Modulation of GTPase activity of G proteins by fluid shear stress and phospholipid composition

Shear stress induces a time- and position-dependent increase in endothelial cell membrane fluidity

Blood flow-associated shear stress may modulate cellular processes through its action on the plasma membrane. We quantified the spatial and temporal aspects of the effects of shear stress (tau) on the lipid fluidity of 1,1'-dihexadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate [DiIC(16)(13)]-stained plasma membranes of bovine aortic endothelial cells in a flow chamber. A confocal microscope was used to determine the DiI diffusion coefficient (D) by fluorescence recovery after photobleaching on cells under static conditions, after a step-tau of 10 or 20 dyn/cm(2), and after the cessation of tau. The method allowed the measurements of D on the upstream and downstream sides of the cell taken midway between the respective cell borders and the nucleus. In <10 s after a step-tau of 10 dyn/cm(2), D showed an upstream increase and a downstream decrease, and both changes disappeared rapidly. There was a secondary, larger increase in upstream D, which reached a peak at 7 min and decreased thereafter, despite the maintenance of tau. D returned to near control values within 5 s after cessation of tau. Downstream D showed little secondary changes throughout the 10-min shearing, as well as after its cessation. Further investigations into the early phase, with simultaneous measurements of upstream and downstream D, confirmed that a step-tau of 10 dyn/cm(2) elicited a rapid (5-s) but transient increase in upstream D and a concurrent decrease in downstream D, yielding a significant difference between the two sites. A step-tau of 20 dyn/cm(2) caused D to increase at both sites at 5 s, but by 30 s and 1 min the upstream D became significantly higher than the downstream D. These results demonstrate shear-induced changes in membrane fluidity that are time dependent and spatially heterogeneous. These changes in membrane fluidity may have important implications in shear-induced membrane protein modulation.

Role of blood shear stress in the regulation of vascular smooth muscle cell migration

Smooth muscle cell (SMC) migration from the media to the intima of blood vessels contributes to neointimal formation and atherogenesis. Here, we demonstrate how blood shear stress regulates vascular SMC migration in the encapsulating tissue of a micro-cylinder implanted in the center of the rat vena cava with the micro-cylinder perpendicular to blood flow. In this model, the micro-cylinder was exposed to a laminar flow with a known shear stress field in the leading region and a vortex flow in the trailing region. After surgery, the micro-cylinder was encapsulated by a thrombus-like tissue within one day, followed by SMC migration from the vena cava to the encapsulating tissue from day 3 to 20. SMC migration was time-dependent with a peak migration speed at day 5. At each given time (excluding day 1), blood shear stress exerts an inhibitory effect on SMC migration with significantly suppressed SMC migration in the laminar flow region than in the stagnation, separation, and vortex flow regions. SMCs were relatively parallel to the shear stress direction in high shear stress regions, whereas perpendicular to the shear stress direction in low shear stress regions. These results suggest that blood shear stress plays a role in regulating SMC migration and orientation in this model.

Segregation of heterotrimeric G proteins in cell surface microdomains. G(q) binds caveolin to concentrate in caveolae, whereas G(i) and G(s) target lipid rafts by default

Select lipid-anchored proteins such as glycosylphosphatidylinositol (GPI)-anchored proteins and nonreceptor tyrosine kinases may preferentially partition into sphingomyelin-rich and cholesterol-rich plasmalemmal microdomains, thereby acquiring resistance to detergent extraction. Two such domains, caveolae and lipid rafts, are morphologically and biochemically distinct, contain many signaling molecules, and may function in compartmentalizing cell surface signaling. Subfractionation and confocal immunofluorescence microscopy reveal that, in lung tissue and in cultured endothelial and epithelial cells, heterotrimeric G proteins (G(i), G(q), G(s), and G(betagamma)) target discrete cell surface microdomains. G(q) specifically concentrates in caveolae, whereas G(i) and G(s) concentrate much more in lipid rafts marked by GPI-anchored proteins (5' nucleotidase and folate receptor). G(q), apparently without G(betagamma) subunits, stably associates with plasmalemmal and cytosolic caveolin. G(i) and G(s) interact with G(betagamma) subunits but not caveolin. G(i) and G(s), unlike G(q), readily move out of caveolae. Thus, caveolin may function as a scaffold to trap, concentrate, and stabilize G(q) preferentially within caveolae over lipid rafts. In N2a cells lacking caveolae and caveolin, G(q), G(i), and G(s) all concentrate in lipid rafts as a complex with G(betagamma). Without effective physiological interaction with caveolin, G proteins tend by default to segregate in lipid rafts. The ramifications of the segregated microdomain distribution and the G(q)-caveolin complex without G(betagamma) for trafficking, signaling, and mechanotransduction are discussed.

