effects, also in relation to multidrug-resistant bacteria. Membrane interactions of naringenin oxime (NO) and its 7-O-alkyl (7-alkoxy) derivatives, such as methyl (7MENO), ethyl (7ETNO), isopropyl (7IPNO), n-butyl (7BUNO) and n-pentyl (7PENO) were studied. Thermotropic properties of model membranes were investigated via differential scanning calorimetry (DSC), the influence on lipid raft mimicking giant unilamellar vesicles (GUVs) via fluorescence microscopy, and membrane permeability via measuring calcein leakage from liposomes. Molecular calculations supplemented the study. The influence of naringenin oximes on two strains of multidrug resistant bacteria: Staphylococcus aureus KJ and Enterococcus faecalis 37VRE was also investigated. In DSC studies all compounds reduced the temperature and enthalpy of main phase transition and caused disappearing of the pretransition. NO was the least active. The reduction in the area of surface domains in GUVs was observed for NO. Compounds NO and 7BUNO resulted in very low secretion of calcein from liposomes (permeability < 3 %). The highest results were observed for 7MENO (88.4 %) and 7IPNO (78.5 %). When bacterial membrane permeability was investigated all compounds caused significant release of propidium iodide from S. aureus (31.6-87.0 % for concentration 128 µg/mL). In the case of E. faecalis, 7ETNO (75.7 %) and NO (28.8 %) were the most active. The rest of the tested compounds showed less activity (permeability < 13.9 %). The strong evidence was observed that antibacterial activity of the tested compounds may be associated with their interaction with bacterial membrane.

Methoxychalcones as potential anticancer agents for colon cancer: Is membrane perturbing potency relevant?

Chalcones are naturally produced by many plants, and constitute precursors for the synthesis of flavons and flavanons. They were shown to possess antibacterial, antifungal, anti-cancer, and anti- inflammatory properties. The goal of the study was to assess the suitability of three synthetic methoxychalcones as potential anticancer agents. In a panel of colon cancer cell lines they were demonstrated to be cytotoxic, proapoptotic, causing cell cycle arrest, and increasing intracellular level of reactive oxygen species. Anticancer activity of the compounds was not diminished in the presence of stool extract containing microbial enzymes that could change the structure of chalcones. Moreover, methoxychalcones interacted strongly with model phosphatidylcholine membranes as detected by differential scanning calorimetry. Metohoxychalcones particularly affected the properties of lipid domains in giant unilamellar liposomes formed from raft-mimicking lipid composition. This may be of importance since many molecular targets for therapy of metastatic colon cancer are raft-associated receptors (e.g., receptor tyrosine kinases). The importance of membrane perturbing potency of methoxychalcones for their biological activity was additionally corroborated by the results obtained by molecular modelling.

Modulators of cellular cholesterol homeostasis as antiproliferative and model membranes perturbing agents.

Cholesterol is an important component of mammalian cell membranes affecting their fluidity and permeability. Together with sphingomyelin, cholesterol forms microdomains, called lipid rafts. They play important role in signal transduction forming platforms for interaction of signal proteins. Altered levels of cholesterol are known to be strongly associated with the development of various pathologies (e.g., cancer, atherosclerosis and cardiovascular diseases). In the present work, the group of compounds that share the property of affecting cellular homeostasis of cholesterol was studied. It contained antipsychotic and antidepressant drugs, as well as the inhibitors of cholesterol biosynthesis, simvastatin, betulin, and its derivatives. All compounds were demonstrated to be cytotoxic to colon cancer cells but not to non-cancerous cells. Moreover, the most active compounds decreased the level of free cellular cholesterol. The interaction of drugs with raft-mimicking model membranes was visualized. All compounds reduced the size of lipid domains, however, only some affected their number and shape. Membrane interactions of betulin and its novel derivatives were characterized in detail. Molecular modeling indicated that high dipole moment and significant lipophilicity were characteristic for the most potent antiproliferative agents. The importance of membrane interactions of cholesterol homeostasis-affecting compounds, especially betulin derivatives, for their anticancer potency was suggested.

