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

Protein Kinase C Epsilon Overexpression Is Associated With Poor Patient Outcomes in AML and Promotes Daunorubicin Resistance Through p-Glycoprotein-Mediated Drug Efflux

The protein kinase C (PKC) family of serine/threonine kinases are pleiotropic signaling regulators and are implicated in hematopoietic signaling and development. Only one isoform however, PKCϵ, has oncogenic properties in solid cancers where it is associated with poor outcomes. Here we show that PKCϵ protein is significantly overexpressed in acute myeloid leukemia (AML; 37% of patients). In addition, PKCϵ expression in AML was associated with a significant reduction in complete remission induction and disease-free survival. Examination of the functional consequences of PKCϵ overexpression in normal human hematopoiesis, showed that PKCϵ promotes myeloid differentiation, particularly of the monocytic lineage, and decreased colony formation, suggesting that PKCϵ does not act as an oncogene in hematopoietic cells. Rather, in AML cell lines, PKCϵ overexpression selectively conferred resistance to the chemotherapeutic agent, daunorubicin, by reducing intracellular concentrations of this agent. Mechanistic analysis showed that PKCϵ promoted the expression of the efflux pump, P-GP (ABCB1), and that drug efflux mediated by this transporter fully accounted for the daunorubicin resistance associated with PKCϵ overexpression. Analysis of AML patient samples also showed a link between PKCϵ and P-GP protein expression suggesting that PKCϵ expression drives treatment resistance in AML by upregulating P-GP expression.

The complexities of PKCα signaling in cancer

Protein kinase C α (PKCα) is a ubiquitously expressed member of the PKC family of serine/threonine kinases with diverse functions in normal and neoplastic cells. Early studies identified anti-proliferative and differentiation-inducing functions for PKCα in some normal tissues (e.g., regenerating epithelia) and pro-proliferative effects in others (e.g., cells of the hematopoietic system, smooth muscle cells). Additional well documented roles of PKCα signaling in normal cells include regulation of the cytoskeleton, cell adhesion, and cell migration, and PKCα can function as a survival factor in many contexts. While a majority of tumors lose expression of PKCα, others display aberrant overexpression of the enzyme. Cancer-related mutations in PKCα are uncommon, but rare examples of driver mutations have been detected in certain cancer types (e. g., choroid gliomas). Here we review the role of PKCα in various cancers, describe mechanisms by which PKCα affects cancer-related cell functions, and discuss how the diverse functions of PKCα contribute to tumor suppressive and tumor promoting activities of the enzyme. We end the discussion by addressing mutations and expression of PKCα in tumors and the clinical relevance of these findings.

Molecular Regulation of Canalicular ABC Transporters

The ATP-binding cassette (ABC) transporters expressed at the canalicular membrane of hepatocytes mediate the secretion of several compounds into the bile canaliculi and therefore play a key role in bile secretion. Among these transporters, ABCB11 secretes bile acids, ABCB4 translocates phosphatidylcholine and ABCG5/G8 is responsible for cholesterol secretion, while ABCB1 and ABCC2 transport a variety of drugs and other compounds. The dysfunction of these transporters leads to severe, rare, evolutionary biliary diseases. The development of new therapies for patients with these diseases requires a deep understanding of the biology of these transporters. In this review, we report the current knowledge regarding the regulation of canalicular ABC transporters' folding, trafficking, membrane stability and function, and we highlight the role of molecular partners in these regulating mechanisms.

Acute regulation of apical ABC transporters in the gut. Potential influence on drug bioavailability

The extensive intestinal surface offers an advantage regarding nutrient, ion and water absorptive capacity but also brings along a high exposition to xenobiotics, including drugs of therapeutic use and food contaminants. After absorption of these compounds by the enterocytes, apical ABC transporters play a key role in secreting them back to the intestinal lumen, hence acting as a transcellular barrier. Rapid and reversible modulation of their activity is a subject of increasing interest for pharmacologists. On the one hand, a decrease in transporter activity may result in increased absorption of therapeutic agents given orally. On the other hand, an increase in transporter activity would decrease their absorption and therapeutic efficacy. Although of less relevance, apical ABC transporters also contribute to disposition of drugs systemically administered. This review article summarizes the present knowledge on the mechanisms aimed to rapidly regulate the activity of the main apical ABC transporters of the gut: multidrug resistance protein 1 (MDR1), multidrug resistance-associated protein 2 (MRP2) and breast cancer resistance protein (BCRP). Regulation of these mechanisms by drugs, drug delivery systems, drug excipients and nutritional components are particularly considered. This information could provide the basis for controlled regulation of bioavailability of therapeutic agents and at the same time would help to prevent potential drug-drug interactions.

Linker Domains: Why ABC Transporters 'Live in Fragments no Longer'

ATP-binding cassette (ABC) transporters are membrane proteins present in all kingdoms of life. We have considered the disordered region that connects the N- and C-terminal halves in many eukaryotic ABC transporters, allowing all four consensus functional domains to be linked. The recent availability of structures of ABC transporters containing linker regions has allowed us to identify the start and end points of the connectors as well as hinting at their localisation. We address questions such as: Where did the linker regions come from? Why do some ABC transporters have connectors and others not? What are the rules and roles of the linker regions? What are the consequences of mutations in these connector regions for disease in humans?

Regulation of CP-25 on P-glycoprotein in synoviocytes of rats with adjuvant arthritis

OBJECTIVE

Methotrexate (MTX) is a commonly used drug for the treatment of rheumatoid arthritis (RA) and it has been studied in RA resistance recently. P-glycoprotein (P-gp) is one of the important transporters that mediate MTX resistance. This study investigated the effect of Paeoniflorin-6'-O-benzene sulfonate (code: CP-25) in the resistance of P-gp-mediated MTX to RA.

METHODS

Adjuvant arthritis (AA) was induced in rats via complete Freund's adjuvant. The experimental groups were divided into normal group; AA model group; monotherapy groups, including CP-25, MTX and dexamethasone; and CP-25 combined with MTX group. The expression of P-gp in synovial tissue was measured by western blot and histochemistry. Besides, P-gp high expression of human hepatoma cell line Bel7402/5-FU and Bel7402 were chose to study in MTX resistance and the function of P-gp was detected by Flow cytometry.

RESULTS

CP-25 had a good therapeutic effect on AA rats, significantly improved manifestations and reduced the expression of P-gp in synovial tissue, spleen medulla and small intestinal epithelial cells in the apical tissues of AA rats. In addition, CP-25 significantly inhibited the up-regulation of P-gp induced by TNF-α stimulation in synoviocytes. Furthermore, according to the accumulation and efflux of rhodamine 123 in Bel7402/5-FU resistant cells and Bel7402 sensitive cells, CP-25 could reverse the resistance of MTX in Bel7402/5-FU cells compared with Bel7402 cells, which was reflected by the reduced IC50 values of MTX. Further study indicated that CP-25 could decrease P-gp expression and inhibit P-gp function in Bel7402/5-FU cells.

