Clonal strains of hormone-producing pituitary cells

Evidence for multiple intracellular calcium pools in GH4C1 cells: investigations using thapsigargin

The actions of thapsigargin (Tg), a plant sesquiterpene lactone, on Ca2+ homeostasis were investigated in digitonin-permeabilized GH4C1 rat pituitary cells. Tg (1 microM) caused a rapid and sustained increase in ambient Ca2+ concentration [( Ca2+]) and inhibited the rise in [Ca2+] induced by subsequent addition of TRH (100 nM), inositol 1,4,5-trisphosphate (IP3, 10 microM), or the nonhydrolyzable GTP analogue guanosine 5'-0-(3-thiotriphosphate) (GTP gamma S, 10 microM). However, neither IP3 nor GTP gamma S pretreatment, which themselves release sequestered Ca2+, prevented the Ca2+ accumulation induced by Tg. Pretreatment with heparin (100 micrograms/ml, 10 min), an IP3 receptor antagonist, did not affect Ca2+ accumulation induced by Tg, although it abolished the rise in [Ca2+] induced by IP3. The ability of Tg to increase [Ca2+] was dependent on added ATP. We conclude that, in GH4C1 cells, Tg acts, in part, on TRH-, IP3- and GTP gamma S-sensitive Ca2+ pools; however, Tg also acts on an ATP-dependent pool of intracellular Ca2+ which is not sensitive to TRH, IP3 or GTP gamma S, indicating a complexity of intracellular Ca2+ pools not previously appreciated in these cells.

Thyrotropin-releasing hormone regulation of human TSHB expression: role of a pituitary-specific transcription factor (Pit-1/GHF-1) and potential interaction with a thyroid hormone-inhibitory element

Regulation of human thyrotropin beta subunit gene (TSHB) expression by thyrotropin-releasing hormone (TRH) was examined in a clonal rat pituitary-cell line (GH3). Transient expression studies were done with various 5'-flanking DNA sequences of TSHB coupled to reporter gene chloramphenicol acetyltransferase. Deletion analysis defined two discrete regions (-128 to -92 base pairs and -28 to +8 base pairs) that each mediated an approximately 2-fold TRH induction. The upstream site contains a DNA sequence with close homology to the DNA-binding site for a pituitary-specific transcriptional factor Pit-1/GHF-1. DNase I footprinting analysis of mouse thyrotropic tumor extract as well as DNA-transfection studies using an expression vector containing an N-terminal deletion of Pit-1/GHF-1 cDNA suggest that Pit-1/GHF-1 or a closely related protein in the thyrotroph mediates TRH responsiveness of this gene. In addition, the downstream site overlaps with the recently characterized thyroid hormone-inhibitory element of TSHB. In fact, deletion of DNA sequences important in thyroid hormone-receptor binding (c-erbAB/c-ERBA2) from +3 to +8 base pairs, significantly reduced (30%) TRH responsiveness. The location of a TRH-stimulatory element near a thyroid hormone-inhibitory element may allow for fine control of TSHB expression in vivo.

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.

Characterization of a new animal model of chronic hyperprolactinemia

Animal models of chronic prolactin (PRL) excess have included rats bearing transplantable pituitary tumors that have produced other hypophyseal hormones in addition to PRL. We report characterization of a new model, the Buffalo rat implanted with the MMQ tumor, a line developed from the 7315a line. Rats implanted with the MMQ tumor have serum PRL levels that increase with time and correlate with the estimated volume of the subcutaneous tumor. When rats are killed 4 weeks after implantation, serum PRL levels are strikingly higher in tumor-bearing rats compared with controls (females, 2,723 +/- 266 v 192 +/- 46 ng/mL, P less than .0001; males, 1,637 +/- 213 v 99 +/- 11 ng/mL, P less than .0001). Serum PRL levels measured by the Nb2 lymphoma assay were higher than immunoassay measurements in both tumor-bearing and control Buffalo rats. Sephadex chromatography of serum from tumor-bearing rats showed that most of the PRL immunoreactivity co-eluted with 125I-rPRL. Neither serum growth hormone (GH) nor luteinizing hormone (LH) levels were different from controls in tumor-bearing rats. Female MMQ-bearing rats had lower estradiol levels. At death, the wet weights of adrenal glands, kidneys, and gonads were not affected by the presence of tumor. In contrast, tumor-bearing rats had increased spleen weight and histological evidence of white pulp hyperplasia. The Buffalo rat implanted with the PRL-only MMQ tumor is a promising new tool for the study of chronic hyperprolactinemia.

Voltage-dependent calcium channels regulate GH4 pituitary cell proliferation at two stages of the cell cycle

Calcium is an intracellular signal implicated in the regulation of cell proliferation. We have examined the growth regulatory role of voltage-dependent calcium channels (VDCC) in a rat pituitary cell line (GH4C1) that expresses two well-characterized VDCC subtypes (L and T) and is growth-inhibited by several agents known to enhance calcium entry. Thyrotropin-releasing hormone (TRH), tetradecanoylphorbol acetate (TPA), and epidermal growth factor (EGF), each known to enhance calcium entry in GH4 cells, decrease GH4 cell number and incorporation of [3H]-thymidine. The growth inhibitory action of these agents is cytostatic with a predominant effect to block G1 cells from entering S-phase. We next examined the growth regulatory action of pharmacologic agents that interact directly and specifically with type L VDCC. Activation of type L VDCC with the dihydropyridine BAY K8644 inhibits GH4 proliferation as measured by cell number and [3H]-thymidine incorporation. This action of BAY K8644 is enhanced by a submaximal K(+)-maintained depolarization, and the growth inhibitory action of these agents is also cytostatic as evident by the block of G1 cells from entering S-phase. Nimodipine, an antagonist specific for type L VDCC blocks (IC50 = 30 nM) BAY K8644-inhibited cell proliferation by substantially reducing the S-phase block. Taken together these findings indicate that calcium entry through type L VDCC inhibits GH4 cell proliferation by blocking entry into S-phase. By contrast, nimodipine caused only a small reversal of the TRH-induced S-phase block, suggesting that TRH inhibits proliferation by a mechanism that differs at least in part from L-channel activation. Unexpectedly, nimodipine, given alone, caused a substantial inhibition of GH4 cell proliferation. This action of nimodipine was cytostatic, yet differed from calcium channel activators in that the percentage of S-phase cells was unchanged whereas G2-M-phase cells increased with a parallel decrease in G1-phase cells. Similar effects were also observed with other classes of calcium channel blockers. Taken together these results indicate that calcium entry through VDCC regulates GH4 cell proliferation differently depending on the stage of the cell cycle. In G1-phase cells, sustained entry of calcium through type L VDCC blocks entry into S-phase. In G2-M-phase cells entry of calcium promotes progression through mitosis.

