Clonal strains of hormone-producing pituitary cells

Screening an In-House Isoquinoline Alkaloids Library for New Blockers of Voltage-Gated Na+ Channels Using Voltage Sensor Fluorescent Probes: Hits and Biases

Voltage-gated Na⁺ (Na_V) channels are significant therapeutic targets for the treatment of cardiac and neurological disorders, thus promoting the search for novel Na_V channel ligands. With the objective of discovering new blockers of Na_V channel ligands, we screened an In-House vegetal alkaloid library using fluorescence cell-based assays. We screened 62 isoquinoline alkaloids (IA) for their ability to decrease the FRET signal of voltage sensor probes (VSP), which were induced by the activation of Na_V channels with batrachotoxin (BTX) in GH3b6 cells. This led to the selection of five IA: liriodenine, oxostephanine, thalmiculine, protopine, and bebeerine, inhibiting the BTX-induced VSP signal with micromolar IC₅₀. These five alkaloids were then assayed using the Na⁺ fluorescent probe ANG-2 and the patch-clamp technique. Only oxostephanine and liriodenine were able to inhibit the BTX-induced ANG-2 signal in HEK293-hNa_V1.3 cells. Indeed, liriodenine and oxostephanine decreased the effects of BTX on Na⁺ currents elicited by the hNa_V1.3 channel, suggesting that conformation change induced by BTX binding could induce a bias in fluorescent assays. However, among the five IA selected in the VSP assay, only bebeerine exhibited strong inhibitory effects against Na⁺ currents elicited by the hNav1.2 and hNav1.6 channels, with IC₅₀ values below 10 µM. So far, bebeerine is the first BBIQ to have been reported to block Na_V channels, with promising therapeutical applications.

Pharmacological Dissection of the Crosstalk between NaV and CaV Channels in GH3b6 Cells

Thanks to the crosstalk between Na⁺ and Ca²⁺ channels, Na⁺ and Ca²⁺ homeostasis interplay in so-called excitable cells enables the generation of action potential in response to electrical stimulation. Here, we investigated the impact of persistent activation of voltage-gated Na⁺ (Na_V) channels by neurotoxins, such as veratridine (VTD), on intracellular Ca²⁺ concentration ([Ca²⁺]_i) in a model of excitable cells, the rat pituitary GH3b6 cells, in order to identify the molecular actors involved in Na⁺-Ca²⁺ homeostasis crosstalk. By combining RT-qPCR, immunoblotting, immunocytochemistry, and patch-clamp techniques, we showed that GH3b6 cells predominantly express the Na_V1.3 channel subtype, which likely endorses their voltage-activated Na⁺ currents. Notably, these Na⁺ currents were blocked by ICA-121431 and activated by the β-scorpion toxin Tf2, two selective Na_V1.3 channel ligands. Using Fura-2, we showed that VTD induced a [Ca²⁺]_i increase. This effect was suppressed by the selective Na_V channel blocker tetrodotoxin, as well by the selective L-type Ca_V channel (LTCC) blocker nifedipine. We also evidenced that crobenetine, a Na_V channel blocker, abolished VTD-induced [Ca²⁺]_i elevation, while it had no effects on LTCC. Altogether, our findings highlight a crosstalk between Na_V and LTCC in GH3b6 cells, providing a new insight into the mode of action of neurotoxins.

Common tools for pituitary adenomas research: cell lines and primary cells

PURPOSE

Pituitary tumor is the common primary brain tumor in humans. For further studying the pathogenesis and new therapeutic targets of pituitary adenoma, cell lines and primary cells are necessary tools. Different from primary cells that have short survival time and hormone secretion maintenance time, cell lines would be endowed with immortal characteristics under the help of gene modification. This review is to explore whether these cell lines still have similar pathophysiological changes in pituitary adenoma cells and methods to prolong the lifespan of pituitary adenoma primary cells.

RESULTS

In the cell lines summarized in the review, HP75, PDFS, HPA and GX were derived from human pituitary adenomas. It was found that the cell lines commonly used in articles published between January 2014 and July 2019 were GH3, AtT20, MMQ, GH4C1, HP75 and TtT/GF. Besides, it was glad that many methods had been used to prolong the lifespan and maintain characteristics of pituitary adenoma primary cells.

CONCLUSION

The paper reviews most of pituitary adenoma cell lines that have been successfully established since 1968 and the relevant situation of primary culture of pituitary adenoma cells. Obviously, it requires us to make more efforts to obtain human pituitary adenoma cell lines and prolong the lifespan of pituitary adenoma primary cells with maintaining their morphology and ability to secret hormones.

Olfactory marker protein regulates prolactin secretion and production by modulating Ca2+ and TRH signaling in lactotrophs

Olfactory marker protein (OMP) is a marker of olfactory receptor-mediated chemoreception, even outside the olfactory system. Here, we report that OMP expression in the pituitary gland plays a role in basal and thyrotropin-releasing hormone (TRH)-induced prolactin (PRL) production and secretion. We found that OMP was expressed in human and rodent pituitary glands, especially in PRL-secreting lactotrophs. OMP knockdown in GH4 rat pituitary cells increased PRL production and secretion via extracellular signal-regulated kinase (ERK)1/2 signaling. Real-time PCR analysis and the Ca²⁺ influx assay revealed that OMP was critical for TRH-induced PRL secretion. OMP-knockout mice showed lower fertility than control mice, which was associated with increased basal PRL production via activation of ERK1/2 signaling and reduced TRH-induced PRL secretion. However, both in vitro and in vivo results indicated that OMP was only required for hormone production and secretion because ERK1/2 activation failed to stimulate cell proliferation. Additionally, patients with prolactinoma lacked OMP expression in tumor tissues with hyperactivated ERK1/2 signaling. These findings indicate that OMP plays a role in PRL production and secretion in lactotrophs through the modulation of Ca²⁺ and TRH signaling.

Uncovering the regulatory mechanism behind production of the prolactin hormone may help tackle reproductive health problems. As well as triggering milk production in female mammals, prolactin is critical for healthy reproduction in both sexes. An excess of prolactin secreted by cells called lactotrophs in the pituitary gland can cause infertility. While scientists know which hormones stimulate prolactin release, how prolactin levels are regulated is unclear. Eun Jig Lee and Cheol Ryong Ku at Yonsei University in Seoul, Korea, and co-workers demonstrated that the olfactory marker protein (OMP) plays a central role in regulating prolactin production. They found that OMP specifically and highly expressed in lactotrophs. Eliminating OMP expression in mice left a key signalling pathway and calcium ion levels upregulated, resulting in increased prolactin and reduced fertility.

Three-dimensional Alginate-bead Culture of Human Pituitary Adenoma Cells

A three-dimensional culture method is described in which primary pituitary adenoma cells are grown in alginate beads. Alginate is a polymer derived from brown sea algae. Briefly, the tumor tissue is cut into small pieces and submitted to an enzymatic digestion with collagenase and trypsin. Next, a cell suspension is obtained. The tumor cell suspension is mixed with 1.2% sodium alginate and dropped into a CaCl2 solution, and the alginate/cell suspension is gelled on contact with the CaCl2 to form spherical beads. The cells embedded in the alginate beads are supplied with nutrients provided by the culture media enriched with 20% FBS. Three-dimensional culture in alginate beads maintains the viability of adenoma cells for long periods of time, up to four months. Moreover, the cells can be liberated from the alginate by washing the beads with sodium citrate and seeded on glass coverslips for further immunocytochemical analyses. The use of a cell culture model allows for the fixation and visualization of the actin cytoskeleton with minimal disorganization. In summary, alginate beads provide a reliable culture system for the maintenance of pituitary adenoma cells.

Do you know the sex of your cells?

Do you know the sex of your cells? Not a question that is frequently heard around the lab bench, yet thanks to recent research is probably one that should be asked. It is self-evident that cervical epithelial cells would be derived from female tissue and prostate cells from a male subject (exemplified by HeLa and LnCaP, respectively), yet beyond these obvious examples, it would be true to say that the sex of cell lines derived from non-reproductive tissue, such as lung, intestine, kidney, for example, is given minimal if any thought. After all, what possible impact could the presence of a Y chromosome have on the biochemistry and cell biology of tissues such as the exocrine pancreatic acini? Intriguingly, recent evidence has suggested that far from being irrelevant, genes expressed on the sex chromosomes can have a marked impact on the biology of such diverse tissues as neurons and renal cells. It is also policy of AJP-Cell Physiology that the source of all cells utilized (species, sex, etc.) should be clearly indicated when submitting an article for publication, an instruction that is rarely followed (http://www.the-aps.org/mm/Publications/Info-For-Authors/Composition). In this review we discuss recent data arguing that the sex of cells being used in experiments can impact the cell's biology, and we provide a table outlining the sex of cell lines that have appeared in AJP-Cell Physiology over the past decade.