The Use of Endo-Cellulase and Endo-Xylanase for the Extraction of Apple Pectins as Factors Modifying Their Anticancer Properties and Affecting Their Synergy with the Active Form of Irinotecan.

Pectin constitutes an essential component of dietary fiber. Modified pectins from various sources possess potent anticancer and immunomodulatory activities. In this study, two pectins isolated from apple pomace by Trichoderma enzyme treatment, PX (with endo-xylanase) and PCX (with both endo-cellulase and endo-xylanase), were studied in colon cancer cell lines (HCT 116, Caco-2, and HT-29). Both pectins reduced colon cancer cell viability, induced apoptosis, and increased intracellular amounts of reactive oxygen species. Additionally, synergy between pectin and an active form of irinotecan, SN-38, in all aspects mentioned above, was discovered. This drug is a common component of cytotoxic combinations recommended as treatment for colon cancer patients. PX and PCX demonstrated significant anti-inflammatory activity in lipopolysaccharide-stimulated cells. Interaction of apple pectins with galectin-3 and Toll-like Receptor 4 (TLR4) was suggested to be responsible for their anticancer and anti-inflammatory effect. Since PCX was more active than PX in almost all experiments, the role of the enzyme used to obtain the pectin for its biological activity was discussed. It was concluded that co-operation between both enzymes was needed to obtain the molecule of the most beneficial properties. The low molecular mass of PCX together with a high proportion of rhamnogalacturonan I (RG I) regions seemed to be crucial for its superior activity.

Isobavachalcone as an Active Membrane Perturbing Agent and Inhibitor of ABCB1 Multidrug Transporter.

Isobavachalcone (IBC) is an active substance from the medicinal plant Psoralea corylifolia. This prenylated chalcone was reported to possess antioxidative, anti-inflammatory, antibacterial, and anticancer activities. Multidrug resistance (MDR) associated with the over-expression of the transporters of vast substrate specificity such as ABCB1 (P-glycoprotein) belongs to the main causes of cancer chemotherapy failure. The cytotoxic, MDR reversing, and ABCB1-inhibiting potency of isobavachalcone was studied in two cellular models: human colorectal adenocarcinoma HT29 cell line and its resistant counterpart HT29/Dx in which doxorubicin resistance was induced by prolonged drug treatment, and the variant of MDCK cells transfected with the human gene encoding ABCB1. Because MDR modulators are frequently membrane-active substances, the interaction of isobavachalcone with model phosphatidylcholine bilayers was studied by means of differential scanning calorimetry. Molecular modeling was employed to characterize the process of membrane permeation by isobavachalcone. IBC interacted with ABCB1 transporter, being a substrate and/or competitive inhibitor of ABCB1. Moreover, IBC intercalated into model membranes, significantly affecting the parameters of their main phospholipid phase transition. It was concluded that isobavachalcone interfered both with the lipid phase of cellular membrane and with ABCB1 transporter, and for this reason, its activity in MDR cancer cells was presumptively beneficial.

Conjugation with Phospholipids as a Modification Increasing Anticancer Activity of Phenolic Acids in Metastatic Melanoma-In Vitro and In Silico Studies.

Phenolic acids possess many beneficial biological activities, including antioxidant and anti-inflammatory properties. Unfortunately, their low bioavailability restricts their potential medical uses, as it limits the concentration of phenolic acids achievable in the organism. The conjugation with phospholipids constitutes one of the most effective strategies to enhance compounds bioavailability in biological systems. In the present study, the conjugates of anisic (ANISA) and veratric acid (VA) with phosphatidylcholine (PC) were investigated. Since both ANISA and VA are inhibitors of tyrosinase, a melanocyte enzyme, the expression of which increases during tumorigenesis, anticancer potential of the conjugates was tested in several metastatic melanoma cell lines. The conjugates proved to be antiproliferative, apoptosis-inducing and cell-cycle-affecting agents, selective for cancerous cells and not affecting normal fibroblasts. The conjugates substituted by ANISA and VA, respectively, at both the sn-1 and sn-2 positions of PC, appeared the most promising, since they were effective against the vast majority of metastatic melanoma cell lines. Additionally, the conjugation of phenolic acids to PC increased their antioxidant activity. Molecular modeling was employed for the first time to estimate the features of the investigated conjugates relevant to their anticancer properties and membrane permeation. Again, the conjugates substituted by phenolic acid at both the sn-1 and sn-2 positions of PC seemed to be presumably most bioavailable.

