Isoform-selective inhibition of facilitative glucose transporters: elucidation of the molecular mechanism of HIV protease inhibitor binding

Glucose transporter 4: Insulin response mastermind, glycolysis catalyst and treatment direction for cancer progression

The glucose transporter family (GLUT) consists of fourteen members. It is responsible for glucose homeostasis and glucose transport from the extracellular space to the cell cytoplasm to further cascade catalysis. GLUT proteins are encoded by the solute carrier family 2 (SLC2) genes and are members of the major facilitator superfamily of membrane transporters. Moreover, different GLUTs also have their transporter kinetics and distribution, so each GLUT member has its uniqueness and importance to play essential roles in human physiology. Evidence from many studies in the field of diabetes showed that GLUT4 travels between the plasma membrane and intracellular vesicles (GLUT4-storage vesicles, GSVs) and that the PI3K/Akt pathway regulates this activity in an insulin-dependent manner or by the AMPK pathway in response to muscle contraction. Moreover, some published results also pointed out that GLUT4 mediates insulin-dependent glucose uptake. Thus, dysfunction of GLUT4 can induce insulin resistance, metabolic reprogramming in diverse chronic diseases, inflammation, and cancer. In addition to the relationship between GLUT4 and insulin response, recent studies also referred to the potential upstream transcription factors that can bind to the promoter region of GLUT4 to regulating downstream signals. Combined all of the evidence, we conclude that GLUT4 has shown valuable unknown functions and is of clinical significance in cancers, which deserves our in-depth discussion and design compounds by structure basis to achieve therapeutic effects. Thus, we intend to write up a most updated review manuscript to include the most recent and critical research findings elucidating how and why GLUT4 plays an essential role in carcinogenesis, which may have broad interests and impacts on this field.

Multiple facilitated glucose transporters SLC2As are required for normal mouse preimplantation embryo development

BACKGROUND

Glucose metabolism is an essential energy source for mammalian preimplantation embryonic development. Therefore, we aimed to analyze the expression of all 12 known glucose transporters (facilitated solute carrier family 2, Slc2a) during early mouse embryo development.

METHODS

Gene and protein expression of Slc2a transporters in oocytes and embryos were assessed by the TaqMan gene expression assay and confocal immunofluorescence, respectively.

RESULTS

Except for Slc2a2, the other 11 Slc2a transcripts were detected in oocytes. Transcripts of Slc2a1, Slc2a3, Slc2a4, and Slc2a8 were the most enriched and detected in preimplantation embryos. The transcription of other Slc2a isoforms was barely detectable or absent after fertilization; however, they were detected in blastocysts, except for Slc2a10 and Slc2a13. Embryo culture in the simple defined medium caused a reduction in transcription of Slc2a1, Slc2a3, Slc2a4, and Slc2a8 in blastocyst; yet, amino acids partially reversed this impaired transcription of Slc2a1 and Slc2a4. SLC2A1 and SLC2A4 proteins were detected at all embryonic stages with nuclear accumulation in the embryos at the early cleavage stage. SLC2A3 and SLC2A8 were not detected in embryos until the eight-cell stage. The cellular membrane localization of SLC2A1, SLC2A3, and SLC2A8 occurred after compaction and was characterized in blastocysts. SLC2A4 was evenly distributed in the cytoplasm and nuclei without its characteristic membrane localization. Indinavir sulfate (a SLC2A4 inhibitor) decreased the rate of development and prevented glucose utilization in embryos after compaction. These inhibitory activities were partially reversed by exogenous insulin.

CONCLUSION

The results unveil distinct expression patterns of individual Slc2a glucose transporters during early embryo development. Taken together, they provide novel insights into the understanding and management of glucose metabolic infertility in assisted-reproductive technologies (ART).

Identification of druggable small molecule antagonists of the Plasmodium falciparum hexose transporter PfHT and assessment of ligand access to the glucose permeation pathway via FLAG-mediated protein engineering

