The alpha1D-adrenergic receptor is expressed intracellularly and coupled to increases in intracellular calcium and reactive oxygen species in human aortic smooth muscle cells

Transcriptome Response of Differentiating Muscle Satellite Cells to Thermal Challenge in Commercial Turkey

Early muscle development involves the proliferation and differentiation of stem cells (satellite cells, SCs) in the mesoderm to form multinucleated myotubes that mature into muscle fibers and fiber bundles. Proliferation of SCs increases the number of cells available for muscle formation while simultaneously maintaining a population of cells for future response. Differentiation dramatically changes properties of the SCs and environmental stressors can have long lasting effects on muscle growth and physiology. This study was designed to characterize transcriptional changes induced in turkey SCs undergoing differentiation under thermal challenge. Satellite cells from the pectoralis major (p. major) muscle of 1-wk old commercial fast-growing birds (Nicholas turkey, NCT) and from a slower-growing research line (Randombred Control Line 2, RBC2) were proliferated for 72 h at 38 °C and then differentiated for 48 h at 33 °C (cold), 43 °C (hot) or 38 °C (control). Gene expression among thermal treatments and between turkey lines was examined by RNAseq to detect significant differentially expressed genes (DEGs). Cold treatment resulted in significant gene expression changes in the SCs from both turkey lines, with the primary effect being down regulation of the DEGs with overrepresentation of genes involved in regulation of skeletal muscle tissue regeneration and sarcomere organization. Heat stress increased expression of genes reported to regulate myoblast differentiation and survival and to promote cell adhesion particularly in the NCT line. Results suggest that growth selection in turkeys has altered the developmental potential of SCs in commercial birds to increase hypertrophic potential of the p. major muscle and sarcomere assembly. The biology of SCs may account for the distinctly different outcomes in response to thermal challenge on breast muscle growth, development, and structure of the turkey.

Toxic effects of Decabromodiphenyl ether (BDE-209) on thyroid of broiler chicks by transcriptome profile analysis

The wide usage of decabromodiphenyl ether (BDE-209) results in its increasing occurrence in the environment and increasing attention in regard to human and animal health. BDE-209 is an endocrine disruptor for hypothyroidism, but the toxicity mechanism is unclear. Here, the histopathology and transcriptome sequencing of thyroid tissue from broiler chicks were investigated by supplemental feeding with different concentrations of BDE-209 for 42 days (0-4 g/kg in basal diet), followed by determining the levels of thyroid hormones in serum. The results showed ruptured and even hyperplastic follicular epithelial cells in the thyroid, and a total of 501 differentially expressed genes were screened out: 222 upregulated and 279 downregulated. Based on the Kyoto Encyclopedia of Genes and Genomes database, neuroactive ligand-receptor interaction pathway was significantly enriched, and α1D-adrenergic receptor, follicle-stimulating hormone receptor, thyroid stimulating hormone receptor, and somatostatin receptor type 2 were shown to be candidate biomarkers. Thyroxine was a possible biomarker due to clear reduction in serum and significant correlation with exposure concentrations. These results suggested that oral intake of BDE-209 can cause structural injuries and even hyperplasia, and affect gene transcription involved in the neuroactive ligand-receptor interaction pathway of thyroid, as well as thyroid hormones in serum.

N-glycosylation of α1D-adrenergic receptor N-terminal domain is required for correct trafficking, function, and biogenesis

G protein-coupled receptor (GPCR) biogenesis, trafficking, and function are regulated by post-translational modifications, including N-glycosylation of asparagine residues. α1D-adrenergic receptors (α1D-ARs) - key regulators of central and autonomic nervous system function - contain two putative N-glycosylation sites within the large N-terminal domain at N65 and N82. However, determining the glycosylation state of this receptor has proven challenging. Towards understanding the role of these putative glycosylation sites, site-directed mutagenesis and lectin affinity purification identified N65 and N82 as bona fide acceptors for N-glycans. Surprisingly, we also report that simultaneously mutating N65 and N82 causes early termination of α1D-AR between transmembrane domain 2 and 3. Label-free dynamic mass redistribution and cell surface trafficking assays revealed that single and double glycosylation deficient mutants display limited function with impaired plasma membrane expression. Confocal microscopy imaging analysis and SNAP-tag sucrose density fractionation assays revealed the dual glycosylation mutant α1D-AR is widely distributed throughout the cytosol and nucleus. Based on these novel findings, we propose α1D-AR transmembrane domain 2 acts as an ER localization signal during active protein biogenesis, and that α1D-AR N-terminal glycosylation is required for complete translation of nascent, functional receptor.

