Evidence of post-translational regulation of P-glycoprotein associated with the expression of a distinctive multiple drug-resistant phenotype in Chinese hamster ovary cells

Inhibition of the multidrug resistance P-glycoprotein: time for a change of strategy?

P-glycoprotein (P-gp) is a key player in the multidrug-resistant phenotype in cancer. The protein confers resistance by mediating the ATP-dependent efflux of an astonishing array of anticancer drugs. Its broad specificity has been the subject of numerous attempts to inhibit the protein and restore the efficacy of anticancer drugs. The general strategy has been to develop compounds that either compete with anticancer drugs for transport or act as direct inhibitors of P-gp. Despite considerable in vitro success, there are no compounds currently available to "block" P-gp-mediated resistance in the clinic. The failure may be attributed to toxicity, adverse drug interaction, and numerous pharmacokinetic issues. This review provides a description of several alternative approaches to overcome the activity of P-gp in drug-resistant cells. These include 1) drugs that specifically target resistant cells, 2) novel nanotechnologies to provide high-dose, targeted delivery of anticancer drugs, 3) compounds that interfere with nongenomic transfer of resistance, and 4) approaches to reduce the expression of P-gp within tumors. Such approaches have been developed through the pursuit of greater understanding of resistance mediators such as P-gp, and they show considerable potential for further application.

Transport processes of radiopharmaceuticals and -modulators

Radiotherapy and radiology have been indispensable components in cancer care for many years. The detection limit of small tumor foci as well as the development of radio-resistance and severe side effects towards normal tissues led to the development of strategies to improve radio-diagnostic and -therapeutic approaches by pharmaceuticals. The term "radiopharmaceutical" has been used for drugs labeled with radioactive tracers for therapy or diagnosis. In addition, drugs have been described to sensitize tumor cells to radiotherapy (radiosensitizers) or to protect normal tissues from detrimental effects of radiation (radioprotectors). The present review summarizes recent concepts on the transport of radiopharmaceuticals, radiosensitizers, and radioprotectors in cells and tissues, e.g. by ATP-binding cassette transporters such as P-glycoprotein. Strengths and weaknesses of current strategies to improve transport-based processes are discussed.

Selective resistance to different glucocorticoids in severe autoimmune disorders

Resistance to glucocorticoids often occurs in patients with severe inflammatory disorders. Occasionally, this resistance could be overcome by switching to a different glucocorticoid, but the mechanisms of this selectivity are not clear. We studied this condition in three patients with severe inflammatory disorders, who responded satisfactorily to betamethasone, but could not be switched to equipotent doses of methylprednisolone or prednisone. While betamethasone displayed similar activity on lymphocyte proliferation in cells obtained from the three patients and controls, higher concentrations of methylprednisolone were needed to inhibit proliferation in patients' cells. In a competition study, the concentration of methylprednisolone that inhibited 50% of specific [(3)H]dexamethasone binding was increased in patients' lymphocytes. Higher Rhodamine-123 efflux was demonstrated in CD4 T cells from two patients, suggesting that an increased activity of membrane transporters could be responsible for the selective response to different glucocorticoids, even if P-glycoprotein and MRP1 expression was not increased.

Multidrug resistance-1 (MDR-1) in autoimmune disorders IV. P-glycoprotein overfunction in lymphocytes from myasthenia gravis patients

