Download In Silico Salinispora Dinesh Kumar K. Waheeta Hopper

2011 International Conference on Bioscience, Biochemistry and Bioinformatics
IPCBEE vol.5 (2011) © (2011) IACSIT Press, Singapore
In Silico Anti-Cancer Activity Prediction of Secondary Metabolites
from Salinispora spp.
Dinesh Kumar K.
Waheeta Hopper
Research Scholar, Department of Bioinformatics
School of Bioengineering,
SRM University,
Kattankulathur, Tamil Nadu, India
e-mail: [email protected]
Professor & Head, Department of Bioinformatics
School of Bioengineering,
SRM University,
Kattankulathur, Tamil Nadu, India
e-mail: [email protected]
EGFR in lung, head, neck, colon, pancreas, breast, ovary,
bladder, kidneys and in gliomas. Expression of EGFR and
its role in cancer prognosis have been investigated in many
human cancers [6]. The epidermal growth factor (EGF) is
the prototype of a family of peptide ligands that bind to cell
membrane receptors and activate intracellular signaling
pathways to control tumor cell growth, proliferation, survival,
metastasis, and angiogenesis. The EGF receptor (EGFR,
ErbB1, or HER1) is one of a four-membered family of
transmembrane receptors that, similar to HER2, is often
overexpressed in cancer cells, correlating with poor
prognosis. EGFR therefore represents a reasonable target for
the development of novel anticancer therapies [7].
EGFR are single chain transmembrane polypeptide
proteins possessing three different domains: (a) the
extracellular domain, which binds to ligands that activate the
receptor, (b) the transmembrane domain that is involved in
dimerization interaction between receptors, and (c) the
intracellular tyrosine kinase domain that phosphorylates
tyrosine residues on substrate proteins. The crystal structures
of extracellular domains of EGFR, HER-2 and HER-3 show
that they consist of four subdomains (I – IV) [8-9].
Endogenous ligands for EGFR known are: epidermal
growth factor (EGF), transforming growth factor- α,
amphiregulin, betacellulin, heparin-binding EGF, and
epiregulin. Upon ligand binding to the extracellular domain,
EGFR undergoes a conformational change that allows
activation either by homodimerization or heterodimerization
with other members of the erbB family and phosphorylation
of several tyrosine residues [10]. The formation of EGF
receptor homodimers is induced by the binding of EGF to its
receptor, resulting in autophosphorylation of the EGF
receptor, stimulation of down-stream signaling pathways,
and enhanced cell proliferation. This complex network
represents a significant factor for the observed phenotypic
heterogeneity and variable drug responses among cancer
types [11].
The crystal structure of EGFR (PDB 1IVO) contains 2:2
EGF: EGFR complex per asymmetric unit. In the 2:2 EGF:
EGFR complex, each EGF molecule is bound to only one of
the two EGFR molecules, two 1:1 EGF: EGFR complexes
are dimerized (Fig.1). The receptor dimerization occurs
mostly through interactions between each domain II. A 20
Abstract—A molecular docking investigation was carried out
on secondary metabolites from the marine actinomycete,
Salinispora to identify their bioactive targets. Over-expression
of epidermal growth factor receptor (EGFR) is an essential
feature in the Basal-like breast cancer, for which there are
currently no therapies. Hence a docking study was carried out
to check for the anti-cancer activity of the compounds from S.
tropica targeting the EGFR. The compounds were subjected
to virtual screening and Extra precision (XP) docking using
Schrödinger protocol.
To preliminarily investigate the
potential molecular target, the docking was performed using
EGFR. The docking result revealed that the compounds,
arenicolide A, arenamycin A and arenamycin B exhibited good
binding interaction to EGFR. The identified interactions could
be a prelude to perform experimental assays.
Keywords-Salinispora; secondary metabolites; Epidermal
Growth Factor Receptor; virtual screening; molecular docking
I.
INTRODUCTION
Actinomycetes are a prolific source of structurally
diverse secondary metabolites, many of these possess
pharmaceutically relevant biological activities [1]. Around
23,000
bioactive
secondary
metabolites
from
microorganisms have been reported and over 10,000 of these
are from actinomycetes, representing 45% of all bioactive
microbial metabolites [2]. These organisms have evolved
with greatest genomic and metabolic diversity and hence
efforts have been directed towards exploring marine
actinomycetes as a source for the discovery of novel
secondary metabolites [3].
Salinispora are marine
actinomycetes, widespread in tropical and subtropical marine
mud. There are three species of the genus are S. tropica, S.
arenicola and S. pacifica. The secondary metabolites from
Salinispora strains have varied biological activities however,
they have been tested only in limited, biomedical assays, and
therefore many of their activities remain unknown.
Salinosporamide A, a potent proteasome inhibitor from
Salinispora has entered phase I clinical trials as an anticancer
agent [4].
Receptor tyrosine kinases such as epidermal growth
factor receptor (EGFR) and three related proteins (the ERBB
family) play crucial roles in both normal physiological and
cancerous conditions [5]. Many human tumors express
392
arenamycin B (-4.053) and cynosporaside(-4.040) interacted
with domain II residues.
residue region (“dimerization arm”), forming a hairpin in the
middle, protrudes from the domain II globule of each
receptor. Dimerization is a prerequisite for EGF receptor
activation and is driven by interactions between the
extracellular domains of the two monomeric partners.
Mutation of either Tyr-246 or Tyr-251 within this arm
abolished EGF receptor homodimer formation [12]. Some of
the secondary metabolites of Salinispora show moderate
anticancer activity in cancer cell lines in in vitro studies.
Hence this work attempts to identify biological targets for
the secondary metabolites of Salinispora spp. by using in
silico methods.
