P-glycoprotein-independent decrease in drug accumulation by phorbol ester treatment of tumor cells

Protein Kinases C-Mediated Regulations of Drug Transporter Activity, Localization and Expression

Drug transporters are now recognized as major actors in pharmacokinetics, involved notably in drug–drug interactions and drug adverse effects. Factors that govern their activity, localization and expression are therefore important to consider. In the present review, the implications of protein kinases C (PKCs) in transporter regulations are summarized and discussed. Both solute carrier (SLC) and ATP-binding cassette (ABC) drug transporters can be regulated by PKCs-related signaling pathways. PKCs thus target activity, membrane localization and/or expression level of major influx and efflux drug transporters, in various normal and pathological types of cells and tissues, often in a PKC isoform-specific manner. PKCs are notably implicated in membrane insertion of bile acid transporters in liver and, in this way, are thought to contribute to cholestatic or choleretic effects of endogenous compounds or drugs. The exact clinical relevance of PKCs-related regulation of drug transporters in terms of drug resistance, pharmacokinetics, drug–drug interactions and drug toxicity remains however to be precisely determined. This issue is likely important to consider in the context of the development of new drugs targeting PKCs-mediated signaling pathways, for treating notably cancers, diabetes or psychiatric disorders.

Mussel blood cells, resistant to the cytotoxic effects of okadaic acid, do not express cell membrane p-glycoprotein activity (multixenobiotic resistance)

Okadaic acid (OA) is a dinoflagellate toxin, accumulating in shellfish and causing diarrhetic shellfish poisoning (DSP) in humans. OA is a highly cytotoxic agent in most cell lines because of its inhibiting properties of protein phosphatases. So far, the cytotoxicity of OA in mussels, the main vectors of DSP, has not been investigated. In this paper, the viability of mussel (Mytilus edulis) blood cells incubated in 10 nM-1 microM OA was studied. After 72 h of exposure, viability was reduced to 54% in 1 microM OA compared with 88% in control cells. This yielded a LC50 of >1 microM for OA, which is 30-1000-times higher compared with other cell types. It was hypothesised that P-glycoprotein (p-gp) activity (multixenobiotic resistance, MXR) contributed to the resistance to OA. Vincristine and rhodamine B was used as p-gp substrates and verapamil or staurosporine (ST) as inhibitors of p-gp transport. However, no indications of cell membrane p-gp activity were detected. Instead, experimental observations led to the conclusion that a MXR transport system was present within lysosomal membranes. Various concentrations of OA did not affect the dynamics of vincristine in blood cells. As a positive control for the assay, p-gp activity was measured in mussel gill tissue. The efflux of rhodamine B was reduced by verapamil, which is, considered evidence for cell membrane p-gp activity, thus the accuracy of the method was confirmed. Rhodamine B efflux was also reduced by OA in gill tissue, which suggested that OA is either a competitive substrate or inhibitor of p-gp activity. When the volume of the lysosomal compartment was measured in blood cells pre-exposed to OA, a significant increase was detected compared with control cells. It was proposed that uptake and storage of OA within the lysosomal system might protect mussel blood cells from the cytotoxic effects of this compound.

Effects of ATP depletion and phosphate analogues on P-glycoprotein conformation in live cells

P-glycoprotein (Pgp), a membrane pump often responsible for the multidrug resistance of cancer cells, undergoes conformational changes in the presence of substrates/modulators, or upon ATP depletion, reflected by its enhanced reactivity with the UIC2 monoclonal antibody. When the UIC2-shift was elicited by certain modulators (e.g. cyclosporin A or vinblastine, but not with verapamil or Tween 80), the subsequent binding of other monoclonal anti-Pgp Ig sharing epitopes with UIC2 (e.g. MM12.10) was abolished [Nagy, H., Goda, K., Arceci, R., Cianfriglia, M., Mechetner, E. & Szabó Jr, G. (2001) Eur. J. Biochem. 268, 2416-2420]. To further study the relationship between UIC2-shift and the suppression of MM12.10 binding, we compared, on live cells, how ATP depletion and treatment of cells with phosphate analogues (sodium orthovanadate, beryllium fluoride and fluoro-aluminate) that trap nucleotides at the catalytic site, affect the two phenomena. Similarly to modulators or ATP depleting agents, all the phosphate analogues increased daunorubicin accumulation in Pgp-expressing cells. Prelabeling of ATP depleted cells with UIC2 completely abolished the subsequent binding of MM12.10, in accordance with the enhanced binding of the first mAb. Vanadate and beryllium fluoride, but not fluoro-aluminate, reversed the effect of cyclosporin A, preventing UIC2 binding and allowing for labeling of cells with MM12.10. Thus, changes in UIC2 reactivity are accompanied by complementary changes in MM12.10 binding also in response to direct modulation of the ATP-binding site, confirming that conformational changes intrinsic to the catalytic cycle are reflected by both UIC2-related phenomena. These data also fit a model where the UIC2 epitope is available for antibody binding throughout the catalytic cycle including the step of ATP binding, to become unavailable only in the catalytic transition state.

"Pros and cons" on how to measure multidrug resistance in leukemias

Drug resistance is one of the most significant challenges in the treatment of various types of malignancies, however most of the experimental and clinical data in multidrug resistance (MDR) has been obtained in leukemias. MDR is the term that describes innate or acquired resistance of tumor cells to a wide range of anticancer drugs. As its presence determines treatment outcome in several forms of leukemias, it is imperative that clinical laboratories provide the most useful data on its expression. Here, a brief review is provided on the pathomechanism and diagnostics of MDR. From the diagnostic point of view it is fortunate that MDR proteins display similar effluxing activity towards many dissimilar agents some of which can be used in fluorescent assays. These tests mimic the real clinical problem i.e. the extrusion activity of MDR proteins towards xenobiotics. Thus, we believe that functional assays when carried out in a standardized way and particularly combined with labeling for various surface markers can be recommended as a front-line test in MDR measurement.

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.

In vitro transepithelial drug transport by on-line measurement: cellular control of paracellular and transcellular transport

Studies on transcellular transport across epithelial cell layers are performed mostly by discontinuous sampling of the transported compound. This has several drawbacks, e.g., it gives disturbances in volume, it limits the time-resolution, and is often laborious. In this report we introduce a method to measure transepithelial transport of fluorescent compounds continuously. The time-resolution is at the (sub)minute scale, allowing the measurement of the change in transport rate before and after transport modulation. We will describe how we used the method to measure transcellular and paracellular transport. For highly membrane-impermeable compounds, the paracellular transport and the regulation of the tight junctions was studied in wild-type and MDR1 cDNA transfected epithelial canine kidney cells (MDCKII). The effect of the multidrug transporter P-glycoprotein (Pgp) on the transepithelial transport was studied. Addition of the Pgp inhibitor SDZ PSC 833 showed a modulation of the idarubicin (IDA) and daunorubicin (DNR) transport, which was larger during transport from the basolateral to the apical side than in the reverse direction. By modeling the transepithelial transport, we found that in these cells Pgp had more effect on the basolateral to apical transport than vice versa, which can be attributed to a relatively large passive permeation coefficient for the cellular basolateral plasma membrane.