Binding of thyrotropin releasing hormone and prolactin release by synchronized GH3 rat pituitary cell line

Natriuretic Peptide Expression and Function in GH3 Somatolactotropes and Feline Somatotrope Pituitary Tumours

Patients harbouring mutations in genes encoding C-type natriuretic peptide (CNP; NPPC) or its receptor guanylyl cyclase B (GC-B, NPR2) suffer from severe growth phenotypes; loss-of-function mutations cause achondroplasia, whereas gain-of-function mutations cause skeletal overgrowth. Although most of the effects of CNP/GC-B on growth are mediated directly on bone, evidence suggests the natriuretic peptides may also affect anterior pituitary control of growth. Our previous studies described the expression of NPPC and NPR2 in a range of human pituitary tumours, normal human pituitary, and normal fetal human pituitary. However, the natriuretic peptide system in somatotropes has not been extensively explored. Here, we examine the expression and function of the CNP/GC-B system in rat GH3 somatolactotrope cell line and pituitary tumours from a cohort of feline hypersomatotropism (HST; acromegaly) patients. Using multiplex RT-qPCR, all three natriuretic peptides and their receptors were detected in GH3 cells. The expression of Nppc was significantly enhanced following treatment with either 100 nM TRH or 10 µM forskolin, yet only Npr1 expression was sensitive to forskolin stimulation; the effects of forskolin and TRH on Nppc expression were PKA- and MAPK-dependent, respectively. CNP stimulation of GH3 somatolactotropes significantly inhibited Esr1, Insr and Lepr expression, but dramatically enhanced cFos expression at the same time point. Oestrogen treatment significantly enhanced expression of Nppa, Nppc, Npr1, and Npr2 in GH3 somatolactotropes, but inhibited CNP-stimulated cGMP accumulation. Finally, transcripts for all three natriuretic peptides and receptors were expressed in feline pituitary tumours from patients with HST. NPPC expression was negatively correlated with pituitary tumour volume and SSTR5 expression, but positively correlated with D2R and GHR expression. Collectively, these data provide mechanisms that control expression and function of CNP in somatolactotrope cells, and identify putative transcriptional targets for CNP action in somatotropes.

Natriuretic peptide activation of extracellular regulated kinase 1/2 (ERK1/2) pathway by particulate guanylyl cyclases in GH3 somatolactotropes

The natriuretic peptides, Atrial-, B-type and C-type natriuretric peptides (ANP, BNP, CNP), are regulators of many endocrine tissues and exert their effects predominantly through the activation of their specific guanylyl cyclase receptors (GC-A and GC-B) to generate cGMP. Whereas cGMP-independent signalling has been reported in response to natriuretic peptides, this is mediated via either the clearance receptor (Npr-C) or a renal-specific NPR-Bi isoform, which both lack intrinsic guanylyl cyclase activity. Here, we report evidence of GC-B-dependent cGMP-independent signalling in pituitary GH3 cells. Stimulation of GH3 cells with CNP resulted in a rapid and sustained enhancement of ERK1/2 phosphorylation (P-ERK1/2), an effect that was not mimicked by dibutryl-cGMP. Furthermore, CNP-stimulated P-ERK1/2 occurred at concentrations below that required for cGMP accumulation. The effect of CNP on P-ERK1/2 was sensitive to pharmacological blockade of MEK (U0126) and Src kinases (PP2). Silencing of the GC-B1 and GC-B2 splice variants of the GC-B receptor by using targeted short interfering RNAs completely blocked the CNP effects on P-ERK1/2. CNP failed to alter GH3 cell proliferation or cell cycle distribution but caused a concentration-dependent increase in the activity of the human glycoprotein α-subunit promoter (αGSU) in a MEK-dependent manner. Finally, CNP also activated the p38 and JNK MAPK pathways in GH3 cells. These findings reveal an additional mechanism of GC-B signalling and suggest additional biological roles for CNP in its target tissues.

