D B Shennan

WWF United Kingdom, Londinium, England, United Kingdom

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Publications (104)388.93 Total impact

  • D B Shennan · C A R Boyd
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    ABSTRACT: This review describes the properties and regulation of the membrane transport proteins which supply the mammary gland with aminonitrogen to support metabolism under different physiological conditions (i.e. pregnancy, lactation and involution). Early studies focussed on characterising amino acid and peptide transport pathways with respect to substrate specificity, kinetics and hormonal regulation to allow a broad picture of the systems within the gland to be established. Recent investigations have concentrated on identifying the individual transporters at the molecular level (i.e. mRNA and protein). Many of the latter studies have identified the molecular correlates of the transport systems uncovered in the earlier functional investigations but in turn have also highlighted the need for more amino acid transport studies to be performed. The transporters function as either cotransporters and exchangers (or both) and act in a coordinated and regulated fashion to support the metabolic needs of the gland. However, it is apparent that a physiological role for a number of the transport proteins has yet to be elucidated. This article highlights the many gaps in our knowledge regarding the precise cellular location of a number of amino acid transporters within the gland. We also describe the role of amino acid transport in mammary cell volume regulation. Finally, the important role that individual mammary transport proteins may have in the growth and proliferation of mammary tumours is discussed.
    No preview · Article · Oct 2013 · Journal of Mammary Gland Biology and Neoplasia
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    D B Shennan

    Preview · Article · Oct 2013 · AJP Cell Physiology
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    ABSTRACT: The mammary gland acts as a bio-factory to produce in large amount few proteins by transcribing temporally and spatially regulated genes and translating their mRNA. The aim of this chapter is to summarize briefly our knowledge on the structure of the milk protein genes and to put into context the rapid growth of information on the regulatory elements involved in controlling the expression of these genes. It also describes the amino acid supply to the mammary gland and the intracellular transport and sorting of milk proteins in the secretory pathway of mammary cells.
    No preview · Chapter · Jan 2013
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    ABSTRACT: The mammary gland acts as a bio-factory to produce in large amount few proteins by transcribing temporally and spatially regulated genes and translating their mRNA. The aim of this chapter is to summarize briefly our knowledge on the structure of the milk protein genes and to put into context the rapid growth of information on the regulatory elements involved in controlling the expression of these genes. It also describes the amino acid supply to the mammary gland and the intracellular transport and sorting of milk proteins in the secretory pathway of mammary cells.
    No preview · Book · Jan 2013
  • D B Shennan
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    ABSTRACT: Sulphate is required by the feto-placental unit for a number of important conjugation and biosynthetic pathways. Functional studies performed several decades ago established that sulphate transport in human placental microvillus and basal membrane vesicles was mainly via a DIDS-sensitive anion-exchange mechanism. In contrast, no evidence was found for Na(+)-dependent transport. Studies performed using isolated human placental tissue confirmed anion-exchange as the main mechanism. More recently, molecular studies have established the presence of anion-exchange proteins which could play a role in transplacental sulphate movement. However, the presence of transcripts for NaS2 has been reported and has prompted the suggestion that Na(+)-sulphate cotransport may play an important role in maternal-fetal sulphate transport. This article reviews our present knowledge of placental sulphate transport, both functional and molecular, and attempts to form a model based on the available evidence.
    No preview · Article · May 2012 · Placenta
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    David B Shennan

    Preview · Article · Nov 2011 · The Journal of Physiology
  • David B Shennan · Jean Thomson
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    ABSTRACT: It has been shown that cell swelling stimulates the efflux of taurine from MCF-7 and MDA-MB-231 cells via a pathway which has channel-like properties. The purpose of this study was to examine the specificity of the volume-activated taurine efflux pathway in both cell lines. A hyposmotic shock increased the efflux of glycine, L-alanine, AIB (α-aminoisobutyric acid), D-aspartate but not L-leucine from MDA-MB-231 and MCF-7 cells. It was evident that the time course of activation/inactivation of those amino acids whose efflux was affected by cell swelling was similar to that of volume-activated taurine efflux. The effect of exogenous ATP on swelling-induced glycine, AIB and D-aspartate efflux from MDA-MB-231 cells was similar to that found on taurine efflux. In addition, volume-activated AIB efflux from MDA-MB-231 cells, like that of swelling-induced taurine efflux, was inhibited by diiodosalicylate. Tamoxifen inhibited volume-activated taurine release from both MDA-MB-231 and MCF-7 cells. The results suggest that neutral and anionic α-amino acids are able to utilize the volume-activated taurine efflux pathway in both cell lines. The effect of tamoxifen on breast cancer growth may, in part, be related to perturbations in cell volume regulation.
    No preview · Article · Mar 2011 · General Physiology and Biophysics
  • C A R Boyd · D B Shennan

