A new assay to quantify in vivo repair of G:T mispairs by base excision repair.

The double mismatch reversion (DMR) assay quantifies the repair of G:T mispairs exclusively by base excision repair in vivo. Synthetic oligonucleotides containing two G:T mispairs on opposite strands were placed into the suppressor tRNA gene supF in the shuttle plasmid pDMR. Placement of two mispairs on opposite strands of supF creates a one to one correspondence between the number of correct repair events prior to replication in which G:T mispairs are converted to G:C base pairs and the number of post-replication progeny plasmids with functional supF. Replication of unrepaired or incorrectly repaired mispairs cannot produce progeny plasmids containing functional supF. Indeed, direct transformation of Escherichia coli strain MBL50, which reports the functional status of supF, with pDMR constructs containing two G:T or G:G mispairs yielded <0.5% wild-type supF-containing colonies. In contrast, passage of G:T mispair-containing pDMR constructs through human 5637 bladder carcinoma cells for 48h prior to plasmid recovery and transformation of the reporter E. coli strain MBL50 produced 47% wild-type supF-containing colonies. This finding was indicative of repair prior to the onset of replication in 5637 cells. However, passage of G:G mispair-containing pDMR constructs through 5637 cells yielded <0.5% wild-type supF-containing colonies. Moreover, no difference was observed in the rate of G:T mispair repair by HCT 116 colorectal carcinoma cells deficient in long-patch mismatch repair and a long-patch mismatch repair proficient HCT 116 subline. These data demonstrate that repair measured by the DMR assay is exclusively attributable to short-patch pathways. The DMR assay proved useful in the analysis of the effect of the base 5' to a mispaired G on the rate of G:T base excision repair by 5637 cells, indicating the sequence preference CpG approximately 5mCpG>TpG>GpG approximately ApG, and in the comparison of G:T base excision repair rates between cell lines.

A role for phospholipase D in GLUT4 glucose transporter translocation.

Based on recent studies showing that phospholipase D (PLD)1 is associated with intracellular membranes and promotes membrane budding from the trans-Golgi, we tested its possible role in the membrane trafficking of GLUT4 glucose transporters. Using immunofluorescence confocal microscopy, expressed Myc epitope-tagged PLD1 was found to associate with intracellular vesicular structures by a mechanism that requires its N-terminal pleckstrin homology domain. Partial co-localization with expressed GLUT4 fused to green fluorescent protein in both 3T3-L1 adipocytes and Chinese hamster ovary cells was evident. Furthermore, microinjection of purified PLD into cultured adipocytes markedly potentiated the effect of a submaximal concentration of insulin to stimulate GLUT4 translocation to cell surface membranes. Insulin stimulated PLD activity in cells expressing high levels of insulin receptors but no such insulin effect was detected in 3T3-L1 adipocytes. Taken together, these results are consistent with the hypothesis that PLD1 associated with GLUT4-containing membranes acts in a constitutive manner to promote the mechanism of GLUT4 translocation by insulin.

Signaling molecules involved in coupling growth hormone receptor to mitogen-activated protein kinase activation.

We have shown previously that GH stimulates the mitogen-activated protein (MAP) kinases designated ERKs (extracellular signal-regulated kinases) 1 and 2. To examine pathways coupling GH receptor (GHR) to MAP kinase activation, we have determined the effects of GH on SHC-growth factor receptor bound 2-son of Sevenless (SHC-Grb2-SOS) association and activation of Ras, Raf, and MAP-ERK kinase (MEK). GH promoted the rapid, transient association of SHC with the Grb2-SOS complex, which correlated with the time course of Ras, Raf, and MEK activation. Despite the continuous presence of GH, these activation events were transient with Ras, Raf, and MEK returning to near basal activity by 15 or 30 min. The inactivation of Ras, Raf, and MEK directly correlated with the serine/threonine phosphorylation of SOS and dissociation of SOS from Grb2 but not Grb2 from tyrosine-phosphorylated SHC. Phosphorylation was blocked by the MEK inhibitor, PD98059. Based upon the established functions of the MAP kinase pathway, these data indicate that GH stimulation results in the assembly of a SHC-Grb2-SOS complex that serves to activate Ras and thereby engage the Raf-MEK-ERK pathway. Activation of this pathway generates a feedback kinase cascade that phosphorylates SOS resulting in the dissociation of SHC-Grb2 complexes from SOS, thereby causing a more rapid termination of the signaling pathway than would result from SHC dephosphorylation.

