Analysis of P-glycoprotein phosphorylation in HL60 cells isolated for resistance to vincristine

A synergetic screening approach with companion effector for combination therapy: application to retinoblastoma

For many cancers, the lack of potency and the toxicity of current drugs limits the dose achievable in patients and the efficacy of treatment. Among them, retinoblastoma is a rare cancer of the eye for which better chemotherapeutic options are needed. Combination therapy is a compelling approach to enhance the efficacy of current treatment, however clinical trials to test rationally designed combinations of approved drugs are slow and expensive, and limited by our lack of in-depth knowledge of drug specificity. Since many patients already turn to nutraceuticals in hopes of improving their condition, we hypothesized that certain approved drugs could potentially synergize with widely consumed supplements. Following this hypothesis, we devised an alternative screening strategy aimed at taking advantage of a bait compound such as a nutraceutical with potential therapeutic benefits but low potency, by screening chemical libraries for approved drugs that synergize with this companion effector. As a proof of concept, we sought to identify approved drugs with synergetic therapeutic effects toward retinoblastoma cells in combination with the antioxidant resveratrol, popular as a supplement. We systematically tested FDA-approved drugs and known bioactives seeking to identify such pairs, which led to uncovering only a few additive combinations; but to our surprise, we identified a class of anticancer drugs widely used in the clinic whose therapeutic effect is antagonized with resveratrol. Our observations could explain in part why some patients do not respond well to treatment. Our results validate this alternative approach, and we expect that our companion effector strategy could significantly impact both drug discovery and the nutraceutical industry.

The Multixenobiotic Resistance Mechanism in Aquatic Organisms

Many aquatic organisms thrive and reproduce in polluted waters. This fact indicates that they are well equipped with a defense system(s) against several toxic xenobiotics simultaneously because water pollution is typically caused by a mixture of a number of pollutants. We have found that the biochemical mechanism underlying such "multixenobiotic" resistance in freshwater and marine mussel, in several marine sponges, and in freshwater fish is similar to the mechanism of multidrug resistance (MDR) found in tumor cells that became refractory to treatment with a variety of chemotherapeutic agents. All these organisms possess a verapamil-sensitive potential to bind 2-acetylaminofluorene and vincristine onto membrane vesicles. They all express mRNA for mdr1 gene, and mdr1 protein product, the glycoprotein P170. Finally, in in vivo experiments, the accumulation of xenobiotics is enhanced in all investigated organisms in the presence of verapamil, the inhibitor of the P170 extrusion pump. The knowledge that the presence of one xenobiotic may block the pumping out, and hence accelerating accumulation, of others, may help us to understand and interpret our present and past data on different environmental parameters obtained using indicator organisms.

ABCB1 (MDR 1) polymorphisms and progression-free survival among women with ovarian cancer following paclitaxel/carboplatin chemotherapy

PURPOSE

The human ABCB1 gene encodes P-glycoprotein, which transports a broad range of anticancer drugs, including paclitaxel. Although the functional consequences of ABCB1 polymorphisms have been the subject of numerous studies, few have assessed the association with clinical outcome.

EXPERIMENTAL DESIGN

We assessed the association between the 2677G>T/A, 3435C>T, and 1236C>T ABCB1 polymorphisms and progression-free and overall survival in 309 patients from the Australian Ovarian Cancer Study treated with paclitaxel/carboplatin and subsequently tested significant observations in an independent validation set.

RESULTS

Women who carried the minor T/A alleles at the 2677G>T/A polymorphism were significantly less likely to relapse following treatment compared with homozygote GG carriers (P(Log-rank)=0.001) in the Australian Ovarian Cancer Study cohort. Subgroup analyses showed that this effect was limited to cases with residual disease <or=1 cm (P(Log-rank)=0.0004), not for those with residual disease >1 cm (P(Log-rank)=0.3). This effect was not confirmed in an independent validation set of carboplatin/paclitaxel-treated patients (n=278) using a higher residual disease cut point (<or=2 cm). However, analysis of the unrestricted data set expanded to include docetaxel-treated patients (n=914) did support an effect of the 2677T/A allele in patients with no macroscopic residual disease (hazard ratio, 0.70; 95% confidence interval, 0.46-1.04; P(one-sided)=0.039).

CONCLUSION

Our findings indicate that there is an effect of the 2677G>T/A polymorphism on progression-free survival in ovarian cancer patients who are treated with a taxane/carboplatin, which is dependent on the extent of residual disease, with a better prognosis for patients with the 2677T/A allele and minimal residual disease.

Complex interaction of BCRP/ABCG2 and imatinib in BCR-ABL-expressing cells: BCRP-mediated resistance to imatinib is attenuated by imatinib-induced reduction of BCRP expression

Imatinib, a potent tyrosine kinase inhibitor, is effluxed from cells by the breast cancer resistance protein (BCRP/ABCG2), yet published studies to date fail to demonstrate resistance to imatinib cytotoxicity in BCRP-overexpressing cells in vitro. We investigated cellular resistance to imatinib in BCR-ABL-expressing cells transduced and selected to overexpress BCRP (K562/BCRP-MX10). These cells exhibited a 2- to 3-fold increase in resistance to imatinib (P < .05) and a 7- to 12-fold increase in resistance to mitoxantrone, a known BCRP substrate. Resistance to imatinib was completely abolished by the specific BCRP inhibitor fumitremorgin C. Studies of the mechanism of the diminished resistance to imatinib compared with mitoxantrone revealed that imatinib decreased the expression of BCRP in K562/BCRP-MX10 cells without affecting mRNA levels. BCRP levels in cells that do not express BCR-ABL were not affected by imatinib. Loss of BCRP expression was accompanied by imatinib-induced reduction of phosphorylated Akt in the BCRP-expressing K562 cells. The phosphoinositol-3 kinase (PI3K) inhibitor LY294002 also decreased BCRP levels in K562/BCRP-MX10 cells. These studies show that BCRP causes measurable imatinib resistance, but this effect is attenuated by imatinib-mediated inhibition of BCR-ABL, which in turn downregulates overall BCRP levels posttranscriptionally via the PI3K-Akt pathway.

Up-regulation of P-glycoprotein expression in small intestine under chronic serotonin-depleted conditions in rats

To investigate the role of serotonin (5-HT), an important neurotransmitter and hormone/paracrine agent in the small intestine, in the transport activity of P-glycoprotein (P-gp), the intestinal transport of quinidine, a P-gp substrate, was examined in 5-HT-depleted rats prepared by intraperitoneal administration of p-chlorophenylalanine, a specific inhibitor of tryptophan hydroxylase in 5-HT biosynthesis. In the in vitro transport study, quinidine transport across rat jejunum was significantly enhanced in both the secretory and absorptive directions under 5-HT-depleted conditions, although the secretory transport was still predominant. The electrophysiological study suggested that the quinidine transport via passive diffusion was enhanced presumably through a paracellular route. This might be due to looser tight junctions under 5-HT-depleted conditions. The voltage-clamp technique clearly indicated that the secretory transport of quinidine through the transcellular pathway was also enhanced by the depletion of 5-HT. Furthermore, 5-HT depletion increased verapamil-sensitive secretory transport of quinidine in rat jejunum. These results indicate that the secretory transport of quinidine via P-gp was significantly enhanced under 5-HT-depleted conditions. The level of ATP, an energy source for functioning P-gp, wet weight of jejunum, and total protein level in rat jejunal mucosa were not changed by 5-HT depletion, but the expression of P-gp in the brush-border membrane of rat jejunum was significantly induced, which is partly responsible for the enhancement of P-gp activity under the 5-HT-depleted condition.