Biomedical advances from tissue culture

The demonstration that the "dedifferentiation" of cells commonly observed in the early days of tissue culture was due to selective overgrowth of fibroblasts led to enrichment culture techniques (alternate animal and culture passage) designed to give a selective advantage to functionally differentiated tumor cells. These experiments resulted in the derivation of a large number of functionally differentiated clonal strains of a range of cell types. These results gave rise to the hypothesis that cells in culture accurately represent cells in vivo but without the complex in vivo environment. This concept has been strengthened with the development of hormonally defined culture media in combination with functionally differentiated clonal cell lines, which have augmented the potential of tissue culture studies. The use of hormonally defined media in place of serum-supplemented media demonstrates that hormonal responses and dependencies can be discovered in culture. Discoveries of hormonal dependencies of cancer cells has led to therapies targeting intracellular signaling pathways while discoveries of hormonal responses of pluripotent cells are helping to identify the potential application of stem cells. In these and other ways tissue culture technology will continue to contribute to solving problems of human health.

Aurora kinase B activity is modulated by thyroid hormone during transcriptional activation of pituitary genes

Covalent histone modifications clearly play an essential role in ligand-dependent transcriptional regulation by nuclear receptors. One of the predominant mechanisms used by nuclear receptors to activate or repress target-gene transcription is the recruitment of coregulatory factors capable of covalently modify the amino terminal ends of histones. Here we show that the thyroid hormone (T3) produces a rapid increase in histone H3Ser10 phosphorylation (H3Ser10ph) concomitant to the rapid displacement of the heterochromatin protein 1β (HP1β) to the nuclear periphery. Moreover, we found that T3-mediated pituitary gene transcription is associated with an increase in H3Ser10ph. Interestingly, the Aurora kinase B inhibitor ZM443979 abolishes the effect of T3 on H3Ser10ph, blocks HP1β delocalization, and significantly reduces ligand-dependent transactivation. Similar effects were shown when Aurora kinase B expression was abrogated in small interfering RNA assays. In an effort to understand the underlying mechanism by which T3 increases H3Ser10ph, we demonstrate that liganded thyroid hormone receptor directly interacts with Aurora kinase B, increasing its kinase activity. Moreover, using chromatin immunoprecipitation assays, we have shown that Aurora kinase B participates of a mechanism that displaces HP1β from promoter region, thus preparing the chromatin for the transcriptional activation of T3 regulated genes. Our findings reveal a novel role for Aurora kinase B during transcriptional initiation in GO/G1, apart from its well-known mitotic activity.

Distinct pools of cAMP centre on different isoforms of adenylyl cyclase in pituitary-derived GH3B6 cells

Microdomains have been proposed to explain specificity in the myriad of possible cellular targets of cAMP. Local differences in cAMP levels can be generated by phosphodiesterases, which control the diffusion of cAMP. Here, we address the possibility that adenylyl cyclases, the source of cAMP, can be primary architects of such microdomains. Distinctly regulated adenylyl cyclases often contribute to total cAMP levels in endogenous cellular settings, making it virtually impossible to determine the contribution of a specific isoform. To investigate cAMP dynamics with high precision at the single-isoform level, we developed a targeted version of Epac2-camps, a cAMP sensor, in which the sensor was tagged to a catalytically inactive version of the Ca(2+)-stimulable adenylyl cyclase 8 (AC8). This sensor, and less stringently targeted versions of Epac2-camps, revealed opposite regulation of cAMP synthesis in response to Ca(2+) in GH(3)B(6) pituitary cells. Ca(2+) release triggered by thyrotropin-releasing hormone stimulated the minor endogenous AC8 species. cAMP levels were decreased by inhibition of AC5 and AC6, and simultaneous activation of phosphodiesterases, in different compartments of the same cell. These findings demonstrate the existence of distinct adenylyl-cyclase-centered cAMP microdomains in live cells and open the door to their molecular micro-dissection.

Tissue culture: the unlimited potential

Lack of tissue-specific differentiated functions of cells in tissue culture, once thought to be due to "dedifferentiation", was shown to be due to selective overgrowth of fibroblasts by a series of simple experiments that challenged the prevailing dogma. Following this insight, enrichment culture techniques (alternate animal and culture passage) were designed to give functionally differentiated tumor cells selective advantage over the fibroblasts. These experiments resulted in the derivation of a large number of functionally differentiated clonal strains of a range of cell types, providing the final point of destruction of the dogma of "dedifferentiation." Instead, the hypothesis was proposed that cells in culture accurately represent cells in vivo, but without the complex in vivo environment. With the development of hormonally defined media and its combination with functionally differentiated clonal cell lines, this concept has been strengthened and the potential of tissue culture studies has been greatly augmented. Hormonally defined media allow the culture of cell types that cannot be grown in conventional, serum-supplemented media. These approaches demonstrate that hormonal responses and dependencies can be discovered in culture. Following this thinking and the discovery of hormonal dependencies of cancer cells has led to a new rationale for therapy. Tissue culture and cell technology continue to play an important role in solving human health problems.

Thyroid hormone antagonizes tumor necrosis factor-alpha signaling in pituitary cells through the induction of dual specificity phosphatase 1

Pituitary function has been shown to be regulated by an increasing number of factors, including cytokines and hormones, such as TNFalpha and T(3). Both the proinflammatory cytokine TNFalpha and T(3) have been suggested to be involved in the maintenance of tissue homeostasis in the anterior pituitary gland. In this report we show that T(3) negatively interferes with MAPK p38 and nuclear factor-kappaB (NF-kappaB) activation by TNFalpha in GH4C1 cells. Our data demonstrate that MAPK p38 is specifically activated upon exposure to TNFalpha and that T(3) abolishes this activation in a time-dependent manner by a mechanism that involves the induction of the MAPK phosphatase, DUSP1. Our data show that the pool of up-regulated DUSP1 by T(3) is mainly localized to the cytosol, and that TNFalpha does not affect this localization. On the other hand, we show that T(3) impairs the activation of the NF-kappaB pathway induced by TNFalpha, producing a significant decrease in NF-kappaB-dependent transcription, phosphorylation of IkappaBalpha, translocation of p65/NF-kappaB to the nucleus, and p65/NF-kappaB transactivation potential. Interestingly, the overexpression of DUSP1 inhibits the NF-kappaB activation achieved by either TNFalpha or ectopic expression of the upstream inducer of MAPK p38. Conversely, DUSP1 depletion abrogates the inhibitory effect of T(3) on the induction of NF-kappaB-dependent transcription by TNFalpha. Overall, our results indicate that T(3) antagonizes TNFalpha signaling in rat pituitary tumor cells through the induction of DUSP1.

Thyroid hormone-mediated activation of the ERK/dual specificity phosphatase 1 pathway augments the apoptosis of GH4C1 cells by down-regulating nuclear factor-kappaB activity

Thyroid hormone (T3) plays a crucial role in processes such as cell proliferation and differentiation, whereas its implication on cellular apoptosis has not been well documented. Here we examined the effect of T3 on the apoptosis of GH4C1 pituitary cells and the mechanisms underlying this effect. We show that T3 produced a significant increase in apoptosis in serum-depleted conditions. This effect was accompanied by a decrease in nuclear factor-kappaB (NF-kappaB)-dependent transcription, IkappaBalpha phosphorylation, translocation of p65/NF-kappaB to the nucleus, phosphorylation, and transactivation. Moreover, these effects were correlated with a T3-induced decrease in the expression of antiapoptotic gene products, such as members of the inhibitor of apoptosis protein and Bcl-2 families. On the other hand, ERK but not c-Jun N-terminal kinase or MAPK p38, was activated upon exposure to T3, and inhibition of ERK alone abrogated T3-mediated apoptosis. In addition, T3 increased the expression of the MAPK phosphatase, dual specificity phosphatase 1 (DUSP1), in an ERK-dependent manner. Interestingly, the suppression of DUSP1 expression abrogated T3-induced inhibition of NF-kappaB-dependent transcription and p65/NF-kappaB translocation to the nucleus, as well as T3-mediated apoptosis. Overall, our results indicate that T3 induces apoptosis in rat pituitary tumor cells by down-regulating NF-kappaB activity through a mechanism dependent on the ERK/DUSP1 pathway.

Tissue culture: the unrealized potential

Lack of differentiated functions of the tissue of origin in tissue culture thought to be due to dedifferentiation was shown to be due to selective overgrowth of fibroblasts. Enrichment culture techniques, (alternate animal and culture passage), designed to give the functionally differentiated cells selective advantage over the fibroblasts resulted in a large number of functionally differentiated clonal strains. Thus the dogma of dedifferentiation was destroyed. It is proposed to substitute the dedifferentiation hypothesis with the hypothesis that cells in culture accurately represent cells in vivo without the complex in vivo environment. With the development of hormonally defined media, combined with functionally differentiated clonal cell lines, the potential of tissue culture studies is greatly augmented. Hormonal responses and dependencies can be discovered in culture and the discovery of dependencies of cancer cells has led to a new rationale for therapy.

