Redistribution of G(q/11)alpha in the pituitary gonadotrope in response to a gonadotropin-releasing hormone agonist

Ca2+ responses to thyrotropin-releasing hormone and angiotensin II: the role of plasma membrane integrity and effect of G11alpha protein overexpression on homologous and heterologous desensitization

The molecular mechanisms involved in GPCR-initiated signaling cascades where the two receptors share the same signaling cascade, such as thyrotropin-releasing hormone (TRH) and angiotensin II (ANG II), are still far from being understood. Here, we analyzed hormone-induced Ca(2+) responses and the process of desensitization in HEK-293 cells, which express endogenous ANG II receptors. These cells were transfected to express exogenously high levels of TRH receptors (clone E2) or both TRH receptors and G(11)alpha protein (clone E2M11). We observed that the characteristics of the Ca(2+) response, as well as the process of desensitization, were both strongly dependent on receptor number and G(11)alpha protein level. Whereas treatment of E2 cells with TRH or ANG II led to significant desensitization of the Ca(2+) response to subsequent addition of either hormone, the response was not desensitized in E2M11 cells expressing high levels of G(11)alpha. In addition, stimulation of both cell lines with THR elicited a clear heterologous desensitization to subsequent stimulation with ANG II. On the other hand, ANG II did not affect a subsequent response to TRH. ANG II-mediated signal transduction was strongly dependent on plasma membrane integrity modified by cholesterol depletion, but signaling through TRH receptors was altered only slightly under these conditions. It may be concluded that the level of expression of G-protein-coupled receptors and their cognate G-proteins strongly influences not only the magnitude of the Ca(2+) response but also the process of desensitization and resistance to subsequent hormone addition.

Disruption of the plasma membrane integrity by cholesterol depletion impairs effectiveness of TRH receptor-mediated signal transduction via G(q)/G(11)alpha proteins

We monitored the radioligand-binding characteristics of thyrotropin-releasing hormone (TRH) receptors, functional activity of G(q/11)alpha proteins, and functional status of the whole signaling cascade in HEK293 expressing high levels of TRH receptors and G(11)alpha. Our analyses indicated that disruption of plasma membrane microdomains by cholesterol depletion did not markedly influence the binding parameters of TRH receptors, but it altered efficacy of signal transduction. The functional coupling between TRH receptor and G(q/11)alpha was assessed by agonist-stimulated [(35)S]GTPgammaS binding, and results of these measurements pointed out to significantly lower potency of TRH to mediate G protein activation in the plasma membrane fraction isolated from cholesterol-depleted cells; there was a shift in sensitivity by one order of magnitude to the higher concentrations. A markedly lower sensitivity to stimulation with TRH was also observed in our experiments dealing with determination of hormone-induced Ca(2+) response. These data suggest that the intact structure of plasma membranes is an important optimum signal transduction initiated by TRH receptors and mediated by G(q/11)alpha proteins.

A mathematical model for LH release in response to continuous and pulsatile exposure of gonadotrophs to GnRH

In a previous study, a model was developed to investigate the release of luteinizing hormone (LH) from pituitary cells in response to a short pulse of gonadotropin-releasing hormone (GnRH). The model included: binding of GnRH to its receptor (R), dimerization and internalization of the hormone receptor complex, interaction with a G protein, production of inositol 1,4,5-trisphosphate (IP₃), release of calcium from the endoplasmic reticulum (ER), entrance of calcium into the cytosol via voltage gated membrane channels, pumping of calcium out of the cytosol via membrane and ER pumps, and release of LH. The extended model, presented in this paper, also includes the following physiologically important phenomena: desensitization of calcium channels; internalization of the dimerized receptors and recycling of some of the internalized receptors; an increase in G_q concentration near the plasma membrane in response to receptor dimerization; and basal rates of synthesis and degradation of the receptors. With suitable choices of the parameters, good agreement with a variety of experimental data of the LH release pattern in response to pulses of various durations, repetition rates, and concentrations of GnRH were obtained. The mathematical model allows us to assess the effects of internalization and desensitization on the shapes and time courses of LH response curves.