Erythroid cell growth and differentiation in vitro in the simulated microgravity environment of the NASA rotating wall vessel bioreactor

Prolonged exposure of humans and experimental animals to the altered gravitational conditions of space flight has adverse effects on the lymphoid and erythroid hematopoietic systems. Although some information is available regarding the cellular and molecular changes in lymphocytes exposed to microgravity, little is known about the erythroid cellular changes that may underlie the reduction in erythropoiesis and resultant anemia. We now report a reduction in erythroid growth and a profound inhibition of erythropoietin (Epo)-induced differentiation in a ground-based simulated microgravity model system. Rauscher murine erythroleukemia cells were grown either in tissue culture vessels at 1 x g or in the simulated microgravity environment of the NASA-designed rotating wall vessel (RWV) bioreactor. Logarithmic growth was observed under both conditions; however, the doubling time in simulated microgravity was only one-half of that seen at 1 x g. No difference in apoptosis was detected. Induction with Epo at the initiation of the culture resulted in differentiation of approximately 25% of the cells at 1 x g, consistent with our previous observations. In contrast, induction with Epo at the initiation of simulated microgravity resulted in only one-half of this degree of differentiation. Significantly, the growth of cells in simulated microgravity for 24 h prior to Epo induction inhibited the differentiation almost completely. The results suggest that the NASA RWV bioreactor may serve as a suitable ground-based microgravity simulator to model the cellular and molecular changes in erythroid cells observed in true microgravity.

Flow-induced expression of endothelial Na-K-Cl cotransport: dependence on K(+) and Cl(-) channels

Steady laminar shear stress has been shown previously to markedly increase Na-K-Cl cotransporter mRNA and protein in human umbilical vein endothelial cells and also to rapidly increase endothelial K(+) and Cl(-) channel conductances. The present study was done to evaluate the effects of shear stress on Na-K-Cl cotransporter activity and protein expression in bovine aortic endothelial cells (BAEC) and to determine whether changes in cotransporter expression may be dependent on early changes in K(+) and Cl(-) channel conductances. Confluent BAEC monolayers were exposed in a parallel-plate flow chamber to either steady shear stress (19 dyn/cm(2)) or purely oscillatory shear stress (0 +/- 19 dyn/cm(2)) for 6-48 h. After shearing, BAEC monolayers were assessed for Na-K-Cl cotransporter activity or were subjected to Western blot analysis of cotransporter protein. Steady shear stress led to a 2- to 4-fold increase in BAEC cotransporter protein levels and a 1.5- to 1.8-fold increase in cotransporter activity, increases that were sustained over the longest time periods studied. Oscillatory flow, in contrast, had no effect on cotransporter protein levels. In the presence of flow-sensitive K(+) and Cl(-) channel pharmacological blockers, the steady shear stress-induced increase in cotransporter protein was virtually abolished. These results suggest that shear stress modulates the expression of the BAEC Na-K-Cl cotransporter by mechanisms that are dependent on flow-activated ion channels.

Endothelial cell response to different mechanical forces

PURPOSE

Endothelial cells (ECs) are subjected to the physical forces induced by blood flow. The aim of this study was to directly compare the EC signaling pathway in response to cyclic strain and shear stress in cultured bovine aortic ECs.