Newly Obtained Apple Pectin as an Adjunct to Irinotecan Therapy of Colorectal Cancer Reducing E. coli Adherence and ß-Glucuronidase Activity.

Colorectal cancer (CRC) is the second cause of cancer death worldwide. The composition and enzymatic activity of colonic microbiota can significantly affect the effectiveness of CRC chemotherapy. Irinotecan is a drug widely used to treat colon cancer. However, the transformation of a drug-glucuronide (SN-38G) back to its active form (SN-38) by bacterial ß-glucuronidase (GUS) constitutes the primary reason for the observed intestinal toxicity of irinotecan. It was demonstrated that novel enzymatically extracted apple pectin (PC) might be a promising candidate for an adjunct to irinotecan therapy. PC itself reduced the viability of HCT 116 and Caco-2 colorectal cancer cells, induced apoptosis, and increased intracellular reactive oxygen species production. Moreover, PC enhanced the cytotoxic and proapoptotic effect of irinotecan (at concentrations below its IC50), i.e., synergistic effect was recorded. Additionally, PC exhibited potent anti-inflammatory properties and prevented adhesion of prototype adherent-invasive E. coli (AIEC) LF82 strain and laboratory K-12C600 strain to colon cancer cells. PC was also identified to be an effective inhibitor of bacterial GUS activity. Altogether, novel apple pectin was identified as a promising candidate for a supplement to irinotecan therapy that might alleviate its side-effects via inhibition of bacterial GUS and thus increasing its therapeutic efficacy.

TMPE Derived from Saffron Natural Monoterpene as Cytotoxic and Multidrug Resistance Reversing Agent in Colon Cancer Cells.

Terpenes constitute one of the largest groups of natural products. They exhibit a wide range of biological activities including antioxidant, anticancer, and drug resistance modulating properties. Saffron extract and its terpene constituents have been demonstrated to be cytotoxic against various types of cancer cells, including breast, liver, lung, pancreatic, and colorectal cancer. In the present work, we have studied anticancer properties of TMPE, a newly synthesized monoterpene derivative of ß-cyclocitral-the main volatile produced by the stigmas of unripe crocuses. TMPE presented selective cytotoxic activity to doxorubicin-resistant colon cancer cells and was identified to be an effective MDR modulator in doxorubicin-resistant cancer cells. Synergy between this derivative and doxorubicin was observed. Most probably, TMPE inhibited transport activity of ABCB1 protein without affecting its expression level. Analysis of TMPE physicochemical parameters suggested it was not likely to be transported by ABCB1. Molecular modeling showed TMPE being more reactive molecule than the parental compound-ß-cyclocitral. Analysis of electrostatic potential maps of both compounds prompted us to hypothesize that reduced reactivity as well as susceptibility to electrophilic attack were related to the lower general toxicity of ß-cyclocitral. All of the above pointed to TMPE as an interesting candidate molecule for MDR reversal in cancer cells.

Organosilicon Compounds, SILA-409 and SILA-421, as Doxorubicin Resistance-Reversing Agents in Human Colon Cancer Cells.

