A peptide nucleic acid-aminosugar conjugate targeting transactivation response element of HIV-1 RNA genome shows a high bioavailability in human cells and strongly inhibits tat-mediated transactivation of HIV-1 transcription

Article abstract of DOI:10.1021/jm300253q

The 6-aminoglucosamine ring of the aminoglycoside antibiotic neomycin B (ring II) was conjugated to a 16-mer peptide nucleic acid (PNA) targeting HIV-1 TAR RNA. For this purpose, we prepared the aminoglucosamine monomer 15 and attached it to the protected PNA prior to its cleavage from the solid support. We found that the resulting PNA-aminoglucosamine conjugate is stable under acidic conditions, efficiently taken up by the human cells and fairly distributed in both cytosol and nucleus without endosomal entrapment because cotreatment with endosome-disrupting agent had no effect on its cellular distribution. The conjugate displayed very high target specificity in vitro and strongly inhibited Tat mediated transactivation of HIV-1 LTR transcription in a cell culture system. The unique properties of this new class of PNA conjugate suggest it to be a potential candidate for therapeutic application.

Full text of DOI:10.1021/jm300253q

Article
pubs.acs.org/jmc
A Peptide Nucleic Acid−Aminosugar Conjugate Targeting
Transactivation Response Element of HIV-1 RNA Genome Shows a
High Bioavailability in Human Cells and Strongly Inhibits Tat-
Mediated Transactivation of HIV-1 Transcription
Indrajit Das,† Jerome Desire,†,§ Dinesh Manvar,‡ Isabelle Baussanne,† Virendra N. Pandey,*,‡
́
̂
́
́
and Jean-Luc Decout*,†
́
†Universite
BP 53, F-38041 Grenoble, France
́ ́ ́
de Grenoble I/CNRS, UMR 5063, Departement de Pharmacochimie Moleculaire, ICMG FR 2607, 470 rue de la Chimie
‡Center for the Study of Emerging and Re-emerging Pathogens, UMDNJNew Jersey Medical School, Department of Biochemistry
and Molecular Biology, 185 South Orange Avenue, Newark, New Jersey 07103, United States
S
* Supporting Information
ABSTRACT: The 6-aminoglucosamine ring of the aminoglycoside antibiotic neomycin
B (ring II) was conjugated to a 16-mer peptide nucleic acid (PNA) targeting HIV-1 TAR
RNA. For this purpose, we prepared the aminoglucosamine monomer 15 and attached it
to the protected PNA prior to its cleavage from the solid support. We found that the
resulting PNA−aminoglucosamine conjugate is stable under acidic conditions, efficiently
taken up by the human cells and fairly distributed in both cytosol and nucleus without
endosomal entrapment because cotreatment with endosome-disrupting agent had no
effect on its cellular distribution. The conjugate displayed very high target specificity in
vitro and strongly inhibited Tat mediated transactivation of HIV-1 LTR transcription in a
cell culture system. The unique properties of this new class of PNA conjugate suggest it
to be a potential candidate for therapeutic application.
unprotonated amino groups in the neamine core, an acido-basic
catalysis of hydrolysis of the target RNA could be expected.44
INTRODUCTION
■
A
Aminoglycosides such as neomycin B 1 (Figure 1) are highly
potent and broad-spectrum pseudo-oligosaccharidic antibiotics
which act by binding to 16S rRNA, causing mRNA decoding
errors, mRNA and tRNA translocation blockage, ribosome
recycling inhibition, and in fine protein synthesis alteration.1−5
The strong affinities of aminoglycosides for other RNA targets
such as several ribozymes6−16 and important RNA motifs of
HIV, the transactivation response element (TAR), the rev
responsive element (RRE), and the dimerization initiation site
(DIS),17−20 led to intensive works for their modifications in the
search for RNA-targeting drugs.21−25 Amphiphilic aminoglyco-
sides have been also reported as antimicrobial agents26−34
targeting bacterial rRNA and/or membranes33,34 and as
efficient gene35,36 and siRNA37 delivery vectors.
number of aminoglycoside antibiotics, and in particular
neomycin B, were demonstrated to promote strand cleavage
of RNA oligonucleotides (minimized HIV-1 TAR element and
prokaryotic ribosomal A-site) by binding and causing sufficient
distortion to the RNA backbone to render it more susceptible
to intramolecular transphosphorylation.45,46 Regarding the
conjugation of aminoglycosides to oligo-2′-deoxyribonucleo-
tides (ODN), neomycin has been covalently attached to the 5′-
end of an ODN complementary to a seven-bases long α-sarcin
loop RNA sequence, and its ability to enhance duplex
formation has been studied.47 A set of neamine- and
ribostamycin-2′-O-methyl oligoribonucleotide conjugates has
been prepared in order to study their nuclease activity.48
Neamine and paromamine−ODN conjugates were also
synthesized using click chemistry, and DNA duplex stabilization
was observed with a paromamine conjugate.49,50 More recently,
aminoglycoside (neomycin, ribostamycin, and methyl neobios-
amine) conjugated to the 3′-end of 2′-O-methyl oligoribonu-
cleotides were synthesized to target a 19F labeled HIV-1 TAR
RNA model which allows monitoring of the invasion by 19F
NMR spectrometry.51 A remarkably enhanced invasion,
In the antisense approach for RNA targeting,38,39 the
conjugation of aminoglycoside to oligonucleotides or peptide
(polyamide) nucleic acids (PNA) is attractive because the
strong binding of the covalently attached aminoglycoside may
enhance hybridization and direct the drug design to shorter
oligonucleotide sequences. Aminoglycosides are able to
stabilize DNA- and RNA-triplexes and DNA−RNA hybrid
triplexes and hybrid duplexes.40−42 Improvements in the
cellular uptake also may be provided because neomycin has
been shown to assist the lipid-mediated delivery of
oligonucleotides.43 Because of the presence of protonated and
Received: December 6, 2011
Published: June 14, 2012
© 2012 American Chemical Society
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Figure 1. Structure of neomycin B, neamine, the anti-TAR PNA, and the corresponding neamine and 6-aminoglucosamine conjugates 4, 5a, and 5b.
compared to that resulting from the corresponding unmodified
2′-O-methyl oligoribonucleotide was observed with the neo-
mycin conjugate. The conjugation of neamine to dinucleotides
has been reported to remarkably decrease affinity to HIV-1
TAR RNA due to electrostatic repulsion between the
phosphates, whereas conjugates of the corresponding PNA
show about 2-fold binding affinities compared to that of
neamine.52 In the antisense approach with ODN conjugates,
the strong binding of aminoglycosides to nucleic acids also
probably leads to intramolecular and intermolecular charge−
charge interaction between the protonated aminoglycoside core
and the phosphodiester backbone of the ODN that can disturb
the binding to the RNA target.
Peptide nucleic acids (PNAs) are a class of antisense DNA
analogues first synthesized by Nielsen and colleagues in 1991.53
The PNA molecules, devoid of sugar phosphate backbone and
charges under physiological conditions, have been shown to
display high affinity for complementary sequences on RNA and
DNA both in single- and double-stranded forms.54−56 PNA are
highly stable and remained uncleaved when incubated with
blood or cell lysate from human and bacterial cells.57 Initial
expectations held that PNAs would quickly enter the field of
antisense as genespecific, nontoxic, and nonimmunogenic
agents. However, problems associated with solubility and
poor cellular uptake of this class of compounds hampered
developments in this direction.58 The synthesis of modified
PNAs or PNA conjugates presents a new means of improving
their solubility and cellular uptake.58
A number of strategies have been used for improving the
biodelivery of naked or modified forms of PNA in cell culture
systems or in vivo.58−61 The major advance in biodelivery of
PNA was made by conjugating cell-transducing agents,
including cell penetrating peptides (CPP). A number of
reviews on CPP and their therapeutic application have been
reported.62−65 A triphenyl-phosphonium-tagged PNA targeted
to HIV-1 TAR was found to be efficiently taken up by cells and
to block HIV-1 production in cell culture.66 The major
drawback in the PNA biodelivery system based on cell
penetrating peptides is the endosomal entrapment of PNA in
the cytosol, which limits its target efficacy.67
penetratin, transportan, and HIV-1 Tat-derived peptide as a
vehicle to deliver anti-HIV-1 PNA into cells.68−70 CPP vehicle
specially the cationic CPP such as Tat and oligo-Arg carrying
PNA cargoes has been shown to enter the cell via endosomal
pathway.65,71 Another major deterrent is that peptide carriers
are unsuitable for oral biodelivery as they are prone to cleavage
by gut proteases prior to their entry into the cells.
