Combined expression of caveolin-1 and an activated AKT/mTOR pathway predicts reduced disease-free survival in clinically confined renal cell carcinoma.

We previously reported that tumour-associated caveolin-1 is a potential biomarker in renal cell carcinoma (RCC), whose overexpression predicts metastasis following surgical resection for clinically confined disease. Much attention has recently focused on the AKT/mTOR pathway in a number of malignancies, including RCC. Since caveolin-1 and the AKT/mTOR signalling cascade are independently shown to be important regulators of tumour angiogenesis, we hypothesised that caveolin-1 interacts with the AKT/mTOR pathway to drive disease progression and metastasis in RCC. The aims of this study were to determine (i) the expression status of the activated AKT/mTOR pathway components (phosphorylated forms) in RCC and (ii) their prognostic value when combined with caveolin-1. Immunohistochemistry for caveolin-1, pAKT, pmTOR, pS6 and p4E-BP1 was performed on tissue microarrays from 174 clinically confined RCCs. Significantly decreased mean disease-free survival was observed when caveolin-1 was coexpressed with either pAKT (2.95 vs 6.14 years), pmTOR (3.17 vs 6.28 years), pS6 (1.45 vs 6.62 years) or p4E-BP1 (2.07 vs 6.09 years) than when neither or any one single biomarker was expressed alone. On multivariate analysis, the covariate of 'caveolin-1/AKT' (neither alone were influential covariates) was a significant influential indicator of poor disease-free survival with a hazard ratio of 2.13 (95% CI: 1.15-3.92), higher than that for vascular invasion. Tumours that coexpressed caveolin-1 and activated mTOR components were more likely to be larger, higher grade and to show vascular invasion. Our results provide the first clinical evidence that caveolin-1 cooperates with an activated AKT/mTOR pathway in cancer and may play an important role in disease progression. We conclude that evaluation of the 'caveolin-1/AKT/mTOR axis' in primary kidney tumours will identify subsets of RCC patients who require greater postoperative surveillance and more intensive treatment.

Caveolin-1 overexpression predicts poor disease-free survival of patients with clinically confined renal cell carcinoma.

Renal cell carcinomas, although usually apparently fully resected at surgery, commonly recur as distant metastasis. New markers are needed to predict which patients may relapse especially as novel methods of treatment (e.g. laproscopic resection) may make it impossible to assess conventional pathological prognostic markers. The caveolins are a family of proteins that represent the major structural components of caveolae; recent work suggests that these may have influence on several signalling pathways and they are thus potential prognostic markers. Immunohistochemistry for caveolin-1 was performed on sections of peripheral tumour from 114 consecutative nonmetastatic RCCs. Cytoplasmic caveolin-1 immunohistochemical (ICC) reaction was scored on a semiquantative scale of 1-3. Immunohistochemical score was tested for impact on disease-free survival by Kaplan-Meier and Cox regression methods. A total of 50 tumours had ICC score 1; 43 had score 2 and 21 score 3. Larger, higher grade and tumours with vascular invasion had significantly higher scores. On univariate survival analysis (Kaplan-Meier), patients with tumours scoring 1 had a mean disease-free survival of 6.61 years (95% CI 5.76-7.46) compared with 5.4 years (4.53-6.30) and 3.15 years (1.87-4.44) for scores 2 and 3, respectively. This is a significant difference (P=0.0017 log rank test). On multivariate analysis with size, grade and caveolin ICC score as independent covariates, caveolin ICC score 3 was an influential predictor of poor disease-free survival with a hazard ratio of 2.6 (P=0.03). We conclude that cytoplasmic overexpression of caveolin-1 predicts a poor prognosis in RCC; that this is likely to be a useful prognostic marker and that it may have importance in tumour progression.

Downregulation and altered spatial pattern of caveolin-1 in chronic plaque psoriasis.

