Inhibition of choriogonadotropin-activated steroidogenesis in cultured Leydig tumor cells by the Rp diastereoisomer of adenosine 3′,5′-cyclic phosphorothioate

Postendocytotic trafficking of the follicle-stimulating hormone (FSH)-FSH receptor complex

Although the fates of the internalized hormone-receptor complexes formed by the lutropin/choriogonadotropin and the TSH receptors have been examined in some detail, much less is known about the fate of the internalized FSH-FSH receptor (FSHR) complex. Using biochemical and imaging approaches we show here that the majority of the internalized FSH-FSHR complex accumulates in endosomes and subsequently recycles back to the cell surface where the bound, intact hormone dissociates back into the medium. Only small amounts of FSH and the FSHR are routed to a lysosomal degradation pathway, and the extent of FSH-induced down-regulation of the cell surface and total FSHR is minimal. This pathway was detected in heterologous (human kidney 293T) cells transfected with the rat (r) or human (h) FSHR as well as in a mouse Sertoli cell line (MSC-1) or a mouse granulosa cell line (KK-1) transfected with the rFSHR.Additional experiments using a series of C-terminal deletions of the rFSHR and the hFSHR showed that the recycling of the internalized FSH-FSHR complex and the extent of hFSH-induced down-regulation is dictated by a short stretch of amino acids present at the extreme C-terminal end of the receptor.We conclude that most of the internalized FSH-FSHR complex is recycled back to the cell surface, that this recycling pathway is highly dependent on amino acid residues present near the C terminus of the FSHR, and that it is an important determinant of the extent of down-regulation of the FSHR.

Identification of a short linear sequence present in the C-terminal tail of the rat follitropin receptor that modulates arrestin-3 binding in a phosphorylation-independent fashion

The rat follitropin receptor (rFSHR) is an unusual G protein-coupled receptor in that agonist-induced activation leads to the phosphorylation of the first and third intracellular loops instead of the C-terminal tail. To determine regions of G protein-coupled receptors that affect internalization independently of phosphorylation we examined the effects of truncations of the C-terminal tail of the rFSHR on agonist-induced internalization. Our studies show that progressive truncations of a region flanked by residues 642 and 651 enhance the internalization of human follicle-stimulating hormone (hFSH). Further characterization of a mutant truncated at residue 649 (designated rFSHR-t649) and another mutant in which the 642-651 region was deleted in the context of the full-length rFSHR, designated rFSHR(Delta642-651), showed that both of them internalized hFSH at rates that were 2-3 times faster than rFSHR-wild type (wt). Like rFSHR-wt, however, the internalization of hFSH mediated by rFSHR-t649 and rFSHR(Delta642-651) can be inhibited with dominant-negative mutants of the non-visual arrestins or dynamin. Alanine-scanning mutagenesis of the 642-651 region suggests that the effects on internalization are not mediated by a single residue, however. In an attempt to understand the molecular basis of the enhanced internalization of hFSH mediated by these mutants we used an assay that can be readily used to assess the association of the rFSHR with the arrestin-3 in co-transfected cells. Using this assay we were able to show that, when compared with rFSHR-wt, rFSHR(Delta642-651) displays an approximately 4-fold enhancement in binding affinity for arrestin-3 and an approximately 1.7-fold reduction in maximal arrestin-3 binding capacity. We conclude that a short linear sequence present in the C-terminal tail of the rFSHR (642SATHNFHARK651) that is not phosphorylated limits internalization by lowering the affinity of the rFSHR for the endogenous non-visual arrestins.

MA-10 cells transfected with the human lutropin/choriogonadotropin receptor (hLHR): a novel experimental paradigm to study the functional properties of the hLHR

MA-10 cells are a clonal strain of mouse Leydig tumor cells that retain many of the properties of Leydig cells including expression of the endogenous lutropin/choriogonadotropin receptor (LHR) and the ability to respond to LH/CG with increased steroidogenesis. Recently we noted a dramatic decrease in expression of the endogenous LHR. Although we do not have an explanation for this decline, we took advantage of it to devise a method that allows for the expression of the recombinant human LHR (hLHR) in a Leydig cell model that is now practically devoid of endogenous LHR. We show that the recombinant hLHR can be expressed at variable densities in MA-10 cells and that it can stimulate cAMP and steroid synthesis as well as activate the inositol phosphate and MAPK cascades. We also show that two naturally occurring mutants of the hLHR associated with Leydig cell hyperplasia and one mutant associated with Leydig cell adenomas are constitutively active when assayed for activation of cAMP, inositol phosphate, progesterone, and MAPK. Our ability to express the hLHR in MA-10 cells (now practically devoid of endogenous LHR) provides a novel paradigm to study the cellular and molecular basis of the functions of the LHR in Leydig cells.

Mechanisms of cytokine effects on G protein-coupled receptor-mediated signaling in airway smooth muscle

Numerous in vitro and in vivo studies have implicated the cytokines interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha) as mediators of airway inflammation and therefore potentially important substances in the pathogenesis of asthma. In this study, we examined the mechanisms by which IL-1 beta and TNF-alpha affect inhibition of cell growth, G protein-coupled receptor (GPCR) desensitization, and the recently reported adenylyl cyclase sensitization in human airway smooth muscle (HASM) cultures. Our findings demonstrate that adenylyl cyclase sensitization is independent of cytokine-mediated cyclooxygenase type 2 (COX-2) and prostaglandin E(2) (PGE(2)) induction, whereas COX-2 induction appears to be required for both growth inhibition and GPCR desensitization. However, GPCR desensitization was highly dependent on the presence of EGF during chronic treatment with cytokines, which could be explained by a synergistic effect of EGF on cytokine-mediated COX-2 and PGE(2) induction. Interestingly, various agents (including inhibitors of p42/p44 and p38 mitogen-activated protein kinase signaling) were significantly more effective in inhibiting cytokine-mediated PGE(2) induction, GPCR desensitization, and cell growth inhibition than in inhibiting COX-2 induction. These data demonstrate disparity in the requirement and sufficiency of COX-2 induction in promoting different functional effects of IL-1 beta and TNF-alpha in HASM.

Novel 3',5'-cyclic nucleotide analogue: adenosine 3',5'-cyclic boranomonophosphate

A general procedure for the first synthesis of a 3',5'-cyclic boranomonophosphate was established. Specifically, adenosine 3',5'-cyclic boranomonophosphosphate (cyclic AMPB, 4c), a P-borane (BH(3)) analogue of adenosine 3',5'-cyclic monophosphate (cAMP), was synthesized via a phosphite approach in good yield. The method is also applicable for syntheses of natural cAMP and its phosphorothioate analogue. The two diasteromers of cyclic AMPB 4c were separated, and their chemical structures were established via spectroscopic methods.

Transforming growth factor-beta1 induces endothelin-1 in a bovine pulmonary artery endothelial cell line and rat lungs via cAMP

We investigated the mechanism of Endothelin-1 regulation by transforming growth factor-beta1 (TGF-beta1) in bovine pulmonary artery endothelial cells (BPAECs) and in isolated perfused rat lungs. Our data show that TGF-beta1 induces ET-1 gene expression and ET-1 peptide synthesis in BPAECs. The induction of preproET-1 mRNA level was due to de novo transcription, as well as mRNA stabilization, and new protein synthesis was not required for this induction. To investigate the role of cAMP-protein kinase A pathway in TGF-beta1-stimulated-ET-1 induction, we exposed BPAECs to various compounds which modulate this pathway. Dibutyryl-cAMP led to an increase in preproET-1 mRNA and Rp-cAMP abolished the induction of preproET-1 mRNA and ET-1 peptide by TGF-beta1. TGF-beta1 increased cAMP in BPAECs. Dexamethasone up-regulated preproET-1 mRNA expression and ET-1 peptide synthesis under basal and TGF-beta1-stimulated conditions. In isolated perfused rat lungs, TGF-beta1 increased preproET-1 mRNA abundance whereas Rp-cAMP inhibited the TGF-beta1-induced ET-1 gene activation. Thus our data suggest that TGF-beta1 stimulates ET-1 gene expression in BPAECs and in rat lungs via a cAMP dependent mechanism.