Platelet-activating factor affects cytosolic free calcium concentration and prolactin secretion in GH4C1 rat pituitary cells

Platelet-activating factor (PAF) is a naturally occurring pleiotropic mediator which acts via specific membrane receptors. In certain target cells, PAF causes elevations in cytosolic free Ca2+ concentration ([Ca2+]i); however, little is known of the effects of PAF on endocrine cells. Therefore, we have investigated the actions of PAF on [Ca2+]i in prolactin-secreting GH4C1 cells and have compared the effects with the well documented actions on these cells of thyrotropin-releasing hormone (TRH). GH4C1 cells were loaded with quin2/AM and fluorescence was measured in suspended populations. PAF induced a dose-dependent (10-100 microM) rise in [Ca2+]i which was slower in onset than that caused by TRH, peaking (200 to 400% above basal [Ca2+]i) at about 12 sec, and decaying over about 3 min to basal [Ca2+]i. Unlike TRH, PAF did not cause a secondary plateau phase of rise in [Ca2+]i. The terpene PAF receptor antagonist BN52021 inhibited the action of PAF on [Ca2+]i. Voltage-dependent Ca2+ channel blocker, verapamil (200 microM), antagonized the action of PAF on [Ca2+]i as did chelation of extracellular Ca2+. PAF also stimulated the secretion of prolactin in a dose-dependent manner (10 to 50 microM). The concentrations of PAF required to evoke responses in GH4C1 cells were considerably higher than those required in several other known PAF target cell types. The high concentration requirement in GH4C1 cells may be due to rapid degradation of PAF or the presence of low affinity receptors. We conclude that PAF can act, via cell surface receptors, on pituitary GH4C1 cells to alter [Ca2+]i by a pathway that enhances influx of extracellular Ca2+ through voltage-gated channels and then to enhance the secretion of prolactin.

Disruption of the Golgi apparatus with brefeldin A does not destabilize the associated detyrosinated microtubule network

Stable subsets of microtubules (MTs) are often enriched in detyrosinated alpha-tubulin. Recently it has been found that the Golgi apparatus is associated with a subset of relatively stable MTs and that detyrosinated MTs colocalize spatially and temporally with the Golgi apparatus in several cell lines. To determine whether the Golgi apparatus actively stabilizes associated MTs and thus allows their time-dependent detyrosination, we have used the drug brefeldin A (BFA) to disrupt the Golgi apparatus and have monitored changes in the Golgi apparatus and MT populations using simultaneous immunofluorescence and fluorescent lectin microscopy. We found that although BFA caused the Golgi apparatus to completely redistribute to the endoplasmic reticulum (ER), the detyrosinated MTs were not disrupted and remained in a juxtanuclear region. By Western blot analysis we found that even after 6 h of continuous exposure of cells to BFA, there was no detectable reduction in the level of detyrosinated alpha-tubulin. Simultaneous treatment with nocodazole and BFA led to a complete disruption of all MTs and normal Golgi structure/organization. Upon removal of nocodazole in the continued presence of BFA, we found that the detyrosinated MTs reformed in a compact juxtanuclear location in the absence of an intact Golgi complex. Finally, we found that the detyrosinated MTs colocalized precisely with a BFA-resistant structure that binds to the lectin, wheat germ agglutinin. We conclude that the juxtanuclear detyrosinated MTs are not actively stabilized by association with BFA-sensitive Golgi membranes. However, another closely associated structure which binds wheat germ agglutinin may serve to stabilize the juxtanuclear MTs. Alternatively, the MT organizing center (MTOC) and/or MT-associated proteins (MAPs) may organize and stabilize the juxtanuclear detyrosinated MTs.

Spatial and temporal colocalization of the Golgi apparatus and microtubules rich in detyrosinated tubulin

The integrity and intracellular distribution of the Golgi apparatus appear to depend upon microtubules. We have found that the microtubules rich in detyrosinated tubulin are located preferentially in the vicinity of the Golgi. Cells were double-stained with antibodies specific for either tyrosinated or detyrosinated tubulin and an antibody to prolactin or wheat germ agglutinin (Golgi markers). Microtubules rich in detyrosinated tubulin showed a close codistribution with the Golgi in three different cultured cell lines GH3, BS-C-1, and AtT20. Disruption of microtubules with nocodazole in GH3 cells resulted in fragmentation and dispersal of the Golgi apparatus as reported previously. During recovery of the microtubules and the Golgi complex after removal of the nocodazole, there was a spatial and temporal colocalization of the Golgi apparatus and microtubules rich in detyrosinated tubulin. Our results suggest that a functional relationship may exist between the structure and organization of the Golgi complex and the detyrosination of alpha-tubulin in microtubules.

Priming effect of hyperosmotic stress on TRH-induced activation of Na+/H+ exchange in GH4C1 rat pituitary cells

The effect of mild hyperosmotic stress on cytosolic pH (pHi) alone, and in combination with thyrotropin-releasing hormone (TRH) or the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) was investigated in GH4C1 cells at resting pHi. Hyperosmotic stress induced by addition of 50 mM choline was without an effect on pHi. In cells stimulated with either TRH or TPA after choline, pHi increased 0.15 +/- 0.05 and 0.14 +/- 0.03 pH units, respectively (mean +/- SD). A similar response was obtained if TRH or TPA was added prior to choline. The effect was abolished by replacing extracellular Na+ with choline+, and by pretreatment of the cells with amiloride, indicating that the change in pHi probably was dependent on activation of Na+/H+ exchange. The results thus indicate that, in GH4C1 cells, hyperosmotic stress in combination with TRH or TPA can activate Na+/H+ exchange at resting pHi levels.

Thyrotropin-releasing hormone receptor occupancy determines the fraction of the responsive pool of inositol lipids hydrolysed in rat pituitary tumour cells

We report that there are distinct thyrotropin-releasing hormone (TRH)-responsive and -unresponsive pools of inositol (Ins) lipids in rat pituitary tumour (GH3) cells, and present evidence that the size of the responsive pool is determined by the number of activated TRH-receptor complexes. By use of an experimental protocol in which cycling of [3H]Ins is inhibited and resynthesis occurs with unlabelled Ins only, we were able to measure specifically the effects of TRH on the hydrolysis of the Ins lipids present before stimulation. A maximally effective dose of TRH (1 microM) caused a time-dependent decrease in 3H-labelled Ins lipids that attained a steady-state value of 42 +/- 1% of the initial level between 1.5 and 2 h. After 2 h, even though there was no further decrease in 3H-labelled Ins lipids, and no increase in [3H]Ins or [3H]Ins phosphates, turnover of Ins lipids, as assessed as incorporation of [32P]Pi into PtdIns, continued at a rate similar to that in cells incubated without LiCl or unlabelled Ins. These data indicate that Ins lipid turnover was not desensitized during prolonged TRH stimulation. Depletion of lipid 3H radioactivity by TRH occurred at higher TRH doses on addition of the competitive antagonist chlordiazepoxide. Addition of 1 microM-TRH after 3 h of stimulation by a sub-maximal (0.3 nM) TRH dose caused a further decrease in 3H radioactivity to the minimum level (40% of initial value). We propose that the TRH-responsive pool of Ins lipids in GH3 cells is composed of the complement of Ins lipids that are within functional proximity of activated TRH-receptor complexes.