BACKGROUND: Caveolin-1 is a key structural and functional protein for plasmalemmal invaginations termed caveolae. Caveolin-1 is known to modulate multiple signal-transducing pathways involved in cell differentiation and proliferation. Psoriasis is viewed as a multifactorial pathology characterized by keratinocyte hyperproliferation and abnormal cell maturation. We hypothesized that loss of caveolin-1 within epidermal keratinocytes may contribute to the development and/or progression of the psoriatic phenotype. OBJECTIVES: To examine the expression and spatial distribution of caveolin-1 in skin biopsies from normal subjects and in patients with psoriasis. METHODS: Using immunohistochemical methods caveolin-1 protein expression was assayed in two independent patient groups. Firstly, a retrospective analysis was conducted on archival skin samples obtained from nine normal subjects and from involved tissue of 12 patients with psoriasis. Following this, a prospectively designed study was conducted in 10 further patients with active psoriasis and involving caveolin-1 staining of biopsy tissue from the uninvolved, advancing edge and lesional skin tissue from within the same subject. RESULTS: In normal skin or uninvolved skin from psoriasis patients intense caveolin-1 staining was present throughout full-thickness epidermis. In 20 of the 22 patient cases (combined retrospective and prospective samples) caveolin-1 protein was significantly reduced and consistently showed very weak or absent staining within the hyperproliferative basal cell layers of the psoriatic plaque (P < 0.002 for retrospective archival study and P < 0.01 for prospectively designed study). Comparisons between caveolin-1 staining in uninvolved tissue and at the advancing edge of a migrating plaque were more equivocal (P > 0.05). CONCLUSIONS: The findings of this study are consistent with a downregulation of caveolin-1 that may serve as an aetiological factor in the development and/or progression of psoriasis. Further mechanistic investigations are required with the potential that caveolin-1 protein may be a novel target for therapy of psoriasis.

Lung surfactant phospholipids inhibit the uptake of respirable microspheres by the alveolar macrophage NR8383.

Fluorescent poly(lactic-co-glycolic acid) microspheres of a respirable size were fabricated for use in a fluorescent activated cell sorting assay utilizing the continuous alveolar macrophage NR8383. This is a suitable model of alveolar phagocytosis, which permitted an investigation of the influence of phospholipid structure on the inhibition of phagocytosis of microspheres. Phospholipid inhibition was found to be independent of phosphatidylcholine alkyl chain length. Head group effects were investigated by studies employing phosphatidyl-choline, -serine, and -ethanolamine, and inhibition was shown to be independent of head group. Closer modelling of the lung environment by co-culturing NR8383 on A549 alveolar epithelium showed type II secretions to also down-regulate phagocytosis. In addition, pre-incubation with microspheres coated with dipalmitoylphosphatidylcholine reduced the uptake of a second microsphere (fluorescein isothiocyanate-labelled latex).

Polylysine and polyornithine gene transfer complexes: a study of complex stability and cellular uptake as a basis for their differential in-vitro transfection efficiency.

Gene transfer vectors formed between the cationic polyamino acid, poly-(L)-omithine (PLO) and plasmid DNA (pDNA) have demonstrated superior transfection efficiency (up to x 10-fold) compared to equivalent polylysine-based systems in-vitro. The mechanism(s) underlying this observation remains to be elucidated. We previously reported no significant difference in colloidal particle size or zeta potential of polycation/pDNA complexes formed with poly-(L)-lysine (PLL), poly-(D)-lysine (PDL) or PLO. Here we report spectrofluorometric analysis indicating that PLO condenses pDNA at lower charge (+/-) ratios than PLL or PDL (cf. 0.8:1, 1.2:1 and 1.5:1). Moreover, PLO/pDNA complexes proved more stable to disruption by the polyanions, poly-(L)-aspartic acid (PAA) and heparin. There were no qualitative differences in the ability of the polycations to protect complexed pDNA from enzymatic degradation both in the presence and in the absence of polyanions. The superior transfection efficiency of PLO/pDNA complexes did not appear to be mediated by an increased cellular delivery of pDNA. The data suggests a greater affinity of PLO for pDNA as an important parameter for the observed superior in-vitro transfection efficiency.

