G-protein-mediated signaling in cholesterol-enriched arterial smooth muscle cells. 1. Reduced membrane-associated G-protein content due to diminished isoprenylation of G-gamma subunits and p21ras

Atorvastatin desensitizes beta-adrenergic signaling in cardiac myocytes via reduced isoprenylation of G-protein gamma-subunits

Statins exert pleiotropic, cholesterol-independent effects by reducing isoprenylation of monomeric GTPases. Here we examined whether statins also reduce isoprenylation of gamma-subunits of heterotrimeric G-proteins and thereby affect beta-adrenergic signaling and regulation of force in cardiac myocytes. Neonatal rat cardiac myocytes (NRCM) were treated with atorvastatin (0.1-10 micromol/l; 12-48 h) and examined for adenylyl cyclase regulating G-protein alpha- (Galpha), beta- (Gbeta), and gamma- (Ggamma) subunits and cAMP accumulation. Engineered heart tissue (EHT) from NRCM was used to evaluate contractile consequences. In atorvastatin-treated NRCM, a second band of Ggamma3 with a lower apparent molecular weight appeared in cytosol and particulate fractions that was absent in vehicle-treated NRCM, but also seen after GGTI-298, a geranylgeranyl transferase inhibitor. In parallel, Gbeta accumulated in the cytosol and total cellular content of Galphas was reduced. In atorvastatin-treated NRCM, the cAMP-increasing effect of isoprenaline was reduced. Likewise, the positive inotropic effect of isoprenaline was desensitized and reduced after treatment with atorvastatin. The effects of atorvastatin were abolished by mevalonate and/or geranylgeranyl pyrophosphate, but not by farnesyl pyrophosphate or squalene. Taken together, the results of this study show that atorvastatin desensitizes NRCM to beta-adrenergic stimulation by a mechanism that involves reduced isoprenylation of Ggamma and subsequent reductions in the cellular content of Galphas.

Hypercholesterolemia Impairs Transduction of Vasodilator Signals Derived From Ischemic Myocardium

OBJECTIVE

Coronary microvessels are functionally coupled to the myocardial metabolic state. In hypercholesterolemia, the coronary vascular dysfunction extends to microvascular levels. We hypothesized that the vasodilator signal transduction from ischemic heart is impaired in the coronary microvascular wall of hypercholesterolemia.

METHODS AND RESULTS

Rabbits were fed with normal chow (control group) or 2% high-cholesterol diet (hypercholesterolemia group) for 8 weeks. Coronary microvessels isolated from rabbit hearts were pressurized and gently placed on a beating canine heart. Myocardial ischemia was produced in the beating heart and the diameter of the isolated microvessel was observed using an intravital microscope with a floating objective. In control group, the isolated microvessels significantly dilated 2 minutes after the onset of ischemia, and a plateau was observed at 10 minutes. In contrast, the microvessels from hypercholesterolemia group did not dilate during ischemia. Dihydroethidium fluorescence microscopy revealed an elevated superoxide level in the microvessels of hypercholesterolemia group. The application of tiron (free radical scavenger) significantly dilated the isolated microvessels only from hypercholesterolemic animals.

CONCLUSIONS

We conclude that the transduction of vasodilator signals derived from ischemic myocardium is impaired in the coronary microvascular wall of hypercholesterolemia. Enhanced oxidative stress in hypercholesterolemia may alter the microvascular function.

Effect of atorvastatin on adhesive phenotype of human endothelial cells activated by tumor necrosis factor alpha

