Circumvention of multi-drug resistance of cancer
cells by Chinese herbal medicines
Stella Chai, Kenneth KW To, Ge Lin*
School of Biomedical Sciences, Faculty of Medicine, The Chinese University
of Hong Kong, Shatin, Hong Kong SAR, China
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.
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- Multi-drug resistance (MDR)
Multi-drug resistance (MDR) in cancer chemotherapy refers to the ability of cancer cells to survive from treatment of a wide range of drugs . In addition to the MDR induced by drugs in early exposure, the MDR cancer cells may subsequently develop cross-resistance to several unexposed and structurally unrelated chemotherapeutic agents . Mechanisms of MDR include decreased uptake of drugs, alterations in cellular pathways and increased active efflux of drugs [3-5] (Figure 1). Overexpression of ATP-binding cassette (ABC) transporters is one of the most common mechanisms. ABC transporters are large membrane-bound proteins consisting of two nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs) which mediate the active transport of substrate drugs out of the cell (Figure 2). Overexpression of the three major ABC transporters, i.e. P-glycoprotein (Pgp), multidrug-resistance-associated protein 1 (MRP1) and breast cancer resistance protein (BCRP/ABCG2) is frequently observed in cancer cell lines selected with chemotherapeutic drugs  and critical to clinical drug resistance . P-glycoprotein (Pgp) P-glycoprotein (Pgp) , which is also referred to as ABCB1 and MDR1, is the most studied ABC transporter. Pgp transports a wide range of chemotherapeutic agents including the anthracyclines, vincas, taxanes, etoposide and mitoxantrone . Pgp is expressed in various tissues in the body. Remarkably high expression can be found in endothelial cells of capillary blood vessels in the brain as well as other organs including intestines, testes and skin [9,10]. Pgp expression is often detected in renal carcinoma, colon carcinoma, adrenal carcinoma and teratocarcinoma . Substrate drugs can bind to Pgp through multiple binding sites, thereby allowing flexibility in the mechanism of transport [11,12]. Multidrug-resistance-associated protein 1 (MRP1) The second major MDR transporter, multidrug-resistance- associated protein (MRP), was first discovered in a doxorubicin-selected lung cancer cell line . A member of the ABCC subfamily, MRP1 is encoded by the ABCC1 gene . Physiologically, MRP1 tends to pump drugs into the body, rather than excreting them into the bile, urine or gut [15,16]. MRP1 was highly expressed in skeletal muscles . Overexpression of MRP1 is in cancer types such as lung, colon and various forms of leukaemia .
How to tackle the MDR cells in chemotherapy is a pressing issue in cancer treatments. Verapamil was the first known Pgp inhibitor to increase the intracellular concentration of anticancer agents in MDR cells by binding to Pgp and inhibiting the Pgp-mediated efflux . It was believed that anticancer drug resistance could be reversed by drug efflux inhibition. Researchers developed and tested a range of Pgp inhibitors to improve the pharmacological effects of chemotherapy in cancer patients [27-29]. However, none of these Pgp inhibitors was further developed for clinical use. Many researchers are looking into Chinese medicine (CM) for potential MDR reversing agents. This article reviews some of the recent findings on the circumvention of ABC transporters-mediated MDR by various ingredients and extracts of CM and their formulae based on whether the MDR reversal involved Pgp alteration.
MDR reversal involving Pgp inhibition
- Active ingredients - alkaloids
Tetrandrine, a calcium channel blocker, is a bisbenzylisoquinoline alkaloid isolated from the root of Stephania tetrandra (Fenfangji) . Tetrandrine reversed MDR in vitro and modulated Pgp-mediated drug efflux [30-33]. A combination of tetrandrine with doxorubicin or vincristine in vitro demonstrated synergistic anticancer effects . Tetrandrine reduced Pgp expression . In mice bearing resistant MCF-1/DOX cells, tetrandrine potentiated the antitumor activities of doxorubicin without significantly increasing toxicity . A synthetic halogenated form of tetrandrine increased vinblastine accumulation in a dose-dependent manner in resistant P388/DOX cell line and prolonged the life-span of tumour-bearing mice up to 25% without any side effects . In a recent clinical trial, a combination of tetrandrine with daunorubicin, etoposide and cytarabine demonstrated antileukaemic effects in 38 patients with acute myeloid leukaemia . Matrine is a quinolizidine alkaloid from Sophora alopecuroides (Kudouzi). In resistant K562/DOX cell line, matrine (up to 50 μg/mL, non-toxic) increased the intracellular accumulation of doxorubicin and induced its apoptotic effects . Matrine enhanced the cytotoxicity of vincristine in resistant K562/VCR cell line . It was proposed that matrine circumvented MDR by reducing Pgp expression . Tetramethylpyrazine is an active alkaloid from Ligusticum chuangxiong (Chuanxiong) and a calcium channel blocker . In Pgp-overexpressing resistant HL-60/ VCR cell line, tetramethylpyrazine significantly reversed MDR towards various drugs such as vincristine, duanorubicin and doxorubicin . Tetramethylpyrazine reduced drug efflux (up to 50%) in Pgp-overexpressing resistant MCF-7/DOX cell line . When used together with b-elemene, tetramethylpyrazine exhibited stronger MDR reversal effects in resistant K562/DOX cell line . Tetramethylpyrazine decreased Pgp expression in resistant HepG2/DOX cell line . However, the reduction in Pgp expression was not universally observed. For instance, Pgp level was not altered despite MDR reversal in tetramethylpyrazine-treated K562/DOX cells . Peimine (also known as verticine) is a cevanine type isosteroidal alkaloid from the bulbs of Fritillaria thunbergii (Zhebeimu) and other Fritillaria species [47,48]. In resistant K562/DOX and HL-60/DOX cell lines, peimine increased intracellular concentration of daunorubicin and reversed MDR probably through inhibition of Pgp expression . Berbamine is a calcium channel blocker from Mahonia fortunei (Shidagonglao). In K562/DOX cell line, berbamine inhibited cell growth by inducing apoptosis in a dose-dependent manner and reduced Pgp expression thereby increasing the intracellular concentration of rhodamine- 123 and doxorubicin [50,51]. In MCF-7/DOX cell line, O-(4-ethoxyl-butyl)-berbamine, a derivative of berbamine, reversed MDR by enhancing G2/M arrest and increasing the intracellular accumulation of doxorubicin .