Multidrug resistance (MDR) mechanisms have been widely studied in cancer. Among them, P-glycoprotein (P-gp) overfunction has been associated with resistance to several antineoplastic agents. The physiological role of P-gp involves hormone and metabolite secretion, bacterial product detoxification, and transport of several drugs to the extracellular space, thus inhibiting their toxic or therapeutic effects. The study of MDR-1 in diseases of autoimmune origin has just recently emerged. Corticosteroids remain the mainstay therapy for autoimmune diseases. As prednisone (PDN) is transported by P-gp, the aim of this study was to evaluate the P-gp function in lymphocytes from myasthenia gravis (MG) patients. Thirty MG patients and 25 healthy controls were studied. Peripheral blood mononuclear cells were isolated by gradient centrifugation and incubated with daunorubicin (DNR) (a fluorescent drug extruded by P-gp). Functional activity of P-gp was analyzed by flow cytometry. Results were expressed as percentage of gated lymphocytes able to efflux DNR. Overall, MG patients showed increased numbers of lymphocytes with functional P-gp activity when compared with controls (x = 4.92 +/- 5.26% vs. x = 0.7 +/- 0.48%, respectively) (P < 0.0001). When patients were classified as responders (n = 21) or refractory (n = 9) to treatment, the latter group exhibited higher values of functional P-gp (x = 10.18 +/- 6.39%) when compared to the responder group (x = 2.66 +/- 2.45%) (P = 0.0076). These data suggest, on the one hand, that drug resistance may be induced by long-term treatment or by high PDN doses and, on the other, emphasize the need for the study of P-gp antagonists in order to improve the current therapeutical schemes for the treatment of MG.

Multidrug resistance-1 (MDR-1) in autoimmune disorders III: increased P-glycoprotein activity in lymphocytes from immune thrombocytopenic purpura patients

OBJECTIVE

P-glycoprotein (P-gp) expression has been widely observed in normal and neoplastic cells. The physiologic role of P-gp involves hormone and metabolite secretion, bacterial product detoxification, and transport of several drugs to the extracellular space. Multidrug resistance-1 is characterized by drug extrusion through P-gp, reducing the intracellular levels of drugs and diminishing their pharmacological effects. Treatment of immune thrombocytopenic purpura (ITP) includes agents that are substrates of P-gp; hence, the objective of this study was to analyze the functional activity of P-gp in lymphocytes from patients with ITP.

PATIENTS AND METHODS

30 ITP patients (9 refractory, 5 dependent, 14 responders to treatment, and 2 with stable disease) and 25 healthy controls were studied. Peripheral blood mononuclear cells were isolated by gradient centrifugation and incubated with daunorubicin (a fluorescent drug extruded by P-gp). Functional activity of P-gp was analyzed by flow cytometry. Results were expressed as the percentage of lymphocytes able to extrude daunorubicin.

RESULTS

ITP patients showed an increased number of lymphocytes with P-gp activity (mean=12.3%+/-16%) when compared to controls (mean=0.87%+/-0.72%) (p<0.05). P-gp function was higher in the refractory group (median=9.4%) than in the treatment-dependent (median=5.4%), responder (median=6.4%), and stable disease (median=5.2%) groups, although no statistical differences were found among them.

CONCLUSION

Enhanced P-gp activity in ITP may be related to an unfavorable clinical outcome and poor response to treatment. Furthermore, P-gp function might affect therapeutic requirements for disease control.

EhPgp5 mRNA stability is a regulatory event in the Entamoeba histolytica multidrug resistance phenotype

The multidrug resistance (MDR) phenotype in Entamoeba histolytica is characterized by the overexpression of the EhPgp5 gene in trophozoites grown in high drug concentrations. Here we evaluated the role of EhPgp5 mRNA stability on MDR using actinomycin D. EhPgp5 mRNA from trophozoites growing without emetine had a half-life of 2.1 h, which augmented to 3.1 h in cells cultured with 90 microM and to 7.8 h with 225 microM emetine. Polyadenylation sites were detected at 118-, 156-, and 189-nucleotide (nt) positions of the EhPgp5 mRNA 3'-untranslated region. Interestingly, trophozoites grown with 225 microM emetine exhibited an extra polyadenylation site at 19 nt. The 3'-untranslated region sequence is AU-rich and has putative consensus sequences for RNA-binding proteins. We detected a RNA-protein complex in a region that contains a polypyrimidine tract (142-159 nt) and a cytoplasmic polyadenylation element (146-154 nt). A longer poly(A) tail in the EhPgp5 mRNA was seen in trophozoites grown with 225 microM emetine. Emetine stress may affect factors involved in mRNA turnover, including polyadenylation/deadenylation proteins, which could induce changes in the EhPgp5 mRNA half-life and poly(A) tail length. Novel evidence on mechanisms participating in E. histolytica MDR phenotype is provided.