We studied the effect of atorvastatin on the adhesive phenotype of human endothelial cells (HUVEC) stimulated by tumor necrosis factor (TNF)-alpha. Surface expression of adhesion molecules on HUVEC was examined by flow cytometry and confocal microscopy, and adhesion of monocytes (human THP-1 cell line) was measured in vitro under flow conditions. In TNF-alpha-activated HUVEC, atorvastatin significantly enhanced surface expression of vascular cell adhesion molecule (VCAM)-1, intercellular adhesion molecule (ICAM)-1, E-selectin, and fractalkine, when compared with TNF-alpha stimulation alone. This enhancement was reversed by mevalonate or geranylgeranyl pyrophosphate (GGPP) and was mimicked by an inhibitor of geranylgeranylation. The enhancing effect of atorvastatin was restricted to TNF-alpha-inducible adhesion molecule and was the reflect of an increased protein synthesis (mRNA and protein) and not of a reduced shedding. Confocal microscopy examination showed that atorvastatin also altered the surface distribution of adhesion molecules. Adhesion of human THP-1 cells on TNF-alpha-activated HUVEC was significantly reduced by atorvastatin (-42% at 1 microM). Mevalonate or GGPP restored the TNF-alpha-induced adhesive potential. These results show that atorvastatin, by inhibiting prenylation of G proteins, enhances the TNF-alpha-induced expression of adhesion molecules at the endothelial cell surface and also alters their surface distribution which may account for the reduced binding of monocytes.

Cholesterol cell content modulates GTPase activity of G proteins in GH4C1 cell membranes

Previous results from our laboratory showed that GH(4)C(1) cells with low-cholesterol cell content had increased adenylyl cyclase (AC) activity with a parallel increase in G protein alpha subunits associated to the plasma membrane. This effect was directly related to mevalonate availability. In the present report, we characterized the high-affinity GTPase activity present in GH(4)C(1) cell membranes and studied its regulation by cholesterol cell content. The high-affinity GTPase activity, measured as the [gamma32P]GTP hydrolysis rate, was both time-dependent and protein concentration-dependent. Cultured cells with lipoprotein-deficient serum (LPDS) showed decreased cholesterol cell content and decreased GTPase activity. The kinetic analysis, as interpreted by Lineweaver-Burk plots, indicated that low-cholesterol cell content had no effect on the apparent affinity for GTP, but resulted in a 47% decrease in the maximal velocity of the reaction. Addition of 25-hydroxycholesterol (25-HC), an inhibitor of the expression of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and synthetase to cells in LPDS, further decreased GTPase activity in a dose-dependent manner. This effect was reverted by exogenous cholesterol, but not by mevalonate. Studies with bacterial toxins revealed that neither cholera toxin (CTX) nor pertussis toxins (PTX) were able to revert the inhibition produced by low-cholesterol cell content. These results allowed us to postulate that cholesterol modulates GTPase activity in both Gs and Gi protein families. To analyse further the mechanism of modulation of GTPase activity by cholesterol cell content, [35S]GTPgammaS binding in membranes of GH(4)C(1) cells was studied. Changes in cholesterol cell content did not have any effect on GTP binding. Data demonstrated that high-affinity GTPase activity in plasma membrane of GH(4)C(1) cells is direct stimulated by cholesterol cell content and not by mevalonate availability. This example provides a mechanism by which cholesterol cell content can modulate signal transduction mediating by G proteins.

Exposure to oxidized low-density lipoprotein reduces activable Ras protein in vascular endothelial cells

Oxidized low-density lipoprotein (ox-LDL) has been shown to alter the migratory and proliferative activities of the vascular endothelial cells (EC) in response to serum and growth factors. The mechanism underlying the antiproliferative effect of ox-LDL on vascular EC has not been fully elucidated. In this report, we show that exposure of vascular EC to ox-LDL results in a marked reduction of the membrane-associated Ras protein. Further study shows that in ox-LDL-treated EC, reduction of the membrane-associated Ras protein is correlated with a reduced amount of active Ras (Ras-guanosine triphosphate), indicating that the Ras signaling pathway is attenuated. The attenuation of the Ras signaling pathway in ox-LDL-treated EC may thus be responsible for the retarded response to the mitogenic stimulation of serum and growth factors.

