Thyrotropin-releasing hormone rapidly activates the phosphodiester hydrolysis of polyphosphoinositides in GH3 pituitary cells. Evidence for the role of a polyphosphoinositide-specific phospholipase C in hormone action

Ontogeny and effects of hypothalamic pituitary disconnection on formation of inositol trisphosphate in fetal sheep pituitary cells

In late gestation fetal sheep, the pituitary becomes increasingly responsive to stimulation by arginine vasopressin (AVP). This change appears to be one important factor mediating the plasma cortisol surge, a critical developmental event. It is not known precisely why pituitary corticotropes become more responsive at this time. In this study we examined the possibility that changes in second messenger generation [inositol trisphosphate (IP(3))] are responsible. Two studies were undertaken. The first was an ontogeny study, where pituitaries were isolated from 100-, 120-, and 140-d gestational age (dGA) fetal sheep. Cells were cultured, stimulated with AVP, and the formation of IP(3) assessed. The amount of IP(3) generated increased with gestational age (percent increases from unstimulated controls were 4.6, 11.5, and 21.5 for 100, 120, and 140 dGA, respectively), with significant differences between the 140-dGA group and both earlier groups apparent. The second study examined the impact of 120-dGA hypothalamo-pituitary disconnection (HPD), which prevents corticotrope maturation, on responsiveness of pituitary cells isolated from 140-dGA fetuses. Cells were stimulated with AVP, and the formation of IP(3) and secretion of ACTH were assessed. Significantly less IP(3) was formed, and ACTH secreted in cells from HPD compared with control fetuses (IP(3) and ACTH levels were 50% and 35% lower, respectively). Results from the HPD study demonstrate that the ontogenic changes in IP(3) after AVP require an intact hypothalamic-pituitary-adrenal axis. These findings suggest that heightened second messenger generation may be a key reason for increased ACTH secretory responsiveness to AVP in the late gestation sheep fetus.

Ras and Rap1 activation of PLCepsilon lipase activity

Phosphoinositide-specific phospholipase C (PLC) plays a pivotal role in signal transduction from various receptor molecules on the plasma membrane. PLCepsilon is characterized by possession of two Ras/Rap-associating (RA) domains and a CDC25 homology domain acting as a guanine nucleotide exchange factor for Rap1. Our recent studies using PLCepsilon-deficient mice have suggested that PLCepsilon plays crucial roles in cardiac semilunar valvulogenesis downstream of the EGF receptor, as well as in chemical carcinogen-induced skin tumor development downstream of Ha-Ras. Stimulation of cultured mammalian cells with growth factors induces translocation of PLCepsilon from the cytoplasm to the plasma membrane and to the Golgi apparatus through direct association at its RA domains with the GTP-bound forms of Ras and Rap1, respectively. These results suggest that growth factor stimulation activates PLCepsilon by means of Ras and/or Rap1. However, growth factor-induced activation of the PLCepsilon lipase activity cannot be measured accurately because of simultaneous activation of PLCgamma through receptor-dependent phosphorylation. In this article, we introduce two methods to assay Ras- or Rap1-dependent activation of PLCepsilon lipase activity, with special emphasis on the use of cells expressing a mutant platelet-derived growth factor receptor lacking the PLCgamma-binding sites.

Cellular trafficking of human alpha1a-adrenergic receptors is continuous and primarily agonist-independent

Alpha1a-adrenergic receptors (alpha1aARs) are present intracellularly and at the cell surface in cultured and natural cell models, where they are subject to agonist-mediated desensitization and internalization. To explore alpha1aAR trafficking, a hemagglutinin (HA)-tagged alpha1aAR/enhanced green fluorescent protein (EGFP) fusion protein was expressed in rat-1 fibroblasts and tracked by EGFP fluorescence and antibody labeling of surface receptors. Confocal analysis of antibody-labeled surface receptors revealed unexpected constitutive internalization in the absence of agonist stimulation. In partial agreement, the inverse agonist prazosin also caused a modest 20 +/- 2% increase in surface receptor levels, suggesting a partial block of constitutive internalization caused by decreased basal activation. However, prazosin was unable to prevent internalization of antibody-tagged surface receptors observed by confocal microscopy or cause obvious redistribution of intracellular receptor to the surface, suggesting that the alpha1aAR is internalizing even in a basal-inactive state. In contrast to the alpha1aAR, surface labeling of an HA-tagged alpha1b-EGFP fusion protein did not result in any apparent constitutive internalization. Constitutive internalization of the alpha1aAR seems to occur alongside reversible agonist-induced internalization, and both seem to involve clathrin-mediated endocytosis but not degradation in lysozymes. Surface receptor density must be maintained by recycling, because the protein synthesis inhibitor cycloheximide has no effect on total or surface receptor density in agonist-treated or untreated cells for 6 h. Constitutive agonist-independent trafficking of alpha1aARs may provide a novel mechanism by which an internal pool of alpha1aARs are maintained and recycled to allow continuous agonist-induced signaling.

Acute agonist-mediated desensitization of the human alpha 1a-adrenergic receptor is primarily independent of carboxyl terminus regulation: implications for regulation of alpha 1aAR splice variants

Despite important roles in myocardial hypertrophy and benign prostatic hyperplasia, little is known about acute effects of agonist stimulation on alpha(1a)-adrenergic receptor (alpha(1a)AR) signaling and function. Regulatory mechanisms are likely complex since 12 distinct human alpha(1a)AR carboxyl-terminal splice variants have been isolated. After determining the predominance of the alpha(1a-1)AR isoform in human heart and prostate, we stably expressed an epitope-tagged alpha(1a-1)AR cDNA in rat-1 fibroblasts and subsequently examined regulation of signaling, phosphorylation, and internalization of the receptor. Human alpha(1a)AR-mediated inositol phosphate signaling is acutely desensitized in response to both agonist and phorbol 12-myristate 13-acetate (PMA) exposure. Concurrent with desensitization, alpha(1a)ARs in (32)P(i)-labeled cells are rapidly phosphorylated in response to both NE and PMA stimulation. Despite the ability of PKC to desensitize alpha(1a)ARs when directly activated with PMA, inhibitors of PKC have no effect on agonist-mediated desensitization. In contrast, involvement of GRK kinases is suggested by the ability of GRK2 to desensitize alpha(1a)ARs. Internalization of cell surface alpha(1a)ARs also occurs in response to agonist stimulation (but not PKC activation), but is initiated more slowly than receptor desensitization. Significantly, deletion of the alpha(1a)AR carboxyl terminus has no effect on receptor internalization or either agonist-induced or GRK-mediated receptor desensitization. Because mechanisms underlying acute agonist-mediated regulation of human alpha(1a)ARs are primarily independent of the carboxyl terminus, they may be common to all functional alpha(1a)AR isoforms.

beta-Arrestin scaffolding of the ERK cascade enhances cytosolic ERK activity but inhibits ERK-mediated transcription following angiotensin AT1a receptor stimulation

beta-Arrestins are cytosolic proteins that mediate homologous desensitization of G protein-coupled receptors (GPCRs) by binding to agonist-occupied receptors and by uncoupling them from heterotrimeric G proteins. The recent finding that beta-arrestins bind to some mitogen-activated protein (MAP) kinases has suggested that they might also function as scaffolds for GPCR-stimulated MAP kinase activation. To define the role of beta-arrestins in the regulation of ERK MAP kinases, we examined the effect of beta-arrestin overexpression on ERK1/2 activation and nuclear signaling in COS-7 cells expressing angiotensin II type 1a receptors (AT1aRs). Expression of either beta-arrestin1 or beta-arrestin2 reduced angiotensin-stimulated phosphatidylinositol hydrolysis but paradoxically increased angiotensin-stimulated ERK1/2 phosphorylation. The increase in ERK1/2 phosphorylation in beta-arrestin-expressing cells correlated with activation of a beta-arrestin-bound pool of ERK2. The beta-arrestin-dependent increase in ERK1/2 phosphorylation was accompanied by a significant reduction in ERK1/2-mediated, Elk1-driven transcription of a luciferase reporter. Analysis of the cellular distribution of phospho-ERK1/2 by confocal immunofluorescence microscopy and cellular fractionation revealed that overexpression of beta-arrestin resulted in a significant increase in the cytosolic pool of phospho-ERK1/2 and a corresponding decrease in the nuclear pool of phospho-ERK1/2 following angiotensin stimulation. beta-Arrestin overexpression resulted in formation of a cytoplasmic pool of beta-arrestin-bound phospho-ERK, decreased nuclear translocation of phospho-ERK1/2, and inhibition of Elk1-driven luciferase transcription even when ERK1/2 was activated by overexpression of cRaf-1 in the absence of AT1aR stimulation. These data demonstrate that beta-arrestins facilitate GPCR-mediated ERK activation but inhibit ERK-dependent transcription by binding to phospho-ERK1/2, leading to its retention in the cytosol.