Electroporation of rat pituitary (GH) cell lines: optimal parameters and effects on endogenous hormone production

An efficient electroporation procedure was established for the genetic transformation of two clonal strains of hormone producing rat pituitary cells (GH12C1 and GH3). We used the bacterial chloramphenicol acetyltransferase (CAT) gene as reporter gene to determine optimal conditions for electroporation. The conditions found to be optimal, measured as expression of the highest CAT activity, were 240-300 V and a DNA concentration of 30-60 micrograms/ml in sucrose buffer. Cell viability was then about 50 per cent. Maximum CAT activity was seen 24 hours after electroporation. The electroporation procedure, in the presence or absence of DNA, caused a transient decrease in endogenous growth hormone (GH) and prolactin (PRL) production.

Inhibitory effect of the somatostatin analog octreotide on rat pituitary tumor cell (GH3) proliferation in vitro

The effects of somatostatin-14 (SS-14) and the somatostatin-analog octreotide (SMS 201-995, Sandostatin) on proliferation of GH3 pituitary tumor cells were investigated in vitro. SMS 201-995 exerted a significant, but transient, inhibition on GH3 cell growth which reached a maximum at 24 h and was no longer detectable at 48 h. The concentration that evoked the strongest inhibitory effect was 10 nM SMS 201-995, while lower and higher doses resulted in a less pronounced effect. The inhibitory effect SMS 201-995 exerted on cell proliferation was associated with a dose- and time-related reduction in both c-myc and c-fos mRNA levels. SS-14 had no noteworthy influence on either cell proliferation or c-myc and c-fos protooncogene expression. These data demonstrate that SS-analogs transiently inhibit pituitary tumor cell proliferation in vitro.

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.

Specific antisense RNA inhibition of growth hormone production in differentiated rat pituitary tumour cells

An expression vector that carried an inverted 800 base pair insert of the rat growth hormone (rGH) cDNA downstream of the SV40 promotor was used to transfect two different growth hormone (GH) producing rat pituitary cell strains, GH12C1 and GH3. This resulted in a specific transient inhibition of growth hormone production up to 75 percent in the course of 72 hours. GH synthesis reduction occurred parallel to a decrease of GH cytoplasmic mRNA levels. Levels of beta-actin and guanine nucleotide-binding regulatory protein (G protein) mRNAs were unaltered, but PRL mRNA levels were increased. Transfection with a control vector did not affect GH production.

Reflex splanchnic nerve stimulation increases levels of carboxypeptidase E mRNA and enzymatic activity in the rat adrenal medulla

Carboxypeptidase E (CPE; EC 3.4.17.10) is a carboxypeptidase B-like enzyme involved with the biosynthesis of numerous peptide hormones and neurotransmitters, including the enkephalins. Reflex splanchnic stimulation of the rat adrenal medulla, which has previously been found to substantially increase enkephalin mRNA and enkephalin peptide levels, was examined for an influence on CPE mRNA and enzymatic activity. Several hours after insulin-induced reflex splanchnic stimulation, the levels of CPE activity in rat adrenal medulla are reduced to 40-60% of control. CPE activity returns to the control level 2 days after the treatment and then continues to increase, reaching approximately 200% of control 1 week after the treatment. The time course of the changes in CPE activity is different from those of the changes in epinephrine levels and the previously reported changes in enkephalin peptide levels. CPE mRNA is also influenced by the insulin shock, with levels increasing to 155% of the control level after 6 h and 170% after 2 days. The time course of the change in CPE mRNA levels is similar to that previously found for proenkephalin mRNA. However, the magnitude of the change is much different: Proenkephalin mRNA has been reported to increase by 1,600%. The changes in CPE mRNA and enzymatic activity are consistent with the proposal that CPE is not a rate-limiting enzyme in the biosynthesis of enkephalin.

Potent and cooperative feedback inhibition of adenylate cyclase activity by calcium in pituitary-derived GH3 cells

Calcium (Ca2+) ion concentrations that are achieved intracellularly upon membrane depolarization or activation of phospholipase C stimulate adenylate cyclase via calmodulin (CaM) in brain tissue. In the present study, this range of Ca2+ concentrations produced unanticipated inhibitory effects on the plasma membrane adenylate cyclase activity of GH3 cells. Ca2+ concentrations ranging from 0.1 to 0.8 microM exerted an increasing inhibition on enzyme activity, which reached a plateau (35-45% inhibition) at around 1 microM. This inhibitory effect was highly cooperative for Ca2+ ions, but was neither enhanced nor dependent upon the addition of CaM (1 microM) to EGTA-washed membranes. The inhibition was greatly enhanced upon stimulation of the enzyme by vasoactive intestinal peptide (VIP) and/or GTP. Prior exposure of cultured cells to pertussis toxin did not affect the inhibition of plasma membrane adenylate cyclase activity by Ca2+, although in these membranes, hormonal (somatostatin) inhibition was significantly attenuated. Maximally effective concentrations of Ca2+ and somatostatin produced additive inhibitory effects on adenylate cyclase. The addition of phosphodiesterase inhibitors demonstrated that inhibitory effects of Ca2+ were not mediated by Ca2(+)-dependent stimulation of a phosphodiesterase activity. These observations provide a mechanism for the feedback inhibition by elevated intracellular Ca2+ levels on cAMP-facilitated Ca2+ entry into GH3 cells, as well as inhibitory crosstalk between Ca2(+)-mobilizing signals and adenylate cyclase activity.

