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

GH3 and RC-4BC cell lines are not suitable as in vitro models to study prolactin modulation and AHR responsiveness in rat pituitary

Some environmental contaminants and pharmaceuticals increase the incidence of uterine tumors in toxicological studies with rats. These tumors can result from a hormonal imbalance due to rat-specific disrupted pituitary prolactin regulation, and are therefore of questionable relevance for humans. In this study we compared in vitro prolactin regulation in rat primary pituitary cells to that in pituitary cell lines, GH3 and RC-4BC. Moreover, we assessed the potential effects of aryl hydrocarbon receptor (AHR) activation on prolactin regulation by using two different AHR agonists, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and DELAQ, the N-deethylated minor metabolite of the pharmaceutical laquinimod. In rat primary pituitary cells, known prolactin stimulant thyrotropin-releasing hormone (TRH) marginally increased prolactin secretion (1.2-fold) and gene expression (1.3-fold). In contrast, synthetic dopamine receptor agonist quinpirole, a known inhibitor of prolactin release, significantly inhibited prolactin secretion (2.6-fold) and gene expression (3.6-fold). In GH3 cells, TRH strongly increased prolactin secretion (6.8-fold) and gene expression (30.8-fold), whereas quinpirole did not affect prolactin secretion nor gene expression. In RC-4BC cells, both TRH and quinpirole did not modulate prolactin secretion nor gene expression. Prolactin secretion and gene expression did not significantly change upon exposure to TCDD or DELAQ. However, DELAQ, but not TCDD, attenuated quinpirole-inhibited prolactin gene expression by 51% in primary pituitary cells. This study shows that pituitary prolactin regulation in rat primary pituitary cells in vitro is distinctly different from rat pituitary cell lines GH3 and RC-4BC. Therefore, effects on pituitary prolactin regulation in vitro should best be performed using rat primary pituitary cells. Additionally, AHR ligands may interact with rat pituitary prolactin regulation, but this appears to depend on the ligand and constitutive prolactin secretion. However, interpretation of the in vitro results with respect to occurrence of uterine tumors in rats should take the complex regulation of prolactin release in the pituitary into account as well as the in vivo hypothalamus-pituitary-gonadal (HPG) axis and its feedback loops.

Cross-talk between the estrogen receptor-, protein kinase A-, and mitogen-activated protein kinase-mediated signaling pathways in the regulation of lactotroph proliferation in primary culture

Using pharmacological means we investigated the functional interactions between the estrogen receptor (ER)-, protein kinase A (PKA)-, and mitogen-activating protein kinase (MAPK)-mediated pathways in the regulation of lactotroph proliferation in primary culture. Treatment of lactotrophs for 28 h with the PKA inhibitor H89 or KT5720, an effective inhibitor of forskolin-induced proliferation, inhibited both insulin- and estradiol-induced proliferation. Inhibition of the MAPK activity by PD98059 or U0126 suppressed not only insulin-induced proliferation but also forskolin- and estradiol-induced proliferation. However, treatment for 28 h with the antiestrogens 4-hydroxy tamoxifen and ICI182780 failed to antagonize estradiol-induced lactotroph proliferation but instead enhanced it. Prolonging the antiestrogen treatment time from 28 to 88 h was effective in antagonizing estradiol-induced proliferation with this long-term treatment also inhibiting insulin- and forskolin-induced proliferation. There was no decrease in these mitogen-induced proliferations following treatment with a progesterone antagonist or protein kinase C inhibitor. These results suggest that cross-talk occurs between the ER-, PKA-, and MAPK-mediated signaling pathways in the regulation of lactotroph proliferation, and that antiestrogens stimulate and inhibit estradiol-induced proliferation in a time-dependent manner.

Thyrotropin-releasing hormone and epidermal growth factor regulate iron-regulatory protein binding in pituitary cells via protein kinase C-dependent and -independent signaling pathways

