Cross-regulation between G-protein-mediated pathways. Activation of the inhibitory pathway of adenylylcylclase increases the expression of beta 2-adrenergic receptors

Cardiac function, structural, and electrical remodeling by microgravity exposure

Space medicine is key to the human exploration of outer space and pushes the boundaries of science, technology, and medicine. Because of harsh environmental conditions related to microgravity and other factors and hazards in outer space, astronauts and spaceflight participants face unique health and medical challenges, including those related to the heart. In this review, we summarize the literature regarding the effects of spaceflight on cardiac structure and function. We also provide an in-depth review of the literature regarding the effects of microgravity on cardiac calcium handling. Our review can inform future mechanistic and therapeutic studies and is applicable to other physiological states similar to microgravity such as prolonged horizontal bed rest and immobilization.

Modulation of intracellular calcium transient in response to β-adrenoceptor stimulation in the hearts of 4-wk-old rats during simulated weightlessness

Modulation of intracellular calcium ([Ca2+]i) transient in response to β-adrenoceptor stimulation in the hearts of hindlimb unweighted (HLU) rats during simulated weightlessness has not been reported. In the present study, we adopted the rat tail suspension for 4 wk to simulate weightlessness. Effects of simulated microgravity on β-adrenoceptor responsiveness were then studied. Mean arterial blood pressure, left ventricular pressure (LVP), systolic function [maximum positive change in pressure over time (+dP/d tmax)], and diastolic function [maximum negative change in pressure over time (−dP/d tmax)] were monitored during the in vivo experiment. β-Adrenoceptor density was quantitated by radioactive ligand binding. Single rat ventricular myocyte was obtained by enzymatic dissociation method. ±dP/d tmax, myocyte contraction, intracellular [Ca2+]i transient, and L-type calcium current in response to β-adrenoceptor stimulation with isoproterenol were measured. Compared with the control group, no significant changes were found in heart weight, body weight, and mean arterial blood pressure, whereas LVP and ±dP/d tmax were significantly reduced. LVP and ±dP/d tmax were significantly attenuated in the HLU group in response to isoproterenol administration. In the in vitro study, the β-adrenoceptor density was unchanged. Effects of isoproterenol on electrically induced single-cell contraction and [Ca2+]i transient in myocytes of ventricles in HLU rats were significantly attenuated. The enhanced L-type Ca2+ current elicited by isoproterenol in cardiomyocytes was significantly decreased in the HLU group. The above results indicate that impaired function of L-type Ca2+ current and decreased [Ca2+]i transient cause the depressed responsiveness of the β-adrenoceptor stimulation, which may be partially responsible for the depression of cardiac function.

Modulation of {beta}-adrenoceptor signaling in the hearts of 4-wk simulated weightlessness rats

The modulation of beta-adrenoceptor signaling in the hearts of hindlimb unweighting (HU) simulated weightlessness rats has not been reported. In the present study, we adopted the rat tail suspension for 4 wk to simulate weightlessness; then the effects of simulated microgravity on beta-adrenoceptor signaling were studied. Mean arterial blood pressure (ABP), left ventricular pressure (LVP), systolic function (+dP/dtmax), and diastolic function (-dP/dtmax) were monitored in the course of the in vivo experiment. Single rat ventricular myocyte was obtained by the enzymatic dissociation method. Hemodynamics, myocyte contraction, and cAMP production in response to beta-adrenoceptor stimulation with isoproterenol or adenylyl cyclase stimulation with forskolin were measured, and Gs protein was also determined. Compared with the control group, no significant changes were found in heart weight, body weight and ABP, while LVP and +/-dP/dtmax were significantly reduced. The ABP decrease, LVP increase, and +/-dP/dtmax in response to isoproterenol administration were significantly attenuated in the HU group. The effects of isoproterenol on electrically induced single-cell contraction and cAMP production in myocytes of ventricles in the HU rats were significantly attenuated. The biologically active isoform, Gsalpha (45 kDa) in the heart, was unchanged. Both the increased electrically induced contraction and cAMP production in response to forskolin were also significantly attenuated in the simulated weightlessness rats. Above results indicated that impaired function of adenylyl cyclase causes beta-adrenoceptor desensitization, which may be partly responsible for the depression of cardiac function.

Regulation of type V adenylate cyclase by Ric8a, a guanine nucleotide exchange factor

In the present study, we demonstrate that AC5 (type V adenylate cyclase) interacts with Ric8a through directly interacting at its N-terminus. Ric8a was shown to be a GEF (guanine nucleotide exchange factor) for several alpha subunits of heterotrimeric GTP binding proteins (Galpha proteins) in vitro. Selective Galpha targets of Ric8a have not yet been revealed in vivo. An interaction between AC5 and Ric8a was verified by pull-down assays, co-immunoprecipitation analyses, and co-localization in the brain. Expression of Ric8a selectively suppressed AC5 activity. Treating cells with pertussis toxin or expressing a dominant negative Galphai mutant abolished the suppressive effect of Ric8a, suggesting that interaction between the N-terminus of AC5 and a GEF (Ric8a) provides a novel pathway to fine-tune AC5 activity via a Galphai-mediated pathway.

Lysophosphatidic acid regulates trafficking of beta2-adrenergic receptors: the Galpha13/p115RhoGEF/JNK pathway stimulates receptor internalization

Lysophosphatidic acid is an important lipid ligand regulating many aspects of cell function, including proliferation and migration. Operating via heterotrimeric G proteins to downstream effectors, lysophosphatidic acid was shown to regulate the function and trafficking of the G protein-coupled beta(2)-adrenergic receptor. C3 exotoxin, expression of dominant negative RhoA, and inhibition of c-Jun N-terminal kinase blocked the ability of lysophosphatidic acid to sequester the beta(2)-adrenergic receptor, whereas expression of constitutively active Galpha(13), p115RhoGEF, or RhoA mimicked lysophosphatidic acid (LPA) action, stimulating the internalization of the Galpha(s)-coupled beta(2)-adrenergic receptor. This study revealed a novel cross-talk exerted from the LPA/Galpha(13)/p115RhoGEF/RhoA pathway to the beta(2)-adrenergic receptor/Galpha(s)/adenylyl cyclase pathway, attenuating the ability of beta-adrenergic agonists to act following stimulation of cells by LPA as may occur during beta-adrenergic therapy of an inflammatory response.

Chicken RFamide-related Peptide (GnIH) and Two Distinct Receptor Subtypes: Identification, Molecular Characterization, and Evolutionary Considerations

RFamide-related peptides (RFRPs) regulate the release of various pituitary hormones in vertebrates. It is completely unknown how the functions of RFRPs vary among animal classes and whether vertebrate RFRPs are orthologous to each other and belong to the same peptide family. This report concerns identification of avian RFRP (gonadotropin-inhibitory hormone, GnIH) from the chicken. Chromosome-wide synteny conservation demonstrated the orthologous relationships among vertebrate RFRPs. The consensus motif for RFRP was modified to Pro-Xaa-Arg-Phe-NH2. We also describe the first identification of two distinct types of receptors for non-mammalian RFRP (RFRPR and NPFFR) from the chicken. Amino acid comparison revealed substantial differences in both termini of receptors among classes of vertebrates. The 5'-flanking regions of chicken RFRPR and NPFFR suggested their expressions in the pituitary gland, and this was confirmed by the RT-PCR analysis. Localizations of both chicken RFRP and its receptors were distinct from those of mammals. These results indicated that avian RFRP, unlike the mammalian one, directly acts on the pituitary gland via receptors to regulate gonadotropin release. It was also suggested that functional differences, especially between avian and mammalian RFRPs, are substantially due to divergences in the structures and expression sites of their receptors.

