Phorbol ester-induced P-glycoprotein phosphorylation and functionality in the HTB-123 human breast cancer cell line

Phorbol ester TPA modulates chemoresistance in the drug sensitive breast cancer cell line MCF-7 by inducing expression of drug efflux transporter ABCG2

Recent studies have indicated a link between levels of cyclooxygenase-2 (COX-2) and development of the multidrug resistance (MDR) phenotype. The ATP-binding cassette sub-family G member 2 (ABCG2) is a major MDR-related transporter protein that is frequently overexpressed in cancer patients. In this study, we aimed to evaluate any positive correlation between COX-2 and ABCG2 gene expression using the COX-2 inducer 12-O-tetradecanoylphorbol-13-acetate (TPA) in human breast cancer cell lines. ABCG2 mRNA and protein expression was studied using real-time RT-PCR and flow cytometry, respectively. A significant increase of COX-2 mRNA expression (up to 11-fold by 4 h) was induced by TPA in MDA-MB-231 cells, this induction effect being lower in MCF-7 cells. TPA caused a considerable increase up to 9-fold in ABCG2 mRNA expression in parental MCF-7 cells, while it caused a small enhancement in ABCG2 expression up to 67 % by 4 h followed by a time-dependent decrease in ABCG2 mRNA expression in MDA-MB-231 cells. TPA treatment resulted in a slight increase of ABCG2 protein expression in MCF-7 cells, while a time-dependent decrease in ABCG2 protein expression was occurred in MDA-MB-231 cells. In conclusion, based on the observed effects of TPA in MDA-Mb-231 cells, it is proposed that TPA up-regulates ABCG2 expression in the drug sensitive MCF-7 breast cancer cell line through COX-2 unrelated pathways.

Potential role of cyclooxygenase-2 on the regulation of the drug efflux transporter ABCG2 in breast cancer cell lines

PURPOSE

ABCG2 (BCRP) implicated as a member of the multidrug resistance (MDR) proteins in tumors, mediating efflux of a wide spectrum of anticancer drugs. In recent years, there has been an increasing tendency toward the exploring of the potential link between cyclooxygenase-2 (COX-2) expression and development of multidrug resistance phenotype in patients with cancer. The aim of this study was to investigate the role of the COX-2 in modulating drug efflux by ABCG2 in a group of breast cancer cell lines.

METHODS

The cytotoxicity of COX-2 inducer (TPA, tetradecanoyl phorbol acetate) and its inhibitor (celecoxib) was determined by an MTT assay. ABCG2 activity was measured by flow cytometric mitoxantrone efflux assay.

RESULTS

TPA exhibited very little inhibitory activity in all cell lines, while long-term treatment with celecoxib significantly inhibited the growth of all cell lines. Furthermore, using mitoxantrone efflux assay was shown that TPA could increase ABCG2 activity in all the cell lines with the greatest stimulatory effects in MCF7-MX (more than 6 times the control level). It seemed that celecoxib inverted the effects of TPA on ABCG2 activity. This was more obvious in MCF7-MX.

CONCLUSION

The results suggest a probable causal link between COX-2 and ABCG2 activity. The use of celecoxib for adjuvant therapy in cancer treatment may contribute to decreased resistance to chemotherapeutic drugs transported by ABCG2.

Anticancer effects of amooranin in human colon carcinoma cell line in vitro and in nude mice xenografts

Amooranin (AMR), a natural triterpenoid drug isolated and characterized from Amoora rohituka stem bark, is cytotoxic to SW620 human colon carcinoma cell line with an IC(50) value of 2.9 microg/ml. This novel compound caused depolarization of mitochondrial membrane and decrease of membrane potential, indicating initial signal of apoptosis induction. The percentage of cells with decreased mitochondrial potential ranged from 7.4% at 1 microg/ml to 60.5% at 100 microg/ml AMR. Flow cytometric analysis of apoptosis using Annexin-V-FITC staining showed that the percentage of apoptotic cells ranged from 7.5% at 1 microg/ml to 59.2% at 100 microg/ml AMR. AMR-induced apoptosis was accompanied by redistribution of cytochrome c from mitochondria to cytosol as well as down regulation of Bcl-2 and Bcl-X(L) proteins in a dose-dependent manner. SW620 human colon carcinoma xenograft mice treated with AMR showed significant reduction in tumor growth rates compared to saline- and doxorubicin-treated groups. The reduction in tumor growth rate was better in xenografts treated with 2 mg/kg AMR than 5 and 10 mg/kg treated mice. The analysis of global gene expression changes induced by AMR in xenograft tumors by microarray hybridization revealed that several genes involved in energy pathways, transport, apoptosis, immune response, nucleic acid metabolism, protein metabolism, cell growth and/or maintenance, signal transduction and cell communication, were affected by this natural cancer drug. These results suggest that the anticancer properties of AMR in SW620 human colon carcinoma cell line are mediated through its effects on functional genomics, targeting the apoptotic process.

