Protein phosphatase-protein kinase interplay modulates alpha 1b-adrenoceptor phosphorylation: effects of okadaic acid

Functional activation of Gαq/11 protein via α1 -adrenoceptor in rat cerebral cortical membranes

Although it is recognized that α₁ -adrenoceptors are coupled to diverse intracellular signalling pathways, its primary transduction mechanisms are evoked by activating phospholipase C in the cell membrane through Gα_q/11 , resulting in production of inositol 1,4,5-trisphosphate and diacylglycerol. However, there have been few studies that indicate directly the involvement of Gα_q/11 proteins in this signalling pathway in the central nervous system. In the current study, we tried to pharmacologically characterize (-)-adrenaline-stimulated [³⁵ S]GTPγS binding to Gα_q/11 in rat brain membranes. Functional activation of Gα_q/11 coupled to α₁ -adrenoceptor was investigated by using [³⁵ S]GTPγS binding/immunoprecipitation assay in the membranes prepared from rat cerebral cortex, hippocampus, and striatum. The specific [³⁵ S]GTPγS binding to Gα_q/11 was stimulated by (-)-adrenaline in a concentration-dependent and saturable manner in rat cerebral cortical membranes. In hippocampal or striatal membranes, the stimulatory effects of (-)-adrenaline were scarce. The effect of (-)-adrenaline was potently inhibited by prazosin, a potent and selective α₁ -adrenoceptor antagonist, but not by yohimbine, a selective α₂ -adrenoceptor antagonist. The response was mimicked by cirazoline, but not by R(-)-phenylephrine. Although oxymetazoline also stimulated the specific [³⁵ S]GTPγS binding to Gα_q/11 as an apparent "super-agonist", detailed pharmacological characterization revealed that its agonistic properties in this experimental system were derived from off-target effects on 5-HT_2A receptors, but not via α₁ -adrenoceptors. In conclusion, functional coupling of α₁ -adrenoceptors to Gα_q/11 proteins are detectable in rat brain membranes by means of [³⁵ S]GTPγS binding/immunoprecipitation assay. It is necessary to interpret the experimental data with caution when oxymetazoline is included as an agonist at α₁ -adrenoceptors.

The relationship between the MMP system, adrenoceptors and phosphoprotein phosphatases

The MMPs and their inhibitors [tissue inhibitor of MMPs (TIMPs)] form the mainstay of extracellular matrix homeostasis. They are expressed in response to numerous stimuli including cytokines and GPCR activation. This review highlights the importance of adrenoceptors and phosphoprotein phosphatases (PPP) in regulating MMPs in the cardiovascular system, which may help explain some of the beneficial effects of targeting the adrenoceptor system in tissue remodelling and will establish emerging crosstalk between these three systems. Although α- and β-adrenoceptor activation increases MMP but decreases TIMP expression, MMPs are implicated in the growth stimulatory effects of adrenoceptor activation through transactivation of epidermal growth factor receptor. Furthermore, they have recently been found to catalyse the proteolysis of β-adrenoceptors and modulate vascular tone. While the mechanisms underpinning these effects are not well defined, reversible protein phosphorylation by kinases and phosphatases may be key. In particular, PPP (Ser/Thr phosphatases) are not only critical in resensitization and internalization of adrenoceptors but also modulate MMP expression. The interrelationship is complex as isoprenaline (ISO) inhibits okadaic acid [phosphoprotein phosphatase type 1/phosphoprotein phosphatase type 2A (PP2A) inhibitor]-mediated MMP expression. While this may be simply due to its ability to transiently increase PP2A activity, there is evidence for MMP-9 that ISO prevents okadaic acid-mediated expression of MMP-9 through a β-arrestin, NF-κB-dependent pathway, which is abolished by knock-down of PP2A. It is essential that crosstalk between MMPs, adrenoceptors and PPP are investigated further as it will provide important insight into how adrenoceptors modulate cardiovascular remodelling, and may identify new targets for pharmacological manipulation of the MMP system.

