Large variations in absolute wall shear stress levels within one species and between species

Wall shear stress (WSS), the frictional force between blood and endothelium, is an important determinant of vascular function. It is generally assumed that WSS remains constant at a reference value of 15 dyn/cm(2). In a study of small rodents, we realized that this assumption could not be valid. This review presents an overview of recent studies in large and small animals where shear stress was measured, derived from velocity measurements or otherwise, in large vessels. The data show that large variations exist within a single species (human: variation of 2-16 N/m(2)). Moreover, when we compared different species at the same location within the arterial tree, an inverse relationship between animal size and wall shear stress was noted. When we related WSS to diameter, a unique relationship was derived for all species studied. This relationship could not be described by the well-known r(3) law of Murray, but by the r(2) law introduced by Zamir et al. in 1972. In summary, by comparing data from the literature, we have shown that: (i) the assumption of a physiological WSS level of approximately 15 dyn/cm(2) for all straight vessels in the arterial tree is incorrect; (ii) WSS is not constant throughout the vascular tree; (iii) WSS varies between species; (iv) WSS is inversely related to the vessel diameter. These data support an "r(2) law" rather than Murray's r(3) law for the larger vessels in the arterial tree.

Atherogenic, enlarged, and dysfunctional HDL in human PLTP/apoA-I double transgenic mice

In low density lipoprotein receptor (LDLR)-deficient mice, overexpression of human plasma phospholipid transfer protein (PLTP) results in increased atherosclerosis. PLTP strongly decreases HDL levels and might alter the antiatherogenic properties of HDL particles. To study the potential interaction between human PLTP and apolipoprotein A-I (apoA-I), double transgenic animals (hPLTPtg/hApoAItg) were compared with hApoAItg mice. PLTP activity was increased 4.5-fold. Plasma total cholesterol and phospholipid were decreased. Average HDL size (analyzed by gel filtration) increased strongly, hPLTPtg/hApoAItg mice having very large, LDL-sized, HDL particles. Also, after density gradient ultracentrifugation, a substantial part of the apoA-I-containing lipoproteins in hPLTPtg/hApoAItg mice was found in the LDL density range. In cholesterol efflux studies from macrophages, HDL isolated from hPLTPtg/hApoAItg mice was less efficient than HDL isolated from hApoAItg mice. Furthermore, it was found that the largest subfraction of the HDL particles present in hPLTPtg/hApoAItg mice was markedly inferior as a cholesterol acceptor, as no labeled cholesterol was transferred to this fraction. In an LDLR-deficient background, the human PLTP-expressing mouse line showed a 2.2-fold increased atherosclerotic lesion area. These data demonstrate that the action of human PLTP in the presence of human apoA-I results in the formation of a dysfunctional HDL subfraction, which is less efficient in the uptake of cholesterol from cholesterol-laden macrophages.

Effect of shear stress on vascular inflammation and plaque development

PURPOSE OF REVIEW

This review describes evidence that shear stress acts through modulation of inflammation and by that process affects atherogenesis and plaque composition.

RECENT FINDINGS

In low shear stress regions antiatherogenic transcription factors are downregulated and pro-atherogenic transcription factors are upregulated. Consequently, inflammatory cells may home low shear stress regions more easily to the plaques because of increased expression of adhesion factors, a decreased rolling speed and an increased expression of chemokines, thereby changing the composition of the plaques into a more vulnerable phenotype. In contrast, in advanced plaque development vascular lumen decreases and shear stress increases, especially upstream of the plaques. The predominant upstream location of lipids induces a prevalent upstream location of inflammatory cells leading to localized plaque rupture.

SUMMARY

Shear stress has been shown to play a role in plaque induction, plaque progression and plaque rupture. The mechanism for plaque induction seems to differ from the role of shear stress for plaque rupture, whereby the former mechanism is induced by low shear stress and the latter by high shear stress.