II.
B. Glide XP Docking
XP docking was performed for the compounds that
showed interaction. Arenicolide A interacted within EGF
binding site amino acid residues with a maximum glide score
-8.398. Arenamycin A (-5.083) interacted with dimerization
site and Arenamycin B (-8.686) interacted with domain II
residues (Table 1). The Hydroxyl group of arenicolide A
interacted with oxygen of Tyr64, carbonyl group of Ser11
and carbonyl group of Asn12 ( Fig. 3 ) in the EGF binding
region. Carbonyl group of arenimycin B interacted with
carbonyl group of Gly410 and carbonyl group of Gln408
(Fig. 4) in the domain II region. Hydroxyl group of
arenamycin A interacted with oxygen of Gln258 (Fig. 5) in
the dimerization site.
Salinispora species posses structurally diverse secondary
metabolites. In silico activity prediction of these compounds
were performed using virtual screening and molecular
docking protocol. The molecular docking results presented
in this study revealed that the three secondary metabolites of
Salinispora showed strong binding affinities for the EGFR.
In the receptor-dimerization interface, Tyr251 of one
monomer forms hydrogen bonds with Arg285 and Phe263
hydrophobically interacts with other monomer. In the crystal
structure 1IVO the replacement of Arg285 to the Ser285 in
receptor- dimerization interface reduce the bioactivity of
EGFR (12). Studies show that Arg285 play an important
role for structure stability. Here we report that the
compounds bind to the Arg285, EGF binding site and
Domain II residues which inhibit the dimerization and
prevent the binding of EGF to the EGFR receptor.
METHODS
A. Ligand Preparation
A compound database of the 23 metabolites of
Salinispora were created and energy minimized using
LigPrep module of Schrödinger suite version 9 [13] keeping
one conformer per ligand, and the rest of the parameters
were kept as default.
B. Protein Preparation
The crystal structure of EGFR was retrieved from Protein
Data Bank [14] that contains 2:2 EGF: EGFR complex per
asymmetric unit (PDB ID: 1IVO) (Fig.1). All water
molecules were removed, and multimeric complexes were
simplified from the PDB structure. Prior to molecular
docking, receptor structure was preprocessed and prepared
by adding the missing hydrogen, correcting the bond orders
followed by optimization and energy minimization with
force field: OPLS (‘optimized potential for liquid
simulations’) using the protein preparation wizard of the
Schrodinger’s suite. The grid generation was carried out
using three separate sets of regions, the EGF binding site and
its interaction with EGFR, observed using PDBSum database
(Fig. 2) [15]; the Domain II; and the amino acids involved in
the protein dimerization. Three separate receptor grid files
of EGFR were generated by using the centroid of selected
residues option.
IV. CONCLUSION
Screening methods are extensively used to reduce cost
and time of drug discovery. It has been clearly demonstrated
that the approach utilized in this study is successful in
finding novel anticancer inhibitors from marine
actinomycetes. Some of the compounds from Salinispora
species showed high binding affinity against EGFR. The
conformation of the docked compounds fits exactly into the
active site region of the receptor. This study indicates the
importance of small molecules from Salinispora species and
their use as bioactive molecules. Further, work can be
extended towards experimental studies and evaluation of
their biological activity. The findings suggest that these
compounds could be developed as lead compounds for
designing of anti-cancer drugs.
C. Virtual Screening Workflow
The prepared protein receptor grid files and the
minimized ligand database were given as input in the virtual
screening workflow protocol. The docking was done using
High Throughput Virtual Screening (HTVS), Standard
precision (SP) and Extra-Precision (XP) methods. The
“write XP descriptor information” option was enabled and
the rest of the parameters were set as default.
III.
RESULTS AND DISCUSSION
ACKNOWLEDGMENT
A. Virtual Screening Workflow
Of the 23 compounds virtually screened by HTVS, three
compounds arenicolide A, rifamycin and lymphostin
interacted by binding within EGF binding site in the
neighbour of the amino acid residues with a maximum glide
score of -5.578, -4.381and -4.371. Lymphostin (-5.711),
pacificanones (-5.625) and arenamycin A (-5.618) interacted
with dimerization site and arenamycin A (-4.120),
The authors are thankful to SRM University for their
financial assistance and support.
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Schrödinger,
LLC,
New
York,
NY,
2010,
(http://www.schrodinger.com/)
Protein Data Bank (http://www.pdb.org/pdb/home/home.do)
PDBSum Database (http://www.ebi.ac.uk/pdbsum/)
TABLE I.
Targeted
site of
EGFR
receptor
EGF
binding
Site
Domain II
Dimerizati
on site
Figure 1. Structure of EGFR receptor and their domains
XP DOCKING OF SECONDARY METABOLITES OF
SALINISPORA SPP. ON EGFR RECEPTOR
Compound
Arenicolide A
Glide
score
(Kcal/m
ol)
-8.398
Arenimycin B
-8.686
Arenimycin
A
-5.083
Interaction
Distan
ce
(Å)
OH---O (SER 11)
OH---O (TYR 64)
OH---O (ASN 12)
OH---O(GLY
410)
(ARG285)NH---O
OH--O(GLN408)
(GLU258) HO--O
HO---H
(ASN210)
2.230
2.427
1.823
1.676
2.307
1.797
2.494
2.321
Figure 2. Residue interactions of EGFR (chain A) with EGF (chain C)
394
Figure 3. Docking interaction of Arenicolide A with EGFR at the EGF
binding site
Figure 4. Docking interaction of Arenamycin A with EGFR at the
dimerization site
Figure 5. Docking interaction of Arenamycin B with EGFR at domain II
395