A self-assembled π-conjugated system as an anti-proliferative agent in prostate cancer cells and a probe for intra-cellular imaging

Article abstract of DOI:10.1039/c4ra07710e

Multifunctional π-conjugated systems derived from renewable resource that self-assemble into supramolecular structures are reported. The aggregation of compounds in different solvents strongly influences their optical properties. These π-conjugated molecules can be used for live cell imaging applications. They also show low cytotoxicity in fibroblasts and suppress proliferation in PC3 prostate cancer cells.

Full text of DOI:10.1039/c4ra07710e

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Abstract
Multifunctional π-conjugated systems derived from renewable resource that self-assemble
into supramolecular structures are reported. The aggregation of compounds in different
solvents strongly influences its optical properties. These π-conjugated molecules can be
used for live cell imaging applications. It also show low cytotoxicity in normal cells and
suppresses the proliferation in PC3 prostate cancer cells.

RSC Advances
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DOI: 10.1039/C4RA07710E
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ARTICLE TYPE
Selfꢀassembled πꢀconjugated system as an antiꢀproliferative agent in
prostate cancer cells and probe for intraꢀcellular imaging
a
a
a
a
b
Krishnamoorthy Lalitha, Preethi Jenifer, Y. Siva Prasad, Kumarasamy Muthusamy, George John,
Subbiah Nagarajan,*
a
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Multifunctional πꢀconjugated systems derived from
renewable resource that selfꢀassemble into supramolecular
decades, there has been considerable focus on establishing and
optimizing efficient materials, biologically relevant molecules,
structures are reported. The aggregation of compounds in 50 and largeꢀscale production of chemicals and fuels that address the
st
8
needs of the 21 century. Among the large number of renewable
1
0
different solvents strongly influences its optical properties.
These πꢀconjugated molecules can be used for live cell
imaging applications. It also show low cytotoxicity in
fibroblast and suppresses the proliferation in PC3 prostate
cancer cells.
resources, cashew nutshell liquid (CNSL) is an important byꢀ
product obtained from the cashew nut industry. The major
component of CNSL being cardanol, a bio based nonꢀisoprene
lipid, comprising of rich mixture of phenolic lipids: 5% of 3ꢀnꢀ
pentadecylphenol (3ꢀPDP), 50% of 3ꢀ(8Zꢀpentadecenyl)phenol,
6% of 3ꢀ(8Z,11Zꢀpentadecadienyl)phenol and 29% of 3ꢀ
8Z,11Z,14ꢀpentadecatrienyl)phenol. The naturally occurring
varying degree of cisꢀdouble bonds and an odd number of carbon
chain with easily accessible saturated and unsaturated
9
5
6
6
5
0
5
1
(
1
2
2
3
3
4
4
5
0
5
0
5
0
5
The selfꢀassembly of low molecular weight building blocks to
form diverse supramolecular architectures has attracted
substantial interest due to their versatile applications in the field
of drug delivery, gene therapy, tissue engineering, enzyme
immobilization, biosensors and construction of novel nanoꢀ or
9
,10
hydrocarbon chains are the unique features of cardanol.
By
1
ꢀ3
harnessing electrophilic aromatic substitution reaction on phenol,
we have synthesized both cardanolꢀaldehyde 2b and PDPꢀ
aldehyde 2c. The Knoevenagel reaction of compounds 2aꢀc with
ethyl acetoacetate under optimized reaction condition led to the
desired 3ꢀacetylcoumarins 3aꢀc in good yields. πꢀGelators has
been synthesized in good yields by aldol condensation of 3aꢀc
with 1ꢀpyrenecarboxaldehyde 4 (Scheme 1).