Single nucleotide polymorphism of human platelet-activating factor receptor impairs G-protein activation

Various proinflammatory and vasoactive actions of platelet-activating factor (PAF) are mediated through a specific G-protein-coupled PAF receptor (PAFR). We identified a novel DNA variant in the human PAFR gene, which substitutes an aspartic acid for an alanine residue at position 224 (A224D) in the putative third cytoplasmic loop. This mutation was observed in a Japanese population at an allele frequency of 7.8%. To delineate the functional consequences of this structural alteration, Chinese hamster ovary cells were stably transfected with constructs encoding either wild-type or A224D mutated PAFR. No significant difference was observed in the expression level of the receptor or the affinity to PAF or to an antagonist, WEB2086, between the cells transfected with wild-type and mutant PAFR. Chinese hamster ovary cells expressing A224D mutant PAFR displayed partial but significant reduction of PAF-induced intracellular signals such as calcium mobilization, inositol phosphate production, inhibition of adenylyl cyclase, and chemotaxis. These findings suggest that this variant receptor produced by a naturally occurring mutation exhibits impaired coupling to G-proteins and may be a basis for interindividual variation in PAF-related physiological responses, disease predisposition or phenotypes, and drug responsiveness.

Production and characterization of an antiserum which recognizes the native receptor for thyrotropin-releasing hormone

Despite attempts in several laboratories, it has been difficult to prepare antiserum to the thyrotropin-releasing hormone receptor (TRHR). We have prepared a polyclonal anti-rat TRHR antiserum by immunization of rabbits with a synthetic peptide corresponding to the C-terminus of the TRHR. The specificity of the antiserum was assessed by enzyme-linked immunosorbent assay. The affinity-purified antibody recognized a major broad band at 50-60 kDa and a minor broad band at 100-120 kDa in Western blot analysis of membrane proteins from TRHR-transfected, but not control, HEK293t cells. Binding to both bands was abolished by preincubation with the immunizing peptide but not control peptide. The approach was repeated with rat pituitary F4C1 cells, which lack endogenous TRHRs; membranes from F4C1 cells transfected with TRHR cDNA, but not control cells, showed specific binding by Western blot. Using laser confocal microscopy, the TRHR was visualized on the plasma membrane of transfected, but not control, F4C1 cells. Similar confocal findings were observed in TRHR-transfected HEK293t cells. Within 5 min after TRH addition, the TRHR signal translocated from the plasma membrane to the cytoplasm of F4C1 cells transfected with TRHR cDNA. Ten minutes after TRH addition, the TRHR signal formed aggregates in the cytoplasm. Thirty minutes after TRH treatment, both cytoplasmic and plasma membrane localizations were observed, suggesting recycling of some TRHRs back to the plasma membrane. These observations are consistent with our previous findings using an epitope-tagged TRHR. In conclusion, we have prepared an antiserum that recognizes the native TRHR by Western blot analysis and confocal microscopy.

A novel bifunctional phospholipase c that is regulated by Galpha 12 and stimulates the Ras/mitogen-activated protein kinase pathway

Three families of phospholipase C (PI-PLCbeta, gamma, and delta) are known to catalyze the hydrolysis of polyphosphoinositides such as phosphatidylinositol 4,5-bisphosphate (PIP(2)) to generate the second messengers inositol 1,4,5 trisphosphate and diacylglycerol, leading to a cascade of intracellular responses that result in cell growth, cell differentiation, and gene expression. Here we describe the founding member of a novel, structurally distinct fourth family of PI-PLC. PLCepsilon not only contains conserved catalytic (X and Y) and regulatory domains (C2) common to other eukaryotic PLCs, but also contains two Ras-associating (RA) domains and a Ras guanine nucleotide exchange factor (RasGEF) motif. PLCepsilon hydrolyzes PIP(2), and this activity is stimulated selectively by a constitutively active form of the heterotrimeric G protein Galpha(12). PLCepsilon and a mutant (H1144L) incapable of hydrolyzing phosphoinositides promote formation of GTP-Ras. Thus PLCepsilon is a RasGEF. PLCepsilon, the mutant H1144L, and the isolated GEF domain activate the mitogen-activated protein kinase pathway in a manner dependent on Ras but independent of PIP(2) hydrolysis. Our findings demonstrate that PLCepsilon is a novel bifunctional enzyme that is regulated by the heterotrimeric G protein Galpha(12) and activates the small G protein Ras/mitogen-activated protein kinase signaling pathway.

An asn to lys polymorphism in the third intracellular loop of the human alpha 2A-adrenergic receptor imparts enhanced agonist-promoted Gi coupling

alpha(2A)-Adrenergic receptors (alpha(2A)AR) are presynaptic autoinhibitory receptors of noradrenergic neurons in the central and peripheral sympathetic nervous systems, which act to dynamically regulate neurotransmitter release. Signaling through the G(i)/G(o) family of G-proteins, the receptor subserves numerous homeostatic and central nervous system functions. A single nucleotide polymorphism of this receptor, which results in an Asn to Lys substitution at amino acid 251 of the third intracellular loop, was identified in the human population. The frequency of Lys-251 was 10-fold greater in African-Americans than in Caucasians, but was not associated with essential hypertension. To determine the consequences of this substitution, wild-type and Lys-251 receptors were expressed in CHO and COS-7 cells. Expression, ligand binding, and basal receptor function were unaffected by the substitution. However, agonist-promoted [(35)S]GTPgammaS binding was approximately 40% greater with the Lys-251 receptor. This enhanced agonist function was observed with catecholamines, azepines, and imidazolines albeit to different degrees. In studies of agonist-promoted functional coupling to G(i), the polymorphic receptor displayed enhanced inhibition of adenylyl cyclase (60 +/- 4. 4 versus 46 +/- 4.1% inhibition) and markedly enhanced stimulation of MAP kinase (57 +/- 9 versus 15- +/- 2-fold increase over basal) compared with wild-type alpha(2A)AR. The potency of epinephrine in stimulating inositol phosphate accumulation was increased approximately 4 fold with the Lys-251 receptor. Unlike previously described variants of G-protein-coupled receptors, where the minor species causes either a loss of function or increased non-agonist function, Lys-251 alpha(2A)AR represents a new class of polymorphism whose phenotype is a gain of agonist-promoted function.

