Reconstitution of an insulin signaling pathway in Xenopus laevis oocytes: coexpression of a mammalian insulin receptor and three different mammalian hexose transporters

A GC-MS/Single-Cell Method to Evaluate Membrane Transporter Substrate Specificity and Signaling

Amino acid transporters play a vital role in metabolism and nutrient signaling pathways. Typically, transport activity is investigated using single substrates and competing amounts of other amino acids. We used GC-MS and LC-MS for metabolic screening of Xenopus laevis oocytes expressing various human amino acid transporters incubated in complex media to establish their comprehensive substrate profiles. For most transporters, amino acid selectivity matched reported substrate profiles. However, we could not detect substantial accumulation of cationic amino acids by SNAT4 and ATB0,+ in contrast to previous reports. In addition, comparative substrate profiles of two related sodium neutral amino acid transporters known as SNAT1 and SNAT2, revealed the latter as a significant leucine accumulator. As a consequence, SNAT2, but not SNAT1, was shown to be an effective activator of the eukaryotic cellular growth regulator mTORC1. We propose, that metabolomic profiling of membrane transporters in Xe nopus laevis oocytes can be used to test their substrate specificity and role in intracellular signaling pathways.

Metformin (Glucophage) inhibits tyrosine phosphatase activity to stimulate the insulin receptor tyrosine kinase

Metformin is a commonly used anti-diabetic but whether its mechanism involves action on the insulin receptor or on downstream events is still controversial. With a time course that was slow compared with insulin action, metformin increased tyrosine phosphorylation of the regulatory domain of the insulin receptor (specifically, tyrosine residues 1150 and 1151). In a direct action, therapeutic levels of metformin stimulated the tyrosine kinase activity of the soluble intracellular portion of the beta subunit of the human insulin receptor toward a substrate derived from the insulin receptor regulatory domain. However, metformin did not alter the order of substrate phosphorylation by the insulin receptor kinase. Using a Xenopus oocyte preparation, we simultaneously recorded tyrosine kinase and phosphatase activities that regulate the insulin receptor by measuring the tyrosine phosphorylation and dephosphorylation of peptides derived from the regulatory domain of the human insulin receptor. In an indirect stimulation of the insulin receptor, metformin inhibited endogenous tyrosine phosphatases and purified human protein tyrosine phosphatase 1B that dephosphorylate and inhibit the insulin receptor kinase. Thus, there was evidence that metformin acted directly upon the insulin receptor and indirectly through inhibition of tyrosine phosphatases.

Regulatory motifs are present in the ITS1 of some flatworm species

Particular sequence motifs can act as transcription regulators. Because the total regulatory effects of such motifs can be related to their abundance, their presence might be expected at locations within the genome where sequences are repeated. Multiple repeats that vary in number among individuals occur within the ribosomal first internal transcribed spacer (ITS1) in some species in three trematode genera: Paragonimus, Schistosoma and Dolichosaccus. In all of these genera we found in ITS1, sequences identical to known enhancer motifs. We also searched for, and identified, known regulatory motifs in published ITS1 sequences of other parasitic flatworms including Echinostoma spp. (Trematoda) and Echinococcus spp. (Cestoda) which lack multiple repeats in ITS1. We present three lines of evidence that this widespread occurrence of such motifs within the ITS1 of parasitic flatworms may indicate a functional role in regulating tissue- or stage-specific transcription of ribosomal genes. First, these motifs are identical to ones whose functional roles have been established using in vitro assays of transcriptional rates. Second, in all 18 species investigated here, between one and three different regulatory motifs were identified. In 14 of these 18 species, the probability that at least one of these motifs occurred because of the random assortment of bases within the regions investigated was 10% or less. In 12 of these 14 species, the probability was 5% or less. Third, the evolutionary divergence of flatworm species investigated is quite ancient. Therefore, the interspecific distribution of motifs observed here, in a rapidly evolving region such as ITS1, is unlikely to be attributable solely to shared evolutionary histories. These results, therefore, suggest a broader functional role for the ITS1 than previously thought.

Metformin interaction with insulin-regulated glucose uptake, using the Xenopus laevis oocyte model expressing the mammalian transporter GLUT4

The primary goal of this work was to better define, in molecular terms, the impact of metformin on hexose carriers. The methodology consisted of determining the zero-trans kinetics of 2-deoxy-D-glucose uptake for the mammalian insulin-sensitive glucose transporter (GLUT4) expressed in Xenopus laevis oocytes. These cells possessed the specialized protein and, when treated with insulin (2 microM) plus metformin (20 microM), showed a markedly enhanced hexose transport activity (2.4-fold increase over basal) as compared to that of cells incubated in the presence of insulin alone (1.8-fold increase over basal). Kinetic analysis of this process revealed that insulin induced a similar response to that observed for the native carrier, i.e., a higher Vmax. When metformin was added together with insulin, we mainly recorded a significant decrease in apparent Km for the sugar transported, Vmax being only marginally modified. Parathyroid hormone (PTH), which is known to impair the intrinsic activity of GLUT4, prevented the stimulatory effect of metformin in both kinds of oocytes whereas cytochalasin D, which interferes with the translocation of carriers, was without effect. These results suggest that metformin combined with insulin can maintain glucose homeostasis by increasing the catalytic activity of some hexose carriers or by improving the affinity of GLUT4 for glucose.

