Differential activation of protein kinase C delta and epsilon gene expression by gonadotropin-releasing hormone in alphaT3-1 cells. Autoregulation by protein kinase C

Chronic activation of adrenal Gq signaling induces Cyp11b2 expression in the zona fasciculata and hyperaldosteronism

Hyperaldosteronism is often associated with inappropriate aldosterone production and aldosterone synthase (Cyp11b2) expression. Normally, Cyp11b2 expression is limited to the adrenal zona glomerulosa (ZG) and regulated by angiotensin II which signals through Gq protein-coupled receptors. As cells migrate inwards, they differentiate into 11β-hydroxylase-expressing zona fasciculata (ZF) cells lacking Cyp11b2. The mechanism causing ZG-specific aldosterone biosynthesis is still unclear. We investigated the effect of chronic Gq signaling using transgenic mice with a clozapine N-oxide (CNO)-activated human M3 muscarinic receptor (DREADD) coupled to Gq (hM3Dq) that was expressed throughout the adrenal cortex. CNO raised circulating aldosterone in the presence of a high sodium diet with greater response seen in females compared to males. Immunohistochemistry and transcriptomics indicated disrupted zonal Cyp11b2 expression while Wnt signaling remained unchanged. Chronic Gq-DREADD signaling also induced an intra-adrenal RAAS in CNO-treated mice. Chronic Gq signaling disrupted adrenal cortex zonal aldosterone production associated with ZF expression of Cyp11b2.

Regulation of prostate cancer by hormone-responsive G protein-coupled receptors

Regulation of prostate cancer by androgen and androgen receptor (AR), and blockade of AR signaling by AR antagonists and steroidogenic enzyme inhibitors have been extensively studied. G protein-coupled receptors (GPCRs) are a family of membrane receptors that regulate almost all physiological processes. Nearly 40% of FDA-approved drugs in the market target GPCRs. A variety of GPCRs that mediate reproductive function have been demonstrated to be involved in the regulation of prostate cancer. These GPCRs include gonadotropin-releasing hormone receptor, luteinizing hormone receptor, follicle-stimulating hormone receptor, relaxin receptor, ghrelin receptor, and kisspeptin receptor. We highlight here GPCR regulation of prostate cancer by these GPCRs. Further therapeutic approaches targeting these GPCRs for the treatment of prostate cancer are summarized.

Differential roles of PKC isoforms (PKCs) in GnRH stimulation of MAPK phosphorylation in gonadotrope derived cells

The role of protein kinase C (PKC) isoforms (PKCs) in GnRH-stimulated MAPK [ERK1/2, JNK1/2 and p38) phosphorylation was examined in gonadotrope derived cells. GnRH induced a protracted activation of ERK1/2 and a slower and more transient activation of JNK1/2 and p38MAPK. Gonadotropes express conventional PKCα and PKCβII, novel PKCδ, PKCε and PKCθ, and atypical PKC-ι/λ. The use of green fluorescent protein (GFP)-PKCs constructs revealed that GnRH induced rapid translocation of PKCα and PKCβII to the plasma membrane, followed by their redistribution to the cytosol. PKCδ and PKCε localized to the cytoplasm and Golgi, followed by the rapid redistribution by GnRH of PKCδ to the perinuclear zone and of PKCε to the plasma membrane. The use of dominant negatives for PKCs and peptide inhibitors for the receptors for activated C kinase (RACKs) has revealed differential role for PKCα, PKCβII, PKCδ and PKCε in ERK1/2, JNK1/2 and p38MAPK phosphorylation in a ligand-and cell context-dependent manner. The paradoxical findings that PKCs activated by GnRH and PMA play a differential role in MAPKs phosphorylation may be explained by persistent vs. transient redistribution of selected PKCs or redistribution of a given PKC to the perinuclear zone vs. the plasma membrane. Thus, we have identified the PKCs involved in GnRH stimulated MAPKs phosphorylation in gonadotrope derived cells. Once activated, the MAPKs will mediate the transcription of the gonadotropin subunits and GnRH receptor genes.

Differential roles of PKC isoforms (PKCs) and Ca2+ in GnRH and phorbol 12-myristate 13-acetate (PMA) stimulation of p38MAPK phosphorylation in immortalized gonadotrope cells

We examined the role of PKCs and Ca²⁺ in GnRH-stimulated p38MAPK phosphorylation in the gonadotrope derived αT3-1 and LβT2 cell lines. GnRH induced a slow and rapid increase in p38MAPK phosphorylation in αT3-1 and LβT2 cells respectively, while PMA gave a slow response. The use of dominant negatives for PKCs and peptide inhibitors for the receptors for activated C kinase (RACKs), has revealed differential role for PKCα, PKCβII, PKCδ and PKCε in p38MAPK phosphorylation in a ligand-and cell context-dependent manner. The paradoxical findings that PKCs activated by GnRH and PMA play a differential role in p38MAPK phosphorylation may be explained by differential localization of the PKCs. Basal, GnRH- and PMA- stimulation of p38MAPK phosphorylation in αT3-1 cells is mediated by Ca²⁺ influx via voltage-gated Ca²⁺ channels and Ca²⁺ mobilization, while in the differentiated LβT2 gonadotrope cells it is mediated only by Ca²⁺ mobilization. p38MAPK resides in the cell membrane and is relocated to the nucleus by GnRH (∼5 min). Thus, we have identified the PKCs and the Ca²⁺ pools involved in GnRH stimulated p38MAPK phosphorylation.

Intrapituitary mechanisms underlying the control of fertility: key players in seasonal breeding

Recent studies have shown that, in conjunction with dynamic changes in the secretion of GnRH from the hypothalamus, paracrine interactions within the pituitary gland play an important role in the regulation of fertility during the annual reproductive cycle. Morphological studies have provided evidence for close associations between gonadotropes and lactotropes and gap junction coupling between these cells in a variety of species. The physiological significance of this cellular interaction was supported by subsequent studies revealing the expression of prolactin receptors in both the pars distalis and pars tuberalis regions of the pituitary. This cellular interaction is critical for adequate gonadotropin output because, in the presence of dopamine, prolactin can negatively regulate the LH response to GnRH. Receptor signaling studies showed that signal convergence at the level of protein kinase C and phospholipase C within the gonadotrope underlies the resulting inhibition of LH secretion. Although this is a conserved mechanism present in all species studied so far, in seasonal breeders such as the sheep and the horse, this mechanism is regulated by photoperiod, as it is only apparent during the long days of spring and summer. At this time of year, the nonbreeding season of the sheep coincides with the breeding season of the horse, indicating that this inhibitory system plays different roles in short- and long-day breeders. Although in the sheep, it contributes to the complete suppression of the reproductive axis, in the horse, it is likely to participate in the fine-tuning of gonadotropin output by preventing gonadotrope desensitization. The photoperiodic regulation of this inhibitory mechanism appears to rely on alterations in the folliculostellate cell population. Indeed, electron microscopic studies have recently shown increased folliculostellate cell area together with upregulation of their adherens junctions during the spring and summer. The association between gonadotropes and lactotropes could also underlie an interaction between the gonadotropic and prolactin axes in the opposite direction. In support of this alternative, a series of studies have demonstrated that GnRH stimulates prolactin secretion in sheep through a mechanism that does not involve the mediatory actions of LH or FSH and that this stimulatory effect of GnRH on the prolactin axis is seasonally regulated. Collectively, these findings highlight the importance of intercellular communications within the pituitary in the control of gonadotropin and prolactin secretion during the annual reproductive cycle in seasonal breeders.

Differential signaling of the GnRH receptor in pituitary gonadotrope cell lines and prostate cancer cell lines

The GnRH receptor (GnRHR) mediates the pituitary functions of GnRH, as well as its anti-proliferative effects in sex hormone-dependent cancer cells. Here we compare the signaling of GnRHR in pituitary gonadotrope cell lines vs. prostate cancer cell lines. We first noticed that the expression level of PKCα, PKCβII and PKCε is much higher in αT3-1 and LβT2 gonadotrope cell lines vs. LNCaP and DU-145 cell lines, while the opposite is seen for PKCδ. Activation of PKCα, PKCβII and PKCε by GnRH is relatively transient in αT3-1 and LβT2 gonadotrope cell lines and more prolonged in LNCaP and DU-145 cell lines. On the otherhand, the activation and re-distribution of the above PKCs by PMA was similar for both gonadotrope cell lines and prostate cancer cell lines. Activation of ERK1/2 by GnRH and PMA was robust in the gonadotrope cell lines, with a smaller effect observed in the prostate cancer cell lines. The Ca(2+) ionophore A23187 stimulated ERK1/2 in gonadotrope cell lines but not in prostate cancer cell lines. GnRH, PMA and A23187 stimulated JNK activity in gonadotrope cell lines, with a more sustained effect in prostate cancer cell lines. Sustained activation of p38 was observed for PMA and A23187 in Du-145 cells, while p38 activation by GnRH, PMA and A23187 in LβT2 cells was transient. Thus, differential expression and re-distribution of PKCs by GnRH and the transient vs. the more sustained nature of the activation of the PKC-MAPK cascade by GnRH in gonadotrope cell lines vs. prostate cancer cell lines respectively, may provide the mechanistic basis for the cell context-dependent differential biological responses observed in GnRH interaction with pituitary gonadotropes vs. prostate cancer cells.