Differential regulation of granule-to-granule and granule-to-plasma membrane fusion during secretion from rat pituitary lactotrophs

We used fluorescence imaging of individual exocytic events together with electron microscopy to study the regulation of dense core granule-to-plasma membrane fusion and granule-to-granule fusion events that occur during secretion from rat pituitary lactotrophs. Stimulating secretion with elevated extracellular potassium, with the calcium ionophore ionomycin, or with thyrotropin releasing hormone or vasoactive intestinal polypeptide resulted in abundant exocytic structures. Approximately 67% of these structures consisted of multiple granules fused together sharing a single exocytic opening with the plasma membrane, i.e., compound exocytosis. For all of these stimulation conditions there appeared to be a finite number of plasma membrane fusion sites, approximately 11 sites around each cellular equator. However, a granule could fuse directly with another granule that had already fused with the plasma membrane even before all plasma membrane sites were occupied. Granule-to-plasma membrane and granule-to-granule fusion events were subject to different regulations. Forskolin, which can elevate cAMP, increased the number of granule-to-granule fusion events without altering the number of granule-to-plasma membrane fusion events. In contrast, the phorbol ester PMA, which activates protein kinase C increased both granule-to-granule and granule-to-plasma membrane fusion events. These results provide a cellular mechanism that can account for the previously demonstrated potentiation of secretion from lactotrophs by cAMP- and PKC-dependent pathways.

A four amino acid deletion polymorphism in the third intracellular loop of the human alpha 2C-adrenergic receptor confers impaired coupling to multiple effectors

The alpha(2)-adrenergic receptors (alpha(2)ARs) play a critical role in modulating neurotransmitter release in the central and peripheral sympathetic nervous systems. A polymorphism of the alpha(2)AR subtype localized to human chromosome 4 (the pharmacologic alpha(2C)AR subtype) within an intracellular domain has been identified in normal individuals. The polymorphism (denoted Del322-325) is because of an in-frame 12-nucleic acid deletion encoding a receptor lacking Gly-Ala-Gly-Pro in the third intracellular loop. To delineate the functional consequences of this structural alteration, Chinese hamster ovary cells were permanently transfected with constructs encoding wild-type human alpha(2C)AR and the polymorphic receptor. The Del322-325 variant had decreased high affinity agonist binding (K(H) = 7.3 +/- 0.95 versus 3.7 +/- 0.43 nm; %R(H) = 31 +/- 4 versus 49 +/- 4) compared with wild-type indicating impaired formation of the agonist-receptor-G protein complex. The polymorphic receptor displayed markedly depressed epinephrine-promoted coupling to G(i), inhibiting adenylyl cyclase by 10 +/- 4.3% compared with 73 +/- 2.4% for wild-type alpha(2C)AR. This also was so for the endogenous ligand norepinephrine and full and partial synthetic agonists. Depressed agonist-promoted coupling to the stimulation of MAP kinase ( approximately 71% impaired) and inositol phosphate production ( approximately 60% impaired) was also found with the polymorphic receptor. The Del322-325 receptor was approximately 10 times more frequent in African-Americans compared with Caucasians (allele frequencies 0.381 versus 0.040). Given this significant loss of function phenotype in several signal transduction cascades and the skewed ethnic prevalence, Del322-325 represents a pharmacoethnogenetic locus and may also be the basis for interindividual variation in cardiovascular or central nervous system pathophysiology.

Membrane targeting and cytoplasmic sequestration in the spatiotemporal localization of human protein kinase C alpha

In order to map the molecular determinants that dictate the subcellular localization of human protein kinase C alpha (hPKCalpha), full-length and deletion mutants of hPKCalpha were tagged with the green fluorescent protein (GFP) and transiently expressed in GH3B6 cells. We found that upon thyrotropin-releasing hormone (TRH) or phorbol 12-myristate 13-acetate stimulation, hPKCalpha-GFP was localized exclusively in regions of cell-cell contacts. Surprisingly, PKCalpha failed to translocate in single cells despite the presence of TRH receptors, as attested by the TRH-induced rise in intracellular calcium concentration in these cells. TRH-stimulated translocation of hPKCalpha-GFP from the cytoplasm to cell-cell contacts was a biphasic process: a fast (measured in seconds) and transient phase, followed by a slower (approximately 1 hour) and long lasting phase. The latter and the translocation induced by phorbol 12-myristate 13-acetate absolutely required the N-terminal V1 region. In contrast to the full-length hPKCalpha, the N-terminal regulatory domain alone or associated with the V3 hinge region was spontaneously and uniformly localized at the plasma membrane of single and apposed cells. However, treatment with the calcium chelator BAPTA/AM induced a differential cytoplasmic/nuclear redistribution of the regulatory domain, depending on its association with V3, which suggests the existence of a mechanism controlling the cytoplasmic sequestration of inactive hPKCalpha and involving the V3 region. By using other deletion mutants, we were able to map the sequence required for this sequestration to the C2+V3 regions. This work points to the existence of a complex interplay between membrane targeting and cytoplasmic sequestration in the control of the spatiotemporal localization of hPKCalpha.

Prolactin-releasing peptide activation of the prolactin promoter is differentially mediated by extracellular signal-regulated protein kinase and c-Jun N-terminal protein kinase

Regulation of the mitogen-activated protein kinase (MAPK) family by prolactin-releasing peptide (PrRP) in both GH3 rat pituitary tumor cells and primary cultures of rat anterior pituitary cells was investigated. PrRP rapidly and transiently activated extracellular signal-regulated protein kinase (ERK) in both types of cells. Both pertussis toxin, which inactivates G(i)/G(o) proteins, and exogenous expression of a peptide derived from the carboxyl terminus of the beta-adrenergic receptor kinase I, which specifically blocks signaling mediated by the betagamma subunits of G proteins, completely blocked the PrRP-induced ERK activation, suggesting the involvement of G(i)/G(o) proteins in the PrRP-induced ERK activation. Down-regulation of cellular protein kinase C did not significantly inhibit the PrRP-induced ERK activation, suggesting that a protein kinase C-independent pathway is mainly involved. PrRP-induced ERK activation was not dependent on either extracellular Ca(2+) or intracellular Ca(2+). However, the ERK cascade was not the only route by which PrRP communicated with the nucleus. JNK was also shown to be significantly activated in response to PrRP. JNK activation in response to PrRP was slower than ERK activation. Moreover, to determine whether a MAPK family cascade regulates rat prolactin (rPRL) promoter activity, we transfected the intact rPRL promoter ligated to the firefly luciferase reporter gene into GH3 cells. PrRP activated the rPRL promoter activity in a time-dependent manner. Co-transfection with a catalytically inactive form of a MAPK construct or a dominant negative JNK, partially but significantly inhibited the induction of the rPRL promoter by PrRP. Furthermore, co-transfection with a dominant negative Ets completely abolished the response of the rPRL promoter to PrRP. These results suggest that PrRP differentially activates ERK and JNK, and both cascades are necessary to elicit rPRL promoter activity in an Ets-dependent mechanism.

Cell signaling in bovine ciliary epithelial organ culture

The ciliary epithelium secretes aqueous humor, an intraocular fluid whose production is regulated in part by transmembrane signaling pathways including those mediated by G protein-coupled receptors. Many drugs, such as beta-adrenergic receptor (AR) antagonists and alpha2-AR agonists, are used to lower intraocular pressure by presumably decreasing fluid transport across this epithelium. Hence, our purpose was to establish a ciliary epithelial organ culture system suitable for the study of cell signaling pathways. A trypsin-mediated dissection method was established to isolate bovine ciliary epithelial sheets. These sheets were cultured in a 5% CO2 incubator. The quality was assessed by light microscopy, by protein analysis, and by the evaluation of epinephrine-mediated phosphoinositide turnover. The cultured epithelial explants were viable as evidenced by minimal trypan blue staining. The explants were composed primarily of nonpigmented cells and some pigmented cells, but no other ciliary body tissues were present on histology. Membrane preparations showed proteins with a distribution from 31 to 116 kDa. Epinephrine caused a dose-dependent increase in [3H]inositol phosphates (InsPs) accumulation with a maximal increase of two- to three-fold over basal levels. This epinephrine-mediated increase was inhibited by prazosin. We established an organ culture system of isolated bovine ciliary epithelium suitable for the study of transmembrane signaling pathways.

