Hematopoietic stem cell differentiation affects expression and function of MRP4 (ABCC4), a transport protein for signaling molecules and drugs

Ceefourin-1, a MRP4/ABCC4 inhibitor, induces apoptosis in AML cells enhanced by histamine

BACKGROUND

Ceefourin-1 is a specific MRP4/ABCC4 inhibitor with potential antileukemic activity. In this study, we evaluate the ability of ceefourin-1 alone or in combination with histamine, an approved antileukemic agent, to induce cell differentiation or apoptosis in human acute myeloid leukemic cells. We also examine ceefourin-1 toxicity in mice.

METHODS

U937, HL-60, and KG1a cells were used as models for human acute myeloid leukemia. Cyclic AMP efflux was estimated by measuring intracellular and extracellular cAMP levels. Cell differentiation was assessed by levels of CD14 and CD11b by FACS, and CD88 by western blot, and by cell morphology. Apoptosis was evaluated by cleavage of caspase-3 and PARP by western blot, and by annexin V binding assay. Subacute toxicity study of ceefourin-1 was carried out in BALB/c mice.

RESULTS

Ceefourin-1 inhibits cAMP exclusion in AML cells and promotes intracellular signaling via CREB. Ceefourin-1 leads AML cells to apoptosis and histamine potentiates this effect, without evidence of cell differentiation. Intraperitoneal administration of ceefourin-1 shows no important alterations in mice blood parameters, hepatic, and renal functions, nor signs of histologic damage.

CONCLUSIONS

These results show that ceefourin-1 promotes apoptosis in AML cells that is enhanced by histamine.

GENERAL SIGNIFICANCE

This work indicates that ceefourin-1 represents a promising molecule that could be used alone or in combination with histamine for in vivo evaluation in acute myeloid leukemia malignancies.

The Role of Tumor-Stroma Interactions in Drug Resistance Within Tumor Microenvironment

Cancer cells resistance to various therapies remains to be a key challenge nowadays. For a long time, scientists focused on tumor cells themselves for the mechanisms of acquired drug resistance. However, recent evidence showed that tumor microenvironment (TME) is essential for regulating immune escape, drug resistance, progression and metastasis of malignant cells. Reciprocal interactions between cancer cells and non-malignant cells within this milieu often reshape the TME and promote drug resistance. Therefore, advanced knowledge about these sophisticated interactions is significant for the design of effective therapeutic approaches. In this review, we highlight cancer-associated fibroblasts (CAFs), tumor-associated macrophages (TAMs), tumor-associated neutrophils (TANs), myeloid-derived suppressor cells (MDSCs), T-regulatory lymphocytes (Tregs), mesenchymal stem cells (MSCs), cancer-associated adipocytes (CAAs), and tumor endothelial cells (TECs) existing in TME, as well as their multiple cross-talk with tumor cells, which eventually endows tumor cells with therapeutic resistance.

Role of ATP-Binding Cassette Transporter Proteins in CNS Tumors: Resistance- Based Perspectives and Clinical Updates

Despite gigantic advances in medical research and development, chemotherapeutic resistance remains a major challenge in complete remission of CNS tumors. The failure of complete eradication of CNS tumors has been correlated with the existence of several factors including overexpression of transporter proteins. To date, 49 ABC-transporter proteins (ABC-TPs) have been reported in humans, and the evidence of their strong association with chemotherapeutics' influx, dissemination, and efflux in CNS tumors, is growing. Research studies on CNS tumors are implicating ABC-TPs as diagnostic, prognostic and therapeutic biomarkers that may be utilised in preclinical and clinical studies. With the current advancements in cell biology, molecular analysis of genomic and transcriptomic interplay, and protein homology-based drug-transporters interaction, our research approaches are streamlining the roles of ABC-TPs in cancer and multidrug resistance. Potential inhibitors of ABC-TP for better clinical outcomes in CNS tumors have emerged. Elacridar has shown to enhance the chemo-sensitivity of Dasatanib and Imatinib in various glioma models. Tariquidar has improved the effectiveness of Temozolomide's in CNS tumors. Although these inhibitors have been effective in preclinical settings, their clinical outcomes have not been as significant in clinical trials. Thus, to have a better understanding of the molecular evaluations of ABC-TPs, as well as drug-interactions, further research is being pursued in research labs. Our lab aims to better comprehend the biological mechanisms involved in drug resistance and to explore novel strategies to increase the clinical effectiveness of anticancer chemotherapeutics, which will ultimately improve clinical outcomes.

Discovery of Novel Symmetrical 1,4-Dihydropyridines as Inhibitors of Multidrug-Resistant Protein (MRP4) Efflux Pump for Anticancer Therapy

Despite the development of targeted therapies in cancer, the problem of multidrug resistance (MDR) is still unsolved. Most patients with metastatic cancer die from MDR. Transmembrane efflux pumps as the main cause of MDR have been addressed by developed inhibitors, but early inhibitors of the most prominent and longest known efflux pump P-glycoprotein (P-gp) were disappointing. Those inhibitors have been used without knowledge about the expression of P-gp by the treated tumor. Therefore the use of inhibitors of transmembrane efflux pumps in clinical settings is reconsidered as a promising strategy in the case of the respective efflux pump expression. We discovered novel symmetric inhibitors of the symmetric efflux pump MRP4 encoded by the ABCC4 gene. MRP4 is involved in many kinds of cancer with resistance to anticancer drugs. All compounds showed better activities than the best known MRP4 inhibitor MK571 in an MRP4-overexpressing cell line assay, and the activities could be related to the various substitution patterns of aromatic residues within the symmetric molecular framework. One of the best compounds was demonstrated to overcome the MRP4-mediated resistance in the cell line model to restore the anticancer drug sensitivity as a proof of concept.