Induction of distinct phenotypes in clonal and variant GH4 pituitary cells

GH cells are a widely used cell strain for the investigation of mechanisms regulating hormone release and synthesis. This report identifies two inducible phenotypes of the GH4 clone (epithelioid and motile) which may extend studies of this well-characterized cell line to different stages of pituitary cell development. GH4C1 cells treated in suspension with epidermal growth factor plus tetradecanoylphorbol acetate aggregate to form large epithelioid colonies with extensive cell-to-cell and cell-to-substratum adhesion. These cells cease replicating within 48 h, increase 50% in cell volume, and synthesize 40-fold more prolactin. A GH4C1 variant with enhanced substratum adhesion and little or no cell-to-cell adhesion (GH4S1), responds differently to this treatment. These cells cease replicating immediately, show increased cell separation, develop leading lamellae, and display locomotory activity. Each phenotype coexists in mixed cultures of GH4C1 and GH4S1 cells. This indicates that the different inducible response of the variant does not result from autocrine secretion. A molecular basis for cell-to-cell adhesion in GH4 cells was investigated. GH4C1, but not the variant cells, express a 180 kDa immunoreactive protein indistinguishable from an isoform of the neural cell adhesion molecule. Therefore the absence of cell-to-cell adhesion and inability to develop extensive cell-to-cell adhesion characteristic of the epithelioid phenotype may result from altered expression of the neural cell adhesion molecule. These findings are important because they have defined an in vitro approach to investigate genetic and cellular changes associated with the development and progression of pituitary cell phenotype.

GH4 pituitary cell variants selected as nonresponsive to thyrotropin-releasing hormone-enhanced substratum adhesion are nonresponsive to epidermal growth factor: evidence for a common signaling defect

Thyrotropin-releasing hormone (TRH) and epidermal growth factor both enhance prolactin synthesis and substrate adhesion (a morphological change called stretching) of GH4 rat pituitary cells. We have examined TRH- and EGF-induced cell stretching using genetic and pharmacologic approaches. We selected and isolated a series of GH4 cell variants nonresponsive to TRH-induced cell stretching (str-). This selection yielded several variants that were nonresponsive to both TRH- and EGF-induced stretching but were still responsive to stretching induced by several other agents (tetradecanoylphorbol acetate [TPA], butyrate, and Neplanocin A). One of the str- variants (a14) was examined in detail. TRH, EGF, and TPA each enhanced prolactin synthesis in a14 cells, indicating that the a14 variant contained functional receptor binding sites for all 3 ligands as well as the capacity to generate those intracellular signals required for enhanced prolactin synthesis. Because the str- variants were isolated without selective pressure for EGF-induced stretching and because the possibility of more than one selectable mutation in all the variants is unlikely, we suggest that TRH and EGF share a common mechanism to induce cell stretching. We next examined whether the str- variants had a defect in a signaling pathway or in the biochemical endpoint for TRH- and EGF-induced cell stretching. A pharmacologic approach was utilized to investigate the biochemical basis for induced cell stretching. A synthetic Arg-Gly-Asp-Ser tetrapeptide (RGDS), specific for fibronectin and vitronectin adhesion receptors, inhibited TRH-, EGF-, and TPA-induced GH4 cell stretching and attachment to fibronectin- and vitronectin-coated dishes. These results suggest that the interaction between fibronectin and/or vitronectin and their receptor(s) may be a biochemical endpoint by which several agonists induced stretching of GH4 cells. Because the str- variant has RGDS-specific binding sites for fibronectin and vitronectin and responds to some agents that induce cell stretching via an RGDS receptor, we conclude that the a14 str- variant has a defect in an intracellular signaling pathway, shared by TRH and EGF, which induces cell stretching.

Suppression of prolactin gene expression in GH cells correlates with site-specific DNA methylation

Prolactin- (PRL) producing and nonproducing subclones of the GH line of (rat) pituitary tumor cells have been compared to elucidate the regulatory mechanisms of PRL gene expression. Particular emphasis was placed on delineating the molecular basis of the suppressed state of the PRL gene in the prolactin-nonproducing (PRL-) GH subclone (GH(1)2C1). We examined six methylatable cytosine residues (5, -CCGG- and 1, -GCGC-) within the 30-kb region of the PRL gene in these subclones. This analysis revealed that -CCGG-sequences of the transcribed region, and specifically, one in the fourth exon of the PRL gene, were heavily methylated in the PRL-, GH(1)2C1 cells. Furthermore, the inhibition of PRL gene expression in GH(1)2C1 was reversed by short-term treatment of the cells with a sublethal concentration of azacytidine (AzaC), an inhibitor of DNA methylation. The reversion of PRL gene expression by AzaC was correlated with the concurrent demethylation of the same -CCGG- sequences in the transcribed region of PRL gene. An inverse correlation between PRL gene expression and the level of methylation of the internal -C- residues in the specific -CCGG-sequence of the transcribed region of the PRL gene was demonstrated. The DNase I sensitivity of these regions of the PRL gene in PRL+, PRL-, and AzaC-treated cells was also consistent with an inverse relationship between methylation state, a higher order of structural modification, and gene expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine A1 receptors inhibit both adenylate cyclase activity and TRH-activated Ca2+ channels by a pertussis toxin-sensitive mechanism in GH3 cells

The present study has examined the effects of adenosine A1 receptors on second messenger processes in GH3 cells. A1 receptors are present which are shown to inhibit adenylate cyclase in a GTP-requiring manner. Hormone (VIP) stimulation is also absolutely required for the observation of inhibition. Adenosine A1 receptor analogues also inhibit TRH-stimulated [Ca2+]i-mobilization in GH3 cells. Both effects of the adenosine receptor agonists are apparently mediated by pertussis toxin substrates, of which there are two--41,000 and 40,000 daltons respectively--in these cells. Somatostatin exerts analogous effects to the adenosine agonists in GH3 cells. Thus it may turn out that a general property of 'cyclase inhibitory receptors' is also to inhibit [Ca2+]i-mobilization in the same cells, when such mechanisms are present.

Autocrine-paracrine inhibition of growth hormone and prolactin production by GH3 cell-conditioned medium

In previous work we have shown that perifused GH3 cells exhibit spontaneously accelerating growth hormone (GH) and prolactin (PRL) secretory rates. This behavior contrasts with GH and PRL secretion rates that are decreasing or stable over the same 3-d period in static cell culture. We now report that GH3 cells maintained in serum-supplemented medium produce an autocrine-paracrine factor(s) which inhibits GH secretion in plate culture; PRL release is frequently reduced as well. The inhibitory effect of conditioned medium on GH secretion was concentration dependent, whereas PRL release was stimulated at low and inhibited at high concentrations over the same range. Extensive dialysis of conditioned medium using membranes with a molecular weight cut-off of 12,000-14,000 did not remove GH inhibition but produced a retentate that stimulated PRL secretion. Heat-inactivation of conditioned medium did not abolish inhibition of GH release but did remove the PRL-stimulatory effect. IGF-I added to fresh culture medium did not reproduce the GH-inhibitory effects of conditioned medium. We conclude that GH3 cell secretory behavior in perifusion and plate culture systems may be partially explained by the production of an autocrine-paracrine factor: its accumulation in plate culture inhibits GH and PRL secretion whereas its removal, by perifusing medium, allows GH and PRL secretion to accelerate.