Ubiquitous expression of the rtTA2S-M2 inducible system in transgenic mice driven by the human hnRNPA2B1/CBX3 CpG island

Background

A sensitive, ubiquitously expressed tetracycline inducible system would be a valuable tool in mouse transgenesis. However, this has been difficult to obtain due to position effects observed at different chromosomal sites of transgene integration, which negatively affect expression in many tissues. The aim of this study was to test the utility of a mammalian methylation-free CpG island to drive ubiquitous expression of the sensitive doxycycline (Dox) inducible rtTA2S-M2 Tet-transactivator in transgenic mice.

Results

An 8 kb genomic fragment from the methylation-free CpG island of the human hnRNPA2B1-CBX3 housekeeping gene locus was tested. In a number of transgenic mouse lines obtained, rtTA2S-M2 expression was detected in many tissues examined. Characterisation of the highest expressing rtTA2S-M2 transgenic mouse line demonstrated Dox-inducible GFP transgene expression in many tissues. Using this line we also show highly sensitive quantitative induction with low doses of Dox of an assayable plasma protein transgene under the control of a Tet Responsive Element (TRE). The utility of this rtTA2S-M2 line for inducible expression in mouse embryos was also demonstrated using a GATA-6 Tet-inducible transgene to show specific phenotypes in the embryonic lung, as well as broader effects resulting from the inducible widespread overexpression of the transgene.

Conclusion

The ubiquitously expressing rtTA2S-M2 transgenic mouse line described here provides a very useful tool for studying the effects of the widespread, inducible overexpression of genes during embryonic development and in adult mice.

Endothelial nitric oxide synthase activity is essential for vasodilation during blood flow recovery but not for arteriogenesis

OBJECTIVE

Arteriogenesis is the major mechanism of vascular growth, which is able to compensate for blood flow deficiency after arterial occlusion. Endothelial nitric oxide synthase (eNOS) activity is essential for neovascularization, however its specific role in arteriogenesis remains unclear. We studied the role of eNOS in arteriogenesis using 3 mouse strains with different eNOS expression.

METHODS AND RESULTS

Distal femoral artery ligation was performed in eNOS-overexpressing mice (eNOStg), eNOS-deficient (eNOS-/-) mice, and wild type (WT) controls. Tissue perfusion and collateral-dependent blood flow were significantly increased in eNOStg mice compared with WT only immediately after ligation. In eNOS-/- mice, although tissue perfusion remained significantly decreased, collateral-dependent blood flow was only decreased until day 7, suggesting normal, perhaps delayed collateral growth. Histology confirmed no differences in collateral arteries of eNOStg, eNOS-/-, and WT mice at 1 and 3 weeks. Administration of an NO donor induced vasodilation in collateral arteries of eNOS-/- mice, but not in WT, identifying the inability to vasodilate collateral arteries as main cause of impaired blood flow recovery in eNOS-/- mice.

CONCLUSIONS

This study demonstrates that eNOS activity is crucial for NO-mediated vasodilation of peripheral collateral vessels after arterial occlusion but not for collateral artery growth.

