Secretion-stimulating and secretion-inhibiting hormones stimulate high-affinity pertussis-toxin-sensitive GTPases in membranes of a pituitary cell line

Function and regulation of somatostatin receptor subtypes

The five known somatostatin receptors serve unique biological roles by virtue of their tissue-specific expression and particular biochemical properties. However, the function of any individual receptor in its normal physiological milieu is not understood. Studies to address this problem have been difficult because tissues and cell lines often express multiple somatostatin receptors and, in the absence of receptor-selective somatostatin analogues, the actions of individual receptors cannot be identified. Moreover, the biological and biochemical actions of somatostatin receptors depend on their cellular environment, so that the behaviour of a receptor expressed in heterologous cells does not necessarily mimic that of endogenous receptors. We have developed two approaches to examine somatostatin receptors which circumvent these problems. Using a biotinylated somatostatin analogue for affinity purification, we isolated somatostatin receptors together with associated G proteins. Subsequent analysis of the purified complex with G protein-specific antibodies showed that the somatostatin receptors in AR42J cells preferentially couple with two pertussis toxin-sensitive G proteins: Gi alpha 1 and Gi alpha 3. To examine individual receptor types, we developed receptor-specific antibodies and used them to show that both sstr1 and sstr2 proteins were present in the GH4C1 pituitary cell line whereas AR42J cells contained sstr2 but not sstr1. Immunoprecipitation of receptor-G protein complexes with GH4C1 cells showed that sstr1 and sstr2 are both coupled to pertussis toxin-sensitive G proteins, in contrast to the results observed when these receptors are overexpressed in some non-endocrine cells. We also showed that the somatostatin receptors in GH4C1 cells are subject to both homologous and heterologous hormonal regulation. The mechanisms involved in the regulation of different receptor types are now being characterized using the receptor-specific antibodies to isolate the individual receptor proteins. Elucidating signal transduction by endogenous somatostatin receptors as well as their hormonal regulation will be critical for understanding the functions of these receptors in the different physiological targets of somatostatin.

Reduction in cholesterol and sialic acid content protects cells from the toxic effects of beta-amyloid peptides

beta-Amyloid (Abeta) is the primary protein component of senile plaques associated with Alzheimer's disease and has been implicated in the neurotoxicity associated with the disease. A variety of evidence points to the importance of Abeta-membrane interactions in the mechanism of Abeta neurotoxicity and indicates that cholesterol and gangliosides are particularly important for Abeta aggregation and binding to membranes. We investigated the effects of cholesterol and sialic acid depletion on Abeta-induced GTPase activity in cells, a step implicated in the mechanism of Abeta toxicity, and Abeta-induced cell toxicity. Cholesterol reduction and depletion of membrane-associated sialic acid residues both significantly reduced the Abeta-induced GTPase activity. In addition, cholesterol and membrane-associated sialic acid residue depletion or inhibition of cholesterol and ganglioside synthesis protected PC12 cells from Abeta-induced toxicity. These results indicate the importance of Abeta-membrane interactions in the mechanism of Abeta toxicity. In addition, these results suggest that control of cellular cholesterol and/or ganglioside content may prove useful in the prevention or treatment of Alzheimer's disease.

The role of G protein activation in the toxicity of amyloidogenic Abeta-(1-40), Abeta-(25-35), and bovine calcitonin

More than 16 different proteins have been identified as amyloid in clinical diseases; among these, beta-amyloid (Abeta) of Alzheimer's disease is the best characterized. In the present study, we performed experiments with Abeta and calcitonin, another amyloid-forming peptide, to examine the role of G protein activation in amyloid toxicity. We demonstrated that the peptides, when prepared under conditions that promoted beta-sheet and amyloid fibril (or protofibril) formation, increased high affinity GTPase activity, but the nonamyloidogenic peptides had no discernible effects on GTP hydrolysis. These increases in GTPase activity were correlated to toxicity. In addition, G protein inhibitors significantly reduced the toxic effects of the amyloidogenic Abeta and calcitonin peptides. Our results further indicated that the amyloidogenic peptides significantly increased GTPase activity of purified Galpha(o) and Galpha(i) subunits and that the effect was not receptor-mediated. Collectively, these results imply that the amyloidogenic structure, regardless of the actual peptide or protein sequence, may be sufficient to cause toxicity and that toxicity is mediated, at least partially, through G protein activation. Our abilities to manipulate G protein activity may lead to novel treatments for Alzheimer's disease and the other amyloidoses.