Ceramide 1-phosphate enhances calcium entry through voltage-operated calcium channels by a protein kinase C-dependent mechanism in GH4C1 rat pituitary cells

Sphingomyelin derivatives modulate a multitude of cellular processes, including the regulation of [Ca2+]i (the intracellular free calcium concentration). Previous studies have shown that these metabolites often inhibit calcium entry through VOCCs (voltage-operated calcium channels). In the present study, we show that, in pituitary GH4C1 cells, C1P (C2-ceramide 1-phosphate) enhances calcium entry in a dose-dependent manner. The phospholipase C inhibitor U73122 attenuated the response. C1P invoked a small, but significant, increase in the formation of inositol phosphates. Pre-treatment of the cells with pertussis toxin was without an effect on the C1P-evoked increase in [Ca2+]i. The effect of C1P was critically dependent on extracellular calcium, since no increase in [Ca2+]i was observed when cells in a calcium-free buffer were stimulated with C1P. Furthermore, if the cells were retreated with 300 nM of the VOCC inhibitor nimodipine, the effect of C1P was almost totally abolished. In addition, ceramide C8-1-phosphate evoked an increase in [Ca2+]i, but the onset of the response was slow compared with that of C1P. In cells treated with 1 mM thapsigargin for 15 min, C1P still evoked an increase in [Ca2+]i. In patch-clamp experiments in the whole-cell mode, C1P enhanced calcium entry through the VOCCs compared with vehicle-treated cells. Dialysis of the cells with C1P did not enhance the calcium current. On-cell patch-clamp experiments showed an enhanced probability of the VOCCs being open (P(open)) in the presence of C1P. Inhibition of PKC (protein kinase C) with GF109203X and down-regulation of PKC with PMA attenuated the C1P-evoked increase in [Ca2+]i. Furthermore, down-regulation of PKC abolished the effect of C1P on P(open). This is the first report showing that a sphingomyelin derivative enhances calcium entry through VOCCs.

HPLC micro-fractionation coupled to a cell-based assay for automated on-line primary screening of calcium antagonistic components in plant extracts

High performance liquid chromatography (HPLC) micro-fractionation was successfully coupled to an automated 45Ca2+ uptake assay using GH4C1 cells for the separation of natural product extracts and for the primary detection of their calcium antagonistic components. The reliability of the procedure was first established with a reference solution consisting of pure compounds with a known effect on the Ca2+ uptake. No loss of activity was observed to occur after HPLC micro-fractionation. Extracts of Peucedanum palustre and Pinus sylvestris, showing high and no inhibition of Ca2+ uptake as total extracts, respectively, were analysed and the inhibitory activity of the P. palustre extract could be traced to two components, identified as columbianadin and isoimperatorin. As expected, no significant inhibition was observed with the micro-fractionated P. sylvestris samples. In summary, the procedure was found to be applicable for primary detection of calcium antagonistic components in complex matrices and to significantly reduce the time previously needed for bioactivity-guided isolation.

The novel antipsychotic aripiprazole is a partial agonist at short and long isoforms of D2 receptors linked to the regulation of adenylyl cyclase activity and prolactin release

Aripiprazole is a novel antipsychotic with a unique mechanism of action, which differs from currently marketed typical and atypical antipsychotics. Aripiprazole has been shown to be a partial agonist at the D(2) family of dopamine (DA) receptors in biochemical and pharmacological studies. To demonstrate aripiprazole's action as a partial D(2) agonist in pituitary cells at the molecular level, we retrovirally transduced the short (D(2S)) and the long (D(2L)) form of the human DA D(2) receptor gene into a rat pituitary cell line, GH4C1. [(3)H]-raclopride saturation binding analyses revealed a B(max) value approximately four-fold higher at D(2S) receptor-expressing GH4C1 cells than at D(2L) receptor-expressing GH4C1 cells, while a K(d) value was similar. Aripiprazole inhibited forskolin-stimulated release of prolactin in both D(2S) and D(2L) receptor-expressing GH4C1 cells, whereas the maximal inhibition of prolactin release was less than that of DA. Similarly, aripiprazole partially inhibited forskolin-induced cAMP accumulation in both D(2) receptor-expressing cells. Aripiprazole antagonized the suppression attained by DA (10(-7) M) in both D(2) receptor-expressing cells and, at the maximal blockade of cAMP, yielded residual cAMP levels equal to those produced by aripiprazole alone. These results indicate that aripiprazole acts as a partial agonist at both D(2S) and D(2L) receptors expressed in GH4C1 cells. These data may explain, at least in part, the observations that aripiprazole shows a novel antipsychotic activity with minimal potential for adverse events including no significant increase of serum prolactin levels in clinical studies.

Miniaturisation and validation of a cell-based assay for screening of Ca2+ channel modulators

Voltage-operated calcium channels (VOCCs) play a significant role in the regulation of intracellular calcium concentrations in cardiovascular, neuronal and skeletal tissues. Therefore, physiologically relevant screening methods for calcium channel modulators are required. A 45Ca2+ uptake assay based on clonal rat pituitary cell line GH4C1, possessing L-type VOCCs, was miniaturised into a 96-well plate format. The assay was validated by known Ca2+ channel blockers, verapamil and nimodipine (IC50 values 3.4 and 0.007 microM, respectively) and by a set of natural compounds and their synthetic derivatives. The results were consistent with our previous data and demonstrated the reliability of the assay. The signal-to-background ratio was 3.9 +/- 0.4, signal-to-noise ratio 10.3 +/- 2.3, Z' factor 0.59 +/- 0.10, and day-to-day variability in positive control values 5%. Furthermore, experiments were also made on a Biomek FX workstation to evaluate the suitability of the assay for automation. With minor modifications the assay is applicable, e.g. for studying possible Ca2+ channel activators in detail. The established 96-well plate assay modification for screening of calcium channel modulators reduces considerably the time, labour and resources needed for cell culture and experiments, and has significant advantages in terms of automation suitability and overall cost-efficiency.

Agonist properties of putative small-molecule somatostatin sst2 receptor-selective antagonists

The availability of antagonist ligands for somatostatin receptors is very limited, with those that are available often displaying agonist properties or limited receptor subtype selectivity. Hay et al. [Bioorg. Med. Chem. Lett. 11 (2001) 2731] recently described the development of small-molecule somatostatin receptor subtype 2 (sst(2)) selective compounds. This study investigates the binding affinity and functional characteristics of two of those antagonists (2 and 3) and the agonist compound, from which they were derived (1). In radioligand binding studies using the agonist radioligands [125I][Tyr(11)]SRIF-14 (Ala-Gly-c[Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-(125I-Tyr)-Thr-Ser-Cys]-OH), [125I]LTT-SRIF-28 ([Leu(8),DTrp(22),125I-Tyr(25)]SRIF-28; Ser-Ala-Asn-Ser-Asn-Pro-Ala-Leu-Ala-Pro-Arg-Glu-Arg-Lys-Ala-Gly-c[Cys-Lys-Asn-Phe-Phe-DTrp-Lys-Thr-(125I-Tyr)-Thr-Ser-Cys]-OH), [125I]CGP 23996 (c[Lys-Asu-Phe-Phe-Trp-Lys-Thr-(125I-Tyr)-Thr-Ser]), [125I][Tyr(3)]octreotide (DPhe-c[Cys-(125I-Tyr)-DTrp-Lys-Thr-Cys]-Thr-OH) and [125I][Tyr(10)]cortistatin-14 (Pro-c[Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-(125I-Tyr)-Ser-Ser-Cys]-Lys) at human recombinant somatostatin receptors expressed in Chinese hamster lung fibroblast (CCL39) cells and native rat cortex, the compounds bound with high affinity (pK(d) 6.8-9.7) and selectivity to human sst(2) receptors. Some affinity was also observed for sst(5) labelled by [125I][Tyr(3)]octreotide and [125I]CGP 23996. In functional studies at human sst(2) receptors expressed in Chinese hamster ovary (CHO) cells, both the agonist 1 and the two putative antagonists 2 and 3 concentration dependently inhibited forskolin-stimulated adenylate cyclase and stimulated luciferase reporter gene expression, with similar efficacy to the natural ligand somatotropin release inhibiting factor (SRIF)-14. Compound 1 had similar potency to SRIF-14, which was in the nanomolar range, whereas 2 and 3 were 10-100-fold less potent. The intrinsic activity of 2 and 3 was too high to allow antagonist studies to be carried out. In conclusion, in contrast to previous findings, all three compounds are potent agonists at recombinant human sst(2) receptors.