The amino terminus of insulin-responsive aminopeptidase causes Glut4 translocation in 3T3-L1 adipocytes.

The insulin-responsive aminopeptidase (IRAP) is a constituent of the vesicles that contain the insulin-regulated glucose transporter (Glut4). Like Glut4, IRAP translocates to the cell surface in response to insulin. Microinjection into 3T3-L1 adipocytes of a glutathione S-transferase (GST) fusion protein containing the cytosolic portion of IRAP (GST-IRAP-(1-109)), resulted in translocation of Glut4 to the cell surface. Immunostaining of 3T3-L1 adipocytes for Glut4 showed that the percentage of cells with substantial cell surface Glut4 was 10% in unstimulated cells, 8% following injection of GST, and 27% following injection of GST-IRAP-(1-109). Increased cell surface Glut4 occurred within 5-10 min following injection and was maintained for at least 4 h. A fusion protein containing only 28 amino acids from IRAP (GST-IRAP-(55-82)) was as effective in increasing cell surface Glut4 as stimulation with 100 nM insulin (44% versus 43%, respectively). In contrast to insulin-stimulated Glut4 translocation, the redistribution of Glut4 following injection of GST-IRAP-(55-82) was not blocked by wortmannin or co-injection with a SH2 domain from the regulatory subunit of phosphatidylinositol 3-kinase. These data suggest that the amino terminus of IRAP interacts with a retention/sorting protein that also regulates the distribution of Glut4 in insulin-responsive cells.

Insulin stimulates the phosphorylation of the 66- and 52-kilodalton Shc isoforms by distinct pathways.

In contrast to the 52-kDa Shc isoform, insulin stimulation caused a quantitative, time-dependent decrease in the SDS-PAGE mobility of 66-kDa Shc in both Chinese hamster ovary/IR cells and 3T3L1 adipocytes. Alkaline phosphatase treatment and direct phosphoamino acid analysis demonstrated that insulin stimulated an increase in serine phosphorylation of the 66-kDa isoform but not 52-kDa Shc, although the latter displayed a marked increase in tyrosine phosphorylation. To identify the responsible kinase pathway, we compared the effects on 66-kDa Shc serine phosphorylation by insulin, anisomycin, and osmotic shock, agents that specifically activate the ERK, JNK, or both pathways, respectively. Insulin and osmotic shock both stimulated a decrease in 66-kDa Shc mobility, whereas anisomycin had no effect. Furthermore, expression of a dominant-interfering Ras mutant (N17Ras) prevented the insulin-stimulated, but not the osmotic shock-induced serine phosphorylation of 66-kDa Shc. Consistent with a MEK-dependent pathway mediating 66-kDa Shc serine phosphorylation, the specific MEK inhibitor (PD98059) and expression of a dominant-interfering MEK mutant partially inhibited both the insulin and osmotic shock-induced reduction in 66-kDa Shc mobility. In contrast, expression of the MAP kinase phosphatase (MKP-1) completely prevented ERK activation but did not inhibit the serine phosphorylation of 66-kDa Shc. These data demonstrate that insulin stimulates the serine phosphorylation of the 66-kDa Shc isoform through a MEK-dependent mechanism.

CD40 ligation results in protein kinase C-independent activation of ERK and JNK in resting murine splenic B cells.

CD40 is a 45- to 50-kDa transmembrane glycoprotein that plays an important role in B cell proliferation, survival, memory, and Ig isotype switching. How CD40 engagement couples to these distal events in B cell activation remains poorly understood. In this study, we have examined signal transduction events mediated by CD40 cross-linking in resting murine splenic B cells. In comparison to signaling via the B cell Ag receptor (BCR), CD40 cross-linking was less effective at activating protein tyrosine kinases. Interestingly, however, CD40 engagement resulted in the phosphorylation of both extracellular signal-regulated protein kinase (ERK) and the Ras guanine nucleotide exchange factor, Son of sevenless. In addition, both ERK and c-Jun NH2-terminal kinase activities were increased after both CD40 and BCR ligation. Overnight treatment of cells with phorbol ester as well as pharmacologic inhibitors of protein kinase C abrogated these signaling events after BCR treatment; however, no effect was seen on CD40-mediated activation of ERK or c-Jun NH2-terminal kinase, suggesting that the BCR and CD40 differentially utilize protein kinase C to couple with these signaling pathways.

