The nucleotide-binding domain 2 of the human transporter protein MRP6

Investigation on the mechanism of the combination of eremias multiocellata and cisplatin in reducing chemoresistance of gastric cancer based on in vitro and in vivo experiments

BACKGROUND

Cisplatin (DDP) is one of the important chemotherapy drugs for patients with advanced gastric cancer and metastasis, but its resistance is a bottleneck problem that affects clinical efficacy and patient survival. Eremias multiocellata (EM) is a traditional Chinese herbal medicine, which has been used in the treatment of precancerous lesions, gastric cancer, liver fibrosis, and other digestive diseases. However, the mechanism of reducing chemotherapy resistance to gastric cancer is still unclear.

METHODS

We used the MTT assay to evaluate the proliferative viability of gastric cancer parental cell line MKN45 and its drug-resistant cell line MKN45/DDP, and compared their drug-resistance indices. The migration and invasion abilities of MKN45/DDP drug-resistant cells were evaluated using the Transwell assay. Apoptosis in MKN45/DDP drug-resistant cells was detected using flow cytometry. The effect of a combination of EM and cisplatin on the levels of reactive oxygen species (ROS) and lipid peroxides (LPO) in cisplatin-resistant gastric cancer cells was detected using ROS fluorescent probes and a lipid peroxidation assay kit in conjunction with flow cytometry. The effect of EM combined with cisplatin on the level of iron ions was detected by fluorescence probe and confocal laser technique. Hematoxylin-eosin staining (HE staining) was used to detect the histopathologic morphology of drug-resistant gastric cancer in nude mice. Ferroptosis-related proteins were measured using immunohistochemistry. Real-time fluorescence quantitative polymerase chain reaction (RT-qPCR) was used to detect tumor drug resistance-related genes. The NF-κB/Snail pathway-related proteins, PI3K/AKT/mTOR pathway-related proteins, and drug resistance-related proteins were detected by Western blot.

RESULTS AND CONCLUSIONS

The results of in vitro and in vivo experiments showed that EM combined with DDP could effectively inhibit the migration and invasive ability of MKN45/DDP cells, as well as induce apoptosis of MKN45/DDP cells; the combination of the two drugs could significantly increase the levels of ROS, lipid peroxidation and divalent ferric ions in MKN45/DDP cells, at the same time reducing the levels of Ferroptosis-related proteins, which could induce Ferroptosis. In addition, EM combined with DDP can also exert the effect of reversing DDP resistance and increasing the sensitivity of gastric cancer drug-resistant cells to DDP by regulating the NF-κB/Snail signaling pathway, PI3K/AKT/mTOR signaling pathway, and the expression of drug resistance-related proteins and genes.

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.

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.

From membrane to mineralization: the curious case of the ABCC6 transporter

ATP-binding cassette subfamily C member 6 gene/protein (ABCC6) is an ATP-dependent transmembrane transporter predominantly expressed in the liver and the kidney. ABCC6 first came to attention in human medicine when it was discovered in 2000 that mutations in its encoding gene, ABCC6, caused the autosomal recessive multisystemic mineralization disease pseudoxanthoma elasticum (PXE). Since then, the physiological and pathological roles of ABCC6 have been the subject of intense research. In the last 20 years, significant findings have clarified ABCC6 structure as well as its physiological role in mineralization homeostasis in humans and animal models. Yet, several facets of ABCC6 biology remain currently incompletely understood, ranging from the precise nature of its substrate(s) to the increasingly complex molecular genetics. Nonetheless, advances in our understanding of pathophysiological mechanisms causing mineralization lead to several treatment options being suggested or already tested in pilot clinical trials for ABCC6 deficiency. This review highlights current knowledge of ABCC6 and the challenges ahead, particularly the attempts to translate basic science into clinical practice.

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.

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.

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.

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.