CONCLUSION

CP-25 regulates the expression of P-gp and inhibits the function of P-gp, thereby improving the resistance of MTX.

Post-translational modifications of transporters

Drug transporter proteins are critical to the distribution of a wide range of endogenous compounds and xenobiotics such as hormones, bile acids, peptides, lipids, sugars, and drugs. There are two classes of drug transporters- the solute carrier (SLC) transporters and ATP-binding cassette (ABC) transporters -which predominantly differ in the energy source utilized to transport substrates across a membrane barrier. Despite their hydrophobic nature and residence in the membrane bilayer, drug transporters have dynamic structures and adopt many conformations during the translocation process. Whereas there is significant literature evidence for the substrate specificity and structure-function relationship for clinically relevant drug transporters proteins, there is less of an understanding in the regulatory mechanisms that contribute to the functional expression of these proteins. Post-translational modifications have been shown to modulate drug transporter functional expression via a wide range of molecular mechanisms. These modifications commonly occur through the addition of a functional group (e.g. phosphorylation), a small protein (e.g. ubiquitination), sugar chains (e.g. glycosylation), or lipids (e.g. palmitoylation) on solvent accessible amino acid residues. These covalent additions often occur as a result of a signaling cascade and may be reversible depending on the type of modification and the intended fate of the signaling event. Here, we review the significant role in which post-translational modifications contribute to the dynamic regulation and functional consequences of SLC and ABC drug transporters and highlight recent progress in understanding their roles in transporter structure, function, and regulation.

Mycobacterium tuberculosis universal stress protein Rv2623 interacts with the putative ATP binding cassette (ABC) transporter Rv1747 to regulate mycobacterial growth

We have previously shown that the Mycobacterium tuberculosis universal stress protein Rv2623 regulates mycobacterial growth and may be required for the establishment of tuberculous persistence. Here, yeast two-hybrid and affinity chromatography experiments have demonstrated that Rv2623 interacts with one of the two forkhead-associated domains (FHA I) of Rv1747, a putative ATP-binding cassette transporter annotated to export lipooligosaccharides. FHA domains are signaling protein modules that mediate protein-protein interactions to modulate a wide variety of biological processes via binding to conserved phosphorylated threonine (pT)-containing oligopeptides of the interactors. Biochemical, immunochemical and mass spectrometric studies have shown that Rv2623 harbors pT and specifically identified threonine 237 as a phosphorylated residue. Relative to wild-type Rv2623 (Rv2623WT), a mutant protein in which T237 has been replaced with a non-phosphorylatable alanine (Rv2623T237A) exhibits decreased interaction with the Rv1747 FHA I domain and diminished growth-regulatory capacity. Interestingly, compared to WT bacilli, an M. tuberculosis Rv2623 null mutant (ΔRv2623) displays enhanced expression of phosphatidyl-myo-inositol mannosides (PIMs), while the ΔRv1747 mutant expresses decreased levels of PIMs. Animal studies have previously shown that ΔRv2623 is hypervirulent, while ΔRv1747 is growth-attenuated. Collectively, these data have provided evidence that Rv2623 interacts with Rv1747 to regulate mycobacterial growth; and this interaction is mediated via the recognition of the conserved Rv2623 pT237-containing FHA-binding motif by the Rv1747 FHA I domain. The divergent aberrant PIM profiles and the opposing in vivo growth phenotypes of ΔRv2623 and ΔRv1747, together with the annotated lipooligosaccharide exporter function of Rv1747, suggest that Rv2623 interacts with Rv1747 to modulate mycobacterial growth by negatively regulating the activity of Rv1747; and that Rv1747 might function as a transporter of PIMs. Because these glycolipids are major mycobacterial cell envelope components that can impact on the immune response, our findings raise the possibility that Rv2623 may regulate bacterial growth, virulence, and entry into persistence, at least in part, by modulating the levels of bacillary PIM expression, perhaps through negatively regulating the Rv1747-dependent export of the immunomodulatory PIMs to alter host-pathogen interaction, thereby influencing the fate of M. tuberculosis in vivo.

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.

Enzastaurin inhibits ABCB1-mediated drug efflux independently of effects on protein kinase C signalling and the cellular p53 status

The PKCβ inhibitor enzastaurin was tested in parental neuroblastoma and rhabdomyosarcoma cell lines, their vincristine-resistant sub-lines, primary neuroblastoma cells, ABCB1-transduced, ABCG2-transduced, and p53-depleted cells. Enzastaurin IC₅₀s ranged from 3.3 to 9.5 μM in cell lines and primary cells independently of the ABCB1, ABCG2, or p53 status. Enzastaurin 0.3125 μM interfered with ABCB1-mediated drug transport. PKCα and PKCβ may phosphorylate and activate ABCB1 under the control of p53. However, enzastaurin exerted similar effects on ABCB1 in the presence or absence of functional p53. Also, enzastaurin inhibited PKC signalling only in concentrations ≥ 1.25 μM. The investigated cell lines did not express PKCβ. PKCα depletion reduced PKC signalling but did not affect ABCB1 activity. Intracellular levels of the fluorescent ABCB1 substrate rhodamine 123 rapidly decreased after wash-out of extracellular enzastaurin, and enzastaurin induced ABCB1 ATPase activity resembling the ABCB1 substrate verapamil. Computational docking experiments detected a direct interaction of enzastaurin and ABCB1. These data suggest that enzastaurin directly interferes with ABCB1 function. Enzastaurin further inhibited ABCG2-mediated drug transport but by a different mechanism since it reduced ABCG2 ATPase activity. These findings are important for the further development of therapies combining enzastaurin with ABC transporter substrates.

Distinct conformational spectrum of homologous multidrug ABC transporters

ATP-binding cassette (ABC) exporters are ubiquitously found in all kingdoms of life and their members play significant roles in mediating drug pharmacokinetics and multidrug resistance in the clinic. Significant questions and controversies remain regarding the relevance of their conformations observed in X-ray structures, their structural dynamics, and mechanism of transport. Here, we used single particle electron microscopy (EM) to delineate the entire conformational spectrum of two homologous ABC exporters (bacterial MsbA and mammalian P-glycoprotein) and the influence of nucleotide and substrate binding. Newly developed amphiphiles in complex with lipids that support high protein stability and activity enabled EM visualization of individual complexes in a membrane-mimicking environment. The data provide a comprehensive view of the conformational flexibility of these ABC exporters under various states and demonstrate not only similarities but striking differences between their mechanistic and energetic regulation of conformational changes.

