Regulation of G protein mRNA levels by thyroliberin, vasoactive intestinal peptide and somatostatin in prolactin-producing rat pituitary adenoma cells

Specific combinations of G-protein subunits discriminate hormonal signalling in rat pituitary (GH(3)) cells in culture

It was previously shown that hormone receptor coupling to voltage-dependent calcium channels in prolactin and growth hormone-producing GH(3) cells was heavily dependent on the specific heterotrimeric combinations of alpha, beta, and gamma subunits of the guanosine triphosphate (GTP)-binding protein family. Consequently, we assessed whether this was also the case for hormonal modulation of the adenylate cyclase (AC) and phospholipase C (PL-C) effector enzymes in GH(3) cells in culture. By employing polyclonal antibodies directed towards C-terminal decapeptides of various alpha subunits in membrane assays, as well as antisense oligonucleotides towards certain beta- and gamma-subunit genes in whole-cell incubations, it was possible to unravel a tentative profile of heterotrimers preferred by some of the seven-transmembrane-stretch receptors in their modulation of AC and PL-C activities. Vasoactive intestinal peptide (VIP) and thyroliberin (TRH) activate membrane-bound AC through alpha(s)beta(2)gamma(2), while somatostatin (SRIH) and dopamine (DA) inhibited the AC through alpha(i2)beta(1)gamma(3). TRH activated membrane-bound PL-C through alpha(q/11)beta(4)gamma(2), while DA inhibition of the PL-C was accomplished via alpha(o)beta(3)gamma(4). Hence, it seems that not only the specificity of alpha subunits determines the coupling between G protein-associated receptors in GH cells, the receptor binding to G proteins also requires certain combinations of beta and gamma subunits.

Cyclic AMP regulates G(omicronalpha) protein and mRNA levels by modulating the transcriptional rate of G(omicronalpha) gene

In rat astroglial cells, four G(omicronalpha) transcripts were found: G(omicron2alpha) mRNA (5.7 kb) and three G(omicron1alpha) mRNAs (4.0, 3.0 and 2.3 kb). However, G(omicron2alpha) but little G(omicronalpha1) proteins were present in membrane-enriched fractions. Culturing astroglial cells with forskolin (10 microM) or isoproterenol (10 microM) a beta-adrenergic agonist increased transiently in a time-dependent manner the levels of G(omicronalpha) proteins. The degradation rate of G(omicronalpha) proteins was slightly decreased by the cAMP treatment. In parallel, forskolin (10 microM) treatment increased transiently the amounts of both G(omicron1alpha) and G(omicron2alpha) mRNAs. The relative transcription rate of G(omicronalpha) gene was increased by 1.7-fold in forskolin-treated cells whereas the half-lives of G(omicron1alpha) and G(omicron2alpha) mRNAs were not significantly changed. These results suggest that cAMP regulates the transcription rate of G(omicronalpha) gene and this is compatible with the existence of a cAMP responsive element in the promoter of the G(omicronalpha) gene.

Galpha(i2)-mRNA and -protein regulation as a mechanism for heterologous sensitization and desensitization of insulin secretion

Prolonged exposure of cells to an agonist of a G-protein-coupled receptor usually results in an attenuation of the cellular response. To elucidate the cellular mechanisms of sensitization or desensitization in an insulin secretory cell system (INS-1 cells), we investigated a regulatory link between G-protein alpha(s)- and alpha(i2)-subunits mRNA, their protein levels and insulin secretion as the biological effect using various compounds. Incubation with epinephrine (50 microM) for 8 h decreased alpha(s)- and alpha(i2)-mRNA levels to 58% and 72%, respectively, which is reversed after a longer incubation. From results using isoprenaline and the alpha2-agonist UK 14,304 epinephrine is shown to mediate its actions via alpha2- but not beta-adrenoceptors. The insulin inhibitory neuropeptide galanin (50 nM) caused a decrease of alpha(s)- and alpha(i2)-mRNA levels, whereas insulinotropic compounds (incretin hormones) such as GIP or GLP-1 (both 10 nM) led to an increase of alpha(s)- and alpha(i2)-mRNA levels. By using the Ca2+ channel blocker verapamil (50 microM) alpha(i2)-mRNA changes clearly depend on Ca2+ influx. The effects on alpha(i2)-mRNA were accompanied by a parallel, albeit weaker effect on the protein level (only GIP and UK 14,304 were investigated). The changes in alpha(i2)-mRNA levels by either compound were paralleled by inverse changes in insulin secretion: preincubation with UK 14,304 for 8 h led to an increased insulin secretion when challenged by either GLP-1, GIP or glucose (8.3 mM). This was similar for galanin, another potent inhibitor of insulin release. On the other hand, exposure to the incretins GIP or GLP-1 for 8 h induced a smaller insulin release when challenged afterwards by either UK 14,304, galanin, GIP, GLP-1, or glucose. Thus the influence on insulin secretion of various compounds is reciprocal to the regulation of alpha(i2)-mRNA levels but not alpha(s)-mRNA levels. There is, therefore, evidence from all the manoeuvres used that alpha(i2)-mRNA regulation may play a role in heterologous sensitization and desensitization of insulin secretion.

