Receptors & signal transduction最新文献

Protein tyrosine phosphatases are important components of signal transduction pathways. The authors have used reverse transcription/polymerase chain reactions to accomplish a comprehensive examination of the RNA expression for 58 distinct mammalian protein tyrosine and dual specificity phosphatase (PTPase) and PTPase-like genes in the normal human diploid fibroblast cell line WI-38. Thirty-seven of these PTPase genes express easily measurable RNA, and four simultaneously express the RNA for two or more isoforms. Messages for an additional eight PTPase genes are detectable at low levels. Only 14 known PTPase genes do not express measurable RNA under our conditions. For purposes of comparison, the authors also assessed the PTPases expressed in the WI-38 cell line using highly degenerate primers to conserved motifs found in the classical tyrosine-specific PTPases. Only eight of the 22 classic tyrosine-specific PTPases detected using the specific primers were detected using these degenerate primers. Our panel of specific PTPase primers should be very useful for semiquantitatively assessing the repertoire of PTPases expressed by cells.

The goal of this study was to investigate the synergistic effects between G-protein-coupled receptor agonists and forskolin-induced activation of adenylyl cyclases, in cultured human corpus cavernosum smooth-muscle cells. Treatment of human corpus cavernosum smooth-muscle cells with forskolin (0.1-10 microM) produced an increase in cAMP synthesis in a concentration-dependent manner. Forskolin-induced adenylyl cyclase activity was markedly augmented by prostaglandin E1 (PGE1) and its metabolite, PGE0, isoproterenol, carbachol, and phenylephrine. Augmentation of forskolin-induced cAMP by PGE1, and PGE0 is probably mediated by prostaglandin E receptors (EP). Enhancement of forskolin-induced cAMP synthesis by isoproterenol is mediated by beta-adrenergic receptors (beta-AR), since this activity was inhibited by propranolol. Stimulation of forskolin-induced cAMP synthesis by carbachol is attributed to activation of muscarinic acetylcholine receptors (mAChR), as demonstrated by inhibition with atropine. The augmentation of forskolin-induced cAMP synthesis by phenylephrine, an alpha1-adrenergic receptor (AR) agonist, however, was unexpected and cannot be attributed to increased intracellular Ca2+, since treatment of cells with either the Ca2+ ionophore, A23187, or 80 mM KCl did not affect forskolin-induced cAMP synthesis. Stimulation of forskolin-induced cAMP synthesis by phenylephrine is explained by its binding to beta-AR and activation of Gs protein, since this augmentation was inhibited by the beta-AR antagonist, propranolol. This observation was further supported by physiological studies in organ bath chambers, in which forskolin-induced relaxation of precontracted corpus cavernosum strips was enhanced by phenylephrine. These studies suggest that synergism between agonist-induced cAMP synthesis and forskolin is attributed to increased conformational stabilization of activated adenylyl cyclase catalytic domains by forskolin and the Gs(alpha)-subunit of activated Gs proteins.

Recently it has been shown that purified complexes of major histocompatibility (MHC) class II and antigenic peptide can recognize T-cell receptors (TCRs) on virally transformed CD4+ T-cells in vitro. It is not clearly understood whether peptide bound to purified MHC II molecules (MHC-P), or to MHC II molecules on the surface of antigen-presenting cells (APC-peptide), initiate similar or different signals in transformed human T-cells. To address this question, the expression of protein tyrosine kinases (PTKs), their phosphorylation, and the effect of various kinase inhibitors were investigated using transformed T- and B-cells. HLA-DR2- and MBP(84-102)-restricted cloned T-cells (SS8T) immortalized with herpes saimiri virus (HSV) and DR2-expressing lymphoblastoid B cells transformed with Epstein-Barr virus (EBV) were utilized in this study. The expression and phosphorylation of three major PTKs (1ck-56, fyn-59, zap-70) involved in signaling through the TCR were analyzed by enhanced chemiluminescence blots. T-cells exposed to soluble MHC-P complex did not show altered expression of 1ck-56 protein. In contrast, a decrease in 1ck expression was observed in SS8T cells when TCRs were engaged with APC-peptide. Upon interaction with the TCR, both MHC-P complex and APC-peptide showed increased fyn-59 protein expression and phosphorylation. In our experiments using immortalized T- and B-cells, the expression of zap-70 protein remained unchanged. When T-cells were exposed to herbimycin and H-7, inhibitors of PTKs and protein kinase C (PKC) pathways, respectively, a dose-dependent decrease in gamma-IFN levels was observed with both systems. However, in the presence of genestein, another PTK inhibitor, such decrease in gamma-IFN was observed only in the case in which T-cells were exposed to MHC-P complexes. These results together suggest that the occupancy of TCRs in transformed T-cells by soluble MHC-P complex and APC-peptide differs with respect to the 1ck expression, although both can induce signals that lead to increased fyn activity and its phosphorylation. In addition, genestein showed differential inhibitory effect on gamma-IFN production by T-cells exposed to APC-peptide and MHC-P complexes, suggesting that the TCR occupancy by MHC-P complex and APC-peptide have subtle differences in PTK pathways.