Insulin and epidermal growth factor receptors regulate distinct pools of Grb2-SOS in the control of Ras activation.

Insulin and epidermal growth factor (EGF) stimulate a rapid but transient increase in the amount of GTP bound to Ras that returns to the basal GDP-bound state within 10-30 min. Although insulin stimulation resulted in a dissociation of the Grb2.SOS complex, EGF did not affect the Grb2.SOS complex but instead induced dissociation of Grb2-SOS from tyrosine-phosphorylated Shc. The dissociation of Grb2-SOS from Shc was not due to dephosphorylation as Shc remained persistently tyrosine-phosphorylated during this time. Furthermore, there was no decrease in the extent of insulin receptor substrate 1, insulin receptor, or EGF receptor tyrosine phosphorylation. Surprisingly, however, despite the EGF-induced decrease in the amount of Grb2-SOS bound to Shc, the extent of Grb2 associated with Shc remained constant, and there was a concomitant increase in the amount of SOS associated with Grb2. In addition, after the insulin-stimulated dissociation of Grb2 from SOS, EGF treatment induced the reassociation of the Grb2.SOS complex. Quantitative immunoprecipitation demonstrated that only a small fraction of the total cellular pool of Grb2 was associated with SOS. Similarly, only a small fraction of SOS and Grb2 were co-immunoprecipitated with Shc. Together, these data suggest the presence of distinct Grb2-SOS pools that are independently utilized by insulin and EGF in their recruitment to tyrosine-phosphorylated Shc.

Epidermal growth factor receptor targeting prevents uncoupling of the Grb2-SOS complex.

Insulin stimulates the Ras/Raf/MEK/ERK pathway leading to feedback phosphorylation of the Ras guanylnucleotide exchange protein SOS and dissociation of Grb2 from SOS. Even though epidermal growth factor (EGF) also stimulates ERK activity and phosphorylation of SOS similar to insulin, EGF induces a dissociation of the Grb2-SOS complex from Shc. To determine the molecular basis for this difference, we examined the signaling properties of a mutant EGF receptor lacking the five major autophosphorylation sites. Although EGF stimulation of the mutant EGF receptor activates ERK and phosphorylation of both Shc and SOS, it fails to directly associate with either Shc or Grb2. However, under these conditions EGF induces a dissociation of the Grb2-SOS complex suggesting a role for receptor and/or plasma membrane targeting in the stabilization of Grb2-SOS interaction. Consistent with this hypothesis, expression of an SH2 domain Grb2 mutant which is unable to mediate plasma membrane targeting of the Grb2-SOS complex results in both insulin- and EGF-stimulated uncoupling of Grb2 from SOS. Furthermore, a plasma membrane-bound Grb2 fusion protein remains constitutively associated with SOS. Together, these data demonstrate that EGF stimulation prevents the feedback uncoupling of Grb2 from SOS by inducing a persistent plasma membrane receptor targeting of the Grb2-SOS complex.

SOS phosphorylation and disassociation of the Grb2-SOS complex by the ERK and JNK signaling pathways.

Insulin activation of Ras is mediated by the plasma membrane targeting of the guanylnucleotide exchange factor SOS associated with the small adapter protein Grb2. SOS also lies in an insulin-stimulated feedback pathway in which the serine/threonine phosphorylation of SOS results in disassociation of the Grb2-SOS complex thereby limiting the extent of Ras activation. To examine the relative role of the mitogen-activated protein kinases in the feedback phosphorylation of SOS we determined the signaling specificity of insulin, osmotic shock, and anisomycin to activate the ERK (extracellular-signal regulated kinase) and JNK (c-Jun kinase) pathways. In Chinese hamster ovary cells expressing the human insulin receptor and murine 3T3L1 adipocytes, insulin specifically activated ERK with no significant effect on JNK, whereas anisomycin specifically activated JNK but was unable to activate ERK. In contrast, osmotic shock was equally effective in the activation of both kinase pathways. Insulin and osmotic shock, but not anisomycin, resulted in SOS phosphorylation and disassociation of the Grb2-SOS complex, demonstrating that the JNK pathway was not involved in the insulin-stimulated feedback uncoupling of the Grb2- SOS complex. Both the insulin and osmotic shock-induced activation of ERK was prevented by treatment of cells with the specific MEK inhibitor (PD98059). However, expression of dominant-interfering Ras (N17Ras) inhibited the insulin- but not osmotic shock-stimulated phosphorylation of ERK and SOS. These data demonstrate that activation of the ERK pathway, but not JNK, is responsible for the feedback phosphorylation and disassociation of the Grb2-SOS complex.

