Antibody-directed therapy of multidrug-resistant tumor cells

Cytotoxicity and modes of action of five Cameroonian medicinal plants against multi-factorial drug resistance of tumor cells

ETHNOPHARMACOLOGICAL RELEVANCE

Beilschmiedia acuta Kosterm, Clausena anisata (Willd) Hook, Fagara tessmannii Engl., Newbouldia laevis Seem., and Polyscias fulva (Hiern) Harms. are medicinal plants used in Cameroonian traditional medicine in the treatment of various types of cancers. The present study aims at investigating 11 methanolic extracts from the above Cameroonian medicinal plants on a panel of human cancer cell lines, including various drug-resistant phenotypes. Possible modes of action were analyzed for two extracts from Beilschmiedia acuta and Polyscia fulva and alpha-hederin, the representative constituent of Polyscia fulva.

MATERIALS AND METHODS

Cytotoxicity was determined using a resazurin assay. Cell cycle, apoptosis, mitochondrial membrane potential (MMP), and reactive oxygen species (ROS) were measured by flow cytometry. Cellular response to alpha-hederin was investigated by a mRNA microarray approach.

RESULTS

Prescreening of extracts (40µg/mL) showed that three of eleven plant extracts inhibited proliferation of CCRF-CEM cells by more than 50%, i.e. BAL (73.65%), the bark extract of Beilschmiedia acuta (78.67%) and PFR (68.72%). Subsequent investigations revealed IC50 values below or around 30µg/mL of BAL and PFR in 10 cell lines, including drug-resistant models, i.e. P-glycoprotein-overexpressing CEM/ADR5000, breast cancer resistance protein-transfected MDA-MB-231-BCRP, TP53 knockout cells (HCT116 p53(-/-)), and mutation-activated epidermal growth factor receptor-transfected U87MG.ΔEGFR cells. IC50 values below 5µg/mL of BAL were obtained for HCT116 (p53(-/-)) cells. IC50 values below 10µM of alpha-hederin were found for sensitive CCRF-CEM and multidrug-resistant CEM/ADR5000 cells. The BAL and PFR extracts induced cell cycle arrest between G0/G1 and S phases. PFR-induced apoptosis was associated with increased ROS generation and MMP breakdown. Microarray-based cluster analysis revealed a gene expression profile that predicted cellular response to alpha-hederin.

CONCLUSION

BAL, PFL and alpha-hederin, an exemplarily taken constituent of Beilschmiedia acuta and Polyscia fulva extracts revealed cytotoxicity towards cancer cell lines. Hence, Beilschmiedia acuta and Polyscia fulva may be valuable to develop drugs against otherwise drug-resistant cancer cells.

P-glycoprotein and its inhibition in tumors by phytochemicals derived from Chinese herbs

P-glycoprotein belongs to the family of ATP-binding cassette (ABC) transporters. It functions in cellular detoxification, pumping a wide range of xenobiotic compounds, including anticancer drugs out of the cell. In cancerous cells, P-glycoprotein confers resistance to a broad spectrum of anticancer agents, a phenomenon termed multidrug resistance. An attractive strategy for overcoming multidrug resistance is to block the transport function of P-glycoprotein and thus increase intracellular concentrations of anticancer drugs to lethal levels. Efforts to identify P-glycoprotein inhibitors have led to numerous candidates, none of which have passed clinical trials with cancer patients due to their high toxicity. The search for naturally inhibitory products from traditional Chinese medicine may be more promising because natural products are frequently less toxic than chemically synthesized substances. In this review, we give an overview of molecular and clinical aspects of P-glycoprotein and multidrug resistance in the context of cancer as well as Chinese herbs and phytochemicals showing inhibitory activity towards P-glycoprotein.

Immunothérapie anti-tumorale contre la multidrogue résistance

Overexpression of a membrane glycoprotein (P170) represents the most common multidrug resistance (MDR) mechanism in cancer therapy. Specific autoantibodies to extracellular loops 1, 2 and 4 of murine P170 are elicited in mice using palmitoylated synthetic peptides reconstituted in liposomes with or without Lipid A and resuspended in alum. IgM antibodies are detected 14 days following the first injection and IgG1 become predominant after the third challenge. Animals do not show any autoimmunity symptoms or induced toxicity up to 18 months after the immunization. Previous immunizations of mice using liposomes with mdr1 peptides efficiently improve chemotherapy with doxorubicin and vinblastine against P388 R cells with a 77% increase of survival half time in the immunized group. Sera from immunized mice are also effective in reducing cellular resistance to vinblastine and doxorubicin in vitro. Taken together these data suggest that this immunization approach might have potential clinical applications.

