The Imipridone ONC213 Targets α-Ketoglutarate Dehydrogenase to Induce Mitochondrial Stress and Suppress Oxidative Phosphorylation in Acute Myeloid Leukemia

Eradication of acute myeloid leukemia (AML) is therapeutically challenging; many patients succumb to AML despite initially responding to conventional treatments. Here, we showed that the imipridone ONC213 elicits potent antileukemia activity in a subset of AML cell lines and primary patient samples, particularly in leukemia stem cells, while producing negligible toxicity in normal hematopoietic cells. ONC213 suppressed mitochondrial respiration and elevated α-ketoglutarate by suppressing α-ketoglutarate dehydrogenase (αKGDH) activity. Deletion of OGDH, which encodes αKGDH, suppressed AML fitness and impaired oxidative phosphorylation, highlighting the key role for αKGDH inhibition in ONC213-induced death. ONC213 treatment induced a unique mitochondrial stress response and suppressed de novo protein synthesis in AML cells. Additionally, ONC213 reduced the translation of MCL1, which contributed to ONC213-induced apoptosis. Importantly, a patient-derived xenograft from a relapsed AML patient was sensitive to ONC213 in vivo. Collectively, these findings support further development of ONC213 for treating AML.

SIGNIFICANCE

In AML cells, ONC213 suppresses αKGDH, which induces a unique mitochondrial stress response, and reduces MCL1 to decrease oxidative phosphorylation and elicit potent antileukemia activity. See related commentary by Boët and Sarry, p. 950.

Tumor-acquired somatic mutation affects conformation to abolish ABCG2-mediated drug resistance

ABCG2 is an important ATP-binding cassette transporter impacting the absorption and distribution of over 200 chemical toxins and drugs. ABCG2 also reduces the cellular accumulation of diverse chemotherapeutic agents. Acquired somatic mutations in the phylogenetically conserved amino acids of ABCG2 might provide unique insights into its molecular mechanisms of transport. Here, we identify a tumor-derived somatic mutation (Q393K) that occurs in a highly conserved amino acid across mammalian species. This ABCG2 mutant seems incapable of providing ABCG2-mediated drug resistance. This was perplexing because it is localized properly and retained interaction with substrates and nucleotides. Using a conformationally sensitive antibody, we show that this mutant appears "locked" in a non-functional conformation. Structural modeling and molecular dynamics simulations based on ABCG2 cryo-EM structures suggested that the Q393K interacts with the E446 to create a strong salt bridge. The salt bridge is proposed to stabilize the inward-facing conformation, resulting in an impaired transporter that lacks the flexibility to readily change conformation, thereby disrupting the necessary communication between substrate binding and transport.

ABCC4 impacts megakaryopoiesis and protects megakaryocytes against 6-mercaptopurine induced cytotoxicity

The role of ABCC4, an ATP-binding cassette transporter, in the process of platelet formation, megakaryopoiesis, is unknown. Here, we show that ABCC4 is highly expressed in megakaryocytes (MKs). Mining of public genomic data (ATAC-seq and genome wide chromatin interactions, Hi-C) revealed that key megakaryopoiesis transcription factors (TFs) interacted with ABCC4 regulatory elements and likely accounted for high ABCC4 expression in MKs. Importantly these genomic interactions for ABCC4 ranked higher than for genes with known roles in megakaryopoiesis suggesting a role for ABCC4 in megakaryopoiesis. We then demonstrate that ABCC4 is required for optimal platelet formation as in vitro differentiation of fetal liver derived MKs from Abcc4-/- mice exhibited impaired proplatelet formation and polyploidization, features required for optimal megakaryopoiesis. Likewise, a human megakaryoblastic cell line, MEG-01 showed that acute ABCC4 inhibition markedly suppressed key processes in megakaryopoiesis and that these effects were related to reduced cAMP export and enhanced dissociation of a negative regulator of megakaryopoiesis, protein kinase A (PKA) from ABCC4. PKA activity concomitantly increased after ABCC4 inhibition which was coupled with significantly reduced GATA-1 expression, a TF needed for optimal megakaryopoiesis. Further, ABCC4 protected MKs from 6-mercaptopurine (6-MP) as Abcc4-/- mice show a profound reduction in MKs after 6-MP treatment. In total, our studies show that ABCC4 not only protects the MKs but is also required for maximal platelet production from MKs, suggesting modulation of ABCC4 function might be a potential therapeutic strategy to regulate platelet production.

The net electrostatic potential and hydration of ABCG2 affect substrate transport

ABCG2 is a medically important ATP-binding cassette transporter with crucial roles in the absorption and distribution of chemically-diverse toxins and drugs, reducing the cellular accumulation of chemotherapeutic drugs to facilitate multidrug resistance in cancer. ABCG2's capacity to transport both hydrophilic and hydrophobic compounds is not well understood. Here we assess the molecular basis for substrate discrimination by the binding pocket. Substitution of a phylogenetically-conserved polar residue, N436, to alanine in the binding pocket of human ABCG2 permits only hydrophobic substrate transport, revealing the unique role of N436 as a discriminator. Molecular dynamics simulations show that this alanine substitution alters the electrostatic potential of the binding pocket favoring hydration of the transport pore. This change affects the contact with substrates and inhibitors, abrogating hydrophilic compound transport while retaining the transport of hydrophobic compounds. The N436 residue is also required for optimal transport inhibition of ABCG2, as many inhibitors are functionally impaired by this ABCG2 mutation. Overall, these findings have biomedical implications, broadly extending our understanding of substrate and inhibitor interactions.

