Development and Validation of a Fast and Homogeneous Cell-Based Fluorescence Screening Assay for Divalent Metal Transporter 1 (DMT1/SLC11A2) Using the FLIPR Tetra

Managing Iron Overload: A Gut Check

Divalent metal transporter 1 (DMT1) is the major importer of ferrous iron at the apical surface of enterocytes in the duodenum. Multiple groups have tried to design specific inhibitors for DMT1 both to study its contributions to iron (and metal ion) homeostasis and to provide a pharmacological means to treat iron overload disorders like hereditary hemochromatosis and thalassemias. This task faces challenges because many tissues express DMT1 and DMT1 transports other metals adding to standard risks in making specific inhibitors. Xenon Pharmaceuticals have published several papers on their efforts. Their latest paper in this issue of the journal culminates their efforts with compounds named XEN601 and XEN602 but implies that these very effective inhibitors have sufficient toxicity for them to halt development. This Viewpoint evaluates their efforts and briefly considers alternative routes to the goal. SIGNIFICANCE STATEMENT: This Viewpoint briefly reviews the paper on inhibitors of DMT1 that appears in this issue of the journal and commends the effort and research utility of those developed by Xenon. The inhibitors have proven to be valuable research tools for studying metal ion homeostasis particularly for iron. If Xenon is ceasing to try to develop them for treatment of iron overload disorders, then new alternatives need to come to the fore.

HIV-1 gp120-Induced Endolysosome de-Acidification Leads to Efflux of Endolysosome Iron, and Increases in Mitochondrial Iron and Reactive Oxygen Species

The HIV-1 coat protein gp120 continues to be implicated in the pathogenesis of HIV-1 associated neurocognitive disorder (HAND); a condition known to affect ~50% of people living with HIV-1 (PLWH). Autopsy brain tissues of HAND individuals display morphological changes to mitochondria and endolysosomes, and HIV-1 gp120 causes mitochondrial dysfunction including increased levels of reactive oxygen species (ROS) and de-acidification of endolysosomes. Ferrous iron is linked directly to ROS production, ferrous iron is contained in and released from endolysosomes, and PLWH have elevated iron and ROS levels. Based on those findings, we tested the hypothesis that HIV-1 gp120-induced endolysosome de-acidification and subsequent iron efflux from endolysosomes is responsible for increased levels of ROS. In U87MG glioblastoma cells, HIV-1 gp120 de-acidified endolysosomes, reduced endolysosome iron levels, increased levels of cytosolic and mitochondrial iron, and increased levels of cytosolic and mitochondrial ROS. These effects were all attenuated significantly by the endolysosome-specific iron chelator deferoxamine, by inhibitors of endolysosome-resident two-pore channels and divalent metal transporter-1 (DMT-1), and by inhibitors of mitochondria-resident DMT-1 and mitochondrial permeability transition pores. These results suggest that oxidative stress commonly observed with HIV-1 gp120 is downstream of its ability to de-acidify endolysosomes, to increase the release of iron from endolysosomes, and to increase the uptake of iron into mitochondria. Thus, endolysosomes might represent early and upstream targets for therapeutic strategies against HAND.

Inhibitors of Human Divalent Metal Transporters DMT1 (SLC11A2) and ZIP8 (SLC39A8) from a GDB-17 Fragment Library

Solute carrier proteins (SLCs) are membrane proteins controlling fluxes across biological membranes and represent an emerging class of drug targets. Here we searched for inhibitors of divalent metal transporters in a library of 1,676 commercially available 3D-shaped fragment-like molecules from the generated database GDB-17, which lists all possible organic molecules up to 17 atoms of C, N, O, S and halogen following simple criteria for chemical stability and synthetic feasibility. While screening against DMT1 (SLC11A2), an iron transporter associated with hemochromatosis and for which only very few inhibitors are known, only yielded two weak inhibitors, our approach led to the discovery of the first inhibitor of ZIP8 (SLC39A8), a zinc transporter associated with manganese homeostasis and osteoarthritis but with no previously reported pharmacology, demonstrating that this target is druggable.

Mechanistic basis of the inhibition of SLC11/NRAMP-mediated metal ion transport by bis-isothiourea substituted compounds

In humans, the divalent metal ion transporter-1 (DMT1) mediates the transport of ferrous iron across the apical membrane of enterocytes. Hence, its inhibition could be beneficial for the treatment of iron overload disorders. Here we characterize the interaction of aromatic bis-isothiourea-substituted compounds with human DMT1 and its prokaryotic homologue EcoDMT. Both transporters are inhibited by a common competitive mechanism with potencies in the low micromolar range. The crystal structure of EcoDMT in complex with a brominated derivative defines the binding of the inhibitor to an extracellular pocket of the transporter in direct contact with residues of the metal ion coordination site, thereby interfering with substrate loading and locking the transporter in its outward-facing state. Mutagenesis and structure-activity relationships further support the observed interaction mode and reveal species-dependent differences between pro- and eukaryotic transporters. Together, our data provide the first detailed mechanistic insight into the pharmacology of SLC11/NRAMP transporters.