MATERIALS AND METHODS

The ECs were seeded on flexible collagen I-coated silicone membranes to examine the effect of cyclic strain. The membranes were deformed with a 150-mm Hg vacuum at a rate of 60 cycle/min for up to 120 minutes. For a comparison of the effect of shear stress, ECs from the same batch as used in the strain experiments were seeded on collagen I-coated silicone sheets. The ECs were then subjected to 10 dyne/cm(2) shear with the use of a parallel flow chamber for up to 120 minutes. Activation of the mitogen- activated protein kinases was assessed by determining phosphorylation of extracellular signal-regulated kinase (ERK), c-jun N-terminal kinase (JNK), and p38 with immunoblotting.

RESULTS

ERK, JNK, and p38 were activated by both cyclic strain and shear stress. Both cyclic strain and shear stress activated JNK with a similar temporal pattern and magnitude and a peak at 30 minutes. However, shear stress induced a more robust and rapid activation of ERK and p38, compared with cyclic strain.

CONCLUSIONS

Our results indicate that different mechanical forces induced differential activation of mitogen-activated protein kinases. This suggests that there may be different mechanoreceptors in ECs to detect the different forces or alternative coupling pathways from a single receptor.

Mechanical control of cyclic AMP signalling and gene transcription through integrins

This study was carried out to discriminate between two alternative hypotheses as to how cells sense mechanical forces and transduce them into changes in gene transcription. Do cells sense mechanical signals through generalized membrane distortion or through specific transmembrane receptors, such as integrins? Here we show that mechanical stresses applied to the cell surface alter the cyclic AMP signalling cascade and downstream gene transcription by modulating local release of signals generated by activated integrin receptors in a G-protein-dependent manner, whereas distortion of integrins in the absence of receptor occupancy has no effect.

Mechanical stress-initiated signal transductions in vascular smooth muscle cells

Mechanical force is an important modulator of cellular morphology and function in a variety of tissues, and is particularly important in cardiovascular systems. Vascular smooth muscle cell (VSMC) hypertrophy and proliferation contribute to the development of atherosclerosis, hypertension, and restenosis, where mechanical forces are largely disturbed. How VSMCs sense and transduce the extracellular mechanical signals into the cell nucleus resulting in quantitative and qualitative changes in gene expression is an interesting and important research field. Recently, it has been demonstrated that mechanical stress rapidly induced phosphorylation of platelet-derived growth factor (PDGF) receptor, activation of integrin receptor, stretch-activated cation channels, and G proteins, which might serve as mechanosensors. Once mechanical force is sensed, protein kinase C and mitogen-activated protein kinases (MAPKs) were activated, leading to increased c-fos and c-jun gene expression and enhanced transcription factor AP-1 DNA-binding activity. Interestingly, physical forces also rapidly resulted in expression of MAPK phosphatase-1 (MKP-1), which inactivates MAPKs. Thus, mechanical stresses can directly stretch the cell membrane and alter receptor or G protein conformation, thereby initiating signalling pathways, usually used by growth factors. These findings have significantly enhanced our knowledge of the pathogenesis of arteriosclerosis and provided promising information for therapeutic interventions for vascular diseases.

Mechanical stress-induced heat shock protein 70 expression in vascular smooth muscle cells is regulated by Rac and Ras small G proteins but not mitogen-activated protein kinases

-Previous studies have documented that acute elevation in blood pressure results in heat shock protein (hsp) 70-mRNA expression followed by hsp70-protein production in rat aortas. In this article, we provide evidence that mechanical forces evoke rapid activation of heat shock transcription factor (HSF) and hsp70 accumulation. In our study, Western blot analysis demonstrated that hsp70-protein induction peaked between 6 and 12 hours after treatment with cyclic stain stress (60 cycles/minute, up to 30% elongation). Elevated protein levels were preceded by hsp70-mRNA transcription, which was associated with HSF1 phosphorylation and activation stimulated by mechanical forces, suggesting that the response was regulated at the transcriptional level. Conditioned medium from cyclic strain-stressed vascular smooth muscle cells (VSMCs) did not result in HSF-DNA-binding activation. Furthermore, mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinases, c-Jun NH(2)-terminal protein kinases or stress-activated protein kinases, and p38 MAPKs, were also highly activated in response to cyclic strain stress. Inhibition of extracellular signal-regulated kinase and p38-MAPK activation by their specific inhibitors (PD 98059 and SB 202190) did not influence HSF1 activation. Interestingly, VSMC lines stably expressing dominant-negative rac (rac N17) abolished hsp-protein production and HSF1 activation induced by cyclic strain stress, whereas a significant reduction of hsp70 expression was seen in ras N17-transfected VSMC lines. Thus, our findings demonstrate that cyclic strain stress-induced hsp70 expression is mediated by HSF1 activation and regulated by rac and ras GTP-binding proteins. Induction of hsp70 could be important in maintaining VSMC homeostasis during vascular remodeling in response to hemodynamic stimulation.

