Platelet-activating factor modulates microvascular dynamics through phospholipase C in the hamster cheek pouch

Phospholipase C

Phospholipase C (PLC) family members constitute a family of diverse enzymes. Thirteen different family members have been cloned. These family members have unique structures that mediate various functions. Although PLC family members all appear to signal through the bi-products of cleaving phospholipids, it is clear that each family member, and at times each isoform, contributes to unique cellular functions. This chapter provides a review of the current literature on PLC. In addition, references have been provided for more in-depth information regarding areas that are not discussed including tyrosine kinase activation of PLC. Understanding the roles of the individual PLC enzymes, and their distinct cellular functions, will lead to a better understanding of the physiological roles of these enzymes in the development of diseases and the maintenance of homeostasis.

Phospholipase C

Phospholipase C (PLC) family members constitute a family of diverse enzymes. Thirteen different family members have been cloned. These family members have unique structures that mediate diverse functions. Although PLC family members all appear to signal through the bi-products of cleaving phospholipids, it is clear that each family member, and at times each isoform, contributes to unique cellular functions. This chapter provides a review of the current literature. In addition, references have been provided for more in depth information regarding areas that are discussed. Ultimately, understanding the roles of the individual PLC enzymes, and their distinct cellular functions, will lead to a better understanding of the development of diseases and the maintenance of homeostasis.

Influence of experimental subarachnoid hemorrhage on nicotine-induced contraction of the rat basilar artery in relation to arachidonic acid metabolites signaling pathway

BACKGROUND

Smoking is one of the most important risk factors for cerebral circulatory disorders. The purpose of this study was to investigate the influence of experimental subarachnoid hemorrhage (SAH) on nicotine-induced contraction (arachidonic acid metabolites) in the basilar arteries of rats.

METHODS

Rats were killed at 1 hour and 1 week after blood injection, and the basilar artery was isolated and cut into a spiral strip.

RESULTS

Testing of cyclooxygenase-1 (COX-1) and 5-lipoxygenase (5-LOX) inhibitors revealed no significant differences in their effects on normal and SAH (1 hour and 1 week). Phospholipase C (PLC) inhibitor (1-(6-((17beta-3-methoxyestra-1,3,5(10)-trien-17yl)amino)hexyl)-1H-pyrrole-2,5,-dione [U-73122]) slightly inhibited contraction of SAH (1 hour and 1 week) when compared to controls. Phospholipase A2 (PLA2) inhibitor (manoalide) and cytosolic PLA2 (cPLA2) inhibitor (arachidonyltrifluoromenthylketone [AACOCF3]) more strongly attenuated contraction in SAH (1 hour and 1 week) than in controls. Secreted PLA2 (sPLA2) inhibitor (indoxam), PLC inhibitor (2-nitro-4-carboxyphenyl N, N-diphenylcarbamate [NCDC]), and COX-2 inhibitors (nimesulide, (5-methanesulfonamido-6-(2,4-difluorothiophenyl)-1-indanone) [L-745337], and celecoxib) only slightly inhibited contraction of SAH (1 week) when compared to normal and SAH (1 hour). The calcium-independent PLA2 (iPLA2) inhibitor bromoenol lactone (BEL) showed greater inhibition of contraction in SAH (1 hour) when compared to normal and SAH (1 week).

CONCLUSIONS

One week after exposure to SAH, PLC, sPLA2, and COX-2 activity were enhanced and cPLA2 activity was inhibited. One hour after exposure to SAH, PLC activity was enhanced and cPLA2 and iPLA2 activity was inhibited. Such changes of inflammatory arachidonic acid metabolites by smoking after SAH may play important roles in fatal cerebral circulatory disorders, suggesting important implications for the etiology and pathogenesis of SAH.

Rapamycin inhibits VEGF-induced microvascular hyperpermeability in vivo

OBJECTIVE

To test the hypothesis that rapamycin inhibits induced microvascular hyperpermeability directly in vivo.