A PPIX-binding probe facilitates discovery of PPIX-induced cell death modulation by peroxiredoxin

While heme synthesis requires the formation of a potentially lethal intermediate, protoporphyrin IX (PPIX), surprisingly little is known about the mechanism of its toxicity, aside from its phototoxicity. The cellular protein interactions of PPIX might provide insight into modulators of PPIX-induced cell death. Here we report the development of PPB, a biotin-conjugated, PPIX-probe that captures proteins capable of interacting with PPIX. Quantitative proteomics in a diverse panel of mammalian cell lines reveal a high degree of concordance for PPB-interacting proteins identified for each cell line. Most differences are quantitative, despite marked differences in PPIX formation and sensitivity. Pathway and quantitative difference analysis indicate that iron and heme metabolism proteins are prominent among PPB-bound proteins in fibroblasts, which undergo PPIX-mediated death determined to occur through ferroptosis. PPB proteomic data (available at PRIDE ProteomeXchange # PXD042631) reveal that redox proteins from PRDX family of glutathione peroxidases interact with PPIX. Targeted gene knockdown of the mitochondrial PRDX3, but not PRDX1 or 2, enhance PPIX-induced death in fibroblasts, an effect blocked by the radical-trapping antioxidant, ferrostatin-1. Increased PPIX formation and death was also observed in a T-lymphoblastoid ferrochelatase-deficient leukemia cell line, suggesting that PPIX elevation might serve as a potential strategy for killing certain leukemias.

Development and validation of an LC-MS/MS method to quantify the bromodomain and extra-terminal (BET) inhibitor JQ1 in mouse plasma and brain microdialysate: Application to cerebral microdialysis study

JQ1, is a cell-permeable small-molecule inhibitor of bromodomain and extra-terminal protein (BET) function with reportedly good CNS penetration, however, unbound and pharmacologically active CNS JQ1 exposures have not been characterized. Additionally, no quantitative bioanalytical methods for JQ1 have been described in the literature to support the CNS penetration studies. In the present article, we discuss the development and validation of a sensitive and reliable liquid chromatography-tandem mass spectrometry (LC-MS/MS) quantitative methods to determine JQ1 in mouse plasma and brain microdialysate. JQ1 and the internal standard, dabrafenib (ISTD), were extracted from plasma and microdialysate samples using a simple solid phase extraction protocol performed on an Oasis HLB μElution plate. Chromatographic separation of JQ1 and ISTD was achieved on a reversed phase C₁₂ analytical column with gradient elution profile of mobile phases (MP A: water containing 0.1 % formic acid and MP B: acetonitrile containing 0.1 % formic acid) at a flow rate of 0.6 mL/min. The mass spectrometric detection was performed in the positive MRM ion mode by monitoring the transitions 457.40 > 341.30 (JQ1) and 520.40 > 307.20 (ISTD). The calibration curves demonstrated good linearities over the concentration range of 5-1000 ng/mL for the mouse plasma method (r² ≥ 0.99) and 0.5-500 ng/mL for the microdialysate method (r² ≥ 0.99). The experimental limit of quantification obtained was 5 and 0.5 ng/mL for the mouse plasma and microdialysate method, respectively, with the coefficient of variation less than 10 % for the analyte peak area. All the other validation parameters, including intra-and inter-day accuracy and precision, matrix effect, selectivity, carryover effect, and stability, were within the USFDA bioanalytical guidelines acceptance limits. The LC-MS/MS method was successfully applied to a mouse pharmacokinetic and cerebral microdialysis study to characterize the unbound JQ1 exposure in brain extracellular fluid and plasma.

Multidrug resistance-associated protein 4 (Mrp4) is a novel genetic factor in the pathogenesis of obesity and diabetes

Multidrug resistance protein 4 (Mrp4) is an efflux transporter known to transport several xenobiotics and endogenous molecules. We recently identified that the lack of Mrp4 increases adipose tissue and body weights in mice. However, the role of Mrp4 in adipose tissue physiology are unknown. The current study aimed at characterizing these specific roles of Mrp4 using wild-type (WT) and knockout (Mrp4^-/- ) mice. Our studies determined that Mrp4 is expressed in mouse adipose tissue and that the lack of Mrp4 expression is associated with adipocyte hypertrophy. Furthermore, the lack of Mrp4 increased blood glucose and leptin levels, and impaired glucose tolerance. Additionally, in 3T3-L1 cells and human pre-adipocytes, pharmacological inhibition of Mrp4 increased adipogenesis and altered expression of adipogenic genes. Lack of Mrp4 activity in both of our in vivo and in vitro models leads to increased activation of adipose tissue cAMP response element-binding protein (Creb) and decreased plasma prostaglandin E (PGE) metabolite levels. These changes in Creb activation, coupled with decreased PGE levels, together promoted the observed metabolic phenotype in Mrp4^-/- mice. In conclusion, our results indicate that Mrp4 as a novel genetic factor involved in the pathogenesis of metabolic diseases, such as obesity and diabetes.

Metabolomic and transcriptomic analysis reveals endogenous substrates and metabolic adaptation in rats lacking Abcg2 and Abcb1a transporters

Abcg2/Bcrp and Abcb1a/Pgp are xenobiotic efflux transporters limiting substrate permeability in the gastrointestinal system and brain, and increasing renal and hepatic drug clearance. The systemic impact of Bcrp and Pgp ablation on metabolic homeostasis of endogenous substrates is incompletely understood. We performed untargeted metabolomics of cerebrospinal fluid (CSF) and plasma, transcriptomics of brain, liver and kidney from male Sprague Dawley rats (WT) and Bcrp/Pgp double knock-out (dKO) rats, and integrated metabolomic/transcriptomic analysis to identify putative substrates and perturbations in canonical metabolic pathways. A predictive Bayesian machine learning model was used to predict in silico those metabolites with greater substrate-like features for either transporters. The CSF and plasma levels of 169 metabolites, nutrients, signaling molecules, antioxidants and lipids were significantly altered in dKO rats, compared to WT rats. These metabolite changes suggested alterations in histidine, branched chain amino acid, purine and pyrimidine metabolism in the dKO rats. Levels of methylated and sulfated metabolites and some primary bile acids were increased in dKO CSF or plasma. Elevated uric acid levels appeared to be a primary driver of changes in purine and pyrimidine biosynthesis. Alterations in Bcrp/Pgp dKO CSF levels of antioxidants, precursors of neurotransmitters, and uric acid suggests the transporters may contribute to the regulation of a healthy central nervous system in rats. Microbiome-generated metabolites were found to be elevated in dKO rat plasma and CSF. The altered dKO metabolome appeared to cause compensatory transcriptional change in urate biosynthesis and response to lipopolysaccharide in brain, oxidation-reduction processes and response to oxidative stress and porphyrin biosynthesis in kidney, and circadian rhythm genes in liver. These findings present insight into endogenous functions of Bcrp and Pgp, the impact that transporter substrates, inhibitors or polymorphisms may have on metabolism, how transporter inhibition could rewire drug sensitivity indirectly through metabolic changes, and identify functional Bcrp biomarkers.