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.

Selective modulation of endothelial cell [Ca2+]i response to flow by the onset rate of shear stress

The response of endothelial cells (ECs) to their hemodynamic environment strongly influences normal vascular physiology and the pathogenesis of atherosclerosis. Unique responses to the complex flow patterns in lesion-prone regions imply that the temporal and spatial features of the mechanical stimuli modulate the cellular response to flow. We report the first systematic study of the effects of temporal gradients of shear stress on ECs. Flow was applied to cultured ECs using a novel cone-and-plate device allowing precise and independent control of the shear stress magnitude and the onset rate. Intracellular free calcium concentration ([Ca2+]i) increased rapidly following the onset of flow, and the characteristics of the transient were modulated by both the shear stress magnitude and onset rate. ECs were most sensitive to shear stress applied at physiological onset rates. Furthermore, the relative contribution of extracellular calcium and IP3-mediated release were dependent upon the specific flow regime.

Activation of ERK and p38 MAP kinases in human fibroblasts during collagen matrix contraction

Studies were carried out to characterize changes in MAP kinase activation during contraction of collagen matrices by fibroblasts under isometric tension. We found that both ERK and p38 MAP kinases were activated during contraction, as determined by immunoblotting and in vitro kinase assays. ERK activation was biphasic, with peaks at 10 min and 2 h; whereas p38 activation was monophasic, with a single peak at 10 min. Activation of ERK, but not p38, appeared to depend at least in part on the Gi class of heterotrimeric G proteins. The results show that ERK and p38 cooperate in contraction-stimulated activation of c-fos transcription.

Rapid displacement of vimentin intermediate filaments in living endothelial cells exposed to flow

Hemodynamic shear stress at the endothelial cell surface induces acute and chronic intracellular responses that regulate vessel wall biology. The cytoskeleton is implicated by acting both as a direct connector to local surface deformation and as a distribution network for mechanical forces throughout the cell; however, direct observation and measurement of its position during flow have only recently become possible. In this study, we directly demonstrate rapid deformation of the intermediate filament (IF) network in living endothelial cells subjected to changes in hemodynamic shear stress. Time-lapse optical sectioning and deconvolution microscopy were performed within the first 3 minutes after the introduction of flow (shear stress, 12 dyn/cm(2)). Spatial and temporal dynamics of green fluorescent protein-vimentin IFs in confluent endothelial cells were analyzed. The imposition of shear stress significantly increased the variability of IF movement throughout the cell in the x-, y-, and z-directions compared with the constitutive dynamics noted in the absence of flow. Acute polymerization and depolymerization of the IF network were absent. The magnitude and direction of flow-induced IF displacement were heterogeneous at the subcellular level. These qualitative and quantitative data demonstrate that shear stress acting at the luminal surface of the endothelium results in rapid deformation of a stable IF network.

Signalling pathways in vascular endothelium activated by shear stress: relevance to atherosclerosis

Major advances in our understanding of how endothelial cells sense and respond to haemodynamic forces and, more specifically, to fluid shear stress have been achieved during the past 3 years. These include definition of potential shear stress receptors and multiple signalling pathways that mediate shear stress regulation of gene expression. A few studies have also pointed to the unique effects of complex shear stress on endothelial activation, thus leading to better understanding of the mechanisms that lead to the development of atherosclerosis.

