Institut für Pharmakologie, Universitätsklinikum Essen, D-45122 Essen, Germany
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ABSTRACT |
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Somatostatin (SST) and somatostatin
receptors (SSTR) are widely distributed in lymphoid tissues. Here, we
report on the stimulatory effects of SST in Epstein-Barr
virus-immortalized B lymphoblasts. By RT-PCR, we demonstrated the
exclusive expression of the somatostatin receptor isoform 2A (SSTR2A)
in B lymphoblasts. Addition of SST rapidly increased the cytosolic free
calcium concentration [Ca2+]i maximally by
about 200 nM, with an EC50 of 1.3 nM, and stimulated the
formation of inositol phosphates. Furthermore, SST increased binding of
guanosine 5'-O-(3-thiotriphosphate) by 50% above basal. These effects were partly inhibited by pertussis toxin (PTX), which
indicates the involvement of PTX-sensitive G proteins. We provide
further evidence that G16, a PTX-insensitive G protein confined to lymphohematopoietic cells, is involved in the otherwise unusual coupling of SSTR2A to phospholipase C activation. In addition, SST activated extracellular regulated kinases and induced a 3.5-fold stimulation of DNA synthesis and a 4.4-fold stimulation of B
lymphoblast proliferation, which was accompanied by an enhanced
immunoglobulin formation. Thus SST exerts a growth factor-like activity
on human B lymphoblasts.
G protein; immunoglobulin formation; MAP kinase; pertussis toxin; phospholipase C
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INTRODUCTION |
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B LYMPHOCYTES ARE THE PRINCIPAL mediators of adaptive humoral immunity. The differentiation and selection processes from quiescent B lymphocytes to antibody-secreting plasma cells and memory cells are governed by specific antigen receptors and a complex network of modulating signals in a specialized lymphoid environment (10). These modulating signals are generated by selective cytokines, B cell coreceptors, and cell adhesion molecules confined to the lymphoid system. In addition, hormones and neurotransmitters that regulate many different physiological systems are also involved in B cell control (9). Such agonists include platelet-activating factor (PAF; Ref. 28), lysophosphatidic acid (LPA; Ref. 44), vasoactive intestinal polypeptide (VIP; Refs. 17 and 18), and catecholamines (26), to name but a few.
Somatostatin (SST), a cyclic tetradecapeptide, was first described as a potent inhibitor of growth hormone secretion (39). Subsequent studies have shown that SST has widespread physiological functions in hormone release, regulation of exocrine secretion, modulation of neural activity, and the inhibition of tumor growth (reviewed in Refs. 34, 39, and 46).
SST binding sites were first detected on circulating human blood lymphocytes (5) but also on many lymphoid tissues (42, 43). Somatostatin-receptor (SSTR)-based imaging has been widely used for the diagnosis of malignant lymphomas and hyperplastic or granulomatous nonmalignant lymph nodes (31, 42, 43).
Despite this widespread distribution in lymphoid tissues, only limited
information on the physiological roles of SST and SSTRs in the immune
system exists (reviewed in Refs. 53 and 54). Several groups of investigators have used lymphocytes to study SST
effects on intracellular effector systems, including adenylyl cyclase
and the Na+/H+ exchanger (23, 34).
Both antiproliferative and growth-promoting activities of SST have been
reported (27, 36, 37). More recent reports have shown that
SST regulates T cell interferon- and interleukin (IL)-2 release
(7, 11). However, for the B cell system, such functional
data on the role of SST and its receptors are lacking.
Five different human SST receptor subtypes (SSTR1-SSTR5) have been cloned (16, 33, 34, 46, 58), and splice variants of SSTR2 exist (35). All SSTRs belong to the family of G protein-coupled or heptahelical receptors but differ with respect to their tissue-specific distribution and pharmacological properties (16, 33).
