P‐Glycoprotein, Multidrug Resistance and Protein Kinase C

2,4,6-Tribromophenol Exposure Decreases P-Glycoprotein Transport at the Blood-Brain Barrier

2,4,6-Tribromophenol (TBP, CAS No. 118-79-6) is a brominated chemical used in the production of flame-retardant epoxy resins and as a wood preservative. In marine environments, TBP is incorporated into shellfish and consumed by predatory fish. Food processing and water treatment facilities produce TBP as a byproduct. 2,4,6-Tribromophenol has been detected in human blood and breast milk. Biologically, TBP interferes with estrogen and thyroid hormone signaling, which regulate important transporters of the blood-brain barrier (BBB). The BBB is a selectively permeable barrier characterized by brain microvessels which are composed of endothelial cells mortared by tight-junction proteins. ATP-binding cassette (ABC) efflux transporters on the luminal membrane facilitate the removal of unwanted endobiotics and xenobiotics from the brain. In this study, we examined the in vivo and ex vivo effects of TBP on two important transporters of the BBB: P-glycoprotein (P-gp, ABCB1) and Multidrug Resistance-associated Protein 2 (MRP2, ABCC2), using male and female rats and mice. 2,4,6-Tribromophenol exposure ex vivo resulted in a time- (1-3 h) and dose- (1-100 nM) dependent decrease in P-gp transport activity. MRP2 transport activity was unchanged under identical conditions. Immunofluorescence and western blotting measured decreases in P-gp expression after TBP treatment. ATPase assays indicate that TBP is not a substrate and does not directly interact with P-gp. In vivo dosing with TBP (0.4 µmol/kg) produced decreases in P-gp transport. Co-treatment with selective protein kinase C (PKC) inhibitors prevented the TBP-mediated decreases in P-gp transport activity.

Novel features in the structure of P-glycoprotein (ABCB1) in the post-hydrolytic state as determined at 7.9 Å resolution

BACKGROUND

P-glycoprotein (ABCB1) is an ATP-binding cassette transporter that plays an important role in the clearance of drugs and xenobiotics and is associated with multi-drug resistance in cancer. Although several P-glycoprotein structures are available, these are either at low resolution, or represent mutated and/or quiescent states of the protein.

RESULTS

In the post-hydrolytic state the structure of the wild-type protein has been resolved at about 8 Å resolution. The cytosolic nucleotide-binding domains (NBDs) are separated but ADP remains bound, especially at the first NBD. Gaps in the transmembrane domains (TMDs) that connect to an inner hydrophilic cavity are filled by density emerging from the annular detergent micelle. The NBD-TMD linker is partly resolved, being located between the NBDs and close to the Signature regions involved in cooperative NBD dimerization. This, and the gap-filling detergent suggest steric impediment to NBD dimerization in the post-hydrolytic state. Two central regions of density lie in two predicted drug-binding sites, implying that the protein may adventitiously bind hydrophobic substances even in the post-hydrolytic state. The previously unresolved N-terminal extension was observed, and the data suggests these 30 residues interact with the headgroup region of the lipid bilayer.

CONCLUSION

The structural data imply that (i) a low basal ATPase activity is ensured by steric blockers of NBD dimerization and (ii) allocrite access to the central cavity may be structurally linked to NBD dimerization, giving insights into the mechanism of drug-stimulation of P-glycoprotein activity.

RC. Novel features in the structure of P-glycoprotein (ABCB1) in the post-hydrolytic state as determined at 7.9Å resolution.. [DOI: 10.1101/308114]

P-glycoprotein (ABCB1) is a ATP-binding cassette transporter that plays an important role in the removal of drugs and xenobiotic compounds from the cell. It is also associated with multi-drug resistance in cancer. Here we report novel features of the cryo-EM-derived structure of P-glycoprotein in the post-hydrolytic state: The cytosolic nucleotide-binding domains (NBDs) are separated despite ADP remaining bound to the NBDs. Gaps in the TMDs that connect to the inner hydrophilic cavity are back-filled by detergent head-groups from the annular detergent micelle and are close to two regions predicted to delineate two pseudo-symmetry-related drug-binding sites. In this conformation, the (newly-resolved) N-terminal extension, NBD-TMD linker region and gap-filling detergents all appear to impede NBD dimerisation. We propose a model for the mechanism of action of the exporter where ATP will be bound to the protein for most of the time, consistent with the high physiological ATP concentrationsin vivo.

