Role of voltage-sensitive calcium channels in [Ca2+]i and secretory responses to activators of protein kinase C in pituitary gonadotrophs

Divergent signaling pathways requiring discrete calcium signals mediate concurrent activation of two mitogen-activated protein kinases by gonadotropin-releasing hormone

Receptors coupled to heterotrimeric G proteins are linked to activation of mitogen-activated protein kinases (MAPKs) via receptor- and cell-specific mechanisms. We have demonstrated recently that gonadotropin-releasing hormone (GnRH) receptor occupancy results in activation of extracellular signal-regulated kinase (ERK) through a mechanism requiring calcium influx through L-type calcium channels in alphaT3-1 cells and primary rat gonadotropes. Further studies were undertaken to explore the signaling mechanisms by which the GnRH receptor is coupled to activation of another member of the MAPK family, c-Jun N-terminal kinase (JNK). GnRH induces activation of the JNK cascade in a dose-, time-, and receptor-dependent manner in clonal alphaT3-1 cells and primary rat pituitary gonadotrophs. Coexpression of dominant negative Cdc42 and kinase-defective p21-activated kinase 1 and MAPK kinase 7 with JNK and ERK indicated that specific activation of JNK by GnRH appears to involve these signaling molecules. Unlike ERK activation, GnRH-stimulated JNK activity does not require activation of protein kinase C and is not blocked after chelation of extracellular calcium with EGTA. GnRH-induced JNK activity was reduced after treatment with the intracellular calcium chelator BAPTA-AM (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester), whereas activation of ERK was not affected. Chelation of intracellular calcium also reduced GnRH-induced activation of JNK in rat pituitary cells in primary culture. GnRH-induced induction and activation of the JNK target c-Jun was inhibited after chelation of intracellular calcium, whereas induction of c-Fos, a known target of ERK, was unaffected. Therefore, although activation of ERK by GnRH requires a specific influx of calcium through L-type calcium channels, JNK activation is independent of extracellular calcium but sensitive to chelation of intracellular calcium. Our results provide novel evidence that GnRH activates two MAPK superfamily members via strikingly divergent signaling pathways with differential sensitivity to activation of protein kinase C and mobilization of discrete pools of calcium.

Calcium influx through L-type channels is required for selective activation of extracellular signal-regulated kinase by gonadotropin-releasing hormone

The hypothalamic decapeptide gonadotropin-releasing hormone stimulates mobilization of two discrete pools of calcium in clonal (alphaT3-1) and primary pituitary gonadotropes. A multidisciplinary approach was implemented to investigate the effects of discrete calcium fluctuations on the signaling pathways linking the gonadotropin-releasing hormone receptor to activation of mitogen-activated protein kinases and immediate early genes. Blockade of calcium influx through nifedipine-sensitive voltage-gated calcium channels reduced buserelin-induced activation of extracellular signal-regulated kinase (ERK) and c-Fos while activation of c-Jun N-terminal kinase and c-Jun was unaffected. Inhibition of buserelin-stimulated ERK activity by nifedipine was also observed in rat pituitary cells in primary culture. Direct activation of alphaT3-1 cell L-type calcium channels with the agonist Bay-K 8644 resulted in phosphorylation of ERK and induction of c-Fos. However, simple voltage-induced channel activation did not produce a sufficient calcium signal, since depolarization with 35 mM KCl failed to induce activation of ERK. Depletion of intracellular calcium stores with thapsigargin did not affect buserelin-induced ERK activation. An inhibitor of protein kinase C decreased calcium influx through nifedipine-sensitive calcium channels and phosphorylation of ERK induced by buserelin. Pharmacological inhibition of protein kinase C did not block Bay-K 8644-induced ERK activation. These observations suggest that calcium influx through L-type channels is required for GnRH-induced activation of ERK and c-Fos and that the influence of calcium lies downstream of protein kinase C.

Protein kinase C as a signal for exocytosis

We have studied signaling mechanisms that stimulate exocytosis and luteinizing hormone secretion in isolated male rat pituitary gonadotropes. As judged by reverse hemolytic plaque assays, phorbol-12-myristate-13-acetate (PMA) stimulates as many gonadotropes to secrete as does gonadotropin-releasing hormone (GnRH). However, PMA and GnRH use different signaling pathways. The secretagogue action of GnRH is not very sensitive to bisindolylmaleimide I, an inhibitor of protein kinase C, but is blocked by loading cells with a calcium chelator, 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. The secretagogue action of PMA is blocked by bisindolylmaleimide I and is not very sensitive to the intracellular calcium chelator. GnRH induces intracellular calcium elevations, whereas PMA does not. As judged by amperometric measurements of quantal catecholamine secretion from dopamine- or serotonin-loaded gonadotropes, the secretagogue action of PMA develops more slowly (in several minutes) than that of GnRH. We conclude that exocytosis of secretory vesicles can be stimulated independently either by calcium elevations or by activation of protein kinase C.

