Effect of auranofin and other gold complexes on the activity of phospholipase C

Auranofin (AF) is an orally active chrysotherapeutic agent used for the treatment of rheumatoid arthritis, a self-perpetuating inflammatory disease. Because of reports suggesting that AF and other gold complexes can, under certain circumstances, exacerbate rheumatoid inflammatory lesions in humans and adjuvant arthritic rats and that phospholipase C (PLC) and phospholipase A2 activities are increased in rheumatoid patients, the effects of AF and a related gold complex on in situ mammalian and purified Bacillus cereus PLC were examined. Results of our studies show that 1) AF and triethylphosphine gold chloride (TEPG), an AF analog, stimulated PLC activity in the sonicate of RAW 264.7 macrophages; 2) AF and TEPG stimulated B. cereus PLC activity in a concentration-dependent manner, but the pattern of stimulation and concentrations of drugs required to stimulate the purified enzyme differ from those seen with the macrophage PLC; 3) metals (cobalt and zinc) and sulfhydryl reagents (N-ethylmaleimide, iodoacetic acid, and glutathione), tested at the same concentrations of AF that enhanced PLC activity, had no effect on the enzyme. These data suggest that stimulation of PLC may be a generic phenomenon since two divergent PLCs are affected by gold complexes. Additionally, these studies may provide one potential explanation for rheumatoid lesion flares seen in patients and animals on chrysotherapy.

Immuno cross-reactivity suggests that catecholamine biosynthesis enzymes and beta-adrenergic receptors may be related

Turkey red blood cell, beta 1-adrenergic receptors (BARs) were prepared to electrophoretic homogeneity by affinity chromatography, size exclusion high performance liquid chromatography, and preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis and used to prepare rabbit polyclonal anti-BAR antibodies. Anti-BAR activity was confirmed by immunoadsorption of [125I]cyanopindolol-labeled BAR to a protein A affinity column using the anti-BAR antibodies. BAR was compared to the catecholamine biosynthetic enzyme dopamine B-hydroxylase (DBH) by anti-BAR antibody cross-reactivity. DBH was purified from bovine adrenal medullae chromaffin vesicles by ion exchange, size exclusion, and concanavalin A-Sepharose chromatography. Final DBH specific activities were 42 +/- 4 units/mg of protein. Homogeneity was confirmed by nondenaturing polyacrylamide gel electrophoresis. Both DBH and BAR were recognized by the anti-BAR antibodies on Western transfer and immunoblotting. No interactions were observed with preimmune controls. Similar results were obtained with glycosylated and deglycosylated DBH, suggesting that the anti-BAR antibodies recognize specific portions of DBH amino acid sequence and not associated carbohydrate. DBH-cross-reactive antibodies were also purified by affinity chromatography using immobilized DBH and shown to immunoadsorb [125I]cyanopindolol-labeled BAR by protein A affinity chromatography. These results suggest that the catecholamine biosynthetic enzyme DBH and BAR may be related in structure.

SKF 104353, a high affinity antagonist for human and guinea pig lung leukotriene D4 receptor, blocked phosphatidylinositol metabolism and thromboxane synthesis induced by leukotriene D4

SKF 104353 (2(S)-hydroxyl-3(R)-carboxyethylthio)-3-[2-(8-phenyloctyl) phenyl] propanoic acid) is a synthetic structural analog of leukotrienes D4 and E4 (LTD4, LTE4). This compound binds to guinea pig and human lung LTD4 receptors with affinities (Ki) of 5 +/- 2 and 10 +/- 3 nM, respectively. The Ki values of a reference compound, FPL 55712, were 2200 and 4500 nM, respectively, approximately 400- and 500-fold less effective than SKF 104353. LTD4- and LTE4-induced biosynthesis of thromboxane B2 has been shown to be mediated by LTD4 receptors in guinea pig lung in vitro. SKF 104353 did not induce synthesis of TxB2 in this system at concentrations of 1-20 microM. When SKF 104353 and increasing concentrations of LTD4 were incubated with guinea pig lung, the dose response curve of LTD4-induced TxB2 biosynthesis was shifted to the right with a -log[KB] = 8.4 +/- 0.2. LTD4-induced phosphatidylinositol (PI) hydrolysis in guinea pig lung has been shown to be the major signal transduction mechanism. In this system, SKF 104353 (1-20 microM) did not promote PI hydrolysis. Pretreatment of the [3H]myo-inositol-labeled guinea pig lung with SKF 104353 shifted the LTD4-induced PI hydrolysis dose response curve to the right, indicating that SKF 104353 inhibited LTD4 receptor-mediated intracellular second messenger formation. These results demonstrate that SKF 104353 is a high affinity, specific LTD4 receptor antagonist. It inhibited LTD4-induced PI hydrolysis and TxB2 biosynthesis in guinea pig lung. SKF 104353 may prove to be an important research tool for research on the activities of leukotrienes and of value therapeutically in the treatment of leukotriene-mediated diseases.

