Comparison of western blot analysis and immunocytochemical detection of P-glycoprotein in multidrug resistant cells

Cloning, expression analysis, and sequence diversity of genes encoding two different immunodominant membrane proteins in poinsettia branch-inducing phytoplasma (PoiBI)

Poinsettia branch-inducing phytoplasma (PoiBI) is a phytopathogenic bacterium that infects poinsettia, and is associated with the free-branching morphotype (characterized by many axillary shoots and flowers) of many commercially grown poinsettias. The major membrane proteins of phytoplasmas are classified into three general types, that is, immunodominant membrane protein (Imp), immunodominant membrane protein A (IdpA), and antigenic membrane protein (Amp). These membrane proteins are often used as targets for the production of antibodies used in phytoplasma detection. Herein, we cloned and sequenced the imp and idpA genes of PoiBI strains from 26 commercial poinsettia cultivars. Although the amino acid sequences of the encoded IdpA proteins were invariant, those of the encoded Imp varied among the PoiBI isolates, with no synonymous nucleotide substitution. Western blotting and immunohistochemical analyses revealed that the amount of Imp expressed exceeded that of IdpA, in contrast to the case of a related phytoplasma-disease, western X-disease, for which the major membrane protein appears to be IdpA, not Imp. These results suggest that even phylogenetically close phytoplasmas express different types of major membrane proteins.

Investigation of the influence of modulation of P-glycoprotein by a multiple dosing regimen of tamoxifen on the pharmacokinetics and toxicodynamics of doxorubicin

PURPOSE

The in vivo effect of modulators of P-glycoprotein (Pgp) on organ accumulation of substrates of Pgp has not been fully investigated. We investigated the influence of a Pgp modulator (tamoxifen, TAM) on the pharmacokinetics and toxicodynamics of a Pgp substrate (doxorubicin, DOX) in rats.

METHODS

TAM was administered daily for 11 days before the administration of DOX in male Sprague-Dawley rats, with all doses being clinically relevant. The experimental design of the project consisted of two different protocols. One was to investigate the effect of DOX on the time course of Pgp-ATPase activity, sarcoplasmic reticulum Ca(2+) -ATPase (SERCA) activity, and DOX concentration in the heart, liver, and kidneys of TAM-pretreated animals; the other protocol was to study the effect of TAM pretreatment on the disposition of DOX in the body by investigating its time course in plasma, urine and bile.

RESULTS

The simultaneous curve fitting of plasma data with urine and bile data with the help of the related pharmacokinetic equations provided the calculated parameters and constants. The first-order rate constants between the central and the myocardial compartments (k(1H) and k(H1)) were decreased in the TAM-treated group. The treatment also significantly reduced the k(1H)/k(H1) ratio in comparison to that of the control group. The first-order biliary elimination rate constant (k(b)) was significantly decreased (29%) in the TAM-treated group. The reduction was estimated in comparison with that of the control group. This reduction could be attributed to the inhibitory effect of TAM on Pgp located on biliary canicular membranes. The initial reduction of Pgp activity in TAM-treated group was at 60% of the basal level. The activity declined and reached a plateau at 20% of the basal activity after 6 h and remained at that level for 24 h. The area under the curves of Pgp-ATPase activity time (AUC(Activity 0-24)) following DOX administration in TAM-treated group was significantly lower than that of the control group, indicating an overall inhibitory effect of TAM on Pgp-ATPase activity under the protocol of this study. The area under the curves of the SERCA activity-time curve following DOX administration in TAM-treated group demonstrated a 15% reduction in AUC(Activity 0-24) in comparison with that of the control group, an indication of increased toxicity. The amount of myocardial Pgp in the 24-h period following DOX administration was comparable to the control group and showed no significant deviation from the basal levels of the protein.

CONCLUSIONS

The effect of TAM on DOX accumulation in the myocardial tissue and the increase in cardiotoxicity can be related to the net inhibitory effect of TAM on the efflux activity of Pgp in the heart. The results of the present study supported the hypothesis of the project that multiple regimen pretreatment with Pgp modulator TAM increases the DOX accumulation in the heart and promotes DOX-induced cardiotoxicity.

Fractionated irradiation of H69 small-cell lung cancer cells causes stable radiation and drug resistance with increased MRP1, MRP2, and topoisomerase IIalpha expression

PURPOSE

After standard treatment with chemotherapy and radiotherapy, small-cell lung cancer (SCLC) often develops resistance to both treatments. Our aims were to establish if fractionated radiation treatment alone would induce radiation and drug resistance in the H69 SCLC cell line, and to determine the mechanisms of resistance.