Diurnal expression of MRP4 in bone marrow cells underlies the dosing-time dependent changes in the oxaliplatin-induced myelotoxicity

The expression and function of some xenobiotic transporters varies according to the time of day, causing the dosing time-dependent changes in drug disposition and toxicity. Multidrug resistance-associated protein-4 (MRP4), an ATP­binding cassette (ABC) efflux transporter encoded by the Abcc4 gene, is highly expressed in bone marrow cells (BMCs) and protects them against xenobiotics, including chemotherapeutic drugs. In this study, we demonstrated that MRP4 was responsible for the extrusion of oxaliplatin (L-OHP), a platinum (Pt)-based chemotherapeutic drug, from BMCs of mice, and that the efflux transporter expression exhibited significant diurnal variation. Therefore, we investigated the relevance of the diurnal expression of MRP4 in BMCs for L-OHP-induced myelotoxicity in mice maintained under standardized light/dark cycle conditions. After intravenous injection of L-OHP, the Pt content in BMCs varied according to the injection time. Lower Pt accumulation in BMCs was detected in mice after injection of L-OHP at the mid-dark phase, during which the expression levels of MRP4 increased. Consistent with these observations, the myelotoxic effects of L-OHP were attenuated when mice were injected with L-OHP during the dark phase. This dosing schedule also alleviated the L-OHP-induced reduction of the peripheral white blood cell count. The present results suggest that the myelotoxicity of L-OHP is attenuated by optimizing the dosing schedule. Diurnal expression of MRP4 in BMCs is associated with the dosing time-dependent changes in L-OHP-induced myelotoxicity.

Multidrug resistance protein 4 (MRP4/ABCC4) is overexpressed in clear cell renal cell carcinoma (ccRCC) and is essential to regulate cell proliferation

Kidney cancer accounts for 2.5% of all cancers, with an annual global incidence of almost 300,000 cases leading to 111,000 deaths. Approximately 85% of kidney tumors are renal cell carcinoma (RCC) and their major histologic subtype is clear cell renal cell carcinoma (ccRCC). Although new therapeutic treatments are being designed and applied based on the combination of tyrosine kinase inhibitors and immunotherapy, no major impact on the mortality has been reported so far. MRP4 is a pump efflux that transporters multiple endogenous and exogenous substances. Recently it has been associated with tumoral persistence and cell proliferation in several types of cancer including pancreas, lung, ovary, colon, ostesarcoma, etc. Herein, we demonstrate for the first time, that MRP4 is overexpressed in ccRCC tumors, compared to control renal tissues. In addition, using cell culture models, we observed that MRP4 pharmacological inhibition produces an imbalance in cAMP metabolism, induces cell arrest, changes in lipid composition, increase in cytoplasmic lipid droplets and finally apoptosis. These data provide solid evidence for the future evaluation of MRP4 as a possible new therapeutic target in ccRCC.

MRP4/ABCC4 expression is regulated by histamine in acute myeloid leukemia cells, determining cAMP efflux

Intracellular cAMP (i-cAMP) levels play an important role in acute myeloid leukemia (AML) cell proliferation and differentiation. Its levels are the result of cAMP production, degradation, and exclusion. We have previously described histamine H₂ receptors and MRP4/ABCC4 as two potential targets for AML therapy. Acting through histamine H₂ receptors, histamine increases cAMP production/synthesis, while MRP4/ABCC4 is responsible for the exclusion of this cyclic nucleotide. In this study, we show that histamine treatment induces MRP4/ABCC4 expression, augmenting cAMP efflux, and that histamine, in combination with MRP inhibitors, is able to reduce AML cell proliferation. Histamine, through histamine H₂ receptor, increases i-cAMP levels and induces MRP4 transcript and protein levels in U937, KG1a, and HL-60 cells. Moreover, histamine induces MRP4 promoter activity in HEK293T cells transfected with histamine H₂ receptor (HEK293T-H₂ R). Our results support that the cAMP/Epac-PKA pathway, and not MEK/ERK nor PI3K/AKT signaling cascades, is involved in histamine-mediated upregulation of MRP4 levels. Finally, the addition of histamine potentiates the inhibition of U937, KG1a, and HL-60 cell proliferation induced by MRP4 inhibitors. Our data highlight that the use of a poly-pharmacological approach aimed at different molecular targets would be beneficial in AML treatment.

Drug repurposing for targeting cyclic nucleotide transporters in acute leukemias - A missed opportunity

While current treatment regimens for acute leukemia can dramatically improve patient survival, there remains room for improvement. Due to its roles in cell differentiation, cell survival, and apoptotic signaling, modulation of the cyclic AMP (cAMP) pathway has provided a meaningful target in hematological malignancies. Several studies have demonstrated that gene expression profiles associated with increased pro-survival cAMP activity or downregulation of various pro-apoptotic factors associated with the cAMP pathway are apparent in acute leukemia patients. Previous work to increase leukemia cell intracellular cAMP focused on the use of cAMP analogs, stimulating cAMP production via transmembrane-associated adenylyl cyclases, or decreasing cAMP degradation by inhibiting phosphodiesterase activity. However, targeting cyclic nucleotide efflux by ATP-binding cassette (ABC) transporters represents an unexplored approach for modulation of intracellular cyclic nucleotide levels. Preliminary studies have shown that inhibition of cAMP efflux can stimulate leukemia cell differentiation, cell growth arrest, and apoptosis, indicating that targeting cAMP efflux may show promise for future therapeutic development. Furthermore, inhibition of cyclic nucleotide transporter activity may also contribute multiple anticancer benefits by reducing extracellular pro-survival signaling in malignant cells. Hence, several opportunities for drug repurposing may exist for targeting cyclic nucleotide transporters.