The inhibition of phagocytosis of respirable microspheres by alveolar and peritoneal macrophages.

Respirable poly(lactic co-glycolic acid) (PLGA) microspheres (2-3 microm diameter), were fabricated as a model drug delivery system whose uptake by macrophages could be quantified by fluorescent activated cell sorting. The microspheres exhibited minimal release of the entrapped flourophore (rhodamine B) and thus avoided possible fluid phase uptake of the flourophore. Externally bound microspheres were removed from the cell membrane by acid washing. The fluorescent intensity associated with the cells arose, therefore, from the internalised microspheres. NR8383 continuous culture alveolar macrophages were verified against primary cultures as a good model of alveolar phagocytosis. Peritoneal macrophages were also isolated and systemic and alveolar phagocytosis compared. Poloxamer 338 adsorbed at the microsphere surface did not reduce phagocytosis by NR8383 macrophages. It did, however, reduce the number of microspheres contained in primary alveolar macrophages but did not reduce the percentage of phagocytic cells. Poloxamer coatings did not reduce phagocytosis by peritoneal macrophages once the ratio of five microspheres per cell was exceeded. Dipalmitoylphosphatidylcholine (DPPC), the major component of lung surfactant, was added to cultures to model the alveolar environment where it was observed to reduce phagocytosis. In light of this finding, microspheres were coated in DPPC, which reduced their uptake by all cell types at all microsphere to cell ratios.

Progress and limitations in the use of in vitro cell cultures to serve as a permeability screen for the blood-brain barrier.

A relatively simple, widely applicable, and robust in vitro method of predicting blood-brain barrier (BBB) permeability to central nervous system-acting drugs is an increasing need. A cell-based model offers the potential to account for transcellular and paracellular drug diffusional processes, metabolism, and active transport processes, as well as nondefined interactions between a drug and cellular material that may impact upon a membrane's overall permeability profile. Any in vitro BBB cell model to be utilized for the transendothelial BBB permeability screening of potential central nervous system drugs must display reproducible solute permeability, and a number of other general criteria including: a restrictive paracellular barrier; a physiologically realistic cell architecture; the functional expression of key transporter mechanisms; and allow ease of culture to meet the technical and time constraints of a screening program. This article reviews the range of in vitro cell-based BBB models available, including the primary/low passage bovine and porcine brain endothelial cultures as well as the spectrum of immortalized brain endothelial cell lines that have been established. The article further discusses the benefits and limitations of exploiting such systems as in vitro BBB permeability screens.

Statistical modelling of the formulation variables in non-viral gene delivery systems.

Traditionally, optimisation of a gene delivery formulation utilises a study design that involves altering only one formulation variable at any one time whilst keeping the other variables constant. As gene delivery formulations become more complex, e.g. to include multiple cellular and sub-cellular targeting elements, there will be an increasing requirement to generate and analyse data more efficiently and allow examination of the interaction between variables. This study aims to demonstrate the utility of multifactorial design, specifically a Central Composite Design, in modelling the responses size, zeta potential and in vitro transfection efficiency of some prototypic non-viral gene delivery vectors. i.e. cationic liposome-pDNA complexes, and extending the application of the design strategy to more complex vectors, i.e. tri-component lipid:polycation:DNA (LPD). The modelled predictions of how the above responses change as a function of formulation show consistency with an extensive literature base of data obtained using more traditional approaches, and highlight the robustness and utility of the Central Composite Design in examining key formulation variables in non-viral gene delivery systems. The approach should be further developed to maximise the predictive impact of data across the full range of pharmaceutical sciences.

Caveolae as potential macromolecule trafficking compartments within alveolar epithelium.