The predominantly inhibitory cellular actions of SST are mediated by multiple effector pathways (23, 33, 34), including the inhibition of adenylyl cyclase (19) and voltage-dependent Ca2+ channels (25), a reduced mobilization of intracellular Ca2+ (19), and the attenuation of Na+/H+ exchange activity. Furthermore, SST stimulates voltage-dependent K+ channels (59) and protein tyrosine phosphatases (6), the activation of which coincides with an antiproliferative effect via proapoptotic pathways (34). SST has also been reported to activate phospholipase A2 and the MAP kinase cascade (34). In some cell lines, predominantly on overexpression of SSTRs, SST stimulates phospholipase C (PLC) activity (2, 8, 29, 47, 51).
Most effects of SST are transmitted via pertussis toxin
(PTX-)-sensitive G proteins, although partially PTX-insensitive effects including activation of PLC have been reported (8, 48,
51). B lymphoblasts express the PTX-sensitive G proteins
Gi2 and G
i3 but not G
i1
and G
o (44). G proteins of the
G
q family are predominantly involved in activation of
PLC
isoforms (40), and G
q and
G
11 are expressed in B lymphoblasts. Highly restricted to lymphohematopoietic cells, an additional G
q class
protein exists, G
16 (and its murine homolog
G
15; Refs. 3 and 57), which is
able to link "promiscuously" numerous heptahelical receptors to PLC
activation, a peculiarity that is not observed with other G
q class proteins (32, 57).
Here, we investigated the effects of SST on proliferation and
immunoglobulin formation of human B lymphoblasts, and we characterized the early signal transduction of SST in these cells. We report the
novel observation that SST stimulates PLC activity and increases [Ca2+]i in B lymphoblasts, events that most
likely involve the coupling of SSTR2A to G16 and
PLC
2. These stimulatory effects of SST on early signal transduction
are accompanied by an increased cell proliferation and immunoglobulin formation.
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MATERIALS AND METHODS |
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Materials.
Fura 2-AM was purchased from Molecular Probes (Eugene, OR).
[3H]-methylthymidine was bought from Hartmann Analytics
(Braunschweig, Germany). [35S]guanosine
5'-O-(3-thiotriphosphate
([35S]GTPS) (specific activity
1,200-1,400 Ci/mmol) was from DuPont-NEN (Bad Homburg, Germany),
and myo-[3H]-inositol was from American Radiolabed
Chemicals (St. Louis, MO). Somatostatin (SST-14) and PAF were purchased
from Calbiochem (Bad Soden, Germany), and
12-O-tetradecanoylphorbol 13-acetate (TPA) was from Sigma
(Deisenhofen, Germany). Primary and secondary antibodies for
immunoglobulin ELISA were purchased from Tago Immunologicals (Burlingame, CA). Unlabeled nucleotides were from Boehringer Mannheim, and PTX was from List Biological Laboratories (Campbell, CA). The
phosphospecific p42/44 MAP kinase antibody was from New England Biolabs
(Schwalbach, Germany), and the polyclonal anti-ERK1/ERK2 antiserum was
from Santa Cruz Biotechnology (Heidelberg, Germany). Reverse
transcriptase (RT; Superscript) was from Life Technologies (Eggenstein,
Germany). Taq polymerase, RNasin, RNase-free DNase, and
restriction enzymes were purchased from Fermentas (St. Leon-Rot, Germany), and Pfu-Taq polymerase was from Promega
(Heidelberg, Germany).
B lymphoblast cell lines and cell culture. Human B lymphoblast cell lines were derived from peripheral blood lymphocytes and immortalized with Epstein-Barr virus as described (44, 48). They were cultured in RPMI 1640 medium containing 2 mM L-glutamine, which was supplemented with 100 U/ml penicillin, 100 µg/ml streptomycin, and 10% FCS (all from Life Technologies). If not indicated otherwise, cells were routinely subcultured into fresh RPMI 1640 medium with 10% FCS at a density of 106 cells/ml 1 day before the experiment.
