Homologous regulation of the alpha2C-adrenoceptor subtype in human hepatocarcinoma, HepG2

Wip1 contributes to the adaptation of HepG2 human liver cancer cells to stress hormone-induced DNA damage

Numerous studies have shown that the release of stress hormones resulting from repeated exposure to chronic psychological stress increases DNA damage and promotes tumorigenesis. However, the mechanisms that enable cancerous cells adapt to stress hormone-induced DNA damage and survive remain unclear. The present study aimed to investigate the impact of stress hormones on the survival of liver cancer cells and the underlying mechanism. HepG2 human liver cancer cells were treated with dexamethasone (DEX), epinephrine (EPI) and norepinephrine (NE) and subjected to the testing of DNA damage, cell survival and cell apoptosis by alkaline comet assay, CCK-8 viability assay and flow cytometry, respectively. The protein expression levels of DNA damage response factors were determined by western blotting analysis. The results revealed that treatment of HepG2 cells with DEX, EPI and NE induced DNA damage without affecting cell survival or inducing apoptosis. The protein levels of wild-type p53-induced phosphatase 1 (Wip1), a type 2C family serine/threonine phosphatase, were increased, and the dephosphorylation of DNA damage response factors, including phosphorylated (p-)ataxia-telangiectasia mutated and p-checkpoint kinase 2, occurred following treatment with DEX, EPI and NE. In addition, a cycloheximide chase assay was performed to explore the protein stability under treatment with stress hormones. Compared with vehicle-treated cells, Wip1 exhibited increased protein stability in stress hormone-treated HepG2 cells. Eventually, the depletion of Wip1 using small interfering RNA verified the role of Wip1 in the modulation of stress hormone-induced DNA damage. These findings suggest that cancerous cells likely adapt to stress hormone-induced DNA damage via Wip1 upregulation. The present study provides an insight into the underlying mechanism that links chronic psychological stress with tumor growth and progression.

A Systematic Review of Inverse Agonism at Adrenoceptor Subtypes

As many, if not most, ligands at G protein-coupled receptor antagonists are inverse agonists, we systematically reviewed inverse agonism at the nine adrenoceptor subtypes. Except for β3-adrenoceptors, inverse agonism has been reported for each of the adrenoceptor subtypes, most often for β2-adrenoceptors, including endogenously expressed receptors in human tissues. As with other receptors, the detection and degree of inverse agonism depend on the cells and tissues under investigation, i.e., they are greatest when the model has a high intrinsic tone/constitutive activity for the response being studied. Accordingly, they may differ between parts of a tissue, for instance, atria vs. ventricles of the heart, and within a cell type, between cellular responses. The basal tone of endogenously expressed receptors is often low, leading to less consistent detection and a lesser extent of observed inverse agonism. Extent inverse agonism depends on specific molecular properties of a compound, but inverse agonism appears to be more common in certain chemical classes. While inverse agonism is a fascinating facet in attempts to mechanistically understand observed drug effects, we are skeptical whether an a priori definition of the extent of inverse agonism in the target product profile of a developmental candidate is a meaningful option in drug discovery and development.

Adrenal adrenoceptors in heart failure: fine-tuning cardiac stimulation

Chronic heart failure (HF) is characterized by sympathetic hyperactivity reflected by increased circulating catecholamines (CAs), which contributes significantly to its morbidity and mortality. Therefore, sympatholytic treatments, that is, treatments that reduce sympathetic hyperactivity, are being pursued currently for the treatment of HF. Secretion of CAs from the adrenal gland, which is a major source of CAs, is regulated by alpha(2)-adrenoceptors (alpha(2)ARs), which inhibit, and by beta-adrenoceptors (betaARs), which enhance CA secretion. All ARs are G-protein-coupled receptors (GPCRs), whose signaling and function are regulated tightly by the family of GPCR kinases (GRKs). Despite the enormous potential of adrenal ARs for the regulation of sympathetic outflow, elucidation of their properties has only begun recently. Here, recent advances regarding the roles of adrenal ARs in the regulation of sympathetic outflow in HF and the regulatory properties of ARs are discussed, along with the potential benefits and challenges of harnessing their function for HF therapy.