Mutations of gonadotropins and gonadotropin receptors: elucidating the physiology and pathophysiology of pituitary-gonadal function

The recent unraveling of structures of genes for the gonadotropin subunits and gonadotropin receptors has provided reproductive endocrinologists with new tools to study normal and pathological functions of the hypothalamic-pituitary-gonadal axis. Rare inactivating mutations that produce distinctive phenotypes of isolated LH or FSH deficiency have been discovered in gonadotropin subunit genes. In addition, there is a common polymorphism in the LHbeta subunit gene with possible clinical significance as a contributing factor to pathologies of LH-dependent gonadal functions. Both activating and inactivating mutations have been detected in the gonadotropin receptor genes, a larger number in the LH receptor gene, but so far only a few in the gene for the FSH receptor. These mutations corroborate and extend our knowledge of clinical consequences of gonadotropin resistance and inappropriate gonadotropin action. The information obtained from human mutations has been complemented by animal models with disrupted or inappropriately activated gonadotropin ligand or receptor genes. These clinical and experimental genetic disease models form a powerful tool for exploring the physiology and pathophysiology of gonadotropin function and provide an excellent example of the power of molecular biological approaches in the study of pathogenesis of diseases.

Characterization and regulation of prolactin receptors in MA-10 Leydig cells

The aim of this study is to further characterize the prolactin receptors (PRL-R) previously reported in the murine Leydig tumor MA-10 cell line, as well as to study their homologous and heterologous regulation. Two forms of PRL-R, a high and a low molecular weight form, were revealed by studies of covalent crosslinking of 125I-human GH to cultured MA-10 cells or cell membranes and immunoprecipitation of the solubilized PRL-R complexes with polyclonal anti PRL-R antibody, followed by SDS-PAGE and autoradiography. The long form had a molecular weight of 101 kDa and was predominant when the study was performed in the presence of protease inhibitors. The short form, with a molecular weight of 39 kDa, appeared, at least in part, to be a proteolytic product of the longer form. The same size forms of PRL-R were detected by crosslinking studies in the parental C57BL/6 mouse testicular Leydig cells, indicating the physiological relevance of the MA-10 cell model to the study of Leydig cell PRL-R. Homologous down-regulation of PRL-R was demonstrated in cultured MA-10 cells exposed for 24 h to increasing concentrations of PRL. In contrast, heterologous, 3 5-fold up-regulation of PRL-R was induced by various cAMP-elevating agents, including 8-bromo-cAMP (10(-4) -10(-3) M), dibutyryl cAMP (3 x 10(-3) M) and cholera toxin (1-10 ng/ml), although not by hCG (up to 100 ng/ml). This up-regulatory effect was apparently the result of a change in affinity, since cholera toxin caused a 2.4-fold increase in PRL-R affinity, with no change in the number of binding sites. In summary, these studies provide further evidence that MA-10 Leydig cells can serve as a physiologically relevant model for the study of PRL and PRL-R interactions, both at the functional level, as shown in our previous study, and at the structural and regulatory levels as shown in the current study.

Mechanisms of acute desensitization of the beta2AR-adenylyl cyclase pathway in human airway smooth muscle

beta2-Adrenergic receptors (beta2ARs) are important regulators of airway smooth muscle tone, and beta-sympathomimetic drugs are the most widely used agents in asthma therapy and are universally recognized as the treatment of choice for acute asthma attacks. Despite the clinical importance of beta-agonists and a good understanding of their mechanism of action in airway smooth muscle relaxation, surprisingly little is known about the manner in which the beta2AR signaling pathway is regulated in human airway smooth muscle (HASM). In this communication, we characterize mechanisms underlying rapid desensitization of the HASM beta2AR-adenylyl cyclase (AC) pathway. Acute homologous desensitization of beta2AR-mediated cyclic adenosine monophosphate (cAMP) production was characterized by an approximately 60% loss of maximal responsiveness to isoproterenol (ISO) when cells were pretreated for 30 min with 1 microM ISO. Acute heterologous beta2AR desensitization was characterized by an approximately 20% and 30% loss of maximal responsiveness to ISO challenge when cells were pretreated with forskolin and prostaglandin E2 (PGE2), respectively. Each form of desensitization was also characterized by an increase in the EC50 for ISO. beta2AR sequestration was associated with but not required for homologous desensitization. However, sequestration was required for rapid resensitization. Minimal alterations in inherent AC activity were observed with both modes of desensitization, suggesting that the beta2AR is the principal locus of regulation. Protein kinase inhibition by staurosporine largely reversed heterologous beta2AR desensitization and had a small but significant effect on homologous desensitization. In contrast, bisindolylmaleimide IX, a specific PKC-inhibitor, had no effect on heterologous or homologous beta2AR desensitization, suggesting that staurosporine effects were mediated by PKA inhibition. Overexpression of the G protein-coupled receptor kinase GRK2 in HASM cultures enhanced homologous desensitization. These data suggest that HASM beta2ARs are highly susceptible to rapid desensitization by multiple agents, and identify both GRKs and PKA as important mediators of acute beta2AR desensitization.

Molecular cloning and functional expression of chicken luteinizing hormone receptor

A complementary DNA for chicken luteinizing hormone (LH) receptor containing the entire coding region was isolated from chicken F1 granulosa cell cDNA library. Nucleotide sequence analysis revealed that there are characteristic GC-rich regions around the N-terminal part. Chicken LH receptor consists of a 19-residue signal peptide, a 366-residue extracellular domain, a 267-residue region containing seven transmembrane segments, and a 76-residue cytoplasmic C-terminal tail. The deduced amino acid sequence of the chicken LH receptor shares 67%, 69%, and 69% identity with the human, rat and porcine LH receptor sequences, respectively, and 51% with chicken FSH receptor. However, an insertion of about 30 amino acid residues is found in chicken LH receptor in the extracellular domain about 44 amino acid residues upstream of the first transmembrane segment. In addition, alternative splicing seems likely to occur at the point where the insertion starts (nucleotide position 933), resulting in the truncated forms of chicken LH receptor with only the extracellular domain. Northern blot analysis revealed the presence of multiple transcripts of LH receptor, a major 3.0-kb and minor 7-kb and 1.5-kb bands, in chicken F1 to F3 granulosa cells. The full length chicken LH receptor cDNA was transiently expressed in COS-7 cells and the transfected cells displayed a concentration-dependent increase in cAMP production when exposed to varying concentrations of chicken LH. This clearly indicates that the cloned cDNA encodes a functional chicken LH receptor protein.

Prostaglandins induce vascular endothelial growth factor in a human monocytic cell line and rat lungs via cAMP

Prostaglandins have emerged as a therapeutic option for patients with peripheral vascular disease as well as pulmonary hypertension as a means to increase blood flow. We tested the hypothesis that prostaglandins regulate vascular endothelial growth factor (VEGF) expression in the human monocytic THP-1 cell line and in isolated perfused rat lungs. Our data show that the stable PGI2-analogue iloprost induces VEGF gene expression (predominantly VEGF121, but also VEGF165 isoforms) and VEGF protein synthesis in THP-1 cells. This effect is abolished by dexamethasone and by Rp-cAMP, a specific inhibitor of cAMP-dependent protein kinase (PKA) activation. The calcium channel blocker diltiazem has no effect on the iloprost-induced VEGF gene expression, and depletion of intracellular Ca2+ stores by long-term exposure (16 h) of THP-1 cells to thapsigargin does not inhibit iloprost-induced VEGF gene expression, suggesting that an increase in intracellular Ca2+ is not essential for VEGF gene induction by iloprost. However, an increase of intracellular Ca2+ by a short-term (2 h) exposure of THP-1 cells to thapsigargin or to the calcium-ionophore A23187 increases VEGF mRNA levels, indicating that a change in intracellular Ca2+ by itself can alter VEGF gene expression. The effects of thapsigargin or A23187 on VEGF gene expression are also mediated via cAMP-PKA since they are inhibited by Rp-cAMP. In isolated perfused rat lungs, PGI2 and PGE2 increases VEGF mRNA abundance whereas Rp-cAMP inhibits the prostaglandin-induced VEGF gene activation. Thus, our data suggest that prostaglandins stimulate VEGF gene expression in monocytic cells and in rat lungs via a cAMP-dependent mechanism.