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.

Molecular analysis of the multidrug transporter, P-glycoprotein

Inherent or acquired resistance of tumor cells to cytotoxic drugs represents a major limitation to the successful chemotherapeutic treatment of cancer. During the past three decades dramatic progress has been made in the understanding of the molecular basis of this phenomenon. Analyses of drug-selected tumor cells which exhibit simultaneous resistance to structurally unrelated anti-cancer drugs have led to the discovery of the human MDR1 gene product, P-glycoprotein, as one of the mechanisms responsible for multidrug resistance. Overexpression of this 170 kDa N-glycosylated plasma membrane protein in mammalian cells has been associated with ATP-dependent reduced drug accumulation, suggesting that P-glycoprotein may act as an energy-dependent drug efflux pump. P-glycoprotein consists of two highly homologous halves each of which contains a transmembrane domain and an ATP binding fold. This overall architecture is characteristic for members of the ATP-binding cassette or ABC superfamily of transporters. Cell biological, molecular genetic and biochemical approaches have been used for structure-function studies of P-glycoprotein and analysis of its mechanism of action. This review summarizes the current status of knowledge on the domain organization, topology and higher order structure of P-glycoprotein, the location of drug- and ATP binding sites within P-glycoprotein, its ATPase and drug transport activities, its possible functions as an ion channel, ATP channel and lipid transporter, its potential role in cholesterol biosynthesis, and the effects of phosphorylation on P-glycoprotein activity.

Regulation of ABCB1/PGP1-catalysed auxin transport by linker phosphorylation

Polar transport of the plant hormone auxin is controlled by PIN- and ABCB/PGP-efflux catalysts. PIN polarity is regulated by the AGC protein kinase, PINOID (PID), while ABCB activity was shown to be dependent on interaction with the FKBP42, TWISTED DWARF1 (TWD1). Using co-immunoprecipitation (co-IP) and shotgun LC-MS/MS analysis, we identified PID as a valid partner in the interaction with TWD1. In-vitro and yeast expression analyses indicated that PID specifically modulates ABCB1-mediated auxin efflux in an action that is dependent on its kinase activity and that is reverted by quercetin binding and thus inhibition of PID autophosphorylation. Triple ABCB1/PID/TWD1 co-transfection in tobacco revealed that PID enhances ABCB1-mediated auxin efflux but blocks ABCB1 in the presence of TWD1. Phospho-proteomic analyses identified S634 as a key residue of the regulatory ABCB1 linker and a very likely target of PID phosphorylation that determines both transporter drug binding and activity. In summary, we provide evidence that PID phosphorylation has a dual, counter-active impact on ABCB1 activity that is coordinated by TWD1-PID interaction.

Regulation of ABC transporter function via phosphorylation by protein kinases

ATP-binding cassette (ABC) transporters are multispanning membrane proteins that utilize ATP to move a broad range of substrates across cellular membranes. ABC transporters are involved in a number of human disorders and diseases. Overexpression of a subset of the transporters has been closely linked to multidrug resistance in both bacteria and viruses and in cancer. A poorly understood and important aspect of ABC transporter biology is the role of phosphorylation as a mechanism to regulate transporter function. In this review, we summarize the current literature addressing the role of phosphorylation in regulating ABC transporter function. A comprehensive list of all the phosphorylation sites that have been identified for the human ABC transporters is presented, and we discuss the role of individual kinases in regulating transporter function. We address the potential pitfalls and difficulties associated with identifying phosphorylation sites and the corresponding kinase(s), and we discuss novel techniques that may circumvent these problems. We conclude by providing a brief perspective on studying ABC transporter phosphorylation.

Rapid loss of blood-brain barrier P-glycoprotein activity through transporter internalization demonstrated using a novel in situ proteolysis protection assay

Blood-brain barrier (BBB) P-glycoprotein activity is rapidly reduced by vascular endothelial growth factor (VEGF) acting via Src and by tumor necrosis factor-alpha acting via protein kinase C (PKC)beta1. To probe underlying mechanism(s), we developed an in vivo, immunoblot-based proteinase K (PK) protection assay to assess the changes in the P-glycoprotein content of the BBB's luminal membrane. Infusion of PK into the brain vasculature selectively cleaved luminal membrane P-glycoprotein, leaving intracellular proteins intact. Intracerebroventricular injection of VEGF partially protected P-glycoprotein from proteolytic cleavage, consistent with transporter internalization. Activation of PKCbeta1 did not protect P-glycoprotein. Thus, VEGF and PKCbeta1 reduce P-glycoprotein activity by distinct mechanisms.

Pim-1 kinase protects P-glycoprotein from degradation and enables its glycosylation and cell surface expression

The oncogenic serine/threonine kinase Pim-1 phosphorylates and activates the ATP-binding cassette transporter breast cancer resistance protein (ABCG2). The ABC transporter P-glycoprotein (Pgp; ABCB1) also contains a Pim-1 phosphorylation consensus sequence, and we hypothesized that Pim-1 also regulates Pgp. Pgp is exported from the endoplasmic reticulum (ER) as a 150-kDa species that is glycosylated to 170-kDa Pgp, translocates to the cell surface, and mediates drug efflux; alternatively, 150-kDa Pgp is cleaved to a 130-kDa proteolytic product by ER proteases or undergoes ubiquitination and proteasomal degradation. Pim-1 and Pgp interaction was studied in GST pull-down and phosphorylation in in vitro kinase assays. Pim-1 knockdown and inhibition effects on Pgp expression were studied by immunoblotting and flow cytometry and on Pgp stability by immunoblotting after cycloheximide treatment. Pim-1 directly interacted with and phosphorylated Pgp in intact cells and in vitro. Pim-1 knockdown or inhibition decreased cellular and cell surface 170-kDa Pgp, in association with both transient increase in 130-kDa Pgp and increased Pgp ubiquitination and proteasomal degradation. Pim-1 inhibition also decreased expression of 150-kDa Pgp in the presence of the glycosylation inhibitor 2-deoxy-d-glucose. Finally, Pim-1 inhibition sensitized Pgp-overexpressing cells to doxorubicin. Thus, Pim-1 regulates Pgp expression by protecting 150-kDa Pgp from proteolytic and proteasomal degradation and enabling Pgp glycosylation and cell surface translocation and thus Pgp-mediated drug efflux. Pim-1 inhibitors are entering clinical trials and may provide a novel approach to abrogating drug resistance.