Effect of chronic lithium treatment on the turnover of alpha2-adrenoceptors after chemical inactivation in rats

One of the most effective psychotherapeutic agents in the treatment of bipolar disease is lithium. Chronic lithium treatment affects some signal transduction mechanisms such as cAMP, cGMP, inositol 1,4,5 P(3), Gi protein, protein kinase C and can also modify gene expression in rat brain. In a previous study, we observed a greater inhibitory effect of lithium on cAMP production after blockade of alpha(2)-adrenoceptors in rat cerebral cortex. Here we examine the influence of chronic lithium treatment on turnover of alpha(2)-adrenoceptors after their inactivation by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) in rat cerebral cortex. After treatment with lithium for 10 days (120 mg/kg/day, i.p.), there was a significant increase in the appearance and disappearance rate constants of these adrenoceptors and a significant reduction of their half-life. These results suggest that chronic lithium administration alters the alpha(2)-adrenoceptor turnover in rat brain.

Desensitization of alpha 2A-adrenoceptor signalling by modest levels of adrenaline is facilitated by beta 2-adrenoceptor-dependent GRK3 up-regulation

(1) Adrenaline (ADR) and noradrenaline (NA) can simultaneously activate inhibitory alpha(2)- and stimulatory beta-adrenoceptors (AR). However, ADR and NA differ significantly in that ADR is a potent beta(2)-AR agonist while NA is not. Only recently has the interaction resulting from the simultaneous activation of alpha(2)- and beta(2)-AR been examined at the cellular level to determine the mechanisms of alpha(2)-AR regulation following concomitant activation of both alpha(2)- and beta(2)-ARs by chronic ADR. (2) This study evaluates beta(2)-AR regulation of alpha(2A)-AR signalling following chronic ADR (300 nM) and NA (1 and 30 micro M) treatments of BE(2)-C human neuroblastoma cells that natively express both beta(2)- and alpha(2A)-ARs. (3) Chronic (24 h) treatment with ADR (300 nM) desensitized the response to the alpha(2A)-AR agonist, brimonidine, in BE(2)-C cells. Addition of the beta-AR antagonist, propranolol, blocked the ADR-induced alpha(2A)-AR desensitization. Unlike ADR, chronic NA (1 micro M) treatment had no effect on the alpha(2A)-AR response. However if NA was increased to 30 micro M for 24 h, alpha(2A)-AR desensitization was observed; this desensitization was partially reversed by propranolol. (4) Chronic ADR (300 nM) treatment reduced alpha(2A)-AR binding levels, contributing to the alpha(2A)-AR desensitization. This decrease was prevented by addition of propranolol during ADR treatment. Chronic NA (30 micro M), like ADR, treatment lowered specific binding, whereas 1 micro M NA treatment was without effect. (5) Chronic ADR treatment produced a significant increase in GRK3 levels and this was blocked by propranolol or GRK2/3 antisense DNA treatment. This antisense DNA, common to both GRK2 and GRK3, also blocked chronic ADR-induced alpha(2A)-AR desensitization and down-regulation. (6) Acute (1 h) ADR (300 nM) or NA treatment (1 micro M) produced alpha(2A)-AR desensitization. The desensitization produced by acute treatment was beta-AR independent, as it was not blocked by propranolol. (7) We conclude that chronic treatment with modest levels of ADR produces alpha(2A)-AR desensitization by mechanisms that involve up-regulation of GRK3 and down-regulation of alpha(2A)-AR levels through interactions with the beta(2)-AR.

Adenosine A1 receptor agonist treatment up-regulates rat brain metabotropic glutamate receptors

Chronic R-N(6)-phenylisopropiladenosine (R-PIA) subcutaneous injection for 6 days significantly increased total glutamate receptor number (180% of control) in rat brain synaptic plasma membranes (SPM), without affecting receptor affinity. A higher increase in metabotropic glutamate (mGlu) receptor number (258% of control) was also detected, indicating that mGlu is the main type of glutamate receptor affected by this treatment. On the other hand, the observed increase in basal and calcium- and Gpp(NH)p-stimulated phospholipase C (PLC) activity after treatment was associated with a significant increase in PLC beta(1) isoform, detected in SPM by immunoblotting assays. Moreover, an increase in PLC activity stimulation with trans-ACPD, in the absence and in the presence of Gpp(NH)p, was detected after R-PIA treatment. These results show that mGlu receptors and its effector system, PLC activity, are up-regulated by chronic exposure to an adenosine A(1) receptor agonist and suggest the existence of a cross-talk mechanism between both signal transduction pathways in rat brain.

Diabetes reduces right atrial β-adrenergic signaling but not agonist stimulation of heart rate in swine

This study assessed the effects of streptozotocin diabetes in swine on the heart rate response to β-adrenergic stimulation the adenylyl cyclase signal transduction pathway. Diabetic animals (n = 9) were hyperglycemic compared to the control group (n = 10) (12.6 ± 1.0 vs. 3.53 ± 0.29 mM). There were no significant differences between the diabetic and nondiabetic groups in the heart rate response to isoproterenol, however, there was a significant reduction (14%) in β-adrenergic receptor density in the right atrium in the diabetic (61 ± 3 fmol/mg protein) versus the nondiabetic group (71 ± 3) (P < 0.05). The content of guanosine triphosphate binding regulatory proteins (Gs and Gi) in the right atrium was not affected by diabetes, nor was adenylyl cyclase activity under unstimulated conditions or with receptor-dependent stimulation with isoproterenol. On the other hand, adenylyl cyclase activity was 34% lower when directly stimulated with forskolin, and it was reduced by 23% when stimulated through Gs with Gpp(NH)p. In conclusion, beta-adrenergic stimulation of heart rate with isoproteronol and the receptor-dependent signal transduction pathway remained intact in the right atrium of diabetic swine despite reduced beta-adrenergic receptor density, G-protein content, and direct stimulation of adenylyl cyclase activity.Key words: diabetes, G-proteins, heart rate, receptors, signal transduction.

