Photoaffinity labeling of the alpha 1-adrenergic receptor using an 125I-labeled aryl azide analogue of prazosin

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.

The cyano-NNO-azoxy function in the design of an irreversible label for alpha 1 adrenoreceptors

A potential alpha 1-adrenergic irreversible antagonist 6, containing the cyano-NNO-azoxy function was synthesized and tested. The effects of norepinephrine on rat thoracic aorta were irreversibly blocked by this compound at the concentration of 1 x 10(-5) M after 60 minutes. Binding studies showed that 6, at 1 x 10(-6) M, did not modify the KD of Prazosin and caused a 30% decrease of the Bmax. Substitution in 6 of the bis (2-chloroethyl)amino moiety for the cyano-NNO-azoxy function afforded 7 which behaves as an irreversible antagonist able to change KD of Prazosin without influencing Bmax.

Glycerol, sodium phosphate, and sodium chloride permit the solubilization and partial purification of rat hepatic alpha 1-receptors by 3-(3-cholamidylpropyl)-dimethylammonio-1-propanesulfonate

CHAPS [3-(3-cholamidylpropyl)-dimethylammonio-1-propanesulfonate], a zwitterionic detergent, has been used to solubilize the rat hepatic alpha 1-adrenergic receptor. Although the use of this detergent alone permitted a poor receptor solubilization, the inclusion of sodium phosphate, sodium chloride, and glycerol to the medium allowed 30% of the binding activity observed in plasma membranes to be recovered. Binding of the selective alpha 1-adrenergic antagonist, [3H]prazosin, by the solubilized preparation was saturable and of high affinity. In addition, binding of the radioligand was inhibited by a variety of adrenergic agents with affinity, specificity, and stereoselectivity comparable to that observed in plasma membranes. The use of glycerol in the solubilization medium permitted recovery of the solubilized receptor in a stable form (T1/2 = 72 h at 4 degrees C). Sequential affinity and size-exclusion gel chromatography allowed a 1000-fold purification of the solubilized receptor. The Stokes' radius and the apparent molecular mass of the purified receptor-Chaps complex (48.4 A and 160,000 Da, respectively), determined by gel filtration chromatography, were similar to those previously obtained for the rat hepatic alpha 1-receptor purified after solubilization with the nonionic detergent digitonin. These data indicate that the combination of Chaps, sodium phosphate, sodium chloride, and glycerol permitted the solubilization and partial purification of hepatic alpha 1-receptor in an active and stable form. The use of this technique might be useful for the solubilization of other membrane-bound proteins by Chaps whose biophysical characteristics make it an ideal detergent for reconstitution experiments.

The oligosaccharide component of alpha 1-adrenergic receptors from BC3H1 and DDT1 muscle cells. Studies with glycosidases and photoaffinity labelling of intact cells

In this study, we clarify the structural aspects of the oligosaccharides associated with the alpha 1-adrenergic receptor in two muscle cell lines. Photoaffinity labelling of intact BC3H1 or DDT1 muscle cells with 2-[4-(4-azido-3-[125I]iodobenzoyl)piperazin-1-yl]-4-amino-6, 7-dimethoxyquinazoline ([125I]azidoprazosin) followed by SDS/polyacrylamide-gel electrophoresis (PAGE) and autoradiography revealed specifically labelled proteins of molecular mass = 87,000 and 81,000, respectively. Treatment of photoaffinity-labelled receptors in DDT1 cells with 33 u. of endoglycosidase F/ml for 24 h resulted in the loss of the 81 kDa receptor and the appearance of a 52.5 kDa protein. When lower concentrations of glycosidase or shorter incubation times were used, the 81 kDa receptor was converted to a 66 kDa protein. Treatment of the photoaffinity-labelled BC3H1 receptor with endoglycosidase F resulted in the appearance of a 50.5 kDa protein. Neither alpha-mannosidase nor endoglycosidase H had an effect on the photoaffinity labelling patterns of the receptor from the two cell types. alpha 1-Adrenergic receptors, solubilized from membranes prepared from BC3H1 and DDT1 cells, bound to wheat germ agglutinin-Sepharose and were displaced by N-acetylglucosamine. Taken together, these results indicate that alpha 1-adrenergic receptors in BC3H1 and DDT1 cells contain complex, but not high, mannose oligosaccharide chains; differences in the composition or number of chains partially accounts for the different molecular mass of the receptor in the two cell lines. The results further indicate that the oligosaccharide chains contribute substantially to the apparent molecular mass of alpha 1-adrenergic receptors, as detected by SDS/PAGE, and that the protein backbone of these receptors is likely to be approximately 50 kDa.