Characterization of a subline of P388 leukemia resistant to amsacrine: evidence of altered topoisomerase II function

Sensitive (P388/S) and amsacrine-resistant (P388/amsacrine) sublines of P388 leukemia were cloned in vitro and tested for differential chemosensitivity against a panel of drugs. P388/amsacrine, resistant both in vivo and in vitro to amsacrine, was cross-resistant to other putative topoisomerase II inhibitors including teniposide, etoposide, bisantrene, and doxorubicin. P388/amsacrine, was however, as sensitive as cloned P388/S to camptothecin, an inhibitor of topoisomerase I. The pattern of cross-resistance suggested that an alteration in topoisomerase II may be involved in the resistance of P388/amsacrine to these drugs. No differences in the uptake of amsacrine were detected between the two sublines. Cross-resistance to vinblastine was evident in P388/amsacrine; however resistance to vinblastine was associated with alterations in uptake or efflux of the drug. The number of protein-concealed single-strand breaks induced in whole cells by amsacrine, teniposide, bisantrene, and camptothecin was measured. Diminished numbers of strand breaks in the resistant subline were consistent with decreases in DNA-protein crosslinks. In the absence of drug treatment, resistant cells sustained approximately one-half as many single-strand breaks and DNA-protein crosslinks as the sensitive cells during preparation of nuclei. As measured by the P4 phage DNA unknotting assay, 0.35 M NaCl nuclear extracts from P388/S contained approximately 2.3-fold more topoisomerase II catalytic activity than did extracts from P388/amsacrine. The amount of protein that immunoreacted with a specific antibody to calf thymus topoisomerase II was also decreased in the resistant cells. These data suggest that alterations in topoisomerase II which lead to differential drug sensitivities are partially responsible for the resistance of P388/amsacrine to a specific group of drugs.

Solubilization and reconstitution of vasopressin V1 receptors of rat liver

The observation that vasopressin receptors loose their binding capacity on solubilization suggests that lipid-milieu may be a requirement for the binding of vasopressin to these receptors. To preserve the ligand binding capacity, we have developed reconstitution procedures to study solubilized rat liver vasopressin receptors. We report here that vasopressin V1 receptors were efficiently solubilized from rat liver membranes with egg lysolecithin and quantitatively reconstituted into phospholipid vesicles resulting in a 4-fold purification. Binding of [3H]arginine vasopressin to the reconstituted receptors was saturable and of high affinity (Kd = 0.6 +/- 0.1 nM). In competition binding experiments the solubilized, reconstituted receptors exhibited the same pharmacological profile as was observed with vasopressin V1 receptors in membranes.

Effects of N-ethylmaleimide on arginine vasopressin-induced responses in an established smooth muscle cell line

We have previously reported that 8-arginine vasopressin (AVP) stimulates phosphatidylinositol turnover and Ca++ mobilization in rat aortic smooth muscle cells (A10). In the present study, N-ethylmaleimide (NEM) was used to further characterize the putative guanine nucleotide binding protein that transduces the V1 receptor effects on phosphatidylinositol turnover and Ca++ efflux in these cells. Pretreatment of the cells with low concentrations of NEM did not affect the basal levels of the inositol phosphates and Ca++ efflux but significantly inhibited the AVP-induced increases. NEM pretreatment did not significantly affect [3H]AVP binding to intact cells. Guanyl-5'-yl imidodiphosphate reduced the apparent binding affinity of AVP to cell membranes. NEM pretreatment abolished this guanyl-5'-yl imidodiphosphate effect. AVP stimulated a specific GTPase activity in cell membranes; this effect was also abolished by NEM pretreatment. The results suggest that in A10 cells a guanine nucleotide binding protein sensitive to NEM couples vasopressin receptors to phospholipase C.