The role of alpha-MSH, its agonists, and C-AMP in in vitro avian melanocytes

Little is known about the effect of alpha-MSH and other melanogenic stimulators on avian melanocytes. Tissue cultures of Barred Plymouth Rock regenerating feather melanocytes were established and the culture medium contained selected concentrations of alpha-MSH and other melanogenic stimulators in Ham's F-10 medium supplemented with antibiotics and 10% new born calf serum. Cultures were maintained at 37 degrees C in 95% air/5% CO2. No increase in melanogenesis over control levels due to the addition of 10(-5) M Forskolin, 10(-4) M IBMX, 10(-3) M c-GMP, and 10(-3) M db-c-AMP was observed in the cultures on days 5 and 7. However, 2.5 (optimum), 5, and 10 micrograms/ml alpha-MSH and 10(-3) M 8-bromo-c-AMP significantly increased melanogenesis over control levels on days 5 and 7. The stimulation of melanogenesis was detectable by a significantly increased number of melanocytes containing numerous stage IV melanosomes. No increase in melanocyte cell number was observed in any of the experimental cultures. The addition of 1, 2 (optimum), or 3 mM calcium did enhance the increased pigmentation effect of 2.5 micrograms/ml alpha-MSH. Two very convincing experiments showed that c-AMP was the second messenger for alpha-MSH in these birds. First, the c-AMP inhibitor, 10(-3) M Rp-c-AMPS, completely inhibited the stimulatory effect of alpha-MSH in these in vitro melanocytes. Second, direct measurements of c-AMP levels in feather tissue showed a significant increase in c-AMP levels 10.min after alpha-MSH treatment. Controls received no alpha-MSH. The results showed that these avian melanocytes have alpha-MSH receptors and were able to respond to the hormone. C-AMP was the second messenger in this system. Apparently db-c-AMP was not able to enter these mature, highly-differentiated cells and c-AMP agonists, Forskolin and IBMX, were also either unable to enter these older cells or, if they did enter the cells, were unable to stimulate c-AMP production. Evidently the more lipophilic 8-bromo-c-AMP was able to enter these cells and stimulate melanogenesis.

Parathyroid hormone-related protein detection and interaction with NO and cyclic AMP in the renovascular system

The presence of parathyroid hormone-related protein (PTHrP) in human kidney vasculature and the signal transduction pathways stimulated during PTHrP-induced vasodilation of the rabbit kidney were investigated. Immunostaining of human kidney revealed the abundant presence of PTHrP in media and intima of all microvessels as well as in macula densa. In isolated perfused rabbit kidney preconstricted with noradrenaline, 10(-5) M Rp-cAMPS, a direct inhibitor of protein kinase A, produced comparable inhibition of 2.5 x 10(-7) M forskolin- and 10(-7) M PTHrP-induced vasorelaxations. Renal vasorelaxation and renal microvessel adenylyl cyclase stimulation underwent comparable desensitization following exposure to PTHrP. Nitric oxide (NO)-synthase inhibition by L-NAME (10(-4) M), NO scavenging by an imidazolineoxyl N-oxide (10(-4) M) and guanylyl cyclase inhibition by methylene blue (10(-4) M) decreased PTHrP-induced vasorelaxation by 27 to 53%, abolished bradykinin-induced vasorelaxation and did not affect forskolin-induced vasorelaxation. The effects of Rp-cAMPS and L-NAME were not additive on PTHrP-induced vasorelaxation. Damaging endothelium by treating the kidney with either anti-factor VIII-related antibody and complement, gossypol or detergent, did not affect PTHrP- or forskolin-induced vasorelaxations but reduced bradykinin-induced vasorelaxation by 53 to 92%. Conversely, endothelial damage did not alter the inhibitory action of L-NAME on PTHrP-induced vasorelaxation. In conclusion, PTHrP is present throughout the human renovascular tree and juxtaglomerular apparatus. Activation of both adenylyl cyclase/protein kinase A and NO-synthase/guanylyl cyclase pathways are directly linked to the renodilatory action of PTHrP in a way that does not require an intact endothelium in the isolated rabbit kidney.

A new cyclic AMP-independent, Gs-mediated stimulatory mechanism via the adenosine A2a receptor in the intact cardiac cell

The objectives of this study were to investigate the mechanism underlying the adenosine A2a receptor (A2aR)-mediated positive inotropic response and to define its contractile function using chick embryo ventricular cells as a model. Activation of the A2aR caused a marked stimulation of calcium entry and cell contractility, which were blocked by verapamil or nifedipine. The effects elicited by maximal concentrations of the A2aR agonist 2-[4-(2-carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenos ine and the beta-adrenergic agonist isoproterenol were additive, indicating that the two receptors do not share a common stimulatory mechanism. The cAMP antagonist (Rp)-adenosine cyclic 3':5'-monophosphorothioate was ineffective in inhibiting the A2aR-mediated stimulation of contractility or the L-type calcium channel, while it completely abolished the isoproterenol effects. Activation of the A2aR had no effect on Na+/Ca2+ exchange or inositol 1,4,5-trisphosphate accumulation. Blocking of the A2aR resulted in unopposed A1 receptor-mediated inhibitory effects and led to an inhibition of basal contractility and an enhanced anti-adrenergic effect by A1 agonist. The adenosine A2a receptor mediates a new cyclic AMP-independent mechanism and a new contractile function in the cardiac cell.

Triggering of CD23b antigen by anti-CD23 monoclonal antibodies induces interleukin-10 production by human macrophages

The aim of this study was to evaluate the capacity of human macrophages to produce interleukin (IL)-10 upon stimulation of membrane CD23. An anti-CD23 monoclonal antibody (mAb) was found to elicit the expression of the specific mRNA for IL-10 in CD23-bearing macrophages, and to induce a time-dependent production of this cytokine with a maximal effect reached after 12 h. Inasmuch as we previously reported that CD23 ligation evoked the generation of nitric oxide and of cAMP, the effect of the Rp diastereoisomer of adenosine 3', 5'-cyclic phosphorothioate (Rp-cAMP, an inhibitor of the cAMP pathway) and of NG-monomethyl-L-arginine (L-NMMA, an inhibitor of the nitric oxide pathway) were evaluated on CD23-induced IL-10 production. In the presence of Rp-cAMP, the CD23-induced production of IL-10 and of the pro-inflammatory cytokine tumor necrosis factor-alpha (TNF-alpha) was totally abrogated, whereas, in the presence of L-NMMA, IL-10 production was enhanced and TNF-alpha production was suppressed. In addition, neutralization of IL-10 with an anti-IL-10 mAb increased both the magnitude and duration of CD23-driven TNF-alpha production. Such an inducing effect was observed with different anti-CD23 mAb (clone 135, MHM6 and 25), indicating that the triggering of the CD23 molecule at the surface of human macrophages induced the generation of IL-10 through a cAMP-dependent mechanism. Concomitantly this generation of IL-10 was down-regulated by nitric oxide, which was also produced after triggering of the CD23 antigen. Taken together these data indicated that human macrophages produced IL-10 after triggering of the CD23 molecule and that this production could regulate the inflammatory state of these cells.

Cyclic AMP is a requisite messenger in the action of big PTTH in the prothoracic glands of pupal Manduca sexta

Prothoracicotropic hormone (PTTH), a peptide produced by the insect brain, stimulates the prothoracic glands to secrete ecdysteroids. The big form of this peptide (25.5 kDa) has been postulated to act through cyclic AMP in larval Manduca sexta, but the role of the cyclic nucleotide in the action of PTTH in pupal glands has been less clear. Results of the present study indicate that PTTH-stimulated ecdysteroid secretion and protein phosphorylation by glands removed from pupal Manduca sexta are blocked by two inhibitors of cAMP-dependent protein kinase: Rp-cAMPS, an antagonist of cAMP binding to the regulatory subunit of the kinase, and H-89, an inhibitor of the catalytic subunit of the kinase. Further, PTTH stimulates significant accumulation of cAMP in pupal glands, although less than that previously seen in PTTH-stimulated larval glands. Cyclic AMP-dependent protein kinase is found in cytoplasmic and membrane-associated glandular subfractions, as measured by incorporation of [32P]8-N3cAMP into the regulatory subunit of the kinase. PTTH enhances cytoplasmic cAMP content and appears to increase the amount of cAMP bound to a cytoplasmic type II regulatory subunit of cAMP-dependent protein kinase. The results indicate that cAMP plays a requisite role in PTTH action in pupal glands, thus arguing in favor of a uniform mechanism of action for the peptide during Manduca development.