Increased expression of alpha 1-adrenergic receptors in the hypothalamus of spontaneously hypertensive rats

The specificity and molecular weights of alpha 1-adrenergic receptors in various tissues of spontaneously hypertensive (SH) rat were compared with normotensive controls (Wistar-Kyoto; WKY) with the use of [125I]HEAT and [125I]azidoprazosin, specific alpha 1-adrenergic receptor antagonists. Binding of [125I]HEAT to membranes prepared from SH rat brain hypothalamus was significantly higher, due to a 75% increase in the Bmax, than the WKY control. In contrast, the Bmax and Kd of [125I]HEAT binding to brainstem and liver membranes from SH rats were not significantly different from those of WKY controls. Competition-inhibition data suggested similar pharmacological specificity with potencies in the order of prazosin greater than yohimbine greater than propranolol for both WKY and SH rat membranes prepared from liver, hypothalamus, brainstem and neuronal cultures. Photoaffinity labeling of alpha 1-adrenergic receptors from hypothalamus, brainstem and neuronal cultures using [125I]azidoprazosin followed by SDS-PAGE and autoradiography showed the presence of one major band with a molecular weight (MW) of 105,000 Da for both WKY and SH rats. In contrast, labeling of liver alpha 1-adrenergic receptors revealed one major band with a MW of 60,000 Da. Quantitation of the 105,000-Da band from SH rat hypothalamic membranes demonstrated a 52% higher intensity compared with WKY controls. Neuronal cultures prepared from 1-day-old SH rats showed a similarly greater intensity of the 105,000-Da band compared with WKY controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Coupling of the alpha 1-adrenergic receptor to a guanine nucleotide-binding regulatory protein by a discrete domain distinct from its ligand recognition site

At rat hepatic membrane alpha 1-adrenergic receptors, the nonhydrolyzable GTP analogue p[NH]ppG causes a rightward shift of agonist competition curves and a loss of high-affinity binding. This p[NH]ppG effect is consistent with the involvement of a guanine nucleotide-binding regulatory protein (G-protein) in alpha 1-adrenergic receptor signalling. Although readily apparent in membranes prepared to avoid retention of endogenous nucleotides and activation of Ca2+-sensitive proteinases (+pi), this p[NH]ppG effect is not observed in membranes prepared without proteinase inhibitors (-pi), or in -pi membranes treated with Ca2+ (-pi, +Ca2+). In these various membrane preparations, different Mr forms of the receptor are also identified by photoaffinity labeling with [125I]CP65526, an aryl azide analog of the alpha 1-selective antagonist, prazosin, followed by SDS-polyacrylamide gel electrophoresis and autoradiography. Whereas a predominant Mr = 80,000 subunit is identified in +pi membranes, in -pi membranes a proteolytic Mr = 59,000 fragment is also observed. In -pi, +Ca2+ membranes, only this latter peptide is detected. To evaluate the ability of each of these forms of the receptor to couple with a G-protein, the effect of p[NH]ppG on the agonist-inhibition of [125I]CP65526 labelling was determined by laser densitometry scanning and computer analysis. At the Mr = 80,000 subunit, p[NH]ppG causes a rightward shift of agonist competition curves and a loss of high-affinity binding, even in -pi membranes. By contrast, agonist-binding at the Mr = 59,000 subunit is of low-affinity and was not affected by p[NH]ppG. These data indicate that the cleaved Mr = 59,000 fragment, while retaining hormone binding activity is unable to undergo G-protein coupling. Thus, the alpha 1-adrenergic receptor appears to contain a discrete domain necessary for G-protein coupling that is distinct from its ligand recognition site.

N-(3-(p-azido-m-[125I]iodophenyl)propionyl)-succinimide--a heterobifunctional reagent for the synthesis of radioactive photoaffinity ligands: synthesis of a carrier-free 125I-labeled cardiac glycoside photoaffinity label

A new heterobifunctional reagent, N-(3-(p-azido-m-iodophenyl)propionyl)-succinimide (AIPPS), was synthesized and chemically characterized. The radiochemical form of the reagent, [125I]AIPPS, should be of general use as a photoactive reagent for the derivatization of free amino groups on a large variety of biologically active compounds, including many hormones. Amino-containing ligands can be derivatized with [125I]AIPPS in a method which is similar to that used for the 125I-labeled Bolton-Hunter reagent (N-(3-(p-hydroxyphenyl)propionyl)-succinimide). The added advantage with [125I]AIPPS, however, is that the ligand derivative is made both photoactive and radioactive in a single step. As an example of how this reagent can be used, we have prepared carrier-free [125I]AIPPS and reacted it with the amino-containing cardiac glycoside, 4-amino-4,6-dideoxyglucosyl digitoxigenin (GluD). The radioiodinated cardiac glycoside, [125I]AIPP-GluD, was purified by thin-layer chromatography and was carrier-free with a specific radioactivity of 2175 Ci/mmol. [125I]AIPP-GluD was an effective photoaffinity label for Na,K-ATPase as shown by specific photoaffinity labeling of purified canine kidney enzyme and human erythrocyte enzyme.