Detection of SSTRs by reverse transcription-polymerase chain reaction (RT-PCR). Lymphoblast RNA was prepared and reverse transcribed using Superscript reverse transcriptase as described (44). Before reverse transcription, RNA was treated with RNase-free DNase according to the manufacturer's instructions. PCR primer sequences were taken from the report by Kubota et al. (22). According to the published sequence for human SSTR2 (58), the reverse primer sequence was modified to 5'-TCACCATGATCTGTCTTTGC-3'. PCR amplicons flanked by these primers contained at least one single restriction site, and the specificity of the synthesized PCR products was confirmed by restriction analysis. To discriminate the splice variant SSTR2A from SSTR2B, RT-PCRs were repeated using the oligonucleotide primers 5'-CTTCCGTCTCCATGGCCATCAGC-3' and 5'-GGTAATGCCTATACAGAGAATAAATA GG-3'. The specific amplicons generated with these primers comprise 686 base pair (bp) for SSTR2A transcripts and for genomic DNA. In SSTR2B, a fragment of 341 bp is removed by alternative splicing, resulting in an amplicon of only 345 bp flanked by these oligonucleotide primers (38). PCRs were carried out in reaction volumes of 50 µl containing 200 µM of each dNTP, 10 pmol of each oligonucleotide primer, cDNA corresponding to 100 ng RNA, and 2.5 U Taq polymerase in reaction buffer supplied by the manufacturer. The amplification profile involved denaturation at 94°C (30 s), annealing at 55°C (45 s), and extension at 72°C (90 s) for 35 cycles. A negative control containing 100 ng of RNA instead of cDNA was included in each experiment. Because all SSTRs lack introns, genomic DNA was used as positive control. PCR products were analyzed by electrophoresis on 1.5% agarose gels and stained with ethidium bromide. Primer sequences were derived from the following GenBank entries: M81829 (SSTR1), M81830 (SSTR2A), L13033 (SSTR2B), AF184174 (SSTR2B), XM_009963 (SSTR3), NT_011387 (SSTR4), and NT_010552 (SSTR5).
Measurement of cytoplasmic free
Ca2+.
Changes in intracellular free Ca2+ concentration
([Ca2+]i) were measured, as described in
detail (44, 48), using the calcium-sensitive fluorescent
dye fura 2. One day before the experiment, B lymphoblasts were
subcultured at a density of 2-5 × 105 cells/ml
in RPMI 1640 medium containing antibiotics and 0.5% FCS. Dye loading,
measurements of fluorescence, and calibration of emission ratios
( = 495 nm) after rapid alternating excitation wavelengths from
= 340 to 380 nm were performed, exactly as described, in a
Perkin-Elmer LS5B spectrofluorimeter (44, 48). For each
experiment, ~1 × 107 dye-loaded cells were measured
at 37°C in HEPES-buffer containing 1 mM CaCl2 (44,
48). For experiments in Ca2+-free medium, EGTA (5 mM
final) was added ~5 s before addition of agonist.
Analysis of inositol phosphate formation. Formation of inositol phosphate (IP) was quantified in lymphoblasts grown at a density of 2 × 106/ml and labeled with 5 µCi/ml myo-[3H]inositol in serum- and inositol-free RPMI 1640 medium as described (44, 48). Lymphoblasts were stimulated, and water-soluble IP were fractionated by anion-exchange chromatography on AG 1-X8 formate resin (Bio-Rad, Munich, Germany) as detailed previously (44).
GTPS binding assay.
Agonist-induced activation of G proteins was determined by measurement
of [35S]GTP
S binding to digitonin-permeabilized
lymphoblasts as described (44, 48, 56). Single
measurements were conducted on 1 × 106 lymphoblasts
for 10 min at 30°C. Nonspecific binding was defined as the fraction
of bound [35S]GTP
S not competed for by 10 µM
unlabeled GTP
S. Measurements were carried out in triplicate.
Cloning, sequencing, and expression of G subunits and SSTR2 in
COS-7 cells.
