Differential activation of mitogen-activated protein kinase pathways in PC12 cells by closely related alpha1-adrenergic receptor subtypes

Adrenoceptors: Receptors, Ligands and Their Clinical Uses, Molecular Pharmacology and Assays

The nine G protein-coupled adrenoceptor subtypes are where the endogenous catecholamines adrenaline and noradrenaline interact with cells. Since they are important therapeutic targets, over a century of effort has been put into developing drugs that modify their activity. This chapter provides an outline of how we have arrived at current knowledge of the receptors, their physiological roles and the methods used to develop ligands. Initial studies in vivo and in vitro with isolated organs and tissues progressed to cell-based techniques and the use of cloned adrenoceptor subtypes together with high-throughput assays that allow close examination of receptors and their signalling pathways. The crystal structures of many of the adrenoceptor subtypes have now been determined opening up new possibilities for drug development.

Dammarane Sapogenins Ameliorates Neurocognitive Functional Impairment Induced by Simulated Long-Duration Spaceflight

Increasing evidence indicates the occurrence of cognitive impairment in astronauts under spaceflight compound conditions, but the underlying mechanisms and countermeasures need to be explored. In this study, we found that learning and memory abilities were significantly reduced in rats under a simulated long-duration spaceflight environment (SLSE), which includes microgravity, isolation confinement, noises, and altered circadian rhythms. Dammarane sapogenins (DS), alkaline hydrolyzed products of ginsenosides, can enhance cognition function by regulating brain neurotransmitter levels and inhibiting SLSE-induced neuronal injury. Bioinformatics combined with experimental verification identified that the PI3K-Akt-mTOR pathway was inhibited and the MAPK pathway was activated during SLSE-induced cognition dysfunction, whereas DS substantially ameliorated the changes in brain. These findings defined the characteristics of SLSE-induced cognitive decline and the mechanisms by which DS improves it. The results provide an effective candidate for improving cognitive function in spaceflight missions.

Isoproterenol acts as a biased agonist of the alpha-1A-adrenoceptor that selectively activates the MAPK/ERK pathway

The α_1A-AR is thought to couple predominantly to the Gα_q/PLC pathway and lead to phosphoinositide hydrolysis and calcium mobilization, although certain agonists acting at this receptor have been reported to trigger activation of arachidonic acid formation and MAPK pathways. For several G protein-coupled receptors (GPCRs) agonists can manifest a bias for activation of particular effector signaling output, i.e. not all agonists of a given GPCR generate responses through utilization of the same signaling cascade(s). Previous work with Gα_q coupling-defective variants of α_1A-AR, as well as a combination of Ca²⁺ channel blockers, uncovered cross-talk between α_1A-AR and β₂-AR that leads to potentiation of a Gα_q-independent signaling cascade in response to α_1A-AR activation. We hypothesized that molecules exist that act as biased agonists to selectively activate this pathway. In this report, isoproterenol (Iso), typically viewed as β-AR-selective agonist, was examined with respect to activation of α_1A-AR. α_1A-AR selective antagonists were used to specifically block Iso evoked signaling in different cellular backgrounds and confirm its action at α_1A-AR. Iso induced signaling at α_1A-AR was further interrogated by probing steps along the Gα_q /PLC, Gα_s and MAPK/ERK pathways. In HEK-293/EBNA cells transiently transduced with α_1A-AR, and CHO_α_1A-AR stable cells, Iso evoked low potency ERK activity as well as Ca²⁺ mobilization that could be blocked by α_1A-AR selective antagonists. The kinetics of Iso induced Ca²⁺ transients differed from typical Gα_q- mediated Ca²⁺ mobilization, lacking both the fast IP₃R mediated response and the sustained phase of Ca²⁺ re-entry. Moreover, no inositol phosphate (IP) accumulation could be detected in either cell line after stimulation with Iso, but activation was accompanied by receptor internalization. Data are presented that indicate that Iso represents a novel type of α_1A-AR partial agonist with signaling bias toward MAPK/ERK signaling cascade that is likely independent of coupling to Gα_q.

Adrenaline rush: the role of adrenergic receptors in stimulant-induced behaviors

Psychostimulants, such as cocaine and amphetamines, act primarily through the monoamine neurotransmitters dopamine (DA), norepinephrine, and serotonin. Although stimulant addiction research has largely focused on DA, medication development efforts targeting the dopaminergic system have thus far been unsuccessful, leading to alternative strategies aimed at abating stimulant abuse. Noradrenergic compounds have shown promise in altering the behavioral effects of stimulants in rodents, nonhuman primates, and humans. In this review, we discuss the contribution of each adrenergic receptor (AR) subtype (α1, α2, and β) to five stimulant-induced behaviors relevant to addiction: locomotor activity, conditioned place preference, anxiety, discrimination, and self-administration. AR manipulation has diverse effects on these behaviors; each subtype profoundly influences outcomes in some paradigms but is inconsequential in others. The functional neuroanatomy and intracellular signaling mechanisms underlying the impact of AR activation/blockade on these behaviors remain largely unknown, presenting a new frontier for research on psychostimulant-AR interactions.

α1D-Adrenoceptors are responsible for the high sensitivity and the slow time-course of noradrenaline-mediated contraction in conductance arteries

The objective of this study was to determine whether the different time-course characteristics of α₁-adrenoceptor-mediated contraction in arteries can be related to the subtypes involved. Contractile responses to noradrenaline (NA) were compared with inositol phosphate accumulation and extracellular signal-regulated kinase (ERK)1/2 phosphorylation after α₁-agonist stimuli in the same vessels in the presence or absence of α₁-antagonists in rat or in α₁-subtype knockout (KO) mice. Aorta, where α_1D-AR is the main functional subtype, had higher sensitivity to NA (in respect of inositol phosphate [IP], pERK1/2, and contractile response) than tail artery, where the α_1A-adrenoceptor subtype is predominant. Furthermore, the contraction in aorta exhibited a slower decay after agonist removal and this was consistent in all strains harboring α_1D-adrenoceptors (from rat, α_1B-KO, and wild-type [WT] mice) but was not observed in the absence of the α_1D-adrenoceptor signal (α_1D-adrenoceptor blocked rat aorta or aorta from α_1D-KO). IP formation paralleled α₁-adrenoceptor-mediated contraction (agonist present or postagonist) in aorta and tail artery. High sensitivity to agonist and persistence of response after agonist removal is a property of α_1D-adrenoceptors. Therefore, the preponderance of this subtype in noninnervated conductance arteries such as aorta allows responsiveness to circulating catecholamines and prevents abrupt changes in vessel caliber when the stimulus fluctuates. Conversely, in innervated distributing arteries, high local concentrations of NA are required to activate α_1A-adrenoceptors for a response that is rapid but short lived allowing fine adjustment of the contractile tone by perivascular sympathetic nerves.