Effect of S(-) perillic acid on protein prenylation and arterial smooth muscle cell proliferation

A number of proteins post-translationally modified by the covalent attachment of mevalonate-derived isoprene groups farnesol (FOH) or geranylgeraniol (GGOH), play a role in cell proliferation. For this reason, protein farnesyltransferase (PFTase) and protein geranylgeranyltransferases (PGGTases) I and II have gained attention as novel targets for the development of antiproliferative agents. Monoterpenes [limonene, perillic acid (PA) and its derivatives] have been shown to inhibit cell growth and protein prenylation in cancer cells. In the present study, we evaluated the effect of S(-) PA on diploid rat aorta smooth muscle cell (SMC) proliferation as related to protein prenylation. S(-) PA (1-3.5 mM) decreased, in a concentration-dependent manner, rat SMC proliferation as evaluated by cell counting and DNA synthesis. Morphological criteria and flow cytometry analysis excluded the induction of apoptosis as a potential antiproliferative mechanism of S(-) PA on SMC and confirmed a block of the cell cycle progression in G(0)/G(1) phase. The antiproliferative effect of S(-) PA could not be prevented by the addition of mevalonate, FOH, and GGOH to the culture medium and was independent of cholesterol biosynthesis. Densitometric analysis of fluorographed gels, after sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the cell lysates, further supported that S(-) PA (1-3.5 mM), under the same experimental conditions, concentration-dependently inhibited FOH (up to 70%) and GGOH (up to 70%) incorporation into cellular proteins. We provide evidence that S(-) PA affects protein prenylation, an effect that may contribute to its inhibition of SMC proliferation.

Inhibition of geranylgeranylation blocks agonist-induced actin reorganization in human airway smooth muscle cells

To determine whether RhoA isoprenylation (geranylgeranylation) is required for agonist-induced actin cytoskeleton reorganization (measured by an increase in the filamentous F- to monomeric G-actin ratio), human airway smooth muscle cells were treated for 72 h with inhibitors of geranylgeranyltransferase I. Geranylgeranyltransferase inhibitor (GGTI)-2147 or -286 pretreatment completely blocked the increase in the F- to G-actin fluorescence ratio when cells were stimulated with lysophosphatidic acid (LPA), endothelin, or carbachol. In contrast, LPA or endothelin induced actin cytoskeletal reorganization in cells treated with farnesyltransferase inhibitor (FTI)-277 to inactivate Ras. Forskolin-induced adenylyl cyclase activity was inhibited by carbachol in GGTI-2147-pretreated cells, demonstrating that the effect of geranylgeranyltransferase I inhibition on stress fiber formation was not due to uncoupling of signaling between the heterotrimeric G(i) protein (the Ggamma subunit is isoprenylated) and distal effectors. These results demonstrate that selective GGTIs can inhibit agonist-induced actin reorganization.

Rho-dependent inhibition of the induction of connective tissue growth factor (CTGF) by HMG CoA reductase inhibitors (statins)

It was supposed that inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG CoA) reductase (statins) might inhibit the expression of the fibrosis-related factor CTGF (connective tissue growth factor) by interfering with the isoprenylation of Rho proteins. The human renal fibroblast cell line TK173 was used as an in vitro model system to study the statin-mediated modulation of the structure of the actin cytoskeleton and of the expression of CTGF mRNA. Incubation of the cells with simvastatin or lovastatin time-dependently and reversibly changed cell morphology and the actin cytoskeleton with maximal effects observed after about 18 h. Within the same time period, statins reduced the basal expression of CTGF and interfered with CTGF induction by lysophosphatidic acid (LPA) or transforming growth factor beta. Simvastatin and lovastatin proved to be much more potent than pravastatin (IC(50) 1 - 3 microM compared to 500 microM). The inhibition of CTGF expression was prevented when the cells were incubated with mevalonate or geranylgeranylpyrophosphate (GGPP) but not by farnesylpyrophosphate (FPP). Specific inhibition of geranylgeranyltransferase-I by GTI-286 inhibited LPA-mediated CTGF expression whereas an inhibitor of farnesyltransferases FTI-276 was ineffective. Simvastatin reduced the binding of the small GTPase RhoA to cellular membranes. The effect was prevented by mevalonate and GGPP, but not FPP. These data are in agreement with the hypothesis that interference of statins with the expression of CTGF mRNA is primarily due to interference with the isoprenylation of RhoA, in line with previous studies, which have shown that RhoA is an essential mediator of CTGF induction. The direct interference of statins with the synthesis of CTGF, a protein functionally related to the development of fibrosis, may thus be a novel mechanism underlying the beneficial effects of statins observed in renal diseases.

