ATP binding cassette transporters and drug resistance in breast cancer.

Resistance to chemotherapy is a critical issue in the management of breast cancer patients. The nature of clinical drug resistance is likely to be multifactorial. However, in the last decade considerable attention has been dedicated to the role played by membrane transporter proteins belonging to the ATP binding cassette protein superfamily, and in particular by the MDR1 product P-glycoprotein (Pgp) and the multidrug resistance protein (MRP1). Heterogeneity of results is a common feature of studies evaluating the expression and prognostic role of these proteins, due to both methodological and biological factors. Nonetheless, Pgp and MRP1 are detected in a significant proportion of untreated breast cancers (on average 40 and 50% respectively, by immunohistochemistry), without a clear and consistent association with cancer stage. Exposure to chemotherapy increases the expression of both proteins. In vitro studies on primary cultures of breast cancer cells obtained at surgery consistently show an association between Pgp (protein) or MDR1 (mRNA) expression and resistance to chemotherapy. However, the correlation with clinical drug resistance is not as well defined. A stronger association of Pgp/MDR1 with response rates has been observed when expression or an increase in expression are detected immediately following chemotherapy. Correlations with prognosis appear more evident in studies using immunohistochemistry, in adjuvant and neoadjuvant settings. Evidence of clinical reversal of drug resistance by verapamil suggests a functional role of Pgp in drug resistance, although the significance of the evidence is generally weakened by poor trial designs. Future studies should take into account the multifactorial nature of drug resistance in breast cancer and use standardized approaches with adequate controls. Expression studies should be complemented by well-designed trials of drug-resistance reversal using target-specific chemosensitizing agents, and relating the results to the levels of expression of the target proteins.

Molecular and pharmacological aspects of antiestrogen resistance.

Endocrine therapy is effective in approximately one-third of all breast cancers and up to 80% of tumors that express both estrogen and progesterone receptors. Despite the low toxicity, good overall response rates, and additional benefits associated with its partial agonist activity, most Tamoxifen-responsive breast cancers acquire resistance. The development of new antiestrogens, both steroidal and non-steroidal, provides the opportunity for the development of non-cross-resistant therapies and the identification of additional mechanisms of action and resistance. Drug-specific pharmacologic mechanisms may confer a resistance phenotype, reflecting the complexities of both tumor biology/pharmacology and the molecular endocrinology of steroid hormone action. However, since all antiestrogens will be effective only in cells that express estrogen receptors (ER), many mechanisms will likely be directly related to ER expression and signaling. For example, loss of ER expression/function is likely to confer a cross-resistance phenotype across all structural classes of antiestrogens. Altered expression of ERalpha and ERbeta, and/or signaling from transcription complexes driven by these receptors, may produce drug-specific resistance phenotypes. We have begun to study the possible changes in gene expression that may occur as cells acquire resistance to steroidal and non-steroidal antiestrogens. Our preliminary studies implicate the altered expression of several estrogen-regulated genes. However, resistance to antiestrogens is likely to be a multigene phenomenon, involving a network of interrelated signaling pathways. The way in which this network is adapted by cells may vary among tumors, consistent with the existence of a highly plastic and adaptable genotype within breast cancer cells.

Cellular and molecular pharmacology of antiestrogen action and resistance.

Antiestrogen therapy remains one of the most widely used and effective treatments for the management of endocrine responsive breast cancers. This reflects the ability of antiestrogens to compete with estrogens for binding to estrogen receptors. Whereas response rates of up to 70% are reported in patients with tumors expressing estrogen and progesterone receptors, most responsive tumors will eventually acquire resistance. The most important factor in de novo resistance is lack of expression of these receptors. However, the mechanisms driving resistance in tumors that express estrogen and/or progesterone receptors are unclear. A tamoxifen-stimulated phenotype has been described, but seems to occur only in a minority of patients. Most tumors (>80%) may become resistant through other, less well defined, resistance mechanisms. These may be multifactorial, including changes in immunity, host endocrinology, and drug pharmacokinetics. Significant changes within the tumor cells may also occur, including alterations in the ratio of the estrogen receptor alpha:beta forms and/or other changes in estrogen receptor-driven transcription complex function. These may lead to perturbations in the gene network signaling downstream of estrogen receptors. Cells may also alter paracrine and autocrine growth factor interactions, potentially producing a ligand-independent activation of estrogen receptors by mitogen-activated protein kinases. Antiestrogens can affect the function of intracellular proteins and signaling that may, or may not, involve estrogen receptor-mediated events. These include changes in oxidative stress responses, specific protein kinase C isoform activation, calmodulin function, and cell membrane structure/function.

