Comparison of the Ca2+ movement by activation of alpha1-adrenoceptor subtypes in HEK-293 cells

Development of a Generic Dual-Reporter Gene Assay for Screening G-Protein-Coupled Receptors

Multiple assay formats have been developed for the pharmacological characterization of G-protein-coupled receptors (GPCRs) and for screening orphan receptors. However, the increased pace of target identification and the rapid expansion of compound libraries present the need to develop novel assay formats capable of screeningmultipleGPCRs simultaneously. To address this need, the authors have developed a generic dual-reporter gene assay that can detect ligand activity at 2 GPCRs within the same assay. Two stableHEK293 cell lineswere generated expressing either a firefly ( Photinus) luciferase gene under the control ofmultiple cAMP-response elements (CREs) or a Renillaluciferase gene under the control ofmultiple 12-Otetradecanoylphorbol-13-acetate (TPA)-responsive elements (TREs). Coseeded reporter cells were used to assess ligandbinding activity at bothGβ s-and Gβ q-coupled receptors. By selectively coexpressing receptors with a chimeric G-protein, agonist activitywas assessed atGβ i/o-coupled receptors in combinationwith eitherGβ s-or Gβ q-coupled receptors. The dual-reporter gene assaywas shown to be capable of simultaneously performing duplexed screens for a variety of agonist and/or antagonist combinations. The data generated from the duplexed reporter assays were pharmacologically relevant, and Zβ factor analysis indicated the suitability of both agonist and antagonist screens for use in high-throughput screening.

Ginsenoside Rd prevents glutamate-induced apoptosis in rat cortical neurons

1. The role of voltage-independent Ca(2+) entry in cell apoptosis has recently received considerable attention. It has been found that ginsenoside Rd significantly inhibits voltage-independent Ca(2+) entry. The aim of the present study was to investigate the protective effects of ginsenoside Rd against glutamate-induced apoptosis of rat cortical neurons. 2. Ginsenoside Rd significantly reduced glutamate-induced apoptotic morphological changes and DNA laddering. In comparison, nimodipine only had a weak effect. 3. Ginsenoside Rd (1, 3 and 10 micromol/L) concentration-dependently inhibited caspase 3 activation and expression of the p20 subunit of active caspase 3 (by 30 +/- 10%, 41 +/- 9% and 62 +/- 19%, respectively, compared with glutamate alone; P < 0.05), whereas 1 micromol/L nimodipine had no effect. 4. Glutamate decreased cell viability to 37.4 +/- 4.7 (n = 8) and evoked cell apoptosis. Ginsenoside Rd (1, 3, 10 and 30 micromol/L) concentration-dependently inhibited glutamate-induced cell death, increased cell viability and reduced apoptotic percentage (from 47.5 +/- 4.9% to 37.4 +/- 6.9%, 28.3 +/- 5.2% and 22.5 +/- 5.6%, respectively; P < 0.05). At 1 micromol/L, nimodipine had no effect on cell viability. Furthermore, although 1, 3, 10, 30 and 60 micromol/L ginsenoside Rd concentration-dependently inhibited glutamate-induced Ca(2+) entry by 8 +/- 2%, 24 +/- 4%, 40 +/- 7%, 49 +/- 8% and 50 +/- 8% (P < 0.05), respectively, nimodipine had no effect. 5. In conclusion, the results indicate that ginsenoside Rd prevents glutamate-induced apoptosis in rat cortical neurons and provide further evidence of the potential of voltage-independent Ca(2+) channel blockers as new neuroprotective drugs for the prevention of neuronal apoptosis and death induced by cerebral ischaemia.

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.

Ginsenoside-Rd from panax notoginseng blocks Ca2+ influx through receptor- and store-operated Ca2+ channels in vascular smooth muscle cells

Previously, it was found that total saponins from panax notoginseng inhibited Ca2+ influx coupling to activation of alpha1-adrenoceptor. This study was designed to investigate the effects of ginsenoside-Rd from total saponins of panax notoginseng on receptor-operated (ROCC) and store-operated (SOCC) Ca2+ channels in vascular smooth muscle cells using fura-2 fluorescence, whole cell patch clamp ion channel recording, radio-ligand-receptor binding, 45Ca2+ radio-trace and organ bath techniques. It was found that ginsenoside-Rd reduced phenylephrine-induced contractile responses and Ca2+ influx in normal media without significant effect on these responses in Ca2+ -free media. Ginsenoside-Rd also decreased phenylephrine- and thapsigargin-induced inward Ca2+ currents, and attenuated thapsigargin- and 1-oleoy-2-acetyl-sn-glycerol (OAG)-induced cation entries that are coupled to ROCC and SOCC respectively. Ginsenoside-Rd failed to inhibit KCl-induced contraction of rat aortal rings and Ca2+ influx, and did not alter voltage-dependent inward Ca2+ current (VDCC) which was blocked by nifedipine. Also, ginsenoside-Rd did not change binding site and affinity of [3H]-prazosin for alpha1-adrenoceptor in the vascular plasma membrane. These results suggest that ginsenoside-Rd, as an inhibitor, remarkably inhibits Ca2+ entry through ROCC and SOCC without effects on VDCC and Ca2+ release in vascular smooth muscle cells.