Vasomotor control in mice overexpressing human endothelial nitric oxide synthase

Nitric oxide (NO) plays a key role in regulating vascular tone. Mice overexpressing endothelial NO synthase [eNOS-transgenic (Tg)] have a 20% lower systemic vascular resistance (SVR) than wild-type (WT) mice. However, because eNOS enzyme activity is 10 times higher in tissue homogenates from eNOS-Tg mice, this in vivo effect is relatively small. We hypothesized that the effect of eNOS overexpression is attenuated by alterations in NO signaling and/or altered contribution of other vasoregulatory pathways. In isoflurane-anesthetized open-chest mice, eNOS inhibition produced a significantly greater increase in SVR in eNOS-Tg mice compared with WT mice, consistent with increased NO synthesis. Vasodilation to sodium nitroprusside (SNP) was reduced, whereas the vasodilator responses to phosphodiesterase-5 blockade and 8-bromo-cGMP (8-Br-cGMP) were maintained in eNOS-Tg compared with WT mice, indicating blunted responsiveness of guanylyl cyclase to NO, which was supported by reduced guanylyl cyclase activity. There was no evidence of eNOS uncoupling, because scavenging of reactive oxygen species (ROS) produced even less vasodilation in eNOS-Tg mice, whereas after eNOS inhibition the vasodilator response to ROS scavenging was similar in WT and eNOS-Tg mice. Interestingly, inhibition of other modulators of vascular tone [including cyclooxygenase, cytochrome P-450 2C9, endothelin, adenosine, and Ca-activated K(+) channels] did not significantly affect SVR in either eNOS-Tg or WT mice, whereas the marked vasoconstrictor responses to ATP-sensitive K(+) and voltage-dependent K(+) channel blockade were similar in WT and eNOS-Tg mice. In conclusion, the vasodilator effects of eNOS overexpression are attenuated by a blunted NO responsiveness, likely at the level of guanylyl cyclase, without evidence of eNOS uncoupling or adaptations in other vasoregulatory pathways.

High Pressure Promotes Monocyte Adhesion to the Vascular Wall

Hypertension is a known risk factor for the development of atherosclerosis. To assess how mechanical factors contribute to this process, mouse carotid arteries were maintained in organ culture at normal (80 mm Hg) or high (150 mm Hg) intraluminal pressure for 1, 6, 12, or 24 hours. Thereafter, fluorescent human monocytic cells (U937) were injected intraluminally and allowed to adhere for 30 minutes before washout. U937 adhesion was increased in vessels kept at 150 mm Hg 12 hours (23.5+/-5.7 versus 9.9+/-2.2 cells/mm at 80 mm Hg; P<0.05) or 24 hours (26.7+/-5.7 versus 8.8+/-1.5 cells/mm; P<0.05). At 24 hours, high pressure was associated with increased mRNA expression of monocyte chemoattractant protein-1, interleukin-6, keratinocyte-derived chemokine, and vascular cell adhesion molecule-1 (6.9+/-2.1, 4.4+/-0.1, 9.8+/-2.8, and 2.4+/-0.1-fold respectively; P<0.05), as assessed by quantitative RT-PCR and corroborated by immunohistochemistry, which also revealed an increase in intracellular adhesion molecule-1 expression. Nuclear factor kappaB inhibition using SN50 peptide abolished the overexpression of chemokines and adhesion molecules and reduced U937 adhesion in vessels at 150 mm Hg. Moreover, treatment of vessels and cells with specific neutralizing antibodies established that monocyte chemoattractant protein-1, interleukin-6, and keratinocyte-derived chemokine released from vessels at 150 mm Hg primed the monocytes, increasing their adhesion to vascular cell adhesion molecule-1 but not intracellular adhesion molecule-1 via alpha4beta1 integrins. The additive effect of chemokines on the adhesion of U937 cells to vascular cell adhesion molecule-1 was confirmed by in vitro assay. Finally, pressure-dependent U937 adhesion was blunted in arteries from mice overexpressing endothelial NO synthase. Hence, high intraluminal pressure induces cytokine and adhesion molecule expression via nuclear factor kappaB, leading to monocytic cell adhesion. These results indicate that hypertension may directly contribute to the development of atherosclerosis through nuclear factor kappaB induction.

Inducible expression of phospholipid transfer protein (PLTP) in transgenic mice: acute effects of PLTP on lipoprotein metabolism