Increased DMT1 and FPN1 expression with enhanced iron absorption in ulcerative colitis human colon

Iron deficiency anemia is a common complication of ulcerative colitis (UC) that can profoundly impact quality of life. Most iron absorption occurs in the duodenum via divalent metal transporter 1 (DMT1)-mediated uptake and ferroportin-1 (FPN1)-mediated export across the apical and basolateral membranes, respectively. However, the colon also contains iron transporters and can participate in iron absorption. Studies have shown increased duodenal DMT1 and FPN1 in patients with UC, but there is conflicting evidence about whether expression is altered in UC colon. We hypothesized that expression of colonic DMT1 and FPN1 will also increase to compensate for iron deficiency. Quantitative RT-PCR and Western blot analyses were performed on duodenal and colonic segmental (right colon, transverse colon, left colon, and rectum) biopsies obtained during colonoscopy. DMT1 mRNA and protein abundances in colonic segments were approximately equal to those in the duodenum, whereas colonic FPN1 mRNA and protein abundances of colonic segments were about one-quarter of those of the duodenum. DMT1 specific mRNA and protein abundances were increased twofold, whereas FPN1 mRNA and protein expressions were increased fivefold in UC distal colon. Immunofluorescence studies revealed enhanced expression of apical membrane- and basolateral membrane-localized DMT1 and FPN1 in UC human colon, respectively. Increased DMT1 expression was associated with enhanced 2-(3-carbamimidoylsulfanylmethyl-benzyl)-isothiourea (CISMBI, DMT1 specific inhibitor)-sensitive ⁵⁹Fe uptake in UC human colon. We conclude from these results that patients with active UC have increased expression of colonic iron transporters and increased iron absorption, which may be targeted in the treatment of UC-related anemia.

Reassessment of the Transport Mechanism of the Human Zinc Transporter SLC39A2

The human zinc transporter SLC39A2, also known as ZIP2, was shown to mediate zinc transport that could be inhibited at pH <7.0 and stimulated by HCO₃^-, suggesting a Zn²⁺/HCO₃^- cotransport mechanism [Gaither, L. A., and Eide, D. J. (2000) J. Biol. Chem. 275, 5560-5564]. In contrast, recent experiments in our laboratory indicated that the functional activity of ZIP2 increases at acidic pH [Franz, M. C., et al. (2014) J. Biomol. Screening 19, 909-916]. The study presented here was therefore designed to reexamine the findings about the pH dependence and to extend the functional characterization of ZIP2. Our current results show that ZIP2-mediated transport is modulated by extracellular pH but independent of the H⁺ driving force. Also, in our experiments, ZIP2-mediated transport is not modulated by extracellular HCO₃^-. Moreover, a high extracellular [K⁺], which induces depolarization, inhibited ZIP2-mediated transport, indicating that the transport mechanism is voltage-dependent. We also show that ZIP2 mediates the uptake of Cd²⁺ ( K_m ∼ 1.57 μM) in a pH-dependent manner ( K_H⁺ ∼ 66 nM). Cd²⁺ transport is inhibited by extracellular [Zn²⁺] (IC₅₀ ∼ 0.32 μM), [Cu²⁺] (IC₅₀ ∼ 1.81 μM), and to a lesser extent [Co²⁺], but not by [Mn²⁺] or [Ba²⁺]. Fe²⁺ is not transported by ZIP2. Accordingly, the substrate selectivity of ZIP2 decreases in the following order: Zn²⁺ > Cd²⁺ ≥ Cu²⁺ > Co²⁺. Altogether, we propose that ZIP2 is a facilitated divalent metal ion transporter that can be modulated by extracellular pH and membrane potential. Given that ZIP2 expression has been reported in acidic environments [Desouki, M. M., et al. (2007) Mol. Cancer 6, 37; Inoue, Y., et al. (2014) J. Biol. Chem. 289, 21451-21462; Tao, Y. T., et al. (2013) Mol. Biol. Rep. 40, 4979-4984], we suggest that the herein described H⁺-mediated regulatory mechanism might be important for determining the velocity and direction of the transport process.

Reduction in Prevalence of Hypertension and Blood Heavy Metals among Curry-Consumed Korean

Curcuma longa is rich in curcumin and used to treat disease and in food as a spice, especially in curry dishes. The objective of the present study was to determine whether curry intake reduces blood levels of heavy metals and hypertension (HTN) using Korean National Health and Nutrition Examination Survey 2013 data (n = 1,350). Study subjects (n = 1,350) were divided into two groups: 1) a curry intake group (n = 603), members of which had consumed a curry dish more than once a month over the previous year, and 2) a non-curry intake group (n = 747). Analysis showed the curry intake group had significantly lower heavy metal blood concentrations (Pb, Hg, and Cd) and blood concentrations of heavy metals were significantly associated with prevalence of HTN (P < 0.001 for Pb, Hg, and Cd). Curry intake also reduced the risk of HTN prevalence (odd ratios for curry intake vs. non-curry intake; Pb, 1.503 vs. 1.862; Hg, 1.112 vs. 1.149; Cd, 1.676 vs. 1.988). Logistic regression analysis was used to confirm the association between blood heavy metal levels and HTN. After adjusting for age, sex, lifetime smoking, and body mass index, the odd ratio of HTN was significant in the non-curry intake group, but not in the curry intake group, implying other factors influenced relations in the curry intake group. In view of the importance of curry consumption with reduced concentrations of heavy metals in blood and the prevalence of HTN, we suggest further well-designed clinical trials be conducted.