Although the Plasmodium falciparum hexose transporter PfHT has emerged as a promising target for anti-malarial therapy, previously identified small-molecule inhibitors have lacked promising drug-like structural features necessary for development as clinical therapeutics. Taking advantage of emerging insight into structure/function relationships in homologous facilitative hexose transporters and our novel high throughput screening platform, we investigated the ability of compounds satisfying Lipinksi rules for drug likeness to directly interact and inhibit PfHT. The Maybridge HitFinder chemical library was interrogated by searching for compounds that reduce intracellular glucose by >40% at 10 μM. Testing of initial hits via measurement of 2-deoxyglucose (2-DG) uptake in PfHT over-expressing cell lines identified 6 structurally unique glucose transport inhibitors. WU-1 (3-(2,6-dichlorophenyl)-5-methyl-N-[2-(4-methylbenzenesulfonyl)ethyl]-1,2-oxazole-4-carboxamide) blocked 2-DG uptake (IC50 = 5.8 ± 0.6 μM) with minimal effect on the human orthologue class I (GLUTs 1-4), class II (GLUT8) and class III (GLUT5) facilitative glucose transporters. WU-1 showed comparable potency in blocking 2-DG uptake in freed parasites and inhibiting parasite growth, with an IC50 of 6.1 ± 0.8 μM and EC50 of 5.5 ± 0.6 μM, respectively. WU-1 also directly competed for N-[2-[2-[2-[(N-biotinylcaproylamino)ethoxy)ethoxyl]-4-[2-(trifluoromethyl)-3H-diazirin-3-yl]benzoyl]-1,3-bis(mannopyranosyl-4-yloxy)-2-propylamine (ATB-BMPA) binding and inhibited the transport of D-glucose with an IC50 of 5.9 ± 0.8 μM in liposomes containing purified PfHT. Kinetic analysis revealed that WU-1 acts as a non-competitive inhibitor of zero-trans D-fructose uptake. Decreased potency for WU-1 and the known endofacial ligand cytochalasin B was observed when PfHT was engineered to contain an N-terminal FLAG tag. This modification resulted in a concomitant increase in affinity for 4,6-O-ethylidene-α-D-glucose, an exofacially directed transport antagonist, but did not alter the Km for 2-DG. Taken together, these data are consistent with a model in which WU-1 binds preferentially to the transporter in an inward open conformation and support the feasibility of developing potent and selective PfHT antagonists as a novel class of anti-malarial drugs.

Development of GLUT4-selective antagonists for multiple myeloma therapy

Cancer cells consume more glucose to fuel metabolic programs fundamental to sustaining their survival, growth and proliferation. Among the fourteen SLC2A family members, GLUTs 1 and 4 are high-affinity glucose transporters. GLUT4 (SLC2A4) is highly expressed in muscle and adipose tissue. Basally retained within the cell, GLUT4 traffics to the plasma membrane (PM) in response to insulin and exercise-stimulation. The plasma cell malignancy multiple myeloma (MM) exhibits increased constitutive expression of GLUT4 on the PM, co-opting use of GLUT4 for survival and proliferation. GLUT4 inhibition by knockdown or treatment with the FDA-approved HIV protease inhibitor ritonavir leads to cytostatic and/or cytotoxic and chemosensitizing effects in tumor cells both in vitro and in vivo. We recently reported our generation of GLUT4 homology models and virtual high-throughput screening (vHTS) to identify multiple series of novel GLUT4 antagonists. In this report, we describe our initial hit-to-lead optimization to synthesize new analogs with improved potency and selectivity for GLUT4, and the biological characterization of these compounds in a variety of assays. We show that our lead compound (compound 20) decreases glucose uptake and cell proliferation as well as inhibits the expression of pro-survival MCL-1 in MM similar to the effect observed via knockdown of GLUT4 expression. Compound 20 is also effective at chemosensitizing multiple myeloma cell lines and patient samples to venetoclax, dexamethasone and melphalan. In sum, we report development of selective GLUT4 inhibitors lacking inhibitory activity against GLUT1 and GLUT8. We show that selective pharmacological inhibition of GLUT4 is feasible and this may represent a novel strategy for the treatment and chemosensitization of multiple myeloma to standard therapeutics.

Regulation of GLUT4 activity in myotubes by 3-O-methyl-d-glucose

The rate of glucose influx to skeletal muscles is determined primarily by the number of functional units of glucose transporter-4 (GLUT4) in the myotube plasma membrane. The abundance of GLUT4 in the plasma membrane is tightly regulated by insulin or contractile activity, which employ distinct pathways to translocate GLUT4-rich vesicles from intracellular compartments. Various studies have indicated that GLUT4 intrinsic activity is also regulated by conformational changes and/or interactions with membrane components and intracellular proteins in the vicinity of the plasma membrane. Here we show that the non-metabolizable glucose analog 3-O-methyl-d-glucose (MeGlc) augmented the rate of hexose transport into myotubes by increasing GLUT4 intrinsic activity without altering the content of the transporter in the plasma membrane. This effect was not a consequence of ATP depletion or hyperosmolar stress and did not involve Akt/PKB or AMPK signal transduction pathways. MeGlc reduced the inhibitory potency (increased K_i) of indinavir, a selective inhibitor of GLUT4, in a dose-dependent manner. Kinetic analyses indicate that MeGlc induced changes in GLUT4 or GLUT4 complexes within the plasma membrane, which enhanced the hexose transport activity and reduced the potency of indinavir inhibition. Finally, we present a simple kinetic analysis for screening and discovering low molecular weight compounds that augment GLUT4 activity.