Identification of MRP4/ABCC4 as a Target for Reducing the Proliferation of Pancreatic Ductal Adenocarcinoma Cells by Modulating the cAMP Efflux

Pancreatic cancer is one of the most lethal types of tumors with no effective therapy available; is currently the third leading cause of cancer in developed countries; and is predicted to become the second deadliest cancer in the United States by 2030. Due to the marginal benefits of current standard chemotherapy, the identification of new therapeutic targets is greatly required. Considering that cAMP pathway is commonly activated in pancreatic ductal adenocarcinoma (PDAC) and its premalignant lesions, we aim to investigate the multidrug resistance-associated protein 4 (MRP4)-dependent cAMP extrusion process as a cause of increased cell proliferation in human PDAC cell lines. Our results from in silico analysis indicate that MRP4 expression may influence PDAC patient outcome; thus, high MRP4 levels could be indicators of poor survival. In addition, we performed in vitro experiments and identified an association between higher MRP4 expression levels and more undifferentiated and malignant models of PDAC and cAMP extrusion capacity. We studied the antiproliferative effect and the overall cAMP response of three MRP4 inhibitors, probenecid, MK571, and ceefourin-1 in PDAC in vitro models. Moreover, MRP4-specific silencing in PANC-1 cells reduced cell proliferation (P < 0.05), whereas MRP4 overexpression in BxPC-3 cells significantly incremented their growth rate in culture (P < 0.05). MRP4 pharmacological inhibition or silencing abrogated cell proliferation through the activation of the cAMP/Epac/Rap1 signaling pathway. Also, extracellular cAMP reverted the antiproliferative effect of MRP4 blockade. Our data highlight the MRP4-dependent cAMP extrusion process as a key participant in cell proliferation, indicating that MRP4 could be an exploitable therapeutic target for PDAC. SIGNIFICANCE STATEMENT: ABCC4/MRP4 is the main transporter responsible for cAMP efflux. In this work, we show that MRP4 expression may influence PDAC patient outcome and identify an association between higher MRP4 expression levels and more undifferentiated and malignant in vitro models of PDAC. Findings prove the involvement of MRP4 in PDAC cell proliferation through a novel extracellular cAMP mitogenic pathway and further support MRP4 inhibition as a promising therapeutic strategy for PDAC treatment.

The Role of Platelets in the Tumor-Microenvironment and the Drug Resistance of Cancer Cells

Besides the critical functions in hemostasis, thrombosis and the wounding process, platelets have been increasingly identified as active players in various processes in tumorigenesis, including angiogenesis and metastasis. Once activated, platelets can release bioactive contents such as lipids, microRNAs, and growth factors into the bloodstream, subsequently enhancing the platelet⁻cancer interaction and stimulating cancer metastasis and angiogenesis. The mechanisms of treatment failure of chemotherapeutic drugs have been investigated to be associated with platelets. Therefore, understanding how platelets contribute to the tumor microenvironment may potentially identify strategies to suppress cancer angiogenesis, metastasis, and drug resistance. Herein, we present a review of recent investigations on the role of platelets in the tumor-microenvironment including angiogenesis, and metastasis, as well as targeting platelets for cancer treatment, especially in drug resistance.

ABC Family Transporters

The transport of specific molecules across lipid membranes is an essential function of all living organisms. The processes are usually mediated by specific transporters. One of the largest transporter families is the ATP-binding cassette (ABC) family. More than 40 ABC transporters have been identified in human, which are divided into 7 subfamilies (ABCA to ABCG) based on their gene structure, amino acid sequence, domain organization, and phylogenetic analysis. Of them, at least 11 ABC transporters including P-glycoprotein (P-GP/ABCB1), multidrug resistance-associated proteins (MRPs/ABCCs), and breast cancer resistance protein (BCRP/ABCG2) are involved in multidrug resistance (MDR) development. These ABC transporters are expressed in various tissues such as the liver, intestine, kidney, and brain, playing important roles in absorption, distribution, and excretion of drugs. Some ABC transporters are also involved in diverse cellular processes such as maintenance of osmotic homeostasis, antigen processing, cell division, immunity, cholesterol, and lipid trafficking. Several human diseases such as cystic fibrosis, sitosterolemia, Tangier disease, intrahepatic cholestasis, and retinal degeneration are associated with mutations in corresponding transporters. This chapter will describe function and expression of several ABC transporters (such as P-GP, BCRP, and MRPs), their substrates and inhibitors, as well as their clinical significance.

Estrogen induces cell proliferation by promoting ABCG2-mediated efflux in endometrial cancer cells

Recently, it has reported that overeating of lipid-food has led to increase the amount of estrogen in vivo and the incidence of endometrial carcinomas. It is well-known that ATP-binding cassette transporter sub-family G2 (ABCG2) is highly expressed in cancer stem cells (CSCs). CSCs possess the ability for differentiation, tumorigenesis, stem cell self-renewal, and the efflux of anti-cancer drug and these abilities affect malignancy of cancer cells. However, little is known about the relationship between the expression of ABCG2 and malignancy of cancer cells. The present study aimed at understanding the regulatory mechanism underlying 17-β-estradiol (E2)-induced cell proliferation under the control of ABCG2. E2 increased cell viability with a peak at 1 μM and facilitated ABCG2 mRNA expression followed by the increase of ABCG2 expression level at plasma membrane. E2-induced cell proliferation was inhibited by reserpine, an inhibitor of ABCG2, and the ABCG2 siRNA treatment. Thus, these results imply that ABCG2 plays an important role in the promotion of E2-induced cell proliferation in Ishikawa cells.

Pharmacogenetics of inflammatory bowel disease: a focus on Crohn's disease

Crohn's disease is an inflammatory bowel disease showing a high heterogeneity in phenotype and a strong genetic component. The treatment is complex, due to different severity of clinical parameters and to the fact that therapies only permit to control symptoms and to induce remission for short periods. Moreover, all categories of drugs present a great interindividual variability both in terms of efficacy and side effects appearance. For this reason, the identification of specific genomic biomarkers involved in drugs response will be of great clinical utility in order to foresee drug's efficacy and to prevent adverse reactions, permitting a more personalized therapeutic approach. In this review, we focus the attention on the pharmacogenetic studies regarding drugs commonly utilized in Crohn's disease treatment.