A protein kinase Cbeta inhibitor attenuates multidrug resistance of neuroblastoma cells

BACKGROUND

The acquisition of drug resistance is a major reason for poor outcome of neuroblastoma. Protein kinase C (PKC) has been suggested to influence drug resistance in cancer cells. The aim of this study was to elucidate whether inhibition of PKCbeta isoforms influences drug-resistance of neuroblastoma cells.

METHODS

The effect of the PKCbeta inhibitor LY379196 on the growth-suppressing effects of different chemotherapeutics on neuroblastoma cells was analyzed with MTT assays. The effect of LY379196 on the accumulation of [3H]vincristine was also investigated

RESULTS

The PKCbeta inhibitor LY379196 suppressed the growth of three neuroblastoma cell lines. LY379196 also augmented the growth-suppressive effect of doxorubicin, etoposide, paclitaxel, and vincristine, but not of carboplatin. The effect was most marked for vincristine and for the cell-line (SK-N-BE(2)) that was least sensitive to vincristine. No effect was observed on the non-resistant IMR-32 cells. Two other PKC inhibitors, Gö6976 and GF109203X, also enhanced the vincristine effect. The PKC inhibitors caused an increased accumulation of [3H]vincristine in SK-N-BE(2) cells.

CONCLUSIONS

This indicates that inhibition of PKCbeta could attenuate multidrug resistance in neuroblastoma cells by augmenting the levels of natural product anticancer drugs in resistant cells.

Effect of PKC412, a selective inhibitor of protein kinase C, on lung metastasis in mice injected with B16 melanoma cells

PKC412, a selective inhibitor of protein kinase C (PKC), is currently in clinical trials as an anti-tumor drug. In the present study, we investigated the anti-metastatic effect of PKC412 using an experimental metastatic mouse model intravenously injected with melanoma cells. One-hour exposure to various concentrations of PKC412 (0.5, 5 and 50 microM) dose-dependently reduced the lung-metastatic potential of highly metastatic B16-F10 and -BL6 mouse melanoma cells in syngeneic mice. Following the exposure, PKC activities in B16-F10 and -BL6 cells were significantly decreased, but growth curves were not influenced. To elucidate the mechanism of the anti-metastatic effect of PKC412, we examined the activity to invade the extracellular matrix and the platelet-aggregating activity of the melanoma cells incubated with PKC412 (0.5, 5 and 50 microM) for 1 hour. PKC412 significantly reduced both the invasive and platelet-aggregating activities. These results suggest that PKC412 shows an anti-metastatic function through the inhibition of the invasive and/or platelet-aggregating activities of melanoma cells. PKC412 is potentially a promising candidate for an anti-metastatic agent.

Anticancer drug resistance in primary human brain tumors

The difficult clinical situation still associated with most types of primary human brain tumors has fostered significant interest in defining novel therapeutic modalities for this heterogeneous group of neoplasms. Beginning in the 1980s chemotherapy has been incorporated into the treatment protocol of a number of intractable brain tumors. However, it has predominantly failed to improve patient outcome. The unsatisfactory results with chemotherapeutic intervention have chiefly been attributed to tumor cell resistance. In recent years, there has been a literal explosion in our understanding about the mechanisms by which cancer cells become chemoresistant. During the course of their evolution (intrinsic resistance) or in response to chemotherapy (acquired resistance) these cells may follow a number of pathways of genetic alterations to possess a common (multidrug) or drug-specific (individual drug) resistant phenotype. Genomic aberrations, deregulation of membrane transporting proteins and cellular enzymes, and an altered susceptibility to commit to apoptosis are among the steps on the way that contribute to the genesis of chemotherapeutic treatment failure. Although, through the years we have come to yield information and inferences as to the roles that different molecular events may have in the resistance phenotype of cancer cells, the actual involvement of single genetic alterations in conferring drug resistance in primary brain tumors remains debatable. This uncertainty and, besides, the lack of proper drug resistance diagnostics, in a vicious circle, hinder the development of effective resistance-modulation strategies. Clinical non-responsiveness to chemotherapy remains a formidable obstacle to the successful treatment of brain tumors and one of the most serious problems to be solved in the therapy of these lesions. Future advances in the chemotherapeutic management of these neoplasms will come with an improved understanding of the significance and interrelationship of the multiple biological systems operative in promoting resistance to this treatment modality. The focus of this review is to summarize current knowledge concerning major drug resistance-related markers, to describe their functional interaction en route to chemoresistance, and to discuss their implication in rendering human brain tumor cells resistant to chemotherapy.

Modulation of protein kinase C in antitumor treatment

PKC isoenzymes were found to be involved in proliferation, antitumor drug resistance and apoptosis. Therefore, it has been tried to exploit PKC as a target for antitumor treatment. PKC alpha activity was found to be elevated, for example, in breast cancers and malignant gliomas, whereas it seems to be underexpressed in many colon cancers. So it can be expected that inhibition of PKC activity will not show similar antitumor activity in all tumors. In some tumors it seems to be essential to inhibit PKC to reduce growth. However, for inhibition of tumor proliferation it may be an advantage to induce apoptosis. In this case an activation of PKC delta should be achieved. The situation is complicated by the facts that bryostatin leads to the activation of PKC and later to a downmodulation and that the PKC inhibitors available to date are not specific for one PKC isoenzyme. For these reasons, PKC modulation led to many contradicting results. Despite these problems, PKC modulators such as miltefosine, bryostatin, safingol, CGP41251 and UCN-01 are used in the clinic or are in clinical evaluation. The question is whether PKC is the major or the only target of these compounds, because they also interfere with other targets. PKC may also be involved in apoptosis. Oncogenes and growth factors can induce cell proliferation and cell survival, however, they can also induce apoptosis, depending on the cell type or conditions in which the cells or grown. PKC participates in these signalling pathways and cross-talks. Induction of apoptosis is also dependent on many additional factors, such as p53, bcl-2, mdm2, etc. Therefore, there are also many contradicting results on PKC modulation of apoptosis. Similar controversial data have been reported about MDR1-mediated multidrug resistance. At present it seems that PKC inhibition alone without direct interaction with PGP will not lead to successful reversal of PGP-mediated drug efflux. One possibility to improve chemotherapy would be to combine established antitumor drugs with modulators of PKC. However, here also very contrasting results were obtained. Many indicate that inhibition, others, that activation of PKC enhances the antiproliferative activity of anticancer drugs. The problem is that the exact functions of the different PKC isoenzymes are not clear at present. So further investigations into the role of PKC isoenzymes in the complex and interacting signalling pathways are essential. It is a major challenge in the future to reveal whether modulation of PKC can be used for the improvement of cancer therapy.

Nontraditional microbial bioactive metabolites

Microorganisms produce low-molar-mass secondary metabolites exhibiting different biological activities, which are used. e.g., in medicine as antimicrobial and antifungal agents, alkaloids and toxins. Some of these substances have highly diverse biological activities and unusual structures. They are produced by streptomycetes, fungi, and bacilli, but interesting products have also been obtained from microorganisms growing in extreme conditions. Several thousands of microbial products have so far been discovered and many other, which can be potentially useful and/or prospective for human use, can still be in the offing.