Thiol competition for Et3PAuS-albumin: a nonenzymatic mechanism for Et3PO formation

Thiols (RSH = 2,3,4,6-tetra-O-acetyl-beta-1-D-thioglucose, beta-1-D-thioglucose, and glutathione) can displace either the albumin or the triethylphosphine from the protein-gold complex, AlbSAuPEt3. The albumin is displaced in preference to triethylphosphine, but irreversible oxidation of the latter eventually shifts the equilibria toward Et3PO and AlbSAuSR. Albumin disulfide bonds are the probable oxidants. Neither O2 nor oxidized glutathione substantially enhanced the rate or extent of Et3PO formation. The labilization of the phosphine in AlbSAuPEt3 is attributed to a strong trans effect of the albumin thiolate, Cys-34. The 31P NMR chemical shifts of various thiolato(triethylphosphine)gold(I) complexes are correlated directly with the affinity of the thiols for gold and inversely with their pKSH values. Deacetylated auranofin (1-thio-beta-D-glucopyranosato-S) (triethylphosphine)gold(I) reacts with the mercaptalbumin and oxidized mercaptalbumin (putatively AlbSOH) forms of bovine serum albumin to form AlbSAuPEt3 with displacement of the thioglucose ligand.

An activator of protein kinase C (phorbol dibutyrate) attenuates atrial-natriuretic-factor-stimulated cyclic GMP accumulation in smooth-muscle cells

Rat thoracic aortic smooth-muscle cells (A-10; A.T.C.C. CRL 1476) displays a high density of vasopressin and atrial-natriuretic-factor (ANF) receptors and a low density of beta-adrenergic receptors. ANF stimulates cGMP (cyclic GMP) accumulation in a time- and dose-dependent fashion. Pretreatment of these cells with phorbol dibutyrate (PDBu), a known activator of protein kinase C, attenuated ANF-stimulated cGMP accumulation without affecting basal cGMP concentrations. This effect was concentration-dependent and was observed as early as 2 min after treatment. 4 alpha-Phorbol 12, 13-didecanoate (alpha PDD), which does not activate protein kinase C, did not inhibit the cGMP accumulation. PDBu pretreatment did not affect the density and affinity of ANF receptors. These data suggest that PDBu, presumably via activation of protein kinase C, might stimulate phosphorylation of a key regulatory protein in the ANF/cGMP pathway.

Relationship between the intracellular effects of camptothecin and the inhibition of DNA topoisomerase I in cultured L1210 cells

Results of filter elution assays of lesions produced in the DNA of cultured L1210 cells by the antineoplastic alkaloid camptothecin support the notion that topoisomerase I is an intracellular target of this drug. One to 10 microM camptothecin induced DNA single-strand, but not double-strand, breaks when incubated with intact cells or with their isolated nuclei. Approximately one half of the strand breakage was protein concealed, as judged by filter elution. Camptothecin-induced, protein-concealed DNA strand breaks disappeared rapidly after drug removal. DNA-protein cross-links were generated by camptothecin with frequencies approximately equal to those of protein-concealed DNA strand breaks. It is likely that camptothecin can inhibit topoisomerase I in intact cells in a manner similar to that in which other antineoplastic agents such as amsacrine or teniposide inhibit topoisomerase II. DNA-breaking lesions other than those resulting from trapped topoisomerase I-DNA complexes may also be generated by camptothecin. The yields of DNA strand breaks induced by camptothecin, amsacrine, or teniposide were approximately doubled when cells were incubated for 16 h with 3-aminobenzamide, an inhibitor of poly(ADP ribosylation) of proteins, prior to 1-h exposure to the antineoplastic compounds. 3-Aminobenzamide also enhanced the cytotoxic action of camptothecin, amsacrine, and teniposide. These results suggest that protein-concealed strand breaks can be lethal lesions and that intracellular topoisomerase I and II activity may be regulated coordinately through poly(ADP ribosylation).