Functional Analysis of Mature Hematopoietic Cells From Mice Lacking the βc Chain of the Granulocyte-Macrophage Colony-Stimulating Factor Receptor

Mice with a null mutation of the βc chain of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors (βc-null mice) develop an alveolar proteinosis-like lung disease. The pathogenesis of this disease is uncertain and, although a defect in alveolar macrophage function has been postulated, no previous analysis of mature hematopoietic cells in mice with alveolar proteinosis has been reported. Therefore, we undertook a functional analysis of the mature hematopoietic cell compartment in βc-null mice. In addition, we reexamined the roles of the GM-CSF receptor  chain and the βc chain in signaling by GM-CSF. Neutrophils and macrophages from βc-null mice were capable of normal survival and phagocytosis in the absence of stimulus and of similar levels of nitric oxide production in response to interferon-γ and lipopolysaccharide. GM-CSF–mediated augmentation of survival, phagocytosis, and hydrogen-ion production were absent in neutrophils from βc-null mice. Interestingly, we were unable to show any ability of the GM-CSF receptor -chain alone to mediate glucose transport in these cells. In keeping with the βc-null mice lung pathology, examination of lavage fluid from the lungs of βc-null mice showed increased cellularity. This was caused by an increase in the number of lymphocytes, neutrophils, and macrophages. Large foamy cells in the lavage fluid from βc-null mice were identified as macrophages using immunohistochemistry. Functional analysis showed that these βc-null alveolar macrophages were capable of phagocytosis but uptake of colloidal carbon and cellular adhesion were reduced. In summary, mature hematopoietic cells with a null mutation of the βc receptor were unable to perform GM-CSF–mediated hematopoietic cell functions including glucose transport, but responded normally to a range of other ligands.

Functional Analysis of Mature Hematopoietic Cells From Mice Lacking the βc Chain of the Granulocyte-Macrophage Colony-Stimulating Factor Receptor

Mice with a null mutation of the βc chain of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors (βc-null mice) develop an alveolar proteinosis-like lung disease. The pathogenesis of this disease is uncertain and, although a defect in alveolar macrophage function has been postulated, no previous analysis of mature hematopoietic cells in mice with alveolar proteinosis has been reported. Therefore, we undertook a functional analysis of the mature hematopoietic cell compartment in βc-null mice. In addition, we reexamined the roles of the GM-CSF receptor  chain and the βc chain in signaling by GM-CSF. Neutrophils and macrophages from βc-null mice were capable of normal survival and phagocytosis in the absence of stimulus and of similar levels of nitric oxide production in response to interferon-γ and lipopolysaccharide. GM-CSF–mediated augmentation of survival, phagocytosis, and hydrogen-ion production were absent in neutrophils from βc-null mice. Interestingly, we were unable to show any ability of the GM-CSF receptor -chain alone to mediate glucose transport in these cells. In keeping with the βc-null mice lung pathology, examination of lavage fluid from the lungs of βc-null mice showed increased cellularity. This was caused by an increase in the number of lymphocytes, neutrophils, and macrophages. Large foamy cells in the lavage fluid from βc-null mice were identified as macrophages using immunohistochemistry. Functional analysis showed that these βc-null alveolar macrophages were capable of phagocytosis but uptake of colloidal carbon and cellular adhesion were reduced. In summary, mature hematopoietic cells with a null mutation of the βc receptor were unable to perform GM-CSF–mediated hematopoietic cell functions including glucose transport, but responded normally to a range of other ligands.

Adaptation of intestinal nutrient transport in health and disease. Part I

Why is it important to understand the mechanisms controlling intestinal adaptation? There are two major answers to this question. Firstly, in establishing the cellular and molecular events associated with intestinal adaptation, we will formulate a general framework that may be applied to the understanding of adaptation of other cell membranes. For example, alterations in the synthesis of glucose carriers and their subsequent insertion into membranes may alter sugar entry across the intestinal brush border membrane (BBM) using the sodium-dependent D-glucose transporter, SGLT1, or the BBM sodium-independent facultative fructose transporter, GLUT5, and may alter facilitated sugar exit across the basolateral membrane (BLM) using GLUT2. The precise role of transcriptional and translational processes in the up- or down-regulation of sugar transport requires further definition. Alterations in enterocyte microsomal lipid metabolic enzyme expression occurring during the course of intestinal adaptation will direct the synthesis of lipids destined for trafficking to the BBM and BLM domains of the enterocyte. This will subsequently alter the passive permeability properties of these membranes and ultimately influence lipid absorption. Therefore, establishing the physiological, cellular and molecular mechanisms of adaptation in the intestine will define principles that may be applied to other epithelia. Secondly, enterocyte membrane adaptation is subject to dietary modification, and these may be exploited as a means to enhance a beneficial or to reduce a detrimental aspect of the intestinal adaptive process in disease states. Alterations in membrane function occur in association with changes in dietary lipids, and these are observed in a variety of cells and tissues including lymphocytes, testes, liver, adipocytes, nerve tissue, nuclear envelope and mitochondria. Therefore, the elucidation of the mechanisms of intestinal adaptation and the manner whereby dietary manipulation modulates these processes affords the future possibility of dietary engineering aimed at using food as a therapeutic agent. It is hoped this approach will form the centerpiece for future investigation that would focus on disease prevention, as well as on the development of better therapeutic strategies to prevent the development or to treat the complications of conditions such as diabetes mellitus, obesity, hyperlipidemia and inflammatory bowel diseases. This review deals with the physiology of glucose transport with specific emphasis on transporters of the brush border membrane (BBM) and the basolateral membrane (BLM). On the BBM the sodium (Na)/glucose transporters (SGLT1 and SGLT2), the Na-independent transporter (GLUT5), and on the BLM the hexose transporter (GLUT2) are discussed. The molecular biology of these transporters is also reviewed.

