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

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.

Mechanisms involved in α1B-adrenoceptor desensitization

α(1B)-Adrenergic receptors mediate many of the actions of the natural catecholamines, adrenaline and noradrenaline. They belong to the seven transmembrane domains G protein-coupled receptor superfamily and exert their actions mainly through activation of Gq proteins and phosphoinositide turnover/calcium signaling. Many hormones and neurotransmitters are capable of inducing α(1B)-adrenergic receptor phosphorylation and desensitization; among them: adrenaline and noradrenaline, phorbol esters, endothelin-I, bradykinin, lysophosphatidic acid, insulin, EGF, PDGF, IGF-I, TGF-β, and estrogens. Key protein kinases for these effects are G protein coupled receptor kinases and protein kinase C. The lipid/protein kinase, phosphoinositide-3 kinase also appears to play a key role, acting upstream of protein kinase C. In addition to the agents employed for cells stimulation, we observed that paracrine/autocrine mediators also participate; these processes include EGF transactivation and sphingosine-1-phosphate production and action. The complex regulation of these receptors unlocks opportunities for therapeutic intervention.

Polyphosphates from Mycobacterium bovis--potent inhibitors of class III adenylate cyclases

cAMP generation in bacteria is often stimulated by sudden, but lasting, changes in extracellular conditions, whereas intracellular cAMP concentrations quickly settle at new levels. As bacteria lack G-proteins, it is unknown how bacterial adenylate cyclase (AC) activities are modulated. Mycobacterium tuberculosis has 15 class III AC genes; therefore, we examined whether mycobacteria contain a factor that may regulate AC activities. We identified mycobacterial polyphosphates with a mean chain length of 72 residues as highly potent inhibitors of dimeric class IIIa, class IIIb and class IIIc ACs from M. tuberculosis and other bacteria. The identity of the inhibitor was established by phosphatase degradation, 31P-NMR, acid or base hydrolysis, PAGE and comparisons with commercial standards, and functional substitution by several polyphosphates. The data indicate that each AC dimer occupies 8-15 phosphate residues on a polyphosphate strand. Other polyionic polymers such as polyglutamate, polylysine and hyaluronic acid do not affect cyclase activity. Notably, the structurally unrelated class I AC Cya from Escherichia coli is unaffected. Bacterial polyphosphate metabolism is generally viewed in the context of stress-related regulatory networks. Thus, regulation of bacterial class III ACs by polyphosphates could be a component of the bacterial stress response.

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.

Roles of c-Src in alpha1B-adrenoceptor phosphorylation and desensitization

1 The role of the protein tyrosine kinase, c-Src, on the function and phosphorylation of alpha1B-adrenoceptors (alpha1B-AR) and their association with G-protein-coupled receptor kinase (GRK) isozymes was studied. 2 Inhibitors of this kinase (PP2 and Src Inhibitor II) decreased ( approximately 50-75%) noradrenaline- (NA) and phorbol myristate acetate-mediated receptor phosphorylation. Expression of a dominant-negative mutant of c-Src similarly reduced receptor phosphorylation induced by the natural agonists, active phorbol esters and endothelin-1 (ET-1). 3 c-Src, GRK2, GRK3 and GRK5 coimmunoprecipitate with alpha1B-ARs in the basal state. In cells treated with NA or phorbol myristate acetate the amount of coimmunoprecipitated GRK2 and GRK3 increased ( approximately 2- to 3-fold), while treatment with ET-1 only augmented the amount of coimmunoprecipitated GRK2 ( approximately 2-fold). The Src inhibitor, PP2, markedly attenuated all these increases. 4 Cell pretreatment with PP2 amplified the increase in intracellular-free calcium observed with NA, in the basal state and after the stimulation (desensitization) induced by ET-1. 5 The data suggest a role of c-Src in alpha1B-AR desensitization/phosphorylation and in the interaction of these ARs with GRKs.

Different kinases desensitize the human delta-opioid receptor (hDOP-R) in the neuroblastoma cell line SK-N-BE upon peptidic and alkaloid agonists

In a previous work, we described a differential desensitization of the human delta-opioid receptor (hDOP-R) by etorphine (a non-selective and alkaloid agonist) and delta-selective and peptidic agonists (DPDPE ([D-Pen(2,5)]enkephalin) and deltorphin I (Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH(2))) in the neuroblastoma cell line SK-N-BE (Allouche et al., Eur. J. Pharmacol., 371, 235, 1999). In the present study, we explored the putative role of different kinases in this differential regulation. First, selective chemical inhibitors of PKA, PKC and tyrosine kinases were used and we showed a significant reduction of etorphine-induced opioid receptor desensitization by the bisindolylmaleimide I (PKC inhibitor) while genistein (tyrosine kinase inhibitor) was potent to impair desensitization induced by the different agonists. When the PKA was inhibited by H89 pretreatment, no modification of opioid receptor desensitization was observed whatever the agonist used. Second, we further studied the role of G protein-coupled receptor kinases (GRKs) and by using western-blot experiments we observed that only the GRK2 isoform was expressed in the SK-N-BE cells. Next, the neuroblastoma cells were transfected with the wild type GRK2 or its dominant negative mutant GRK2-K220R and the inhibition on cAMP level was determined in naïve and agonist-pretreated cells. We showed that over-expression of GRK2-K220R totally abolished etorphine-induced receptor desensitization while no effect was observed with peptidic agonists and over-expression of GRK2 selectively impaired cAMP inhibition promoted by etorphine suggesting that this kinase was involved in the regulation of hDOP-R activated only by etorphine. Third, correlation between functional experiments and phosphorylation of the hDOP-R after agonist activation was assessed by western-blot using the specific anti-phospho-DOP-R Ser(363) antibody. While all agonists were potent to increase phosphorylation of opioid receptor, we showed no impairment of receptor phosphorylation level after PKC inhibitor pretreatment. Upon agonist activation, no enhancement of receptor phosphorylation was observed when the GRK2 was over-expressed while the GRK2-K220R partially reduced the hDOP-R Ser(363) phosphorylation only after peptidic agonists pretreatment. In conclusion, hDOP-R desensitization upon etorphine exposure relies on the GRK2, PKC and tyrosine kinases while DPDPE and deltorphin I mediate desensitization at least via tyrosine kinases. Although the Ser(363) was described as the primary phosphorylation site of the mouse DOP-R, we observed no correlation between desensitization and phosphorylation of this amino acid.

Role of epidermal growth factor receptor transactivation in alpha1B-adrenoceptor phosphorylation

Phosphorylation of G protein-coupled receptors is one of the earliest events that regulate their function. Current evidence indicates that homologous desensitization of these receptors mainly involves G protein-coupled receptor kinases whereas in heterologous desensitization second messenger-activated kinases play key roles. Recent data show that transactivation of EGF (epidermal growth factor) receptors may also play a role in receptor phosphorylation. The role of this process was studied for the alpha1B-adrenoceptor phosphorylation induced by agents acting through different processes using inhibitors to block the EGF receptor transactivation process at different levels. Experiments were performed using transfected rat-1 fibroblasts that express alpha1B-adrenoceptors in a stably fashion. A metalloproteinase inhibitor, an anti-heparin-binding-EGF-selective antibody, and a selective EGF-receptor kinase inhibitor blocked the alpha1B-adrenoceptor phosphorylation induced by noradrenaline or endothelin-1. Our results indicate that shedding of heparin-binding-EGF, transactivation of EGF receptors plays a more general role in alpha1B-adrenoceptor phosphorylation than previously anticipated. It is possible that other receptors/channels could be modulated through a similar pathway.

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.