L1210 cells cultivated under the selection pressure of doxorubicin or vincristine express common mechanisms of multidrug resistance based on the overexpression of P-glycoprotein

Detection of the Mitochondrial Membrane Potential by the Cationic Dye JC-1 in L1210 Cells with Massive Overexpression of the Plasma Membrane ABCB1 Drug Transporter

JC-1, a cationic fluorescent dye when added to living cells, is known to be localized exclusively in mitochondria, particularly in good physiological conditions characterized by sufficient mitochondrial membrane potential (ΔΨ). The accumulation of JC-1 in these organelles leads to the formation J-aggregates (with a specific red fluorescence emission maximum at 590 nm), which is in addition to the typical green fluorescence of J-monomers (emission maximum of ∼529 nm). The lack of mitochondrial ΔΨ leads to the depression of JC-1 mitochondrial accumulation and a decrease in J-aggregate formation. Therefore, the ratio between the red and green fluorescence of cells loaded with JC-1 is often used for the detection of the mitochondrial membrane potential. However, JC-1 represents a suitable substrate of the multidrug transporter P-glycoprotein (P-gp). Therefore, the depression of the JC-1 content in intracellular space and particularly in the mitochondria to a level that is inefficient for J-aggregate formation could be expected in P-gp-positive cells. In the current paper, we proved this behavior on parental P-gp-negative L1210 (S) cells and their P-gp-positive variants obtained by either selection with vincristine (R) or transfection with the human gene encoding P-gp (T). P-glycoprotein inhibitors cyclosporine A and verapamil fail to restore JC-1 loading of the R and T cells to an extent similar to that observed in S cells. In contrast, the noncompetitive high affinity P-gp inhibitor tariquidar fully restored JC-1 accumulation and the presence of the typical red fluorescence of J-aggregates. In the presence of tariquidar, measurement of the JC-1 fluorescence revealed similar levels of mitochondrial membrane potential in P-gp-negative (S) and P-gp-positive cells (R and T).

Silver pyridine-2-sulfonate complex - its characterization, DNA binding, topoisomerase I inhibition, antimicrobial and anticancer response

In the current study the ability of silver pyridine-2-sulfonate complex to exert multiple biological activities is compared with the pharmacological action of silver sulfadiazine (AgSD). Polymeric form of {[Ag(py-2-SO₃)]}_n (AgPS) was synthesized and characterized by analytical techniques (IR, CHN, TG/DTA, MS) and its molecular formula was established. The crystal structure was determined by X-ray diffraction method and the polymeric complex crystallizes in the triclinic P-1 space group. The stability of Ag(I) complex was verified by ¹H and ¹³C NMR measurements and the interaction with calf thymus DNA through UV-VIS and fluorescence quenching experiments was studied. The Ag(I) complex was able to interact with DNA by dual binding mode: partial intercalation along groove binding. The binding constants were calculated to be in the order of 10³ M^-1. Topoisomerase I inhibition study have shown that silver complex is inhibiting its activity at concentration of 30 μM. The cytotoxic activity of AgPS and AgSD against mouse leukaemia L1210 S, R and T cell line was also evaluated. AgPS showed higher cytotoxicity than AgSD after 48 h incubation. The results suggest that mechanism of cell death is necrosis with a contribution of late apoptosis. Antimicrobial testing indicates higher growth inhibition effect of AgPS with comparison to commercially available AgSD.

Sesquiterpene lactone 6-O-angeloylplenolin reverses vincristine resistance by inhibiting YB-1 nuclear translocation in colon carcinoma cells

Multidrug resistance (MDR) is a major obstacle to cancer chemotherapy efficacy. In the present study, 6-O-angeloylplenolin repressed the overexpression of ATP binding cassette subfamily B member 1 (MDR1) and increasing the intracellular concentration of anticancer drugs. A reduction in P-glycoprotein expression (encoded by MDR1) was observed in parallel with a decline in mRNA expression in vincristine-resistant HCT (HCT-8/VCR) cells treated with 6-O-angeloylplenolin. In addition, 6-O-angeloylplenolin suppressed the activity of the MDR1 gene promoter. Treatment with 6-O-angeloylplenolin also decreased the amount of the specific protein complex that interacted with the MDR1 gene promoter in HCT-8/VCR cells, potentially leading to the suppression of MDR1 expression. Treatment with 6-O-angeloylplenolin inhibited the nuclear translocation of Y-box binding protein-1 in HCT-8/VCR cells treated with 6-O-angeloylplenolin, contributing to the negative regulation of MDR1. Finally, 6-O-angeloylplenolin reversed VCR resistance in an HCT/VCR xenograft model. In conclusion, 6-O-angeloylplenolin exhibited a MDR-reversing effect by downregulating MDR1 expression and could represent a novel adjuvant agent for chemotherapy.

Acute myeloid leukemia cells MOLM-13 and SKM-1 established for resistance by azacytidine are crossresistant to P-glycoprotein substrates

Establishment of the acute myeloid leukemia cells SKM-1 and MOLM-13 for resistance by azacytidine (AzaC) resulted in SKM-1/AzaC and MOLM-13/AzaC cell variants with reduced sensitivity to AzaC. Despite the fact that AzaC is not substrate of P-glycoprotein (P-gp), the adaptation procedure resulted in an induction in P-gp expression/efflux activity that confers crossresistance to P-gp substrates in both resistant cell variants. While the resistance to P-gp substrates in SKM-1/AzaC and MOLM-13/AzaC cells could be reversed by the P-gp inhibitors, resistance to AzaC was insensitive to these inhibitors in both resistant cell variants. In addition, NF-κB and the antiapoptotic protein Bcl-2 were downregulated and the proapoptotic proteins Bax and p53 were upregulated in both resistant cell variants when compared with their sensitive counterparts. Moreover, at least five times the elevation in overall glutathione S-transferase activity was measured with 1-chloro-2, 5-dinitrobenzene as a substrate in the resistant variant of both cell lines. Taken together, the findings of the present study indicate that the treatment of AML cells with AzaC might lead to a drug resistance phenotype that may be associated with cross resistance to P-gp substrates and substrates of glutathione S-transferases.