Activation of PKC isoform beta(I) at the blood-brain barrier rapidly decreases P-glycoprotein activity and enhances drug delivery to the brain

P-glycoprotein is an ATP (adenosine triphosphate)-driven drug efflux transporter that is highly expressed at the blood-brain barrier (BBB) and is a major obstacle to the pharmacotherapy of central nervous system diseases, including brain tumors, neuro-AIDS, and epilepsy. Previous studies have shown that P-glycoprotein transport activity in rat brain capillaries is rapidly reduced by the proinflammatory cytokine, tumor necrosis factor-alpha (TNF-alpha) acting through protein kinase C (PKC)-dependent signaling. In this study, we used isolated rat brain capillaries to show that the TNF-alpha-induced reduction of P-glycoprotein activity was prevented by a PKCbeta(I/II) inhibitor, LY333531, and mimicked by a PKCbeta(I/II) activator, 12-deoxyphorbol-13-phenylacetate-20-acetate (dPPA). Western blotting of brain capillary extracts with phospho-specific antibodies showed that dPPA activated PKCbeta(I), but not PKCbeta(II). Moreover, in intact rats, intracarotid infusion of dPPA potently increased brain accumulation of the P-glycoprotein substrate, [(3)H]-verapamil without compromising tight junction integrity. Thus, PKCbeta(I) activation selectively reduced P-glycoprotein activity both in vitro and in vivo. Targeting PKCbeta(I) at the BBB may prove to be an effective strategy for enhancing the delivery of small molecule therapeutics to the brain.

Correlation between apical localization of Abcc2/Mrp2 and phosphorylation status of ezrin in rat intestine

The multidrug resistance-associated protein 2/ATP-binding cassette transporter family C2 (Mrp2/Abcc2) is an ATP-dependent export pump that mediates the transport of a variety of organic anions. Abcc2 is mainly expressed on the canalicular membrane of hepatocytes and also the brush-border membrane of intestinal epithelial cells. We have previously reported that Abcc2 is rapidly internalized from the canalicular membrane during acute oxidative stress, which induces protein kinase C (PKC) activation in rat liver. However, it has not been elucidated whether PKC is involved in the regulation of Abcc2 localization in other tissues. In this study, we investigated this issue in rat intestinal epithelia. Exposure to thymeleatoxin, a conventional PKC (cPKC) activator, for 20 min reduced the cumulative glutathione S-bimane efflux for 40 min via Abcc2 from 30.3 +/- 2.1 nmol/cm to 18.1 +/- 1.6 nmol/cm. Likewise, the Abcc2 expression in the brush-border membrane of the small intestine was reduced to half that of the control without changing the total amount of Abcc2 present in the homogenate. Immunoprecipitation analysis suggested an interaction between Abcc2 and ezrin, a scaffolding protein that is dominantly expressed in the intestine. Thymeleatoxin treatment decreased the amount of the active form (C-terminally phosphorylated form) of ezrin and the amount of Abcc2 that coimmunoprecipitated with ezrin. These results indicate that cPKC activation diminishes the protein-protein interaction between ezrin and Abcc2. In conclusion, the phosphorylation status of ezrin correlates with the cell surface expression of Abcc2 in the rat small intestine, which may be regulated by cPKC.

Curcumin as chemosensitizer

This overview presents curcumin as a significant chemosensitizer in cancer chemotherapy. Although the review focuses on curcumin and its analogues on multidrug resistance (MDR) reversal, the relevance of curcumin as a nuclear factor (NF)-KB blocker and sensitizer of many chemoresistant cancer cell lines to chemotherapeutic agents will also be discussed. One of the major mechanisms of MDR is the enhanced ability of tumor cells to actively efflux drugs, leading to a decrease in cellular drug accumulation below toxic levels. Active drug efflux is mediated by several members of the ATP-binding cassette (ABC) superfamily of membrane transporters, which have now been subdivided into seven families designated A through G. Among these ABC families, the classical MDR is attributed to the elevated expression of ABCB1 (Pgp), ABCC1 (MRP1), and ABCG2 (MXR). The clinical importance of Pgp, MRP1, and MXR for MDR and cancer treatment has led to the investigation of the inhibiting properties of several compounds on these transporters. At present, due in part to the disappointing results associated with the many side effects of synthetic modulators that have been used in clinical trials, current research efforts are directed toward the identification of novel compounds, with attention to dietary natural products. The advantage is that they exhibit little or virtually no side effects and do not further increase the patient's medication burden.

Targeting the protein kinase C family: are we there yet?

Protein kinase C (PKC) comprises a family of serine/threonine kinases that are involved in the transduction of signals for cell proliferation, differentiation, apoptosis and angiogenesis. Unsurprisingly, disruption of PKC regulation is implicated in tumorigenesis and drug resistance. PKC function is complex in this context owing to the differing roles of individual isozymes within the cell and across tumour types. Therapeutically targeting PKC isozymes is not new; however, with many of the early PKC inhibitor cytotoxic drug combinations being discarded at the phase II level, and recent phase III studies in non-small-cell lung cancer proving negative, what's going wrong?

Human multidrug resistance ABCB and ABCG transporters: participation in a chemoimmunity defense system

In this review we give an overview of the physiological functions of a group of ATP binding cassette (ABC) transporter proteins, which were discovered, and still referred to, as multidrug resistance (MDR) transporters. Although they indeed play an important role in cancer drug resistance, their major physiological function is to provide general protection against hydrophobic xenobiotics. With a highly conserved structure, membrane topology, and mechanism of action, these essential transporters are preserved throughout all living systems, from bacteria to human. We describe the general structural and mechanistic features of the human MDR-ABC transporters and introduce some of the basic methods that can be applied for the analysis of their expression, function, regulation, and modulation. We treat in detail the biochemistry, cell biology, and physiology of the ABCB1 (MDR1/P-glycoprotein) and the ABCG2 (MXR/BCRP) proteins and describe emerging information related to additional ABCB- and ABCG-type transporters with a potential role in drug and xenobiotic resistance. Throughout this review we demonstrate and emphasize the general network characteristics of the MDR-ABC transporters, functioning at the cellular and physiological tissue barriers. In addition, we suggest that multidrug transporters are essential parts of an innate defense system, the "chemoimmunity" network, which has a number of features reminiscent of classical immunology.

Therapeutic resistance in lung cancer

Despite considerable progress over the last 25 years in the systemic therapy of lung cancer, intrinsic and acquired resistance to chemotherapeutic agents and radiation remains a vexing problem. The number of mechanisms of therapeutic resistance in lung cancer has expanded considerably over the past three decades, and the crucial role of stress resistance pathways is increasingly recognised as a cause of intrinsic and acquired chemo- and radiotherapy resistance. This paper reviews recent evidence for stress defence proteins, particularly RALBP1/RLIP76, in mediating intrinsic and acquired chemotherapy and radiation resistance in human lung cancer.