Equivalent death of P-glycoprotein expressing and nonexpressing cells induced by the protein kinase C inhibitor staurosporine

P-glycoprotein (P-gp) is an ATP-dependent drug pump that confers multidrug resistance. In addition to its ability to efflux toxins P-gp can also inhibit apoptosis induced by a wide array of cell death stimuli that rely on activation of intracellular caspases for full function. We have previously demonstrated that stimuli including drugs such as hexamethylene bisacetamide (HMBA), the cytotoxic lymphocyte granule protein granzyme B, and pore-forming proteins such as perforin, kill P-gp positive cells in a caspase-independent manner. We therefore hypothesised that drugs that are not effluxed by P-gp and which induce cell death in the absence of caspase activation could induce death of P-gp expressing cells. Staurosporine has been previously shown to kill cells in the absence of caspase activation. Consistent with our hypothesis, we demonstrate here that staurosporine can equivalently kill P-gp(+ve) and P-gp(-ve) tumor cell lines in a caspase-independent manner.

Effect of bioflavonoids on vincristine transport across blood-brain barrier

Several grapefruit juice bioflavonoids, including quercetin, are reported to stimulate P-glycoprotein-mediated drug efflux from cultured tumor cells. To see whether these bioflavonoids alter the permeation of vincristine across the blood-brain barrier, we conducted experiments with cultured mouse brain capillary endothelial cells (MBEC4 cells) in vitro and ddY mice in vivo. The steady-state uptake of [3H]vincristine by MBEC4 cells was decreased by 10 microM quercetin, but increased by 50 microM quercetin. Similarly, the in vivo brain-to-plasma concentration ratio of [3H]vincristine in ddY mice was decreased by coadministration of 0.1 mg/kg quercetin, but increased by 1.0 mg/kg quercetin. Kaempferol had a similar biphasic effect on the in vitro uptake of [3H]vincristine. Other aglycones tested (chrysin, flavon, hesperetin, naringenin) increased [3H]vincristine uptake in the 10-50 microM range, and glycosides (hesperidin, naringin, rutin) were without effect. We then addressed the mechanism of the concentration-dependent biphasic action of quercetin. Verapamil, a P-glycoprotein inhibitor, inhibited the efflux of [3H]vincristine from MBEC4 cells, while 10 microM quercetin significantly stimulated it. The uptake of [3H]vincristine by MBEC4 cells was increased by inhibitors of protein kinase C, but decreased by phorbol 12-myristate-13-acetate (PMA), as well as by 10 microM quercetin. The phosphorylation level of P-glycoprotein was increased in the presence of 5 microM quercetin or 100 nM PMA, but decreased by the protein kinase C inhibitor H7 (1-(5-isoquinolinesulfonyl)-2-methylpiperazine, 30 microM). We conclude that low concentrations of quercetin indirectly activate the transport of [3H]vincristine by enhancing the phosphorylation (and hence activity) of P-glycoprotein, whereas high concentrations of quercetin inhibit P-glycoprotein. Our results indicate that patients taking drugs which are P-glycoprotein substrates may need to restrict their intake of bioflavonoid-containing foods and beverages, such as grapefruit juice.

Multixenobiotic resistance as a cellular defense mechanism in aquatic organisms

Multixenobiotic resistance in aquatic organisms exposed to natural toxins or anthropogenic contaminants is a phenomenon analogous to multidrug resistance in mammalian tumor cell lines tolerant of anti-cancer drugs. Multidrug resistance is commonly due to the elevated expression of transmembrane P-glycoproteins (P-gp) which actively transport a wide variety of structurally and functionally diverse compounds. The purpose of this review is to place aquatic ecotoxicological data in context of the larger multidrug resistance field of study. Information on P-glycoproteins structure, mechanism of transport, and substrate specificity gained through traditional mammalian and cell culture models is examined in conjunction with recent work on aquatic species exposed to xenobiotics both in the field and in the laboratory. The physiological function of P-glycoproteins is explored through studies of gene knockout models and expression patterns in normal tissues and tumors. The effect of xenobiotic exposures on P-gp activity and protein titer is examined in wild and captive populations of aquatic invertebrates and vertebrates. Substrate overlap and evidence of co-expression of phase I detoxification enzymes (e.g. cytochromes P450) and P-gp are presented. The role of P-gp chemosensitizers as environmental pollutants and the ecotoxicological consequences of P-gp inhibition are highlighted. The overwhelming evidence suggests that P-glycoproteins provide aquatic organisms with resistance to a wide range of natural and anthropogenic toxins.

Mechanism of inhibition of P-glycoprotein-mediated drug transport by protein kinase C blockers

P-glycoprotein is a membrane ATPase that transports drugs out of cells and confers resistance to a variety of chemically unrelated drugs (multidrug resistance). P-glycoprotein is phosphorylated by protein kinase C (PKC), and PKC blockers reduce P-glycoprotein phosphorylation and increase drug accumulation. These observations suggest that phosphorylation of P-glycoprotein stimulates drug transport. However, there is evidence that PKC inhibitors directly interact with P-glycoprotein, and therefore the mechanism of their effects on P-glycoprotein-mediated drug transport and the possible role of phosphorylation in the regulation of P-glycoprotein function remain unclear. In the present work, we studied the effects of different kinds of PKC inhibitors on drug transport in cells expressing wild-type human P-glycoprotein and a PKC phosphorylation-defective mutant. We demonstrated that PKC blockers inhibit drug transport hy mechanisms independent of P-glycoprotein phosphorylation. Inhibition by the blockers occurs by (i) direct competition with transported drugs for binding to P-glycoprotein, and (ii) indirect inhibition through a pathway that involves PKC inhibition, but is independent of P-glycoprotein phosphorylation. The effects of the blockers on P-glycoprotein phosphorylation do not seem to play an important role, but the PKC-signaling pathway regulates P-glycoprotein-mediated drug transport.

Reversal of multidrug resistance by the staurosporine derivatives CGP 41251 and CGP 42700

It has been shown previously that the staurosporine derivative CGP 41251, a specific inhibitor of protein kinase C (IC50 = 50 nM), exhibits antitumor activity and reverses mdr1 mediated multidrug resistance. At present, the compound is evaluated as an anticancer drug in clinical phase I trials. We compared the effects of CGP 41251 with CGP 42700, another staurosporine derivative, which exhibits low protein kinase C inhibiting activity (IC50 = > 100 microM). We found that in contrast to CGP 41251, CGP 42700 does not show antiproliferative activity in HeLa and KB cells in tissue culture (up to a concentration of 10 microM). We compared both compounds for their ability to reverse mdr1-mediated resistance in KB-C1 and in HeLa-MDR1 cells (transfected with the mdr1 gene). CGP 42700 is able to reverse mdr1-mediated resistance to a similar extent as CGP 41251. The intracellular accumulation of rhodamine 123 in KB-C1 cells following pretreatment with CGP 41251 for 30 min was higher than that following treatment with CGP 42700 if determined in medium without serum. However, quantitation of rhodamine efflux in an ex vivo assay using human CD8+ cells in serum showed that CGP 42700 is more effective in inhibiting the efflux of rhodamine 123 than CGP 41251. We conclude from our results that (1) CGP 42700 is more effective in reversal of multidrug resistance in serum than CGP 41251, indicating that the compound may be useful for treatment of patients, and (2) CGP 42700 does not inhibit protein kinase C and cell proliferation and, therefore, may be less toxic and elicit less side effects in humans than other chemosensitizers.