microscopic materials and devices. In this context molecular
gels are known as distinct class of soft materials. The gels are
formed by the hierarchical assembly of low molecular weight
organic gelators in a suitable solvent to structures such as fibrils,
11
4
tapes, rods and tubes. The specific nonꢀcovalent interactions
such as hydrogen bonding, ꢀꢀꢀ interaction, electrostatic and van
der Waals interactions, hydrophilicꢀlipophilic balance (HLB) and
other supramolecular weak forces are the driving forces for the
5
selfꢀassembly. Fluorescent supramolecular gels derived from
biologically relevant molecules have received much attention
because of their wide range of applications. Coumarin, a class of
naturally occurring benzopyrone derivative has been used as an
important pharmacophore, as it displays biological activities such
as anticancer, antibacterial, antifungal, anticoagulant and antiꢀ
6
HIV, to name a few. The high fluorescence quantum yield and
sensitivity toward the small changes in microenvironment are
unique to the selfꢀassembled pyrene derivatives, which enables
7
them in applications in biomedical and biological research.
These prospects drive us through extensive synthetic efforts to
obtain more diverse pyreneꢀcoupled coumarin based πꢀgelator
with various hydrophobic tails, which could be used as drug
carrier under high concentration, cell imaging agent under lower
concencentration and may exhibit therapeutic value too.
Organogels derived from “πꢀgelators” are called “πꢀgels” which
are selfꢀassembled soft materials obtained from gelators with
7
0
Scheme 1. Synthesis of multifunctional coumarinꢀcoupled pyrene
derivatives 5aꢀc.
7
more than one aromatic πꢀunit. In the present studies, πꢀgelators
Most of the pyrene based low molecular weight organogelator
were developed from renewable plantꢀderived resource, cashew
nut shell liquid. Renewable resources have been used for several
(
LMOG) were prone to gelate solvents by using weak bonding
mechanism in the presence of suitable solvents, even in the
This journal is © The Royal Society of Chemistry [year]
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absence of hydrogen bonding. The supramolecular interaction
of 5a by means of πꢀπ stacking is inferred by NMR spectral
analysis (Figure S1). Selfꢀassembly of such an efficient gelator
through nonꢀcovalent interactions into fibrillar aggregate that
enunciates coumarin coupled pyrene, intercalated free decanol
and hydrogen bonded decanol with the carbonyl group of
coumarinꢀcoupled pyrene derivative (Figure S2). This reflection
is approximately equal to the molecular length of 5a, which was
5
0
5
0
5
0
could immobilize the solvent molecule by capillary force to form 50 confirmed by molecular modelling studies using energy
4
a gel. Gelation studies using aromatic solvents, alcohols and
vegetable oils were carried out (Table S1). πꢀGelators 5a and 5b
exhibit excellent organogelation ability, showing critical gelation
concentrations (CGCs) of 0.28 and 1.0 % (wt/v) respectively in
minimized calculations. Sharp peaks observed between 0.98ꢀ0.62
nm may arise from packing of dodecanol (both free and hydrogen
bonded dodecanol) due to van der Waals interaction in gel
network. The broad peak at 25º is assignable to the (001) aspect
13
1
1
2
2
3
higher alcohols such as decanol and dodecanol. πꢀConjugated 55 of πꢀπ stacking of pyrene units.
molecule 5c did not form gel because of its enhanced
The UVꢀvis spectrum of compound 5a in acetonitrile shows three
bands centred at 307, 392 and 427 nm, which are attributed to
coumarin and pyrene unit under unꢀaggregated form. By
changing the solvent to DMSO, the peak observed at 307 nm
hydrophobicity. GelꢀSol transition temperature (T ) was
g
4
determined by typical test tube inversion method. In fact
organogel formed by these compounds experience a gel to sol
transition upon heatingꢀcooling cycles (T = 65 ̊C ). Morphology 60 shifted to 352 nm and other peaks remained as same. This result
g
of the aggregates constitutive of the organogel in dodecanol has
been identified using optical microscopy imaging deposited on
glass slide. It shows thin fiber and twisted fiberꢀlike structures of
implies the weak interaction of DMSO with coumarin core of 5a.