Coronary microcirculation: physiology and pharmacology

Coronary microvessels play a pivotal role in determining the supply of oxygen and nutrients to the myocardium by regulating the coronary flow conductance and substance transport. Direct approaches analyzing the coronary microvessels have provided a large body of knowledge concerning the physiological and pharmacological characteristics of the coronary circulation, as has the rapid accumulation of biochemical findings about the substances that mediate vascular functions. Myogenic and flow-induced intrinsic vascular controls that determine basal tone have been observed in coronary microvessels in vitro. Coronary microvascular responses during metabolic stimulation, autoregulation, and reactive hyperemia have been analyzed in vivo, and are known to be largely mediated by metabolic factors, although the involvement of other factors should also be taken into account. The importance of ATP-sensitive K(+) channels in the metabolic control has been increasingly recognized. Furthermore, many neurohumoral mediators significantly affect coronary microvascular control in endothelium-dependent and -independent manners. The striking size-dependent heterogeneity of microvascular responses to all of these intrinsic, metabolic, and neurohumoral factors is orchestrated for optimal perfusion of the myocardium by synergistic and competitive interactions. The regulation of coronary microvascular permeability is another important factor for the nutrient supply and for edema formation. Analyses of collateral microvessels and subendocardial microvessels are important for understanding the pathophysiology of ischemic hearts and hypertrophied hearts. Studies of the microvascular responses to drugs and of the impairment of coronary microvessels in diseased conditions provide useful information for treating microvascular dysfunctions. In this article, the endogenous regulatory system and pharmacological responses of the coronary circulation are reviewed from the microvascular point of view.

Ras/Rac-Dependent activation of p38 mitogen-activated protein kinases in smooth muscle cells stimulated by cyclic strain stress

p38, a subfamily of the mitogen-activated protein kinases (MAPKs), is a crucial signal transducer between a variety of extracellular stimuli and gene expression in mammalian cells. This kinase is activated in cultured cells stimulated by heat shock, osmotic stress, and proinflammatory cytokines, but a similar activation of p38 MAPKs in vascular smooth muscle cells (SMCs) stimulated by mechanical stress has yet to be studied. We studied signal pathways leading to time- and strength-dependent p38 activation in rat SMCs in response to cyclic strain stress. p38 phosphorylation in stressed SMCs showed maximal activation at 10 minutes. This activation was significantly inhibited by pretreatment of the SMCs with pertussis toxin, a G-protein antagonist, and enhanced by treatment with suramin, a growth factor receptor antagonist, but opposite effects in the activation of extracellular signal-regulated kinases stimulated by mechanical forces were found. p38 activation was markedly reduced in stressed SMCs after protein kinase C depletion. Interestingly, SMC lines stably expressing dominant-negative ras (ras N17) or rac1 (rac1 N17) almost abolished p38 phosphorylation induced by cyclic strain stress. When p38 activation was inhibited by the specific inhibitor SB 202190, SMC migration, determined in a Boyden chamber in response to stimulation with platelet-derived growth factor-BB, and SMC proliferation, stimulated by cyclic strain stress, were abrogated. Thus, we provide the first evidence that cyclic strain stress rapidly activates p38 MAPKs via activation of protein kinase C ras/rac signal pathways, suggesting that p38 MAPKs are important signal transducers mediating the mechanical stress-induced cell responses essential for SMC migration and proliferation.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Volatile anesthetics modulate the binding of guanine nucleotides to the alpha subunits of heterotrimeric GTP binding proteins