Tamoxifen modulation of etoposide cytotoxicity involves inhibition of protein kinase C activity and insulin-like growth factor II expression in brain tumor cells

Tamoxifen, a non-steroidal anti-estrogen widely used against breast cancer, is also useful for treatment of other malignancies, due to its sensitizing effect on other chemotherapeutic agents and radiation. We have investigated the advantages of combining tamoxifen with one of the commonly used cancer chemotherapeutic drug, etoposide (VP-16) in brain tumor cell lines. While tamoxifen (10 microM) increased etoposide cytotoxicity 8.3-fold in the human glioma cell line (HTB-14), it increased etoposide cytotoxicity 47.5- and 40-fold in two primary cell lines established from pediatric medulloblastoma patients (MCH-BT-31 and MCH-BT-39), respectively. Similarly, in the pediatric ependymoma cell lines (MCH-BT-30 and MCH-BT-52), tamoxifen enhanced etoposide cytotoxicity 6- and 2.68-fold, respectively. CalcuSyn analysis of cytotoxicity data showed that tamoxifen and etoposide combinations were synergistic with combination index values ranging from 0.243 to 0.369 at IC50 level among different pediatric brain tumor cell lines. Tamoxifen is also cytotoxic at higher concentrations (> 20 microM) in brain tumor cells. To understand the mechanism underlying the tamoxifen modulation of etoposide cytotoxicity, we analyzed expression of P-glycoprotein (P-gp), insulin-like growth factor-I receptor (IGF-IR), IGF-I, IGF-II and estrogen receptor as well as protein kinase C (PKC) activity. While P-gp, IGF-IR and IGF-I were not affected, enhanced inhibition of PKC, and IGF-II were observed in brain tumor cells treated with tamoxifen and etoposide combination as compared to cells treated with either drug alone. Tamoxifen at 10 microM when combined with etoposide at 0-100 microM concentrations reduced PKC activity 77% compared to only 58% without tamoxifen. IGF-II expression decreased to 48.6% of the untreated control in the combination treatment as compared to 31.2% for etoposide alone and 26.2% for tamoxifen alone treatments. These results suggest that inhibitory effect of tamoxifen on brain tumor cells manifest through different mechanisms involving inhibition of targets such as PKC and IGF-II.

Novel plant triterpenoid drug amooranin overcomes multidrug resistance in human leukemia and colon carcinoma cell lines

Amooranin (AMR), a plant terpenoid, isolated from Amoora rohituka, was investigated for its ability to overcome multidrug resistance in human leukemia and colon carcinoma cell lines. AMR IC(50) values of multidrug-resistant leukemia (CEM/VLB) and colon carcinoma (SW620/Ad-300) cell lines were higher (1.9- and 6-fold) than parental sensitive cell lines (CEM and SW620). AMR induced G(2)+M phase-arrest during cell cycle traverse in leukemia and colon carcinoma cell lines and the percentage of cells in G(2)+M phase increased in a dose-dependent manner. Coincubation of tumor cells with both DOX and AMR reversed DOX resistance in 104-fold DOX-resistant CEM/VLB and 111-fold DOX-resistant SW620/Ad-300 cell lines with a dose modification factor of 50.9 and 99.6, respectively. Flow cytometric assay showed that AMR causes enhanced cellular DOX accumulation in a dose-dependent manner. AMR inhibits photolabeling of P-glycoprotein (P-gp) with [(3)H]-azidopine and the blocking effect enhanced with increasing concentrations of AMR. Our results show that AMR competitively inhibits P-gp-mediated DOX efflux, suggestive of a mechanism underlying the enhanced DOX accumulation and reversal of multidrug resistance by AMR.

Molecular studies in pediatric medulloblastomas

Ten pediatric medulloblastoma patients were analyzed for DNA content, cell cycle, expression of drug resistance, apoptosis, cell proliferation, and N-myc genes to determine their prognostic significance. Medulloblastoma patients with progressive disease had fourth ventricle foraminal extension and larger tumors in the imaging studies. Patients with aneuploid tumors responded well to treatment regimens as compared with those with diploid tumors. Cell cycle analysis showed that the patients with progressive disease had a high S-phase fraction in the tumor cell population as compared with patients with favorable response to treatment. The correlation coefficients between Bcl-2 and MRP, Bcl-2 and Bax, p53 and p21, as well as Ki67 and PCNA were positive and significant, indicating their possible coregulated expression. The relationship between these markers indicates their relative and cumulative effect on cellular drug resistance, apoptosis, and/or cell proliferation in pediatric medulloblastomas.