Multidrug resistance (MDR) that occurs in cancer cells constitutes one of the major reasons for chemotherapy failure. The main molecular mechanism of MDR is overexpression of protein transporters from the ATP-binding cassette (ABC) superfamily, such as ABCB1 (multidrug resistance protein 1 (MDR1), P-glycoprotein). At the expense of ATP hydrolysis, ABCB1 pumps a diverse range of substrates (including anticancer drugs) out of the cell, thereby reducing their intracellular concentration. In the present study, the ability of two patented disiloxanes (SILA-409 and SILA-421) to reverse drug resistance in human colon adenocarcinoma cell lines LoVo and LoVo/Dx was investigated. It was demonstrated that both compounds in concentrations of 0.5-1 µM strongly increased the sensitivity of LoVo/Dx cells to doxorubicin. By means of an accumulation test in which rhodamine 123 was used as an ABCB1 substrate analogue, both organosilicon compounds were also shown to inhibit ABCB1 transport activity. The intracellular accumulation of doxorubicin was also increased, and more drug entered the cellular nuclei of resistant cells in the presence of the studied compounds. In conclusion, both SILA-409 and SILA-421 were demonstrated to be effective MDR reversal agents in resistant human colon cancer cells.

The Combined Use of Phenothiazines and Statins Strongly Affects Doxorubicin-Resistance, Apoptosis, and Cox-2 Activity in Colon Cancer Cells.

Since none of the multidrug resistance (MDR) modulators tested so far found their way into clinic, a novel approach to overcome the MDR of cancer cells has been proposed. The combined use of two MDR modulators of dissimilar mechanisms of action was suggested to benefit from the synergy between them. The effect of three phenothiazine derivatives that were used as single agents and in combination with simvastatin on cell growth, apoptosis induction, activity, and expression of cyclooxygenase-2 (COX-2) in doxorubicin-resistant colon cancer cells (LoVo/Dx) was investigated. Treatment of LoVo/Dx cells by phenothiazine derivatives combined with simvastatin resulted in an increase of doxorubicin cytotoxicity and its intracellular accumulation as compared to the treatment with phenothiazine derivatives that were used as single agents. Similarly, LoVo/Dx cells treated with two-component mixture of modulators showed the reduced expression of ABCB1 (P-glycoprotein) transporter and COX-2 enzyme, both on mRNA and protein level. Reduced expression of anti-apoptotic Bcl-2 protein and increased expression of pro-apoptotic Bax were also detected. Additionally, COX-2 activity was diminished, and caspase-3 activity was increased to a higher extent by phenothiazine derivative:simvastatin mixtures than by phenothiazine derivatives themselves. Therefore, the introduction of simvastatin strengthened the anti-MDR, anti-inflammatory, and pro-apoptotic properties of phenothiazines in LoVo/Dx cells.

Simvastatin Strongly Augments Proapoptotic, Anti-inflammatory and Cytotoxic Activity of Oxicam Derivatives in Doxorubicin-resistant Colon Cancer Cells.

BACKGROUND: Incidence of cancer is still increasing. Chemotherapy is often unsuccessful; moreover, anticancer drugs cause serious side-effects. It is necessary to develop effective agents for combination therapies that would increase antitumor effects of treatment and reduce its side-effects. MATERIALS AND METHODS: Anticancer activity of oxicam derivatives (PR17 and PR18) alone and in combination with simvastatin on doxorubicin-resistant colon cancer cells was studied. Apoptosis was investigated via caspase-3 activation assay as well as via western blot analysis of expression of apoptotic components, B-cell lymphoma 2 protein (BCL2) and BCL2-associated X protein (BAX). Expression and activity of cyclo-oxygenase-2 (COX2) was also assessed. RESULTS: Oxicam derivatives induced apoptosis through a caspase-3-dependent pathway, up-regulated BAX expression, and down-regulated BCL2 expression. Additionally, oxicam derivatives reduced expression and activity of COX2. Effect of oxicam derivatives on these processes was strongly potentiated by simvastatin. CONCLUSION: Oxicam derivatives at low concentrations effectively inhibit growth of cancer cells after co-administration with simvastatin.

Cytotoxic and multidrug resistance reversal activity of phenothiazine derivative is strongly enhanced by theobromine, a phytochemical from cocoa.

