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Department of Cell and Cancer Biology, National Cancer Institute, National Institutes of Health (A.M., M.J.M., Y.W., F.C.), Bethesda, Maryland 20892; and Molecular Signaling Group, Clinical Sciences Research Center, St. Bartholomews and the Royal London School of Medicine and Dentistry (S.K.), London, United Kingdom E1 2AT
Address all correspondence and requests for reprints to: Dr. Alfredo Martínez, Department of Cell and Cancer Biology, National Cancer Institute, National Institutes of Health, Building 10, Room 13N262, 9000 Rockville Pike, Bethesda, Maryland 20892. E-mail: martineza{at}bprb.nci.nih.gov
| Abstract |
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-cells,
respectively), which are located at the periphery of the islets.
Interestingly, the smooth muscle cells in the pancreatic vasculature
expressed only RDC1, which is in agreement with physiological data
showing that AM functions in the cardiovascular system are mainly
mediated through a CGRP1 receptor. These data further implicate AM and
the other components of its peptide family as important regulators of
insulin release. | Introduction |
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In the pancreas, AM is located in the islet cells, with predominant expression in the F cells, which also contain pancreatic polypeptide (6). The expression of AM in endocrine pancreatic cells seems to be a highly conserved feature from both the phylogenetic (7) and the ontogenetic (8) perspective. The presence of AM in the pancreas is justified by its function as a regulator of ß-cell physiology (6, 9). AM acts as a tonic inhibitor of insulin release, which has been shown in both isolated islets and through glucose tolerance tests in vivo (6). Recently, AM has been implicated in the inhibition of amylase secretion by pancreatic acini (10), thereby establishing AM as a multifunctional pancreatic hormone.
Several receptors with different affinities for AM have been cloned and sequenced. All of them belong to the seven-transmembrane domain G protein-coupled receptor superfamily and were previously characterized as orphan receptors. The first molecule to be identified for its ability to bind AM was L1, a 395-amino acid polypeptide isolated from rat lung that was able to elevate cAMP in COS-7 cells after exposure to AM, with a Kd of 8.2 x 10-9 M (11). Another molecule able to bind AM is RDC1, a CGRP1 receptor initially isolated from a dog thyroid library and later shown to bind both CGRP and AM, although with different affinities (3 x 10-9 and 1 x 10-7 M, respectively) (12). Another player in this complex physiology is the calcitonin receptor-like receptor (CRLR), which was cloned in 1993 (13) and characterized as a CGRP receptor 3 yr later (14). Interestingly, this receptor requires the presence of modulating proteins with a single transmembrane domain known as receptor activity-modifying proteins (RAMPs). When CRLR and RAMP1 were cotransfected into Xenopus oocytes, a CGRP1 pharmacological response was acquired (CGRP > AM). On the other hand, cotransfection of CRLR and RAMP2 or RAMP3 resulted in a typical AM receptor pharmacology (AM >> CGRP) (15). Recently, a fourth putative receptor for AM has been isolated from human tissue and shown to be expressed by hemopoietic cells (16). This receptor presents a high sequence homology to the rat L1, but, unfortunately, when it was expressed in COS-7 cells it was not functional (17). None of these receptors has a measurable affinity for amylin, the third member of this peptide family, but recently Muff et al. (18) observed that cotransfection of human calcitonin receptor isotype 2 with either RAMP1 or RAMP3 evoked selective binding of amylin in rabbit endothelial cells. There are still several discrepancies between the pharmacological properties observed in many organs and the characteristics of the identified receptors. Given these controversies, many researchers in the field believe that new receptor molecules will be found in the near future. In fact, we recently reported the existence of an AM binding protein in the bloodstream of numerous species that might be a soluble receptor (19).
To better understand the physiological actions of the AM peptide family in the pancreas, we developed antibodies and molecular probes for the three best characterized AM receptors and some related proteins and studied their presence and distribution in the pancreas.
| Materials and Methods |
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Antibodies
Three peptides were selected from the extracellular regions of
L1, RDC1, and CRLR (Table 1
) and synthesized.