Homologous and heterologous desensitization of guanylyl cyclase-B signaling in GH3 somatolactotropes

The guanylyl cyclases, GC-A and GC-B, are selective receptors for atrial and C-type natriuretic peptides (ANP and CNP, respectively). In the anterior pituitary, CNP and GC-B are major regulators of cGMP production in gonadotropes and yet mouse models of disrupted CNP and GC-B indicate a potential role in growth hormone secretion. In the current study, we investigate the molecular and pharmacological properties of the CNP/GC-B system in somatotrope lineage cells. Primary rat pituitary and GH3 somatolactotropes expressed functional GC-A and GC-B receptors that had similar EC50 properties in terms of cGMP production. Interestingly, GC-B signaling underwent rapid homologous desensitization in a protein phosphatase 2A (PP2A)-dependent manner. Chronic exposure to either CNP or ANP caused a significant down-regulation of both GC-A- and GC-B-dependent cGMP accumulation in a ligand-specific manner. However, this down-regulation was not accompanied by alterations in the sub-cellular localization of these receptors. Heterologous desensitization of GC-B signaling occurred in GH3 cells following exposure to either sphingosine-1-phosphate or thyrotrophin-releasing hormone (TRH). This heterologous desensitization was protein kinase C (PKC)-dependent, as pre-treatment with GF109203X prevented the effect of TRH on CNP/GC-B signaling. Collectively, these data indicate common and distinct properties of particulate guanylyl cyclase receptors in somatotropes and reveal that independent mechanisms of homologous and heterologous desensitization occur involving either PP2A or PKC. Guanylyl cyclase receptors thus represent potential novel therapeutic targets for treating growth-hormone-associated disorders.

Growth and cell cycle regulation of mRNA levels in GH3 cells

In the rat anterior pituitary gland, estrogen increases both prolactin (PRL) mRNA levels and stimulates the proliferation of PRL-producing cells. The temporal sequence of these events suggests that PRL gene expression may be coordinated with cell proliferation. We investigated the relationship between cell cycle progression and the accumulation of the PRL mRNA, as well as several other mRNAs, in the rat pituitary tumor GH3 cell line. Serum-deprived cells progressed from G0 to S phase in 20-24 h following serum stimulation. During this time, beta-actin mRNA levels increased 7-fold in 5 h, then returned to basal levels prior to the beginning of S phase. Histone H1 mRNA levels increased approximately 3-fold as cells entered S phase. These data are consistent with the cell cycle-dependent regulation of beta-actin and histone H1 gene expression reported for other cell types. Glucocorticoid receptor mRNA levels were barely detectable in serum-deprived cells but rapidly increased 3- to 5-fold following serum stimulation. This increase resulted in glucocorticoid receptor mRNA levels that were equivalent to those seen in cells maintained in serum-containing medium, suggesting that serum factors regulate glucocorticoid receptor gene expression. In contrast to these changes in gene expression, the levels of PRL and growth hormone (GH) mRNAs gradually increased 2-fold while the cells progressed through G1 phase. Similarly, in cells synchronized to progress through S and G2 phases following aphidicolin treatment, histone H1 gene expression showed a specific increase in S phase cells, whereas PRL and GH mRNA levels changed little with cell cycle progression. These results indicate that the levels of PRL and GH mRNAs are not regulated in a cell cycle-dependent manner. When changes in estrogen responsiveness were determined during the cell cycle, we found that estradiol treatment was capable of increasing PRL mRNA accumulation independent of cell cycle progression and cell cycle distribution in synchronized GH3 cells. These results support the hypothesis that the hormonal regulation of PRL gene expression is not significantly affected by cell growth.

