Demonstration of a heterogenous population of binding sites for thyroliberin in prolactin-producing tumour cells and their possible functional significance

Specificity of TRH receptor coupling to G-proteins for regulation of ERG K+ channels in GH3 rat anterior pituitary cells

The identity of the G-protein coupling thyrotropin-releasing hormone (TRH) receptors to rat ether-à-go-go related gene (r-ERG) K+ channel modulation was studied in situ using perforated-patch clamped adenohypophysial GH(3) cells and dominant-negative variants (Galpha-QL/DN) of G-protein alpha subunits. Expression of dominant-negative Galpha(q/11) that minimizes the TRH-induced Ca2+ signal had no effect on r-ERG current inhibition elicited by the hormone. In contrast, the introduction of dominant-negative variants of Galpha13 and the small G-protein Rho caused a significant loss of the inhibitory effect of TRH on r-ERG. A strong reduction of this TRH effect was also obtained in cells expressing either dominant-negative Galpha(s) or transducin alpha subunits, an agent known to sequester free G-protein betagamma dimers. As a further indication of specificity of the dominant-negative effects, only the dominant-negative variants of Galpha13 and Rho (but not Galpha(s)-QL/DN or Galpha(t)) were able to reduce the TRH-induced shifts of human ERG (HERG) activation voltage dependence in HEK293 cells permanently expressing HERG channels and TRH receptors. Our results demonstrate that whereas the TRH receptor uses a G(q/11) protein for transducing the Ca2+ signal during the initial response to TRH, this G-protein is not involved in the TRH-induced inhibition of endogenous r-ERG currents in pituitary cells. They also identify G(s) (or a G(s)-like protein) and G13 as important contributors to the hormonal effect in these cells and suggest that betagamma dimers released from these proteins may participate in modulation of ERG currents triggered by TRH.

The human neuroendocrine thyrotropin-releasing hormone receptor promoter is activated by the haematopoietic transcription factor c-Myb

Thyrotropin-releasing hormone (TRH) receptor (TRHR) is a G-protein-coupled receptor playing a crucial role in the anterior pituitary where it controls the synthesis and secretion of thyroid-stimulating hormone and prolactin. Its widespread presence not only in the central nervous system, but also in peripheral tissues, including thymus, indicates other important, but unknown, functions. One hypothesis is that the neuropeptide TRH could play a role in the immune system. We report here that the human TRHR promoter contains 11 putative response elements for the haematopoietic transcription factor c-Myb and is highly Myb-responsive in transfection assays. Analysis of Myb binding to putative response elements revealed one preferred binding site in intron 1 of the receptor gene. Transfection studies of promoter deletions confirmed that this high-affinity element is necessary for efficient Myb-dependent transactivation of reporter plasmids in CV-1 cells. The Myb-dependent activation of the TRHR promoter was strongly suppressed by expression of a dominant negative Myb-Engrailed fusion. In line with these observations, reverse transcriptase PCR analysis of rat tissues showed that the TRHR gene is expressed both in thymocytes and bone marrow. Furthermore, specific, high-affinity TRH agonist binding to cell-surface receptors was demonstrated in thymocytes and a haematopoietic cell line. Our findings imply a novel functional link between the neuroendocrine and the immune systems at the level of promoter regulation.

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.

Transcription of the human thyrotropin-releasing hormone receptor gene-analysis of basal promoter elements and glucocorticoid response elements

