Elevation of glucosylceramide in multidrug-resistant cancer cells and accumulation in cytoplasmic droplets

Lipid metabolism as a target for cancer drug resistance: progress and prospects

Cancer is the world's leading cause of human death today, and the treatment process of cancer is highly complex. Chemotherapy and targeted therapy are commonly used in cancer treatment, and the emergence of drug resistance is a significant problem in cancer treatment. Therefore, the mechanism of drug resistance during cancer treatment has become a hot issue in current research. A series of studies have found that lipid metabolism is closely related to cancer drug resistance. This paper details the changes of lipid metabolism in drug resistance and how lipid metabolism affects drug resistance. More importantly, most studies have reported that combination therapy may lead to changes in lipid-related metabolic pathways, which may reverse the development of cancer drug resistance and enhance or rescue the sensitivity to therapeutic drugs. This paper summarizes the progress of drug design targeting lipid metabolism in improving drug resistance, and providing new ideas and strategies for future tumor treatment. Therefore, this paper reviews the issues of combining medications with lipid metabolism and drug resistance.

Cholesterol-Depletion-Induced Membrane Repair Carries a Raft Conformer of P-Glycoprotein to the Cell Surface, Indicating Enhanced Cholesterol Trafficking in MDR Cells, Which Makes Them Resistant to Cholesterol Modifications

The human P-glycoprotein (P-gp), a transporter responsible for multidrug resistance, is present in the plasma membrane's raft and non-raft domains. One specific conformation of P-gp that binds to the monoclonal antibody UIC2 is primarily associated with raft domains and displays heightened internalization in cells overexpressing P-gp, such as in NIH-3T3 MDR1 cells. Our primary objective was to investigate whether the trafficking of this particular P-gp conformer is dependent on cholesterol levels. Surprisingly, depleting cholesterol using cyclodextrin resulted in an unexpected increase in the proportion of raft-associated P-gp within the cell membrane, as determined by UIC2-reactive P-gp. This increase appears to be a compensatory response to cholesterol loss from the plasma membrane, whereby cholesterol-rich raft micro-domains are delivered to the cell surface through an augmented exocytosis process. Furthermore, this exocytotic event is found to be part of a complex trafficking mechanism involving lysosomal exocytosis, which contributes to membrane repair after cholesterol reduction induced by cyclodextrin treatment. Notably, cells overexpressing P-gp demonstrated higher total cellular cholesterol levels, an increased abundance of stable lysosomes, and more effective membrane repair following cholesterol modifications. These modifications encompassed exocytotic events that involved the transport of P-gp-carrying rafts. Importantly, the enhanced membrane repair capability resulted in a durable phenotype for MDR1 expressing cells, as evidenced by significantly improved viabilities of multidrug-resistant Pgp-overexpressing immortal NIH-3T3 MDR1 and MDCK-MDR1 cells compared to their parents when subjected to cholesterol alterations.

Application of Lipidomics for Assessing Tissue Lipid Profiles of Patients With Squamous Cell Carcinoma

Background: Lipid metabolism disorders play a key role in the pathogenesis of squamous cell carcinoma (SqCC). Herein we used lipidomics to study the tissue lipid profiles of 40 patients with SqCC. Methods: Lipidomics, based on ultrahigh-performance liquid chromatography-Q Exactive hybrid quadrupole-orbitrap high-resolution accurate mass spectrometry, was applied to identify altered lipid metabolites between tumor and adjacent noninvolved tissues (ANIT), and partial least squares-discriminant analysis model facilitated the identification of differentially abundant lipids. The area under the receiver operator characteristic curve and variable importance in projection scores of the aforementioned model were calculated to select lipid profiles. Metabolic pathway analyses were completed using Kyoto Encyclopedia of Genes and Genomes and MetaboAnalyst. Results: Differences in lipid profiles were found between tumor and ANIT, early- and advanced-stage SqCC, and positive and negative lymph node metastases. The lipid profile panel was composed of five lipids-PC(44:4), diacylglycerol(36:5), sphingomyelin(d18:1/20:0), phosphatidylinositol(46:7), and HexCer-AP(t8:0/32:2 + O)-and could effectively differentiate between tumor and ANIT. Further, pathway analyses revealed alterations in several lipid metabolism pathways, including glycerophospholipid metabolism, glycosylphosphatidylinositol anchor biosynthesis, linoleic acid metabolism, glycerolipid metabolism, and sphingolipid metabolism. Conclusion: Our data revealed several changes in the tissue lipid profiles of patients with SqCC; moreover, we identified a lipid profile panel that could effectually distinguish tumor tissues from ANIT. We believe that our results provide new insights into the biological behavior of lung SqCC.

Metabolic reprograming confers tamoxifen resistance in breast cancer

Breast cancer is the most common cancer among females and the leading cause of cancer-related deaths. Approximately 70 % of breast cancers are estrogen receptor (ER) positive. An ER antagonist such as tamoxifen is used as adjuvant therapy in ER-positive patients. The major problem with endocrine therapy is the emergence of acquired resistance in approximately 40 % of patients receiving tamoxifen. Metabolic alteration is one of the hallmarks of cancer cells. Rapidly proliferating cancer cells require increased nutritional support to fuel various functions such as proliferation, cell migration, and metastasis. Recent studies have established that the metabolic state of cancer cells influences their susceptibility to chemotherapeutic drugs and that cancer cells reprogram their metabolism to develop into resistant phenotypes. In this review, we discuss the major findings on metabolic pathway alterations in tamoxifen-resistant (TAMR) breast cancer and the molecular mechanisms known to regulate the expression and function of metabolic enzymes and the respective metabolite levels upon tamoxifen treatment. It is anticipated that this in-depth analysis of specific metabolic pathways in TAMR cancer might be exploited therapeutically.

An overview of therapeutic potential of N-alkylated 1-deoxynojirimycin congeners

Polyhydroxylated alkaloids display a wide range of biological activities, suggesting their use in the treatment of various diseases. Their most famous representative, 1-deoxynojirimycin (DNJ), is a natural product that shows α- and β-glucosidase inhibition. This molecule has been since converted into two clinically approved drugs i.e., Zavesca® and Glyset®, targeting type I Gaucher's disease and type II diabetes mellitus, respectively. This review examines the therapeutic potential of important DNJ congeners reported in last decade and presents concise mechanism of glycosidase inhibition. A brief overview of substituents conjugation's impact on DNJ scaffold (including N-alkylated DNJ derivatives, mono-valent, di-valent and multivalent DNJ congeners, N-[5-(adamantan-1-yl-methoxy)-pentyl]-1-deoxynojirimycin (AMP-DNM) look alike DNJ based lipophilic derivatives, AMP-DNM based neoglycoconjugates, DNJ click derivatives with varying carboxylic acids and aromatic moieties, conjugates of DNJ and glucose, and N-bridged DNJ analogues) towards various enzymes such as α/β glucosidase, porcine trehalase, as F508del-CFTR correctors, α-mannosidase, human placental β-glucocerebrosidase, N370S β-GCase, α-amylase and insect trehalase as potent and selective inhibitors have been discussed with potential bioactivities, which can provide inspiration for future studies.