They were coupled to keyhole limpet hemocyanin
(Calbiochem-Behring, La Jolla, CA) via glutaraldehyde
cross-linkage, and the conjugate was used to hyperimmunize New Zealand
White rabbits as previously described (20).
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Immunocytochemistry
The avidin-biotin-peroxidase complex method was performed in
paraffin sections as previously described (6). Optimal concentrations
for the primary antibodies were 1:500 (anti-RDC1), 1:1000 (anti-L1),
and 1:5000 (anti-CRLR). Negative controls included substitution of the
primary antibody by preimmune rabbit serum and preabsorption of the
antibody with 10 nmol/ml homologous or heterologous synthetic
peptides.
Confocal microscopy
To better estimate putative hormone colocalizations, triple
labeling followed by confocal micoscopy were performed as previously
described (7). Paraffin sections were dewaxed, rehydrated, and exposed
to donkey normal serum (Jackson ImmunoResearch Laboratories, Inc.; 1:30) in PBS. Then, sections were exposed to a mixture of
three primary antisera obtained in different species, overnight at 4 C.
The mixture consisted of guinea pig antibovine insulin (1:2,000;
Jackson ImmunoResearch Laboratories, Inc.), a rabbit
antibody against one of the receptors (in-house), and one monoclonal
antibody at a time. Optimal concentrations for the receptor antibodies
were previously described for immunocytochemistry. Monoclonal
antibodies used in this study include antisomatostatin (1:10,000; UC
102L, CURE, UCLA), antiglucagon (1:1,000; CURE, UCLA; with permission
from Prof. Gregor, Department of Internal Medicine, Eberhard-Karls
University, Tubingen, Germany), and antirat pancreatic polypeptide
(1:500; CURE, UCLA) (22). The next day, the sections were incubated for
1 h in the second layer, which was a mixture of Cy5-donkey
antiguinea pig (Jackson ImmunoResearch Laboratories, Inc.), Bodipy goat antirabbit (Molecular Probes, Inc., Eugene, OR), and biotinylated goat antimouse (Dakopatts,
Glostrup, Denmark), all of them at a final concentration of 1:200. A
third layer was added for an additional hour, containing lissamine
rhodamine streptavidin (1:200; Jackson ImmunoResearch Laboratories, Inc.). After thorough washes, the slides were
mounted in SlowFade solution (Molecular Probes, Inc.) and
observed with a Carl Zeiss Laser Scanning Microscope 510
(New York, NY) equipped with four lasers.
Cell culture
Two well-characterized cell lines originated from pancreatic ß
cells: CRL 1777 (hamster), and CRL 2055 (mouse) were obtained from the
American Type Culture Collection (Rockville, MD). They
were maintained in F12K medium (Life Technologies, Inc.,
Gaithersburg, MD) supplemented with either 10% horse serum plus 2.5%
FCS (CRL 1777) or 10% FCS (CRL 2055), at 37 C in a 5%
CO2 atmosphere.
RT-PCR and Southern blot
The MicroFast Track kit (Invitrogen, San Diego, CA)
was used to extract messenger RNA (mRNA) from the cell lines. RT, PCR,
and Southern blot were carried out as previously described (20).
Primers specific for the three AM/CGRP receptors, the human calcitonin
receptor, and the RAMP proteins were designed and are shown in Table 2
. mRNA from mouse liver and the human tumor
cell line NCI-H209 were used as controls.
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| Results |
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| Discussion |
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It became clear that the other pancreatic endocrine cell types do not express any of the AM receptors. Therefore these results indicate that the inhibitory effect of AM in insulin secretion (6) is caused by a direct action on the ß-cell, rather than through an indirect mechanism involving the secretion of secondary mediators. This is in agreement with our previous study on rat isolated islets in which we did not observe any change in the secretion of glucagon, somatostatin, or pancreatic polypeptide after exposure of the islets to AM, but we saw a clear dose-dependent inhibition of insulin secretion (6). In the same study we observed an increase in cAMP after stimulation with AM (6). cAMP is the second messenger for the three AM receptors (12, 13, 14, 15), and its elevation indicates the presence of physiologically active AM receptors in the rat islets.