Lack of Multidrug Resistance-associated Protein 4 Prolongs Partial Hepatectomy-induced Hepatic Steatosis

Multidrug resistance-associated protein 4 (Mrp4) is an efflux transporter involved in the active transport of several endogenous and exogenous chemicals. Previously, we have shown that hepatic Mrp4 expression increases following acetaminophen overdose. In mice, these increases in Mrp4 expression are observed specifically in hepatocytes undergoing active proliferation. From this, we hypothesized that Mrp4 plays a key role in hepatocyte proliferation and that lack of Mrp4 impedes liver regeneration following liver injury and/or tissue loss. To evaluate the role of Mrp4 in these processes, we employed two-third partial hepatectomy (PH) as an experimental liver regeneration model. In this study, we performed PH-surgery on male wildtype (C57BL/6J) and Mrp4 knockout mice. Plasma and liver tissues were collected at 24, 48, and 72 h postsurgery and evaluated for liver injury and liver regeneration endpoints, and for PH-induced hepatic lipid accumulation. Our results show that lack of Mrp4 did not alter hepatocyte proliferation and liver injury following PH as evaluated by Ki-67 antigen staining and plasma alanine aminotransferase levels. To our surprise, Mrp4 knockout mice exhibited increased hepatic lipid content, in particular, di- and triglyceride levels. Gene expression analysis showed that lack of Mrp4 upregulated hepatic lipin1 and diacylglycerol O-acyltransferase 1 and 2 gene expression, which are involved in the synthesis of di- and triglycerides. Our observations indicate that lack of Mrp4 prolonged PH-induced hepatic steatosis in mice and suggest that Mrp4 may be a novel genetic factor in the development of hepatic steatosis.

Influence of ABC transporters on the excretion of ciprofloxacin assessed with PET imaging in mice

Ciprofloxacin is a commonly prescribed fluoroquinolone antibiotic which is cleared by active tubular secretion and intestinal excretion. Ciprofloxacin is a known substrate of the ATP-binding cassette (ABC) transporters breast cancer resistance protein (BCRP) and multidrug resistance-associated protein 4 (MRP4). In this work, we used positron emission tomography (PET) imaging to investigate the influence of BCRP, MRP4, MRP2 and P-glycoprotein (P-gp) on the excretion of [¹⁸F]ciprofloxacin in mice. Dynamic 90-min PET scans were performed after intravenous injection of [¹⁸F]ciprofloxacin in wild-type mice without and with pre-treatment with the broad-spectrum MRP inhibitor MK571. Moreover, [¹⁸F]ciprofloxacin PET scans were performed in Abcc4^(-/-), Abcc2^(-/-), Abcc4^(-/-)Abcg2^(-/-) and Abcb1a/b^(-/-)Abcg2^(-/-) mice. In addition to non-compartmental pharmacokinetic (PK) analysis, a novel three-compartment PK model was developed for a detailed assessment of the renal disposition of [¹⁸F]ciprofloxacin. In MK571 pre-treated mice, a significant increase in the blood exposure to [¹⁸F]ciprofloxacin was observed along with a significant reduction in the renal and intestinal clearances. PK modelling revealed a significant reduction in renal radioactivity uptake (CL₁) and in the rate constants for transfer of radioactivity from the corticomedullary renal region into blood (k₂) and urine (k₃), respectively, after MK571 administration. No changes in the renal clearance or in the estimated kidney PK model parameters were observed in any of the studied knockout models, while a significant reduction in the intestinal clearance was observed in Abcc2^(-/-) and Abcc4^(-/-)Abcg2^(-/-) mice. Our data failed to reveal a role of any of the studied ABC transporters in the tubular secretion of ciprofloxacin. This may indicate that ciprofloxacin is handled in the kidneys by more than one transporter family, most likely with a great degree of mutual functional redundancy. Our study highlights the potential of PET imaging for an assessment of transporter-mediated renal excretion of radiolabelled drugs.

The Heme-Regulated Inhibitor Pathway Modulates Susceptibility of Poor Prognosis B-Lineage Acute Leukemia to BH3-Mimetics

Antiapoptotic MCL1 is one of the most frequently amplified genes in human cancers and elevated expression confers resistance to many therapeutics including the BH3-mimetic agents ABT-199 and ABT-263. The antimalarial, dihydroartemisinin (DHA) translationally represses MCL-1 and synergizes with BH3-mimetics. To explore how DHA represses MCL-1, a genome-wide CRISPR screen identified that loss of genes in the heme synthesis pathway renders mouse BCR-ABL⁺ B-ALL cells resistant to DHA-induced death. Mechanistically, DHA disrupts the interaction between heme and the eIF2α kinase heme-regulated inhibitor (HRI) triggering the integrated stress response. Genetic ablation of Eif2ak1, which encodes HRI, blocks MCL-1 repression in response to DHA treatment and represses the synergistic killing of DHA and BH3-mimetics compared with wild-type leukemia. Furthermore, BTdCPU, a small-molecule activator of HRI, similarly triggers MCL-1 repression and synergizes with BH3-mimetics in mouse and human leukemia including both Ph⁺ and Ph-like B-ALL. Finally, combinatorial treatment of leukemia bearing mice with both BTdCPU and a BH3-mimetic extended survival and repressed MCL-1 in vivo. These findings reveal for the first time that the HRI-dependent cellular heme-sensing pathway can modulate apoptosis in leukemic cells by repressing MCL-1 and increasing their responsiveness to BH3-mimetics. This signaling pathway could represent a generalizable mechanism for repressing MCL-1 expression in malignant cells and sensitizing them to available therapeutics. IMPLICATIONS: The HRI-dependent cellular heme-sensing pathway can modulate apoptotic sensitivity in leukemic cells by repressing antiapoptotic MCL-1 and increasing their responsiveness to BH3-mimetics.