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.

Development and characterization of P-glycoprotein 1 (Pgp1, ABCB1)-mediated doxorubicin-resistant PLHC-1 hepatoma fish cell line

The development of the multidrug resistance (MDR) phenotype in mammals is often mediated by the overexpression of the P-glycoprotein1 (Pgp, ABCB1) or multidrug resistance-associated protein (MRP)-like ABC transport proteins. A similar phenomenon has also been observed and considered as an important part of the multixenobiotic resistance (MXR) defence system in aquatic organisms. We have recently demonstrated the presence of ABC transporters in the widely used in vitro fish model, the PLHC-1 hepatoma cell line. In the present study we were able to select a highly resistant PLHC-1 sub-clone (PLHC-1/dox) by culturing the wild-type cells in the presence of 1 microM doxorubicin. Using quantitative PCR a 42-fold higher expression of ABCB1 gene was determined in the PLHC-1/dox cells compared to non-selected wild-type cells (PLHC-1/wt). The efflux rates of model fluorescent Pgp1 substrates rhodamine 123 and calcein-AM were 3- to 4-fold higher in the PLHC-1/dox in comparison to the PLHC-1/wt cells. PLHC-1/dox were 45-fold more resistant to doxorubicin cytotoxicity than PLHC-1/wt. Similarly to mammalian cell lines, typical cross-resistance to cytotoxicity of other chemotherapeutics such as daunorubicin, vincristine, vinblastine, etoposide and colchicine, occurred. Furthermore, cyclosporine A, verapamil and PSC833, specific inhibitors of Pgp1 transport activity, completely reversed resistance of PLHC-1/dox cells to all tested drugs, resulting in EC50 values similar to the EC50 values found for PLHC-1/wt. In contrast, MK571, a specific inhibitor of MRP type of efflux transporters, sensitized PLHC-1/dox cells, neither to doxorubicin, nor to any other of the chemotherapeutics used in the study. These data demonstrate for the first time that a specific Pgp1-mediated doxorubicin resistance mechanism is present in the PLHC-1 fish hepatoma cell line. In addition, the fact that low micromolar concentrations of specific inhibitors may completely reverse a highly expressed doxorubicin resistance points to the fragility of Pgp1-mediated MXR defence mechanism in fish.

Combined treatment of P-gp-positive L1210/VCR cells by verapamil and all-trans retinoic acid induces down-regulation of P-glycoprotein expression and transport activity

The development of the most common multidrug resistance (MDR) phenotype associated with a massive overexpression of P-glycoprotein (P-gp) in neoplastic cells may result in more than one hundred fold higher resistance of these cells to several drugs. L1210/VCR is a P-gp-positive drug resistant cell line in which P-gp overexpression was achieved by repeated cultivation of parental cells with a stepwise increasing concentration of vincristine. Relatively little is known about regulation of P-gp expression. Therefore, serious efforts have been made to recognize all aspects involved in regulation of P-gp expression. Retinoic acid nuclear receptors are involved in regulating expression of a large number of different proteins. Several authors have described that all-trans retinoic acid (ATRA, ligand of retinoic acid receptors, RARs) may induce alterations in P-gp expression and/or activity in drug resistant malignant cell lines. There are also other nuclear receptors for retinoids--retinoid X receptors (RXRs)--that may be involved in the development of the P-gp-mediated MDR phenotype. The topic of the present paper is a study of the relationship, if any, between the regulatory pathways of nuclear receptors for retinoids and P-glycoprotein expression. Increased levels of mRNAs encoding the retinoic acid nuclear receptors RARalpha and gamma, as well as decreased levels of the mRNAs encoding RARbeta and the retinoid X receptor RXRgamma or slightly decreased levels of RXRbeta mRNA, were observed in L1210/VCR cells in comparison with parental L1210 cells. Neither L1210 cells nor L1210/VCR cells contained measurable amounts of mRNA encoding the RXRalpha receptor. ATRA did not influence the viability of L1210/VCR cells differently from L1210 cells. A combined treatment of L1210/VCR cells with vincristine (1.08 micromol/l) and ATRA induced slightly higher cell death than that observed with ATRA alone. When applied alone, ATRA did not influence P-gp expression (monitored by anti P-gp antibody c219 using western blot analysis) or transport activity (monitored by use of calcein/AM as a P-gp substrate by FACS) in L1210/VCR cells. In contrast, when ATRA was applied together with verapamil (an often used P-gp inhibitor), a significant decrease in P-gp expression and transport activity were observed. However, no significant differences in [11, 12-(3)H]-ATRA uptake were observed in either sensitive or resistant cells, in the latter case in the absence or presence of vincristine. Moreover, verapamil did not influence ATRA uptake under any conditions. Thus, we can conclude that the combined treatment of L1210/VCR cells with ATRA and verapamil is able to depress P-gp expression, and consequently its activity. ATRA is not a P-gp-transportable substance, and thus this effect could not be attributed to verapamil-induced inhibition of P-gp that would allow ATRA to reach retinoic acid nuclear receptors and activate them.