The cDNAs encompassing the coding sequences of the human
G
q, G
11, G
16, and SSTR2
were cloned by RT-PCR from B lymphoblast RNA using the oligonucleotide
primers 5'-CCACCGCACCATGGCCCGCTCG-3' and 5'-TGGGGCCTGGGTCACAGCAGG-3 for
the amplification of G
16 (accession no. NM_002068),
5'-GGAAGAATGACTCTGGAGTCCATCATGGCG-3' and
5'-CAGGCACAATTAGACCAGATTGTACTCC-3' for G
q (accession
no. NM_002072), 5'- GGCCGGGACGATGACTCTGG-3' and
5'-GCGAAGTGTACGGAGGGAGAGATG-3' for G
11 (accession
no. AF011497), 5'-GTCTTTTCTTTCCACACCCCTGTG-3' and
5'-GAAGCACTTGCAAATAAAACAAGG AG-3' for G
14 (accession no.
XM_005478), and the oligonucleotides 5'-AAAGCAGCCATGGACATGGCGG-3' and
5'-CCCCAAGCAGTTCAGATACTGG-3' for the amplification of SSTR2
(accession no. M81830), respectively. Reverse transcription was
performed as described above. For PCR amplification, Pfu-Taq
(Promega) was used according to the manufacturer's instructions. PCR
products were cloned into pGEM-T easy vector (Promega), sequenced, and
further subcloned into the mammalian expression vector
pcDNA3.1+ (Invitrogen). Human brain RNA was used for the
amplification of G
14. The PLC
2 expression vector was
a kind gift of Dr. M. Simon (Pasadena, CA).
Measurement of MAP kinase activity. B lymphoblasts (~1 × 107) were incubated in the presence or absence of SST (100 nM) for 2 min at 37°C, rapidly spun down, and lysed exactly as described (44). MAP kinase activity was determined by immunoblotting with an antibody that recognizes the phosphorylated, and thereby activated, MAP kinase isoforms p44/42, i.e., ERK1/ERK2, as described (44). Aliquots of these lysates were analyzed in a second control Western blot for equal expression and handling of ERK1/ERK2 proteins during preparation with an anti-ERK1/ERK2 antiserum.
Determination of cell proliferation, thymidine incorporation, and immunoglobulin synthesis. Cells were seeded at an initial density of 2 × 105/ml in serum-free RPMI 1640 medium, stimulated with SST, and propagated for 4 days (44). Cells were counted daily using a CASY cell analyzer system (Schärfe, Reutlingen, Germany). For determination of DNA synthesis, B lymphoblasts were subcultured in 24-well dishes under identical experimental conditions as described above for analysis of cell proliferation. They were propagated for two days and prepulsed with 1 µCi of [3H]methylthymidine for 18 h. The incorporation of radioactivity was determined exactly as described (44). Supernatants of SST-stimulated and control cells from the proliferation assay described above were harvested, and immunoglobulin concentrations were quantified by ELISA as described (44).
Statistical analysis and presentation of data. Data were analyzed using two-tailed Student's t-tests and regarded significantly different at P < 0.05. If not indicated otherwise, all experiments were performed in triplicate using at least two different cell lines. Data represent means ± SE if not indicated otherwise.
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RESULTS |
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Expression of SSTR type 2A transcripts in B lymphoblasts.
In the first series of experiments, we examined the expression of the
various SSTR isoforms in human B lymphoblasts by RT-PCR using
established oligonucleotide primers (22), which do not flank intron sequences. By using human genomic DNA as control, all five
SSTRs were amplified (Fig.
1A), and the specificity of the PCR fragments was confirmed by restriction analysis (not shown). However, in RT-PCR studies, we only detected mRNA transcripts encoding
for the isoform SSTR2 (Fig. 1A). To exclude fortuitous amplification of potentially contaminating genomic DNA in RT-PCRs, we
included controls of the RNA without reverse transcription. Furthermore, all RNA specimens were treated with RNase-free DNase before reverse transcription. No PCR amplicons were detected in these
samples.
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Effects of SST on
[Ca2+]i, IP formation, and
binding of GTPS.