METHODS

Male golden Syrian hamsters (80-120 g) were treated with either rapamycin (at 0.1, 0.5, 2, and 10 mg/kg i.p.) or vehicle at 24 hours and at 1 hour prior to preparation of the cheek pouch. Caveolin-1 scaffolding (1 mg/kg; positive inhibitory control) was injected i.p. 24 hours prior to the experiment. 10(-8) M vascular endothelial growth factor (VEGF) or 10(-7) M platelet-activating factor (PAF) were topically applied to the cheek pouch. Microvascular permeability and arteriolar diameter were assessed using integrated optical intensity (IOI) and vascular wall imaging, respectively.

RESULTS

Rapamycin at 0.1 and 0.5 mg/kg significantly reduced VEGF-stimulated mean IOI from 63.0 +/- 4.2 to 9.7 +/- 5.0 (85% reduction, P < 0.001) and 3.6 +/- 2.7 (95% reduction, P < 0.001), respectively. Rapamycin at 2 mg/kg also lowered VEGF-stimulated hyperpermeability (40% reduction, P < 0.05). However, 10 mg/kg rapamycin increased VEGF-induced microvascular hyperpermeability. Rapamycin at 0.5 mg/kg attenuated VEGF-induced vasodilation and PAF-induced hyperpermeability, but did not inhibit PAF-induced vasoconstriction.

CONCLUSIONS

At therapeutically relevant concentrations, rapamycin inhibits VEGF- and PAF-induced microvascular permeability. This inhibition is (i) a direct effect on the endothelial barrier, and (ii) independent of arteriolar vasodilation. Rapamycin at 10 mg/kg stimulates effectors that increase microvascular permeability.

The NO cascade, eNOS location, and microvascular permeability

The nitric oxide (NO) cascade and endothelial NO synthase (eNOS) are best known for their role in endothelium-mediated relaxation of vascular smooth muscle. Activation of eNOS by certain inflammatory stimuli and enhanced NO release have also been shown to promote increased microvascular permeability. However, it is not entirely clear why activation of eNOS by certain vasodilatory agents, like acetylcholine, does not affect microvascular permeability, whereas activation of eNOS by other inflammatory agents that increase permeability, like platelet-activating factor, does not cause vasodilation. In this review, we discuss the evidence demonstrating the role of eNOS in the elevation of microvascular permeability. We also examine the relative importance of eNOS phosphorylation and localization in its function to promote elevated microvascular permeability as well as emerging topics with regard to eNOS and microvascular permeability regulation.

Nitroglycerin-induced S-nitrosylation and desensitization of soluble guanylyl cyclase contribute to nitrate tolerance

Nitrates such as nitroglycerin (GTN) and nitric oxide donors such as S-nitrosothiols are clinically vasoactive through stimulation of soluble guanylyl cyclase (sGC), which produces the second messenger cGMP. Development of nitrate tolerance, after exposure to GTN for several hours, is a major drawback to a widely used cardiovascular therapy. We recently showed that exposure to nitric oxide and to S-nitrosothiols causes S-nitrosylation of sGC, which directly desensitizes sGC to stimulation by nitric oxide. We tested the hypothesis that desensitization of sGC by S-nitrosylation is a mechanism of nitrate tolerance. Our results established that vascular tolerance to nitrates can be recapitulated in vivo by S-nitrosylation through exposure to cell membrane-permeable S-nitrosothiols and that sGC is S-nitrosylated and desensitized in the tolerant, treated tissues. We next determined that (1) GTN treatment of primary aortic smooth muscle cells induces S-nitrosylation of sGC and its desensitization as a function of GTN concentration; (2) S-nitrosylation and desensitization are prevented by treatment with N-acetyl-cysteine, a precursor of glutathione, used clinically to prevent development of nitrate tolerance; and (3) S-nitrosylation and desensitization are reversed by cessation of GTN treatment. Finally, we demonstrated that in vivo development of nitrate tolerance and crosstolerance by 3-day chronic GTN treatment correlates with S-nitrosylation and desensitization of sGC in tolerant tissues. These results suggest that in vivo nitrate tolerance is mediated, in part, by desensitization of sGC through GTN-dependent S-nitrosylation.

