Download 100 Marine organisms in somatostatin receptor scintillation proximity

S636 Biochemical Society Transactions (1997) 25
100 Marine organisms in somatostatin receptor scintillation
proximity assays
KIRSTEN MILLS*, PAUL STEAD’ and MARTYN BANKS*
*Lead Discovery Unit “Compound Diversity Unit
Glaxo Wellcome R&D, Gunnel’s Wood Road, Stevenage,
SG12NY, England.
Somatostatin is a small endogenous peptide first discovered for
its ability to inhibit growth hormone secretion (1). It has been
described as the “universal endocrine off switch” (2), and is now
known to be widely distributed in the body, and involved in many
disease states, such as cancer and diabetes mellitis. The actions of
somatostatin are complex, involving two types of peptide ligand
which bind to 5 receptor types, coupled through various G proteins
to several signal transduction pathways.
Cellular effects of somatostatin are mediated by 7transmembrane receptors. The majority of these metabotropic
receptors are negatively coupled to adenylyl cyclase vial Gi or Go;
somatostatin binds to reduce CAMPproduction and thus decrease
cellular activation. Five receptor subtypes have been described,
known as SST,., (3). SST,, and are classified as SRIF-1
receptors, and SST, and are classified as SRIF-2 receptors.
Within each class, there is 70 - 80% sequence homology and a
similar pharmacological profile (3), although SRIF-I receptors
bind the truncated somatostatin analogue, octreotide, with high
affinity, whereas SRIF-2 receptors do not (4). At least one of the
receptors exists in 2 forms: SST,, and SSTzB are splice variants of
the human SST, receptor which differ only in the C-terminal
amino acid sequence (5). Overall, SSTs share 4550% homology,
only slightly higher than their 40% homology with the opioid
receptor family (6). The families show similar pharmacology,
with many compounds shown to display affinity for both receptor
types (7).
The effects of somatostatin are decreased in many disease states
including cancer, CNS disorders and the increased perception of
pain, although conditions, such as diabetes rnellitus, show an
increased concentration of somatostatin. Characterisation of the
pathophysiological role of somatostatin in these diseases makes it
a valuable pharmacologicaltarget.
Most of the peptides known to act as agonists at SSTs in vivo
also cause down-regulation of the receptors following repeated
administration, an effect commonly seen with G protein coupled
receptors (8).
Automated laboratory analysis systems now routinely perform
receptor-ligand binding assays such as scintillation proximity
assays for high throughput screening, where large numbers of
samples such as compounds and combinatorial libraries can be
screened within a short time period. This mechanistic, in vitro
approach also allows the screening of large numbers of crude
natural product extracts, regardless of cytotoxicity. Nonkelective
extracts can be quickly discarded, allowing functional cell assays
to be carried out only on those extracts which have selective high
affinity for the receptor of interest.
Natural products are the source, or inspiration, for the majority
of the world’s medicines, and marine species are responsible for
approximately half of total global biodiversity (9). This vast
resource is being exploited by pharmaceutical companies in the
search for novel therapies, yet to date, no marine-derived drugs are
on the market.
Some marine invertebrates have been reported to show
somatostatin-like activity, and there are published reports
describing active somatostatin receptor antagonists which have
.,
been isolated from sponges (1 0). This literature precedence for
marine-derived active compounds led the Lead Discovery Unit at
Glaxo Wellcome to test a variety of crude marine extracts from
Harbor Branch Oceanographic Institute in the SST, and assays.
The majority of these samples were sponges from the phylum
Porifera and coelenterates from the phylum Cnidaria.
Samples were tested initially at single concentration, and active
samples were retested in duplicate at a single concentration. Any
confirmed positives were tested in a dose response, and progressed
to functional whole cell assays. For crude marine extracts,
samples were submitted for extraction after active ICso values
were obtained in the receptor binding assay. Fractionated samples
were tested in the SPA.
This identified a SST, selective genus of soft corals which had
no previously reported biological activity against SSTs.
,
References
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