Differential role of PKC isoforms in GnRH and phorbol 12-myristate 13-acetate activation of extracellular signal-regulated kinase and Jun N-terminal kinase

GnRH is the first key hormone of reproduction. The role of protein kinase C (PKC) isoforms in GnRH-stimulated MAPK [ERK and Jun N-terminal kinase (JNK)] was examined in the αT3-1 and LβT2 gonadotrope cells. Incubation of the cells with GnRH resulted in a protracted activation of ERK1/2 and a slower and more transient activation of JNK1/2. Gonadotropes express conventional PKCα and conventional PKCβII, novel PKCδ, novel PKCε, and novel PKCθ, and atypical PKC-ι/λ. The use of green fluorescent protein-PKC constructs revealed that GnRH induced rapid translocation of PKCα and PKCβII to the plasma membrane, followed by their redistribution to the cytosol. PKCδ and PKCε localized to the cytoplasm and Golgi, followed by the rapid redistribution by GnRH of PKCδ to the perinuclear zone and of PKCε to the plasma membrane. Interestingly, PKCα, PKCβII, and PKCε translocation to the plasma membrane was more pronounced and more prolonged in phorbol-12-myristate-13-acetate (PMA) than in GnRH-treated cells. The use of selective inhibitors and dominant-negative plasmids for the various PKCs has revealed that PKCβII, PKCδ, and PKCε mediate ERK2 activation by GnRH, whereas PKCα, PKCβII, PKCδ, and PKCε mediate ERK2 activation by PMA. Also, PKCα, PKCβII, PKCδ, and PKCε are involved in GnRH and PMA stimulation of JNK1 in a cell-context-dependent manner. We present preliminary evidence that persistent vs. transient redistribution of selected PKCs or redistribution of a given PKC to the perinuclear zone vs. the plasma membrane may dictate its selective role in ERK or JNK activation. Thus, we have described the contribution of selective PKCs to ERK and JNK activation by GnRH.

The GnRH receptor and the response of gonadotrope cells to GnRH pulse frequency code. A story of an atypical adaptation of cell function relying on a lack of receptor homologous desensitization

Brain control of the reproductive system is mediated through hypothalamic gonadotropin-releasing hormone (GnRH) which activates specific receptors (GnRHR) present at the surface of the pituitary gonadotropes to trigger secretion of the two gonadotropins LH and FSH. A unique feature of this system is the high dependence on the secretion mode of GnRH, which is basically pulsatile but undergoes considerable fluctuations in pulse frequency pattern in response to endogenous or external factors. How the physiological fluctuations of GnRH secretion that orchestrate normal reproduction are decoded by the gonadotrope cell machinery to ultimately control gonadotropin release and/or subunit gene transcription has been the subject of intensive studies during the past decades. Surprisingly, the mammalian GnRHR is unique among G protein-coupled receptor family as it lacks the carboxy-terminal tail usually involved in classical endocytotic process. Accordingly, it does not desensitize properly and internalizes very poorly. Both this atypical intrinsic property and post-receptor events may thus contribute to decode the GnRH signal. This includes the participation of a network of signaling pathways that differently respond to GnRH together with a growing amount of genes differentially sensitive to pulse frequency. Among these are two pairs of genes, the transcription factors EGR-1 and NAB, and the regulatory factors activin and follistatin, that function as intracellular autoregulatory feedback loops controlling respectively LHbeta and FSHbeta gene expression and hence, LH and FSH synthesis. Pituitary gonadotropes thus represent a unique model of cells functionally adapted to respond to a considerably fluctuating neuroendocrine stimulation, from short individual pulses to sustained GnRH as observed at the proestrus of ovarian cycle. Altogether, the data emphasize the adaptative reciprocal complementarity of hypothalamic GnRH neurones and pituitary gonadotropes to function as an original unit.

A preformed signaling complex mediates GnRH-activated ERK phosphorylation of paxillin and FAK at focal adhesions in L beta T2 gonadotrope cells

Most receptor tyrosine kinases and G protein-coupled receptors (GPCRs) operate via a limited number of MAPK cascades but still exert diverse functions, and therefore signal specificity remains an enigma. Also, most GPCR ligands utilize families of receptors for mediation of diverse biological actions; however, the mammalian type I GnRH receptor (GnRHR) seems to be the sole receptor mediating GnRH-induced gonadotropin synthesis and release. Signaling complexes associated with GPCRs may thus provide the means for signal specificity. Here we describe a signaling complex associated with the GnRHR, which is a unique GPCR lacking a C-terminal tail. Unlike other GPCRs, this signaling complex is preformed, and exposure of L beta T2 gonadotropes to GnRH induces its dynamic rearrangement. The signaling complex includes c-Src, protein kinase C delta, -epsilon, and -alpha, Ras, MAPK kinase 1/2, ERK1/2, tubulin, focal adhesion kinase (FAK), paxillin, vinculin, caveolin-1, kinase suppressor of Ras-1, and the GnRHR. Exposure to GnRH (5 min) causes MAPK kinase 1/2, ERK1/2, tubulin, vinculin, and the GnRHR to detach from c-Src, but they reassociate within 30 min. On the other hand, FAK, paxillin, the protein kinase Cs, and caveolin-1 stay bound to c-Src, whereas kinase suppressor of Ras-1 appears in the complex only 30 min after GnRH stimulation. GnRH was found to activate ERK1/2 in the complex in a c-Src-dependent manner, and the activated ERK1/2 subsequently phosphorylates FAK and paxillin. In parallel, caveolin-1, FAK, vinculin, and paxillin are phosphorylated on Tyr residues apparently by GnRH-activated c-Src. Receptor tyrosine kinases and GPCRs translocate ERK1/2 to the nucleus to phosphorylate and activate transcription factors. We therefore propose that the role of the multiprotein signaling complex is to sequester a cytosolic pool of activated ERK1/2 to phosphorylate FAK and paxillin at focal adhesions.

Signaling by G-protein-coupled receptor (GPCR): studies on the GnRH receptor

Gonadotropin-releasing hormone (GnRH) is the first key hormone of reproduction. GnRH analogs are extensively used in in vitro fertilization, and treatment of sex hormone-dependent cancers, due to their ability to bring about 'chemical castration'. The interaction of GnRH with its cognate type I receptor (GnRHR) in pituitary gonadotropes results in the activation of Gq/G(11), phospholipase Cbeta (PLCbetaI), PLA(2), and PLD. Sequential activation of the phospholipases generates the second messengers inositol 1, 4, 5-trisphosphate (IP(3)), diacylglycerol (DAG), and arachidonic acid (AA), which are required for Ca(2+) mobilization, the activation of various protein kinase C isoforms (PKCs), and the production of prostaglandin (PG) and other metabolites of AA, respectively. PKC isoforms are the major mediators of the downstream activation of a number of mitogen-activated protein kinase (MAPK) cascades by GnRH, namely: extracellular signal-regulated kinase (ERK), jun-N-terminal kinase (JNK), and p38MAPK. The activated MAPKs phosphorylate both cytosolic and nuclear proteins to initiate the transcriptional activation of the gonadotropin subunit genes and the GnRHR. While Ca(2+) mobilization has been found to initiate rapid gonadotropin secretion, Ca(2+), together with various PKC isoforms, MAPKs and AA metabolites also serve as key nodes, in the GnRH-stimulated signaling network that enables the gonadotropes to decode GnRH pulse frequencies and translating that into differential gonadotropin synthesis and release. Even though pulsatility of GnRH is recognized as a major determinant for differential gonadotropin subunit gene expression and gonadotropin secretion very little is yet known about the signaling circuits governing GnRH action at the 'Systems Biology' level. Direct apoptotic and metastatic effects of GnRH analogs in gonadal steroid-dependent cancers expressing the GnRHR also seem to be mediated by the activation of the PKC/MAPK pathways. However, the mechanisms dictating life (pituitary) vs. death (cancer) decisions made by the same GnRHR remain elusive. Understanding these molecular mechanisms triggered by the GnRHR through biochemical and 'Systems Biology' approaches would provide the basis for the construction of the dynamic connectivity maps, which operate in the various cell types (endocrine, cancer, and immune system) targeted by GnRH. The connectivity maps will open a new vista for exploring the direct effects of GnRH analogs in tumors and the design of novel combined therapies for fertility control, reproductive disorders and cancers.