A role for divalent metal transporter (DMT1) in mitochondrial uptake of iron and manganese

Much of iron and manganese metabolism occurs in mitochondria. Uptake of redox-active iron must be tightly controlled, but little is known about how metal ions enter mitochondria. Recently, we established that the divalent metal transporter 1 (DMT1) is present in the outer mitochondrial membrane (OMM). Therefore we asked if it mediates Fe²⁺ and Mn²⁺ influx. Mitochondria were isolated from HEK293 cells permanently transfected with inducible rat DMT1 isoform 1 A/+IRE (HEK293-rDMT1). Fe²⁺-induced quenching of the dye PhenGreen™SK (PGSK) occurred in two phases, one of which reflected OMM DMT1 with stronger Fe²⁺ uptake after DMT1 overexpression. DMT1-specific quenching showed an apparent affinity of ~1.5 µM for Fe²⁺and was blocked by the DMT1 inhibitor CISMBI. Fe²⁺ influx reflected an imposed proton gradient, a response that was also observed in purified rat kidney cortex (rKC) mitochondria. Non-heme Fe accumulation assayed by ICPOES and stable ⁵⁷Fe isotope incorporation by ICPMS were increased in HEK293-rDMT1 mitochondria. HEK293-rDMT1 mitochondria displayed higher ⁵⁹Fe²⁺ and ⁵⁴Mn²⁺ uptake relative to controls with ⁵⁴Mn²⁺ uptake blocked by the DMT1 inhibitor XEN602. Such transport was defective in rKC mitochondria with the Belgrade (G185R) mutation. Thus, these results support a role for DMT1 in mitochondrial Fe²⁺ and Mn²⁺ acquisition.

A novel proton transfer mechanism in the SLC11 family of divalent metal ion transporters

In humans, the H+-coupled Fe2+ transporter DMT1 (SLC11A2) is essential for proper maintenance of iron homeostasis. While X-ray diffraction has recently unveiled the structure of the bacterial homologue ScaDMT as a LeuT-fold transporter, the exact mechanism of H+-cotransport has remained elusive. Here, we used a combination of molecular dynamics simulations, in silico pK a calculations and site-directed mutagenesis, followed by rigorous functional analysis, to discover two previously uncharacterized functionally relevant residues in hDMT1 that contribute to H+-coupling. E193 plays a central role in proton binding, thereby affecting transport properties and electrogenicity, while N472 likely coordinates the metal ion, securing an optimally "closed" state of the protein. Our molecular dynamics simulations provide insight into how H+-translocation through E193 is allosterically linked to intracellular gating, establishing a novel transport mechanism distinct from that of other H+-coupled transporters.

Discovery and characterization of a novel non-competitive inhibitor of the divalent metal transporter DMT1/SLC11A2

Divalent metal transporter-1 (SLC11A2/DMT1) uses the H(+) electrochemical gradient as the driving force to transport divalent metal ions such as Fe(2+), Mn(2+) and others metals into mammalian cells. DMT1 is ubiquitously expressed, most notably in proximal duodenum, immature erythroid cells, brain and kidney. This transporter mediates H(+)-coupled transport of ferrous iron across the apical membrane of enterocytes. In addition, in cells such as to erythroid precursors, following transferrin receptor (TfR) mediated endocytosis; it mediates H(+)-coupled exit of ferrous iron from endocytic vesicles into the cytosol. Dysfunction of human DMT1 is associated with several pathologies such as iron deficiency anemia hemochromatosis, Parkinson's disease and Alzheimer's disease, as well as colorectal cancer and esophageal adenocarcinoma, making DMT1 an attractive target for drug discovery. In the present study, we performed a ligand-based virtual screening of the Princeton database (700,000 commercially available compounds) to search for pharmacophore shape analogs of recently reported DMT1 inhibitors. We discovered a new compound, named pyrimidinone 8, which mediates a reversible linear non-competitive inhibition of human DMT1 (hDMT1) transport activity with a Ki of ∼20μM. This compound does not affect hDMT1 cell surface expression and shows no dependence on extracellular pH. To our knowledge, this is the first experimental evidence that hDMT1 can be allosterically modulated by pharmacological agents. Pyrimidinone 8 represents a novel versatile tool compound and it may serve as a lead structure for the development of therapeutic compounds for pre-clinical assessment.