The idea of the use of anticancer drugs together with a chemosensitizer emerged as the strategy of reversal of multidrug resistance (MDR) of cancer cells expressing ABC proteins many years ago. The approaches relying on the use of a single chemosensitizer have never resulted in a clinical success. Therefore, the application of drug combinations of two or more compounds with different mechanisms of action might be an alternative approach to increase the success rate. In the present study the cytotoxic and NF-κB inhibition potential of the phenothiazine derivative, MAE-TPR, was evaluated. MAE-TPR was demonstrated to be an effective doxorubicin-resistance modulator in human adenocarcinoma cell line LoVo/Dx. In the presence of MAE-TPR cytotoxicity of doxorubicin was elevated, and its intracellular accumulation increased. Strong synergism occurred between MAE-TPR and Dox. MAE-TPR diminished also the expression of ABCB1 transporter (P-glycoprotein) by affecting NF-κB pathway. Theobromine, a phytochemical from cocoa, which was barely active itself, strongly augmented MDR reversal potency of MAE-TPR. The effect of the combination of phenothiazine derivative with theobromine on cancer cells was studied for the first time in the present work. It was concluded that the use of the proposed combination of two modulators might be a promising strategy for MDR reversal since modulators could be used in concentrations much lower than in case of their single application and in that way the risk of intolerable side-effects could be reduced.

MDR reversal and pro-apoptotic effects of statins and statins combined with flavonoids in colon cancer cells.

The resistance of cancer cells to a variety of structurally non-related cytotoxic drugs is known as multidrug resistance phenomenon (MDR). In cellular membranes an activity of MDR transporters such as P-glycoprotein (ABCB1) is affected by their lipid environment. Many various compounds have been examined for their ability to restore drug-sensitivity of resistant cancer cells. Statins, inhibitors of the key enzyme of mevalonate pathway HMG-CoA (3-hydroxy-3-methyl-glutaryl-coenzyme A) reductase are drugs commonly prescribed in order to reduce serum level of cholesterol and to diminish the risk of cardiovascular disease. Statins as drugs that influence lipid composition of cell membrane and in that way they also exert influence on lipid bilayer properties appear to be good candidates as MDR modulators. In this work it was shown that statins - mevastatin and simvastatin exert antiproliferative, pro-apoptotic and reversing drug resistance effect in human colon adenocarcinoma cell line LoVo and its drug-resistant subline LoVo/Dx. A hypothesis was also checked whether flavones, which as it is well known are able to influence the biosynthesis of cholesterol, may change the anticancer activity of statins. Our investigations have revealed that combined use of statins and studied flavonoids results in enhanced cell growth inhibition and apoptosis and lower cancer cell proliferation as compared to the application only statins alone. Moreover, in drug resistant LoVo/Dx cells a stronger decrease of resistance to doxorubicine was observed in the presence of statins in combination with flavones as compared to the effect observed for statins only.

Increased lipid peroxidation, apoptosis and selective cytotoxicity in colon cancer cell line LoVo and its doxorubicin-resistant subline LoVo/Dx in the presence of newly synthesized phenothiazine derivatives.

Cancer cells often develop the resistance to pro-apoptotic signaling that makes them invulnerable to conventional treatment. Therapeutic strategies that make cancer cells enter the path of apoptosis are desirable due to the avoidance of inflammatory reaction that usually accompanies necrosis. In the present study phenothiazines (fluphenazine and four recently synthesized derivatives) were investigated in order to identify compounds with a potent anticancer activity. Since phenothiazines are known as multidrug resistance modulators the sensitive human colorectal adenocarcinoma cell line (LoVo) and its doxorubicin-resistant, ABCB1 overexpressing, subline (LoVo/Dx) have been employed as a model system. In studied cancer cells cytotoxic effect of the phenothiazine derivatives was accompanied by apoptosis and autophagy induction as well as by the increase of cellular lipid peroxidation and intracellular reactive oxygen species generation. Molecular modelling revealed that reactivity of phenothazines (manifested by their low energy gap) but not lipophilicity was positively correlated with their anticancer potency, pro-oxidant properties and apoptosis induction ability. Additionally, some of the studied compounds turned out to be more potent cytotoxic and pro-apoptotic agents in doxorubicin-resistant (LoVo/Dx) cells than in sensitive ones (LoVo). The hypothesis was assumed that studied phenothiazine derivatives induced apoptotic cell death by increasing the production of reactive oxygen species.