Partial agonist clonidine mediates alpha(2)-AR subtypes specific regulation of cAMP accumulation in adenylyl cyclase II transfected DDT1-MF2 cells

alpha2-Adrenergic receptor (alpha(2)-AR) activation in the pregnant rat myometrium at midterm potentiates beta(2)-AR stimulation of adenylyl cyclase (AC) via Gbetagamma regulation of the type II isoform of adenylyl cyclase. However, at term, alpha(2)-AR activation inhibits beta(2)-AR stimulation of AC. This phenomenon is associated with changes in alpha(2)-AR subtype expression (midterm alpha(2A/D)-AR >> alpha(2B)-AR; term alpha(2B) >or =alpha(2A/D)-AR), without any change in ACII mRNA, suggesting that alpha(2A/D)- and alpha(2B)-AR differentially regulate beta(2)-cAMP production. To address this issue, we have stably expressed the same density of alpha(2A/D)- or alpha(2B)-AR with AC II in DDT1-MF2 cells. Clonidine (partial agonist) increased beta(2)-AR-stimulated cAMP production in alpha(2A/D)-AR-ACII transfectants but inhibited it in alpha(2B)-AR-ACII transfectants. In contrast, epinephrine (full agonist) enhanced beta(2)-stimulated ACII in both alpha(2A)- and alpha(2B)-ACII clonal cell lines. 4-Azidoanilido-[alpha-(32)P]GTP-labeling of activated G proteins indicated that, in alpha(2B)-AR transfectants, clonidine activated only Gi(2), whereas epinephrine, the full agonist, effectively coupled to Gi(2) and Gi(3). Thus, partial and full agonists selectively activate G proteins that lead to drug specific effects on effectors. Moreover, these data indicate that Gi(3) activation is required for potentiation of beta(2)-AR stimulation of AC by alpha(2A/D) and alpha(2B)-AR in DDT1-MF2 cells. This may reflect an issue of the amount of Gbetagamma released upon receptor activation and/or betagamma composition of Gi(3) versus Gi(2).

Impaired G(s)alpha and adenylyl cyclase cause beta-adrenoceptor desensitization in chronically hypoxic rat hearts

The effects of beta-adrenoceptor stimulation with isoproterenol on electrically induced contraction and intracellular calcium ([Ca(2+)](i)) transient, and cAMP in myocytes from both hypertrophied right and nonhypertrophied left ventricles of rats exposed to 10% oxygen for 4 wk, were significantly attenuated. The increased [Ca(2+)](i) transient in response to cholera toxin was abolished, whereas increased cAMP after NaF significantly attenuated. The biologically active isoform, G(s)alpha-small (45 kDa), was reduced while the biologically inactive isoform, G(s)alpha-large (52 kDa), increased. The increased electrically induced [Ca(2+)](i) transient and cAMP with 10-100 microM forskolin were significantly attenuated in chronically hypoxic rats. The content of G(i)alpha(2), the predominant isoform of G(i) protein in the heart, was unchanged. Results indicate that impaired functions of G(s) protein and adenylyl cyclase cause beta-adrenoceptor desensitization. The impaired function of the G(s) protein may be due to reduced G(s)alpha-small and/or increased G(s)alpha-large, which does not result from changes in G(i) protein. Responses to all treatments were the same for right and left ventricles, indicating that the impaired cardiac functions are not secondary to cardiac hypertrophy.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Galpha(i2)-mRNA and -protein regulation as a mechanism for heterologous sensitization and desensitization of insulin secretion

Prolonged exposure of cells to an agonist of a G-protein-coupled receptor usually results in an attenuation of the cellular response. To elucidate the cellular mechanisms of sensitization or desensitization in an insulin secretory cell system (INS-1 cells), we investigated a regulatory link between G-protein alpha(s)- and alpha(i2)-subunits mRNA, their protein levels and insulin secretion as the biological effect using various compounds. Incubation with epinephrine (50 microM) for 8 h decreased alpha(s)- and alpha(i2)-mRNA levels to 58% and 72%, respectively, which is reversed after a longer incubation. From results using isoprenaline and the alpha2-agonist UK 14,304 epinephrine is shown to mediate its actions via alpha2- but not beta-adrenoceptors. The insulin inhibitory neuropeptide galanin (50 nM) caused a decrease of alpha(s)- and alpha(i2)-mRNA levels, whereas insulinotropic compounds (incretin hormones) such as GIP or GLP-1 (both 10 nM) led to an increase of alpha(s)- and alpha(i2)-mRNA levels. By using the Ca2+ channel blocker verapamil (50 microM) alpha(i2)-mRNA changes clearly depend on Ca2+ influx. The effects on alpha(i2)-mRNA were accompanied by a parallel, albeit weaker effect on the protein level (only GIP and UK 14,304 were investigated). The changes in alpha(i2)-mRNA levels by either compound were paralleled by inverse changes in insulin secretion: preincubation with UK 14,304 for 8 h led to an increased insulin secretion when challenged by either GLP-1, GIP or glucose (8.3 mM). This was similar for galanin, another potent inhibitor of insulin release. On the other hand, exposure to the incretins GIP or GLP-1 for 8 h induced a smaller insulin release when challenged afterwards by either UK 14,304, galanin, GIP, GLP-1, or glucose. Thus the influence on insulin secretion of various compounds is reciprocal to the regulation of alpha(i2)-mRNA levels but not alpha(s)-mRNA levels. There is, therefore, evidence from all the manoeuvres used that alpha(i2)-mRNA regulation may play a role in heterologous sensitization and desensitization of insulin secretion.

Human brain metabolic response to caffeine and the effects of tolerance

OBJECTIVE

Since there is limited information concerning caffeine's metabolic effects on the human brain, the authors applied a rapid proton echo-planar spectroscopic imaging technique to dynamically measure regional brain metabolic responses to caffeine ingestion. They specifically measured changes in brain lactate due to the combined effects of caffeine's stimulation of glycolysis and reduction of cerebral blood flow.

METHOD

Nine heavy caffeine users and nine caffeine-intolerant individuals, who had previously discontinued or substantially curtailed use of caffeinated products because of associated anxiety and discomforting physiological arousal, were studied at baseline and then during 1 hour following ingestion of caffeine citrate (10 mg/kg). To assess state-trait contributions and the effects of caffeine tolerance, five of the caffeine users were restudied after a 1- to 2-month caffeine holiday.

RESULTS

The caffeine-intolerant individuals, but not the regular caffeine users, experienced substantial psychological and physiological distress in response to caffeine ingestion. Significant increases in global and regionally specific brain lactate were observed only among the caffeine-intolerant subjects. Reexposure of the regular caffeine users to caffeine after a caffeine holiday resulted in little or no adverse clinical reaction but significant rises in brain lactate which were of a magnitude similar to that observed for the caffeine-intolerant group.

CONCLUSIONS

These results provide direct evidence for the loss of caffeine tolerance in the human brain subsequent to caffeine discontinuation and suggest mechanisms for the phenomenon of caffeine intolerance other than its metabolic effects on elevating brain lactate.

Cross-talk between beta-adrenergic and metabotropic glutamate receptors in rat C6 glioma cells

Exposure of rat C6 glioma cells to the beta-adrenergic receptor agonist isoproterenol potentiates basal and metabotropic glutamate receptor-stimulated phospholipase C activity in rat C6 glioma cells. After treatment of cells for 24 h with 10 microM isoproterenol, metabotropic glutamate receptors and phospholipase C activity were determined in C6 plasma membranes. Isoproterenol treatment caused an increase of 67% in the total number of binding sites (Bmax=12.1+/-1. 8 pmol/mg protein versus Bmax=20.27+/-0.88 pmol/mg protein) with Kd values of the same order (Kd=1250+/-101 nM versus Kd=1401+/-211 nM), using l-[3H]glutamate as radioligand. On the other hand, basal, guanylyl imidodiphosphate (Gpp[NH]p)- and trans-aminocyclopentane-1, 3-dicarboxylic acid (trans-ACPD)-stimulated phospholipase C activities were also significantly increased in membranes from isoproterenol-treated cells compared to control cells, by 337%, 33% and 40% respectively. Moreover, a significant increase of 94% in the steady-state level of phospholipase C beta1 in membranes from isoproterenol-treated cells compared to control was also detected by immunoblot. These results show that metabotropic glutamate receptors and its effector system, phospholipase C, are affected by isoproterenol treatment, showing the existence of cross-talk between these signal transduction pathways.