Methods for conjugation of PNA with aminoglycosides have
also been reported.52,72−76 In one unique study in the antisense
approach, we have conjugated the neamine core 2 (Figure 1)
corresponding to rings I and II of neomycin B 1 with the 16-
mer PNA 3 targeting HIV-1 TAR RNA (anti-TAR PNA).75 In
the resulting conjugate 4 (Figure 1), the presence of the
neamine core not only imparts greater solubility and enhanced
cellular uptake of PNA, it also confers a unique metal-ion-
independent target specific RNA cleavage property to the
conjugate.75,76 We also noted that unlike PNA−CPP conjugate,
PNA−neamine is compatible with oral administration in mice
and efficiently absorbed in the gut (unpublished data).
Probably, the presence of positive charges is necessary for the
penetration of the conjugate 4 in human cells through
interaction with the membrane phospholipids and disruption
of the membrane integrity. To decrease the number of amine
functions in conjugate 4 that may be protonated under
physiological conditions and could be a source of toxicity and
nonspecific interaction with the nontarget nucleic acid, our
strategy was to conjugate the anti-TAR PNA sequence 3 to the
small aminoglycosidic moiety 6-amino-6-deoxyglucosamine
(neosamine) which corresponds to ring II of neamine (and
neomycin).
N-Acetyl-D-glucosamine is the major constituent of the
peptidoglycan polymer (murein) forming the bacterial cell wall
and of the chitin polymer forming the exoskeleton of
arthropods such as crustaceans (e.g., crabs). The chitosan
polymers obtained by hydrolysis of chitin under basic
conditions have been intensively used for preparing gene
delivery vectors.77 Aminosugars such as D-glucosamine have
been also found to be able to weakly induce single-strand
breakage in DNA that was increased in the presence of
copper(II) ions.78−82 In the search for antibacterial agents, 6-
aminoglucosamine derivatives incorporating at the 1-position
acyclic deoxystreptamine mimetics (ring I of neomycin B) have
The most efficient cellular uptake of anti-HIV-1 PNA was
noted with the use of cell-penetrating peptides (CPP) including
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a
Scheme 1. Synthesis of the Glucosamine Derivative 15 for Further Coupling to the Anti-TAR PNAs
a
Reagents and conditions: (a) BnBr, K2CO3, DMF, rt, 10 h, 84%; (b) Et3SiH, BF3·OEt2, CH2Cl2, 0 °C, 2 h, 81%; (c) 6-bromohexanoic acid ethyl
ester, NaH, DMF, rt, 8 h, 92%; (d) H2, Pd/C (10%), EtOH, rt, 12 h, 98%; (e) TsCl, pyridine; rt, 10 h, 96%; (f) NaN3, DMF, 80 °C, 3 h, 91%; (g) 1
M NaOH, dioxane/H2O (4/1), 100 °C, 14 h, 74%; (h) HCO2NH4, Pd(OH)2/C (20%), MeOH/H2O (9/1), reflux, 1.5 h, 95%; (i) TrCl, DMF,
Et3N, rt, 8 h, 68%; (j) 1 M NaOH/dioxane (1/1), 80 °C, 6 h, 85%.
been synthesized but none of them shows a high potency
comparable to that of the natural aminoglycosides.83 Here, we
report on the synthesis of a 6-amino-6-deoxyglucosamine
derivative and its conjugation to sequence specific PNA
targeted to TAR element on HIV-1 genome. The resulting
conjugate 5a (Figure 1) is rapidly taken up by the cells and
exhibits potent antiviral activity, thus suggesting a strong
therapeutic potential for this class of molecules.
The prepared conjugate 5a and the corresponding
fluorescently tagged conjugate 5b (Figure 1) were designed
to carry only two positive charges under physiological
conditions, whereas in the neamine conjugate 4, four positive
charges can be present. In 5a and 5b, the PNA and the linking
chain were attached at the 4-oxygen atom of the neosamine
moiety for a maximal distance of both protonated amine
functions in regard to the possible crucial role of these
functions for an efficient cellular uptake. A methyl group was
introduced at the anomeric position of the neosamine moiety
(1-methylneosamine) in order to stabilize the sugar ring and to
minimize the steric hindrance at this position as well as the
lipophily for clearly defining a charged polar head in the
conjugate.
protection of the 4- and 6-hydroxyl functions by formation of
the benzilidene derivative, and (iii) deacetylation to release the
2-amine function.
To protect the 3-hydroxyl function allowing the selective
introduction of the linking chain from the free 4-hydroxyl
group, the 2-amine function was protected in two steps as an
oxazolidinone spanning the N-2 and O-3 atoms and bearing a
benzyl group on the nitrogen atom.85 Treatment of the
resulting compound 7 with triethylsilane and BF3 diethyl
etherate allowed the selective deprotection of the 4-hydroxyl
function to lead to 8, which was alkylated with ethyl 6-
bromohexanoate in order to introduce the protected linking
arm. From the prepared ester 9, the 6-O-benzyl group was
removed by catalytic hydrogenation to release the 6-hydroxyl
group.
A 6-azido group precursor of the additional 6-amine function
was introduced through tosylation and then substitution of the
tosyl group by reaction with sodium azide to lead to the
derivative 12. Treatment of 12 with aqueous NaOH opened the
oxazolidinone ring, releasing the 3-hydroxyl function and the
benzylated 2-amino function as well as a carboxylate function at
the end of the linking chain in compound 13. In the presence of
palladium hydroxide and ammonium formate, it was possible to
reduce concomitantly the 6-azido group and to debenzylate the
2-amino function of 13 to lead to the methyl-2,6-
diaminoglucopyranoside 14 bearing the linking chain with a
terminal carboxylate function.
To (i) link the methyl-2,6-diaminoglucopyranoside ring to
the anti-TAR PNA sequence, (ii) deprotect the obtained
conjugate under the conditions used to remove the
benzhydryloxycarbonyl (Bhoc) groups employed in the PNA
synthesis for protection of the amine functions of the bases, and
(iii) cleave the conjugate from the solid support, trityl acidic-
labile protecting groups were introduced to protect the 2- and
6-amino functions of 14 and afforded compound 15 in two
steps. It was not possible to introduce a 4-methoxybenzyl group
to protect the remaining free 3-hydroxyl group of compound
13 probably due to a strong steric hindrance generated by the
SYNTHESIS
■
To link the 6-amino-D-glucosamine ring to the anti-TAR PNA
sequence, inexpensive N-acetyl-D-glucosamine was used as a
starting compound. The target glucosamine derivative 15 used
for further coupling to the protected PNAs 3a and 3b was
prepared in 14 steps (Scheme 1).
In comparison to the glucosamine moiety, a second amine
function should be added at the 6-position and a methyl group
at the 1-hemicetal function as well as a linking arm at the 4-
position for attachment to the PNA. This arm should carry a
terminal carboxylate group for coupling through an amide
junction to the N-terminus of the protected anti-TAR PNAs.
The first steps in the synthesis were performed according to
the literature for obtaining the intermediate 684 through (i)
methylation of the anomeric hemiacetal function, (ii)
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a
Scheme 2. Synthesis of the PNA−Aminoglucosamine Conjugates 5a and 5b
a
Reagents and conditions: (a) EDC, HOBt, DMF, protected PNA 3a or 3b; (b) TFA/anisole (1/1), rt, 2 h.
Figure 2. (A) Uptake of FITC labeled PNA−aminoglucosamine (5b) in CEM cells: cells were incubated with varying concentration of 5b (50 nM to
1 μM) in complete RPMI medium at 37 °C for 14 h. Cells were washed and resuspended in PBS containing 2% FBS, and fluorescence positive cells
were measured by FACScan. (B) The graphical presentation of percent uptake of 5b as a function of time and concentration. (C) Cellular
localization of FITC labeled PNA−glucosamine 5b in CEM cells: Cells were incubated in the absence of PNA or in the presence of 1 μM FITC
labeled naked PNA or 0.5 and 1 μM of 5b. After 12 h, cells were washed and images were acquired. DAPI was used to stain (blue) cellular nuclei;
FITC indicates the fluorescence of FITC labeled PNA or its glucosamine conjugate 5b, measured at 488 nm. MERGE shows the distribution of 5b
in the cell. (D) Cellular localization of 5b in Huh7.5 cells: Cells were incubated with 1 μM of 5b in the absence (control) and presence of
chloroquine as described by Turner et al.67 Images were acquired after 10 h of incubation, washing, and staining with WGA-rhodamine and DAPI to
label the membrane glycoproteins (red) and nuclear DNA (blue) respectively. The FITC labeled PNA−glucosamine 5b is shown in green
fluorescence. The merge indicate superimposition of blue, red, and green.