The chemosensitizers of multixenobiotic resistance mechanism in aquatic invertebrates: a new class of pollutants

Mechanism of multixenobiotic resistance (MXR), identical to multidrug resistance (MDR) in tumor cells, has been found in aquatic invertebrates. The presence of this ATP-dependent membrane P-glycoprotein (Pgp) pump was confirmed by biochemical ('binding'), molecular (immunohistochemical, Western, Northern), physiological (verapamil-sensitivity) and toxicological (modulation of toxicity) methods. The inducibility of MXR in the presence of xenobiotics and its wide taxonomic distribution suggests its role as a general biological defense mechanism that rescues organisms by pumping potentially toxic xenobiotics out of the cells. Some xenobiotics, the chemosensitizers, can inhibit this defense mechanism. The presence of these MXR-inhibitors has important implications on environmental parameters like exposure, uptake, internal dose, bioaccumulation, response, synergism and toxicity. Such MXR-inhibitors, for example, enhance the accumulation of carcinogenic aromatic amines in mussel, with subsequent enhancement in production of their mutagenic metabolites, in induction of single strand breaks in DNA, and in induction of DNA-adducts. The property to inhibit defense mechanism of organisms classifies MXR-inhibitors among top-hazardous environmental chemicals. Therefore, we measured the concentration of chemosensitizers in water concentrates or sediment extracts as their potential to modulate the accumulation of fluorescent dyes in a cell-culture of NIH 3T3 mouse fibroblasts stable transfected with human MDR1 gene, or as the potential of native waters to decrease the efflux-rate of Rhodamine B from gills of mussels. We found significantly higher concentrations of MXR-inhibitors in samples from polluted marine sites or from polluted rivers than in samples from corresponding unpolluted sites. These concentrations were able to enhance the accumulation of fluorescent dyes or carcinogenic aromatic amines in clams, mussels, snails and sponges exposed to these xenobiotics, demonstrating the ecotoxicological relevance of MXR-inhibitors present in polluted waters.

The role of protein kinase C (PKC) in the evolution and proliferation of malignant gliomas, and the application of PKC inhibition as a novel approach to anti-glioma therapy

The present article reviews the role of the second messenger enzyme protein kinase C (PKC) in the growth regulation of high-grade gliomas, and evaluates the efficacy of therapeutic strategies directed against PKC for blocking the proliferation of these malignancies in in vitro and in vivo models. The translation of such strategies to the treatment of patients with malignant gliomas may provide a novel approach for improving the otherwise grim outlook associated with these neoplasms.

Can tumour cell drug resistance be reversed by essential fatty acids and their metabolites?

Tumour cell drug resistance is a major problem in cancer chemotherapy. Essential fatty acids have been shown to be cytotoxic to a variety of tumour cells in vitro. But, the effect of these fatty acids on tumour cell drug resistance has not been well characterized. Gamma-linolenic acid (GLA) of the n-6 series and eicosapentaenoic acid (EPA) of the n-3 series potentiated the cytotoxicity of anti-cancer drugs: vincristine, cis-platinum and doxorubicin on human cervical carcinoma (HeLa) cells in vitro. Alpha-linolenic acid (ALA), GLA, EPA and docosahexaenoic acid (DHA) enhanced the uptake of vincristine by HeLa cells. In addition, DHA, EPA, GLA and DGLA were found to be cytotoxic to both vincristine-sensitive (KB-3-1) and -resistant (KB-ChR-8-5) human cervical carcinoma cells in vitro. Pre-incubation of vincristine-resistant cells with sub-optimal doses of fatty acids enhanced the cytotoxic action of vincristine. GLA, DGLA, AA, EPA and DHA enhanced the uptake and inhibited the efflux of vincristine and thus, augmented the intracellular concentration of the anti-cancer drug(s). Fatty acid analysis of KB-3-1 and KB-ChR-8-5 cells showed that the latter contained low amounts of ALA, GLA, 22:5 n-3 and DHA in comparison to the vincristine-sensitive cells. The concentrations of GLA and DHA were increased 10-15 fold in the phospholipid, free fatty acid and ether lipid cellular lipid pools of GLA and DHA treated cells. These results coupled with the observation that various fatty acids can alter the activity of cell membrane bound enzymes such as sodium-potassium-ATPase and 5'-nucleotidase, levels of various anti-oxidants, p53 expression and the concentrations of protein kinase C suggest that essential fatty acids and their metabolites can reverse tumour cell drug-resistance at least in vitro.

Genes encoding multiple drug resistance-like proteins in Aspergillus fumigatus and Aspergillus flavus

Polymerase chain reaction using degenerate primers was used to identify genes encoding proteins of the ATP-binding cassette superfamily in Aspergillus fumigatus and Aspergillus flavus. In A. fumigatus, two genes (AfuMDR1 and AfuMDR2) encoding proteins of the ATP-binding cassette superfamily were identified. One gene (AflMDR1) was isolated from A. flavus and is the apparent homologue to AfuMDR1. AfuMDR1 and AflMDR1 encode proteins of molecular weights 148,000 and 143,000, respectively, each containing 12 putative transmembrane regions and two ATP-binding sites. These proteins are arranged in two homologous halves, each half consisting of a hydrophobic region (encoding six putative transmembrane domains) and an ATP-binding site. The AfuMDR1 and AflMDR1-encoded proteins bear a high degree of similarity to the Schizosaccharomyces pombe leptomycin B resistance protein and to human MDR1. The second gene identified in A. fumigatus, AfuMDR2, encodes a protein of molecular weight 85,000, containing four putative transmembrane domains and an ATP binding domain. The encoded protein is similar to those encoded by MDL1 and MDL2, two MDR-like genes of Saccharomyces cerevisiae. Expression of AFUMDR1 in S. cerevisiae conferred increased resistance to the antifungal agent cilofungin (LY121019), an echinocandin B analog.

Regulation of the function of P-glycoprotein by epidermal growth factor through phospholipase C

Many multidrug-resistant (MDR) cell lines overexpress the epidermal growth factor receptor (EGFR) as well as P-glycoprotein (P-gp). However, the role of the increased EGFR in P-gp-mediated drug resistance remains unclear. Since recent studies suggest that activation of phospholipase C (PLC) could increase the phosphorylation of P-gp, and activation of the EGFR would also activate PLC, we investigated whether the effect of epidermal growth factor (EGF) on the phosphorylation of P-gp was mediated through PLC. Treatment of the human MDR breast cancer cell line, MCF-7/AdrR, with EGF increased the phosphorylation of P-gp by 20-50%. The increased phosphorylation of P-gp was accompanied by stimulation of PLC activity, as measured by the production of inositol, 1,4,5-trisphosphate and diacylglycerol, products of phosphatidylinositol-4,5-bisphosphate hydrolysis. Treatment of MDR cells with EGF also had detectable effects on P-gp function. For example, following incubation of MCF-7/AdrR cells with ECF, we observed a consistent decrease in total vinblastine (VBL) accumulation. Kinetic analysis revealed this change to be due to an increase in membrane efflux. The latter was measured by the initial uptake velocity, which was inhibited by EGF. VBL uptake measured at 0-320 sec was inhibited by 20-40%, which was associated with a similar increase in VBL efflux. EGF had no effect on drug accumulation, uptake, or efflux in sensitive MCF-7 cells. These data indicate that EGF can modulate the phosphorylation and function of P-gp, and suggest that this effect may be initiated by the activation of PLC.