Histamine H2 Receptor in Blood Cells: A Suitable Target for the Treatment of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) consists in a cancer of early hematopoietic cells arising in the bone marrow, most often of those cells that would turn into white blood cells (except lymphocytes). Chemotherapy is the treatment of choice for AML but one of the major complications is that current drugs are highly toxic and poorly tolerated. In general, treatment for AML consists of induction chemotherapy and post-remission therapy. If no further post-remission is given, almost all patients will eventually relapse. Histamine, acting at histamine type-2 (H₂) receptors on phagocytes and AML blast cells, helps prevent the production and release of oxygen-free radicals, thereby protecting NK and cytotoxic T cells. This protection allows immune-stimulating agents, such as interleukin-2 (IL-2), to activate cytotoxic cells more effectively, enhancing the killing of tumor cells. Based on this mechanism, post-remission therapy with histamine and IL-2 was found to significantly prevent relapse of AML. Alternatively, another potentially less toxic approach to treat AML employs drugs to induce differentiation of malignant cells. It is based on the assumption that many neoplastic cell types exhibit reversible defects in differentiation, which upon appropriate treatment results in tumor reprogramming and the induction of terminal differentiation. There are promissory results showing that an elevated and sustained signaling through H₂ receptors is able to differentiate leukemia-derived cell lines, opening the door for the use of H₂ agonists for specific differentiation therapies. In both situations, histamine acting through H₂ receptors constitutes an eligible treatment to induce leukemic cell differentiation, improving combined therapies.

Several adaptor proteins promote intracellular localisation of the transporter MRP4/ABCC4 in platelets and haematopoietic cells

The multidrug resistance protein 4 (MRP4/ABCC4) has been identified as an important transporter for signalling molecules including cyclic nucleotides and several lipid mediators in platelets and may thus represent a novel target to interfere with platelet function. Besides its localisation in the plasma membrane, MRP4 has been also detected in the membrane of dense granules in resting platelets. In polarised cells it is localised at the basolateral or apical plasma membrane. To date, the mechanism of MRP4 trafficking has not been elucidated; protein interactions may regulate both the localisation and function of this transporter. We approached this issue by searching for interacting proteins by in vitro binding assays, followed by immunoblotting and mass spectrometry, and by visualising their co-localisation in platelets and haematopoietic cells. We identified the PDZ domain containing scaffold proteins ezrin-binding protein 50 (EBP50/NHERF1), postsynaptic density protein 95 (PSD95), and sorting nexin 27 (SNX27), but also the adaptor protein complex 3 subunit β3A (AP3B1) and the heat shock protein HSP90 as putative interaction partners of MRP4. The knock-down of SNX27, PSD95, and AP3B1 by siRNA in megakaryoblastic leukaemia cells led to a redistribution of MRP4 from intracellular structures to the plasma membrane. Inhibition of HSP90 led to a diminished expression and retention of MRP4 in the endoplasmic reticulum. These results indicate that MRP4 localisation and function are regulated by multiple protein interactions. Changes in the adaptor proteins can hence lead to altered localisation and function of the transporter.

Cyclic AMP efflux inhibitors as potential therapeutic agents for leukemia

Apoptotic evasion is a hallmark of cancer. We propose that some cancers may evade cell death by regulating 3'-5'-cyclic adenosine monophosphate (cAMP), which is associated with pro-apoptotic signaling. We hypothesize that leukemic cells possess mechanisms that efflux cAMP from the cytoplasm, thus protecting them from apoptosis. Accordingly, cAMP efflux inhibition should result in: cAMP accumulation, activation of cAMP-dependent downstream signaling, viability loss, and apoptosis. We developed a novel assay to assess cAMP efflux and performed screens to identify inhibitors. In an acute myeloid leukemia (AML) model, several identified compounds reduced cAMP efflux, appropriately modulated pathways that are responsive to cAMP elevation (cAMP-responsive element-binding protein phosphorylation, and deactivation of Very Late Antigen-4 integrin), and induced mitochondrial depolarization and caspase activation. Blocking adenylyl cyclase activity was sufficient to reduce effects of the most potent compounds. These compounds also decreased cAMP efflux and viability of B-lineage acute lymphoblastic leukemia (B-ALL) cell lines and primary patient samples, but not of normal primary peripheral blood mononuclear cells. Our data suggest that cAMP efflux is a functional feature that could be therapeutically targeted in leukemia. Furthermore, because some of the identified drugs are currently used for treating other illnesses, this work creates an opportunity for repurposing.

Pharmacokinetics and pharmacogenomics of β-lactam-induced neutropenia

AIM

Determine if individuals with β-lactam induced neutropenia have polymorphisms that impair function of MRP4 or OAT1/OAT3.

METHODS

Subjects with β-lactam induced neutropenia were compared to controls for the presence of MRP4 and OAT1/OAT3 polymorphisms, estimated plasma trough concentrations and area under the curve.

RESULTS

Subjects with a homozygous polymorphism at MRP4 3348 A to G were 5.3 times more likely to develop neutropenia (p = 0.171). No statistical differences were noted in pharmacokinetic parameters. Contingency analysis of children greater than 5 years of age showed neutropenia in subjects who were homozygous wild type at MRP4 3348 A to G was significantly associated with standard or high dosing (p = 0.03).

CONCLUSION

MRP4 3348 A to G should be further studied for potential contribution to the development of β-lactam induced neutropenia.

ABCC4 Is a Determinant of Cytarabine-Induced Cytotoxicity and Myelosuppression

Resistance to cytarabine remains a major challenge in the treatment of acute myeloid leukemia (AML). Based on previous studies implicating ABCC4/MRP4 in the transport of nucleosides, we hypothesized that cytarabine is sensitive to ABCC4‐mediated efflux, thereby decreasing its cytotoxic response against AML blasts. The uptake of cytarabine and its monophosphate metabolite was found to be facilitated in ABCC4‐expressing vesicles and intracellular retention was significantly impaired by overexpression of human ABCC4 or mouse Abcc4 (P < 0.05). ABCC4 was expressed highly in AML primary blasts and cell lines, and cytotoxicity of cytarabine in cells was increased in the presence of the ABCC4 inhibitors MK571 or sorafenib, as well as after ABCC4 siRNA. In Abcc4‐null mice, cytarabine‐induced hematological toxicity was enhanced and ex vivo colony‐forming assays showed that Abcc4‐deficiency sensitized myeloid progenitors to cytarabine. Collectively, these studies demonstrate that ABCC4 plays a protective role against cytarabine‐mediated insults in leukemic and host myeloid cells.