O-4-linking arm and the N-2-trityl group. Thus 15 was used
without protection of the 3-hydroxyl function for the coupling
to the protected PNAs after exchange of the sodium counterion
for a triethylammonium cation (Scheme 2).
This salt was coupled at the N-terminus of the protected 16-
mer anti-TAR PNA 3a attached to its solid support of synthesis
using EDC/HOBt as coupling agents. Concomitant depro-
tection of the neosamine moiety, of the PNA bases, and
cleavage from the solid support were performed in one step
using TFA/anisole (1:1). The resulting conjugate 5a was
purified by HPLC on C18 reversed phase (50% coupling yield)
and its structure was confirmed by mass spectrometry
(MALDI: calcd 4574.88, found 4576.00).
To study the cellular uptake of the anti-TAR PNA−
neosamine conjugate, a second anti-TAR conjugate incorporat-
ing at its C-terminus extremity, a fluorescent probe was
prepared from a protected lysine residue functionalized with
fluorescein isothiocyanate (FITC) and attached to a solid
support. For this, the neosamine derivative 15 was coupled to
the protected anti-TAR PNA 3b bearing fluorescein at the C-
terminus. The resulting conjugate was deprotected and cleaved
from the solid support with TFA/anisole. It was purified by
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HPLC to give rise to the fluorescent conjugate 5b (Scheme 2);
its structure was confirmed by mass spectrometry (MALDI:
calcd 5206.09, found 5207.28).
It has been recognized that cellular delivery of PNAs
conjugated with cell penetrating peptides occurs mainly
through endocytotic pathways and thus retaining a major
fraction of internalized PNA in the endosome−lysosome
compartments.87 Endosome disrupting agents such as chlor-
oquine have shown to improve the release of the endosomal
entrapped PNA into the cytosol. Confocal microscopy studies
carried out by Turner et al.67 have demonstrated that
fluorescently labeled PNA−peptide conjugates sequestered in
endosomes or membrane-bound compartments in HeLa cells
are significantly reduced when cotreated with chloroquine and
thus result in improved distribution of the conjugates in the
cytosol.67
We therefore resorted to examine if there is any difference in
the cellular distribution of 5b upon cotreatment with
chloroquine. We used Huh7.5 cells, which have much larger
cytoplasmic space as compared to CEM cells, and incubated
these cells with 1 μM of the conjugate 5b in the absence and
presence of chloroquine as described before.67 After 10 h of
incubation, cells were washed by PBS and fixed with 4%
paraformaldehyde. The cellular nucleus and the membrane
glycoproteins were stained by DAPI and wheat germ agglutinin
conjugated with rhodamine (WGA), respectively, and examined
by confocal microscopy. The result shown in Figure 2D
indicates that cellular distribution of 5b remained unaffected
upon cotreatment with chloroquine, suggesting the absence of
endosomal entrapment of 5b in the cell.
STABILITY OF THE PNA−GLUCOSAMINE
CONJUGATE (5a) UNDER ACIDIC CONDITIONS
■
The DNA oligonucleotides based antisenses are susceptible to
apurination under acidic condition and hence not suitable for
oral administration. Although naked PNAs are highly stable
molecule under acidic conditions, we expected that its
glucosamine conjugate should also be stable under similar
conditions. To examine this, we incubated 5a with 0.1 M
aqueous HCl at 37 °C for 3 days and then compared its HPLC
elution profile with untreated samples. We found no difference
in the retention time of treated and untreated samples and thus
confirming its stability at high acidic conditions (data not
shown).
CELLULAR UPTAKE, LOCALIZATION, AND
ANTIVIRAL ACTIVITY
■
Cellular Uptake. Since in our earlier studies we have shown
that fluorescein labeled PNA−neamine when incubated with
the cells at a 2 μM concentration, approximately 33% cells
became fluorescence positive after overnight incubation.76 We
therefore examined if conjugate 5b carrying one 6-amino-
glucosamine ring is a better candidate for biodelivery. The
CEM cells were incubated with increasing concentration of 5b
at 37 °C. Cells were harvested at different time points (30 min,
3 and 14 h) and measured for fluorescence by FACScan. We
noted that 5b is efficiently taken up by the cells in a time and
concentration dependent manner (Figure 2A,B). At 0.2 μM
concentration, 10% of the cells were fluorescence positive
within 30 min, which reached to 85% within 14 h (Figure 2B).
By increasing the concentration to 0.3 μM, nearly 100% cells
were positive within 14 h of incubation (Figure 2B). We further
noted that the cellular uptake of 5b was independent of
temperature, suggesting that its uptake is not via endocytosis,
which has been shown to be temperature dependent and occurs
at physiological temperature.86 We observed that the extent of
cellular uptake of 5b remained unchanged at both 4 and 37 °C
(results not shown). We also noted that PNA−aminoglucos-
amine is not cytotoxic because greater than 95% cells remained
viable at all the concentrations (0.2−1 μM) tested. Thus, the
cellular uptake of 5b which carries only ring II of the neamine
core was found to be improved several fold as compared to that
observed with PNA−neamine conjugate 4.76 As against
conjugate 4, which displayed only 30% uptake at 2 μM
concentration after overnight incubation, 5b displayed 100%
cellular uptake at 0.3 μM concentration within 14 h of
incubation.
This result is exciting because most of the biodelivery system
so far developed for PNA have limitations due to their
endosomal entrapment in the cystosol.67 Our results with 5b
indicate that the problem associated due to endosomal
compartmentalization could be circumvented by conjugating
target specific PNA to the 6-aminoglucosamine ring. The
localization and distribution of 5b in both cytosol and nucleus
indicate that 5a could efficiently block Tat mediated trans-
ctivation of HIV-1 transcription in the nucleus and abort
reverse transcription process of viral RNA in the cytosol.
TARGET SPECIFICITY
■
Because cellular uptake of PNA conjugated with 6-amino-
glucosamine was significantly enhanced, we examined if this
improvement in uptake is not compromised by target
specificity. We then examined the binding specificity of the
PNA−aminoglucosamine conjugate (5a) to the TAR RNA by
gel mobility shift assay using the 32P-labeled 82 bp long TAR
RNA transcript. We also used mutated TAR RNA carrying a
deletion in the stem (TAR-BS) and a 98 base long RNA
fragment from hepatitis C virus 3′ nontranslated region (HCV-
3′NTR) as the nontarget RNA and scrambled PNA−glucos-
amine as the negative controls. As shown in Figure 3, a distinct
shift in the mobility of TAR RNA was observed due to the
formation of PNA−RNA complex. The extent of gel shift of the
target RNA by 5a was stochiometric. At 0.5:1 molar ratio of 5a
to TAR RNA, 50% gel shift was seen while complete shift was
noted at 1:1 molar ratio (lane 5). We have earlier demonstrated
that PNA−neamine conjugate 4 exhibits metal independent
RNA nuclease activity which specifically cleaves the target RNA
at the binding site upon prolonged incubation.75 However, we
could not demonstrate similar target specific RNA cleavage
activity with conjugate 5a either in the absence or presence of
different metals ions (results not shown).
To further confirm the cellular uptake of 5b in CEM cells, we
captured the images of cells treated with FITC labeled naked
PNA and FITC labeled PNA−aminoglucosamine (5b) using
confocal microscope. The cells were incubated with the
indicated amount of FITC labeled naked PNA or FITC
labeled PNA−aminoglucosamine 5b. After 12 h, cells were
harvested and visualized by confocal microscope (Figure 2C).
The cells incubated in the presence of 0.5 and 1 μM of 5b
showed approximately 90% and 99% fluorescence positive cells,
respectively. In contrast, FITC labeled naked PNA was unable
to enter in the cell even after prolonged incubation.