Glucosylceramide and galactosylceramide, small glycosphingolipids with significant impact on health and disease

Numerous clinical observations and exploitation of cellular and animal models indicate that glucosylceramide (GlcCer) and galactosylceramide (GalCer) are involved in many physiological and pathological phenomena. In many cases, the biological importance of these monohexosylcermides has been shown indirectly as the result of studies on enzymes involved in their synthesis and degradation. Under physiological conditions, GalCer plays a key role in the maintenance of proper structure and stability of myelin and differentiation of oligodendrocytes. On the other hand, GlcCer is necessary for the proper functions of epidermis. Such an important lysosomal storage disease as Gaucher disease (GD) and a neurodegenerative disorder as Parkinson’s disease are characterized by mutations in the GBA1 gene, decreased activity of lysosomal GBA1 glucosylceramidase and accumulation of GlcCer. In contrast, another lysosomal disease, Krabbe disease, is associated with mutations in the GALC gene, resulting in deficiency or decreased activity of lysosomal galactosylceramidase and accumulation of GalCer and galactosylsphingosine. Little is known about the role of both monohexosylceramides in tumor progression; however, numerous studies indicate that GlcCer and GalCer play important roles in the development of multidrug-resistance by cancer cells. It was shown that GlcCer is able to provoke immune reaction and acts as a self-antigen in GD. On the other hand, GalCer was recognized as an important cellular receptor for HIV-1. Altogether, these two molecules are excellent examples of how slight differences in chemical composition and molecular conformation contribute to profound differences in their physicochemical properties and biological functions.

Dynamic Signatures of Lipid Droplets as New Markers to Quantify Cellular Metabolic Changes

The metabolic properties of live cells are very susceptible to intra- or extracellular perturbations, making their measurements challenging tasks. We show that the dynamics of lipid droplets (LDs) carry information to measure the lipid metabolism of live cells. Coherent anti-Stokes Raman scattering microscopy was used to statistically quantify LD dynamics in living cells in a label-free manner. We introduce dynamic signatures of cells derived from the LD displacement, speed, travel length, and directionality, which allows for the detection of cellular changes induced by stimuli such as fluorescent labeling, temperature change, starvation, and chemical treatment. Histogram fittings of the dynamic signatures using log-normal distribution functions provide quantification of changes in cellular metabolic states. The LD dynamics also enable separation of subpopulations of LDs correlated with different functions. We demonstrate that LD dynamics measured by chemical imaging are new markers to quantify the metabolic changes in live cells.

On the nature of ceramide-mitochondria interactions - Dissection using comprehensive mitochondrial phenotyping

Sphingolipids are a unique class of lipids owing to their non-glycerol-containing backbone, ceramide, that is constructed from a long-chain aliphatic amino alcohol, sphinganine, to which a fatty acid is attached via an amide bond. Ceramide plays a star role in the initiation of apoptosis by virtue of its interactions with mitochondria, a control point for a downstream array of signaling cascades culminating in apoptosis. Many pathways converge on mitochondria to elicit mitochondrial outer membrane permeabilization (MOMP), a step that corrupts bioenergetic service. Although much is known regarding ceramides interaction with mitochondria and the ensuing cell signal transduction cascades, how ceramide impacts the elements of mitochondrial bioenergetic function is poorly understood. The objective of this review is to introduce the reader to sphingolipid metabolism, present a snapshot of mitochondrial respiration, elaborate on ceramides convergence on mitochondria and the upstream players that collaborate to elicit MOMP, and introduce a mitochondrial phenotyping platform that can be of utility in dissecting the fine-points of ceramide impact on cellular bioenergetics.

Glucosylceramide production maintains colon integrity in response to Bacteroides fragilis toxin-induced colon epithelial cell signaling

Enterotoxigenic Bacteroides fragilis (ETBF) is a commensal bacterium of great importance to human health due to its ability to induce colitis and cause colon tumor formation in mice through the production of B. fragilis toxin (BFT). The formation of tumors is dependent on a pro-inflammatory signaling cascade, which begins with the disruption of epithelial barrier integrity through cleavage of E-cadherin. Here, we show that BFT increases levels of glucosylceramide, a vital intestinal sphingolipid, both in mice and in colon organoids (colonoids) generated from the distal colons of mice. When colonoids are treated with BFT in the presence of an inhibitor of glucosylceramide synthase (GCS), the enzyme responsible for generating glucosylceramide, colonoids become highly permeable, lose structural integrity, and eventually burst, releasing their contents into the extracellular matrix. By increasing glucosylceramide levels in colonoids via an inhibitor of glucocerebrosidase (GBA, the enzyme that degrades glucosylceramide), colonoid permeability was reduced, and bursting was significantly decreased. In the presence of BFT, pharmacological inhibition of GCS caused levels of tight junction protein 1 (TJP1) to decrease. However, when GBA was inhibited, TJP1 levels remained stable, suggesting that BFT-induced production of glucosylceramide helps to stabilize tight junctions. Taken together, our data demonstrate a glucosylceramide-dependent mechanism by which the colon epithelium responds to BFT.

Pisum sativum Defensin 1 Eradicates Mouse Metastatic Lung Nodules from B16F10 Melanoma Cells

Psd1 is a pea plant defensin which can be actively expressed in Pichia pastoris and shows broad antifungal activity. This activity is dependent on fungal membrane glucosylceramide (GlcCer), which is also important for its internalization, nuclear localization, and endoreduplication. Certain cancer cells present a lipid metabolism imbalance resulting in the overexpression of GlcCer in their membrane. In this work, in vitroassays using B16F10 cells showed that labeled fluorescein isothiocyanate FITC-Psd1 internalized into live cultured cells and targeted the nucleus, which underwent fragmentation, exhibiting approximately 60% of cells in the sub-G0/G1 stage. This phenomenon was dependent on GlcCer, and the participation of cyclin-F was suggested. In a murine lung metastatic melanoma model, intravenous injection of Psd1 together with B16F10 cells drastically reduced the number of nodules at concentrations above 0.5 mg/kg. Additionally, the administration of 1 mg/kg Psd1 decreased the number of lung inflammatory cells to near zero without weight loss, unlike animals that received melanoma cells only. It is worth noting that 1 mg/kg Psd1 alone did not provoke inflammation in lung tissue or weight or vital signal losses over 21 days, inferring no whole animal cytotoxicity. These results suggest that Psd1 could be a promising prototype for human lung anti-metastatic melanoma therapy.

Imaging of cancer lipid metabolism in response to therapy

Lipids represent a diverse array of molecules essential to the cell's structure, defense, energy, and communication. Lipid metabolism can often become dysregulated during tumor development. During cancer therapy, targeted inhibition of cell proliferation can likewise cause widespread and drastic changes in lipid composition. Molecular imaging techniques have been developed to monitor altered lipid profiles as a biomarker for cancer diagnosis and treatment response. For decades, MRS has been the dominant non-invasive technique for studying lipid metabolite levels. Recent insights into the oncogenic transformations driving changes in lipid metabolism have revealed new mechanisms and signaling molecules that can be exploited using optical imaging, mass spectrometry imaging, and positron emission tomography. These novel imaging modalities have provided researchers with a diverse toolbox to examine changes in lipids in response to a wide array of anticancer strategies including chemotherapy, radiation therapy, signal transduction inhibitors, gene therapy, immunotherapy, or a combination of these strategies. The understanding of lipid metabolism in response to cancer therapy continues to evolve as each therapeutic method emerges, and this review seeks to summarize the current field and areas of unmet needs.

Valspodar-modulated chemotherapy in human ovarian cancer cells SK-OV-3 and MDAH-2774

Overcoming drug resistance in ovarian cancer is the overarching goal in gynecologic oncology. One way to increase drug cytotoxicity without increasing the drug dose is to simultaneously apply multidrug resistance modulator. Valspodar is the second generation P-glycoprotein 1 modulator capable of reversing multidrug resistance in different cancers. In this study, we evaluated the effect of valspodar and cisplatin co-treatment on cell viability, cell death and oxidative status in ovarian cancer cells. Two human ovarian cancer cell lines SK-OV-3 and MDAH-2774 were treated with cisplatin, valspodar, or cisplatin + valspodar for 24 or 48 hours. Untreated cells were used as control group. Cell viability was evaluated by MTT assay. Cell death was assessed by TUNEL and comet assay. Lipid peroxidation (malondialdehyde) and protein thiol groups were analyzed as oxidative stress markers. The expression of mitochondrial superoxide dismutase (MnSOD) was assessed by immunocytochemistry. Valspodar effectively reduced the resistance of SK-OV-3 cells to cisplatin, as demonstrated by increased oxidative stress, decreased cell viability and increased apoptosis in SK-OV-3 cells co-treated with valspodar and cisplatin compared to other groups. However, valspodar did not significantly affect the resistance of MDAH-2774 cells to cisplatin. Stronger staining for MnSOD in MDAH-2774 vs. SK-OV-3 cells after co-treatment with cisplatin and valspodar may determine the resistance of MDAH-2774 cell line to cisplatin.