    No preview · Article · Oct 2010 · Molecular Genetics and Metabolism
  • David B Shennan · Jean Thomson
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    ABSTRACT: It has been suggested that system L (LAT1/CD98hc) is up-regulated in cancer cells, including breast tumour cells, and is therefore a promising molecular target to inhibit or limit tumour cell growth. In view of this, we have examined the effect of BCH and other inhibitors of system L on the growth of MCF-7, ZR-75-1 and MDA-MB-231 cells. Treating cells with BCH markedly inhibited the metabolism of WST-1 in a dose-dependent fashion. Similarly, melphalan and D-leucine inhibited the growth of cultured breast cancer cells whereas MeAIB, an inhibitor of system A, was without effect. The effects of BCH and melphalan on cell growth were non-additive suggesting that both compounds were acting at a single locus. The results indicate that system L is required to maintain MCF-7, ZR-75-1 and MDA-MB-231 cell growth and support the notion that LAT1/CD98hc may be a suitable target to inhibit breast cancer progression.
    No preview · Article · Nov 2008 · Oncology Reports
  • David B Shennan
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    ABSTRACT: The secretion of calcium into milk by mammary epithelial cells is a fundamentally important process. Despite this, the mechanisms which underlie the movement of calcium across the lactating mammary gland are still poorly understood. There are, however, two models which describe the handling of calcium by mammary epithelial cells. On the one hand, a model which has existed for several decades, suggests that the vast majority of calcium enters milk via the Golgi secretory vesicle route. On the other hand, a new model has recently been proposed which implies that the active transport of calcium across the apical membrane of mammary secretory cells is central to milk calcium secretion. This short review examines the strengths and weaknesses of both models and suggests some experiments which could add to our understanding of mammary calcium transport.
    No preview · Article · Jun 2008 · Cellular & Molecular Biology Letters
  • David B Shennan
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    ABSTRACT: Cells have to regulate their volume in order to survive. Moreover, it is now evident that cell volume per se and the membrane transport processes which regulate it, comprise an important signalling unit. For example, macromolecular synthesis, apoptosis, cell growth and hormone secretion are all influenced by the cellular hydration state. Therefore, a thorough understanding of volume-activated transport processes could lead to new strategies being developed to control the function and growth of both normal and cancerous cells. Cell swelling stimulates the release of ions such as K(+) and Cl(-) together with organic osmolytes, especially the beta-amino acid taurine. Despite being the subject of intense research interest, the nature of the volume-activated taurine efflux pathway is still a matter of controversy. On the one hand it has been suggested that osmosensitive taurine efflux utilizes volume-sensitive anion channels whereas on the other it has been proposed that the band 3 anion-exchanger is a swelling-induced taurine efflux pathway. This article reviews the evidence for and against a role of anion channels and exchangers in osmosensitive taurine transport. Furthermore, the distinct possibility that neither pathway is involved in taurine transport is highlighted. The putative relationship between swelling-induced taurine transport and volume-activated anionic amino acid, alpha-neutral amino acid and K(+) transport is also examined.
    No preview · Article · Feb 2008 · Cellular Physiology and Biochemistry
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    David B Shennan · Jean Thomson
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    ABSTRACT: It has been reported that estrogen receptor-positive MCF-7 cells express TauT, a Na+-dependent taurine transporter. However, there is a paucity of information relating to the characteristics of taurine transport in this human breast cancer cell line. Therefore, we have examined the characteristics and regulation of taurine uptake by MCF-7 cells. Taurine uptake by MCF-7 cells showed an absolute dependence upon extracellular Na+. Although taurine uptake was reduced in Cl- free medium a significant portion of taurine uptake persisted in the presence of NO3 -. Taurine uptake by MCF-7 cells was inhibited by extracellular β-alanine but not by L-alanine or L-leucine. 17β-estadiol increased taurine uptake by MCF-7 cells: the Vmax of influx was increased without affecting the Km. The effect of 17β-estradiol on taurine uptake by MCF-7 cells was dependent upon the presence of extracellular Na+. In contrast, 17β-estradiol had no significant effect on the kinetic parameters of taurine uptake by estrogen receptor-negative MDA-MB-231 cells. It appears that estrogen regulates taurine uptake by MCF-7 cells via TauT. In addition, Na+-dependent taurine uptake may not be strictly dependent upon extracellular Cl-.
    Preview · Article · Apr 2007 · Cellular & Molecular Biology Letters
  • David B Shennan · Jean Thomson · James Davidson · Iain F Gow