Using Pharmacology to Squeeze the Life Out of Childhood Leukemia, and Potential Strategies to Achieve Breakthroughs in Medulloblastoma Treatment

Eliminating cancer was once thought of as a war. This analogy is still apt today; however, we now realize that cancer is a much more formidable enemy than scientists originally perceived, and in some cases, it harbors a profound ability to thwart our best efforts to defeat it. However, before we were aware of the complexity of cancer, chemotherapy against childhood acute lymphoblastic leukemia (ALL) was successful because it applied the principles of pharmacology. Herein, we provide a historic perspective of the experience at St. Jude Children's Research Hospital. In 1962, when the hospital opened, fewer than 3% of patients experienced durable cure. Through judicious application of pharmacologic principles (e.g., combination therapy with agents using different mechanisms of action) plus appropriate drug scheduling, dosing, and pharmacodynamics, the survival of patients with ALL now exceeds 90%. We contrast this approach to treating ALL with the contemporary approach to treating medulloblastoma, in which genetics and molecular signatures are being used to guide the development of more-efficacious treatment strategies with minimal toxicity. Finally, we highlight the emerging technologies that can sustain and propel the collaborative efforts to squeeze the life out of these cancers. SIGNIFICANCE STATEMENT: Up until the early 1960s, chemotherapy for childhood acute lymphoblastic leukemia was mostly ineffective. This changed with the knowledge and implementation of rational approaches to combination therapy. Although the therapeutics of brain cancers such as medulloblastoma are not as refined (in part because of the blood-brain barrier obstacle), recent extraordinary advances in knowledge of medulloblastoma pathobiology has led to innovations in disease classification accompanied with strategies to improve therapeutic outcomes. Undoubtedly, additional novel approaches, such as immunological therapeutics, will open new avenues to further the goal of taming cancer.

ABCG2 requires a single aromatic amino acid to "clamp" substrates and inhibitors into the binding pocket

ATP-binding cassette sub-family G member 2 (ABCG2) is a homodimeric ATP-binding cassette (ABC) transporter that not only has a key role in helping cancer cells to evade the cytotoxic effects of chemotherapy, but also in protecting organisms from multiple xeno- and endobiotics. Structural studies indicate that substrate and inhibitor (ligands) binding to ABCG2 can be differentiated quantitatively by the number of amino acid contacts, with inhibitors displaying more contacts. Although binding is the obligate initial step in the transport cycle, there is no empirical evidence for one amino acid being primarily responsible for ligand binding. By mutagenesis and biochemical studies, we demonstrated that the phylogenetically conserved amino acid residue, F439, was critical for both transport and the binding of multiple substrates and inhibitors. Structural modeling implied that the π-π interactions from each F439 monomer mediated the binding of a surprisingly diverse array of structurally unrelated substrates and inhibitors and that this symmetrical π-π interaction "clamps" the ligand into the binding pocket. Key molecular features of diverse ABCG2 ligands using the π-π clamp along with structural studies created a pharmacophore model. These novel findings have important therapeutic implications because key properties of ligands interacting with ABCG2 have been disovered. Furthermore, mechanistic insights have been revealed by demonstrating that for ABCG2 a single amino acid is essential for engaging and initiating transport of multiple drugs and xenobiotics.

An ABC Transporter Drives Medulloblastoma Pathogenesis by Regulating Sonic Hedgehog Signaling

Mutations in Sonic hedgehog (SHH) signaling promote aberrant proliferation and tumor growth. SHH-medulloblastoma (MB) is among the most frequent brain tumors in children less than 3 years of age. Although key components of the SHH pathway are well-known, we hypothesized that new disease-modifying targets of SHH-MB might be identified from large-scale bioinformatics and systems biology analyses. Using a data-driven systems biology approach, we built a MB-specific interactome. The ATP-binding cassette transporter ABCC4 was identified as a modulator of SHH-MB. Accordingly, increased ABCC4 expression correlated with poor overall survival in patients with SHH-MB. Knockdown of ABCC4 expression markedly blunted the constitutive activation of the SHH pathway secondary to Ptch1 or Sufu insufficiency. In human tumor cell lines, ABCC4 knockdown and inhibition reduced full-length GLI3 levels. In a clinically relevant murine SHH-MB model, targeted ablation of Abcc4 in primary tumors significantly reduced tumor burden and extended the lifespan of tumor-bearing mice. These studies reveal ABCC4 as a potent SHH pathway regulator and a new candidate to target with the potential to improve SHH-MB therapy. SIGNIFICANCE: These findings identify ABCC4 transporter as a new target in SHH-MB, prompting the development of inhibitors or the repurporsing of existing drugs to target ABCC4.

Lack of multidrug resistance-associated protein 4 (Mrp4) alters the kinetics of acetaminophen toxicity

Acetaminophen (APAP) overdose is the most frequent cause of drug-induced liver injury in humans and a common chemical model to investigate genetic determinants of susceptibility to drug-induced liver injury (DILI). Previous studies performed in our laboratory identified the efflux transporter multidrug resistance-associated protein 4 (Mrp4) as an inducible gene in the liver following toxic APAP exposure in both humans and rodents. In mice, blockade of hepatic Mrp4 induction following APAP administration increases susceptibility towards APAP hepatotoxicity. Collectively, these findings suggest that Mrp4 plays an important role in tolerance to APAP-induced liver injury. To further study the role of Mrp4 in APAP-induced hepatotoxicity, we treated 10–12 weeks old male wild type (WT, C57BL/6J) and Mrp4 knockout (Mrp4^−/−) mice with APAP (400 mg/Kg in saline, i.p.) or vehicle. Liver injury endpoints and hepatic gene expression were analyzed at 12, 24 and 48 h post-APAP injections. Unexpectedly, the kinetics of histologically measured liver damage and plasma ALT revealed that Mrp4^−/ mice had decreased ALT levels and hepatic necrosis compared to WT mice only at 12 h. Notably, hepatic non-protein sulfhydryl (NPSH) levels were increased in the APAP treated Mrp4^−/− mice at intervals less than 24 h, consistent with the capability of Mrp4 to export glutathione. Further gene expression analysis revealed that hepatic drug metabolism genes were downregulated in Mrp4^−/− mice at earlier time points post-APAP administration. However, despite significant decreases in endpoints of liver injury detected at an early time point after APAP treatment, these changes were not sustained at later time points as Mrp4^−/− mice ultimately had hepatic toxicity at levels comparable to WT mice. In conclusion, our data indicate that lack of Mrp4 by itself in mice does not alter susceptibility to APAP toxicity.