Alzheimer's-specific effects of soluble beta-amyloid on protein kinase C-alpha and -gamma degradation in human fibroblasts

Alzheimer's disease (AD) is a multifactorial disease in which beta-amyloid peptide (betaAP) plays a critical role. We report here that the soluble fraction 1-40 of betaAP differentially degrades protein kinase C-alpha and -gamma (PKCalpha and PKCgamma) isoenzymes in normal (age-matched controls, AC) and AD fibroblasts most likely through proteolytic cascades. Treatment with nanomolar concentrations of betaAP(1-40) induced a 75% decrease in PKCalpha, but not PKCgamma, immunoreactivity in AC fibroblasts. In the AD fibroblasts, a 70% reduction of the PKCgamma, but not PKCalpha, immunoreactivity was observed after betaAP treatment. Preincubation of AC or AD fibroblasts with 50 microM lactacystine, a selective proteasome inhibitor, prevented beta-AP(1-40)-mediated degradation of PKCalpha in the AC cells, and PKCgamma in the AD fibroblasts. The effects of betaAP(1-40) on PKCalpha in AC fibroblasts were prevented by inhibition of protein synthesis and reversed by PKC activation. A 3-hr treatment with 100 nM phorbol 12-myristate 13-acetate restored the PKCalpha signal in treated AC cells but it did not reverse the effects of betaAP(1-40) on PKCgamma in the AD fibroblasts. Pretreatment with the protein synthesis inhibitor, cycloheximide (CHX, 100 microM), inhibited the effects of betaAP(1-40) on PKCalpha and blocked the rescue effect of phorbol 12-myristate 13-acetate in AC fibroblasts but did not modify PKCgamma immunoreactivity in AD cells. These results suggest that betaAP(1-40) differentially affects PKC regulation in AC and AD cells via proteolytic degradation and that PKC activation exerts a protective role via de novo protein synthesis in normal but not AD cells.

Prolonging the delayed phase of myocardial protection: repetitive adenosine A1 receptor activation maintains rabbit myocardium in a preconditioned state

OBJECTIVES

This study was designed to examine whether the myocardium can be maintained in a protected state by extending the delayed phase of cardioprotection with chronic, intermittent adenosine A1 receptor activation.

BACKGROUND

Several recent studies have explored the temporal characteristics of the protective effects of ischemic preconditioning. Two distinct phases of myocardial protection have been described: the short-lived immediate phase, or "classic" preconditioning, and the delayed phase, or "second window of protection" (SWOP). Previous studies have examined the potential for extending the duration of classic preconditioning by repeated application of the preconditioning stimulus. Pretreatment with either multiple episodes of ischemia or continuous infusion of a selective adenosine A1 receptor agonist, 2-chloro-N6-cyclopentyladenosine (CCPA), resulted in attenuation of the protective effects of preconditioning, implying downregulation of the receptors involved in triggering classic preconditioning.

METHODS

Male New Zealand White rabbits were treated with repeated intravenous boluses of CCPA, 100 microg/kg body weight, or 0.9% saline at 48-h intervals. Forty-eight hours after the fifth dose (day 10), the animals were anesthetized and subjected to 30 min of coronary occlusion, followed by 120 min of reperfusion. Infarct size was determined as a percentage of myocardial risk volume using tetrazolium staining. To further explore whether the rabbits had developed tolerance to the effects of adenosine A1 receptor activation, a subgroup of animals were treated with a further bolus of CCPA, 100 microg/kg, at the end of the reperfusion period, and the hemodynamic response was monitored for 10 min before excision of the heart.

RESULTS

Pretreatment with intermittent doses of CCPA resulted in a 42% reduction in the infarct to risk ratio compared with vehicle pretreatment (26.6+/-3.7% vs. 45.9+/-5.5%, p < 0.01). Furthermore, CCPA treatment at the end of reperfusion resulted in identical hypotension and bradycardia in both groups.

CONCLUSIONS

We conclude that rabbits can be maintained in a protected state against myocardial infarction by repeated activation of adenosine A1 receptors, with no evidence of tachyphylaxis to the infarct-limiting or hemodynamic effects of CCPA. This finding suggests that adenosine A1 receptor activation may hold promise as a new approach to long-term cardioprotection.

Expression and regulation of G alpha q and G alpha 11 mRNAs and proteins in bovine adrenal cells

Bovine adrenal cortical cells (BAC) express corticotropin (ACTH) and angiotensin II (AngII) receptors (AT1 subtype), which are coupled to adenylate cyclase and phosphoinositide pathways, respectively. The coupling of AT1 to phosphoinositide breakdown is mainly pertussis toxin-insensitive suggesting that this receptor is coupled to Gaeq/Gae11. In the present work we have demonstrated that BAC express G alpha q and G alpha 11 mRNA and proteins, and their variation during culture as well as their regulation by ACTH and AngII is different. ACTH enhanced G alpha q mRNA levels mainly by increasing the transcription rate. In addition, ACTH increased both G alpha q and G alpha 11 proteins without changing their half-lives. In contrast, AngII reduced both G alpha q mRNA and protein and increased G alpha 11 mRNA but not G alpha 11 protein. The decrease of G alpha q mRNA levels was mainly due to a marked reduction of its half-life. These changes in G alpha q/G alpha 11 proteins induced by both hormones were associated with an enhanced AngII-induced inositol phosphate accumulation, more marked after stimulation with ACTH than after AngII pretreatment. In summary, the present results demonstrated that BAC express both G alpha q and G alpha 11 and their regulations are different and in contrast to other cell types these regulations do not involve changes in the half-life of G alpha q/G alpha 11 proteins.

Altered oxidation and signal transduction systems in fibroblasts from Alzheimer patients

Abnormalities in calcium regulation, amyloid-beta-protein (A beta) production and oxidative metabolism have been implicated in Alzheimer's disease (AD). The use of cultured fibroblasts complement post-mortem and genetic approaches in clarifying the interaction of these processes and the underlying mechanism for the changes in AD. Definition of gene defects in particular Alzheimer families (FAD) permits elucidation of the role of those genetic abnormalities in altered signal transduction in cell lines from those families. Abnormalities in calcium regulation, ion channels, cyclic AMP, the phosphatidylinositide cascade and oxidative metabolism are well documented in fibroblasts from patients with primary genetic defects in the presenilins. Recent studies in AD fibroblasts that demonstrate abnormal secretion of A beta, a protein known to form the characteristic extracellular amyloid deposits in AD brain, further supports the use of these cells in AD research. Comparison of changes in calcium signaling, mitochondrial oxidation and A beta production in these cells suggests that changes in signal transduction including calcium may be a more consistent observation than altered A beta production in fibroblasts from some FAD families. An understanding of these abnormalities in fibroblasts may provide further insights into the pathophysiology of AD, new diagnostic measures and perhaps innovative therapeutic approaches.