Competitive and allosteric interactions in ligand binding to P-glycoprotein as observed on an immobilized P-glycoprotein liquid chromatographic stationary phase.

A liquid chromatographic stationary phase containing immobilized P-glycoprotein (Pgp) was synthesized using cell membranes obtained from Pgp-expressing cells. The resulting Pgp-stationary phase was used in frontal and zonal chromatographic studies to investigate the binding of vinblastine (VBL), doxorubicin (DOX), verapamil (VER), and cyclosporin A (CsA) to the immobilized Pgp. The compounds were added individually to the chromatographic system with or without ATP in the running buffer. Using this approach, dissociation constants were calculated for VBL (23.5 +/- 7.8 nM), DOX (15.0 +/- 3.2 microM), VER (54.2 +/- 4.7 microM), and CsA [97.9 +/- 19.4 nM (without ATP) and 62.5 +/- 4.6 nM (with ATP)]. The compounds were also added in pairs using standard competitive chromatography procedures. The results of the study demonstrate that competitive interactions occurred between VBL and DOX, cooperative allosteric interactions occurred between VBL and CsA and ATP and CsA, and anticooperative allosteric interactions occurred between ATP and VBL and VER. The chromatographic studies indicate that the immobilized Pgp was modified by ligand and cofactor binding and that the stationary phase can be used to study drug-drug binding interactions on the Pgp molecule.

Development of an immobilized P-glycoprotein stationary phase for on-line liquid chromatographic determination of drug-binding affinities.

The membrane transporter P-glycoprotein (PGP) has been immobilized on an immobilized artificial membrane (IAM) LC stationary phase. The resulting PGP-IAM phase retained the ability of the native PGP to bind the known PGP-ligand vinblastine. Displacement studies using other known PGP ligands, verapamil and cyclosporin A, demonstrated that there was selective binding between vinblastine and the immobilized PGP transporter. The binding affinity (Kd value) of vinblastine for the PGP-IAM was determined to be 19+/-20 and 71+/-11 nM on two separate columns. These values are consistent with previously reported values of 9+/-2, 8+/-2, and 37+/-10 nM, which were obtained using native membranes. The Kd values obtained on the PGP-IAM for cyclosporin A and verapamil were 492+/-21 and 172+/-29 microM, respectively. These results were higher than the corresponding Kd values obtained using native membranes, but the relative affinities vinblastine > cyclosporin A >> verapamil are consistent in both approaches. During several months of experiments and storage, the PGP-IAM was found to be reproducible and stable. The stationary phase appears to be useful in the on-line screening for PGP ligands.

Use of ERE and reporter gene constructs to assess putative estrogenic activity.

Estrogens primarily function through the activation of their receptors, which subsequently function as nuclear transcription factors. There are two estrogen receptor (ER) genes, now designated ERa (the classic ER gene) and ER/3. The key consequence of the activation of either gene product is the regulation of gene transcription. The extent and nature of transcription appear to be regulated by a series of coregulator proteins. One of the most sensitive assays for detection of potential estrogenic activity is measurement of the ability of a test compound to influence the transcription of reporter genes. In this regard, many investigators use promoter-reporter constructs. To assess putative estrogenic activity, an estrogen-responsive promoter is generally placed upstream of a reporter gene and transiently transfected into a target cell. When exposed to an estrogenic compound, expression of the reporter gene would normally be induced. We briefly discuss several issues pertinent to the use of these assays and the interpretation of resulting data, including estrogen-responsive, promoter-reporter constructs, reporter genes and measurements of activity, choice of target cell or cell line, transient introduction of promoter-reporter constructs into cells, basic statistical approaches to data analysis, and definitions of agonist, partial agonist, and antagonist.

Multidrug resistance in breast cancer: a meta-analysis of MDR1/gp170 expression and its possible functional significance.