With inhalational delivery the alveolar epithelium appears to be the appropriate lung surface to target for the systemic delivery of macromolecules, such as therapeutic proteins. The existence of a high numerical density of smooth-coated or non-coated plasma membrane vesicles or invaginations within the alveolar epithelial type I cell has long been recognised. The putative function of these vesicles in macromolecule transport remains the focus of research in both pulmonary physiology and pharmaceutical science disciplines. These vesicles, or subpopulations thereof, have been shown to biochemically possess caveolin, a marker protein for caveolae. This review considers the morphometric and biochemical studies that have progressed the characterisation of the vesicle populations within alveolar type I epithelium. Parallel research findings from the endothelial literature have been considered to contrast the state of progress of caveolae research in alveolar epithelium. Speculation is made on a model of caveolae vesicle-mediated transport that may satisfy some of the pulmonary pharmacokinetic data that has been generated for macromolecule absorption. The putative transport function of caveolae within alveolar epithelium is reviewed with respect to in-situ tracer studies conducted within the alveolar airspace. Finally, the functional characterisation of in-vitro alveolar epithelial cell cultures is considered with respect to the role of caveolae in macromolecule transport. A potentially significant role for alveolar caveolae in mediating the alveolar airspace to blood transport of macromolecules cannot be dismissed. Considerable research is required, however, to address this issue in a quantitative manner. A better understanding of the membrane dynamics of caveolae in alveolar epithelium will help resolve the function of these vesicular compartments and may lead to the development of more specific drug targeting approaches for promoting pulmonary drug delivery.

Caveolae and the caveolins in human disease.

There has been an exponential growth in caveolae research since the early 1990s. The caveolae membrane system comprises unique lipid and protein domains, and fulfills a role in a wide range of processes. At the plasma membrane caveolae serve to compartmentalise and integrate a wide range of signal transduction processes. A key structural and functional protein for caveolae is caveolin. Caveolin proteins possess a 'scaffolding' domain which for caveolins-1 and -3 appear central to many of the reported signal regulation functions for caveolae. Caveolae or caveolin protein are increasingly implicated in the molecular pathology of a number of diseases. Opportunities exist for basic and applied investigators working within the pharmaceutical sciences to exploit the caveolae membrane system to identify novel pharmacological targets and therapeutic strategies, including the delivery of pharmacologically active caveolin based peptides.

Caveolae: an alternative membrane transport compartment.

Caveolae are omega-shaped invaginations of the plasma membrane with a diameter of 50-100 nm. Caveolae invaginations can detach from the plasma membrane to form discrete functional caveolae vesicles within the cell cytoplasm. Caveolae are most prominent in adipocytes, fibroblasts, muscle cells (skeletal, smooth and cardiac), capillary endothelium and type I pneumocytes, although other cell types also display these structures but at a lower numerical density. The key structural and functional protein for caveolae is caveolin. At the plasma membrane caveolae serve to compartmentalize and integrate a wide range of signal transduction processes. Caveolae also serve transport functions including that of the vesicular internalisation of small molecules by the process of potocytosis, and the endocytic and transcytotic movements of macromolecules. Opportunities exist for basic and applied investigators working within the pharmaceutical sciences to exploit caveolae membrane interactions with the aim to develop novel cellular or transcellular drug delivery strategies.

Physical stability and in-vitro gene expression efficiency of nebulised lipid-peptide-DNA complexes.