Expression analysis of the thyrotropin-releasing hormone receptor (TRHR) in the immune system using agonist anti-TRHR monoclonal antibodies

Monoclonal anti-rat thyrotropin-releasing hormone (TRH) receptor (TRHR)-specific antibodies (mAb) were generated by immunization with synthetic peptides of rat TRHR partial amino acid sequences; one (TRHR01) was directed against a sequence (84-98) in the extracellular portion of the rat TRHR reported to be constant among different species, including man, and the second (TRHR02) recognizes the C-terminal region sequence 399-412. In lysates from GH4C1 cells, a clonal rat pituitary cell line, both mAb recognize the TRHR in Western blot analysis, and TRHR02 immunoprecipitates the TRHR. Incubation of GH4C1 cells with the mAb causes a fluorescence shift in fluorescence-activated cell sorting analysis. The cells were stained specifically by both mAb using immunocytochemical techniques. Furthermore, TRHR01 is agonistic in its ability to trigger Ca2+ flux, and desensitizes the TRH receptor. We tested for TRHR in several rat organs and found expression in lymphoid tissues. TRHR01 recognizes the human TRHR, and analysis of human peripheral blood lymphocyte and tonsil-derived leukocyte populations showed receptor expression in non-activated and phytohemagglutinin-activated T and B cells.

Gi2 and protein kinase C are required for thyrotropin-releasing hormone-induced stimulation of voltage-dependent Ca2+ channels in rat pituitary GH3 cells

In rat pituitary GH3 cells, thyrotropin-releasing hormone (TRH) and other secretion-stimulating hormones trigger an increase in the cytosolic Ca2+ concentration by two mechanisms. Ca2+ is released from intracellular stores in response to inositol 1,4,5-trisphosphate and can enter the cell through voltage-dependent L-type Ca2+ channels. Stimulation of these channels is sensitive to pertussis toxin, indicating that a pertussis toxin-sensitive heterotrimeric guanine nucleotide-binding regulatory protein (G protein) is involved in functional coupling of the receptor to the Ca2+ channel. We identified the G protein involved in the stimulatory effect of TRH on the Ca2+ channel by type-selective suppression of G-protein synthesis. Antisense oligonucleotides were microinjected into GH3 cell nuclei, and 48 h after injection the TRH effect was tested. Whereas antisense oligonucleotides hybridizing to the mRNA of G(o) or Gi1 alpha-subunit sequences did not affect stimulation by TRH, oligonucleotides suppressing the expression of the Gi2 alpha subunit abolished this effect, and oligonucleotides directed against the mRNA of the Gi3 alpha subunit had less effect. The requirement of a concurrent inositol phospholipid degradation and subsequent protein kinase C (PKC) activation for the TRH effect on Ca(2+)-channel activity was demonstrated by inhibitory effects of antisense oligonucleotides directed against Gq/G11/Gz alpha-subunit sequences and treatment of GH3 cells with PKC inhibitors, respectively. Our results suggest that TRH elevates the cytosolic Ca2+ concentration in GH3 cells transiently via Ca2+ release from internal stores, followed by a phase of sustained Ca2+ influx through voltage-dependent Ca2+ channels stimulated by the concerted action of Gi2 (and Gi3) plus PKC.