BACKGROUND: P-glycoprotein (gp170; encoded by the MDR1 gene [also known as PGY1]) is a membrane protein capable of exporting a variety of anticancer drugs from cells. MDR1/gp170 expression has been studied in breast cancer, but the prevalence of this expression and its role in breast tumor drug resistance are unclear. PURPOSE: We conducted a critical review and meta-analysis of studies examining MDR1/gp170 expression in breast cancer to estimate the likely prevalence and clinical relevance of this expression. We also explored reasons for differences in the findings from individual studies. METHODS: Published papers on MDR1/gp170 expression in breast cancer were identified by searching several literature databases and reviewing the bibliographies of identified papers. Variability across the studies in the proportion of tumors expressing MDR1/gp170 was assessed by use of chi-squared tests of homogeneity, weighted means, and weighted linear regression. Pooled relative risks (RRs) for the association between the induction of MDR1/gp170 expression and prior chemotherapy and associations between MDR1/gp170 expression and several clinical outcomes were estimated by use of Mantel-Haenszel methods. Heterogeneity among the pooled RRs was explored by use of chi-squared tests. Reported P values are two-sided. RESULTS: Thirty-one studies were identified and evaluated. The proportion of breast tumors expressing MDR1/gp170 in all of the studies was 41.2%, but there was substantial heterogeneity in the values across individual studies (P<.0001). Regression analyses demonstrated that a considerable portion of the observed heterogeneity was a consequence of the change, over time, from RNA hybridization-based assays to immunohistochemistry-based assays of MDR1/gp170 expression. Measuring MDR1/gp170 expression before versus after chemotherapy and use of cytotoxic drugs that are not substrates for gp170 also contributed to the heterogeneity. Treatment with chemotherapeutic drugs or hormonal agents was associated with an increase in the proportion of tumors expressing MDR1/gp170 (RR = 1.77; 95% confidence interval [CI] = 1.46-2.15). Patients with tumors expressing MDR1/gp170 were three times more likely to fail to respond to chemotherapy than patients whose tumors were MDR1/gp170 negative (RR = 3.21; 95% CI = 2.28-4.51); this RR increased to 4.19 (95% CI = 2.71-6.47) when considering only patients whose tumor expression of MDR1/gp170 was measured after chemotherapy. MDR1/gp170 expression was not associated with lymph node metastases, estrogen receptor status, tumor size, tumor grade, or tumor histology. CONCLUSIONS AND IMPLICATIONS: MDR1/gp170 expression in breast tumors is associated with treatment and with a poor response to chemotherapy. The data are consistent with a contributory role for MDR1/gp170 in the multidrug resistance in some breast tumors.

MDA435/LCC6 and MDA435/LCC6MDR1: ascites models of human breast cancer.

We have established a novel ascites tumour model (MDA435/LCC6) from the oestrogen receptor-negative, invasive and metastatic MDA-MB-435 human breast cancer cell line. MDA435/LCC6 cells grow as both malignant ascites and solid tumours in vivo in nude mice and nude rats, with a tumour incidence of approximately 100%. Untreated mice develop ascites following i.p. inoculation of 1 x 10(6) cells and have a reproducible life span of approximately 30 days, with all animals dying within a 48 h period. The in vivo response of MDA435/LCC6 ascites to several cytotoxic drugs, including doxorubicin, etoposide (VP-16), BCNU and mitomycin C, closely reflects the activity of these single agents in previously untreated breast cancer patients. MDA435/LCC6 cells also retain the anchorage-dependent and anchorage-independent in vitro growth properties of the parental MDA-MB-435 cells, and can be used in standard in vitro drug screening assays. The drug resistance pattern of the MDA435/LCC6 cells suggests that they may have few active endogenous drug resistance mechanisms. To generate a model for the screening of MDR1-reversing agents, MDA435/LCC6 were transduced with a retroviral vector directing the constitutive expression of the MDR1 cDNA, producing a cell line with a classical MDR1 resistance pattern (MDA435/LCC6MDR1). THese ascites models may be a viable alternative to the murine leukaemia ascites (L1210, P388) and, in conjunction with other breast cancer cell lines, facilitate the in vitro and in vivo screening of new cytotoxic drugs and drug combinations.