The involvement of dihydropyridine-sensitive calcium channels in phorbol ester-induced luteinizing hormone and growth hormone release

We examined the role of voltage-activated, L-type, Ca2+ channels in phorbol ester-induced luteinizing hormone (LH) and growth hormone (GH) release from rat anterior pituitary tissue. The L-type Ca2+ channel inhibitor, nimodipine (NMD), inhibited phorbol 12,13-dibutyrate (PDBu)-induced GH release but had no significant effect on LH release. The L-type Ca2+ channel activator BAY K 8644 had no effect on PDBu-induced GH release but potentiated PDBu-induced LH release. In contrast, 60 mM K(+)-induced LH and GH release were inhibited by NMD, whereas BAY K 8644 had no effect. When PDBu and either K+ or BAY K 8644 were used together, they acted synergistically to evoke levels of LH release greater than addition of release caused by each secretagogue alone. However, the release of GH was additive with PDBu and either K+, BAY K 8644. The protein kinase C (PKC) inhibitor staurosporine inhibited both PDBu-induced LH release and GH release. A structurally different PKC inhibitor, H7, significantly inhibited PDBu-induced LH release but had no effect on PDBu-induced GH release. Both staurosporine and H7 inhibited LH release induced by PDBu and BAY K 8644 together. In contrast, although staurosporine inhibited GH release induced by PDBu and BAY K 8644, H7 significantly potentiated this response. A difference in the action of these two inhibitors was also apparent on K(+)-induced hormone release where staurosporine partially blocked K(+)-induced LH and GH release but H7 had no effect on the release of either hormone. Data obtained in 45Ca2+ influx experiments further suggested that a staurosporine-sensitive, but H7-resistant, PKC-like kinase may tonically maintain L-channels in a voltage-sensitive state, as down-regulation of PKC in dispersed anterior pituitary cells by long term PDBu treatment caused a significant reduction in K(+)-induced 45Ca2+ influx. We conclude that phorbol ester-induced GH release, but not LH release, is a result of L-type Ca2+ channel activation which may occur by means of alterations in the channel itself to increase its responsiveness to a given depolarisation.

Modulatory actions of estradiol and progesterone on phorbol ester-stimulated LH secretion from cultured rat pituitary cells

We compared the ability of estradiol and progesterone to modulate gonadotropin-releasing hormone (GnRH) and protein kinase C (PKC)-mediated luteinizing hormone (LH) secretion. Long-term (48 h) treatment of rat pituitary cells with 1 nM estradiol enhanced GnRH and phorbol ester (TPA)-stimulated LH secretion. This positive effect was facilitated by additional short-term (4 h) treatment with progesterone (100 nM). However, long-term progesterone treatment, which inhibited GnRH-stimulated LH secretion, did not influence TPA-stimulated gonadotropin release. These steroid actions occurred without an effect on the total amount of LH in the cell cultures (total LH = LH secreted + LH remaining in the cell) and neither the secretagogues nor the steroids altered total LH. Since GnRH or TPA-induced LH secretion depends on Ca2+ influx into the gonadotroph, we also analyzed the effects of estradiol and progesterone under physiological extracellular Ca2+ concentrations and in the absence of extracellular Ca2+. The steroids were able to influence GnRH or TPA-induced LH secretion under both conditions. However, when TPA was used as stimulus in Ca(2+)-deficient medium the relative changes induced by estradiol and progesterone were more pronounced, possibly indicating that the extracellular Ca(2+)-independent component of PKC-mediated LH secretion is more important for the regulation of the steroid effects. It is concluded that estradiol and progesterone might mediate their modulatory actions on GnRH-stimulated LH secretion via an influence on PKC. This effect can occur independently from de novo synthesis of LH and Ca2+ influx into gonadotrophs.