Scribble co-operatively binds multiple α1D-adrenergic receptor C-terminal PDZ ligands

Many G protein-coupled receptors (GPCRs) are organized as dynamic macromolecular complexes in human cells. Unraveling the structural determinants of unique GPCR complexes may identify unique protein:protein interfaces to be exploited for drug development. We previously reported α_1D-adrenergic receptors (α_1D-ARs) – key regulators of cardiovascular and central nervous system function – form homodimeric, modular PDZ protein complexes with cell-type specificity. Towards mapping α_1D-AR complex architecture, biolayer interferometry (BLI) revealed the α_1D-AR C-terminal PDZ ligand selectively binds the PDZ protein scribble (SCRIB) with >8x higher affinity than known interactors syntrophin, CASK and DLG1. Complementary in situ and in vitro assays revealed SCRIB PDZ domains 1 and 4 to be high affinity α_1D-AR PDZ ligand interaction sites. SNAP-GST pull-down assays demonstrate SCRIB binds multiple α_1D-AR PDZ ligands via a co-operative mechanism. Structure-function analyses pinpoint R1110^PDZ4 as a unique, critical residue dictating SCRIB:α_1D-AR binding specificity. The crystal structure of SCRIB PDZ4 R1110G predicts spatial shifts in the SCRIB PDZ4 carboxylate binding loop dictate α_1D-AR binding specificity. Thus, the findings herein identify SCRIB PDZ domains 1 and 4 as high affinity α_1D-AR interaction sites, and potential drug targets to treat diseases associated with aberrant α_1D-AR signaling.

Endogenous N-terminal Domain Cleavage Modulates α1D-Adrenergic Receptor Pharmacodynamics

The α1D-adrenergic receptor (ADRA1D) is a key regulator of cardiovascular, prostate, and central nervous system functions. This clinically relevant G protein-coupled receptor has proven difficult to study, as it must form an obligate modular homodimer containing the PDZ proteins scribble and syntrophin or become retained in the endoplasmic reticulum as non-functional protein. We previously determined that targeted removal of the N-terminal (NT) 79 amino acids facilitates ADRA1D plasma membrane expression and agonist-stimulated functional responses. However, whether such an event occurs in physiological contexts was unknown. Herein, we report the ADRA1D is subjected to innate NT processing in cultured human cells. SNAP near-infrared imaging and tandem-affinity purification revealed the ADRA1D is expressed as both full-length and NT truncated forms in multiple human cell lines. Serial truncation mapping identified the cleavage site as Leu(90)/Val(91) in the 95-amino acid ADRA1D NT domain, suggesting human cells express a Δ1-91 ADRA1D species. Tandem-affinity purification MS/MS and co-immunoprecipitation analysis indicate NT processing of ADRA1D is not required to form scribble-syntrophin macromolecular complexes. Yet, label-free dynamic mass redistribution signaling assays demonstrate that Δ1-91 ADRA1D agonist responses were greater than WT ADRA1D. Mutagenesis of the cleavage site nullified the processing event, resulting in ADRA1D agonist responses less than the WT receptor. Thus, we propose that processing of the ADRA1D NT domain is a physiological mechanism employed by cells to generate a functional ADRA1D isoform with optimal pharmacodynamic properties.