Inhibitors of GLUT/SLC2A Enhance the Action of BCNU and Temozolomide against High-Grade Gliomas

Glucose transport across glioblastoma membranes plays a crucial role in maintaining the enhanced glycolysis typical of high-grade gliomas and glioblastoma. We tested the ability of two inhibitors of the glucose transporters GLUT/SLC2A superfamily, indinavir (IDV) and ritonavir (RTV), and of one inhibitor of the Na/glucose antiporter type 2 (SGLT2/SLC5A2) superfamily, phlorizin (PHZ), in decreasing glucose consumption and cell proliferation of human and murine glioblastoma cells. We found in vitro that RTV, active on at least three different GLUT/SLC2A transporters, was more effective than IDV, a specific inhibitor of GLUT4/SLC2A4, both in decreasing glucose consumption and lactate production and in inhibiting growth of U87MG and Hu197 human glioblastoma cell lines and primary cultures of human glioblastoma. PHZ was inactive on the same cells. Similar results were obtained when cells were grown in adherence or as 3D multicellular tumor spheroids. RTV treatment but not IDV treatment induced AMP-activated protein kinase (AMPKα) phosphorylation that paralleled the decrease in glycolytic activity and cell growth. IDV, but not RTV, induced an increase in GLUT1/SLC2A1 whose activity could compensate for the inhibition of GLUT4/SLC2A4 by IDV. RTV and IDV pass poorly the blood brain barrier and are unlikely to reach sufficient liquoral concentrations in vivo to inhibit glioblastoma growth as single agents. Isobologram analysis of the association of RTV or IDV and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) or 4-methyl-5-oxo-2,3,4,6,8-pentazabicyclo[4.3.0]nona-2,7,9-triene-9-carboxamide (TMZ) indicated synergy only with RTV on inhibition of glioblastoma cells. Finally, we tested in vivo the combination of RTV and BCNU on established GL261 tumors. This drug combination increased the overall survival and allowed a five-fold reduction in the dose of BCNU.

SLC2A8 (GLUT8) is a mammalian trehalose transporter required for trehalose-induced autophagy

Trehalose is a disaccharide demonstrated to mitigate disease burden in multiple murine neurodegenerative models. We recently revealed that trehalose rapidly induces hepatic autophagy and abrogates hepatic steatosis by inhibiting hexose transport via the SLC2A family of facilitative transporters. Prior studies, however, postulate that intracellular trehalose is sufficient to induce cellular autophagy. The objective of the current study was to identify the means by which trehalose accesses the hepatocyte cytoplasm, and define the distal signaling mechanisms by which trehalose induces autophagy. We provide gas chromatographic/mass spectrometric, fluorescence microscopic and radiolabeled uptake evidence that trehalose traverses the plasma membrane via SLC2A8 (GLUT8), a homolog of the trehalose transporter-1 (Tret1). Moreover, GLUT8-deficient hepatocytes and GLUT8-deficient mice exposed to trehalose resisted trehalose-induced AMP-activated protein kinase (AMPK) phosphorylation and autophagic induction in vitro and in vivo. Although trehalose profoundly attenuated mTORC1 signaling, trehalose-induced mTORC1 suppression was insufficient to activate autophagy in the absence of AMPK or GLUT8. Strikingly, transient, heterologous Tret1 overexpression reconstituted autophagic flux and AMPK signaling defects in GLUT8-deficient hepatocyte cultures. Together, these data suggest that cytoplasmic trehalose access is carrier-mediated, and that GLUT8 is a mammalian trehalose transporter required for hepatocyte trehalose-induced autophagy and signal transduction.