Endothelins (ETs) are 21-amino acid peptides that bind to membrane receptors to initiate pathophysiological effects. This report characterizes ET receptors in benign prostatic hyperplasia-1 (BPH-1) cells, a prostate cell line isolated from a specimen of a 60-yr-old man with benign prostatic hyperplasia. [(125)I]ET-1 or -3 binding was of high affinity, with B(max) and K(d) values of 48 fmol/1 x 10(6) cells and 0.16 nM for ET-1, and 2.9 fmol/1 x 10(6) cells and 0.033 nM for ET-3, respectively. ET-1, ET-3, FR139317, Ro 46-2005, and IRL1620 inhibited [(125)I]ET-1 binding to these cells with IC50 values of 0.22, 186, 0.20, 52.8, and 772.3 nM, respectively. Reverse transcription-polymerase chain reaction confirmed that BPH-1 cells expressed more ET(A) than ET(B) receptors. ET-1 did not have any effect on arachidonic acid release, but caused a modest stimulation of phosphatidylinositol hydrolysis, and induced a prominent, sustained elevation in intracellular Ca2+ concentrations. The functional effects of ET-1 were completely inhibited by the ET(A)-selective antagonists FR139317 and A-127722, suggesting that the effects were mediated by the ET(A) receptor. These results suggest that ET may play functional roles in benign prostatic hyperplasia.

The binding of oxidized low-density lipoprotein (Ox LDL) by monocyte-macrophages causes pleiotropic effects, including changes in gene expression, and is thought to represent an early event in atherogenesis. The integral membrane glycoprotein CD36 appears to play a physiological role in binding and uptake of Ox LDL by monocyte-macrophages, although the molecular events associated with CD36-Ox LDL interaction are unknown. To approach this issue, we used CD36 transfected Chinese hampster ovary (CHO) cells, exposed them to Ox LDL, and determined changes in the activity of the transcription factor NF-kappaB. We report here that Ox LDL enhanced DNA binding activity of nuclear extracts to an NF-kappaB sequence following activation of CD36-producing CHO cells with the proinflammatory cytokine tumor necrosis factor-alpha (TNF-alpha). This enhanced DNA binding activity was inhibited by coincubation of CD36 transfected cells with the human CD36-specific antibody OKM5. We also determined that activation of NF-kappaB DNA binding activity required an intact carboxyl-terminal cytoplasmic segment on CD36. Our results support the idea that human CD36 mediates signal transduction events in response to Ox LDL.

In this study, we have identified and characterized functional alpha2-adrenergic receptor (alpha2-AR) subtypes in human corpus cavernosum and in cultured human corpus cavernosum smooth muscle cells. Analysis of total RNA, isolated from whole corpus cavernosum tissue and smooth muscle cells, by RNase protection assays, demonstrated expression of mRNA for alpha2A, alpha2B, and alpha2C adrenergic receptor subtypes in whole tissue and alpha2A and alpha2C subtypes in cultured smooth muscle cells. Binding studies with [3H]RX821002 (a highly selective and specific ligand for alpha2-adrenergic receptor) in isolated membrane fractions of human corpus cavernosum smooth muscle cells, demonstrated specific alpha2-AR binding sites with high affinity (Kd = 0.63 nM) and limited capacity (25-30 fmol/mg protein). Binding of [3H]RX821002 was displaced with the nonselective alpha-AR antagonist, phentolamine, and with the alpha-AR agonist, norepinephrine, in a dose-dependent manner, but not by the selective alpha1-AR agonist, phenylephrine. Binding of [3H]rauwolscine was also displaced by phentolamine. UK 14,304, a selective alpha2-AR agonist, inhibited forskolin-induced cyclic adenosine monophosphate (cAMP) synthesis in cultured human corpus cavernosum smooth muscle cells and induced dose-dependent contractions of tissue strips in organ bath chambers. UK 14,304-induced contractions were inhibited with alpha2-AR selective antagonists, rauwolscine and delquamine (RS 15385-197). These observations suggest that in human corpus cavernosum, norepinephrine (NE) and epinephrine may activate postsynaptic alpha2-AR subtypes, in addition to activating alpha1-AR subtypes, on smooth muscle cells, contributing to local control of human corpus cavernosum smooth muscle tone, in vivo.