Apparent membrane pore-formation by Portuguese Man-of-war (Physalia physalis) venom in intact cultured cells

Intracellular, ratiometric microfluorimetry with fura-2 reveals that low doses of Portuguese Man-of-war (Physalia physalis) venom cause a linear increase in intracellular calcium accumulation by cultured L-929 cells. The influx of calcium is preceded by a lag period that is relatively independent of venom concentration, except at very low concentrations. Electron micrographs of negatively stained preparations of membranes from venom-treated L-929 and GH(4)C(1) cells exhibit 10-80 nm diameter lesions. The number and diameter of these lesions correlate with venom concentration. The venom forms lesions in GH(4)C(1) cells at much lower concentrations than in L-929 cells. Osmotic protectants such as sucrose and polyethylene glycol (PEG), reduce the extent of lactate dehydrogenase (LDH) release from venom-treated cells with the higher molecular weight PEG causing a greater inhibition of LDH release than sucrose. These results imply that Man-of-war venom produces pore-like structures in the membranes of target cells, which leads to colloid osmotic swelling with subsequent release of intracellular proteins and cell lysis.

G protein preferences for dopamine D2 inhibition of prolactin secretion and DNA synthesis in GH4 pituitary cells

Dopamine is the primary inhibitory regulator of lactotroph proliferation and prolactin (PRL) secretion in vivo, acting via dopamine D2 receptors (short D2S and long D2L forms). In GH4C1 pituitary cells transfected with D2S or D2L receptor cDNA, dopamine inhibits PRL secretion and DNA synthesis. These actions were blocked by pertussis toxin, implicating G(i)/G(o) proteins. To address roles of specific G(i)/G(o)4 proteins in these actions a series of GH4C1 cell lines specifically depleted of individual Galpha subunits was examined. D2S-mediated inhibition of BayK8644-stimulated PRL secretion was primarily dependent on G(o) over G(i), as observed for BayK8644-induced calcium influx. By contrast, inhibitory coupling of the D2S receptor to TRH-induced PRL secretion was partially impaired by depletion of any single G protein, but especially G(i)3. Inhibitory coupling of D2L receptors to PRL secretion required G(o), but not G(i)2, muscarinic receptor coupling was resistant to depletion of any G(i)/G(o) protein, whereas the 5-HT1A and somatostatin receptors required G(i)2 or G(i)3 for coupling. The various receptors also demonstrated distinct G protein requirements for inhibition of DNA synthesis: depletion of any G(i)/G(o) subunit completely uncoupled the D2S receptor, the D2L receptor was uncoupled by depletion of G(i)2, and muscarinic and somatostatin receptors were resistant to depletion of G(i)2 only. These results demonstrate distinct receptor-G protein preferences for inhibition of TRH-induced PRL secretion and DNA synthesis.

Conserved mammalian gonadotropin-releasing hormone receptor carboxyl terminal amino acids regulate ligand binding, effector coupling and internalization

The mammalian gonadotropin-releasing hormone receptor (GnRHR), with 327 amino acids, is among the smallest G protein coupled receptors identified. Absent from this receptor is the cytoplasmic tail, characteristic of other members of this superfamily, which frequently mediates desensitization and down-regulation. The fifteen carboxyl terminal residues in the mammalian GnRHR are absolutely conserved, suggesting important roles for these residues. In the current study, mutations of the mammalian GnRHR were made to study the carboxyl terminus. The receptor mutant GnRHR(Ser(326)Ala) was reduced in ligand affinity (117% reduction compared to wild type (wt)), while receptor numbers and internalization remained unchanged. GnRHR(Ser(326)Tyr) was decreased in effector coupling, while ligand affinity remained unchanged compared to wt. These studies also show that, while mutation of Ser(326) caused a change in ligand binding and effector coupling, truncation at this residue (GnRHR[des(326-327)]) had no measurable effect on GnRHR ligand binding, effector coupling or internalization, functions which appear to require different structural determinants than expression and routing. Removal of all three carboxyl terminal residues (Phe(325), Ser(326) and Leu(327)) or mutation of the receptor (GnRHR[Phe(325)Ala]) caused a complete loss of measurable ligand binding and effector coupling, clearly suggesting an unexplained role for Phe(325).

Map kinase phosphatase-1 gene expression and regulation in neuroendocrine cells

Long-term cellular processes like proliferation, differentiation, and adaptive responses (e.g. neuronal plasticity) are initiated by the synthesis of immediate early gene (IEG) products which control the expression of late response genes. Immediate early genes encode for transcription factors, structural proteins, cytokines, and other regulatory proteins. One of the latter category of IEG products is the mitogen-activated protein (MAP) kinase phosphatase-1 (MKP-1), a dual specificity tyrosine phosphatase which inactivates the MAP kinase ERK in the nucleus. In GH4C1 neuroendocrine cells, MKP-1 is rapidly synthesised and translocated to the nucleus in response thyrotropin-releasing hormone (TRH) or epidermal growth factor (EGF). Regulation of MKP-1 gene expression in this cell line is controlled at the transcriptional level via a strong block to elongation in the exon I of the gene. After stimulation with TRH the block to elongation is released and gene transcription is completed. Nuclear run-on is traditionally used to identify blocks to elongation and to determine endogeneous levels of transcriptional activities, but this method has severe technical limitations. An alternative approach to nuclear run-on is presented here for the MKP-1 gene, which involves the purification and analysis of nascent and free nuclear RNA fractions. [1] This method may be helpful to study in more detail the mechanisms of transcriptional elongation in mammalian cells.

MAP kinase phosphatase-1 gene transcription in rat neuroendocrine cells is modulated by a calcium-sensitive block to elongation in the first exon

Transcriptional elongation of many eukaryotic, prokaryotic, and viral genes is tightly controlled, which contributes to gene regulation. Here we describe this phenomenon for the MAP kinase phosphatase 1 (MKP-1) immediate early gene. In rat GH4C1 pituitary cells, MKP-1 mRNA is rapidly and transiently induced by the thyrotropin-releasing hormone (TRH) and the epidermal growth factor EGF via transcriptional activation of the gene. Ca(2+) signals are necessary for the induction of MKP-1 in response to TRH but not to EGF. Reporter gene analysis with the newly cloned rat promoter sequence shows only limited induction in response to various stimuli, including TRH or EGF. By nuclear run-on assays we demonstrate that in basal conditions, a strong block to elongation in the first exon regulates the MKP-1 gene and that stimulation with either TRH or EGF overcomes the block. Ca(2+) signals are important to release the MKP-1 elongation block in a manner similar to the c-fos oncogene. These results suggest that a common mechanism of intragenic regulation may be conserved between MKP-1 and c-fos in mammalian cells.

Dominant negative ER induces apoptosis in GH(4) pituitary lactotrope cells and inhibits tumor growth in nude mice

The ER plays an important role in the proliferation and differentiation of lactotrope tumor cells. GH(4) cells were infected with adenoviral vectors (AdL540Q and Ad1-536) to investigate the ability of dominant negative ER mutants to affect the regulation of gene expression and cell growth by endogenous ER. The dominant negative mutants suppressed estradiol stimulation of an estrogen-responsive reporter gene and the PRL promoter in these cells. AdL540Q or Ad1--536 infection also inhibited GH(4) cell growth and induced apoptosis, increasing the expression of the proapoptotic Bax protein and decreasing the expression of antiapoptotic Bcl-2. AdwtER-infected cells also showed decreased Bcl-2 protein. E2-induced activation of p38 MAPK, an enzyme that may participate in apoptosis, was observed in cells infected with AdwtER, AdL540Q, and Ad1--536. Consistent with the apoptotic effects in vitro, infection of GH(4) cells with AdL540Q or Ad1--536 inhibited the ability of the cells to form tumors in nude mice. These results indicate that dominant negative ER mutants induce apoptosis of GH(4) cells and suppress tumor formation and development. The delivery of dominant negative ERs by adenoviral vectors may provide an alternative modality for the targeted therapy of pituitary lactotrope adenomas and other estrogen-responsive tumors.

Involvement of mitogen-activated protein kinase in cyclic adenosine 3',5'-monophosphate-induced hormone gene expression in rat pituitary GH(3) cells

We examined whether mitogen-activated protein (MAP) kinase was activated by stimulation of the cAMP pathway and whether MAP kinase activation was involved in synthesis of PRL and GH in GH(3) cells. Treatment of the cells with a cAMP analog, 8-(4-chlorophenylthio)cAMP (CPT-cAMP), activated MAP kinase and increased PRL at both the protein and messenger RNA levels. The protein and messenger RNA of GH were decreased by the treatment. We constructed the luciferase reporter genes after the promoters of PRL and GH and found the activation of both promoters by the CPT-cAMP treatment. We confirmed that overexpression of the catalytic subunit of cAMP-dependent protein kinase had essentially the same effects on MAP kinase activation and synthesis of PRL and GH as the CPT-cAMP treatment. Furthermore, treatment of the cells with pituitary adenylate cyclase-activating polypeptide 27 activated MAP kinase. The activation of PRL promoter by CPT-cAMP and pituitary adenylate cyclase-activating polypeptide 27 was abolished by pretreatment with PD098059 and H89. Although the increase in PRL and GH secretion by CPT-cAMP was inhibited by H89, PD098059 had no effect on secretion. These results suggest that cAMP-induced MAP kinase activation is essential for PRL gene expression, but not for secretion of PRL and GH.