Biphasic changes in the density of muscarinic and beta-adrenergic receptors in cardiac atria of rats treated with diisopropylfluorophosphate

Chronic treatment with organophosphate inhibitors of cholinesterases is known to bring about down-regulation of muscarinic acetylcholine receptors in the heart while its effect on the functionally antagonistic beta-adrenergic receptors is not known. We describe experiments in which rats were exposed to daily injections of diisopropylflurophosphate (DFP) and the density of muscarinic and beta-adrenergic binding sites in their cardiac atria was measured according to the binding of (3H)quinuclidinyl benzilate ((3H)QNB) and (-)-4-(3-tert-butylamino-2-hydroxy)-propoxy-(5,7-3H) benzimidazol-2-one ((3H)CGP 12177) as subtype non-specific muscarinic and beta-adrenergic ligands, respectively. Biphasic course of changes was discovered with both ligands. With the dosage scheme applied, the density of beta-adrenoceptors was augmented 24 h after the first dose of DFP and decreased to about one half of control values after 5 days of treatment with DFP. The density of muscarinic receptors was augmented after two days of treatment with DFP and decreased to about one half of control values after 5 days of treatment. Comparatively small changes in the heart rate were observed during the treatment, they reflected changes in the muscarinic and beta-adrenergic receptor density. The finding of DFP-induced changes in the binding of (3H)CGP 12177 suggests that the mechanisms responsible for the control of the density of muscarinic and beta-adrenergic receptors in the heart cells are interconnected but potential roles of other factors involved in in vivo experiments deserve further analysis. The transient increase in the density of muscarinic receptors after two days of DFP treatment appears related to published data on transient stimulation by cholinergic agonists of the transcription of mRNAs for muscarinic receptors.

Chronic exposure of NG 108-15 cells to inhibitory acting drugs reduces stimulatory prostaglandin E1 receptor number

Prolonged exposure of neuroblastoma x glioma (NG 108-15) hybrid cells to inhibitory acting drugs results in sensitization of adenylate cyclase. We now report that chronic activation (3 days) of either inhibitory delta-opioid receptors, alpha 2B-adrenoceptors, or muscarinic M4 receptors significantly decreases the number of stimulatory, adenylate cyclase-coupled prostaglandin E1 receptors. Pharmacological characterization further revealed that the loss of [3H]prostaglandin E1-binding sites most likely corresponds to a reduction of the number of high-affinity, G protein-coupled prostaglandin E1 receptors. The decline in functionally active prostaglandin E1 receptors developed in a time- and dose-dependent manner and could be prevented by pretreatment of the cells with pertussis toxin. Heterologous prostaglandin E1 receptor regulation was blocked by concomitant exposure of the cells to antagonists for inhibitory receptors and was rapidly reversed (t 1/2 < 30 min) upon termination of chronic inhibitory drug treatment. The decrease in high-affinity prostaglandin E1 receptors developed regardless of whether full or partial agonists were used for pretreatment. In addition, the concentrations of inhibitory drugs required to maximally affect prostaglandin E1 receptor number closely resembled those mediating maximal adenylate cyclase inhibition. The data demonstrate that chronic inhibitory drug treatment of NG 108-15 hybrid cells reduces the number of functionally active, excitatory prostaglandin E1 receptors. Thus, it is proposed that adaptations at the level of stimulatory receptor systems contribute to the regulatory mechanisms associated with drug dependence.

Regulation of the mRNA-binding protein AUF1 by activation of the beta-adrenergic receptor signal transduction pathway

In both cell culture based model systems and in the failing human heart, beta-adrenergic receptors ( beta-AR) undergo agonist-mediated down-regulation. This decrease correlates closely with down-regulation of its mRNA, an effect regulated in part by changes in mRNA stability. Regulation of mRNA stability has been associated with mRNA-binding proteins that recognize A + U-rich elements within the 3'-untranslated regions of many mRNAs encoding proto-oncogene and cytokine mRNAs. We demonstrate here that the mRNA-binding protein, AUF1, is present in both human heart and in hamster DDT1-MF2 smooth muscle cells and that its abundance is regulated by beta-AR agonist stimulation. In human heart, AUF1 mRNA and protein was significantly increased in individuals with myocardial failure, a condition associated with increases in the beta-adrenergic receptor agonist norepinephrine. In the same hearts, there was a significant decrease (approximately 50%) in the abundance of beta1-AR mRNA and protein. In DDT1-MF2 cells, where agonist-mediated destabilization of beta2-AR mRNA was first described, exposure to beta-AR agonist resulted in a significant increase in AUF1 mRNA and protein (approximately 100%). Conversely, agonist exposure significantly decreased (approximately 40%) beta2-adrenergic receptor mRNA abundance. Last, we demonstrate that AUF1 can be immunoprecipitated from polysome-derived proteins following UV cross-linking to the 3'-untranslated region of the human beta1-AR mRNA and that purified, recombinant p37AUF1 protein also binds to beta1-AR 3'-untranslated region mRNA.

Prolonged in vitro exposure of broiler adipocytes to somatostatin enhances lipolysis and induces desensitization of antilipolysis

Adipocytes isolated from abdominal fat of female broilers and maintained in primary culture were used to characterize acute and chronic effects of somatostatin (SRIF) on lipolysis and antilipolysis and to determine whether desensitization and cross-regulation phenomena are involved in the regulation of adipocyte metabolism by SRIF and glucagon. Acute exposure of adipocytes to SRIF resulted in a dose-dependent inhibition of basal and glucagon-stimulated lipolysis. The potency and extent of lipolysis inhibition by SRIF were inversely related to the dose of glucagon used to stimulate lipolysis. Preincubation of adipocytes with SRIF induced a dose-response and time-dependent increase in the set-point of lipolysis. Adipocyte sensitivity to glucagon was not altered by SRIF pretreatment. In contrast, preincubation with SRIF reduced adipocyte sensitivity and maximal responsiveness to SRIF. The pattern and extent of antilipolysis attenuation were dependent on the dose and time of preincubation with SRIF as well as on the concentration of glucagon used to acutely stimulate lipolysis. In two experiments, the attenuation of antilipolysis induced by SRIF pretreatment was observed in the absence of enhanced lipolysis, whereas in one experiment enhanced lipolysis was observed in the absence of attenuated antilipolysis. These results indicated that persistent activation of antilipolysis by SRIF increased the set-point of lipolysis by sensitizing (i.e., cross-regulating) lipolysis and by desensitizing antilipolysis.

Chronic treatment with carbachol sensitizes the myocardium to cAMP-induced arrhythmia

BACKGROUND

The present study investigated biochemical and functional consequences of chronic activation of the inhibitory Gi alpha-coupled adenylyl cyclase pathway in the heart.