[Multidrug resistance in uveal melanoma]

In spite of important progress in the local treatment of uveal melanoma, the most frequent primitive intraocular tumor, 15%-30% of patients still die because of tumor metastasis. This tumor is characterized by constitutive chemoresistance, thwarting any attempt to control it using the usual chemotherapy protocols. The chemoresistance of uveal melanoma is mainly due to the typical multidrug resistance phenotype (MDR), which is linked to overexpression of membrane proteins that actively extrude anticancer drugs from the cell. Typical MDR is particularly complex in this tumor since several chemoresistance-related proteins are simultaneously produced. The negative prognostic significance of the overexpression of P-glycoprotein, the main representative among the typical MDR-related proteins, was shown in uveal melanoma. The atypical MDR phenotype, which refers to other chemoresistance mechanisms such as resistance to apoptosis also contributes to the chemoresistance of uveal melanoma. Thanks to the recent progress in molecular biology, the chemosensitization strategies of gene therapy approaches, which aim at weakening the pathological activity of MDR genes in cancer cells, are currently on the rise. This approach will disrupt current therapeutic strategies and necessarily improve and standardize the methods used to characterize the chemoresistance profile of this cancer. Indeed, we will have to know the genes to be targeted for each melanoma in order to induce cell chemosensitivity.

The role of PKCα and RLIP76 in transport-mediated doxorubicin-resistance in lung cancer

In deletion mutant analyses of potential phosphorylation sites in RLIP76, we identified T297 and S509 as targets for phosphorylation by PKCalpha. Phosphorylation at T297 increased doxorubicin (DOX)-transport activity approximately 2-fold for RLIP76 purified from recombinant source, or from three small (H69, H1417, H1618) and three non-small cell, one each derived from H226 (squamous), H358 (bronchio alveolar), and H1395 (adenocarcinoma) lung cancer cell lines. T297 phosphorylation conferred sensitivity to tryptic digestion at R293. The specific activity for DOX-transport by RLIP76 purified from non-small cell, which was primarily in the phosphorylated form, was approximately twice that in small cell lung cancer cell lines. These finding offer a novel explanation for the observed intrinsic differences in sensitivity to DOX between non-small cell and small cell lung cancer cell lines.

Absence of N-linked glycosylation does not affect plasma membrane localization of breast cancer resistance protein (BCRP/ABCG2)

Breast cancer resistance protein (BCRP/ABCG2) is an ATP-binding cassette (ABC) multidrug transporter that confers resistance to various anticancer drugs like topotecan and mitoxantrone. To obtain more insight in its cellular functioning, we investigated phosphorylation and N-linked glycosylation of BCRP. In the epithelial Madin-Darby canine kidney (MDCK) cell line, we did not detect phosphorylation of BCRP, in contrast to MRP2, which was phosphorylated. In the ovarian carcinoma cell line T8 also no phosphorylated BCRP was detected. As BCRP in both lines effectively transports drugs, it appears that phosphorylation of BCRP (if it occurs at all) is not needed for drug transport. We further mutated the asparagine residues 418, 557 and 596 in three putative N-linked glycosylation motifs of BCRP to alanines. Mutant proteins were expressed in CHO9 and MDCKII cells by transient transfection and characterized by Western blot and immunofluorescence analysis. We found that only BCRP-N596A and a mutant with all three asparagines mutated (triple mutant) were not glycosylated anymore, indicating that only asparagine 596 is normally glycosylated. The mutation of asparagine 596 (or 418) had little effect on the subcellular localization of BCRP, indicating that N-linked glycosylation is not essential for routing to the plasma membrane. However, BCRP-N557A and the triple mutant were mainly localized intracellularly, probably in the endoplasmic reticulum, suggesting that this mutation disrupted proper routing of BCRP.

Mrp2/Abcc2 transport activity is stimulated by protein kinase Calpha in a baculo virus co-expression system

Cholestatic and choleretic effect are well known for protein kinase C activator and inhibitor, respectively. However, post-translational regulation, especially the effect of phosphorylation status of the biliary transporters on their intrinsic transport activity has not been fully understood. In this study, effect of phosphorylation on the transport activity of Mrp2, a biliary organic anion transporter, was examined in membrane vesicles isolated from Sf9 cells co-expressing excess amount of protein kinase Calpha (PKCalpha). Mrp2-mediated transport activity was enhanced to three-fold by co-expressing PKCalpha. At the same time, phosphorylation of Mrp2 was also detected. The Km and Vmax values for the transport of [3H]estradiol-17beta-D-glucuronide exhibited a 1.5-fold decrease and a 1.9-fold increase, respectively. Probenecid (100 microM) and benzylpenicillin (1 mM), both are activator of Mrp2, did not stimulated the transport activity of phosphorylated Mrp2. On the other hand, transport activity was further stimulated by Estron-3-sulfate and taurocholic acid. Similar mechanism that occurred in the presence of probenecid and benzylpenicillin, but different from that occurred in the presence of Estron-3-sulfate and taurocholic acid seems to be involved in the stimulation. Considering the discrepancy between the previous in vivo inhibitory effect of PKC activators and our in vitro stimulatory effect of PKCalpha on Mrp2 transport activity, direct modulation of Mrp2-transport activity may be minor if any under in vivo condition.

Phosphorylation by Protein Kinase CK2 Modulates the Activity of the ATP Binding Cassette A1 Transporter

In a previous characterization of the ABCA subfamily of the ATP-binding cassette (ABC) transporters, we identified potential protein kinase 2 (CK2) phosphorylation sites, which are conserved in eukaryotic and prokaryotic members of the ABCA transporters. These phosphorylation residues are located in the conserved cytoplamic R1 and R2 domains, downstream of the nucleotide binding domains NBD1 and NBD2. To study the possible regulation of the ABCA1 transporter by CK2, we expressed the recombinant cytoplasmic domains of ABCA1, NBD1+R1 and NBD2+R2. We demonstrated that in vitro ABCA1 NBD1+R1, and not NBD2+R2, is phosphorylated by CK2, and we identified Thr-1242, Thr-1243, and Ser-1255 as the phosphorylated residues in the R1 domain by mass spectrometry. We further investigated the functional significance of the threonine and serine phosphorylation sites in NBD1 by site-directed mutagenesis of the entire ABCA1 followed by transfection into Hek-293 Tet-Off cells. The ABCA1 flippase activity, apolipoprotein AI and AII binding, and cellular phospholipid and cholesterol efflux were enhanced by mutations preventing CK2 phosphorylation of the threonine and serine residues. This was confirmed by the effect of specific protein kinase CK2 inhibitors upon the activity of wild type and mutant ABCA1 in transfected Hek-293 Tet-Off cells. The activities of the mutants mimicking threonine phosphorylation were close to that of wild type ABCA1. Our data, therefore, suggest that besides protein kinase A and C, protein kinase CK2 might play an important role in vivo in regulating the function and transport activity of ABCA1 and possibly of other members of the ABCA subfamily.