Suppression of catechol-O-methyltransferase activity through blunting of alpha2-adrenoceptor can explain hypertension in Dahl salt-sensitive rats

Catecholamines have been reported to be involved in the development of salt-sensitive hypertension. We investigated the relation between catechol-O-methyltransferase (COMT) and salt-sensitivity. In the first experiment, Dahl salt-sensitive (DS) rats were given a normal-salt (NS), high-salt (HS), or HS+hydralazine (80 mg/l water) diet for 4 or 13 weeks, and Dahl salt-resistant (DR) rats were given a NS or HS diet. COMT activities in both the kidneys and liver and urinary norepinephrine (NE) and dopamine (DA) excretion were measured. In the second experiment, HepG2 cells were used to investigate the role of NE in regulating COMT activity. In the third experiment, we investigated the reactivity of pre- and postsynaptic alpha(2)-adrenoceptor (AR) in DS rats. HS loading significantly suppressed the activities of membrane-bound COMT (MB-COMT) and, consistent with this finding, increased the urinary NE level in DS rats, but not in DR rats. Hydralazine did not restore the MB-COMT activities, which suggested that HS loading rather than elevated blood pressure suppressed the MB-COMT activities. The in vitro experiment using HepG2 cells revealed that NE increased the MB-COMT activity via the alpha(2)-AR. However, both the pre- and postsynaptic alpha(2)-AR reactivity was decreased by HS loading in DS rats. In conclusion, HS intake suppresses the MB-COMT activities in DS rats, presumably by blunting alpha(2)-AR signaling. The suppression of MB-COMT activities, consequent decrease in degradation of NE, and increase in NE release by blunting of alpha(2)-AR function may be involved in the development of salt-sensitive hypertension in DS rats, in whom DA-dependent natriuresis may be suppressed.

Involvement of G protein-coupled receptor kinase (GRK) 3 and GRK2 in down-regulation of the alpha2B-adrenoceptor

Increasing the cellular levels of G protein-coupled receptor kinase (GRK) 2 or GRK3 renders the alpha2B-adrenoceptor (AR) more sensitive to agonist-induced down-regulation (J Pharmacol Exp Ther 312:767-773, 2005). However, an absolute requirement of GRK3 and GRK2 for alpha2B-AR down-regulation is controversial. In this study, using NG108 cells (endogenous alpha2B-AR), we provide strong evidence for a critical role of both GRK3 and GRK2 in down-regulation of the alpha2B-AR. Pretreatment of NG108 cells with 20 microM epinephrine (EPI) begins down-regulating the alpha2B-AR by 2 h. The translocation of GRK3 and GRK2 to the membrane peaks at 30 min, decreasing by 1 h. Although these results may implicate GRK3 and GRK2 in alpha2B-AR down-regulation, significant receptor down-regulation is not observed until 2 h, after GRK3 and GRK2 translocation has peaked and is declining. To more directly establish a role for GRK3 and GRK2 in alpha2B-AR down-regulation, NG108 cells were transfected to express GRK3ct, which binds to liberated Gbetagamma subunits, preventing GRK3 and GRK2 translocation to the membrane. Overexpression of GRK3ct prevented not only the translocation of GRK3 and GRK2 but also the down-regulation of the alpha2B-AR caused by 24-h pretreatment with 20 microM EPI. Taken together, these data provide direct evidence for a role of GRK3 and GRK2 in the down-regulation of the alpha2B-AR and contribute significantly to the increasing evidence in the literature for a pivotal role of GRKs in modulating the agonist-induced down-regulation of the alpha2-AR.

Cellular G protein-coupled receptor kinase levels regulate sensitivity of the {alpha}2b-adrenergic receptor to undergo agonist-induced down-regulation

Chronic coactivation of alpha(2B)- and beta(2)-adrenoceptors (AR) was recently reported to down-regulate the alpha(2B)-AR at a lower threshold epinephrine (EPI) concentration compared with the activation of alpha(2B)-AR alone. This is the result of a modest beta(2)-AR-dependent up-regulation of G protein-coupled receptor kinase 3 (GRK3). In the present study, we determined that increasing GRK2 or GRK3 levels, independent of beta(2)-AR activation, decreases the EC(50) concentration for agonist-induced down-regulation of the alpha(2B)-AR using NG108 cells with or without overexpression (2- to 10-fold) of GRK2 or GRK3. In parental NG108 cells, the EC(50) concentration of EPI required for down-regulation of the alpha(2B)-AR is 30 microM. A 2- to 3-fold overexpression of GRK3 in NG108 cells, however, reduces the EC(50) to 0.2 microM (a 150-fold decrease), whereas a comparable overexpression of GRK2 reduces it to 1 microM (a 30-fold decrease). However, when GRK3 or GRK2 in NG108 cells are overexpressed 8- to 10-fold, the EC(50) concentration (0.02 microM EPI) for alpha(2B)-AR down-regulation is reduced 1000-fold. These data clearly suggest that a modest (2- to 3-fold) up-regulation of GRK3 is more effective at enhancing the sensitivity of alpha(2B)-AR to down-regulation after exposure to EPI than a modest up-regulation of GRK2, but that both GRK2 and GRK3 are equally effective at inducing alpha(2B)-AR down-regulation when up-regulated 8- to 10-fold. To our knowledge, this is the first report to systematically demonstrate that GRKs, particularly GRK3, play a pivotal role in modulating the agonist EC(50) concentration that down-regulates the alpha(2B)-AR and thus adds a new dimension to an already intricate signaling network.

