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Current
Drug Targets
ISSN: 1389-4501
Current Drug Targets
Volume 9, Number 4, April 2008
Contents
Glycosyltransferases-Part-I
Guest Editor: Subhash Basu
Editorial Pp. 261
Unique Structural Motif Supports Mannosylphospho
Dolichol Synthase: An Important Angiogenesis Regulator
Pp. 262-271
K. Baksi, J.J. Tavárez-Pagán, J.A. Martínez
and D.K. Banerjee
[Abstract]
Regulation of Lactosylceramide Synthase (Glucosylceramide
β1→4
Galactosyltransferase); Implication as A Drug Target
Pp. 272-281
S. Chatterjee, A. Kolmakova and M. Rajesh
[Abstract]
Golgi Localization of Glycosyltransferases Involved
in Ganglioside Biosynthesis Pp. 282-291
G. van Echten-Deckert and M. Guravi
[Abstract]
Structure and Function of β-1,4-Galactosyltransferase
Pp. 292-309
P.K. Qasba, B. Ramakrishnan and E. Boeggeman
[Abstract]
Promoter Structure and Transcriptional Regulation
of Human β-Galactoside
α2,
3-Sialyltransferase Genes Pp. 310-316
A. Taniguchi
[Abstract]
Cloning and Transcriptional Regulation of Genes Responsible
for Synthesis of Gangliosides Pp. 317-324
G. Zeng and R.K. Yu
[Abstract]
General Articles
Aging and Remodeling During Healing of the Wounded Heart:
Current Therapies and Novel Drug Targets Pp. 325-344
B.I. Jugdutt
[Abstract]
Cellular and Molecular Mechanisms Involved in the
Action of Vitamin D Analogs Targeting Vitiligo Depigmentation
Pp. 345-359
S.A. Birlea, G.-E. Costin and D.A. Norris
[Abstract]
Abstracts
[Back to top]
Editorial
Glycoconjugates (glycolipids, glycoproteins and proteoglycans)
are ubiquitous on eukaryotic cell surfaces. Isolation, chemical
structure determinations, and characterization of individual
Glycosyl-transferases for their biosynthesis took almost 4
decades (until the early 1990’s) of hard work by endless
glycobiochemists and glycobiologists around the world. From
1986 until now putative gene sequences of almost 300 glycosyltransferases
have been established.
Each glycoconjugate is characterized by its unique structure
synthesized by activities of a group of specific glycosyltrans-
ferases. It is nearly established that a specific linkage
(between two sugars) is established by the catalysis of a
specific gene product (or a glycosyltransferase) as proposed
by Professor Saul Roseman in early 1970’s; sometimes
with little variation of gene sequence, a different linkage
formation is also catalyzed as observed recently. However,
many questions remain unanswered: i) what are the active sites
of these enzymes present in the protein sequences? ii) How
are these enzymes transcriptionally regulated? iii) How are
these glycosyltransferases post-translationally modified and
regulated? iv) How are these glycosyltransferases regulated
in apoptotic and metastatic cells?
When our chief editor contacted me two years ago to edit an
issue on ”Glycosyltransferases” in this new well-rated
journal, I agreed to do so, keeping these questions in my
mind. At first I had to identify all those excellent scientists
who are actively contributing in this field of “Glycosyltransferases”
and secondly to propose to each individual author (also with
their active collaborators) to write an article wherein, with
their expertise, they could write for the interested readers
the latest words in which that area in which they are working.
Finally, after one year of negotiations I received 13 well-written
articles from the experts in the field, which covered some
of the questions raised earlier. We decided to publish these
thirteen articles in two volumes. Of course many active researchers
in this field were unable to contribute in these two volumes.
To maintain the high standard of this journal I went through
every line of each article and checked almost every reference
so that it is correctly quoted in the right place; of course
this was a time consuming task for one person. Finally, I
am able to release the first volume of the ”Glycosyltransferases”
in this journal, with six papers to the press, and we are
almost ready within a few months to send to press the all
other 7 articles to be printed in the second volume. A total
list of all the accepted all 13 articles will be printed in
both the volumes. I am happy to accept the responsibility
as a special editor for these two volumes. The chief editor
is my good friend for 37 years who requested me to edit and
I am also expecting to have a few more new friends for rest
of my life after these two volumes are published.