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Activation of transcriptional elongation occurs following the
recruitment of Tat to the transcription machinery via a specific
interaction with TAR, a 59-residue RNA leader sequence in the
HIV-1 LTR.89 The main advantage of targeting the TAR
element is that it is conserved and folds into a stable stem−loop
structure. It was expected that binding of 5a to sequences on
the stem−loop region of TAR may destabilize the stem−loop
structure of TAR and abolish Tat−TAR interaction. If this
mode of action is the case, then 5a should be able to negatively
impact Tat/LTR driven reporter while should have no effect in
the absence of Tat.
To examine this, we used a reporter plasmid construct
(pHIV-1 LTR-Luc) expressing firefly luciferase under the
control of the HIV-1 LTR and pRL-SV40 expressing Renilla
luciferase under the control of SV40 as an internal control. The
reporter plasmids were cotransfected in CEM cells with and
without pCMV-Tat. The transfected cells were grown in RPMI
medium supplemented with or without different concentrations
of 5a or scrambled PNA−glucosamine conjugate. After 24 h,
cells were harvested, lysed, and assayed for the determination of
relative expression of firefly and Renilla luciferase activity
(Figure 4). Expression of Renilla luciferase driven by the CMV
promoter did not change in the absence or presence of the Tat
expression clone or in the presence or absence of 5a and was
therefore used as a control to normalize for transfection
efficiency. On the other hand, expression of the firefly luciferase
driven by the HIV-1 LTR directly correlated with the
concentration of 5a, thus indicating its promoter specificity.
These results are presented in Figure 4 as the percent inhibition
of Tat-mediated transactivation of the HIV-1 LTR at indicated
concentrations of 5a or scrambled PNA−glucosamine con-
jugate calculated from the respective ratios of the firefly and
Renilla luciferase activities.
Figure 3. (A) Binding specificity of PNATAR−aminoglucosamine
(conjugate 5a) to its target sequence. The affinity of the conjugate 5a
(A) for its target sequence on the TAR RNA was assessed by gel
mobility shift analysis. (B) TAR mutant RNA carrying a deletion at the
top of the stem (TAR BS). (C) The 98-mer HCV 3′NTR RNA
fragment as the nontarget RNA. (D) TAR RNA incubated with
scrambled PNA−glucosamine conjugate 5a as a negative control. Lane
1 through 7 represents molar ratios of PNATAR−glucosamine or
scrambled PNA−aminoglucosamine to TAR RNA of 0.0, 0.2, 0.5, 0.8,
1.0, 2.0, and 5.0, respectively.
INHIBITION OF TAT-MEDIATED
TRANSACTIVATION OF HIV-1-LTR
■
Because the PNA−glucosamine conjugate 5a targeted to HIV-1
TAR is efficiently taken up by the cells, we examine if 5a is able
to block Tat-mediated transactivation of transcription of HIV-1
long terminal repeat (HIV-1 LTR) when supplemented in the
culture medium. The significance of Tat−TAR interaction in
regulating HIV-1 gene expression is well documented.88,89
Figure 4. Effect of the PNA−glucosamine conjugate 5a on Tat-mediated transactivation: CEM cells (0.5 × 106 cells per well in 12 well plate) were
transfected with the reporter plasmids pHIV-1 LTR-Luc (2 μg) and pRL-SV40 (500 ng) in the presence and absence of Tat expression vector,
pCMV-Tat (2 μg). After 2 h of transfection, cells were supplemented with PNA conjugates in the concentration as indicated in μM. The transfected
cells were harvested 24 h post-transfection, and the luciferase activities were measured. Expression of Renilla luciferase was considered as a control for
normalization of transfection efficiency. The results are presented as the relative ratio of firefly to Renilla luciferase activity in percentage by keeping
the relative activity of untreated cells as 100%. Error bars represents the standard error of the mean of two independent experiments.
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As shown in Figure 4, relative luciferase activity was barely
detectable in the absence of Tat (lane 2) as compared to that
observed in the presence of Tat (lane 1). The expression of
firefly luciferase was strongly inhibited when transfected cells
were grown in the presence of 5a (lanes 7−10). Approximately
75% inhibition of Tat-mediated transactivation of firefly
luciferase was noted at 1 μM concentration of 5a. In contrast,
only a basal level of LTR driven firefly luciferase expression was
observed in the absence of Tat (lane 2), which remained
unaffected by 5a (lanes 3−6).
The naked PNA (lanes 11, 12) or scrambled PNA−
glucosamine conjugate (lanes 13, 14) had no effect on the
expression of firefly luciferase when supplemented in the
culture medium. These results clearly indicated that 5a
effectively blocks Tat-mediated transactivation of the HIV-1
LTR.
amine. The cells grown in the presence of 0.1 and 0.5 mM
PNA−aminoglucosamine (5a) showed similar extent of
[3H]thymidine incorporation, suggesting that the cellular
proliferation was not affected in the presence of 5a.
CONCLUSION
■
In conclusion, the PNA−aminoglucosamine conjugate 5a
carrying a reduced number of amino groups as compared to
the PNA−neamine conjugate 4 displayed several-fold enhanced
cellular uptake and target efficacy at subnanomolar concen-
tration. It efficiently enters the cells, strongly binds to its target
sequence on HIV-1 genome, blocks Tat-mediated trans-
activation of HIV-1 LTR, and inhibits virus production in
HIV-1 infected cells. The most exciting results was that, unlike
peptide based PNA biodelivery system, PNA conjugated with
glucosamine is uniformly localized and distributed in the
cytosol and nucleus and not subjected to endosomal compart-
ment in the cytosol. Using this novel class of small molecule to
conjugate sequence specific PNA for sequestering critical
regulatory elements in the viral RNA genome may have great
potential in PNA based therapeutics. A strategy to enhance oral
biodelivery of PNAs will certainly improve their therapeutic
because PNA−aminoglucosamine conjugates being highly
stable under acidic conditions corresponding to the stomachal
environment may also be expected to be compatible with oral
administration.
ANTIVIRAL ACTIVITY AND CYTOTOXICITY
■
Because PNA−aminoglucosamine conjugate efficiently inter-
nalized in the cell and binds the target sequence in HIV-1 TAR
RNA, we examined if the conjugate is able to block TAR
mediated transactivation of HIV-1 transcription in HIV-1
infected lymphocytes. To evaluate this, we used pseudotyped
HIV-1 particles carrying luciferase reporter gene to infect CEM
T-lymphocyte cells. The infected cells were grown at 37 °C for
48 h and then supplemented with increasing concentration of
5a. After 76 h, cells were lysed and assayed for firefly luciferase
activity. The results shown in Figure 5 indicate that 5a strongly
EXPERIMENTAL SECTION
■
General. All reagents were used as purchased from suppliers
without further purification. The protected 16-mer PNA oligomers
were purchased from Eurogentec. DMF was distilled in the presence of
CaH2 and stored under argon atmosphere prior to use. Thin layer
chromatographies were performed on silica gel (Alugram Sil G/UV254
)
or alumina gel (Alugram Alox N/UV254) from Macherey-Nagel, and
spots were detected either by UV absorption or by charring with
ninhydrin. HPLC purifications were carried out on a C18 reversed-
phase column (Macherey-Nagel, 10.0 mm × 25.0 mm) with a Dionex
Ultimate 3000 apparatus. Melting points were determined with a
1
BUCHI 510 apparatus and are reported uncorrected. H NMR (400
Figure 5. (A) Inhibition of HIV-1 transcription by 5a: The CEM cells
were infected with pseudotyped HIV-1 virus expressing the firefly
luciferase reporter. After 48 h postinfection, indicated amount of
PNA−aminoglucosamine was supplemented in the culture medium.
After 76 h, cells were harvested and assayed for firefly luciferase activity
in the cell extract. The results are expressed as mean value of two
independent experiments with error bar. (B) Effect of 5a on cellular
proliferation: CEM cells were grown in the absence or presence of 0.1
mM and 0.5 mM of 5a dissolved in DMSO. The final concentration of
DMSO supplemented through 5a in the medium was 5%. The culture
medium was supplemented with 5 μCi of [methyl-3H]thymidine/mL.
After 48 h, cells were harvested and [3H]-thymidine incorporated into
the DNA was determined. The results expressed as percent of
thymidine incorporation with respect to the control are the averages of
three sets of independent experiments.
MHz) and 13C NMR (100 MHz) spectra were recorded with a
BRUKER ADVANCE 400 spectrometer using the residual solvent
signal as internal standard. HRMS were obtained from the Mass
Spectrometry Service, CRMPO, at the University of Rennes I, France,
using a MICROMASS ZABSPEC-TOF spectrometer and a VARIAN
MAT311 spectrometer or with a 4700 reflector spectrometer. The
purities of the conjugates 5a and 5b determined by C18 reversed phase
HPLC were found to be 96.2% and 98.5%, respectively (see
Supporting Information).
