Potentiation of agonist-stimulated cyclic AMP accumulation by tyrosine kinase inhibitors in rat pinealocytes

Daily rhythm in pineal phosphodiesterase (PDE) activity reflects adrenergic/3',5'-cyclic adenosine 5'-monophosphate induction of the PDE4B2 variant

The pineal gland is a photoneuroendocrine transducer that influences circadian and circannual dynamics of many physiological functions via the daily rhythm in melatonin production and release. Melatonin synthesis is stimulated at night by a photoneural system through which pineal adenylate cyclase is adrenergically activated, resulting in an elevation of cAMP. cAMP enhances melatonin synthesis through actions on several elements of the biosynthetic pathway. cAMP degradation also appears to increase at night due to an increase in phosphodiesterase (PDE) activity, which peaks in the middle of the night. Here, it was found that this nocturnal increase in PDE activity results from an increase in the abundance of PDE4B2 mRNA (approximately 5-fold; doubling time, approximately 2 h). The resulting level is notably higher (>6-fold) than in all other tissues examined, none of which exhibit a robust daily rhythm. The increase in PDE4B2 mRNA is followed by increases in PDE4B2 protein and PDE4 enzyme activity. Results from in vivo and in vitro studies indicate that these changes are due to activation of adrenergic receptors and a cAMP-dependent protein kinase A mechanism. Inhibition of PDE4 activity during the late phase of adrenergic stimulation enhances cAMP and melatonin levels. The evidence that PDE4B2 plays a negative feedback role in adrenergic/cAMP signaling in the pineal gland provides the first proof that cAMP control of PDE4B2 is a physiologically relevant control mechanism in cAMP signaling.

Differential action of a protein tyrosine kinase inhibitor, genistein, on the positive inotropic effect of endothelin-1 and norepinephrine in canine ventricular myocardium

Experiments were carried out in isolated canine ventricular trabeculae and acetoxymethylester of indo-1-loaded single myocytes to elucidate the role of protein tyrosine kinase (PTK) in the inotropic effect of endothelin-1 (ET-1) induced by crosstalk with norepinephrine (NE). The PTK inhibitor genistein was used as a pharmacological tool. Genistein but not daidzein inhibited the positive inotropic effect and the increase in Ca(2+) transients induced by ET-1 by crosstalk with NE at low concentrations. Genistein and daidzein antagonized the negative inotropic effect and the decrease in Ca(2+) transients induced by ET-1 by crosstalk with NE at high concentrations, but genistein did not affect the antiadrenergic effect of carbachol. Genistein but not daidzein enhanced the positive inotropic effect and the increase in Ca(2+) transients induced by NE via beta-adrenoceptors, while the enhancing effect of genistein was abolished by the protein tyrosine phosphatase inhibitor vanadate. These findings indicate that genistein (1) induces a positive inotropic effect in association with an increase in Ca(2+) transients, (2) inhibits the positive inotropic effect of ET-1 induced by crosstalk with NE, and (3) enhances the positive inotropic effect of NE induced via beta-adrenoceptors by inhibition of PTK. In addition, genistein inhibits the negative inotropic effect of ET-1 induced by crosstalk with NE through a PTK-unrelated mechanism. PTK may play a crucial role in the receptor-mediated regulation of cardiac contractile function in canine ventricular myocardium.

Epidermal growth factor receptor tyrosine kinase inhibition is not protective in PCK rats

BACKGROUND

Advances in the understanding of cystogenesis, identification of the PKHD1 gene and availability of a rat model (the PCK rat) caused by a Pkhd1 mutation facilitate testing of therapies for autosomal-recessive polycystic kidney disease (ARPKD). Considerable support exists for the importance of the epidermal growth factor (EGF)/transforming growth factor-alpha (TGF-alpha)/EGF receptor (EGFR) axis and of the adenylyl cyclase-adenosine 3',5'-cyclic monophosphate (cAMP) pathway in the pathogenesis of cyst formation and progressive enlargement.

METHODS

To determine whether EGFR tyrosine kinase inhibition is protective in the PCK rat, male and female animals were treated with EKI-785 or EKB-569 or with vehicle alone between 3 and 10 weeks of age. Biochemical and histomorphometric analysis, immunohistochemistry, immunoblotting, enzyme immunoassay, and quantitative reverse transcription-polymerase chain reaction (RT-PCR) were used to ascertain the effects of treatment.