Insulin receptor substrate 1 and 2 (IRS1 and IRS2): what a tangled web we weave.

The insulin receptor is a transmembrane tyrosine kinase that is essential for mediating multiple intracellular signalling cascades that lead ultimately to the biological actions of insulin Tyrosine phosphorylation o f the cytosolic proteins insulin receptor substrate 1 and 2 (IRS1 and IRS2) produces protein 'scaffolding' for the assembly of effector proteins containing Src homology 2 (SH2) domains, thereby generating multisubunit signalling complexes. Although IRS1 was originally isolated as a specific insulin receptor substrate, both IRS1 and IRS2 appear to play a broader role, functioning also as proximal substrates in growth hormone and cytokine receptor signalling. Current data establish IRS1 and IRS2 as critical effectors integrating various cell-type-specific signals into distinct, but overlapping, biological responses.

Aqueous soluble tetrazolium/formazan MTS as an indicator of NADH- and NADPH-dependent dehydrogenase activity.

Recently a new tetrazolium was described for the use of monitoring cell viability in culture. This tetrazolium, commonly referred to as MTS [3-(4,5-dimethylthiazol-2-yl)- 5-(3-carboxymethonyphenol)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt], has the unusual property that it can be reduced to a water-soluble formazan. beta-Nicotinamide adenine dinucleotide/reduced (NADH) and beta-nicotinamide adenine dinucleotide phosphate/reduced (NADPH) are examples of physiologically important reducing agents. In cell-free studies, MTS was reduce to the soluble formazan in the presence of NADH and NADPH, and reaction were compared to those with dithiothreitol (DTT) or 2-mercaptoethanol (2-ME). The efficiency of these reactions was enhanced 1000-fold by the presence of phenazine methosulfate. Selectivity in the electron transfer from NADPH was slightly greater than NADH, and NADPH or NADH was much greater than the thiols DTT or 2-ME. Generation of either NADH or NADPH in solution by malate dehydrogenase or isocitrate dehydrogenase, respectively, was monitored by the MTS reduction reaction. The rate of formazan formation was comparable to the formation of NADH or NADPH. This system represents a useful tool for evaluating reaction kinetics in solutions of NAD- or NADP-dependent dehydrogenase enzymes, and these reactions can be performed in typical biological buffers containing reducing agents without significant interference to the MTS/formazan system.

Desensitization of Ras activation by a feedback disassociation of the SOS-Grb2 complex.

Activation of Ras by the exchange of bound GDP for GTP is predominantly catalyzed by the guanylnucleotide exchange factor SOS. Receptor tyrosine kinases increase Ras-GTP loading by targeting SOS to the plasma membrane location of Ras through the small adaptor protein Grb2. However, despite the continuous stimulation of receptor tyrosine kinase activity, Ras activation is transient and, in the case of insulin, begins returning to the GDP-bound state within 5 min. We report here that the cascade of serine kinases activated directly by Ras results in a mitogen-activated protein kinase kinase (MEK)-dependent phosphorylation of SOS and subsequent disassociation of the Grb2-SOS complex, thereby interrupting the ability of SOS to catalyze nucleotide exchange on Ras. These data demonstrate a molecular feedback mechanism accounting for the desensitization of Ras-GTP loading following insulin stimulation.

Mitogen-activated protein kinase kinase inhibition does not block the stimulation of glucose utilization by insulin.