METHODS AND RESULTS

Rats (220 to 260 g) were treated with 4-day infusions of the M-cholinoceptor agonist carbachol (9.6 mg/kg per day) or vehicle. An additional group that received the beta-adrenoceptor agonist isoprenaline (2.4 mg/kg per day) served as control. The main finding was that chronic infusion of carbachol led to a marked increase in isoprenaline- or forskolin-induced arrhythmia in electrically driven papillary muscles (in vitro). Compared with control, the potency of isoprenaline and forskolin to induce arrhythmia in cardiac preparations from carbachol-treated rats was increased 36- and 2.2-fold and the efficacy was increased 7.3- and 2.3-fold, respectively. The potency of carbachol to antagonize the isoprenaline- and forskolin-induced arrhythmia was decreased 30-fold. These changes were accompanied by a decrease in left ventricular M-cholinoceptor density by 15% (P < .05) and a decrease in pertussis toxin-sensitive G proteins (Gi alpha) by 26% (P < .05) without a decrease in the corresponding mRNAs. beta-Adrenoceptor density and basal and stimulated adenylyl cyclase activity remained unchanged. In contrast, isoprenaline infusion induced a decrease in arrhythmogenic potency of forskolin (P = NS), which was accompanied by a decrease in beta-adrenoceptor density, an increase in Gi alpha protein and mRNA levels, and a decrease in basal and stimulated adenylyl cyclase activity.

CONCLUSIONS

Chronic parasympathetic activation sensitizes the myocardium to cAMP-induced arrhythmia. These changes may be due to quantitative alterations in functional Gi alpha.

Morphine dependence in human neuroblastoma SH-SY5Y cells is associated with adaptive changes in both the quantity and functional interaction of PGE1 receptors and stimulatory G proteins

Chronic exposure of all-trans-retinoic acid-differentiated SH-SY5Y cells to morphine (10 mu M; 2 days) results in sensitization of adenylate cyclase as characterized by a significant increase in both PGE1 receptor-mediated as well as receptor-independent (NaF, 10 mM; forskolin, 100 mu M) stimulation of effector activity. To investigate the underlying biochemical alterations, chronic opioid regulation of each of the components comprising the stimulatory PGE1 receptor system was examined. On receptor level, chronic morphine treatment was found to reduce PGE1 receptor number (Bmax) by approximately 40%, whereas their affinity slightly increased. Binding experiments performed in the presence of GTPgammaS (100 mu M) further indicate that the decrease in PGE1 receptor density is associated with a loss of functionally G protein-coupled receptors. On post-receptor level, chronic morphine treatment substantially increased the abundance and functional activity of stimulatory G proteins, as assessed by cholera toxin-catalyzed ADP-ribosylation of GSalpha and S49 cyc- reconstitution assays. No changes were found on the level of adenylate cyclase. Evaluation of the functional interaction between PGE1 receptors and GS in situ by application of a C-terminal anti-GSalpha antibody revealed a more intense coupling efficiency between these two entities, since a significant higher amount of antibody (2.3-fold) was required in morphine dependent cell membranes to half-maximally attenuate PGE1 receptor-stimulated adenylate cyclase activity. In addition, limitation of the amount of functionally available GSalpha within the PGE1 receptor/adenylate cyclase signal transduction cascade abolished the generation of a supersensitive adenylate cyclase response during the state of naloxone (100 mu M)-precipitated withdrawal. These data demonstrate that in human neuroblastoma SH-SY5Y cells chronic morphine-induced sensitization of adenylate cyclase is associated with distinct quantitative and qualitative adaptations within the stimulatory adenylate cyclase-coupled PGE1 receptor system. Thus, alterations in the functional activity of stimulatory receptor systems are suggested to contribute to the cellular mechanisms underlying opioid dependence.

Effects of theophylline and dibutyryl-cAMP on adenosine receptors and heart rate in cultured cardiocytes

The effects of chronic exposure to the adenosine antagonist theophylline (Theo) and dibutyryl cyclic-AMP, a membrane-permeant derivative of the second messenger 3', 5'-cyclic-AMP (cAMP), on contractions and adenosine receptor levels in cultured cardiocytes were studied. Binding of the A1-adenosine receptor antagonist [3H]8-cyclopentyl-1,3-dipropylxanthine ([3H]CPX) was used to monitor the level of the receptors in intact cardiocytes. Both Theo and cAMP stimulated the rate of contraction and also increased the density of adenosine receptors. The Bmax value for [3H]CPX binding to intact cardiocytes was increased by 45-47% following 4 days of exposure to either 50 microM Theo or 100 microM cAMP. Scatchard analysis indicated that the affinity of the A1 receptors for [3H]CPX remained unchanged (Kd 0.1-0.2 nM). No significant differences were observed in protein content or in cell number. A linear correlation was achieved between the level of A1-adenosine receptors and heart rate at various Theo and dibutyryl-cAMP concentrations, although Theo was more efficient in elevation of the receptor density. Increases of 82, 78, 138 and 235% in A1 receptor density and increases of 63, 59, 66 and 150% in heart rate were obtained following 5 days of treatment with 1, 10, and 1000 microM of Theo, respectively. It is concluded that there is a linkage between the rate of cardiac contractions and the level of adenosine receptors. Thus, changes in the density of adenosine receptors may compensate for chronic drug-induced changes in cardiac contractile activity so as to restore conditions to the normal state.

Adrenergic desensitization in left ventricle from streptozotocin diabetic swine

Patients with diabetes mellitus that exhibit cardiac pump failure display compromised stroke volume, ejection fraction, and slower rates of rise and fall of left ventricular (LV) dP/dt in the absence of ischemic injury. We hypothesized that diabetic cardiomyopathy may involve decrements in adrenergic sensitivity, with specific molecular alterations in the beta-adrenergic receptor (beta AR)- G protein- adenylyl cyclase (AC) signal transduction system. We assessed the effects of 3 months of streptozotocin-induced diabetes (125 mg/kg i.v.; DIAB, n = 10) on myocardial signal transduction in mini-pigs. DIAB were hyperglycemic compared to controls (CON, n = 10; 20.92 +/- 2.64 v 5.24 +/- 0.35 mM glucose), and had lower fasting insulin levels (6.46 +/- 0.97 v 13.68 +/- 3.91 microU/ml). Transmural LV free wall homogenates from DIAB exhibited similar beta AR density as CON, but decreased cAMP production (pmol cAMP/mg prot.min) using these pharmacological stimulators: 10 microM Isoproterenol plus 100 microM GTP (74 +/- 5 v 97 +/- 11); 100 microM Gpp(NH)p (116 +/- 7 v 161 +/- 17); 10 mM fluoride ion (266 +/- 16 v 324 +/- 25). No differences between DIAB and CON were observed when stimulated by 100 microM forskolin (440 +/- 20 v 429 +/- 33), suggesting no alterations in the catalytic subunit of AC. In DIAB, quantitative immunoblotting indicated slightly depressed levels of Gs (552 +/- 44 v 630 +/- 59 pmol/g ww; NS), but a significant redistribution of alpha s from the sarcolemma to the cytosol (32.7 +/- 0.82% v 25.9 +/- 1.7%). Significantly elevated levels of cardiac Gi were seen in DIAB homogenates compared to CON ventricles (2326 +/- 145 v 1522 +/- 181 pmol/g ww), with no alpha i subunit redistribution. We conclude that despite maintained beta AR density, receptor-dependent and G protein-dependent stimulation of AC is depressed so that streptozotocin-induced diabetic LV is affected by increased cardiac Gi, redistribution of Gs alpha to the cytosol, and an increase in the Gi/Gs ratio. These results help explain depressed catecholamine responsiveness and cardiac performance exhibited by diabetic patients.