RESULTS

Contrary to other murine models of ARPKD, overexpression and apical mislocalization of EGFR were not detected in the PCK rats. Consistent with these expression results, EKI-785 or EKB-569 administration had no effect or worsened PKD, and had no effect on the development of fibrocystic liver disease. Increased renal cAMP and vasopressin V2 receptor expression were observed in the EKI-785-treated animals.

CONCLUSION

EGFR tyrosine kinase inhibition did not protect PCK rats from the development of PKD. This may be due to effects on collecting duct cAMP that counteract possible beneficial effects on the extracellular-regulated protein kinase (ERK)/mitogen-activated protein kinase (MAPK) pathway, particularly in the absence of EGFR overexpression or mislocalization. The relevance of these observations to the treatment of human cystic kidney diseases deserves further study.

Circadian signaling in the chick pineal organ

The chick pineal organ is recognized to contain an endogenous circadian oscillator as well as having direct photic input pathways and the capability of synthesizing melatonin. Despite its interesting circadian cell biology, far less is known about the chick pineal as compared to mammalian pineal glands. The goals of our research were to identify and characterize novel components of the circadian system in this photoneuroendocrine organ. Using a subtractive screening strategy of a nocturnal chick pineal cDNA library, we identified numerous genes whose expression in the chick pineal has never been reported. Among these, we focused our attention on a homologue to the regulatory subunit of the mammalian serine/threonine protein phosphatase (STPP) 2A. The expression of this gene in the chick pineal is highly circadian both in vivo and in vitro. Analysis of the PP2A enzyme in this tissue revealed that it is predominantly cytosolic in localization, sensitive to classical PP2A inhibitors, and far more active during the subjective night. Interestingly, the acute pharmacological inhibition of PP2A leads to elevated phosphoCREB levels and concomitant melatonin secretion, indicating that this enzyme participates at some level in the control of nocturnal pineal melatonin synthesis. In a second aspect of our research, we examined the mechanisms underlying the circadian rhythmicity of cyclic GMP in the chick pineal. This signaling molecule is poorly understood, despite its well-known, high-amplitude circadian rhythms and the presence of many cGMP-dependent targets in this tissue. Our work has shown that although both soluble (sGC) and membrane-bound (mGC) forms of guanylyl cyclase are present, the primary contributor to the circadian rhythms of cGMP is the mGC-B enzyme, which is activated only by the natriuretic peptide CNP. As pharmacological blockade of mGC-B (but not sGC) suppresses nocturnal cGMP levels, we conclude that CNP-dependent mechanisms are involved. Hence, the circadian clock in the chick pineal appears to drive either CNP secretion or mGC-B expression (or synthetic efficiency) in order to elevate nocturnal cGMP. Conversely, light may inhibit cGMP by uncoupling this drive. These data provide new strategies for understanding both photic input pathways (presumed to depend on cGMP) and cGMP-dependent cellular function in the chick pineal organ.

Potentiation of cyclic AMP and cyclic GMP accumulation by p38 mitogen-activated protein kinase (p38MAPK) inhibitors in rat pinealocytes

The effects of p38 mitogen-activated protein kinase (p38MAPK) inhibitors on the adrenergic-stimulated cyclic nucleotide production in rat pinealocytes were investigated. Treatment with SB202190 [4-(4-fluorophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)IH-imidazole] and SB203580 [4-(4-fluoropheny)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)IH-imidazole] (1-100 microM), two pyridinyl imidazole compounds that inhibit p38MAPK, as well as SB202474 [4-(ethyl)-2-(4-methoxyphenyl)-5-(4-pyridyl)IH-imidazole], an inactive analog, was effective in potentiating norepinephrine- and isoproterenol-stimulated cyclic AMP (cAMP) and cyclic GMP (cGMP) accumulation in a concentration-dependent manner. All three compounds caused a greater increase in the cGMP than the cAMP response, with SB202474 being substantially more potent than the two active analogs. At 100 microM, SB202474 potentiated the isoproterenol-stimulated cAMP and cGMP accumulation by 65 and 500%, respectively. Pharmacological studies indicated that the potentiating effect of SB202474 was independent of protein kinase C activation, intracellular calcium elevation, or serine/threonine phosphatase inhibition, three pathways known to potentiate the beta-adrenergic-stimulated cyclic nucleotide responses in rat pinealocytes. In contrast, the potentiating effect of SB202474 was abolished in the presence of a phosphodiesterase inhibitor, isobutylmethylxanthine. At 100 microM, all three compounds inhibited cAMP- and cGMP-phosphodiesterase activities by 50 and 80%, respectively. These results suggest that the commonly used p38MAPK inhibitors can modulate cyclic nucleotide responses through phosphodiesterase inhibition, a mechanism that appears to be independent of p38MAPK inhibition.