The coexpression of calcitonin receptor and RAMP1 and/or RAMP3 has been shown to generate an amylin receptor physiology in endothelial cells (18). In the mouse cell line CRL 2055 we have demonstrated the presence of these three molecules, providing molecular evidence for the existence of the amylin receptor in ß-cells. Amylin is produced by the ß-cells of the pancreas and also influences insulin secretion (23, 24, 25), indicating that a receptor for this peptide must exist in the islets. In consequence, the action of amylin in the pancreas may be mediated through an autocrine mechanism. Nevertheless, physiological experiments must be conducted to ascertain this. The lack of calcitonin receptor, RAMP1, and RAMP2 signal in the cell line CRL 1777 may be due to the absence of these molecules from the hamster ß-cells or to a low homology of the hamsters genes with the human sequences used to generate the probes.
The presence of the receptors for the three components of this peptide
family: AM (L1, RDC1, and CRLR plus RAMP3), CGRP (RDC1 and CRLR plus
RAMP1), and amylin (calcitonin receptor plus RAMP3) in the same cell
type suggests the existence of a redundant mechanism for the reception
of the three peptide signals. This seems to be the rule rather than the
exception in ß-cells, which are known for the implementation of
several similar fail-safe mechanisms (26). Although L1 and CRLR in the
presence of RAMP2 or RAMP3 are able to bind AM at low concentrations
(
10-9 M), RDC1 has a lower affinity
(
10-7 M). The expression of all of these
molecules in the same cell may implicate a variety of responses
depending on the local concentration of AM. On the other hand, RDC1 and
CRLR coexpressed with RAMP1 are excellent CGRP1 receptors. CGRP has
been shown to inhibit insulin secretion as well and to be present in
the somatostatin-producing cells (22, 23).
The observation of two populations of ß-cells, the one toward the
periphery with higher immunoreactivity for AM/CGRP receptors and the
other closer to the center with lower positivity, may relate to the
expression patterns for AM and CGRP. AM is produced by F cells, which
are always in the periphery of the islets and also express pancreatic
polypeptide (6), whereas CGRP is released from
-cells, which are
also at the periphery (22, 23). If we assume a paracrine secretion for
these hormones, it makes sense that the ß-cells closer to the source
of AM and CGRP should contain a higher concentration of the
receptors.
Another interesting finding is the expression of RDC1, but not the other receptors, in the smooth muscle cells of the pancreatic vessels. It has been repeatedly reported that the vasodilatory action of AM is mediated through a CGRP1 receptor, as it can be inhibited by CGRP-(837), a specific peptide antagonist of such pharmacology (27, 28). Our results clearly agree with these reports and suggest that RDC1 may be the only AM receptor present in the vascular system, at least in the pancreas. This is in agreement with a recent report that found RDC1 to be the only AM receptor present in rat aortic vascular smooth muscle cells (29).
Surprisingly, none of the receptors studied was expressed by the exocrine acini, even though Tsuchida et al. (10) showed an inhibitory role for AM in the secretion of amylase. These observations might suggest the existence of a different AM receptor in the exocrine cells of the pancreas.
Many factors have been shown to modulate AM expression in different cell types (30). In bovine pancreas it has been demonstrated that a mild infection and/or lipopolysaccharide injection results in an impressive overexpression of inducible nitric oxide synthase and AM, which, in turn, reduces circulating insulin and elevates glucose (31). Therefore, any compound or physiological state that modulates AM expression may have a direct impact on insulin release and glucose homeostasis. In fact, it has been recently suggested that AM may be responsible for the onset of type 2 diabetes in some individuals (32).
In summary, the three AM receptors are restricted to ß-cells of the islets, constituting a complex system designed to regulate insulin secretion. Unraveling this physiological puzzle may contribute to a better understanding of the normal function of the endocrine pancreas as well as the diseases produced by dysregulation of insulin secretion.
| Acknowledgments |
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| Footnotes |
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Received August 11, 1999.
| References |
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