The effects of volatile anesthetics on guanine nucleotide binding to the purified alpha subunits of heterotrimeric GTP binding (G) proteins were studied. At sub-anesthetic doses, halothane, isoflurane, enflurane and sevoflurane inhibit exchange of GTPgammaS for GDP bound to Galpha subunits and markedly enhance the dissociation of GTPgammaS, but fail to suppress GDPbetaS release. Nucleotide exchange from non-myristoylated Galpha(i1) is similarly inhibited in the absence of any membrane lipid or detergent. The degrees of inhibition of GDP/GTPgammaS exchange and enhancement of GTPgammaS dissociation are in the same order: alpha(i2)alpha(i1)alpha(i3)alpha(s). By contrast, Galpha(o), which is closely related to Galpha(i), is completely insensitive to anesthetics. We conclude that volatile agents, at clinically relevant doses, have a direct effect on the conformation and stability of the GTP/Mg(2+) bound state of some, but not all Galpha subunits. By destabilizing this state, volatile agents may uncouple metabotropic and other heptahelical receptors from pathways modulating neuronal excitation.

Cyclic strain stress-induced mitogen-activated protein kinase (MAPK) phosphatase 1 expression in vascular smooth muscle cells is regulated by Ras/Rac-MAPK pathways

Recently, we demonstrated that mechanical stress results in rapid phosphorylation or activation of platelet-derived growth factor receptors in vascular smooth muscle cells (VSMCs) followed by activation of mitogen-activated protein kinases (MAPKs) and AP-1 transcription factors (Hu, Y., Bock, G., Wick, G., and Xu, Q. (1998) FASEB J. 12, 1135-1142). Herein, we provide evidence that VSMC responses to mechanical stress also include induction of MAPK phosphatase-1 (MKP-1), which may serve as a negative regulator of MAPK signaling pathways. When rat VSMCs cultivated on a flexible membrane were subjected to cyclic strain stress (60 cycles/min, 5-30% elongation), induction of MKP-1 proteins and mRNA was observed in time- and strength-dependent manners. Concomitantly, mechanical forces evoked rapid and transient activation of all three members of MAPKs, i.e. extracellular signal-regulated kinases (ERKs), c-Jun NH(2)-terminal protein kinases (JNKs), or stress-activated protein kinases (SAPKs), and p38 MAPKs. Suramin, a growth factor receptor antagonist, completely abolished ERK activation, significantly blocked MKP-1 expression, but not JNK/SAPK and p38 MAPK activation, in response to mechanical stress. Interestingly, VSMC lines stably expressing dominant negative Ras (Ras N17) or Rac (Rac N17) exhibited a marked decrease in MKP-1 expression; the inhibition of ERK kinases (MEK1/2) by PD 98059 or of p38 MAPKs by SB 202190 resulted in a down-regulation of MKP-1 induction. Furthermore, overexpressing MKP-1 in VSMCs led to the dephosphorylation and inactivation of ERKs, JNKs/SAPKs, and p38 MAPKs and inhibition of DNA synthesis. Taken together, our findings demonstrate that mechanical stress induces MKP-1 expression regulated by two signal pathways, including growth factor receptor-Ras-ERK and Rac-JNK/SAPK or p38 MAPK, and that MKP-1 inhibits VSMC proliferation via MAPK inactivation. These results suggest that MKP-1 plays a crucial role in mechanical stress-stimulated signaling leading to VSMC growth and differentiation.

Effects of lovastatin on expression of cell cycle regulatory proteins in vascular smooth muscle cells

BACKGROUND

The sequential appearance of cyclins D and E is thought to initiate subsequent DNA synthesis in proliferating cells. Previous studies have reported that DNA synthesis in cultured rat vascular smooth muscle cells (VSMCs) was suppressed by the HMG-CoA reductase inhibitor lovastatin. The effects of lovastatin on cell cycle regulatory proteins in proliferating VSMCs, however, are largely unknown. Thus, we investigated the sequential expression of cyclin D1, cyclin E, cyclin-dependent kinase (CDK) 4, CDK2, and p27Kip1 in cultured rat VSMCs stimulated by platelet-derived growth factor (PDGF)-BB in the presence or absence of lovastatin.