Distinguishing the differences in β-glycosylceramidase folds, dynamics, and actions informs therapeutic uses

Glycosyl hydrolases (GHs) are carbohydrate-active enzymes that hydrolyze a specific β-glycosidic bond in glycoconjugate substrates; β-glucosidases degrade glucosylceramide, a ubiquitous glycosphingolipid. GHs are grouped into structurally similar families that themselves can be grouped into clans. GH1, GH5, and GH30 glycosidases belong to clan A hydrolases with a catalytic (β/α)8 TIM barrel domain, whereas GH116 belongs to clan O with a catalytic (α/α)6 domain. In humans, GH abnormalities underlie metabolic diseases. The lysosomal enzyme glucocerebrosidase (family GH30), deficient in Gaucher disease and implicated in Parkinson disease etiology, and the cytosol-facing membrane-bound glucosylceramidase (family GH116) remove the terminal glucose from the ceramide lipid moiety. Here, we compare enzyme differences in fold, action, dynamics, and catalytic domain stabilization by binding site occupancy. We also explore other glycosidases with reported glycosylceramidase activity, including human cytosolic β-glucosidase, intestinal lactase-phlorizin hydrolase, and lysosomal galactosylceramidase. Last, we describe the successful translation of research to practice: recombinant glycosidases and glucosylceramide metabolism modulators are approved drug products (enzyme replacement therapies). Activity-based probes now facilitate the diagnosis of enzyme deficiency and screening for compounds that interact with the catalytic pocket of glycosidases. Future research may deepen the understanding of the functional variety of these enzymes and their therapeutic potential.

The Onus of Sphingolipid Enzymes in Cancer Drug Resistance

Chemotherapy resistance, inherent or acquired, represents a serious barrier to the successful treatment of cancer. Although drug efflux, conducted by plasma membrane-resident proteins, detoxification enzymes, cell death inhibition, and DNA damage repair are ensemble players in this unwanted biology, a full understanding of the many in concert molecular mechanisms driving drug resistance is lacking. Recent discoveries in sphingolipid (SL) metabolism have provided significant insight into the role of these lipids in cancer growth; however, considerably less is known with respect to SLs and the drug-resistant phenotype. One exception here is enhanced ceramide glycosylation, a hallmark of multidrug resistance that is believed responsible, in part, for diminishing ceramides tumor-suppressor potential. This chapter will review various aspects of SL biology that relate to chemotherapy resistance and extend this topic to acknowledge the role of chemotherapy selection pressure in promoting dysregulated SL metabolism, a characteristic in cancer and an exploitable target for therapy.

Single gold nanoparticle plasmonic spectroscopy for study of chemical-dependent efflux function of single ABC transporters of single live Bacillus subtilis cells

ATP-binding cassette (ABC) membrane transporters serve as self-defense transport apparatus in many living organisms and they can selectively extrude a wide variety of substrates, leading to multidrug resistance (MDR). The detailed molecular mechanisms remain elusive. Single nanoparticle plasmonic spectroscopy highly depends upon their sizes, shapes, chemical and surface properties. In our previous studies, we have used the size-dependent plasmonic spectra of single silver nanoparticles (Ag NPs) to study the real-time efflux kinetics of the ABC (BmrA) transporter and MexAB-OprM transporter in single live cells (Gram-positive and Gram-negative bacterium), respectively. In this study, we prepared and used purified, biocompatible and stable (non-aggregated) gold nanoparticles (Au NPs) (12.4 ± 0.9 nm) to study the efflux kinetics of single BmrA membrane transporters of single live Bacillus subtillis cells, aiming to probe chemical dependent efflux functions of BmrA transporters and their potential chemical sensing capability. Similar to those observed using Ag NPs, accumulation of the intracellular Au NPs in single live cells (WT and ΔBmrA) highly depends upon the cellular expression of BmrA and the NP concentration (0.7 and 1.4 nM). The lower accumulation of intracellular Au NPs in WT (normal expression of BmrA) than ΔBmrA (deletion of bmrA) indicates that BmrA extrudes the Au NPs out of the WT cells. The accumulation of Au NPs in the cells increases with NP concentration, suggesting that the Au NPs most likely passively diffuse into the cells, similar to antibiotics. The result demonstrates that such small Au NPs can serve as imaging probes to study the efflux function of the BmrA membrane transporter in single live cells. Furthermore, the dependence of the accumulation rate of intracellular Au NPs in single live cells upon the expression of BmrA and the concentration of the NPs is about twice higher than that of the same sized Ag NPs. This interesting finding suggests the chemical-dependent efflux kinetics of BmrA and that BmrA could distinguish nearly identical sized Au NPs from Ag NPs and might possess chemical sensing machinery.

The expanding role of sphingolipids in lipid droplet biogenesis

Sphingolipids are a diverse class of lipids that have regulatory, structural, and metabolic functions. Although chemically distinct from the neutral lipids and the glycerophospholipids, which are the main lipid components of the lipid droplets, sphingolipids have nonetheless been shown to influence lipid droplet formation. The goal of this article is to review the available information and provide a cohesive picture of the role sphingolipids play in lipid droplet biogenesis. The following topics are discussed: (i) the abundance of sphingolipids in lipid droplets and their functional significance; (ii) cross-talk between the synthetic pathways of sphingolipids, glycerophospholipids, and neutral lipids; (iii) the impact of bioactive sphingolipids on TAG synthesis and degradation; (iv) interactions between sphingolipids and other lipid droplet components, like cholesterol esters and proteins; (v) inhibition/genetic deletion of specific sphingolipid metabolic enzymes and the resulting effects on lipid droplet formation in mouse models. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Sphingolipid abnormalities in cancer multidrug resistance: Chicken or egg?

The cancer multidrug resistance (MDR) phenotype encompasses a myriad of molecular, genetic and cellular alterations resulting from progressive oncogenic transformation and selection. Drug efflux transporters, in particular the MDR P-glycoprotein ABCB1, play an important role in MDR but cannot confer the complete phenotype alone indicating parallel alterations are prerequisite. Sphingolipids are essential constituents of lipid raft domains and directly participate in functionalization of transmembrane proteins, including providing an optimal lipid microenvironment for multidrug transporters, and are also perturbed in cancer. Here we postulate that increased sphingomyelin content, developing early in some cancers, recruits and functionalizes plasma membrane ABCB1 conferring a state of partial MDR, which is completed by glycosphingolipid disturbance and the appearance of intracellular vesicular ABCB1. In this review, the independent and interdependent roles of sphingolipid alterations and ABCB1 upregulation during the transformation process and resultant conferment of partial and complete MDR phenotypes are discussed.