Active ingredients - saponins
Ginsenosides are the major active components from Panax ginseng (Renshen). Ginsenosides are mainly triterpenoid dammarane derivatives. Several ginsenosides, namely Rg1, Rg3, Re, Rc and Rd inhibited drug efflux . A combination of purified saponins containing Rb1, Rb2, Rc, Rd, Re and Rg1 reversed MDR whereas individual ginsenosides did not produce any effect . Ginsenosides reversed MDR of several chemotherapeutic drugs such as homoharringtonine, cytarabine, doxorubicin and etoposide in K562/VCR and in a dose-dependent manner in K562/DOX . Pgp expression decreased but bcl-2 expression remained the same . Rb1 reversed MDR of harringtonolide and vincristine in K562/HHT and HL60/VCR cell lines respectively [57,58]. Panax notoginseng (Sanqi) total saponins reversed MDR of doxorubicin in MCF-7/DOX and K562/VCR cell lines. The mechanism may be related to the decrease of Pgp expression [59,60].
- Active ingredients - flavonoids
Quercetin is one of the most widely distributed flavonoids in natural products including Chinese medicinal herbs such as Sophora japonica (Huai). Quercetin inhibited the binding of heat shock factor at the MDR1 promoter, thereby decreasing MDR1 transcription and reducing Pgp expression . Quercetin also inhibited the overexpression of Pgp mediated by arsenite . In HL-60/DOX and K562/DOX cell lines, quercetin enhanced the anticancer sensitivity to daunorubicin and decreased Pgp expression [63,64]. MDR reversal effect of quercetin was probably mediated by its action on mitochondrial membrane potential and the induction of apoptosis. Furthermore, quercetin derivatives rather than quercetin itself reversed MDR . Quercetin increased the sensitivity of Pgp-overexpressing KBV1 cell line towards vinblastine and paclitaxel in a dosedependent manner. Among many active flavonoids, quercetin was less potent than kaempferol but more effective than genistein and daidzein in reversing MDR. Genistein and daidzein had no effect on Pgp expression . Although quercetin may be a potential MDR reversing agent, lethal drug-drug interaction between quercetin and digoxin has been reported. Quercetin (40 mg/ kg) elevated the peak blood concentration of digoxin and caused sudden death of tested animals . Curcumin, the major component in Curcuma longa (Jianghuang), inhibited the transport activity of all three major ABC transporters, i.e. Pgp, MRP1 and ABCG2 . Curcumin reversed MDR of doxorubicin or daunorubicin in K562/DOX cell line and decreased Pgp expression in a time-dependent manner . Curcumin enhanced the sensitivity to vincristine by the inhibition of Pgp in SGC7901/VCR cell line . Moreover, curcumin was useful in reversing MDR associated with a decrease in bcl-2 and survivin expression but an increase in caspase-3 expression in COC1/DDP cell line . The cytotoxicity of vincristine and paclitaxel were also partially restored by curcumin in resistant KBV20C cell line . Curcumin derivatives reversed MDR by inhibiting Pgp efflux . A chlorine substituent at the meta-or para-position on benzamide improved MDR reversal . Bisdemethoxycurcumin modified from curcumin resulted in greater inhibition of Pgp expression . Tetrahydrocurcumin, the major metabolite of curcumin, inhibited all three major ABC transporters . Curcumin induced atypical and caspase-independent cell death in MDR cells . In leukaemic cells collected from 78 childhood leukaemia patients, curcumin reduced Pgp expression . A specialized nanoemulsion of curcumin is better than conventional solution form drugs in enhancing the efficiency of drug delivery into the cells, down-regulating Pgp expression, inhibiting the NF_B pathway and promoting apoptotic response [68,77].
This article is an Open Access article published in Chinese Medicine 2010, 5:26.