Degradation of G11 alpha/Gq alpha is accelerated by agonist occupancy of alpha 1A/D, alpha 1B, and alpha 1C adrenergic receptors

Cells of clones of rat 1 fibroblasts transfected to express the molecularly defined alpha 1A/D, alpha 1B, or alpha 1C adrenoreceptors and prelabeled with myo-[3H]inositol were each shown to generate high levels of inositol phosphates when exposed to the alpha 1 adrenoreceptor agonist phenylephrine. Maintained exposure of each of these cells to phenylephrine resulted in a large down-regulation of the receptors and also a marked down-regulation of cellular levels of both of the phosphoinositidase C-linked G-proteins Gq alpha and G11 alpha. To examine the mechanism of phenylephrine-induced down-regulation of Gq alpha and G11 alpha, pulse-chase 35S-amino acid labeling experiments were performed with each of the alpha 1A/D, alpha 1B, and alpha 1C adrenoreceptor-expressing cell lines. The rate of degradation of G11 alpha/Gq alpha, which was adequately modeled by a monoexponential with half-life between 33 and 40 h in each of the cell lines in the absence of agonist, was accelerated substantially (some 4-fold) in the presence of phenylephrine. By contrast, the rate of degradation of the G-protein Gi2 alpha, which would not be anticipated to be activated by members of the alpha 1 adrenoreceptor family, was unaltered by the presence of phenylephrine. Levels of mRNA encoding Gq alpha and G11 alpha were not substantially altered by exposure of the cells to phenylephrine in any of the cell lines studied.

Thrombin, phorbol ester, and cAMP regulate thrombin receptor protein and mRNA expression by different pathways

Human mesangial cells have been used to study the regulation of thrombin receptor protein and mRNA expression during cross-talk between different signal transduction pathways. Persistent activation of thrombin receptor by thrombin led to homologous down-regulation of thrombin receptor protein. However, thrombin receptor mRNA expression was not affected, suggesting that increased receptor degradation is responsible for homologous down-regulation. Chronic activation of protein kinase C by phorbol 12-myristate 13-acetate (PMA) and of adenylylcyclase by prostaglandin E1 (PGE1) resulted in heterologous down-regulation of thrombin receptor protein. In contrast to thrombin, PMA and PGE1 reduced in parallel thrombin receptor mRNA levels to 51% and 24% of control, respectively, indicating that heterologous down-regulation of thrombin receptor protein is, at least in part, due to inhibition of receptor mRNA expression. The mechanisms of heterologous down-regulation of thrombin receptor protein have been studied in detail and compared to homologous down-regulation. PMA-induced down-regulation was completely blocked by GF 109 203 X, an inhibitor of protein kinase C. However, the loss of thrombin receptor induced by thrombin was not prevented by GF 109 203 X, indicating that homologous regulation is not dependent on protein kinase C activation. The heterologous effect of PGE1 was mimicked by 8-bromo-cAMP, isobutylmethylxanthine, and forskolin, suggesting that an increase in intracellular cAMP level is involved in heterologous regulation. Interestingly, heterologous down-regulation induced by PGE1 seems not to require previous internalization of thrombin receptor. These data indicate that thrombin receptor protein and mRNA expression can be regulated in homologous and heterologous ways by different mechanisms.

Forskolin-induced up-regulation and functional supersensitivity of dopamine D2long receptors expressed by Ltk- cells

Mouse fibroblast Ltk- cells, stably expressing the human dopamine D2long receptor, were grown in the presence of forskolin (100 microM) for 4 or 16 h. The 16 h treatment resulted in a significant up-regulation of the dopamine D2long receptors by 43-96% as measured with [3H]raclopride with no change in the Kd value. A significant increase in the maximal inhibition of acute forskolin-stimulated cAMP accumulation by dopamine (0.1 nM-3 microM) was found both at 4 and 16 h. No such D2long-receptor-coupled response to dopamine could be detected in wild-type, untransfected, Ltk- cells with or without forskolin treatment. Furthermore, basal cAMP levels as well as the maximal response to acute forskolin stimulation decreased in the D2long receptor expressing cells with the treatment, by 33% and 23% respectively. The results indicate that persistent maintenance of high cAMP levels in transfected Ltk- cells may lead to adaptive quantitative and functional changes of the dopamine D2long receptor reminiscent of receptor supersensitivity induced by chronic antagonist treatment in vivo where the receptor targeted is inhibitorily coupled to adenylyl cyclase, as is the D2long receptor. This may provide a model for studying mechanisms underlying dopamine D2 receptor up-regulation and receptor supersensitivity, not readily induced in cell lines.

Liver beta-adrenergic receptors, G proteins, and adenylyl cyclase activity in obesity-diabetes syndromes

The ob and db genes produce similar hormonal anomalies in mice. Although the expression of the syndromes diverges with age, at 8-12 wk both ob/ob and db/db mice are hyperglycemic and hyperinsulinemic and show evidence of hypercorticoidism. Nevertheless, membranes isolated from livers of ob/ob and db/db mice behave differently in terms of adenylyl cyclase activity and beta-adrenergic receptor function. There are three times as many beta 2-adrenergic receptor binding sites and a threefold increase in the response to catecholamines in ob/ob mouse liver membranes than in comparable preparations from normal controls or db/db mice. By contrast, the two main G proteins of liver membranes (Gs alpha and Gi alpha 2) are less abundant in the mutants, ob/ob and db/db, than in their respective lean controls. Adrenalectomy normalizes the exaggerated response to beta-adrenergic agonists and the number of beta-adrenergic binding sites in the ob/ob mouse. This shows that the enhanced beta-adrenergic receptor response is linked to hypercorticoidism. Cellular maturation and differentiation (D. C. Watkins, J. K. Northrup, and C. C. Malbon, J. Biol. Chem. 262: 10651-10657, 1987) and diseases such as obesity and diabetes (cf. N. McFarlane-Anderson, J. Bailly, and N. Bégin-Heick, Biochem. J. 282: 15-23, 1992) have been associated with modifications in the complement of G proteins detected in cells. However, the relationship among levels, types, and intracellular localization of G proteins in tissues and their influence on the transduction of the message to an effector system, such as adenylyl cyclase, are not yet well understood.

Brain G-protein proteolysis by calpain: enhancement by lithium

Heterotrimeric G-proteins mediate many receptor-coupled signal transduction processes and the cellular concentrations of G-proteins are modulated by several factors, including development, activity, and drugs. The mechanisms causing changes in G-protein concentrations are mostly unknown. The purpose of this study was to determine if G-proteins could be proteolyzed by calpain, a calcium-activated neutral protease that has been linked with neuronal plasticity. In membranes prepared from rat cerebral cortex, calpain rapidly cleaved the alpha-subunit of Go but did not hydrolyze beta-subunits. Comparisons of the proteolysis of different alpha-subunits revealed that they were differentially susceptible to calpain-induced proteolysis in the order of alpha s > alpha o > alpha q > alpha i. Preincubation of cortical membranes with GTP gamma S, which binds to G alpha and causes its dissociation from the beta gamma dimer, reduced calpain-mediated proteolysis of alpha o. Lithium, the primary treatment for mania, enhanced the calpain-mediated proteolysis of alpha o in the heterotrimeric state but did not affect proteolysis of dissociated, GTP gamma S-bound alpha o. These results demonstrate that proteolysis by calpain is a potential mechanism by which cellular G-protein concentrations can be regulated.