Synthesis. Glucosamine Derivative 7. To a solution of compound
6
84 (3.30 g, 10.7 mmol) in dry DMF (15 mL) were added successively
K2CO3 (7.4 g, 54 mmol, 5 equiv) and benzyl bromide (3.18 mL, 26.7
mmol, 2.5 equiv). The solution was stirred at room temperature for 10
h under argon atmosphere. A saturated NH4Cl solution was added,
and the mixture was extracted with ethyl acetate (3 × 20 mL). The
combined organic layers were dried over MgSO4, filtered, and
concentrated. The residue obtained was purified over silica gel column
(EtOAc/cyclohexane, 1/3) to afford 7 (3.57 g, 84%) as a white solid;
mp 186−188 °C. 1H NMR (400 MHz, CDCl3) δ 3.19 (s, 3H, OMe),
3.35 (dd, J = 12.0 Hz, 1H, H-2), 3.78−3.85 (m, 2H, H-5, H-6′), 3.98
(q, J = 8.0, 12.0 Hz, 1H, H-4), 4.21−4.24 (m, 1H, H-6), 4.45 (s, 2H,
NCH2Ph), 4.58 (d, J = 4.0 Hz, 1H, H-1), 4.69 (q, J = 8.0, 12.0 Hz, 1H,
H-3), 5.57 (s, 1H, CHPh), 7.33−7.38 (m, 8H, aromatic), 7.47−7.49
(m, 2H, aromatic). 13C NMR (100 MHz, CDCl3) δ 49.0 (NCH2Ph),
55.8 (OMe), 62.3 (C-2), 65.6 (C-5), 68.7 (C-6), 73.2 (C-3), 80.4 (C-
4), 97.4 (C-1), 101.6 (CHPh), 126.3, 128.5, 128.9, 129.0, 129.2, 129.4,
135.3, 136.7, 158.9 (CO). HRMS (ESI) Calcd for C22H23NO6Na [M
inhibited HIV-1 transcription by blocking the TAR, as evident
by the low level of expression of luciferase activity.
Approximately 50% reduction in luciferase activity was
observed at 800 nM concentration of 5a. We further confirmed
that dose-dependent reduction in luciferase expression is not
due to cellular cytotoxicity of 5a. As shown in Figure 5B, we did
not find any difference in the [3H]thymidine incorporation
when infected CEM cells were grown in the absence or
presence of indicated concentration of PNA−aminoglucos-
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Journal of Medicinal Chemistry
Article
+ Na]+: 420.14231, found 420.1411, Calcd for C22H23NO6K [M +
K]+: 436.1162, found 436.1160.
Glucosamine Derivative 11. To a solution of compound 10 (1.60
g, 3.54 mmol) in dry pyridine (10 mL) was added TsCl (1.69 g, 8.85
mmol, 2.5 equiv), and the mixture was stirred at ambient temperature
under argon atmosphere. After completion of the reaction (TLC),
saturated NaHCO3 solution was added and the resulting aqueous
mixture was extracted with ethyl acetate (3 × 10 mL). The combined
organic layers were dried over anhydrous MgSO4, filtered, and
concentrated. The residue was purified over silica gel column (EtOAc/
pentane, 1/2) to afford the desired compound 11 (2.06 g, 96%) as a
Glucosamine Derivative 8. To a stirred solution of compound 7
(2.20 g, 5.54 mmol) in dry dichloromethane (10 mL), under argon
atmosphere, was added Et3SiH (5.47 mL, 66.5 mmol, 12 equiv), and
the mixture was cooled to 0 °C. To this mixture, BF3·OEt2 (1.39 mL,
11.08 mmol, 2 equiv) was added dropwise, and then the reaction
mixture was stirred at 0 °C for 2 h. A saturated NaHCO3 solution was
added, and the resulting aqueous mixture was extracted with
dichloromethane (3 × 20 mL). The combined organic layers were
dried over MgSO4, filtered, and concentrated. The residue was purified
over silica gel column (EtOAc/cyclohexane, 1/1) to afford 8 (1.79 g,
1
yellow oil. H NMR (400 MHz, CDCl3) δ 1.25 (t, J = 8.0 Hz, 3H,
Me), 1.27−1.30 (m, 2H, H-3′), 1.41−1.46 (m, 2H, H-2′), 1.57−1.61
(m, 2H, H-4′), 2.26 (t, J = 8.0 Hz, 2H, CH2CO), 2.43 (s, 3H, Me),
3.05 (s, 3H, OMe), 3.17 (dd, J = 4.0, 12.0 Hz, 1H, H-2), 3.31−3.33
(m, 1H, OCH2), 3.54−3.57 (m, 2H, H-4, H-5), 3.74−3.77 (m, 1H,
OCH2), 4.12 (q, J = 8.0, 16.0 Hz, 2H, CH2-ethyl ester), 4.16 (d, J = 4.0
Hz, 1H, H-6′), 4.24 (dd, J = 4.0, 8.0 Hz, 1H, H-6), 4.30 (d, J = 16.0
Hz, 1H, NCH2Ph), 4.40−4.47 (m, 3H, H-1, H-3, NCH2), 7.27−7.34
(m, 7H, aromatic), 7.75 (d, J = 8.0 Hz, 2H, aromatic). 13C NMR (100
MHz, CDCl3) δ 14.4 (Me), 21.8 (Me), 24.8 (H-4′), 25.6 (H-3′), 29.6
(H-2′), 34.3 (CH2CO), 48.9 (NCH2Ph), 55.7 (OMe), 60.3 (CH2-
ethyl ester), 61.2 (C-2), 68.1 (C-6), 70.9 (C-5), 71.2 (OCH2), 75.2
(C-4), 76.8 (C-3), 96.3 (C-1), 128.1, 128.4, 128.6, 128.8, 128.9, 130.0,
133.0, 135.2, 145.1, 158.9 (CO-amide), 173.7 (CO-ester). HRMS
(ESI) Calcd for C30H39NO10NaS [M + Na]+: 628.2192, found
628.2187. Calcd for C30H39NO10SK [M + K]+: 644.1932, found
644.1927.
Glucosamine Derivative 12. To a solution of compound 11 (1.90
g, 3.14 mmol) in DMF (10 mL) was added NaN3 (2.04 g, 31.4 mmol,
10 equiv), and then the solution was heated at 80 °C for 3 h under
argon atmosphere. A saturated NH4Cl solution was added, and the
resulting aqueous mixture was extracted with ethyl acetate (3 × 10
mL). The combined organic layers were dried over anhydrous MgSO4,
filtered, and concentrated under reduced pressure. The residue was
purified over a silica gel column (EtOAc/pentane, 1/3) to afford 12
(1.36 g, 91%) as a yellow gummy liquid. 1H NMR (400 MHz, CDCl3)
δ 1.24 (t, J = 8.0 Hz, 3H, Me), 1.32−1.36 (m, 2H, H-3′), 1.53−1.56
(m, 2H, H-2′), 1.58−1.64 (m, 2H, H-4′), 2.28 (t, J = 8.0 Hz, 2H,
CH2CO), 3.13 (s, 3H, OMe), 3.24 (dd, J = 4.0, 12.0 Hz, 1H, H-2),
3.43−3.47 (m, 3H, H-6, H-6′, OCH2), 3.55 (t, J = 8.0, 12.0 Hz, 1H, H-
4), 3.62−3.65 (m, 1H, H-5), 3.81−3.87 (m, 1H, OCH2), 4.11 (q, J =
8.0, 16.0 Hz, 2H, CH2-ethyl ester), 4.34 (d, J = 16.0 Hz, 1H,
NCH2Ph), 4.45−4.51 (m, 3H, H-1, H-3, NCH2), 7.30−7.36 (m, 5H,
aromatic). 13C NMR (100 MHz, CDCl3) δ 14.4 (Me), 24.8 (H-4′),
25.7 (H-3′), 29.6 (H-2′), 34.3 (CH2CO), 48.9 (NCH2Ph), 51.0 (C-6),
55.7 (OMe), 60.4 (CH2-ethyl ester), 61.5 (C-2), 71.2 (OCH2), 72.5
(C-5), 76.5 (C-4), 76.9 (C-3), 96.3 (C-1), 128.4, 128.8, 128.9, 135.3,
159.0 (CO-amide), 173.7 (CO-ester). HRMS (ESI) Calcd for
C23H32N4O7Na [M + Na]+: 499.2169, found 499.2183. Calcd for
C23H32N4O7K [M + K]+: 515.1908, found 515.1916.