Zebrafish abcb11b mutant reveals strategies to restore bile excretion impaired by bile salt export pump deficiency

Bile salt export pump (BSEP) adenosine triphosphate-binding cassette B11 (ABCB11) is a liver-specific ABC transporter that mediates canalicular bile salt excretion from hepatocytes. Human mutations in ABCB11 cause progressive familial intrahepatic cholestasis type 2. Although over 150 ABCB11 variants have been reported, our understanding of their biological consequences is limited by the lack of an experimental model that recapitulates the patient phenotypes. We applied CRISPR/Cas9-based genome editing technology to knock out abcb11b, the ortholog of human ABCB11, in zebrafish and found that these mutants died prematurely. Histological and ultrastructural analyses showed that abcb11b mutant zebrafish exhibited hepatocyte injury similar to that seen in patients with progressive familial intrahepatic cholestasis type 2. Hepatocytes of mutant zebrafish failed to excrete the fluorescently tagged bile acid that is a substrate of human BSEP. Multidrug resistance protein 1, which is thought to play a compensatory role in Abcb11 knockout mice, was mislocalized to the hepatocyte cytoplasm in abcb11b mutant zebrafish and in a patient lacking BSEP protein due to nonsense mutations in ABCB11. We discovered that BSEP deficiency induced autophagy in both human and zebrafish hepatocytes. Treatment with rapamycin restored bile acid excretion, attenuated hepatocyte damage, and extended the life span of abcb11b mutant zebrafish, correlating with the recovery of canalicular multidrug resistance protein 1 localization.

CONCLUSIONS

Collectively, these data suggest a model that rapamycin rescues BSEP-deficient phenotypes by prompting alternative transporters to excrete bile salts; multidrug resistance protein 1 is a candidate for such an alternative transporter. (Hepatology 2018;67:1531-1545).

Beyond Competitive Inhibition: Regulation of ABC Transporters by Kinases and Protein-Protein Interactions as Potential Mechanisms of Drug-Drug Interactions

ATP-binding cassette (ABC) transporters are transmembrane efflux transporters mediating the extrusion of an array of substrates ranging from amino acids and lipids to xenobiotics, and many therapeutic compounds, including anticancer drugs. The ABC transporters are also recognized as important contributors to pharmacokinetics, especially in drug-drug interactions and adverse drug effects. Drugs and xenobiotics, as well as pathologic conditions, can influence the transcription of ABC transporters, or modify their activity or intracellular localization. Kinases can affect the aforementioned processes for ABC transporters as do protein interactions. In this review, we focus on the ABC transporters ABCB1, ABCB11, ABCC1, ABCC4, and ABCG2 and illustrate how kinases and protein-protein interactions affect these transporters. The clinical relevance of these factors is currently unknown; however, these examples suggest that our understanding of drug-drug interactions will benefit from further knowledge of how kinases and protein-protein interactions affect ABC transporters.

ABCB6, an ABC Transporter Impacting Drug Response and Disease

Recent findings have discovered how insufficiency of ATP-binding cassette (ABC) transporter, ABCB6, can negatively impact human health. These advances were made possible by, first, finding that ABCB6 deficiency was the genetic basis for some severe transfusion reactions and by, second, determining that functionally impaired ABCB6 variants enhanced the severity of porphyria, i.e., diseases associated with defects in heme synthesis. ABCB6 is a broad-spectrum porphyrin transporter that is capable of both exporting and importing heme and its precursors across the plasma membrane and outer mitochondrial membrane, respectively. Biochemical studies have demonstrated that while ABCB6 influences the antioxidant system by reducing the levels of reactive oxygen species, the exact mechanism is currently unknown, though effects on heme synthesis are likely. Furthermore, it is unknown what biochemical or cellular signals determine where ABCB6 localizes in the cell. This review highlights the major recent findings on ABCB6 and focuses on details of its structure, mechanism, transport, contributions to cellular stress, and current clinical implications.

A novel role for OATP2A1/SLCO2A1 in a murine model of colon cancer

Prostaglandin E₂ (PGE₂) is associated with proliferation and angiogenesis in colorectal tumours. The role of prostaglandin transporter OATP2A1/SLCO2A1 in colon cancer tumorogenesis is unknown. We evaluated mice of various Slco2a1 genotypes in a murine model of colon cancer, the adenomatous polyposis (APC) mutant (Apc^∆716/+) model. Median lifespan was significantly extended from 19 weeks in Slco2a1^+/+/Apc^Δ716/+ mice to 25 weeks in Slco2a1^−/−/Apc^Δ716/+ mice. Survival was directly related to a reduction in the number of large polyps in the Slco2a1^−/−/Apc^∆716/+ compared to the Slco2a1^+/+/Apc^Δ716/+ or Slco2a1^+/−/Apc^Δ716/+mice. The large polyps from the Slco2a1^−/−/Apc^∆716/+ mice had significant reductions in microvascular density, consistent with the high expression of Slco2a1 in the tumour-associated vascular endothelial cells. Chemical suppression of OATP2A1 function significantly reduced tube formation and wound-healing activity of PGE₂ in human vascular endothelial cells (HUVECs) although the amount of extracellular PGE₂ was not affected by an OATP2A1 inhibitor. Further an in vivo model of angiogenesis, showed a significant reduction of haemoglobin content (54.2%) in sponges implanted into Slco2a1^−/−, compared to wildtype mice. These studies indicate that OATP2A1 is likely to promote tumorogenesis by PGE₂ uptake into the endothelial cells, suggesting that blockade of OATP2A1 is an additional pharmacologic strategy to improve colon cancer outcomes.