Tyrosine kinase effects on adrenoceptor-stimulated cyclic AMP accumulation in preoptic area and hypothalamus of female rats: modulation by estradiol

These studies examined the functional interactions between adrenergic G-protein coupled receptors and protein tyrosine kinases in the preoptic area and hypothalamus, brain regions that regulate reproductive function in female rats, and evaluated whether in vivo treatment with estradiol for 2 days modulates the cross-talk between these two signaling pathways. In hypothalamic slices genistein, a general tyrosine kinase inhibitor, enhances norepinephrine-stimulated cAMP synthesis independent of estradiol treatment. Genistein appears to act by increasing beta-adrenoceptor signaling. At high norepinephrine concentrations, estradiol potentiates genistein enhancement of the cAMP response in hypothalamic slices. This interaction between estradiol and genistein appears to involve modification of alpha(2)-adrenoceptor signaling mechanisms. In preoptic area slices, genistein enhancement of norepinephrine-stimulated cAMP synthesis is only observed in estradiol-treated rats. In this brain region, genistein enhances cAMP accumulation by modifying alpha(1)- and/or alpha(2)-adrenoceptor rather than beta-adrenoceptor signaling. Genistein amplification of norepinephrine-stimulated cAMP synthesis is not mediated by interactions with estrogen receptors, or by regulation of adenylyl cyclase or phosphodiesterase activities. At the concentration used, genistein inhibits tyrosine phosphorylation in slices from both brain regions. Daidzein, an inactive analogue of genistein, fails to enhance the norepinephrine-stimulated cAMP response in either brain region independent of hormone treatment. These results suggest that protein tyrosine kinases regulate adrenergic responses in the hypothalamus and preoptic area. Moreover, the functional interaction between adrenergic G-protein coupled receptor signaling and protein tyrosine kinases is modified in a brain region and receptor subtype specific manner by estradiol.

Genistein potentiates the relaxation induced by beta1- and beta2-adrenoceptor activation in rat aortic rings

In rat aortic rings, genistein, an inhibitor of tyrosine kinase, but not daidzein, an inactive analogue of genistein, potentiated the relaxation induced by isoproterenol. Atenolol, a beta1-adrenoceptor antagonist, or ICI-118,551, a beta2-adrenoceptor antagonist, inhibited the relaxation induced by isoproterenol. The potentiating effect of genistein on the relaxation induced by isoproterenol in the presence of ICI-118,551 was apparently greater than that in the presence of atenolol. In the presence of ICI-118,551, theophylline, an inhibitor of cyclic adenosine monophosphate (cAMP)-dependent phosphodiesterase (cAMP-PDE), markedly inhibited the potentiating effect of genistein on the isoproterenol-induced relaxation, whereas in the presence of atenolol, theophylline only partly inhibited the potentiating effect of genistein. The relaxation induced by forskolin, an activator of adenylyl cyclase, was potentiated by genistein or theophylline. In the presence of theophylline, the relaxation induced by forskolin was not further affected by genistein. Genistein also inhibited the activities of cAMP-PDE. In the presence of atenolol, but not ICI-118,551, iberiotoxin, an inhibitor of Ca-activated K channels, inhibited the relaxation induced by isoproterenol and the potentiating effect of genistein. In the presence of atenolol, quinacrine, an inhibitor of phospholipase A2, and metyrapone, an inhibitor of P-450 enzymes, but not alpha-naphthoflavone, an inhibitor of P-450 enzymes, indomethacin, a cyclooxygenase inhibitor, or AA861, a 5-lipoxygenase inhibitor, inhibited the potentiating effect of genistein. These results suggest that the potentiation of the beta1-adrenoceptor-induced relaxation by activation of genistein may mostly be due to inhibition of cAMP-PDE activities. In addition, the potentiation of the relaxation induced by activation of beta2-adrenoceptors by genistein may be related to the inhibition of arachidonic acid metabolism and cAMP-PDE activities.