Pharmacological nature of nicotine-induced contraction in the rat basilar artery: Involvement of arachidonic acid metabolites

The pharmacological nature of nicotine-induced contraction in the rat basilar artery is poorly understood. The purpose of this study was to investigate the endothelium dependency and involvement of arachidonic acid metabolites in nicotine-induced contraction in the rat basilar artery. The rat basilar artery was removed from the brain and cut into a spiral preparation. Nicotine (3x10(-5) to 10(-2) M) induced the concentration-dependent contraction in the rat basilar artery, and the maximal contraction was obtained at 3x10(-3) M. The contraction induced by nicotine (3x10(-3) M) was significantly attenuated by the presence of saponin (0.05 mg/ml, 15 min). Phospholipase C (PLC) inhibitors (NCDC and U-73122), calcium-independent phospholipase A(2) (iPLA(2)) inhibitor (BEL), cyclooxygenase-2 (COX-2) inhibitors (nimesulide, L-745,337 and celecoxib), and a 5-lipoxygenase (5-LOX) inhibitor (ZM-230487) concentration-dependently attenuated the nicotine-induced contraction. A cytosolic phospholipase A(2) (cPLA(2)) inhibitor (AACOCF3), secretory phospholipase A(2) (sPLA(2)) inhibitor (indoxam), and cyclooxygenase-1 (COX-1) inhibitors (flurbiprofen and ketoprofen) did not affect the nicotine-induced contraction. From these results, it was suggested that nicotine-induced contraction in the rat basilar artery is endothelium-dependent and is due to arachidonic acid metabolites.

Endothelial nitric oxide synthase is a molecular vascular target for the Chinese herb Danshen in hypertension

Danshen, a Chinese herb, reduces hypertension in Oriental medicine. We hypothesized that Danshen acts partially through endothelial nitric oxide synthase (eNOS) signaling mechanisms. We tested the hypothesis using tanshinone II(A), an active ingredient of Danshen, and the two-kidney, one-clip renovascular hypertension model in hamsters. Oral tanshinone (50 microg/100 g body wt) reduced mean arterial pressure (MAP) from 161.2 +/- 6.9 to 130.0 +/- 7.8 mmHg (mean +/- SE; P < 0.05) in hypertensive hamsters. MAP in sham-operated hamsters was 114.3 +/- 9.2 mmHg. Topical tanshinone at 1 microg/ml and 5 microg/ml increased normalized arteriolar diameter from 1.00 to 1.25 +/- 0.08 and 1.57 +/- 0.11, respectively, and increased periarteriolar nitric oxide concentration from 87.1 +/- 11.3 to 146.9 +/- 23.1 nM (P < 0.05) at 5 microg/ml in hamster cheek pouch. N(G)-monomethyl-L-arginine inhibited tanshinone-induced vasodilation. Hypertension reduced eNOS protein relative to sham-operated control. Tanshinone prevented the hypertension-induced reduction of eNOS and increased eNOS expression to levels higher than sham-operated control in hamster cheek pouch. Topical tanshinone increased normalized arteriolar diameter from 1.0 to 1.47 +/- 0.08 in the cremaster muscle of control mice and to 1.12 +/- 0.13 in cremasters of eNOS knockout mice. In ECV-304 cells transfected with eNOS-green fluorescent protein, tanshinone increased eNOS protein expression 1.35 +/- 0.05- and 1.85 +/- 0.07-fold above control after 5-min and 1-h application, respectively. Tanshinone also increased eNOS phosphorylation 1.19 +/- 0.07- and 1.72 +/- 0.20-fold relative to control after 5-min and 1-h application. Our data provide a basis to understand the action of a Chinese herb used in alternative medicine. We conclude that eNOS stimulation is one mechanism by which tanshinone induces vasodilation and reduces blood pressure.

Acupuncture reduces experimental renovascular hypertension through mechanisms involving nitric oxide synthases

OBJECTIVE

To test the hypothesis that acupuncture on stomach 36 point (ST-36) reduces hypertension by activating nitric oxide synthase signaling mechanisms.