Inhibition of Human Drug Transporter Activities by the Pyrethroid Pesticides Allethrin and Tetramethrin

Pyrethroids are widely-used chemical insecticides, to which humans are commonly exposed, and known to alter functional expression of drug metabolizing enzymes. Limited data have additionally suggested that drug transporters, that constitute key-actors of the drug detoxification system, may also be targeted by pyrethroids. The present study was therefore designed to analyze the potential regulatory effects of these pesticides towards activities of main ATP-binding cassette (ABC) and solute carrier (SLC) drug transporters, using transporter-overexpressing cells. The pyrethroids allethrin and tetramethrin were found to inhibit various ABC and SLC drug transporters, including multidrug resistance-associated protein (MRP) 2, breast cancer resistance protein (BCRP), organic anion transporter polypeptide (OATP) 1B1, organic anion transporter (OAT) 3, multidrug and toxin extrusion transporter (MATE) 1, organic cation transporter (OCT) 1 and OCT2, with IC50 values however ranging from 2.6 μM (OCT1 inhibition by allethrin) to 77.6 μM (OAT3 inhibition by tetramethrin) and thus much higher than pyrethroid concentrations (in the nM range) reached in environmentally pyrethroid-exposed humans. By contrast, allethrin and tetramethrin cis-stimulated OATP2B1 activity and failed to alter activities of OATP1B3, OAT1 and MATE2-K, whereas P-glycoprotein activity was additionally moderately inhibited. Twelve other pyrethoids used at 100 μM did not block activities of the various investigated transporters, or only moderately inhibited some of them (inhibition by less than 50%). In silico analysis of structure-activity relationships next revealed that molecular parameters, including molecular weight and lipophilicity, are associated with transporter inhibition by allethrin/tetramethrin and successfully predicted transporter inhibition by the pyrethroids imiprothrin and prallethrin. Taken together, these data fully demonstrated that two pyrethoids, i.e., allethrin and tetramethrin, can act as regulators of the activity of various ABC and SLC drug transporters, but only when used at high and non-relevant concentrations, making unlikely any contribution of these transporter activity alterations to pyrethroid toxicity in environmentally exposed humans.

Drug Transporter Expression and Activity in Human Hepatoma HuH-7 Cells

Human hepatoma cells may represent a valuable alternative to the use of human hepatocytes for studying hepatic drug transporters, which is now a regulatory issue during drug development. In the present work, we have characterized hepatic drug transporter expression, activity and regulation in human hepatoma HuH-7 cells, in order to determine the potential relevance of these cells for drug transport assays. HuH-7 cells displayed notable multidrug resistance-associated protein (MRP) activity, presumed to reflect expression of various hepatic MRPs, including MRP2. By contrast, they failed to display functional activities of the uptake transporters sodium taurocholate co-transporting polypeptide (NTCP), organic anion-transporting polypeptides (OATPs) and organic cation transporter 1 (OCT1), and of the canalicular transporters P-glycoprotein and breast cancer resistance protein (BCRP). Concomitantly, mRNA expressions of various sinusoidal and canalicular hepatic drug transporters were not detected (NTCP, OATP1B1, organic anion transporter 2 (OAT2), OCT1 and bile salt export pump) or were found to be lower (OATP1B3, OATP2B1, multidrug and toxin extrusion protein 1, BCRP and MRP3) in hepatoma HuH-7 cells than those found in human hepatocytes, whereas other transporters such as OAT7, MRP4 and MRP5 were up-regulated. HuH-7 cells additionally exhibited farnesoid X receptor (FXR)- and nuclear factor erythroid 2-related factor 2 (Nrf2)-related up-regulation of some transporters. Such data indicate that HuH-7 cells, although expressing rather poorly some main hepatic drug transporters, may be useful for investigating interactions of drugs with MRPs, notably MRP2, and for studying FXR- or Nrf2-mediated gene regulation.

Nanoparticle-based drug delivery in cancer: the role of cell membrane structures

Development of novel drug-delivery systems aims to specifically deliver anticancer drugs to tumor tissues and improve the efficiency of chemotherapy, while minimizing side effects of drugs on healthy tissues and organs. However, drug-delivery systems are confronted by membrane barriers and multiple drug resistance in cancer cells. In recent years, the obtained results indicate an important role of lipids, proteins and carbohydrates in apoptosis, drug transport and the process of cellular uptake of nanoparticles via endocytosis. This article discusses the hypothesis of the relationship between cell membrane structure and nanoparticles in cancer cells.

Single Nanoparticle Plasmonic Spectroscopy for Study of Charge-Dependent Efflux Function of Multidrug ABC Transporters of Single Live Bacillus subtilis Cells

Multidrug membrane transporters can selectively extrude a wide variety of structurally and functionally unrelated substrates, and they are responsible for ineffective treatment of a wide range of diseases (e.g., infection and cancer). Their underlying molecular mechanisms remain elusive. In this study, we functionalized Ag NPs (11 nm in diameter) with two biocompatible peptides (CALNNK, CALNNE) to prepare positively and negatively charged Ag-peptide NPs (Ag-CALNNK NPs+ζ, Ag-CALNNE NPs-4ζ), respectively. We used them as photostable plasmonic imaging probes to study charge-dependent efflux kinetics of BmrA (ABC) membrane transporter of single live Bacillus (B.) subtilis cells. Two strains of the cells, normal expression of BmrA (WT) or devoid of BmrA (ΔBmrA), were used to study the charge-dependent efflux kinetics of single NPs upon the expression of BmrA. The NPs (1.4 nM) were stable (non-aggregated) in a PBS buffer and biocompatible to the cells. We found the high dependent accumulation of the intracellular NPs in both WT and ΔBmrA upon the charge and concentration of NPs. Notably, the accumulation rates of the positively charged NPs in single live WT cells are nearly identical to those in ΔBmrA cells, showing independence upon the expression of BmrA. In contrast, the accumulation rates of the negatively charged NPs in WT are much lower than in ΔBmrA, showing high dependence upon the expression of BmrA and suggesting that BmrA extrude the negatively charged NPs, but not positively charged NPs, out of the WT. The accumulation of positively charged NPs in both WT and ΔBmrA increases nearly proportionally to the NP concentration. The accumulation of negatively charged NPs in ΔBmrA, but not in WT, also increases nearly proportionally to the NP concentration. These results suggest that both negatively and positively charged NPs enter the cells via passive diffusion driven by concentration gradients across the cellular membrane, and BmrA can only extrude the negatively charged NPs out of the WT. This study shows that single NP plasmon spectroscopy can serve as a powerful tool to identify single plasmonic NPs and to probe the charge-dependent efflux kinetics and function of single membrane transporters in single live cells in real time.

Lipid Droplets: A Key Cellular Organelle Associated with Cancer Cell Survival under Normoxia and Hypoxia

The Warburg effect describes the phenomenon by which cancer cells obtain energy from glycolysis even under normoxic (O₂-sufficient) conditions. Tumor tissues are generally exposed to hypoxia owing to inefficient and aberrant vasculature. Cancer cells have multiple molecular mechanisms to adapt to such stress conditions by reprogramming the cellular metabolism. Hypoxia-inducible factors are major transcription factors induced in cancer cells in response to hypoxia that contribute to the metabolic changes. In addition, cancer cells within hypoxic tumor areas have reduced access to serum components such as nutrients and lipids. However, the effect of such serum factor deprivation on cancer cell biology in the context of tumor hypoxia is not fully understood. Cancer cells are lipid-rich under normoxia and hypoxia, leading to the increased generation of a cellular organelle, the lipid droplet (LD). In recent years, the LD-mediated stress response mechanisms of cancer cells have been revealed. This review focuses on the production and functions of LDs in various types of cancer cells in relation to the associated cellular environment factors including tissue oxygenation status and metabolic mechanisms. This information will contribute to the current understanding of how cancer cells adapt to diverse tumor environments to promote their survival.