The lower respiratory tract provides a number of disease targets for gene therapy. Nebulisation is the most practical system for the aerosolisation of non-viral gene delivery systems. The aerosolisation process represents a significant challenge to the maintenance of the physical stability and biological activity of the gene vector. In this study we investigate the role of a condensing polycationic peptide on the stability and efficiency of nebulised lipid-DNA complexes. Complexes prepared from the cationic lipid 1, 2-dioleoyl-3-trimethylammonium propane (DOTAP) and plasmid DNA (pDNA) at mass (w/w) ratios of 12:1, 6:1 and 3:1, and complexes prepared from DOTAP, the polycationic peptide, protamine, and pDNA (LPD) at 3:2:1 w/w ratio were nebulised using a Pari LC Plus jet nebuliser. Samples from the nebuliser reservoir (pre- and post-nebulisation) and from the aerosol mist were collected and investigated for changes, including: particle diameter, retention of in-vitro transfection activity and the relative concentration and nature of the complexed pDNA remaining after the nebulisation procedure. The process of jet nebulisation adversely affected the physical stability of lipid:pDNA complexes with only those formulated at 12:1 w/w DOTAP:pDNA able to maintain their pre-nebulisation particle size distribution (145+/-3 nm pre-nebulisation vs. 142+/-2 nm aerosol mist) and preserve significant pDNA integrity in the reservoir (35% of pre-nebulisation pDNA band intensity). The LPD complexes were smaller (102+/-1 nm pre-nebulisation vs. 113+/-2 nm aerosol mist) with considerably greater retention of pDNA integrity in the reservoir (90% of pre-nebulisation pDNA band intensity). In contrast the concentration of pDNA in the aerosol mist for both the 12:1 w/w DOTAP:pDNA and LPD complexes were significantly reduced (10 and 12% of pre-nebulised values, respectively). Despite reduced pDNA concentration the transfection (% cells transfected) mediated by aerosol mist for the nebulised complexes was comparatively efficient (LPD aerosol mist 26 vs. 40% for pre-nebulised complex; the respective values for 12: 1 w/w DOTAP:pDNA were 12 vs. 28%). The physical stability and biological activity of nebulised lipid:pDNA complexes can be improved by inclusion of a condensing polycationic peptide such as protamine. The incorporation of the peptide precludes the use of potentially toxic excesses of lipid and charge and may act as a platform for the covalent attachment of peptide signals mediating sub-cellular targetting.

Gene expression in an intact ex-vivo skin tissue model following percutaneous delivery of cationic liposome-plasmid DNA complexes.

The skin represents an attractive site for the localised gene therapy of dermatological pathologies and as a potential antigen bioreactor following transdermal delivery. Potential also exists for the gene therapy of skin as a cosmetic intervention. The most exploited non-viral gene delivery system involves the complexation of cationic liposomes with plasmid DNA (pDNA) to form lipid:pDNA vectors that protect the DNA from nuclease-mediated degradation and improve transgene-cell interactions. Despite numerous studies examining the potential for these vectors in delivering genes to a variety of keratinocyte models, investigations into the topical application of such complexes to intact skin tissue is limited. This ex-vivo study, conducted with intact skin tissue derived from hairless mice, provides quantitative confirmation that topical administration of cationic lipid:pDNA complexes can mediate uptake and expression of reporter pDNA (33-fold higher compared with control) in viable epidermal tissue. The ex-vivo study design provides for intact skin tissue that has not been subjected to depilatory procedures of potential detriment to stratum corneum barrier function, and can be utilised for the quantitative and efficient examination of a potentially wide range of non-viral gene vectors designed for epidermal expression.

Microcalorimetry does not predict the cellular phagocytosis of latex microspheres.

Current literature highlights the potential suitability of microcalorimetry for the investigation of cell-drug interactions. Previous work using bacteria or antigens derived from infectious organisms yielded conclusions that heat production is a quantitative means of measuring phagocytosis. In this study we evaluated the potential of flow-through microcalorimetry as a method of quantifying the phagocytosis of microsphere particulates. The technique avoids the need to incorporate radioactive or fluorescent markers into the particulate formulation, and would be widely applicable in biopharmaceutical research. Using the monocyte cell line Mono Mac 6 a power output of 9.00 microW per million cells was increased significantly on addition of zymosan, lipopolysaccaride (LPS) and phorbol myristate acetate but not following exposure to FITC labelled latex microspheres (LM). TNFalpha production increased on exposure to zymosan, LPS and LPS-phorbol myristate acetate, though not on exposure to LB. An assay was developed which allowed the quantification of internalised particulates in phagocytic cells using fluorescent activated cell sorting (FACS). In contrast to the microcalorimetric and TNFalpha data FACS revealed that 20% of the MM6 population phagocytosed a mean of 1.35 LM. Microcalorimetry and measurements of TNFalpha production are assays of cellular activation a phenomenon not necessarily associated with phagocytosis. FACS, however, serves as a specific and quantitative measure of phagocytosis. Microcalorimetry may not be a suitable technique for the quantitative assessment of the phagocytosis of drug delivery particulates.