Calcitriol attenuates the thyrotropin-releasing hormone-stimulated inositol phosphate production in clonal rat pituitary (GH4C1) cells

Three days pretreatment of the prolactin (PRL) secreting GH4C1 cells with 10 nM calcitriol attenuated both the basal and thyrotropin-releasing hormone (TRH)-stimulated (1 microM, 5 s) inositol trisphosphate (IP3) production by 30 and 26%, respectively. The effect was detectable at 10 nM (basal) and 1 pM (TRH-stimulated), and maximal at 1 microM (basal) and 10 nM (TRH), respectively. Calcitriol was at least 100 times more potent than calcidiol and 24-hydroxycalcidiol, and the effect was reversible upon cessation of pretreatment. Calcitriol pretreatment (1 microM, 5 days) also decreased the levels of phosphatidyl-inositol, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate by 23, 55 and 32%, respectively. GTP gamma S-stimulated (100 microM, 30 s) IP3 production was decreased by 45% after calcitriol pretreatment (10 nM, 5 days). Pertussis toxin (1 nM, 4 h) attenuated both the basal and TRH-stimulated IP3 production, but this effect was omitted by calcitriol pretreatment. Thus, calcitriol specifically attenuates both the basal and TRH-stimulated inositol phosphate production in GH4C1 cells. The mechanism, at least partly, involves decreased availability of phosphoinositides for phospholipase C. Calcitriol regulation of a pertussis toxin-sensitive G-protein might also play some role.

Neuropeptides and inflammation. A somatostatin analog as a selective antagonist of neutrophil activation by substance P

OBJECTIVE

Substance P and somatostatin are neuropeptides found in peripheral sensory nerves. In vitro, these have opposing effects on inflammatory cells. We compared the effects of these peptides on the activation of neutrophils.

METHODS

Neutrophils were isolated from healthy volunteers, and chemotaxis, superoxide anion generation, aggregation, and changes in cytosolic calcium and GTPase activity were measured in the presence of substance P, somatostatin, and the chemoattractant FMLP.

RESULTS

Substance P was an effective chemoattractant, 20% as potent as FMLP at equimolar concentrations. Substance P also stimulated GTPase activity in neutrophil plasma membranes. Somatostatin did not activate neutrophils; however, it effectively inhibited neutrophil chemotaxis and GTPase activity provoked by substance P, but not by FMLP.

CONCLUSIONS

These studies demonstrate that substance P can effectively stimulate chemotaxis, possibly via effects on a GTP-binding protein distinct from that triggered by FMLP, and that somatostatin is a selective antagonist of substance P. The biochemical specificities of these peptides on cells may modulate neurogenic inflammation at the local level.

Incomplete functional differentiation of HL-60 leukemic cells by synthetic lipopeptides. Partial inhibition by pertussis toxin of enhanced superoxide formation