Protein kinase C-independent correlation between P-glycoprotein expression and volume sensitivity of Cl- channel

The possible correlation between P-glycoprotein (PGP) and volume-sensitive Cl- channel was examined in a pair of cell lines: a subline of the human epidermoid KB cell (KB-3-1) and the corresponding MDR1-transfected cell line (KB-G2). Western blot analysis and indirect immunofluorescence studies indicated that KB-G2, but not KB-3-1, exhibits the PGP expression. Patch-clamp whole-cell recordings showed that osmotic swelling activates Cl- currents not only in PGP-expressing but also in PGP-lacking cells. The amplitude of the maximal current was indistinguishable between both cells. Activation of protein kinase C (PKC) or loading with a PKC inhibitor failed to affect the swelling-induced activation of the Cl- currents in both cells. The relation between whole-cell Cl- currents and cell size measured simultaneously showed that volume sensitivity of the Cl- channel was augmented by the PGP expression irrespective of the activity of PKC on the plasma membrane. A similar increase in volume sensitivity of the Cl- channel was also induced by the expression of the ATP hydrolysis-deficient PGP mutant, K433M. We conclude that P-glycoprotein does not represent the volume-sensitive Cl- channel but that its expression modulates volume sensitivity of the Cl- channel in a manner independent of its ATPase activity or of the protein kinase C activity.

Identification of the in vivo phosphorylation sites for acidic-directed kinases in murine mdr1b P-glycoprotein

P-glycoprotein, the multidrug resistance transporter, is phosphorylated in vivo and the major phosphorylation domain has been identified as the linker region (amino acids 629-686). The linker region is a highly charged segment of the transporter in which the negative and positive amino acid side chains are spatially segregated. Both of these charged domains contain several consensus phosphorylation sites for protein kinases. Three of the consensus phosphorylation sites for basic-directed kinases in murine mdr1b P-glycoprotein are utilized in vivo and have been identified as serines 665, 669, and 681. Mutagenesis of all the consensus basic-directed kinase phosphorylation sites in the linker region of human MDR1 P-glycoprotein did not alter the ability of the mutated transporter to confer the multidrug resistance phenotype in stably transfected cell lines. These studies would suggest that phosphorylation/dephosphorylation within the basic domain of the linker region is not directly involved in regulation of drug transporter activity. We now report that the linker region of mdr1b P-glycoprotein is also phosphorylated in vivo within the acidic domain (amino acids 631-658). These sites have been mapped using casein kinase II, a prototypic acidic-directed kinase, and a recombinant mdr1b linker region peptide (amino acids 621-687). Electrospray mass spectrometry demonstrated that casein kinase II could introduce up to five phosphates into the recombinant peptide. Two-dimensional phosphopeptide mapping indicated that all the phosphates were contained in a tryptic peptide consisting of amino acids 631-658. Phosphopeptide mapping of in vivo labeled P-glycoprotein, isolated from either J7.V1-1, a murine vinblastine-resistant cell line, or HeLa cells stably transfected with mdr1b P-glycoprotein cDNA, revealed that this tryptic peptide was phosphorylated in both proteins.

Chemosensitization of cancer cells by the staurosporine derivative CGP 41251 in association with decreased P-glycoprotein phosphorylation

The multidrug resistance (MDR) phenotype of cancer cells often correlates with the level and activity of protein kinase C (PKC). We studied the ability of the staurosporine derivative PKC inhibitor CGP 41251 to reverse the MDR phenotype in MCF-7 human breast carcinoma and CT-26 murine colon adenocarcinoma cells and their doxorubicin (DXR)-selected MDR variants. Nontoxic concentrations of CGP 41251 significantly enhanced the cytotoxic properties of DXR, actinomycin D, vinblastine, and vincristine but not those of 5-fluorouracil. CGP 41251 increased intracellular concentrations of [14C]DXR but did not cause significant differences in P-glycoprotein (P-gp) expression. Pretreatment of MCF-7adr cells with phorbol 12-myristate 13-acetate reduced the CGP 41251 mediated intracellular accumulation of [14C]DXR. At concentrations that induced drug uptake, CGP 41251 significantly decreased the level of P-gp phosphorylation in the cells but did not compete with [3H]azidopine for photoaffinity labeling of P-gp. These data provide evidence that CGP 41251 reverses the MDR phenotype by modulating the phosphorylation of P-gp and/or other PKC substrates critical to the maintenance of the MDR phenotype.

Inhibition of protein kinase C in multidrug-resistant cells by modulators of multidrug resistance

We have evaluated the protein kinase C (PKC) activity in two series of cultured cell lines presenting the multidrug-resistance (MDR) phenotype and in the corresponding wild-type cells: the human KB 3.1, KB A1 and KB 8.5 cell lines, and the rat C6, C6 0.5 and C6 1V cell lines. We have observed an increase in PKC activity in the MDR cell lines of the KB cell lineage, proportional to their degree of resistance to doxorubicin. In contrast, the MDR cell lines of the C6 cell lineage presented no change (C6 0.5) or even decrease (C6 1V) in PKC activity; the basal level of PKC activity in C6 cells was, however, 50-fold higher than in KB 3.1 cells. We have tested, in these lines, the effect of four modulators of MDR: verapamil, cyclosporin A, quinine and S-9788, on doxorubicin acytotoxicity and on PKC activity. We observed that cyclosporin A and S-9788, which were the most active on MDR reversal, were able to inhibit PKC activity in the KB resistant lines as well as in all C6 lines, whereas verapamil and quinine had only marginal effects on PKC activity. The distribution of PKC isoenzymes was studied by Western blots. The PKC alpha, gamma and delta isoforms were increased in the KB resistant lines as compared to wild-type cells, which could account for the increase PKC activity we observed. In contrast, PKC alpha and gamma were decreased in C6 1V cells, as expected from the results obtained for total PKC activity, but we also noticed an important decrease in PKC delta in the C6 0.5 line. Our results suggest that an increase in PKC activity is not an absolute requirement for expression of MDR, provided that the basal level be high enough; and that some modulators may act on MDR, not only through direct P-glycoprotein interaction, but also through P-glycoprotein phosphorylation or expression. The distribution of PKC isoenzymes revealed that the modifications encountered between sensitive and resistant cells mainly concerned alpha, gamma and delta isoenzymes of PKC.