Human bile acid transporter ASBT (SLC10A2) forms functional non-covalent homodimers and higher order oligomers

The human apical sodium-dependent bile acid transporter, hASBT/SLC10A2, plays a central role in cholesterol homeostasis via the efficient reabsorption of bile acids from the distal ileum. hASBT has been shown to self-associate in higher order complexes, but while the functional role of endogenous cysteines has been reported, their implication in the oligomerization of hASBT remains unresolved. Here, we determined the self-association architecture of hASBT by site-directed mutagenesis combined with biochemical, immunological and functional approaches. We generated a cysteine-less form of hASBT by creating point mutations at all 13 endogenous cysteines in a stepwise manner. Although Cysless hASBT had significantly reduced function correlated with lowered surface expression, it featured an extra glycosylation site that facilitated its differentiation from wt-hASBT on immunoblots. Decreased protein expression was associated with instability and subsequent proteasome-dependent degradation of Cysless hASBT protein. Chemical cross-linking of wild-type and Cysless species revealed that hASBT exists as an active dimer and/or higher order oligomer with apparently no requirement for endogenous cysteine residues. This was further corroborated by co-immunoprecipitation of differentially tagged (HA-, Flag-) wild-type and Cysless hASBT. Finally, Cysless hASBT exhibited a dominant-negative effect when co-expressed with wild-type hASBT which validated heterodimerization/oligomerization at the functional level. Combined, our data conclusively demonstrate the functional existence of hASBT dimers and higher order oligomers irrespective of cysteine-mediated covalent bonds, thereby providing greater understanding of its topological assembly at the membrane surface.

ABCC6 knockdown in HepG2 cells induces a senescent-like cell phenotype

Background

Pseudoxanthoma elasticum (PXE) is characterized by progressive ectopic mineralization of elastic fibers in dermal, ocular and vascular tissues. No effective treatment exists. It is caused by inactivating mutations in the gene encoding for the ATP-binding cassette, sub-family C member 6 transporter (ABCC6), which is mainly expressed in the liver. The ABCC6 substrate (s) and the PXE pathomechanism remain unknown. Recent studies have shown that overexpression of ABCC6 in HEK293 cells results in efflux of ATP, which is rapidly converted into nucleoside monophosphates and pyrophosphate (PPi). Since the latter inhibits mineralization, it was proposed that the absence of circulating PPi in PXE patients results in the characteristic ectopic mineralization. These studies also demonstrated that the presence of ABCC6 modifies cell secretory activity and suggested that ABCC6 can change the cell phenotype.

Methods

Stable ABCC6 knockdown HepG2 clones were generated using small hairpin RNA (shRNA) technology. The intracellular glutathione and ROS levels were determined. Experiments using cell cycle analysis, real-time PCR and western blot were performed on genes involved in the senescence phenotype.

Results

To shed light on the physiological role of ABCC6, we focused on the phenotype of HepG2 cells that lack ABCC6 activity. Interestingly, we found that ABCC6 knockdown HepG2 cells show: 1) intracellular reductive stress; 2) cell cycle arrest in G1 phase; 3) upregulation of p21^Cip p53 independent; and 4) downregulation of lamin A/C.

Conclusions

These findings show that the absence of ABCC6 profoundly changes the HepG2 phenotype, suggesting that the PXE syndrome is a complex metabolic disease that is not exclusively related to the absence of pyrophosphate in the bloodstream.

Mutation spectrum in the ABCC6 gene and genotype-phenotype correlations in a French cohort with pseudoxanthoma elasticum

PURPOSE

Pseudoxanthoma elasticum (PXE) is an autosomal recessive disorder caused by variants in the ABCC6 gene. Ectopic mineralization of connective tissues leads to skin, eye, and cardiovascular manifestations with considerable phenotypic variability of unknown cause. We aimed to identify genotype-phenotype correlations in PXE.

METHODS

A molecular analysis was performed on 458 French PXE probands clinically evaluated using the Phenodex score (PS). Variant topographic analysis and genotype-phenotype correlation analysis were performed according to the number and type of identified variants.

RESULTS

Complete molecular analysis of 306 cases allowed the identification of 538 mutational events (88% detection rate) with 142 distinct variants, of which 66 were novel. Missense variant distribution was specific to some regions and residues of ABCC6. For the 220 cases with a complete PS, there was a higher prevalence of eye features in Caucasian patients (P = 0.03) and more severe eye and vascular phenotype in patients with loss-of-function variants (P = 0.02 and 0.05, respectively). Nephrolithiases and strokes, absent from the PS, were prevalent features of the disorder (11 and 10%, respectively).