Having demonstrated the expression of SSTR2A-specific transcripts in B
lymphoblasts, we examined the effects of agonist stimulation on
different effector systems. In the presence of 1 mM extracellular Ca2+, basal [Ca2+]i amounted to
111 ± 13 nM (n = 31). On addition of 100 nM SST, [Ca2+]i increased by 191 ± 26 nM above
baseline values (Fig. 2A). SST-evoked Ca2+
signals consisted of an initial peak followed by a sustained plateau
(Fig. 3A). When extracellular
Ca2+ was chelated by addition of EGTA (5 mM) before
stimulation with the agonist, SST-induced Ca2+ increases
were markedly reduced and amounted to 45 ± 12 nM above basal
(Fig. 3B). Thus SST evoked both Ca2+ influx and
mobilization from intracellular stores. The increases in
[Ca2+]i were concentration dependent in the
range of 10
10 to 10
6 M SST (Fig.
3C), and the EC50 for SST-induced
Ca2+ signals was 1.3 ± 0.9 nM. Maximum changes in
[Ca2+]i were observed at 10
7 M
SST. Because SSTRs couple to PTX-sensitive Gi and
Go proteins, we examined the effects of PTX (50 ng/ml;
16 h) on SST-induced [Ca2+]i transients.
PTX treatment resulted in a distinct inhibition of SST-stimulated
Ca2+ signals by 71 ± 12%. (Figs. 2A and
3A), which indicates that transmembrane signaling of SSTR2A
in human B lymphoblasts involves PTX-sensitive G proteins.
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Expression of transcripts encoding G subunits of the
G
q family and reconstitution of SSTR2A-mediated IP
generation.
Our data presented so far suggest that both PTX-sensitive and
PTX-insensitive G proteins are involved in SST-stimulated IP generation, which is in accordance with results of other groups (2, 8, 47). The PTX-sensitive fraction of SST-stimulated IP formation is attributable most likely to the well-known activation of G
i proteins by SSTRs (23, 34). This
results in the liberation of G
subunits, which in turn activate
phospholipase C
(PLC
) isoforms (40). However,
because G
i proteins are expressed ubiquitously, addition
of SST should result in IP generation in all cells expressing SSTR2A,
which is obviously not the case. Therefore, we wondered which
components of the signaling cascade in lymphoblasts contribute to their
peculiar behavior. Interestingly, the expression patterns of
G
16 and PLC
2, two proteins involved in IP generation,
are restricted to lymphohematopoietic cells (3, 40).
Studies with other G protein-coupled receptors expressed in blood cells suggested that a basal activation of a G
q family
protein, most likely G
16, is required for activation of
PLC
2, which is then further potentiated by free G
subunits
(4). Furthermore, G
16 is able to couple
nonspecifically to a wide variety of different heptahelical receptors
(32). By RT-PCR, we detected G
16-specific transcripts together with transcripts encoding the widely expressed G
q family members G
q and
G
11 in human B lymphoblasts (Fig. 5,
A-C). The
identities of these transcripts were verified by cloning and
sequencing. Transcripts of the fourth member of this G
q
family, G
14, were not detectable in B lymphoblasts but
were present in RNA from human brain (Fig. 5D).
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MAP kinase activation, DNA synthesis, cell proliferation, and
synthesis of immunoglobulins.
The experiments presented so far provided evidence for stimulatory
actions of SST in B lymphoblasts, as shown for IP formation and G
protein activation. Next, we studied whether SST affects typical
cellular functions of B lymphoblasts, i.e., proliferation and
immunoglobulin synthesis. Furthermore, we investigated whether SST
stimulates additional signaling pathways in these cells and examined
the effects of SST on MAP kinase activation. Available evidence
suggests that MAP kinase activation by heptahelical receptors that
couple to PTX-sensitive G proteins involves a complex array of adaptor
proteins, GTPases, and kinases (for review, see Ref. 45).