Hormonal carcinogenesis in breast cancer: cellular and molecular studies of malignant progression.

We have established and characterized a series of variant cell lines in which to identify the critical factors associated with E2-induced malignant progression, and the acquisition to tamoxifen resistance in human breast cancer. Sublines of the hormone-dependent MCF-7 cell line (MCF7/MIII and MCF7/LCC1) form stable, invasive, estrogen independent tumors in the mammary fat pads of ovariectomized athymic nude mice. These cells retain expression of both estrogen (ER) and progesterone receptors (PGR), but retain sensitivity to each of the major structural classes of antiestrogens. The tamoxifen-resistant MCF7/LCC2 cells retain sensitivity to the inhibitory effects of the steroidal antiestrogen ICI 182780. By comparing the parental hormone-dependent and variant hormone-independent cells, we have demonstrated an altered expression of some estrogen regulated genes (PGR, pS2, cathepsin D) in the hormone-independent variants. Other genes remain normally estrogen regulated (ER, laminin receptor, EGF-receptor). These data strongly implicate the altered regulation of a specific subset or network of estrogen regulated genes in the malignant progression of human breast cancer. Some of the primary response genes in this network may exhibit dose-response and induction kinetics similar to pS2, which is constitutively upregulated in the MCF7/MIII, MCF7/LCC1 and MCF7/LCC2 cells.

Effect of tamoxifen on the multidrug-resistant phenotype in human breast cancer cells: isobologram, drug accumulation, and M(r) 170,000 glycoprotein (gp170) binding studies.

We have performed isobologram analyses of the ability of tamoxifen (TAM) to alter the response to Adriamycin (ADR) and vinblastine (VBL) in human breast cancer cells. MCF-7 cells express functional receptors for estrogen and progesterone but do not express detectable levels of M(r) 170,000 glycoprotein (gp170). CL 10.3 and MCF-7ADR cells are MCF-7 variants which express gp170. CL 10.3 but not MCF-7ADR cells express functional steroid hormone receptors. Tamoxifen (1-2.5 microM) interacts synergistically with ADR and VBL in CL 10.3 and MCF-7ADR cells. TAM increases the cytotoxicity of VBL and ADR and the intracellular levels of [3H]VBL by approximately 2-3-fold. TAM also prevents the binding of [3H]azidopine to gp170. The ability of TAM to concurrently increase the cytotoxic effects of ADR and VBL, increase VBL accumulation, and inhibit the binding of azidopine to gp170 strongly implies that the synergistic effects of TAM are mediated through its effects on gp170. TAM produces an antagonistic to additive interaction with ADR and VBL in MCF-7 cells, and at high concentrations (5 microM) the synergy apparent in CL 10.3 and MCF-7ADR cells is lost. While TAM clearly has a significant potential for use as a chemosensitizing agent, the design of clinical trials may require careful consideration.

Hormone resistance, invasiveness, and metastatic potential in breast cancer.

Critical phenotypic changes that occur during the progression of breast cancer include the loss of hormone-dependence, acquired resistance to systemic therapies, and increased metastatic potential. We have isolated a series of MCF-7 human breast cancer variants which exhibit hormone-independent growth, antiestrogen resistance, and increased metastatic potential. Analysis of the phenotypes of these variants strongly suggests that changes in the expression of specific genes may be critical to the generation of phenotypic diversity in the process of malignant progression in breast cancer. Epigenetic changes may contribute significantly to the generation of these phenotypic changes observed during breast cancer progression. Many of the characteristics of the progressed phenotypes appear to have arisen in response to appropriate selective pressures (growth in ovariectomized nude mice; growth in the presence of antiestrogens). These observations are consistent with the concept of clonal selection and expansion in the process of malignant progression.

The biology of breast tumor progression. Acquisition of hormone independence and resistance to cytotoxic drugs.