Laminin induces formation of neurite-like processes and potentiates prolactin secretion by GH3 rat pituitary cells

Tumor-derived GH3 rat pituitary cell lines are widely utilized to study mechanisms of prolactin secretion and responsiveness to secretagogues. These cells served here as a model with which to study relationships between shape and function. When GH3 cells were routinely grown in serum-supplemented medium, they exhibited the polygonal phenotype of epithelial cells, with scarce secretory granules. In contrast, when seeded in a serum-free medium, they attached loosely and contained more secretory granules. In both cases, they released prolactin in a nonpolarized manner. We show in the present work that laminin extracted from Englebreth-Holm-Swarm (EHS) tumors was a potent attachment and spreading factor for GH3/B6 cells seeded in serum-free medium. Moreover, it induced the formation of neurite-like processes, which were increased in number and length by chronic treatment with a specific secretagogue, thyroliberin (TRH). These changes in cell shape were correlated with a potentiation of prolactin secretion, both basal and TRH-stimulated. Furthermore, using immunocytochemistry and electron microscopy, we revealed--at the dilated tip of processes--an accumulation not only of prolactin, but also of synaptophysin, a vesicle membrane marker, and of several organelles, such as secretory granules, smooth vesicles, dense bodies and mitochondria. The cytoplasmic processes contained long parallel bundles of microtubules and showed a strong immunoreactivity for beta 2-tubulin. In addition, we found immunocyto-chemical evidence for the presence of 200-k Da neurofilament protein in GH3/B6 cell processes as well as in neurites of cultured hypothalamic neurons. We conclude that, in GH3/B6 cells, laminin induced the differentiation of neurite-like processes, which were the site of polarized organelle transport and exhibited some neuronal markers.

Activity-dependent regulation of gene expression in muscle and neuronal cells

In both the central and the peripheral nervous systems, impulse activity regulates the expression of a vast number of genes that code for synaptic proteins, including neuropeptides, enzymes involved in neurotransmitter biosynthesis and degradation, and membrane receptors. In recent years, the mechanisms involved in these regulations became amenable to investigation by the methods of recombinant DNA technology. The first part of this review focuses on the activity-dependent control of nicotinic acetylcholine receptor biosynthesis in vertebrate muscle, a model case for the regulation of synaptic protein biosynthesis at the postsynaptic level. The second part summarizes some examples of neuronal proteins whose biosynthesis is under the control of transsynaptic impulse activity. The first, second, and third intracellular messengers involved in membrane-to-gene signaling are discussed, as are possible posttranscriptional control mechanisms. Finally, models are proposed for a role of neuronal activity in the genesis and stabilization of the synapse.

Rat pituitary tumor cells in serum-free culture. I. Selection of thyroid hormone-responsive and autonomous cells

The growth of GH4C1, GH3, GH1, and GH3C15 rat pituitary tumor cell lines was studied in a serum-free medium (designated TRM-1) formulated with 1:1 (vol/vol) mixture of Ham's F12 nutrient mixture and Dulbecco's modified Eagle's medium (F12-DME) containing 15 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), 50 micrograms/ml gentamicin supplemented with 10 micrograms/ml bovine insulin, 10 micrograms/ml human transferrin (Tf), 10 ng/ml selenous acid, 10 nM 3,5,3'-triiodothyronine (T3), 50 microM ethanolamine (Etn), and 500 micrograms/ml bovine serum albumin. Of the lines evaluated, only the GH1 failed to grow in TRM-1. Passage of the GH4C1 and GH3 lines from serum-containing medium into TRM-1 caused an initial selection resulting in cells that grew progressively at higher rates and finally were maintained indefinitely in TRM-1. These populations showed a requirement for supraphysiologic concentrations of T3 (1.0 to 10 nM). After adaptation of the GH4C1 line in TRM-1 for greater than or equal to 20 generations, removal of components gave a less complex mixture containing 15 mM HEPES, 50 micrograms/ml gentamicin, 10 micrograms/ml Tf, 10 nM T3, and 50 microM Etn (designated TRM-2) that supported serial passage of the cells. Under these conditions, thyroid hormone dependence was lost progressively. When T3 was removed from TRM-2 adapted cells, a third population was selected that no longer required thyroid hormones and was only slightly stimulated by T3. These studies demonstrated that the combination of serum-containing and serum-free conditions can be used to select pituitary cell populations that a) required both serum-factor(s) and T3 for optimum growth, b) required supraphysiologic concentrations of T3 without serum proteins other than Tf and albumin, and c) were completely autonomous in that they proliferated in medium supplemented only with Tf and nutrients without necessity of other serum factor(s) or T3.

Anomalous increase of Ca2+ current by high concentration K+ stimulation in whole cell clamped GH3 cells

We examined the effect of high concentration K+ (50 mM K+) stimulation to neurosecretory GH3 cells under voltage clamp control and unexpectedly found a considerable increase in the inward current evoked by depolarizing pulses. This augmented current was present in Na+-free solution containing Ca2+, tetraethylammonium+ and tetrodotoxin and showed similarity in its voltage dependence to the Ca+ channel current in the control (5 mM K+) solution. The augmented current was significantly reduced by Ca2+ channel blockers, Co2+ (5 mM) and nifedipine (2.5 microM), and was increased by the raise of external Ca2+ concentration. Correspondingly, Quin-2 experiments in GH3 cells showed that the rise in cytosolic free Ca2+ concentration in response to high K+ stimulation was suppressed by the same concentration of nifedipine. These data suggest that, in addition to its depolarizing effect, high K+ may modify voltage-sensitive Ca2+ channels such that they exhibit increased permeability although their voltage dependence of activation and pharmacological sensitivity remain largely unchanged.

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.

Rapid transient elevations of cytosolic calcium triggered by thyrotropin releasing hormone in individual cells of the pituitary line GH3B6

The kinetic features of the changes in the cytosolic free Ca2+ concentration, [Ca2+]i, following stimulation by thyrotropin releasing hormone (TRH) were analysed in single cells of a pituitary line (GH3B6) by dual excitation microfluorimetry [Tsien, Rink & Poenie (1985) Cell Calcium 6, 145-157], using fura 2 as intracellular Ca2+ probe. Two phases were observed: initially, [Ca2+]i is raised in a single rapid transient to a maximum averaging 8.0 microM, and in a second phase TRH causes a series of rapid [Ca2+]i oscillations with maxima around 1.0 microM, which are probably due to the enhanced firing of action potentials. TRH triggers both phases independently, i.e. it can elicit either the first or the second phase exclusively. This is also the case in those cells in which [Ca2+]i undergoes rhythmic oscillations due to the firing of spontaneous action potentials [Schlegel, Winiger, Mollard, Vacher, Wuarin, Zahnd, Wolheim & Dufy (1987) Nature (London) 329, 719-721]. The sudden onset of the first phase of TRH action on [Ca2+]i shows that Ca2+ mobilization due to enhanced production of inositol phosphate may occur as rapidly as the firing of action potentials, i.e. in the ms time range. Due to a marked response type heterogeneity and to the randomness of the rapid events, previous monitoring of [Ca2+]i in cell populations had misleadingly suggested small and maintained changes due to TRH. It is concluded that stimulatory regulation of secretion is provided by the generation of rapid [Ca2+]i transients, the frequency of which determines secretory rate. Furthermore, it is demonstrated that the regulation of [Ca2+]i by hormones and neurotransmitters in pituitary and many other cell types will have to be studied at the single cell level in order to appreciate its role in cell activation.