A Novel Fluorescence Resonance Energy Transfer-Based Screen in High-Throughput Format To Identify Inhibitors of Malarial and Human Glucose Transporters

The glucose transporter PfHT is essential to the survival of the malaria parasite Plasmodium falciparum and has been shown to be a druggable target with high potential for pharmacological intervention. Identification of compounds against novel drug targets is crucial to combating resistance against current therapeutics. Here, we describe the development of a cell-based assay system readily adaptable to high-throughput screening that directly measures compound effects on PfHT-mediated glucose transport. Intracellular glucose concentrations are detected using a genetically encoded fluorescence resonance energy transfer (FRET)-based glucose sensor. This allows assessment of the ability of small molecules to inhibit glucose uptake with high accuracy (Z' factor of >0.8), thereby eliminating the need for radiolabeled substrates. Furthermore, we have adapted this assay to counterscreen PfHT hits against the human orthologues GLUT1, -2, -3, and -4. We report the identification of several hits after screening the Medicines for Malaria Venture (MMV) Malaria Box, a library of 400 compounds known to inhibit erythrocytic development of P. falciparum Hit compounds were characterized by determining the half-maximal inhibitory concentration (IC₅₀) for the uptake of radiolabeled glucose into isolated P. falciparum parasites. One of our hits, compound MMV009085, shows high potency and orthologue selectivity, thereby successfully validating our assay for antimalarial screening.

Severe dyslipidemia and immune activation in HIV patients with dysglycemia

BACKGROUND AND OBJECTIVE

Diabetes mellitus (DM) is common in human immunodeficiency virus (HIV)-infected patients. However, the relationship between dysglycemia, lipid metabolism, and immune activation in HIV patients is poorly understood.

METHODS

We retrospectively analyzed the clinical data of 180 HIV patients, including 153 patients undergoing highly active antiretroviral therapy (HAART) and 27 HAART-naive patients. DM was defined as fasting serum glucose levels ≥126 mg/dl, and impaired fasting glucose (IFG) was defined as serum glucose levels of 101-125 mg/dl at two different time points. Lipid metabolic indexes were measured. CD4+, CD8+, and CD8+ HLA-DR+ T cells were determined by flow cytometry.

RESULTS

IFM and DM percentages were higher in the HAART group than in the HAART-naive group (59.5% vs. 48.1% and 21.6% vs. 7.4%, respectively; p < 0.01). Additionally, DM percentage was high in patients receiving HAART containing protease inhibitors. Serum levels of triglycerides and very low-density lipoprotein cholesterol were higher in IFG and DM HAART patients than in euglycemic HAART patients (p < 0.05). Serum triglyceride levels were higher in HAART-naive DM patients than in other patients (p < 0.05). CD8+ and CD8+ HLA-DR+ cell counts were higher in IFG and DM HAART patients than in euglycemic HAART patients (p < 0.05). Ordinal logistic regression analysis suggested that TRIG, VLDL, CD8, and HAART were predictors of glucose metabolic disorders.

CONCLUSION

HIV patients with hyperglycemia have severe dyslipidemia and immune activation, and HAART is an important impact factor of glucose and lipid metabolic disorders.

Trehalose inhibits solute carrier 2A (SLC2A) proteins to induce autophagy and prevent hepatic steatosis

Trehalose is a naturally occurring disaccharide that has gained attention for its ability to induce cellular autophagy and mitigate diseases related to pathological protein aggregation. Despite decades of ubiquitous use as a nutraceutical, preservative, and humectant, its mechanism of action remains elusive. We showed that trehalose inhibited members of the SLC2A (also known as GLUT) family of glucose transporters. Trehalose-mediated inhibition of glucose transport induced AMPK (adenosine 5'-monophosphate-activated protein kinase)-dependent autophagy and regression of hepatic steatosis in vivo and a reduction in the accumulation of lipid droplets in primary murine hepatocyte cultures. Our data indicated that trehalose triggers beneficial cellular autophagy by inhibiting glucose transport.

Expression, purification, and functional characterization of the insulin-responsive facilitative glucose transporter GLUT4

The insulin-responsive facilitative glucose transporter GLUT4 is of fundamental importance for maintenance of glucose homeostasis. Despite intensive effort, the ability to express and purify sufficient quantities of structurally and functionally intact protein for biophysical analysis has previously been exceedingly difficult. We report here the development of novel methods to express, purify, and functionally reconstitute GLUT4 into detergent micelles and proteoliposomes. Rat GLUT4 containing FLAG and His tags at the amino and carboxy termini, respectively, was engineered and stably transfected into HEK-293 cells. Overexpression in suspension culture yielded over 1.5 mg of protein per liter of culture. Systematic screening of detergent solubilized GLUT4-GFP fusion protein via fluorescent-detection size exclusion chromatography identified lauryl maltose neopentyl glycol (LMNG) as highly effective for isolating monomeric GLUT4 micelles. Preservation of structural integrity and ligand binding was demonstrated via quenching of tryptophan fluorescence and competition of ATB-BMPA photolabeling by cytochalasin B. GLUT4 was reconstituted into lipid nanodiscs and proper folding was confirmed. Reconstitution of purified GLUT4 with amphipol A8-35 stabilized the transporter at elevated temperatures for extended periods of time. Functional activity of purified GLUT4 was confirmed by reconstitution of LMNG-purified GLUT4 into proteoliposomes and measurement of saturable uptake of D-glucose over L-glucose. Taken together, these data validate the development of an efficient means to generate milligram quantities of stable and functionally intact GLUT4 that is suitable for a wide array of biochemical and biophysical analyses.