cAMP regulates expression of the cyclic nucleotide transporter MRP4 (ABCC4) through the EPAC pathway

Multidrug resistance protein 4 (MRP4/ABCC4) has been established as an independent regulator of cyclic AMP (cAMP) levels particularly in vascular smooth muscle cells and in hematopoietic cells. Here, we assessed whether cAMP in turn regulates MRP4. A significant upregulation of MRP4 mRNA and protein by long-term treatment with cAMP-enhancing agents was observed in HeLa cells, smooth muscle cells, and megakaryoblastic leukemia M07e cells. This upregulation was not affected by inhibition of protein kinase A, but could be reverted by inhibitors and siRNA of an alternative cAMP-signaling route involving exchange proteins activated by cyclic AMP (EPAC) and mitogen-activated protein kinases. A selective EPAC activator could equally induce MRP4. The transcriptional regulation was confirmed in a luciferase reporter gene assay using a vector containing a 1494-bp fragment of the promoter region of the MRP4/ABCC4 gene. Our results suggest that enhanced cAMP levels upregulate MRP4 expression, which can result in increased cAMP efflux.

Multidrug resistance-associated protein 4 is a determinant of arsenite resistance

Although arsenic trioxide (arsenite, As^III) has shown a remarkable efficacy in the treatment of acute promyelocytic leukemia patients, multidrug resistance is still a major concern for its clinical use. Multidrug resistance-associated protein 4 (MRP4), which belongs to the ATP-binding cassette (ABC) superfamily of transporters, is localized to the basolateral membrane of hepatocytes and the apical membrane of renal proximal tubule cells. Due to its characteristic localization, MRP4 is proposed as a candidate in the elimination of arsenic and may contribute to resistance to As^III. To test this hypothesis, stable HEK293 cells overexpressing MRP4 or MRP2 were used to establish the role of these two transporters in As^III resistance. The IC₅₀ values of As^III in MRP4 cells were approximately 6-fold higher than those in MRP2 cells, supporting an important role for MRP4 in resistance to As^III. The capacity of MRP4 to confer resistance to As^III was further confirmed by a dramatic decrease in the IC₅₀ values with the addition of MK571, an MRP4 inhibitor, and cyclosporine A, a well-known broad-spectrum inhibitor of ABC transporters. Surprisingly, the sensitivity of the MRP2 cells to As^III was similar to that of the parent cells, although insufficient formation of glutathione and/or Se conjugated arsenic compounds in the MRP2 cells might limit transport. Given that MRP4 is a major contributor to arsenic resistance in vitro, further investigation into the correlation between MRP4 expression and treatment outcome of leukemia patients treated with arsenic-based regimens is warranted.

Impaired platelet activation and cAMP homeostasis in MRP4-deficient mice

Molecules that reduce the level of cyclic adenosine 5'-monophosphate (cAMP) in the platelet cytosol, such as adenosine 5'-diphosphate (ADP) secreted from dense granules, trigger platelet activation. Therefore, any change in the distribution and/or availability of cyclic nucleotides or ADP may interfere with platelet reactivity. In this study, we evaluated the role of multidrug resistance protein 4 (MRP4, or ABCC4), a nucleotide transporter, in platelet functions in vivo and in vitro by investigating MRP4-deficient mice. MRP4 deletion resulted in a slight increase in platelet count but had no impact on platelet ultrastructure. In MRP4-deficient mice, the arterial occlusion was delayed and the tail bleeding time was prolonged. In a model of platelet depletion and transfusion mimicking a platelet-specific knockout, mice injected with MRP4(-/-) platelets also showed a significant increase in blood loss compared with mice injected with wild-type platelets. Defective thrombus formation and platelet activation were confirmed in vitro by studying platelet adhesion to collagen in flow conditions, integrin αIIbβ3 activation, washed platelet secretion, and aggregation induced by low concentrations of proteinase-activated receptor 4-activating peptide, U46619, or ADP. We found no role of MRP4 in ADP dense-granule storage, but MRP4 redistributed cAMP from the cytosol to dense granules, as confirmed by increased vasodilator-stimulated phosphoprotein phosphorylation in MRP4-deficient platelets. These data suggest that MRP4 promotes platelet aggregation by modulating the cAMP-protein kinase A signaling pathway, suggesting that MRP4 might serve as a target for novel antiplatelet agents.

Role of drug transport and metabolism in the chemoresistance of acute myeloid leukemia

Acute myeloid leukemia is a clonal but heterogeneous disease differing in molecular pathogenesis, clinical features and response to chemotherapy. This latter frequently consists of a combination of cytarabine and anthracyclines, although etoposide, demethylating agents, and other drugs are also used. Unfortunately, chemoresistance is a common and serious problem. Multiple mechanisms account for impaired effectiveness of drugs and reduced levels of active agents in target cells. The latter can be due to lower drug uptake, increased export or decreased intracellular proportion of active/inactive agent due to changes in the expression/function of enzymes responsible for the activation of pro-drugs and the inactivation of active agents. Characterization of the "resistome", or profile of expressed genes accounting for multi-drug resistance (MDR) phenotype, would permit to predict the lack of response to chemotherapy and would help in the selection of the best pharmacological regime for each patient and moment, and to develop strategies of chemosensitization.