Agonist-dependent phosphorylation of the G protein-coupled receptor kinase 2 (GRK2) by Src tyrosine kinase

GRK2 is a member of the G protein-coupled receptor kinase (GRK) family, which phosphorylates the activated form of a variety of G protein-coupled receptors (GPCR) and plays an important role in GPCR modulation. It has been recently reported that stimulation of the mitogen-activated protein kinase cascade by GPCRs involves tyrosine phosphorylation of docking proteins mediated by members of the Src tyrosine kinase family. In this report, we have investigated the possible role of c-Src in modulating GRK2 function. We demonstrate that c-Src can directly phosphorylate GRK2 on tyrosine residues, as shown by in vitro experiments with purified proteins. The phosphorylation reaction exhibits an apparent K(m) for GRK2 of 12 nM, thus suggesting a physiological relevance in living cells. Consistently, overexpression of the constitutively active c-Src Y527F mutant in COS-7 cells leads to tyrosine phosphorylation of co-expressed GRK2. In addition, GRK2 can be detected in phosphotyrosine immunoprecipitates from HEK-293 cells transiently transfected with this Src mutant. Interestingly, phosphotyrosine immunoblots reveal a rapid and transient increase in GRK2 phosphorylation upon agonist stimulation of beta(2)-adrenergic receptors co-transfected with GRK2 and wild type c-Src in COS-7 cells. This tyrosine phosphorylation is maximal within 5 min of isoproterenol stimulation and reaches values of approximately 5-fold over basal conditions. Furthermore, GRK2 phosphorylation on tyrosine residues promotes an increased kinase activity toward its substrates. Our results suggest that GRK2 phosphorylation by c-Src is inherent to GPCR activation and put forward a new mechanism for the regulation of GPCR signaling.

Cyclic nucleotide phosphodiesterases (PDE) 3 and 4 in normal, malignant, and HTLV-I transformed human lymphocytes

Intracellular cyclic AMP, determined in part by cyclic nucleotide phosphodiesterases (PDEs), regulates proliferation and immune functions in lymphoid cells. Total PDE, PDE3, and PDE4 activities were measured in phytohemagglutinin (PHA)-activated peripheral blood mononuclear cells (PBMC-PHA), normal natural killer (NK) cells, Jurkat and Kit225-K6 leukemic T-cells, T-cell lines transformed with human T-lymphotropic virus (HTLV)-I (a retrovirus that causes adult T-cell leukemia/lymphoma) and HTLV-II (a nonpathogenic retrovirus), normal B-cells, and B-cells transformed with Epstein-Barr virus (EBV). All cells exhibited PDE3 and PDE4 activities but in different proportions. In EBV-transformed B cells, PDE4 was much higher than PDE3. HTLV-I+ T-cells differed significantly from other T-lymphocyte-derived cells in also having a higher proportion of PDE4 activities, which apparently were not related to selective induction of any one PDE4 mRNA (judged by reverse transcription-polymerase chain reaction) or expression of the HTLV-I regulatory protein Tax. In MJ cells (an HTLV-I+ T-cell line), Jurkat cells, and PBMC-PHA cells, the tyrosine kinase inhibitor herbimycin A strongly inhibited PDE activity. Growth of MJ cells was inhibited by herbimycin A and a protein kinase C (PKC) inhibitor, and was arrested in G1 by rolipram, a specific PDE4 inhibitor. Proliferation of several HTLV-I+ T-cell lines, PBMC-PHA, and Jurkat cells was inhibited differentially by forskolin (which activates adenylyl cyclase), the selective PDE inhibitors cilostamide and rolipram, and the nonselective PDE inhibitors pentoxifylline and isobutyl methylxanthine. These results suggest that PDE4 isoforms may be functionally up-regulated in HTLV-I+ T-cells and may contribute to the virus-induced proliferation, and that PDEs could be therapeutic targets in immune/inflammatory and neoplastic diseases.