Many breast tumors appear to follow a predictable clinical pattern, being initially responsive to endocrine therapy and to cytotoxic chemotherapy but ultimately exhibiting a phenotype resistant to both modalities. Using the MCF-7 human breast cancer cell line as an example of an 'early' phenotype (estrogen and progesterone receptor positive, steroid responsive, low metastatic potential), we have isolated and characterized a series of hormone-independent but hormone-responsive variants (MIII and MCF7/LCC1). However, these variants remain responsive to both antiestrogens and cytotoxic drugs (methotrexate and colchicine). MIII and MCF7/LCC1 cells appear to mimic some of the critical aspects of the early progression to a more aggressive phenotype. An examination of the phenotype of these cells suggests that some hormone-independent breast cancer cells are derived from hormone-dependent parental cells. The development of a hormone-independent phenotype can arise independently of acquisition of a cytotoxic drug resistant phenotype.

Predictive value of some clinical and pathological parameters on upper level axillary lymph node involvement in breast cancer.

The role of radical axillary dissection in breast cancer management is presently under discussion. In this study we have evaluated the relationship between the pattern of metastatic axillary lymph node involvement by level and some of the main prognostic factors (age of the patient, size, grading, estrogen receptor and progesterone receptor status of the primary tumor) in 185 patients with operable breast cancer. The III level appeared to be involved in 31 (16.8%) out the 108 patients with axillary lymph nodes positive for metastases. A discontinuous pattern of axillary involvement (skip metastases) was observed in about 10% of cases. Logistic regression analysis of the data shows that only G3 is significantly correlated with the risk of III level invasion (p less than 0.05). We conclude that, at present, a selection of possible candidates for a less than radical axillary dissection is not as yet feasible. Since the risk for III level invasion cannot be sufficiently defined.

Effect of 17-beta-estradiol on doxorubicin cytotoxicity in human breast cancer cell culture.

Pre-treatment with 17-beta-estradiol appeared to improve the cytotoxic efficacy of doxorubicin on MCF-7 but not on ZR-75-1 and EVSA-T human breast cancer cell lines. MCF-7 and ZR-75-1 are both estrogen receptor-positive cell lines: however, only ZR-75-1 showed improved proliferation in the presence of estradiol. On the other hand MCF-7 appeared basically more resistant to doxorubicin compared to the other cell lines. The results indicate that estrogenic pre-treatment is a potential tool for partially overcoming human breast cell resistance to doxorubicin; moreover, they suggest that the mechanism of interaction could be not exclusively related to actual cytokinetics modulation.

Different techniques for drug cytotoxicity evaluation on MCF-7 human breast cancer cell line.

Contrasting results have been reported following the use of different in vitro techniques for the evaluation of drug cytotoxicity on cultured cell lines. Our interest focuses on the evaluation of drug cytotoxicity on a ER+ breast cancer cell line (MCF-7). The present study compares the effect of Doxorubicin using the following different techniques: a dye exclusion test; cell growth after treatment, expressed either as slope of cell growth curves or as the number of cells in treated cultures as a percentage of the number of cells in control cultures at different time intervals after treatment; a clonogenic assay in liquid medium. The dye exclusion assay failed to demonstrate drug-related killing of cells. Our data from the present study support the relative superiority of clonogenic assay compared to other methods. However, the occasional use of different methods could by-pass the limitations of this assay, and, depending on the specific experimental conditions, could lead to a better definition of drug cell killing.

The effects of therapy on estrogen receptors in breast cancer.

The importance of estrogen receptors (ER) in predicting the results of therapy in advanced-stage breast carcinoma is now generally accepted. It is, therefore, important to know whether therapy itself, besides other factors, could affect ER status. The aim of the authors was to investigate this problem by reviewing the data from the literature. They have taken into account the effects of hormonal and/or chemotherapy and of radiotherapy, moreover, they have considered the importance of the time elapsed since the suspension of treatment. Hormonal therapy appears to be the kind of treatment more clearly correlated with a loss of ER: the authors have reported some hypotheses about the possible mechanisms of this action. The effect of chemotherapy is much less clear; the data about radiotherapy are few, unhomogeneous and, often, insufficient. Instead, it appears quite clear that ER tend to regain their original status after the suspension of therapy. More studies, are needed before any definitive conclusion can be drawn; it will be necessary to take into account also the possible effect of the different criteria for the preselection of patients. The actual data appear, anyway, to confirm the importance of routine receptor assay on breast tumors, especially after systemic treatment and independently of the kind of therapy itself.