Hormone-mediated repression: a negative glucocorticoid response element from the bovine prolactin gene

We have defined and characterized a region upstream of the bovine prolactin gene that confers repression by glucocorticoids. This 'negative glucocorticoid response element' (nGRE) contains multiple footprinting sites for purified glucocorticoid receptor protein between -51 and -562 bp. A strong consensus sequence for receptor binding within the nGRE has not yet been defined, but it is apparent that nGRE sequences differ from the GRE consensus elements that confer positive glucocorticoid regulation. Unlike 'positive' GREs, the nGRE enhances promoter activity in the absence of glucocorticoids or receptor, presumably through the action of a protein that binds in the same region and activates transcription. The hormone-receptor complex appears to negate this enhancement by competing or inactivating the second factor. As with positive GREs, nGRE sequences confer hormonal regulation upon linked heterologous promoters within various cell types; a 34-bp subfragment containing a single receptor binding site is sufficient for nGRE activity. We speculate that nGRE sequences might alter the structure of bound receptor, thereby preventing it from functioning as a positive regulator when bound at those sites.

Somatostatin inhibits prolactin secretion by multiple mechanisms involving a site of action distal to increased cyclic adenosine 3',5'-monophosphate and elevated cytosolic Ca2+ in rat lactotrophs

The release of prolactin (PRL) from a clonal cell-line of anterior pituitary cells (GH4C1) was inhibited by somatostatin (SRIH) in a dose-dependent manner (ED50 nM). The inhibition (20% of control levels) was detectable within 50 s and maximal within 90 s. Thyroliberin (TRH) enhancement of PRL secretion was biphasic. SRIH inhibited both phases equally. Ionomycin in combination with the phorbol ester, TPA, mimics the TRH-elicited PRL release, and SRIH partly inhibited this effect. SRIH had no effect on TRH-stimulated formation of inositol trisphosphate, and only small effects on TRH-activated adenylate cyclase. Vasoactive intestinal peptide (VIP) and forskolin stimulated cAMP formation and PRL release potently. SRIH inhibited both effects of VIP and forskolin, and there was a close correlation between the inhibition of PRL secretion and cAMP accumulation. 8-Bromo-cAMP enhanced PRL release, an effect that was also partly reduced by SRIH. The Ca2+ channel activator, BAY-K-8644 and high extracellular K+ increased PRL release, and SRIH caused a partial reduction in the release response to both secretagogues. SRIH lowered [Ca2+]i, and markedly reduced the rise in [Ca2+]i elicited by TRH, VIP and K+. SRIH did not influence the Ca2+ spikes recorded in Na+-free solution, and had no effect on the TRH-induced membrane potential changes. Our results demonstrate that SRIH may inhibit PRL release from GH4C1 cells by (1) inhibiting hormone-sensitive adenylate cyclase, (2) blocking the effect of cAMP and (3) lowering [Ca2+]i. None of these effects is, however, sufficient to explain all the effects of SRIH, suggesting that SRIH also exerts a major action at a step subsequent to cAMP accumulation and [Ca2+]i elevation. Since the GH4C1 cells possess one single class of binding sites, this implies that the same SRIH receptor is coupled to several cellular signalling systems.

Modulation by 1,25-dihydroxycholecalciferol of the acute change in cytosolic free calcium induced by thyrotropin-releasing hormone in GH4C1 pituitary cells

Receptor-mediated regulation of prolactin synthesis by 1,25-dihydroxycholecalciferol (1,25(OH)2D3) in the pituitary cell strain GH4C1 is dependent on the concentration of extracellular calcium. We have now investigated the actions of 1,25(OH)2D3 on cytosolic free calcium concentrations [( Ca2+]i) in these cells using the fluorescent indicator quin2. Basal resting [Ca2+]i was unchanged in cells treated with 1 nM 1,25(OH)2D3 either acutely (from 0 to 15 min) or for periods of up to 48 h. However, the initial peak of the biphasic change in [Ca2+]i induced by thyrotropin-releasing hormone (TRH) was enhanced more than twofold in cells pretreated for 24 or 48 h with 1,25(OH)2D3. This 1,25(OH)2D3-enhanced calcium response was restricted to the initial phase of TRH action; the secondary plateau phase was unaffected. Neither the affinity nor number of TRH receptors nor the early time course of [3H]MeTRH binding to GH4C1 cells were affected by pretreatment with 1,25(OH)2D3. Because TRH binding was not altered, four sites along the intracellular signal transduction pathway of TRH action were examined. Neither protein kinase C activation nor inositol polyphosphate accumulation were enhanced in response to TRH, in 1,25(OH)2D3 pretreated cells, indicating that phosphatidylinositol hydrolysis was unchanged by pretreatment. A low concentration of ionomycin was used to probe the size of the nonmitochondrial intracellular calcium pool that is sensitive to TRH. Ionomycin was not able to mobilize more calcium from 1,25(OH)2D3 pretreated cells, indicating that TRH-responsive intracellular calcium stores were probably not enhanced by pretreatment. Chelation of extracellular calcium, however, did eliminate enhancement of the TRH response in 1,25(OH)2D3-pretreated cells. We conclude that 1,25(OH)2D3 modulates acute dynamic changes in [Ca2+]i induced by TRH without affecting basal [Ca2+]i. The mechanism of the enhanced response of 1,25(OH)2D3-pretreated cells to TRH appears to depend upon a postreceptor event independent of phosphatidylinositol hydrolysis that involves increased calcium conductance at the level of the plasma membrane. A less likely explanation involves enhancement of intracellular calcium stores in an ionomycin-resistant, EGTA-sensitive, TRH-mobilizable reservoir.