Stimulation of α1a adrenergic receptors induces cellular proliferation or antiproliferative hypertrophy dependent solely on agonist concentration

Stimulation of α_1aAdrenergic Receptors (ARs) is known to have anti-proliferative and hypertrophic effects; however, some studies also suggests this receptor can increase cell proliferation. Surprisingly, we find the α_1aAR expressed in rat-1 fibroblasts can produce either phenotype, depending exclusively on agonist concentration. Stimulation of the α_1aAR by high dose phenylephrine (>10⁻⁷ M) induces an antiproliferative, hypertrophic response accompanied by robust and extended p38 activation. Inhibition of p38 with SB203580 prevented the antiproliferative response, while inhibition of Erk or Jnk had no effect. In stark contrast, stimulation of the α_1aAR with low dose phenylephrine (∼10⁻⁸ M) induced an Erk-dependent increase in cellular proliferation. Agonist-induced Erk phosphorylation was preceded by rapid FGFR and EGFR transactivation; however, only EGFR inhibition blocked Erk activation and proliferation. The general matrix metalloprotease inhibitor, GM6001, blocked agonist induced Erk activation within seconds, strongly suggesting EGFR activation involved extracellular triple membrane pass signaling. Erk activation required little Ca²⁺ release and was blocked by PLCβ or PKC inhibition but not by intracellular Ca²⁺ chelation, suggesting Ca²⁺ independent activation of novel PKC isoforms. In contrast, Ca²⁺ release was essential for PI3K/Akt activation, which was acutely maximal at non-proliferative doses of agonist. Remarkably, our data suggests EGFR transactivation leading to Erk induced proliferation has the lowest activation threshold of any α_1aAR response. The ability of α_1aARs to induce proliferation are discussed in light of evidence suggesting antagonistic growth responses reflect native α_1aAR function.

Induction of neurite-outgrowth in PC12 cells by alpha-toxin from Clostridium perfringens

Alpha-toxin-induced phosphorylation of PDK1 via the tyrosine kinase A (TrkA) receptor signaling pathway plays an important role in the activation of rabbit neutrophils. The relation between the toxin and TrkA, however, remains poorly understood. Here, we show that the toxin-induced phosphorylation of TrkA is closely related to the induction of neurite-outgrowth in PC12 cells. The toxin induced neurite-outgrowth and phosphorylation of TrkA in the cells in a dose-dependent manner. K252a, a TrkA inhibitor, and shRNA for TrkA inhibited the toxin-induced neurite-outgrowth, and phosphorylation of TrkA and ERK1/2. PD98059, an inhibitor of the ERK1/2 cascade, inhibited phosphorylation of ERK1/2 and the neurite-outgrowth induced by alpha-toxin. The wild-type toxin induced the formation of diacylglycerol, and neurite-outgrowth, but H148G, a variant toxin which binds to cell membranes and has lost the enzymatic activity did not. We demonstrated that the phosphorylation of TrkA through the phospholipid metabolism induced by the toxin synergistically play a key role in neurite-outgrowth.

alpha(1)-Adrenergic receptor subtype function in fetal and adult cerebral arteries

In the developing fetus, cerebral artery (CA) contractility demonstrates significant functional differences from that of the adult. This may be a consequence of differential activities of alpha(1)-adrenergic receptor (alpha(1)-AR) subtypes. Thus we tested the hypothesis that maturational differences in adrenergic-mediated CA contractility are, in part, a consequence of differential expression and/or activities of alpha(1)-AR subtypes. In CA from fetal ( approximately 140 days) and nonpregnant adult sheep, we used wire myography and imaging, with simultaneous measurement of tension and intracellular Ca(2+) concentration ([Ca(2+)](i)), radioimmunoassay, and Western immunoblots to examine phenylephrine (Phe)-induced contractile responses. The alpha(1A)-AR antagonists (5-MU and WB-4101) completely inhibited Phe-induced contraction in adult but not fetal CA; however, [Ca(2+)](i) increase was reduced significantly in both age groups. The alpha(1D)-AR antagonist (BMY-7378) blocked both Phe-induced contractions and Ca(2+) responses to a significantly greater extent in adult compared with fetal CA. In both age groups, inhibition of alpha(1A)-AR and alpha(1B)-AR, but not alpha(1D)-AR, significantly reduced inositol 1,4,5-trisphosphate responses to Phe. Western immunoblots demonstrated that the alpha(1)-AR subtype expression was only approximately 20% in fetal CA compared with the adult. Moreover, in fetal CA, the alpha(1D)-AR was expressed significantly greater than the other two subtypes. Also, in fetal but not adult CA, Phe induced a significant increase in activated ERK1/2; this increase in phosphorylated ERK was blocked by alpha(1B)-AR (CEC) and alpha(1D)-AR (BMY-7378) inhibitors, but not by alpha(1A)-AR inhibitors (5-MU or WB-4101). In conclusion, in the fetal CA, alpha(1B)-AR and alpha(1D)-AR subtypes play a key role in contractile response as well as in ERK activation. We speculate that in fetal CA alpha(1B)-AR and alpha(1D)-AR subtypes may be a critical factor associated with cerebrovascular growth and function.

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.

Haloperidol regulates the phosphorylation level of the MEK-ERK-p90RSK signal pathway via protein phosphatase 2A in the rat frontal cortex

Haloperidol, a classical antipsychotic drug, affects the extracellular signal-regulated kinase (ERK) pathway in the brain. However, findings are inconsistent and the mechanism by which haloperidol regulates ERK is poorly understood. Therefore, we examined the ERK pathway and the related protein phosphatase 2A (PP2A) in detail after haloperidol administration. Haloperidol (0.5 and 1 mg/kg) induced biphasic changes in the phosphorylation level of mitogen-activated protein kinase kinase (MEK), ERK, and p90 ribosomal S6 kinase (p90RSK) without changing Raf-1 phosphorylation. Fifteen minutes after haloperidol administration, MEK-ERK-p90RSK phosphorylation increased, whilst PP2A activity decreased. At 60 min, the reverse was observed and the binding of PP2A to MEK and ERK increased. Higher dosages of haloperidol (2 and 4 mg/kg), affected neither MEK-ERK-p90RSK phosphorylation nor PP2A activity. Accordingly, PP2A regulates acute dose- and time-dependent changes in MEK-ERK-p90RSK phosphorylation after haloperidol treatment. These findings suggest the involvement of a dephosphorylating mechanism in the acute action of haloperidol.