Intracellular iron homeostasis is regulated, in part, by interactions between iron-regulatory proteins (IRP1 and IRP2) and iron-responsive elements (IREs) in ferritin and transferrin receptor mRNAs. In addition to iron, cellular oxidative stress induced by H(2)O(2), nitric oxide, and hypoxia, and hormonal activation by thyroid hormone and erythropoeitin have each been shown to regulate IRP binding to IREs. Hormonal signals, in particular mediated through protein kinase C (PKC), play a central role in the modulation of IRP/IRE interactions since phorbol esters were shown to activate IRP binding (Eisenstein, R. S., Tuazon, P. T., Schalinske, K. L., Anderson, S. A., and Traugh, J. A. (1993) J. Biol. Chem. 268, 27363-27370). In pituitary thyrotrophs (TtT97), we found that thyrotropin releasing hormone (TRH) and epidermal growth factor (EGF) increased IRP binding to a ferritin IRE, dependent on PKC and mitogen-activated protein kinase (MAPK) activity. In contrast, TRH and EGF decreased IRP binding in pituitary lactotrophs (GH3), despite activation of PKC and MAPK. IRP1 and IRP2 levels remained constant and IRP2 binding was predominant throughout. TRH and EGF markedly decreased IRP binding in MAPK kinase inhibitor-treated GH3 cells, whereas, they increased IRP binding in phosphatase inhibitor-treated GH3 cells. IRE-dependent CAT reporter translational expression closely reflected IRP binding to the ferritin IRE in both GH3 and TtT97 cells. Interestingly, ferritin protein levels were regulated similarly by TRH in both cell lines. These data link two different cell receptor systems to common signaling pathways that regulate IRP binding and ferritin expression. Remarkably, for TRH and EGF, these effects may be PKC-dependent or -independent determined by the cell type.

Proliferative effects of cholecystokinin in GH3 pituitary cells mediated by CCK2 receptors and potentiated by insulin

1. Proliferative effects of CCK peptides have been examined in rat anterior pituitary GH3 cells, which express CCK2 receptors. 2. CCK-8s, gastrin(1-17) and its glycine-extended precursor G(1-17)-Gly, previously reported to cause proliferation via putative novel sites on AR4-2J and Swiss 3T3 cells, elicited significant dose dependent increases of similar magnitude in [3H]thymidine incorporation over 3 days in serum-free medium of 39 +/- 10% (P < 0.01, n = 20), 37 +/- 8% (P < 0.01, n = 27) and 41 +/- 6% (P < 0.01, n = 36) respectively. 3. CCK-8s and gastrin potentially stimulated mitogenesis (EC50 values 0.12 nM and 3.0 nM respectively), whilst G-Gly displayed similar efficacy but markedly lower potency. L-365,260 consistently blocked each peptide. The CCK2 receptor affinity of G-Gly in GH3 cells was 1.09 microM (1.01;1.17, n = 6) and 5.53 microM (3.71;5.99, n = 4) in guinea-pig cortex. 4. 1 microM G-Gly weakly stimulated Ca2+ increase, eliciting a 104 +/- 21% increase over basal Ca2+ levels, and was blocked by 1 microM L-365,260 whilst CCK-8s (100 nM) produced a much larger Ca2+ response (331 +/- 14%). 5. Insulin dose dependently enhanced proliferative effects of CCK-8s with a maximal leftwards shift of the CCK-8s curve at 100 ng ml(-1) (17 nM) (EC50 decreased 500 fold, from 0.1 nM to 0.2 pM; P < 0.0001). 10 microg ml(-1) insulin was supramaximal reducing the EC50 to 5 pM (P = 0.027) whilst 1 ng ml(-1) insulin was ineffective. Insulin weakly displaced [125I]BHCCK binding to GH3 CCK2 receptors (IC50 3.6 microM). 6. Results are consistent with mediation of G-Gly effects via CCK2 receptors in GH3 cells and reinforce the role of CCK2 receptors in control of cell growth. Effects of insulin in enhancing CCK proliferative potency may suggest that CCK2 and insulin receptors converge on common intracellular targets and indicates that mitogenic stimuli are influenced by the combination of extracellular factors present.

Inhibitory effect of thyrotropin releasing hormone on spontaneous proliferation of mouse spleen lymphocytes in vitro

The effects of thyrotropin releasing hormone (TRH) and of TRH-like tripeptide pGlu-His-Gly-OH (colon mitosis inhibitor, CMI) on spontaneous proliferation of murine splenocytes were investigated in vitro. The 3H-thymidine incorporation into splenocyte DNA was used as an index of proliferation. It was found that TRH suppressed the proliferation of splenocytes. In contrast, CMI was ineffective by itself but used together with TRH blocked the effect of the latter.

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.

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

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

A comparison of protein synthetic patterns of MDCK cells grown in serum and hormonally defined serum-free medium

The development of hormonally defined serum-free media (HDM) for the culture of mammalian cells has allowed the study of specific cellular functions in a totally defined environment. Recently, several reports have indicated that there are differences in the basic cellular physiology of cells cultured in HDM when compared to cells cultured by the more traditional method of using serum-supplemented culture media (SM). We report here that there are significant changes in the protein synthetic pattern in MDCK cells grown in HDM. There were no changes exhibited during the first passage in HDM, but following 10 passages in HDM there was an increased isotope ratio of 1) plasma membrane proteins with molecular weights of 12,000, 36,000 and 68,000 and 2) endoplasmic reticulum proteins with molecular weights of 12,000 and 37,000. Additionally, the incorporation of methionine and uridine were significantly increased in cells cultured in HDM for 10 passages. At present, we believe that these changes in the protein synthetic patterns are due at least partially to increased protein synthesis as indicated by monosome/polysome ratios. Therefore, though the use of HDM offers a completely defined system for studying cellular function, results obtained using HDM must be interpreted with caution when comparing them to previous studies that have used SM.