    No preview · Article · Feb 2006 · Advances in Experimental Medicine and Biology
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    David B Shennan · Jean Thomson · Iain F Gow
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    ABSTRACT: The properties and regulation of volume-activated taurine efflux from MDA-MB-231 and MCF-7 cells have been investigated. Volume-activated taurine release from both cell lines was almost completely inhibited by diidosalicylate. DIDS , was more effective at inhibiting swelling-induced taurine release from MCF-7 than from MDA-MB-231 cells. On the basis of comparing taurine, Cl(-) and I(-) efflux time courses, it appears that volume-activated taurine efflux does not utilize volume-sensitive anion channels in MDA-MB- 231 and MCF-7 cells. Extracellular ATP stimulated volume-activated taurine release from MDA-MB-231 cells but not from MCF-7 cells. The effect of ATP was mimicked by UTP and was dependent upon external calcium and inhibited by suramin. However, suramin inhibited volume-activated taurine efflux from both MDA-MB-231 and MCF-7 cells even in the absence of exogenously added ATP suggesting that it acts directly on the taurine efflux pathway and/or is inhibiting the effect of ATP released from the cells. Volume-activated taurine efflux from MDA-MB-231 cells was stimulated by ionomycin. In contrast, ionomycin had no effect on taurine release from MCF-7 cells. Adenosine also stimulated volume-activated taurine efflux from MDA-MB-231 cells. The results suggest that purines regulate taurine transport in MDA-MB- 231 cells via more than one type of receptor.
    Full-text · Article · Feb 2006 · Cellular Physiology and Biochemistry
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    I F Gow · J Thomson · J Davidson · D B Shennan
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    ABSTRACT: The effect of a hyposmotic shock and extracellular ATP on the efflux of K(+)(Rb(+)) from human breast cancer cell lines (MDA-MB-231 and MCF-7) has been examined. A hyposmotic shock increased the fractional efflux of K(+)(Rb(+)) from MDA-MB-231 cells via a pathway which was unaffected by Cl(-) replacement. Apamin, charybdotoxin or removing extracellular Ca(2+) had no effect on volume-activated K(+)(Rb(+)) efflux MDA-MB-231 cells. An osmotic shock also stimulated K(+)(Rb(+)) efflux from MCF-7 cells but to a much lesser extent than found with MDA-MB-231 cells. ATP-stimulated K(+)(Rb(+)) efflux from MDA-MB-231 cells in a dose-dependent fashion but had little effect on K(+)(Rb(+)) release from MCF-7 cells. ATP-stimulated K(+)(Rb(+)) efflux was only inhibited slightly by replacing Cl(-) with NO(3)(-). Removal of external Ca(2+) during treatment with ATP reduced the fractional efflux of K(+)(Rb(+)) in a manner suggesting a role for cellular Ca(2+) stores. Charybdotoxin, but neither apamin nor iberiotoxin, inhibited ATP-stimulated K(+)(Rb(+)) release from MDA-MB-231 cells. Suramin inhibited the ATP-activated efflux of K(+)(Rb(+)). UTP also stimulated K(+)(Rb(+)) efflux from MDA-MB-231 cells whereas ADP, AMP and adenosine were without effect. A combination of an osmotic shock and ATP increased the fractional efflux of K(+)(Rb(+)) to a level greater than the sum of the individual treatments. It appears that the hyposmotically-activated and ATP-stimulated K(+) efflux pathways are separate entities. However, there may be a degree of 'crosstalk' between the two pathways.
    Full-text · Article · Jul 2005 · Biochimica et Biophysica Acta
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    D B Shennan · J Thomson · I F Gow · M T Travers · M C Barber