Compound 5a in dodecanol show bands at 324 and 448 nm, red
shift in all these peaks are due to molecular aggregation involving
the formation of hydrogen bonding between carbonyl carbons
1
00ꢀ200 nm thickness that bundle to form 3D network (Figure 1).
Morphology and properties of the πꢀgel resembles the selfꢀ 65 coumarin moiety and dodecanol, and πꢀπ stacking of pyrene
7
assembly mechanism of πꢀconjugated molecule. Detailed
gelation test indicated that compound 5aꢀ5c do not form gel in
any of the aromatic solvents tested and form stable gel in long
chain alcohols and vegetable oils. Increasing the lipophilicity of
(Figure 2a). Based on the results, we propose molecular
14
arrangement of 5a in higher alcohol (Figure 1). The UV
ꢀ
5
titration of compound 5a dissolved in DMSO (1X10 M) with
PBS buffer solution were then conducted. As expected, with the
the coumarin coupled pyrene derivative by introducing 70 continuous addition of 100ꢀL of PBS buffer, absorbance band at
unsaturated and saturated alkyl chain decreases the gelation
427 nm decreases with gradual increase in volume of PBS buffer
solution. This property is attributed to the formation of nanoꢀ
structures by selfꢀassembly of pyrene and coumarin moieties
(Figure 2b). After the investigation of the selfꢀassembly features
ability. At lower concentration, 5aꢀc in DMSOꢀwater mixture
ꢀ
3
(
1X10 M solution) form selfꢀassembled nanoꢀsheets and nanoꢀ
flakes (Figure 1). Nanoparticle formation was further confirmed
using particle size analyser (Zetasizer). The average sizes of selfꢀ 75 of 5a in solution using UVꢀvis spectroscopy, we have evaluated
ꢀ
3
assembled aggregates of 5aꢀc in DMSOꢀwater mixture (1X10 M
solution) are 194, 21 and 274 nm respectively (See ESI).
the fluorescence property of 5a in solution and selfꢀassembled
state. The intense fluorescence observed in the gel state
stimulated us to explore the emission property of compound 5aꢀc
under different experimental conditions to confirm the influence
of aggregation of the material. In selfꢀassembled state, emission
spectrum of 5a in dodecanol shows three peaks at 389, 409 and
8
8
9
9
0
5
0
5
5
54, which on further titration using dodecanol, the intensity of
peak observed at 409 nm got decreased and 554 nm show blue
shift and is attributed to the disassembly behaviour of selfꢀ
assembled pyrene moiety (Figure 2c). Similarly, compound 5a in
DMSO shows three intense peaks at 397, 409 and 478 nm (Figure
S3). The molecular aggregation in supergelator 5a dissolved in
DMSO was induced by stepwise addition of 100ꢀL of PBS buffer
and its emission behaviour was also followed. In the aggregated
state, the emission spectrum covers a broad range of visible
spectral range and exhibit vibronic coupling maximum at 414 and
5
76 nm. The drastic increase in emission intensity with a red shift
in the aggregated state implies the formation of selfꢀassembled
Figure 1. (a&b) Optical microscopy image of gel, 5a in dodecanol (0.28
% wt/v) under white light and fluorescence light respectively, inset show
the formation of twisted fibers; (c) Pictures of gel under UV light [leftꢀ5a
in decanol, middleꢀ5b in hazelnut oil and rightꢀ5a in dodecanol]; (d&e)
HRTEM images of the selfꢀassembly of 5a and 5b in DMSOꢀwater
mixture respectively; (f) Picture of selfꢀassembled solution of 5a in
15
structure (Figure 2). We determined fluorescence quantum
3
5
0
ꢀ
5
yields of 5aꢀc in different solvents (1X10 M). Compound 5aꢀc
exhibited a very low fluorescence quantum yield ranging from 9ꢀ
1
2 % in DMSOꢀPBS buffer (1:1 ratio), which could be due to the
selfꢀaggregation of molecules. By using DMSO alone as a solvent
increase in quantum yields (73%) was observed.