The Glucose Transporter PfHT1 Is an Antimalarial Target of the HIV Protease Inhibitor Lopinavir

Malaria and HIV infection are coendemic in a large portion of the world and remain a major cause of morbidity and mortality. Growing resistance of Plasmodium species to existing therapies has increased the need for new therapeutic approaches. The Plasmodium glucose transporter PfHT is known to be essential for parasite growth and survival. We have previously shown that HIV protease inhibitors (PIs) act as antagonists of mammalian glucose transporters. While the PI lopinavir is known to have antimalarial activity, the mechanism of action is unknown. We report here that lopinavir blocks glucose uptake into isolated malaria parasites at therapeutically relevant drug levels. Malaria parasites depend on a constant supply of glucose as their primary source of energy, and decreasing the available concentration of glucose leads to parasite death. We identified the malarial glucose transporter PfHT as a target for inhibition by lopinavir that leads to parasite death. This discovery provides a mechanistic basis for the antimalarial effect of lopinavir and provides a direct target for novel drug design with utility beyond the HIV-infected population.

Antipsychotics inhibit glucose transport: Determination of olanzapine binding site in Staphylococcus epidermidis glucose/H(+) symporter

The antipsychotic drug olanzapine is widely prescribed to treat schizophrenia and other psychotic disorders. However, it often causes unwanted side effects, including diabetes, due to disruption of insulin-dependant glucose metabolism through a mechanism yet to be elucidated. To determine if olanzapine can affect the first step in glucose metabolism - glucose transport inside cells - we investigated the effect of this drug on the transport activity of a model glucose transporter. The glucose transporter from Staphylococcus epidermidis (GlcPSe) is specific for glucose, inhibited by various human glucose transporter (GLUT) inhibitors, has high sequence and structure homology to GLUTs, and is readily amenable to transport assay, mutagenesis, and computational modeling. We found that olanzapine inhibits glucose transport of GlcPSe with an IC50 0.9 ± 0.1 mM. Computational docking of olanzapine to the GlcPSe structure revealed potential binding sites that were further examined through mutagenesis and transport assay to identify residues important for olanzapine inhibition. These investigations suggest that olanzapine binds in a polar region of the cytosolic part of the transporter, and interacts with residues R129, strictly conserved in all GLUTs, and N136, conserved in only a few GLUTs, including the insulin-responsive GLUT4. We propose that olanzapine inhibits GlcPSe by impeding the alternating opening and closing of the substrate cavity necessary for glucose transport. It accomplishes this by disrupting a key salt bridge formed by conserved residues R129 and E362, that stabilizes the outward-facing conformation of the transporter.

In Silico Modeling-based Identification of Glucose Transporter 4 (GLUT4)-selective Inhibitors for Cancer Therapy

Tumor cells rely on elevated glucose consumption and metabolism for survival and proliferation. Glucose transporters mediating glucose entry are key proximal rate-limiting checkpoints. Unlike GLUT1 that is highly expressed in cancer and more ubiquitously expressed in normal tissues, GLUT4 exhibits more limited normal expression profiles. We have previously determined that insulin-responsive GLUT4 is constitutively localized on the plasma membrane of myeloma cells. Consequently, suppression of GLUT4 or inhibition of glucose transport with the HIV protease inhibitor ritonavir elicited growth arrest and/or apoptosis in multiple myeloma. GLUT4 inhibition also caused sensitization to metformin in multiple myeloma and chronic lymphocytic leukemia and a number of solid tumors suggesting the broader therapeutic utility of targeting GLUT4. This study sought to identify selective inhibitors of GLUT4 to develop a more potent cancer chemotherapeutic with fewer potential off-target effects. Recently, the crystal structure of GLUT1 in an inward open conformation was reported. Although this is an important achievement, a full understanding of the structural biology of facilitative glucose transport remains elusive. To date, there is no three-dimensional structure for GLUT4. We have generated a homology model for GLUT4 that we utilized to screen for drug-like compounds from a library of 18 million compounds. Despite 68% homology between GLUT1 and GLUT4, our virtual screen identified two potent compounds that were shown to target GLUT4 preferentially over GLUT1 and block glucose transport. Our results strongly bolster the utility of developing GLUT4-selective inhibitors as anti-cancer therapeutics.