Regulation of galpha i palmitoylation by activation of the 5-hydroxytryptamine-1A receptor

Nearly all alpha subunits of heterotrimeric GTP-binding regulatory proteins (G proteins) are palmitoylated at cysteine residues near the N terminus. A regulated cycle of palmitoylation could provide a mechanism for modulating G protein signaling by affecting protein interactions and localization of the subunit. In the present studies we utilized both [(3)H]palmitate incorporation and pulse-chase techniques to address the dynamics of alpha(i) palmitoylation in Chinese hamster ovary cells. Both techniques demonstrated a dose- and time-dependent change in [(3)H]palmitate labeling of alpha(i) upon activation of stably expressed 5-hydroxytryptamine-1A receptors by the agonist (+/-)-2-dipropylamino-8-hydroxy-1,2,3, 4-tetrahydronaphthalene hydrobromide (DPAT), with an EC(50) of approximately 10 nm. For the incorporation assay, DPAT elicited an approximate doubling in labeling at the earliest time point measured. For the pulse-chase assay, DPAT promoted a significant loss of radiolabel almost equally as fast. These data demonstrate that the exchange of palmitate on alpha(i) is increased upon stimulation of 5-hydroxytryptamine-1A receptors through the combined processes of depalmitoylation and palmitoylation. These results provide the basis for extending the concept of regulated exchange of palmitate beyond G(s) and provide a framework for exploring the specific functional attributes of the palmitoylated and depalmitoylated forms of subunit.

Gonadotropin-releasing hormone receptor concentration differentially regulates intracellular signaling pathways in GGH3 cells

Pituitary cell lines (GGH3) expressing the GnRH receptor (GnRHR) were used to investigate the effect of GnRHR concentration on the ability of a GnRH agonist to activate second messenger systems. Four different strategies were utilized to generate cells expressing functionally different concentrations of receptors: (1) transient transfection with different concentrations of wild type GnRHR into GH3 cells, (2) utilization of two cell lines derived from a common stably transfected line expressing high (4,209 +/- 535 receptors/cell) or low (1,031 +/- 36 receptors/cell) concentrations of GnRHR, (3) co-incubation of GGH3-1' cells with a GnRH agonist (Buserelin) and a GnRH antagonist to compete for binding sites, and (4) photo-affinity binding to GnRHR with a GnRH antagonist to change effective receptor concentration. A range of receptor concentrations (1,000-8,000 receptors/cell) were generated by these techniques. Inositol phosphate (IP) and cAMP accumulation were quantified to assess the effect of receptor concentration on receptor-effector coupling. Under all four paradigms, the efficacy and potency of Buserelin stimulated IP production was dependent on receptor concentration. In contrast, Buserelin stimulated cAMP release was relatively unchanged at varying concentrations of GnRHR. This suggests that the cellular concentration of GnRHR affects the induction of cell signaling pathways. These results demonstrate that a single ligand-receptor-complex can differentially activate second messenger systems and present a mechanism by which multiple physiological endpoints can be differentially regulated by a single hormone/receptor interaction.

Distribution of signaling molecules involved in vasopressin-induced Ca2+ mobilization in rat hepatocyte multiplets

In freshly isolated rat hepatocyte multiplets, Ca2+ signals in response to vasopressin are highly organized. In this study we used specific probes to visualize, by fluorescence and confocal microscopy, the main signaling molecules involved in vasopressin-mediated Ca2+ responses. V1a receptors were detected with a novel fluorescent antagonist, Rhm8-PVA. The Galphaq/Galpha11, PLCbeta3, PIP2, and InsP3 receptors were detected with specific antibodies. V1a vasopressin receptors and PIP2 were associated with the basolateral membrane and were not detected in the bile canalicular domain. Galphaq/Galpha11, PLCbeta3, and InsP3 receptors were associated with the basolateral membrane and also with other intracellular structures. We used double labeling, Western blotting, and drugs (cytochalasin D, colchicine) known to disorganize the cytoskeleton to demonstrate the partial co-localization of Galphaq/Galpha11 with F-actin.