Evaluation of P-Glycoprotein Inhibitory Potential Using a Rhodamine 123 Accumulation Assay

In vitro evaluation of P-glycoprotein (P-gp) inhibitory potential is now a regulatory issue during drug development, in order to predict clinical inhibition of P-gp and subsequent drug-drug interactions. Assays for this purpose, commonly based on P-gp-expressing cell lines and digoxin as a reference P-gp substrate probe, unfortunately exhibit high variability, raising thus the question of developing alternative or complementary tests for measuring inhibition of P-gp activity. In this context, the present study was designed to investigate the use of the fluorescent dye rhodamine 123 as a reference P-gp substrate probe for characterizing P-gp inhibitory potential of 16 structurally-unrelated drugs known to interact with P-gp. 14/16 of these P-gp inhibitors were found to increase rhodamine 123 accumulation in P-gp-overexpressing MCF7R cells, thus allowing the determination of their P-gp inhibitory potential, i.e., their half maximal inhibitor concentration (IC50) value towards P-gp-mediated transport of the dye. These IC50 values were in the range of variability of previously reported IC50 for P-gp and can be used for the prediction of clinical P-gp inhibition according to Food and Drug Administration (FDA) criteria, with notable sensitivity (80%). Therefore, the data demonstrated the feasibility of the use of rhodamine 123 for evaluating the P-gp inhibitory potential of drugs.

Single Nanoparticle Plasmonic Spectroscopy for Study of the Efflux Function of Multidrug ABC Membrane Transporters of Single Live Cells

ATP-binding cassette (ABC) membrane transporters exist in all living organisms and play key roles in a wide range of cellular and physiological functions. The ABC transporters can selectively extrude a wide variety of structurally and functionally unrelated substrates, leading to multidrug resistance. Despite extensive study, their efflux molecular mechanisms remain elusive. In this study, we synthesized and characterized purified silver nanoparticles (Ag NPs) (97 ± 13 nm in diameter), and used them as photostable optical imaging probes to study efflux kinetics of ABC membrane transporters (BmrA) of single live cells (B. subtillis). The NPs with concentrations up to 3.7 pM were stable (non-aggregated) in a PBS buffer and biocompatible with the cells. We found a high dependence of accumulation of the intracellular NPs in single live cells (WT, Ct-BmrA-EGFP, ΔbmrA) upon the cellular expression level of BmrA and NP concentration (0.93, 1.85 and 3.7 pM), showing the highest accumulation of intracellular NPs in ΔbmrA (deletion of BmrA) and the lowest ones in Ct-BmrA-EGFP (over-expression of BmrA). Interestingly, the accumulation of intracellular NPs in ΔbmrA increases nearly proportionally with the NP concentration, while those in WT and Ct-BrmA-EGFP do not. This suggests that the NPs enter the cells via passive diffusion driven by concentration gradients and are extruded out of cells by BmrA transporters, similar to conventional pump substrates (antibiotics). This study shows that such large substrates (84-100 nm NPs) can enter into the live cells and be extruded out of the cells by BmrA, and the NPs can serve as nm-sized optical imaging probes to study the size-dependent efflux kinetics of membrane transporters in single live cells in real time.

Glucosylceramide synthase inhibitors differentially affect expression of glycosphingolipids

Glucosylceramide synthase (GCS) catalyzes the first committed step in the biosynthesis of glucosylceramide (GlcCer)-related glycosphingolipids (GSLs). Although inhibitors of GCS, PPMP and PDMP have been widely used to elucidate their biological function and relevance, our comprehensive literature review revealed that the available data are ambiguous. We therefore investigated whether and to what extent GCS inhibitors affect the expression of lactosylceramide (LacCer), neolacto (nLc4 and P1), ganglio (GM1 and GD3) and globo (Gb3 and SSEA3) series GSLs in a panel of human cancer cell lines using flow cytometry, a commonly applied method investigating cell-surface GSLs after GCS inhibition. Their cell-surface GSL expression considerably varied among cell lines and more importantly, sublethal concentrations (IC10) of both inhibitors preferentially and significantly reduced the expression of Gb3 in the cancer cell lines IGROV1, BG1, HT29 and T47D, whereas SSEA3 was only reduced in BG1. Unexpectedly, the neolacto and ganglio series was not affected. LacCer, the precursor of all GlcCer-related GSL, was significantly reduced only in BG1 cells treated with PPMP. Future research questions addressing particular GSLs require careful consideration; our results indicate that the extent to which there is a decrease in the expression of one or more particular GSLs is dependent on the cell line under investigation, the type of GCS inhibitor and exposure duration.

FTIR spectroscopy reveals lipid droplets in drug resistant laryngeal carcinoma cells through detection of increased ester vibrational bands intensity

The major obstacle to successful chemotherapy of cancer patients is drug resistance. Previously we explored the molecular mechanisms of curcumin cross-resistance in carboplatin resistant human laryngeal carcinoma 7T cells. Following curcumin treatment we found a reduction in curcumin accumulation, and reduced induction of reactive oxygen species (ROS) and their downstream effects, compared to parental HEp-2 cells. In order to shed more light on mechanisms involved in drug resistance of 7T cells, in the present study we applied Fourier transform infrared (FTIR) spectroscopy, a technique that provides information about the nature and quantities of all molecules present in the cell. By comparing the spectra from parental HEp-2 cells and their 7T subline, we found an increase in the intensity of ester vibrational bands in 7T cells. This implied an increase in the amount of cholesteryl esters in resistant cells, which we confirmed by an enzymatic assay. Since cholesteryl esters are localized in lipid droplets, we confirmed their higher quantity and serum dependency in 7T cells compared to HEp-2 cells. Moreover, treatment with oleic acid induced more lipid droplets in 7T when compared to HEp-2 cells, as shown by flow cytometry. We can conclude that along with previously determined molecular mechanisms of curcumin resistance in 7T cells, these cells exhibit an increased content of cholesteryl esters and lipid droplets, suggesting an alteration in cellular lipid metabolism as a possible additional mechanism of drug resistance. Furthermore, our results suggest the use of FTIR spectroscopy as a promising technique in drug resistance research.

Dynamics of ceramide generation and metabolism in response to fenretinide--Diversity within and among leukemia

Fenretinide, N-(4-hydroxyphenyl)retinamide, (4-HPR), a synthetic retinoid, owes its cancer-toxic effects in part to the generation of ceramide, a potent tumor-suppressing sphingolipid. As such, 4-HPR has garnered considerable interest as a chemotherapeutic. Cancer cells, however, via various metabolic routes, inactivate ceramide, and this can limit 4-HPR efficacy. As relatively little is known regarding 4-HPR-induced ceramide management in acute myelogeneous leukemia (AML), we undertook the present study to evaluate the impact of 4-HPR on ceramide production, metabolism, and cytotoxicity. In KG-1, HL-60, and HL-60/VCR (multidrug resistant) human leukemia cells, 4-HPR induced 15-, 2-, and 20-fold increases in ceramide (measured using [3H]palmitic acid), respectively. By use of specific inhibitors we show that ceramide was produced by sphingomyelinase and de novo pathways in response to 4-HPR exposure. HL-60/VCR cells metabolized ceramide to glucosylceramide (GC). 4-HPR exposure (1.25-10 μM) reduced viability in all cell lines, with approximate IC50's ranging from 1 to 8.0 μM. Reactive oxygen species (ROS) were generated in response to 4-HPR treatment, and the concomitant cytotoxicity was reversed by addition of vitamin E. 4-HPR was not cytotoxic nor did it elicit ceramide formation in K562, a chronic myeloid leukemia cell line; however, K562 cells were sensitive to a cell-deliverable form of ceramide, C6-ceramide. Treatment of Molt-3, an acute lymphoblastic leukemia cell line, with 4-HPR revealed moderate ceramide production (5-fold over control), robust conversion of ceramide to GC and sphingomyelin, and resistance to 4-HPR and C6-ceramide. In conclusion, this work demonstrates diversity within and among leukemia in 4-HPR sensitivity and ceramide generation and subsequent metabolism. As such, knowledge of these metabolic pathways can provide guidance for enhancing ceramide-driven effects of 4-HPR in treatment of leukemia.