Multidrug resistance protein 4/ ATP binding cassette transporter 4: a new potential therapeutic target for acute myeloid leukemia

Less than a third of adults patients with acute myeloid leukemia (AML) are cured by current treatments, emphasizing the need for new approaches to therapy. We previously demonstrated that besides playing a role in drug-resistant leukemia cell lines, multidrug resistance protein 4 (MRP4/ABCC4) regulates leukemia cell proliferation and differentiation through the endogenous MRP4/ABCC4 substrate, cAMP. Here, we studied the role of MRP4/ABCC4 in tumor progression in a mouse xenograft model and in leukemic stem cells (LSCs) differentiation. We found a decrease in the mitotic index and an increase in the apoptotic index associated with the inhibition of tumor growth when mice were treated with rolipram (PDE4 inhibitor) and/or probenecid (MRPs inhibitor). Genetic silencing and pharmacologic inhibition of MRP4 reduced tumor growth. Furthermore, MRP4 knockdown induced cell cycle arrest and apoptosis in vivo. Interestingly, when LSC population was isolated, we observed that increased cAMP levels and MRP4/ABCC4 blockade resulted in LSCs differentiation. Taken together, our findings show that MRP4/ABCC4 has a relevant role in tumor growth and apoptosis and in the eradication of LSCs, providing the basis for a novel promising target in AML therapy.

Influence of SLCO1B1 polymorphism on maintenance therapy for childhood leukemia

BACKGROUND

Management of the adverse effects of chemotherapy is essential to improve outcome of children with leukemia. Some genetic polymorphisms can predict treatment-related toxicity, and be used individually in dose modification of 6-mercaptopurine (6-MP) and methotrexate (MTX) in maintenance therapy for childhood acute lymphoblastic leukemia (ALL). We investigated associations between clinical course and candidate gene polymorphisms less evaluated in Japanese patients.

METHODS

Fifty-three children who received maintenance chemotherapy were enrolled in this study. The scheduled dose of oral 6-MP was 40 mg/m(2) daily and that of oral MTX was 25 mg/m(2) weekly. The doses were adjusted according to white blood cell count (target range, 2.5-3.5 × 10(9) /L) and aspartate aminotransferase and alanine aminotransferase level (< 750 IU/L). Eight polymorphisms in six candidate genes, TPMT, ITPA, MRP4, MTHFR, RFC1, and SLCO1B1, were genotyped using the Taqman PCR method. Clinical course was reviewed retrospectively from medical records.

RESULTS

The average dose of 6-MP was lower in the patients with at least one variant allele at SLCO1B1 c.521 T > C than in the patients with wild homozygous genotype. The other analyzed polymorphisms were not associated with toxicity, 6-MP, or MTX dose.

CONCLUSIONS

Polymorphism of SLCO1B1 c.521 T > C could be a strong predictor of 6-MP dose reduction in maintenance chemotherapy in childhood ALL.

The Pharmacological and Physiological Role of Multidrug-Resistant Protein 4

Multidrug-resistant protein 4 (MRP4), a member of the C subfamily of ATP-binding cassette transporters, is distributed in a variety of tissues and a number of cancers. As a drug transporter, MRP4 is responsible for the pharmacokinetics and pharmacodynamics of numerous drugs, especially antiviral drugs, antitumor drugs, and diuretics. In this regard, the functional role of MRP4 is affected by a number of factors, such as genetic mutations; tissue-specific transcriptional regulations; post-transcriptional regulations, including miRNAs and membrane internalization; and substrate competition. Unlike other C family members, MRP4 is in a pivotal position to transport cellular signaling molecules, through which it is tightly connected to the living activity and physiologic processes of cells and bodies. In the context of several cancers in which MRP4 is overexpressed, MRP4 inhibition shows striking effects against cancer progression and drug resistance. In this review, we describe the role of MRP4 more specifically in both healthy conditions and disease states, with an emphasis on its potential as a drug target.

Adverse events associated with azathioprine treatment in korean pediatric inflammatory bowel disease patients

PURPOSE

This study was aimed to evaluate the frequency and course of adverse events associated with azathioprine treatment in Korean pediatric patients with inflammatory bowel disease.

METHODS

Total of 174 pediatric patients (age range, 1 to 19 years) with inflammatory bowel disease who received azathioprine in order to maintain remission at Samsung Medical Center (Seoul, Korea) from January 2002 through December 2012 were included in this study. Medical records of these subjects were retrospectively reviewed regarding the development of adverse events associated with azathioprine treatment.

RESULTS

Ninety-eight patients (56.3%) of 174 patients experienced 136 episodes of adverse events, requiring dose reduction in 31 patients (17.8%), and discontinuation in 18 patients (10.3%). The mean dose of azathioprine that had been initially administered was 1.32±0.42 mg/kg/day. Among the adverse reactions, bone marrow suppression developed in 47 patients (27.0%), requiring dose reduction in 22 patients (12.6%) and discontinuation in 8 patients (4.6%). Other adverse events that occurred were gastrointestinal disturbance (15.5%), hair loss (12.1%), pancreatitis (7.5%), arthralgia (6.9%), hepatotoxicity (2.9%), skin rash/allergic reactions (2.9%), headache/dizziness (2.3%), sepsis (0.6%), and oral mucositis (0.6%).

CONCLUSION

Bone marrow suppression, especially leukopenia was most commonly associated with azathioprine treatment in Korean pediatric inflammatory bowel disease patients. Close observation for possible adverse events is required in this population with inflammatory bowel diseases who are under treatment with azathioprine.

A process-based review of mouse models of pulmonary hypertension

Genetically modified mouse models have unparalleled power to determine the mechanisms behind different processes involved in the molecular and physiologic etiology of various classes of human pulmonary hypertension (PH). Processes known to be involved in PH for which there are extensive mouse models available include the following: (1) Regulation of vascular tone through secreted vasoactive factors; (2) regulation of vascular tone through potassium and calcium channels; (3) regulation of vascular remodeling through alteration in metabolic processes, either through alteration in substrate usage or through circulating factors; (4) spontaneous vascular remodeling either before or after development of elevated pulmonary pressures; and (5) models in which changes in tone and remodeling are primarily driven by inflammation. PH development in mice is of necessity faster and with different physiologic ramifications than found in human disease, and so mice make poor models of natural history of PH. However, transgenic mouse models are a perfect tool for studying the processes involved in pulmonary vascular function and disease, and can effectively be used to test interventions designed against particular molecular pathways and processes involved in disease.