One main determinant in high-density lipoprotein (HDL) metabolism is phospholipid transfer protein (PLTP), a plasma protein that is associated with HDL. In transgenic mice overexpressing human PLTP we found that elevated plasma PLTP levels dose-dependently increased the susceptibility to diet-induced atherosclerosis. This could be mainly due to the fact that most functions of PLTP are potentially atherogenic, such as decreasing plasma HDL levels. To further elucidate the role of PLTP in lipoprotein metabolism and atherosclerosis we generated a novel transgenic mouse model that allows conditional expression of human PLTP. In this mouse model a human PLTP encoding sequence is controlled by a Tet-On system. Upon induction of PLTP expression, our mouse model showed a strongly increased PLTP activity (from 3.0 +/- 0.6 to 11.4 +/- 2.8 AU, p < 0.001). The increase in PLTP activity resulted in an acute decrease in plasma cholesterol of 33% and a comparable decrease in phospholipids. The decrease in total plasma cholesterol and phospholipids was caused by a 35% decrease in HDL-cholesterol level and a 41% decrease in HDL-phospholipid level. These results demonstrate the feasibility of our mouse model to induce an acute elevation of PLTP activity, which is easily reversible. As a direct consequence of an increase in PLTP activity, HDL-cholesterol and HDL-phospholipid levels strongly decrease. Using this mouse model, it will be possible to study the effects of acute elevation of PLTP activity on lipoprotein metabolism and pre-existing atherosclerosis.

Early exercise training normalizes myofilament function and attenuates left ventricular pump dysfunction in mice with a large myocardial infarction

The extent and mechanism of the cardiac benefit of early exercise training following myocardial infarction (MI) is incompletely understood, but may involve blunting of abnormalities in Ca(2+)-handling and myofilament function. Consequently, we investigated the effects of 8-weeks of voluntary exercise, started early after a large MI, on left ventricular (LV) remodeling and dysfunction in the mouse. Exercise had no effect on survival, MI size or LV dimensions, but improved LV fractional shortening from 8+/-1 to 12+/-1%, and LVdP/dt(P30) from 5295+/-207 to 5794+/-207 mm Hg/s (both P<0.05), and reduced pulmonary congestion. These global effects of exercise were associated with normalization of the MI-induced increase in myofilament Ca(2+)-sensitivity (DeltapCa(50)=0.037). This effect of exercise was PKA-mediated and likely because of improved beta(1)-adrenergic signaling, as suggested by the increased beta(1)-adrenoceptor protein (48%) and cAMP levels (36%; all P<0.05). Exercise prevented the MI-induced decreased maximum force generating capacity of skinned cardiomyocytes (F(max) increased from 14.3+/-0.7 to 18.3+/-0.8 kN/m(2) P<0.05), which was associated with enhanced shortening of unloaded intact cardiomyocytes (from 4.1+/-0.3 to 7.0+/-0.6%; P<0.05). Furthermore, exercise reduced diastolic Ca(2+)-concentrations (by approximately 30%, P<0.05) despite the unchanged SERCA2a and PLB expression and PLB phosphorylation status. Importantly, exercise had no effect on Ca(2+)-transient amplitude, indicating that the improved LV and cardiomyocyte shortening were principally because of improved myofilament function. In conclusion, early exercise in mice after a large MI has no effect on LV remodeling, but attenuates global LV dysfunction. The latter can be explained by the exercise-induced improvement of myofilament function.

Shear stress-induced changes in atherosclerotic plaque composition are modulated by chemokines

We previously found that low shear stress (LSS) induces atherosclerotic plaques in mice with increased lipid and matrix metalloproteinase content and decreased vascular smooth muscle and collagen content. Here, we evaluated the role of chemokines in this process, using an extravascular device inducing regions of LSS, high shear stress, and oscillatory shear stress (OSS) in the carotid artery. One week of shear stress alterations induced expression of IFN-gamma-inducible protein-10 (IP-10) exclusively in the LSS region, whereas monocyte chemoattractant protein-1 (MCP-1) and the mouse homolog of growth-regulated oncogene alpha (GRO-alpha) were equally upregulated in both LSS and OSS regions. After 3 weeks, GRO-alpha and IP-10 were specifically upregulated in LSS regions. After 9 weeks, lesions with thinner fibrous caps and larger necrotic cores were found in the LSS region compared with the OSS region. Equal levels of MCP-1 expression were observed in both regions, while expression of fractalkine was found in the LSS region only. Blockage of fractalkine inhibited plaque growth and resulted in striking differences in plaque composition in the LSS region. We conclude that LSS or OSS triggers expression of chemokines involved in atherogenesis. Fractalkine upregulation is critically important for the composition of LSS-induced atherosclerotic lesions.