Effects of simple aromatic compounds and flavonoids on Ca2+ fluxes in rat pituitary GH(4)C(1) cells

The biological activity of phenolic compounds from plants is well documented in vitro, but little is known about the possible effect of simple aromatic compounds and flavonoids on voltage-operated Ca2+ channels (VOCCs). In pituitary cells, several intracellular pathways may regulate the activity of VOCCs. In this study, we investigated the effect of nine phenylpropanes and metanes, and 20 flavonoids on high K(+)-induced 45Ca2+ entry in clonal rat pituitary GH(4)C(1) cells. At the highest dose tested (20 microg/ml), flavone (a flavone) inhibited 45Ca2+ entry by 63.5%, naringenin (a flavanone) by 56.3% and genistein (an isoflavone) by 54.6%. The phenylmetane derivative octyl gallate was the most potent compound tested, with an IC(50) value of 15.0 microg/ml. The IC(50) value for the reference compound verapamil hydrochloride was 3.0 microg/ml. In sharp contrast to the above, the flavonols quercetin and morin potentiated 45Ca2+ entry. At 20 microg/ml, quercetin increased 45Ca2+ entry by 54.1% and morin by 48.0%. Quercetin increased the cellular cAMP content in a concentration-dependent manner. H 89, an inhibitor of protein kinase A, inhibited the effect of quercetin on 45Ca2+ entry. The results thus suggest that the effect of quercetin is the result of a protein kinase A-mediated activation of VOCCs. Quercetin induced a rapid and marked increase in both the transient (143.1+/-4.2%) and delayed (198.8+/-10.0%) Ca2+ currents, measured by the whole cell patch clamp technique. The onset of the inhibitory effect of octyl gallate was slow, but resulted in an almost complete inhibition of both Ca2+ currents.

Pituitary Ets-1 and GABP bind to the growth factor regulatory sites of the rat prolactin promoter

Ets factors play a critical role in oncogenic Ras- and growth factor-mediated regulation of the proximal rat prolactin (rPRL) promoter in pituitary cells. The rPRL promoter contains two key functional Ets binding sites (EBS): a composite EBS/Pit-1 element located at -212 and an EBS that co-localizes with the basal transcription element (BTE, or A-site) located at -96. Oncogenic Ras exclusively signals to the -212 site, which we have named the Ras response element (RRE); whereas the response of multiple growth factors (FGFs, EGF, IGF, insulin and TRH) maps to both EBSs. Although Ets-1 and GA binding protein (GABP) have been implicated in the Ras and insulin responses, respectively, the precise identity of the pituitary Ets factors that specifically bind to the RRE and BTE sites remains unknown. In order to identify the Ets factor(s) present in GH4 and GH3 nuclear extracts (GH4NE and GH3NE) that bind to the EBSs contained in the RRE and BTE, we used EBS-RRE and BTE oligonucleotides in electrophoretic mobility shift assays (EMSAs), antibody supershift assays, western blot analysis of partially purified fractions and UV-crosslinking studies. EMSAs, using either the BTE or EBS-RRE probes, identified a specific protein-DNA complex, designated complex A, which contains an Ets factor as determined by oligonucleotide competition studies. Using western blot analysis of GH3 nuclear proteins that bind to heparin-Sepharose, we have shown that Ets-1 and GABP, which are MAP kinase substrates, co-purify with complex A, and supershift analysis with specific antisera revealed that complex A contains Ets-1, GABPalpha and GABPbeta1. In addition, we show that recombinant full-length Ets-1 binds equivalently to BTE and EBS-RRE probes, while recombinant GABPalpha/beta preferentially binds to the BTE probe. Furthermore, comparing the DNA binding of GH4NE containing both Ets-1 and GABP and HeLa nuclear extracts devoid of Ets-1 but containing GABP, we were able to show that the EBS-RRE preferentially binds Ets-1, while the BTE binds both GABP and Ets-1. Finally, UV-crosslinking experiments with radiolabeled EBS-RRE and BTE oligonucleotides showed that these probes specifically bind to a protein of approximately 64 kDa, which is consistent with binding to Ets-1 (54 kDa) and/or the DNA binding subunit of GABP, GABPalpha (57 kDa). These studies show that endogenous, pituitary-derived GABP and Ets-1 bind to the BTE, whereas Ets-1 preferentially binds to the EBS-RRE. Taken together, these data provide important insights into the mechanisms by which the combination of distinct Ets members and EBSs transduce differential growth factor responses.

GH mRNA levels are elevated by forskolin but not GH releasing hormone in GHRH receptor-expressing MtT/S somatotroph cell line

The MtT/S somatotroph cell line should be a growth hormone-releasing hormone (GHRH)-responsive model system for the study of physiological control of growth hormone (GH) transcription because GH secretion from these cells is stimulated by GHRH. To examine the GH transcriptional activity of these cells, endogenous GH mRNA levels were measured using a ribonuclease protection assay following treatment under a variety of hormonal conditions. While omission of serum led to reduction of GH mRNA to 22% of control levels by 2 days and to 8% by 5 days (P<0.05 for both), GH mRNA levels were maintained at control values in serum-free medium containing 5 nM dexamethasone and 30 pM triiodothyronine (TDM). However, the addition of 10 nM GHRH under any treatment condition did not significantly alter GH mRNA levels. Characterization of the MtT/S cells showed that GHRH-receptor (GHRH-R) mRNA was detectable by reverse transcription-polymerase chain reaction (RT-PCR) amplification. Measurement of extracellular cAMP showed that the MtT/S cells have basal levels of > or =20 nmol/10(6) cells per h in both serum-containing and serum-free media, and that GHRH had no effect on cAMP levels, suggesting constitutive activation. To rule out the possibility of autocrine stimulation by GHRH produced endogenously, GHRH mRNA was not detectable in MtT/S cells using RT-PCR amplification. The stimulatory G-protein alpha subunit, mutations of which are known to activate adenylate cyclase constitutively in acromegaly, was sequenced but found not to differ from normal pituitary in the regions most commonly mutated. Finally, treatment with 10 microM forskolin, to directly activate adenylate cyclase, increased GH mRNA to 140% of controls in TDM, and to 163% in serum-free medium after 2 days, and to 166% in TDM-treated cells and 174% in serum-free culture after 5 days (all P<0.05). Taken together, these data indicate that although MtT/S cells express the GHRH-R, GHRH cannot stimulate adenylate cyclase to increase GH transcription due to constitutive elevation of cAMP levels, by a means that may be similar to that in cases of acromegaly not caused by oncogenic gsp mutations.

Partial transcriptome of the developing pituitary gland

The anterior lobe of the pituitary gland is composed of five hormone-producing cell types and develops from Rathke's pouch, an invagination of oral ectoderm. In mice, rapid cell proliferation occurs in the pouch from embryonic day 12.5 (e12.5) to e14.5, preceding the appearance of most hormone transcripts. Cell-type-specific commitment probably occurs prior to e14.5, but cell differentiation can be demonstrated only by detection of hormone transcripts. Although several transcription factors critical for pouch expansion are known, few of their target genes have been identified. To identify putative transcription factor target genes and cell-type-specific markers, we used differential display PCR analysis of RNA prepared from e12.5 and e14.5 Rathke's pouches. We present an expression profile of the developing pituitary gland including 83 transcripts, 40% of which are novel. The tissue distribution, cell specificity, and developmental regulation were determined for a subset of the transcripts.

Development and characterization of pituitary GH3 cell clones stably transfected with a human proinsulin cDNA

Successful beta-cell replacement therapy in insulin-dependent (type I) diabetes is hindered by the scarcity of human donor tissue and by the recurrence of autoimmune destruction of transplanted beta cells. Availability of non-beta cells, capable of releasing insulin and escaping autoimmune recognition, would therefore be important for diabetes cell therapy. We developed rat pituitary GH3 cells stably transfected with a furin-cleavable human proinsulin cDNA linked to the rat PRL promoter. Two clones (InsGH3/clone 1 and 7) were characterized in vitro with regard to basal and stimulated insulin release and proinsulin transgene expression. Mature insulin secretion was obtained in both clones, accounting for about 40% of total released (pro)insulin-like products. Immunocytochemistry of InsGH3 cells showed a cytoplasmic granular insulin staining that colocalized with secretogranin II (SGII) immunoreactivity. InsGH3 cells/clone 7 contained and released in vitro significantly more insulin than clone 1. Secretagogue-stimulated insulin secretion was observed in both InsGH3 clones either under static or dynamic conditions, indicating that insulin was targeted also to the regulated secretory pathway. Proinsulin mRNA levels were elevated in InsGH3 cells, being significantly higher than in betaTC3 cells. Moreover, proinsulin gene expression increased in response to various stimuli, thereby showing the regulation of the transfected gene at the transcriptional level. In conclusion, these data point to InsGH3 cells as a potential beta-cell surrogate even though additional engineering is required to instruct them to release insulin in response to physiologic stimulations.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Portuguese Man-of-war (Physalia physalis) venom induces calcium influx into cells by permeabilizing plasma membranes

Portuguese Man-of-war (Physalia physalis) nematocyst venom dose-dependently stimulates calcium (45Ca(2+)) influx into L-929, GH(4)C(1), FRL, and embryonic chick heart cells. Venom-induced calcium influx is not blocked by ouabain, vanadate, nor organic calcium channel blockers, but is blocked by transition metal cations, such as lanthanum and zinc. Venom-induced calcium influx is accompanied in a dose-dependent manner by the release of intracellular lactate dehydrogenase, indicating a loss in plasma membrane integrity and cytolysis. Concentrations of zinc that block 45Ca(2+) influx also block lactate dehydrogenase release. Lanthanum, which also blocks 45Ca(2+) uptake, does not neutralize the cytolytic activity of the venom, but rather inhibits the venom's cytolytic action at the level of the target cell plasma membrane. Our findings indicate that Man-of-war venom causes an influx of calcium into several different cells types, not just those of the cardiovascular system, and this influx likely occurs by permeabilizing the plasma membranes of cells.