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    ABSTRACT: The transport of L-leucine by two human breast cancer cell lines has been examined. L-leucine uptake by MDA-MB-231 and MCF-7 cells was via a BCH-sensitive, Na+ -independent pathway. L-leucine uptake by both cell lines was inhibited by L-alanine, D-leucine and to a lesser extent by L-lysine but not by L-proline. Estrogen (17beta-estradiol) stimulated L-leucine uptake by MCF-7 but not by MDA-MB-231 cells. L-leucine efflux from MDA-MB-231 and MCF-7 cells was trans-stimulated by BCH in a dose-dependent fashion. The effect of external BCH on L-leucine efflux from both cell types was almost abolished by reducing the temperature from 37 to 4 degrees C. There was, however, a significant efflux of L-leucine under zero-trans conditions which was also temperature-sensitive. L-glutamine, L-leucine, D-leucine, L-alanine, AIB and L-lysine all trans-stimulated L-leucine release from MDA-MB-231 and MCF-7 cells. In contrast, D-alanine and L-proline had little or no effect. The anti-cancer agent melphalan inhibited L-leucine uptake by MDA-MB-231 cells but had no effect on L-leucine efflux. Quantitative real-time PCR revealed that LAT1 mRNA was approximately 200 times more abundant than LAT2 mRNA in MCF-7 cells and confirmed that MDA-MB-231 cells express LAT1 but not LAT2 mRNA. LAT1 mRNA levels were higher in MCF-7 cells than in MDA-MB-231 cells. Furthermore, LAT1 mRNA was more abundant than CD98hc mRNA in both MDA-MB-231 and MCF-7 cells. The results suggest that system L is the major transporter for L-leucine in both MDA-MB-231 and MCF-7 cells. It is possible that LAT1 may be the major molecular correlate of system L in both cell types. However, not all of the properties of system L reflected those of LAT1/LAT2/CD98hc.
    Full-text · Article · Sep 2004 · Biochimica et Biophysica Acta
  • D.B. Shennan · J Thomson
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    ABSTRACT: A knowledge of volume-sensitive solute transport in mammary cells is important in light of evidence that mammary cell metabolism is regulated by the cellular hydration state. In this report we have examined volume-sensitive taurine and K+ (Rb+) transport by lactating rat mammary tissue. A hyposmotic shock increased taurine efflux from rat mammary tissue: taurine release returned to a basal level upon transferring the tissue back to an isosmotic medium. However, the time taken to activate taurine efflux was less than the time taken to inactivate taurine release. A second subsequent osmotic challenge also increased taurine release but to a lesser extent than the first osmotic shock. A similar pattern was observed for bumetanide-insensitive, volume-activated K+ (Rb+) release from mammary tissue explants suggesting that taurine and K+ efflux are acting in concert to regulate mammary cell volume. An abrupt hyposmotic shock increased taurine efflux from mammary explants to a greater extent than a gradual reduction in the osmolality of the incubation medium. Increasing extracellular K+ increased taurine release via a pathway sensitive to niflumic acid, which suggests that activation of volume-sensitive taurine efflux does not require a change in the ionic strength of the incubation medium or a decrease in intracellular osmolality. A hyposmotic shock also stimulated taurine efflux from rat mammary acini. In contrast, a hyposmotic challenge had no effect on taurine uptake measured under sodium-free conditions. Hyposmotically induced taurine efflux was not dependent upon extracellular calcium. The results suggest that taurine and K+ transport may allow mammary cells to volume-regulate and consequently help to control mammary metabolism.
    No preview · Article · Aug 2004 · Molecular and Cellular Biochemistry
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    M T Travers · I F Gow · M C Barber · J Thomson · D B Shennan