Inward transport of [3H]-1-methyl-4-phenylpyridinium in rat isolated hepatocytes: putative involvement of a P-glycoprotein transporter

1. The liver has an important role in the detoxification of organic cations from the circulation. [3H]-1-methyl-4-phenylpyridinium ([3H]-MPP+), a low molecular weight organic cation, is efficiently taken up and accumulated by rat hepatocytes through mechanisms partially unknown. 2. The aim of the present work was to characterize further the uptake of MPP+ by rat isolated hepatocytes. The putative interactions of a wide range of drugs, including inhibitors/substrates of P-glycoprotein, were studied. 3. The uptake of MPP+ was investigated in rat freshly isolated hepatocytes (incubated in Krebs-Henseleit medium with 200 nM [3H]-MPP+ for 5 min) and in the rat liver in situ (perfused with Krebs-Henseleit/BSA medium with 200 nM [3H]-MPP+ for 30 min). [3H]-MPP+ accumulation in the cells and in tissue was determined by liquid scintillation counting. 4. Verapamil (100 microM), quinidine (100 microM), amiloride (1 mM), (+)-tubocurarine (100 microM), vecuronium (45 microM), bilirubin (200 microM), progesterone (200 microM), daunomycin (100 microM), vinblastine (100 microM), cyclosporin A (100 microM) and cimetidine (100 microM) had a significant inhibitory effect on the accumulation of [3H]-MPP+ in isolated hepatocytes. Tetraethylammonium (100 microM) had no effect. 5. In the rat perfused liver, both cyclosporin A (100 microM) and verapamil (100 microM) had much less marked inhibitory effects as compared to their effects on isolated hepatocytes (0% against 35% and 45% against 96% of inhibition, respectively). 6. Inhibition of alkaline phosphatase activity by increasing or decreasing the pH of the incubation medium or by the presence of vanadate (1 mM) or homoarginine (500 microM) led to a significant increase in the accumulation of [3H]-MPP+ in isolated hepatocytes. 7. It was concluded that, in addition to the type I organic cation hepatic transporter, [3H]-MPP+ is taken up by rat hepatocytes through P-glycoprotein, a canalicular transport system that usually excretes endobiotics and xenobiotics. We proposed that the reversal of transport through P-glycoprotein may be related to the loss of efficacy of alkaline in isolated hepatocytes.

Effects of sphingosine stereoisomers on P-glycoprotein phosphorylation and vinblastine accumulation in multidrug-resistant MCF-7 cells

To investigate the role of protein kinase C (PKC) in the regulation of multidrug resistance and P-glycoprotein (P-gp) phosphorylation, the natural isomer of sphingosine (SPH), D-erythro sphingosine (De SPH), and its three unnatural stereoisomers were synthesized. The SPH isomers showed similar potencies as inhibitors of in vitro PKC activity and phorbol binding, with IC50 values of approximately 50 microM in both assays. Treatment of multidrug-resistant MCF-7ADR cells with SPH stereoisomers increased vinblastine (VLB) accumulation up to 6-fold at 50 microM but did not alter VLB accumulation in drug-sensitive MCF-7 wild-type (WT) cells or accumulation of 5-fluorouracil in either cell line. Phorbol dibutyrate treatment of MCF-7ADR cells increased phosphorylation of P-gp, and this increase was inhibited by prior treatment with SPH stereoisomers. Treatment of MCF-7ADR cells with SPH stereoisomers decreased basal phosphorylation of the P-gp, suggesting inhibition of PKC-mediated phosphorylation of P-gp. Most drugs that are known to reverse multidrug resistance, including several PKC inhibitors, have been shown to directly interact with P-gp and inhibit drug binding. SPH stereoisomers did not inhibit specific binding of [3H] VLB to MCF-7ADR cell membranes or [3H]azidopine photoaffinity labeling of P-gp or alter P-gp ATPase activity. These results suggest that SPH isomers are not substrates of P-gp and suggest that modulation of VLB accumulation by SPH stereoisomers is associated with inhibition of PKC-mediated phosphorylation of P-gp.

Protein kinase C-mediated phosphorylation does not regulate drug transport by the human multidrug resistance P-glycoprotein

P-glycoprotein (P-gp) is an active transporter that can confer multidrug resistance by pumping cytotoxic drugs out of cells and tumors. P-gp is phosphorylated at several sites in the "linker" region, which separates the two halves of the molecule. To examine the role of phosphorylation in drug transport, we mutated P-gp such that it could no longer be phosphorylated by protein kinase C (PKC). When expressed in yeast, the ability of the mutant proteins to confer drug resistance, or to mediate [3H]vinblastine accumulation in secretory vesicles, was indistinguishable from that of wild type P-gp. A matched pair of mammalian cell lines were generated expressing wild type P-gp and a non-phosphorylatable mutant protein. Mutation of the phosphorylation sites did not alter P-gp expression or its subcellular localization. The transport properties of the mutant and wild type proteins were indistinguishable. Thus, phosphorylation of the linker of P-gp by PKC does not affect the rate of drug transport. In light of these data, the use of agents that alter PKC activity to reverse multidrug resistance in the clinic should be considered with caution.

Characterization of phosphorylation-defective mutants of human P-glycoprotein expressed in mammalian cells

To assess the role of phosphorylation of the human multidrug resistance MDR1 gene product P-glycoprotein for its drug transport activity, phosphorylation sites within its linker region were subjected to mutational analysis. We constructed a 5A mutant, in which serines at positions 661, 667, 671, 675, and 683 were replaced by nonphosphorylatable alanine residues, and a 5D mutant carrying aspartic acid residues at the respective positions to mimic permanently phosphorylated serine residues. Transfection studies revealed that both mutants were targeted properly to the cell surface and conferred multidrug resistance by diminishing drug accumulation. In contrast to wild-type P-glycoprotein, the overexpressed 5A and the 5D mutants exhibited no detectable levels of phosphorylation, either in vivo following metabolic labeling of cells with [32P]orthophosphate or in vitro in phosphorylation assays with protein kinase C, cAMP-dependent protein kinase, or a P-glyco-protein-specific protein kinase purified from multidrug-resistant KB-V1 cells. These results reconfirm that the major P-glycoprotein phosphorylation sites are located within the linker region. Furthermore, the first direct evidence is provided that phosphorylation/dephosphorylation mechanisms do not play an essential role in the establishment of the multidrug resistance phenotype mediated by human P-glycoprotein.

P-glycoprotein, multidrug resistance and protein kinase C

The multidrug resistant (MDR) phenotype is a well-studied subject that has been recognized as a determinant underlying specific types of drug resistance in human cancer. Although it is clear that the P-glycoprotein plays a major role in MDR, it is not clear whether post-translational modifications such as phosphorylation have any major impact on its modulation. The laboratory of Dr. Bruce Chabner was one of the first to describe increased expression and activity of protein kinase C (PKC) associated with the MDR phenotype. Since that time, a similar correlation has been observed in many other MDR cell lines. Most of these studies have been performed with doxorubicin-selected cells that have acquired MDR and have shown increased PKC activity, mainly for PKC-alpha isoenzyme. Intrinsic MDR in human renal cell carcinoma lines has been shown to correlate directly with PKC activity, but further studies with intrinsic MDR cell lines are needed before any conclusions can be drawn. More recent evidence suggests that there is a complex biochemical process by which PKC isoenzymes differentially phosphorylate specific serine residues in the linker region of P-glycoprotein which may lead to alterations in P-glycoprotein ATPase and drug-binding functions. To further complicate matters, PKC plays an important role in anti-apoptotic pathways, which can confound the dissection and elucidation of drug-resistance mechanisms. However, these areas are still under active investigation and not fully answered. Further studies are needed to specifically answer the question of whether PKC directly modulates basal and/or drug-stimulated P-glycoprotein function. This manuscript reviews the majority of the literature on PKC and MDR, as well as offers caveats for interpretation of these studies to answer the above questions.