Reciprocal cross talk between gonadotropin-releasing hormone (GnRH) and prostaglandin receptors regulates GnRH receptor expression and differential gonadotropin secretion

The asynchronous secretion of gonadotrope LH and FSH under the control of GnRH is crucial for ovarian cyclicity but the underlying mechanism is not fully resolved. Because prostaglandins (PG) are autocrine regulators in many tissues, we determined whether they have this role in gonadotropes. We first demonstrated that GnRH stimulates PG synthesis by induction of cyclooxygenase-2, via the protein kinase C/c-Src/phosphatidylinositol 3'-kinase/MAPK pathway in the LbetaT2 gonadotrope cell line. We then demonstrated that PGF(2alpha) and PGI2, but not PGE2 inhibited GnRH receptor expression by inhibition of phosphoinositide turnover. PGF(2alpha), but not PGI2 or PGE2, reduced GnRH-induction of LHbeta gene expression, but not the alpha-gonadotropin subunit or the FSHbeta subunit genes. The prostanoid receptors EP1, EP2, FP, and IP were expressed in rat gonadotropes. Incubations of rat pituitaries with PGF(2alpha), but not PGI2 or PGE2, inhibited GnRH-induced LH secretion, whereas the cyclooxygenase inhibitor, indomethacin, stimulated GnRH-induced LH secretion. None of these treatments had any effect on GnRH-induced FSH secretion. The findings have thus elaborated a novel GnRH signaling pathway mediated by PGF(2alpha)-FP and PGI2-IP, which acts through an autocrine/paracrine modality to limit autoregulation of the GnRH receptor and differentially inhibit LH and FSH release. These findings provide a mechanism for asynchronous LH and FSH secretions and suggest the use of combination therapies of GnRH and prostanoid analogs to treat infertility, diseases with unbalanced LH and FSH secretion and in hormone-dependent diseases such as prostatic cancer.

Endocrine signaling in ovarian surface epithelium and cancer

Ovarian cancer is the sixth most common cancer and the fifth leading cause of cancer-related death among women in developed countries. Greater than 85% of human ovarian cancer arises within the ovarian surface epithelium (OSE), with the remainder derived from granulosa cells or, rarely, stroma or germ cells. The pathophysiology of ovarian cancer is the least understood among all major human malignancies because of a poor understanding of the aetiological factors and mechanisms of ovarian cancer progression. There is increasing evidence suggesting that several key reproductive hormones, such as GnRH, gonadotrophins and sex steroids, regulate the growth of normal OSE and ovarian cancer cells. The objective of this review was to highlight the effects of these endocrine factors on ovarian cancer cell growth and to summarize the signalling mechanisms involved in normal human OSE and its neoplastic counterparts.

Activation of mitogen-activated protein kinase (MAPK) by GnRH is cell-context dependent

The interaction of GnRH with its cognate receptor (GnRHR) in pituitary gonadotropes includes activation of Gq/G11 and phospholipase Cbeta (PLCbeta), which generates the second messengers inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), which are required for Ca2+ mobilization and PKC isoforms activation. Activation of PKC in pituitary gonadotropes leads to the activation of the major members of the mitogen-activated protein kinase superfamily (MAPK), namely: extracellular signal-regulated kinase (ERK), jun-N-terminal Kinase (JNK) and p38MAPK. The above pathways mediate GnRH-induced gonadotropin release and synthesis. Here we summarise the diverse mechanisms utilized by GnRH to activate the MAPK members and show that they depend on "cell-context".

PKC and ERK are differentially involved in gonadotropin-releasing hormone-induced growth hormone gene expression in the goldfish pituitary

Gonadotropin-releasing hormone (GnRH) is produced by the hypothalamus and stimulates the synthesis and secretion of gonadotropin hormones. In addition, GnRH also stimulates the production and secretion of growth hormone (GH) in some fish species and in humans with certain clinical disorders. In the goldfish pituitary, GH secretion and gene expression are regulated by two endogenous forms of GnRH known as salmon GnRH and chicken GnRH-II. It is well established that PKC mediates GnRH-stimulated GH secretion in the goldfish pituitary. In contrast, the signal transduction of GnRH-induced GH gene expression has not been elucidated in any model system. In this study, we demonstrate, for the first time, the presence of novel and atypical PKC isoforms in the pituitary of a fish. Moreover, our results indicate that conventional PKCα is present selectively in GH-producing cells. Treatment of primary cultures of dispersed goldfish pituitary cells with PKC activators (phorbol ester or diacylglycerol analog) did not affect basal or GnRH-induced GH mRNA levels, and two different inhibitors of PKC (calphostin C and GF109203X) did not reduce the effects of GnRH on GH gene expression. Together, these results suggest that, in contrast to secretion, conventional and novel PKCs are not involved in GnRH-stimulated increases in GH mRNA levels in the goldfish pituitary. Instead, PD98059 inhibited GnRH-induced GH gene expression, suggesting that the ERK signaling pathway is involved. The results presented here provide novel insights into the functional specificity of GnRH-induced signaling and the regulation of GH gene expression.

Gonadotropin-releasing hormone induces apoptosis of prostate cancer cells: role of c-Jun NH2-terminal kinase, protein kinase B, and extracellular signal-regulated kinase pathways

A standard therapy used today for prostate cancer is androgen ablation by gonadotropin-releasing hormone analogs (GnRH-a). Although most patients respond to androgen ablation as an initial systemic therapy, nearly all cases will develop androgen resistance, the management of which is still a major challenge. Here, we report that GnRH-a can directly induce apoptosis of the androgen-independent prostate cancer-derived DU145 and PC3 cell lines. Using specific inhibitors, we found that the apoptotic effect of GnRH-a is mediated by c-Jun NH2-terminal kinase (JNK) and inhibited by the phosphatidylinositol 3'-kinase (PI3K)-protein kinase B (PKB) pathway. Indeed, in DU145 cells, GnRH-a activates the JNK cascade in a c-Src- and MLK3-dependent manner but does not involve protein kinase C and epidermal growth factor receptor. Concomitantly, GnRH-a reduces the activity of the PI3K-PKB pathway, which results in the dephosphorylation of PKB mainly in the nucleus. The reduction of PKB activity releases PKB-induced inhibition of MLK3 and thus further stimulates JNK activity and accelerates the apoptotic effect of GnRH-a. Interestingly, extracellular signal-regulated kinase is also activated by GnRH-a, and this occurs via a pathway that involves matrix metalloproteinases and epidermal growth factor receptor, but its activation does not affect JNK activation and the GnRH-a-induced apoptosis. Our results support a potential use of GnRH-a for the treatment of advanced prostate cancer and suggest that the outcome of this treatment can be amplified by using PI3K-PKB inhibitors.

Signal pathway involved in increased expression of neutral endopeptidase 24.11 by gonadotropin releasing hormone in choriocarcinoma cells

Neutral endopeptidase 24.11 (NEP) is known to regulate cellular functions by degrading several bioactive peptides, such as gonadotropin-releasing hormone (GnRH). The present study was performed to clarify the mechanisms of NEP expression by GnRH in human choriocarcinoma (BeWo) cells. GnRH increased NEP expression and enzyme activity in a dose- and time-dependent manner in BeWo cells. The phosphorylation levels of protein kinase C (PKC) delta, p38 mitogen-activated protein kinase (MAPK), and c-Jun N-terminal kinase (JNK1 and 2) were enhanced after 10 min exposure of 10(-6)m GnRH. The effect of GnRH on both NEP expression and enzyme activity was completely inhibited by inhibitors of PKC, PKC delta, and p38MAPK. Cell number was reduced by 54.4 per cent of the control by culture with 10(-6)m GnRH for 24 h. However, phosphoramidon, a NEP specific inhibitor, inhibited antiproliferative effect of GnRH and reverted to the control level. In conclusion, GnRH induces NEP expression by PKC delta and p38MAPK, and increased NEP expression may be involved in antiproliferative effect in BeWo cells.

Genomics view of gonadotrope signaling circuits

Gonadotropin-releasing hormone (GnRH) binds to the pituitary GnRH receptor to activate signal transduction cascades that ultimately modulate gonadotropin biosynthesis. Comprehensive studies of the GnRH-activated gene program in the LbetaT2 gonadotrope cell line have greatly increased our knowledge of the number of early and intermediate gene transcripts that are modulated by GnRH. Among the classes of gene induced are several whose protein products provide feedback to various levels of signaling pathways, suggesting that gene induction forms an integral component of signal transduction and contributes to longer-timescale feedback and feedforward loops. High-throughput quantitative genomic studies, mathematical modeling and biochemical studies are beginning to delineate the organization and function of the signal-decoding and logic circuit modules of the gonadotrope's signal transduction network.