Sphingosine 1-phosphate-mediated α1B-adrenoceptor desensitization and phosphorylation. Direct and paracrine/autocrine actions

Sphingosine-1-phosphate-induced α1B-adrenergic receptor desensitization and phosphorylation were studied in rat-1 fibroblasts stably expressing enhanced green fluorescent protein-tagged adrenoceptors. Sphingosine-1-phosphate induced adrenoceptor desensitization and phosphorylation through a signaling cascade that involved phosphoinositide 3-kinase and protein kinase C activities. The autocrine/paracrine role of sphingosine-1-phosphate was also studied. It was observed that activation of receptor tyrosine kinases, such as insulin growth factor-1 (IGF-I) and epidermal growth factor (EGF) receptors increased sphingosine kinase activity. Such activation and consequent production of sphingosine-1-phosphate appear to be functionally relevant in IGF-I- and EGF-induced α1B-adrenoceptor phosphorylation and desensitization as evidenced by the following facts: a) expression of a catalytically inactive (dominant-negative) mutant of sphingosine kinase 1 or b) S1P1 receptor knockdown markedly reduced this growth factor action. This action of sphingosine-1-phosphate involves EGF receptor transactivation. In addition, taking advantage of the presence of the eGFP tag in the receptor construction, we showed that S1P was capable of inducing α1B-adrenergic receptor internalization and that its autocrine/paracrine generation was relevant for internalization induced by IGF-I. Four distinct hormone receptors and two autocrine/paracrine mediators participate in IGF-I receptor-α1B-adrenergic receptor crosstalk.

Effect of inhibitors of mitogen-activated protein kinase kinase on alpha(1B)-adrenoceptor phosphorylation

1 Mitogen-activated protein kinases mediate hormone/neurotransmitter action on proliferation and differentiation and participate in receptor regulation. The effect of inhibitors of mitogen-activated kinase kinase (MEK) on alpha(1B)-adrenoceptor phosphorylation state and function was studied using different cell lines. It was observed that at nanomolar concentrations the MEK inhibitors, PD98059 (2'-amino-3'-methoxyflavone) and UO126 [1,4-(diamino-2,3-dicyano/1,4-bis-(2-aminophenylthio)-butadiene], increased alpha(1B)-adrenoceptor phosphorylation and diminished the functional response of this receptor to noradrenaline. These agents did not alter the action of lysophosphatidic acid. 2 Staurosporine (IC(50) approximately 0.8 nm) (a general protein kinase inhibitor) and bis-indolyl-maleimide I (IC(50) approximately 200 nm) (a selective protein kinase C inhibitor) inhibited PD98059-induced alpha(1B)-adrenoceptor phosphorylation. In contrast, neither wortmannin (phosphoinositide 3-kinase inhibitor) nor genistein (protein tyrosine kinase inhibitor) had any effect. The data suggest the possibility that MEK might exert control on the activity of the enzymes that regulate receptor phosphorylation, such as G-protein-coupled receptor kinases, protein kinase C or serine/threonine protein phosphatases. 3 Coimmunoprecipitation studies showed a constant association of total extracellular signal-regulated kinase 2 (ERK2) with alpha(1B)-adrenoceptors. Association of phospho-ERK 1/2 to alpha(1B)-adrenoceptors increased not only in response to agonist but also in response to agents that increase alpha(1B)-adrenoceptor and ERK1/2 phosphorylation [such as endothelin-1, phorbol 12-myristate-13-acetate (PMA) and epidermal growth factor (EGF)]; not surprisingly, PD98059 decreased this effect. 4 Our data show that blockade of MEK activity results in increased alpha(1B)-adrenoceptor phosphorylation, diminished adrenoceptor function and perturbation of receptor-ERK1/2 interaction.

Regulation of LPA receptor function by estrogens

17beta-Estradiol induced LPA(1) receptor desensitization in C9 cells stably expressing LPA(1) receptors and transiently expressing estrogen receptor alpha. Such desensitization was evidenced by a reduction in lysophosphatidic acid-mediated Ca(2+)mobilization and it was associated to receptor phosphorylation and internalization. These effects of 17beta-estradiol were rapid (taking place over 5 min) and were blocked by the estrogen receptor antagonist ICI 182780. Similarly, inhibitors of phosphoinositide 3-kinase (wortmannin and LY294002) and of protein kinase C (staurosporine and Gö 6976) blocked 17beta-estradiol-induced LPA(1) receptor desensitization and phosphorylation. Confocal microscopy evidenced LPA(1) receptor internalization in response to 17beta-estradiol treatment. Association between LPA(1) receptors and protein kinase C alpha was suggested by co-immunoprecipitation assays. Protein kinase C alpha was associated with LPA(1) receptors in the absence of stimulus and such association further increased in a dynamic fashion in response to 17beta-estradiol. The results demonstrated that in C9 cells estrogens modulate LPA(1) action through estrogen receptor alpha with the participation of protein kinase C alpha and phosphoinositide 3-kinase.