P-glycoprotein-mediated multidrug resistance: experimental and clinical strategies for its reversal

The study of the cellular, biochemical, and molecular biology and pharmacology of MDR has provided one of the most active and exciting areas within cancer research and one that holds great promise for translation into clinical benefit. While convincing evidence for the functional role of P-gp in mediating clinical drug resistance in humans remains elusive, studies of the clinical expression of P-gp and trials of chemosensitizers with cancer chemotherapy suggest "resistance modification" strategies may be effective in some tumors with intrinsic or acquired drug resistance. However, even if P-gp-associated MDR proves to be a relevant and reversible cause of clinical drug resistance, numerous problems remain to be solved before effective clinical chemosensitization may be achieved. Such factors as absorption, distribution, and metabolism; the effect of chemosensitizers on chemotherapeutic drug clearance; toxicity to normal tissues expressing P-gp; and the most efficacious modulator regimens all remain to be defined in vivo. Clearly, the identification of more specific, potent, and less clinically toxic chemosensitizers for clinical use remains critical to the possible success of this approach. Nonetheless, the finding that a number of pharmacological agents can antagonize a well-characterized form of experimental drug resistance provides promise for potential clinical applications. Further study of chemosensitizers in humans and the rational design of novel chemosensitizers with improved activity should define the importance of MDR in clinically resistant cancer.

Circumvention of P-glycoprotein-mediated multiple drug resistance by phosphorylation modulators is independent of protein kinases

Expression of P-glycoprotein by tumor cells confers resistance to multiple natural product drugs because of its ability to export these compounds. This transporter is a substrate for several protein kinases; however, the functional significance of its phosphorylation is not defined. We examined the effects of many activators and inhibitors of protein kinases on the activity of P-glycoprotein in drug-resistant human breast carcinoma cells (MCF-7/ADR). Several phorbol esters sensitized these cells to P-glycoprotein substrate drugs; however, there was no correlation with activation of protein kinase C. The 4 alpha- and 4 beta-isomers of phorbol 12-myristate 13-acetate were equally potent in sensitizing the cells to actinomycin D and daunomycin and in increasing the intracellular accumulation of [3H]vinblastine. These effects of 4 beta-phorbol myristate acetate required much higher concentrations than were needed to increase P-glycoprotein phosphorylation and were not antagonized by staurosporine. Similar to verapamil, the phorbol esters did not sensitize MCF-7/ADR cells to cisplatin, nor parental MCF-7 cells to any of the anticancer drugs. Mezerein, K-252a, and H-89 sensitized MCF-7/ADR cells, increased intracellular accumulation of [3H]vinblastine, and antagonized photolabeling of P-glycoprotein by [3H]azidopine. Therefore, phosphorylation does not appear to play a significant role in regulating P-glycoprotein activity in MCF-7/ADR cells.

Volume-sensitive chloride channel activity does not depend on endogenous P-glycoprotein

To determine whether endogenous P-glycoprotein, the MDR1 gene product that functions as a drug transport pump, is a volume-sensitive Cl- channel molecule or a protein kinase C-mediated regulator of the Cl- channel, whole-cell patch-clamp and molecular biological experiments were carried out in a human small intestinal epithelial cell line. Endogenous expression of P-glycoprotein was confirmed by Northern blot analysis, reverse transcription-polymerase chain reaction, Western blot analysis, and immunostaining. The P-glycoprotein expression was abolished by the antisense (but not sense) oligonucleotide for the MDR1 gene, whereas the magnitude of the Cl- current activated by osmotic swelling was not distinguishable between both antisense- and sense-treated cells. The volume-sensitive Cl- currents were not specifically affected by the anti-P-glycoprotein monoclonal antibodies, MRK16, C219, and UIC2. An inhibitor of P-glycoprotein-mediated pump activity, verapamil, was found to never affect the Cl- current. A substrate for the P-glycoprotein-mediated drug pump, vincristine or daunomycin, did not prevent swelling-induced activation of the Cl- current. Furthermore, the Cl- current was not affected by an activator of protein kinase C (12-O-tetradecanoylphorbol-13-acetate or 1-oleoyl-2-acetyl-sn-glycerol). Thus, it is concluded that the endogenous P-glycoprotein molecule is not itself a volume-sensitive Cl- channel nor a protein kinase C-mediated regulator of the channel in the human epithelial cells.

Partial inhibition of multidrug resistance by safingol is independent of modulation of P-glycoprotein substrate activities and correlated with inhibition of protein kinase C

Safingol is a lysosphingolipid protein kinase C (PKC) inhibitor that competitively interacts at the regulatory phorbol binding domain of PKC. We investigated the effects of safingol on antineoplastic drug sensitivity and PKC activity of MCF-7 tumor cell lines. Safingol treatment of 32P-labeled MCF-7 WT and MCF-7 DOXR cells inhibited phosphorylation of the myristoylated alanine-rich protein kinase C substrate in both cell lines, suggesting inhibition of cellular PKC. However, only in MCF-7 DOXR cells did safingol treatment increase accumulation of [3H]vinblastine and enhance toxicity of Vinca alkaloids and anthracyclines. Drug accumulation changes in MCF-7 DOXR cells treated with safingol were accompanied by inhibition of basal and phorbol 12,13-dibutyrate-stimulated phosphorylation of P-glycoprotein (P-gp). Expression of P-gp and levels of mdr1 message in MCF-7 DOXR cells were not altered by safingol treatment alone or in combination with vinblastine. Treatment of MCF-7 DOXR cell membranes with safingol did not inhibit [3H]vinblastine binding or [3H]azidopine photoaffinity labeling of P-gp. Furthermore, safingol did not stimulate P-gp ATPase activity in membranes prepared from MCF-7 DOXR cells. We conclude that enhanced drug accumulation and sensitivity in MCF-7 DOXR cells treated with safingol are correlated with inhibition of PKC rather than competitive interference with P-gp drug binding through direct interaction with P-glycoprotein.

Protein kinase C inhibitors reduce phorbol ester-induced resistance to methotrexate in Chinese hamster ovary cells

Phorbol 12-myristate 13-acetate (TPA) increases the number of colonies surviving methotrexate (MTX) exposure in a dose-dependent manner upon short incubation with Chinese hamster ovary (CHO) cells. Seventy percent of the isolated colonies showed increased copy number for the dihydrofolate reductase gene. EGTA prevents the increase in resistance triggered by TPA. Calcium ionophore A23187 and angiotensin II also increase this resistance, suggesting that calcium is involved in this process. Protein kinase C (PKC) from CHO cells is rapidly activated by TPA, A23187 and angiotensin II. PKC inhibitors, 1-(5-Isoquinolinylsulphonyl)-2-methyl-piperazine (H-7), glycyrrhetinic acid, staurosporine and calphostin C decrease the generation of resistant colonies to MTX upon incubation with TPA. However, 5 nM staurosporine on its own increases resistance to MTX while having the ability to translocate CHO PKC. In vitro, H-7, staurosporine and calphostin C inhibit PKC activity translocated by TPA incubation with CHO cells. We conclude that PKC, the activity of which is dependent on calcium and phospholipids, is part of the pathway that leads to development of increased resistance to MTX. Thus, inhibition of PKC prevents the appearance of this resistance. Our results suggest the possibility of using non-toxic PKC inhibitors as resistance modulators in MTX chemotherapy.