Is protein kinase C activity required for the N-methyl-D-aspartate-evoked rise in cytosolic Ca2+ in mouse striatal neurons?

The present study investigates the roles of protein kinase C (PKC) and A (PKA) activities in NMDA-mediated Ca2+ entry in primary cultures of mouse striatal neurons. Inhibitors of protein kinases, such as sphingosine, RO 31-8220 and staurosporine inhibited the NMDA- but also the KCl-induced rise in cytosolic Ca2+. However, the PKA antagonist Rp-adenosine-3',5'monophosphothioate (Rp-cAMPS) did not alter the NMDA+D-serine response, whereas it completely suppressed the KCl response. The NMDA+D-serine-evoked rise in cytosolic Ca2+, observed in the absence of external Mg2+, was potentiated by the PKC activator phorbol 12-myristate 13-acetate (PMA) only when submaximal effective concentrations of this agonist and co-agonist were used. In addition, the PKC activator did not alter the NMDA+D-serine-evoked response in the presence of varying concentrations of Mg2+. Confirming the dependence on PKC activity, desensitization of PKC resulting from long-term PMA treatment led to an impairment of the NMDA response, leaving the KCl-induced response intact. We therefore propose that PKC not only potentiates but is also required for the NMDA-evoked elevation in cytosolic Ca2+ in mouse striatal neurons.

Regulation of C6 glioma cell steroidogenesis by adenosine 3',5'-cyclic monophosphate

Incubation of C6 glioma cells in the presence of aminoglutethimide, an inhibitor of cholesterol metabolism, together with either adenosine 3',5'-cyclic monophosphate (cAMP) analogues or agents that increase cAMP synthesis, such as cholera toxin, forskolin, and isoproterenol, stimulated the rate of pregnenolone formation by their isolated mitochondria. This effect of cAMP was blocked by the antagonist (Rp)-cAMPS. The incorporation rate of mevalonolactone into pregnenolone was also increased by the stimulation of adenylyl cyclase activity in intact C6 cells. It is concluded that cAMP stimulates glial cell steroidogenesis by increasing the movement of the substrate, cholesterol, to the mitochondria, where it will be metabolized to pregnenolone by the side chain cleavage cytochrome P450 enzyme.

Effects of hydroxyurea and cyclic adenosine monophosphate/protein kinase A inhibitors on the expression of the human chorionic gonadotropin alpha subunit and c-myc genes in choriocarcinoma

We have previously shown that treatment of choriocarcinoma cells with methotrexate or hydroxyurea leads to both cessation of cell growth, accompanied by repression of c-myc oncogene expression, and induction of genes associated with the placental phenotype, including both subunits of human chorionic gonadotropin (hCG) and placental alkaline phosphatase. Since the genes induced by these antimetabolites are also cyclic AMP inducible, we hypothesized that these antimetabolites may cause activation of the cyclic AMP/protein kinase A pathway, suppressing genes associated with cellular proliferation and inducing placental gene expression. Three inhibitors of the cyclic AMP/protein kinase A pathway were assayed for their ability to inhibit the induction of the human chorionic gonadotropin alpha gene by hydroxyurea, and none of these inhibitors eliminated this induction. In addition, blockade of the cyclic AMP/protein kinase A pathway did not reverse the suppression of c-myc by hydroxyurea. The results of the inhibitor studies suggest that hydroxyurea acts independently of the protein kinase A pathway to stimulate gene expression, and that suppression of c-myc is insufficient to cause the induction of the human chorionic gonadotropin alpha gene by hydroxyurea.

Second messenger signaling of PTH- and PTHRP-stimulated osteoclast-like cell formation from hemopoietic blast cells

The second messenger signaling mechanisms of parathyroid hormone (PTH)- and PTH-related peptide (PTHRP)-stimulated osteoclast-like cell formation were investigated in mouse hemopoietic blast cells that possessed PTH binding sites. Human (h) PTH-(1-34) or hPTHRP-(1-34) resulted in a dose-dependent stimulation of tartrate-resistant acid phosphatase-positive multinucleated cells (MNC) formation. Pretreatment with [Nle8,18Tyr34]hPTH-(3-34) significantly blocked hPTH-(1-34)- and hPTHRP-(1-34)-stimulated MNC formation. Dibutyryladenosine 3',5'-cyclic monophosphate (10(-4) M) and forskolin (10(-5) M) as well as the stimulatory diastereoisomer of adenosine 3',5'-cyclic phosphorothioate (Sp-cAMPS), a direct activator of adenosine 3',5'-cyclic monophosphate (cAMP)-dependent protein kinase (PKA) (10(-4) M), stimulated MNC formation, and Rp-cAMPS, an inhibitor of PKA activation (10(-4) M), almost completely inhibited MNC formation stimulated by the aforementioned agents but not by 1,25-dihydroxyvitamin D3. Moreover, Rp-cAMPS significantly blocked PTH- and PTHRP-stimulated MNC formation. Treatment with calcium ionophores (10(-8) and 10(-7) M) and phorbol 12-myristate 13-acetate, a protein kinase C (PKC) activator (10(-8) to 10(-6) M), but not 4 alpha-phorbol 12,13-didecanoate, a phorbol incapable of activating PKC, stimulated MNC formation. Two PKC inhibitors [1-(5-isoquinolinylsulfonyl)-2-methylpiperazine dihydrochloride and staurosporine] equally blocked PTH- and PTHRP-stimulated MNC formation. The combined pretreatment with Rp-cAMPS and PKC inhibitors completely blocked PTH- and PTHRP-stimulated MNC formation. Present findings indicate that the activation of PKA and PKC is directly linked to PTH- and PTHRP-stimulated osteoclast-like cell formation from hemopoietic blast cells.

Stimulation of ecdysteroidogenesis by small prothoracicotropic hormone: role of cyclic AMP

Prothoracicotropic hormones (PTTHs) stimulate synthesis and secretion of ecdysteroids by insect prothoracic glands. In Manduca sexta, PTTH exists as two size variants, small and big PTTH. Experiments were performed to assess the possible role of cyclic AMP in small PTTH signal transduction. cAMP analogs, or agents that increase intracellular cAMP, stimulated ecdysteroidogenesis. Small PTTH enhanced glandular cAMP levels; the rise in cAMP preceded an increase in ecdysteroid secretion. Prothoracic glands accumulated less cAMP when treated with small PTTH than when treated with big PTTH. A phosphodiesterase inhibitor (1-methyl-3-isobutylxanthine) (MIX) increased the amount of cAMP in glands treated with small but not big PTTH, suggesting that glandular phosphodiesterase activity may be elevated in the presence of small PTTH. PTTH-stimulated ecdysteroid secretion was suppressed by a cAMP antagonist (Rp-cAMPS). The effects of small and big PTTH on ecdysteroidogenesis were non-additive. The combined results suggest that cAMP is employed as a second messenger by both prothoracicotropins, and that there may be subtle differences in their respective mechanisms of action.