ABCC4/MRP4: a MYCN-regulated transporter and potential therapeutic target in neuroblastoma

Resistance to cytotoxic drugs is thought to be a major cause of treatment failure in childhood neuroblastoma, and members of the ATP-binding cassette (ABC) transporter superfamily may contribute to this phenomenon by active efflux of chemotherapeutic agents from cancer cells. As a member of the C subfamily of ABC transporters, multidrug resistance-associated protein MRP4/ABCC4 has the ability to export a variety of endogenous and exogenous substances across the plasma membrane. In light of its capacity for chemotherapeutic drug efflux, MRP4 has been studied in the context of drug resistance in a number of cancer cell types. However, MRP4 also influences cancer cell biology independently of chemotherapeutic drug exposure, which highlights the potential importance of endogenous MRP4 substrates in cancer biology. Furthermore, MRP4 is a direct transcriptional target of Myc family oncoproteins and expression of this transporter is a powerful independent predictor of clinical outcome in neuroblastoma. Together, these features suggest that inhibition of MRP4 may be an attractive therapeutic approach for neuroblastoma and other cancers that rely on MRP4. In this respect, existing options for MRP4 inhibition are relatively non-selective and thus development of more specific anti-MRP4 compounds should be a major focus of future work in this area.

c-MYC oncoprotein dictates transcriptional profiles of ATP-binding cassette transporter genes in chronic myelogenous leukemia CD34+ hematopoietic progenitor cells

Resistance to chemotherapeutic agents remains one of the major impediments to a successful treatment of chronic myeloid leukemia (CML). Misregulation of the activity of a specific group of ATP-binding cassette transporters (ABC) is responsible for reducing the intracellular concentration of drugs in leukemic cells. Moreover, a consistent body of evidence also suggests that ABC transporters play a role in cancer progression beyond the efflux of cytotoxic drugs. Despite a large number of studies that investigated the function of the ABC transporters, little is known about the transcriptional regulation of the ABC genes. Here, we present data showing that the oncoprotein c-MYC is a direct transcriptional regulator of a large set of ABC transporters in CML. Furthermore, molecular analysis carried out in CD34+ hematopoietic cell precursors of 21 CML patients reveals that the overexpression of ABC transporters driven by c-MYC is a peculiar characteristic of the CD34+ population in CML and was not found either in the population of mononuclear cells from which they had been purified nor in CD34+ cells isolated from healthy donors. Finally, we describe how the methylation state of CpG islands may regulate the access of c-MYC to ABCG2 gene promoter, a well-studied gene associated with multidrug resistance in CML, hence, affecting its expression. Taken together, our findings support a model in which c-MYC-driven transcriptional events, combined with epigenetic mechanisms, direct and regulate the expression of ABC genes with possible implications in tumor malignancy and drug efflux in CML.

Inhibition of MRP4 prevents and reverses pulmonary hypertension in mice

Multidrug resistance-associated protein 4 (MRP4, also known as Abcc4) regulates intracellular levels of cAMP and cGMP in arterial SMCs. Here, we report our studies of the role of MRP4 in the development and progression of pulmonary arterial hypertension (PAH), a severe vascular disease characterized by chronically elevated pulmonary artery pressure and accompanied by remodeling of the small pulmonary arteries as a prelude to right heart failure and premature death. MRP4 expression was increased in pulmonary arteries from patients with idiopathic PAH as well as in WT mice exposed to hypoxic conditions. Consistent with a pathogenic role for MRP4 in PAH, WT mice exposed to hypoxia for 3 weeks showed reversal of hypoxic pulmonary hypertension (PH) following oral administration of the MRP4 inhibitor MK571, and Mrp4-/- mice were protected from hypoxic PH. Inhibition of MRP4 in vitro was accompanied by increased intracellular cAMP and cGMP levels and PKA and PKG activities, implicating cyclic nucleotide-related signaling pathways in the mechanism underlying the protective effects of MRP4 inhibition. Our data suggest that MRP4 could represent a potential target for therapeutic intervention in PAH.

Expression of multidrug resistance proteins is localized principally to the Malpighian tubules in larvae of the cabbage looper moth, Trichoplusia ni

The multidrug resistance proteins (MRPs) serve a number of important roles in development, physiological homeostasis and metabolic resistance. In insects, they may also contribute to resistance against xenobiotics including insecticides and plant secondary metabolites. To investigate their contribution to xenobiotic resistance, we have examined the tissue distribution of gene and protein expression of the multidrug resistance proteins TrnMRP1 and TrnMRP4 of the lepidopteran insect, Trichoplusia ni. Using quantitative PCR and immunohistochemistry, we have identified high expression levels of both transporters in the Malpighian tubules relative to levels in other major tissues of the body, where they probably contribute to excretion of metabolic wastes or ingested xenobiotics. We have specifically located TrnMRP protein expression in a subpopulation of Malpighian tubule secondary cells. Expression of TrnMRP1 was also detected both at a high level in specific cortical neurons of larval ganglia and at a lower level throughout the cortex, where it may act in signaling or protective functions, respectively. In contrast, expression of TrnMRP4 was low to absent in larval ganglia, with the exception of single cells in the central connective. We discuss the potential implications of this TrnMRP activity on insect development and metabolic resistance.

Multidrug resistance protein 4 (MRP4/ABCC4) regulates cAMP cellular levels and controls human leukemia cell proliferation and differentiation

Increased intracellular cAMP concentration plays a well established role in leukemic cell maturation. We previously reported that U937 cells stimulated by H2 receptor agonists, despite a robust increase in cAMP, fail to mature because of rapid H2 receptor desensitization and phosphodiesterase (PDE) activation. Here we show that intracellular cAMP levels not only in U937 cells but also in other acute myeloid leukemia cell lines are also regulated by multidrug resistance-associated proteins (MRPs), particularly MRP4. U937, HL-60, and KG-1a cells, exposed to amthamine (H2-receptor agonist), augmented intracellular cAMP concentration with a concomitant increase in the efflux. Extrusion of cAMP was ATP-dependent and probenecid-sensitive, supporting that the transport was MRP-mediated. Cells exposed to amthamine and the PDE4 inhibitor showed enhanced cAMP extrusion, but this response was inhibited by MRP blockade. Amthamine stimulation, combined with PDE4 and MRP inhibition, induced maximal cell arrest proliferation. Knockdown strategy by shRNA revealed that this process was mediated by MRP4. Furthermore, blockade by probenecid or MRP4 knockdown showed that increased intracellular cAMP levels induce maturation in U937 cells. These findings confirm the key role of intracellular cAMP levels in leukemic cell maturation and provide the first evidence that MRP4 may represent a new potential target for leukemia differentiation therapy.