Euphorbia Species-derived Diterpenes and Coumarins as Multidrug Resistance Modulators in Human Colon Carcinoma Cells.

BACKGROUND: Recently, many new potent multidrug resistance (MDR) reversal agents have been discovered, among them lathyrane and jatrophane diterpenes isolated from various Euphorbia species. In the present study, the cytotoxicity, P-glycoprotein inhibition activity, and MDR reversal potency of six diterpenes and two coumarins from two Euphorbia species were studied in human colon carcinoma LoVo cells, and doxorubicin-resistant, LoVo/Dx cells. MATERIALS AND METHODS: Cytotoxicity of the studied compounds (alone and in combination with doxorubicin) was investigated. Inhibition of P-glycoprotein transport activity was monitored by flow cytometry. Changes in intracellular doxorubicin accumulation were observed by means of fluorescence microscopy. RESULTS: Latilagascene B was demonstrated to be an effective P-glycoprotein inhibitor, able to increase doxorubicin accumulation in resistant cells, however not able to restore doxorubicin cytotoxicity in LoVo/Dx cells. CONCLUSION: The structure of latilagascene B seems to be an interesting candidate for further synthesis of new derivatives of reduced cytotoxicity and high anti-MDR potency.

Effect of new oxicam derivatives on efflux pumps overexpressed in resistant a human colorectal adenocarcinoma cell line.

BACKGROUND: Oxicams are non-steroidal anti-inflammatory drugs (NSAIDs). Antitumor potential of NSAIDs has often been reported in literature. We studied antitumor activity of newly synthesized oxicam derivatives (PR17 and PR18) against doxorubicin-sensitive and resistant human colorectal adenocarcinoma cells (LoVo and LoVo/Dx). MATERIALS AND METHODS: The cytotoxicity of oxicam derivatives alone and in combination with doxorubicin was assessed. Inhibition of P-glycoprotein (ABCB1) transport activity was monitored by flow cytometry. Expression of ABCB1 gene was analyzed by semi-quantitative reverse transcription PCR, while ABCB1 protein expression was assessed by western blotting. RESULTS: Oxicam derivative PR18 was more cytotoxic to cancer cells than PR17. PR18 was observed to sensitize LoVo/Dx cells to doxorubicin and was identified as an effective multidrug resistance modulator. Additionally, ABCB1 expression was reduced in the presence of PR18. CONCLUSION: PR18 was identified as an effective modulator in LoVo/Dx resistant human colorectal adenocarcinoma cells which overexpressed ABCB1 efflux pump.

Interaction of prenylated chalcones and flavanones from common hop with phosphatidylcholine model membranes.

Common hop (Humulus lupulus) constitutes a source of numerous prenylated chalcones such as xanthohumol (XH) and flavanones such as 8-prenylnaringenin (8-PN) and isoxanthohumol (IXH). Range of their biological activities includes estrogenic, anti-inflammatory, anti-infective, anti-cancer, and antioxidant activities. The aim of the present work was to characterize the influence of prenylated polyphenols on model 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) membranes by means of differential scanning calorimetry (DSC), fluorescence and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopies. All studied compounds intercalated into DPPC bilayers and decreased its melting temperature as recorded by DSC, Laurdan and Prodan fluorescence, and ATR-FTIR. Polyphenols interacted mainly with glycerol backbone and acyl chain region of membrane. Magnitude of the induced effect correlated both with lipophilicity and molecular shape of the studied compounds. Elbow-shaped 8-PN and IXH were locked at polar-apolar region with their prenyl chains penetrating into hydrophobic part of the bilayer, while relatively planar XH molecule adopted linear shape that resulted in its deeper insertion into hydrophobic region. Additionally, by means of DSC and Laurdan fluorescence IXH was demonstrated to induce lateral phase separation in DPPC bilayers in gel-like state. It was assumed that IXH-rich and IXH-poor microdomains appeared within membrane. Present work constitutes the first experimental report describing interactions of prenylated hop polyphenols with phospholipid model membranes.