The effects of epidermal growth factor on cell proliferation and prolactin production by GH3 rat pituitary cells

The effects of epidermal growth factor (EGF) were studied in rat pituitary tumor cells, GH3, grown in serum-supplemented and serum-free chemically defined media. EGF (1 nM) increased the cell number to 132% of the control cultured in the defined medium during a 6-day incubation period, while it decreased the cell number to 60% of the control in the serum-supplemented medium. EGF altered the morphology of the cells grown in the defined medium more markedly to an elongated conformation than that of cells grown in the serum-supplemented medium. EGF also stimulated prolactin (PRL) production by culture in the presence or absence of serum. The effects of the cell density of GH3 on the action of EGF were shown to appear in two ways. The mitogenic influence of EGF was more effective on, and more responsive to, high-density cells, whereas the stimulatory action on PRL production was less effective on high-density cells. However, the inhibitory effects on cellular growth appeared independently of cell densities. The results obtained with 125I-EGF binding experiments indicated that the number of binding sites, affinity, and internalization of EGF receptors were similar in either serum-supplemented or serum-free culture. At low cell density, the number of available 125I-EGF binding sites per cell was larger than at high cell density. These results suggested that there was no apparent correlation between EGF binding and its differing effects on the growth of GH3 cultured in the serum-supplemented and the defined medium.

Serum-stimulated cyclic-AMP production in S49 lymphoma cells grown in serum-free medium

Growth of S49 lymphoma cells with horse serum leads to an increase in cellular cAMP phosphodiesterase activity and a resultant loss of hormone- and cholera-toxin-stimulated cAMP accumulation. We now show that the serum requires protein synthesis to produce these effects. Further, we show that acute addition of serum to wild-type S49 cells, grown in serum-free medium, rapidly (under 2 min) and transiently (under 30 min) stimulates cellular cAMP, 10-fold over basal levels. This 'acute' effect of serum was not observed in UNC S49 cells, suggesting that a functional Ns, the guanine nucleotide regulatory component that mediates stimulation of adenylate cyclase, is required for the serum-mediated stimulation of cellular cAMP. Serum added acutely to wild-type S49 cells also augmented cAMP accumulation in response to isoproterenol and forskolin. The half-maximally effective concentrations of horse serum that acutely stimulated or more slowly decreased the cAMP accumulation were approx. 0.2% and 2.0%, respectively. Preliminary attempts to characterize further the serum factor indicate that it has a high (250 000-300 000) molecular weight and is insensitive to boiling; chromatography on Sepharose CL-6B yields a 100-fold purification. Thus, the serum contains one or more components that activate adenylate cyclase, increase cellular cAMP levels and ultimately induce cAMP phosphodiesterase in S49 lymphoma cells.

Effects of TRH on cell proliferation of rat pituitary cells (GH3)

Chronic treatment (more than 3 d) of GH3 cells, cloned rat pituitary cells producing prolactin, with 100 nM TRH resulted in a 41% reduction in the rate of cell growth in a medium containing 0.5% fetal bovine serum. These effects of TRH appeared both in the medium containing a higher concentration of serum and in that containing six growth factors, i.e. insulin, transferrin, parathyroid hormone, fibroblast growth factor, triiodothyronine, and multiplication-stimulating activity (MSA) instead of serum. TRH stimulated prolactin production by GH3 cells in a dose-dependent manner both in the serum-supplemented and serum-free media. On the other hand, TRH, at 1 nM, elicited a 130% stimulation in the cellular growth, whereas, at concentrations of more than 10 nM, it inhibited the growth significantly. In the defined culture system, it was demonstrated that TRH stimulated prolactin production in the presence or absence of six growth factors, whereas its inhibitory effects on cellular growth appeared only in the presence of MSA regardless of the presence or absence of the other five factors. Furthermore, it was shown that a dose-dependent stimulatory effect of MSA on the growth of GH3 cells was suppressed by TRH. TRH exhibited only a stimulatory effect on cellular growth in the medium containing the five factors other than MSA. In conclusion, TRH could inhibit cell growth of GH3 in the presence of MSA in the defined medium or MSA-like factor(s) in the serum-supplemented medium.