ꢀ3
4
DMSOꢀwater (1:1 ratio; 1X10 M solution) and (g) Schematic
representation of gel formation.
Smallꢀangle Xꢀray diffraction (SAXD) was employed to acquire
an additional insight into the structures that constitute the gel
formed from supergelator 5a. XRD of the wet gel provides a
Bragg’s reflection at 1.35 nm, 1.30 nm, 1.26 nm which
45
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| Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C4RA07710E
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and perinuclear region of the cells. The green selfꢀfluorescence
arising from 5aꢀc can be readily observed. Fluorescence intensity
decreases with increase in hydrophobicity of πꢀconjugated
systems (Figure 3). Nanoparticles derived from 5aꢀc were
uniformly located into the cytoplasm of the cells. Endocytosis of
nanoparticles involves four different mechanisms: Clathrinꢀ
mediated endocytosis, caveolae mediated endocytosis,
macropinocytosis and phagocytosis. Inhibitors such as sucrose
and chlorpromazine (blocking agents of clathrinꢀcoated pit
formation) and filipin (an inhibitor of caveolaeꢀassociated
endocytosis) had no significant inhibition effect on the
nanoparticle
uptake.
Nocodazole,
an
inhibitor
of
macropinocytosis decreases the uptake of nanoparticle up to 60%.
The prominent cell uptake pathway for selfꢀassembled
1
6
nanoparticles are macropinocytosis and phagocytosis.
In
fibroblast no damage in cells were observed, which implies the
low cytotoxicity of fibroblast and also identified from
cytotoxicity assay. In addition, the death of majority of PC3 cells
were observed, which is due to anticancer activity of 5aꢀc, as
identified based on cell membrane rupture and the overflow of
cytoplasm (Figure S7). Coumarin based antiꢀcancer drug,
decursin inhibits Wnt/βꢀcatenin pathway and cellular
17
proliferation. We hypothesize that 5aꢀc might also follow
similar of mechanism. This selfꢀfluorescent probe would
potentially facilitate a simultaneous combination of optical
diagnosis and treatment for prostate cancer.
Figure 2. (a) UVꢀVis spectra of 5a in different solvents; (b & c) UV
titration of 5a in DMSO with PBS buffer and its corresponding plot of
absorbance intensity vs concentration; (d) Emission spectra of 5a in
ꢀ
5
5
0
dodecanol (1X10 ) and its response with respect to dilution [λex
25nm]; (e) plot of emission intensity vs concentration of 5a in
dodecanol; (f & g) Fluorescence titration of 5a in DMSO with PBS buffer
ex = 325nm] and its corresponding plot of wavelength vs concentration .
=
3
[
λ
In titration experiments, direction of arrow show the response of
absorption and emission intensity with piecemeal addition of 100ꢀL of
corresponding solvent. 2mL of initial volume of solution (1X10 ) was
taken for titration experiments.
1
ꢀ5
Figure 3. LCSM images of (aꢀc) fibroblast incubated with 5aꢀc and (dꢀf)
PC3 prostate cancer cells incubated with 5aꢀc for 24h respectively. Green
from selfꢀfluorescent πꢀconjugated systems 5aꢀc and blue from Hoechst
strain used to differentiate nucleus.
Similarly selfꢀassembly of 5b and 5c was also identified by using
UV and fluorescence studies (Figure S4 and S5). From these
result, we resolve that at higher concentration, 5a and 5b form gel
in decanol and dodecanol, and at lower concentration 5aꢀc in
DMSOꢀwater (1:1 mixture) forms selfꢀassembled nanostructures
60
1
2
2
5
0
5
In order to verify the biocompatibility of the fluorescent nanoꢀ
18
structures, it is necessary to evaluate its in vitro cytotoxicity.