Nature and uses of fluorescent dyes for drug transporter studies

INTRODUCTION

Drug transporters are now recognized as major players involved in pharmacokinetics and toxicology. Methods for assessing their activity are important to consider, particularly owing to regulatory requirements with respect to inhibition of drug transporter activity and prediction of drug-drug interactions. In this context, the use of fluorescent-dye-based transport assays is likely to deserve attention.

AREAS COVERED

This review provides an overview of the nature of fluorescent dye substrates for ATP-binding cassette and solute carrier drug transporters. Their use for investigating drug transporter activity in cultured cells and clinical hematological samples, drug transporter inhibition, drug transporter imaging and drug transport at the organ level are summarized.

EXPERT OPINION

A wide range of fluorescent dyes is now available for use in various aspects of drug transporter studies. The use of these dyes for transporter analyses may, however, be hampered by classic pitfalls of fluorescence technology, such as quenching. Transporter-independent processes such as passive diffusion of dyes through plasma membrane or dye sequestration into subcellular compartments must also be considered, as well as the redundant handling by various distinct transporters of some fluorescent probes. Finally, standardization of dye-based transport assays remains an important on-going issue.

Targeting glucosylceramide synthase induction of cell surface globotriaosylceramide (Gb3) in acquired cisplatin-resistance of lung cancer and malignant pleural mesothelioma cells

BACKGROUND

Acquired resistance to cisplatin treatment is a caveat when treating patients with non-small cell lung cancer (NSCLC) and malignant pleural mesothelioma (MPM). Ceramide increases in response to chemotherapy, leading to proliferation arrest and apoptosis. However, a tumour stress activation of glucosylceramide synthase (GCS) follows to eliminate ceramide by formation of glycosphingolipids (GSLs) such as globotriaosylceramide (Gb3), the functional receptor of verotoxin-1. Ceramide elimination enhances cell proliferation and apoptosis blockade, thus stimulating tumor progression. GSLs transactivate multidrug resistance 1/P-glycoprotein (MDR1) and multidrug resistance-associated protein 1 (MRP1) expression which further prevents ceramide accumulation and stimulates drug efflux. We investigated the expression of Gb3, MDR1 and MRP1 in NSCLC and MPM cells with acquired cisplatin resistance, and if GCS activity or MDR1 pump inhibitors would reduce their expression and reverse cisplatin-resistance.

METHODS

Cell surface expression of Gb3, MDR1 and MRP1 and intracellular expression of MDR1 and MRP1 was analyzed by flow cytometry and confocal microscopy on P31 MPM and H1299 NSCLC cells and subline cells with acquired cisplatin resistance. The effect of GCS inhibitor PPMP and MDR1 pump inhibitor cyclosporin A for 72h on expression and cisplatin cytotoxicity was tested.

RESULTS

The cisplatin-resistant cells expressed increased cell surface Gb3. Cell surface Gb3 expression of resistant cells was annihilated by PPMP whereas cyclosporin A decreased Gb3 and MDR1 expression in H1299 cells. No decrease of MDR1 by PPMP was noted in using flow cytometry, whereas a decrease of MDR1 in H1299 and H1299res was indicated with confocal microscopy. No certain co-localization of Gb3 and MDR1 was noted. PPMP, but not cyclosporin A, potentiated cisplatin cytotoxicity in all cells.

CONCLUSIONS

Cell surface Gb3 expression is a likely tumour biomarker for acquired cisplatin resistance of NSCLC and MPM cells. Tumour cell resistance to MDR1 inhibitors of cell surface MDR1 and Gb3 could explain the aggressiveness of NSCLC and MPM. Therapy with GCS activity inhibitors or toxin targeting of the Gb3 receptor may substantially reduce acquired cisplatin drug resistance of NSCLC and MPM cells.

Tamoxifen regulation of sphingolipid metabolism--Therapeutic implications

Tamoxifen, a triphenylethylene antiestrogen and one of the first-line endocrine therapies used to treat estrogen receptor-positive breast cancer, has a number of interesting, off-target effects, and among these is the inhibition of sphingolipid metabolism. More specifically, tamoxifen inhibits ceramide glycosylation, and enzymatic step that can adventitiously support the influential tumor-suppressor properties of ceramide, the aliphatic backbone of sphingolipids. Additionally, tamoxifen and metabolites N-desmethyltamoxifen and 4-hydroxytamoxifen, have been shown to inhibit ceramide hydrolysis by the enzyme acid ceramidase. This particular intervention slows ceramide destruction and thereby depresses formation of sphingosine 1-phosphate, a mitogenic sphingolipid with cancer growth-promoting properties. As ceramide-centric therapies are becoming appealing clinical interventions in the treatment of cancer, agents like tamoxifen that can retard the generation of mitogenic sphingolipids and buffer ceramide clearance via inhibition of glycosylation, take on new importance. In this review, we present an abridged, lay introduction to sphingolipid metabolism, briefly chronicle tamoxifen's history in the clinic, examine studies that demonstrate the impact of triphenylethylenes on sphingolipid metabolism in cancer cells, and canvass works relevant to the use of tamoxifen as adjuvant to drive ceramide-centric therapies in cancer treatment. The objective is to inform the readership of what could be a novel, off-label indication of tamoxifen and structurally-related triphenylethylenes, an indication divorced from estrogen receptor status and one with application in drug resistance.

Regulation of human hepatic drug transporter activity and expression by diesel exhaust particle extract

Diesel exhaust particles (DEPs) are common environmental air pollutants primarily affecting the lung. DEPs or chemicals adsorbed on DEPs also exert extra-pulmonary effects, including alteration of hepatic drug detoxifying enzyme expression. The present study was designed to determine whether organic DEP extract (DEPe) may target hepatic drug transporters that contribute in a major way to drug detoxification. Using primary human hepatocytes and transporter-overexpressing cells, DEPe was first shown to strongly inhibit activities of the sinusoidal solute carrier (SLC) uptake transporters organic anion-transporting polypeptides (OATP) 1B1, 1B3 and 2B1 and of the canalicular ATP-binding cassette (ABC) efflux pump multidrug resistance-associated protein 2, with IC50 values ranging from approximately 1 to 20 μg/mL and relevant to environmental exposure situations. By contrast, 25 μg/mL DEPe failed to alter activities of the SLC transporter organic cation transporter (OCT) 1 and of the ABC efflux pumps P-glycoprotein and bile salt export pump (BSEP), whereas it only moderately inhibited those of sodium taurocholate co-transporting polypeptide and of breast cancer resistance protein (BCRP). Treatment by 25 μg/mL DEPe was next demonstrated to induce expression of BCRP at both mRNA and protein level in cultured human hepatic cells, whereas it concomitantly repressed mRNA expression of various transporters, including OATP1B3, OATP2B1, OCT1 and BSEP. Such changes in transporter expression were found to be highly correlated to those caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a reference activator of the aryl hydrocarbon receptor (AhR) pathway. This suggests that DEPe, which is enriched in known ligands of AhR like polycyclic aromatic hydrocarbons, alters drug transporter expression via activation of the AhR cascade. Taken together, these data established human hepatic transporters as targets of organic chemicals containing in DEPs, which may contribute to their systemic effects through impairing hepatic transport of endogenous compound or drug substrates of these transporters.