Okadaic acid increases the phosphorylation state of alpha1A-adrenoceptors and induces receptor desensitization

Okadaic acid, a protein phosphatase inhibitor, and phorbol myristate acetate, an activator of protein kinase C, increased the phosphorylation state of alpha1A-adrenergic receptors. The effects of these agents were of similar magnitude but that of okadaic acid developed more slowly. Wortmannin (inhibitor of phosphoinositide 3-kinase), but not staurosporine (inhibitor of protein kinase C), abolished the effect of okadaic acid on the alpha1A-adrenoceptor phosphorylation state. The effect of phorbol myristate acetate on this parameter was blocked by staurosporine and only partially inhibited by wortmannin. Okadaic acid markedly increased the co-immunoprecipitation of both the catalytic and regulatory subunits of phosphatidylinositol 3-kinase and of Akt/protein kinase B with the adrenoceptor and only marginally increases receptor association with protein kinase C epsilon. Okadaic acid induced desensitization of alpha1A-adrenoceptors as evidenced by a decreased ability of noradrenaline to increase intracellular calcium. Such desensitization was fully reverted by wortmannin. Our data indicate that inhibition of serine/threonine protein phosphatases increases the phosphorylation state of alpha1A-adrenergic receptor and alters the adrenoceptor function.

Phosphorylation and desensitization of the lysophosphatidic acid receptor LPA1

In C9 cells, LPA (lysophosphatidic acid) induced inositol phosphate production, increased intracellular calcium concentration and inhibited adenylate cyclase activity. These responses were abolished in cells challenged with active phorbol esters. Action of phorbol esters was blocked by inhibitors of PKC (protein kinase C) and by its down-regulation. LPA1 receptor phosphorylation was observed in response to phorbol esters. The effect was rapid (t1/2 approximately 1 min), intense (2-fold) and sustained (at least 60 min). PKC inhibitors markedly decreased the LPA1 receptor phosphorylation induced by phorbol esters. LPA1 receptor tagged with the green fluorescent protein internalized in response to PKC activation. In addition, LPA and angiotensin II were also capable of inducing LPA1 receptor phosphorylation, showing that LPA1 receptor can be subjected to homologous and heterologous desensitization.

The elusive alpha(1D)-adrenoceptor: molecular and cellular characteristics and integrative roles

alpha(1)-Adrenoceptors seem to play key roles in cardiovascular, genitourinary, and central nervous system functions. This review will be focused on alpha(1D)-adrenoceptors. These receptors have intrinsic activity, and many of the more commonly used antagonists are in reality inverse agonists. alpha(1D)-Adrenoceptors are phosphorylated in the basal state, and the natural agonists, adrenaline and noradrenaline, increase their phosphorylation; similar effects are induced by direct activation of protein kinase C and through activation of nonadrenergic receptors. Interestingly, a large proportion of alpha(1D)-adrenoceptors are located in intracellular vesicles. Such intracellular location can be changed to surface expression through the use of inverse agonists and coexpression of alpha(1B)-adrenoceptors, which seem to act as pharmacological chaperons for proper plasma membrane insertion. The alpha(1D)-adrenoceptor amino terminus seems to contain a signal that keeps the receptor intracellularly, but interaction with other proteins may also contribute. The precise relationship between the intrinsic activity, phosphorylation, and intracellular location is currently unknown. alpha(1D)-Adrenoceptor activation induces contraction in a variety of vessels, and a role in the control of blood pressure has been suggested. Studies using young prehypertensive and adult spontaneously hypertensive rats as well as knockout mice suggest that vascular alpha(1D)-adrenoceptors are involved in the genesis/maintenance of hypertension.

