Membrane insertion and topology of the amino-terminal domain TMD0 of multidrug-resistance associated protein 6 (MRP6)

The Expression Level of ABCC6 Transporter in Colon Cancer Cells Correlates with the Activation of Different Intracellular Signaling Pathways

The ATP-binding cassette sub-family C member 6 transporter (ABCC6) is mainly found in the basolateral plasma membrane of hepatic and kidney cells. In hepatocarcinoma HepG2 cells, ABCC6 was involved in cell migration. In the present study, we investigated the role of ABCC6 in colon cancer evaluating the effect of Quercetin and Probenecid, inhibitors of the ectonucleotidase NT5E and ABCC6, respectively, on migration rate of Caco2 and HT29 cell lines. Both drugs reduced cell migration analyzed by scratch test. Gene and protein expression were evaluated by quantitative reverse-transcription PCR (RT-qPCR) and Western blot, respectively. In Caco2 cells, in which ABCC6 is significantly expressed, the addition of ATP restored motility, suggesting the involvement of P2 receptors. Contrary to HT29 cells, where the expression of ABCC6 is negligible but remarkable to the level of NT5E, no effect of ATP addition was detected, suggesting a main role on their migration by the phosphatidylinositol 3′-kinase (PI3K)/Akt system. Therefore, in some colon cancers in which ABCC6 is overexpressed, it may have a primary role in controlling the extracellular purinergic system by feeding it with ATP, thus representing a potential target for a therapy aimed at mitigating invasiveness of those type of cancers.

Effect of Quercetin on ABCC6 Transporter: Implication in HepG2 Migration

Quercetin is a member of the flavonoid group of compounds, which is abundantly present in various dietary sources. It has excellent antioxidant properties and anti-inflammatory activity and is very effective as an anti-cancer agent against various types of tumors, both in vivo and in vitro. Quercetin has been also reported to modulate the activity of some members of the multidrug-resistance transporters family, such as P-gp, ABCC1, ABCC2, and ABCG2, and the activity of ecto-5′-nucleotidase (NT5E/CD73), a key regulator in some tumor processes such as invasion, migration, and metastasis. In this study, we investigated the effect of Quercetin on ABCC6 expression in HepG2 cells. ABCC6 is a member of the superfamily of ATP-binding cassette (ABC) transporters, poorly involved in drug resistance, whose mutations cause pseudoxanthoma elasticum, an inherited disease characterized by ectopic calcification of soft connective tissues. Recently, it has been reported that ABCC6 contributes to cytoskeleton rearrangements and HepG2 cell motility through purinergic signaling. Gene and protein expression were evaluated by quantitative Reverse-Transcription PCR (RT-qPCR) and western blot, respectively. Actin cytoskeleton dynamics was evaluated by laser confocal microscopy using fluorophore-conjugated phalloidin. Cell motility was analyzed by an in vitro wound-healing migration assay. We propose that ABCC6 expression may be controlled by the AKT pathway as part of an adaptative response to oxidative stress, which can be mitigated by the use of Quercetin-like flavonoids.

Circulating Tumour Cells as Prognostic Biomarkers in Colorectal Cancer: A Systematic Review

Despite therapeutic advances, colorectal cancer (CRC) is still one of the deadliest cancers, partly due to local recurrence and metastatic disease. Tumour cells that spread by gaining access to peripheral blood are called circulating tumour cells (CTCs). These may be present before there are any clinical signs, but can be detected within blood samples. CTCs from patients with CRC may be isolated in a laboratory for characterization and multiple analyses. In this review, we focus on the prognostic potential of CTCs detection, by evaluating the reported progress and applications of such analyses. Our search found 77 relevant studies that reported CTC detection in CRC. Both cell count and features were reported as promising prognosis biomarkers. Since CTCs are rare and can lose their differentiation, new tools are being developed to improve detection. CTCs may have potential as prognostic biomarkers for CRC in terms of survival prediction, anticipating chemotherapy resistance, and surgical planning. CTCs are not yet used in clinical practice, and further investigations are required in order to better frame their practical value.