Examination of the biophysical interaction between plasmid DNA and the polycations, polylysine and polyornithine, as a basis for their differential gene transfection in-vitro.

The impetus to develop non-viral gene delivery vectors has led to examination of synthetic polycationic polymers as plasmid DNA (pDNA) condensing agents. Previous reports have highlighted superiority (up to x 10-fold) in the in-vitro transfection of pDNA complexes formed by poly-(L)-ornithine (PLO) compared to those formed with poly-(L)-lysine (PLL). The apparent basis for this consistent superiority of PLO complexes remains to be established. This comparative study investigates whether physico chemical differences in the supramolecular properties of polycation:pDNA complexes provide a basis for their observed differential gene transfection. Specifically, particle size distribution and zeta potential of the above complexes formulated over a wide range of polycation:pDNA ratios were found to be consistent with a condensed (150-200 nm) cationic ( + 30-40 mV) system but not influenced by the type of cationic polymer used. A spectrofluorimetric EtBr exclusion assay showed that polycation:pDNA complexes display different pDNA condensation behaviour, with PLO able to condense pDNA at a lower polycation mass compared to both polylysine isomers, and form complexes that were more resistant to disruption following challenge with anionic counter species, i.e. poly-(L)-aspartic acid and the glycosaminoglycan molecule. heparin. We conclude that particle size and surface potential as gross supramolecular properties of these complexes do not represent, at least in a non-biological system, the basis for the differential transfection behaviour observed between these condensing polymers. However, differences in the ability of the polylysine and polyornithine polymers to interact with pDNA and to stabilise the polymer-pDNA assembly could have profound effects upon the cellular and sub-cellular biological processing of pDNA molecules and contribute to the disparity in cell transfection efficiency observed between these complexes.

The effect of fatty acids and analogues upon intracellular levels of doxorubicin in cells displaying P-glycoprotein mediated multidrug resistance.

Multidrug resistance mediated by overexpression of P-glycoprotein (P-gp) is a major obstacle in the chemotherapeutic management of cancer. The objectives of the current work were to examine if fatty acids affect the intracellular transport and dynamics of doxorubicin in drug-resistant cancer cell lines, and to assess if such effects were mediated through modulation of P-gp efflux pump activity. Among the range of fatty acids tested in this study, eicosapentaenoic acid diester (EPADI) increased doxorubicin accumulation [A] to 137% and retention [R] to 212% in doxorubicin-resistant MCF-7/ADR breast carcinoma cells, and [A] to 147% and [R] to 163% in vinblastine-resistant KBVI nasopharyngeal carcinoma cells. Consistent with EPADI-induced increases in intracellular doxorubicin concentrations, EPADI (10 microg/ml) sensitized MCF-7/ADR cells to the cytotoxic effects of doxorubicin (1 microg/ml) as assessed by MTT assay (viability < 50% of control), while EPADI itself displayed no cytotoxicity. The combination of EPADI (10 microg/ml) with verapamil (1 microM) resulted in a considerable increase in the [A] and [R] of the model P-gp substrate rhodamine-123 within drug-resistant cells compared to when either agent were used alone. KBV1 cells treated with combination of EPADI (10 microg/ml) and verapamil (1 microM) achieved 160% and 1120% greater [A] and [R] of rhodamine-123, respectively, compared to untreated cells. The P-gp modulatory effects of EPADI either alone, or as part of a combination with more potent inhibitors, should be further investigated.