Immunophenotyping as a guide for targeted therapy

Immunophenotyping of acute and chronic leukaemias has revealed many lineage- and differentiation-specific antigens. It has now become possible to classify leukaemias according to their unique antigenic expression pattern. Among many lineage- and differentiation-specific antigens, disease-specific antigens are increasingly recognized because of their specific prognostic or therapeutic relevance. Expression of the multidrug resistance proteins of the ABC transporter family is associated with a poor response to treatment and a grave clinical prognosis. Recently, attempts to reverse refractory disease by using P-glycoprotein inhibitors have been performed in acute myeloid leukaemia, so far without evidence of clinical benefit. Other new leads to use antigen expression as a way of designing tumour-specific therapy have resulted in imatinib and Flt3 inhibitors which target tyrosine kinases in the leukaemic cell. Clinical trials are underway to investigate the effect of these new agents. The development of an antibody-calicheamycin complex directed against the myeloid-specific antigen CD33 has shown clinical activity in patients with relapsed acute myeloid leukaemia. The further development of these approaches is discussed.

Modulating protein kinase C (PKC) to increase the efficacy of chemotherapy: stepping into darkness

The identification of molecules that promote chemotherapeutic resistance would allow rationally designed approaches to abrogate this resistance, thereby possibly improving clinical outcomes for patients with cancer. In this regard, the PKC family is attractive for targeting, because it is comprised of a family of isoforms that play key roles in multiple cellular processes and can contribute to cellular transformation. Encouraging in vitro data originally showed that approaches to modulate PKC activity through small-molecule inhibitors or genetic manipulation could affect tumor cell survival. Recently, some of these approaches have begun clinical testing. Early-stage clinical trials revealed scattered clinical responses to these agents, but the most recent clinical trials have shown that combining modulators of PKC with standard chemotherapy does not improve outcome over chemotherapy alone. In this review, we will trace the development of these approaches, and discuss possible explanations for the recent negative results. Importantly, we will suggest guidelines for the clinical evaluation of PKC modulators.

Apolipoprotein A-I activates cellular cAMP signaling through the ABCA1 transporter

It has been suggested that the signal transduction pathway initiated by apoA-I activates key proteins involved in cellular lipid efflux. We investigated apoA-I-mediated cAMP signaling in cultured human fibroblasts induced with (22R)-hydroxycholesterol and 9-cis-retinoic acid (stimulated cells). Treatment of stimulated fibroblasts with apoA-I for short periods of time (<or=45 min) increased ATP binding cassette A1 (ABCA1) phosphorylation in a concentration-dependent manner. Concomitantly, apoA-I increased the intracellular level of cAMP in a concentration- and time-dependent manner. The maximal cAMP level was reached within 10 min at 10 microg/ml apoA-I representing a 1-fold increase. The ability of apoA-I to mediate cAMP production was only observed in stimulated fibroblasts. Furthermore, overexpression of ABCA1 in Chinese hamster ovary cells resulted in a 1.5-fold increase in apoA-I-mediated cAMP accumulation as compared with untransfected cells. In contrast, forskolin increased cAMP production significantly in unstimulated fibroblasts as well as in untransfected Chinese hamster ovary cells. Pharmacological inhibition of protein kinase A (H89) completely blocked apoA-I-mediated ABCA1 phosphorylation. Naturally occurring mutations of ABCA1 associated with Tangier disease (C1477R, 2203X, and 2145X) severely reduced apoA-I-mediated cAMP production, ABCA1 phosphorylation, (125)I-apoA-I binding, and lipid efflux, without affecting forskolin-mediated cAMP elevation. In contrast, the protein kinase A catalytic subunit was able to phosphorylate ABCA1 similarly from mutant and normal cell lines in vitro. Together, our results indicate that apoA-I activates ABCA1 phosphorylation through the cAMP/protein kinase A-dependent pathway, apoA-I-mediated cAMP production required high level expression of functional ABCA1, and Tangier disease mutants have defective apoA-I-mediated cAMP signaling. These findings suggest that apoA-I may activate cAMP signaling through G protein-coupled ABCA1 transporter.

Potent Killing of Paclitaxel- and Doxorubicin-resistant Breast Cancer Cells by Calphostin C Accompanied by Cytoplasmic Vacuolization

Drug resistance is a major impediment to the successful treatment of breast cancer using chemotherapy. The photoactivatable drug calphostin C has shown promise in killing select drug-resistant tumor cells lines in vitro. To assess the effectiveness of this agent in killing doxorubicin- or paclitaxel-resistant breast tumor cells and to explore its mode of action, MCF-7 cells were exposed to increasing concentrations of either doxorubicin or paclitaxel until maximum resistance was obtained. This resulted in the creation of isogenic drug-resistant MCF-7TAX and MCF-7DOX cell lines, which were approximately 50- and 65-fold resistant to paclitaxel and doxorubicin, respectively. Interestingly, calphostin C was able to kill MCF-7TAX cells as efficiently as wildtype MCF-7 cells (IC50s were 9.2 and 13.2 nM, respectively), while MCF-7DOX cells required a 5-fold higher concentration of calphostin C to achieve the same killing (IC50 = 64.2 nM). Consistent with their known mechanisms of action, paclitaxel killed tumor cells by inducing mitotic arrest and cell multinucleation, while doxorubicin induced plasma membrane blebbing and decreased nuclear staining with propidium iodide. In contrast, cytoplasmic vacuolization accompanied cell killing by calphostin C in these cell lines, without the induction of caspase-8 or PARP cleavage or the release of cytochrome c from mitochondria. Calphostin C had little effect on the uptake of either paclitaxel or doxorubicin by the cells. Taken together, the above data suggests that calphostin C is able to potently kill drug-resistant breast tumor cells through a mechanism that may involve the induction of cytoplasmic vacuolization, without activation of typical apoptotic pathways. Consequently, calphostin C may prove useful clinically to combat tumor growth in breast cancer patients whose tumors have become unresponsive to anthracyclines or taxanes, particularly in association with photodynamic therapy.

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.

Multidrug resistance transporters in the olfactory receptor neurons of Xenopus laevis tadpoles

Olfactory receptor neurons (ORNs) are the only class of neurons that is directly exposed to the environment. Therefore, they need to deal with xenobiotic and potentially cytotoxic substances. Here we show for the first time that ORNs possess transporter systems that expel xenobiotics across the plasma membrane. Using calcein and calcium-indicator dyes as xenobiotics, we demonstrate that ORNs appear to express the multidrug resistance P-glycoprotein (MDR1) and multidrug resistance-associated proteins (MRP). This endows ORNs with the ability to transport a large number of substrates including calcium-indicator dyes and calcein across their plasma membranes. Conversely, blocking P-glycoprotein and MRP increases the net uptake of these dyes.