METHODS AND MATERIALS

H69 SCLC cells were treated with fractionated X-rays to an accumulated dose of 37.5 Gy over 8 months to produce the H69/R38 subline. Drug and radiation resistance was determined using the MTT (3,-4,5 dimethylthiazol-2,5 diphenyltetrazolium bromide) cell viability assay. Protein expression was analyzed by Western blot.

RESULTS

The H69/R38 subline was resistant to radiation (2.0 +/- 0.2-fold, p < 0.0001), cisplatin (14 +/- 7-fold, p < 0.001), daunorubicin (6 +/- 3-fold, p < 0.05), and navelbine (1.7 +/- 0.15-fold, p < 0.02). This was associated with increased expression of the multidrug resistance-associated proteins, MRP1 and MRP2, and topoisomerase IIalpha and decreased expression of glutathione-S-transferase pi (GSTpi) and bcl-2 and decreased cisplatin accumulation. Treatment with 4 Gy of X-rays produced a 66% decrease in MRP2 in the H69 cells with no change in the H69/R38 cells. This treatment also caused a 5-fold increase in topoisomerase IIalpha in the H69/R38 cells compared with a 1.5-fold increase in the H69 cells.

CONCLUSIONS

Fractionated radiation alone can lead to the development of stable radiation and drug resistance and an altered response to radiation in SCLC cells.

Induction of broad drug resistance in small cell lung cancer cells and its reversal by paclitaxel

The H82 "variant" and the H69 "classic" small cell lung cancer (SCLC) cell lines were treated with low levels of epirubicin (69 and 14 nM) which caused little cell death but produced the H82/E8 and H69/E8 extended-multidrug resistant sublines. Both were resistant to drugs associated with multidrug resistance (MDR), and to chlorambucil (9.5- and 5.6-fold, respectively) and cisplatin (2.3- and 8.5-fold, respectively). There was increased expression of the multidrug resistance-associated protein (MRP1) in the H82/E8 subline while P-glycoprotein expression was not detected in any cells or sublines. Treatment of the H82 cells for 1 hr with 69 nM epirubicin increased MRP1-mRNA expression within 4 hr and this was associated with an increase in the resistance to epirubicin, chlorambucil, cisplatin and paclitaxel. Further, a 1 hr treatment with non-cytotoxic doses of chlorambucil (2.5 microM), cisplatin (1.3 microM) or paclitaxel (13 nM), drugs not normally associated with MRP1-mediated MDR, also increased MRP1-mRNA expression in the H82 cells with paclitaxel causing the highest increase (4.5-fold). For chlorambucil treatment, this increased MRPI-mRNA expression was accompanied by increased drug resistance while paclitaxel treatment had no effect on drug resistance in the H82 cells. For the drug resistant H82/E8 subline, these drug treatments had no effect on the MRP1-mRNA expression and little effect on increasing the subline drug resistance. However, pretreatment with paclitaxel sensitised the H82/E8 subline to chlorambucil and cisplatin returning the subline to the sensitivity of the H82 cell line. We conclude that treatment with low levels of MDR and non-MDR drugs can induce extended-multidrug resistance in SCLC cells, a process that probably involves the co-ordinate upregulation of MRP1 and other resistance mechanisms. The results also suggest paclitaxel may have a role as a response modifier in the treatment of refractory SCLC.

Increased MRP expression is associated with resistance to radiation, anthracyclines and etoposide in cells treated with fractionated gamma-radiation

The failure of chemotherapy is often associated with the failure of radiotherapy in the treatment of cancer. To investigate this relationship, the CCRF-CEM (CEM) human T-cell leukaemia cell line was treated with fractionated gamma-radiation totalling 75 Gy (10 cycles of 1.5 Gy daily for 5 days). This produced the CEMRR subline which was 1.5-fold resistant to radiation compared with the parental CEM cells. The CEMRR subline was also resistant to daunorbicin, idarubicin and etoposide but not to paclitaxel, cis-platinum or chlorambucil. Treatment with 50 microM buthionine sulphoximine, an inhibitor of glutathione synthesis, reversed the daunorubicin resistance in the CEMRR subline. Multidrug resistance-associated protein (MRP) mRNA was 6-fold higher in the CEMRR subline than in the CEM cells, and there was no detectable expression of P-glycoprotein in either the CEM cells or the CEMRR subline. Treatment of the CEM cells with 2 Gy of gamma-radiation caused an increase in MRP-mRNA within 4 hr which, by 24 hr, was greater than 5-fold that of the untreated CEM cells. No change in MRP mRNA was observed in the CEMRR subline with similar treatment. We conclude that MRP is involved in the immediate response to radiation and it may account for the drug resistance that often develops following radiation treatment.