We have evaluated the cytotoxicity of compounds 5aꢀc by MTS
assay on fibroblast and PCꢀ3 cells. The maximum absorbance
(λabs) of formazan, produced by the cleavage of MTS by
dehydrogenases in living cells, at a wavelength of 490 nm is
directly proportional to the number of live cells in the MTS assay.
The λabs for compound 5aꢀc in DMSOꢀwater or DMSOꢀPBS
buffer (1:1 mixture) was observed between 340ꢀ360nm, which
would not interfere with MTS assay. Cytotoxicity assay shows
that the fluorescent lipophilic compounds 5aꢀc showed no
significant cytotoxic effect on fibroblast. After 24h incubation,
suppression in the proliferation of PC3 cells were observed and
not in fibroblast. Inhibition in the proliferation of PC3 cells
increases with increase in concentration of coumarinꢀcoupled
(
nanoꢀsheet and nanoꢀflakes). Fluorescence of selfꢀassembled
system was not quenched even in extreme pH conditions (pH 4 &
10), and thus this system can be applied for cell imaging under
physiologically important conditions at various pH values (Figure
S6). In order to take the advantage of the utility of selfꢀassembly
of 5aꢀc in DMSOꢀwater mixture for live cell imaging application,
6
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0
5
16
normal (fibroblast) cells and PC3 human prostate cancer cells
were incubated with medium containing 5aꢀc [5a: 0.6X10 M,
ꢀ
3
ꢀ
3
ꢀ3
5
b: 0.4 X10 M and 5c: 0.4 X10 M (250µg/1000µL)] for 24h
and cellular localization was traced using Laser Confocal
Scanning Microscopy (LCSM). Selfꢀassembled coumarinꢀ
coupled pyrenes 5aꢀc were uniformly located in the cytoplasm
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pyrene derivatives 5aꢀc (Figure S8).
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7
5
0
In conclusion, πꢀconjugated systems derived from renewable
resource that selfꢀassemble into supramolecular structures
through hydrogen bonding and πꢀπ stacking of pyrene units are
reported. The aggregation of compounds in different solvents
strongly influences its optical properties resulting in a redshift
and increase of emission intensity. Under higher concentration, it
5
0
5
0
5
1
; (p) M. Kumar, N. Kumar, V. Bhalla, P. R. Sharma and T. Kaur,
form
a gel and in lower concentration, selfꢀassembled
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proliferation of PC3 cells might be due to the inhibition of Wnt/βꢀ
17
catenin pathway. These selfꢀassembled soft materials provide a
promising platform for direct cell imaging and disease
therapeutics. Further detailed investigation on mode of action is
in progress in our research laboratory.
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This work was financially supported by the Department of
Science and Technology (IFAꢀCHꢀ04 and #SB/FT/CSꢀ024/2013),
India and Board of Research in Nuclear Science
(#37(1)/20/47/2014), Department of Atomic Energy, India. S.N
thank DST, New Delhi for spectrofluorometer under the FIST
sensor project scheme to SASTRA University. GJ acknowledges
the support from the Gulf of Mexico Research Initiative (GoMRI)
through the Consortium for the Molecular Engineering of
Dispersant Systems (CꢀMEDS) subcontract (TULꢀ626ꢀ11/12).
We also thank SASTRA University for TRR research fund.
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a
Organic Synthesis Group, Department of Chemistry & The Centre for
Nanotechnology and Advanced Biomaterials, School of Chemical and
Biotechnology, SASTRA University, Thanjavur - 613401, Tamil Nadu,
INDIA Fax: 04362264120; Tel: 04362304270; E-mail:
nagarajan@scbt.sastra.edu.
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$
We thank CARISM, SASTRA University for NMR facility.
†
Electronic Supplementary Information (ESI) available: [Experimental
details, figures, tables and copy of NMR spectras]. See
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