Snapin, a new regulator of receptor signaling, augments alpha1A-adrenoceptor-operated calcium influx through TRPC6

Activation of G(q)-protein-coupled receptors, including the alpha(1A)-adrenoceptor (alpha(1A)-AR), causes a sustained Ca(2+) influx via receptor-operated Ca(2+) (ROC) channels, following the transient release of intracellular Ca(2+). Transient receptor potential canonical (TRPC) channel is one of the candidate proteins constituting the ROC channels, but the precise mechanism linking receptor activation to increased influx of Ca(2+) via TRPCs is not yet fully understood. We identified Snapin as a protein interacting with the C terminus of the alpha(1A)-AR. In receptor-expressing PC12 cells, co-transfection of Snapin augmented alpha(1A)-AR-stimulated sustained increases in intracellular Ca(2+) ([Ca(2+)](i)) via ROC channels. By altering the Snapin binding C-terminal domain of the alpha(1A)-AR or by reducing cellular Snapin with short interfering RNA, the sustained increase in [Ca(2+)](i) in Snapin-alpha(1A)-AR co-expressing PC12 cells was attenuated. Snapin co-immunoprecipitated with TRPC6 and alpha(1A)-AR, and these interactions were augmented upon alpha(1A)-AR activation, increasing the recruitment of TRPC6 to the cell surface. Our data suggest a new receptor-operated signaling mechanism where Snapin links the alpha(1A)-AR to TRPC6, augmenting Ca(2+) influx via ROC channels.

Catecholaminergic control of mitogen-activated protein kinase signaling in paraventricular neuroendocrine neurons in vivo and in vitro: a proposed role during glycemic challenges

Paraventricular hypothalamic (PVH) corticotropin-releasing hormone (CRH) neuroendocrine neurons mount neurosecretory and transcriptional responses to glycemic challenges [intravenous 2-deoxyglucose (2-DG) or insulin]. Although these responses require signals from intact afferents originating from hindbrain CA (catecholaminergic) neurons, the identity of these signals and the mechanisms by which they are transduced by PVH neurons during glycemic challenge remain unclear. Here, we tested whether the prototypical catecholamine, norepinephrine (NE), can reproduce PVH neuroendocrine responses to glycemic challenge. Because these responses include phosphorylation of p44/42 mitogen-activated protein (MAP) kinases [extracellular signal-regulated kinases 1/2 (ERK1/2)], we also determined whether NE activates ERK1/2 in PVH neurons and, if so, by what mechanism. We show that systemic insulin and 2-DG, and PVH-targeted NE microinjections, rapidly elevated PVH phospho-ERK1/2 levels. NE increased Crh and c-fos expression, together with circulating ACTH/corticosterone. However, because injections also increased c-Fos mRNA in other brain regions, we used hypothalamic slices maintained in vitro to clarify whether NE activates PVH neurons without contribution of inputs from distal regions. In slices, bath-applied NE triggered robust phospho-ERK1/2 immunoreactivity in PVH (including CRH) neurons, which attenuated markedly in the presence of the alpha1 adrenoceptor antagonist, prazosin, or the MAP kinase kinase (MEK) inhibitor, U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene). Therefore, at a systems level, local PVH delivery of NE is sufficient to account for hindbrain activation of CRH neuroendocrine neurons during glycemic challenge. At a cellular level, these data provide the first demonstration that MAP kinase signaling cascades (MEK-->ERK) are intracellular transducers of noradrenergic signals in CRH neurons, and implicate this transduction mechanism as an important component of central neuroendocrine responses during glycemic challenge.

Internalization and distribution of three alpha1-adrenoceptor subtypes in HEK293A cells before and after agonist stimulation

AIM

To examine the subcellular distribution of the 3 alpha1-adrenoceptor (alpha1-AR) subtypes and their internalization and trafficking upon agonist stimulation in human embryonic kidney 293A cells.

METHODS

Confocal real-time imaging, enzyme linked immunosorbent assay (ELISA) and whole cell [3H]-prazosin binding assay were applied to detect the distribution and localization of the 3 alpha1-AR subtypes.

RESULTS

alpha1A-AR was found both on the cell surface and in the cytoplasm; alpha1BAR, however, was predominantly detected on the cell surface, while alpha1D-AR was detected mainly in the intracellular compartments. After stimulation with phenylephrine, localization changes were detected by confocal microscopy for alpha1A- and alpha1B-AR,but the localization of alpha1D-AR were unaffected. Phenylephrine stimulation promoted a more rapid internalization of alpha1B-AR than alpha1A-AR. alpha1D-AR internalization was detected only by ELISA. Whole cell [3H]-prazosin binding assay showed that alpha1A-AR functional receptors were detected both on the cell surface and in the cytoplasm; alpha1B-AR, however, were detected predominantly on the cell surface, while alpha1D-AR were detected mainly in intracellular compartments. Phenylephrine stimulation promoted internalization of alpha1A- and alpha1B-AR.

CONCLUSION

Phenylephrine stimulation induced changes in the localization of the 3 alpha1-AR.

Signal transduction and regulation: are all alpha1-adrenergic receptor subtypes created equal?

The current manuscript reviews the evidence whether and how subtypes of alpha(1)-adrenergic receptors, i.e. alpha(1A)-, alpha(1B)- and alpha(1D)-adrenergic receptors, differentially couple to signal transduction pathways and exhibit differential susceptibility to regulation. In both regards studies in tissues or cells natively expressing the subtypes are hampered because the relative expression of the subtypes is poorly controlled and the observed effects may be cell-type specific. An alternative approach, i.e. transfection of multiple subtypes into the same host cell line overcomes this limitation, but it often remains unclear whether results in such artificial systems are representative for the physiological situation. The overall evidence suggests that indeed subtype-intrinsic and cell type-specific factors interact to direct alpha(1)-adrenergic receptor signaling and regulation. This may explain why so many apparently controversial findings have been reported from various tissues and cells. One of the few consistent themes is that alpha(1D)-adrenergic receptors signal less effectively upon agonist stimulation than the other subtypes, most likely because they exhibit spontaneous internalization.

Mammalian Tolloid Alters Subcellular Localization, Internalization, and Signaling of α1a-Adrenergic Receptors

In the present study, we identified the CUB5 domain of mammalian Tolloid (mTLD) as a novel protein binding to alpha(1A)-adrenergic receptor (AR) using the yeast two-hybrid system. Whereas CUB5 did not couple to either alpha(1B)-AR or alpha(1D)-AR. It was determined that amino acids 322 to 359 of alpha(1A)-AR were the major binding region for CUB5. The direct interaction between alpha(1A)-AR cytoplasmic tail and CUB5 was discovered by glutathione S-transferase pull-down assay. We confirmed the interaction of mTLD with alpha(1A)-AR in human embryonic kidney (HEK) 293 cells by immunoprecipitation, immunofluorescence, and fluorescence resonance energy transfer. Although mTLD did not affect the density and affinity of receptors in crudely prepared membranes from HEK293 cells stably expressing alpha(1A)-AR, it significantly altered the subcellular localization of the receptors. Moreover, mTLD reduced the level of cell surface alpha(1A)-ARs, delayed the initial rate of agonist-induced receptor internalization, and facilitated agonist-induced calcium transient. We have demonstrated that mTLD interacts with alpha(1A)-AR directly, alters the subcellular localization of receptor, and influences agonist-induced alpha(1A)-AR internalization and calcium signaling.