Caveolin-1 regulates shear stress-dependent activation of extracellular signal-regulated kinase

Fluid shear stress activates a member of the mitogen-activated protein (MAP) kinase family, extracellular signal-regulated kinase (ERK), by mechanisms dependent on cholesterol in the plasma membrane in bovine aortic endothelial cells (BAEC). Caveolae are microdomains of the plasma membrane that are enriched with cholesterol, caveolin, and signaling molecules. We hypothesized that caveolin-1 regulates shear activation of ERK. Because caveolin-1 is not exposed to the outside, cells were minimally permeabilized by Triton X-100 (0.01%) to deliver a neutralizing, polyclonal caveolin-1 antibody (pCav-1) inside the cells. pCav-1 then bound to caveolin-1 and inhibited shear activation of ERK but not c-Jun NH(2)-terminal kinase. Epitope mapping studies showed that pCav-1 binds to caveolin-1 at two regions (residues 1-21 and 61-101). When the recombinant proteins containing the epitopes fused to glutathione-S-transferase (GST-Cav(1-21) or GST-Cav(61-101)) were preincubated with pCav-1, only GST-Cav(61-101) reversed the inhibitory effect of the antibody on shear activation of ERK. Other antibodies, including m2234, which binds to caveolin-1 residues 1-21, had no effect on shear activation of ERK. Caveolin-1 residues 61-101 contain the scaffolding and oligomerization domains, suggesting that binding of pCav-1 to these regions likely disrupts the clustering of caveolin-1 or its interaction with signaling molecules involved in the shear-sensitive ERK pathway. We suggest that caveolae-like domains play a critical role in the mechanosensing and/or mechanosignal transduction of the ERK pathway.

Fluid shear stress increases membrane fluidity in endothelial cells: a study with DCVJ fluorescence

Fluid shear stress (FSS) has been shown to be an ubiquitous stimulator of mammalian cell metabolism. Although many of the intracellular signal transduction pathways have been characterized, the primary mechanoreceptor for FSS remains unknown. One hypothesis is that the cytoplasmic membrane acts as the receptor for FSS, leading to increased membrane fluidity, which in turn leads to the activation of heterotrimetric G proteins (13). 9-(Dicyanovinyl)-julolidine (DCVJ) is a fluorescent probe that integrates into the cell membrane and changes its quantum yield with the viscosity of the environment. In a parallel-plate flow chamber, confluent layers of DCVJ-labeled human endothelial cells were exposed to different levels of FSS. With increased FSS, a reduced fluorescence intensity was observed, indicating an increase of membrane fluidity. Step changes of FSS caused an approximately linear drop of fluorescence within 5 s, showing fast and almost full recovery after shear cessation. A linear dose-response relationship between shear stress and membrane fluidity changes was observed. The average fluidity increase over the entire cell monolayer was 22% at 26 dyn/cm(2). This study provides evidence for a link between FSS and membrane fluidity, and suggests that the membrane is an important flow mechanosensor of the cell.

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.

Mechanisms for regulation of fluid shear stress response in circulating leukocytes

We have shown that leukocytes retract their pseudopods and detach from substrates after exposure to physiological fluid shear stresses ( approximately 1.5 dyn/cm(2)). In inflammation, however, pseudopod projection during spreading and firm adhesion on endothelium is observed even in microvessels with normal blood flow and fluid shear stresses. Thus, we examined mechanisms that may serve to regulate the shear stress response of circulating leukocytes. In the presence of inflammatory mediators (platelet-activating factor [PAF] f-met-leu-phe), a subgroup of cells ceases to respond to shear stress. cGMP analogs and nitric oxide (NO) donors enhance the shear stress response and reverse the inhibitory effect of inflammatory mediators on the shear stress response, whereas depletion of cGMP leads to cessation of the shear stress response even in unstimulated leukocytes. The ability of cGMP to enhance the shear stress response is not associated with CD18 expression, because cGMP has no effect on CD18 expression in response to shear stress. The shear stress response of leukocytes in endothelial nitric oxide synthase (-/-) mice, in which NO level in blood is decreased, is attenuated compared with that in wild-type mice. In rat mesentery venules stimulated by PAF under normal blood flow, a cGMP analog diminishes pseudopod projection of leukocytes, whereas inhibition of NO leads to enhanced pseudopod projection and spreading. The evidence suggests that inflammatory mediators suppress the shear stress response of leukocytes leading to spreading even under normal physiological shear stress, whereas cGMP may serve to maintain shear stress response even in inflammation.