Synthesis and characterization of arylamine derivatives of rauwolscine as molecular probes for alpha 2-adrenergic receptors

The selective alpha 2-adrenergic receptor antagonist rauwolscine was structurally modified to yield a series of arylamine carboxamide derivatives, which were investigated as potential molecular probes for the localization and structural characterization of alpha 2-adrenergic receptors. The arylamine carboxamides differ in the number of carbon atoms separating the reactive phenyl moiety from the fused ring structure of the parent compound, rauwolscine carboxylate. Competitive inhibition studies with [3H]rauwolscine in rat kidney membranes indicate that the affinity for the carboxamide derivatives is inversely related to the length of the carbon spacer arm with rauwolscine 4-aminophenyl carboxamide (zero carbon spacer arm; rau-AMPC) exhibiting the highest affinity (Kd = 2.3 +/- 0.2 nM). Radioiodination of rau-AMPC yields a ligand, 125I-rau-AMPC, which binds to rat kidney alpha 2-adrenergic receptors with high affinity, as determined by both kinetic analysis (Kd = k2/k1 = 0.016 min-1/2.1 X 10(7) M-1 min-1 = 0.76 nM) and equilibrium binding studies (Kd = 0.78 +/- 0.16 nM). 125I-rau-AMPC was quantitatively converted to the photolabile arylazide derivative 17 alpha-hydroxy-20 alpha-yohimban-16 beta-(N-4-azido-3-[125I]iodophenyl) carboxamide (125I-rau-AZPC). In a partially purified receptor preparation from porcine brain, this compound photolabels a major (Mr = 62,000) peptide. The labeling of this peptide is inhibited by adrenergic agonists and antagonists with a rank order of potency consistent with an alpha 2-adrenergic receptor binding site. Both 125I-rau-AMPC and the photolabile arylazide derivative, 125I-rau-AZPC, should prove useful as molecular probes for the structural and biochemical characterization of alpha 2-adrenergic receptors.

Photoaffinity labeling of the porcine brain alpha 2-adrenergic receptor using a radioiodinated arylazide derivative of rauwolscine: identification of the hormone-binding subunit

A functionalized derivative of the alpha 2-selective antagonist rauwolscine formed the basis for a photoaffinity adduct that has allowed identification of the hormone-binding subunit of the brain alpha 2-adrenergic receptor protein. Rauwolscine carboxylate underwent reaction with 4-N-t-butyloxycarbonyl-aminoaniline, leading to the synthesis of rauwolscine 4-aminophenyl carboxamide (Rau-AmPC). Rau-AmPC was radioiodinated and converted to the arylazide derivative, 17 alpha-hydroxy-20 alpha-yohimban-16 beta-[N-(4-azido-3-[125I]iodo)phenyl] carboxamide (125I-Rau-AzPC), via a diazonium salt intermediate. The characterization of 125I-Rau-AzPC as a photolabile probe employed alpha 2-adrenergic receptors, which were first solubilized from porcine brain membranes and partially purified by affinity chromatography utilizing a yohimbine-agarose affinity matrix. In the partially purified receptor preparation incubated with 125I-Rau-AzPC, photolysis resulted in covalent labeling of a major (Mr, 62,000) peptide as determined by NaDodSO4/PAGE and autoradiography. Labeling of this peptide was inhibited by the alpha 2-selective antagonist, yohimbine, and the non-subtype-selective alpha-antagonist, phentolamine, but not by the alpha 1-antagonist, prazosin, or the beta-receptor antagonist, (-)-alprenolol. The alpha-adrenergic agonist epinephrine also inhibited labeling in a stereoselective manner. These data indicate that the photolabeled Mr 62,000 peptide is the hormone-binding subunit of the alpha 2-adrenergic receptor protein. The availability of this radioiodinated photoaffinity probe for the alpha 2-adrenergic receptor should facilitate further structural and biophysical characterization of the receptor protein.