The mechanism for calcium (Ca2+) release in heart and skeletal muscle during excitation-contraction coupling is currently unknown. A widely held hypothesis is that a small amount of Ca2+ enters the cell and elicits a larger intracellular release of Ca2+ from the sarcoplasmic reticulum (SR), termed "Ca2+-induced Ca2+-release" (CICR). In addition to its role in excitation-contraction coupling, Ca2+ is also known to activate the cysteine protease calpain, which has been recently found to specifically cleave the ryanodine receptor in vitro. The authors investigated the question of whether Ca2+ sensitive protease activation could account for an apparent CICR. The authors first reproduced the phenomenon of CICR using detergent treated L6 myotubes ("skinned cells"). Leupeptin, a cysteine protease inhibitor, reduced the initial velocity and extent of Ca2+ release from the SR; a similar result was obtained when skinned cells were treated with iodoacetate, a sulfhydryl alkylating agent. Dithiothreitol enhanced both the rate and extent of Ca2+ release. Caffeine-induced Ca2+-release was unaffected by the thiol protease inhibitors or activators. This suggests that a cysteine protease may be responsible, in part, for CICR in vitro. The authors also found that vesicles exposed to Ca2+ to induce CICR were unable to fully reaccumulate Ca2+ a second time. Yet, when caffeine released comparable amounts of Ca2+, the initial Ca2+ level was fully restored. Similarly, leupeptin protected the vesicles from the reaccumulation deficit induced by Ca2+. The authors' findings suggest that proteolysis activated by a Ca2+-sensitive protease may account for the direct in vitro demonstration of CICR; such an effect may more likely reflect a role in apoptosis than excitation-contraction coupling.

Endothelins (ETs) are 21 amino acid peptides with vasoactive and mitogenic properties. The three isopeptides (ET-1, -2, and -3) and their receptors (E1A and ETB subtypes) display expression in numerous tissues and possibly mediate autocrine/paracrine actions. The present investigation shows that ET-1 triggers biphasic increases of the concentration of cytoplasmic Ca2+ ([Ca2+]i) in pathological human parathyroid cells. Both the peak and sustained [Ca2+]i increase, as well as the proportion of responding cells, are dose-dependent in the 10(-10)-10(-7) mol/L range of ET-1. In absence of external Ca2+, the ET-1-induced [Ca2+]i peak is attenuated. ET-3 has no effect on [Ca2+]i indicating functional dominance of the ETA receptor subtype. ET-1 (10 nmol/L) lowers parathyroid hormone secretion in 0.5 mmol/L but not in higher external Ca2+ concentrations, and parathyroid cell ET release is inhibited by increases of external Ca2+. Fibroblasts overgrowing the parathyroid chief cells during monolayer culture respond to ET-1 with biphasic [Ca2+]i increases or repetitive [Ca2+]i spikes, but show no response to elevation of external Ca2+. These findings imply that ET secretion and ET receptor expression may constitute an autocrine/paracrine mechanism in the regulation of human PTH secretion.

Among the cellular actions of vanadate ions are several that have the potential to be of significance in the regulation of protein phosphorylation. The effects of vanadate on adenosine 3',5' cyclic monophosphate (cAMP)-dependent and independent, alkali-resistant protein phosphorylation in a synaptosomal preparation from rat cortex were examined in this study. Three major vanadate-stimulated, cAMP-independent phosphoproteins (58-, 50-, and 39-kDa) and two cAMP-dependent species (37- and 32-kDa) were detectable. The potentiation between vanadate and cAMP in stimulating the phosphorylation of the latter two proteins is in contrast to the nonadditive combined effect of both on the phosphorylation of other synaptosomal proteins. The two cAMP-dependent, 32P-labeled proteins possess identical or very similar physicochemical properties to two previously cited neuronal phosphoproteins, namely, dopamine- and adenosine 3',5'-monophosphate-regulated phosphoprotein-32 (DARPP-32) and inhibitor-1 (I-1). Such properties include phosphorylation by cAMP-dependent protein kinase, the presence of an alkali-resistant phosphothreonine residue, comigration on two-dimensional gel electrophoresis, dephosphorylation by type-2B protein phosphatase, and crossreactivity with specific antibodies. Costimulation by cAMP and vanadate of phosphorylation of the latter two proteins on threonine residues, at concentrations of vanadate consistent with the regulation of protein tyrosine phosphatase activity, indicates a unique interaction between these two regulators of protein phosphorylation at the nerve terminus.

Metavanadate, orthovanadate, and pervanadate all inhibited [3H]QNB antagonist binding to the human brain muscarinic acetylcholine receptor (mAChR) in the presence of glutathione, with the order of decreasing potency and the concentration required for 50% inhibition (I[50]) being: pervanadate (95 microM) > orthovanadate (132 microM) > metavanadate (452 microM). Omission of glutathione decreased the inhibition of the vanadium compounds 2-6-fold. Preincubating the vanadium compounds with the mAChR in the presence of glutathione at 37 degrees for 1 h markedly decreased the I(50) values as follows: pervanadate (13 microM) > orthovanadate (46 microM) > metavanadate (118 microM). Inhibition by the vanadium compounds was blocked by EDTA, Mn2+, and Trolox, a water-soluble vitamin E analog. Vanadium use in treating diabetes is discussed regarding its inhibition of mAChR function.