Ginseng pharmacology: multiple constituents and multiple actions

Ginseng is a highly valued herb in the Far East and has gained popularity in the West during the last decade. There is extensive literature on the beneficial effects of ginseng and its constituents. The major active components of ginseng are ginsenosides, a diverse group of steroidal saponins, which demonstrate the ability to target a myriad of tissues, producing an array of pharmacological responses. However, many mechanisms of ginsenoside activity still remain unknown. Since ginsenosides and other constituents of ginseng produce effects that are different from one another, and a single ginsenoside initiates multiple actions in the same tissue, the overall pharmacology of ginseng is complex. The ability of ginsenosides to independently target multireceptor systems at the plasma membrane, as well as to activate intracellular steroid receptors, may explain some pharmacological effects. This commentary aims to review selected effects of ginseng and ginsenosides and describe their possible modes of action. Structural variability of ginsenosides, structural and functional relationship to steroids, and potential targets of action are discussed.

A flow-activated chloride-selective membrane current in vascular endothelial cells

Shear stress-induced activation of endothelial ion channels, one of the earliest responses to flow, is implicated in mechano-signal transduction that results in the regulation of vascular tone. The effects of laminar flow on endothelial membrane potential were studied in vitro using both fluorescent potentiometric dye measurements and whole-cell patch-clamp recordings. The application of flow stimulated membrane hyperpolarization, which was reversed to depolarization within 35 to 160 seconds. The depolarization was caused by a Cl(-)-selective membrane current activated by flow independently of the K(+) channel-mediated hyperpolarization. Thus, flow activated both K(+) and Cl(-) currents, with the net membrane potential being determined by the balance of the responses. Membrane potential sensitivity to flow was unchanged by flow preconditioning that elongated and aligned the cells.

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.

Modulation by pathophysiological stimuli of the shear stress-induced up-regulation of endothelial nitric oxide synthase expression in endothelial cells

OBJECTIVE

Fluid shear stress (the frictional force resulting from blood flow) is a principal regulator of endothelial nitric oxide synthase (eNOS) expression. We examined the responses of eNOS messenger ribonucleic acid (mRNA) levels to dynamic shear stimuli in the presence of pathological risk modifiers.

METHODS

Confluent bovine aortic endothelial cells were subjected in vitro to shear stress (using a cone-plate viscometer) and to hydrostatic pressure (using a custom-built pressure chamber device). eNOS mRNA levels were quantitated by densitometric analysis of Northern blots.

RESULTS

In contrast to steady laminar shear stress, which elevated eNOS mRNA levels in a time- and dose-dependent manner (2.9- and 3.6-fold after 6 h at 4 and 20 dyn/cm2, respectively), steady hydrostatic pressure of 150 mm Hg decreased eNOS mRNA levels by 46%. eNOS mRNA up-regulation by shear stress was reversible after cessation of flow, although it was not influenced by previous shear exposure, and it was not mediated by a stable transferable factor. eNOS mRNA up-regulation by sinusoidal shear stress was frequency-dependent, with a moderate response at 1-Hz oscillating shear and no change at 0.3 Hz. Hypoxia (3% O2) suppressed eNOS mRNA expression by 78% under static conditions and by 72% under shear conditions but did not alter the fold induction by shear. Elevated glucose concentrations reduced eNOS mRNA levels in both resting and shear stress-exposed cells but did not reduce the fold induction by shear; the protein kinase C inhibitor calphostin C was without effect. Shear-induced up-regulation of eNOS mRNA was unaffected by changes in the medium partial pressure of CO2/pH, by the Na+/H+-exchanger inhibitor HOE694, or by aspirin. In contrast, the shear response was potentiated by homocysteine.

CONCLUSION

Both physical and chemical stimuli regulate eNOS mRNA levels in endothelial cells. Although eNOS mRNA expression is increased by shear stress, it is decreased by hydrostatic pressure, hypoxia, and elevated glucose levels. The effect of shear on eNOS mRNA expression involves a reversible, frequency-dependent process. These in vitro findings suggest possible contributions of the eNOS flow response to atherosclerosis, in the presence of systemic risk factors.