Gelatinolytic Activity in Atherosclerotic Plaques Is Highly Localized and Is Associated With Both Macrophages and Smooth Muscle Cells In Vivo

Background— Atherosclerosis is considered an inflammatory disease. Recent studies provided evidence for a predominant upstream location of plaque inflammation. The present study introduces a novel technique that evaluates the underlying mechanism of this spatial organization.

Methods and Results— In hypercholesterolemic rabbits, atherosclerosis of the infrarenal aorta was induced by a combination of endothelial denudation and a high-cholesterol diet (2% cholesterol for 2 months). At the time of death, aortic vessel segments were dissected and reconstructed with a new technique that preserved the original intravascular ultrasound–derived lumen geometry. This enabled us to study the spatial relation of histological markers like macrophages, smooth muscle cells, lipids, gelatinolytic activity, and oxidized low-density lipoprotein. Results showed a predominant upstream localization of macrophages and gelatinase activity. Colocalization studies indicated that gelatinase activity was associated with macrophages and smooth muscle cells. Further analysis revealed that this was caused by subsets of smooth muscle cells and macrophages, which were associated with oxidized low-density lipoprotein accumulation.

Conclusions— Upstream localization of a vulnerable plaque phenotype is probably due to an accumulation of oxidized low-density lipoprotein, which activates/induces subsets of smooth muscle cells and macrophages to gelatinase production.

Sex differences in atherosclerosis in mice with elevated phospholipid transfer protein activity are related to decreased plasma high density lipoproteins and not to increased production of triglycerides

Plasma phospholipid transfer protein (PLTP) has atherogenic properties in genetically modified mice. PLTP stimulates hepatic triglyceride secretion and reduces plasma levels of high density lipoproteins (HDL). The present study was performed to relate the increased atherosclerosis in PLTP transgenic mice to one of these atherogenic effects. A humanized mouse model was used which had decreased LDL receptor expression and was transgenic for human cholesterylester transfer protein (CETP) in order to obtain a better resemblance to the plasma lipoprotein profile present in humans. It is well known that female mice are more susceptible to atherosclerosis than male mice. Therefore, we compared male and female mice expressing human PLTP. The animals were fed an atherogenic diet and the effects on plasma lipids and lipoproteins, triglyceride secretion and the development of atherosclerosis were measured. The development of atherosclerosis was sex-dependent. This effect was stronger in PLTP transgenic mice, while PLTP activity levels were virtually identical. Also, the rates of hepatic secretion of triglycerides were similar. In contrast, plasma levels of HDL were about 2-fold lower in female mice than in male mice after feeding an atherogenic diet. We conclude that increased atherosclerosis caused by overexpression of PLTP is related to a decrease in HDL, rather than to elevated hepatic secretion of triglycerides.

Atherosclerotic Lesion Size and Vulnerability Are Determined by Patterns of Fluid Shear Stress

Background— Atherosclerotic lesions are predominantly observed in curved arteries and near side branches, where low or oscillatory shear stress patterns occur, suggesting a causal connection. However, the effect of shear stress on plaque vulnerability is unknown because the lack of an appropriate in vivo model precludes cause-effect studies.

Methods and Results— We developed a perivascular shear stress modifier that induces regions of lowered, increased, and lowered/oscillatory (ie, with vortices) shear stresses in mouse carotid arteries and studied plaque formation and composition. Atherosclerotic lesions developed invariably in the regions with lowered shear stress or vortices, whereas the regions of increased shear stress were protected. Lowered shear stress lesions were larger (intima/media, 1.38±0.68 versus 0.22±0.04); contained fewer smooth muscle cells (1.9±1.6% versus 26.3±9.7%), less collagen (15.3±1.0% versus 22.2±1.0%), and more lipids (15.8±0.9% versus 10.2±0.5%); and showed more outward vascular remodeling (214±19% versus 117±9%) than did oscillatory shear stress lesions. Expression of proatherogenic inflammatory mediators and matrix metalloproteinase activity was higher in the lowered shear stress regions. Spontaneous and angiotensin II–induced intraplaque hemorrhages occurred in the lowered shear stress regions only.