Rapid induction of long-lasting drug efflux activity in brain vascular endothelial cells but not malignant glioma following irradiation

The influence of radiotherapy on malignant glioma multidrug resistance to chemotherapy was evaluated because patients with glioma often are treated with a combination of radiotherapy and chemotherapy. Multidrug resistance gene (MDR1, mdr1a, and mdr1b) transcripts were found in human and rat glioma cell lines. P-Glycoprotein (Pgp) was immunohistochemically detected in glioma cell lines and in the rat brain vascular endothelial cell line (RBE4). A multidrug resistance pump efflux activity assay demonstrated increased calcein efflux of RBE4 endothelial cells, but not glioma cells, 2 h after irradiation and still increased 14 d after irradiation. The increased efflux was equally inhibited by verapamil with or without irradiation. In the rat intracranial glioma model (BT4C), Pgp was demonstrated in capillary endothelial cells of the tumor tissue and surrounding normal brain, but not in tumor cells. The expression of gene transcripts or Pgp was not affected by irradiation. The results indicate that long-lasting verapamil-resistant drug efflux mechanisms are activated in brain endothelial cells after irradiation. The results might explain the poor efficacy of chemotherapy following radiotherapy and contribute to consideration of new treatment strategies in the management of malignant glioma.

Cell-cycle-dependent turnover of P-glycoprotein in multidrug-resistant cells

Regulation of P-glycoprotein (Pgp) expression occurs not only at the DNA and mRNA level but also at the protein level. We showed previously that Pgp was stabilized when multidrug-resistant CH(R)C5 and SKVCR 2.0 ovarian cell lines were subjected to serum-starved or high-cell-density growth conditions, whereas Pgp turnover in a leukemic multidrug-resistant cell line, CEMVLB0.1, was not affected by serum starvation (Muller et al., 1995). On further analysis, we have observed that the majority of the CH(R)C5 and SKVCR 2.0 cells under these conditions were in the G1/G0 phase of the cell cycle, whereas the cell cycle of CEMVLB0.1 cells was not affected. Pgp in CEMVLB0.1 cells was stabilized only when the cell cycle was delayed in the G1/G0 phase by using amino acid-deficient growth medium. In CH(R)C5 cells, Pgp half-life was also considerably increased when the cell cycle of these ovary-derived cells was delayed in the G1/G0 phase by using high concentrations of progesterone under normal serum growth conditions. In contrast, Pgp stability was not greatly affected if these cells were delayed in the S or G2/M phase of the cell cycle with Ara-C, cisplatin, or colchicine under the same conditions. Insulin-like growth factors could release the serum-starved CH(R)C5 and SKVCR2.0 cells from the G1/G0 phase and destabilized Pgp. These results indicate that Pgp turnover is a cell-cycle-related process in MDR cells.

BBB transport and P-glycoprotein functionality using MDR1A (-/-) and wild-type mice. Total brain versus microdialysis concentration profiles of rhodamine-123

PURPOSE

The effect of P-glycoprotein (Pgp) on brain distribution using mdr1a (-/-) mice was investigated.

METHODS

Fluorescein (Flu) and FD-4 were used to check whether blood-brain barrier (BBB) integrity was maintained in mdr1a (-/-) mice. The Pgp substrate rhodamine-123 (R123) was infused and total brain, blood and brain microdialysate concentrations in mdr1a (-/-) mice and wild-type mice were compared.