Functional analysis of candidate ABC transporter proteins for sitosterol transport

Two ATP-binding cassette (ABC) proteins, ABCG5 and ABCG8, have recently been associated with the accumulation of dietary cholesterol in the sterol storage disease sitosterolemia. These two 'half-transporters' are assumed to dimerize to form the complete sitosterol transporter which reduces the absorption of sitosterol and related molecules in the intestine by pumping them back into the lumen. Although mutations altering ABCG5 and ABCG8 are found in affected patients, no functional demonstration of sitosterol transport has been achieved. In this study, we investigated whether other ABC transporters implicated in lipid movement and expressed in tissues with a role in sterol synthesis and absorption, might also be involved in sitosterol transport. Transport by the multidrug resistance P-glycoprotein (P-gp; Abcb1), the multidrug resistance-associated protein (Mrp1; Abcc1), the breast cancer resistance protein (Bcrp; Abcg2) and the bile salt export pump (Bsep; Abcb11) was assessed using several assays. Unexpectedly, none of the candidate proteins mediated significant sitosterol transport. This has implications for the pathology of sitosterolemia. In addition, the data suggest that otherwise broad-specific ABC transporters have acquired specificity to exclude sitosterol and related sterols like cholesterol presumably because the abundance of cholesterol in the membrane would interfere with their action; in consequence, specific transporters have evolved to handle these sterols.

Protein kinase A site-specific phosphorylation regulates ATP-binding cassette A1 (ABCA1)-mediated phospholipid efflux

ATP-binding cassette A1 (ABCA1) is a key mediator of cholesterol and phospholipid efflux to apolipoprotein particles. We show that ABCA1 is a constitutively phosphorylated protein in both RAW macrophages and in a human embryonic kidney cell line expressing ABCA1. Furthermore, we demonstrate that phosphorylation of ABCA1 is mediated by protein kinase A (PKA) or a PKA-like kinase in vivo. Through site-directed mutagenesis studies of consensus PKA phosphorylation sites and in vitro PKA kinase assays, we show that Ser-1042 and Ser-2054, located in the nucleotide binding domains of ABCA1, are major phosphorylation sites for PKA. ApoA-I-dependent phospholipid efflux was decreased significantly by mutation of Ser-2054 alone and Ser-1042/Ser-2054 but was not significantly impaired with Ser-1042 alone. The mechanism by which ABCA1 phosphorylation affected ApoA-I-dependent phospholipid efflux did not involve either alterations in ApoA-I binding or changes in ABCA1 protein stability. These studies demonstrate a novel serine (Ser-2054) on the ABCA1 protein crucial for PKA phosphorylation and for regulation of ABCA1 transporter activity.

Nucleotide triphosphatase activity of the N-terminal nucleotide-binding domains of the multidrug resistance proteins P-glycoprotein and MRP1

The multidrug resistance proteins P-glycoprotein (Pgp) and MRP1 are drug-efflux pumps. In this study, we compared the nucleotide triphosphatase activities of the isolated N-terminal nucleotide binding domains (NBD1) of Pgp and MRP1, and explored the potential role of the phosphorylation target domain of Pgp on the regulation of Pgp NBD1 ATPase activity. We found that: (1) the NBD1s of Pgp and MRP1 have ATPase and GTPase activities, (2) the K(m)s of Pgp NBD1 for ATP and GTP hydrolysis are identical, while the K(m) of MRP1 NBD1 for ATP is lower than that for GTP, and (3) phosphorylation of MLD by PKA or PKC produces a marginal increase of V(max) for ATP hydrolysis, without affecting the affinity for ATP. These results show efficient GTP hydrolysis by the NBD1s of Pgp and MRP1, and a minor role of phosphorylation in the control of Pgp NBD1 ATPase activity.

Protein kinase C inhibitors

Protein kinase C (PKC) is a family of serine-threonine protein kinases that are involved in signal transduction pathways that regulate growth factor response, proliferation, and apoptosis. Its central role in these processes, which are closely involved in tumor initiation, progression, and response to antitumor agents, makes it an attractive therapeutic target in cancer. Despite initial activity seen in melanoma (bryostatin and UCN-01), non-Hodgkin's lymphoma (ISIS 3521, bryostatin, and UCN-01), and ovarian carcinoma (ISIS 3521 and bryostatin) in phase I studies, single-agent activity in those phase II studies reported to date has been limited. Preclinical data highlight a role for PKC in modulation of drug resistance and synergy with conventional cytotoxic drugs. A randomized phase III study of ISIS 3521 in combination with carboplatin and paclitaxel, compared with chemotherapy alone, in advanced non-small-cell lung cancer is underway. This paper reviews the rationale for using PKC inhibitors in cancer therapy, the challenges for clinical trial design, and the recent clinical experience with modulators of PKC activity.

Strategies to overcome simultaneous P-glycoprotein mediated efflux and CYP3A4 mediated metabolism of drugs

Cytochrome P450 3A4 (CYP3A4), abundant in both the liver and upper intestinal enterocytes, limits the systemic bioavailability of xenobiotics. P-glycoprotein (P-gp), the MDR1 gene product, is also known to reduce the oral bioavailability of the drug molecules. High cellular expression of P-gp and CYP3A4 in mature intestinal enterocytes and their similar substrate specificity suggest that the function of these proteins may be complementary and may form a co-ordinated intestinal barrier. Various ongoing preclinical and clinical studies have demonstrated that the oral bioavailability of various P-gp and/or CYP3A4 substrates can be increased by simultaneous administration of P-gp and/or CYP3A4 inactivators. The current review describes the background and summarises several proposed hypotheses in modifying oral bioavailability by various drug-inhibitor interactions.

The multidrug resistance P-glycoprotein. Oligomeric state and intramolecular interactions

The human multidrug resistance P-glycoprotein (P-gp), a member of the ATP-binding cassette (ABC) superfamily of transporters, is frequently responsible for the failure of chemotherapy by virtue of its ability to export hydrophobic cytotoxic drugs from cells. Elucidating the inter- and intramolecular interactions of this protein is critical to understanding its cellular function and mechanism of action. Toward this end, we have used both biochemical and genetic techniques to probe potential oligomerization interactions of P-gp. Differentially epitope-tagged P-gp molecules did not co-immunoprecipitate when co-expressed in HEK293 cells or when co-translated in vitro, demonstrating that P-gp is monomeric in both the presence and absence of detergents. The two cytoplasmic domains of P-gp did not interact with each other in vivo when co-expressed as gene fusions in yeast. In contrast, the homologous domains of the transporter associated with antigen processing (TAP), which reside on separate polypeptides and must form a heterodimeric transporter (TAP1/TAP2), did interact in this system, suggesting a role for these domains in TAP dimerization. Implications for understanding the subunit organization of ABC transporters are discussed.