Glucocorticoid regulation of beta-adrenergic receptors in 3T3-L1 preadipocytes

Treatment of 3T3-L1 preadipocytes (fibroblasts) with 250 nM dexamethasone for 48 hr caused a doubling of total beta-adrenergic receptors and an increase in beta 2-adrenergic receptor subtype proportion from approximately 50% in controls to 85% in treated cells. The responses to epinephrine and norepinephrine in a whole cell cAMP accumulation assay reflected these changes. The effects of dexamethasone on beta-adrenergic receptors were mediated through the glucocorticoid receptor and were time and dose dependent with an EC50 of 2.77 +/- 0.73 nM for an increase in the proportion of beta 2-adrenergic receptors. The rank order of potency of steroids to effect these changes (betamethasone = dexamethasone greater than fludrocortisone greater than hydrocortisone = triamcinolone greater than aldosterone) correlated with their glucocorticoid potency. [3H]Dexamethasone binding to intact cells yielded a KD value of 3.47 +/- 0.38 nM for binding to the glucocorticoid receptor which correlated well with the EC50 for dexamethasone to alter beta-adrenergic receptors. Inhibition of [3H]dexamethasone binding by other steroids confirmed that the ability of steroids to regulate beta-adrenergic receptors correlated with the affinity of each compound for the 3T3-L1 glucocorticoid receptor. Progesterone, which can bind to the glucocorticoid receptor but has only weak agonist activity, competitively inhibited the ability of dexamethasone to alter beta-adrenergic receptors. Protein synthesis, RNA synthesis, and N-linked glycosylation appeared to be necessary for the change in receptor subtype expression and the increase in beta-adrenergic receptor number induced by dexamethasone. The present study suggests that regulation of beta-adrenergic receptor expression in 3T3-L1 preadipocytes by dexamethasone is a glucocorticoid-specific effect which may require gene activation.

Structure-activity relationships of beta-adrenergic receptor-coupled adenylate cyclase: implications of a redox mechanism for the action of agonists at beta-adrenergic receptors

The present studies have tested the hypothesis that agonists at beta-adrenergic receptors activate the beta-receptors by reducing them. This was examined by analyzing the interactions of 41 beta-agonists and antagonists with the receptors. The structural features which determined binding affinity (KD) were shown to be distinct from those which determined intrinsic, activity (IA). The IA was shown to be related to the oxidation-reduction properties which were determined primarily by the nature of the substituents on the phenyl ring. Thus, the parent compound phenylethanolamine, having no phenolic substituent, acted as an antagonist (IA = 0) and was also redox inactive. All of the antagonists tested (19) exhibited EP (peak potential for the first oxidative wave) values greater than 0.75 V, suggesting that they were difficult to oxidize. Agonists, however, exhibited a wide range of EP (0.25-0.7 V) with values lower than those of the antagonists. The agonists tested include catecholamines, catecholamine analogs bearing meta-substituted amino functionalities (such as amino, methylamino, formanilide, sulfonamide, urea, and carbamate), resorcinol, and hydroxymethyl congeners. It is proposed that the oxidizing tendency of the substituent on the phenyl ring is one of the factors that influences IA. To test the hypothesis further, we electrolytically oxidized isoproterenol to adrenochrome or to the o-quinone intermediate and tested for activity. The 4e-, 4H+-oxidation product adrenochrome did not bind to or stimulate adenylate cyclase, suggesting that the reducing ability to isoproterenol is important for its agonistic activity. A cyclic redox mechanism for the action of agonists at beta-adrenergic receptors is presented. We propose that agonist are electron donors. Their interactions with receptors result in reduction leading to activation of the receptors.

Identification and isolation of a mammalian protein which is antigenically and functionally related to the phospholipase A2 stimulatory peptide melittin

Antibodies prepared against the phospholipase A2 stimulatory peptide melittin were used to identify and isolate a novel mammalian protein with similar functional and antigenic properties. The mammalian protein of Mr 28,000 was isolated from cell sonicates by high performance immunoaffinity chromatography and size exclusion chromatography. This stimulatory protein was stable for several months when frozen at -70 degrees C. The purified protein selectively stimulated phospholipase A2 when phosphatidylcholine was used as a substrate but had no effect on phospholipase A2 activity when phosphatidylethanolamine was used as a substrate. Furthermore, this protein had no effect on phospholipase C activity or on pancreatic or snake venom phospholipase A2. The stimulatory activity was unaffected by RNase or DNase treatment. However, boiling or trypsin digestion inactivated the phospholipase stimulatory activity. The mechanism of phospholipase A2 stimulation appeared to result from an increase in the apparent Vmax of the enzyme.