Thyrotropin-releasing hormone increases the levels of c-fos and beta-actin mRNA in GH3/B6 pituitary tumor cells

The effects of thyrotropin-releasing hormone (TRH) on the mRNA levels of c-fos, N-myc, beta-actin and prolactin (PRL) were studied in GH3/B6 cells, a rat pituitary cell line. TRH has previously been shown to increase biosynthesis and release of PRL, and to stimulate PRL gene transcription (13,19) in these cells. All experiments were performed on quiescent serum-deprived cells, and under these conditions, addition of TRH stimulated PRL production but did not alter cellular proliferation. Simultaneously, TRH induced time- and concentration-dependent increases in c-fos and beta-actin mRNA levels, with peak responses at 30 min and 4 h, respectively, in a dose range from 1 nM to 100 nM. The TRH effects on N-myc mRNA levels were less consistent. Addition of serum to quiescent GH3/B6 cells induced (3-5 fold) increases in c-fos, beta-actin and PRL mRNAs which differed in magnitude and kinetics when compared to TRH stimulation. Serum did not alter N-myc mRNA levels. The TRH-induced increases in c-fos and beta-actin mRNA may play a role in the secretory response.

Influence of cell cycle phases on the electrical activity and hormone release in a transformed line of anterior pituitary cells

Electrophysiological experiments have shown that about 50% of cultured GH3 cells (tumoral cell line, from the anterior pituitary gland) are inexcitable i.e. they do not display action potentials either spontaneously or when depolarized by a current pulse. We report here this inexcitability may be related to cellular kinetics. Thus we have studied the relationship between the various phases of the cell cycle, the electrophysiological properties of GH3/B6 cells and spontaneous or induced Prolactin and Growth Hormone (GH) release rates. Asynchronous populations of viable cells were stained with Hoechst 33 342 DNA fluorescent dye, and sorted using a flow cytometer into G1 and S phases. After selection intracellular potentials were recorded using a single glass micro-electrode; the basal or TRH stimulated rates of PRL and GH secretions were determined by RIA. Electrical properties of the cells i.e. resting potentials, input membrane resistance and excitability, reached a maximum for cells in G2+M phases. Only cells in G2+M displayed action potentials and TRH increased their secretion by 5 times for GH and by 6 times for PRL. In G1 and S phases the cells were electrically inactive and secretion rates remained at their basal levels. These findings demonstrate that the mechanism of stimulus secretion coupling is dependent upon the phases of the cell cycle.

Prolactin storage in a clonal strain of rat pituitary tumor cells is cell-cycle dependent

GH4C1 cells (CH cells) are a clonal strain of rat pituitary tumor cells which secrete prolactin. We measured intracellular prolactin at different stages of the cell cycle using flow microfluorometry. Prolactin was stained by an indirect immunocytochemical technique using fluorescein isothiocyanate (FITC)-conjugated antiserum, and DNA was stained simultaneously with propidium iodide. We found that prolactin storage in GH cells was cell-cycle dependent; prolactin storage increased as cells passed from G1 to S to G2 + M. We have shown previously that insulin and 17 beta-estradiol act synergistically to increase intracellular prolactin three- to sevenfold and slow the rate of cell growth to approximately 70% of control cells. In this study we observed that insulin and estradiol increased prolactin storage at each stage of the cell cycle but did not affect the cell-cycle distribution of the population even though cell growth was slowed. We conclude that insulin and estradiol did not increase prolactin storage by affecting the cell-cycle distribution of the population.

Effects of thyroliberin (TRH) on cell proliferation and prolactin secretion by GH3/B6 rat pituitary cells: a comparison between serum-free and serum-supplemented media

Numerous studies have shown that prolactin (PRL) production by GH3 cells grown in serum supplemented media is regulated by several hormones including thyroliberin (TRH). The recent availability of hormonally defined, serum-free media for the growth of GH3 cells has made it possible to determine the effect of TRH in absence of other prolactin regulating hormones. Here we demonstrate that transfer of GH3/B6 cells from serum-supplemented medium to serum-free media results in several important changes: (1) altered growth response to TRH, (2) altered cell attachment and morphology, (3) greatly reduced prolactin production, and (4) greater stimulation of prolactin production by TRH. After 4 days in serum-free medium, TRH stimulates prolactin production by as much as 5-fold instead of approximately 2-fold in serum-supplemented medium. Furthermore, this increased responsiveness to TRH in serum-free medium is accompanied by a 10-fold decrease in the ED50 for TRH (concentration needed for half-maximal response) and paradoxically by a 2-fold reduction in the number of high-affinity TRH binding sites without significant change of their association constant.