The potentiating effect of genistein on the relaxation induced by isoproterenol in rat aortic rings

In rat aortic rings, the mechanism of potentiating effect of genistein, a tyrosine kinase inhibitor, on the relaxation induced by isoproterenol was examined. Pretreatment of the aortic rings by genistein, but not by daidzein, an inactive analogue of genistein, potentiated the relaxation induced by isoproterenol. Genistein also potentiated the relaxation induced by forskolin, an activator of guanylyl cyclase, and dibutyryl cyclic AMP. In addition, theophylline, an inhibitor of phosphodiesterase, potentiated the relaxation induced by isoproterenol and forskolin. Theophylline partly inhibited the potentiation of isoproterenol-induced relaxation by genistein while it completely inhibited the potentiation of forskolin-induced relaxation by genistein. Iberiotoxin, an inhibitor of Ca-activated K (KCa) channels, partly inhibited the isoproterenol-induced relaxation and the potentiating effect of genistein on the relaxation induced by isoproterenol. Quinacrine (an inhibitor of phospholipase A2), alpha-naphthoflavone (an inhibitor of cytochrome P-450 enzymes), and 8-methoxypsoralen (an inhibitor of cytochrome P-450 enzymes), partly inhibited the potentiating effect of genistein on the isoproterenol-induced relaxation, but metyrapone (an inhibitor of cytochrome P-450 enzymes), indomethacin (an inhibitor of cyclooxygenase), and AA861 (an inhibitor of 5-lipoxygenase) did not. These results suggest that the potentiation of isoproterenol-induced relaxation by genistein may be related to the activities of phosphodiesterase, KCa channels, and cytochrome P-450 enzymes.

3',5'-cyclic guanosine monophosphate activates mitogen-activated protein kinase in rat pinealocytes

The role of 3',5'-cyclic guanosine monophosphate (cGMP) in the activation of mitogen-activated protein kinases (MAPKs) was investigated in rat pinealocytes. Treatment with dibutyryl cGMP (DBcGMP) dose-dependently increased the phosphorylation of both p44 and p42 isoforms of MAPK. This effect of DBcGMP was abolished by PD98059 (a MAPK kinase inhibitor), H7 (a nonspecific protein kinase inhibitor), and KT5823 [a selective cGMP-dependent protein kinase (PKG) inhibitor]. Elevation of cellular cGMP content by treatment with norepinephrine, zaprinast (a cGMP phosphodiesterase inhibitor), or nitroprusside was effective in activating MAPK. Natriuretic peptides that were effective in elevating cGMP levels in this tissue were also effective in activating MAPK. Our results indicate that, in this neuroendocrine tissue, the cGMP/PKG signaling pathway is an important mechanism used by hormones and neurotransmitters in activating MAPK.

Platelet-activating factor stimulation of p125(FAK) and p130(Cas) tyrosine phosphorylation in brain

The effect of platelet-activating factor (PAF) on protein tyrosine phosphorylation was studied in rat brain slices. PAF induced a time- and concentration-dependent increase in tyrosine phosphorylation of a doublet of approximately 125 kDa. These proteins were identified by immunoprecipitation as p125(FAK) and p130(Cas), using monoclonal antibodies. This effect was mediated by PAF receptors, as shown by its inhibition by the action of a PAF antagonist. The tyrosine phosphorylation evoked by PAF was dependent, at least in part, on external calcium. The involvement of protein kinase C was demonstrated by the synergistic effect of TPA on PAF-stimulated tyrosine phosphorylation. The finding that PAF stimulates tyrosine phosphorylation of both focal adhesion protein p125(FAK) and p130(Cas) suggests that PAF might modulate the integrin mediated signal transduction in the brain.

Increased nitroglycerin-induced relaxation by genistein in rat aortic rings

The effect of genistein, a tyrosine kinase inhibitor, on nitroglycerin-induced relaxation was examined in rat aortic rings contracted by phenylephrine. In rat aortic rings, genistein (10(-5) M and 3x10(-5) M), a tyrosine kinase inhibitor, but not daidzein, an analogue of genistein, increased relaxation induced by nitroglycerin in a concentration-dependent manner. Iberiotoxin, an inhibitor of Ca2+ -activated K+ channels, inhibited the relaxation induced by nitroglycerin, but it did not affect the effect of genistein. Glibenclamide, an inhibitor of ATP-sensitive K+ channels, did not affect the relaxation induced by nitroglycerin. Theophylline, an inhibitor of cyclic AMP-dependent phosphodiesterase, increased the relaxation induced by nitroglycerin, and genistein (10(-5) M) failed to affect the relaxation induced by nitroglycerin in the presence of theophylline. Genistein also inhibited the activity of cyclic AMP-dependent phosphodiesterase. In addition, 6-[4-(4'-pyridyl)amino phenyl]-4,5-dihydro-3(2H)-pyridazinone hydrochloride, an inhibitor of cyclic GMP-inhibitable cyclic AMP phosphodiesterase, inhibited the relaxation induced by nitroglycerin. These results suggest that, in the rat aortic rings, genistein inhibits cyclic AMP-dependent phosphodiesterase activities, resulting in the increase of the relaxation induced by nitroglycerin.