Glycosylated and phosphorylated proteins--expression in yeast and oocytes of Xenopus: prospects and challenges--relevance to expression of thermostable proteins

Phosphorylation and glycosylation are important posttranslational events in the biosynthesis of proteins. The different degrees of phosphorylation and glycosylation of proteins have been an intriguing phenomenon. Advances in genetic engineering have made it possible to control the degree of glycosylation and phosphorylation of proteins. Structural biology of phosphorylated and glycosylated proteins has been advancing at a much slower pace due to difficulties in using high-resolution NMR studies in solution phase. Major difficulties have arisen from the inherent mobilities of phosphorylated and glycosylated side chains. This paper reviews molecular and structural biology of phosphorylated and glycosylated proteins expressed in eukaryotic expression systems which are especially suited for large-scale production of these proteins. In our laboratory, we have observed that eukaryotic expression systems are particularly suited for the expression of thermostable light-activated proteins, e.g., bacteriorhodopsins and plastocyanins.

Regulation of MDR1 promoter activity in human breast carcinoma cells by protein kinase C isozymes alpha and theta

Increased levels of the protein kinase C (PKC) isoenzymes alpha and theta occur in conjunction with MDR1 gene expression in cells and tissues that have acquired a multidrug resistance (MDR) phenotype. Studies using PKC activators or antisense strategies against PKC suggest that activation of PKC engenders MDR1 gene transcription. In this study the potential roles of PKC-alpha and PKC-theta in MDR1 gene transcriptional regulation were explored. Human-derived MCF-7 breast cancer cells that lack constitutive expression of PKC-alpha or PKC-theta at detectable levels were transfected with full-length PKC-alpha or PKC-theta genes driven by the ecdysone promoter. Stable transfectants were selected by use of the appropriate antibiotics. Treatment of these cells with ponasterone A induced expression of PKC that was catalytically active and underwent translocation and down-regulation on exposure to 12-O-tetradecanoyl-13-phorbol acetate (TPA). These cells were used to analyse PKC-mediated regulation of the MDR1 promoter by further transient transfection with either 1073 bp of the MDR1 gene promoter or deletion fragments thereof to -8 bp, each linked to a chloramphenicol acetyl transferase (CAT) reporter gene. In PKC-alpha expressing cells TPA caused activation of all promoter fragments to -29 bp. This finding suggests that TPA-inducible MDR1 transcription mediated through the TPA responsive factor early growth response 1 (EGR-1) in this region of the promoter may be due to activation of PKC-alpha. In contrast, PKC-theta activated only two MDR1 fragments, -982 and -612 bp. The effect of TPA on reporter gene expression was attenuated by the PKC inhibitor GF 109203X. These data suggest that MDR1 promoter transcription can be regulated by PKC-alpha and PKC-theta. The results support the search for therapeutic strategies directed specifically against PKC-alpha to ameliorate resistance of tumours against cytotoxic agents.

Cysteines 431 and 1074 are responsible for inhibitory disulfide cross-linking between the two nucleotide-binding sites in human P-glycoprotein

Human wild-type and Cys-less P-glycoproteins were expressed in Pichia pastoris and purified in high yield in detergent-soluble form. Both ran on SDS gels as a single 140-kDa band in the presence of reducing agent and showed strong verapamil-stimulated ATPase activity in the presence of added lipid. The wild type showed spontaneous formation of higher molecular mass species in the absence of reducing agent, and its ATPase was activated by dithiothreitol. Oxidation with Cu(2+) generated the same higher molecular mass species, primarily at 200 and approximately 300 kDa, in high yield. Cross-linking was reversed by dithiothreitol and prevented by pretreatment with N-ethylmaleimide. Using proteins containing different combinations of naturally occurring Cys residues, it was demonstrated that an inhibitory intramolecular disulfide bond forms between Cys-431 and Cys-1074 (located in the Walker A sequences of nucleotide-binding sites 1 and 2, respectively), giving rise to the 200-kDa species. In addition, dimeric P-glycoprotein species ( approximately 300 kDa) form by intermolecular disulfide bonding between Cys-431 and Cys-1074. The ready formation of the intramolecular disulfide between Cys-431 and Cys-1074 establishes that the two nucleotide-binding sites of P-glycoprotein are structurally very close and capable of intimate functional interaction, consistent with available information on the catalytic mechanism. Formation of such a disulfide in vivo could, in principle, underlie a regulatory mechanism and might provide a means of intervention to inhibit P-glycoprotein.

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.

Chemotherapy resistance in acute myeloid leukaemia

The development of refractory disease in acute myeloid leukaemia (AML) is frequently associated with the expression of one or several multidrug resistance (MDR) genes. MDR1, MRP1 and LRP have been identified as important adverse prognostic factors in AML. Recently it has become possible to reverse clinical multidrug resistance by blocking P-glycoprotein-mediated drug efflux. The potential relevance of MDR and new approaches to treat refractory disease, are discussed.

Protein kinase C effectors bind to multidrug ABC transporters and inhibit their activity

P-Glycoprotein and homologous multidrug transporters contain a phosphorylatable linker sequence that was proposed to control drug efflux on the basis that it was indeed phosphorylated in vitro and in vivo, and that inhibitors of protein kinase C (PKC) inhibited both P-glycoprotein phosphorylation and activity. However, site-directed mutagenesis of all phosphorylatable residues did not alter the drug resistance. The present work shows that PKC effectors are able to bind directly to multidrug transporters, from either cancer cells (mouse P-glycoprotein), yeast (Saccharomyces cerevisiae Pdr5p), or protozoan parasite (Leishmania tropica ltmdr1), and to inhibit their energy-dependent drug-efflux activity. The binding of staurosporine and derivatives such as CGP 41251 is prevented by preincubation with ATP, suggesting at least partial interaction at the ATP-binding site. In contrast, more hydrophobic compounds such as calphostin C and CGP 42700 bind outside the ATP-binding site and strongly interfere with drug interaction. A direct correlation is obtained between the efficiencies of PKC effectors to inhibit energy-dependent interaction of rhodamine 6G with yeast Pdr5p, to promote intracellular drug accumulation in various multidrug resistant cells, and to chemosensitize growth of resistant cells. The noncompetitive inhibition by PKC effectors of rhodamine 6G interaction with Pdr5p suggests that the binding might interfere with signal transduction between nucleotide hydrolysis and drug interaction. The overall results indicate that the multidrug transporters from different species display common features for interaction with PKC inhibitors. The hydrophobic derivative of staurosporine, CGP 42700, constitutes a potentially powerful modulator of P-glycoprotein-mediated multidrug resistance.