Estrogen mitogenic action. ii. negative regulation of the steroid hormone-responsive growth of cell lines derived from human and rodent target tissue tumors and conceptual implications

In an accompanying report (Moreno-Cuevas, J. E.; Sirbasku, D. A., In Vitro Cell. Dev. Biol.; 2000), we demonstrated 80-fold estrogen mitogenic effects with MTW9/PL2 rat mammary tumor cells in cultures supplemented with charcoal-dextran-treated serum. All sera tested contained an estrogen reversible inhibitor(s). The purpose of this report is to extend those observations to additional sex steroid-responsive human and rodent cell lines. Every line tested showed a biphasic response to hormone-depleted serum. Concentrations of < or = 10% (v/v) promoted substantive growth. At higher concentrations, serum was progressively inhibitory. With estrogen receptor-positive (ER+) human breast cancer cells, rat pituitary tumor cells, and Syrian hamster kidney tumor cells, 50% (v/v) serum caused significant inhibition, which was reversed by very low physiologic concentrations of estrogens. This same pattern was observed with the steroid hormone-responsive LNCaP human prostatic carcinoma cells. Because steroid hormone mitogenic effects are now easily demonstrable using our new methods, the identification of positive results has nullified our original endocrine estromedin hypothesis. We also evaluated autocrine/paracrine growth factor models of estrogen-responsive growth. We asked if insulin-like growth factors I and II, insulin, transforming growth factor alpha, or epidermal growth factor substituted for the positive effects of estrogens. Growth factors did not reverse the serum-caused inhibition. We asked also if transforming growth factor beta (TGFP) substituted for the serum-borne inhibitor. TGFbeta did not substitute. Altogether, our results are most consistent with the concept of a unique serum-borne inhibitor as has been proposed in the estrocolyone model. However, the aspect of the estrocolyone model related to steroid hormone mechanism of action requires more evaluation. The effects of sex steroids at picomolar concentrations may reflect mediation via inhibitor "activated" intracellular signaling pathways.

Estrogen mitogenic action. III. is phenol red a "red herring"?

The reported estrogenic action of phenol red and/or its lipophilic contaminants has led to the widespread use of indicator-free culture medium to conduct endocrine studies in vitro. Because we have recently developed methods to measure large-magnitude estrogen effects in the tissue culture medium containing phenol red, we concluded that the indicator issue required further evaluation. To do this, we selected nine estrogen receptor positive (ER+) cell lines representing four target tissues and three species. We investigated phenol red using five different experimental protocols. First, 17beta-estradiol (E2) responsive growth of all nine ER+ cells lines was compared in the medium with and without the indicator. Second, using representative lines we asked if phenol red was mitogenic in the indicator-free medium. The dose-response effects of phenol red were compared directly to those of E2. Third, we asked if tamoxifen-inhibited growth equally in phenol red-containing and indicator-free medium. This study was based on a report indicating that antiestrogen effects should be seen only in phenol red-containing medium. Fourth, we asked if phenol red displaced the binding of 3H-E2 using ERK intact human breast cancer cells. Fifth, we compared E2 and phenol red as inducers of the progesterone receptor using a human breast cancer cell line. All the experiments presented in this report support the conclusion that the concentration of phenol red contaminants in a standard culture medium available today is not sufficient to cause estrogenic effects. In brief, our studies indicate that the real issue of how to demonstrate estrogenic effects in culture resides elsewhere than phenol red. We have found that the demonstration of sex steroid hormone-mitogenic effects in culture depends upon conditions that maximize the effects of a serum-borne inhibitor(s). When the effects of the inhibitor are optimized, the presence or absence of phenol red makes no everyday difference to the demonstration of estrogen mitogenic effects with several target cell types from diverse species.

Stimulation of mitogen-activated protein kinase pathway in rat somatotrophs by growth hormone-releasing hormone

Growth hormone-releasing hormone (GHRH) is an important regulator of somatotroph development and function. However, GHRH signaling is still not completely understood. Signaling through the mitogen-activated protein kinase (MAPK) pathway has been observed in a wide variety of cell types but has not been explored as a mediator of GHRH action. In this study, we examined the phosphorylation of MAPK pathway intermediates in response to GHRH. After treatment of the GH4 rat somatotroph cell line with rGHRH (10(7) M) for 2.5 min, there was robust phosphorylation of MAPK not seen in vehicle-treated cells. Treatment of HeLa cells with GHRH resulted in no activation of MAPK, but activation was conferred by transfection with the GHRH receptor cDNA. MAPK activation by GHRH was dose dependent from 1 to 100 nM, was evident at 2.5 min, peaked at 5 min, and returned to baseline by 20 min. Pretreatment of GH4 cells with somatostatin analog BIM23014 or the MEK1 inhibitor PD98095 prevented the activation of MAPK. Finally, treatment with GHRH increased GH4 proliferation in culture, and this response was prevented by pretreatment with BIM23014 and PD98095. These results indicate that GHRH activates the MAPK pathway. Furthermore, activation of MAPK may mediate, at least in part, the effects of GHRH on somatotroph cell line proliferation. The findings support the concept that multiple pathways mediate the effects of GHRH.

Novel action of pituitary adenylate cyclase-activating polypeptide. Stimulation of extracellular acidification in rat pituitary GH4C1 cells

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a member of the vasoactive intestinal peptide/secretin family. Using microphysiometry, we have found that PACAP acutely (1 min) increased the extracellular acidification rate (ECAR) in GH4C1 cells approximately 40% above basal in a concentration-dependent manner. ECAR, maximally induced by PACAP, can be increased further by thyrotropin-releasing hormone (TRH), indicating that the signalling pathways for these two neuropeptides are not identical. In studies on the mechanism of PACAP-enhanced ECAR, we found that maximum stimulation of the cAMP/PKA pathway by treatment with FSK, or the PKC pathway with PMA, did not inhibit the ECAR response to PACAP. The PKC inhibitor calphostin C and the MAP kinase inhibitor PD98059 had no effect on the ECAR response to PACAP. Furthermore, PACAP induced little or no change in cytosolic Ca(2+) ([Ca(2+)](i)), while TRH induced a large increase in [Ca(2+)](i). However, the tyrosine kinase inhibitor genistein completely blocked PACAP-induced ECAR, suggesting involvement of tyrosine kinase(s). We conclude that PACAP causes an increase in ECAR in GH4C1 rat pituitary cells, which is not dependent on the PKA, PKC, MAP kinase or Ca(2+) signalling pathways, but does require tyrosine kinase activity.

L-type calcium channel activity regulates sodium channel levels in rat pituitary GH3 cells

1. The effects of chronic pharmacological modulation of L-type Ca2+ channel activity on the cell surface expression of Na+ channels were examined in GH3 cells. 2. Prolonged inhibition (4-5 days) of L-channels with nimodipine caused a 50-60 % decrease in the peak amplitude of whole-cell Na+ currents recorded with the patch-clamp technique. On the contrary, prolonged exposure to the L-channel agonist Bay K 8644 induced an approximately 2.5-fold increase in peak Na+ current. In both cases, there were only minor changes in cell capacitance and no significant changes in Na+ channel gating properties. 3. Measurements of the specific binding of radiolabelled saxitoxin to intact cells showed that nimodipine treatment reduced the number of cell surface Na+ channels, whereas treatment with Bay K 8664 produced the opposite effect. The dual regulation of Na+ channel abundance explained the mentioned changes in Na+ current amplitude. 4. Plasma membrane Na+ channels had a half-life of approximately 17 h both in control cells and in cells treated with Bay K 8644, as estimated from the rate of decay of peak Na+ current after inhibition of protein synthesis with cycloheximide. Actinomycin D, an inhibitor of gene transcription, and also cycloheximide, occluded the stimulatory effect of Bay K 8644 on Na+ current density when measured over a 24 h period. 5. These findings indicate that the entry of Ca2+ through L-type channels influences in a positive way the number of functional Na+ channels in GH3 cells, and suggest that Ca2+ influx stimulates either Na+ channel gene expression or the expression of a regulatory protein that promotes translocation of pre-assembled Na+ channels into the plasma membrane.