Insulin stimulates the activity of mitogen-activated protein kinase (MAPK) via its upstream activator, MAPK kinase (MEK), a dual specificity kinase that phosphorylates MAPK on threonine and tyrosine. The potential role of MAPK activation in insulin action was investigated with the specific MEK inhibitor PD98059. Insulin stimulation of MAPK activity in 3T3-L1 adipocytes (2.7-fold) and L6 myotubes (1.4-fold) was completely abolished by pretreatment of cells with the MEK inhibitor, as was the phosphorylation of MAPK and pp90Rsk, and the transcriptional activation of c-fos. Insulin receptor autophosphorylation on tyrosine residues and activation of phosphatidylinositol 3'-kinase were unaffected. Pretreatment of cells with PD98059 had no effect on basal and insulin-stimulated glucose uptake, lipogenesis, and glycogen synthesis. Glycogen synthase activity in extracts from 3T3-L1 adipocytes and L6 myotubes was increased 3-fold and 1.7-fold, respectively, by insulin. Pretreatment with 10 microM PD98059 was without effect. Similarly, the 2-fold activation of protein phosphatase 1 by insulin was insensitive to PD98059. These results indicate that stimulation of the MAPK pathway by insulin is not required for many of the metabolic activities of the hormone in cultured fat and muscle cells.

Insulin-stimulated disassociation of the SOS-Grb2 complex.

Insulin stimulation of differentiated 3T3-L1 adipocytes or Chinese hamster ovary cells expressing high levels of the insulin receptor resulted in a time-dependent decrease in the electrophoretic mobility of SOS on sodium dodecyl sulfate-polyacrylamide gels. The reduction in SOS mobility was completely reversed by alkaline phosphatase treatment, and the in vitro phosphorylation of SOS by mitogen-activated protein kinase resulted in a decrease of electrophoretic mobility identical to that following in vivo insulin stimulation. Immunoprecipitation of Grb2 followed by SOS immunoblotting demonstrated a disassociation of the SOS-Grb2 complex that paralleled the decrease in SOS electrophoretic mobility. Similarly, SOS immunoprecipitation followed by Grb2 immunoblotting also indicated an uncoupling of the SOS-Grb2 complex. Further, incubation of whole-cell extracts with glutathione-S-transferase-Grb2 fusion proteins demonstrated that insulin stimulation resulted in a decreased affinity of SOS for Grb2. In contrast, the dissociation of SOS from Grb2 did not affect the interactions between Grb2 and tyrosine-phosphorylated Shc. In addition to insulin, several other agents which activate the mitogen-activated protein kinase pathway (platelet-derived growth factor, serum, and phorbol ester) also resulted in the uncoupling of the SOS-Grb2 complex. Consistent with these results, expression of v-ras and v-raf resulted in a constitutive decrease in the association between SOS and Grb2. Together, these data suggest a molecular mechanism accounting for the transient activation of ras due to the uncoupling of the SOS-Grb2 complex following SOS phosphorylation.

Functional expression of insulin receptor substrate-1 is required for insulin-stimulated mitogenic signaling.

To examine the role of the insulin receptor substrate-1 (IRS-1) in mediating insulin biological responsiveness, we generated Chinese hamster ovary cell lines expressing antisense IRS-1 RNA. These cells displayed morphological alterations as well as markedly reduced growth rates compared to the parental cells. Furthermore, the antisense IRS-1 cell lines had decreased insulin-stimulated IRS-1 tyrosine phosphorylation, reduced phosphatidylinositol 3-kinase activation, and decreased thymidine incorporation relative to the parental cell line. Insulin-dependent transcriptional regulation of a serum response element/luciferase reporter construct (SRE-Luc) was also reduced in the antisense IRS-1-expressing cell lines. However, co-transfection with a plasmid directing the expression of rat IRS-1 fully restored insulin-stimulated SRE-Luc activity in the IRS-1 antisense cell lines. Thus, the inhibition in insulin signaling was a specific effect of decreased IRS-1 tyrosine phosphorylation. Taken together, these data demonstrate that insulin regulation of mitogenic signaling requires the functional expression of IRS-1 and documents its central importance in the insulin intracellular signaling pathway.

Gonadotropin-releasing hormone-stimulated intracellular Ca2+ fluctuations and luteinizing hormone release can be uncoupled from inositol phosphate production.