RESULTS

Maintenance of BBB integrity was indicated by equal total brain/blood ratios of Flu and FD-4 in both mice types. R123 concentrations in brain after i.v. infusion were about 4-fold higher in mdr1a (-/-) than in wild-type mice (P < 0.05), without changes in blood levels. After microdialysis experiments the same results were found, excluding artifacts in the interpretation of Pgp functionality by the use of this technique. However the 4-fold ratio in brain was not reflected in corresponding microdialysates. No local differences of R123 in the brain were found. By the no-net-flux method in vivo recovery appeared to 4.6-fold lower in mdrla (-/-) mice compared with wild-type mice.

CONCLUSIONS

Pgp plays an important role in R123 distribution into the brain. Using intracerebral microdialysis, changes in in vivo recovery by the absence or inhibition of Pgp (or active efflux in general) need to be considered carefully.

Enhanced expression of neuropeptides in human breast cancer cell lines following irradiation

Previously, we have observed that the expression of the neuropeptides bombesin (BN-), the mammalian counterpart being gastrin-releasing peptide (GRP), and substance P (SP) in intact normal tissues, such as salivary and laryngeal glands, increases in response to irradiation. In the present study, the aim was to evaluate whether irradiation can have effects on individual cells that normally synthesize neuropeptides. In addition, since these neuropeptides are potentially mitogenic, we studied tumor cells. Therefore, the estrogen receptor-negative human breast cancer cell line MDA-MB-231 and its subline, with acquired doxorubicin resistance, MDA-MB-231 Dox were examined before irradiation and 4, 10, and 15 days after irradiation with 4 Gy (195 kV, 2 Gy fractions with 4 hours interval). Potential dose related changes were studied by delivering single doses of 2 or 9 Gy with the same technique. Immunohistochemical and radioimmunoassay (RIA) methods were used for detection of the SP and BN/GRP. Before, and at all time points following irradiation, a subpopulation in both cell lines displayed an intense immunostaining of SP and BN/GRP. A partial reorganization of the immunoreactive material was observed 10 days after irradiation. The RIA-analyses displayed signs of a dose-related increase, and a time-dependent transient and significant increase in the content of both peptides. The pattern of changes differed between the two peptides, and was especially pronounced in the doxorubicin resistant cells with regard to SP. Another neuropeptide, calcitonin gene related peptide (CGRP), was not detected in the cells used. The results suggest that irradiation has effects on a population of cultured neuropeptide-synthesizing cells. The occurrence and the specific changes obtained in the levels of neuropeptides, in response to irradiation, might imply an importance in the growth of breast cancer cells and in explaining repair processes following irradiation.

P-glycoprotein expression: critical determinant in the response to osteosarcoma chemotherapy

BACKGROUND

Fewer than 20% of patients with bone cancer who are treated with surgery alone are cured. Even with the best current treatment, surgery combined with chemotherapy, only 60%-80% of patients with nonmetastatic bone cancer and 10% of patients with metastatic bone cancer are cured. Thus far, the reason for treatment failure in the nonresponding subset has not been identified. It has been hypothesized that P-glycoprotein, which confers multidrug resistance, might be the cause. We sought to determine whether the expression of P-glycoprotein is associated with poor treatment outcome in osteosarcoma.

METHODS

In a retrospective study, we correlated P-glycoprotein expression with the outcome of conventional chemotherapy in 62 consecutive, clinically staged patients diagnosed as having osteosarcoma between 1980 and 1989.

RESULTS

P-glycoprotein was overexpressed in 27 patients but not in another 34 patients, and expression was ambiguous in the sample from one patient. At a median follow-up of 8.9 years, the 34 patients whose tumors did not express P-glycoprotein had significantly better relapse-free rates than the 27 subjects whose tumors expressed the protein (87% versus 0%; P<.00001) and had improved survival rates (94% versus 35%; P<.00001). Among the 46 patients who received chemotherapy before surgery, the 23 whose tumors were negative for P-glycoprotein showed significantly better long-term outcomes (P<.00002), although differences in tumor necrosis in response to therapy were only of borderline significance (P = .057).