Transcriptional and post-transcriptional regulation of the PKC delta gene by etoposide in L1210 murine leukemia cells: implication of PKC delta autoregulation

Protein kinase C delta (PKC delta) plays an important role in the regulation of apoptosis in response to diverse anticancer agents. PKC delta is cleaved irreversibly to a catalytically active fragment in response to apoptotic stimuli; however, little information is available about the regulation of PKC delta gene expression. In this study, we found that the amount of steady-state PKC delta mRNA and protein was increased by etoposide in mouse L1210 leukemia cells. The transcriptional rate of the PKC delta gene and the stability of PKC delta mRNA were increased by treatment with etoposide, resulting in the accumulation of PKC delta protein. Rottlerin inhibited etoposide-induced PKC delta gene expression significantly, while Go6976, LY294002 and PD98059 had no effect. Further, both stable and adenovirus-mediated expression of a dominant negative PKC delta(KR) abrogated etoposide-induced PKC delta expression. Etoposide-stimulated PKC delta transcription but not PKC delta mRNA stability was blocked completely by pretreatment with rottlerin. Our data reveal a novel mechanism whereby PKC delta gene is regulated at the transcriptional and post-transcriptional level in the L1210 leukemia cell line.

Protein kinase Cdelta as gonadotropin-releasing hormone target isoenzyme in the alphaT3-1 gonadotrope cell line

We investigated the kinetics of gonadotropin-releasing hormone (GnRH)-induced activation of the protein kinase C (PKC) delta isoform in alphaT3-1 gonadotrope cells. Results were evaluated in subcellular fractions and whole-cell lysates using specific antibodies recognizing either non- or (trans- and auto-)phosphorylated forms of the kinase at Thr505 and Ser643 residues modulating stability and/or activation of the enzyme. Under basal conditions, and in contrast to PKC epsilon, PKC delta was mainly associated with the membrane compartment. GnRH (10(-7)M) elicited further and rapid membrane translocation and time-dependent phosphorylation at both sites of PKC delta. The neuropeptide's effects did not show a refractory period after short but successive GnRH stimulation and were abolished by the GnRH antagonist, antide. Sustained GnRH stimulation (2-6 h) provoked rapid down-regulation of PKC delta. Antide, by inhibiting the initial processes (translocation, phosphorylation), counteracted the degradation of the enzyme. Proteolytic processing of PKC delta was shown to mainly involve proteasome activity. Indeed, specific proteasome inhibitors prevented GnRH-elicited kinase depletion and induced membrane accumulation of the enzyme in a phosphorylated (Thr505, Ser643) form. Thus, GnRH may regulate time-dependent cell responses by modulating the phosphorylation/activation state of its signal transduction effector proteins, and by maintaining their appropriate expression balance via proteolytic processes involving the proteasome system.

The biology of gonadotropin hormone-releasing hormone: role in the control of tumor growth and progression in humans

It is now well known that different forms of GnRH coexist in the same vertebrate species. In humans, two forms of GnRH have been identified so far. The first form corresponds to the hypophysiotropic decapeptide, and is now called GnRH-I. The second form has been initially identified in the chicken brain, and it is referred to as GnRH-II. GnRH-I binds to and activates specific receptors, belonging to the 7 transmembrane (7TM) domain superfamily, present on pituitary gonadotropes. These receptors (type I GnRH receptors) are coupled to the Gq/11/PLC intracellular signalling pathway. A receptor specific for GnRH-II (type II GnRH receptor) has been identified in non-mammalian vertebrates as well as in primates, but not yet in humans. In the last 10-15 years experimental evidence has been accumulated indicating that GnRH-I is expressed, together with its receptors, in tumors of the reproductive tract (prostate, breast, ovary, and endometrium). In these hormone-related tumors, activation of type I GnRH receptors consistently decreases cell proliferation, mainly by interfering with the mitogenic activity of stimulatory growth factors (e.g., EGF, IGF). Recent data seem to suggest that GnRH-I might also reduce the migratory and invasive capacity of cancer cells, possibly by affecting the expression and/or activity of cell adhesion molecules and of enzymes involved in the remodelling of the extracellular matrix. These observations point to GnRH-I as an autocrine negative regulatory factor on tumor growth progression and metastatization. Extensive research has been performed to clarify the molecular mechanisms underlying the peculiar antitumor activity of GnRH-I. Type I GnRH receptors in hormone-related tumors correspond to those present at the pituitary level in terms of cDNA nucleotide sequence and protein molecular weight, but do not share the same pharmacological profile in terms of binding affinity for the different synthetic GnRH-I analogs. Moreover, the classical intracellular signalling pathway mediating the stimulatory activity of the decapeptide on gonadotropin synthesis and secretion is not involved in its inhibitory activity on hormone-related tumor growth. In these tumors, type I GnRH receptors are coupled to the Gi-cAMP, rather than the Gq/11-PLC, signal transduction pathway. Recently, we have reported that GnRH-I and type I GnRH receptors are expressed also in tumors not related to the reproductive system, such as melanoma. Also in melanoma cells, GnRH-I behaves as a negative regulator of tumor growth and progression. Interestingly, the biochemical and pharmacological profiles of type I GnRH receptors in melanoma seem to correspond to those of the receptors at pituitary level. The data so far reported on the expression and on the possible functions of GnRH-II in humans are still scanty. The decapeptide has been identified, together with a 'putative' type II GnRH receptor, both in the central nervous system and in peripheral structures, such as tissues of the reproductive tract (both normal and tumoral). The specific biological functions of GnRH-II in humans are presently under investigation.

Structure of the GnRH receptor-stimulated signaling network: insights from genomics

The GnRH receptor influences gene expression in the gonadotrope through activating signaling cascades that modulate transcription factor expression and activity. A longstanding question in neuroendocrinology is how instructions received at the membrane in the form of the pattern of receptor stimulation are processed into specific biosynthetic changes at each gonadotropin promoter. Signal transduction from the membrane to preformed transcription factors relies on recognition of altered conformations. Signal transduction through the layers of the gene network also requires the biosynthesis of new transcription factors. The signal processing of this system depends on its molecular connectivity map and its feedback and feed-forward loops. Review of signal transduction, gene control, and genomic studies provide evidence of key loops that cross between cellular and nuclear compartments. Genomic studies suggest that the signal transduction and gene network form a continuum. We propose that information transfer in the gonadotrope depends on robust signaling modules that serve to integrate events at different time scales across cytoplasmic and nuclear compartments.

Activation and nuclear translocation of PKCdelta, Pyk2 and ERK1/2 by gonadotropin releasing hormone in HEK293 cells

The mechanism of agonist-induced activation of Pyk2 and its relationship with ERK1/2 phosphorylation was analyzed in HEK293 cells stably expressing the gonadotropin releasing hormone (GnRH) receptor. GnRH stimulation caused rapid and sustained phosphorylation of ERK1/2 and Pyk2 that was accompanied by their nuclear translocation. Pyk2 was also localized on cell membranes and at focal adhesions. Dominant negative Pyk2 (PKM) had no effect on GnRH-induced ERK1/2 phosphorylation and c-fos expression. These actions of GnRH on ERK1/2 and Pyk2 were mimicked by activation of protein kinase C (PKC) and were abolished by its inhibition. GnRH caused translocation of PKCalpha and delta, but not of epsilon, iota and lambda, to the cell membrane, as well as phosphorylation of Raf at Ser338, a major site in the activation of MEK/ERK1/2. Stimulation of HEK293 cells by EGF caused marked ERK1/2 phosphorylation that was attenuated by the selective EGFR receptor (EGF-R) kinase inhibitor, AG1478. However, GnRH-induced ERK1/2 activation was independent of EGF-R activation. These results indicate that activation of PKC is responsible for GnRH-induced phosphorylation of both ERK1/2 and Pyk2, and that Pyk2 activation does not contribute to GnRH signaling. Moreover, GnRH-induced phosphorylation of ERK1/2 and expression of c-fos in HEK293 cells is independent of Src and EGF-R transactivation, and is mediated through the PKC/Raf/MEK cascade.

Gonadotropin-releasing hormone activates the 12-lipoxygenase pathway in the LbetaT2 gonadotrope cell line

Previous studies have indicated that arachidonic acid and its lipoxygenase (LO) metabolites play a role in the post-receptor effects of gonadotropin-releasing hormone (GnRH) but the exact role and nature of these putative eicosanoids remain unclear. The potential role of arachidonic acid and LO in GnRH receptor-mediated signaling was investigated in the LbetaT2 gonadotrope cell line, which expresses gonadotropins (LH and FSH) and GnRH-receptor mRNAs. Western immunobloting of LbetaT2 cell extracts, performed with a murine leukocyte polyclonal antibody against 12-LO, showed a 70-kD band, suggesting the presence of 12-LO protein in these cells. GnRH nearly doubled the release of 12-hydroeicosatetraenoic acid, a product of the 12-LO enzyme, within 10 min. A specific reverse transcriptase polymerase chain reaction with a set of primers based on the reported sequence of rat brain 12-LO yielded a 170-bp band which showed 100% homology with the expected rat brain 12-LO sequence. Exposure of LbetaT2 cells to pulsatile GnRH treatment (10 nM, 90-min interpulse, one and three pulses) led to a approximately 3-fold increase in 12-LO mRNA levels. In conclusion, we provide evidence for the presence of a 12-LO enzyme in LbetaT2 cells, the expression and activity of which are increased by short-term/pulsatile exposure to GnRH. LbetaT2 cells represent a potential model to further study the involvement of 12-LO in GnRH receptor signaling.