The gene for the human thyrotropin-releasing hormone receptor (TRHR) spans 35 kb and contains three exons and two introns (Matre et al. (1999) J. Neurochem. 72, 1-11). Despite a reported transcription start site (TSS) mapped to position -885 upstream of the translation initiation codon (Iwasaki et al. (1996) J. Biol. Chem. 271, 22183-8), we found cell type specific promoter activity directed by a fragment downstream of this site (-770 to +1). To elucidate the basis for this unexpected activity, we analyzed basal promoter elements in this region of the gene. One divergent TATA box, TTTAAA in position -759, was found by mutational analysis to be critical for promoter activity, providing a likely explanation for the basal activity observed. This proximal region apparently contains several promoter elements, including Pit-1 binding sequences within the first intron of the TRHR gene as previously reported. Here we describe the analysis of two putative glucocorticoid response elements (GREs) that we identified in this region, one (distal) half site overlapping the proposed TSS at -885 and one (proximal) full site within the first intron at position -624. Accordingly, stimulation of rat pituitary GH3 and GH4C1 cells with dexamethasone strongly enhanced transcription activity of a reporter construct containing the distal GRE half site and the proximal GRE site. Both sites bound the glucocorticoid receptor (GR) in a specific manner. Deletion of the distal GRE half site abolished the dexamethasone induction of CAT transcription, as did mutations in the proximal site. We therefore conclude that both sites are necessary for regulation of the TRHR gene transcription by glucocorticoids.

Direct effects of vitamin D3 analogues on G-protein mediated signalling systems in rat osteosarcoma cells and rat pituitary adenoma cells

In normal rats treated with 1,25(OH)2D3 or 24,25(OH)2D3, serum Ca2+, ALP, PRL and GH are significantly altered. In order to study the primary effect of vitamin D3 analogues on target organ function, rat UMR 106 osteosarcoma and GH3 pituitary adenoma cells in monolayer culture were exposed accordingly. Surprisingly, prolonged exposure of these cell lines to physiological levels of either 1,25(OH)2D3 or 24,25(OH)2D3 did not significantly affect the secretory parameters (ALP, PRL or GH) tested. However, 1,25(OH)2D3 exposure significantly reduced PTH- and Gpp(NH)p-elicited AC as well as Gpp(NH)p-stimulated PLC activities in the UMR 106 cells. These changes were accompanied by an increase and decrease in the membrane contents of the G-protein subunits G36 beta and Gq/11 alpha, respectively. In contrast, 24,25(OH)2D3 remained without significant biological effect on these signalling systems despite concomitantly augmented levels of G36 beta. TRH- and Gpp(NH)p-elicited PLC activities in the GH3 cells were significantly reduced by 1,25(OH)2D3 with a concurrent reduction in cellular amounts of Gq/11 alpha, however, 24,25(OH)2D3 did not significantly alter any signalling systems nor G-proteins analyzed. It is concluded that the osteoblastic and pituitary cell secretion of ALP, PRL and GH remain unaffected by the presence of 1,25(OH)2D3 and 24,25(OH)2D3, despite distinct alterations in components of G-protein mediated signalling pathways. Hence, other factors like ambient Ca2+ may be responsible for the perturbed secretory patterns of ALP and PRL seen in vitamin D3 treated rats.

Cell-specific expression and function of adenylyl cyclases in rat pituitary tumour cell lines