Liberated G subunits from activated PTX-sensitive G proteins are
thought to initiate this pathway (45). As shown in Fig. 7, SST (100 nM; 2 min) induced an
increase in phospho-ERK1/2, which predominantly affected
p42ERK2. This activation was stronger than that induced by
PAF (100 nM; 2 min), another well-characterized B lymphocyte activator
(28). SST-induced MAP kinase activation varied between
three- and sevenfold compared with the nonstimulated controls, as
determined by image analysis. Treatment of B lymphoblasts with PTX (50 ng/ml; 16 h) completely blocked SST-induced MAP kinase activation
(not shown).
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DISCUSSION |
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SST is a widely expressed neuropeptide that affects numerous organ systems and cell types in a prevailingly inhibitory fashion. Increasing evidence, predominantly from binding, nuclear imaging, and expression studies, suggests that SST also plays a role in the immune system (53, 54). On the basis of observations that SST can evoke Ca2+ signals in human B lymphoblasts (48), we investigated this signaling pathway in more detail. Specifically, we tried to unravel which components of the B lymphoblast signaling network are responsible for this unusual effector activation by SST. A second and independent focus of this work was to elucidate the significance of SST for B lymphoblast function, and we examined two typical B cell responses, proliferation and immunoglobulin formation. Our results suggest that SST acts as a B cell growth factor. Both issues, early effector activation and cell proliferation, deserve a more detailed discussion. Finally, we have to address the question of potential physiological roles for SST in B cell regulation, an issue which will remain mostly speculative.
SSTR expression and early signaling.
The only SSTR isoform we detected in B lymphoblasts is SSTR2A, which is
the predominant SSTR in nonneural tissues. Although this notion is
drawn from RT-PCR studies, it is in accord with other expression,
binding, and imaging studies (52, 53). SSTR2A is a typical
heptahelical receptor that couples to heterotrimeric G proteins. This
is also true for B lymphoblasts, as shown here by an increased GTPS
binding on SST stimulation. Direct biochemical interaction studies
provided evidence for coupling of G
i3 and G
o to SSTR2 (24). Because B lymphoblasts
lack G
o (44), which is predominantly
expressed in neural tissues, SSTR2 most likely couples to
G
i subunits in this cell type. This notion is further corroborated by the inhibitory effects of PTX on SST-stimulated GTP
S
binding, IP formation, increases in [Ca2+]i,
and MAP kinase activation. Of note, these effects of PTX varied considerably between different effector systems. Whereas SST-stimulated MAP kinase activation and GTP
S binding were totally blocked and Ca2+ signals were inhibited by ~70%, PTX diminished
SST-induced lymphoblast proliferation by only 20%. Because
PTX-sensitive G proteins are most abundantly expressed in mammalian
cells, and because G
i proteins have a considerable rate
of spontaneous GDP/GTP exchanges, a concomitant activation of
PTX-insensitive G proteins most likely escapes gross binding studies as
performed here by GTP
S binding to whole cells, a method which has
even been optimized for the analysis of PTX-sensitive G proteins
(56). Thus our results from GTP
S binding analyses do
not exclude a simultaneous activation of, for example,
G
q class G proteins, which is in accord with the partial
PTX-insensitivity of SST-stimulated Ca2+ signals observed
here. At this point, we have to consider further that PTX is a highly
sensitive but not an entirely specific tool for the analysis of
G
i-dependent signaling pathways, especially if immune
cells are concerned. There is evidence that long-term treatment with
PTX stimulates proliferation of T and B lymphocytes (20),
an effect that has been attributed to the B oligomer of the toxin and
does not involve ADP ribosylation (20). Although we have
verified in previous studies that our PTX treatment conditions result
in a complete modification of G
i proteins in B
lymphoblasts (48), we also observed that long-term
treatment with the toxin inconsistently evoked minor increases in cell
number (44), which could also explain the limited effect
of PTX on SST-stimulated B lymphoblast proliferation shown here.
SSTR2A-mediated Ca2+ signals in B
lymphoblasts.
Although there is evidence from one tumor cell line that SSTR2 can
activate Ca2+ channels (50), in most cell
types SSTR2 stimulation is associated with decreased Ca2+
signals caused by an inhibition of Ca2+ channels or an
activation of K+ channels (25, 33, 34, 59).