N-glycosylation of the human granulocyte-macrophage colony-stimulating factor receptor alpha subunit is essential for ligand binding and signal transduction

The alpha subunit of the receptor for human granulocyte-macrophage colony-stimulating factor (GM-CSF) is a glycoprotein containing 11 potential N-glycosylation sites in the extracellular domain. We examined the role of N-glycosylation on alpha subunit membrane localization and function. Tunicamycin, an N-glycosylation inhibitor, markedly inhibited GM-CSF binding, GM-CSF-induced deoxyglucose uptake, and protein tyrosine phosphorylation in HL-60(eos) cells but did not affect cell surface expression of the alpha subunit as detected by an anti-alpha subunit monoclonal antibody. In COS cells expressing the alpha subunit and treated with tunicamycin, N-unglycosylated alpha subunit was expressed and transported to the cell surface but was not capable of binding GM-CSF. High affinity binding in COS cells expressing both alpha and beta subunits was also blocked by tunicamycin treatment. These studies indicate that N-linked oligosaccharides are essential for alpha subunit ligand binding and signaling by the human GM-CSF receptor.

The 3' untranslated region of bovine preprolactin contains a transferable non-poly(A) mRNA sequence that prolongs translation

Preprolactin transcripts, synthesized in vitro, were actively translated for a prolonged period when injected into Xenopus oocytes. As a result, prolactin continued to be secreted into the media for up to 6 days after injection of the transcript. To investigate the role of the preprolactin 3' untranslated sequence in stabilizing transcripts, it was fused to coding regions derived from signal recognition particle receptor alpha-subunit or preproinsulin receptor. The translational half-life of the chimeric RNA was increased for both coding regions, suggesting that a sequence within the preprolactin 3' untranslated region that prolongs translation is transferable. Deletion mutagenesis of this untranslated region demonstrated that a sequence of 98 nucleotides immediately following the prolactin stop codon was sufficient to prolong translation of RNAs injected into Xenopus oocytes.

GTP analogs suppress uptake but not transport of D-glucose analogs in Glut1 glucose transporter-expressing Xenopus oocytes

A Xenopus oocyte expression-co-injection system was used to study the influence of guanine nucleotides on D-glucose uptake. GTP analogs like GTP gamma S and GppNHp had no effect on 3-O-methylglucose transport determined by zero-trans uptake or equilibrium exchange, but suppressed 2-deoxyglucose uptake into Glut1 glucose transporter-expressing oocytes by up to 86%. Both GTP analogs showed concentration dependence of their effectiveness, with GTP gamma S being more potent than GppNHp. No statistically significant differences were observed between groups of oocytes co-injected with water or GDP beta S (250 and 500 microM intracellular concentration). Glut1 transporter expression in plasma membrane was not different between water or GTP gamma S-co-injected oocytes. Thus, inhibition of hexokinase catalytic activity is the most likely causative factor for down-regulation of 2-deoxyglucose uptake.

Mammalian facilitative hexose transporters mediate the transport of dehydroascorbic acid

Although vitamin C is critical to human physiology, it is not clear how it is taken up into cells. The kinetics of cell and tissue accumulation of ascorbic acid in vitro indicate that the process is mediated by specific transporters at the cell membrane. Some experimental observations have linked the transport of ascorbic acid with hexose transport systems in mammalian cells, although no clear information is available regarding the specific role(s) of these transporters, if any, in this process. Here we use the Xenopus laevis oocyte expression system to show that the mammalian facilitative hexose transporters are efficient transporters of the oxidized form of vitamin C (dehydroascorbic acid). Two transport pathways, one with low affinity and one with high affinity for dehydroascorbic acid, were found in oocytes expressing the mammalian transporters, and these oocytes accumulated vitamin C against a concentration gradient when supplied with dehydroascorbic acid. We obtained similar results in experiments using normal human neutrophils. These observations indicate that mammalian facilitative hexose transporters are a physiologically significant pathway for the uptake and accumulation of vitamin C by cells, and suggest a mechanism for the accumulation of ascorbic acid against a concentration gradient.