P-glycoprotein-mediated acquired multidrug resistance of human lung cancer cells in vivo

We examined whether the increased expression of P-glycoprotein (P-gp) encoded by the human multidrug resistance gene MDR1 is related to the acquired multidrug resistance of lung cancer in vivo. We estimated the chemosensitivity of lung cancer xenografts (LC-6, adenocarcinoma; Lu-24, small-cell cancer) by calculation of relative tumour growth (T/C%, treated/control) in vivo, based on statistical significance determined by the Mann-Whitney U test (P < 0.01, one-sided). MDR1 gene expression levels were evaluated by reverse transcription-polymerase chain reaction (RT-PCR) assay. P-gp production and P-gp localisation were examined by Western blotting and by immunohistochemical analysis respectively. LC-6 and Lu-24 were initially sensitive to both vincristine (VCR, 1.6 mg kg-1: LC-6, 45%; Lu-24, 39%) and doxorubicin (DOX, 12 mg kg-1: LC-6, 26%; Lu-24, 27%) in vivo. VCR-resistant variants (LC-6R, 66% and Lu-24R, 68%) selected with VCR (0.4 mg kg-1, x 9) significantly acquired cross-resistance to DOX (LC-6R, 55% and Lu-24R, 55% respectively). RT-PCR assay showed increased levels of MDR1 expression in LC-6R and Lu-24R with stable MDR1 expression levels. P-gp expression levels were elevated, and the percentage of P-gp-positive tumour cells increased in both LC-6R and Lu-24R. These results suggest that P-gp/MDR1 overexpression is related to acquired multidrug resistance in lung cancer in vivo.

P-glycoprotein: clinical significance and methods of analysis

Multidrug resistance (MDR) is responsible for a decrease in sensitivity of tumor cells tumor cells to unrelated, naturally occurring anticancer drugs. This resistance is correlated with expression and activity of a membrane protein, P-gp 170, functioning as a drug-extruding pump. It has been well described in in vitro situations; however, the clinical detection and implications are not yet clear. Multiple detection assays have been developed based on the discovery of the MDR gene family and the corresponding protein. Southern, Northern, or Western blot analysis, S1 nuclease protection or PCR-based assays, immunohistochemical detection or functionality tests by flow cytometry have been used extensively. However, by use of these techniques on clinical material, both normal and malignant, contradictory results have emerged. The sensitivity and specificity of a certain technique are always limited by unavoidable parameters, for example, skill of the technician. Moreover, the complexity of the development of resistance against anticancer agents (external determinants), such as the diversity of tumor tissues, the simultaneous presence of other resistance mechanisms, and the low expression level, make MDR detection equivocal and can lead to contradictory results. Previous treatment influencing the MDR profile and inappropriate timing of the test make a possible correlation between MDR expression and chemotherapeutic resistance difficult to establish and can lead to discordant results. In this review, the need for proper criteria is stressed. No single detection technique provides the ideal test to detect MDR. Tandem testing could give more certainty, although small sample size limit this application. Formulation of a standard assay with better definition of a positivity is essential before clinical trials are started.

P-glycoprotein-mediated multidrug resistance in normal and neoplastic hematopoietic cells

The multidrug transporter, P-glycoprotein (P-gp), is expressed by CD34-positive bone marrow cells, which include hematopoietic stem cells, and in other cells in the bone marrow and peripheral blood, including some lymphoid cells. Multidrug resistance mediated by P-gp appears to be a major impediment to successful treatment of acute myeloid leukemias and multiple myelomas. However, the impact of P-gp expression on prognosis has to be confirmed in several other hematopoietic neoplasms. The role of P-gp in normal and malignant hematopoiesis and clinical attempts to circumvent multidrug resistance in hematopoietic malignancies are reviewed. The recent transduction of the MDR1 gene into murine hematopoietic cells, which protects them from toxic effects of chemotherapy, suggests that MDR1 gene therapy may help prevent myelosuppression following chemotherapy.