Simultaneous alpha2B- and beta2-adrenoceptor activation sensitizes the alpha2B-adrenoceptor for agonist-induced down-regulation

We recently reported that alpha(2A)-adrenoceptor (AR) desensitization and down-regulation occurs after 24-h treatment with epinephrine (EPI) (0.3 microM) in BE(2)-C cells that express both alpha(2)- and beta(2)-ARs. The same concentration of norepinephrine (NE) has no effect. The effect of EPI is prevented by beta(2)-AR blockade and is associated with an increase in G protein-coupled receptor kinase 3 (GRK3) expression. Because differences in agonist-induced down-regulation of the alpha(2A)-versus alpha(2B)-ARs have been reported, the present study examines the effects of simultaneous activation of alpha(2B)- and beta(2)-ARs on alpha(2B)-AR number and signaling. We studied NG108 cells that naturally express alpha(2B)-ARs, and BN17 cells, NG108 cells transfected to express the human beta(2)-AR. In NG108 cells, alpha(2B)-AR desensitization and down-regulation require treatment with 20 microM EPI or NE; GRK expression was not changed. In BN17 cells expressing beta(2)-ARs, the threshold EPI concentration for alpha(2B)-AR desensitization and down-regulation was reduced to 0.3 microM; 10 microM NE was required for the same effect. Furthermore, 24-h EPI or NE treatments that produced desensitization also resulted in a selective 2-fold up-regulation of GRK3; GRK2 was unchanged. The beta-AR antagonist alprenolol (1 microM) and GRK3 antisense (but not sense) DNA blocked 0.3 microM EPI- and 10 microM NE-induced desensitization and down-regulation of the alpha(2B)-AR as well as GRK3 up-regulation. In conclusion, simultaneous activation of alpha(2B)- and beta(2)-ARs results in a 67-fold decrease in the threshold concentration of EPI required for alpha(2B)-AR down-regulation. This lower threshold for down-regulation is associated with alpha(2B)- and beta(2)-AR dependent up-regulation of GRK3 expression.

Efficacy as a Vector: the Relative Prevalence and Paucity of Inverse Agonism

This article describes the expected phenotypic behavior of all types of ligands in constitutively active receptor systems and, in particular, the molecular mechanisms of inverse agonism. The possible physiological relevance of inverse agonism also is discussed. Competitive antagonists with the molecular property of negative efficacy demonstrate inverse agonism in constitutively active receptor systems. This is a phenotypic behavior that can only be observed in the appropriate assay; a lack of observed inverse agonism is evidence that the ligand does not possess negative efficacy only if it can be shown that constitutive receptor activity is present. In the absence of constitutive activity, inverse agonists behave as simple competitive antagonists. A survey of 105 articles on the activity of 380 antagonists on 73 biological G-protein-coupled receptor targets indicates that, in this sample dataset, 322 are inverse agonists and 58 (15%) are neutral antagonists. The predominance of inverse agonism agrees with theoretical predictions which indicate that neutral antagonists are the minority species in pharmacological space.

Ligand modulation of [35S]GTPgammaS binding at human alpha(2A), alpha(2B) and alpha(2C) adrenoceptors

Affinities and efficacies of chemically diverse ligands--some of them used as clinical agents--were examined, employing [3H]RX821,002 and [35S]GTPgammaS binding assays, respectively, at human (h) cloned, halpha(2A), halpha(2B) and halpha(2C) adrenoceptors (AR) expressed in Chinese hamster ovary (CHO) cells. As compared to noradrenaline (NA, efficacy defined as 100%), the majority of the 13 agonists tested generally behaved as partial agonists. Amongst 18 antagonists, pK(B) and pK(i) values, which were highly correlated for each alpha(2)-AR subtype, failed to reveal any strikingly selective agents. Inverse agonist properties were not detected for any antagonist, consistent with a lack of constitutive activity suggested by the monophasic inhibition of [35S]GTPgammaS binding by GTPgammaS. These data should facilitate interpretation of experimental and clinical actions of adrenergic agonists. Moreover, they emphasize the continuing need for alpha(2)-AR subtype-selective antagonists in order to define further the roles and therapeutic relevance of halpha(2A)-, halpha(2B)-, and halpha(2C)-AR.