Dynamic mass redistribution reveals diverging importance of PDZ-ligands for G protein-coupled receptor pharmacodynamics

G protein-coupled receptors (GPCRs) are essential membrane proteins that facilitate cell-to-cell communication and co-ordinate physiological processes. At least 30 human GPCRs contain a Type I PSD-95/DLG/Zo-1 (PDZ) ligand in their distal C-terminal domain; this four amino acid motif of X-[S/T]-X-[φ] sequence facilitates interactions with PDZ domain-containing proteins. Because PDZ protein interactions have profound effects on GPCR ligand pharmacology, cellular localization, signal-transduction effector coupling and duration of activity, we analyzed the importance of Type I PDZ ligands for the function of 23 full-length and PDZ-ligand truncated (ΔPDZ) human GPCRs in cultured human cells. SNAP-epitope tag polyacrylamide gel electrophoresis revealed most Type I PDZ GPCRs exist as both monomers and multimers; removal of the PDZ ligand played minimal role in multimer formation. Additionally, SNAP-cell surface staining indicated removal of the PDZ ligand had minimal effects on plasma membrane localization for most GPCRs examined. Label-free dynamic mass redistribution functional responses, however, revealed diverging effects of the PDZ ligand. While no clear trend was observed across all GPCRs tested or even within receptor families, a subset of GPCRs displayed diminished agonist efficacy in the absence of a PDZ ligand (i.e. HT2RB, ADRB1), whereas others demonstrated enhanced agonist efficacies (i.e. LPAR2, SSTR5). These results demonstrate the utility of label-free functional assays to tease apart the contributions of conserved protein interaction domains for GPCR signal-transduction coupling in cultured cells.

The potential of metabolomic analysis techniques for the characterisation of α1-adrenergic receptors in cultured N1E-115 mouse neuroblastoma cells

Several studies of neuropathic pain have linked abnormal adrenergic signalling to the development and maintenance of pain, although the mechanisms underlying this are not yet fully understood. Metabolomic analysis is a technique that can be used to give a snapshot of biochemical status, and can aid in the identification of the mechanisms behind pathological changes identified in cells, tissues and biological fluids. This study aimed to use gas chromatography-mass spectrometry-based metabolomic profiling in combination with reverse transcriptase-polymerase chain reaction and immunocytochemistry to identify functional α1-adrenergic receptors on cultured N1E-115 mouse neuroblastoma cells. The study was able to confirm the presence of mRNA for the α1D subtype, as well as protein expression of the α1-adrenergic receptor. Furthermore, metabolomic data revealed changes to the metabolite profile of cells when exposed to adrenergic pharmacological intervention. Agonist treatment with phenylephrine hydrochloride (10 µM) resulted in altered levels of several metabolites including myo-inositol, glucose, fructose, alanine, leucine, phenylalanine, valine, and n-acetylglutamic acid. Many of the changes observed in N1E-115 cells by agonist treatment were modulated by additional antagonist treatment (prazosin hydrochloride, 100 µM). A number of these changes reflected what is known about the biochemistry of α1-adrenergic receptor activation. This preliminary study therefore demonstrates the potential of metabolomic profiling to confirm the presence of functional receptors on cultured cells.

GABAB receptors are expressed in human aortic smooth muscle cells and regulate the intracellular Ca(2+) concentration

The aim of this study was to investigate the expression of GABAB receptors, a subclass of receptors to the inhibitory neurotransmitter gamma-aminobutyric acid (GABAB), in human aortic smooth muscle cells (HASMCs), and to explore if altering receptor activation modified intracellular Ca(2+) concentration ([Ca(2+)]i) of HASMCs. Real-time PCR, western blots and immunofluorescence were used to determine the expression of GABABR1 and GABABR2 in cultured HASMCs. Immunohistochemistry was used to localize the two subunits in human left anterior descending artery (LAD). The effects of the GABAB receptor agonist baclofen on [Ca(2+)]i in cultured HASMCs were demonstrated using fluo-3. Both GABABR1 and GABABR2 mRNA and protein were identified in cultured HASMCs and antibody staining was also localized to smooth muscle cells of human LAD. 100 μM baclofen caused a transient increase of [Ca(2+)]i in cultured HASMCs regardless of whether Ca(2+) was added to the medium, and the effects were inhibited by pre-treatment with CGP46381 (selective GABAB receptor antagonist), pertussis toxin (a Gi/o protein inhibitor), and U73122 (a phospholipase C blocker). GABAB receptors are expressed in HASMCs and regulate the [Ca(2+)]i via a Gi/o-coupled receptor pathway and a phospholipase C activation pathway.