Extracellular signal-regulated kinase and c-Src, but not Jun N-terminal kinase, are involved in basal and gonadotropin-releasing hormone-stimulated activity of the glycoprotein hormone alpha-subunit promoter

Addition of a GnRH agonist (GnRH-A) to alphaT3-1 cells stimulates different MAPK cascades: ERK, Jun N-terminal kinase (JNK), and p38. Activation of JNK, ERK, and p38 shows a unique fold activation ratio of 25:12:2, which might encode signal specificity. ERK is translocated to the nucleus within 20 min with a peak at 120 min of GnRH-A stimulation. We used the human alpha-subunit promoter linked to chloramphenicol acetyl transferase (alphaCAT) to examine the role of ERK, JNK, and c-Src, which is implicated in MAPK activation, in basal and GnRH-stimulated alphaCAT. Addition of GnRH-A resulted in a 3-fold increase in alphaCAT, whereas the Ca(2+) ionophore ionomycin and the protein kinase C (PKC) activator 12-O-tetradecanoylphorbol-13-acetate (TPA) had no effect. Addition of GnRH-A and TPA, but not GnRH-A and ionomycin, produced a synergistic response, whereas removal of Ca(2+), but not down-regulation of TPA-sensitive PKCs, abolished GnRH-A-stimulated alphaCAT. Thus, regulation of alpha-promoter activity by GnRH is Ca(2+) dependent and is further augmented by PKC. Cotransfection of alphaCAT and constitutively active or dominant negative plasmids of ERK and JNK cascade members, or the use of the ERK inhibitor PD98059, revealed that ERK, but not JNK, is involved in basal and GnRH-A-stimulated alphaCAT. Because c-Src participates in MAPK activation by GnRH, we also studied its role. Cotransfection of alphaCAT and the dominant negative form of c-Src or incubation with the c-Src inhibitor PP1 reduced GnRH-A-stimulated alphaCAT. The 5'-deletion analysis revealed that the -846/-420 region participated in basal alpha-transcription. In addition, the -346/-156 region containing the pituitary glycoprotein hormone basal element, alpha-basal elements, glycoprotein-specific element, and upstream response element is involved in basal and GnRH-A-stimulated alphaCAT. ERK contribution to GnRH maps to -346/-280 containing the pituitary glycoprotein hormone basal element and alpha-basal elements 1/2. Surprisingly, although c-Src is involved in GnRH-A-stimulated ERK, its involvement is mapped to another region (-280/-180) containing the glycoprotein-specific element. Thus, ERK and c-Src but not JNK are involved in basal and GnRH-A-stimulated-alphaCAT, whereas c-Src contribution is independent of ERK activation.

Dependence of gonadotropin-releasing hormone-induced neuronal MAPK signaling on epidermal growth factor receptor transactivation

The hypothalamic decapeptide, gonadotropin-releasing hormone (GnRH), utilizes multiple signaling pathways to activate extracellularly regulated mitogen-activated protein kinases (ERK1/2) in normal and immortalized pituitary gonadotrophs and transfected cells expressing the GnRH receptor. In immortalized hypothalamic GnRH neurons (GT1-7 cells), which also express GnRH receptors, GnRH, epidermal growth factor (EGF), and phorbol 12-myristate 13-acetate (PMA) caused marked phosphorylation of ERK1/2. This action of GnRH and PMA, but not that of EGF, was primarily dependent on activation of protein kinase C (PKC), and the ERK1/2 responses to all three agents were abolished by the selective EGF receptor kinase inhibitor, AG1478. Consistent with this, both GnRH and EGF increased tyrosine phosphorylation of the EGF receptor. GnRH and PMA, but not EGF, caused rapid phosphorylation of the proline-rich tyrosine kinase, Pyk2, at Tyr(402). This was reduced by Ca(2+) chelation and inhibition of PKC, but not by AG1478. GnRH stimulation caused translocation of PKC alpha and -epsilon to the cell membrane and enhanced the association of Src with PKC alpha and PKC epsilon, Pyk2, and the EGF receptor. The Src inhibitor, PP2, the C-terminal Src kinase (Csk), and dominant-negative Pyk2 attenuated ERK1/2 activation by GnRH and PMA but not by EGF. These findings indicate that Src and Pyk2 act upstream of the EGF receptor to mediate its transactivation, which is essential for GnRH-induced ERK1/2 phosphorylation in hypothalamic GnRH neurons.

An early growth response protein (Egr) 1 cis-element is required for gonadotropin-releasing hormone-induced mitogen-activated protein kinase phosphatase 2 gene expression

In pituitary gonadotropes, gonadotropin-releasing hormone (GnRH) activates all three major mitogen-activated protein kinase (MAPK) cascades. The MAPKs play key roles in transcriptional activation of GnRH-responsive genes. MAPK phosphatases (MKPs) are dual specificity protein phosphatases involved in feedback regulation of MAPK activity. Previous studies indicate that GnRH activates MKP-2 expression in gonadotropes, dependent upon activation of multiple MAPKs and discrete Ca(2+) signals. To further understand the transcriptional mechanism(s) of MKP-2 induction by GnRH, we studied the activity of a 198-nucleotide MKP-2 proximal promoter region that supports GnRH responsiveness in reporter gene assays. Functional analysis of the MKP-2 promoter confirmed a requirement for the protein kinase C-extracellular signal-regulated kinase (ERK) pathway and VGCC-derived Ca(2+) signals in transcriptional activation of the MKP-2 gene. However, the inhibitory effect of thapsigargin on MKP-2 protein expression previously identified was not mediated at the level of promoter activation, suggesting a distinct mechanism for the action of thapsigargin-sensitive Ca(2+) signals. MGRE (MKP-2 GnRH response element) within the MKP-2 promoter mediated promoter activation through the protein kinase C-ERK pathway. The zinc finger transcription factor Egr-1 was identified in the MGRE-binding complex. Egr-1/MGRE binding was induced by GnRH in an ERK-dependent manner. Transcriptional activity of Egr-1 protein was enhanced by GnRH treatment. In addition, overexpression of the Egr-interacting protein, NAB1, resulted in increased GnRH-stimulated MKP-2 gene transcription. Consistent with the putative role of Egr-1 in MKP-2 promoter regulation, Egr-1 protein expression closely correlated with the expression of MKP-2 protein in alpha T3-1 cells. Together, these data suggest that Egr-1 may be a key factor in mediating GnRH-dependent transcriptional activation of the MKP-2 gene.

Intracellular signaling pathways mediated by the gonadotropin-releasing hormone (GnRH) receptor

The hypothalamic gonadotropin-releasing hormone (GnRH) is a key regulator of the reproductive system, triggering the synthesis and release of LH and FSH in the pituitary. GnRH transmits its signal via two specific serpentine receptors that belong to the large group of G-protein coupled receptors (GPCRs). Here we review the intracellular signaling pathways mediated by the GnRH receptor (GnRHR). In pituitary-derived alpha T3-1 cells, a widely used model for GnRH action, GnRHR signaling includes activation of mitogen-activated protein kinase (MAPK) cascades, which provide an important link for the transmission of signals from the cell surface to the nucleus and play a role in the regulation of gonadotropin transcription. Activation of ERK--one of the MAPK cascades--by GnRH in these cells depends mainly on the phosphorylation of Raf1 by PKC, supported by a pathway involving c-Src, dynamin, and Ras. On the other hand, the activation of JNK, another MAPK cascade, involves PKC, c-Src, CDC42/Rac1, and probably MEKK1. The GnRHR is also expressed in non-pituitary cells and was found to be involved in the inhibition of cell proliferation in certain cells. Therefore, GnRHR represents a potential target for GnRH-analogs used for cancer treatment. Interestingly, the signaling mechanism of the GnRHR in other cell types significantly differs from that in pituitary cells. Studies conducted in GnRHR-expressing COS7 cells have shown that GnRHR transmits its signals mainly via Gi, EGF receptor, c-Src, and is not dependent on PKC. Understanding the signaling mechanisms elicited by GnRHR can shed light on the mechanism of action of GnRH in pituitary and extra-pituitary tissues.