Multidrug resistance in the laboratory and clinic

Multidrug resistance represents a major obstacle in the successful therapy of neoplastic diseases. Studies have demonstrated that this form of drug resistance occurs in cultured tumor cell lines as well as in human cancers. P-glycoprotein appears to play an important role in such cells by acting as an energy-dependent efflux pump to remove various natural-product drugs from the cell before they have a chance to exert their cytotoxic effects. Using the tools of molecular biology, studies are beginning to reveal the true incidence of multidrug resistance, as mediated by the MDR1 gene, in the clinical setting. It has been demonstrated, at least in the laboratory, that resistance mediated by P-glycoprotein may be modulated by a wide variety of compounds, including verapamil and cyclosporine A. These are compounds which, by themselves, generally have little or no effect on the tumor cells, but when used in conjunction with antineoplastic agents act to decrease, and in some instances eliminate, drug resistance. The mechanism(s) by which these agents act to reverse resistance is not fully understood. Clinical trials to modulate P-glycoprotein activity are now under way to determine whether such strategies will be feasible. The detection of the P-glycoprotein in patient samples is very important in the design of these studies, as it appears that drug-resistant cells lacking P-glycoprotein will be unaffected by agents such as verapamil. Clinical studies are needed in which patients are stratified into chemotherapy protocols based on levels of MDR1 mRNA or P-glycoprotein expression in the primary tumors. Several research areas have been identified that are important to the transfer of the discovery of the MDR1 gene and its protein product from the research laboratory to the clinical environment. There is an immediate need for comprehensive information on the prevalence and levels of expression of the human MDR genes and their protein products in human organs and tissues. Data are needed on P-glycoprotein levels in specific subpopulations (e.g., according to age, sex, race, and diet), and the study of the heterogeneity and variability of expression of P-glycoprotein in normal human tissues should be given high priority. Since early studies have indicated some successes in identifying patients with classic multidrug resistance who might be responsive to chemosensitization, it can be anticipated that clinical research will accelerate in this area. The next wave of clinical studies will provide clinical investigators with opportunities to develop and evaluate P-glycoprotein tests and correlate test results with clinical outcomes.

Comparison of an immunoperoxidase "sandwich" staining method and western blot detection of P-glycoprotein in human cell lines and sarcomas

The applicability of a multilayer immunoperoxidase "sandwich" method (IpS) developed by Chan14 for the amplified detection of P-glycoprotein (Pgp) was investigated. The authors examined 15 formalin-fixed cell lines, as well as formalin-fixed, paraffin-embedded sections from single biopsies of 46 sarcomas. The cell lines included sensitive and multidrug resistant sublines (KB, A2780, MCF-7, HeLa) with various relative degrees of resistance to doxorubicin (Dox). The sarcoma biopsy specimens were selected on the basis of the results obtained in Western blot (WB) detection of Pgp (22 positive and 24 negative by WB) using C219 and C494 monoclonal antibodies to Pgp. The IpS method employed C219. The least resistant cell line in which Pgp could be detected by IpS was fivefold resistant to doxorubicin, whereas Pgp was detected by WB in cells greater than twofold resistant. Cell lines having greater than fivefold resistance to Dox were positive by both IpS and WB analyses. The less resistant cell lines contained more nonreactive cells whereas the highly resistant cell lines showed more homogeneous strong membrane reactions. Among the six cell lines determined to be Pgp negative by WB analysis, no false positive immunostaining by IpS existed. One of 22 WB positive and 7 of 24 WB-negative sarcoma biopsy specimens were positive by IpS methods. Reaction varied and was always focal (a minimum of 3-5 cells, ranging up to 3-4 high power fields) indicating pronounced heterogeneous distribution of Pgp. Thus, WB can detect low average (overall) levels of Pgp in tumor samples but such low concentrations of PgP at the single cell are not detectable by IpS methods. However, IpS can discern among many Pgp-negative cells small subpopulations of immunoreactive cells, which are not detected by WB analysis due to Pgp dilution by the membrane protein of numerous Pgp negative cells. IpS and WB used together as complementary methods can provide more complete information about Pgp distribution and content, particularly in the case of heterogeneous human tumors. The IpS method is more suitable for less drastically treated (not embedded) cell line specimens than for paraffin-embedded (routine) sections. Some modification of the present IpS protocol seems necessary to increase its sensitivity and reduce the disparity with WB results.

Multidrug resistance

Resistance of malignant cells to cytotoxic agents is often a limiting factor to successful chemotherapy. The classical multidrug resistance is characterised by overexpression of a membrane protein, P-glycoprotein, which acts like a drug extruding pump reducing accumulation of cytotoxic agents inside malignant cells, thereby preventing their function. Resistance is expressed simultaneously towards several structurally unrelated drugs. P-glycoprotein is also expressed in many normal human tissues, e.g., in the gastrointestinal tract, and this may be the reason for intrinsic resistance observed clinically in cancers derived from certain tissues. More often multidrug resistance is acquired during chemotherapy. The physiological function of P-glycoprotein is still unknown but it may have a role in cellular detoxification and secreting mechanisms. Interest in the phenomenon of multidrug resistance centres on the correlation of P-glycoprotein expression to clinical drug resistance. Another goal is to find mechanisms by which the function of P-glycoprotein as a multidrug transporter is prevented and drug resistance reversed.