The inhibitory effect of pseudolaric acid B on gastric cancer and multidrug resistance via Cox-2/PKC-α/P-gp pathway

Aim

To investigate the inhibitory effect of pseudolaric acid B on subcutaneous xenografts of human gastric adenocarcinoma and the underlying molecular mechanisms involved in its multidrug resistance.

Methods

Human gastric adenocarcinoma SGC7901 cells and drug-resistant SGC7901/ADR cells were injected into nude mice to establish a subcutaneous xenograft model. The effects of pseudolaric acid B with or without adriamycin treatment were compared by determining the tumor size and weight. Cyclo-oxygenase-2, protein kinaseC-α and P-glycoprotein expression levels were determined by immunohistochemistry and western blot.

Results

Pseudolaric acid B significantly suppressed the tumor growth induced by SGC7901 cells and SGC7901/ADR cells. The combination of pseudolaric acid B and the traditional chemotherapy drug adriamycin exhibited more potent inhibitory effects on the growth of gastric cancer in vivo than treatment with either pseudolaric acid B or adriamycin alone. Protein expression levels of cyclo-oxygenase-2, protein kinaseC-α and P-glycoprotein were inhibited by pseudolaric acid B alone or in combination with adriamycin in SGC7901/ADR cell xenografts.

Conclusion

Pseudolaric acid B has a significant inhibitory effect and an additive inhibitory effect in combination with adriamycin on the growth of gastric cancer in vivo, which reverses the multidrug resistance of gastric neoplasm to chemotherapy drugs by downregulating the Cox-2/PKC-α/P-gp/mdr1 signaling pathway.

FLT3-ITD and MLL-PTD influence the expression of MDR-1, MRP-1, and BCRP mRNA but not LRP mRNA assessed with RQ-PCR method in adult acute myeloid leukemia

Fms-like tyrosine kinase 3-internal tandem duplication (FLT3-ITD) and mixed-lineage leukemia gene-partial tandem duplication (MLL-PTD) are aberrations associated with leukemia which indicate unsatisfactory prognosis. Downstream regulatory targets of FLT3-ITD and MLL-PTD are not well defined. We have analyzed the expression of MDR-1, multidrug resistant protein-1 (MRP-1), breast cancer resistance protein (BCRP), and lung resistance protein (LRP) messenger RNA (mRNA) in relation to the mutational status of FLT3-ITD and MLL-PTD in 185 acute myeloid leukemia (AML) adult patients. The real-time quantitative polymerase chain reaction method was performed to assess the expression of the MDR-1, MRP-1, BCRP, and LRP mRNA, and the results were presented as coefficients calculated using an intermediate method according to Pfaffl's rule. Significantly higher expressions of MDR-1 mRNA were found in patients who did not harbor FLT3-ITD (0.20 vs. 0.05; p = 0.0001) and MRP-1 mRNA in patients with this mutation (0.96 vs. 0.70; p = 0.002) and of BCRP mRNA in patients with MLL-PTD (0.61 vs. 0.38; p = 0.03). In univariate analysis, the high expression of MDR-1 mRNA (≥0.1317) negatively influenced the outcome of induction therapy (p = 0.05), whereas the high expression of BCRP mRNA (≥1.1487) was associated with a high relapse rate (RR) (p = 0.013). We found that the high expression of MDR-1 (≥0.1317), MRP-1 (≥0.8409), and BCRP mRNA (≥1.1487) significantly influenced disease-free survival (DFS; p = 0.059, 0.032, and 0.009, respectively) and overall survival (0.048, 0.014, and 0.059, respectively). Moreover, a high expression of BCRP mRNA (≥1.1487) proved to be an independent prognostic factor for RR (p = 0.01) and DFS (p = 0.002) in multivariate analysis. The significant correlation between the expression of MDR-1, MRP-1, and BCRP mRNA and FLT3-ITD or MLL-PTD in AML patients requires further investigation.