Evolving concepts in cancer therapy through targeting sphingolipid metabolism

Traditional methods of cancer treatment are limited in their efficacy due to both inherent and acquired factors. Many different studies have shown that the generation of ceramide in response to cytotoxic therapy is generally an important step leading to cell death. Cancer cells employ different methods to both limit ceramide generation and to remove ceramide in order to become resistant to treatment. Furthermore, sphingosine kinase activity, which phosphorylates sphingosine the product of ceramide hydrolysis, has been linked to multidrug resistance, and can act as a strong survival factor. This review will examine several of the most frequently used cancer therapies and their effect on both ceramide generation and the mechanisms employed to remove it. The development and use of inhibitors of sphingosine kinase will be focused upon as an example of how targeting sphingolipid metabolism may provide an effective means to improve treatment response rates and reduce associated treatment toxicity. This article is part of a Special Issue entitled Tools to study lipid functions.

Regulation of hepatic drug transporter activity and expression by organochlorine pesticides

Organochlorine (OC) pesticides constitute a major class of persistent and toxic organic pollutants, known to modulate drug-detoxifying enzymes. In the present study, OCs were demonstrated to also alter the activity and expression of human hepatic drug transporters. Activity of the sinusoidal influx transporter OCT1 (organic cation transporter 1) was thus inhibited by endosulfan, chlordane, heptachlor, lindane, and dieldrine, but not by dichlorodiphenyltrichloroethane isomers, whereas those of the canalicular efflux pumps MRP2 (multidrug resistance-associated protein 2) and BCRP (breast cancer resistance protein) were blocked by endosulfan, chlordane, heptachlor, and chlordecone; this latter OC additionally inhibited the multidrug resistance gene 1 (MDR1)/P-glycoprotein (P-gp) activity. OCs, except endosulfan, were next found to induce MDR1/P-gp and MRP2 mRNA expressions in hepatoma HepaRG cells; some of them also upregulated BCRP. By contrast, expression of sinusoidal transporters was not impaired (organic anion-transporting polypeptide (OATP) 1B1 and OATP2B1) or was downregulated (sodium taurocholate co-transporting polypeptide (NTCP) and OCT1). Such regulations of drug transporter activity and expression, depending on the respective nature of OCs and transporters, may contribute to the toxicity of OC pesticides.

Sphingolipid metabolic pathway: an overview of major roles played in human diseases

Sphingolipids, a family of membrane lipids, are bioactive molecules that participate in diverse functions controlling fundamental cellular processes such as cell division, differentiation, and cell death. Given that most of these cellular processes form the basis for several pathologies, it is not surprising that sphingolipids are key players in several pathological processes. This review discusses the role of the sphingolipid metabolic pathway in diabetes, Alzheimer's disease, and hepatocellular carcinoma, with a special emphasis on the changes in gene expression pattern in these disease conditions. For convenience, the sphingolipid metabolic pathway is divided into hypothetical compartments (modules) with each compartment representing a physiological process and changes in gene expression pattern are mapped to each of these modules. It appears that alterations in the gene expression pattern in these disease conditions are biased to manipulate the system in order to result in a particular disease.

Use of acid sphingomyelinase for cancer therapy

Acid sphingomyelinase (ASM) is a lipid hydrolase that cleaves the sphingolipid, sphingomyelin, into ceramide. Mutations in the ASM gene (SMPD1) result in the rare lysosomal storage disorder, Niemann-Pick disease (NPD). In addition to its role in NPD, over the past two decades, the importance of sphingolipids, and ASM in particular, in normal physiology and the pathophysiology of numerous common diseases also has become known. For example, altered sphingolipid metabolism occurs in many cancers, generally reducing the levels of the pro-apoptotic lipid, ceramide, and/or elevating the levels of the proliferative lipid, sphingosine-1-phosphate (S1P). These changes likely contribute to the tumorigenicity and/or metastatic capacity of the cancer. In addition, many cancer therapies induce ceramide-mediated death, and cancer cells have evolved novel mechanisms to overcome this effect. In the present review, we discuss sphingolipid metabolism in cancer, and specifically the potential for pharmacological modulation using ASM. Of note, recombinant human ASM (rhASM) has been produced for human use and is being evaluated as a treatment for NPD. Thus, its use for cancer therapy could be rapidly evaluated in the clinic after appropriate animal model studies have been completed. As this enzyme was initially studied in the context of NPD, we start with a brief overview of the history of ASM and NPD, followed by a discussion of the role of ASM in cancer biology, and then summarize emerging preclinical efficacy studies using rhASM as an adjunct in the treatment of solid tumors.

Natural products as platforms for the design of sphingolipid-related anticancer agents

Modulation of sphingolipid metabolism is a promising strategy for cancer therapy that has already opened innovative approaches for the development of pharmacological tools and rationally designed new drugs. On the other hand, natural products represent a classical and well-established source of chemical diversity that has guided medicinal chemists on the development of new chemical entities with potential therapeutic use. Based on these premises, the aim of this chapter is to provide the reader with a general overview of some of the most representative families of sphingolipid-related natural products that have been described in the recent literature as lead compounds for the design of new modulators of sphingolipid metabolism. Special emphasis is placed on the structural aspects of natural sphingoids and synthetic analogs that have found application as anticancer agents. In addition, their cellular targets and/or their mode of action are also considered.

Sphingolipids and response to chemotherapy

Chemotherapy is frequently used to treat primary or metastatic cancers, but intrinsic or acquired drug resistance limits its efficiency. Sphingolipids are important regulators of various cellular processes including proliferation, apoptosis, differentiation, angiogenesis, stress, and inflammatory responses which are linked to various aspects of cancer, like tumor growth, neoangiogenesis, and response to chemotherapy. Ceramide, the central molecule of sphingolipid metabolism, generally mediates antiproliferative and proapoptotic functions, whereas sphingosine-1-phosphate and other derivatives have opposing effects. Among the variety of enzymes that control ceramide generation, acid or neutral sphingomyelinases and ceramide synthases are important targets to allow killing of cancer cells by chemotherapeutic drugs. On the contrary, glucosylceramide synthase, ceramidase, and sphingosine kinase are other targets driving cancer cell resistance to chemotherapy. This chapter focuses on ceramide-based mechanisms leading to cancer therapy sensitization or resistance which could have some impacts on the development of novel cancer therapeutic strategies.

Shiga toxins

Shiga toxins are virulence factors produced by the bacteria Shigella dysenteriae and certain strains of Escherichia coli. There is currently no available treatment for disease caused by these toxin-producing bacteria, and understanding the biology of the Shiga toxins might be instrumental in addressing this issue. In target cells, the toxins efficiently inhibit protein synthesis by inactivating ribosomes, and they may induce signaling leading to apoptosis. To reach their cytoplasmic target, Shiga toxins are endocytosed and transported by a retrograde pathway to the endoplasmic reticulum, before the enzymatically active moiety is translocated to the cytosol. The toxins thereby serve as powerful tools to investigate mechanisms of intracellular transport. Although Shiga toxins are a serious threat to human health, the toxins may be exploited for medical purposes such as cancer therapy or imaging.

Microfluidity mapping using fluorescence correlation spectroscopy: a new way to investigate plasma membrane microorganization of living cells

Diffusion time distribution analysis has been employed to highlight the microfluidity fingerprint of plasma membrane of living cells. Diffusion time measurements were obtained through fluorescence correlation spectroscopy performed at the single cell level, over various eukaryotic cell lines (MCF7, LR73, KB3.1, MESSA and MDCKII). The nonsymmetric profile of the diffusion time distributions established experimentally, is discussed according to Monte Carlo simulations, which reproduce the diffusion of the fluorescent probe in heterogeneous membrane.