Affinity labeling of the DDT1 MF-2 cell alpha 1-adrenergic receptor with [3H]phenoxybenzamine

In this study, we used phenoxybenzamine to label the alpha 1-adrenergic receptor of a smooth muscle cell line. Our results demonstrate a dose-dependent occupancy of alpha 1-adrenergic receptors by phenoxybenzamine determined by competition for the [3H]prazosin binding site. Following incorporation of [3H]phenoxybenzamine, partially purified membranes were solubilized and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing conditions. Despite numerous Coomassie blue-stained bands, only three bands, Mr = 80,000 +/- 500, Mr = 33,000 +/- 2,000, and Mr = 21,000 +/- 400 (N = 4), were labeled with [3H]phenoxybenzamine as determined by autofluorography. Incorporation of [3H]phenoxybenzamine into the Mr = 80,000 band, but not the Mr = 33,000 and Mr = 21,000 bands, was affected by adrenergic agonists and antagonists in a manner consistent with an alpha 1-adrenergic interaction. Labeling of the Mr = 33,000 and Mr = 21,000 bands was partially blocked by phenoxybenzamine. We conclude that [3H]phenoxybenzamine can be used as an affinity probe for the alpha 1-adrenergic receptor and that the ligand binding site of the alpha 1-adrenergic receptor resides in a Mr = 80,000 protein.

A new photoaffinity probe, 4-amino-2-[4-(4-azidocinnamoyl)piperazino]-6,7-dimethoxyqu inazoline, for alpha 1-adrenoceptors

A photoaffinity probe for alpha 1-adrenoceptors was synthesized and its properties examined on rat brain membrane preparations. The binding of 4-amino-2-[4-(4-azidocinnamoyl)piperazino]-6,7-dimethoxyquin azoline (ACP) to these receptors was of high affinity (KD = 1.05 nM) and reversible in the dark. A dose-dependent decrease in the concentration of [3H]prazosin binding sites without a change in KD was observed when membranes were preincubated with ACP, photolyzed, and then extensively washed prior to assay. This reduction in receptor concentration was prevented by alpha 1-adrenergic ligands. The specificity of ACP for alpha 1-receptors was further demonstrated by its inability to compete with [3H]dihydroalprenolol and [3H]yohimbine binding in these same membranes. Also, the concentrations and affinity constants of beta-adrenoceptors and alpha 2-adrenoceptors were unaffected in membranes which had been photolyzed after preincubation with ACP. No reduction in concentration of alpha 1-adrenoceptors was detected if ACP was photolyzed prior to incubation with receptors or if ACP was maintained in darkness throughout the experiment. The results suggest that ACP is a specific and sensitive photoprobe that may be useful for further studies on alpha 1-adrenoceptor coupled systems and that may be particularly suited for use in cell culture work.

Identification of the subunit structure of rat pineal adrenergic receptors by photoaffinity labeling

The adrenergic receptors of rat pineal gland were investigated using radiolabeled ligand binding and photoaffinity labeling techniques. 125I-2-[beta-(4-hydroxyphenyl)ethylaminomethyl]tetralone (125I-HEAT) and 125I-cyanopindolol (125I-CYP) labeled specific sites on rat pineal gland membranes with equilibrium dissociation constants (KD) of 48 (+/- 5) pM and 30 (+/- 5) pM, respectively. Binding site maxima were 481 (+/- 63) and 1,020 (+/- 85) fmol/mg protein. The sites labeled by 125I-HEAT had the pharmacological characteristics of alpha 1-adrenergic receptors. 125I-CYP-labeled beta-adrenergic receptors were characterized as a homogeneous population of beta 1-adrenergic receptors. The alpha 1- and beta 1-adrenergic receptors were covalently labeled with the specific photoaffinity probes 4-amino-6,7-dimethoxy-2-(4-[5-(4-azido-3-[125I]iodophenyl) pentanoyl]-1-piperazinyl) quinazoline (125I-APDQ) and 125I-p-azidobenzylcarazolol (125I-pABC). 125I-APDQ labeled an alpha 1-adrenergic receptor peptide of Mr = 74,000 (+/- 4,000), which was similar to peptides labeled in rat cerebral cortex, liver, and spleen. 125I-pABC labeled a single beta 1-adrenergic receptor peptide with a Mr = 42,000 (+/- 1,500), which differed from the 60-65,000 peptide commonly seen in mammalian tissues. Possible reasons for these differences are discussed.