Long-Term Exposure to the Atypical Antipsychotic Olanzapine Differently Up-Regulates Extracellular Signal-Regulated Kinases 1 and 2 Phosphorylation in Subcellular Compartments of Rat Prefrontal Cortex

Antipsychotics are the drugs of choice for the treatment of schizophrenia. Besides blocking monoamine receptors, these molecules affect intracellular signaling mechanisms, resulting in long-term synaptic alterations. Western blot analysis was used to investigate the effect of long-term administration (14 days) with the typical antipsychotic haloperidol and the atypical olanzapine on the expression and phosphorylation state of extracellular signal-related kinases (ERKs) 1 and 2 (ERK1/2), proteins involved in the regulation of multiple intracellular signaling cascades. A single injection of both drugs produced an overall decrease in ERK1/2 phosphorylation in different subcellular compartments. Conversely, long-term treatment with olanzapine, but not haloperidol, increased ERK1/2 phosphorylation in the prefrontal cortex in a compartment-specific and time-dependent fashion. In fact, ERK1/2 phosphorylation was elevated in the nuclear and cytosolic fractions 2 h after the last drug administration, whereas it was enhanced only in the membrane fraction when the animals were killed 24 h after the last injection. This effect might be the result of an activation of the mitogen-activated protein kinase pathway, because the phosphorylation of extracellular signal-regulated kinase kinase 1/2 was also increased by long-term olanzapine administration. Our data demonstrate that long-term exposure to olanzapine dynamically regulates ERK1/2 phosphorylation in different subcellular compartments, revealing a novel mechanism of action for this atypical agent and pointing to temporally separated locations of signaling events mediated by these kinases after long-term olanzapine administration.

Functional genomics of sex hormone-dependent neuroendocrine systems: specific and generalized actions in the CNS

Sex hormone effects on hypothalamic neurons have been worked out to a point where receptor mechanisms are relatively well understood, a neural circuit for a sex steroid-dependent behavior has been determined, and several functional genomic regulations have been discovered and conceptualized. With that knowledge in hand, we approach deeper problems of explaining sexual arousal and generalized CNS arousal. After a brief summary of arousal mechanisms, we focus on three chemical systems which signal generalized arousal and impact hormone-dependent hypothalamic neurons of behavioral importance: histamine, norepinephrine and enkephalin.

The N Terminus of the Human α1D-Adrenergic Receptor Prevents Cell Surface Expression

We previously reported that truncation of the N-terminal 79 amino acids of alpha(1D)-adrenoceptors (Delta(1-79)alpha(1D)-ARs) greatly increases binding site density. In this study, we determined whether this effect was associated with changes in alpha(1D)-AR subcellular localization. Confocal imaging of green fluorescent protein (GFP)-tagged receptors and sucrose density gradient fractionation suggested that full-length alpha(1D)-ARs were found primarily in intracellular compartments, whereas Delta(1-79)alpha(1D)-ARs were translocated to the plasma membrane. This resulted in a 3- to 4-fold increase in intrinsic activity for stimulation of inositol phosphate formation by norepinephrine. We determined whether this effect was transplantable by creating N-terminal chimeras of alpha(1)-ARs containing the body of one subtype and the N terminus of another (alpha(1A)NT-D, alpha(1B)NT-D, alpha(1D)NT-A, and alpha(1D)NT-B). When expressed in human embryonic kidney 293 cells, radioligand binding revealed that binding densities of alpha(1A)-or alpha(1B)-ARs containing the alpha(1D)-N terminus decreased by 86 to 93%, whereas substitution of alpha(1A)- or alpha(1B)-N termini increased alpha(1D)-AR binding site density by 2- to 3-fold. Confocal microscopy showed that GFP-tagged alpha(1D)NT-B-ARs were found only on the cell surface, whereas GFP-tagged alpha(1B)NT-D-ARs were completely intracellular. Radioligand binding and confocal imaging of GFP-tagged alpha(1D)- and Delta(1-79)alpha(1D)-ARs expressed in rat aortic smooth muscle cells produced similar results, suggesting these effects are generalizable to cell types that endogenously express alpha(1D)-ARs. These findings demonstrate that the N-terminal region of alpha(1D)-ARs contain a transplantable signal that is critical for regulating formation of functional bindings, through regulating cellular localization.

Intravenous 2-deoxy-D-glucose injection rapidly elevates levels of the phosphorylated forms of p44/42 mitogen-activated protein kinases (extracellularly regulated kinases 1/2) in rat hypothalamic parvicellular paraventricular neurons

CRH neurons within the medial parvicellular part of the hypothalamic paraventricular nucleus (PVHmp) can respond to afferent inputs encoding stress-related information by initiating peptide synthesis (signaling cascades, transcription, and translation) and/or peptide release. However, understanding these cellular events is hampered by three outstanding issues: 1) neural inputs that activate CRH neurons remain incompletely identified; 2) the identity and temporal dynamics of signaling pathways within CRH neurons are poorly understood; and 3) the precise coupling of the first two issues has not been established. Here, we report that the phosphorylated forms of p44/p42 MAPKs (pERK1/2) are rapidly detected in PVHmp cells after i.v. infusion of the antimetabolite, 2-deoxy-D-glucose (2-DG). Combined immunocytochemistry and in situ hybridization revealed that pERK1/2 immunoreactivity is detectable 10 min after 2-DG infusion not only within most PVHmp neurons containing CRH mRNA (78.6% of mean total CRH cells counted) but also in many non-CRH neurons (45.5% of mean total sampled cells). In contrast, Fos protein in the PVHmp was not detected within this time period, consistent with the known time course for its translation. Stress associated with halothane exposure also robustly elevated pERK1/2 levels in PVHmp neurons approximately 10 min after exposure. Our results implicate pERK1/2 in stress-induced activation of CRH neurosecretory cells and underscore their utility as indices of rapid cellular activation. Because 2-DG-induced activation of CRH gene transcription in these neurons requires a catecholaminergic input, our data also suggest that pERK1/2 could couple afferent catecholaminergic signals with CRH gene expression in these neurons.