Multi-modal strategies for overcoming tumor drug resistance: hypoxia, the Warburg effect, stem cells, and multifunctional nanotechnology

Inefficiencies in systemic drug delivery and tumor residence as well as micro-environmental selection pressures contribute to the development of multidrug resistance (MDR) in cancer. Characteristics of MDR include abnormal vasculature, regions of hypoxia, up-regulation of ABC-transporters, aerobic glycolysis, and an elevated apoptotic threshold. Nano-sized delivery vehicles are ideal for treating MDR cancer as they can improve the therapeutic index of drugs and they can be engineered to achieve multifunctional parameters. The multifunctional ability of nanocarriers makes them more adept at treating heterogeneous tumor mass than traditional chemotherapy. Nanocarriers also have preferential tumor accumulation via the EPR effect; this accumulation can be further enhanced by actively targeting the biological profile of MDR cells. Perhaps the most significant benefit of using nanocarrier drug delivery to treat MDR cancer is that nanocarrier delivery diverts the effects of ABC-transporter mediated drug efflux; which is the primary mechanism of MDR. This review discusses the capabilities, applications, and examples of multifunctional nanocarriers for the treatment of MDR. This review emphasizes multifunctional nanocarriers that enhance drug delivery efficiency, the application of RNAi, modulation of the tumor apoptotic threshold, and physical approaches to overcome MDR.

The effect of glucosylceramide synthase on P-glycoprotein function in K562/AO2 leukemia drug-resistance cell line

Previous work from our laboratory demonstrated that glucosylceramide synthase (GCS) and multidrug resistance 1 gene (MDR1) are co-overexpressed in drug-resistant leukemia cells. We hypothesized that GCS and MDR1 may interact. In this study, we used RNA interference (RNAi) to silence the GCS or MDR1 gene in K562/AO2 drug-resistant cells. The sensitivity of cells to different treatments with doxorubicin was evaluated. We used Taqman probe fluorescence real-time quantitative PCR, and detected expression of GCS and MDR1 mRNAs in different interfering groups. Intracellular mean fluorescence intensity (MFI), which represents rhodamine123 (rh123) retention, was determined by flow cytometry (FCM). An MTT cytotoxicity assay showed that the 50% inhibition concentration (IC50) of doxorubicin of K562/AO2 cells (138.25 ± 3.75 µg/ml) was significantly higher than that of K562 drug-sensitive cells (2.125 ± 0.125 µg/ml), and that IC50 was evidently lower in K562/AO2 cells, whether it was transfected with a small interfering RNA (siRNA) targeting GCS (GCSsiRNA) or one targeting MDR1 (MDR1siRNA). Compared with untreated K562/AO2 cells, the inhibition rates of GCS mRNA in the cells transfected with GCSsiRNA for 9 and 36 h were 56.67 ± 9.29% (p < 0.05) and 74 ± 6.38% (p < 0.05), respectively. Interestingly, the expression of MDR1 mRNA was also inhibited to 51.7 ± 4.5% (p < 0.05) 36 h after transfection with GCSsiRNA, but there was no significant difference in MDR1 expression at 9 h post-transfection in cells treated with GCSsiRNA and a negative control. It is well known that rh123 retention in cells results from an efflux function of P-glycoprotein (P-gp). In K562 cells, rh123 retention was much higher than in K562/AO2 cells (p < 0.01). We also noted that rh123 retention in the K562/AO2 cells transfected with GCSsiRNA for 48 h was significantly higher than in the negative control group. In conclusion, we show in the present study that inhibition of the GCS gene affects the expression of MDR1 mRNA and P-gp function.

Design and probing of efflux functions of EGFP fused ABC membrane transporters in live cells using fluorescence spectroscopy

We have designed and constructed fusion genes of C-terminal (Ct) or N-terminal (Nt) bmrA with EGFP vectors and successfully expressed them in ΔBmrA (BmrA deletion strain of Bacillus subtilis), generating two new strains of B. subtilis (Ct-BmrA-EGFP and Nt-BmrA-EGFP). The fusion genes were characterized using gel electrophoresis and DNA sequencing. Their expression in live cells was determined by measuring the fluorescence of EGFP in single live cells using fluorescence microscopy and spectroscopy. The efflux function of the new strains was studied by measuring their accumulation kinetics of intracellular Hoechst dye molecules (a pump substrate) using fluorescence spectroscopy, which were compared with wild-type (WT-BmrA) and ΔBmrA strains. Both new strains show lower accumulation rates than ΔBmrA, and their efflux kinetics are inhibited by a pump inhibitor (orthovanadate). The results suggest that both strains extrude the dye molecules and the fusion proteins retain the efflux function of BmrA (ATP-binding cassette, ABC, transporter). Notably, Nt-BmrA-EGFP strain shows lower accumulation rates (higher efflux rates) than Ct-BmrA-EGFP. Modeled structures of the fusion proteins illustrate a highly flexible linker region connecting EGFP with BmrA, suggesting a minimal obstruction of EGFP to the BmrA. A closer distance of two C termini (~14 Å) than two N termini (47.9 Å) of the "closed" BmrA dimer depicts the larger steric effect of C-terminal fusion. This study also shows that glucose affects the fluorescence study of efflux function of BmrA, suggesting that efflux kinetics of ABC membrane transporters in live cells must be characterized in the absence of glucose.

Ceramide glucosyltransferase of the nematode Caenorhabditis elegans is involved in oocyte formation and in early embryonic cell division

Ceramide glucosyltransferase (Ugcg) [uridine diphosphate (UDP)-glucose:N-acylsphingosine D-glucosyltransferase or UDP-glucose ceramide glucosyltransferase (GlcT): EC 2.4.1.80] catalyzes formation of glucosylceramide (GlcCer) from ceramide and UDP-glucose. There is only one Ugcg gene in the mouse genome, which is essential in embryogenesis and brain development. The nematode Caenorhabditis elegans has three Ugcg genes (cgt-1, cgt-2 and cgt-3), and double RNAi of the cgt-1 and cgt-3 genes results in lethality at the L1 larval stage. In this study, we isolated knockout worms for the three genes and characterized the gene functions. Each gene product showed active enzymatic activity when expressed in GM95 cells deficient in glycosphingolipids (GSLs). When each gene function was disrupted, the brood size of the animal markedly decreased, and abnormal oocytes and multinucleated embryos were formed. The CGT-3 protein had the highest Ugcg activity, and knockout of its gene resulted in the severest phenotype. When cgt-3 RNAi was performed on rrf-1 worms lacking somatic RNAi machinery but with intact germline RNAi machinery, a number of abnormal oocytes and multinucleated eggs were observed, although the somatic phenotype, i.e., L1 lethal effects of cgt-1/cgt-3 RNAi, was completely suppressed. Cell surface expression of GSLs and sphingomyelin, which are important components of membrane domains, was affected in the RNAi-treated embryos. In the embryos, an abnormality in cytokinesis was also observed. From these results, we concluded that the Ugcg gene is indispensable in the germline and that an ample supply of GlcCer is needed for oocytes and fertilized eggs to maintain normal membranes and to proceed through the normal cell cycle.

Apoptotic sphingolipid ceramide in cancer therapy

Apoptosis, also called programmed cell death, is physiologically and pathologically involved in cellular homeostasis. Escape of apoptotic signaling is a critical strategy commonly used for cancer tumorigenesis. Ceramide, a derivative of sphingolipid breakdown products, acts as second messenger for multiple extracellular stimuli including growth factors, chemical agents, and environmental stresses, such as hypoxia, and heat stress as well as irradiation. Also, ceramide acts as tumor-suppressor lipid because a variety of stress stimuli cause apoptosis by increasing intracellular ceramide to initiate apoptotic signaling. Defects on ceramide generation and sphingolipid metabolism are developed for cancer cell survival and cancer therapy resistance. Alternatively, targeting ceramide metabolism to correct these defects might provide opportunities to overcome cancer therapy resistance.