Conclusions— Lowered shear stress and oscillatory shear stress are both essential conditions in plaque formation. Lowered shear stress induces larger lesions with a vulnerable plaque phenotype, whereas vortices with oscillatory shear stress induce stable lesions.

Fenofibrate reverses the decline in HDL cholesterol in mice overexpressing human phospholipid transfer protein

In humans, fibrates are used to treat dyslipidemia, because these drugs lower plasma triglycerides and raise HDL cholesterol. Treatment with fibrates lowers plasma phospholipid transfer protein (PLTP) activity in humans, but increases PLTP activity in mice, without a consistent effect on HDL-cholesterol concentration. Earlier, we found that PLTP overexpression in transgenic mice results in decreased plasma HDL levels and increased diet-induced atherosclerosis. So it seems that the interplay between fibrates, PLTP and HDL is different in mice and man, which may be important for atherosclerosis development. In the present study, we measured the effects of fibrates on PLTP expression in cultured human hepatocytes and effects of fibrate treatment on human PLTP expression, plasma PLTP activity and HDL levels in human PLTP transgenic mice. Fibrate treatment did not influence PLTP mRNA levels in human hepatocytes. Hepatic human PLTP mRNA levels and PLTP activity were both moderately elevated by fenofibrate treatment in human PLTP transgenic mice. In wild-type mice, however, feeding fenofibrate resulted in a strong induction of PLTP mRNA in the liver and a more than 4-fold increase of plasma PLTP activity. Plasma triglycerides were reduced in all mice by 48% or more by fenofibrate treatment. HDL-cholesterol concentrations were substantially increased by fenofibrate in PLTP overexpressing mice (+72%), but unaffected in wild-type mice. We conclude that fenofibrate treatment reverses the HDL-lowering effect of PLTP overexpression in human PLTP transgenic mice.

Shear stress affects the intracellular distribution of eNOS: direct demonstration by a novel in vivo technique

The focal location of atherosclerosis in the vascular tree is correlated with local variations in shear stress. We developed a method to induce defined variations in shear stress in a straight vessel segment of a mouse. To this end, a cylinder with a tapered lumen was placed around the carotid artery, inducing a high shear stress field. Concomitantly, regions of low shear stress and oscillatory shear stress were created upstream and down-stream of the device, respectively. This device was used in mice transgenic for an eNOS3GFP fusion gene. We observed a strong induction of endothelial nitric oxide synthase-green fluorescent protein (eNOS-GFP) mRNA expression in the high shear stress region compared with the other regions (P < .05). Quantification of eNOS-GFP fluorescence or of immunoreactivity to the Golgi complex or to platelet endothelial cell adhesion molecule 1 (PECAM-1) showed an increase in the high shear stress region (P < .05) compared with nontreated carotid arteries. Colocalization of eNOS-GFP with either the Golgi complex or PECAM-1 also responded to alterations of shear stress. In conclusion, we showed a direct response of mRNA and protein expression in vivo to induced variations of shear stress. This model provides the opportunity to study the relationship between shear stress alterations, gene expression, and atherosclerosis.