Direct visualization of receptors for thyrotropin-releasing hormone with a fluorescein-labeled analog

Thyrotropin-releasing hormone (TRH) binds to specific receptors on GH4C1 pituitary tumor cells. A fluorescently labeled analog of TRH was synthesized by coupling pGlu-His-ProNH(CH2)6NH2 to fluorescein isothiocyanate. The fluorescein-labeled peptide (FL-TRH) stimulated prolactin synthesis and release by GH4C1 cultures and bound to TRH receptors with an apparent Kd of 400 nM. Binding of FL-TRH to unfixed, viable GH4C1 cells was followed by fluorescence microscopy. After incubation with 1.4 microM FL-TRH for 1 hr at 37 degrees C, the surface of all cells was fluorescent and patches of intense fluorescence were evident. Control cultures incubated with FL-TRH and excess TRH were not fluorescent, and a line of pituitary tumor cells which lacks TRH receptors displayed little fluorescence after incubation with FL-TRH. When GH4C1 cells were incubated with FL-TRH for 1 hr at 37 degrees C and then with excess TRH for an additional 1 hr, the fluorescence associated with the cells was diminished to control levels. The results demonstrate that the fluorescein-labeled peptide labels specific TRH receptors.

Effect of 17 beta-estradiol on thyroliberin responsiveness in GH3/B6 rat prolactin cells

GH3/B6 rat prolactin cells were used to analyse at the cellular level the mechanisms by which 17 beta-estradiol (E2) regulates TRH responsiveness of prolactin cells. Before experiments, cells were grown for up to 7 days in 3 different media: normal medium (N) containing 15% horse serum and 2.5% fetal calf serum, CD medium prepared with charcoal-dextran extracted serum and CDE medium supplemented with 4 x 10(-8) M E2. The binding of 3H-TRH (30 min at 37 degrees C) and the TRH-induced percent increase of prolactin release as a function of TRH doses were compared in the 3 conditions. Preculture in E2 enriched medium increased by 50% the number of TRH high-affinity binding sites without modifying their affinity, increased by up to 3 times the percent of the TRH-induced stimulation of prolactin release and improved by one order of magnitude the ED50 of the TRH effect on prolactin release. The presence of HEPES (10 mM) during TRH challenge masked the effect of E2 on the increase in number of binding sites but respected its potentiating effect on prolactin release.

Immunostaining of dog pituitary somatotrophs with anti-somatostatin serum: identification of receptors for somatostatin

An antiserum to cyclic somatostatin was applied to tissues of the dog anterior pituitary gland in association with the immunohistochemical procedure. A positive immunoreaction was observed in cells identified as somatotrophs. Since the latter staining was not abolished by preadsorbtion of the anti-somatostatin serum with excess antigen, it is proposed that the positive stain represents pituitary receptor sites for the inhibiting factor. Using a similar protocol, receptors in the anterior pituitary of the rat could not be visualized even after preincubation with somatostatin.

Increased binding of concanavalin A at the cell surface following exposure to thyroliberin

Thyroliberin (THR) binds specifically to SD1 rat prolactin cells and increases prolactin release. THR-induced modifications of surface membrane of intact SD1 cells were looked for, using concanavalin A (Con A) as a probe. At the electron microscope level the binding was restricted to the cell surface. Preexposure of the cells to TRH (27 nM) for 30 min at 37 degrees C increased the binding of Con A by 28--120%. Such an increase was not observed with low doses of TRH (13.5 and 2.7 nm) nor after only a 10-min exposure to 27 nM TRH. This effect is specific for TRH; it was not observed with other peptiDES. Simultaneous exposure to Con A and [3H] TRH did not alter [3H]TRH binding, but preexposure to Con A reduced the [3H]TRH binding by 10%, which may be due to steric hindrance. It is concluded that TRH induces an increased exposure of surface membrane glycoproteins in intact SD1 cells.