Obstacles to Brain Tumor Therapy: Key ABC Transporters

The delivery of cancer chemotherapy to treat brain tumors remains a challenge, in part, because of the inherent biological barrier, the blood-brain barrier. While its presence and role as a protector of the normal brain parenchyma has been acknowledged for decades, it is only recently that the important transporter components, expressed in the tightly knit capillary endothelial cells, have been deciphered. These transporters are ATP-binding cassette (ABC) transporters and, so far, the major clinically important ones that functionally contribute to the blood-brain barrier are ABCG2 and ABCB1. A further limitation to cancer therapy of brain tumors or brain metastases is the blood-tumor barrier, where tumors erect a barrier of transporters that further impede drug entry. The expression and regulation of these two transporters at these barriers, as well as tumor derived alteration in expression and/or mutation, are likely obstacles to effective therapy.

Upregulated heme biosynthesis, an exploitable vulnerability in MYCN-driven leukemogenesis

The increased heme biosynthesis long observed in leukemia was previously of unknown significance. Heme, synthesized from porphyrin precursors, plays a central role in oxygen metabolism and mitochondrial function, yet little is known about its role in leukemogenesis. Here, we show increased expression of heme biosynthetic genes, including UROD, only in pediatric AML samples that have high MYCN expression. High expression of both UROD and MYCN predicts poor overall survival and unfavorable outcomes in adult AML. Murine leukemic progenitors derived from hematopoietic progenitor cells (HPCs) overexpressing a MYCN cDNA (MYCN-HPCs) require heme/porphyrin biosynthesis, accompanied by increased oxygen consumption, to fully engage in self-renewal and oncogenic transformation. Blocking heme biosynthesis reduced mitochondrial oxygen consumption and markedly suppressed self-renewal. Leukemic progenitors rely on balanced production of heme and heme intermediates, the porphyrins. Porphyrin homeostasis is required because absence of the porphyrin exporter, ABCG2, increased death of leukemic progenitors in vitro and prolonged the survival of mice transplanted with Abcg2-KO MYCN-HPCs. Pediatric AML patients with elevated MYCN mRNA display strong activation of TP53 target genes. Abcg2-KO MYCN-HPCs were rescued from porphyrin toxicity by p53 loss. This vulnerability was exploited to show that treatment with a porphyrin precursor, coupled with the absence of ABCG2, blocked MYCN-driven leukemogenesis in vivo, thereby demonstrating that porphyrin homeostasis is a pathway crucial to MYCN leukemogenesis.

Suppression of the ATP-binding cassette transporter ABCC4 impairs neuroblastoma tumour growth and sensitises to irinotecan in vivo

The ATP-binding cassette transporter ABCC4 (multidrug resistance protein 4, MRP4) mRNA level is a strong predictor of poor clinical outcome in neuroblastoma which may relate to its export of endogenous signalling molecules and chemotherapeutic agents. We sought to determine whether ABCC4 contributes to development, growth and drug response in neuroblastoma in vivo. In neuroblastoma patients, high ABCC4 protein levels were associated with reduced overall survival. Inducible knockdown of ABCC4 strongly inhibited the growth of human neuroblastoma cells in vitro and impaired the growth of neuroblastoma xenografts. Loss of Abcc4 in the Th-MYCN transgenic neuroblastoma mouse model did not impact tumour formation; however, Abcc4-null neuroblastomas were strongly sensitised to the ABCC4 substrate drug irinotecan. Our findings demonstrate a role for ABCC4 in neuroblastoma cell proliferation and chemoresistance and provide rationale for a strategy where inhibition of ABCC4 should both attenuate the growth of neuroblastoma and sensitise tumours to ABCC4 chemotherapeutic substrates.

Differential effects of thiopurine methyltransferase (TPMT) and multidrug resistance-associated protein gene 4 (MRP4) on mercaptopurine toxicity

PURPOSE

Mercaptopurine plays a pivotal role in treatment of acute lymphoblastic leukemia (ALL) and autoimmune diseases, and inter-individual variability in mercaptopurine tolerance can influence treatment outcome. Thiopurine methyltransferase (TPMT) and multi-drug resistant Protein 4 (MRP4) have both been associated with mercaptopurine toxicity in clinical studies, but their relative contributions remain unclear.

METHODS

We studied the metabolism of and tolerance to mercaptopurine in murine knockout models of Tpmt, Mrp4, and both genes simultaneously.

RESULTS

Upon mercaptopurine treatment, Tpmt ^-/- Mrp4 ^-/- mice had the highest concentration of bone marrow thioguanine nucleotides (8.5 pmol/5 × 10⁶ cells, P = 7.8 × 10^-4 compared with 2.7 pmol/5 × 10⁶ cells in wild-types), followed by those with Mrp4 or Tpmt deficiency alone (6.1 and 4.3 pmol/5 × 10⁶ cells, respectively). Mrp4-deficient mice accumulated higher concentrations of methylmercaptopurine metabolites compared with wild-type (76.5 vs. 23.2 pmol/5 × 10⁶ cells, P = 0.027). Mice exposed to a clinically relevant mercaptopurine dosing regimen displayed differences in toxicity and survival among the genotypes. The double knock-out of both genes experienced greater toxicity and shorter survival compared to the single knockout of either Tpmt (P = 1.7 × 10^-6) or Mrp4 (P = 7.4 × 10^-10).

CONCLUSIONS

We showed that both Tpmt and Mrp4 influence mercaptopurine disposition and toxicity.