Flow dependence and time constant of the change in nitric oxide concentration measured in the vascular media

It has been considered that the concentration of endothelium-derived nitric oxide (NO) in the arterial vascular wall changes in response to flow-induced shear stress. In the present study, using an NO-sensitive electrode, the aim was to directly evaluate the relationship between perfusion rate and NO concentration in the arterial vascular wall. The NO microelectrode (diameter: 100 microns) was inserted into the vascular media of isolated canine femoral arteries, and the vessel was perfused with a Krebs-Henseleit buffer solution. A flow-related change in NO concentration in the vascular media was then evaluated by changing perfusion rate. NO concentration attained a peak value with a first-order time delay by a stepwise increase in perfusion rate, and the peak-level NO concentration was linearly correlated with perfusion rate in each vessel (10-154 pA at 2.1-72.3 ml min-1; n = 7, r2 = 0.89-0.99, p < 0.03). The average time constant for an increase in NO current with a stepwise increase in perfusion rate was 24 +/- 3 s (n = 5). NO production was increased by perfusing a solution containing 1 mmol l-1 L-arginine and was attenuated by 100 mumol l-1 NG-nitro-L-arginine, indicating the intactness of the endothelium, proper insertion of the NO electrode and selective detection of NO by the electrode. It is concluded that the NO microelectrode is applicable to NO measurement in the vascular media where NO controls vascular tone and that the concentration of NO in the arterial vascular media changes with perfusion rate in a rate-dependent manner as well as with a time constant of about 24 s for a stepwise increase in flow.

Secretory phospholipase A2 induces phospholipase Cgamma-1 activation and Ca2+ mobilization in the human astrocytoma cell line 1321N1 by a mechanism independent of its catalytic activity

The effect of secretory phospholipase A2 (sPLA2) on intracellular Ca2+ signaling in human astrocytoma cells was studied. sPLA2 increased cytosolic [Ca2+] ([Ca2+]c) in both Ca2+-containing and Ca2+-free medium, thus suggesting Ca2+ release from intracellular stores. The activation by sPLA2 of arachidonate release via cytosolic PLA2 (cPLA2) was also independent of extracellular Ca2+. As sPLA2 requires Ca2+ for activity, these results indicate that both Ca2+ mobilization and cPLA2 activation induced by sPLA2 are unrelated to phospholipase activity but dependent on signaling mechanisms. The sPLA2-induced [Ca2+]c peak was sensitive to Bordetella pertussis toxin and inhibited by caffeine, suggesting its mediation by inositol 1,4,5-trisphosphate (IP3). sPLA2 induced tyrosine phosphorylation and membrane targeting of phospholipase Cgamma-1 (PLCgamma-1). Moreover, the Ca2+ peak was sensitive to protein tyrosine kinase inhibitors. sPLA2 activates two signaling pathways: one leading to the activation of the MAP kinase/cPLA2 cascade and another leading to PLCgamma activation and Ca2+ release.

Stress factors acting at the level of the plasma membrane induce transcription via the stress response element (STRE) of the yeast Saccharomyces cerevisiae

A variety of stress factors induces transcription via the stress response element (STRE) present in control regions of a number of genes of the yeast Saccharomyces cerevisiae. Induction of transcription involves nuclear translocation of the STRE-binding transcription activators Msn2p and Msn4p. The primary cellular events triggering this translocation are presently not well understood. In this investigation, we have observed that a number of factors acting at the level of the yeast plasma membrane, including the antifungal agent nystatin, the steroidal alkaloid tomatine, benzyl alcohol, a number of detergents and the plasma membrane H+-ATPase inhibitor diethylstilbestrol or mutations in the PMA1 gene encoding the plasma membrane ATPase, induce Msn2p nuclear accumulation and STRE-dependent transcription. At least some of the stress factors acting via STREs cause an increase in plasma membrane permeability, leading to a decrease in membrane potential, which might be a primary cellular stress signal. A decrease in internal pH triggered by permeabilization of the plasma membrane or a change in cAMP levels are at least not obligatory factors in intracellular stress signal transduction. The signal transduction pathway transmitting the signal generated at the plasma membrane to Msn2p is still unknown.