Location-dependent signaling of the group 1 metabotropic glutamate receptor mGlu5

Although G protein-coupled receptors are primarily known for converting extracellular signals into intracellular responses, some receptors, such as the group 1 metabotropic glutamate receptor, mGlu5, are also localized on intracellular membranes where they can mediate both overlapping and unique signaling effects. Thus, besides "ligand bias," whereby a receptor's signaling modality can shift from G protein dependence to independence, canonical mGlu5 receptor signaling can also be influenced by "location bias" (i.e., the particular membrane and/or cell type from which it signals). Because mGlu5 receptors play important roles in both normal development and in disorders such as Fragile X syndrome, autism, epilepsy, addiction, anxiety, schizophrenia, pain, dyskinesias, and melanoma, a large number of drugs are being developed to allosterically target this receptor. Therefore, it is critical to understand how such drugs might be affecting mGlu5 receptor function on different membranes and in different brain regions. Further elucidation of the site(s) of action of these drugs may determine which signal pathways mediate therapeutic efficacy.

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.

Nuclear GPCRs in cardiomyocytes: an insider's view of β-adrenergic receptor signaling

In recent years, we have come to appreciate the complexity of G protein-coupled receptor signaling in general and β-adrenergic receptor (β-AR) signaling in particular. Starting originally from three β-AR subtypes expressed in cardiomyocytes with relatively simple, linear signaling cascades, it is now clear that there are large receptor-based networks which provide a rich and diverse set of responses depending on their complement of signaling partners and the physiological state. More recently, it has become clear that subcellular localization of these signaling complexes also enriches the diversity of phenotypic outcomes. Here, we review our understanding of the signaling repertoire controlled by nuclear β-AR subtypes as well our understanding of the novel roles for G proteins themselves in the nucleus, with a special focus, where possible, on their effects in cardiomyocytes. Finally, we discuss the potential pathological implications of alterations in nuclear β-AR signaling.

Alpha-dystrobrevin-1 recruits alpha-catulin to the alpha1D-adrenergic receptor/dystrophin-associated protein complex signalosome

α(1D)-Adrenergic receptors (ARs) are key regulators of cardiovascular system function that increase blood pressure and promote vascular remodeling. Unfortunately, little information exists about the signaling pathways used by this important G protein-coupled receptor (GPCR). We recently discovered that α(1D)-ARs form a "signalosome" with multiple members of the dystrophin-associated protein complex (DAPC) to become functionally expressed at the plasma membrane and bind ligands. However, the molecular mechanism by which the DAPC imparts functionality to the α(1D)-AR signalosome remains a mystery. To test the hypothesis that previously unidentified molecules are recruited to the α(1D)-AR signalosome, we performed an extensive proteomic analysis on each member of the DAPC. Bioinformatic analysis of our proteomic data sets detected a common interacting protein of relatively unknown function, α-catulin. Coimmunoprecipitation and blot overlay assays indicate that α-catulin is directly recruited to the α(1D)-AR signalosome by the C-terminal domain of α-dystrobrevin-1 and not the closely related splice variant α-dystrobrevin-2. Proteomic and biochemical analysis revealed that α-catulin supersensitizes α(1D)-AR functional responses by recruiting effector molecules to the signalosome. Taken together, our study implicates α-catulin as a unique regulator of GPCR signaling and represents a unique expansion of the intricate and continually evolving array of GPCR signaling networks.