Subhash Basu
Editor for special issues
Department of Chemistry and Biochemistry
University of Notre Dame
USA
[Back to top]
Unique Structural Motif Supports Mannosylphospho Dolichol
Synthase: An Important Angiogenesis Regulator
K. Baksi, J.J. Tavárez-Pagán, J.A. Martínez
and D.K. Banerjee
Mannosylphospho dolichol synthase (DPMS) catalyzes the
transfer reaction GDP-mannose + Dol-P ↔
Dol-P-Man + GDP, a ‘key step’ in the assembly
of lipid-linked oligosaccharide (LLO) and a pre-requisite
for asparagine-linked (N-linked) protein glycosylation. DPMS
is present from a protozoan parasite to human, and its sequence
carries a cAMP-dependent phosphorylation motif. We have evaluated
the involvement of DPMS in angiogenesis, an essential physiological
event during the growth of breast and other solid tumors.
It has been observed that enhancers of intracellular cAMP
accelerated the capillary endothelial cell proliferation by
reducing the cell cycle duration. Reduced Con A to WGA fluorescence
ratio indicated high level complex type N-glycans on the cell
surface. This was supported by upregulated LLO biosynthesis
in cells stimulated either with a β-agonist
isoproterenol or other cAMP enhancer, such as 8Br-cAMP, forskolin,
cholera toxin, or prostaglandin E1. The turnover (t1/2)
of LLO was also increased. Increased LLO biosynthesis correlated
extremely well with the DPMS activity in cells treated with
8Br-cAMP. High DPMS activity in isoproterenol-treated cells
was not due to an increased gene expression because actinomycin
D failed to block the upregulation. cDNA cloning of capillary
endothelial cell Dpm1 gene and the deduced amino acid sequence
identified a PKA motif in capillary endothelial cell DPMS.
Thus, it has been concluded that increased DPMS activity through
protein phosphorylation is a driving force for angiogeneis.
Its abolition, however, led to cell arrest in G1 and induction
of apoptosis.
[Back to top]
Regulation of Lactosylceramide Synthase (Glucosylceramide
β1→4
Galactosyltransferase); Implication as A Drug Target
S. Chatterjee, A. Kolmakova and M. Rajesh
Lactosylceramide is a ubiquitously present glycosphingolipid
in mammalian tissues and has been implicated in cell proliferation,
adhesion, migration and angiogenesis. This glycosphingolipid
is synthesized by Golgi-localized enzyme LacCer synthase.
According to recent nomenclature and gene mapping studies,
two LacCer synthases β1,4GalT-V
and β1,4GalT-VI
have been identified and characterized. In addition, β1,4GalT-V
has been implicated in the synthesis of N-glycans of cell
surface glycoproteins. During the past two decades data have
accumulated suggesting that the cellular level of LacCer can
be regulated by various growth factors, cytokines, lipids,
lipoproteins and hemodynamic factors, such as fluid shear
stress, by altering the activity of LacCer synthase. An interesting
feature is that a nuclear regulating factor (SP1) plays a
critical role in transcriptional regulation of this enzyme
in cancer cells. Moreover, in human umbilical vein endothelial
cells, NF-κB
has been also shown to regulate this enzyme which, in turn,
regulates the gene/protein expression of platelet endothelial
cell adhesion molecule, intercellular cell adhesion molecule
and angiogenesis. Since new blood supply via formation
of capillaries is critical in tumor growth, metastasis, and
atherogenesis, these findings expand the role of enzyme in
these pathologies.
Additional studies are warranted to understand the molecular
and biochemical basis of how LacCer synthases are regulated.
These studies will facilitate advances in discovery of drugs
which mitigate diseases, such as atherosclerosis and cancer
due to an aberrant regulation of these LacCer synthases.
[Back to top]
Golgi Localization of Glycosyltransferases Involved in Ganglioside
Biosynthesis
G. van Echten-Deckert and M. Guravi
Gangliosides make up a group of sialic acid-containing
complex glycosphingolipids particularly abundant in the central
nervous system. The finding indicating gangliosides are stored
in certain hereditary diseases affecting the central nervous
system opened the interest in studying their metabolism. The
initial in vitro pioneering work on the glycosyltransferases
involved in ganglioside biosynthesis was done by Roseman and
his associates primarily in embryonic chick brains almost
forty years ago. Since that time enzymes catalyzing the formation
of main human gangliosides have been successfully purified
and cloned. Their specificity has been determined and their
subcellular localization and topology has been established.