Inhibitors of serine/threonine protein phosphatases antagonize the antinociception induced by agonists of alpha 2 adrenoceptors and GABAB but not kappa-opioid receptors in the tail flick test in mice

We previously reported that serine/threonine protein phosphatases (PPs) play a role in the antinociception induced by the mu-opioid receptor agonist morphine. In this study we evaluated the possible involvement of PPs on the antinociception induced by agonists of others G protein-coupled receptors in the tail flick test in mice. The subcutaneous administration of clonidine (0.25-4 mg/kg), baclofen (2-32 mg/kg) or U50,488H (2-16 mg/kg) (agonists of alpha(2) adrenoceptors, GABA(B) and kappa-opioid receptors, respectively) produced dose-dependent antinociception. The antinociceptive effects of clonidine and baclofen were antagonized in a dose-dependent way by the protein phosphatase inhibitors okadaic acid (0.001-10 pg/mouse, i.c.v.) and cantharidin (0.001-10 ng/mouse, i.c.v.), and okadaic acid was 1000 times more potent than cantharidin in producing this effect. The effects of these drugs appear to be specifically due to the blockade of PPs, since L-norokadaone (an analogue of okadaic acid that has no effect on PPs) did not modify clonidine- or baclofen-induced antinociception over the wide range of doses used (0.001-1000 pg/mouse, i.c.v.). On the other hand, the antinociception induced by activation of kappa-opioid receptors with U50,488H was not modified by okadaic acid or cantharidin. In conclusion, our data support the idea that serine/threonine PPs are differentially involved in the antinociceptive effects of several agonists of G protein-coupled receptors in mice.

Role of Ca2+ feedback on single cell inositol 1,4,5-trisphosphate oscillations mediated by G-protein-coupled receptors

The dynamics of inositol 1,4,5-trisphosphate (Ins (1,4,5)P3) production during periods of G-protein-coupled receptor-mediated Ca2+ oscillations have been investigated using the pleckstrin homology (PH) domain of phospholipase C (PLC) delta1 tagged with enhanced green fluorescent protein (eGFP-PHPLCdelta1). Activation of noradrenergic alpha1B and muscarinic M3 receptors recombinantly expressed in the same Chinese hamster ovary cell indicates that Ca2+ responses to these G-protein-coupled receptors are stimulus strength-dependent. Thus, activation of alpha1B receptors produced transient base-line Ca2+ oscillations, sinusoidal Ca2+ oscillations, and then a steady-state plateau level of Ca2+ as the level of agonist stimulation increased. Activation of M3 receptors, which have a higher coupling efficiency than alpha1B receptors, produced a sustained increase in intracellular Ca2+ even at low levels of agonist stimulation. Confocal imaging of eGFP-PHPLCdelta1 visualized periodic increases in Ins(1,4,5)P3 production underlying the base-line Ca2+ oscillations. Ins(1,4,5)P3 oscillations were blocked by thapsigargin but not by protein kinase C down-regulation. The net effect of increasing intracellular Ca2+ was stimulatory to Ins(1,4,5)P3 production, and dual imaging experiments indicated that receptor-mediated Ins(1,4,5)P3 production was sensitive to changes in intracellular Ca2+ between basal and approximately 200 nM. Together, these data suggest that alpha1B receptor-mediated Ins(1,4,5)P3 oscillations result from a positive feedback effect of Ca2+ onto phospholipase C. The mechanisms underlying alpha1B receptor-mediated Ca2+ responses are therefore different from those for the metabotropic glutamate receptor 5a, where Ins(1,4,5)P3 oscillations are the primary driving force for oscillatory Ca2+ responses (Nash, M. S., Young, K. W., Challiss, R. A. J., and Nahorski, S. R. (2001) Nature 413, 381-382). For alpha1B receptors the Ca2+-dependent Ins(1,4,5)P3 production may serve to augment the existing regenerative Ca2+-induced Ca2+-release process; however, the sensitivity to Ca2+ feedback is such that only transient base-line Ca2+ spikes may be capable of causing Ins(1,4,5)P3 oscillations.