Identification and characterization of leukotriene D4 receptors and signal transduction processes in rat basophilic leukemia cells

The leukotriene D4 (LTD4) receptor on rat basophilic leukemia (RBL-1) cell membranes was characterized using a radioligand binding assay. [3H]LTD4 binding to RBL-1 membrane receptors was stereoselective, specific, and saturable. The binding affinity and maximum binding density of [3H]LTD4 to RBL-1 membrane receptors were 0.9 +/- 0.2 nM and 800 +/- 125 fmol/mg protein, respectively. Binding of [3H]LTD4 to the receptors was enhanced by divalent cations (Ca2+, Mg2+, and Mn2+) and inhibited by guanine nucleotides and sodium ions, specifically, indicating that a guanine nucleotide-binding protein may regulate the agonist-receptor interaction. LTD4, LTE4 agonist and antagonist analogs competed with the radioligand in binding to the RBL-1 LTD4 receptors. The binding affinities of these analogs correlated with (a) those determined from the guinea pig lung LTD4 receptors and (b) the pharmacological activities in smooth muscle contraction. LTD4 and related agonists also induced time- and concentration-dependent phosphatidylinositol hydrolysis in RBL-1 cells. The LTD4 induction of inositol 1-phosphate was potent, stereoselective, specific, and was blocked by LTD4 receptor antagonists. The rank order potency of agonist-induced inositol 1-phosphate formation in RBL-1 cells was equivalent to the receptor binding affinity determined using either RBL-1 cell or guinea pig lung membranes. These studies have demonstrated the G protein coupled LTD4 receptors on RBL-1 cell membranes. Binding of agonists to the receptor may activate the G protein-regulated phospholipase C to induce hydrolysis of phosphatidylinositol. The hydrolytic products of phosphatidylinositol, possibly inositol trisphosphate and diacylglycerol, may be the intracellular messengers for LTD4 receptors in RBL-1 cells.

Chemical synthesis and expression of a cassette adapted ubiquitin gene

A gene encoding the yeast ubiquitin was chemically synthesized and expressed in yeast under regulatory control of the copper metallothionein (CUP1) promoter. The gene was assembled in a one-step ligation reaction from eight oligonucleotide fragments ranging in length from 50 to 64 nucleotides. To facilitate mutagenesis and gene fusion studies, eight unique 6-base-cutting restriction enzyme sites were placed in the reading frame which did not alter the encoded protein sequence or force the utilization of rare codons. In a copper-resistant yeast strain (CUP1r), expression of the gene was induced by copper to approximately 5% of the total yeast proteins, as determined by Coomassie-stained polyacrylamide gels. The protein, purified from yeast, reacted with ubiquitin-specific antibodies and was found to be biologically active in supporting ubiquitin-dependent protein degradation in vitro.

Cellular interactions of auranofin and a related gold complex with RAW 264.7 macrophages

Auranofin (AF) is an orally active chrysotherapeutic agent whose precise mechanism of action with its putative target cell, the macrophage, is not known. In a previous paper, we described a sequential thiol exchange mechanism that explained auranofin's molecular mechanism of interaction with RAW 264.7 cells. To further understand the mode of action of AF and to test the validity of the thiol exchange model, we have continued to study the interactions with macrophages of AF and a related gold complex, triethylphosphine gold chloride (TEPG). Evaluation of the effects of AF and TEPG on RAW 264.7 cells demonstrated that: more gold from TEPG than AF associated with cells over time and with a variety of concentrations; and cellular association of AF and TEPG was temperature dependent. The energy of activation for cell association, the rate-limiting step in the thiol exchange process, was lower for TEPG than AF; cellular association and uptake of both compounds did not require metabolic energy; and efflux of both AF and TEPG was time, temperature, and thiol dependent. Based on these and previous data, we conclude that sequential thiol exchange may be a generic phenomenon for cellular uptake and distribution of thiol reactive gold compounds and that the rate-limiting step is the exchange of either tetraacetylthioglucose (TATG) or chloride for a membrane-localized sulfhydryl group.

A vector for construction of gene libraries and the expression of heterologous genes in Saccharomyces cerevisiae

We have constructed a convenient new vector, YEp-DE, for the construction of gene libraries and the expression of heterologous genes in Saccharomyces cerevisiae. The vector contains the yeast LEU2 gene, the 2 mu origin of replication, and a region from pUC18 that includes the ampr gene, the Escherichia coli origin of replication (ori), and the LacZ gene with multiple cloning sites. Five sites (Sac1, Sma1, BamH1, Sal1, Sph1) in this region are unique. This vector has advantages over similar yeast-E. coli shuttle vectors: small size (7291 bp, entirely sequenced), convenient cloning sites, and lacZ selection for detecting recombinant plasmids.