Antitumour effects and pharmacokinetics of combination of vinblastine with a staurosporine derivative, NA-382, in P388/ADR-bearing mice

The effects of a staurosporine derivative, N-ethoxycarbonyl-7-oxostaurosporine (NA-382), on the pharmacokinetics of vinblastine were evaluated, compared with those of verapamil, in multidrug-resistant P388/ADR-bearing mice. At first, the in-vitro experiments indicated that NA-382 permeated into the cells better and were more effective in combined cytotoxicity with vinblastine and on accumulation of vinblastine than with verapamil in P388/ADR cells. In combined intraperitoneal injection with vinblastine (200 micrograms kg-1) into P388/ADR-bearing mice, NA-382 in a suspension form (10 mg kg-1) prolonged the life-span of the mice near to that of P388/S-bearing mice treated with vinblastine alone, but verapamil even at the maximum tolerated dosage (30 mg kg-1) barely affected the in-vivo antitumour effect of vinblastine. When simultaneously administered with vinblastine to P388/ADR-bearing mice, NA-382 maintained significantly higher vinblastine levels in the tumour cells for 24 h and gave a larger area under the time-intracellular vinblastine concentration curve (0 to 24 h) than those receiving vinblastine alone, with long retention of the agent in ascitic fluid. Verapamil increased the cellular vinblastine content for only 6 h, accompanying a rapid elimination of the agent from the ascitic fluid. This study indicates that NA-382 is more effective against multidrug-resistance than verapamil, and its suspension is also advantageous for cancer chemotherapy of multidrug-resistant tumours.

Increased genotoxicity of acetylaminofluorene by modulators of multixenobiotic resistance mechanism: studies with the fresh water clam Corbicula fluminea

The presence of a 'multixenobiotic resistance' [MXR] mechanism in gills of the freshwater clam Corbicula fluminea was investigated. Western blot analyses of membrane vesicles from gills, applying antibodies to vertebrate P170 multidrug resistance (MDR) protein, revealed a 135 kDa immunoreactive protein. Verapamil caused a reduction of 3H-vincristine (3H-VCR) binding onto vesicles from clam. Exposure of clams to 3H-VCR in the presence of verapamil or staurosporine (STP) enhanced the accumulation of 3H-VCR over control values. Furthermore, clams were exposed instead to VCR, to a model carcinogen, 2-acetylaminofluorene (AAF), to determine the verapamil- and STP-dependent increase of single-strand breaks (SSBs) in DNA from gills of this organism. Verapamil caused no or little increase of SSBs induced by exposure to 0.01 or 0.10 microM AAF, respectively, as measured by the alkaline elution technique. In contrast, in the presence of STP a highly significant and dose-dependent enhancement of AAF-mediated SSBs was measured already at exposure to 0.01 microM AAF. These data indicate (i) that the clam C. fluminea is provided with a P-glycoprotein-like element of the MDR-mechanism, (ii) that this system can be poisoned by chemosensitizers such as verapamil and STP, (iii) the role of protein kinase C in the regulation of MXR function and (iv) the importance of the MXR modulators for the assessment of ecotoxicological effects of pollutants.

Effects of protein kinase C modulators on multidrug resistance in human glioma cells

To identify the role of protein kinase C (PKC) in multidrug resistance, the effects of phorbol-12-myristate-13-acetate (PMA), a PKC activator, or calphostin C, a PKC inhibitor, on intracellular vincristine accumulation and expression of P-glycoprotein phosphorylation were studied in one multidrug-resistant and three multidrug-sensitive human glioma cell lines. Basal PKC activities and immunoreactivities of PKC-alpha and -zeta were higher in multidrug-resistant cells than in multidrug-sensitive cells. There was no significant difference in the immunoreactivity of PKC-delta between multidrug-resistant and -sensitive cells, and immunoreactive PKC-beta, -gamma, and -epsilon were not detected in either multidrug-resistant or -sensitive cells. The treatment of multidrug-resistant cells with 100 nM PMA for 2 hours resulted in the activation not of PKC-zeta but of PKC-alpha, with concomitant decrease in vincristine accumulation and increase in P-glycoprotein phosphorylation. The exposure of multidrug-resistant cells to 100 nM PMA for 24 hours induced down-regulation not of PKC-zeta but of PKC-alpha, with concurrent decrease in vincristine accumulation, and reduced but still increased P-glycoprotein phosphorylation. The treatment of multidrug-resistant cells with 100 nM calphostin C for 2 hours decreased immunoreactive PKC-zeta and not immunoreactive PKC-alpha, inducing increase in vincristine accumulation, with concomitant decrease in P-glycoprotein phosphorylation. There was no evidence of significant change in vincristine accumulation in multidrug-sensitive cells treated with PMA or calphostin C. This may suggest that at least two isozymes of PKC, PKC-alpha and -zeta, are involved in P-glycoprotein phosphorylation and that vincristine efflux function in multidrug-resistant human glioma cells is closely associated with P-glycoprotein phosphorylation and is decreased by PKC inhibitor.

Effects of phosphorylation of P-glycoprotein on multidrug resistance

Cells expressing elevated levels of the membrane phosphoprotein P-glycoprotein exhibit a multidrug resistance phenotype. Studies involving protein kinase activators and inhibitors have implied that covalent modification of P-glycoprotein by phosphorylation may modulate its biological activity as a multidrug transporter. Most of these reagents, however, have additional mechanisms of action and may alter drug accumulation within multidrug resistant cells independent of, or in addition to, their effects on the state of phosphorylation of P-glycoprotein. The protein kinase(s) responsible for P-glycoprotein phosphorylation has(ve) not been unambiguously identified, although several possible candidates have been suggested. Recent biochemical analyses demonstrate that the major sites of phosphorylation are clustered within the linker region that connects the two homologous halves of P-glycoprotein. Mutational analyses have been initiated to confirm this finding. Preliminary data obtained from phosphorylation- and dephosphorylation-defective mutants suggest that phosphorylation of P-glycoprotein is not essential to confer multidrug resistance.

Regulation of protein kinase C and role in cancer biology

Protein kinase C (PKC) is a family of closely related lipid-dependent and diacyglycerol-activated isoenzymes known to play an important role in the signal transduction pathways involved in hormone release, mitogenesis and tumor promotion. Reversible activation of PKC by the second messengers diacylglycerol and calcium is an established model for the short term regulation of PKC in the immediate events of signal transduction. PKC can also be modulated long term by changes in the levels of activators or inhibitors for a prolonged period or by changes in the levels of functional PKC isoenzymes in the cell during development or in response to hormones and/or differentiation factors. Indeed, studies have indicated that the sustained activation or inhibition of PKC activity in vivo may play a critical role in regulation of long term cellular events such as proliferation, differentiation and tumorigenesis. In addition, these regulatory events are important in colon cancer, where a decrease in PKC activators and activity suggests PKC acts as an anti-oncogene, in breast cancer, where an increase in PKC activity suggests an oncogenic role for PKC, and in multidrug resistance (MDR) and metastasis where an increase in PKC activity correlates with increased resistance and metastatic potential. These studies highlight the importance and significance of regulation of PKC activity in vivo.