In human neutrophils, the synthetic lipopeptide, N-palmitoyl-S-[2,3- bis(palmitoyloxy-(2RS)-propyl]-(R)-cysteinyl-(S)-seryl-(S)-lysyl-( S)-lysyl-(S) -lysyl-(S)-lysine [Pam3CysSer(Lys)4], activates NADPH-oxidase catalyzed superoxide (O2-) formation through pertussis-toxin-sensitive and pertussis-toxin-insensitive mechanisms (Seifert, R., Schultz, G., Richter-Freund, M., Metzger, J., Wiesmüller, K.-H., Jung, G., Bessler, W. G. & Hauschildt, S. (1990) Biochem. J. 267, 795-802). We studied the effects of lipopeptides on differentiation of HL-60 leukemic cells. Pam3CysSer(Lys)4 enhanced phorbol-12-myristate-13-acetate-induced O2- formation (presumably through the expression of components of NADPH oxidase) in a concentration-dependent manner with a half-maximal effect at 100 ng/ml and a maximum at 1 microgram/ml. The effect of the lipopeptide was evident after 24 h and reached a plateau after 48 h. (2S,6S)-2-Palmitoylamino-6,7- bis(palmitoyloxy)heptanoyl-(S)-seryl-(S)-lysyl-(S)-lysyl-(S) -lysyl-(S)-lysine enhanced O2- formation as well. The effects of Pam3CysSer(Lys)4 were potentiated by dibutyryl cAMP, dimethyl sulfoxide, retinoic acid, 1,25-dihydroxyvitamin D3, interferon-gamma and tumor-necrosis-factor-alpha. Pertussis toxin, but not its B-oligomer, partially inhibited enhanced O2- formation induced by Pam3CysSer(Lys)4. O2- formation induced by arachidonic acid and gamma-hexachlorocyclohexane were more sensitive to inhibition by pertussis toxin than O2- formation induced by phorbol 12-myristate 13-acetate. Enhanced O2- formation induced by dibutyryl cAMP was not affected by pertussis toxin. Unlike ATP, histamine, prostaglandin E1 and the beta-adrenergic agonist, isoproterenol, Pam3CysSer(Lys)4 did not increase cytosolic Ca2+ [( Ca2+]i) in undifferentiated HL-60 cells. Histamine but not lipopeptides stimulated high-affinity GTPase of guanine-nucleotide-binding proteins in membranes of undifferentiated HL-60 cells. In Pam3CysSer(Lys)4-differentiated HL-60 cells, the responsiveness to the [Ca2+]i-increasing agonists, N-formyl-L-methionyl-L-leucyl-L-phenylalanine, C5a and leukotriene B4, was increased, whilst the responsiveness to prostaglandin E1 and isoproterenol was decreased. Pam3CysSer(Lys)4 did not inhibit proliferation of HL-60 cells but decreased transferrin receptor expression and increased C3bi receptor expression. Pertussis toxin did not affect proliferation and expression of transferrin and C3bi receptors. Dibutyryl cAMP was considerably more effective than Pam3CysSer(Lys)4 at inducing alterations in the above parameters. Our results suggest that (a) Pam3CysSer(Lys)4 induces incomplete functional differentiation of HL-60 cells through a mechanism which does not depend on a rise in [Ca2+]i and is different from that of other differentiation-inducing substances and (b) the mechanism by which Pam3CysSer(Lys)4 induces differentiation involves pertussis-toxin-sensitive and pertussis-toxin-insensitive mechanisms.

Phospholipase C-beta 1 is regulated by a pertussis toxin-insensitive G-protein

Regulation of phospholipase C (PLC) by receptors is mediated either through protein tyrosine phosphorylation or by activation of GTP-binding proteins (Gp). For the latter, pertussis toxin (PT)-sensitive and -insensitive pathways have been described, indicating PLC regulation by at least two types of G-proteins. The identity of PLC isoenzymes which are regulated by either type of Gp remains to be determined. Thyrotropin-releasing hormone stimulates a PLC in GH3 cells via a PT-insensitive Gp. Reconstitution methods for the assay of the GH3-cell Gp were developed. Previously, the membrane PLC was found to be reversibly extracted from membranes by high salt and to be activated by guanosine 5'-[gamma-thio]triphosphate (GTP[S]) only when membrane-associated, suggesting that Gp was retained in salt-extracted membranes. In the present work, Gp was cholate-solubilized from PLC-deficient membranes and incorporated into phospholipid vesicles, which were found to confer GTP[S]- and AlF4(-)-stimulated activity on a solubilized membrane PLC. The reconstitution provided a direct assay for the GH3-cell Gp which was shown to be distinct from Gi, Go and Gs proteins by immunodepletion studies. Incorporation of G-protein beta-gamma subunits into phospholipid vesicles with Gp inhibited GTP[S]-stimulated activity in the reconstitution. The results indicated that Gp is a heterotrimeric G-protein with the properties expected for the PT-insensitive GH3-cell Gp protein. PLC-beta 1 was fully purified and shown to be regulated by Gp in the reconstitution. In contrast, PT-sensitive G-proteins failed to affect the activity of PLC-beta 1. The results indicate (1) that a PT-insensitive Gp regulates PLC-beta 1 and (2) that PT-sensitive and -insensitive pathways of PLC regulation employ different PLC isoenzymes as well as different G-proteins.