Fibroblast growth factor activation of the rat PRL promoter is mediated by PKCdelta

Fibroblast growth factors play a critical role in cell growth, development, and differentiation and are also implicated in the formation and progression of tumors in a variety of tissues including pituitary. We have previously shown that fibroblast growth factor activation of the rat PRL promoter in GH4T2 pituitary tumor cells is mediated via MAP kinase in a Ras/Raf-1-independent manner. Herein we show using biochemical, molecular, and pharmacological approaches that PKCdelta is a critical component of the fibroblast growth factor signaling pathway. PKC inhibitors, or down-regulation of PKC, rendered the rat PRL promoter refractory to subsequent stimulation by fibroblast growth factors, implying a role for PKC in fibroblast growth factor signal transduction. FGFs caused specific translocation of PKCdelta from cytosolic to membrane fractions, consistent with enzyme activation. In contrast, other PKCs expressed in GH4T2 cells (alpha, betaI, betaII, and epsilon) did not translocate in response to fibroblast growth factors. The PKCdelta subtype-selective inhibitor, rottlerin, or expression of a dominant negative PKCdelta adenoviral construct also blocked fibroblast growth factor induction of rat PRL promoter activity, confirming a role for the novel PKCdelta isoform. PKC inhibitors selective for the conventional alpha and beta isoforms or dominant negative PKCalpha adenoviral expression constructs had no effect. Induction of the endogenous PRL gene was also blocked by adenoviral dominant negative PKCdelta expression but not by an analogous dominant negative PKCalpha construct. Finally, rottlerin significantly attenuated FGF-induced MAP kinase phosphorylation. Together, these results indicate that MAP kinase-dependent fibroblast growth factor stimulation of the rat PRL promoter in pituitary cells is mediated by PKCdelta.

Mechanism of GnRH receptor signaling on gonadotropin release and gene expression in pituitary gonadotrophs

Gonadotropin releasing hormone (GnRH), the first key hormone of reproduction, is synthesized and secreted from the hypothalamus in a pulsatile manner and stimulates pituitary gonadotrophs (5-10% of the pituitary cells) to synthesize and release gonadotropin luteinizing hormone (LH) and follicle stimulating hormone (FSH). Gonadotrophs consist of 60% multihormonal cells (LH+FSH) and 18% LH- and 22% FSH-containing cells. LH and FSH, members of the glycoprotein hormone family, stimulate spermatogenesis, folliculogenesis, and ovulation. Although GnRH plays a pivotal role in gonadotropin synthesis and release, other factors such as gonadal steroids and gonadal peptides exert positive and negative feedback mechanisms, which affect GnRH actions. GnRH actions include activation of phosphoinositide turnover as well as phospholipase D and A2, mobilization and influx of Ca2+, activation of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK). A complex crosstalk between the above messenger molecules mediates the diverse actions of GnRH. Understanding the signaling mechanisms involved in GnRH actions is the basis for our understanding of basic reproductive functions in general and gonadotropin synthesis and release in particular.

Role of dynamin, Src, and Ras in the protein kinase C-mediated activation of ERK by gonadotropin-releasing hormone

G-protein-coupled receptors are a large group of integral membranal receptors, which in response to ligand binding initiate diverse downstream signaling. Here we studied the gonadotropin-releasing hormone (GnRH) receptor, which uses Gq for its downstream signaling. We show that extracellular signal-regulated kinase (ERK) activation is fully dependent on protein kinase C (PKC), but only partially dependent on Src, dynamin, and Ras. Receptor tyrosine kinases, FAK, Gbetagamma, and beta-arrestin, which were implicated in some G-protein-coupled receptor signaling to MAPK cascades, do not play a role in the GnRH to ERK pathway. Our results suggest that the activation of ERK by GnRH involves two distinct signaling pathways, which converge at the level of Raf-1. The main pathway involves a direct activation of Raf-1 by PKC, and this step is partially dependent on a second pathway consisting of Ras activation, which occurs in a dynamin-dependent manner, downstream of Src.

Rapid down-regulation of the type I inositol 1,4,5-trisphosphate receptor and desensitization of gonadotropin-releasing hormone-mediated Ca2+ responses in alpha T3-1 gonadotropes

Despite no evidence for desensitization of phospholipase C-coupled gonadotropin-releasing hormone (GnRH) receptors, we previously reported marked suppression of GnRH-mediated Ca(2+) responses in alphaT3-1 cells by pre-exposure to GnRH. This suppression could not be accounted for solely by reduced inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) responses, thereby implicating uncoupling of Ins(1,4,5)P(3) production and Ca(2+) mobilization (McArdle, C. A., Willars, G. B., Fowkes, R. C., Nahorski, S. R., Davidson, J. S., and Forrest-Owen, W. (1996) J. Biol. Chem. 271, 23711-23717). In the current study we demonstrate that GnRH causes a homologous and heterologous desensitization of Ca(2+) signaling in alphaT3-1 cells that is coincident with a rapid (t((12)) < 20 min), marked, and functionally relevant loss of type I Ins(1,4,5)P(3) receptor immunoreactivity and binding. Furthermore, using an alphaT3-1 cell line expressing recombinant muscarinic M(3) receptors we show that the unique resistance of the GnRH receptor to rapid desensitization contributes to a fast, profound, and sustained loss of Ins(1,4,5)P(3) receptor immunoreactivity. These data highlight a potential role for rapid Ins(1,4,5)P(3) receptor down-regulation in homologous and heterologous desensitization and in particular suggest that this mechanism may contribute to the suppression of the reproductive system that is exploited in the major clinical applications of GnRH analogues.

Transcriptional down-regulation of human gonadotropin-releasing hormone (GnRH) receptor gene by GnRH: role of protein kinase C and activating protein 1

Clinical applications of GnRH agonists (GnRHa) are based primarily on the decrease in gonadotropin release after down-regulation of the GnRH receptor (GnRHR) by continuous GnRHa administration. However, the molecular mechanisms underlying the transcriptional regulation of the human GnRHR gene after prolonged GnRH treatment remain poorly understood. In the present study GnRHa-mediated regulation of human GnRHR gene transcription was studied by transiently transfecting the mouse gonadotrope-derived (alphaT3-1) cells with a 2297-bp human GnRHR promoter-luciferase construct (p2300-LucF). A dose- and time-dependent decrease in human GnRHR promoter activity was observed after GnRHa treatment. An average 71% decrease in promoter activity was observed after 24-h treatment with 0.1 microM GnRHa, which was blocked by cotreatment of the GnRH antagonist, antide. This effect was mimicked by phorbol 12-myristate 13-acetate (TPA) administration. In addition, the GnRHa- and TPA-mediated decrease in the human GnRHR promoter activity was reversed by a specific protein kinase C (PKC) inhibitor, GF109203X, or depletion of PKC by TPA pretreatment. These findings indicate that the activation of the PKC pathway is important in regulating the human GnRHR gene expression. By progressive 5'-deletion studies, we have identified a 248-bp DNA fragment (-1018 to -771, relative to the translation start site) at the 5'-flanking region of the human GnRHR gene that is responsible for the GnRHa-mediated down-regulation of human GnRHR promoter activity. Analysis of this sequence reveals the existence of two putative activating protein-1 (AP-1) sites with 87% homology to the consensus sequence (5'-TGA(G/C)T(C/A)A-3'), located at -1000 to -994 (5'-TTAGACA-3', in complementary orientation) and -943 to 937 (5'-TGAATAA-3'). Using competitive gel mobility shift assays, AP-1 binding was observed within this 248-bp region. Site-directed mutation of the putative AP-1-binding site located at -1000 to -994 abolished the GnRHa-induced inhibition. Further competitive GMSA and supershift experiments confirmed the identity of AP-1 binding in this region. By the use of Western blot analysis, a significant increase in c-Jun (100%; P < 0.05) and c-Fos (50%; P < 0.05) protein levels was observed after GnRHa treatment in alphaT3-1 cells. In addition, our data suggested that a change in AP-1 composition, particularly c-Fos, was important in mediating GnRHa-induced inhibition of human GnRHR gene expression. We conclude that activation of the PKC pathway by GnRH is important in controlling human GnRHR gene expression. In addition, the putative AP-1-binding site located at -1000 to -994 of the human GnRHR5'-flanking region has been functionally identified to be involved in mediating this down-regulatory effect.