The present study demonstrates cell-specific distribution and describes distinct functional regulation of different adenylyl cyclases (AC, types I-VI) in rat pituitary cell tumor cell lines (GH12C1, GH3 and GH4C1 cells) and pituitary tissue. Northern-blot analysis revealed a distinct pattern of cell-specific expression of the different AC types; Ca2+/calmodulin (CaM)-insensitive AC type II was found in all cell lines tested except GH(1)2C1 cells. The Ca(2+)-inhibitable AC type VI was found in all cell types tested. We observed a lack of the Ca2+/CaM-sensitive AC type I in GH3 and GH4C1 cells. GH(1)2C1 cells exclusively contained both Ca2+/CaM-sensitive AC types I and III, the latter previously believed to be specific for olfactory tissue. An additional transcript of AC type III was found in rat brain and rat liver tissue. AC type IV, which is Ca2+/CaM insensitive, could be detected in the prolactin-producing GH3 and GH4C1 cells and pituitary tissue but not in growth-hormone-producing GH(1)2C1 cells. Basal and vasoactive-intestinal-peptide-(VIP)-releasing-hormone, somatostatin (SRIF) and thyrotropin-releasing-hormone (TRH)-modulation of AC activity was measured in the presence of 100 microM EGTA, anti-CaM serum (dilution 1:2000) or 10 microM trifluoroperazine. Antisera against guanine-nucleotide-binding protein (G-protein) alpha subunits (G(i)-2 alpha, Gs alpha) and beta subunits (G beta 35/36) and CaM were added for functional studies of the SRIF and VIP-modulated AC in GH(1)2C1 and GH3 cells. These experiments indicate that the VIP and the SRIF receptors are coupled to a Ca2+/CaM-sensitive AC in GH(1)2C1 cells, different from the AC involved in the regulation of cAMP levels in GH3 and GH4C1 cells. In addition, the beta gamma-complex is possibly able to modulate SRIF-inhibited AC activity by potentiating the inhibitory effect. The TRH receptor in GH3 and GH4C1 cells is coupled to a Ca2+/CaM-sensitive AC which is different from the already cloned forms of AC types I and III. We, therefore, conclude that hormone regulation of pituitary tumour cell functions differs between the GH cell lines, due to specific utilisation of AC types.

Identification of thyrotropin-releasing hormone receptor messenger RNA in the rat central nervous system and eye

TRH exerts a wide variety of neuropharmacological actions by interacting with specific receptors in the central nervous system (CNS). Specific binding sites for TRH have been identified also in the mammalian retina. However, whether TRH receptors (TRH-R) in the brain and retina are identical in structure with those in the anterior pituitary gland is presently unknown. In this study, TRH-R gene expression was examined by Northern blot analysis in the CNS and eye using a cloned rat pituitary TRH-R cDNA. Northern analysis demonstrated a specific hybridization band of approximately 3.8 kb in hypothalamus, cerebrum, cerebellum, brain stem, spinal cord, and eye, indistinguishable from that characterized in pituitary gland. These data strongly support the hypothesis that a TRH receptor similar or identical to that cloned from the pituitary occurs in the retina and throughout the CNS.

The thyroliberin receptor interacts directly with a stimulatory guanine-nucleotide-binding protein in the activation of adenylyl cyclase in GH3 rat pituitary tumour cells. Evidence obtained by the use of antisense RNA inhibition and immunoblocking of the stimulatory guanine-nucleotide-binding protein

The thyroliberin receptor in GH3 pituitary tumour cells is known to couple to phospholipase C via a guanine-nucleotide-binding protein (G protein). Thyroliberin is postulated also to activate adenylyl cyclase, via the stimulatory G protein (Gs). In order to study this coupling, we constructed an antisense RNA expression vector that contained part of the Gs alpha-subunit cDNA clone (Gs alpha) in an inverted orientation relative to the mouse metallothionein promoter. The cDNA fragment included part of the coding region and all of the 3' non-translated region. Transient expression of Gs alpha antisense RNA in GH3 cells resulted in the specific decrease of Gs alpha mRNA levels, followed by decreased Gs alpha protein levels. Thyroliberin-elicited adenylyl cyclase activation in membrane preparations showed a reduction of up to 85%, whereas phospholipase C stimulation remained unaffected. Activation of adenylyl cyclase by vasoactive intestinal peptide was reduced by 30-40%. Investigation of the effects of thyroliberin and vasoactive intestinal peptide on adenylyl cyclase in GH3 cell membranes pretreated with antisera against Gs alpha and Gi-1 alpha/Gi-2 alpha support the results obtained by the use of the antisense technique. We conclude that thyroliberin has a bifunctional effect on GH3 cells, in activating adenylyl cyclase via Gs or a Gs-like protein in addition to the coupling to phospholipase C.