METHODS

The authors used the two-kidney, one-clip renal hypertension (2K1C) hamster model with electroacupuncture treatment.

RESULTS

Thirty-minute daily electroacupuncture treatment for 5 days reduced mean arterial pressure from 160.0 +/- 7.6 to 128.0 +/- 4.3 mmHg (mean +/- SEM), compared to 115.0 +/- 7.2 mmHg in sham-operated hamsters. Electroacupuncture increased periarteriolar NO concentration from 309.0 +/- 21.7 nM to 417.9 +/- 20.9 nM in the 2K1C hamster cheek pouch microcirculation when measured with NO-sensitive microelectrodes. Hypertension reduced endothelial nitric oxide synthase (eNOS) and neuronal nitric oxide synthase (nNOS) proteins relative to the sham-operated control, as measured by Western blotting. Electroacupuncture prevented the reduction of eNOS and nNOS associated with hypertension and showed even higher eNOS and nNOS expressions than sham-operated control in stomach and cheek pouch tissues, which are on the stomach meridian. Analysis of liver tissue, a non-stomach-meridian organ, indicated that electroacupuncture did not have a significant benefit in terms of enhanced expressions of eNOS and nNOS in the treated 2K1C hypertensive group.

CONCLUSIONS

Activation of eNOS and nNOS is one of the mechanisms through which ST-36 electroacupuncture reduces blood pressure; this reduction works through the stomach meridian.

Mitogen-activated protein kinases regulate platelet-activating factor-induced hyperpermeability

OBJECTIVE

The authors tested the hypothesis that p42/44- (ERK-1/2) and/or p38-mitogen-activated protein kinases (MAPK) are in vivo regulatory elements in the platelet-activating factor (PAF) activated signaling cascade that stimulates microvascular hyperpermeability.

METHODS

FITC-dextran 70 was used as the macromolecular tracer for microvascular permeability in the mouse mesenteric fat tissue. Interstitial integrated optical intensity (IOI) was used as an index of permeability.

RESULTS

An application of 10(-7) M PAF increased IOI from 23.1 +/- 3.6 to 70.8 +/- 7.4 (mean +/- SEM). Inhibition of ERK-1/2 with 3 microM and 30 microM AG126 reduced IOI to 32.3 +/- 2.5. Similarly, inhibition of p38-MAPK with 6 nM, 60 nM and 600 nM SB203580 lowered IOI to 29.1 +/- 2.4.

CONCLUSIONS

The results demonstrate that ERK-1/2 and p38MAPK participate in the signaling cascade that regulates PAF-induced microvascular hyperpermeability in vivo.

Endothelial focal adhesions and barrier function

Focal adhesions composed of integrins provide an important structural basis for anchoring the endothelial lining to its surrounding matrices in the vascular wall. Complex molecular reactions occur at the endothelial cell-matrix contact sites in response to physical and chemical stress present in the circulatory system. Recent experimental evidence points to the importance of focal adhesions in the regulation of microvascular barrier function. On one hand, the adhesive interaction between integrins and their extracellular ligands is essential to the maintenance of endothelial barrier properties, and interruption of integrin-matrix binding leads to leaky microvessels. On the other hand, focal adhesion assembly and activation serve as important signalling events in modulating endothelial permeability under stimulatory conditions in the presence of angiogenic factors, inflammatory mediators, or physical forces. The focal responses show distinctive patterns with different temporal characteristics, whereas focal adhesion kinase (FAK) plays a central role in initiating and integrating various signalling pathways that ultimately affect the barrier function. The molecular basis of focal adhesion-dependent microvascular permeability is currently under investigation, and advances in the technologies of computerized quantitative microscopy and intact microvessel imaging should aid the establishment of a functional significance for focal adhesions in the physiological regulation of microvascular permeability.