Modulation of gonadotropin levels by peptides acting at the anterior pituitary gland

There are several lines of evidence that point to peptides participating in the regulation of LH and/or FSH levels by action at the pituitary. This evidence includes altered secretion of gonadotropins from the anterior pituitary cells or tissue in vitro when exposed to the peptide. Additionally, modification of GnRH-stimulated LH/FSH secretion has been observed. Furthermore, there is potential for a separately modulated interaction with the primed response. Another potential of action is by interaction among non-GnRH peptides on gonadotropin-regulating processes, although there are no good data available on this aspect. Other observations, consistent with a pituitary role for the peptides in modulation of LH, include detection of the peptides in portal blood, detection of high-affinity receptors or receptor mRNA in the pituitary, and detection of intrapituitary peptide or peptide mRNA in the pituitary. The modulation by steroids of both concentrations and type of activities provides a further level of physiological refinement. There is, however, some confusion regarding the involvement of these peptides in gonadotropin control. The reasons can be seen by considering aspects of investigations. There are experimental variations such as 1) species studied, e.g., NPY has been reported to have an effect on LH secretion from rat cells (168) but not on sheep anterior pituitary tissue (64), and substance P inhibits GnRH-stimulated release from rat cells (182) but potentiates the response in prepubertal porcine cells (92); 2) the steroidal conditions under which the study is performed, e.g., NPY has opposite effects in certain endocrine environments, augmenting GnRH-stimulated LH release in proestrus-like conditions (168), and inhibiting in metestrus-like environment (66); 3) the type of cell preparation, e.g., responsiveness to substance P might depend on whether cells in overnight culture were in separated or clustered state (91); 4) the time course considered, e.g., oxytocin that might induce marked LH release from pituitary cells after a longer length of incubation than GnRH requires (68); 5) length of exposure to peptide, e.g., endothelin that augmented or inhibited GnRH-stimulated LH release (50); 6) In addition, it is possible that the traditional endpoint selected in such studies, namely, observation of gonadotropin secretion, is not necessarily the most important for these peptides (56, 81, 117). Unfortunately, at this stage a definitive answer to the question "What do the peptides actually do?" cannot be provided and we remain tantalized by the glimpses of potential roles. Perhaps in a few years an updated review will be able to include a more complete answer. It is necessary for the full understanding of LH control that not only the properties of the peptides in isolation be characterized but also their interactions.

Gonadotropin-releasing hormone receptor couples to multiple G proteins in rat gonadotrophs and in GGH3 cells: evidence from palmitoylation and overexpression of G proteins

There is evidence in several cell systems suggesting that the GnRH receptor couples to multiple G proteins. Presently there are no published studies showing GnRH receptor coupling to Gialpha, Gsalpha, and Gq/11alpha in a single cell type. To examine this possibility we measured palmitoylation of G proteins in response to GnRH receptor occupancy, since this event is a measure of G-protein activation by cognate receptors. GnRH stimulated time (0-120 min)- and dose (10(-12)-10(-6) g/ml)-dependent palmitoylation of both Gialpha and Gsalpha. Palmitoylation is G-protein activation dependent; accordingly, pertussis toxin (100 ng/ml; PTX), phorbol myristic acid (100 ng/ml), and Antide (50 nM; a GnRH antagonist) did not stimulate palmitoylation of Gialpha or Gsalpha above basal levels. However, cholera toxin (5 microgram/ml), an activator of Gsalpha, stimulated palmitoylation of Gsalpha but not Gialpha. We used a lactotrope-derived cell line expressing the GnRH receptor (GGH3) to examine whether the ability of the receptor to couple multiple G proteins is gonadotroph specific. GGH3 cells were transfected with specific cDNA coding for different G proteins, and agonist-stimulated second messenger production was assessed. Buserelin (a GnRH agonist) stimulated increased cAMP release in Gsalpha cDNA-transfected GGH3 cells, whereas in Gialpha cDNA-transfected cells, both inositol phosphate (IP) production and cAMP release were decreased in response to buserelin. Transfection of Gqalpha, G11alpha, G14alpha, and G15alpha cDNA into GGH3 cells resulted in an increased IP production in response to buserelin, indicating that GnRH receptor couples to this PTX-insensitive G-protein family. The observations presented in this study provide evidence for GnRH receptor coupling to multiple G proteins in a single cell type.