1
81%) as a yellow foam. H NMR (400 MHz, CD3OD) δ 3.13 (s, 3H,
OMe), 3.34 (dd, J = 4.0, 8.0 Hz, 1H, H-2), 3.57−3.61 (m, 1H, H-5),
3.72−3.73 (m, 2H, H-6, H-6′), 3.89 (t, J = 8.0 Hz, 1H, H-4), 4.29 (d, J
= 12.0 Hz, 1H, NCH2Ph), 4.37−4.57 (m, 4H, H-3, OCH2Ph,
NCH2Ph), 4.61 (d, J = 4.0 Hz, 1H, H-1), 7.31−7.36 (m, 10H,
aromatic). 13C NMR (100 MHz, CD3OD) δ 49.5 (NCH2Ph), 55.9
(OMe), 62.8 (C-2), 69.7 (C-4), 69.9 (C-6), 74.5 (OCH2Ph), 75.7 (C-
5), 78.4 (C-3), 97.7 (C-1), 128.8, 128.9, 129.1, 129.5, 129.7, 129.8,
137.1, 139.6, 161.4 (CO). HRMS (ESI) Calcd for C22H25NO6Na [M
+ Na]+: 422.1580, found 422.1587. Calcd for C22H25NO6K [M + K]+:
438.1319, found 438.1299.
Glucosamine Derivative 9. To a solution of compound 8 (2.80 g,
7.01 mmol) in DMF (10 mL) was added NaH (0.56 g, 14.02 mmol, 2
equiv, 60% suspension), and the mixture was stirred at room
temperature for 10 min under argon atmosphere. Ethyl 6-bromo
hexanoate (2.61 mL, 14.02 mmol, 2 equiv) was added, and the mixture
was stirred at room temperature for 8 h. A saturated NH4Cl solution
was added, and the resulting aqueous mixture was extracted with ethyl
acetate (3 × 20 mL). The combined organic layers were dried over
anhydrous MgSO4, filtered, and concentrated. The residue was
purified over silica gel column (EtOAc/cyclohexane, 1/3) to afford
9 (3.49 g, 92%) as a yellow foam. 1H NMR (400 MHz, CDCl3) δ 1.26
(t, J = 8.0 Hz, 3H, Me), 1.27−1.31 (m, 2H, H-3′), 1.45−1.49 (m, 2H,
H-2′), 1.59−1.62 (m, 2H, H-4′), 2.27 (t, J = 8.0 Hz, 2H, CH2CO),
3.15 (s, 3H, OMe), 3.27 (dd, J = 4.0, 12.0 Hz, 1H, H-2), 3.36−3.37
(m, 1H, OCH2), 3.58−3.61 (m, 2H, H-5, H-6′), 3.68−3.71 (m, 2H, H-
4, H-6), 3.78 (m, 1H, OCH2), 4.13 (q, J = 8.0, 16.0 Hz, 2H, CH2-ethyl
ester), 4.41 (d, 2H, NCH2Ph), 4.45−4.52 (m, 2H, H-3, OCH2Ph),
4.55 (d, J = 4.0 Hz, 1H, H-1), 4.63 (d, J = 12.0 Hz, 1H, OCH2Ph),
7.29−7.37 (m, 10H, aromatic). 13C NMR (100 MHz, CDCl3) δ 14.4
(Me), 24.9 (C-4′), 25.7 (C-3′), 29.7 (C-2′), 34.4 (CH2CO), 48.9
(NCH2Ph), 55.6 (OMe), 60.4 (CH2-ethyl ester), 61.3 (C-2), 67.9 (C-
6), 71.3 (OCH2), 72.7 (C-5), 73.7 (OCH2Ph), 75.5 (C-4), 77.4 (C-3),
96.4 (C-1), 127.9, 128.0, 128.4, 128.6, 128.9, 135.4, 138.0, 159.3 (CO-
amide), 173.8 (CO-ester). HRMS (ESI) Calcd for C30H39NO8Na [M
+ Na]+: 564.2573, found 564.2574. Calcd for C30H39NO8K [M + K]+:
580.2313, found 580.2345. Calcd. for C29H39NO6Na [M − CO2 +
Na]+: 520.2675, found 520.2688.
Glucosamine Derivative 13. A solution of compound 12 (1.10 g,
2.31 mmol) in a mixture of 1 M NaOH/1,4-dioxane (5/5 mL, v/v)
was heated at 80 °C for 20 h. The mixture was diluted with ethyl
acetate and washed with brine. The separated aqueous layer was
repeatedly washed with ethyl acetate. The combined organic layers
were dried over anhydrous MgSO4, filtered, and concentrated. The
residue obtained was purified over a silica gel column (CH2Cl2/
MeOH, 9/1) to afford 13 (0.72 g, 74%) as a yellow gummy liquid. 1H
NMR (400 MHz, CDCl3) δ 1.37−1.43 (m, 2H, H-3′), 1.53−1.64 (m,
4H, H-2′, H-4′), 2.30 (t, J = 4.0, 8.0 Hz, 2H, CH2CO), 2.70 (dd, J =
4.0, 12.0 HZ, 1H, H-2), 3.13 (t, J = 8.0 Hz, 1H, H-4), 3.31 (s, 3H,
OMe), 3.40 (dd, J = 8.0, 12.0 Hz, 1H, H-6′), 3.48 (dd, J = 12.0 Hz, 1H,
H-6), 3.51−3.55 (m, 1H, OCH2), 3.62−3.66 (m, 1H, H-5), 3.76 (t, J =
8.0, 12.0 Hz, 1H, H-3), 3.88−3.94 (m, 1H, OCH2), 3.96 (s, 2H,
NCH2Ph), 4.59 (d, J = 4.0 Hz, 1H, H-1), 6.63 (bs, NH), 7.32−7.37
(m, 5H, aromatic). 13C NMR (100 MHz, CDCl3) δ 24.9 (C-4′), 25.6
(C-3′), 30.0 (C-2′), 35.2 (CH2CO), 51.6 (NCH2Ph), 51.7 (C-6), 55.5
(OMe), 61.6 (C-2), 70.2 (C-5), 72.2 (C-3), 72.3 (OCH2), 78.9 (C-4),
97.1 (C-1), 128.2, 128.8, 129.0, 137.6, 179.5 (CO). HRMS (ESI)
calcd. for C20H30N4O6Na [M + Na]+: 445.2069, found 445.2063.
Glucosamine Derivative 10. To a solution of compound 9 (3.20 g,
5.91 mmol) in a MeOH/H2O (9/1 mL) mixture were added
Pd(OH)2/C (20%) (0.17 g, 1.18 mmol, 0.2 equiv) and HCO2NH4
(1.86 g, 29.5 mmol, 5 equiv). The resulting mixture was refluxed for
1.5 h, filtered through a pad of Celite, and concentrated. The residue
was purified over silica gel column (EtOAc/pentane, 2/1) to afford 10
1
(2.61 g, 98%) as a yellow foam. H NMR (400 MHz, CDCl3) δ 1.25
(t, J = 8.0 Hz, 3H, Me), 1.36−1.38 (m, 2H, H-4′), 1.55−1.65 (m, 4H,
H-2′, H-3′), 2.29 (t, J = 8.0 Hz, 2H, CH2CO), 3.15 (s, 3H, OMe), 3.21
(dd, J = 12 Hz, 1H, H-2), 3.49−3.53 (m, 2H, H-5, OCH2), 3.67 (t, J =
8.0 Hz, 1H, H-4), 3.76 (d, 2H, H-6, H-6′), 3.83−3.87 (m, 1H, OCH2),
4.12 (q, J = 8.0, 16.0 Hz, 2H, CH2-ethyl ester), 4.41 (d, 2H, NCH2Ph),
4.50−4.55 (m, 2H, H-1, H-3), 7.30−7.36 (m, 5H, aromatic). 13C
NMR (100 MHz, CDCl3) δ 14.2 (Me), 24.6 (C-4′), 25.5 (C-3′), 29.4
(C-2′), 34.2 (CH2CO), 48.6 (NCH2Ph), 55.4 (OMe), 60.2 (CH2-
ethyl ester), 61.0 (C-6), 61.2 (C-2), 71.0 (OCH2), 73.3 (C-5), 75.4
(C-4), 77.1 (C-3), 96.2 (C-1), 128.2, 128.6, 128.7, 135.2, 159.1 (CO,
amide), 173.8 (CO, ester). HRMS (ESI) Calcd for C23H33NO8Na [M
+ Na]+: 474.2104, found 474.2117. Calcd for C23H34NO8 [M + H]+:
452.2284, found, 452.2284.