Multidrug resistance reversal and apoptosis induction in human colon cancer cells by some flavonoids present in citrus plants.

Multidrug resistance (MDR) of cancer cells constitutes one of the main reasons for chemotherapy failure. The search for nontoxic modulators that reduce MDR is a task of great importance. An ability to enhance apoptosis of resistant cells would also be beneficial. In the present study, the MDR reversal and apoptosis-inducing potency of three flavonoids produced by Citrus plants, namely, naringenin (1a), aromadendrin (2), and tangeretin (3), and the methylated naringenin derivatives (1b, 1c), have been studied in sensitive (LoVo) and multidrug-resistant (LoVo/Dx) human colon adenocarcinoma cells. Cytotoxicity of methoxylated flavonoids was higher as compared to hydroxylated analogues. Only 3 turned out to inhibit P-glycoprotein, as demonstrated by a rhodamine 123 accumulation assay. It also increased doxorubicin accumulation in LoVo/Dx cells and enabled doxorubicin to enter cellular nuclei. In addition, 3 was found to be an effective MDR modulator in resistant cells by sensitizing them to doxorubicin. Tangeretin-induced caspase-3 activation and elevated surface phosphatidylserine exposure demonstrated its apoptosis-inducing activity in LoVo/Dx cells, while the other flavonoids evaluated were not active. Additionally, 3 was more toxic to resistant rather than to sensitive cancer cells. Its apoptosis-inducing activity was also higher in LoVo/Dx than in LoVo cells. It was concluded that the activity of 3 against multidrug-resistant cancer cells may be enhanced by its apoptosis-inducing activity.

Polyphenols counteract tumor cell chemoresistance conferred by multidrug resistance proteins.

One of the main reasons of cancer resistance to chemotherapeutic treatment is the presence of different ABC multidrug transporters in plasma membranes. The transporters extrude wide spectrum of anticancer agents out of cancer cells at the expense of energy derived from ATP-hydrolysis. Plant-origin polyphenolic compounds, mainly flavonoids and stilbenes or their synthetic derivatives, can modulate the main ABC transporters responsible for cancer drug resistance, including P-glycoprotein, MRP1 and BCRP. The recent studies on different resistant cancer cell lines enabled the discovery of a number of polyphenolic compounds able to reverse drug resistance in vitro and these compounds could be promising candidates for further clinical trials. The review summarizes the recent advances in the field of polyphenols interaction with ATP-binding cassette multidrug transporters. The mechanism of flavonoids interactions with the multidrug transporters and the structure-activity relationship are also discussed.

Interaction of phenothiazines, stilbenes and flavonoids with multidrug resistance-associated transporters, P-glycoprotein and MRP1.

Multidrug resistance (MDR) of cancer cells poses a serious obstacle to successful chemotherapy. The overexpression of multispecific ATP-binding cassette transporters appears to be the main mechanism of MDR. A search for MDR-reversing agents able to sensitize resistant cells to chemotherapy is ongoing in the hope of their possible clinical use. Studies of MDR modulators, although they have not produced clinically beneficial effects yet, may greatly enrich our knowledge about MDR transporters, their specificity and mechanism of action, especially substrate and/or inhibitor recognition. In the present review, interactions of three groups of modulators: phenothiazines, flavonoids and stilbenes with both P-glycoprotein and MRP1 are discussed. Each group of compounds is likely to interact with the MDR transporters by a different mechanism. Phenothiazines probably interact with drug binding sites, but they also could indirectly affect the transporter's activity by perturbing lipid bilayers. Flavonoids mainly interact with ABC proteins within their nucleotide-binding domains, though the more hydrophobic flavonoids may bind to regions within transmembrane domains. The possible mechanism of MDR reversal by stilbenes may result from their direct interaction with the transporter (possibly within substrate recognition sites) but some indirect effects such as stilbene-induced changes in gene expression pattern and in apoptotic pathways should also be considered. Literature data as well as some of our recent results are discussed. Special emphasis is put on cases when the interactions of a given compound with both P-glycoprotein and MRP1 have been studied simultaneously.