Functional study of the novel multidrug resistance gene HA117 and its comparison to multidrug resistance gene 1

BACKGROUND

The novel gene HA117 is a multidrug resistance (MDR) gene expressed by all-trans retinoic acid-resistant HL-60 cells. In the present study, we compared the multidrug resistance of the HA117 with that of the classical multidrug resistance gene 1 (MDR1) in breast cancer cell line 4T1.

METHODS

Transduction of the breast cancer cell line 4T1 with adenoviral vectors encoding the HA117 gene and the green fluorescence protein gene (GFP) (Ad-GFP-HA117), the MDR1 and GFP (Ad-GFP-MDR1) or GFP (Ad-GFP) was respectively carried out. The transduction efficiency and the multiplicity of infection (MOI) were detected by fluorescence microscope and flow cytometry. The transcription of HA117 gene and MDR1 gene were detected by reverse transcription polymerase chain reaction (RT-PCR). Western blotting analysis was used to detect the expression of P-glycoprotein (P-gp) but the expression of HA117 could not be analyzed as it is a novel gene and its antibody has not yet been synthesized. The drug-excretion activity of HA117 and MDR1 were determined by daunorubicin (DNR) efflux assay. The drug sensitivities of 4T1/HA117 and 4T1/MDR1 to chemotherapeutic agents were detected by Methyl-Thiazolyl-Tetrazolium (MTT) assay.

RESULTS

The transducted efficiency of Ad-GFP-HA117 and Ad-GFP-MDR1 were 75%-80% when MOI was equal to 50. The transduction of Ad-GFP-HA117 and Ad-GFP-MDR1 could increase the expression of HA117 and MDR1. The drug resistance index to Adriamycin (ADM), vincristine (VCR), paclitaxel (Taxol) and bleomycin (BLM) increased to19.8050, 9.0663, 9.7245, 3.5650 respectively for 4T1/HA117 and 24.2236, 11.0480, 11.3741, 0.9630 respectively for 4T1/MDR1 as compared to the control cells. There were no significant differences in drug sensitivity between 4T1/HA117 and 4T1/MDR1 for the P-gp substrates (ADM, VCR and Taxol) (P < 0.05), while the difference between them for P-gp non-substrate (BLM) was statistically significant (P < 0.05). DNR efflux assay confirmed that the multidrug resistance mechanism of HA117 might not be similar to that of MDR1.

CONCLUSIONS

These results confirm that HA117 is a strong MDR gene in both HL-60 and 4T1 cells. Furthermore, our results indicate that the MDR mechanism of the HA117 gene may not be similar to that of MDR1.

Multidrug resistance strength of the novel multidrug resistance gene HA117: compared with MRP1

The novel gene HA117 is a multidrug resistance (MDR) gene in all-trans retinoic acid resistance HL-60 cells. The transduction of adenovirus vectors encoding HA117 conferred breast cancer cell line 4T1 MDR not only to MRP1 substrate drugs but also to MRP1 non-substrate drugs and the MDR strength of HA117 was similar to that of multidrug resistance-associated protein-1 (MRP1) for MRP1 substrate, but HA117 had no daunorubicin-excretion function.