The multidrug-resistance protein 4 polymorphism is a new factor accounting for thiopurine sensitivity in Japanese patients with inflammatory bowel disease

BACKGROUND

Multidrug resistance protein 4 (MRP4) functions as an efflux pump of nucleoside monophosphate analogs, such as 6-mercaptopurine (6-MP) and 6-thioguanine nucleotide (6-TGN). A single-nucleotide polymorphism in human MRP4 (rs3765534) dramatically reduces MRP4 function and results in the intracellular accumulation of 6-TGN. In this study, we investigated the association between MRP4 G2269A polymorphism and thiopurine sensitivity in Japanese IBD patients.

METHODS

Direct sequencing of the MRP4 exon 18 was performed. The TPMT A719G and ITPase C94A polymorphisms were determined by polymerase-chain reaction-restriction fragment length polymorphism analyses.

RESULTS

Of the 279 samples analyzed (44 healthy volunteers and 235 IBD patients), 68 samples showed a heterozygote of MRP4 G2269A and 7 carried a homozygote. The allelic frequency of MRP4 G2269A was 14.7%. In 130 IBD patients treated with azathioprine/6-MP, the white blood cell count was significantly lower in patients with theMRP4 variant alone (n = 26) than in patients with a wild allelotype (n = 74) (P = 0.014) or in patients with the ITPase variant alone (n = 22) (P = 0.0095). The 6-TGN levels were significantly higher in patients with the MRP4 variant alone than in patients with the wild allelotype(P = 0.049). Of the 15 patients who experienced leucopenia (<3 x 10⁹/l), 7 patients carried the MRP4 variant.The odds ratio of carrying the MRP4 variant alone and having leukopenia was 3.30 (95% confidence interval 1.03–10.57, P = 0.036).

CONCLUSIONS

These results suggest that MRP4 G2269A might be a new factor accounting for thiopurine sensitivity in Japanese patients with IBD.

Direct and coordinate regulation of ATP-binding cassette transporter genes by Myc factors generates specific transcription signatures that significantly affect the chemoresistance phenotype of cancer cells

Increased expression of specific ATP-binding cassette (ABC) transporters is known to mediate the efflux of chemotherapeutic agents from cancer cells. Therefore, establishing how ABC transporter genes are controlled at their transcription level may help provide insight into the role of these multifaceted transporters in the malignant phenotype. We have investigated ABC transporter gene expression in a large neuroblastoma data set of 251 tumor samples. Clustering analysis demonstrated a strong association between differential ABC gene expression patterns in tumor samples and amplification of the MYCN oncogene, suggesting a correlation with MYCN function. Using expression profiling and chromatin immunoprecipitation studies, we show that MYCN oncoprotein coordinately regulates transcription of specific ABC transporter genes, by acting as either an activator or a repressor. Finally, we extend these notions to c-MYC showing that it can also regulate the same set of ABC transporter genes in other tumor cells through similar dynamics. Overall our findings provide insight into MYC-driven molecular mechanisms that contribute to coordinate transcriptional regulation of a large set of ABC transporter genes, thus affecting global drug efflux.

ABC transporters in cancer: more than just drug efflux pumps

Multidrug transporter proteins are best known for their contributions to chemoresistance through the efflux of anticancer drugs from cancer cells. However, a considerable body of evidence also points to their importance in cancer extending beyond drug transport to fundamental roles in tumour biology. Currently, much of the evidence for these additional roles is correlative and definitive studies are needed to confirm causality. We propose that delineating the precise roles of these transporters in tumorigenesis and treatment response will be important for the development of more effective targeted therapies.

Influence of development, HIV infection, and antiretroviral therapies on the gene expression profiles of ABC transporters in human lymphocytes

The efficacy of drugs acting on lymphocytes like anticancer, immunosuppressive, and antiretroviral drugs depends on their intracellular concentrations, which could be modulated by membrane efflux pumps belonging to the ABC transporter superfamily. The gene expression profiles of 6 main ABC transporters (MDR1, MRP1, MRP3, MRP4, MRP5, and BCRP) were established in lymphocytes from birth to adulthood using blood samples from 57 children and 15 adults (34 and 5 HIV-infected, respectively). Gene expression levels were quantified by quantitative RT-PCR. In adults, the MRP1 gene had the highest expression, followed by the MRP5 gene. BCRP and MRP4 genes were significantly higher expressed at birth than after 1 month of life. Neither HIV infection nor antiretroviral therapies modulated the gene expression profiles of ABC transporters. In conclusion, drugs that are substrates of BCRP and MRP4, like zidovudine, may have an altered efficacy in newborns.

ABC drug transporters and immunity: novel therapeutic targets in autoimmunity and cancer

ABC transporters were identified originally for their contribution to clinical MDR as a result of their capacity to extrude various unrelated cytotoxic drugs. More recent reports have shown that ABC transporters can play important roles in the development, differentiation, and maturation of immune cells and are involved in migration of immune effector cells to sites of inflammation. Many of the currently identified, endogenous ABC transporter substrates have immunostimulating effects. Increasing the expression of ABC transporters on immune cells and thereby enhancing immune cell development or functionality may be beneficial to immunotherapy in the field of oncology. On the contrary, in the treatment of autoimmune diseases, blockade of these transporters may prove beneficial, as it could dampen disease activity by compromising immune effector cell functions. This review will focus on the expression, regulation, and substrate specificity of ABC transporters in relation to functional activities of immune effector cells and discusses implications for the treatment of cancer on the one hand and autoimmune diseases on the other.