METHODS

Quiescent VSMCs, with and without lovastatin (20 microM) pretreatment for nine hours, were stimulated by PDGF-BB (25 ng/ml). The incorporation of tritiated thymidine was done to assess DNA synthesis. VSMC lysates were obtained every 6 hours for up to 36 hours after stimulation and were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot analysis using relevant polyclonal antibodies. Autoradiograms were analyzed using a densitometer.

RESULTS

The peak expression of cyclins D1 and E occurred at 18 and 30 hours of PDGF stimulation, respectively. Concomitant expression of CDK4 and CDK2 was also observed. The expression of p27Kip1, by contrast, was reduced in association with DNA synthesis. Lovastatin suppressed DNA synthesis and reduced the expression of cyclin D1 and cyclin E, whereas p27Kip1 expression was strongly induced by lovastatin pretreatment. CDK4 and CDK2 expression was unaffected by lovastatin treatment.

CONCLUSIONS

PDGF-BB induces cyclins D1 and E prior to the onset of DNA synthesis in VSMCs. Lovastatin may suppress DNA synthesis in VSMCs by inducing p27Kip1 and reducing expression of cyclins D1 and E.

Atherogenic lipoproteins and tyrosine kinase mitogenic signaling in mesangial cells

BACKGROUND

Mesangial hypercellularity is a critical early histopathological finding seen in human and experimental glomerular diseases. Hyperlipidemia and the glomerular deposition of atherogenic lipoproteins [for example, low-density lipoprotein (LDL) and its oxidized variants, minimally oxidized/modified LDL (mm-LDL)] are commonly associated with mesangial hypercellularity and the development of glomerular disease. This article reviews signal transduction pathways involved in cell proliferation and provides evidence for the participation of atherogenic lipoproteins in intracellular signaling pathways for mesangial cell proliferation. The mitogenic intracellular signaling pathways are regulated by the activation of a series of transmembrane and cytoplasmic protein tyrosine kinases that converge into the activation of Ras and downstream mitogen-activated protein (MAP) kinase. Activated MAP kinase, through translocating into the nucleus and the activation of various transcription factors and proto-oncogenes, regulates cellular proliferation.

METHODS

Murine mesangial cells were stimulated with LDL and mm-LDL and were analyzed for the tyrosine kinase activity, phosphorylation of membrane proteins, activation of Ras and MAP kinase, and cell proliferation.

RESULTS

The results indicated that the stimulation of mesangial cells with LDL and, with greater activity, mm-LDL induced the phosphorylation of membrane platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) receptors, activated Ras, and resulted in sustained (up to 24 hr) activation of MAP kinase. LDL/mm-LDL-mediated mesangial cell proliferation and MAP kinase activation were dependent on the activation of tyrosine kinases.

CONCLUSIONS

We suggest that the accumulation of LDL and more potently its oxidized forms within the glomerulus, through the activation of membrane receptor tyrosine kinases, activate the Ras and MAP kinase signaling cascade leading to DNA synthesis and subsequent cell proliferation.

Mechanism of action of cholera toxin on the opossum internal anal sphincter smooth muscle