Construction and characterization of a novel cross-regulation system for regulating cloned gene expression in Escherichia coli

A novel cross-regulation system employing two pairs of interacting promoter-repressor systems was constructed using the tac-lacI and lambda pL-cI promoter-operator-repressor systems. In particular, transcription of the cat gene and the fused cI gene is regulated by the tac promoter, while transcription of the lacI gene is controlled by the lambda pL promoter. In order to compare CAT production from this new system with a currently employed transcription control configuration, a control expression vector utilizing the constitutive repressor synthesis configuration was also constructed. In this construct, cat is under the control of the tac promoter, and lac repressor is provided from a single copy of the lacIq allele included in the plasmid. Induction results using different copy number vectors indicate that induced cat expression levels are at least twofold higher using the cross-regulation system which has very low basal expression. These results match well with previous mathematical modeling predictions indicating excellent control of basal expression and also higher cloned-gene expression post-induction over a broad range of copy numbers for a cross-regulation control configuration. Induction of the cross-regulation system both up-regulated the activation pathway and down-regulated the inhibition pathway, shifting the system steady-state from lac repressor expression to cat and cI expression. The control strategy presented here should be equally applicable to regulate transcription in diverse hosts.

Effects of chronic lithium and carbamazepine treatment on G-protein subunit expression in rat cerebral cortex

Although lithium and carbamazepine (CBZ) are effective in the treatment of bipolar affective disorder, their mechanism of action is still unknown. Recent evidence suggests that lithium and CBZ might exert their therapeutic effects by modulating the function of guanosine triphosphate (GTP)-regulatory (G) proteins associated with central nervous system second messenger systems. In the present study, we showed that chronic lithium administration decreases G alpha s, G alpha i1, and G alpha i2 messenger RNA (mRNA) abundance by 25%-30% in rat cerebral cortex. However, the levels of G alpha s, G alpha i1, and G alpha i2 mRNA were unaffected by chronic CBZ treatment. The effects of lithium on G alpha s, G alpha i1, and G alpha i2 mRNA levels appear to be selective, as the mRNA levels of G alpha o, G alpha x, G beta 1, G beta 2, and G beta 3 subunits remained unchanged. Two days after terminating chronic lithium treatment, changes in G alpha s, G alpha i1, and G alpha i2 mRNA levels were not demonstrable. Short-term administration of lithium (2 days), however, reduced only the G alpha i2 mRNA levels. Surprisingly, there was no significant difference in the amount of immunologically detectable G alpha s-s, G alpha s-1, G alpha i(1 + 2), G alpha 0, and G beta (1 + 2) in the cortex of rats chronically treated with lithium or CBZ, compared with controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Agonist regulation of gene expression of adrenergic receptors and G proteins

Study of transmembrane signaling via G proteins has focused to a large extent upon investigations of individual G protein-linked receptor-effector systems. Agonist-induced desensitization and down-regulation of beta-adrenergic receptors, for example, have been studied extensively and adopted as a general model for G protein-linked receptor regulation. This review focuses not only on agonist regulation of adrenergic receptor gene expression, but also on how agonists regulate opposing adrenergic receptor-mediated pathways. This important feature of G protein-mediated pathways, i.e., cross-regulation and integration of information among several pathways, will be discussed in the context of what has been learned in the adrenergic receptor-coupled pathways.

D2-dopaminergic agonist quinpirole and 8-bromo-cAMP have opposite effects on Go alpha GTP-binding protein mRNA without changing D2 dopamine receptor mRNA levels in striatal neurones in primary culture

Long-term coordinated regulations (during development or by agonists and second messenger molecules) of the expression of mRNAs encoding D2-dopamine (DA) receptors and D2 receptor-linked Go alpha proteins have been studied by Northern blot analysis in mouse embryonic striatal neurones in primary culture. During the course of the cell culture, the levels of both mRNAs increased, in conjunction with the maturation of the neurones. When the preparation was treated with the D2-DA agonist quinpirole (5-15 hrs, 10(-4) M), which decreases cAMP in these neurones, the levels of Go alpha mRNAs were enhanced whereas that of the D2 mRNA remained unchanged. Conversely, the Go alpha mRNAs, but not the D2 mRNA, decreased when the neurones were exposed to 8-bromo-cAMP (16 hrs, 10(-6) M). It is concluded that, in these experimental conditions where neurones have not yet established their connexions, the longterm regulation of the membrane transmission of D2-DA signal might implicate mainly the Go alpha encoding gene.

Serotonergic signalling between thyroid cells: protein kinase C and 5-HT2 receptors in the secretion and action of serotonin

Parafollicular (PF) cells of the thyroid gland are neural crest derivatives, which costore the neurotransmitter, 5-hydroxytryptamine (5-HT) with calcitonin. PF cells are located adjacent to follicular (F) cells within the basement membrane of thyroid follicles. It has been proposed that 5-HT serves an intercellular signalling function in the thyroid and that F cells are its target. This proposal was tested by using cell lines derived from PF (medullary thyroid carcinoma [MTC]) and F (FRTL-5) cells to study the mechanisms that mediate the secretion and action of 5-HT. Secretion of 5-HT by MTC cells was evoked by thyroid stimulating hormone, thyrotropin (TSH), elevated extracellular calcium (increases [Ca2+]e), or by agents that increase intracellular cAMP (increases [cAMP]i). When protein kinase C (PKC) was down-regulated by prolonged treatment of MTC cells with phorbol 12-myristate 13-acetate (PMA), or PKC was inhibited by staurosporin, the TSH- or PMA-evoked secretion of 5-HT was blocked; however, interference with PKC function did not affect 5-HT secretion evoked by increases [Ca2+]e or increases [cAMP]i. In the putative targets, FRTL-5 cells, 5-HT increased the turnover of phosphoinositides (PI), cytosolic calcium (increases [Ca2+]i), increases [cAMP]i, and biphasically modified the effect of TSH on cAMP. All of these 5-HT effects were inhibited by 5-HT2 receptor antagonists (spiperone and ketanserin) and by pertussis toxin (PTx), suggesting that the actions of 5-HT are mediated by 5-HT2 receptors, which are coupled to a G protein. This suggestion was supported by the following additional observations: FRTL-5 membranes bound the 5-HT2 agonist, [125I]2,5-dimethoxy-4-iodophenylisopropylamine ([125I]-DOI), and anti-idiotypic antibodies, which recognize 5-HT2 receptors. [125I]-DOI binding was inhibited by guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma-S) and the antibodies were displaced by spiperone. Data are consistent with the hypothesis that 5-HT serves as a PF to F cell messenger.