Interactions of G(h)/transglutaminase with phospholipase Cdelta1 and with GTP

The inositol phosphate hydrolyzing activity of human phospholipase Cdelta1 (PLCdelta1) is markedly inhibited when the enzyme is coexpressed with the human heart G(h)/transglutaminase (TG) in human embryonic kidney cells. Because the cotransfection does not affect the amount of PLCdelta1 in the cells, the depression of phospholipase activity probably is a result of a direct interaction between the two proteins. An ELISA procedure was employed to document the associations of purified TG preparations from a variety of tissues (human red cells, rabbit lens, guinea pig liver) with PLCdelta1. Nucleotides (GTP > GDP > ATP > GMP = ADP, in order of decreasing efficiency) interfered with the formation of the PLCdelta1:TG complex. A conformational change in the TG partner, occurring with nucleotide binding, is thought to be responsible for dissociating the two proteins. The structural rearrangement produces a remarkable shift in the anodic mobility of TG in electrophoresis: TG(slow) + GTP -->/<-- [TG:GTP](fast). Altogether, our findings indicate that GTP controls PLCdelta1 activity by releasing this protein from an inhibitory association with G(h)/transglutaminase.

Regulation of dense core release from neuroendocrine cells revealed by imaging single exocytic events

Using FM1-43 fluorescence, we have optically detected single exocytic and endocytic events in rat pituitary lactotrophs. About fifty discrete fluorescent spots abruptly appear around the entire surface of a cell bathed in FM1-43 and high-potassium saline. The spots, which also immunostain for prolactin, reflect the labeling of dense cores as well as membranes of exocytosed secretory granules. Stained cores are not released, but remain attached to the cell and are eventually endocytosed. However, in cells exposed to dopamine (or an analog, bromocriptine), the cores dissolve and are secreted after several seconds. Solubilization of dense cores is mediated through a reduction in cytoplasmic cyclic AMP. Thus, the composition of secretions from individual secretory granules is regulated.

Roles of Gi and Gq/11 in mediating desensitization of the luteinizing hormone/choriogonadotropin receptor in porcine ovarian follicular membranes

Although desensitization of most guanine nucleotide-binding (G) protein receptors is triggered by phosphorylation of the receptor, desensitization of the LH/CG receptor (-R) in porcine follicular ovarian membranes appears to be independent of LH/CG-R phosphorylation. We therefore evaluated whether desensitization of the LH/CG-R reflected a direct inhibition of adenylyl cyclase (AC) activity by either the alpha-subunit of Gi or betagamma-subunits derived from any of the membrane G proteins activated in response to LH/CG-R activation or whether desensitization reflected a competition between Gs and a G protein that activated phospholipase C for binding sites on the LH/CG-R. The results showed that follicular membrane AC activity was not inhibited upon activation of the LH/CG-R despite evidence that the ACs in follicular membranes, when maximally activated by forskolin, could be inhibited when membrane G proteins were activated by guanyl-5'-yl imidodiphosphate, and that pertussis toxin pretreatment of membranes raised forskolin-stimulated AC activity, consistent with a tonic inhibition of follicular membrane AC activity. Similarly, agonist-stimulated desensitization of LH/CG-R-stimulated AC activity was not inhibited by pertussis toxin. Therefore, desensitization is not the result of inhibition of AC mediated by an inhibitory Gi subunit. Follicular membrane AC was also not inhibited by Gbetagamma subunits freed with activation of Gs Gq/11, or G13, based on the inabilities of exogenous Gbetagamma to promote desensitization and of a protein that sequesters Gbetagamma to inhibit desensitization. Desensitization was also not inhibited by a Gq/11 C-terminal peptide or antiserum directed toward the C-terminus of Gq/11, nor was it reversed with the addition of Gbetagamma to membranes exhibiting desensitized LH/CG-R, suggesting that desensitization is independent of coupling of the LH/CG-R to Gq/11. These results indicate that agonist-dependent desensitization of LH/CG-R-stimulated AC activity is mediated by a unique mechanism.

Characterization of DNA regions mediating the ability of Ca2+/calmodulin dependent protein kinase II to stimulate prolactin promoter activity

The ability of Ca2+/calmodulin-dependent protein kinases (CaMKs) to regulate transcription of the rat prolactin (PRL) gene has been examined. We found that KN-62, a potent inhibitor of CaM kinases, blunted the ability of TRH to activate the prolactin promoter. Transfection experiments using expression plasmids for constitutively active forms of CaMKI, CaMKII, or CaMKIV show that CaMKII is the most effective activator of prolactin promoter expression. Deletion studies demonstrated that the upstream boundary of sequences necessary to respond to CaMKII is located within the distal enhancer of the prolactin gene. Neither the distal enhancer alone nor the proximal region of the prolactin gene are sufficient to mediate a response to CaMKII. Mutational analysis suggests that several Pit-1 binding sites contribute to CaMKII responsiveness. These findings suggest that CaMKII responsiveness of the prolactin promoter requires multiple factor binding sites in both the distal and proximal regions of the gene.

Structural and functional requirements for agonist-induced internalization of the human platelet-activating factor receptor

The receptor for platelet-activating factor (PAF) is a member of the G-protein-coupled receptor family. To study the structural elements and mechanisms involved in the internalization of human PAF receptor (hPAFR), we used the following mutants: a truncated mutant in the C-terminal tail of the receptor (Cys317 --> Stop) and mutations in the (D/N)P(X)2,3Y motif (Asp289 --> Asn,Ala and Tyr293 --> Phe,Ala). Chinese hamster ovary cells expressing the Cys317 --> Stop mutant exhibited a marked reduction in their capacity to internalize PAF, suggesting the existence of determinants important for endocytosis in the last 26 amino acids of the cytoplasmic tail. Substitution of Asp289 to alanine abolished both internalization and G-protein coupling, whereas substitution of Tyr293 to alanine abolished coupling but not internalization. Inhibition or activation of protein kinase C did not significantly affect the internalization process. Receptor sequestration and ligand uptake was, at least in part, blocked by concanavalin A and blockers of endocytosis mediated by clathrin-coated pits. Our data suggest that the internalization of a G-protein-coupled receptor and coupling to a G-protein can be two independent events. Moreover, the C terminus tail of hPAFR, but not the putative internalization motifs, may be involved in the internalization of hPAFR.

Identification of transmembrane domain residues determinant in the structure-function relationship of the human platelet-activating factor receptor by site-directed mutagenesis

Platelet-activating factor (PAF) is a potent phospholipid mediator that produces a wide range of biological responses. The PAF receptor is a member of the seven-transmembrane GTP-binding regulatory protein-coupled receptor superfamily. This receptor binds PAF with high affinity and couples to multiple signaling pathways, leading to physiological responses that can be inhibited by various structurally distinct PAF antagonists. We have used site-directed mutagenesis and functional expression studies to examine the role of the Phe97 and Phe98 residues located in the third transmembrane helix and Asn285 and Asp289 of the seventh transmembrane helix in ligand binding and activation of the human PAF receptor in transiently transfected COS-7 cells. The double mutant FFGG (Phe97 and Phe98 mutated into Gly residues) showed a 3-4-fold decrease in affinity for PAF, but not for the specific antagonist WEB2086, when compared with the wild-type (WT) receptor. The FFGG mutant receptor, however, displayed normal agonist activation, suggesting that these two adjacent Phe residues maintain the native PAF receptor conformation rather than interacting with the ligand. On the other hand, substitution of Ala for Asp289 increased the receptor affinity for PAF but abolished PAF-dependent inositol phosphate accumulation; it did not affect WEB2086 binding. Substitution of Asn for Asp289, however, resulted in a mutant receptor with normal binding and activation characteristics. When Asn285 was mutated to Ala, the resulting receptor was undistinguishable from the WT receptor. Surprisingly, substitution of Ile for Asn285 led to a loss of ligand binding despite normal cell surface expression levels of this mutant, as verified by flow cytometric analysis. Our data suggest that residues 285 and 289 are determinant in the structure and activation of the PAF receptor but not in direct ligand binding, as had been recently proposed in a PAF receptor molecular model.