Identification and characterization of vascular (V1) vasopressin receptors of an established smooth muscle cell line

We report the identification and characterization of specific vasopressin-binding sites on intact cells and membranes of the established vascular smooth muscle cell line A-10, the fate of vasopressin associated with the cells, the role of guanine nucleotides in the regulation of the affinity of the vasopressin-binding sites, and the determination of the vasopressin receptor subtype. We have found specific vasopressin-binding sites on intact cells in monolayer (110,000 sites per cell during log growth and 60,000 sites per cell in stationary culture) with a KD of 6 nM at 37 degrees. After incubation of [3H]-8-arginine vasopressin ([3H]AVP) and cells for less than 20 min, cell-associated AVP was intact; with longer incubation times, AVP was progressively degraded. The major metabolites included phenylalanine and a fraction that eluted from a C18 reverse phase high performance liquid chromatography column between AVP and 8-arginine, 9-desglycinamide vasopressin. Extensive degradation also occurred when AVP was allowed to dissociate from the cells. With increased time of incubation, the amount of specifically bound AVP that could dissociate decreased, suggesting receptor-mediated endocytosis. In saturation equilibrium binding experiments with plasma membranes, two affinity states with KD of 0.7 nM and 379 nM were observed. The number of high affinity binding sites was similar to the number of receptors found on intact cells. Guanosine 5'-(beta,gamma-imido)triphosphate decreased vasopressin binding to the high affinity sites and did not significantly affect the low affinity sites. Competition binding experiments indicated that the vasopressin-binding sites of A-10 cells belong to the vascular V1 receptor subtype. We conclude that the established vascular smooth muscle cell line A-10 expressed vasopressin receptors of the vascular V1 subtype. Vasopressin bound to the receptors reversibly, but could also be degraded by the cells presumably after receptor-mediated endocytosis. The receptors might exist in different affinity states; guanosine 5'-(beta,gamma-imido)triphosphate decreased the affinity of the high affinity binding state.

Differential effects of manoalide on secreted and intracellular phospholipases

Manoalide, a novel nonsteroidal sesterterpenoid, is a potent inhibitor of phospholipase A2 isolated from bee and cobra venoms. This report compares the inhibition by manoalide of phospholipase A2 in crude cytosol fractions from four mammalian tissues with that of four purified extracellular phospholipase A2's. Phospholipase A2 isolated from bee venom (Apis mellifera) was the most sensitive to inactivation by manoalide (IC50 approximately equal to 0.12 microM). Extracellular phospholipase A2 from rattlesnake and cobra venom was intermediate in sensitivity to manoalide (IC50 values of 0.7 and 1.9 microM respectively). Porcine pancreatic phospholipase A2 was relatively resistant to inactivation by manoalide (IC50 approximately equal to 30 microM). The phospholipase A2 assayed in crude cytosol fractions from four mammalian tissues exhibited IC50 values of 30 microM or greater. Cytosolic proteins as well as bovine serum albumin and poly-L-lysine (Mr = 57,000) protected purified bee venom phospholipase A2 from inactivation by manoalide. In contrast, amino acids such as lysine and alanine failed to protect the purified enzyme from inactivation. Proteins and certain amino acids, such as lysine, formed a chromogenic product when incubated with manoalide. These data suggest that lysine is capable of reacting with manoalide, but only when it is present in macromolecules is it capable of protecting phospholipase A2 from inactivation by manoalide. Because cellular proteins protect PLA2 from inactivation by manoalide, high concentrations of manoalide must be applied topically to produce statistically significant inactivation of intracellular phospholipase A2. Finally, a chemical model is presented which explains the formation of a chromogenic product when manoalide is incubated with proteins and amino acids.