The stimulation of phosphorylation of intracellular proteins in GH3 rat pituitary tumour cells by phorbol esters of distinct biological activity

Using a pituitary tumour cell line (GH3), we have studied the phosphorylation of intracellular proteins induced by phorbol esters of diverse biological activity. All the active phorbol esters, including the weakly tumour-promoting but non-platelet aggregatory compound DOPPA, stimulated the phosphorylation of a cytosolic 80 kDa protein. A protein of this molecular mass has been suggested to be a marker of PKC activity. In contrast, only TPA and the non-tumour promoting but highly active phorbol ester SAP A stimulated the phosphorylation of a 130 kDa membrane protein. The results suggest that these phorbol esters activate PKC, but induce the differential phosphorylation of a variety of intracellular proteins.

Carboxylmethylation of calmodulin in cultured pituitary cells

We have used fast protein liquid chromatography (FPLC) and reverse-phase HPLC to rapidly resolve carboxylmethylated proteins in cultured pituitary GH3 cells. This procedure preserves labile carboxylmethyl esters, which are lost under the usual procedures employed for protein fractionation. GH3 cells were incubated with [methyl-3H]-methionine in culture and incorporation of label into the soluble fraction, total cell protein, and protein carboxylmethyl esters was determined; protein carboxylmethyl ester formation was shown to be resistant to cycloheximide. Fractionation of protein carboxylmethyl esters from GH3 cells by gel permeation FPLC, anion-exchange FPLC, and reverse-phase HPLC in the presence of calcium and in the presence of EGTA identified two proteins that are major substrates for protein carboxylmethyltransferase and indicated that one of these proteins is calmodulin. Similar results were obtained when a cytosolic fraction from GH3 cells was incubated with S-adenosyl-L-[methyl-3H]methionine. These results indicate that rapid chromatography at low temperature and low pH is useful for the analysis of eucaryotic carboxylmethylated proteins and that contrary to reports obtained in other systems, calmodulin is carboxylmethylated in intact pituitary cells.

Oscillations of cytosolic Ca2+ in pituitary cells due to action potentials

Electrical activity in non-neuronal cells can be induced by altering the membrane potential and eliciting action potentials. For example, hormones, nutrients and neurotransmitters act on excitable endocrine cells. In an attempt to correlate such electrical activity with regulation of cell activation, we report here direct measurements of cytosolic free Ca2+ changes coincident with action potentials. This was achieved by the powerful and novel combination of two complex techniques, the patch clamp and microfluorimetry using fura 2 methodology. Changes in intracellular calcium concentration were monitored in single cells of the pituitary line GH3B6. We show that a single action potential leads to a marked transient increase in cytosolic free calcium. The size of these short-lived maxima is sufficient to evoke secretory activity. The striking kinetic features of these transients enabled us to identify oscillations in intracellular calcium concentration in unperturbed cells resulting from spontaneous action potentials, and hence provide an explanation for basal secretory activity. Somatostatin, an inhibitor of pituitary function, abolishes the spontaneous spiking of free cytosolic Ca2+ which may explain its inhibitory effect on basal prolactin secretion. Our data therefore demonstrate that electrical activity can stimulate Ca2+-dependent functions in excitable non-neuronal cells.

GH3 cell secretion of growth hormone and prolactin increases spontaneously during perifusion

GH3 cell secretory activity was studied in long-term perifusion to define previously reported spontaneous increases in growth hormone (GH) and prolactin production (PRL). Mechanically harvested cells (1 X 10(7)/column) were perifused at 4 ml/h for 72 h. A basal period of variable duration (8 to 12 h), during which hormone secretion was stable, was followed by steadily increasing secretion rates. Changes in cell number were not sufficient to account for increased hormone secretion rates: a) there was no significant change in cell count after 72 h (0.97 +/- 0.03 X 10(7); n = 18); b) mean cell column DNA content increased 25.5% above the base value, whereas GH secretion rose 385% and PRL rose 178% (n = 5). Observed differences in the duration of the basal secretion period, the basal secretory rate, and the magnitude of secretory rate increase were associated with several variables: a) variability within a subline was a function of passage number: GH secretion decreased and PRL secretion increased with subculture number; b) cells with identical lot and freeze numbers, but received at different times, behaved differently; c) the presence of an antifungal agent (nystatin) altered hormone secretion reproducibly.

CONCLUSIONS

a) rates of GH and PRL secretion rise spontaneously in perifusion without a proportional increase in GH3 cell number; b) fluctuations in the rate of GH3 cell secretion of GH and PRL are not entirely random but are determined by several definable variables.

Melatonin reduces the production and secretion of prolactin and growth hormone from rat pituitary cells in culture

A culture of clonal tumour cells from rat pituitary gland that secrete both prolactin and growth hormone were used to investigate whether the pineal hormone melatonin can act directly on the pituitary gland to control prolactin production. Melatonin inhibition of prolactin and growth hormone production was significant but mild. Concentrations of between 10(-8) M and 10(-6) M reduced both prolactin and growth hormone production and prolactin secretion by 10-50%. 17 beta-oestradiol (OE) and thyrotrophin-releasing hormone (TRH) stimulated prolactin production but had no significant effect on growth hormone production. Melatonin reduced the effects of both of these compounds. Both TRH and vasoactive intestinal peptide (VIP) stimulated secretion of prolactin, and TRH also of growth hormone. Melatonin reduced these effects significantly. TRH and VIP increased cAMP production two- and 12-fold, respectively. Melatonin had no effect on basal or stimulated cAMP production. The melatonin-induced changes in prolactin and growth hormone production and secretion seen here do not approach the magnitude of the fluctuations seen in plasma in vivo. It is concluded that while melatonin does have a direct effect on the lactotroph in the regulation of prolactin production, its main physiological target must be elsewhere.

Voltage-activated calcium channels that must be phosphorylated to respond to membrane depolarization

Two classes of calcium channels were activated by membrane depolarization in cell-free membrane patches from GH3 cells, an electrically excitable cell line derived from a mammalian pituitary tumor. One class had a conductance of approximately 10 pS in 90 mM barium, had a threshold of activation near -40 mV, and was inactivated rapidly at holding potentials more positive than -80 mV. The other class, with a conductance of approximately 23 pS and a threshold nearer -20 mV, did not inactivate in barium but stopped responding to depolarization altogether when the cytoplasmic side of the patch was exposed to a standard physiological saline solution. Buffering the concentration of calcium ions to less than 10 nM on the cytoplasmic side did not prevent this loss of activity. However, activity was restored and maintained for the duration of the patch when the catalytic subunit of cAMP-dependent protein kinase was added with MgATP to the cytoplasmic side of the membrane. Cell-free patch formation in the presence of the dihydropyridine, BAY K 8644, also delayed the loss of activity, but unlike the catalytic subunit plus ATP, BAY K 8644 alone did not restore activity when it was added after the channels no longer responded to depolarization. Evidently the dihydropyridine-sensitive class of voltage-activated calcium channels must be phosphorylated in order to open when the membrane is depolarized. That hypothesis provides a simple framework for understanding the modulation of calcium channel gating by neurotransmitters, calcium ions, and dihydropyridines.