Cross talk of dual-signal transduction systems in the regulation of DNA synthesis by parathyroid hormone in osteoblastic osteosarcoma cells

There has been recent evidence that calcium/protein kinase C (Ca/PKC) messenger system as well as adenylate cyclase are involved in the signal transduction stimulated by PTH. We therefore examined the role of these dual-signal transduction systems and the interaction of these systems in the regulation of DNA synthesis by PTH in the osteoblastic osteosarcoma cells, UMR-106. As recently reported, 10(-4) M Sp-cAMPS, a direct activator of cAMP-dependent protein kinase (PKA), and 10(-4) M dibutyryl-cAMP, as well as hPTH-(1-34), caused the significant inhibition of [3H]thymidine incorporation (TdR). Both A23187 and ionomycin (10(-8)-10(-6) M) inhibited TdR in a dose-dependent manner, with a minimal effective dose at 10(-7) M. Although 10(-6) M phorbol 12-myristate 13-acetate (PMA) caused slight but significant stimulation of TdR by itself, it augmented not only dibutyryl-cAMP- but also Sp-cAMPS-induced inhibition of TdR. On the other hand, 4 alpha-phorbol 12,13-didecanoate, incapable of activating PKC, failed to augment these cAMP analogs-induced effects. Pretreatment with 50 microM H-7, an inhibitor of PKC, not only abolished the PMA-induced augmentation of effect by cAMP analogs but also significantly blocked the PTH-induced inhibitory effect on TdR. Pretreatment with 10(-6) M PMA, which downregulates PKC, significantly inhibited the PTH-induced suppression of TdR. Combined treatment with cAMP analog (dibutyryl-cAMP or Sp-cAMPS) and calcium ionophore (A23187 or ionomycin) caused additive effects on TdR, and PMA used in combination with both cAMP analog and calcium ionophore induced the further inhibition of TdR.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of testosterone production by atrial natriuretic peptide in isolated mouse Leydig cells results from a promiscuous activation of cyclic AMP-dependent protein kinase by cyclic GMP

The aim of this study was to examine the possibility that atrial natriuretic peptide-stimulated testosterone production by mouse Leydig cells results from an activation of cAMP-dependent protein kinase (kinase A) by cGMP. In these cells, both 8Br-cGMP and 8Br-cAMP could stimulate testosterone production, though the latter was approximately 50-fold more potent. Following the stimulation of the cells with the atrial peptide, a dose-related decrease in the cellular protein-bound cAMP accompanied by a concomitant increase in the protein-bound cGMP was observed. The steroidogenesis stimulated by both human chorionic gonadotrophin (hCG) and atrial peptide was inhibited in a dose-dependent manner by a cAMP antagonist, adenosine 3',5'-cyclic monophosphothioate, Rp-isomer (RpcAMPS). In a cell-free [3H]cAMP binding assay, we have shown that unlabelled cGMP and RpcAMPS could competitively inhibit the [3H]cAMP binding, confirming that cAMP, RpcAMPS and cGMP could bind to the same binding protein. Finally, in a cell-free kinase A assay system, we have demonstrated that in lysates prepared from either atrial peptide or hCG-stimulated cells, the cellular kinase A was activated to an equal extent. We conclude from the data obtained that cGMP can bind to the cAMP-binding sites of kinase A and thereby brings about a promiscuous activation of this kinase. This appears to be an underlying mechanism by which atrial peptide hormone is able to stimulate the steroidogenesis in mouse Leydig cells.

Is forskolin stimulation of rat striatal tyrosine hydroxylase dependent on adenylate cyclase activation?

Because some responses to forskolin are adenosine 3',5'-cyclic phosphate (cyclic AMP) independent, we investigated the involvement of adenylate cyclase in the stimulatory effect of forskolin on synaptosomal tyrosine hydroxylase (TH) activity of rat striatum. The forskolin analogue, 1,9-dideoxyforskolin, which mimics the cyclic AMP-independent effects of forskolin, was a very weak activator of both striatal adenylate cyclase and tyrosine hydroxylase activities, whereas forskolin stimulated the two enzymes effectively and with similar potencies. Moreover, exposure of synaptosomes to the specific cyclic AMP antagonist Rp-adenosine 3',5'-cyclic phosphorothioate reduced basal TH activity and counteracted the stimulatory effect on the enzyme activity by submaximal concentrations of forskolin. These results provide circumstantial evidence that in striatal dopaminergic nerve terminals a presynaptic adenylate cyclase mediates the stimulation of TH activity by forskolin.

Roles of PKA and PKC in facilitation of evoked and spontaneous transmitter release at depressed and nondepressed synapses in aplysia sensory neurons

Two second messenger pathways, one that uses the cAMP-dependent protein kinase A (PKA), the other that uses protein kinase C (PKC), have been found to contribute to the short-term presynaptic facilitation of the connections between the sensory neurons in Aplysia and their target cells, the interneurons and motor neurons of the gill-withdrawal reflex. To study their relative contributions as a function of the previous history of the neuron's activity, we have examined the effects of inhibiting PKA (using Rp-cAMPS) and PKC (using H7) on the short-term facilitation of spontaneous release as well as of the evoked release induced by serotonin at nondepressed, partially depressed, and highly depressed synapses. Our results suggest that whereas activation of PKA is sufficient to trigger the facilitation of nondepressed synapses, activation of both PKA and PKC is required to facilitate depressed synapses, with the contribution of PKC becoming progressively more important as synaptic transmission becomes more depressed.

Second messenger signaling in the regulation of cytosolic pH and DNA synthesis by parathyroid hormone (PTH) and PTH-related peptide in osteoblastic osteosarcoma cells: role of Na+/H+ exchange

The present study was performed to investigate the regulation of cytosolic pH (pHi) and DNA synthesis by parathyroid hormone(PTH) and PTH-related peptide (PTHrP) in osteoblasts, using osteoblastic osteosarcoma cells, UMR-106 which possessed PTH-responsive dual signal transduction systems (cAMP-dependent protein kinase (PKA) and calcium/protein kinase C [Ca/PKC]) and amiloride-inhibitable Na+/H+ exchange system. Both human (h)PTH-(1-34) and hPTHrP-(1-34) caused a progressive decrease in pHi and the inhibition of [3H]thymidine incorporation (TdR) to the same degree in a dose-dependent manner with a minimal effective dose of 10(-10) M. Dibutyryl cAMP (10(-4) M and Sp-cAMPS (10(-4) M), a direct stimulator of PKA also caused a progressive decrease in pHi, and calcium ionophores (A23187 and ionomycin, 10(-6) M) caused a transient decrease in pHi. Pretreatment with amiloride (0.3 mM) mostly blocked dbcAMP- and Sp-cAMPS-induced decrease in pHi but did not affect calcium ionophore-induced decrease in pHi. In the presence of amiloride, PTH and PTHrP caused a transient decrease in pHi, which was similar to the pattern of calcium ionophore-induced change in pHi. Amiloride did not affect the inhibition of TdR by PTH or PTHrP as well as that by cAMP analogues or calcium ionophores. The present study indicated that PTH and PTHrP caused cytosolic acidification through PKA-inhibited Na+/H+ exchange and increased cytosolic calcium-induced pathway and that the regulation of DNA synthesis by PTH and PTHrP was not via Na+/H+ exchange system.

Unexpected growth-stimulatory effect of somatostatin analogue on cultured human pancreatic carcinoid cells

Therapeutic efficacy of a synthetic somatostatin analogue for the treatment of carcinoid tumors is still controversial. In vivo studies performed in our laboratory showed that a somatostatin analogue, SMS 201-995, significantly inhibited growth of human pancreatic carcinoid (BON) tumors xenotransplanted into athymic nude mice. In the present study, however, SMS 201-995 did not inhibit in vitro growth of BON cells, but rather SMS 201-995 stimulated growth in a dose-dependent fashion. The growth-stimulatory effect was likely mediated through the reduction of cyclic AMP production. Unsuccessful treatment of certain types of carcinoid tumor with SMS 201-995 may be partly due to the direct growth-stimulatory effect of SMS 201-995 on carcinoid cells.