Regulation of alpha(1)-adrenoceptor-linked phosphoinositide metabolism in cultured glia: involvement of protein phosphatases and kinases

Noradrenaline-stimulated phosphoinositide breakdown in cultured glia was found to be mediated by alpha(1A)-adrenoceptors. The alpha(1A)-selective agonist A61603 was as effective as noradrenaline in eliciting 3H-inositol phosphate (IP) accumulation but was approximately 50-fold more potent. In addition, the use of selective antagonists revealed a clear rank order of potency in the ability of these drugs to reverse the effect of noradrenaline on phosphoinositide breakdown: RS17053 (alpha(1A)-selective) >>AH11110A (alpha(1B)-selective)>BMY7378 (alpha(1D)-selective). Pre-treatment of cultured glia with the protein phosphatase inhibitor okadaic acid resulted in a concentration- and time-dependent reduction in noradrenaline-evoked 3H-IP accumulation. This effect was mimicked by, but was not additive with, a phorbol ester, was reversed by protein kinase C (PKC) inhibitors and was not evident in cells which had been PKC depleted. The ability of cell extracts to dephosphorylate radiolabelled glycogen phosphorylase revealed the presence of the phosphatases PP1 and PP2A in almost equal abundance. Okadaic acid pre-treatment of intact cultures elicited a marked reduction in total phosphatase activity, particularly that mediated by PP2A. We also determined the effect of okadaic acid pre-treatment on PKC and cyclic AMP-dependent protein kinase (PKA) activities in these cells. PKC and PKA activities in cell extracts were assessed by determining the incorporation of 32P into histone and kemptide, respectively. Okadaic acid elicited increases in both Ca(2+)-dependent and Ca(2+)-independent PKC activity; in addition, increases in both initial and total PKA activities were also recorded. The effect of okadaic acid on noradrenaline-stimulated 3H-IP accumulation were not, however, mimicked by either forskolin or 8-bromo-cyclic AMP, suggesting that this event is not regulated by PKA. Our data point to roles for both PKC and PP2A in the regulation of alpha(1A)-adrenoceptor-linked phosphoinositide metabolism in cultured cortical glia.

alpha1B-Adrenergic receptor phosphorylation and desensitization induced by transforming growth factor-beta

Transforming growth factor-beta (TGF-beta) induced alpha(1B)-adrenergic receptor phosphorylation in Rat-1 fibroblasts stably expressing these adrenoceptors. This effect of TGF-beta was rapid, reaching a maximum within 30 min and decreasing thereafter, and concentration-dependent (EC(50) 0.3 pM). The phosphoinositide 3-kinase inhibitors wortmannin and LY294002, and the protein kinase C inhibitors staurosporine, Ro 318220 and bisindolylmaleimide, blocked the effect of this growth factor. alpha(1B)-Adrenergic receptor phosphorylation was associated with desensitization, as indicated by a reduction in the adrenergic-mediated production of [(3)H]inositol phosphates. Phosphorylation of alpha(1B)-adrenergic receptors by TGF-beta was also observed in Cos-1 cells transfected with the receptor. Co-transfection of the dominant-negative mutant of the regulatory subunit of phosphoinositide 3-kinase (Deltap85) inhibited the phosphorylation of alpha(1B)-adrenergic receptors induced by TGF-beta. Our results indicate that activation of TGF-beta receptors induces alpha(1B)-adrenergic receptor phosphorylation and desensitization. The data suggest that phosphoinositide 3-kinase and protein kinase C play key roles in this effect of TGF-beta.

Receptor phosphorylation mediates estradiol reduction of alpha2-adrenoceptor coupling to G protein in the hypothalamus of female rats

Estrogen increases evoked norepinephrine release in the hypothalamus of female rodents, in part by reducing the ability of alpha2-adrenoceptors to act as negative feed-back inhibitors of norepinephrine release. Estrogen enhancement of norepinephrine release in the hypothalamus correlates with decreased coupling of the alpha2-adrenoceptor to G protein. To determine the mechanism by which estrogen uncouples alpha2-adrenoceptors from G protein, we tested the hypothesis that estrogen increases alpha2-adrenoceptor phosphorylation. Short-term activation of endogenous serine/threonine phosphatases with protamine or treatment with exogenous phosphatase restored alpha2-adrenoceptor coupling to G protein to control levels in hypothalami from estrogen-exposed female rats. Additional experiments examined whether estrogen alters G protein-coupled receptor kinase expression or activity or serine/threonine phosphatase activity. These proteins are involved in G protein-coupled receptor phosphorylation, internalization, and recycling. Estrogen exposure reduced G protein-coupled receptor kinase mRNA, protein, and activity in the hypothalamus. Furthermore, estrogen treatment reduced serine/threonine phosphatase activity in the hypothalamus. Analysis of ligand binding in subcellular fractions demonstrated that estrogen decreases the fraction of internalized alpha2-adrenoceptors in the hypothalamus.Therefore, estrogen promotes norepinephrine release in the hypothalamus by stabilizing alpha2-adrenoceptor phosphorylation, uncoupling the receptor from G protein. Estrogen may stabilize alpha2-adrenoceptor phosphorylation by inhibiting receptor internalization and dephosphorylation.