The original tracings shown in Fig. 3 suggest the contribution of both
Ca2+ influx and Ca2+ liberation from
intracellular stores to the generation of Ca2+ signals in
SST-stimulated B cells. Concomitantly, we observed an activation of PLC
by SST. To our knowledge, the human B lymphoblast is the only
nontransfected cell type for which a direct activation of PLC by SSTR2
stimulation has been observed so far. Hence, we wondered which
mechanisms confined to B lymphoblasts contribute to this unusual PLC
activation. Lymphohematopoietic cells specifically express PLC2 and
G
16, a G
q class G protein (3,
40). G
16 can couple promiscuously to a wide
variety of heptahelical receptors, including the 5-HT1A
receptor, the formyl peptide receptor, the µ-opiod receptor, the
2-adrenergic receptor, the M2-muscarinic acetylcholine receptor, the LTB4 receptor, and the P2U
purinoreceptor (4, 14, 32). Baltensperger and Porzig
(4) provided evidence that heptahelical receptors that
couple to PTX-sensitive G proteins in lymphohematopoietic cells require
a basal activation of a G
q class G protein, e.g.,
G
16, for efficient activation of PLC. Free G
subunits arising from concomitant G
i activation then further potentiate PLC activity in a PTX-sensitive manner
(4). However, sole free G
dimers were unable to
mediate PLC activation in the absence of activatable G
16
in this system (4). Interestingly, G
16
activation has also been related to cell growth stimulation of
hematopoietic cells by heptahelical receptors (15).
Somatostatin-induced B lymphoblast proliferation. The second novel observation of this report is an augmented B lymphoblast proliferation on stimulation with SST, which was further accompanied by an increase in immunoglobulin secretion.
In most cell types, mainly antiproliferative effects of SST have been observed (6, 33, 39). For the immune system, early studies reported growth-inhibiting effects of SST in activated T lymphocytes (27, 37). In B lymphocytes, biphasic effects of SST have been described: SST at 10Somatostatin and the immune system. Immortalized B lymphoblasts are a frequently employed model system for differentiated antibody-secreting B cells (28, 44). However, some caution should be taken when extrapolating data derived from such cell lines. Based primarily on these in vitro studies, it is only possible to speculate on the precise physiological roles of SST for B cells in the immune system.
SSTRs are widely expressed in lymphoid tissues (31, 42, 43), and both peptidergic nerves and autocrine SST secretion are potential sources for SST in the immune system (1, 12, 13, 49). High levels of SST binding sites are found in germinal centers of Peyer's patches, tonsils, appendix, colonic lymphoid tissue, and on thymocytes (41, 49). In quiescent blood lymphocytes, mRNA encoding for SSTRs is found at low levels only (52). On lymphoblastic differentiation and proliferation, beginning at early differentiation stages, B and T cells start to express SSTRs, with SSTR2 being the predominant SSTR isoform (52, 53), which is also confirmed in our study for B lymphoblasts. SSTR2 transcripts are also found in plasma cells (53). Messenger RNAs for SSTR3, SSTR4, and SSTR5 have been detected in a few lymphocyte cell lines, whereas SSTR1 mRNA was undetectable (7, 52, 53). There is evidence for autocrine SST secretion in lymphoid tissues, including thymus and spleen (1, 12, 13, 49). Lipopolysaccharide, interferon- ![]() |
ACKNOWLEDGEMENTS |
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We thank Dr. K. H. Jakobs for valuable comments on the manuscript.
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FOOTNOTES |
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This study was supported by the Deutsche Forschungsgemeinschaft.
Address for reprint requests and other correspondence: D. Rosskopf, Institut für Pharmakologie, Universitätsklinikum Essen, Hufelandstr. 55, D-45122 Essen, Germany (E-mail: dieter.rosskopf{at}uni-essen.de).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
First published September 18, 2002;10.1152/ajpcell.00160.2001
Received 28 March 2001; accepted in final form 11 September 2002.
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