The role of translation in neoplastic transformation from a pathologist's point of view

Increased cell proliferation, which is a hallmark of aggressive malignant neoplasms, requires a general increase in protein synthesis and a specific increase in the synthesis of replication-promoting proteins. Transient increase in the general protein synthesis rate, as well as preferential translation of specific mRNAs coding for growth promoting proteins (e.g. cyclin D1), takes place during normal mitogenic response. A number of extensively studied growth signal transduction pathways (Ras, PI3K, MAPK, mTOR-dependent pathways) activate the function and expression of various components of the translational machinery. In abnormal situations, constitutive activation of signal transduction pathways (e.g. oncogenic activation of Ras or Myc) leads to continuous upregulation of key elements of translational machinery. On the other hand, tumor suppressor genes (p53, pRb) downregulate ribosomal and tRNA synthesis, and their inactivation results in uncontrolled production of these translational components. During recent years, a significant effort has been dedicated to determining whether expression of translation factors is increased in human tumors using clinical biopsy specimens. The results of these studies indicate that expression of particular translation initiation factors is not always increased in human neoplasms. The pattern of expression is characteristic for a particular tumor type. For example, eIF-4E is usually increased in bronchioloalveolar carcinomas but not in squamous cell carcinomas of the lung. Interestingly, in certain highly proliferative and aggressive neoplasms (e.g. squamous cell carcinoma of the lung, melanoma), the expression of eIF-4E is barely detectable. These findings suggest that mechanisms for increasing general protein synthesis in various neoplasms differ significantly. Finally, the possibility of qualitative alterations in the translational machinery, rather than a simple increase in the activity of its components, is discussed along with the possibility of targeting those qualitative differences for tumor therapy.

Monoclonal antibody therapeutics and apoptosis

The potential for disease-specific targeting and low toxicity profiles have made monoclonal antibodies attractive therapeutic drug candidates. Antibody-mediated target cell killing is frequently associated with immune effector mechanisms such as antibody-directed cellular cytotoxicity, but they can also be induced by apoptotic processes. Antibody-directed mechanisms, including antigen crosslinking, activation of death receptors, and blockade of ligand-receptor growth or survival pathways, can elicit the induction of apoptosis in targeted cells. Depending on their mechanism of action, monoclonal antibodies can induce targeted cell-specific killing alone or can enhance target cell susceptibility to chemo- or radiotherapeutics by effecting the modulation of antiapoptotic pathways. This review will focus on the mechanisms by which antibodies are capable of eliciting programmed cell death either directly or indirectly within tumor cells.

Regression of established tumors expressing P-glycoprotein by combinations of adriamycin, cyclosporin derivatives, and MRK-16 antibodies

BACKGROUND

Overexpression of P-glycoprotein, a transmembrane protein capable of transporting a broad spectrum of anticancer drugs out of cells, likely contributes to tumor drug resistance. Strategies for overcoming this resistance include the use of specific compounds, such as cyclosporin derivatives, that modulate P-glycoprotein function and antibodies that bind to the protein, thereby altering its activity.

PURPOSE

We examined the antitumor activity of combination treatment with the anti-P-glycoprotein monoclonal antibody MRK-16, a cyclosporin derivative (either cyclosporin A [CsA] or PSC 833), and the anticancer drug Adriamycin (ADM) against human colorectal carcinoma cells in vitro and established xenografts of these cells in vivo.

METHODS

The human colorectal carcinoma cell line HCT-15 and its ADM-resistant subline HCT-15/ADM2-2 were used in this study. Cellular staining with a tetrazolium dye was used to assess the antitumor (i.e., antiproliferative) effects of treatment in vitro. Caliper measurement of tumor volumes was used to assess the antitumor effects of treatment in vivo. Cell surface binding of MRK-16 was measured by means of an immunofluorescence assay. Differences in the patterns of tumor cell growth in vitro and tumor growth rates in vivo were evaluated by means of repeated measure analysis of variance. Synergy in the combined effects of treatment was evaluated by means of the fractional product method.

RESULTS

HCT-15 cells were found to express P-glycoprotein intrinsically; HCT-15/ADM2-2 cells expressed approximately five times more P-glycoprotein than the parental cells. HCT-15/ADM2-2 cells were also found to be about eight times more resistant to ADM in vitro than the parental cells. Incubation of both cell types in vitro with either MRK-16 and ADM or one of the cyclosporin derivatives and ADM inhibited cell growth minimally; however, ternary treatment with MRK-16, one of the cyclosporin derivatives, and ADM dramatically reduced the growth of both cell types. An analysis of treatment effects indicated that synergistic effects were obtained with ternary treatment. When athymic mice bearing established tumors (either HCT-15 or HCT-15/ADM2-2) were treated similarly with various combinations of the tested agents, the most pronounced antitumor effects were observed with ternary treatment. In some mice bearing HCT-15/ADM2-2 xenografts, ternary treatment led to complete tumor regression. Finally, CsA and PSC 833 were both shown to enhance MRK-16 binding to HCT-15 cells and HCT-15/ADM2-2 cells in vitro.

CONCLUSION

Combination treatment with a cyclosporin derivative and an anti-P-glycoprotein antibody can be effective in circumventing P-glycoprotein-mediated drug resistance.