Mutations of two adjacent amino acids generate inactive and constitutively active forms of the human platelet-activating factor receptor

We have mutated two residues, Ala230 and Leu231, in the C-terminal portion of the third intracellular loop of the human platelet-activating factor (PAF) receptor into Glu230 and Arg231, respectively. The Leu231 --> Arg231 substitution led to two major modifications: 1) increased constitutive activity of the PAF receptor resulting in agonist-independent production of inositol phosphates and 2) increased affinity of the receptor for binding PAF (agonist) but not WEB2086 (antagonist). The L231R mutant was able to adopt at least two conformations: (i) a higher affinity state than the corresponding state of the wild-type receptor (WT), dependent on G protein coupling, and (ii) a low affinity state, higher than the one for the uncoupled WT receptor. The Ala230 --> Glu230 substitution also resulted in two major modifications: 1) unresponsiveness in terms of phosphatidylinositol hydrolysis in response to PAF and 2) a marked decrease in affinity of the receptor for binding the agonist but not the antagonist. Competition binding studies of transient receptor expression in COS-7 cells and the inability of guanosine 5 -O-(3-thiotriphosphate) to modulate the decrease in affinity of a stable A230E mutant in Chinese hamster ovary cells suggest an inherent low affinity conformation for this mutant. Alternatively, mutation of Ala230 to Gln230 suggested that the residue 230 has a fundamental effect on receptor affinity and its charge is determinant in G protein coupling of the PAF receptor. In this report, we show that substitution of two immediately adjacent residues of the PAF receptor, Ala230 and Leu231, surprisingly leads to an inactive and a constitutively active phenotype, respectively. These results further support the concept of constitutively active G protein-coupled receptors as adopting ''active'' state conformations similar to those induced by agonist binding to WT receptors.

The human 5-hydroxytryptamine 1A receptor differentially modulates phospholipase C and adenylyl cyclase activities

In this study, we quantitate and compare the ability of the 5-hydroxytryptamine 1A (5-HT1A) receptor to modulate the activities of phospholipase C and adenylyl cyclase as a function of receptor concentration. We used a single clonal cell line permanently expressing the human 5-HT1A receptor, and progressively depleted the receptor concentration using an alkylating antagonist (N8-bromoacetyl-N1-3'-(4-indolyloxy)-2'-hydroxypropyl-Z-1,8-diamin o-p-methane, (+-) Pindobind). For serotonin-induced phospholipase C stimulation, reductions in receptor number result in dose-response curves that shift downward and rightward, reflecting both a decreasing maximal effect as well as an increasing ED50. In contrast, depletion of more than 95% of the receptors has no effect on the maximal inhibition of forskolin-stimulated adenylyl cyclase activity. Moreover, at all receptor concentrations, the amount of serotonin required to produce half-maximal phospholipase C stimulation is several-fold more than that required to produce half-maximal inhibition of adenylyl cyclase activity. We conclude that the 5-HT1A receptor modulates these two pathways differently, and that the overall response to challenge with serotonin, in terms of both phosphatidyl inositol hydrolysis and cyclic AMP production, is dependent upon receptor number.

Cross-desensitization of chemoattractant receptors occurs at multiple levels. Evidence for a role for inhibition of phospholipase C activity

To define the molecular mechanisms of cross-regulation among chemoattractant receptors, we stably coexpressed, in a rat basophilic leukemia (RBL-2H3) cell line, epitope-tagged receptors for the chemoattractants formylmethionylleucylphenylalanine (fMLP), a peptide of the fifth component of the complement system (C5a), and interleukin-8 (IL-8). All the expressed receptors underwent homologous phosphorylation and desensitization upon agonist stimulation. When co-expressed, epitope-tagged C5a receptor (ET-C5aR) and epitope-tagged IL-8 receptor (ET-IL-8RA) were cross-phosphorylated by activation of the other. Activation of epitope-tagged fMLP receptor (ET-FR) also cross-phosphorylated ET-C5aR and ET-IL-8RA, but ET-FR was totally resistant to cross-phosphorylation. Similarly, C5a and IL-8 stimulation of [35S]guanosine 5'-3-O-(thio) triphosphate (GTP gamma S) binding and Ca2+ mobilization were cross-desensitized by each other and by fMLP. Stimulation of [35S]GTP gamma S binding by fMLP was also not cross-desensitized by C5a or IL-8, however, Ca2+ mobilization was, suggesting a site of inhibition distal to G protein activation. Consistent with this desensitization of Ca2+ mobilization, inositol 1,4,5-trisphosphate release in RBL-2H3 cells expressing both ET-C5aR and ET-FR revealed that fMLP and C5a cross-desensitized each other's ability to stimulate phosphoinositide hydrolysis. Taken together, these results indicate that receptor cross-phosphorylation correlates directly with desensitization at the level of G protein activation. The ET-FR was resistant to this process. Of note, cross-desensitization of ET-FR at the level of phosphoinositide hydrolysis and Ca2+ mobilization was demonstrated in the absence of receptor phosphorylation. This suggests a new form of chemoattractant cross-regulation at a site distal to receptor/G protein coupling, involving the activity of phospholipase C.

Influence of Hepes- and CO2/HCO(3-)-buffer on Ca2+ transients induced by TRH and elevated K+ in rat pituitary GH4C1 cells

The influence of two buffer systems (Hepes and CO2/HCO3-) on intracellular Ca2+ ([Ca2+]i) transients evoked by TRH and by elevated K+ were studied in single, and small clusters of, clonal rat pituitary GH4C1 cells using Fura 2. The steady-state level of [Ca2+]i was virtually identical in Hepes and CO2/HCO3-. In both buffers, addition of TRH induced a transient increase in [Ca2+]i which attained a significantly higher peak in Hepes (357 +/- 43 nM) when compared with values measured in the presence of CO2/HCO3- (184 +/- 21 nM). In Hepes, the basal IP3-level was higher than in CO2/HCO3-. The TRH-evoked increase in IP3 was higher in magnitude in Hepes than in CO2/HCO3-, although the stimulated/basal ratio was not different between the two buffers. The buffer composition had no effect on the specific binding of 3H-TRH to the cells. Furthermore, the amplitude of the increase in [Ca2+]i evoked by 50 mM K+ was identical in both buffers. TRH and K+ had no effect on pHi in either buffer. The present results indicate that HCO3- has an influence on TRH-induced Ca2+ transient, at least in part by modifying the TRH-evoked production of IP3.

Binding, coupling, and mRNA subtype heterogeneity of alpha 1-adrenergic receptors in cultured human RPE

In retinal pigment epithelium, apically applied epinephrine changes the conductance of specific ions which subsequently affects the membrane voltage and stimulates transepithelial fluid transport. In this investigation, myo-[3H]inositol radiotracer studies, radioligand binding with [125I]HEAT, and ribonuclease protection assays were performed to examine the coupling of this receptor to phosphoinositide hydrolysis and the specific mRNA subtypes expressed in cultured human retinal pigment epithelial cells. After labeling second to sixth passage cells with 3-muCi myo-[3H]inositol for 24 hr, epinephrine caused a dose- and time-dependent increase in [3H]inositol phosphate products (EC50 of 0.7 microM). This stimulation was antagonized by prazosin but not by propranolol. The effect of epinephrine was potentiated by the presence of the monoamine oxidase inhibitor, pargyline (10 microM). Pertussis toxin (1 microgram per well) attenuated the stimulatory effect of epinephrine. In the radioligand binding assays, [125I]HEAT binding sites varied among different cell lines, with a range of 44 to 200 fmol (mg protein)-1. Using a ribonuclease protection assay, alpha 1D and alpha 1B, but not alpha 1C, adrenergic mRNA subtypes were detected in cultured human cells. Collectively, these results show that the catecholamines act on a potentially heterogeneous population of alpha 1-adrenergic receptors coupled to phospholipase C by a pertussis toxin-sensitive G protein.