Gene-environment interplay in neurogenesis and neurodegeneration

Factors associated with predisposition and vulnerability to neurodegenerative disorders may be described usefully within the context of gene-environment interplay. There are many identified genetic determinants for so-called genetic disorders, and it is possible to duplicate many elements of recognized human neurodegenerative disorders in either knock-in or knock-out mice. However, there are similarly, many identifiable environmental influences on outcomes of the genetic defects; and the course of a progressive neurodegenerative disorder can be greatly modified by environmental elements. Constituent cellular defense mechanisms responsive to the challenge of increased reactive oxygen species represent only one crossroad whereby environment can influence genetic predisposition. In this paper we highlight some of the major neurodegenerative disorders and discuss possible links of gene-environment interplay. The process of adult neurogenesis in brain is also presented as an additional element that influences gene-environment interplay. And the so-called priming processes (i.e., production of receptor supersensitization by repeated drug dosing), is introduced as yet another process that influences how genes and environment ultimately and co-dependently govern behavioral ontogeny and outcome. In studies attributing the influence of genetic alteration on behavioral phenotypy, it is essential to carefully control environmental influences.

Regulation of alpha-1B adrenergic receptor localization, trafficking, function, and stability

The alpha-1 adrenergic receptors (alpha(1)ARs) play important roles in normal physiology and in many disease states, and understanding their signaling pathways and regulatory mechanisms is thus of considerable relevance, in particular for identifying pharmacological targets for therapeutic modulation. The expression, function, localization, trafficking, and stability of these receptors are all subject to complex regulation by diverse molecular mechanisms. This article highlights recent studies from our laboratory and others focused on the localization and trafficking of the alpha-1B adrenergic receptor (alpha(1B)AR) subtype and on changes in its stability that are likely to be involved in regulating receptor expression. The role(s) of protein kinase C in alpha(1B)AR sequestration, endocytosis, and extracellular signal-regulated kinase (ERK) activation are summarized, and evidence for alpha(1B)AR localization in caveolae/rafts is presented. Receptor structural domains involved in the multiple steps and mechanisms of agonist-induced desensitization are described. Finally, aspects of alpha(1B)AR structural stability that appear to control its drug-induced up- and down-regulation are discussed. Our understanding of regulation for the alpha(1B)AR subtype provides a model for studies of the differential regulation of the other alpha(1)AR subtypes and may lead to identification of new molecular targets for therapeutic intervention in a variety of disease states.

Norepinephrine activates extracellular-regulated kinase in cortical neurons

BACKGROUND

Previous studies demonstrate that indirect activation of monoamine receptors by antidepressant treatment increases neurotrophic factors that activate the mitogen-activated protein kinase cascade; however, it is also possible that these monoamine receptors influence the mitogen-activated protein kinase pathway independent of neurotrophic factors. The influence of norepinephrine on the phosphorylation of extracellular-regulated protein kinase is characterized.

METHODS

Primary cerebral cortical cultures were prepared from embryonic day 18 rat brains and were subsequently incubated with norepinephrine in the absence or presence of agents acting as noradrenergic receptors or as intracellular signaling proteins. Levels of phosphorylated extracellular-regulated protein kinase were determined by immunoblot.

RESULTS

The results demonstrate that incubation with norepinephrine produces a time- and dose-dependent activation of phosphorylated extracellular-regulated protein kinase and that this increase is dependent on activation of alpha(2)- and beta-adrenergic receptor subtypes. In addition, the results demonstrate that norepinephrine activation of phosphorylated extracellular-regulated protein kinase is dependent on a pertussis toxin-sensitive G protein, a receptor tyrosine kinase, and activation of phosphatidylinositol 3-kinase.

CONCLUSIONS

The findings suggest that activation of the mitogen-activated protein kinase cascade by norepinephrine can occur via a tyrosine kinase-dependent signaling pathway but independent of classical second-messenger or Src-dependent kinases.

Norepinephrine activates P44 and P42 MAPK in human prostate stromal and smooth muscle cells but not in epithelial cells

BACKGROUND

In vascular smooth muscle cells, alpha1-adrenergic stimulation increases DNA synthesis and cell proliferation via activation of p44/42 (ERK1/2) MAPK. We examined whether norepinephrine (NE) activates MAPK and stimulates the proliferation of prostatic epithelial and non-epithelial cells.

METHODS

Human prostatic epithelial cells, stromal cells, and smooth muscle cells were purchased from BioWhittaker (Walkersville, MD). After reaching a semi-confluent condition, the cells were cultured in RPMI-1640 without serum for 1 day. At 10 min after adding NE (10(-6) or 10(-7) M) to the medium, the cells were collected. Cell lysate was analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) followed by Western blot using anti-phospho-p44/42 and anti-p44/42 antibodies. The activation of p44/42 was estimated by the ratio of phospho-p44/42 to total p44/42. Cell proliferation was evaluated by (3)H-thymidine uptake assay. After reaching a semi-confluent condition, the cells were cultured in RPMI-1640 containing 0.5% FCS with or without NE (10(-6) or 10(-7) M) for 16 hr followed by a (3)H-thymidine uptake period (24 hr).

RESULTS

P44/42 MAPK was significantly activated by NE in non-epithelial cells (stromal cells and smooth muscle cells) while not in epithelial cells. The uptake of (3)H-thymidine was significantly increased by NE in both non-epithelial cells, which was inhibited by alpha1-adrenoceptor antagonists.

CONCLUSIONS

These results suggest that NE may stimulate the proliferation of non-epithelial prostatic cells, which may be involved in the pathogenesis of BPH.

Mutational uncoupling of alpha1A-adrenergic receptors from G proteins also uncouples mitogenic and transcriptional responses in PC12 cells

Activation of human alpha1A-adrenergic receptors in PC12 cells causes many second messenger, mitogenic, and transcriptional responses. We examined the role of G protein activation in these responses by uncoupling the receptor through deletion of the first three amino acids from the third intracellular loop (Delta208-210). Expression levels of retrovirus-transfected wild-type and Delta(208-210) alpha1A-adrenergic receptors in PC12 cells were similar and showed identical binding affinities for antagonists. However, the potency of the agonist norepinephrine was increased 9-fold by the Delta (208-210) mutation. In PC12 cells expressing the Delta (208-210) construct, calcium and inositol phosphate responses to norepinephrine were essentially abolished. The strong activation of mitogen-activated protein kinase pathways seen upon stimulation of wild-type alpha1A-adrenergic receptors in PC12 cells was abolished by the Delta (208-210) mutation, as was activation of the tyrosine kinase Pyk2. Norepinephrine also activates several transcriptional reporters through alpha1A-adrenergic receptors in PC12 cells, including reporters for activator protein 1, serum response element, cAMP response element, nuclear factor-kappaB, nuclear factor of activated T cells, gamma-interferon-activated sequence, and signal transducer and activator of transcription. All these transcriptional responses were abolished by the Delta (208-210) mutation. Overexpression of Galpha16 did not rescue any of these responses. These data suggest that known second messenger, mitogenic, and transcriptional effects of alpha1A-adrenergic receptors in PC12 cells all require G protein activation.