Structural and pharmacological differences between codfish and rat brain alpha 1-adrenergic receptors revealed by photoaffinity labeling with 125I-APDQ

The photoaffinity probe 125I-APDQ has been used to characterize alpha 1-receptor peptides in the cod and rat brains. In the cod brain a major specific peptide of Mr = 68,000 could be covalently labeled by 125I-APDQ as revealed by SDS-PAGE. In the rat brain a specific peptide with Mr = 77,000 was instead labeled. When a number of adrenergic agonists and antagonists were tested for their ability to protect the labeling by 125I-APDQ their potencies were those expected for alpha 1-receptors in both species. The ligand binding peptide in the cod brain also distinguished between stereoisomers of epinephrine as expected for a physiological receptor. However, there was a distinct difference between the cod and rat alpha 1-receptor in that the beta-agonist 1-isoprenaline was equipotent to 1-norepinephrine in the cod whereas it was less potent in the rat. The protecting ability of the tested agents were also matched by their ability to displace the alpha 1-adrenergic ligand 3H-prazosin from alpha 1-receptor binding sites in brain membranes from both species. Thus, the codfish alpha 1-receptor seems to be different from mammalian alpha 1-receptors both structurally and pharmacologically.

An endogenous Ca2+-sensitive proteinase converts the hepatic alpha 1-adrenergic receptor to guanine nucleotide-insensitive forms

An iodoazido[125I]prazosin analogue was employed to photoaffinity label alpha 1-adrenergic receptors in rat liver plasma membranes. Labeled proteins were separated by gradient polyacrylamide gel electrophoresis in sodium dodecyl sulfate, and (-)-epinephrine displacement of [3H]prazosin binding was concurrently measured in the presence or absence of guanosine 5'-O-(gamma-thiotriphosphate) (GTP[gamma S]). Inclusion of EGTA and/or proteinase inhibitors during membrane preparation and incubation increased the effect of GTP[gamma S] on alpha 1-adrenergic agonist binding and this could be correlated with increased concentrations of a 78 kDa photoaffinity labeled protein. In contrast, omission of EGTA or addition of exogenous Ca2+ diminished or abolished the effect of GTP[gamma S] on binding and caused loss of the 78 kDa form and the appearance of lower molecular weight labeled proteins. Age-dependent differences in GTP[gamma S] effects on alpha 1-adrenergic agonist binding were abolished when membranes were prepared and incubated in the presence of EGTA and proteinase inhibitors. However, the 78 kDa photoaffinity labeled protein observed in adult rats (over 225 g body weight) was not apparent in membranes from younger rats (50-75 g), even when the membranes were prepared and incubated in the presence of EGTA and proteinase inhibitors. Instead, a 68 kDa species was the major labeled protein. These data suggest that GTP effects on alpha 1-adrenergic agonist binding in rat liver membranes require the presence of either a 68 or 78 kDa alpha 1-adrenergic binding protein. Failure to inhibit proteolysis in the membranes leads to the generation of lower-molecular-weight binding proteins and the loss of GTP effects on alpha 1-adrenergic agonist binding, although [3H]prazosin binding characteristics are not changed. It is suggested that either the proteolyzed forms of the alpha 1-adrenergic receptor are unable to couple to a putative guanine nucleotide-binding regulatory protein, or that such a protein is concurrently proteolyzed and is thus unable to couple to the receptor.

Mechanisms involved in alpha-adrenergic phenomena

Epinephrine and norepinephrine exert many important actions by interacting with alpha 1- and alpha 2-adrenergic receptors in their target cells. Activation of alpha 2-adrenergic receptors causes platelet aggregation and other inhibitory cellular responses. Some of these responses are attributable to a decrease in cAMP due to inhibition of adenylate cyclase. Activation of alpha 2-adrenergic receptors promotes their coupling to an inhibitory guanine nucleotide binding protein (Ni). This coupling promotes the binding of GTP to Ni, causing it to dissociate into subunits. This results in inhibition of the catalytic component of adenylate cyclase. Activation of alpha 1-adrenergic receptors stimulates the contraction of most smooth muscles and alters secretion and metabolism in several tissues. The primary event is a breakdown of phosphatidylinositol-4,5-bisphosphate in the plasma membrane to produce two intracellular "messengers": myo-inositol-1,4,5-trisphosphate (IP3) and 1,2-diacylglycerol (DAG). IP3 causes the release of Ca2+ from endoplasmic reticulum, producing a rapid rise in cytosolic Ca2+. Ca2+ binds to the regulatory protein calmodulin, and the resulting complex interacts with specific or multifunctional calmodulin-dependent protein kinases and other calmodulin-responsive proteins, altering their activities and thereby producing a variety of physiological responses. DAG also produces effects by activating a Ca2+-phospholipid-dependent protein kinase (protein kinase C) that phosphorylates and alters the activity of certain cellular proteins. Frequently there is synergism between the IP3 and DAG mechanisms.