Investigation of the Importance of Multidrug Resistance-Associated Protein 4 (Mrp4/Abcc4) in the Active Efflux of Anionic Drugs Across the Blood-Brain Barrier

The importance of multidrug resistance-associated protein 4 (Mrp4/Abcc4) in limiting the penetration of Mrp4 substrate compounds into the central nervous system across the blood-brain barrier was investigated using Mrp4^-/- mice. Significant adenosine triphosphate-dependent uptake by MRP4 was observed for ochratoxin A, pitavastatin, raltitrexed (K_m = 43.7 μM), pravastatin, cyclic guanosine monophosphate, 2,4-dichlorophenoxyacetate, and urate. The defect in the Mrp4 gene did not affect the brain-to-plasma ratio (K_p,brain) of quinidine and dantrolene. Following intravenous infusion in wild-type and Mrp4^-/- mice, the plasma concentrations of the tested compounds (cefazolin, cefmetazole, ciprofloxacin, cyclophosphamide, furosemide, hydrochlorothiazide, methotrexate, pitavastatin, pravastatin, and raltitrexed) were identical; however, Mrp4^-/- mice showed a significantly higher (1.9- to 2.5-fold) K_p,brain than wild-type mice for methotrexate, raltitrexed, and cyclophosphamide. GF120918, a dual inhibitor of P-gp and Bcrp, significantly decreased K_p,cortex and K_p,cerebellum only in Mrp4^-/- mice. Methotrexate and raltitrexed are also substrates of multispecific organic anion transporters such as Oatp1a4 and Oat3. GF120918 showed an inhibition potency against Oatp1a4, but not against Oat3. These results suggest that Mrp4 limits the penetration of methotrexate and raltitrexed into the brain across the blood-brain barrier, which is likely to be facilitated by some uptake transporters.

The severity of hereditary porphyria is modulated by the porphyrin exporter and Lan antigen ABCB6

Hereditary porphyrias are caused by mutations in genes that encode haem biosynthetic enzymes with resultant buildup of cytotoxic metabolic porphyrin intermediates. A long-standing open question is why the same causal porphyria mutations exhibit widely variable penetrance and expressivity in different individuals. Here we show that severely affected porphyria patients harbour variant alleles in the ABCB6 gene, also known as Lan, which encodes an ATP-binding cassette (ABC) transporter. Plasma membrane ABCB6 exports a variety of disease-related porphyrins. Functional studies show that most of these ABCB6 variants are expressed poorly and/or have impaired function. Accordingly, homozygous disruption of the Abcb6 gene in mice exacerbates porphyria phenotypes in the Fech^m1Pas mouse model, as evidenced by increased porphyrin accumulation, and marked liver injury. Collectively, these studies support ABCB6 role as a genetic modifier of porphyria and suggest that porphyrin-inducing drugs may produce excessive toxicities in individuals with the rare Lan(−) blood type.

Accumulation of intermediates of haem biosynthesis, porphyrins, is harmful and usually inherited, but it is unclear how the same mutation may make some individuals more ill than others. Here, the authors show that a porphyrin transporter ABCB6 is a modulator of porphyria, and that patients with functionally defective ABCB6 show more severe symptoms.

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.

Apoptosome activation, an important molecular instigator in 6-mercaptopurine induced Leydig cell death

Leydig cells are crucial to the production of testosterone in males. It is unknown if the cancer chemotherapeutic drug, 6-mercaptopurine (6 MP), produces Leydig cell failure among adult survivors of childhood acute lymphoblastic leukemia. Moreover, it is not known whether Leydig cell failure is due to either a loss of cells or an impairment in their function. Herein, we show, in a subset of childhood cancer survivors, that Leydig cell failure is related to the dose of 6 MP. This was extended, in a murine model, to demonstrate that 6 MP exposure induced caspase 3 activation, and the loss of Leydig cells was independent of Bak and Bax activation. The death of these non-proliferating cells was triggered by 6 MP metabolism, requiring formation of both cytosolic reactive oxygen species and thiopurine nucleotide triphosphates. The thiopurine nucleotide triphosphates (with physiological amounts of dATP) uniquely activated the apoptosome. An ABC transporter (Abcc4/Mrp4) reduced the amount of thiopurines, thereby providing protection for Leydig cells. The studies reported here demonstrate that the apoptosome is uniquely activated by thiopurine nucleotides and suggest that 6 MP induced Leydig cell death is likely a cause of Leydig cell failure in some survivors of childhood cancer.

Maternal bile acid transporter deficiency promotes neonatal demise

Intrahepatic cholestasis of pregnancy (ICP) is associated with adverse neonatal survival and is estimated to impact between 0.4 and 5% of pregnancies worldwide. Here we show that maternal cholestasis (due to Abcb11 deficiency) produces neonatal death among all offspring within 24 h of birth due to atelectasis-producing pulmonary hypoxia, which recapitulates the neonatal respiratory distress of human ICP. Neonates of Abcb11-deficient mothers have elevated pulmonary bile acids and altered pulmonary surfactant structure. Maternal absence of Nr1i2 superimposed on Abcb11 deficiency strongly reduces maternal serum bile acid concentrations and increases neonatal survival. We identify pulmonary bile acids as a key factor in the disruption of the structure of pulmonary surfactant in neonates of ICP. These findings have important implications for neonatal respiratory failure, especially when maternal bile acids are elevated during pregnancy, and highlight potential pathways and targets amenable to therapeutic intervention to ameliorate this condition.

ABCG2 Transporter Expression Impacts Group 3 Medulloblastoma Response to Chemotherapy

While a small number of plasma membrane ABC transporters can export chemotherapeutic drugs and confer drug resistance, it is unknown whether these transporters are expressed or functional in less therapeutically tractable cancers such as Group 3 (G3) medulloblastoma. Herein we show that among this class of drug transporters, only ABCG2 was expressed at highly increased levels in human G3 medulloblastoma and a mouse model of this disease. In the mouse model, Abcg2 protein was expressed at the plasma membrane where it functioned as expected on the basis of export of prototypical substrates. By screening ABC substrates against mouse G3 medulloblastoma tumorspheres in vitro, we found that Abcg2 inhibition could potentiate responses to the clinically used drug topotecan, producing a more than 9-fold suppression of cell proliferation. Extended studies in vivo in this model confirmed that Abcg2 inhibition was sufficient to enhance antiproliferative responses to topotecan, producing a significant survival advantage compared with subjects treated with topotecan alone. Our findings offer a preclinical proof of concept for blockade of ABCG2 transporter activity as a strategy to empower chemotherapeutic responses in G3 medulloblastoma.