Thyrotropin-releasing hormone induces opposite effects on Ca2+ channel currents in pituitary cells by two pathways

Thyrotropin-releasing hormone (TRH) stimulates pituitary secretion by steps involving a cytosolic Ca2+ rise. We examined various pathways of Ca2+ elevation in pituitary GH3 cells. By using the patch clamp technique in the whole-cell configuration and Ba2+ as divalent charge carrier through Ca2+ channels, TRH (1 microM) reversibly reduced the current by about 55%. This hormonal effect was prevented by infusing guanine 5'-[beta-thio]diphosphate (GDP[beta S]) intracellularly but not by pretreating the cell with pertussis toxin (PT). Since PT-insensitive guanine nucleotide-binding regulatory (G) proteins are known to mediate a hormone-stimulated inositol trisphosphate-mediated Ca2+ release from intracellular stores, we assume that the inhibitory effect of TRH on Ba2+ currents through Ca2+ channels is caused by the increased intracellular Ca2+. To prevent a Ca(2+)-release-dependent inhibition of Ca2+ channels, we preincubated GH3 cells in a medium free of divalent charge carriers and measured the Na+ current through Ca2+ channels. When fura-2 was used as indicator for the cytosolic Ca2+, TRH induced a release from intracellular stores only once and had no effect on the intracellular Ca2+ concentration during further applications. In line with this observation, TRH initially reduced the Na+ current through Ca2+ channels but stimulated it during subsequent applications. The stimulation was sensitive to GDP[beta S] and was abolished by pretreatment with PT, suggesting that the stimulatory action of TRH is mediated by a G protein different from the one that functionally couples the receptor to phosphatidylinositol 4,5-bisphosphate hydrolysis. In conclusion, the present data suggest that TRH increases the intracellular Ca2+ concentration by two interacting pathways, that release from intracellular stores causes a secondary blockage of Ca2+ channels, and that, especially with empty intracellular Ca2+ stores, Ca2+ channels are stimulated by a PT-sensitive G protein.

Potentiation of thyrotropin-releasing hormone-stimulated prolactin mRNA levels in GH3 cells by acetylcholine

We have examined the effect of acetylcholine (ACh) pretreatment on the thyrotropin-releasing hormone (TRH) induced prolactin gene expression in GH3 cells, a rat pituitary tumor cell line. Prolonged exposure (greater than 6 h) to ACh enhanced the TRH-induced prolactin mRNA accumulation in a time- and concentration-dependent manner while ACh by itself did not affect the basal prolactin mRNA levels appreciably. Maximal augmentation of the TRH-induced prolactin mRNA accumulation was obtained when cells were pretreated with 10(-5) M ACh for 24 h. The activation was mimicked by carbachol and oxotremorine and was blocked by the simultaneous presence of atropine. Preincubation of GH3 cells with pertussis toxin abolished the augmenting effect of ACh. These results indicate that prolonged exposure to muscarinic receptor agonists may enhance the TRH-stimulated prolactin mRNA expression and a pertussis toxin sensitive G-protein may be involved.

Assignment of G-protein subtypes to specific receptors inducing inhibition of calcium currents

The inhibition of voltage-dependent Ca2+ channels in secretory cells by plasma membrane receptors is mediated by pertussis toxin-sensitive G proteins. Multiple forms of G proteins have been described, differing principally in their alpha subunits, but it has not been possible to establish which G-protein subtype mediates inhibition by a specific receptor. By intranuclear injection of antisense oligonucleotides into rat pituitary GH3 cells, the essential role of the Go-type G proteins in Ca(2+)-channel inhibition is established: the subtypes Go1 and Go2 mediate inhibition through the muscarinic and somatostatin receptors, respectively.