Cell specific distribution of guanine nucleotide-binding regulatory proteins in rat pituitary tumour cell lines

To investigate the effects of guanine nucleotide-binding regulatory proteins (G proteins) on hormonal regulation of prolactin (PRL) synthesis and secretion, the qualitative distribution of G protein alpha-subunits and their mRNAs was studied in three functionally different pituitary tumour cell lines (GH cells) and normal rat pituitary tissue. Levels of basal and modulated adenylyl cyclase (AC) and phospholipase C (PLC) activities are also included. GH cells and pituitary tissue contained various amounts of mRNAs and protein for Gs alpha, Gi-2 alpha, Gi-3 alpha and Go alpha, while mRNA for Gi-1 alpha was only detected in normal pituitary tissue. Gz alpha/Gx alpha mRNA was expressed in all pituitary cell lines as well as in pituitary tissue. Go alpha mRNA and Gz alpha/G x alpha mRNA displayed size heterogeneity. These findings may have importance in the understanding of hormone regulation of second messenger systems.

Different-sized mRNAs from GH4C1 cells induce a TRH-dependent electrical response in Xenopus laevis oocytes

Poly(A)+ RNA from the GH4C1 rat pituitary cell line elicited a thyrotropin releasing hormone response in Xenopus laevis oocytes which could be measured as a change in membrane current by the voltage-clamp method. Oocytes injected with Poly(A)+ RNA from GH12C1 cells which do not bind thyrotropin releasing hormone or with buffer solution alone did not show this response. Size fractionation of total poly(A)+ RNA by sucrose density-gradient centrifugation shows two response maximal representing various mRNA fractions larger than 18S. These results indicate the presence of thyrotropin releasing hormone receptor mRNA heterogeneity where the smallest mRNA is at least 2 kb.

A 64 kDa protein is a candidate for a thyrotropin-releasing hormone receptor in prolactin-producing rat pituitary tumor cells (GH4C1 cells)

A thyrotropin-releasing hormone (TRH) binding protein of 64 kDa has been identified by covalently crosslinking [3H]TRH to GH4C1 cells by ultraviolet illumination. The crosslinkage of [3H]TRH is UV-dose dependent and is inhibited by an excess of unlabeled TRH. A 64 kDa protein is also detected on immunoblots using an antiserum raised against GH4C1 cell surface epitopes. In a closely related cell line (GH12C1) which does not bind [3H]TRH, the 64 kDa protein cannot be demonstrated by [3H]TRH crosslinking nor by immunoblotting. These findings indicate that the 64 kDa protein is a candidate for a TRH-receptor protein in GH4C1 cells.

Hormone-sensitive adenylate cyclase of prolactin-producing rat pituitary adenoma (GH4C1) cells: molecular organization

Hormonal activation and inhibition of the GH4Cl1 cell adenylate cyclase complex is delineated. In the presence of the guanyl nucleotide GTP, enzyme activity was enhanced twofold by thyroliberin, sixfold by vasoactive intestinal peptide (VIP), twofold by prostaglandin E2 and twofold by isoproterenol. The diterpene, forskolin, increased, the activity 14-fold. In the presence of high GTP (400 microM) and NaCl (150 mM) concentrations, somatostatin inhibited (ED50 = 0.5 microM) the cyclase activity by 40%. In the presence of 10 microM somatostatin, the ED50 values (5 nM) for thyroliberin- and VIP-stimulated adenylate cyclase activities were shifted to 20 nM. Forskolin-elicited activation was, however, not affected by somatostatin. Cholera-toxin and pertussis-toxin pretreatment of the enzyme brought about some 20-fold and twofold activation, respectively. Inhibition by somatostatin was abolished upon pre-exposure to pertussis toxin. Mild alkylation by N-ethylmaleimide increased basal and hormone-activated adenylate cyclase while somatostatin again failed to express its inhibitory potential. Further alkylation caused a gradual decline and convergence of hormone-modulated cyclase activities towards zero. The N-ethylmaleimide-induced attenuation of thyroliberin-elicited activity was paralleled by a decrease in [3H]thyroliberin binding. Trifluoperazine and an anti-calmodulin serum reduced basal and net thyroliberin-, VIP- and forskolin-enhanced cyclase activities by some 30%, 100%, 70% and 80%, respectively. The Vmax of basal and thyroliberin-stimulated adenylate cyclase was diminished by 65%, leaving the apparent Km values (7.2 mM and 2.6 mM, respectively) for Mg2+ unaltered. Finally, the phorbol ester 12-O-tetra-decanoyl-phorbol 13-acetate (TPA) doubled the activity. This effect was counteracted by the protein kinase C inhibitor, polymyxin B, while thyroliberin-enhanced adenylate cyclase remained unaffected. In summary, we have described an adenylate cyclase with stimulatory (Rs) and inhibitory (Ri) receptors coupled to a calmodulin-sensitive holoenzyme through the Gs and Gi type of GTP-binding proteins. The ratio of the Gs to Gi is high. It appears that the GH4C1 cell adenylate cyclase is also activated by protein kinase C by interference with Gi. Apparently, thyroliberin activates the cyclase both directly through Gs and indirectly via protein kinase C stimulation.