A novel model of cryoinjury-induced myocardial infarction in the mouse: a comparison with coronary artery ligation

Mouse myocardial infarction (MI) models are frequently used research tools. The most commonly applied model is coronary artery ligation. However, coronary ligation often gives rise to apical aneurysmatic infarcts of variable size. Other infarct models include cryoinfarction, which produces reproducible infarcts of the anterior wall. Thus far, this model has not been extensively described in mice. Therefore, we developed a murine cryoinfarction model and compared it with coronary ligation. Studies were performed under isoflurane anesthesia with a follow-up of 4 and 8 wk. Cryoinfarction was induced using a 2- or 3-mm cryoprobe. Two-dimensional guided M-mode echocardiography was used to assess fractional shortening and left ventricular (LV) dimensions at baseline and end point. At end point, hemodynamics were assessed using a 1.4-Fr Millar catheter. Pressure-diameter relations were constructed by combining echocardiography and hemodynamic data. Histological and morphometric analyses of infarct and remote areas were performed. At 4 wk, 3-mm cryoinfarction resulted in decreased LV fractional shortening as well as decreased global LV contractility and relaxation, which was comparable with coronary ligation. No adverse remodeling was observed at this time point, in contrast with the ligation model. However, progressive LV remodeling occured between 4 and 8 wk after cryoinfarction with a further decline in hemodynamic parameters and LV pump function. Histologically, cryoinfarction resulted in highly reproducible, transmural, cone-shaped infarcts with reperfusion at the macrovascular level. These results indicate that the cryoinfarction model represents the anterior myocardial infarct with modest adverse remodeling and may thus be representative for infarcts encountered in clinical practice.

Elevation of plasma phospholipid transfer protein increases the risk of atherosclerosis despite lower apolipoprotein B-containing lipoproteins

Plasma phospholipid transfer protein (PLTP) transfers phospholipids between lipoproteins and mediates HDL conversion. PLTP-overexpressing mice have increased atherosclerosis. However, mice do not express cholesteryl ester transfer protein (CETP), which is involved in the same metabolic pathways as PLTP. Therefore, we studied atherosclerosis in heterozygous LDL receptor-deficient (LDLR(+/-)) mice expressing both human CETP and human PLTP. We used two transgenic lines with moderately and highly elevated plasma PLTP activity. In LDLR(+/-)/huCETPtg mice, cholesterol is present in both LDL and HDL. Both are decreased in LDLR(+/-)/huCETPtg/huPLTPtg mice (>50%). An atherogenic diet resulted in high levels of VLDL+LDL cholesterol. PLTP expression caused a strong PLTP dose-dependent decrease in VLDL and LDL cholesterol (-26% and -69%) and a decrease in HDL cholesterol (-70%). Surprisingly, atherosclerosis was increased in the two transgenic lines with moderately and highly elevated plasma PLTP activity (1.9-fold and 4.4-fold, respectively), indicating that the adverse effect of the reduction in plasma HDL outweighs the beneficial effect of the reduction in apolipoprotein B (apoB)-containing lipoproteins. The activities of the antiatherogenic enzymes paraoxonase and platelet-activating factor acetyl hydrolase were both PLTP dose-dependently reduced ( approximately -33% and -65%, respectively). We conclude that expression of PLTP in this animal model results in increased atherosclerosis in spite of reduced apoB-containing lipoproteins, by reduction of HDL and of HDL-associated antioxidant enzyme activities.

Endothelial nitric oxide synthase overexpression attenuates myocardial reperfusion injury