Interaction between Cl- channels and CRAC-related Ca2+ signaling during T lymphocyte activation and proliferation

AIM

To test the hypothesis that Cl channel blockers affect T cell proliferation through Ca2+-release-activated Ca2+ (CRAC) signaling and examine the effects of the combination of a CRAC channel blocker and a Cl channel blocker on concanavalin A (ConA; 5 mg/mL)-induced Ca2+ signaling, gene expression and cellular proliferation in human peripheral T lymphocytes.

METHODS

[3H]Thymidine incorporation, Fura-2 fluorescent probe, RNase protection assay, and reverse transcription-polymerase chain reaction were used.

RESULTS

The Cl channel blocker 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) inhibited ConA-induced Ca2+ influx, interleukin-2 mRNA expression and T lymphocyte proliferation in a concentration-dependent manner, and also enhanced the inhibitory effects of 1-[beta-[3-(4-methoxyphenyl)propoxyl]-4-methoxyphenethyl]-1H-imidazole (SK&F96365) on the above key events during T cell activation. A combination of DIDS (1 micromol/L) and SK&F96365 (1 micromol/L) significantly diminished ConA-induced ClC-3 mRNA expression by 64%, whereas DIDS(1 micromol/L) or SK&F96365 (1 micromol/L) alone decreased ConA-induced ClC-3 mRNA expression by only 16% and 9%, respectively.

CONCLUSION

These results suggest that there is an interaction between CRAC-mediated Ca2+ signaling and DIDS-sensitive Cl channels during ConA-induced T cell activation and proliferation. Moreover, the DIDS-sensitive Cl channels may be related to the ClC-3 Cl channels.

Evidence for capacitative and non-capacitative Ca2+ entry pathways coexist in A10 vascular smooth muscle cells

It is generally thought that receptor-operated Ca2+ entry is related to store-operated or capacitative Ca2+ entry mechanism. Recent evidence suggests that non-capacitative Ca2+ entry pathways are also involved in receptor activated Ca2+ influx in many different kinds of cells. In this study, we studied whether alpha1-adrenoreceptor (alpha1-AR)-activated Ca2+ entry is coupled to both capacitative and non-capacitative pathways in A10 vascular smooth muscle cells by fura-2 fluorescence probe and conventional whole-cell patch clamp techniques. We found that both thapsigargin (TG) and phenylephrine (Phe) induced transient increase in cytoplasmic Ca2+ concentration ([Ca2+]i) in Ca2+-free medium, and subsequent addition of Ca2+ evoked a sustained [Ca2+]i rise. When the membrane potential was held at -60 mV, both TG and Phe activated inward currents, which were inhibited by GdCl3(Gd3+), 0Na+/0Ca2+ solution and 1-{beta[3-(4-mehtoxyphenyl)propoxy]-4-methoxypheneth-yl}-1H- imidazole hydro-chloride (SK&F96365), but not by nifedipine. When Ca2+ store was depleted by TG in Ca2+-free solution, Phe failed to further evoke [Ca2+]i rise. However, when capacitative Ca2+ entry was activated by TG in the medium containing Ca2+, 10 microM Phe further increased [Ca2+]i. At the same concentration, TG activated an inward cation current, subsequent addition of Phe also further induced an inward cation current. Furthermore, the amplitudes of [Ca2+]i increase and current density induced by Phe in the presence of TG were less than that induced by Phe alone. Our results suggest that both capacitative and non-capacitative Ca2+ entry pathways are involved in Ca2+ influx induced by activation of alpha1-AR in A10 vascular smooth muscle cells.

Ca2+ participates in alpha1B-adrenoceptor-mediated cAMP response in HEK293 cells

AIM

To investigate the alpha1B-adrenoceptor (alpha1B-AR)-mediated cAMP response and underlying mechanisms in HEK293 cells.

METHODS

Full-length cDNA encoding alpha1B-AR was transfected into HEK293 cells using the calcium phosphate precipitation method, and alpha1B-AR expression and cAMP accumulation were determined by using the saturation radioligand binding assay and ion-exchange chromatography, respectively.