Conditional and unconditional inhibition of calcium-activated potassium channels by reversible protein phosphorylation

Large conductance, calcium-activated potassium channels (BK(Ca) or maxi-K) are important determinants of membrane excitability in many cell types. We used patch clamp techniques to study the biochemical regulation of native BK(Ca) channel proteins by endogenous Ser/Thr-directed protein kinases and phosphatases in cell-free membrane patches from rat pituitary tumor cells (GH(4)C(1)). When protein kinase activity was blocked by removing ATP, endogenous protein phosphatases slowly increased BK(Ca) channel activity approximately 3-fold. Dephosphorylated channels could be activated fully by physiological increases in cytoplasmic calcium or membrane depolarization. In contrast, endogenous protein kinases inhibited BK(Ca) channel activity at two functionally distinct sites. A closely associated, cAMP-dependent protein kinase rapidly reduced channel activity in a conditional manner that could be overcome completely by increasing cytoplasmic free calcium 3-fold or 20 mV further depolarization. Phosphorylation at a pharmacologically distinct site inhibited channel activity unconditionally by reducing availability to approximately half that of maximum at all physiological calcium and voltages. Conditional versus unconditional inhibition of BK(Ca) channel activity through different protein kinases provides cells with a powerful computational mechanism for regulating membrane excitability.

A fast and sensitive technique to study the kinetics and the concentration dependencies of DNA fragmentation during drug-induced apoptosis

Apoptotic cell death with its characteristic coordinated cellular breakdown can be triggered by cytotoxic drugs. One prominent feature that differentiates apoptotic from necrotic cell death is the caspase-mediated activation of an endonuclease that internucleosomally cleaves DNA resulting in the so-called apoptotic DNA ladder. Here we report a new rapid, sensitive and inexpensive column separation technique to study drug-induced DNA fragmentation from 10(6) or less cells. This technique, which is based on a modified plasmid spin column kit, avoids the use of hazardous chemicals. With this procedure and subsequent densitometric analysis it was possible to study the concentration dependencies and the kinetics of drug-induced DNA fragmentation. The applicability of this technique is shown for okadaic acid- and cantharidic-acid-treated pituitary GH(3) cells as well as highly okadaic-acid-resistant sublines. These studies allowed us to compare as well as to differentiate the effects and potencies of these structurally different but functionally quite similar inhibitors of ser/thr phosphatases 1 and 2A.

Structural basis for the binding specificity of a SSTR1-selective analog of somatostatin

The availability of subtype-specific agonists and antagonists for somatostatin (SS) receptors (SSTRs) will be important for elucidation of the function of each receptor isoform in vivo. A SS analog, des-AA1,2,5-[D-Trp8, IAmp9]SS (CH275), has been shown previously to bind preferentially to SSTR1. In this report, we identify structural determinants in the ligand and receptor responsible for the selective binding of CH275 to SSTR1 by modifying both the ligand and the receptor. We propose that IAmp9 in CH275, like Lys9 in SS, interacts with Asp137 in the middle of the third transmembrane domain of SSTR1 to form an ion pair, while other residues unique to SSTR1 conbribute to binding selectivity of CH275 for SSTR1. Replacement of Asp137 with Asn resulted in loss of binding of radiolabeled SS and decreased potencies of both SS and CH275 to induce a change in the extracellular acidification rate measured by microphysiometry. The structural determinants for specific binding to SSTR1 were mapped in chimeric SSTR1/SSTR2 receptors. One chimera, 2beta, with the N-terminus to second transmembrane domain (TM2) from SSTR2 and the remainder of the receptor from SSTR1, had low affinity for CH275. Furthermore, when a single residue, Leu107, in TM2 of SSTR1 was replaced with Phe, the corresponding residue in SSTR2, a 20-fold decrease in affinity for CH275 with no significant change in affinity for SS was observed. A reciprocal change from Phe to Leu in the chimeric receptor 2beta resulted in a 10-fold increase in affinity for CH275. Thus, Leu107 is an important determinant for CH275 binding to SSTR1. To identify the moiety in CH275 which could interact with Leu107, a new analog des-AA1,2,5-[D-Trp8, Amp9]SS was prepared. This analog bound to both SSTR1 and SSTR2 with similar affinities; thus, subtype selectivity was lost. Collectively, these data support a binding model for CH275 in which the positively charged IAmp interacts with the negatively charged Asp137 in TM3 of SSTR1 and the isopropyl group of IAmp forms a hydrophobic interaction with Leu107 in TM2.

Regulation of Ca2+-activated nonselective cationic currents in rat pituitary GH3 cells: involvement in L-type Ca2+ current

Ionic currents were investigated by a patch clamp technique in a clonal strain of pituitary (GH3) cells, using the whole cell configuration with Cs+ internal solution. Depolarizing pulses positive to 0 mV from a holding potential of -50 mV activated the voltage-dependent L-type Ca2+ current (ICa,L) and late outward current. Upon repolarization to the holding potential, a slowly decaying inward tail current was also observed. This inward tail current upon repolarization following a depolarizing pulse was found to be enhanced by Bay K 8644, but blocked by nifedipine or tetrandrine. This current was eliminated by Ba2+ replacement of external Ca2+ as the charge carrier through Ca2+ channels, removal of Ca2+ from the bath solution, or buffering intracellular Ca2+ with EGTA (10 mM). The reversal potential of inward tail current was approximately -25 mV. When intracellular Cl- was changed, the reversal potential of the Ca2+-activated currents was not shifted. Thus, this current is elicited by depolarizing pulses that activate ICa,L and allow Ca2+ influx, and is referred to as Ca2+-activated nonselective cationic current (ICAN). Without including EGTA in the patch pipette, the slowly decaying inward current underlying the long-lasting depolarizing potential after Ca2+ spike was also observed with a hybrid current-voltage protocol. Thus, the present studies clearly indicate that Ca2+-activated nonselective cationic channels are expressed in GH3 cells, and can be elicited by the depolarizing stimuli that lead to the activation of ICa,L.

Calcitonin gene-related peptide as a GH secretagogue in human and rat pituitary somatotrophs

To elucidate the role of calcitonin gene-related peptide (CGRP) in regulating pituitary function, we investigated the effects of CGRP and the related peptide adrenomedullin (AdM) on the secretion of growth hormone (GH) in vitro from human pituitary adenoma cells, rat pituitary tumor (GH3) cells, and normal rat pituitary cells. In 3 of 5 human somatotroph adenomas, GH secretion was stimulated by CGRP (1-100 nM). In one case of somatotroph adenoma, GH release was observed following the addition of 10 nM GHRH and 10 nM CGRP. The addition of CGRP or AdM (1 pM-10 nM) evoked GH secretion from GH3 cells with a bell-shaped distribution curve. CGRP (100 pM) caused the maximum increase of GH secretion (172+/-14 (mean+/-S.D.)% of control). The addition of CGRP8-37, an antagonist of CGRP type 1 receptors, inhibited the stimulatory effect of AdM but did not inhibit the effect of CGRP. The addition of CGRP and AdM evoked moderate GH secretion from normal rat pituitary cells. These results suggested that CGRP is a new GH secretagogue in human and rat pituitary tumor cells.

The thyrotropin-releasing hormone-receptor complex and G11alpha are both internalised into clathrin-coated vesicles

It has been proposed that, after agonist binding, the thyrotropin-releasing hormone receptor (TRHR) becomes internalised associated with Gq, as part of a TRH-TRHR-Gq ternary complex [13]. We tested this hypothesis directly by examining the intracellular distribution of the TRHR and Gq/11 after agonist binding. The localisation of the TRH-TRHR complex and Gq/11alpha was studied by the biochemical isolation of clathrin-coated vesicles (CCVs). The internalised TRH-TRHR complex was localised in CCVs. The CCVs, which had internalised [3H]MeTRH, contained 4-fold higher levels of radiolabelled ligand than did CCVs from cells incubated with [3H]MeTRH at 4 degrees C. Like the receptor-ligand (RL) complex, G11alpha also translocated to these endocytic vesicles. For example, CCVs from cells with internalised TRH-TRHR complexes contained G11alpha, whereas CCVs from cells without internalised RL complexes lacked G11alpha. We conclude that, after agonist-induced TRHR-G11alpha coupling, both the TRH-TRHR complex and G11alpha are internalised in CCVs.