Functional linkage as a direction for studies in oxidative stress: α-adrenergic receptorsThis review is one of a selection of papers published in a Special Issue on Oxidative Stress in Health and Disease

The α-adrenergic receptors (adrenoceptors) are activated by the endogenous agonists epinephrine and norepinephrine. They are G protein-coupled receptors that may be broadly classified into α1 (subclasses α1A, α1B, α1D) and α2 (subclasses α2A, α2B, α2C). The α1-adrenoceptors act by binding to Gαq subunits of the G proteins, causing activation of phospholipase C (PLC). PLC converts phosphatidylinositol 4,5-bisphosphate into inositol trisphosphate (IP3) and diacylglycerol (DAG), which have downstream effects on cytosolic Ca2+ concentration. The α2-adrenoceptors bind to Gαi thus inhibiting adenylyl cyclase and decreasing cAMP levels. DAG alters protein kinase C activity and cAMP activates protein kinase A. The downstream pathways of the two receptors may also interact. Activation of α1- and α2-adrenoceptors in vascular smooth muscle results in vasoconstriction. However, the densities of individual receptor subclasses vary between vessel beds or between vessels of various sizes within the same bed. In vasculature, the densities of adrenoceptor subclasses differ between conduit arteries and arterioles. These differences, along with differences in coupling mechanisms, allow for fine regulation of arterial blood flow. This diversity is enhanced by interactions resulting from homo- and heterodimer formation of the receptors, metabolic pathways, and kinases. Reactive oxygen species generated in pathologies may alter α1- and α2-adrenoceptor cascades, change vascular contractility, or cause remodeling of blood vessels. This review emphasizes the need for understanding the functional linkage between α-adrenoceptor subtypes, coupling, cross talk, and oxidative stress in cardiovascular pathologies.

Subtypes of functional alpha1-adrenoceptor

In this review, subtypes of functional alpha1-adrenoceptor are discussed. These are cell membrane receptors, belonging to the seven-transmembrane-spanning G-protein-linked family of receptors, which respond to the physiological agonist noradrenaline. alpha1-Adrenoceptors can be divided into alpha1A-, alpha1B- and alpha1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A fourth alpha1-adrenoceptor, the alpha1L-, represents a functional phenotype of the alpha1A-adrenoceptor. alpha1-Adrenoceptor subtype knock-out mice have refined our knowledge of the functions of alpha-adrenoceptor subtypes, particuarly as subtype-selective agonists and antagonists are not available for all subtypes. alpha1-Adrenoceptors function as stimulatory receptors involved particularly in smooth muscle contraction, especially contraction of vascular smooth muscle, both in local vasoconstriction and in the control of blood pressure and temperature, and contraction of the prostate and bladder neck. Central actions are now being elucidated.

Functional alpha1- and beta2-adrenergic receptors in human osteoblasts

Central (hypothalamic) control of bone mass is proposed to be mediated through beta2-adrenergic receptors (beta2-ARs). While investigations in mouse bone cells suggest that epinephrine enhances both RANKL and OPG mRNA via both beta-ARs and alpha-ARs, whether alpha-ARs are expressed in human bone cells is controversial. The current study investigated the expression of alpha1-AR and beta2-AR mRNA and protein and the functional role of adrenergic stimulation in human osteoblasts (HOBs). Expression of alpha1B- and beta2-ARs was examined by RT-PCR, immunofluorescence microscopy and Western blot (for alpha1B-ARs). Proliferation in HOBs was assessed by (3)H-thymidine incorporation and expression of RANKL and OPG was determined by quantitative RT-PCR. RNA message for alpha1B- and beta2-ARs was expressed in HOBs and MG63 human osteosarcoma cells. alpha1B- and beta2-AR immunofluorescent localization in HOBs was shown for the first time by deconvolution microscopy. alpha1B-AR protein was identified in HOBs by Western blot. Both alpha1-agonists and propranolol (beta-blocker) increased HOB replication but fenoterol, a beta2-agonist, inhibited it. Fenoterol nearly doubled RANKL mRNA and this was inhibited by propranolol. The alpha1-agonist cirazoline increased OPG mRNA and this increase was abolished by siRNA knockdown of alpha1B-ARs in HOBs. These data indicate that both alpha1-ARs and beta2-ARs are present and functional in HOBs. In addition to beta2-ARs, alpha1-ARs in human bone cells may play a role in modulation of bone turnover by the sympathetic nervous system.