Effect of Quercetin on ABCC6 Transporter: Implication in HepG2 Migration

Quercetin is a member of the flavonoid group of compounds, which is abundantly present in various dietary sources. It has excellent antioxidant properties and anti-inflammatory activity and is very effective as an anti-cancer agent against various types of tumors, both in vivo and in vitro. Quercetin has been also reported to modulate the activity of some members of the multidrug-resistance transporters family, such as P-gp, ABCC1, ABCC2, and ABCG2, and the activity of ecto-5′-nucleotidase (NT5E/CD73), a key regulator in some tumor processes such as invasion, migration, and metastasis. In this study, we investigated the effect of Quercetin on ABCC6 expression in HepG2 cells. ABCC6 is a member of the superfamily of ATP-binding cassette (ABC) transporters, poorly involved in drug resistance, whose mutations cause pseudoxanthoma elasticum, an inherited disease characterized by ectopic calcification of soft connective tissues. Recently, it has been reported that ABCC6 contributes to cytoskeleton rearrangements and HepG2 cell motility through purinergic signaling. Gene and protein expression were evaluated by quantitative Reverse-Transcription PCR (RT-qPCR) and western blot, respectively. Actin cytoskeleton dynamics was evaluated by laser confocal microscopy using fluorophore-conjugated phalloidin. Cell motility was analyzed by an in vitro wound-healing migration assay. We propose that ABCC6 expression may be controlled by the AKT pathway as part of an adaptative response to oxidative stress, which can be mitigated by the use of Quercetin-like flavonoids.

Structural and Functional Characterization of the ABCC6 Transporter in Hepatic Cells: Role on PXE, Cancer Therapy and Drug Resistance

Pseudoxanthoma elasticum (PXE) is a complex autosomal recessive disease caused by mutations of ABCC6 transporter and characterized by ectopic mineralization of soft connective tissues. Compared to the other ABC transporters, very few studies are available to explain the structural components and working of a full ABCC6 transporter, which may provide some idea about its physiological role in humans. Some studies suggest that mutations of ABCC6 in the liver lead to a decrease in some circulating factor and indicate that PXE is a metabolic disease. It has been reported that ABCC6 mediates the efflux of ATP, which is hydrolyzed in PPi and AMP; in the extracellular milieu, PPi gives potent anti-mineralization effect, whereas AMP is hydrolyzed to Pi and adenosine which affects some cellular properties by modulating the purinergic pathway. Structural and functional studies have demonstrated that silencing or inhibition of ABCC6 with probenecid changed the expression of several genes and proteins such as NT5E and TNAP, as well as Lamin, and CDK1, which are involved in cell motility and cell cycle. Furthermore, a change in cytoskeleton rearrangement and decreased motility of HepG2 cells makes ABCC6 a potential target for anti-cancer therapy. Collectively, these findings suggested that ABCC6 transporter performs functions that modify both the external and internal compartments of the cells.

Multidrug resistance proteins (MRPs): Structure, function and the overcoming of cancer multidrug resistance

ATP-binding cassette (ABC) transporters mediate the ATP-driven translocation of structurally and mechanistically distinct substrates against steep concentration gradients. Among the seven human ABC subfamilies namely ABCA-ABCG, ABCC is the largest subfamily with 13 members. In this respect, 9 of the ABCC members are termed "multidrug resistance proteins" (MRPs1-9) due to their ability to mediate cancer multidrug resistance (MDR) by extruding various chemotherapeutic agents or their metabolites from tumor cells. Furthermore, MRPs are also responsible for the ATP-driven efflux of physiologically important organic anions such as leukotriene C₄, folic acid, bile acids and cAMP. Thus, MRPs are involved in important regulatory pathways. Blocking the anticancer drug efflux function of MRPs has shown promising results in overcoming cancer MDR. As a result, many novel MRP modulators have been developed in the past decade. In the current review, we summarize the structure, tissue distribution, biological and pharmacological functions as well as clinical insights of MRPs. Furthermore, recent updates in MRP modulators and their therapeutic applications in clinical trials are also discussed.

Inhibition of ABCC6 Transporter Modifies Cytoskeleton and Reduces Motility of HepG2 Cells via Purinergic Pathway