Mediation of serotonin hyperalgesia by the cAMP second messenger system

In this study we have evaluated the second messenger system that might couple 5-HT1A receptor activation to produce peripheral hyperalgesia. The intradermal injection of the serotonin (5-hydroxytryptamine; 5-HT) receptor agonist for the 1A receptor subset (5-HT1A), (+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthaline hydrobromide (8-OH DPAT) produces a dose-dependent hyperalgesia which was attenuated by a cAMP kinase inhibitor (the R-isomer of cyclic adenosine-3'-5'-monophosphate), but prolonged by the inhibition of endogenous phosphodiesterase by rolipram, supporting a role for the cAMP second messenger system. The 5-HT1A receptor agonist, 8-OH-DPAT, and the adenyl cyclase activator, forskolin administered together, produced an additive hyperalgesia, suggesting that the 5-HT1A receptor in peripheral terminals of the primary afferent neurons is positively coupled to the cAMP second messenger system in producing hyperalgesia. The inability of pertussis toxin to inhibit 8-OH DPAT-induced hyperalgesia further supports this hypothesis. The coupling of the 5-HT1A receptor to the cAMP second messenger system appears to be through guanine regulatory proteins since guanosine 5'-O-(3-thiotriphosphate) and cholera toxin both markedly enhanced 8-OH DPAT hyperalgesia. In further support of the role of guanine nucleotide regulatory proteins, guanosine 5'-O-(2-thiodiphosphate), as well as activators of inhibitory guanine regulatory proteins (the mu-opioid agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin, and the adenosine A1 agonist, N6-cyclopentyladenosine, significantly attenuated 8-OH DPAT hyperalgesia.

The direct involvement of cAMP-dependent protein kinase in the regulation of collagen synthesis by parathyroid hormone (PTH) and PTH-related peptide in osteoblast-like osteosarcoma cells (UMR-106)

The present study was performed to characterize the direct involvement of cAMP-dependent protein kinase (PKA) in the regulation of collagen synthesis by parathyroid hormone (PTH) and PTH-related peptide (PTHrP) in osteoblastic osteosarcoma cells, UMR-106. Sp-cAMPS (10(-4)M), a direct activator of PKA, as well as dibutyryl cAMP (dbcAMP, 10(-4)M) significantly inhibited collagen synthesis. Human (h) PTH-(1-34) (10(-7)M) and hPTHrP (10(-7) M) inhibited collagen synthesis to the same degree. Although Rp-cAMPS, which acted directly as an antagonist in the activation of PKA, did not affect collagen synthesis by itself, it significantly antagonized dbcAMP- and Sp-cAMPS-induced inhibition of collagen synthesis. Moreover, Rp-cAMPS antagonized PTH- and PTHrP-induced inhibition of collagen synthesis to the same degree. The present study first indicated that the activation of PKA was directly linked to the regulation of collagen synthesis by PTH in osteoblast and that PTHrP had the same effect on collagen synthesis presumably through the same mechanism as PTH.

The activation of cAMP-dependent protein kinase is directly linked to the stimulation of bone resorption by parathyroid hormone

The present study was performed to characterize the direct involvement of cAMP in the stimulation of bone resorption by parathyroid hormone (PTH), using Sp-cAMPS and Rp-cAMPS, which were the direct agonist and antagonist in the activation of cAMP-dependent protein kinase (PKA), respectively. Bone resorbing activity was estimated as the number of pits formed on the dentine slice and total area of pits per slice in bone marrow cells derived from 2 week-old mice. Dibutyryl cAMP (dbcAMP)(10(-4)M) and Sp-cAMPS (10(-4)M) caused the remarkable stimulation of bone resorption. Although Rp-cAMPS (10(-4)M) did not affect bone resorption by itself, it significantly inhibited dbcAMP- and Sp-cAMPS-induced stimulation of bone resorption. Moreover, Rp-cAMPS (10(-4)M) antagonized 10(-7)M human PTH-(1-34)-induced stimulation of bone resorption, although it did not affect 10(-8)M 1,25(OH)2D3-induced stimulation of bone resorption. Present study indicates the direct involvement of PKA in the stimulation of bone resorption by PTH.

Receptor-mediated autocrine growth-stimulatory effect of 5-hydroxytryptamine on cultured human pancreatic carcinoid cells

5-hydroxytryptamine (5-HT) is a mitogen for fibroblasts, vascular smooth muscle cells, renal mesangial cells, and jejunal crypt cells. The human carcinoid cell line (termed BON) that we established in our laboratory from a pancreatic carcinoid tumor produces and secretes 5-HT. In this study, therefore, we examined the effect of 5-HT on growth of BON cells. Furthermore, by use of selective 5-HT receptor antagonists, we examined receptor and post-receptor mechanisms by which 5-HT-induced responses were produced. 5-HT stimulated growth of BON cells. 5-HT stimulated phosphatidylinositol (PI) hydrolysis in a dose-dependent fashion and inhibited cyclic AMP production in a dose-dependent fashion. The 5-HT1A/1B receptor antagonist, SDZ 21-009, prevented the reduction of cyclic AMP production evoked by 5-HT and inhibited the mitogenic action of 5-HT. The 5-HT1C/2 receptor antagonist, mesulergine, competitively inhibited PI hydrolysis, but did not affect the mitogenic action of 5-HT. The mitogenic action of 5-HT and the reduction of cyclic AMP production evoked by 5-HT were also inhibited by pertussis toxin. These results suggest that 5-HT is an autocrine growth factor for BON cells and that mitogenic mechanism of 5-HT involves receptor-mediated inhibition of the production of cyclic AMP which may be linked to pertussis toxin-sensitive GTP binding protein. 8-bromo-cyclic AMP inhibited growth of BON cells whereas 8-bromo-cyclic GMP had no effect on cell growth. Involvement of protein kinase A in BON cell growth regulation was confirmed by the observation that a cAMP-dependent protein kinase antagonist (Rp-cAMPS) could stimulate BON cell growth.

Interaction between cyclic nucleotide second messenger systems in murine Leydig cells

Mouse Leydig cell androgen production can be acutely stimulated by atrial natriuretic factor (ANF) via cyclic guanosine 3',5'-monophosphate (cGMP). This stimulation can approach that seen with high concentrations of luteinizing hormone (LH) acting via cyclic adenosine 3',5'-monophosphate (cAMP). To assess the potential for synergistic interaction between LH/cAMP and ANF/cGMP Leydig cells were co-exposed to ANF and LH or ANF/cGMP and site/type-selective cAMP analogues. Co-exposure to 1 nM ANF and 1 ng/ml LH elicited a synergistic increase in androgen production. Both 500 microM 8-bromo-cGMP and ANF (1.0-2.5 nM) synergized with cAMP analogues selective for either of the two major isoenzymes of protein kinase A. Phosphodiesterase (PDE) inhibition was not involved as inclusion of a PDE inhibitor only augmented the response. It appears that ANF/cGMP may interact cooperatively with LH/cAMP in the stimulatory control of androgen production in the mouse Leydig cell and that the site of synergistic interaction may be the activation of the cAMP-dependent protein kinase.

Acute action of luteinizing hormone on mouse Leydig cells: accumulation of mitochondrial phosphoproteins and stimulation of testosterone synthesis

Upon stimulation of Leydig cells with luteinizing hormone (LH) or dibutyryl-3',5'-cyclic AMP (Bt2cAMP) at 37 degrees C, two mitochondrial phosphoproteins accumulate with the same stimulant dose response as the increased rate of testosterone synthesis. The proteins pp32 and pp30 have apparent isoelectric points of 6.6 and 6.5 and molecular weights of approximately 32 30 kDa respectively, as determined by two-dimensional polyacrylamide gel electrophoresis. These two phosphoproteins are not detected in mouse adipose or liver cells nor in the total testicular cell population, of which Leydig cells constitute a small percentage. However, both proteins are also observed in mouse adrenal cells stimulated by ACTH or Bt2cAMP. The appearance of pp32 and pp30 is prevented by inhibitors of cytosolic protein translation, indicating that only newly synthesized protein is available as a substrate for phosphorylation. Proteolytic peptide mapping indicates that both of these mouse Leydig and adrenal proteins have structural similarity to pp30 (formerly denoted as ib), the 30 kDa mitochondrial phosphoprotein that we have observed previously in peptide hormone or Bt2cAMP-stimulated rat adrenal cortex (Pon, L.A., Hartigan, J.A. and Orme-Johnson, N.R. (1986) J. Biol. Chem. 261, 13309-13316; Alberta, J.A., Epstein, L.F., Pon, L.A. and Orme-Johnson, N.R. (1989) J. Biol. Chem. 264, 2368-2372) and rat corpus luteum cells (Pon, L.A. and Orme-Johnson, N.R. (1986) J. Biol. Chem. 261, 6694-6599). Since pp32 is a larger mitochondrial protein of similar primary structure to pp30, it is a potential precursor of this protein. Finally, the detection of the mitochondrial phosphoprotein pp30 in a third steroidogenic tissue type and a third species provides further correlative evidence that the production of pp30 may be an integral part of the subcellular mechanism by which peptide hormones stimulate steroid hormone biosynthesis.