A novel radiolabeled vasopressin antagonist: [3H-Phe]-desGlyd(CH2)5D-Tyr(Et)VAVP, [3H]-SK&F 101926

We report the vasopressin receptor-binding properties of [3H-Phe]-desGlyd(CH2)5D-Tyr(Et)VAVP, [3H]-SK&F 101926, the first radiolabeled vasopressin receptor antagonist. We chose to radiolabel SK&F 101926 because this vasopressin analog is a potent antagonist of vascular V1 and renal V2 vasopressin receptors in all species studied. [3H]-SK&F 101926 bound with a single high affinity to intact vascular smooth muscle cells (A-10; KD = 0.5 nM), and plasma membranes A-10 cells (KD = 0.4 nM) and rat liver (KD = 0.2 nM). In competition experiments with [3H]-SK&F 101926 and [3H]arginine vasopressin ([3H]AVP) using cell and liver membranes, the affinity rank orders of vasopressin analogs were the same and were typical for the V1 receptor subtype. In competition binding experiments with [3H]-SK&F 101926 using cell and liver membranes, guanosine 5'-(beta,gamma-imido)triphosphate did not significantly alter the affinity of the V1 antagonist d(CH2)5Tyr(Me)AVP, but the affinity of AVP was decreased. These data indicate that the V1 receptor can exist in at least two affinity states that are modulated by guanine nucleotides. [3H]-SK&F 101926 also bound specifically and with high affinity to V2 receptors of MDCK cells. We conclude that [3H]-SK&F 101926 binds with high affinity to V1 and V2 vasopressin receptors and is a powerful new tool for the identification of vasopressin receptors and the study of molecular mechanisms involved in the interaction of vasopressin with its receptors.

Phorbol ester-mediated inhibition of vasopressin and beta-adrenergic responses in a vascular smooth muscle cell line

We have reported previously that in the vascular smooth muscle cell line A-10 (ATCC CRL 1476), vasopressin stimulated phosphatidylinositol turnover Ca2+ efflux and inhibited isoproterenol-stimulated cAMP accumulation. Here we report that pretreatment of these cells with phorbol dibutyrate, an activator of protein kinase C, attenuated the responses to vasopressin and isoproterenol. This effect was concentration dependent and could be observed after pretreatment for 2 min. 4 alpha Phorbol 12,13-didecanoate, which does not activate protein kinase C, did not attenuate the responses. These data suggest that activation of protein kinase C by phorbol dibutyrate attenuates the responses of vascular smooth muscle cells to isoproterenol and vasopressin. Although phorbol ester did not affect [3H]-8-arginine vasopressin binding to intact cells, it appeared to uncouple vasopressin receptors from guanine nucleotide-binding protein.

Leukotriene-induced hydrolysis of inositol lipids in guinea pig lung: mechanism of signal transduction for leukotriene-D4 receptors

Addition of leukotriene D4 (LTD4) to [3H]myo-inositol-labeled guinea pig lung induced rapid breakdown of inositol lipids. Formation of [3H]inositol trisphosphate was rapid, with a peak of 140-160% of the control level, 30 sec post-treatment. Formation of [3H]inositol bisphosphate and [3H]inositol monophosphate ([3H]IP1) was also rapid in the presence of LiCl. LTD4-induced [3H]IP1 formation was concentration dependent, stereoselective, and not inhibited by the cyclooxygenase inhibitor, indomethacin. Agonist analogs of LTD4 and leukotriene E4 also induced dose-dependent increases in the synthesis of [3H]IP1. The rank order potency of the agonist-induced [3H]IP1 formation was equivalent to those reported for LTD4 receptor binding, smooth muscle contraction, and thromboxane B2 biosynthesis. Furthermore, a specific receptor antagonist, SKF 102922, inhibited LTD4-induced [3H]IP1 formation in guinea pig lung. These studies suggest that LTD4 may interact with membrane receptor and activate a phospholipase C, which in turn induces the hydrolysis of inositol lipids. The hydrolysis products, diacylglycerol and inositol trisphosphate, can be regarded as the intracellular messengers for LTD4 receptors in guinea pig lung. This concept may explain a variety of pharmacological effects of leukotrienes in different types of target cells or tissues.

Induction of metallothionein is correlated with resistance to auranofin, a gold compound, in Chinese hamster ovary cells

Metallothioneins (MTs) are low molecular weight, thiol-rich, metal-binding proteins. Auranofin (AF) is a gold compound active in the treatment of rheumatoid arthritis. The effects of AF on regulation of MT gene expression in Chinese hamster ovary cells were studied. AF-resistant cells accumulated substantial amounts of MT mRNA and protein, whereas no induction was observed in AF-sensitive cells. Cells capable of inducing MT in the presence of AF were much less sensitive to AF-mediated cytotoxicity. Induction of MT by low concentrations of Cd protected cells from subsequently administered doses of AF. The level of protection correlated with the level of induced MT. These findings indicate that MT plays a central role in the mechanisms underlying cellular resistance to gold compounds.