Vascular endothelial growth factor stimulates differential signaling pathways in in vivo microcirculation

Vascular endothelial growth factor (VEGF) induces mild vasodilation and strong increases in microvascular permeability. Using intravital microscopy and digital integrated optical intensity image analysis, we tested, in the hamster cheek pouch microcirculation, the hypothesis that differential signaling pathways in arterioles and venules represent an in vivo regulatory mechanism in the control of vascular diameter and permeability. The experimental design involved blocking specific signaling molecules and simultaneously assessing VEGF-induced changes in arteriolar diameter and microvascular transport of FITC-Dextran 150. Inhibition of Akt [indirectly via phosphatidylinositol 3-kinase with LY-294002 or wortmannin] or PKC (with bisindolylmaleimide) reduced VEGF-induced hyperpermeability. However, phosphatidylinositol 3-kinase/Akt inhibition enhanced the early phase and attenuated the late phase of VEGF-induced vasodilation, whereas blocking PKC had no effect. Inhibition of extracellular signal-regulated kinase (ERK)-1/2 (with PD-98059 or AG-126) also reduced VEGF-induced hyperpermeability but did not block VEGF-induced vasodilation. Blockade of endothelial nitric oxide synthase (with N(omega)-monomethyl-l-arginine) inhibited VEGF-induced changes in both permeability and diameter. Furthermore, immunofluorescence studies with human umbilical vein endothelial cells revealed that bisindolylmaleimide, PD-98059, and l-NMMA attenuate VEGF-induced reorganization of vascular endothelial cadherin. Our data demonstrate that 1) endothelial nitric oxide synthase is a common convergence pathway for VEGF-induced changes in arteriolar diameter and microvascular permeability; 2) PKC and ERK-1/2 do not play a major role in VEGF-induced vasodilation in the hamster cheek pouch microcirculation; and 3) Akt, PKC, and ERK-1/2 are elements of the signaling cascade that regulates VEGF-stimulated microvascular hyperpermeability. Our data provide evidence for differential signaling as a regulatory step in VEGF-stimulated microvascular dynamics.

PAF- and bradykinin-induced hyperpermeability of rat venules is independent of actin-myosin contraction

We tested the hypothesis that acutely induced hyperpermeability is dependent on actin-myosin contractility by using individually perfused mesentery venules of pentobarbital-anesthetized rats. Venule hydraulic conductivity (Lp) was measured to monitor hyperpermeability response to the platelet-activating factor (PAF) 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine or bradykinin. Perfusion with PAF (10 nM) induced a robust transient high Lp [24.3 +/- 1.7 x 10-7 cm/(s.cmH2O)] that peaked in 8.9 +/- 0.5 min and then returned toward control Lp [1.6 +/- 0.1 x 10-7 cm/(s.cmH2O)]. Reconstruction of venular segments with the use of transmission electron microscopy of serial sections confirmed that PAF induces paracellular inflammatory gaps. Specific inhibition of myosin light chain kinase (MLCK) with 1-10 microM 1-(5-iodonaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine hydrochloride (ML-7) failed to block the PAF Lp response or change the time-to-peak Lp. ML-7 reduced baseline Lp 50% at 40 min of pretreatment. ML-7 also increased the rate of recovery from PAF hyperpermeability measured as the decrease of half-time of recovery from 4.8 +/- 0.7 to 3.2 +/- 0.3 min. Inhibition of myosin ATPase with 5-20 mM 2,3-butanedione 2-monoxime also failed to alter the hyperpermeability response to PAF. Similar results were found using ML-7 to modulate responses. These experiments indicate that an actin-myosin contractile mechanism modulated by MLCK does not contribute significantly to the robust initial increase in permeability of rat venular microvessels exposed to two common inflammatory mediators. The results are consistent with paracellular gap formation by local release of endothelial-endothelial cell adhesion structures in the absence of contraction by the actin-myosin network.