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Calcd for C20H31N4O6 [M + H]+: 423.2239, found 423.2244. Calcd
for C19H27N4O5 [M − CH3OH + H]+: 391.1981, found 391.1972.
Glucosamine Derivative 14. To a solution of compound 13 (0.64
g, 1.44 mmol) in a MeOH/H2O (9/1 mL) mixture were added
Pd(OH)2/C (20%) (0.04 g, 0.29 mmol, 0.2 equiv) and HCO2NH4
(0.45 g, 7.20 mmol, 5 equiv). The mixture was refluxed for 1.5 h,
filtered through a pad of Celite, and concentrated to afford 14 (0.45 g,
95%) as a white crystalline solid; mp 87−89 °C. 1H NMR (400 MHz,
D2O) δ 1.30−1.35 (m, 2H, H-4′), 1.51−1.60 (m, 4H, H-2′, H-3′), 2.16
(t, J = 8.0 Hz, 2H, CH2CO), 2.93 (dd, J = 4.0, 12.0 Hz, 1H, H-2),
3.11−3.22 (m, 2H, H-4, H-6′), 3.37 (dd, J = 4.0, 12.0 Hz, 1H, H-6),
3.40 (s, 3H, OMe), 3.61−3.70 (m, 2H, H-3, OCH2), 3.81−3.87 (m,
2H, H-5, OCH2), 4.85 (s, 1H, H-1). 13C NMR (100 MHz, D2O) δ
25.0 (C-4′), 25.4 (C-3′), 28.9 (C-2′), 37.3 (CH2CO), 40.3 (C-6), 54.4
(C-2), 55.4 (OMe), 67.1 (C-5), 72.0 (C-3), 73.1 (OCH2), 79.7 (C-4),
98.5 (C-1), 183.6 (CO). HRMS (ESI) Calcd for C13H26N2O6Na [M +
Na]+: 329.1689, found 329.1690. Calcd for C13H27N2O6 [M + H]+:
307.1869, found 307.1864. Calcd for C13H25N2O6Na2 [M − H +
2Na]+: 351.1508, found 351.1516.
Glucosamine Derivative 15. A solution of compound 14 (0.22 g,
0.67 mmol) in a DMF/triethylamine (7/1 mL) mixture, under argon
atmosphere, was stirred at room temperature for 30 min, and then a
solution of trityl chloride (0.93 g, 3.35 mmol, 5 equiv) in a DMF/
triethylamine (5/1) mixture was added. The resulting solution was
stirred at room temperature for 8 h. A saturated NH4Cl solution was
added, and the resulting aqueous mixture was extracted with ethyl
acetate (3 × 10 mL). The combined organic layers were dried over
anhydrous MgSO4, filtered, and concentrated. The residue obtained
was purified over silica gel column (EtOAc/cyclohexane, 1/8
containing triethylamine) to give the tritritylated compound (0.47 g,
68%) as a white amorphous solid. HRMS (ESI) Calcd for C70H69N2O6
[M + H]+: 1033.5156, found 1033.5163.
A solution of this compound (0.45 g, 0.43 mmol) in a mixture 1 M
NaOH/dioxane (4/4 mL, v/v) was heated at 80 °C for 6 h and then
diluted with ethyl acetate and washed with a saturated solution of
NaCl. The separated aqueous layer was repeatedly washed with ethyl
acetate. The combined organic layers were dried over anhydrous
MgSO4, filtered, and concentrated. The obtained residue was purified
over silica gel column deactivated with triethylamine (CH2Cl2/MeOH,
9/1) to give 15 (0.33 g, 85%) as a white crystalline solid; mp 107−109
°C. 1H NMR (400 MHz, CDCl3) δ 1.21−1.27 (m, 2H, H-4′), 1.30 (t, J
= 4.0, 8.0 Hz, 9H, Et3N), 1.41−1.44 (m, 2H, H-3′), 1.55−1.57 (m, 2H,
H-2′), 1.97−1.98 (m, 1H, H-6′), 2.19 (t, J = 8.0 Hz, 2H, CH2CO),
2.50−2.53 (m, 1H, H-6), 2.77 (d, J = 4.0 Hz, 1H, H-1), 2.81 (dd, J =
4.0, 8.0 Hz, 1H, H-2), 2.92 (q, J = 4.0, 8.0 Hz, 1H, H-4), 2.99 (s, 3H,
OMe), 3.05 (q, J = 8.0, 12.0 Hz, 6H, Et3N), 3.28−3.31 (m, 1H,
OCH2), 3.64 (m, 1H, H-5), 3.76−3.83 (m, 2H, H-3, OCH2), 7.16−
7.30 (m, 20H, aromatic), 7.40−7.42 (m, 5H, aromatic), 7.56−7.58 (m,
5H, aromatic). 13C NMR (100 MHz, CDCl3) δ 8.9 (Me, Et3N), 25.6
(C-4′), 26.1 (C-3′), 30.3 (C-2′), 36.1 (CH2CO), 45.1 (C-6), 45.4
(CH2, Et3N), 54.2 (OMe), 58.1 (C-2), 66.4 (C trityl), 70.2 (C-5), 70.3
(C trityl), 71.9 (OCH2), 73.8 (C-3), 80.7 (C-4), 97.5 (C-1), 126.3,
126.7, 126.8, 127.8, 127.9, 128.0, 128.1, 128.3, 128.8, 129.0, 129.1,
146.2, 147.0, 148.7, 179.1 (CO). HRMS (ESI) Calcd for
C51H54N2O6Na [M − Na]+: 813.3880, found 813.3889. Calcd for
C51H53N2O6Na2 [M − H + 2Na]+: 835.3699, found 835.3696. Calcd
for C51H52N2O6Na3 [M − 2H + 3Na]+: 857.3519, found 857.3553.
Procedure for the Synthesis of PNA−Aminoglucosamine
Conjugates 5a and 5b. To a solution of the protected glucosamine
derivative 15 (18 mg, 20 μmol) in dry DMF (1 mL), under argon,
were added 1-[3-(dimethylaminopropyl)]-3-ethylcarbodiimide hydro-
chloride (5.2 mg, 30 μmol) and 1-hydroxybenzotriazole (HOBt) (4.0
mg, 30 μmol). The resulting solution was stirred for 15 min, and then
the protected PNA (3a or 3b) on its solid support was added under
argon (PNA synthesis at 2 μmol scale). The mixture was stirred at
room temperature for 1 h and filtered. The neamine PNA conjugate
was then cleaved from the solid support with concomitant
deprotection by treatment with TFA/anisole (1/1) for 1 h. The
mixture was filtered, and then diethyl ether was added to the solution
to precipitate the PNA. HPLC purification was carried out on a C18
reversed-phase column (Macherey-Nagel, 10.0 mm × 250 mm) with a
Dionex Ultimate 3000 (detection at 260 and 280 nm). Elution was
performed at 60 °C at a flow rate of 2 mL/min by building up the
following gradients: 0.1% TFA in acetonitrile/0.1% TFA in water (10/
90 v/v) for 10 min, then 0.1% TFA in acetonitrile/0.1% TFA in water/
methanol (10/85/5 v/v/v) for 5a or 0.1% TFA in acetonitrile/0.1%
TFA in water (10/90 to 40/60 v/v for 30 min) for 5b.
Conjugate 5a: MALDI-TOF MS m/z found, 4576.0088; calcd for
C183H239N91O54, 4574.8753 (noncoupled and free PNA: 4286.7068).
Conjugate 5b: MALDI-TOF MS m/z found, 5207.2808; calcd for
C216H272N94O64, 5206.0919 (noncoupled free PNA: 4917.9234).