Cholera toxin (CTX), an activator of G(s) protein, is an important pharmacological tool in G protein research. The effect and the mechanism of action of CTX in the gastrointestinal smooth muscle, including the internal anal sphincter (IAS), are not known. The present investigation was carried out to examine the effects of CTX on the signal transduction associated with the adenylate cyclase (AC) pathway on the basal tone of the IAS smooth muscle. CTX caused a prompt and dose-dependent fall in the basal tone of the IAS that was not affected by the neurotoxins TTX and omega-conotoxin or the nitric oxide synthase inhibitor N(G)-nitro-L-arginine. The cyclooxygenase inhibitor indomethacin, cAMP-dependent protein kinase inhibitor Rp-8-bromoadenosine 3',5' cyclic monophosphorothioate inhibited CTX-induced IAS smooth muscle relaxation. Furthermore, CTX caused a concentration-dependent relaxation of the isolated smooth muscle cells (SMC) of the IAS, which was blocked by G(s)alpha antibody (G(s)alpha-Ab). The IAS smooth muscle relaxation was accompanied with an increase in the GTPase activity that was also specifically blocked by G(s)alpha-Ab. We conclude that a major part of the inhibitory action of CTX in the IAS is via the direct response of the SMC that is linked with G(s) protein to the AC pathway. A part of the inhibitory action of CTX on the smooth muscle occurs via the activation of cyclooxygenase pathway. The relative contribution of such actions of CTX in the smooth muscle in the gastrointestinal motility disturbances following cholera infection remains to be determined.

Vasomotor responses of coronary stenoses to acetylcholine and their relation to serum lipid levels in stable angina pectoris

The effects of acetylcholine administration on coronary stenoses in relation to serum lipids level were evaluated in 18 patients (15 men, 3 women) with coronary artery disease and stable angina. Intracoronary acetylcholine was infused in concentrations 10(-7), 10(-6), 10(-5) M, followed by intracoronary bolus administration of isosorbide dinitrate. Computerized angiography was used to assess the changes in the diameter of stenoses and of proximal and distal segments. During acetylcholine infusion, at concentrations between 10(-7) to 10(-5) M, there was a significant (p <0.01) dose-dependent constriction of proximal and distal segments and of stenoses reversed by isosorbide dinitrate. There was no correlation between the serum total cholesterol level and the responses of proximal and distal segments to acetylcholine or nitrate. A correlation (p <0.05) was found between the serum total cholesterol level and the response of stenoses to acetylcholine, but there was no correlation with the response to isosorbide dinitrate. In conclusion, in patients with stable angina current serum total cholesterol level correlates with the vasomotor response of coronary stenoses to intracoronary acetylcholine. These findings are consistent with a direct effect of cholesterol, increasing basal coronary vasomotor tone and increasing the stimulated vasoconstrictor response of stenoses.

Coronary arteriolar dilation to acidosis: role of ATP-sensitive potassium channels and pertussis toxin-sensitive G proteins

BACKGROUND

We previously demonstrated that coronary arteriolar dilation in response to acidosis is mediated by the opening of ATP-sensitive potassium (KATP) channels. However, the signal transduction involved in the KATP-channel activation during acidosis has not been elucidated. A recent study in cardiac myocytes implied that pertussis toxin (PTX)-sensitive G proteins may be involved in the signal transduction for KATP-channel activation. However, it remains unclear whether this transduction process also occurs in the vascular tissue and, in particular, whether it exerts functional dilation in response to acidosis.

METHODS AND RESULTS

To examine the signaling pathway for acidosis-induced dilation, porcine coronary arterioles were isolated, cannulated, and pressurized for in vitro study. The GTPase activity in reconstituted G proteins was examined at different levels of pH. Extravascular acidosis (pH 7.3 to 7.0) produced a graded dilation of coronary arterioles. This dilation was not affected by removal of endothelium but was significantly attenuated after inhibition of KATP channels and G proteins by glibenclamide and PTX, respectively. Glibenclamide and PTX attenuated the acidosis-induced arteriolar dilation to the same extent, and combined administration of both inhibitors did not further inhibit the vasodilation. These results indicated that both inhibitors act on the same vasodilatory pathway. Furthermore, vasodilation of coronary arterioles to the KATP-channel opener pinacidil and to the endothelium-independent vasodilator sodium nitroprusside was not affected by PTX. Because PTX inhibited acidosis-induced vasodilation without inhibiting KATP-channel function, it is suggested that PTX inhibits the vasodilatory pathway upstream from KATP channels. GTPase activity in reconstituted G proteins was significantly enhanced by a reduction in pH, indicating that G proteins were directly activated by acidosis.