The intrinsic activity of (-)-3-PPP vis-à-vis prolactin-suppressing dopamine D2 receptors in transfected GH4C1 cells is dependent on which secretagogue that is used to provoke prolactin release

The abilities of dopamine (DA) and the partial DA D2 receptor agonist (-)-(3-hydroxyphenyl)-N-n-propylpiperidine, (-)-3-PPP, to suppress prolactin (PRL) release induced by any of five different PRL secretagogues in GH4C1 cells transfected with the human D2 receptor (short isoform) were investigated. Whereas DA reduced the response to all five secretagogues. (-)-3-PPP reduced the response to vasoactive intestinal peptide (VIP) and thyrotropin-releasing hormone (TRH), but not to high medium potassium (K+) or to the potassium channel antagonist tetraethylammonium (TEA). (-)-3-PPP tended to reduce the PRL release induced by the Ca2+ channel agonist BAY K-8644 (BAY); however, this effect of the partial agonist was modest and not significant. Whereas the effects of both DA and (-)-3-PPP on the PRL response to VIP and TRH were counteracted by co-incubation with the D2 antagonist raclopride, the effects of DA on the PRL response to K+, BAY, and TEA were antagonized by co-incubation with either raclopride or (-)-3-PPP. The results show that, at a given receptor density, the intrinsic activity of a partial D2 agonist with respect to D2-mediated suppression of PRL release may vary from agonism to antagonism depending on which intracellular transduction systems that are being concomitantly activated.

Functional analysis of cloned opioid receptors in transfected cell lines

Opioids modulate numerous central and peripheral processes including pain perception neuroendocrine secretion and the immune response. The opioid signal is transduced from receptors through G proteins to various different effectors. Heterogeneity exists at all levels of the transduction process. There are numerous endogenous ligands with differing selectivities for at least three distinct opioid receptors (mu, delta, kappa). G proteins activated by opioid receptors are generally of the pertussis toxin-sensitive Gi/Go class, but there are also opioid actions that are thought to involve Gq and cholera toxin-sensitive G proteins. To further complicate the issue, the actions of opioid receptors may be mediated by G-protein alpha subunits and/or beta gamma subunits. Subsequent to G protein activation several effectors are known to orchestrate the opioid signal. For example activation of opioid receptors increases phosphatidyl inositol turnover, activates K+ channels and reduces adenylyl cyclase and Ca2+ channel activities. Each of these effectors shows considerable heterogeneity. In this review we examine the opioid signal transduction mechanism. Several important questions arise: Why do opioid ligands with similar binding affinities have different potencies in functional assays? To which Ca2+ channel subtypes do opioid receptors couple? Do opioid receptors couple to Ca2+ channels through direct G protein interactions? Does the opioid-induced inhibition of vesicular release occur through modulation of multiple effectors? We are attempting to answer these questions by expressing cloned opioid receptors in GH3 cells. Using this well characterized system we can study the entire opioid signal transduction process from ligand-receptor interaction to G protein-effector coupling and subsequent inhibition of vesicular release.

Phospholipase C activation in rat pituitary adenoma (GH) cells

The presence of the pertussis toxin (PTX) insensitive GTP-binding proteins (C-proteins) G(q) alpha and/or G(11) alpha has been demonstrated in three different prolactin (PRL) and growth hormone (GH) producing pituitary adenoma cell lines. Immunoblocking of their coupling to hormone receptors indicates that G(q) and/or G(11) confer throliberin (TRH) responsive phospholipase C (PL-C) activity in these cells. The contention was substantiated by immunoprecipitation analyses showing that anti G(q)/11 alpha-sera coprecipitated PL-C activity. In essence, only G(q)/11 (but neither G(12) G(13) nor G(o)) seems to mediate the TRH-sensitive PL-C activity, while G(o) may be coupled to a basal or constitutive PL-C activity. Immunoblocking studies imply that the B gamma-complex also, to some extent, may stimulate GH(3) pituitary cell line PL-C activity. Finally, the steady state levels of G(q)/(11) alpha mRNA and protein were down regulated upon long term exposure of the GH(3) cells to TRH (but not to vasoactive intestinal peptide = VIP).

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.

Gonadotrophin-releasing hormone receptor agonist-mediated down-regulation of Gq alpha/G11 alpha (pertussis toxin-insensitive) G proteins in alpha T3-1 gonadotroph cells reflects increased G protein turnover but not alterations in mRNA levels

Prolonged exposure of alpha T3-1 pituitary gonadotrophs to a gonadotrophin-releasing hormone receptor agonist results in marked down-regulation of the pertussis toxin-insensitive G proteins Gq alpha and G11 alpha. The turnover of Gq alpha/G11 alpha was substantially accelerated in the presence of agonist. By contrast, the rate of degradation of the G protein Gi2 alpha was unaffected by agonist treatment. Analysis of Gq alpha/G11 alpha mRNA levels by reverse transcription-PCR demonstrated no detectable differences between control and agonist-treated cells. These studies indicate that gonadotrophin-releasing hormone receptor agonist-mediated down-regulation of Gq alpha/G11 alpha is a reflection of enhanced proteolysis of the activated G proteins.

D2-dopaminergic agonist quinpirole and 8-bromo-cAMP have opposite effects on Go alpha GTP-binding protein mRNA without changing D2 dopamine receptor mRNA levels in striatal neurones in primary culture

Long-term coordinated regulations (during development or by agonists and second messenger molecules) of the expression of mRNAs encoding D2-dopamine (DA) receptors and D2 receptor-linked Go alpha proteins have been studied by Northern blot analysis in mouse embryonic striatal neurones in primary culture. During the course of the cell culture, the levels of both mRNAs increased, in conjunction with the maturation of the neurones. When the preparation was treated with the D2-DA agonist quinpirole (5-15 hrs, 10(-4) M), which decreases cAMP in these neurones, the levels of Go alpha mRNAs were enhanced whereas that of the D2 mRNA remained unchanged. Conversely, the Go alpha mRNAs, but not the D2 mRNA, decreased when the neurones were exposed to 8-bromo-cAMP (16 hrs, 10(-6) M). It is concluded that, in these experimental conditions where neurones have not yet established their connexions, the longterm regulation of the membrane transmission of D2-DA signal might implicate mainly the Go alpha encoding gene.

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.

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.