ABC transporters as phenotypic markers and functional regulators of stem cells

Characterization of molecules with tightly controlled expression patterns during differentiation represents an approach to understanding regulation of hematopoietic stem cell commitment. The multidrug resistance-1 (MDR1) gene product, P-glycoprotein, and the breast cancer resistance protein (BCRP) are expressed differentially during hematopoiesis, with the highest levels in primitive bone marrow stem cell populations that are CD34(low) and CD34(-), respectively. Roles for ATP-binding cassette (ABC) transporter superfamily members in conferring drug resistance have been extensively described. However, recent hematopoietic overexpression studies have begun to reveal previously unknown roles for ABC transporter function in normal and malignant hematopoiesis. Expression of MDR1 and BCRP transporters in the myeloid lineage has been reported in blasts from acute myeloid leukemia, but very low to undetectable in normal myelomonocytic cells. Retroviral-mediated dysregulated expression of the MDR1 transporter resulted in increased hematopoietic repopulating activity and myeloproliferative disease in mice. A distinct functional role for the BCRP transporter as a negative regulator of hematopoietic repopulating activity has recently been demonstrated using the same approach. Additionally, the presence of BCRP expression specifically on hematopoietic side-population stem cells and neural stem/progenitors, makes BCRP an attractive candidate marker for isolation of stem cells with the ability to respond to diverse environmental cues. Regulation of stem cell biology by ABC transporters has emerged as an important new field of investigation. In light of these findings, it will be critical to further characterize this family of proteins in hematopoietic lineage-restricted stem cells and in pluripotent stem cells capable of crossing lineage barriers.

SB203580, a specific inhibitor of p38-MAPK pathway, is a new reversal agent of P-glycoprotein-mediated multidrug resistance

P-glycoprotein (P-gp) is the plasma membrane transport pump responsible for efflux of chemotherapeutic agents from cells and is one of the systems that secures multidrug resistance (MDR) of neoplastic cells. In the present study, drug sensitive L1210 and multidrug resistant L1210/VCR (characterized by overexpression of P-gp) mouse leukemic cell lines were used as an experimental model. We have found that SB203580, a specific inhibitor of p38-MAPK pathway, significantly reduced the degree of the vincristine resistance in L1210/VCR cells. This phenomenon was accompanied by a decrease in the LC(50) value of vincristine from 3.203+/-0.521 to 0.557+/-0.082 microM. The LC(50) value of sensitive cells for vincristine was about 0.011 microM. The effect of SB203580 on L1210/VCR cells was associated with significantly increased intracellular accumulation of [3H]-vincristine in the concentration dependent manner. Prolonged exposure of resistant cells to 30 microM SB203580 did neither significantly influence the gene expression of P-gp, nor change the protein levels of p38-MAPK. Western blot analysis revealed that the MDR phenotype in L1210/VCR cells was associated with increased level and activity of cytosolic p38-MAPK. In resistant cells, the enhanced phosphorylation of both, p38-MAPK and ATF-2 (endogenous substrate for p38-MAPK) was found as well. In conclusion we could remark that SB203580, an inhibitor of p38 kinase pathway, reversed the MDR resistance of L1210/VCR cells. MDR phenotype of these cells is connected with increased levels and activities of p38-MAPK. These findings point to the possible involvement of the p38-MAPK pathway in the modulation of P-gp mediated multidrug resistance in the L1210/VCR mouse leukemic cell line. However, the mechanisms of SB203580 action should be further investigated.

Pharmacokinetic and pharmacodynamic implications of P-glycoprotein modulation

P-glycoprotein (P-gp) is a cell membrane-associated protein that transports a variety of drug substrates. Although P-gp has been studied extensively as a mediator of multidrug resistance in cancer, only recently has the role of P-gp expressed in normal tissues as a determinant of drug pharmacokinetics and pharmacodynamics been examined. P-glycoprotein is present in organ systems that influence drug absorption (intestine), distribution to site of action (central nervous system and leukocytes), and elimination (liver and kidney), as well as several other tissues. Many marketed drugs inhibit P-gp function, and several compounds are under development as P-gp inhibitors. Similarly, numerous drugs can induce P-gp expression. While P-gp induction does not have a therapeutic role, P-gp inhibition is an attractive therapeutic approach to reverse multidrug resistance. Clinicians should recognize that P-gp induction or inhibition may have a substantial effect on the pharmacokinetics and pharmacodynamics of concomitantly administered drugs that are substrates for this transporter.