N-terminal truncation of human alpha1D-adrenoceptors increases expression of binding sites but not protein

The role of the N-terminus of human alpha(1D)-adrenoceptors was examined by deleting the first 79 amino acids (Delta(1-79)) and epitope-tagging to facilitate immunoprecipitation and detection. Following transfection into HEK293 cells, 6- to 13-fold increases in the density of specific [125I]BE 2254 binding sites were observed for both tagged and untagged Delta(1-79)alpha(1D)- compared to full-length alpha(1D)-adrenoceptors, while agonist and antagonist affinities remained unchanged. In contrast, immunoprecipitation of tagged receptors showed that full-length alpha(1D)-adrenoceptor protein was at least twice as abundant as Delta(1-79)alpha(1D)-adrenoceptor protein. Photoaffinity labelling with [125I]arylazidoprazosin showed much more intense labelling of tagged Delta(1-79)alpha(1D)- than of full-length alpha(1D)-adrenoceptors. Substantial N-linked glycosylation of tagged Delta(1-79)alpha(1D)-adrenoceptors was observed, although full-length alpha(1D)-adrenoceptors contain two consensus glycosylation sites but are not glycosylated. These results suggest that N-terminal truncation of alpha(1D)-adrenoceptors enhances processing of a binding competent form in HEK293 cells; and show a clear dissociation between abundance of receptor protein and density of receptor binding sites.

Stimulation of the ERK pathway by GTP-loaded Rap1 requires the concomitant activation of Ras, protein kinase C, and protein kinase A in neuronal cells

The small GTPases Ras or Rap1 were suggested to mediate the stimulatory effect of some G protein-coupled receptors on ERK activity in neuronal cells. Accordingly, we reported here that pituitary adenylate cyclase-activating polypeptide (PACAP), whose G protein-coupled receptor triggers neuronal differentiation of the PC12 cell line via ERK1/2 activation, transiently activated Ras and induced the sustained GTP loading of Rap1. Ras mediated peak stimulation of ERK by PACAP, whereas Rap1 was necessary for the sustained activation phase. However, PACAP-induced GTP-loading of Rap1 was not sufficient to account for ERK activation by PACAP because 1) PACAP-elicited Rap1 GTP-loading depended only on phospholipase C, whereas maximal stimulation of ERK by PACAP also required the activity of protein kinase A (PKA), protein kinase C (PKC), and calcium-dependent signaling; and 2) constitutively active mutants of Rap1, Rap1A-V12, and Rap1B-V12 only minimally stimulated the ERK pathway compared with Ras-V12. The effect of Rap1A-V12 was dramatically potentiated by the concurrent activation of PKC, the cAMP pathway, and Ras, and this potentiation was blocked by dominant-negative mutants of Ras and Raf. Thus, this set of data indicated that GPCR-elicited GTP loading of Rap1 was not sufficient to stimulate efficiently ERK in PC12 cells and required the permissive co-stimulation of PKA, PKC, or Ras.

Interaction of neuronal nitric oxide synthase with alpha1-adrenergic receptor subtypes in transfected HEK-293 cells

BACKGROUND

The C-terminal four amino acids (GEEV) of human alpha1A-adrenergic receptors (ARs) have been reported to interact with the PDZ domain of neuronal nitric oxide synthase (nNOS) in a yeast two-hybrid system. The other two alpha1-AR subtypes have no sequence homology in this region, raising the possibility of subtype-specific protein-protein interactions.

RESULTS

We used co-immunoprecipitation and functional approaches with epitope-tagged alpha1-ARs to examine this interaction and the importance of the C-terminal tail. Following co-transfection of HEK-293 cells with hexahistidine/Flag (HF)-tagged alpha1A-ARs and nNOS, membranes were solubilized and immunoprecipitated with anti-FLAG affinity resin or anti-nNOS antibodies. Immunoprecipitation of HFalpha1A-ARs resulted in co-immunoprecipitation of nNOS and vice versa, confirming that these proteins interact. However, nNOS also co-immunoprecipitated with HFalpha1B- and HFalpha1D-ARs, suggesting that the interaction is not specific to the alpha1A subtype. In addition, nNOS co-immunoprecipitated with each of the three HFalpha1-AR subtypes which had been C-terminally truncated, suggesting that this interaction does not require the C-tails; and with Flag-tagged beta1- and beta2-ARs. Treatment of PC12 cells expressing HFalpha1A-ARs with an inhibitor of nitric oxide formation did not alter norepinephrine-mediated activation of mitogen activated protein kinases, suggesting nNOS is not involved in this response.

CONCLUSIONS

These results show that nNOS does interact with full-length alpha1A-ARs, but that this interaction is not subtype-specific and does not require the C-terminal tail, raising questions about its functional significance.

Biochemistry and pharmacology of epitope-tagged alpha(1)-adrenergic receptor subtypes

Human alpha(1A)-, alpha(1B)-, and alpha(1D)-adrenergic receptors were tagged at their amino termini with FLAG epitopes and stably expressed in human embryonic kidney (HEK)293 cells. Tagged receptors demonstrated a wild-type pharmacology and mobilization of intracellular Ca(2+). After solubilization and immunoprecipitation, monomers, dimers, and trimers of each subtype were apparent on Western blots. Further denaturation with 6 M urea reduced most oligomers to monomers. Deglycosylation reduced the molecular size of alpha(1A)-, and to a lesser extent alpha(1B)- and alpha(1D)-adrenergic receptors. Radioligand binding site density was highest for alpha(1A)- and much lower for alpha(1B)- and alpha(1D)-adrenergic receptors, but did not correlate with protein expression. Commercial anti-alpha(1)-adrenergic receptor antibodies did not recognize the tagged receptors in Western blots of cell lysates, and substantial cross-reactivity was still observed after solubilization and immunoprecipitation. Surprisingly, only receptor monomers were apparent after photoaffinity labeling with (125)I-arylazidoprazosin, and the intensity of photoaffinity-labeling correlated with the density of radioligand binding sites. We conclude that epitope-tagged alpha(1)-adrenergic receptors exist as both monomers and oligomers in HEK293 cells, but there is substantial discrepancy between protein and binding site expression. Because only monomers are detected by photoaffinity labeling, dimers and trimers observed on Western blots may be pharmacologically inactive.