Evidence that oestradiol attenuates beta-adrenoceptor function in the hypothalamus of female rats by altering receptor phosphorylation and sequestration

Activation of beta-adrenoceptors in the hypothalamus (HYP) and preoptic area (POA) inhibits both gonadotropin release and reproductive behaviour in female rats. Exposure of female rats for 48 h to physiologically relevant doses of oestrogen attenuates beta-adrenoceptor function in the HYP and POA as indicated by reduced isoproterenol (beta-adrenoceptor agonist) stimulation of adenylyl cyclase activity. Reduced beta-adrenoceptor coupling to G protein in the HYP-POA from oestrogen-exposed female rats correlates with attenuation of beta-adrenoceptor function. To examine potential mechanisms underlying receptor-G protein uncoupling, initial experiments tested the hypothesis that oestrogen attenuation of beta-adrenoceptor function in the HYP and POA involves receptor phosphorylation. Activation of endogenous serine/threonine phosphatases with protamine restores agonist-stimulated cAMP accumulation in HYP slices from oestrogen-exposed female rats to control levels. Additional experiments examined whether oestrogen-induced changes in beta-adrenoceptor binding density and/or subcellular localization correlate with the attenuation of beta-adrenoceptor function in the HYP and POA. Oestrogen treatment does not alter total beta-adrenoceptor binding density in the HYP or POA. However, oestrogen significantly reduces cell surface binding of the hydrophilic beta-adrenoceptor antagonist [3H] CGP 12177 to intact HYP and POA slices. At the same time, oestrogen decreases the fraction of beta-adrenoceptors localized in a light vesicle fraction following sucrose density gradient centrifugation. Therefore, oestrogen attenuates beta-adrenoceptor signalling in the HYP-POA by uncoupling the beta-adrenoceptor from G protein, perhaps by promoting receptor phosphorylation. Furthermore, a significant fraction of beta-adrenoceptors in the HYP and POA are no longer accessible to hydrophilic ligands, but are not internalized. Thus, physiological doses of oestrogen may facilitate reproductive behaviour and gonadotropin release, in part, by stabilizing beta-adrenoceptor phosphorylation in the HYP and POA, thereby uncoupling the receptors from G protein.

Alpha 1-adrenoceptors: function and phosphorylation

This review focuses on alpha(1)-adrenoceptor phosphorylation and function. Most of what is currently known is based on studies on the hamster alpha(1B)-adrenoceptor. It is known that agonist stimulation leads to homologous desensitization of these receptors and current evidence indicates that such decrease in receptor activity is associated with receptor phosphorylation. Such receptor phosphorylation seems to involve G protein-receptor kinases and the receptor phosphorylation sites have been located in the carboxyl tail (Ser(404), Ser(408), and Ser(410)). There is also evidence showing that in addition to desensitization, receptor phosphorylation is associated with internalization and roles of beta-arrestins have been observed. Direct activation of protein kinase C leads to receptor desensitization/internalization associated with phosphorylation; the protein-kinase-C-catalyzed receptor phosphorylation sites have been also located in the carboxyl tail (Ser(394) and Ser(400)). Activation of G(q)-coupled receptors, such as the endothelin ET(A) receptor induces alpha(1B)-adrenoceptor phosphorylation and desensitization. Such effect involves protein kinase C and a yet unidentified tyrosine kinase. Activation of G(i)-coupled receptors, such as the lysophosphatidic acid receptor, also induces alpha(1B)-adrenoceptor phosphorylation and desensitization. These effects involve protein kinase C and phosphatidyl inositol 3-kinase. Interestingly, activation of epidermal growth factor receptors also induces alpha(1B)-adrenoceptor phosphorylation and desensitization involving protein kinase C and phosphatidyl inositol 3-kinase. A pivotal role of these kinases in heterologous desensitization is evidenced.