PKA and actin play critical roles as downstream effectors in MRP4-mediated regulation of fibroblast migration

Multidrug resistance protein 4 (MRP4), a member of the ATP binding cassette transporter family, functions as a plasma membrane exporter of cyclic nucleotides. Recently, we demonstrated that fibroblasts lacking the Mrp4 gene migrate faster and contain higher cyclic-nucleotide levels. Here, we show that cAMP accumulation and protein kinase A (PKA) activity are higher and polarized in Mrp4(-/-) fibroblasts, versus Mrp4(+/+) cells. MRP4-containing macromolecular complexes isolated from these fibroblasts contained several proteins, including actin, which play important roles in cell migration. We found that actin interacts with MRP4, predominantly at the plasma membrane, and an intact actin cytoskeleton is required to restrict MRP4 to specific microdomains of the plasma membrane. Our data further indicated that the enhanced accumulation of cAMP in Mrp4(-/-) fibroblasts facilitates cortical actin polymerization in a PKA-dependent manner at the leading edge, which in turn increases the overall rate of cell migration to accelerate the process of wound healing. Disruption of actin polymerization or inhibition of PKA activity abolished the effect of MRP4 on cell migration. Together, our findings suggest a novel cAMP-dependent mechanism for MRP4-mediated regulation of fibroblast migration whereby PKA and actin play critical roles as downstream effectors.

Compartmentalized accumulation of cAMP near complexes of multidrug resistance protein 4 (MRP4) and cystic fibrosis transmembrane conductance regulator (CFTR) contributes to drug-induced diarrhea

Diarrhea is one of the most common adverse side effects observed in ∼7% of individuals consuming Food and Drug Administration (FDA)-approved drugs. The mechanism of how these drugs alter fluid secretion in the gut and induce diarrhea is not clearly understood. Several drugs are either substrates or inhibitors of multidrug resistance protein 4 (MRP4), such as the anti-colon cancer drug irinotecan and an anti-retroviral used to treat HIV infection, 3'-azido-3'-deoxythymidine (AZT). These drugs activate cystic fibrosis transmembrane conductance regulator (CFTR)-mediated fluid secretion by inhibiting MRP4-mediated cAMP efflux. Binding of drugs to MRP4 augments the formation of MRP4-CFTR-containing macromolecular complexes that is mediated via scaffolding protein PDZK1. Importantly, HIV patients on AZT treatment demonstrate augmented MRP4-CFTR complex formation in the colon, which defines a novel paradigm of drug-induced diarrhea.

The arachidonic acid metabolome serves as a conserved regulator of cholesterol metabolism

Cholesterol metabolism is closely interrelated with cardiovascular disease in humans. Dietary supplementation with omega-6 polyunsaturated fatty acids including arachidonic acid (AA) was shown to favorably affect plasma LDL-C and HDL-C. However, the underlying mechanisms are poorly understood. By combining data from a GWAS screening in >100,000 individuals of European ancestry, mediator lipidomics, and functional validation studies in mice, we identify the AA metabolome as an important regulator of cholesterol homeostasis. Pharmacological modulation of AA metabolism by aspirin induced hepatic generation of leukotrienes (LTs) and lipoxins (LXs), thereby increasing hepatic expression of the bile salt export pump Abcb11. Induction of Abcb11 translated in enhanced reverse cholesterol transport, one key function of HDL. Further characterization of the bioactive AA-derivatives identified LX mimetics to lower plasma LDL-C. Our results define the AA metabolomeasconserved regulator of cholesterol metabolism, and identify AA derivatives as promising therapeutics to treat cardiovascular disease in humans.

The role of transporters in toxicity and disease

The significance of transporters in the disposition, metabolism, and elimination of drugs is well recognized. One gap in our knowledge is a comprehensive understanding of how drug transporters change functionality (their amount and activity) in response to disease and how disease and its inevitable pathology change transporter expression. In this issue of Drug Metabolism and Disposition a series of review and primary research articles are presented to highlight the importance of transporters in toxicity and disease. Because of the central role of the liver in drug metabolism, many of the articles in this theme issue focus on transporters in the liver and how pathology or alterations in physiology affects transporter expression. The contributing authors have also considered the role of transporters in drug interactions as well as drug-induced liver injury. Noninvasive approaches to assessing transporter function in vivo are also described. Several articles highlight important issues in oncology where toxicity must be balanced against efficacy. In total, this theme issue will provide a stepping-stone to future studies that will establish a more comprehensive understanding of transporters in disease.

Deficiency of ATP-binding cassette transporter B6 in megakaryocyte progenitors accelerates atherosclerosis in mice

OBJECTIVE

The ATP-binding cassette (ABC) transporter B6 (ABCB6) is highly expressed in megakaryocyte progenitors, but its role in platelet production and disease has not been elucidated.

APPROACH AND RESULTS

Among various ABC transporters, ABCB6 was highly expressed in megakaryocyte progenitors, exhibiting the same pattern of expression of genes involved in heme synthesis pathway. Transplantation of Abcb6 deficient (Abcb6(-/-)) bone marrow into low density lipoprotein receptor deficient recipient mice resulted in expansion and proliferation of megakaryocyte progenitors, attributable to increased reactive oxygen species production in response to porphyrin loading. The enhanced megakaryopoiesis in Abcb6(-/-) bone marrow-transplanted mice was further illustrated by increased platelet counts, mean platelet volume, and platelet activity. Platelets from Abcb6(-/-) bone marrow-transplanted mice had higher levels of chemokine (C-C motif) ligand 5, which was associated with increased plasma chemokine (C-C motif) ligand 5 levels. There were also increased platelet-leukocyte aggregates, which resulted in leukocyte activation. Abcb6(-/-) bone marrow-transplanted mice had accelerated atherosclerosis which was associated with deposition of the chemotactic agent, chemokine (C-C motif) ligand 5 in atherosclerotic plaques, resulting in increased macrophage accumulation.

CONCLUSIONS

Our findings identify a new role of ABCB6 in preventing atherosclerosis development by dampening platelet production, reactivity, and chemokine (C-C motif) ligand 5 deposition in atherosclerotic lesions.