Potentiation of cyclic AMP-mediated vasorelaxation by phenylephrine in pulmonary arteries of the rat

Alpha1-adrenoceptor agonists may potentiate relaxation to beta-adrenoceptor agonists, although the mechanisms are unclear. We compared relaxations induced by beta-adrenoceptor agonists and cyclic AMP-dependent vasodilators in rat pulmonary arteries constricted with prostaglandin F2alpha (PGF2alpha) or the alpha1-adrenoceptor agonist phenylephrine (PE). In addition, we examined whether differences were related to cyclic AMP- or nitric oxide (NO) and cyclic GMP-dependent pathways. Isoprenaline-induced relaxation was substantially potentiated in arteries constricted with PE compared with PGF2alpha. Methoxamine was similar to PE, whereas there was no difference between PGF2alpha and 30 mM KCl. The potentiation was primarily due to a marked increase in the NO-independent component of relaxation, from 9.1+/-1.7% for PGF2alpha to 55.1+/-4.4% for PE. NO-dependent relaxation was also enhanced, but to a lesser extent (50%). Relaxation to salbutamol was almost entirely NO-dependent in both groups, and was potentiated approximately 50% by PE. Relaxation to forskolin (activator of adenylate cyclase) was also enhanced in PE constricted arteries. Part of this relaxation was NO-dependent, but the major effect of PE was to increase the NO-independent component. Propranolol diminished but did not abolish the potentiation. There was no difference in response to CPT cyclic AMP (membrane permeant analogue) between PE and PGF2alpha, suggesting that mechanisms distal to the production of cyclic AMP were unchanged. Relaxation to sodium nitroprusside (SNP) was the same for PE and PGF2alpha, although relaxation to acetylcholine (ACh) was slightly depressed. This implies that potentiation by PE does not involve the cyclic GMP pathway directly. Mesenteric arteries constricted with PE did not show potentiation of isoprenaline-induced relaxation compared to those constricted with PGF2alpha, suggesting that this effect may be specific to the pulmonary circulation. These results clearly show that PE potentiates both the NO-independent and -dependent components of cyclic AMP-mediated relaxation in pulmonary arteries of the rat, although the effect on the former is more profound. We suggest that potentiation of both components is largely due to direct activation of adenylate cyclase via alpha1-adrenoceptors, within the smooth muscle and endothelial cells respectively.

Decreases in cAMP phosphodiesterase activity in hepatocytes cultured with herbimycin A due to cellular microtubule polymerization related to inhibition of tyrosine phosphorylation of alpha-tubulin

The increase in cellular cAMP concentration during 10-min incubation of rat hepatocytes with glucagon or forskolin was enhanced markedly when the hepatocytes had been cultured for several hours with herbimycin A. This effect of herbimycin was accompanied by inhibition of tyrosine-phosphorylation of cellular proteins including alpha-tubulin, antagonized by coaddition of Na3VO4 plus H2O2, which also antagonized the herbimycin-induced tyrosine phosphorylation, and overcome by the addition to the 10-min incubation medium of a certain inhibitor of cAMP phosphodiesterase (PDE), which caused a huge accumulation of cAMP. The effective PDE inhibitors were 4-[3-(cyclopentyloxy)-4-methoxyphenyl]-2-pyrrolidinone (rolipram) and 4-(3-butyloxy-4-methoxyphenyl)-2-imidazolidinone (Ro-20-1724, a PDE4 inhibitor), in addition to 3-isobutyl-1-methylxanthine (a nonselective inhibitor). Rapid breakdown of the once-accumulated cAMP in cultured hepatocytes during the subsequent incubation without PDE inhibitors was progressively prevented when the concentration of herbimycin was increased from 0.3 to 10 microM during prior culture. This effect of herbimycin to inhibit PDE activity in intact cells was abolished by coaddition of a microtubule-disrupting agent, either colchicine or vinblastine, into the culture, but remained unchanged if the vinblastine-containing medium was further supplemented with taxol, a microtubule-stabilizing agent, which by itself mimicked the effect of herbimycin. None of these agents, which thus affected PDE activity in intact cells, inhibited the PDE activity assayable in the cell lysates. The taxol-like and vinblastine-suppressible action of herbimycin to stimulate microtubular assembly was antagonized by Na3VO4/H2O2, as confirmed by confocal microscopic images of the cells stained with fluorescein-bound anti-(alpha-tubulin). Thus, 4-h culture of hepatocytes with herbimycin inhibits phosphorylation of the C-terminal tyrosine residue of alpha-tubulin, thereby stimulating formation of a microtubular network which is responsible for the inhibition of PDE4 in the intact cells by an unknown mechanism.