In order to study the dependence of GnRH-stimulated LH release on inositol phosphate (IP) turnover, this study used an inhibitor of phospholipase C activity, 1-[6-[[17 beta-3- methoxyestra-1,3,5(10)-triene-17-yl]amino]hexyl]-1H-pyrrole-dione (U-73122) and an inactive analog 1-[6[[17 beta-3-methoxyestra-1,3,5(10)- triene-17-yl]amino]hexyl]2,5-pyrrolidine-dione (U-73343). U-73122 (10 microM) decreased GnRH-provoked (1 microM, 45 min) IP accumulation from 873 +/- 61 dpm to 365 +/- 50 dpm (basal accumulation also was decreased from 420 +/- 18 dpm to 207 +/- 16 dpm) while LH release was not inhibited (30.2 +/- 1.4% of cellular LH in control compared to 30.3 +/- 1.1% in U-73122 pretreated cells). GnRH provoked increased IP3 accumulation (123% of basal) after 15 sec of stimulation, IP2 accumulation (131% of basal) after 30 sec, and IP1 (121% of basal) after 1 min. Pretreatment with U-73122 blocked accumulation of IPs at these early timepoints. Sodium fluoride (NaF)-stimulated IP accumulation was also inhibited by U-73122 (from 1539 +/- 132 dpm to 414 +/- 21 dpm) while LH release increased from 22.9 +/- 1.4% total cellular LH to 28.0 +/- 2.2%. In contrast, GnRH- and NaF-stimulated IP accumulation were not significantly decreased in U-73343 pretreated cells (GnRH: 817 +/- 43 dpm compared to 873 +/- 61 dpm in control; NaF: 1133 +/- 74 dpm compared to 1539 +/- 132 dpm in control cells). Results of a perifusion study showed that U-73122 did not block the initial phase of GnRH-stimulated LH release or interfere with the development of desensitization to the releasing hormone. In addition, GnRH-stimulated intracellular Ca2+ fluctuations were similar in magnitude and duration in U-73122-pretreated compared to U-73343-pretreated cells. These results demonstrate that GnRH- as well as NaF-stimulated LH release can be uncoupled from IP production calling to question the role of IP3 as a second messenger for GnRH-stimulated LH release.

Sodium fluoride provokes gonadotrope desensitization to gonadotropin-releasing hormone (GnRH) and gonadotrope sensitization to A23187: evidence for multiple G proteins in GnRH action.

Pretreatment of pituitary cell cultures with GnRH causes altered gonadotrope responsiveness to LH secretagogues. The precise mechanism by which this occurs is not understood. Because a G protein appears to be activated after GnRH stimulation of the gonadotrope, a role for this moiety in GnRH-stimulated alterations in gonadotrope responsiveness was assessed. We show that 3 h pretreatment of pituitary cell cultures with 10 mM NaF (a G protein activator), resulted in decreased gonadotrope responsiveness to subsequent GnRH treatment (3 h, 100 nM; 34.4 +/- 1.6% vs. 23.4 +/- 1.5% of total cellular LH). NaF-provoked gonadotrope desensitization to GnRH also occurred in the presence of 3 mM EGTA and in cells which had been depleted of protein kinase C. Desensitization to GnRH did not occur in response to pretreatment with (Bu)2cAMP (8 h, 1 mM). In addition, neither GnRH nor NaF stimulated inositol phosphate production above basal levels after the NaF pretreatment. GnRH receptor binding also decreased by 30% with NaF pretreatment. In contrast, 3 h NaF (10 mM) pretreatment enhanced responsiveness of the gonadotrope to the Ca2+ ionophore A23187 in a protein kinase C- and cAMP-dependent manner. Responsiveness to the phorbol ester, phorbol 12-myristate 13-acetate, was also increased, whereas responsiveness to the Ca2+ channel activator maitotoxin was unchanged. These data suggest that G protein activation by NaF provokes gonadotrope desensitization to GnRH stimulation by both decreasing receptor numbers and by uncoupling of the receptors from inositol phosphate production. In addition, a distinct G protein action appears to be involved in sensitizing the gonadotrope to A23187 and phorbol 12-myristate 13-acetate.

Maintenance of gonadotropin-releasing hormone (GnRH)-stimulated luteinizing hormone release despite desensitization of GnRH-stimulated cytosolic calcium responses.