Characterization of the gonadotrophin-releasing hormone calcium response in single alpha T3-1 pituitary gonadotroph cells

Intracellular calcium ([Ca2+]i) was measured in single immortalized gonadotroph alpha T3-1 cells using dual wavelength fluorescence microscopy combined with dynamic video imaging. Gonadotrophin-releasing hormone (GnRH, 10(-8) M) produced a biphasic rise in [Ca2+]i which could be abolished by a GnRH antagonist. The initial calcium transient was complete within seconds while the smaller secondary plateau phase lasted several minutes. The calcium spike was reduced by nifedipine (10(-6) M), a calcium channel blocker, and thapsigargin (10(-6) M) which inhibits inositol 1,4,5-trisphosphate (IP3) mediated release of [Ca2+]i but abolished by the intracellular calcium antagonist TMB-8 (10(-6) M). The secondary phase was reduced following pretreatment with either nifedipine or the protein kinase C (PKC) antagonist, H-7 (10(-6) M). The PKC agonist PMA (phorbol 12-myristate 13-acetate, 10(-6) M) produced a small rise in basal [Ca2+]i and abolished the GnRH calcium response. The initial calcium response to GnRH therefore involves both an IP3-mediated rise in cytosolic calcium due to the release from intracellular stores and an influx of extracellular calcium through second messenger-operated calcium channels. In contrast the secondary calcium response mainly involves the influx of extracellular calcium through PKC-activated calcium channels.

Actions of two native GnRHs and protein kinase C modulators on goldfish pituitary cells. Studies on intracellular calcium levels and gonadotropin release

Previous results indicate that the two native gonadotropin (GtH)-releasing hormones of the goldfish, sGnRH and cGnRHII, stimulate GtH secretion in an extracellular Ca2+ ([Ca2+]o) dependent manner. In the present study, sGnRH, cGnRHII, KCI and the protein kinase C (PKC) activators TPA and DiC8, stimulated increases in intracellular Ca2+ ([Ca2+]i) levels in goldfish pituitary cells. Testing in Ca(2+)-deficient medium abolished the [Ca2+]i responses to cGnRHII, TPA and KCI and attenuated responses to sGnRH and DiC8. These results are the first to demonstrate that in teleost pituitary cells both native GnRHs stimulate increases in [Ca2+]i levels via [Ca2+]o entry. sGnRH- and DiC8-stimulated increases in [Ca2+]i also appear to be partially due to mobilization of Ca2+ from intracellular stores. Other results are consistent with a role for PKC in mediating GnRH action especially extracellular Ca2+ entry. Firstly, the PKC inhibitor staurosporine decreased GnRH- and TPA-induced [Ca2+]i responses. Secondly, incubation with Ca(2+)-deficient medium attenuated TPA- and DiC8-stimulated GtH release. Thirdly, GtH release responses to PKC activators were enhanced and reduced by an agonist and an antagonist of Ca2+ channel function, respectively. However, differences in the sensitivity of DiC8- and TPA-elicited responses to manipulations of [Ca2+]o entry indicate that these two PKC activators may have different actions in the goldfish pituitary. A difference in action of the two GnRHs on mobilization of Ca2+ from intracellular stores is also indicated.

Activation of protein kinase C reduces L-type calcium channel activity of GH3 pituitary cells

These studies describe the effect of protein kinase C (PKC) activation on the activity of voltage-sensitive L-type Ca2+ channels of GH3 pituitary cells. The rate of 45Ca2+ uptake was stimulated greater than 25-fold by depolarization in the presence of BAY K 8644; the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) reduced this response by 70% in a concentration-dependent fashion. Phorbol 12,13-dibutyrate (PDBu) inhibited depolarization-induced 45Ca2+ uptake within 1 min and caused a nearly maximal reduction after 1 h; its effects were rapidly reversible. TPA decreased the high K(+)-stimulated increase in intracellular free calcium ion concentration ([Ca2+]i) from 8.5- to 3.2-fold by 5 min and to 2.0-fold after 18 h without altering the peak [Ca2+]i response to the peptide hormone TRH. Ca2+ channel current, measured directly using the whole cell configuration of the patch-clamp technique, declined an average of 6.4% over 5 min for control cells and 28.9% when TPA was added to the bathing medium for 5 min. Treatment with 100 nM TPA for 24 h dramatically reduced peak current without shifting the peak of the current-voltage relationship. The mean peak Ca2+ channel current was reduced from 423 to 128 pA, although a few cells seemed completely resistant. To determine whether the effects of phorbol esters were due to the activation of PKC we tested the potency of several drugs to inhibit L-channel activity and to shift the affinity of the epidermal growth factor (EGF) receptor, an established PKC response.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyrotropin-releasing hormone-mediated Mn2+ entry in perifused rat anterior pituitary cells