Signaling mechanisms underlying the vascular myogenic response

The vascular myogenic response refers to the acute reaction of a blood vessel to a change in transmural pressure. This response is critically important for the development of resting vascular tone, upon which other control mechanisms exert vasodilator and vasoconstrictor influences. The purpose of this review is to summarize and synthesize information regarding the cellular mechanism(s) underlying the myogenic response in blood vessels, with particular emphasis on arterioles. When necessary, experiments performed on larger blood vessels, visceral smooth muscle, and even striated muscle are cited. Mechanical aspects of myogenic behavior are discussed first, followed by electromechanical coupling mechanisms. Next, mechanotransduction by membrane-bound enzymes and involvement of second messengers, including calcium, are discussed. After this, the roles of the extracellular matrix, integrins, and the smooth muscle cytoskeleton are reviewed, with emphasis on short-term signaling mechanisms. Finally, suggestions are offered for possible future studies.

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.

Induction of nitric oxide synthase mRNA by shear stress requires intracellular calcium and G-protein signals and is modulated by PI 3 kinase

We have investigated the signaling pathways by which shear stress induces accumulation of endothelial nitric oxide synthase (eNOS) mRNA in bovine aortic endothelial cells (BAEC). Steady laminar fluid shear stress (20 dyn/cm2) induced a time-dependent increase in eNOS mRNA levels that did not require de novo protein synthesis and was in part transcriptional. Shear responsiveness was conferred on a luciferase reporter by a portion of the eNOS gene promoter encoding the 5'-flanking region between nt -1600 and -779. Shear-mediated induction of eNOS mRNA was abolished by chelation of intracellular calcium ([Ca2+]i) with BAPTA-AM, and inhibited by blockade of calcium entry with SKF96535. In contrast, eNOS mRNA upregulation by shear was potentiated by thapsigargin-mediated depletion of Ca2+i stores. Pertussis toxin (PTX) inhibited both the shear-induced elevation in [Ca2+]i and the subsequent increase in eNOS mRNA, implicating a PTX-sensitive G-protein in both responses. Shear-induced upregulation of eNOS mRNA was unaffected by the calmodulin inhibitor W-7 and by the tyrosine kinase inhibitor herbimycin A, suggesting that neither calmodulin nor tyrosine kinases are required. However, eNOS mRNA upregulation was potentiated by the PI 3-kinase inhibitors wortmannin and LY294002, suggesting that PI 3-kinase inhibits the shear response. Although microtubule integrity is required for the shear-induced regulation of endothelin-1 mRNA and the morphological and cytoskeletal responses to flow, neither microtubule dissolution with nocodazole nor microtubule stabilization with taxol altered shear-induced [Ca2+]i elevation or upregulation of eNOS mRNA. In conclusion, shear stress of BAEC increases eNOS transcriptional rate and upregulates eNOS mRNA levels by a process that requires calmodulin-independent [Ca2+]i signaling and a PTX-sensitive G-protein, is inhibited by PI 3-kinase, and is independent of microtubule integrity and tyrosine kinase activity.

Heterotrimeric G proteins physically associated with the lipopolysaccharide receptor CD14 modulate both in vivo and in vitro responses to lipopolysaccharide

Septic shock induced by lipopolysaccharide (LPS) triggering of cytokine production from monocytes/macrophages is a major cause of morbidity and mortality. The major monocyte/macrophage LPS receptor is the glycosylphosphatidylinositol (GPI)-anchored glycoprotein CD14. Here we demonstrate that CD14 coimmunoprecipitates with Gi/Go heterotrimeric G proteins. Furthermore, we demonstrate that heterotrimeric G proteins specifically regulate CD14-mediated, LPS-induced mitogen-activated protein kinase (MAPK) activation and cytokine production in normal human monocytes and cultured cells. We report here that a G protein binding peptide protects rats from LPS-induced mortality, suggesting a functional linkage between a GPI-anchored receptor and the intracellular signaling molecules with which it is physically associated.