ABCC6, belonging to sub-family C of ATP-binding cassette transporter, is an ATP-dependent transporter mainly present in the basolateral plasma membrane of hepatic and kidney cells. Although the substrates transported are still uncertain, ABCC6 has been shown to promote ATP release. The extracellular ATP and its derivatives di- and mono-nucleotides and adenosine by acting on specific receptors activate the so-called purinergic pathway, which in turn controls relevant cellular functions such as cell immunity, inflammation, and cancer. Here, we analyzed the effect of Abcc6 knockdown and probenecid-induced ABCC6 inhibition on cell cycle, cytoskeleton, and motility of HepG2 cells. Gene and protein expression were evaluated by quantitative Reverse Transcription PCR (RT-qPCR) and western blot, respectively. Cellular cycle analysis was evaluated by flow cytometry. Actin cytoskeleton dynamics was evaluated by laser confocal microscopy using fluorophore-conjugated phalloidin. Cell motility was analyzed by in vitro wound-healing migration assay. Cell migration is reduced both in Abcc6 knockdown HepG2 cells and in probenecid treated HepG2 cells by interfering with the extracellular reserve of ATP. Therefore, ABCC6 could contribute to cytoskeleton rearrangements and cell motility through purinergic signaling. Altogether, our findings shed light on a new role of the ABCC6 transporter in HepG2 cells and suggest that its inhibitor/s could be considered potential anti-metastatic drugs.

Membrane integration and topology of RIFIN and STEVOR proteins of the Plasmodium falciparum parasite

The malarial parasite Plasmodium exports its own proteins to the cell surfaces of red blood cells (RBCs) during infection. Examples of exported proteins include members of the repetitive interspersed family (RIFIN) and subtelomeric variable open reading frame (STEVOR) family of proteins from Plasmodium falciparum. The presence of these parasite-derived proteins on surfaces of infected RBCs triggers the adhesion of infected cells to uninfected cells (rosetting) and to the vascular endothelium potentially obstructing blood flow. While there is a fair amount of information on the localization of these proteins on the cell surfaces of RBCs, less is known about how they can be exported to the membrane and the topologies they can adopt during the process. The first step of export is plausibly the cotranslational insertion of proteins into the endoplasmic reticulum (ER) of the parasite, and here, we investigate the insertion of three RIFIN and two STEVOR proteins into the ER membrane. We employ a well-established experimental system that uses N-linked glycosylation of sites within the protein as a measure to assess the extent of membrane insertion and the topology it assumes when inserted into the ER membrane. Our results indicate that for all the proteins tested, transmembranes (TMs) 1 and 3 integrate into the membrane, so that the protein assumes an overall topology of Ncyt-Ccyt. We also show that the segment predicted to be TM2 for each of the proteins likely does not reside in the membrane, but is translocated to the lumen.

Murine astrotactins 1 and 2 have a similar membrane topology and mature via endoproteolytic cleavage catalyzed by a signal peptidase

Astrotactin 1 (Astn1) and Astn2 are membrane proteins that function in glial-guided migration, receptor trafficking, and synaptic plasticity in the brain as well as in planar polarity pathways in the skin. Here we used glycosylation mapping and protease protection approaches to map the topologies of mouse Astn1 and Astn2 in rough microsomal membranes and found that Astn2 has a cleaved N-terminal signal peptide, an N-terminal domain located in the lumen of the rough microsomal membranes (topologically equivalent to the extracellular surface in cells), two transmembrane helices, and a large C-terminal lumenal domain. We also found that Astn1 has the same topology as Astn2, but we did not observe any evidence of signal peptide cleavage in Astn1. Both Astn1 and Astn2 mature through endoproteolytic cleavage in the second transmembrane helix; importantly, we identified the endoprotease responsible for the maturation of Astn1 and Astn2 as the endoplasmic reticulum signal peptidase. Differences in the degree of Astn1 and Astn2 maturation possibly contribute to the higher levels of the C-terminal domain of Astn1 detected on neuronal membranes of the central nervous system. These differences may also explain the distinct cellular functions of Astn1 and Astn2, such as in membrane adhesion, receptor trafficking, and planar polarity signaling.

The P-glycoprotein inhibitor diltiazem-like 8-(4-chlorophenyl)-5-methyl-8-[(2Z)-pent-2-en-1-yloxy]-8H-[1,2,4]oxadiazolo[3,4-c][1,4]thiazin-3-one inhibits esterase activity and H3 histone acetylation