The activation of cAMP-dependent protein kinase is directly linked to the inhibition of osteoblast proliferation (UMR-106) by parathyroid hormone-related protein

The present study was performed to compare the effect of parathyroid hormone-related protein (PTHrP) on the proliferation of osteoblastic osteosarcoma cells (UMR-106) with that of PTH and characterize the direct involvement of cAMP in the change of osteoblast proliferation by PTHrP. Human(h)PTHrP-(1-34) (10(-11)-10(-7)M) dose-dependently inhibited [3H]thymidine incorporation (TdR) in the same manner as hPTH-(1-34). The simultaneous addition of PTHrP and PTH at a maximal effective dose of 10(-7) M did not cause additive suppressive effect on cell proliferation. Rp-cAMPs, which has been recently shown to act directly as antagonist in the activation of cAMP-dependent protein kinase (PKA), dose-dependently (10(-6)-10(-4)M) antagonized PTHrP-induced suppression of TdR in the same manner as PTH. Present study indicated that PTHrP has the same effect on osteoblast proliferation as PTH and that the activation of PKA is directly linked to the change of osteoblast proliferation by PTHrP.

The activation of cAMP-dependent protein kinase is directly linked to the regulation of osteoblast proliferation (UMR-106) by parathyroid hormone

In order to characterize the direct involvement of cAMP in the change of osteoblast proliferation by parathyroid hormone (PTH), we employed the diastereoisomers of adenosine 3',5'-cyclic phosphorothioate, Sp-cAMPS and Rp-cAMPS, which have been recently shown to act directly as agonist and antagonist, respectively in the activation of cAMP-dependent protein kinase (PKA). Dibutyryl cAMP (dbcAMP) and cholera toxin as well as human(h) PTH-(1-34) significantly inhibited [3H]thymidine incorporation (TdR) in osteoblastic osteosarcoma cells, UMR-106. Sp-cAMPS (10(-6)-10(-4) M) inhibited TdR in a dose-dependent manner. Although Rp-cAMPS (10(-6)-10(-4) M) itself did not affect TdR, it significantly blocked dbcAMP-, cholera toxin- and Sp-cAMPS-induced suppression of TdR. Moreover, Rp-cAMPS (10(-6)-10(-4) M) dose-dependently antagonized hPTH-induced suppression of TdR. Present studies first indicated that the activation of PKA is directly linked to the change of osteoblast proliferation by PTH.

Type 1 and 2 isoenzymes of cAMP-dependent protein kinase in Leydig cell steroidogenesis

This study examined the functional significance of the type 1 (T1) and type 2 (T2) cAMP-dependent protein kinase (PK-A) isoenzymes in androgen production by mouse Leydig cells. Leydig cells were exposed to cAMP analogues selective for either of the two cAMP binding sites on the regulatory subunits of each PK-A isoenzyme. As the two binding sites have been shown to exhibit positive cooperativity, coexposure to the appropriate combination of analogues will synergistically increase androgen production if either T1 or T2 PK-A is present and functional in the cell. We found that both PK-A isoenzymes are present and functionally active, though the T1 kinase predominates. Coexposure to the cAMP analogues and cAMP or luteinizing hormone also synergistically increased androgen production via both isoenzymes while forskolin acted only via the T1 isoenzyme, suggesting that forskolin may instigate cellular events in addition to cAMP synthesis.

Further confirmation of the role of adenyl cyclase and of cAMP-dependent protein kinase in primary afferent hyperalgesia

Recent evidence has suggested that cAMP plays a role as a second messenger in the decrease in nociceptive threshold (or hyperalgesia) produced by agents acting on primary afferent terminals. In support of this hypothesis we report that intradermal injection of a direct activator of adenyl cyclase, forskolin, produces a dose-dependent hyperalgesia in the rat. The duration of this hyperalgesia was prolonged by the phosphodiesterase inhibitors, isobutylmethylxanthine and rolipram. Forskolin hyperalgesia was antagonized by the Rp isomer of cyclic adenosine-3'5'-monophosphothioate, an analog of cAMP that prevents the phosphorylation of the cAMP protein kinase. The Rp isomer of cyclic adenosine-3'5'-monophosphothioate also inhibited the hyperalgesia induced by a membrane-permeable analogue of cAMP, 8-bromocyclic adenosine monophosphate, as well as the hyperalgesia induced by agents that are presumed to act directly on primary afferent nociceptors: prostaglandin E2, prostaglandin I2, (8R,15S)-dihydroxyicosa(5E-9,11,13Z)tetraenoic acid; and the adenosine A2-agonist 2-phenylaminoadenosine. Although the cAMP second messenger system contributes to primary afferent hyperalgesia, we found no evidence for a contribution of protein kinase C. Thus, hyperalgesia induced by prostaglandin E2, prostacyclin (prostaglandin I2), (8R,15S)-dihydroxyicosa(5E-9,11,13Z)tetraenoic acid, the adenosine A2-agonist 2-phenylaminoadenosine, 8-bromocyclic adenosine monophosphate and the direct activator of adenyl cyclase, forskolin, were not significantly attenuated by the selective inhibition of protein kinase C by the 19-31 fragment of protein kinase C. Two other inhibitors of protein kinase C, sphingosine and staurosporine, also failed to attenuate prostaglandin E2-induced hyperalgesia.

Glucose-stimulated insulin secretion is not dependent on activation of protein kinase A

The involvement of cyclic AMP-dependent protein kinase A (PKA) in the exocytotic release of insulin from rat pancreatic islets was investigated using the Rp isomer of adenosine 3',5'-cyclic phosphorothioate (Rp-cAMPS). Preincubation of electrically permeabilised islets with Rp-cAMPS (1 mM, 1 h, 4 degrees C) inhibited cAMP-induced phosphorylation of islet proteins of apparent molecular weights in the range 20-90 kDa, but did not affect basal (50 nM Ca2+) nor Ca2(+)-stimulated (10 microM) protein phosphorylation. Similarly, Rp-cAMPS (500 microM) inhibited both cAMP- (100 microM) and 8BrcAMP-induced (100 microM) insulin secretion from electrically permeabilised islets without affecting Ca2(+)-stimulated (10 microM) insulin release. In intact islets, Rp-cAMPS (500 microM) inhibited forskolin (1 microM, 10 microM) potentiation of insulin secretion, but did not significantly impair the insulin secretory response to a range of glucose concentrations (2-20 mM). These results suggest that cAMP-induced activation of PKA is not essential for either basal or glucose-stimulated insulin secretion from rat islets.