The phorbol ester TPA induces hormone release and electrical activity in clonal rat pituitary cells

The phorbol ester TPA activates the protein kinase C in a similar way as 1,2-diacylglycerol. The effect of TPA on prolactin (PRL) secretion and electrical properties of rat pituitary cells in culture (GH4C1 cells) were compared with the effects of thyroliberin (TRH) on the corresponding parameters. The rate of hormone release was measured using a parafusion system optimized to give high time resolution. Samples for PRL measurements were taken every 4 s. The TRH evoked a biphasic PRL release, with a transient peak after about 30 s followed by a lower but sustained enhancement of the secretion. The TPA mimicked the late phase of the secretory response to TRH. The TPA analogue, 4 alpha-PDD, had no effect on the PRL release. The TRH also evoked biphasic membrane potential changes in the GH4C1 cells; the late phase consisting of membrane depolarisation associated with increased input resistance and enhanced firing of Ca2+ dependent action potentials. The TPA mimicked to a great extent these late phase effects of TRH, whereas the inactive analogue 4 alpha-PDD was ineffective. Continuous exposure to TPA masked the late phase of the electrophysiological response to TRH, suggesting that TPA and TRH share common mechanisms in their action on GH4C1 cells. We suggest that TRH enhances the electrical activity in these cells due to protein phosphorylation induced by diacylglycerol activation of protein kinase C, which in turn suppresses the membrane permeability to K+.

Vasoactive intestinal peptide causes a calcium-dependent stimulation of prolactin secretion in GH4C1 cells

We have studied the effects of vasoactive intestinal peptide (VIP) on PRL secretion in a Bio-Gel column parafusion system containing rat pituitary tumour cells (GH4C1). A dose-dependent increase in PRL release was observed with half-maximal and maximal effect (2.1-fold) at 8 X 10(-8) and 5 X 10(-6) M, respectively. The PRL-stimulatory effect of VIP was instantaneous and maintained during the parafusion experiments (up to 60 min). On a molar basis VIP was always less effective than thyroliberin (THR), and the maximum stimulation of PRL release obtained with TRH was 1.2-3.0-fold higher (n = 12) than the maximum effect seen after VIP administration. The PRL-releasing effects of VIP, THR and high extracellular K+ were almost completely abolished in the presence of two inhibitors of voltage-sensitive Ca2+ channels, CoCl2 (10(-3) M) and verapamil (10(-4) M). In Ca2+-free buffer VIP, TRH and high extracellular K+ had only negligible effects, but the responses were fully restored in the presence of normal concentrations of extracellular Ca2+. In contrast to TRH, VIP had no demonstrable effect on the Ca2+-dependent action potentials of the GH4 cells.