Effects of Ca(2+) influx through nonselective cation channel on noradrenaline-induced mitogenic responses

We have recently shown that noradrenaline induces extracellular Ca(2+) influx through nonselective cation channel (NSCC) in Chinese hamster ovary cells expressing alpha(1A)-adrenoceptors (CHO-alpha(1A)). Moreover, this NSCC is sensitive to (R,S)-(3,4-dihydro-6,7-dimethoxy-isoquinoline-1-yl)-2-phenyl-N,N-di-[2-(2,3,4-trimethoxyphenyl)ethyl]-acetamide (LOE 908) and resistant to 1-[b-(3-[4-Methoxyphenyl]propoxy)-4-methoxyphenethyl]-1H-imidazole hydrochloride (SK&F 96365). In the present study, we characterized the effects of extracellular Ca(2+) influx through NSCC on noradrenaline-induced mitogenic responses and activation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) of CHO-alpha(1A) using LOE 908 and SK&F 96365. Noradrenaline induced a mitogenic response in CHO-alpha(1A). LOE 908 completely inhibited the noradrenaline-induced mitogenesis, whereas SK&F 96365 did not inhibit it. The IC(50) value of LOE 908 for noradrenaline-induced mitogenesis was similar to that for the noradrenaline-induced increase in intracellular free Ca(2+) concentration ([Ca(2+)](i)). Noradrenaline stimulated ERK1/2 activity. The magnitude of noradrenaline-induced ERK1/2 activity in the absence of extracellular Ca(2+) was 40% of that in the presence of extracellular Ca(2+). LOE 908 partially (60%) inhibited the noradrenaline-induced ERK1/2 activity, whereas SK&F 96365 did not inhibit it. The IC(50) value of LOE 908 for noradrenaline-induced ERK1/2 activity was similar to that for the noradrenaline-induced increase in [Ca(2+)](i). Collectively, these results demonstrate that extracellular Ca(2+) influx through LOE 908-sensitive and SK&F 96365-resistant NSCC may be essential for noradrenaline-induced mitogenesis in CHO-alpha(1A). Moreover, the noradrenaline-induced ERK1/2 activity involves two distinct pathways, one dependent on extracellular Ca(2+) influx through NSCC, whereas the other is independent of the influx.

Subtype-specific neuronal differentiation of PC12 cells transfected with alpha2-adrenergic receptors

Cells of the PC12 rat pheochromocytoma cell line acquire characteristics of sympathetic neurons under appropriate treatment. Stably transfected PC12 cells expressing individual alpha2-adrenergic receptor (alpha2-AR) subtypes were used to assess the role of alpha2-ARs in neuronal differentiation and to characterise the signalling pathways activated by the alpha2-AR agonist epinephrine in these cells. The effects of alpha2-AR activation were compared with the differentiating action and the signalling mechanisms of nerve growth factor (NGF). Epinephrine induced neuronal differentiation of PC12alpha2 cells through alpha2-AR activation in a subtype-dependent manner, internalization of all human alpha2-AR subtypes, and activation of mitogen-activated protein kinase (MAPK) and the serine-threonine protein kinase Akt. Epinephrine and NGF showed synergism in their differentiating effects. The MAPK kinase (MEK-1) inhibitor PD 98059 abolished the differentiating effect of epinephrine indicating that the differentiation is dependent on MAPK activation. Activating protein-1 (AP-1) DNA-binding activity was increased after epinephrine treatment in all three PC12alpha2 subtype clones. Evaluation of the potential physiological consequences of these findings requires further studies on endogenously expressed alpha2-ARs in neuronal cells.

alpha(1B)- and alpha(1D)-Adrenergic receptors exhibit different requirements for agonist and mitogen-activated protein kinase activation to regulate growth responses in rat 1 fibroblasts

We compared DNA replication, protein biosynthesis, and mitogen-activated protein kinase (MAPK) activity in Rat 1 fibroblasts stably expressing either the alpha(1B)-adrenergic receptor (AR) or alpha(1D)-AR subtypes. Activation of both the alpha(1B)-AR and alpha(1D)-AR inhibited DNA synthesis (as assessed by [(3)H]thymidine incorporation). In contrast, both receptors stimulated protein biosynthesis (as measured by [(35)S]methionine incorporation) and activated extracellular signal-regulated kinase (ERK)1/2. Importantly, these responses were agonist-dependent for the alpha(1B)-AR, but were agonist-independent for the alpha(1D)-AR. Agonist activation of the alpha(1B)-AR resulted in increased p38 kinase activity, but not c-Jun NH(2)-terminal kinase (JNK) activity, whereas the alpha(1D)-AR activated JNK but not p38 kinase. Unlike ERK1/2, JNK activity was increased by agonist treatment in the alpha(1D)-AR cells. An ERK1/2-pathway inhibitor PD98059 had no effect on phenylephrine-mediated inhibition of DNA synthesis in either cell line but blocked protein biosynthesis mediated by both receptors. The p38 kinase inhibitor SB203580 blocked alpha(1B)-AR effects on [(3)H]thymidine and [(35)S]methionine incorporation in alpha(1B)-AR-expressing cells, but had no effect on alpha(1D)-AR-mediated growth responses, consistent with the inability of the alpha(1D)-AR to activate p38 kinase. Therefore, alpha(1B)- and alpha(1D)-ARs mediated similar growth responses but differ with respect to the MAPK family member involved and the requirement for agonist.

Stress induces increases in IL-6 production by leucocytes of patients with the chronic inflammatory disease juvenile rheumatoid arthritis: a putative role for alpha(1)-adrenergic receptors

Juvenile rheumatoid arthritis (JRA) is characterized by chronic inflammation of the joints. In the present study we demonstrate that exposure of JRA patients to a noradrenergic stressor (cold pressor test) results in enhanced LPS-induced IL-6 production by peripheral blood cells of these patients. Healthy, age-matched controls had the same rise in norepinephrine, but do not respond with changes in IL-6 production after exposure to the cold pressor test. Moreover, PBMC of patients with JRA express mRNA encoding alpha(1)-adrenergic receptors (AR), predominantly of the alpha(1d)-AR subtype. In contrast, we could not detect mRNA encoding for alpha(1)-AR in PBMC of healthy controls. The results of this study suggest that expression of alpha(1)-AR mRNA in PBMC during chronic inflammation is associated with altered responses of the immune system to stress.

Noradrenaline induces phosphorylation of ERK-2 in human peripheral blood mononuclear cells after induction of alpha(1)-adrenergic receptors

alpha(1)-Adrenergic receptors (ARs) are not expressed by peripheral blood mononuclear cells (PBMCs) of healthy human individuals. However, in the present study we show that alpha(1)-ARs can be induced in lymphocytes after culturing with either the mitogen PHA or the glucocorticoid dexamethasone. Moreover, incubation of these activated PBMCs with noradrenaline (NA) results in enhanced phosphorylation of ERK-2, a kinase involved in the activation of many immune functions. Similar induction of alpha(1)-AR mRNA with concomitant NA-induced activation of ERK-2 occurs in monocytes after culture with LPS. Our results demonstrate that functional alpha(1)-ARs can be induced on PBMCs and that these alpha(1)-ARs mediate NA-induced activation of ERK-2.