Specific CD3ε Association of a Phosphodiesterase 4B Isoform Determines Its Selective Tyrosine Phosphorylation After CD3 Ligation

cAMP-specific phosphodiesterases (PDE) comprise an extensive family of enzymes that control intracellular levels of cAMP and thus regulate T cell responses. It is not known how the function of these enzymes is altered by TCR engagement. We have examined this issue by studying one of the PDE isozymes (PDE4B). PDE4B RNA and protein were detected in resting PBLs, and the levels of PDE4B protein increased with cell cycling. In peripheral blood T cells, two previously reported PDE4B isoforms could be detected: one was 75–80 kDa (PDE4B1) and the other was 65–67 kDa (PDE4B2). These two isoforms differed in their N-terminal sequence, with the presence of four potential myristylation sites in the PDE4B2 that are absent in PDE4B1. Consequently, only PDE4B2 was found in association with the CD3ε chain of the TCR. In addition, although both isoforms were phosphorylated in tyrosines in pervanadate-stimulated T cells, only the TCR-associated PDE4B2 was tyrosine-phosphorylated following CD3 ligation. The kinetics of phosphorylation of TCR-associated PDE4B2 correlated with changes in cAMP levels, suggesting that tyrosine phosphorylation of the TCR-associated PDE4B isoform upon engagement of this receptor may be an important regulatory step in PDE4B function. Our results reveal that selectivity of PDE4B activation can be achieved by differential receptor association and phosphorylation of the alternatively spliced forms of this PDE.

Modulation of gonadotropin levels by peptides acting at the anterior pituitary gland

There are several lines of evidence that point to peptides participating in the regulation of LH and/or FSH levels by action at the pituitary. This evidence includes altered secretion of gonadotropins from the anterior pituitary cells or tissue in vitro when exposed to the peptide. Additionally, modification of GnRH-stimulated LH/FSH secretion has been observed. Furthermore, there is potential for a separately modulated interaction with the primed response. Another potential of action is by interaction among non-GnRH peptides on gonadotropin-regulating processes, although there are no good data available on this aspect. Other observations, consistent with a pituitary role for the peptides in modulation of LH, include detection of the peptides in portal blood, detection of high-affinity receptors or receptor mRNA in the pituitary, and detection of intrapituitary peptide or peptide mRNA in the pituitary. The modulation by steroids of both concentrations and type of activities provides a further level of physiological refinement. There is, however, some confusion regarding the involvement of these peptides in gonadotropin control. The reasons can be seen by considering aspects of investigations. There are experimental variations such as 1) species studied, e.g., NPY has been reported to have an effect on LH secretion from rat cells (168) but not on sheep anterior pituitary tissue (64), and substance P inhibits GnRH-stimulated release from rat cells (182) but potentiates the response in prepubertal porcine cells (92); 2) the steroidal conditions under which the study is performed, e.g., NPY has opposite effects in certain endocrine environments, augmenting GnRH-stimulated LH release in proestrus-like conditions (168), and inhibiting in metestrus-like environment (66); 3) the type of cell preparation, e.g., responsiveness to substance P might depend on whether cells in overnight culture were in separated or clustered state (91); 4) the time course considered, e.g., oxytocin that might induce marked LH release from pituitary cells after a longer length of incubation than GnRH requires (68); 5) length of exposure to peptide, e.g., endothelin that augmented or inhibited GnRH-stimulated LH release (50); 6) In addition, it is possible that the traditional endpoint selected in such studies, namely, observation of gonadotropin secretion, is not necessarily the most important for these peptides (56, 81, 117). Unfortunately, at this stage a definitive answer to the question "What do the peptides actually do?" cannot be provided and we remain tantalized by the glimpses of potential roles. Perhaps in a few years an updated review will be able to include a more complete answer. It is necessary for the full understanding of LH control that not only the properties of the peptides in isolation be characterized but also their interactions.