Insulin regulates P-glycoprotein in rat brain microvessel endothelial cells via an insulin receptor-mediated PKC/NF-kappaB pathway but not a PI3K/Akt pathway

Our previous study showed that insulin restored impaired function and expression of P-glycoprotein in diabetic blood-brain barrier, and further study showed that insulin up-regulated P-glycoprotein expression and function in normal blood-brain barrier, so insulin might be one of the factors that regulated the function and expression of P-glycoprotein in blood-brain barrier of diabetes. In this study, the intracellular pathways that insulin regulated the P-glycoprotein were investigated using primarily cultured rat brain microvessel endothelial cells model. The rat brain microvessel endothelial cells were incubated in normal culture medium containing 50 mU/l insulin and different concentrations of inhibitors for 72 h. The P-glycoprotein function and expression in the rat brain microvessel endothelial cells were assessed using the uptake of P-glycoprotein substrate rhodamine 123 and western blot assay, respectively. It was found that treatment of 50 mU/l insulin significantly increased P-glycoprotein function and expression in rat brain microvessel endothelial cells. This induced effect was blocked by insulin receptor antibody, insulin receptor tyrosine kinase inhibitor I-OMe-AG538, PKC inhibitor chelerythrine and NF-kappaB inhibitor pyrrolidine dithiocarbamate ammonium (PDTC). But this induced effect was not inhibited by phosphatidylinositol 3-kinase (PI3K)/Akt inhibitor LY294002. These results indicated that insulin regulated P-glycoprotein function and expression through signal transduction pathways involving activation of PKC/NF-kappaB but not PI3K/Akt pathway.

Impact of intraperitoneal, sustained delivery of paclitaxel on the expression of P-glycoprotein in ovarian tumors

Recently, we developed a novel implantable drug delivery system which can provide sustained intraperitoneal (i.p.) delivery of paclitaxel (PTX). As the impact of local sustained delivery on the development of multidrug resistance (MDR) is unknown, the objective of this study was to determine the impact of this drug delivery system on the in vivo expression of MDR1/P-glycoprotein (PGP) in a human ovarian xenograft tumor model. As compared to controls, intermittent i.p. dosing with PTX formulated in Cremophor EL (PTX(CrEL)) induced a two-fold increase in mRNA levels of MDR1 after a 14-day dosing period. On the other hand, sustained i.p. delivery of PTX with the implant system (PTX(film)) did not significantly affect MDR1 expression. Immunodetection of PGP in isolated xenografts supported the mRNA data. Histological analysis by H&E staining demonstrated a dose-dependent increase in tumor necrosis in the PTX(film) treated animals. Further, in vitro studies in human ovarian carcinoma cells also demonstrated a significant induction in the efflux activity of PGP with intermittent dosing schedules to PTX(CrEL) whereas this was not seen in cells dosed with PTX(film). Our findings suggest that sustained i.p. administration with PTX(film) attenuates development of MDR, suggesting that sustained, localized delivery of chemotherapeutic agents may improve current treatment strategies for ovarian cancer.

Protein kinase C activation and its role in kidney disease (Review Article)

Protein kinase C (PKC) comprises a superfamily of isoenzymes, many of which are activated by cofactors such as diacylglycerol and phosphatidylserine. In order to be capable of activation, PKC must first undergo a series of phosphorylations. In turn, activated PKC phosphorylates a wide variety of intracellular target proteins and has multiple functions in signal transduced cellular regulation. A role for PKC activation had been noted in several renal diseases, but two that have had most investigation are diabetic nephropathy and kidney cancer. In diabetic nephropathy, an elevation in diacylglycerol and/or other cofactor stimulants leads to an increase in activity of certain PKC isoforms, changes that are linked to the development of dysfunctional vasculature. The ability of isoform-specific PKC inhibitors to antagonize diabetes-induced vascular disease is a new avenue for treatment of this disorder. In the development and progressive invasiveness of kidney cancer, increased activity of several specific isoforms of PKC has been noted. It is thought that this may promote the kidney cancer's inherent resistance to apoptosis, in natural regression or after treatments, or it may promote the invasiveness of renal cancers via cellular differentiation pathways. In general, however, a more complete understanding of the functions of individual PKC isoforms in the kidney, and development or recognition of specific inhibitors or promoters of their activation, will be necessary to apply this knowledge for treatment of cellular dysregulation in renal disease.