Pulmonary vascular permeability and ischemic injury in gelsolin-deficient mice

Gelsolin is a potent actin filament regulatory protein that controls cytoskeletal assembly and disassembly. Because cellular gelsolin deficiency leads to pronounced actin stress fiber formation and defective chemotaxis, and similar cytoskeletal remodeling results in endothelial barrier dysfunction, we hypothesized that gelsolin deficient mice would exhibit increased vascular permeability. To test this hypothesis, we compared baseline lung lavage (BAL) protein concentration, wet/dry weight ratio, and osmotic reflection coefficient for albumin (sigma alb) in gelsolin-deficient (gsn-/-) and C57BL/6 (wild-type) mice. In addition, we assessed lung permeability in response to ischemia by evaluating BAL protein concentration after 4, 8, or 24 h of left pulmonary arterial (LPA) occlusion, and lung wet/dry weight ratio and histology after 24 h of LPA occlusion, in gsn-/- and wild-type animals, as compared with control and sham-operated mice. Baseline measurements revealed that BAL protein concentration was 18-fold higher in gsn-/- than in wild-type mice, whereas sigma alb averaged 0.62 + 0.15 in wild-type, as compared with 0.31 + 0.05 in gsn-/- animals, indicating that gelsolin deficiency caused increased pulmonary vascular permeability. Ischemia increased lung permeability (BAL protein and lung wet/dry weight) in both wild-type and gsn-/- mice. However, whereas the fold-increase in BAL protein concentration was less in gsn-/- mice (2- to 4-fold) as compared with wild-type (22- to 34-fold), the duration of ischemia-induced permeability changes was prolonged. Lung wet/dry weight and gross histology following ischemia were comparable in wild-type and gsn-/- animals. These data suggest that gelsolin significantly contributes to maintenance of vascular barrier function in the lung.

p42/44MAPK regulates baseline permeability and cGMP-induced hyperpermeability in endothelial cells

We tested the hypothesis that p42/44MAPK and p38MAPK (mitogen-activated protein kinases; MAPK) signaling pathways regulate endothelial cell permeability to macromolecules. Passage 2-4 human umbilical vein endothelial cells (HUVEC) were grown to confluence on fibronectin-coated Snapwell membranes. The flux of fluorescein isothiocyanate-labeled dextran-70 across the HUVEC monolayers served to determine permeability. Application of 1 mM 8-bromo 3' 5'-cyclic guanosine monophosphate (8-Br-cGMP) increased permeability from 7.0 +/- 1.6 x 10(-6) to 12.5 +/- 2.8 x 10(-6) cm/s (P < 0.05). Pretreatment of HUVEC for 60 min with a selective p42/44MAPK inhibitor (AG126 at 2.7 and 27 microM) blocked 8-Br-cGMP-induced hyperpermeability. However, inhibition of p38MAPK (SB203580 at 0.6 microM) did not influence the cGMP-induced hyperpermeability response. AG126, administered at 27 microM, decreased baseline permeability from 7.9 +/- 0.5 x 10(-6) to 5.9 +/- 0.5 x 10(-6) cm/s (P < 0.05). Our results indicate that the p42/44MAPK signaling pathway is important in the regulation of baseline permeability and cGMP-induced hyperpermeability.

Human urotensin II mediates vasoconstriction via an increase in inositol phosphates

The cyclic peptide urotensin II has recently been cloned from human and reported to potently constrict primate blood vessels. To elucidate the cellular signalling mechanisms of this peptide, we investigated a possible relationship of vasomotor effects of human urotensin II and phosphoinositide turnover in isolated rabbit thoracic aorta. Human urotensin II produced a slowly developing increase in isometric contractile force (pEC(50)=9.0) that was endothelium-independent. The contractile effect of urotensin II was significantly inhibited by the phospholipase C inhibitor, 2-nitro-4-carboxyphenyl-N,N,-diphenylcarbamate (NCDC), but not by the cyclooxygenase inhibitor, indomethacin. In slices of rabbit thoracic aorta, human urotensin II increased phosphoinositide hydrolysis, and this effect was also inhibited by NCDC. The potency of urotensin II (pEC(50)=8.6) was similar to that found in the contractile studies. Thus, vasoconstrictor effects of human urotensin II appear to be mediated by a phospholipase C-dependent increase in inositol phosphates, suggesting that the peptide acts via a G(q) protein-coupled receptor.