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.

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.

Aryl hydrocarbon receptor ligands increase ABC transporter activity and protein expression in killifish (Fundulus heteroclitus) renal proximal tubules

Many widespread and persistent organic pollutants, for example, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and some polychlorinated biphenyls, activate the aryl hydrocarbon receptor (AhR) causing it to translocate to the cell nucleus where it transactivates target genes, increasing expression of a number of xenobiotic metabolizing enzymes as well as some transporters. AhR's ability to target transporters within the kidney is essentially unexplored. We show here that exposing isolated killifish (Fundulus heteroclitus) renal proximal tubules to micromolar β-naphthoflavone (BNF) or nanomolar TCDD roughly doubled the transport activity of Multidrug resistance-associated proteins Mrp2 and Mrp4, P-glycoprotein (P-gp) and Breast cancer resistance protein (Bcrp), all ATP-driven xenobiotic efflux pumps and critical determinants of renal xenobiotic excretion. These effects were abolished by actinomycin D and cycloheximide and by the AhR antagonist, α-naphthoflavone, indicating that increased transport activity was dependent on transcription and translation as well as ligand binding to AhR. Quantitative immunostaining of renal tubules exposed to BNF and TCDD showed increased luminal membrane expression of Mrp2, Mrp4, P-gp and Bcrp. Thus, in these renal tubules, the four ABC transporters are targets of AhR action.

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.

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.

Gender-specific expression of ATP-binding cassette (Abc) transporters and cytoprotective genes in mouse choroid plexus

The choroid plexus (CP) and blood-brain barrier (BBB) control the movement of several drugs and endogenous compounds between the brain and systemic circulation. The multidrug resistance associated protein (Mrp) efflux transporters form part of these barriers. Several Mrp transporters are positively regulated by the transcription factor nuclear factor erythroid-2-related factor (Nrf2) in liver. The Mrps, Nrf2 and Nrf2-dependent genes are cytoprotective and our aim was to examine basal gender differences in expression of Mrp transporters, Nrf2 and Nrf2-dependent genes (Nqo1 and Ho-1) in the brain-barriers. Previous studies have shown higher expression of Mrp1, Mrp2 and Mrp4 in female mouse liver and kidney. We hypothesized that similar renal/hepatic gender-specific patterns are present in the brain-barrier epithelia interfaces. qPCR and immunoblot analyses showed that Mrp4, Ho-1 and Nqo1 expression was higher in female CP. Mrp1, Mrp2 and Nrf2 expression in the CP had no gender pattern. Female Mrp1, Mrp2 and Mrp4 mouse brain expressions in remaining brain areas, excluding CP, were higher than male. Functional analysis of Mrp4 in CP revealed active accumulation of the Mrp4 model substrate fluo-cAMP. WT female CP had 10-fold higher accumulation in the vascular spaces than males and 60% higher than Mrp4^-/- females. Probenecid blocked all transport. Methotrexate did as well except in Mrp4^-/- females where it had no effect, suggesting compensatory induction of transport occurred in Mrp4^-/-. Collectively, our findings indicate significant gender differences in expression of Mrp transporters and cytoprotective genes in the CP and BBB.

Multidrug resistance-associated protein 4 expression in ammonia-treated cultured rat astrocytes and cerebral cortex of cirrhotic patients with hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric syndrome frequently accompanying liver cirrhosis and reflects the clinical manifestation of a low grade cerebral edema associated with cerebral oxidative/nitrosative stress. The multidrug resistance-associated protein (Mrp) 4 is an export pump which transports metabolites that were recently suggested to play a major role in the pathogenesis of HE such as neurosteroids and cyclic nucleotides. We therefore studied Mrp4 expression changes in ammonia-exposed cultured astrocytes and postmortem human brain samples of cirrhotic patients with HE. NH₄ Cl increased Mrp4 mRNA and protein levels in astrocytes in a dose- and time-dependent manner up to threefold after 72 h of exposure and concurrently inhibited N-glycosylation of Mrp4 protein. Upregulation of Mrp4 mRNA and protein as well as impaired N-glycosylation of Mrp4 protein by ammonia were sensitive towards the glutamine-synthetase inhibitor l-methionine-S-sulfoximine and were not induced by CH₃ NH₃ Cl (5 mmol/L). Upregulation of Mrp4 mRNA required ammonia-induced activation of nitric oxide synthases or NADPH oxidase and p38^MAPK -dependent activation of PPARα. Inhibition of Mrp4 by ceefourin 1 synergistically enhanced both, inhibition of astrocyte proliferation as well as transcription of the oxidative stress surrogate marker heme oxygenase 1 by forskolin (10 µmol/L, 72 h) or NH₄ Cl (5 mmol/L, 72 h) in cultured rat astrocytes. Increased Mrp4 mRNA and protein levels were also found in postmortem brain samples from patients with liver cirrhosis with HE but not in those without HE. The data show that Mrp4 is upregulated in HE, which may be relevant for the handling of neurosteroids and cyclic nucleotides in response to ammonia. GLIA 2015;63:2092-2105.

Crucial role for phylogenetically conserved cytoplasmic loop 3 in ABCC4 protein expression

The ABC transporter ABCC4 is recognized as an ATP-dependent exporter of endogenous substances as well as an increasing variety of anionic chemotherapeutics. A loss-of-function variant of zebrafish Abcc4 was identified with a single amino acid substitution in the cytoplasmic loop T804M. Because this substituted amino acid is highly conserved among ABCC4 orthologs and is located in cytoplasmic loop 3 (CL3), we investigated the impact of this mutation on human and zebrafish Abcc4 expression. We demonstrate that zebrafish Abcc4 T804M or human ABCC4 T796M exhibit substantially reduced expression, coupled with impaired plasma membrane localization. To understand the molecular basis for the localization defect, we developed a homology model of zebrafish Abcc4. The homology model suggested that the bulky methionine substitution disrupted side-chain contacts. Molecular dynamic simulations of a fragment of human or zebrafish CL3 containing a methionine substitution indicated altered helicity coupled with reduced thermal stability. Trifluoroethanol challenge coupled with circular dichroism revealed that the methionine substitution disrupted the ability of this fragment of CL3 to readily form an α-helix. Furthermore, expression and plasma membrane localization of these mutant ABCC4/Abcc4 proteins are mostly rescued by growing cells at subphysiological temperatures. Because the cystic fibrosis transmembrane conductance regulator (ABCC7) is closely related to ABCC4, we extended this by engineering certain pathogenic CFTR-CL3 mutations, and we showed they destabilized human and zebrafish ABCC4. Altogether, our studies provide the first evidence for a conserved domain in CL3 of ABCC4 that is crucial in ensuring its proper plasma membrane localization.