Impact of extracellular acidity on the activity of P-glycoprotein and the cytotoxicity of chemotherapeutic drugs

The expression and activity of P-glycoprotein (pGP) play a role in the multidrug resistance of tumors. Because solid-growing tumors often show pronounced hypoxia or extracellular acidosis, this study attempted to analyze the impact of an acidic environment on the expression and activity of pGP and on the cytotoxicity of chemotherapeutic agents. For this, prostate carcinoma cells were exposed to an acidic extracellular environment (pH 6.6) for up to 24 hours. pGP activity was more than doubled after 3 to 6 hours of incubation in acidic medium, whereas cellular pGP expression remained constant, indicating that increased transport rate is the result of functional modulation. In parallel, the cytotoxic efficacy of daunorubicin showed pronounced reduction at low pH, an effect that was reversible on coincubation with a pGP inhibitor. A reduction of intracellular Ca2+ concentration by 35% under acidic conditions induced a higher transport rate of pGP, an effect comparable to that found on inhibition of protein kinase C (PKC). These data indicate that pGP activity is increased by low extracellular pH presumably as a result of lowered intracellular calcium levels and inhibition of PKC. These findings may explain the reduced cytotoxicity of chemotherapeutic agents in hypoxic/acidic tumors.

Cell structure and cytokinesis alterations in multidrug-resistant Leishmania (Leishmania) amazonensis

Multidrug-resistant Leishmania (Leishmania) amazonensis may be obtained by in vitro selection with vinblastine. In order to determine whether this phenotype is linked to structural alterations, we analyzed the cell architecture by electron microscopy. The vinblastine resistant CL2 clone of L. (L.) amazonensis, but not wild-type parasites, showed a cytokinesis dysfunction. The CL2 promastigotes had multiple nuclei, kinetoplasts and flagella, suggesting that vinblastine resistance may be associated with truncated cell division. The subpellicular microtubule plasma membrane connection was also affected. Wild-type parasites treated with vinblastine displayed similar alterations, presenting lobulated and multinucleated cells. Taken together, these data indicate that antimicrotubule drug-selected parasites may show evidence of the mutation of cytoskeleton proteins, impairing normal cell function.

The expression of P-glycoprotein in AML cells with FLT3 internal tandem duplications is associated with reduced apoptosis in response to FLT3 inhibitors

P-glycoprotein (pgp), a membrane efflux pump, is recognized to have an anti-apoptotic function. Internal tandem duplications (ITDs) of the Fms-like tyrosine kinase 3 (FLT3) receptor are the most common mutations in acute myeloid leukaemia (AML). Both ITDs and pgp positivity confer an adverse clinical prognosis. FLT3 inhibitors induce variable apoptosis in cell lines transfected with FLT3 ITDs. We studied the effect of herbimycin A, AG1296 and PKC412 on primary AML blasts. All compounds showed significantly higher cell kill after 48-h incubation in samples with an ITD compared with wild type (Herbimicin P < 0.001; AG1296 P = 0.001, PKC412, P = 0.002). Pgp-positive samples were significantly less sensitive to herbimycin and AG1296 than pgp-negative samples, although neither molecule inhibited the efflux function of pgp. The concurrent incubation with the pgp inhibitor PSC833 resulted in an enhanced cell kill in 4/5 ITD pgp-positive samples versus two of nine ITD pgp-negative samples. PKC412 inhibited pgp function and induced cell death in FLT3 ITD/pgp-positive samples. We conclude that AML samples with a FLT3 ITD are more susceptible to these inhibitors than wild-type samples. However, the expression of pgp in cells with FLT3 ITDs can reduce their sensitivity to FLT3 inhibitors and therefore pgp expression should be assessed in clinical trials of FLT3 inhibitors.

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.