Involvement of ionized cytosolic calcium ([Ca2+]i) and protein kinase-C (PKC) in GnRH-stimulated LH release was assessed by correlating measurable changes in [Ca2+]i and LH release in PKC-depleted and nondepleted gonadotropes. Primary cultures of anterior pituitary cells were loaded with the calcium-sensitive fluorescent dye fura-2 and placed in a perifusion chamber. GnRH pulses were delivered to the cells, and changes in fura-2 fluorescence and LH release were determined. The level of [Ca2+]i (assessed by fura-2) increased rapidly to a maximum within 20-40 sec, followed by a slower decline over the next minute (spike phase) to a sustained intermediate value (plateau phase). GnRH-stimulated LH release was unaffected by loading cells with fura-2. Both LH release and changes in [Ca2+]i were directly dependent on GnRH concentration. Pretreatment with the GnRH antagonist Antide (50 nM; [NAcD2Nal1-DpClPhe2-D3Pal3-Ser4-NicLys5-++ +DNicLys6-Leu7-ILys8-Pro9-DAla10]NH2 ) had no effect on basal [Ca2+]i or basal LH release, but did block both GnRH-stimulated calcium mobilization and GnRH-stimulated LH release. GnRH pretreatment (3.5 nM; 10 min) blocked the calcium spike phase, but not the plateau phase occurring in response to a GnRH pulse (10 nM; 5 min) delivered immediately after pretreatment. Inhibition of the calcium spike phase was transient (recovery within 15 min) and was dependent on pretreatment concentrations of GnRH. Calcium spike phase inhibition by GnRH pretreatment prevented increased LH release from PKC-depleted cells in response to a subsequent pulse of GnRH, but not from gonadotropes with normal levels of PKC. This suggests that initial LH release is dependent on changes in [Ca2+]i, but enhancement of LH release after periods of elevated GnRH concentrations may be dependent on PKC.

Regulation of the pituitary gonadotrope by gonadotropin-releasing hormone: multiple intracellular effectors.

Gonadotropin-releasing hormone (GnRH) stimulates synthesis and release of the pituitary gonadotropins luteinizing hormone (LH) and follicle stimulating hormone (FSH). Other actions of GnRH include gonadotrope sensitization and desensitization as well as stimulation of GnRH receptor synthesis. Gonadotropin release initiated by increased intracellular calcium is a result of calcium mobilization from intracellular stores and influx of extracellular calcium through receptor-operated channels. Increases in intracellular calcium and the presence of both calmodulin and calmodulin binding-proteins in pituitary suggests that formation of Ca(2+)-calmodulin complexes and subsequent alteration of calmodulin-binding protein activity are likely important intermediate steps in the signaling pathway of LH release. Although activation of protein kinase C is not necessary for GnRH-stimulated LH release or gonadotrope desensitization, it appears to be essential for GnRH effects on LH beta gene expression. Therefore gonadotrope responses are apparently mediated by multiple intracellular signaling mechanisms.

Stimulation of luteinizing hormone release by sodium fluoride is independent of protein kinase-C activity and unaffected by desensitization to gonadotropin-releasing hormone.

GnRH stimulates secretion of pituitary LH by increasing intracellular calcium. Increased calcium may result from activation of phospholipase-C, since there is an increase in inositol phosphates and diacylglycerol, and a redistribution of protein kinase-C (PKC) from cytosolic to a particulate cell fraction in GnRH-stimulated pituitary cultures. A GTP-binding protein (G-protein) may mediate GnRH actions, since GTP stimulates LH release in permeabilized gonadotropes and decreases receptor affinity for a GnRH analog. In the present study we have used sodium fluoride, an exogenous activator of G-proteins, to investigate the possibility of a G-protein link between GnRH receptor activation, phospholipase-C activity, and LH release. Treatment of primary pituitary cell cultures from immature female rats with sodium fluoride stimulated the release of 20% total cellular LH and increased inositol phosphate accumulation. Sodium fluoride-stimulated LH release was insensitive to cholera toxin and pertussis toxin. Sodium fluoride-stimulated LH release was additive with a maximally effective concentration of phorbol 12-myristate 13-acetate and was not inhibited by depletion of cellular PKC, suggesting that PKC does not mediate sodium fluoride effects. Treatment of cultures with 3 mM EGTA and 10 nM GnRH for 5 and 16 h reduced pituitary responsiveness to subsequent treatment with GnRH, but had no effect on sodium fluoride-stimulated LH release. Although the precise mechanism of sodium fluoride-stimulated LH release remains to be described, our results support a role for a G-protein in regulation of LH release by the releasing hormone.