RESULTS

Under agonist stimulation, alpha1B-AR mediated cAMP synthesis in HEK293 cells, and blockade by PLC-PKC or tyrosine kinase did not reduce cAMP accumulation induced by NE. Pretreatment with pertussis toxin (PTX) had little effect on basal cAMP accumulation as well as norepinephrine (NE)-stimulated cAMP accumulation. In addition, pretreatment with cholera toxin (CTX) neither mimicked nor blocked the effect induced by NE. The extracellular Ca2+ chelator egtazic acid (EGTA), nonselective Ca2+ channel blocker CdCl2 and calmodulin (CaM) inhibitor W-7 significantly reduced NE-induced cAMP accumulation from 1.59%+/-0.47% to 1.00%+/-0.31%, 0.78%+/-0.23%, and 0.90%+/-0.40%, respectively.

CONCLUSION

By coupling with a PTX-insensitive G protein, alpha1BAR promotes Ca2+ influx via receptor-dependent Ca2+ channels, then Ca2+ is linked to CaM to form a Ca2+-CaM complex, which stimulates adenylyl cyclase (AC), thereby increasing the cAMP production in HEK293 cell lines.

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 Cl- channel blockers on endothelin-1-induced proliferation of rat vascular smooth muscle cells

The effects of Cl- channel blockers on endothelin-1 (ET-1)-induced proliferation of rat aortic vascular smooth muscle cells (VSMC) were examined. We found ET-1 concentration-dependently increased cell count and [3H]-thymidine incorporation into VSMC, with EC50 values of 24.8 and 11.4 nM, respectively. Both nifedipine and SK&F96365 inhibited 10 nM ET-1-induced [3H]-thymidine incorporation into VSMC with the maximal inhibitory concentrations of 1 and 10 microM, respectively. DIDS inhibited 10 nM ET-1-induced increase in cell count and [3H]-thymidine incorporation into VSMC in a concentration-dependent manner, whereas other Cl- channel blockers including IAA-94, NPPB, DPC, SITS and furosemide did not produce these effects. 3 microM DIDS reduced 10 nM ET-1-induced sustained increase in cytoplasmic Ca2+ concentration ([Ca2+]) by 52%. Pretreatment of VSMC with 1 microM nifedipine completely inhibited the DIDS effect on 10 nM ET-1-induced [3H]-thymidine incorporation into VSMC and sustained increase in [Ca2+]i, whereas pretreatment with 10 microM SK&F96365 did not completely block these effects of DIDS. DIDS did not affect ET-1-induced Ca2+ release and 30 mM KCl-induced increase in [Ca2+]i. Our data suggest that DIDS-sensitive Cl- channels mediate VSMC proliferation induced by ET-1 by mechanisms related to membrane depolarization and Ca2+ influx through voltage-dependent Ca2+ channels.

Inverse alpha(1A) and alpha(1D) adrenoceptor mRNA expression during isolation of hepatocytes

It is now well documented that changes in gene expression take place during cell isolation and culture. Here, we report the change in the expression of the mRNAs for alpha(1)-adrenoceptor subtypes, during dissociation of guinea pig liver cells with collagenase. Using Reverse Transcription-Polymerase Chain Reaction (RT-PCR) assays, it was observed that during the isolation procedure, the mRNA for the alpha(1A)-adrenoceptor, normally expressed in whole liver, was degraded and the mRNA for alpha(1D) subtype, barely expressed in whole liver, increased in an actinomycin D-sensitive manner. When the isolation procedure was performed in the presence of cycloheximide, the mRNA for the alpha(1A)-adrenoceptor did not diminish and the induction of the alpha(1D)-adrenoceptor mRNA was even more evident. Our data indicate that cell isolation alters alpha(1)-adrenoceptor mRNA expression.

Ca2+-dependence of vasoconstriction mediated by alpha1A-adrenoceptors in perfused rat hindlimb: a pharmacological approach

We investigated the source of Ca2+ for the vasoconstriction mediated by alpha1a-adrenoceptors in perfused rat hindlimb in functional studies. The noradrenaline (NA)-induced maximum response was decreased by 92% following perfusion with Ca2+-free medium. Depletion of intracellular Ca2+-stores with repeatedly application of caffeine and NA in Ca2+-free medium resulted in complete abolishment of NA-response. Nifedipine concentration-dependently inhibited NA-contraction with a maximum inhibition of 65%. The residual nifedipine-insensitive response was further inhibited by Cd2+. Following depletion of Ca2+ stores with cyclopiazonic acid in Ca2+ free medium for 30 min, the NA-response obtained by re-admission of Ca2+ was decreased by 80%. However, re-introduction of Ca2+ to NA-treated tissues in Ca2+-free medium without prior treatment with cyclopiazonic acid normalizes the NA-response. These results suggest that the NA-contraction in this preparation is mediated largely via an influx of extracellular Ca2+, of which the majority utilizes L-type calcium channels. Only a small portion of the contractile response to NA is derived from intracellular stores, which probably also play a modulatory role on Ca2+ influx.