Beta 3- and alpha1-adrenergic Erk1/2 activation is Src- but not Gi-mediated in Brown adipocytes

A novel signaling pathway for mediation of beta(3)-adrenergic activation of the mitogen-activated protein kinases Erk1/2 (associated with proliferation, differentiation, and apoptosis) has recently been proposed, which implies mediation via constitutively coupled G(i)-proteins and Gbetagamma-subunits, distinct from the classical cAMP pathway of beta-adrenergic stimulation. To verify the significance of this pathway in cells in primary cultures that entopically express beta(3)-adrenoreceptors, we examined the functionality of this pathway in cultured brown adipocytes. Norepinephrine activated Erk1/2 via both beta(3) receptors and alpha(1) receptors but not via alpha(2) receptors. Forskolin induced Erk1/2 activation similarly to beta(3) activation, indicating cAMP-mediation; this induction could be inhibited with H89, implying protein kinase A mediation. The G(i)-pathway was functional in these cells, as pertussis toxin increased agonist-induced cAMP accumulation. However, pertussis toxin was unable to affect adrenergically induced Erk1/2 activation. Also, wortmannin was without effect, implying that Gbetagamma activation of the phosphatidylinositol 3-kinase pathway was not involved. PP1/2, which inhibits Src, abolished both beta(3)- and alpha(1)-induced Erk1/2 activation. Thus, the proposed novel G(i) pathway for beta(3) mediation is not universal, because it is not functional in the untransformed primary cell culture system with entopically expressed beta(3) receptors examined here. Here, the beta(3) signal is mediated classically via cAMP/protein kinase A. beta(3) and alpha(1) signals converge at Src, which thus mediates Erk1/2 activation in both pathways.

Transcriptional responses to growth factor and G protein-coupled receptors in PC12 cells: comparison of alpha(1)-adrenergic receptor subtypes

Transcriptional responses to growth factor and G protein-coupled receptors were compared in PC12 cells using retroviral luciferase reporters. In cells stably expressing alpha(1A)-adrenergic receptors, norepinephrine activated all five reporters [AP1 (activator protein-1), SRE (serum response element), CRE (cyclic AMP response element), NFkappaB) (nuclear factor-kappaB), and NFAT (nuclear factor of activated T cells)], whereas nerve growth factor (NGF) and epidermal growth factor activated only AP1 and SRE. Activation of P2Y2 receptors by UTP did not activate any reporters. Protein kinase C inhibition blocked NFkappaB activation by norepinephrine, but potentiated CRE. Mitogen-activated protein kinase kinase inhibition blocked AP1 activation by norepinephrine, but also potentiated CRE. p38 mitogen-activated protein kinase inhibition reduced most norepinephrine responses, but not NGF responses. inhibition of Src eliminated SRE responses to norepinephrine and NGF, and reduced all responses except CRE. Phosphatidylinositol 3-kinase inhibitors markedly potentiated CRE activation by norepinephrine, with only small effects on the other responses. Comparison of the three human subtypes showed that the alpha(1A) activated all five reporters, the alpha(1B) showed smaller effects, and the alpha(1D) was ineffective. Cell differentiation caused by norepinephrine, but not NGF, was reduced by all inhibitors studied. These experiments suggest that alpha(1A)-adrenergic receptors activate a wider array of transcriptional responses than do growth factors in PC12 cells. These responses are not linearly related to second messenger production, and different subtypes show different patterns of activation.

MAP kinase pathways activated by stress: the p38 MAPK pathway

A stress-activated serine/threonine protein kinase, p38 mitogen-activated protein kinase (p38 MAPK), belongs to the MAP kinase superfamily. Diverse extracellular stimuli, including ultraviolet light, irradiation, heat shock, high osmotic stress, proinflammatory cytokines and certain mitogens, trigger a stress-regulated protein kinase cascade culminating in activation of p38 MAPK through phosphorylation on a TGY motif within the kinase activation loop. p38 MAPK appears to play a major role in apoptosis, cytokine production, transcriptional regulation, and cytoskeletal reorganization, and has been causally implicated in sepsis, ischemic heart disease, arthritis, human immunodeficiency virus infection, and Alzheimer's disease. The availability of specific inhibitors helps to clarify the role that p38 MAPK plays in these processes, and may ultimately offer therapeutic benefit for certain critically ill patients.

Tyrosine kinase inhibitors and Ca2+ signaling: direct interactions with fura-2

Selective inhibitors were used to study the role of tyrosine kinases in alpha(1A)-adrenoceptor-mediated responses in transfected PC12 cells. Ca(2+) responses to noradrenaline were measured using fura-2, and the effects of genistein, tyrphostin A25, and herbimycin A were examined. Neither genistein nor herbimycin A pretreatment altered noradrenaline-induced Ca(2+) responses, although tyrphostin A25 pretreatment caused some reduction. However, acute addition of genistein quickly reversed the apparent noradrenaline response, apparently, through a direct interaction with cytoplasmic fura-2. Both genistein and tyrphostin A25, at concentrations similar to those used to inhibit tyrosine kinases, markedly reduced fluorescence of fura-2 excited by both 340 and 380 nm, and genistein also reduced the 340/380 ratio. Tyrosine kinase inhibitors did not alter noradrenaline stimulated inositol phosphate formation in alpha(1A)-PC12 cells. These results suggest that tyrosine kinases are not involved in second messenger responses to alpha(1A)-adrenoceptors, but that tyrosine kinase inhibitors can interact directly with fura-2.

Purification and characterization of beta-adrenergic receptor mRNA-binding proteins

Beta-adrenergic receptors (beta-ARs), like other G-protein-coupled receptors, can undergo post-transciptional regulation at the level of mRNA stability. In particular, the human beta(1)- and beta(2)-ARs and the hamster beta(2)-AR mRNA undergo beta-agonist-mediated destabilization. By UV cross-linking, we have previously described an approximately M(r) 36,000 mRNA-binding protein, betaARB, that binds to A/C+U-rich nucleotide regions within 3'-untranslated regions. Further, we have demonstrated previously that betaARB is immunologically distinct from AUF1/heterogeneous nuclear ribonucleoprotein (hnRNP) D, another mRNA-binding protein associated with destabilization of A+U-rich mRNAs (Pende, A., Tremmel, K. D., DeMaria, C. T., Blaxall, B. C., Minobe, W., Sherman, J. A., Bisognano, J., Bristow, M. R., Brewer, G., and Port, J. D. (1996) J. Biol. Chem. 271, 8493-8501). In this report, we describe the peptide composition of betaARB. Mass spectrometric analysis of an approximately M(r) 36,000 band isolated from ribosomal salt wash proteins revealed the presence of two mRNA-binding proteins, hnRNP A1, and the elav-like protein, HuR, both of which are known to bind to A+U-rich nucleotide regions. By immunoprecipitation, HuR appears to be the biologically dominant RNA binding component of betaARB. Although hnRNP A1 and HuR can both be immunoprecipitated from ribosomal salt wash proteins, the composition of betaARB (HuR alone versus HuR and hnRNP A1) appears to be dependent on the mRNA probe used. The exact role of HuR and hnRNP A1 in the regulation of beta-AR mRNA stability remains to be determined.