Regulation of adenylyl cyclase activity in human peripheral blood mononuclear cells: effects of protein kinase inhibitors and of a calcium ionophore

In a previous paper we presented evidence for a negative regulation of adenylyl cyclase activity by tyrosine protein kinase(s) in the human leukemic T cell line Jurkat. In order to examine this point in non malignant cells, we conducted the present study in human peripheral blood mononuclear cells (PBMC). In these cells, staurosporine, a broad spectrum protein kinase inhibitor, enhanced not only the receptor-mediated, induced by prostaglandin E2 (PGE2), but also the direct (forskolin-induced) stimulation of adenylyl cyclase activity. Herbimycin A, a specific protein tyrosine kinase inhibitor, reproduced only in part the effect of staurosporine, whereas bisindolylmaleimide, the most specific protein kinase C (PKC) inhibitor known at present time, was ineffective. All these observations were made both in the absence and presence of isobutylmethylxanthine, a phosphodiesterase inhibitor, indicating that the effects of staurosporine and herbimycin A on cAMP accumulation were not due to phosphodiesterase inhibition. The calcium ionophore A 23187 also enhanced the PGE2-induced cAMP accumulation, and this effect was not additive to that of staurosporine, but additive to that of herbimycin A. These results confirm and extend those obtained in Jurkat cells. Taken together, they indicate that in human PBMC the adenylyl cyclase activity is negatively regulated by tyrosine kinase(s) and not by PKC, and positively regulated by Ca2+. They also suggest that the major enhancement by staurosporine of the PGE2-induced cAMP accumulation, although chiefly mediated by protein tyrosine kinase inhibition, also depends on another, presently undetermined, effect of the drug simulating that of Ca2+.

Ceramide selectively inhibits calcium-mediated potentiation of beta-adrenergic-stimulated cyclic nucleotide accumulation in rat pinealocytes

Interaction between sphingomyelin metabolism and cyclic nucleotide synthesis in rat pinealocytes was investigated by determining the effect of ceramide on adrenergic-stimulated cAMP and cGMP accumulation. Although C2-, C6-, and C8-ceramide had no effect on basal, isoproterenol-, or norepinephrine-stimulated cAMP and cGMP accumulation, they inhibited the potentiation caused by depolarising concentrations of K+ or BayK 8644. Similar inhibition was observed when ceramide metabolism was inhibited by a glucosylceramide synthase inhibitor. In contrast, the potentiation of cAMP and cGMP accumulation caused by other intracellular Ca(2+)-elevating agents such as ionomycin or thapsigargin or by an activator of protein kinase C was not affected by ceramide. Taken together, our results suggest that ceramide selectively inhibits cyclic nucleotide synthesis when the nucleotide synthesis is potentiated by an increase in intracellular Ca2+ through L-type Ca2+ channels and that the sphingomyelin cycle probably plays an important role in the regulation of these channels.

Modulation of cyclic AMP levels in a clonal neural cell line by inhibitors of tyrosine phosphorylation

The convergence of tyrosine kinase and cyclic AMP (cAMP) signal transduction pathways was investigated in the HT4.7 neural cell line with inhibitors of tyrosine kinases and tyrosine phosphatases. The protein tyrosine kinase inhibitor genistein inhibited isoproterenol-stimulated cAMP production by 40-60% in whole cells, with no effect on basal cAMP levels. In both whole cells and membranes, genistein also inhibited cAMP produced in response to direct stimulation of adenylyl cyclase with forskolin. However, in the absence of phosphodiesterase inhibitors, genistein presentation resulted in an increase in cAMP levels. Genistein inhibited phosphodiesterase activity by 80-85%, indicating that tyrosine phosphorylation stimulates both cAMP synthesis and degradation. The decrease in cAMP levels by genistein was not merely competitive inhibition of adenylyl cyclase with respect to ATP, since the Km of adenylyl cyclase for ATP remained essentially the same in either the presence or the absence of genistein. Another tyrosine kinase inhibitor, herbimycin A, which inhibits by a different mechanism than genistein, also decreased forskolin-stimulated cAMP in whole cells. As would be expected for the involvement of tyrosine phosphorylation in the control of cAMP production, inhibition of tyrosine phosphatases by vandate increased forskolin-stimulated cAMP production. These results suggest that cAMP production can be regulated by tyrosine phosphorylation, and the simultaneous activation of both cAMP synthesis and degradation may serve to alter the duration of cAMP elevation.