Receptor-mediated Ca2+ influx has been shown to exist in several cell types. Thyrotropin-releasing-hormone (TRH)-stimulated Ca2+ entry has also been postulated to exist in rat anterior pituitary cells, but direct evidence has been lacking. We have measured the fluorescence quenching of indo-1 caused by Mn2+ at a Ca(2+)-insensitive wavelength to investigate the actions of TRH on cation entry in dispersed perifused anterior pituitary cells. In indo-1-loaded cells perifused with Ca(2+)-free medium, Mn2+ caused fluorescence quenching in unstimulated cells; TRH caused further quenching. TRH-stimulated Mn2+ entry was transient, and levelled off within a few minutes in the presence of continuous TRH infusion. TRH-stimulated Mn2+ entry was dependent on the concentration of Mn2+ (50 microM-1 mM). TRH (1 microM) caused a larger effect than TRH (10 nM). La3+ and Ni2+ blocked the quenching stimulated by TRH. The rate of basal quenching was not blocked by dopamine, but TRH-stimulated Mn2+ entry was partially blocked by 1 microM-dopamine and almost completely abolished by 10 microM-dopamine. Thapsigargin (1-5 microM), a tumour promotor which depleted intracellular Ca2+ stores, had little effect on Mn2+. F- (20 mM), which activates G-proteins, also had little effect on Mn2+ entry. We conclude that TRH can transiently stimulate Ca2+ entry through a channel than can pass Mn2+ and be inhibited by dopamine. Depleting Ca2+ stores alone is not sufficient to stimulate Ca2+ entry, and so TRH must do so by other mechanisms.

Calcium signaling and secretory responses in agonist-stimulated pituitary gonadotrophs

In cultured pituitary gonadotrophs, gonadotropin-releasing hormone (GnRH) caused dose-dependent and biphasic increases in cytoplasmic calcium concentration ([Ca2+]i) and LH release. Both extra- and intracellular calcium pools participate in GnRH-induced elevation of [Ca2+]i and LH secretion. The spike phase of the [Ca2+]i response represents the primary signal derived predominantly from the rapid mobilization of intracellular Ca2+. In contrast, the prolonged phase of the Ca2+ signal depends exclusively on Ca2+ entry from the extracellular pool. The influx of Ca2+ occurs partially through dihydropyridine-sensitive calcium channels. Both [Ca2+]i and LH responses to increasing concentrations of GnRH occur over very similar time scales, suggesting that increasing degrees of receptor occupancy are transduced into amplitude-modulated Ca2+ responses, which in turn activate exocytosis in a linear manner. However, several lines of evidence indicated the complexity over the relationship between Ca2+ signaling and LH exocytosis. In contrast to [Ca2+]i measurements in cell suspension, single cell Ca2+ measurements revealed the existence of a more complicated pattern of Ca2+ response to GnRH, with a biphasic response to high agonist doses and prominent oscillatory responses to lower GnRH concentrations, with a log-linear correlation between GnRH dose and the frequency of Ca2+ spiking. In addition, analysis of the magnitudes of the [Ca2+]i and LH responses of gonadotrophs to a wide range of GnRH concentrations in the presence and absence of extracellular Ca2+, and to K+ and phorbol ester stimulation, showed non-linearity between these parameters with amplification of [Ca2+]i-mediated exocytosis. Studies on cell depleted of protein kinase C under conditions that did not change the LH pool suggested the participation of protein kinase C in this amplification, especially during the plateau phase of the secretory response to GnRH.

Medium hyperosmolarity depresses thyrotropin-releasing hormone-induced Ca2+ influx and prolactin secretion in GH4C1 cells

We studied the influence of graded degrees of hyperosmolarity on the dynamics of the thyrotropin-releasing hormone (TRH)-induced rise in cytosol Ca2+ concentration ([Ca2+]i) and prolactin (PRL) secretion in GH4C1 cells. TRH caused two phases of increase in [Ca2+]i that were differentially altered by hyperosmolarity: 100% hyperosmolarity (600 mOsm) depressed only 20% of an initial high-amplitude [Ca2+]i burst (first phase) dependent on Ca2+ mobilized from intracellular pools, but it abolished a sustained low-amplitude second phase dependent on extracellular Ca2+ influx. Low degrees of hyperosmolarity suppressed PRL secretion due to Ca2+ influx while high degrees suppressed secretion due to mobilized Ca2+. These data suggest that in GH4C1 cells hypertonic inhibition of secretion may result from both blocking Ca2+ influx and mechanisms unrelated to [Ca2+]i.