Transcriptional down-regulation of the human alpha2C-adrenergic receptor by cAMP

The heterologous regulation of the alpha2C-adrenergic receptor (alpha2C-AR) was investigated in the HepG2 cell line. Binding of [(3)H]MK912 (alpha2-antagonist) to membranes from cells submitted to various treatments showed that exposure to insulin, phorbol 12-myristate 13-acetate, or dexamethasone did not affect receptor density. On the other hand, treatment with forskolin resulted in a large reduction of alpha2C-AR number. The effect of forskolin was mimicked by 8-br-cAMP and was abolished by the protein kinase A inhibitor, H89. The action of cAMP was slow (t(1/2) = 23 h), dose-dependent, and additive to the receptor down-regulation elicited by the alpha2-agonist, UK14304. Furthermore, the diminution of receptor was not caused by an increased rate of its degradation but resulted from a decrease in the steady state amounts of alpha2C4-mRNA. As assessed by experiments in the presence of actinomycin D, the stability of alpha2C4-mRNA was not affected by 8-br-cAMP or forskolin. By contrast, the activity of a luciferase construct containing the entire promoter region of the alpha2C4 gene (1.9 kilobase pairs) was inhibited, indicating that the primary mechanism of action of the two compounds is at the transcriptional level. Deletions in the 5'-end of this construct showed that the elements responsible for cAMP responsiveness lie within a 242-base-pair fragment of the gene promoter (nucleotides -236/+6 relative to transcription start). Band-shift experiments indicated that nuclear factors bind to this region in a cAMP-dependent manner. The determination of the actual cis- and trans-acting elements involved will be the object of future investigation, but the present study provides evidence for transcriptional regulation of human alpha2C-AR by cAMP.

Inverse agonism at alpha(2)-adrenoceptors in native tissue

Several alpha(2)-adrenoceptor antagonists have inverse agonist properties in cell culture systems, usually expressing high levels or a constitutively active form of alpha(2)-adrenoceptors. In characterizing the binding of alpha(2)-adrenoceptor agonists to rat brain tissue sections, we found that conditions known to alter agonist affinity for these receptors, particularly the addition of 100 microM GTP, altered the binding of the alpha(2)-adrenoceptor antagonist, [3H](1,4-benzodioxan-2-methoxy-2-yl)-2-imidazoline hydrochloride (RX821002). In further studies, we found that under our conditions [3H]RX821002 demonstrates inverse agonist properties at alpha(2)-adrenoceptors. This is the first demonstration of inverse agonism at alpha(2)-adrenoceptors in native tissue. We found that the alpha(2)-adrenoceptor antagonist, (2S,12bS)1', 3'-dimethylspiro(1,3,4,5',6,6',7,12b-octahydro-2H-benzo(b)fu ro(2, 3-a)quinazoline)-2,4'-pyrimidin-2'-one (MK-912), did not have clearly discernible inverse agonist properties and acted as a neutral antagonist in these studies. On the other hand, the antagonist rauwolscine actually displayed partial agonist properties in our studies. These findings indicate that the inverse agonist properties of alpha(2)-adrenoceptor antagonists can be demonstrated in native tissue, as well as in tissue culture, and they strengthen the idea that inverse agonist properties may be of physiological and pharmacological importance.

Silent alpha(2C)-adrenergic receptors enable cold-induced vasoconstriction in cutaneous arteries

Cold constricts cutaneous blood vessels by increasing the reactivity of smooth muscle alpha(2)-adrenergic receptors (alpha(2)-ARs). Experiments were performed to determine the role of alpha(2)-AR subtypes (alpha(2A)-, alpha(2B)-, alpha(2C)-ARs) in this response. Stimulation of alpha(1)-ARs by phenylephrine or alpha(2)-ARs by UK-14,304 caused constriction of isolated mouse tail arteries mounted in a pressurized myograph system. Compared with proximal arteries, distal arteries were more responsive to alpha(2)-AR activation but less responsive to activation of alpha(1)-ARs. Cold augmented constriction to alpha(2)-AR activation in distal arteries but did not affect the response to alpha(1)-AR stimulation or the level of myogenic tone. Western blot analysis demonstrated expression of alpha(2A)- and alpha(2C)-ARs in tail arteries: expression of alpha(2C)-ARs decreased in distal compared with proximal arteries, whereas expression of the glycosylated form of the alpha(2A)-AR increased in distal arteries. At 37 degrees C, alpha(2)-AR-induced vasoconstriction in distal arteries was inhibited by selective blockade of alpha(2A)-ARs (BRL-44408) but not by selective inhibition of alpha(2B)-ARs (ARC-239) or alpha(2C)-ARs (MK-912). In contrast, during cold exposure (28 degrees C), the augmented response to UK-14,304 was inhibited by the alpha(2C)-AR antagonist MK-912, which selectively abolished cold-induced amplification of the response. These experiments indicate that cold-induced amplification of alpha(2)-ARs is mediated by alpha(2C)-ARs that are normally silent in these cutaneous arteries. Blockade of alpha(2C)-ARs may prove an effective treatment for Raynaud's Phenomenon.