CONCLUSIONS

P-glycoprotein expression does correlate with treatment failure in patients with osteosarcoma. This correlation raises the possibility that inhibiting the action of P-glycoprotein as part of therapy for this disease would improve outcome.

Identification of a distinctive P-glycoprotein-mediated resistance phenotype in human ovarian carcinoma cells after their in vitro exposure to fractionated X-irradiation

BACKGROUND

Clinical drug resistance is recognized in patients previously treated with radiotherapy and after chemotherapy. In vitro exposure of mammalian tumor cells to fractionated X-irradiation also resulted in the expression of drug resistance. Analysis of the resistance phenotype of irradiated Chinese hamster ovary sublines revealed P-glycoprotein overexpression, without any concomitant increase in P-glycoprotein messenger RNA, under posttranslational regulation. This study aimed to determine whether this distinctive resistance phenotype could also be identified in irradiated human tumor cells.

METHODS

Irradiated sublines established from two human ovarian tumor cell lines, SK-OV-3 and JA-T, which showed resistance to vincristine and to etoposide, were studied. Protein and RNA expression were quantitated by Western and Northern blotting or RNase protection assays. P-glycoprotein turnover was measured after immunoprecipitation of metabolically labelled cells.

RESULTS

Significant P-glycoprotein overexpression was detected using the C219 and C494 monoclonal antibodies in the two irradiated human ovarian tumor sublines. No concomitant increase in P-glycoprotein messenger RNA was detectable in the SK-OV-3/DXR10 subline, contrasting with the increased message characteristic of vincristine-selected SKVCR sublines. In addition, turnover of P-glycoprotein was significantly reduced in these DXR10 cells when compared with that measured in a vincristine-selected subline. These irradiated sublines showed reduced levels of epidermal growth factor receptors and unchanged levels of topoisomerase II, but they overexpressed c-erbB2 marginally and heat shock protein 27 significantly. These latter elevations in protein levels, however, were associated with concomitant increases in their respective messenger RNAs, implicating regulation at the transcriptional level.

CONCLUSIONS

Exposure of human ovarian tumor cells to fractionated X-irradiation in vitro resulted in the expression of a distinctive multiple drug resistance phenotype unusually involving posttranslational regulation of P-glycoprotein. Monitoring tumor biopsies for P-glycoprotein-associated drug resistance in patients treated with radiotherapy should evaluate protein levels rather than, or as well as, MDR1 mRNA expression.

Modified multiple drug resistance phenotype of Chinese hamster ovary cells selected with X-rays and vincristine versus X-rays only

Exposure of Chinese hamster ovary (CHO) cells to fractionated X-irradiation [ten fractions of 9 Gray (Gy)] resulted in the expression of a multiple drug resistance phenotype which was distinct from that of drug-selected cells in two features: (i) resistance to vinca alkaloids and epipodophyllotoxins but sensitivity to anthracyclines was retained; (ii) overexpression of P-glycoprotein (Pgp) but regulated by post-translational stability rather than by any elevation in Pgp mRNA (Hill et al., 1990). It was also reported that when these cells (designated DXR-10) were subsequently exposed to another ten fractions of 9 Gy (20 x 9 Gy in total), no further increases in drug resistance or in the extent of Pgp expression were observed. To examine this apparent plateauing of the drug resistance phenotype following X-ray pretreatment, DXR-10 cells were instead treated with ten pulsed vincristine exposures. The resultant cell line, designated DXR-10/VCR-10, proved to be more resistant to vincristine, implying that the effect of further drug selection was additive to that of X-ray pretreatment. In addition, these cells showed resistance to doxorubicin and increased Pgp expression which was matched by a concomitant elevation in Pgp mRNA. These findings appear to confirm that Pgp expression is differentially regulated in tumour cells showing drug resistance after drug as opposed to X-ray selection.