Biological Assays. Cellular Uptake of Fluorescein Labeled PNA−
Aminoglucosamine by Flow Cytometry. CEM (T-cell lymphocytes)
cells were used to evaluate the efficiency of cellular uptake of conjugate
5b. The cells were grown at 37 °C in 5% CO2 environment in
complete RPMI1640 medium (GIBCO, Life Technology, USA)
containing 10% fetal bovine serum and 100 units each of penicillin and
streptomycin. The cells gowing at log phase were harvested, washed
with phosphate buffered saline, and resuspended in RPMI media with
2% FBS. Cells were aliquoted in 12-well plate at 0.5 × 106 cells/mL
and incubated for 30 min at 37 °C. Fluorescein labeled PNA−
aminoglucosamine conjugate 5b was added to cells at varying
concentrations (50 nM−1000 nM) and incubated at 37 °C. At
different time points, cells were harvested, washed twice with PBS, and
resuspended in 1 mL of PBS for FACScan analysis on Becton
Dickinson flow cytometer. To exclude the uptake by dead cells, the
scanning was done in the presence of propidium iodide (1 μg/mL).
Cell Quest Pro software (Becton Dickinson) was used to acquire and
analyze events detected by FL1 detector (for FITC), which excluded
FL3 detector (propidium iodide). Graphs were generated using
GraphPad Prism 4.0. We also used Huh7.5 cells, which are relatively
larger and have more cytoplasmic space than CEM cells. Cells were
incubated with 1 μM PNA−glucosamine conjugate 5b in the absence
and presence of an endosome disrupting agent, chloroquine.67 After 10
h of incubation, cells were washed with PBS and stained with wheat
germ agglutinin conjugated with rhodamine to label the oligosacchar-
ides component of the membrane glycoprotein (red) and with DAPI
to stain the nuclear DNA (blue). Uptake of FITC labeled PNA−
glucosamine 5b shows green fluorescence measured at 488 nm. The
images were acquired on Nikon A1R confocal microscope.
Preparation of 32P-Labeled HIV-1 TAR RNA and HCV 3′NTR RNA
Fragment. The plasmid pEM-7 encoding the HIV-1 TAR under the
control of the T7 promoter was linearized using HindIII and
transcribed to generate 82 base long runoff transcript of HIV-1 TAR
RNA using T7 RNA polymerase transcription kit (Roche Dignostics).
For preparing hepatitis C virus RNA fragment corresponding to 3x′ tail
of HCV-3′NTR, we amplified this region by PCR and then
transcribing the PCR product as described before.90 The RNA
transcript was internally labeled using α32P-CTP (3000 Ci/mmol;
Perkin-Elmer Life and Analytical Sciences). The runoff transcript was
purified by phenol:chloroform:isoamyl alcohol extraction followed by
ethanol precipitation. The RNA was air-dried and resuspended in
water pretreated with diethyl pyrocarbonate (DEPC) and finally
stored at −70 °C.
Gel Mobility Shift Assay. The binding affinity of the PNA−
aminoglucosamine for the TAR RNA was examined by gel mobility
shift assay. An aliquot of 32P-labled 82 bp long TAR RNA transcript
(10 nM) was incubated with varying concentrations of conjugate 5a in
a binding buffer containing 50 mM Tris-HCl (pH 7.8), 60 mM KCl,
and 5 mM MgCl2 in a final volume of 10 μL. After incubation at room
temperature for 1 h, 10 μL of gel loading dye (0.27% bromophenol
blue and 20% glycerol) was added to the samples and were run (100
V) on a native 8% polyacrylamide gel using Tris-borate-EDTA buffer
at room temperature for 4 h. The gel was subjected to
PhosphorImager analysis (Molecular Dynamics).
Tat-Mediated Transactivation of HIV-1 LTR. CEM cells (0.5 × 106
cells per well in 12 well plate) were transfected with the plasmid
cocktail containing pHIV-1 LTR Luc (2 μg) and pRL-SV40 (500 ng)
in the presence and absence of pCMV-Tat (2 μg) using X-
tremeGENE 9 DNA transfection reagent (Roche). The transfection
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Article
complexes were prepared in Opti-MEM medium using 6 μL of
transfection reagent. The transfected cells were further grown in
normal growth medium (RPMI medium containing 10% fetal bovine
serum, 4 mM L-glutamine, 100 U of penicillin, and 100 μg of
streptomycin per mL) at 37 °C in 5% CO2 environment. After 2 h,
PNA-conjugates were supplemented in the medium and cells were
further grown. After 24 h, cells were harvested and washed twice with
PBS. The firefly and Renilla luciferase activities were measured using
dual-luciferase reporter assay system (Dual Luciferase Assay kit). In
brief, the cells were lysed using 100 μL of passive lysis buffer on a
rocking shaker for 20 min. The lysate were centrifuged at 15000 rpm
for 1 min, and supernatant was collected in fresh tubes. Luciferase
assay was performed by mixing 10 μL of cell lysate in 100 μL of
luciferase assay regent in a 96-well plate (Fluotrac). The light emission
was measured using Peckard Top count luminescence counter. Firefly
luciferase activities were quenched by the addition of 100 μL of Stop
and Glo reagent to measure the Renilla luciferase activity. Transfection
efficiencies were normalized by the expression of the Renilla luciferase
reporter gene construct cotransfected along with the experimental
plasmid.
Production and Purification of Pseudotyped HIV-1 Virions.
Pseudotyped HIV-1 virions were produced in 293T cells as described
before66 by cotransfection of pNL4−3.Luc.R.E with the pVSV-G
retroviral vector, encoding the vesicular stomatitis virus protein G
under the control of the CMV immediate-early promoter (BD
Biosciences Clontech) using the calcium phosphate transfection
system (Life Technologies). The transfected cells were grown in
complete Dulbecco’s Modified Eagle’s Medium, and the culture
supernatant was saved at 24, 48, and 72 h post transfection, pooled
together, and analyzed for p24 antigen using the ELISA p24 antigen kit
(Zaptometrix Corp.). The pseudotyped HIV-1 virions were isolated
from the culture supernatant by filtration through a 0.45 μm pore size
PVDF membrane (Millipore) followed by ultracentrifugation at
70000g for 45 min. The viral pellet was resuspended in complete
Dulbecco’s medium and stored at −80 °C.
ASSOCIATED CONTENT
■
S
* Supporting Information
General information for the synthesis, 1H and 13C NMR
spectra of the glucosamine derivatives and HPLC profiles of the
conjugates 5a and 5b. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*For V.N.P.: phone, 973-972-0660; fax, 973-972-8657/5594; E-
mail, pandey@umdnj.edu. For J.-L.D.: phone, +33 4 76 63 53
17; fax, +33 4 76 63 52 98; E-mail, Jean-Luc.Decout@ujf-
grenoble.fr.
Present Address
§Universite
́
de Poitiers/CNRS, UMR 7285, Institut de Chimie
des Milieux et Materiaux de Poitiers, 4 rue Michel Brunet,
́
86022 Poitiers, France.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Indo−French Centre for the
Promotion of Advanced Research (IFCPAR, Project no. 3405-
1, J.-L. D.) and by a grant from the NIAID/NIH (AI42520) to
V.N.P.
ABBREVIATIONS USED
■
CPP, cell-penetrating peptides; DAPI, 4′,6-diamidino-2-phenyl-
indole; FACS, fluorescence activated cell sorter; FITC,
fluorescein isothiocyanate; ODN, oligo-2′-deoxyribonucleoti-
des; PNA, peptide (polyamide) nucleic acid; TAR, trans-
activation responsive region; Tat, trans-activator of tran-
scription
Antiviral Efficacy of Conjugate 5a. The CEM CD4+ lymphocytes
12D7 (13) were grown in RPMI-1640 medium supplemented with
10% fetal calf serum, 4 mM L-glutamine at 37 °C in 5% CO2
containing humidified air. Early mid log phase cells were harvested
and washed with an equal volume of PBS w/o Ca2+ and Mg2+.
Approximately 1 × 106 cells in 1 mL of RPMI-1640 medium were
incubated with pseudovirions (14 ng; equiv p24) on a rocker for 2 h.
The cells were centrifuged, and after washing with 1× PBS,
resuspended in fresh complete RPMI media in a 12-well plate. After
48 h post infection, an increasing amount of conjugate 5a was
supplemented in the culture medium. After 76 h, the cells were
harvested, washed with 1× PBS, and lysed in 1× passive lysis buffer
(Promega). The cell lysates were centrifuged at 15000 rpm for 15 min,
and an aliquot of the supernatant was assayed to determine inhibition
of HIV-1 transcription by measuring reporter luciferase activity. The
IC50 value was calculated using CalcuSyn (Biosoft).
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