Sphingosine inhibits voltage-operated calcium channels in GH4C1 cells

In the present study we investigated the mechanism of inhibitory action of sphingosine (SP) on voltage-activated calcium channels (VOCCs) in pituitary GH4C1 cells. Using the patch-clamp technique in the whole-cell mode, we show that SP inhibits Ba2+ currents (IBa) when 0.1 mM BAPTA is included in the patch pipette. However, when the BAPTA concentration was raised to 1-10 mM, SP was without a significant effect. The effect of SP was apparently not mediated via a kinase, as it was not inhibited by staurosporine. By using the double-pulse protocol (to release possible functional inhibition of the VOCCs by G proteins), we observed that G proteins apparently evoked very little functional inhibition of the VOCCs. Furthermore, including GDPbetaS (guanyl-5'-yl thiophosphate) in the patch pipette did not alter the inhibitory effect of SP on the Ba2+ current, suggesting that SP did not modulate the VOCCs via a G protein-dependent pathway. Single-channel experiments with SP in the pipette, and experiments with excised outside-out patches, suggested that SP directly inhibited VOCCs. The main mechanism of action was a dose-dependent prolongation of the closed time of the channels. The results thus show that SP is a potent inhibitor of VOCCs in GH4C1 cells, and that calcium may be a cofactor in this inhibition.

Characterization of two pituitary GH3 cell sublines partially resistant to apoptosis induction by okadaic acid

Pituitary GH3 cells die by apoptosis when treated with okadaic acid, a specific inhibitor of ser/thr phosphatases. Incubations starting at concentrations of 5 and 12.5 nM followed by stepwise rises resulted in two populations (the S1 and S2 sublines) that proliferated at initially lethal 30 nM. Cells were partially resistant to higher concentrations of okadaic acid and its derivative methyl okadaate. Toxicity of the structurally distinct inhibitors cantharidic acid and calyculin A was differently affected in the two resistant lines. The enhanced expression of the P-glycoprotein was one mechanism of resistance in S1 and S2. Resistance was reversed completely (S1) or partially (S2) by the addition of verapamil. In addition, phosphatase activity, presumably PP2A, was increased in S2. Therefore, pharmacokinetic and pharmacodynamic mechanisms can protect pituitary GH3 cells from apoptotic cell death by okadaic acid.

A new action of parathyroid hormone. receptor-mediated stimulation of extracellular acidification in human osteoblast-like SaOS-2 cells

The major physiological function of parathyroid hormone (PTH) is the maintenance of Ca2+/Pi homeostasis via the parathyroid hormone/parathyroid hormone-related protein receptor (PTHR) in kidney and bone. An important consequence of PTHR activation in bone is enhanced local acidification of the extracellular space. Agonist activation of some seven transmembrane-domain receptors increases the extracellular acidification rate (ECAR). We utilized microphysiometry to investigate PTH-stimulated, receptor-mediated increases in ECAR in human osteoblast-like SaOS-2 cells. PTH-(1-34) elicited a large, acute, dose-dependent increase in ECAR with an EC50 of about 2 nM. The PTH-induced increase in ECAR was specific to cells expressing the PTHR and was inhibited by PTHR antagonists. Rapid, partial, homologous desensitization of the PTH-induced increase in ECAR was observed. Incubation of SaOS-2 cells with 8-bromo-cyclic AMP neither mimicked nor abrogated the PTH effect, and PTH stimulated an acute increase in ECAR in cAMP-resistant SaOS-2 Ca#4A cells. Stimulation of ECAR by PTH was independent of transient increases in cytosolic free calcium. Both inhibition and down-regulation of PKC reduced the PTH-induced increase in ECAR. Inhibition of Na+/H+ exchange did not affect the PTH-induced ECAR response. We conclude that PTH caused a receptor-mediated, concentration-dependent, increase in ECAR, which was not dependent on the cAMP/PKA signaling pathway or the Na+/H+ exchanger but involved the action of PKC. Thus, acid production in bone, a physiologically important action of PTH, is not confined to osteoclasts as previously considered but is also mediated by osteoblasts.

Both overlapping and distinct signaling pathways for somatostatin receptor subtypes SSTR1 and SSTR2 in pituitary cells

To elucidate the signaling events mediated by specific somatostatin receptor (SSTR) subtypes, we expressed SSTR1 and SSTR2 individually in rat pituitary GH12C1 and F4C1 cells, which lack endogenous somatostatin receptors. In transfected GH12C1 cells, both SSTR1 and SSTR2 coupled to inhibition of Ca2+ influx and hyperpolarization of membrane potential via a pertussis toxin (PTx)-sensitive mechanism. These effects reflected modulation of ion channel activities which are important for regulation of hormone secretion. Somatostatin analogs MK678 and CH275 acted as subtype selective agonists as expected. In transfected F4C1 cells, both SSTR1 and SSTR2 mediated somatostatin-induced inhibition of adenylyl cyclase via a PTx-sensitive pathway. In addition, activation of SSTR2 in F4C1 cells, but not SSTR1, stimulated phospholipase C (PLC) activity and an increase in [Ca2+]i due to release of Ca2+ from intracellular stores. Unlike adenylyl cyclase inhibition, the PLC-mediated response was only partially sensitive to PTx. To determine the structural determinants in SSTR2 necessary for activation of PLC, we constructed chimeric receptors in which domains of SSTR2 were introduced into SSTR1. Chimeric receptors containing only the third intracellular loop, or all three intracellular loops from SSTR2, mediated inhibition of adenylyl cyclase, but failed to stimulate PLC activity as did wild-type SSTR2. Furthermore, the C-terminal tail of SSTR2 was not required for coupling to PLC. Thus, by expressing individual somatostatin receptor subtypes in pituitary cells, we have identified both overlapping and distinct signaling pathways for SSTR1 and SSTR2, and have shown that sequences other than simply the intracellular domains are required for SSTR2 to couple to the PLC signaling pathway.

Epidermal growth factor and okadaic acid stimulate Sp1 proteolysis

Sp1 nuclear levels have been shown to directly correlate with the proliferative state of the cell. We therefore studied changes in the abundance of Sp1 in a rat pituitary cell line GH4 whose growth rate is regulated by epidermal growth factor (EGF). Nuclear extracts from GH4 cells treated with 10 nM EGF for at least 16 h showed a 50% decrease in Sp1 binding to a GC-rich element present in the gastrin promoter. The decrease in binding correlated with a decrease in cell proliferation, a loss of nuclear Sp1 protein and a 50-60% decrease in Sp1-mediated transactivation through an Sp1 enhancer element in transfection assays. Okadaic acid, a phosphatase inhibitor, was synergistic with the effect of EGF on Sp1 protein levels suggesting that the loss of Sp1 was mediated by phosphorylation events. This result was confirmed by showing a 2-fold increase in orthophosphate-labeled Sp1 with EGF and okadaic acid. Cycloheximide prevented the expected loss of Sp1 mediated by EGF and okadaic acid suggesting that the synthesis of a protease may mediate these events. This hypothesis was tested directly by showing that the cysteine protease inhibitor leupeptin prevented Sp1 degradation. Using the PEST-FIND computer program, the computed PEST score for human and rat Sp1 is 10.4 and 13.7, respectively, indicating that Sp1 has a domain with a high concentration of proline, glutamic acid, serine, and threonine residues as reported for a number of proteins with inducible rates of degradation. Collectively, these results indicate that sustained stimulation of GH4 cells by EGF initiates a cascade of phosphorylation events that promotes Sp1 proteolysis, decreased Sp1 nuclear levels and decreased cellular proliferation.

Lovastatin decreases prolactin and growth hormone gene expression in GH4C1 cells through a cAMP dependent mechanism

The heterotrimeric G protein Gs couples several surface ligand receptors to cAMP production, as well as to both growth hormone (GH) and prolactin (PRL) gene expression in pituitary and GH cells. It has been shown that constitutively active alpha s stimulates transient expression of both PRL- and GH- chloramphenicol acetyl transferase (CAT) constructions, which indicates that both the PRL and GH promoter regions are under the influence of signal pathways mediated by alpha s. We have previously shown that the cholesterol lowering drug lovastatin decreases both the amount of G alpha s subunit in the membrane and the adenylyl cyclase activity in GH4C1 cells. Thus, we tried to verify whether that decrease in alpha s levels could affect PRL and GH secretion, as well as the expression of PRL- and GH-CAT constructions. Since the regulation of these two genes is dependent on the pituitary specific transcription factor Pit-1, the effect of lovastatin on the expression of Pit-1-CAT constructions was also studied. Our results show that lovastatin decreased the basal expression of these three cAMP-responsive genes in GH4C1 cells, being partially reversed by the addition of mevalonate to the culture medium. This effect of lovastatin on the promoter activities of the transfected constructions was also observed in PRL and GH secretion to the medium, suggesting that this drug produces similar changes in the endogenous promoters of both hormones. Moreover, the presence of lovastatin did not prevent the response to the cAMP activator forskolin, indicating that the main effect of this drug could be exerted through upstream adenylyl cyclase. In conclusion, our data indicate that lovastatin decreases the basal expression of Pit-1 and consequently of both GH and PRL genes through a mechanism probably mediated by the decrease of G alpha s levels in the cell membrane. Taken together, these results suggest that the activity of membrane heterotrimeric G proteins regulates the basal transcription of specific cellular genes in GH4C1 cells. Moreover the effects of lovastatin may be taken into account in the study of constitutively endocrine disorders associated with an increased secretion of either PRL or GH.