Regulation of PKC delta expression by estrogen and rat placental lactogen-1 in luteinized rat ovarian granulosa cells

Protein kinase C (PKC) delta is dramatically upregulated in the corpus luteum in the second half of pregnancy in the rat. To gain insight into the hormonal regulation of PKC delta expression, studies were undertaken to analyze the regulation of PKC delta expression in a luteinized rat granulosa cell model. PKC delta protein expression was evaluated in luteinized granulosa cells, isolated from human (h)CG-treated immature female rats 7 h after the injection of an ovulatory dose of hCG and cultured up to 12 days. Cytochrome P450 cholesterol side chain cleavage enzyme expression was observed throughout the culture period, and a majority of the cells expressed steroidogenic acute regulatory protein and responded to rat placental lactogen (rPL)-1 by exhibiting hypertrophy, consistent with maintenance of the luteal phenotype. Both PKC delta protein and mRNA expression increased 3.5-4-fold with time of culture, and PKC delta mRNA expression could be eliminated by treatment of cells with the PKC inhibitor GF109203X. E(2) caused a specific dose- and time-dependent increase in expression of PKC delta protein of twofold, whereas PKC delta mRNA was unaffected by E(2) over a 12-day culture period. Treatment of cells with 500 ng/ml rPL-1 for the final 4 days of a 12-day culture in the absence of E(2) had no effect on PKC delta protein or mRNA expression, while treatment with 500 or 3000 ng/ml rPL-1 in the presence of E(2) significantly enhanced both PKC delta protein and mRNA expression (up to threefold). These results show that two of the major regulators of luteal function in the second half of pregnancy in the rat, E(2) and rPL-1, cooperate to regulate PKC delta expression in luteinized granulosa cells.

Activation of MAPK cascades by G-protein-coupled receptors: the case of gonadotropin-releasing hormone receptor

G-protein-coupled receptors (GPCRs) are a large group of integral membrane receptors that transmit signals from a diverse array of external stimuli, including neurotransmitters, hormones, phospholipids, photons, odorants and taste ligands. In response to ligand binding, the GPCRs initiate diverse downstream signaling pathways through four groups of G proteins and other interacting proteins. Key components in GPCR-induced intracellular signaling are four groups of mitogen-activated protein kinase (MAPK) cascades: extracellular signal-related kinase (ERK), Jun N-terminal kinase (JNK), p38MAPK and big MAPK (BMK). The hallmark of MAPK signaling is the stimulation-dependent nuclear translocation of the involved kinases, which regulate gene expression and the cytoplasmic acute response to mitogenic, stress-related, apoptotic and survival stimuli. A special type of GPCR is the gonadotropin-releasing hormone (GnRH) receptor, which uses primarily the Gq protein for its downstream signaling. GnRH activates all four MAPK cascades by a PKC-dependent mechanism. Common signaling molecules, including the tyrosine kinase c-SRC and the small GTPases CDC42, RAC and RAS, are implicated in various aspects of the GnRH-MAPK pathways. Thus, the activation of MAPK cascades by GnRH opens a new vista in the understanding of the transcriptional regulation of genes encoding gonadotropins. However, additional studies on cell lines and whole animals are required to understand GnRH signaling in the context of other hormones during the reproductive cycle of mouse and human.

Protein kinase C isoforms in pituitary cells displaying differential sensitivity to phorbol ester

Investigations with protein kinase C (PKC) isoform-specific antisera, revealed distinct profiles of PKC isoform content amongst pituitary tissues. Western analysis revealed the alpha, beta, delta, epsilon, zeta and theta isoforms of PKC are present in rat anterior and posterior pituitary tissue as well as in the GH3 somatomammotrophic cell line. AtT-20/D16-V corticotrophic and alphaT3-1 gonadotrophic murine cell lines contained no PKC-delta. The gamma or eta isoforms were undetected in any pituitary tissue. PKC activity measurements revealed Ca2+-independent PKCs in alphaT3-1 and GH3 cells which were more sensitive to activation by phorbol-dibutyrate (PDBu) than the corresponding PKC activity found in COS cells. However, Ca2+-dependent PKC activities were of similar sensitivity to PDBu in GH5, alphaT3-1 and COS cells, indicating that functional differences observed in PDBu-sensitivity in these cells may be due to differential activation of Ca2+-independent PKC isoforms. Moreover, substrate-specificity of these PKCs were also compared indicating that the amount of Ca2+-dependency of the observed PKC activity from the same pituitary tissue is dependent upon the substrate utilized by the PKC isotypes present. These findings explain differential sensitivities of PKC-mediated actions that have previously been observed in a range of pituitary cells.

Differential expression of protein kinase C isoform transcripts in human hematopoietic progenitors undergoing differentiation

Protein kinase C (PKC), a key component of the signaling pathways leading to proliferation and differentiation, consists of a family closely related serine/threonine protein kinases. The mRNA expression of these PKC isoforms has been characterized during hematopoietic differentiation. Using the reverse-transcriptase polymerase chain reaction technique, we have analyzed the levels of isoform transcripts in bone marrow CD34(+) hematopoietic progenitors and their progeny differentiated along erythroid, megakaryocyte, or granulocyte/monocyte lineages, upon exposure to growth factors. In contrast with isoforms alpha, beta(I), beta(II), delta, and epsilon, ubiquitously expressed, isoforms theta, eta/L, zeta, and iota/lambda exhibited a lineage-restricted expression. These qualitative changes, which allow to distinguish the erythroid and megakaryocyte phenotypes from the granulocyte/monocyte phenotype, include zeta exclusively upregulated in granulocytes/monocytes and theta, eta/L, and iota/lambda exclusively expressed in megakaryocytes and erythroblasts. In contrast, erythroblasts and megakaryocytes, which supposedly share a common bipotential progenitor, displayed only quantitative changes. These results evidence the selective expression of PKC isoforms at transcriptional and/or posttranscriptional levels in hematopoietic progenitors induced to differentiate, which may suggest a differential contribution of individual isoforms to cellular signaling.

An autoregulatory loop controlling CYP1A1 gene expression: role of H(2)O(2) and NFI

Cytochrome P450 1A1 (CYP1A1), like many monooxygenases, can produce reactive oxygen species during its catalytic cycle. Apart from the well-characterized xenobiotic-elicited induction, the regulatory mechanisms involved in the control of the steady-state activity of CYP1A1 have not been elucidated. We show here that reactive oxygen species generated from the activity of CYP1A1 limit the levels of induced CYP1A1 mRNAs. The mechanism involves the repression of the CYP1A1 gene promoter activity in a negative-feedback autoregulatory loop. Indeed, increasing the CYP1A1 activity by transfecting CYP1A1 expression vectors into hepatoma cells elicited an oxidative stress and led to the repression of a reporter gene driven by the CYP1A1 gene promoter. This negative autoregulation is abolished by ellipticine (an inhibitor of CYP1A1) and by catalase (which catalyzes H(2)O(2) catabolism), thus implying that H(2)O(2) is an intermediate. Down-regulation is also abolished by the mutation of the proximal nuclear factor I (NFI) site in the promoter. The transactivating domain of NFI/CTF was found to act in synergy with the arylhydrocarbon receptor pathway during the induction of CYP1A1 by 2,3,7,8-tetrachloro-p-dibenzodioxin. Using an NFI/CTF-Gal4 fusion, we show that NFI/CTF transactivating function is decreased by a high activity of CYP1A1. This regulation is also abolished by catalase or ellipticine. Consistently, the transactivating function of NFI/CTF is repressed in cells treated with H(2)O(2), a novel finding indicating that the transactivating domain of a transcription factor can be targeted by oxidative stress. In conclusion, an autoregulatory loop leads to the fine tuning of the CYP1A1 gene expression through the down-regulation of NFI activity by CYP1A1-based H(2)O(2) production. This mechanism allows a limitation of the potentially toxic CYP1A1 activity within the cell.

Gonadotropin-releasing hormone activates the equine luteinizing hormone beta promoter through a protein kinase C/mitogen-activated protein kinase pathway

GnRH regulation of LH secretion is well understood and involves Ca(2+) mobilization. However, the mechanism by which GnRH activates transcription of the LHbeta gene is controversial. GnRH is known to elevate intracellular calcium and activate the protein kinase C (PKC) pathway. The present study evaluated the pathway(s) involved in GnRH induction of LHbeta transcription. We have previously reported that the equine LHbeta (eLHbeta -448/+60) promoter is active in alphaT3-1 cells. Therefore, we created a clonal, stably transfected alphaT3-1 gonadotroph cell line harboring the eLHbeta promoter (-448/+60) fused to the luciferase reporter gene. Administration of a GnRH agonist resulted in induction of promoter activity that was completely inhibited by the antagonist antide. Various calcium-affecting drugs had no effect on the promoter. Administration of phorbol 12-myristate 13-acetate (PMA) elicited an activation similar to, albeit lower than, that with GnRH. Down-regulation or pharmacological inhibition of PKC completely blocked PMA's induction of the promoter, while GnRH induction was only partly attenuated. Treatment with the mitogen-activated protein kinase (MAPK) kinase inhibitor, PD98059, completely inhibited the activation of eLHbeta by PMA but only partly diminished GnRH's induction. Expression of the transcription factor, early growth response protein 1 (Egr1), correlated completely with activation of MAPK, suggesting that Egr1 is the factor through which PKC/MAPK acts. Our data suggest that GnRH induces activity of the eLHbeta promoter by activating a signal transduction cascade involving PKC-MAPK-Egr1 but that has no significant requirement for calcium.