TRH-evoked entry of extracellular calcium in GH4C1 cells: possible importance of arachidonic acid metabolites

Previous studies have shown that stimulating pituitary GH4C1 cells with thyrotropin-releasing hormone (TRH) evoked a biphasic change in cytosolic free Ca2+ concentration ([Ca2+]i): a rapid release of sequestered Ca2+ due to the production of inositol-1,4,5-trisphosphate, and Ca2+ entry via both voltage-operated Ca2+ channels and a presently unknown voltage-independent influx pathway. The aim of the present study was to further evaluate to which extent the TRH-evoked changes in [Ca2+]i were dependent on entry of extracellular Ca2+, and which mechanisms participated in regulating this Ca2+ entry. Pretreatment of the cells with 4-bromophenylacylbromide (an inhibitor of phospholipase A2), nordihydroguaiaretic acid (an inhibitor of lipoxygenase), and econazole (an inhibitor of both lipoxygenase and cytochrome P-450 enzymes), attenuated the TRH-evoked increase in [Ca2+]i, suggesting that noncyclooxygenase metabolites of arachidonic acid or cytochrome P-450 metabolites may participate in regulating the TRH-evoked entry of extracellular Ca2+. Both nordihydroguaiaretic acid and econazole showed a similar inhibition of the Ca2+ entry, as did SKF 96365, a compound previously shown to inhibit receptor-activated Ca2+ entry. We also showed that arachidonic acid per se increased [Ca2+]i, and acidified the cytosol in GH4C1 cells in a dose-dependent manner. The effects of arachidonic acid was reversed by addition of BSA to the cell suspension. The calcium entry and the activation of the metabolism of arachidonic acid may thus be important components of the TRH-evoked signal-transduction pathway in GH4C1 cells.

Roles of second-messenger systems and neuronal activity in the regulation of lordosis by neurotransmitters, neuropeptides, and estrogen: a review

Many neurotransmitters and neuropeptides can affect the rodent feminine sexual behavior, lordosis, when administered in the ventromedial hypothalamus (VMH), midbrain central gray (MCG), or other brain regions. A survey of the electrophysiological and biochemical actions of these neural agents revealed that there is a very consistent association between lordosis facilitation with both the activation of the phosphoinositide (PI) pathway and the excitation of VMH and MCG neurons. In contrast, lordosis inhibition is associated, less consistently, with alterations of the adenylate cyclase (AC) system and the inhibition of neuronal activity. The findings that lordosis could be facilitated by going beyond membrane receptors and directly activating the PI pathway, suggest that this second-messenger pathway is a common mediator for the lordosis-facilitating agents. Furthermore, as in the case of stimulating membrane receptors, direct activation of this common mediator also requires estrogen priming for lordosis facilitation. Therefore, it is likely that the PI pathway is modulated by estrogen in the permissive action of estrogen priming. Indeed, a literature review shows that estrogen can affect selective isozymes of key enzyme families of the PI pathway at various levels. Such selective modulations, at several levels, could easily alter the course of a PI cascade; thence, the eventual functional outcome. These findings prompt us to propose that estrogen enables lordosis to be facilitated by a selective modulation of the PI pathway.

Calcitriol attenuates the basal and vasoactive intestinal peptide-stimulated cAMP production in prolactin-secreting rat pituitary (GH4C1) cells

A clonal strain of prolactin-producing rat pituitary tumour cells (GH4C1 cells) was used to study the effect of calcitriol on cyclic adenosine monophosphate (cAMP) production. Calcitriol (10 nM) attenuated both the basal and vasoactive intestinal peptide (VIP)-stimulated cAMP production after 2 days' pretreatment of the cells. The effect was detectable at 1 nM and maximal at about 10 nM. Calcitriol was at least 100 times more potent than calcidiol and 24-hydroxycalcidiol. Calcitriol (10 nM, 4 days) did not affect the specific binding of 125I-VIP, but attenuated the guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS)-stimulated (100 microM) adenylyl cyclase activity by 25%. Calcitriol (10 nM, 4 days) also attenuated both the Mn2+ (1 mM) and the forskolin-stimulated (10 microM) adenylyl cyclase activity by 43 and 41%, respectively. In conclusion, these data suggest that calcitriol attenuates the basal and VIP-stimulated cAMP production by inhibiting the catalytic subunit of the adenylyl cyclase as well as the amount of the G protein Gs alpha.

MMQ cells: a model for evaluating the role of G proteins in the modulation of prolactin release

It is well known that dopamine (DA) inhibits while vasoactive intestinal peptide (VIP) and angiotensin II (ANG II) stimulate prolactin (PRL) release from normal anterior pituitary lactotrophs; however, elucidation of the intracellular mechanisms involved in these effects has been hindered by the cellular heterogeneity of the anterior pituitary. MMQ cells, isolated from the PRL-secreting rat pituitary tumor 7315a is an interesting model since they only secrete PRL. In order to determine whether and which GTP-binding (G) proteins are involved in the modulation of cyclic 3',5'-adenosine monophosphate (cAMP) accumulation and phospholipids turnover and eventually PRL release, we have performed studies with MMQ cells. For this purpose, the levels of various G proteins (alpha o, alpha s, alpha i, alpha q and beta) and their mRNAs, measured by Western and Northern blots respectively, were correlated with intracellular cAMP accumulation in response to DA, VIP or DA plus VIP, and with inositol phosphates (IPx) formation in response to ANG II, DA or DA plus ANG II. This study shows that, when compared to normal pituitary tissue, the levels of alpha o, alpha o2 and alpha i3 were significantly decreased in MMQ cells; those of alpha o1, alpha i (alpha i1 + alpha i2), alpha s42 and alpha q were very low or undetectable while those of alpha s47 and beta were normal. DA was unable to inhibit basal PRL release and cAMP accumulation. VIP increased both cAMP accumulation and PRL release, while cAMP accumulation elicited by VIP could be suppressed by DA. BAY K 8644-induced PRL release also could be suppressed by DA.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular free sodium concentrations in GH4C1 cells

In the present investigation, intracellular sodium ([Na+]i) levels were determined in GH4C1 cells using the fluorescent probe SBFI. Fluorescence was determined by excitation at 340 nm and 385 nm, and emission was measured at 500 nm. Intracellular free sodium ([Na+]i) was determined by comparing the ratio 340/385 to a calibration curve. The ratio was linear between 10 and 60 mM Na+. Resting [Na+]i in GH4C1 cells was 26 +/- 6.2 mM (mean +/- SD). In cells incubated in Na(+)-free buffer [Na+]i decreased to 3 +/- 3.6 mM. If Na+/K+ ATPase was inhibited by incubating the cells with 1 mM ouabain, [Na+]i increased to 47 +/- 12.8 mM in 15 min. Stimulating the cells with TRH, phorbol myristyl acetate, or thapsigargin had no effect on [Na+]i. Incubating the cells in Ca(2+)-free buffer rapidly increased [Na+]i. The increase was not inhibited by tetrodotoxin. Addition of extracellular Ca2+, nimodipine, or Ni2+ to these cells immediately decreased [Na+]i, whereas Bay K 8644 enhanced the influx of Na+. In cells where [Na+]i was increased the TRH-induced increase in intracellular free calcium ([Ca2+]i) was decreased compared with control cells. Our results suggest that Na+ enters the cells via Ca2+ channels, and [Na+]i may attenuate TRH-induced changes in [Ca2+]i in GH4C1 cells.