CONCLUSION

We propose an updated PS including renal and neurological features and adaptation of follow-up according to the genetic and ethnic status of PXE-affected patients.Genet Med advance online publication 19 January 2017.

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.

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

The function of the ATP-binding cassette transporter MRP6 is unknown but mutations in its gene cause pseudoxanthoma elasticum. We have investigated the membrane topology of the N-terminal transmembrane domain TMD0 of MRP6 and the membrane integration and orientation propensities of its transmembrane segments (TMs) by glycosylation mapping. Results demonstrate that TMD0 has five TMs, an Nout-Cin topology and that the less hydrophobic TMs have strong preference for their orientation in the membrane that affects the neighboring TMs. Two disease-causing mutations changing the number of positive charges in the loops of TMD0 did not affect the membrane insertion efficiencies of the adjacent TMs.

UBXN2A regulates nicotinic receptor degradation by modulating the E3 ligase activity of CHIP

Neuronal nicotinic acetylcholine receptors (nAChRs) containing the α3 subunit are known for their prominent role in normal ganglionic transmission while their involvement in the mechanisms underlying nicotine addiction and smoking-related disease has been emerging only in recent years. The amount of information available on the maturation and trafficking of α3-containing nAChRs is limited. We previously showed that UBXN2A is a p97 adaptor protein that facilitates the maturation and trafficking of α3-containing nAChRs. Further investigation of the mechanisms of UBXN2A actions revealed that the protein interacts with CHIP (carboxyl terminus of Hsc70 interacting protein), whose ubiquitin E3 ligase activity regulates the degradation of several disease-related proteins. We show that CHIP displays E3 ligase activity toward the α3 nAChR subunit and contributes to its ubiquitination and subsequent degradation. UBXN2A interferes with CHIP-mediated ubiquitination of α3 and protects the nicotinic receptor subunit from endoplasmic reticulum associated degradation (ERAD). UBXN2A also cross-talks with VCP/p97 and HSC70/HSP70 proteins in a complex where α3 is likely to be targeted by CHIP. Overall,we identify CHIP as an E3 ligase for α3 and UBXN2A as a protein that may efficiently regulate the stability of CHIP's client substrates.

Multidrug Resistance Proteins (MRPs) and Cancer Therapy

The ATP-binding cassette (ABC) transporters are members of a protein superfamily that are known to translocate various substrates across membranes, including metabolic products, lipids and sterols, and xenobiotic drugs. Multidrug resistance proteins (MRPs) belong to the subfamily C in the ABC transporter superfamily. MRPs have been implicated in mediating multidrug resistance by actively extruding chemotherapeutic substrates. Moreover, some MRPs are known to be essential in physiological excretory or regulatory pathways. The importance of MRPs in cancer therapy is also implied by their clinical insights. Modulating the function of MRPs to re-sensitize chemotherapeutic agents in cancer therapy shows great promise in cancer therapy; thus, multiple MRP inhibitors have been developed recently. This review article summarizes the structure, distribution, and physiological as well as pharmacological function of MRP1-MRP9 in cancer chemotherapy. Several novel modulators targeting MRPs in cancer therapy are also discussed.

Hormonal regulation of hepatic drug biotransformation and transport systems

The human body is constantly exposed to many xenobiotics including environmental pollutants, food additives, therapeutic drugs, etc. The liver is considered the primary site for drug metabolism and elimination pathways, consisting in uptake, phase I and II reactions, and efflux processes, usually acting in this same order. Modulation of biotransformation and disposition of drugs of clinical application has important therapeutic and toxicological implications. We here provide a compilation and analysis of relevant, more recent literature reporting hormonal regulation of hepatic drug biotransformation and transport systems. We provide additional information on the effect of hormones that tentatively explain differences between sexes. A brief discussion on discrepancies between experimental models and species, as well as a link between gender-related differences and the hormonal mechanism explaining such differences, is also presented. Finally, we include a comment on the pathophysiological, toxicological, and pharmacological relevance of these regulations.