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    ABSTRACT: The activity and expression of indoleamine 2,3-dioxygenase together with L-tryptophan transport has been examined in cultured human breast cancer cells. MDA-MB-231 but not MCF-7 cells expressed mRNA for indoleamine 2,3-dioxygenase. Kynurenine production by MDA-MB-231 cells, which was taken as a measure of enzyme activity, was markedly stimulated by interferon-gamma (1000 units/ml). Accordingly, L-tryptophan utilization by MDA-MB-231 cells was enhanced by interferon-gamma. 1-Methyl-DL-tryptophan (1 mM) inhibited interferon-gamma induced kynurenine production by MBA-MB-231 cells. Kynurenine production by MCF-7 cells remained at basal levels when cultured in the presence of interferon-gamma. L-Tryptophan transport into MDA-MB-231 cells was via a Na(+)-independent, BCH-sensitive pathway. It appears that system L (LAT1/CD98) may be the only pathway for l-tryptophan transport into these cells. 1-Methyl-D,L-tryptophan trans-stimulated l-tryptophan efflux from MDA-MB-231 cells and thus appears to be a transported substrate of system L. The results suggest that system L plays an important role in providing indoleamine-2,3-dioxygenase with its main substrate, L-tryptophan, and suggest a mechanism by which estrogen receptor-negative breast cancer cells may evade the attention of the immune system.
    Full-text · Article · Mar 2004 · Biochimica et Biophysica Acta
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    D B Shennan · J Thomson · M C Barber · M T Travers
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    ABSTRACT: The functional and molecular properties of system L in human mammary cancer cells (MDA-MB-231 and MCF-7) have been examined. All transport experiments were conducted under Na(+)-free conditions. alpha-Aminoisobutyric acid (AIB) uptake by MDA-MB-231 and MCF-7 cells was almost abolished by BCH (2-amino-2-norbornane-carboxylic acid). AIB uptake by MDA-MB-231 cells was also inhibited by L-alanine (83.6%), L-lysine (75.6%) but not by L-proline. Similarly, L-lysine and L-alanine, respectively, reduced AIB influx into MCF-7 cells by 45.3% and 63.7%. The K(m) of AIB uptake into MDA-MB-231 and MCF-7 cells was, respectively, 1.6 and 8.8 mM, whereas the V(max) was, respectively, 9.7 and 110.0 nmol/mg protein/10 min. AIB efflux from MDA-MB-231 and MCF-7 cells was trans-stimulated by BCH, L-glutamine, L-alanine, L-leucine, L-lysine and AIB (all at 2 mM). In contrast, L-glutamate, L-proline, L-arginine and MeAIB had no effect. The interaction between L-lysine and AIB efflux was one of low affinity. The fractional release of AIB from MDA-MB-231 cells was trans-accelerated by D-leucine and D-tryptophan but not by D-alanine. MDA-MB-231 and MCF-7 cells expressed LAT1 and CD98 mRNA. MCF-7 cells also expressed LAT2 mRNA. The results suggest that AIB transport in mammary cancer cells under Na(+)-free conditions is predominantly via system L which acts as an exchange mechanism. The differences in the kinetics of AIB transport between MDA-MB-231 and MCF-7 cells may be due to the differential expression of LAT2.
    Preview · Article · May 2003 · Biochimica et Biophysica Acta
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    ABSTRACT: During lactation, mammary epithelial cells secrete large quantities of milk proteins. More than 90% of these proteins are derived from the transcription of a few tissue-specific genes, expression of which is under a complex multi-hormonal regulation that involves both transcriptional and post- transcriptional mechanisms. Furthermore, to fulfil its bioreactor activity, the mammary gland needs an optimal supply of amino acids as well as efficient translation and transport systems during lactation.
    No preview · Chapter · Jan 2003

Publication Stats

2k Citations
388.93 Total Impact Points

Institutions

  • 2013
    • WWF United Kingdom
      Londinium, England, United Kingdom
  • 2007-2011
    • University of Strathclyde
      • Strathclyde Institute of Pharmacy and Biomedical Sciences
      Glasgow, Scotland, United Kingdom
  • 2010
    • UK Department of Health
      Londinium, England, United Kingdom
  • 1995-1997
    • University of Wales
      • Epithelial Function and Development Group
      Cardiff, Wales, United Kingdom
  • 1995-1996
    • University of Reading
      Reading, England, United Kingdom
  • 1994
    • Aberystwyth University
      Aberystwyth, Wales, United Kingdom
  • 1985-1988
    • University of Oxford
      • Department of Physiology, Anatomy and Genetics
      Oxford, England, United Kingdom
  • 1986
    • University of Dundee
      Dundee, Scotland, United Kingdom