Subsecond and second changes in inositol polyphosphates in GH4C1 cells induced by thyrotropin-releasing hormone

It has been demonstrated previously that thyrotropin-releasing hormone (TRH) induces changes in inositol polyphosphates in the GH3 and GH4C1 strains of rat pituitary cells within 2.5-5.0 s. TRH also causes a rapid rise in cytosolic free calcium concentration ([Ca2+]i) in these cells which is due largely to redistribution of cellular calcium stores. Therefore, it has been concluded that TRH acts to release sequestered calcium in these cells via enhanced generation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. If this conclusion were correct, TRH-enhanced accumulation of Ins(1,4,5)P3 should occur at least as rapidly as the increase in [Ca2+]i. We have shown previously that the rise in [Ca2+]i induced by TRH occurs within about 400 ms; thus, it was important to investigate the subsecond time-course of changes in inositol phosphates caused by TRH. Using a rapid mixing device, we have measured changes in inositol polyphosphates on a subsecond time scale in GH4C1 cells prelabelled with myo-[2-3H]inositol. Although TRH did alter inositol polyphosphate metabolism within 500 ms, the changes observed did not reveal a statistically significant increase in Ins(1,4,5)P3 within time intervals of less than 1000 ms. Thus, we have been unable to demonstrate that a TRH-induced rise in Ins(1,4,5)P3 precedes or occurs concomitantly with the rise in [Ca2+]i in GH4C1 cells. Although these results do not disprove the current view that Ins(1,4,5)P3 mediates the action of TRH on intracellular calcium redistribution, we conclude that caution should be exercised in this, and possibly other cell systems, in accepting the dogma that all of the rapid, agonist-induced redistributions of intracellular calcium are mediated by Ins(1,4,5)P3.

Thyrotropin-releasing hormone activates Na+/H+ exchange in rat pituitary cells

The effects of thyrotropin-releasing hormone (TRH) and 12-O-tetradecanoylphorbol 13-acetate (TPA) on cytosolic pH (pHi) were studied on GH4C1 pituitary cells loaded with the fluorescent pH indicator bis(carboxyethyl)carboxyfluorescein (BCECF) and the fluorescent Ca2+ indicator quin2. TRH, which was minimally effective at around 10(-9) M, and TPA, 100 nM, produced very small elevations in pHi of about 0.05 pH units from the normal basal resting pHi of GH4C1 cells of around 7.05. The effects were more marked after acid-loading the cells using 1 micrograms of nigericin/ml. Preincubation with amiloride or replacing the extracellular Na+ with choline+ completely blocked the elevations stimulated by TRH or TPA, consistent with an activation of the Na+/H+ antiport mechanism. The effects were completely independent of the cytoplasmic free calcium concentration ([Ca2+]i). The calcium ionophore ionomycin produced an elevation in [Ca2+]i with no concomitant effect on pHi, and amiloride, although completely inhibiting the pH change stimulated by TRH, failed to affect the initial stimulated [Ca2+]i transient. Although the data are consistent with an elevation in pHi by TRH which is caused by stimulation of a protein kinase C and subsequent activation of the antiporter, the rapidity of the onset of the pHi response to TRH could not be mimicked by a combination of TPA and ionomycin. These results, together with previous findings which show that secretion can be mimicked by TPA and ionomycin, suggest that TRH-stimulated Na+/H+ exchange plays no part in the acute stimulation of secretion, but that TRH increases the pH-sensitivity of the antiport system during increased synthesis of prolactin and growth hormone.

Cell-specific expression of the human parathyroid hormone gene in rat pituitary cells

The promoter region of the human parathyroid hormone gene was fused to the Escherichia coli neo gene and introduced into GH4C1 rat pituitary and human HeLa cells. Both TATA boxes of the human parathyroid hormone gene accurately directed transcription in GH4C1 cells; the parathyroid hormone promoter was inactive in HeLa cells.

Two different cis-active elements transfer the transcriptional effects of both EGF and phorbol esters

Short cis-active sequences of the rat prolactin or Moloney murine leukemia virus genes transfer transcriptional regulation by both epidermal growth factor and phorbol esters to fusion genes. These sequences act in a position- and orientation-independent manner. Competitive binding analyses with nuclear extracts from stimulated and unstimulated cells suggest that different trans-acting factors associate with the regulatory sequence of each gene. A model is proposed suggesting that both epidermal growth factor and phorbol esters stimulate the transcription of responsive genes via discrete classes of hormone-dependent, enhancer-like elements that bind different trans-acting factors, even in the absence of hormone stimulation.

Insulin regulation of rat growth hormone gene transcription

We have previously shown that insulin suppresses growth hormone (GH) messenger (m) RNA levels in rat pituitary cells. To further delineate the molecular mechanism of insulin action, the effect of insulin treatment on GH gene transcription rates was examined in GH3 pituitary cells grown in serum-free defined medium. A transcriptional run-off assay was performed when intact isolated nuclei were allowed to continue RNA synthesis in an in vitro reaction. Specific incorporation of [32P]GTP into RNA was quantified by hybridization to rat GH complementary (c) DNA. Hybridization efficiency was measured with an internal [3H]cRNA standard and ranged from 30 to 48%. Alpha-amanitin (1 microgram/ml) inhibited total transcription, and excess unlabeled rat pituitary mRNA (250 ng) competitively inhibited GH mRNA hybridization by greater than 80%. Insulin (0.7 nM) inhibited new GH mRNA synthesis, and maximal inhibition (30% of control) was observed with 7 nM insulin after 4 h treatment. The inhibitory effects of insulin on new GH mRNA synthesis were abolished by both insulin-receptor-antiserum and by guinea-pig anti-insulin serum. The results show that insulin exerts a rapid suppression of new GH mRNA synthesis. These data provide evidence for the direct transcriptional regulation of the GH gene by insulin.

Novel prolactin related mRNAs in rat pituitary cells

From cytoplasm of rat pituitary GH4C1 tumour cells, anti prolactin anti-serum precipitates a polypeptide with apparent molecular weight of 75.000 in addition to prolactin. In vitro translation of size fractionated RNA shows that a 82.000 molecular weight PRL-like polypeptide is encoded by a mRNA larger than the 1 kb prolactin mRNA. Northern blot analysis shows that a rat prolactin cDNA probe hybridize to a 3.2 kb RNA and a 1.5 kb RNA in addition to the 1 kb PRL mRNA. The 82.000 molecular weight translation product and the 3.2 kb mRNA is also detected in rat anterior pituitary cytoplasm. We conclude that at least one high molecular weight mRNA which code for a prolactin-like polypeptide, is present in normal rat anterior pituitary gland and in GH4C1 cells.