Calciotropic hormones raise the chemically detectable [Pi] in UMR 106-06 osteoblast-like cells

Uptake of orthophosphate (Pi) by osteoblast-like cells is known to be stimulated by parathyroid hormone (PTH), but effects on intracellular [Pi] have not been investigated. Here we show in rat osteoblast-like cells (UMR 106-06) that PTH (10(-11) to 10(-7) M) increases both 32Pi uptake and cellular [Pi] by up to 50 per cent. 1,25 Dihydroxyvitamin D3 (1,25D) (10(-12) to 10(-6) M) and salmon calcitonin (CT) (10(-12) to 10(-6) g ml-1) also increased cellular [Pi] (by up to 60 per cent), but the percentage increases in total cellular 32Pi uptake were smaller. The effects of 1,25D were transient (observable at 80 min and 6 h but not 24 h), and were also observed with 24,25 dihydroxy- and 25 hydroxyvitamin D3. Transient degradation of organic phosphorus pools to Pi might contribute to this increased [Pi]. These pools remain to be identified but were not shown to be phospholipids. Foetal bovine serum also affected cellular [Pi]. Care is therefore needed in distinguishing direct hormonal effects on cellular [Pi] from indirect effects arising from changes in the rate of cell growth.

Somatostatin inhibition of phosphoinositides turnover in isolated rat acinar pancreatic cells: interaction with bombesin

The effects of somatostatin-14 and bombesin on [3H]inositol phosphate accumulation were studied in 24 h myo-[3H]inositol-prelabeled cultured rat acinar cells. Bombesin, 10 nM, stimulated basal formation of phosphatidyl monophosphate (InsP1), phosphatidyl 4,5-biphosphate (InsP2) and inositol 1,4,5-triphosphate (InsP3) by 128 +/- 5.2%, 147 +/- 10% and 155 +/- 5%, respectively. At 5 s, the ED50 value for InsP3 stimulation was 0.70 +/- 0.2 nM. This stimulation was partly blocked (64 +/- 0.04% inhibition) by 10 ng/ml Bordetella pertussis toxin. In contrast to bombesin, somatostatin, 10 nM, inhibited basal InsP1, InsP2 and InsP3 formation. At 5 s, the inhibition degree for InsP3 was 18 +/- 2.5% and the IC50s values 1 +/- 0.09 nM, 1 +/- 0.12 nM and 0.07 +/- 0.005 nM for InsP1, InsP2 and InsP3, respectively. Bombesin-stimulated InsP3 formation was also inhibited by somatostatin. At 5 s, the inhibition degree was 85 +/- 3.5% at 10 nM and the IC50 value, 0.10 +/- 0.05 nM. Furthermore, somatostatin inhibition of bombesin stimulation was partly blocked (66 +/- 4% inhibition) by Bordetella pertussis toxin. These data therefore suggest that the acinar pancreatic cells contain a somatostatin receptor exerting a negative control on basal and bombesin receptor-stimulated phosphatidyl inositol turnover.

Modulation of G proteins and second messenger responsiveness by steroid hormones in GH3rat pituitary tumour cells

We have investigated the modulation of different G protein alpha- and beta-subunit levels in prolactin (PRL) and growth hormone producing rat pituitary adenoma cells (GH3 cells) in culture after prolonged exposure (6-48 h) to the steroid hormones 17 beta-oestradiol and dexamethasone. Gi-3 alpha- and G beta-subunits were the only G protein subunits which increased in response to 10(-6) M oestradiol (to approximately 150 and 200% of controls, respectively), while the other alpha-subunits investigated (Gs alpha, Gi-2 alpha and G(o) alpha) remained relatively unchanged. Thyroliberin (TRH)--and guanosine 5'-[beta gamma-imido]trisphosphate (Gpp(NH)p)-elicited adenylyl cyclase (AC) activities were reduced during 6-12 h of oestradiol treatment (by 60 and 20%, respectively), while the inhibitory effect of somatostatin (SRIF) increased by approximately 100%. Dexamethasone (10(-6) M) increased levels of the stimulatory G protein Gs alpha (to approximately 340%) and decreased levels of Gi-3 alpha (to 25%). After 48 h, the AC response to TRH was reduced by approximately 70%, whereas the effect of the other modulators remained close to controls. We conclude that G protein subunits in GH3 cells are subject to specific regulation by steroid hormones and that this may be important in the tuning of the responsiveness of PRL secretion to hormones in the in vivo situation.

Paradoxical signal transduction mechanism of endothelins and sarafotoxins in cultured pituitary cells: stimulation of phosphoinositide turnover and inhibition of prolactin release

Endothelins (ET-1, ET-2, ET-3 and vasoactive intestinal contractor, VIC) and sarafotoxins (SRTX-b and SRTX-c) appear to bind with high affinity to a homogeneous class of binding sites in cultured rat pituitary cells. All of these ligands seem to interact with the same receptor (ETA-R), except for SRTX-c which apparently binds to a separate receptor. Binding was followed by phosphodiesteric cleavage of phosphoinositides, resulting in the formation of inositol phosphates. No consistent effect on basal or gonadotropin-releasing hormone (GnRH)-induced release of luteinizing hormone (LH) was exerted by ET or SRTX during 2 h of static incubation. On the other hand, both groups of vasoactive peptides inhibited basal and thyrotropin-releasing hormone (TRH)-induced prolactin secretion. Surprisingly, activation of phosphoinositide turnover by TRH in pituitary mammotrophs led to stimulation of prolactin secretion, whereas activation of the same pathway by ET or SRTX resulted in inhibition of prolactin secretion. ET and SRTX stimulated inositol phosphate formation in GH3 cell line and in the gonadotroph-like cell line alpha T-3 (which is capable of producing the alpha subunit of the gonadotrophins), indicating that the peptides interact with both pituitary mammotrophs and gonadotrophs. The very low concentrations (nM range) needed to stimulate phosphoinositide turnover and to inhibit prolactin secretion, as well as the recent finding that ETs are present in the hypothalamo-pituitary axis suggest that ET might participate in the neuroendocrine modulation of pituitary functions. One such possibility is that ETs might be members of the prolactin inhibiting factors (PIFs) family.

Characterization of the TRH-induced activation of Na+/H(+)-exchange in pituitary GH4C1 cells

In the present study in GH4C1 cells, the dependence of TRH-induced activation of Na+/H(+)-exchange on extracellular Na+ and Ca2+ was examined. Furthermore, the effects of both extracellular and intracellular H+ on Na+/H(+)-exchange were investigated. The buffering capacity was 63 +/- 11.8 mM (pH unit)-1 at basal intracellular pH (pHi) of 7.02 +/- 0.02. The initial rate of alkalinization in cells acidified with nigericin increased with increasing concentrations of extracellular Na+ according to simple Michaelis-Menten kinetics. The apparent Km-value for Na+ was 53 +/- 17.5 mM and the Vmax value was 28 +/- 4.5 mM H+/min. Addition of Na+ together with TRH increased Vmax to 56 +/- 6.4 mM H+/min (P < 0.05), while no difference was observed in Km. Decreasing extracellular pH (pHo) decreased the rate of alkalinization of acid-loaded cells, despite a large inward Na+ gradient. Furthermore, a decrease in pHi was necessary to obtain activation of Na+/H+ exchange. At pHi-values close to basal pHi no activation of Na+/H(+)-exchange was obtained. In addition, the results showed that extracellular Ca2+ was necessary for TRH-induced activation of Na+/H+ exchange. Blocking influx of extracellular Ca2+ with Ni2+ abolished the effect of TRH, suggesting that the TRH-induced activation of Na+/H(+)-exchange in GH4C1 cells is dependent on influx of extracellular Ca2+.