Transgenic mouse models deficient in distinct ganglioside-directed
glycosyltransferases are available and represent a vital step
toward understanding the metabolism and function of this challenging
lipid class. In the present review we briefly introduce the
reader in the complex structure of gangliosides, then we summarize
new developments concerning their function especially regarding
neurodegenerative disorders, and in this article we would
like to review on what is known about glycosyltransferases
that catalyze the formation of these complex lipids in the
Golgi apparatus, that was established by Basu and his associates
almost three decades ago.
[Back to top]
Structure and Function of β-1,4-Galactosyltransferase
P.K. Qasba, B. Ramakrishnan and E. Boeggeman
Beta-1,4-galactosylransferase (β4Gal-T1)
participates in the synthesis of Galβ1-4-GlcNAc-disaccharide
unit of glycoconjugates. It is a trans-Golgi glycosyltransferase
(Glyco-T) with a type II membrane protein topology, a short
N-terminal cytoplasmic domain, a membrane-spanning
region, as well as a stem and a C-terminal catalytic domain
facing the trans-Golgi-lumen. Its hydrophobic membrane-spanning
region, like that of other Glyco-T, has a shorter length compared
to plasma membrane proteins, an important feature for its
retention in the trans-Golgi. The catalytic domain
has two flexible loops, a long and a small one. The primary
metal binding site is located at the N-terminal hinge
region of the long flexible loop. Upon binding of metal ion
and sugar-nucleotide, the flexible loops undergo a marked
conformational change, from an open to a closed conformation.
Conformational change simultaneously creates at the C-terminal
region of the flexible loop an oligosaccharide acceptor binding
site that did not exist before. The loop acts as a lid covering
the bound donor substrate. After completion of the transfer
of the glycosyl unit to the acceptor, the saccharide product
is ejected; the loop reverts to its native conformation to
release the remaining nucleotide moiety. The conformational
change in β4Gal-T1
also creates the binding site for a mammary gland-specific
protein, α-lactalbumin
(LA), which changes the acceptor specificity of the enzyme
toward glucose to synthesize lactose during lactation. The
specificity of the sugar donor is generally determined by
a few residues in the sugar-nucleotide binding pocket of Glyco-T,
conserved among the family members from different species.
Mutation of these residues has allowed us to design new and
novel glycosyltransferases, with broader or requisite donor
and acceptor specificities, and to synthesize specific complex
carbohydrates as well as specific inhibitors for these enzymes.
[Back to top]
Promoter Structure and Transcriptional Regulation of Human
β-Galactoside
α2,
3-Sialyltransferase Genes
A. Taniguchi
Six human β-galactoside
α 2,3-sialyltransferase
genes, which are hST3Gal I-VI, have been cloned. Multiple
genes encode enzymes with closely related catalytic specificities
but different patterns of tissue expression. The multiple
genes correspond to the control of various tissue specific
regulators. Several studies have examined the transcriptional
regulation of some human β-galactoside
α2,3-sialyltransferases
genes. Multiple mRNA forms differing only in the 5'-untranslated
regions have been identified in hST3Gal II, hST3Gal III, hST3Gal
IV, hST3Gal V, and hST3Gal VI. These transcripts are produced
by a combination of alternative splicing and promoter utilization,
suggesting the transcriptional regulation of this gene depends
on the use of alternative promoters, further suggesting that
tissue-specific transcriptional regulation of these genes
depends on the use of multiple genes and multiple promoters.
The multiple regulatory pathways of these ubiquitous sialyltransferases
may be differentially modulated in various cell types.