Aberrant caveolin-1 expression in psoriasis: a signalling hypothesis.

A preliminary retrospective immunocytochemical study was conducted examining the expression of caveolin-1 in skin biopsies resected from clinically defined psoriatic subjects. These pilot investigations revealed a dramatic down-regulation of caveolin-1 (a protein product of the caveolin supergene family known to regulate signal transduction events and cell cycle dynamics) in the hyperproliferative basal regions of the epidermis in all psoriatic biopsies examined when compared to normal control samples. These results lead us to hypothesise that caveolin-1 negatively regulates key signal transduction pathways in epidermal keratinocytes and through it's reduced expression in psoriasis, pertubations in keratinocyte cell signalling and abnormal cell differentiation ensue, events fundamental to the development of the psoriatic phenotype. Novel therapeutic strategies for the treatment of psoriasis based upon caveolin-1 protein can be envisaged.

Caveolin-1 expression and caveolae biogenesis during cell transdifferentiation in lung alveolar epithelial primary cultures.

Caveolae are omega-shaped invaginations of the plasmalemma possessing a cytoplasmic membrane protein coat of caveolin. Caveolae are present in the in vivo alveolar epithelial type I (ATI) lung cell, but absent in its progenitor, the alveolar epithelial type II (ATII) cell. In primary culture ATII cells grown on a plastic substratum acquire with time an ATI-"like" phenotype. We demonstrate that freshly isolated rat ATII cells lack caveolae and expression of caveolin-1 (a critical caveolae structural protein). As the ATII cells acquire an ATI-like phenotype in primary culture caveolin-1 expression increases, with caveolin-1 signal at 192 h postseeding up to 50-fold greater than at 60 h; caveolae were morphologically evident only after 132 h. When maintaining the differentiated ATII phenotype with time, i.e., culture upon collagen with an apical interface of air, a temporal increase in caveolin-1 expression was not observed, with only very faint signals evident even at 192 h postseeding; at no time did these cultures display caveolae. In late primary ATII cultures caveolin-1 expression and caveolae biogenesis occur as a function of in vitro transformation from the ATII to the ATI-like phenotype. The results have broad implications for the in vitro study of the role of caveolae and caveolin in alveolar epithelial cell biology.

Transport of phosphatidylcholine in MDR3-negative epithelial cell lines via drug-induced MDR1 P-glycoprotein.

Human MDR1 P-glycoprotein (P-gp) is a membrane efflux pump for cytotoxics, whereas MDR3 P-gp is a phosphatidylcholine transporter. We have examined a role for MDR1 P-gp in phosphatidylcholine transport in MDR3-negative epithelial cells that have been induced to express the MDR1 P-gp by exposure to cytotoxics. The accumulation and retention of the fluorescently labelled phosphatidylcholine analogue, C12-NBD-PC, was studied in resistant, KBV1 and MCFadr, and sensitive, KB3-1 and MCF7, cells. Lower accumulation and decreased retention of C12-NBD-PC was evident in resistant cells, e.g., KBV1 accumulated 56%, and MCFadr accumulated 60%, of C12-NBD-PC levels in KB3-1 and MCF7, respectively. Treatment with the MDR1 P-gp inhibitor, verapamil, altered the kinetics of C12-NBD-PC in the resistant cells to more closely follow the pattern of C12-NBD-PC handling by sensitive cells. Comparison of C12-NBD-PC to that of the model MDR1 P-gp substrate, rhodamine-123, indicated phosphatidylcholine turnover kinetics by MDR1 P-gp to be relatively low. The transport by MDR1 P-gp of phosphatidylcholine from inner to outer membrane leaflet may regulate P-gp function and fulfill a role in the MDR1 multidrug-resistant phenotype.