Alpha 1-adrenoceptors in the corneal endothelium

The potent alpha 1-adrenoceptor antagonist, [3H]prazosin, exhibited high affinity, specific and reversible binding to intact rabbit, bovine and human corneal endothelial cells in culture. The binding of 1 nM [3H]prazosin to rabbit cells reached a steady-state level within 10 min at 37 degrees C. Under these conditions, approximately 50% of the [3H]prazosin bound was specific. The level of specific [3H]prazosin binding was concentration-dependent, but Rosenthal analysis indicated that [3H]prazosin bound to at least two sites. One site exhibited a high affinity for [3H]prazosin (Kd = 0.2 nM), but a relatively low binding capacity (Bmax = 175 fmol bound mg-1 protein); the other site showed a relatively low affinity for the radioligand (Kd = 85 nM), but a much higher binding capacity (1280 fmol mg-1). Several known alpha 1-adrenoceptor antagonists and agonists competitively inhibited [3H]prazosin binding at the high affinity site when incubated with the radioligand. The relative potencies of these competing ligands were generally consistent with their binding affinities for alpha 1-adrenoceptors in other tissues. Phenylephrine stimulated the rate of hydrolysis of phosphatidylinositol 4,5-bisphosphate to inositol 1,4,5-trisphosphate by 63% in these cells. This stimulation was inhibited by 52% if phentolamine was also present during the incubation. These data indicate that corneal endothelial cells have alpha 1-adrenoceptors which can modulate polyphosphoinositide turnover in this tissue.

Hypothalamic hormones modulate G protein levels and second messenger responsiveness in GH3 rat pituitary tumour cells

Thyroliberin (TRH), vasoactive intestinal peptide (VIP) and somatostatin (SRIF) act through receptors that are coupled to guanine nucleotide-binding regulatory proteins (G proteins). Regulation of hormone action may occur at the level of G protein coupling to the receptor or effector systems. In this study we demonstrate that prolonged exposure (for up to 48 hr) of cultured rat pituitary adenoma GH3 cells to these hormones caused homologous and to some extent heterologous attenuation of the adenylyl cyclase (AC) (EC 4.6.1.1) responsiveness. In addition, TRH and SRIF diminished both TRH- and guanosine 5'-[beta gamma-imido]-triphosphate-enhanced phospholipase C (PLC) (EC 3.1.4.3) activity within the same time-course. Measurements of cells membrane levels of Gs protein alpha-subunit (Gs alpha), G(i)-1 alpha/G(i)-2 alpha, G(i)-3 alpha, G(o) alpha and G beta by immunoblotting were performed. TRH and VIP upregulated levels of all G proteins except G(o) alpha and G beta. In contrast, SRIF caused a marked reduction of G beta levels. Thus, TRH and VIP, both acting through Gs, both modulated the alpha-subunit levels of this signal transducer, whereas SRIF, which possibly acts through G(i)-2, did not change the steady state level of G(i)-2 alpha. The actions of TRH, VIP and SRIF are multifaceted at the G protein level, where modulations of subtypes not directly involved in their actions may occur. These findings emphasize the complexity expected to be found in the in vivo situation.

Association of functional alteration in intracellular signal transduction systems with the occurrence of up-regulation of muscarinic receptors in primary cultured neurons

1. Alterations in intracellular biosignaling systems associated with the up-regulated muscarinic receptor were investigated using primary cultured neurons exposured to 10 nM atropine for 5 days. 2. In neurons treated with atropine, the response of PI turnover to muscarinic stimulation decreased in comparison with that in non-treated neurons. 3. The GTP gamma S-stimulated PI turnover also reduced in neurons possessing up-regulated muscarinic receptor, although phospholipase C activity was not different in these two types of neurons. 4. In addition, the long-term exposure (5 days) to atropine induced the increase in GTPase activity and [3H]GppNHp binding. 5. These results suggest that the decreased response of PI turnover to muscarinic stimulation may be attributed to the functional deterioration of G-protein itself and/or coupling between G-protein and phospholipase C. 6. The present results also strongly suggest that the accentuation of the function of G protein may occur in association with the up-regulation of muscarinic receptor.

Changes in cyclic strain increase inositol trisphosphate and diacylglycerol in endothelial cells

The purpose of this study was to assess the effect of cyclic strain on phosphatidylinositol turnover in cultured bovine aortic endothelial cells (EC). Confluent EC grown on flexible membrane bottoms were deformed by vacuum to 24% maximum strain and subjected to two cyclic strain regimens. In the first set of experiments, EC were subjected to deformation at a frequency of 60 cycles/min for either 0 (stationary control), 1, 5, 10, 25, 50, or 100 cycles of stretch. In the second set of experiments, EC were preconditioned by deforming the membranes at 60 cycles/min for 24 h. The cycling frequency was then acutely increased to 100 cycles/min for 0, 1, 5, 10, 25, or 100 cycles. Inositol phosphate and diacylglycerol concentrations were determined at the end of each regimen. The results demonstrate that either the initiation of pulsatile stretch or an acute change in cyclic stretch frequency stimulates a sequential and transient generation of inositol trisphosphate, its metabolites inositol biphosphate and monophosphate, and diacylglycerol. Thus EC may respond to the initiation and change in cyclic stretch frequency by a signal transduction pathway involving inositol lipid metabolism.

Thyroid hormones regulate the formation of inositol phosphate in response to thyrotropin-releasing hormone in rat anterior pituitaries

The effects of thyroid hormones on TSH secretion and inositol phospholipid hydrolysis in response to thyrotropin-releasing hormone (TRH) in rat anterior pituitaries were examined. Experimental hypothyroidism caused a significant increase in [3H]inositol phosphate ([3H]IP) formation in response to TRH in rat anterior pituitaries with a concomitant elevation of blood TSH. In contrast, administration of thyroxine (T4) to hypothyroid rats resulted in a complete restoration of blood TSH and TRH-stimulated [3H]IP formation to the euthyroid control value. Furthermore, in vitro pre-treatment of anterior pituitaries with triiodothyronine (T3) produced a dose-dependent decrease in both TSH secretion and the formation of [3H]IP in response to TRH. These results indicate that thyroid hormones regulate TRH receptor-linked inositol phospholipid hydrolysis in the rat anterior pituitary, suggesting that negative feedback action of thyroid hormone occurs at post receptor event in the rat anterior pituitary, which may, to a certain extent, be responsible for the underlying mechanism of T3 inhibition of TSH secretion.

PMA-sensitive protein kinase C is not necessary in TRH-stimulated prolactin release from female rat primary pituitary cells

In GH3 cells and other clonal rat pituitary tumor cells, TRH has been shown to mediate its effects on prolactin release via a rise of cytosolic Ca2+ and activation of protein kinase C. In this study, we examined the role of protein kinase C in TRH-stimulated prolactin release from female rat primary pituitary cell culture. Both TRH and PMA stimulated prolactin release in a dose-dependent manner. When present together at maximal concentrations, TRH and PMA produced an effect which was slightly less than additive. Pretreatment of rat pituitary cells with 10(-6) M PMA for 24 hrs completely down-regulated protein kinase C, since such PMA-pretreated cells did not release prolactin in response to a second dose of PMA. Interestingly, protein kinase C down-regulation had no effect on TRH-induced prolactin release from rat pituitary cells. In contrast, PMA-pretreated GH3 cells did not respond to a subsequent stimulation by either PMA or TRH. Pretreatment of rat pituitary cells with TRH (10(-7) M, 24 hrs) inhibited the subsequent response to TRH, but not PMA. Forskolin, an adenylate cyclase activator, stimulated prolactin release by itself and in a synergistic manner when incubated together with TRH or PMA. The synergistic effects of forskolin on prolactin release was greater in the presence of PMA than TRH. Down-regulation of protein kinase C by PMA pretreatment abolished the synergistic effect produced by PMA and forskolin but had no effect on those generated by TRH and forskolin. sn-1,2-Dioctanylglycerol (DOG) pretreatment attenuated the subsequent response to DOG and PMA but not TRH. The effect of TRH, but not PMA, on prolactin release required the presence of extracellular Ca2+. In conclusion, the mechanism by which TRH causes prolactin release from rat primary pituitary cells is different from that of GH3 cells; the former is a protein kinase C-independent process whereas the latter is at least partially dependent upon the activation of protein kinase C.