Preferential induction of necrosis in human breast cancer cells by a p53 peptide derived from the MDM2 binding site

p53 is the most frequently altered gene in human cancer and therefore represents an ideal target for cancer therapy. Several amino terminal p53-derived synthetic peptides were tested for their antiproliferative effects on breast cancer cell lines MDA-MB-468 (mutant p53), MCF-7 (overexpressed wild-type p53), and MDA-MB-157 (null p53). p53(15)Ant peptide representing the majority of the mouse double minute clone 2 binding site on p53 (amino acids 12-26) fused to the Drosophila carrier protein Antennapedia was the most effective. p53(15)Ant peptide induced rapid, nonapoptotic cell death resembling necrosis in all breast cancer cells; however, minimal cytotoxicity was observed in the nonmalignant breast epithelial cells MCF-10-2A and MCF-10F. Bioinformatic/biophysical analysis utilizing hydrophobic moment and secondary structure predictions as well as circular dichroism spectroscopy revealed an alpha-helical hydrophobic peptide structure with membrane disruptive potential. Based on these findings, p53(15)Ant peptide may be a novel peptide cancer therapeutic because it induces necrotic cell death and not apoptosis, which is uncommon in traditional cancer therapy.

P-glycoprotein in acute myeloid leukaemia: therapeutic implications of its association with both a multidrug-resistant and an apoptosis-resistant phenotype

P-glycoprotein (Pgp) expression is an independent prognostic factor for response to remission-induction chemotherapy in acute myeloblastic leukaemia, particularly in the elderly. There are several potential agents for modulating Pgp-mediated multi-drug resistance, such as cyclosporin A and PSC833, which are currently being evaluated in clinical trials. An alternative therapeutic strategy is to increase the use of drugs which are unaffected by Pgp. However, in this review, we explain why this may be more difficult than it appears. Evidence from in vitro studies of primary AML blasts supports the commonly held supposition that chemoresistance may be linked to apoptosis-resistance. We have found that Pgp has a drug-independent role in the inhibition of in vitro apoptosis in AML blasts. Modulation of cytokine efflux, signalling lipids and intracellular pH have all been suggested as ways by which Pgp may affect cellular resistance to apoptosis; these are discussed in this review. For a chemosensitising agent to be successful, it may be more important for it to enhance apoptosis than to increase drug uptake.

Impact of BCRP/MXR, MRP1 and MDR1/P-Glycoprotein on thermoresistant variants of atypical and classical multidrug resistant cancer cells

The impact of the ABC transporters breast cancer resistance protein/mitoxantrone resistance associated transporter (BCRP/MXR), multidrug resistance-associated protein 1 (MRP1) and multidrug resistance gene-1/P-glycoprotein (MDR1/PGP) on the multidrug resistance (MDR) phenotype in chemoresistance and thermoresistance was investigated in the parental human gastric carcinoma cell line EPG85-257P, the atypical MDR subline EPG85-257RNOV, the classical MDR subline EPG85-257RDB and their thermoresistant counterparts EPG85-257P-TR, EPG85-257RNOV-TR and EPG85-257RDB-TR. Within the atypical MDR subline EPG85-257RNOV expression of BCRP/MXR and of MRP1 were clearly enhanced (vs. parental and classical MDR lines). MDR1/PGP expression was distinctly elevated in the classical MDR subline EPG85-257RDB (vs. parental and atypical MDR sublines). In all thermoresistant counterparts basal expression of BCRP/MXR, MRP1 and MDR1/PGP was increased relative to thermosensitive sublines. Although it could be shown that the overexpressed ABC transporters were functionally active, however, no decreased drug accumulations of doxorubicin, mitoxantrone and rhodamine 123 were observed. Thus, expression of BCRP/MXR, MRP1 and MDR1/PGP was found to be dependent on the appropriate type of chemoresistance; correlating with a classical or atypical MDR phenotype. Within the thermoresistant variants, however, the increase in ABC transporter expression did obviously not influence the MDR phenotype.