[Back to top]
Cloning and Transcriptional Regulation of Genes Responsible
for Synthesis of Gangliosides
G. Zeng and R.K. Yu
Ganglioside synthases are glycosyltransferases involved
in the biosynthesis of glycoconjugates. A number of ganglioside
synthase genes have been cloned and characterized. They are
classified into different families of glycosyltransferases
based on similarities of their amino acid sequences. Tissue-specific
expression of these genes has been analyzed by hybridization
using cDNA fragments. Enzymatic characterization with the
expressed recombinant enzymes showed these enzymes differ
in their donor and acceptor substrate specificities and other
biochemical parameters. In vitro enzymatic analysis
also showed that one linkage can be synthesized by multiple
enzymes and one enzyme may be responsible for synthesis of
multiple gangliosides. Following the cloning of the ganglioside
synthase genes, the promoters of the key synthase genes in
the ganglioside biosynthetic pathway have been cloned and
analyzed. All of the promoters are TATA-less, lacking a CCAAT
box but containing GC-rich boxes, characteristic of the house-keeping
genes, although transcription of ganglioside synthase genes
is subject to complex developmental and tissue-specific regulation.
A set of cis-acting elements and transcription factors,
including Sp1, AP2, and CREB, function in the proximal promoters.
Negative-regulatory regions have also been defined in most
of the promoters. We present here an overview of these genes
and their transcriptional regulation.
[Back to top]
Aging and Remodeling During Healing of the Wounded Heart:
Current Therapies and Novel Drug Targets
B.I. Jugdutt
Aging has become a major health care problem and socio-economic
burden worldwide. Myocardial infarction (MI) is the major
killer worldwide and coronary reperfusion is the major form
of acute post-MI therapy. The aging population is increasing,
and with it, morbidity and mortality due to impaired healing
after ST-segment elevation MI (STEMI) and its consequences.
Optimal healing of the wounded heart is critical for preservation
of structural and functional integrity of the pumping chambers,
survival, and a favorable outcome irrespective of age. Although
STEMI is more prevalent in the elderly and impaired healing
during aging may promote adverse remodeling and thereby jeopardize
outcome, there is an information gap on post-STEMI healing
and its therapy in the elderly. Current therapies during post-STEMI
healing are aimed primarily at the <65 age-group and preclinical
studies tend to test drugs in mostly young animals. Therapies
over the last decade have improved post-MI survival mainly
in patients aged < 65 years. Novel healing-specific proteins
may provide potential targets for improving healing and limiting
adverse remodeling of the post-STEMI heart in the elderly,
thereby improving outcome.
[Back to top]
Cellular and Molecular Mechanisms Involved in the Action of
Vitamin D Analogs Targeting Vitiligo Depigmentation
S.A. Birlea, G.-E. Costin and D.A. Norris
The active metabolite of vitamin D3
- 1,25-(OH)2 D3
- exerts most of its physiological and pharmacological actions
through its nuclear receptor (VDR), regulating the transcriptional
machinery of a variety of cell types. Basic research motivated
by the detection of VDR in numerous target cells, has indicated
potential therapeutic applications of VDR ligands in osteoporosis,
cancer, secondary hyperparathyroidism and autoimmune diseases
such as psoriasis, systemic lupus erythematosus, rheumatoid
arthritis, type 1 diabetes and multiple sclerosis. In recent
years vitamin D analogs, particularly calcipotriol and tacalcitol,
have been used as topical therapeutic agents in vitiligo,
an autoimmune pigmentary disorder characterized by aberrant
loss of functional melanocytes from involved epidermis. The
presence of cytotoxic T cells targeting melanocyte antigens
and imbalance of the cytokine network were described as characteristics
of the disease, eventually leading to melanocyte damage and
death. Vitamin D ligands are designed to target the local
immune response in vitiligo, acting on specific T cell activation,
mainly by inhibiting the transition of T cells from early
to late G1 phase and by inhibiting the expression of several
pro-inflammatory cytokines genes, such as those encoding tumor
necrosis factor alpha (TNF-α)
and interferon gamma (IFN-γ).
Vitamin D3 compounds are
known to influence melanocyte maturation and differentiation
and also to up-regulate melanogenesis through pathways activated
by specific ligand receptors, such as endothelin receptor
and c-kit. In this review we summarize the complex pathogenetic
rationale of vitamin D analogs in vitiligo depigmentation.
Understanding the cellular and molecular mechanisms through
which vitamin D targets the epidermal melanin unit is of great
interest for identification of new effective therapeutic combination(s)
that might induce repigmentation in vitiligo.
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