Role of protein kinase C in facilitation of luteinizing hormone (LH)-releasing hormone-induced LH surges by neuropeptide Y

In female rats, neuropeptide Y (NPY) facilitates LHRH-induced LH surges without affecting basal LH release. The signal transduction mechanisms mediating this facilitation are unknown. Here, the involvement of PKC in this process was investigated. Anterior pituitaries (APs) were removed from rats at 1400 h proestrus and perifused in vitro with M199 for 5 h. After an equilibration and baseline period, tissue received hourly 5-minute pulses of the PKC inhibitor GF109203X (GFX), 2.5 microM, followed 15 min later by a 5-minute pulse of LHRH (10(-8) M), NPY (10(-6) M), or phorbol 12-myristate 13-acetate (PMA, 50 nM), or some combination. This regimen was repeated hourly for 3 h. As shown previously, NPY had no effect on basal LH release but greatly facilitated LHRH-induced LH release. Treatment with PMA also facilitated LHRH-induced LH release, to approximately the same degree as NPY. Inhibition of PKC activity with GFX completely prevented NPY's and PMA's facilitation of LH release but did not inhibit LH release stimulated by LHRH alone. Because previous work suggested involvement of both NPY and PKC in alterations of LHRH receptor affinity or number, the in vivo effects of NPY on LHRH binding characteristics were also investigated. Although NPY treatment reliably enhanced LHRH-induced LH and FSH surges in proestrous rats, this action was not accompanied by any detectable change in the affinity or concentration of LHRH receptors in anterior pituitary cell membranes. In summary, we have found that NPY's actions are blocked by PKC inhibition, mimicked by PKC stimulation, and not associated with any overt alterations in LHRH receptor affinity or number. We conclude that PKC activation is required for NPY's facilitation of LHRH-induced LH surges, and that this mechanism likely involves PKC targets other than those which may alter LHRH receptor number or affinity.

Heparin down-regulates the phorbol ester-induced protein kinase C gene expression in human endothelial cells: enzyme-mediated autoregulation of protein kinase C-alpha and -delta genes

Overexpression of protein kinase C-alpha and protein kinase C-delta has been shown to modulate a number of biological effects, including the cell growth and differentiation. We hypothesized that heparin, a potent antimitogenic drug, could affect the cell proliferation by inhibiting the expression of specific protein kinase C genes. Heparin, markedly but not completely, inhibited the serum-stimulated protein kinase C-alpha and -delta mRNA expression. Protein kinase C inhibition or down-regulation significantly decreased the serum-induced protein kinase C isoenzyme gene expression. Heparin failed to inhibit the residual effect of serum that was resistant to the above-mentioned treatments. Phorbol 12-myristate 13-acetate elicited an increase of protein kinase C isoenzyme gene expression that was completely prevented by protein kinase C inhibition or down-regulation. Heparin dose-dependently counteracted and ultimately abolished the increase in the protein kinase C isoenzyme gene expression elicited by phorbol 12-myristate 13-acetate. These results suggest that the inhibition of an autoregulatory role wielded by protein kinase C on the protein kinase C-alpha and -delta gene expression might represent a possible mechanism by which glycosaminoglycans modulate the cell growth.

Protein kinase C delta

The protein kinase C (PKC) family consists of 11 isoenzymes that, due to structural and enzymatic differences, can be subdivided into three groups: The Ca(2+)-dependent, diacylglycerol (DAG)-activated cPKCs (conventional PKCs: alpha, beta 1, beta 2, gamma); the Ca(2+)-independent, DAG-activated nPKCs (novel PKCs: delta, epsilon, eta, theta, mu), and the Ca(2+)-dependent, DAG non-responsive aPKCs (atypical PKCs: zeta, lambda/iota). PKC mu is a novel PKC, but with some special structural and enzymatic properties.

GnRH signalling pathways and GnRH-induced homologous desensitization in a gonadotrope cell line (alphaT3-1)

Exposure of the gonadotrope cells to gonadotropin-releasing hormone (GnRH) reduces their responsiveness to a new GnRH stimulation (homologous desensitization). The time frame as well as the mechanisms underlying this phenomenon are yet unclear. We studied in a gonadotrope cell line (alphaT3-1) the effects of short as well as long term GnRH pretreatments on the GnRH-induced phospholipases-C (PLC), -A2 (PLA2) and -D (PLD) activities, by measuring the production of IP3, total inositol phosphates (IPs), arachidonic acid (AA) and phosphatidylethanol (PEt) respectively. We demonstrated that although rapid desensitization of GnRH-induced IP3 formation did not occur in these cells, persistent stimulation of cells with GnRH or its analogue resulted in a time-dependent attenuation of GnRH-elicited IPs formation. GnRH-induced IPs desensitization was potentiated after direct activation of PKC by the phorbol ester TPA, suggesting the involvement of distinct mechanisms in the uncoupling exerted by either GnRH or TPA on GnRH-stimulated PI hydrolysis. The levels of individual phosphoinositides remained unchanged under any desensitization condition applied. Interestingly, while the GnRH-induced PLA2 activity was rapidly desensitized (2.5 min) after GnRH pretreatments, the neuropeptide-evoked PLD activation was affected at later times, indicating an important time-dependent contribution of these enzymatic activities in the sequential events underlying the GnRH-induced homologous desensitization processes in the gonadotropes. Under GnRH desensitization conditions, TPA was still able to induce PLD activation and to further potentiate the GnRH-evoked PLD activity. AlphaT3-1 cells possess several PKC isoforms which, except PKCzeta, were differentially down-regulated by TPA (PKCalpha, betaII, delta, epsilon, eta) or GnRH (PKCbetaII, delta, epsilon, eta). In spite of the presence of PKC inhibitors or down-regulation of PKC isoforms by TPA, the desensitizing effect of the neuropeptide on GnRH-induced IPs, AA and PEt formation remained unchanged. In conclusion, in alphaT3-1 cells the GnRH-induced homologous desensitization affects the GnRH coupling with PLC, PLA2 and PLD by mechanism(s) which do not implicate TPA-sensitive PKC isoforms, but likely reflect time-dependent modification(s) on the activation processes of the enzymes.

Mechanism of GnRH receptor signaling: combinatorial cross-talk of Ca2+ and protein kinase C

Gonadotropin-releasing hormone (GnRH), the first key hormone of reproduction, is synthesized in the hypothalamus and is released in a pulsatile manner to stimulate pituitary gonadotrope-luteinizing hormone (LH) and follicle-stimulating hormone (FSH) synthesis and release. Gonadotropes represent only about 10% of pituitary cells and are divided into monohormonal cells (18% LH and 22% FSH cells) and 60% multihormonal (LH + FSH) cells. GnRH binds to a specific seven transmembrane domain receptor which is coupled to Gq and activates sequentially different phospholipases to provide Ca2+ and lipid-derived messenger molecules. Initially, phospholipase C is activated, followed by activation of both phospholipase A2 (PLA2) and phospholipase D (PLD). Generation of the second messengers inositol 1,4,5-trisphosphate and diacylglycerol (DAG) lead to mobilization of intracellular pools of Ca2+ and activation of protein kinase C (PKC). Early DAG and Ca2+, derived via enhanced phosphoinositide turnover, might be involved in rapid activation of selective Ca(2+)-dependent, conventional PKC isoforms (cPKC). On the other hand, late DAG, derived from phosphatidic acid (PA) via PLD, may activate Ca(2+)-independent novel PKC isoforms (nPKC). In addition, arachidonic acid (AA) which is liberated by activated PLA2, might also support selective activation of PKC isoforms (PKCs) with or without other cofactors. Differential cross-talk of Ca2+, AA, and selective PKCs might generate a compartmentalized signal transduction cascade to downstream elements which are activated during the neurohormone action. Among those elements is the mitogen-activated protein kinase (MAPK) cascade which is activated by GnRH in a PKC-, Ca(2+)-, and protein tyrosine kinase (PTK)-dependent fashion. Transcriptional regulation can be mediated by the activation of transcription factors such as c-fos by MAPK. Indeed, GnRH activates the expression of both c-jun and c-fos which might participate in gene regulation via the formation of AP-1. The signaling cascade leading to gonadotropin (LH and FSH) gene regulation by GnRH is still not known and might involve the above-mentioned cascades. AA and selective lipoxygenase products such as leukotriene C4 also participate in GnRH action, possibly by cross-talk with PKCs, or by an autocrine/paracrine amplification cycle. A complex combinatorial, spatial and temporal cross-talk of the above messenger molecules seems to mediate the diverse effects elicited by GnRH, the first key hormone of the reproductive cycle.