With the aim to reduce multidrug resistance several molecules were synthesized and tested for their ability to inhibit ATP-binding cassette (ABC) proteins, which are responsible for drugs transport out from cells. The compound 8-(4-chlorophenyl)-5-methyl-8-[(2Z)-pent-2-en-1-yloxy]-8H-[1,2,4]oxadiazolo[3,4-c][1,4]thiazin-3-one namely 2c, is structurally related to the myocardial-calcium-channel-modulator diltiazem and is considered one of the most efficient P-glycoprotein inhibitors, able to induce apoptosis at low concentrations of doxorubicin in multidrug resistant ovarian cells. In this study experiments were carried out to evaluate other biological activities of compound 2c. We verified the ability of 2c to inhibit ABC transporters do not involved in drug resistance and considering the inhibitory effect of diltiazem on recombinant human carboxylesterase, we observed its inhibitory effect on the esterase activity. Our findings demonstrated that 2c exhibits broad-spectrum activity as ABC transporters inhibitor being able to inhibit ABCC6, a protein belonging to the ABC family although poorly involved in drug resistance. 2c also inhibits cell esterase activity, acetylcholine esterase activity in vitro and cell histone H3 acetylation according to its structural homology with some known HAT inhibitors. The results obtained provide new knowledge on the biological activities of 2c and represent useful information when it is used as an inhibitor of drug resistance.

Phytochemical Profile, Antioxidant and Antidiabetic Activities of Adansonia digitata L. (Baobab) from Mali, as a Source of Health-Promoting Compounds

Background: Adansonia digitata L. (Malvaceae), also known as baobab, is a tree attracting recent interest especially due to the high nutritional value of the fruit pulp. However, few studies are reported on the secondary metabolite content, showing high variability depending on the geographic region. Methods: In this study, the chemical profiles of Malian commercial baobab fruits and leaves, focused on phenolic content, were investigated by HPLC coupled with a photodiode array (PDA)/UV and an electrospray ionization (ESI) mass spectrometer (MS) and gas chromatography (GC)/MS. In addition, the extracts of fruit pulps obtained from three different markets (Fruits 1, 2, and 3) were evaluated for their total phenolic content (TPC), antioxidant activity and α-glucosidase inhibition. Results: Baobab fruit pulps were found to be rich in procyanidins and flavonol glycosides, with tiliroside as the major constituent. The baobab leaves showed a similar profile respect to the fruits, but with more detected phenolics. All fruit pulp extracts exerted antioxidant activity (highest for Fruit 3) and higher α-glucosidase inhibition than acarbose used as standard. Conclusions: This study confirmed the variability of baobab with different origins and indicated Malian species baobab as a promising source of health-promoting substances.

Structural characterization of the L0 cytoplasmic loop of human multidrug resistance protein 6 (MRP6)

ABCC6 is a member of the C subfamily of ATP-binding cassette transporters whose mutations are correlated to Pseudoxanthoma elasticum, an autosomal recessive, progressive disorder characterized by ectopic mineralization and fragmentation of elastic fibers. Structural studies of the entire protein have been hindered by its large size, membrane association, and domain complexity. Studies previously performed have contributed to shed light on the structure and function of the nucleotide binding domains and of the N-terminal region. Here we report the expression in E. coli of the polypeptide E₂₀₅-G₂₇₉ contained in the cytoplasmic L0 loop. For the first time structural studies in solution were performed. Far-UV CD spectra showed that L0 is structured, assuming predominantly α-helix in TFE solution and turns in phosphate buffer. Fluorescence spectra indicated some flexibility of the regions containing aromatic residues. ¹H NMR spectroscopy identified three helical regions separated by more flexible regions.

Extracellular ATP Regulates CD73 and ABCC6 Expression in HepG2 Cells

The ATP-binding cassette sub-family C member 6 transporter (ABCC6) is an ATP dependent transporter mainly found in the basolateral plasma membrane of hepatic and kidney cells. Mutations in ABCC6 gene were associated to the Pseudoxanthoma elasticum (PXE), an autosomal recessive disease characterized by a progressive ectopic calcification of elastic fibers in dermal, ocular, and vascular tissues. It is reported that the over-expression of ABCC6 in HEK293 cells results in the cellular efflux of ATP and other nucleoside triphosphates, which in turn are rapidly converted into nucleoside monophosphates and pyrophosphate (PPi). Since PPi is an inhibitor of mineralization, it was proposed that the absence of circulating PPi in PXE patients results in the ectopic mineralization, a typical feature of PXE. In the extracellular environment, ATP is converted, not only into pyrophosphate, but also into AMP by an ectonucleosidase, which in turn is transformed into adenosine and phosphate. ABCC6 protein is thus involved in the production of extracellular adenosine and therefore it could have a role in the activation of the purinergic system. In the liver, purinergic signaling has been shown to regulate key basic cellular functions. Our previous studies showed that in ABCC6 knockdown HepG2 cells the expression of some genes, related with the calcification processes, is dysregulated. In this study, experiments have been carried out in order to verify if ABCC6, besides supplying the pyrophosphate required to prevent the mineralization of soft tissues, also plays a role in the activation of the purinergic system. For this purpose, the transport activity of ABCC6 was blocked with Probenecid and the expression of ABCC6 and NT5E was analyzed with real time PCR and western blotting. The results of this study showed that both proteins are downregulated in the presence of Probenecid and upregulated in the presence of adenosine or ATP.