Blockade of granulosa cell differentiation by an antagonistic analog of adenosine 3',5'-cyclic monophosphate (cAMP): central but non-exclusive intermediary role of cAMP in follicle-stimulating hormone action

The intermediary role and relative importance of cAMP in follicle-stimulating hormone (FSH) hormonal action were reinvestigated at the level of the rat granulosa cell employing Rp-cAMPS, a novel antagonistic analog of cAMP. This approach may not only provide for direct documentation of cAMP dependence, but may also, by inference, highlight the potential relative importance of other putative intracellular second messenger systems. Initial cell-free validation studies indicated that Rp-cAMPS is capable of effectively competing with cAMP for binding to and activation of the regulatory subunit of the granulosa cell A-kinase holoenzyme. Subsequent whole-cell studies employed cultured rat granulosa cells, the cAMP-phosphodiesterase activity of which was suppressed with ZK62711. Basal progesterone accumulation was relatively low, remaining unaffected by treatment with a maximally effective dose of Rp-cAMPS by itself (10(-3) M). Whereas treatment with FSH (30 ng/ml) resulted in a substantial increase in progesterone accumulation, concurrent treatment with increasing concentrations (10(-6)-10(-3) M) or Rp-cAMPS brought about dose-dependent decrements in the FSH effect with a median effective dose of 1.8 +/- (SE) 0.4 x 10(-5) M and a maximal, but incomplete inhibitory effect of 70 +/- (SE) 6%. Higher concentrations of FSH (greater than or equal to 100 ng/ml) progressively diminished, but did not abolish the Rp-cAMPS blockade. Removal of Rp-cAMPS resulted in progressive resumption of FSH responsiveness suggesting reversibility of action. Significantly, Rp-cAMPS proved highly effective in blocking the action of its agonistic diastereomer Sp-cAMPS. However, Rp-cAMPS was unable to block the action of the lactogenic receptor agonist prolactin, the second messenger of which remains uncertain. Taken together, these findings provide additional direct support to the notion that cAMP may be an intracellular second messenger of FSH. However, to the extent that Rp-cAMPS is incapable of complete neutralization of FSH action, our findings further suggest that cAMP may play a central, albeit non-exclusive role in FSH-supported granulosa cell differentiation and that other putative second messenger systems may also be at play.

Regulation of steroidogenesis in subclones of the MA-10 mouse Leydig tumor cell line

We have previously reported the isolation of a subclone of the MA-10 mouse Leydig tumor cell line (MA-10 LP) which secretes less than 10% of the steroid synthesized by the parent, accumulates comparable amounts of cAMP and has equivalent cholesterol side-chain cleavage activity as the parent population (Kilgore and Stocco (1989) Endocrinology 124, 1210-1216). In the present study we show that addition of exogenous sterol carrier protein 2 (SCP2) to isolated mitochondria was not able to overcome the deficient steroid response of MA-10 LP. We have also demonstrated that human chorionic gonadotropin (hCG)-stimulated cellular events which activate steroid production by subsequently isolated mitochondria require ongoing protein synthesis, release of intracellular calcium and are mediated through the calcium-calmodulin complex. Additionally, mitochondrial sonicates from hCG-stimulated parent cells were able to stimulate steroid production by intact mitochondria isolated from unstimulated parent cells, whereas sonicates from similarly treated MA-10 LP had no effect on steroid production in these mitochondria. Together these data suggest that hCG induces changes in the mitochondria of the parent stock which are not induced to the same extent in the mitochondria of MA-10 LP.

Structure of the lutropin/choriogonadotropin receptor

In summary, the LH/CG receptor is a single polypeptide which contains a large hydrophilic domain that is situated extracellularly, attached to a region that spans the plasma membrane seven times, the carboxy-terminal region being intracellular. This topology was predicted by the amino acid sequence and has been confirmed by our immunofluorescence studies. The extracellular domain, which is related to a family of leucine-rich glycoproteins, is presumably involved in binding the large glycoprotein hormones hCG and LH. The carboxy-terminal half of the receptor, which is related to the family of rhodopsinlike receptors, is (by analogy with these receptors) presumably involved in the coupling of the receptor to the G protein. Our transfection studies confirm that this single polypeptide is capable of binding hormone and activating adenylyl cyclase. Therefore, not only is the structure of the LH/CG receptor unique compared to other cell surface receptors characterized to date, but also its structure suggests that the mechanism of the translation of hormone binding to G protein coupling in this receptor is different from other G protein-coupled receptors whose ligands are much smaller and intercalcate among the transmembrane helices. We predict that, due to the homology among the glycoprotein hormones, the structures of the FSH and TSH receptors share extensive amino acid and structural homology with the LH/CG receptor. Last, our newly acquired knowledge about the structure of the LH/CG receptor, and the development of a cDNA and antibodies for this receptor, should enable more detailed studies on the function and regulation of the LH/CG receptor, not previously possible.

Evidence for the functional coupling of cyclic AMP in MA-10 mouse Leydig tumour cells

A number of studies have indicated that increased production of steroids can be obtained with doses of tropic hormone which do not result in detectable increases in intracellular cAMP. It has been suggested that this may be a result of compartmentalization or functional coupling of cAMP generated by hormone-receptor interactions to specific steroid producing pathways in the cell. In the present study we have stimulated the MA-10 mouse Leydig tumour cell with hCG, dibutyryl cAMP (dbcAMP) and forskolin to determine if functional coupling of cAMP occurs. Treatment with hCG, dbcAMP and forskolin all resulted in significant increases in the production of progesterone, the major steroid produced in these cells. Stimulation with hCG followed by 2D-PAGE analysis of the proteins resulted in the appearance of two proteins in the 30,000 molecular weight range (pI 6.8 and 6.6) and two in the 25,000-27,000 region (pI 5.9-6.0). Stimulation with dbcAMP or forskolin resulted in the appearance of the same proteins seen with hCG, but also in the appearance of two additional proteins, also having molecular weights of approximately 30,000 (pI 6.3 and 6.1). These data indicate that cAMP generated via hCG stimulation, whilst able to generate similar amounts of progesterone, does not stimulate the synthesis of the same proteins as does cAMP added exogenously or generated through indiscriminate activation of adenylate cyclase activity. Thus, it would appear that the gonadotropin activated pathway generates cAMP which remains functionally compartmentalized within the cell.

Lutropin-choriogonadotropin receptor: an unusual member of the G protein-coupled receptor family

A complementary DNA (cDNA) for the rat luteal lutropin-choriogonadotropin receptor (LH-CG-R) was isolated with the use of a DNA probe generated in a polymerase chain reaction with oligonucleotide primers based on peptide sequences of purified receptor protein. As would be predicted from the cDNA sequence, the LH-CG-R consists of a 26-residue signal peptide, a 341-residue extracellular domain displaying an internal repeat structure characteristic of members of the leucine-rich glycoprotein (LRG) family, and a 333-residue region containing seven transmembrane segments. This membrane-spanning region displays sequence similarity with all members of the G protein-coupled receptor family. Hence, the LH-CG-R gene may have evolved by recombination of LRG and G protein-coupled receptor genes. Cells engineered to express LH-CG-R cDNA bind human choriogonadotropin with high affinity and show an increase in cyclic adenosine monophosphate when exposed to hormone. As revealed by RNA blot analysis and in situ hybridization, the 4.4-kilobase cognate messenger RNA is prominently localized in the rat ovary.

Regulation of the differentiated functions of Leydig tumor cells by epidermal growth factor

The three effects of mEGF on MA-10 Leydig tumor cells that have been discussed here are summarized in TABLE 7. The earliest effect of mEGF on MA-10 cells can be detected within 5 min of addition of mEGF and it lasts for about 60 min. During this time mEGF transiently attenuates hCG-stimulated adenylate cyclase activity. Although the magnitude of this effect is small, it can be correlated with a transient attenuation of the hCG-provoked increase in steroid synthesis. At longer times (i.e., 1-8 h) mEGF activates steroid synthesis by a "cAMP-independent pathway" and it potentiates (in a synergistic fashion) the activation of steroidogenesis by hCG, other compounds that activate adenylate cyclase activity, and cAMP analogues. At even longer times (i.e., 8-48 h) mEGF down-regulates the LH/CG receptors and by doing so, limits the steroidogenic response of the cells to hCG. From a biochemical point of view, our data provide an excellent example of those actions of growth factors that are unrelated to the control of cell multiplication, and of the complexity involved even when dealing with a single cell type and a single growth factor. Admittedly we know very little about the molecular basis of the phenomena described herein. Current work in our laboratory, however, is aimed at filling this gap. Among all the questions that we can address, we believe that it is particularly important to characterize the intracellular signaling system(s) activated by mEGF and to determine if a single signaling system is responsible for the diverse biological actions of mEGF in MA-10 cells. From a physiological point of view, our data may also prove important to the understanding of the regulation of testicular functions. There is increasing evidence for the production of EGF (or related peptides such as transforming growth factor alpha) in several tissues, including the testes and ovaries. These findings, together with the results summarized here suggest that EGF (or related peptides) act within the testes in a paracrine, or autocrine fashion and that they may have important modulatory effects on the activation of Leydig cell steroidogenesis by gonadotropins.