CONCLUSIONS

On the basis of these findings, we conclude that acidosis-induced coronary arteriolar dilation is mediated by the opening of smooth muscle KATP channels through the activation of PTX-sensitive G proteins.

Cholesterol depletion of caveolae causes hyperactivation of extracellular signal-related kinase (ERK)

Previously we showed that activation of Erk in quiescent cells occurs in the caveolae fraction isolated from fibroblasts. Since the structure and function of caveolae is sensitive to the amount of cholesterol in the membrane, it might be that a direct link exists between the concentration of membrane cholesterol and mitogen-activated protein (MAP) kinase activation. We acutely lowered the cholesterol level of the caveolae fraction by incubating Rat-1 cells in the presence of either cyclodextrin or progesterone. Cholesterol-depleted caveolae had a reduced amount of several key protein components of the MAP kinase complex, including Ras, Grb2, Erk2, and Src. Incubation of these cells in the presence of epidermal growth factor (EGF) caused a rapid loss of EGF receptor from the caveolae fraction, but the usual recruitment of c-Raf was markedly inhibited. Despite the reduced amount of c-Raf and Erk2 in the cholesterol-depleted caveolae fraction, EGF caused a hyperactivation of the remaining caveolae Erk isoenzymes. This was followed by an increase in the amount of active Erk in the cytoplasm. The increased amount of activated Erk produced under these conditions was linked to a 2-fold higher level of EGF-stimulated DNA synthesis. Even cholesterol depletion by itself stimulated Erk activation and DNA synthesis. These results suggest that the MAP kinase pathway can connect the cholesterol level of caveolae membrane to the control of cell division.

G-protein-mediated signaling in cholesterol-enriched arterial smooth muscle cells. 2. Role of protein kinase C-delta in the regulation of eicosanoid production

PGI2 generation by the vessel wall is an agonist for cyclic-AMP-dependent cholesteryl ester hydrolysis. The process of enhanced PGI2 synthesis is stimulated, in part, by G-protein-coupled receptor ligands. Cellular cholesterol enrichment has been hypothesized to alter G-protein-mediated PGI2 synthesis. In the studies reported herein, cells generated PGI2 in response to AlF4-, GTPgammaS, and ATP in a dose-dependent manner. G-protein agonists stimulated eicosanoid production principally by activating phospholipase A2, but not phospholipase C. This is in contrast to PDGF, which stimulated phospholipase A2 and PLCgamma activities. Galphai subunits mediate G-protein agonist-induced PGI2 synthesis, since ATP- and PDGF-induced PGI2 synthesis was inhibited by pertussis toxin. Although cholesterol enrichment reduced arachidonic acid- and PDGF-induced PGI2 synthesis, cholesterol enrichment enhanced PGI2 release in response to AlF4-, GTPgammaS, and ATP. The enhancement of PGI2 release in cholesterol-enriched cells was augmented by mevalonate, which inhibits the ability of cholesterol enrichment to reduce membrane-associated G-protein subunits. Since cholesterol enrichment inhibited PDGF and AlF4--induced MAP kinase activity [Pomerantz, K., Lander, H. M., Summers, B., Robishaw, J. D., Balcueva, E. A., & Hajjar, D. P. (1997) Biochemistry 36, 9523-9531] (the major mechanism by which phospholipase A2 is activated), these results suggest that cholesterol enrichment induces other alternative signaling pathways leading to phospholipase A2 activation. A PKC-dependent pathway is described herein that is involved in enhanced eicosanoid production in cholesterol-enriched cells. This conclusion is supported by two observations: (1) G-protein-linked PGI2 production is inhibited by calphostin, and (2) cholesterol enrichment augments the specific translocation of the delta-isoform of PKC from the cytosol to the plasma membrane following treatment of cells with phorbol ester. These data support the concept that, in cells possessing normal levels of cholesterol, MAP-kinase-dependent pathways mediate eicosanoid synthesis in response to G-protein activation; however, under conditions of high cellular cholesterol levels, augmented G-protein-linked eicosanoid production results from enhanced PKCdelta activity.