Expression of some ATP-binding cassette transporters in acute myeloid leukemia

Hematopoietic cells express ATP binding cassette (ABC) transporters in relation to different degrees of differentiation. One of the known multidrug resistance mechanisms in acute myeloid leukemia (AML) is the overexpression of efflux pumps belonging to the superfamily of ABC transporters such as ABCB1, ABCG2 and ABCC1. Although several studies were carried out to correlate ABC transporters expression with drug resistance, little is known about their role as markers of diagnosis and progression of the disease. For this purpose we investigated the expression, by real-time PCR, of some ABC genes in bone marrow samples of AML patients at diagnosis and after induction therapy. At diagnosis, ABCG2 was always down-regulated, while an up regulated trend for ABCC1 was observed. After therapy the examined genes showed a different expression trend and approached the values of healthy subjects suggesting that this event could be considered as a marker of AML regression. The expression levels of some ABC transporters such as ABCC6, seems to be related to gender, age and to the presence of FLT3/ITD gene mutation.

Refined topology model of the STT3/Stt3 protein subunit of the oligosaccharyltransferase complex

The oligosaccharyltransferase complex, localized in the endoplasmic reticulum (ER) of eukaryotic cells, is responsible for the N-linked glycosylation of numerous protein substrates. The membrane protein STT3 is a highly conserved part of the oligosaccharyltransferase and likely contains the active site of the complex. However, understanding the catalytic determinants of this system has been challenging, in part because of a discrepancy in the structural topology of the bacterial versus eukaryotic proteins and incomplete information about the mechanism of membrane integration. Here, we use a glycosylation mapping approach to investigate these questions. We measured the membrane integration efficiency of the mouse STT3-A and yeast Stt3p transmembrane domains (TMDs) and report a refined topology of the N-terminal half of the mouse STT3-A. Our results show that most of the STT3 TMDs are well inserted into the ER membrane on their own or in the presence of the natural flanking residues. However, for the mouse STT3-A hydrophobic domains 4 and 6 and yeast Stt3p domains 2, 3a, 3c, and 6 we measured reduced insertion efficiency into the ER membrane. Furthermore, we mapped the first half of the STT3-A protein, finding two extra hydrophobic domains between the third and the fourth TMD. This result indicates that the eukaryotic STT3 has 13 transmembrane domains, consistent with the structure of the bacterial homolog of STT3 and setting the stage for future combined efforts to interrogate this fascinating system.

New insights into the roles of the N-terminal region of the ABCC6 transporter

ABCC6 is a human ATP binding cassette (ABC) transporter of the plasma membrane associated with Pseudoxanthoma elasticum (PXE), an autosomal recessive disease characterized by ectopic calcification of elastic fibers in dermal, ocular and vascular tissues. Similar to other ABC transporters, ABCC6 encloses the core structure of four domains: two transmembrane domains (TMDs) and two nucleotide binding domains (NBDs) but also an additional N-terminal extension, including a transmembrane domain (TMD0) and a cytosolic loop (L0), which is only found in some members of ABCC subfamily, and for which the function remains to be established. To investigate the functional roles of this N-terminal region, we generated several domain deletion constructs of ABCC6, expressed in HEK293 and polarized LLC-PK1 cells. ABCC6 lacking TMD0 displayed full transport activity as the wild type protein. Unlike the wild type protein, ABCC6 without L0 was not targeted to the basolateral membrane. Moreover, homology modeling of L0 suggests that it forms an ATPase regulatory domain. Furthermore, we show that the expression of ABCC6 is linked to a cellular influx of Ca(2+). The results suggest that TMD0 is not required for transport function and that L0 maintains ABCC6 in a targeting-competent state for the basolateral membrane and might be involved in regulating the NBDs. These findings shed new light on a possible physiological function of ABCC6 and may explain some of the hallmarks of the clinical features associated with PXE that could contribute to the identification of novel pharmacological targets.