Inhibition of vascular smooth muscle G protein-coupled receptor kinase 2 enhances alpha1D-adrenergic receptor constriction

G protein-coupled receptor kinase 2 (GRK2) is a serine/theorinine kinase that phosphorylates and desensitizes agonist-bound G protein-coupled receptors. GRK2 is increased in expression and activity in lymphocytes and vascular smooth muscle (VSM) in human hypertension and animal models of the disease. Inhibition of GRK2 using the carboxyl-terminal portion of the protein (GRK2ct) has been an effective tool to restore compromised beta-adrenergic receptor (AR) function in heart failure and improve outcome. A well-characterized dysfunction in hypertension is attenuation of betaAR-mediated vasodilation. Therefore, we tested the role of inhibition of GRK2 using GRK2ct or VSM-selective GRK2 gene ablation in a renal artery stenosis model of elevated blood pressure (BP) [the two-kidney, one-clip (2K1C) model]. Use of the 2K1C model resulted in a 30% increase in conscious BP, a threefold increase in plasma norepinephrine levels, and a 50% increase in VSM GRK2 mRNA levels. BP remained increased despite VSM-specific GRK2 inhibition by either GRK2 knockout (GRK2KO) or peptide inhibition (GRK2ct). Although betaAR-mediated dilation in vivo and in situ was enhanced, alpha(1)AR-mediated vasoconstriction was also increased. Further pharmacological experiments using alpha(1)AR antagonists revealed that GRK2 inhibition of expression (GRK2KO) or activity (GRK2ct) enhanced alpha(1D)AR vasoconstriction. This is the first study to suggest that VSM alpha(1D)ARs are a GRK2 substrate in vivo.

The alpha1D-adrenergic receptor induces vascular smooth muscle apoptosis via a p53-dependent mechanism

Activation of the endogenous alpha1-adrenergic receptor (AR) associated with human aortic smooth muscle cells resulted in a dose- and time-dependent increase in the levels of mitochondrial reactive oxygen species (ROS). ROS increases were apparent within 10 min and maximal after 45 min. Prolonged activation (>4 h) of the alpha1-AR resulted in smooth muscle cell apoptosis. Both the increase in ROS and apoptotic cell death were blocked by the nonselective alpha1-AR antagonist prazosin as well as the selective alpha1D-AR antagonist 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7, 9-dione (BMY 7378). Increases in ROS and apoptosis produced by alpha1-AR activation were also blocked by the p38 mitogen-activated protein kinase inhibitor 4-(4-fluorophenyl)-2-(4-hydroxyphenyl)-5-(4-pyridyl)-1H-imidazole (SB 202190) and the NAPDH oxidase inhibitor apocynin. The extracellular signal-regulated kinase 1/2 inhibitor 2'-amino-3'-methoxyflavone (PD 98059) or the c-Jun NH2-terminal kinase inhibitor 1, 9-pyrazoloanthrone anthra(1, 9-cd)pyrazol-6(2H)-one (SP 600125) was without effect on increases in ROS levels or apoptosis. Pifithrin-alpha, an inhibitor of the tumor suppressor protein p53, had no effect on ROS generation but did block alpha1D-AR-induced apoptosis. Activation of the alpha1D-AR resulted in translocation of p53 to the mitochondria. The mitochondrial translocation of p53 was blocked by prazosin, BMY 7378, apocynin, SB 202190, and pifithrin-alpha. Apoptosis was also blocked by small interfering RNA directed against p53. These data show that the alpha1D-AR is coupled to the generation of mitochondrial ROS by a pathway involving p38 and NADPH oxidase. Sustained activation of the alpha1D-AR results in smooth muscle cell apoptosis in a pathway that involves the tumor suppressor protein p53 and the mitochondrial translocation of p53. The data also provide evidence of a linkage between the alpha1D-AR and p53.