Previous studies indicate that deficiency of endothelial nitric oxide (NO) synthase (eNOS)-derived NO exacerbates myocardial reperfusion injury. We hypothesized that overexpression of eNOS would reduce the extent of myocardial ischemia-reperfusion (MI/R) injury. We investigated two distinct strains of transgenic (TG) mice overexpressing the eNOS gene (eNOS TG). Bovine eNOS was overexpressed in one strain (eNOS TG-Kobe), whereas the human eNOS gene was overexpressed in the other strain (eNOS TG-RT). Non-TG (NTG) and eNOS TG mice were subjected to 30 min of coronary artery occlusion followed by 24 h of reperfusion, and the extent of myocardial infarction was determined. Myocardial infarct size was reduced by 33% in the eNOS TG-Kobe strain (P < 0.05 vs. NTG) and by 32% in the eNOS TG-RT strain (P < 0.05 vs. NTG). However, postischemic cardiac function (cardiac output, fractional shortening) was not improved in the eNOS TG-Kobe mouse at 24 h of reperfusion [P = not significant (NS) vs. NTG]. In additional studies, eNOS TG-Kobe mice were subjected to 30 min of myocardial infarction and 7 days of reperfusion. Fractional shortening and the first derivative of left ventricular pressure were measured in eNOS TG-Kobe and NTG mice, and no significant differences in contractility were observed (P = NS) between the eNOS TG mice and NTG controls. Left ventricular end-diastolic pressure was significantly (P < 0.05 vs. NTG) reduced in the eNOS TG-Kobe strain at 7 days of reperfusion. The cardioprotective effects of eNOS overexpression on myocardial infarct size were ablated by Nomega-nitro-l-arginine methyl ester (300 mg/kg) pretreatment. Thus genetic overexpression of eNOS in mice attenuates myocardial infarction after MI/R but fails to significantly protect against postischemic myocardial contractile dysfunction in mice.

The Role of Shear Stress in Atherosclerosis: Action Through Gene Expression and Inflammation?

Atherosclerotic lesions preferentially localize near side branches or curved vessels. During the last few decades, research has been shown that low or low and oscillating shear stress is associated with plaque location. Despite ample evidence, the precise mechanism is unknown. This is mainly because of a lack of appropriate animal models. We describe two novel methods to study the hypothesis that shear stress acts through endothelial gene expression or shear stress acts through localizing of inflammation. Both literature evidence and own findings support a role for both mechanisms in atherosclerosis.

Functional expression of endothelial nitric oxide synthase fused to green fluorescent protein in transgenic mice

The activity of endothelial nitric oxide synthase (eNOS) is subject to complex transcriptional and post-translational regulation including the association with several proteins and variations in subcellular distribution. In the present study we describe a transgenic mouse model expressing eNOS fused to green fluorescent protein (GFP), which allows the study of localization and regulation of eNOS expression. We tested the functionality of eNOS in the eNOS-GFP mice. Expression of eNOS was restricted to the endothelial lining of blood vessels in various tissues tested, without appreciable expression in non-endothelial cells. Activity of the enzyme was confirmed by assaying the conversion of L-arginine to L-citrulline. NO production in isolated vessels was increased in transgenic mice when compared to non-transgenic control animals (4.88 +/- 0.59 and 2.48 +/- 0.47 micro mol/L NO, respectively, P < 0.005). Both the mean aortic pressure and the pulmonary artery pressure were reduced in eNOS-GFP mice (both approximately 30%, P < 0.05). Plasma cholesterol levels were also slightly reduced ( approximately 20%, P < 0.05). In conclusion, eNOS-GFP mice express functional eNOS and provide a unique model to study regulation of eNOS activity or eNOS-mediated vascular events, including response to ischemia, response to differences in shear stress, angiogenesis and vasculogenesis, and to study the subcellular distribution in relation with functional responses to these events.

Increased fecal bile acid excretion in transgenic mice with elevated expression of human phospholipid transfer protein

OBJECTIVE

HDL plays a key role in protection against development of atherosclerosis by promoting reverse cholesterol transport from peripheral tissues to the liver for secretion into bile. Phospholipid transfer protein (PLTP) promotes the transfer of phospholipids between lipoproteins and modulates HDL size and composition, thereby having a crucial role in HDL metabolism. We investigated the effect of increased PLTP activity on removal of cholesterol from the body.

METHODS AND RESULTS

On a chow diet, transgenic mice overexpressing human PLTP have a 15-fold increased plasma PLTP activity compared with wild-type mice (572.4+/-59.2 versus 38.6+/-3.6 micromol/mL per h). Plasma cholesterol, mainly present in HDL, is strongly decreased (-92%), caused by a rapid clearance from the circulation by the liver and leading to a 1.8-fold increase in hepatic cholesteryl esters. This results in a 2-fold increase in biliary bile acid secretion without changing the bile saturation index. Consequently, the transgenic mice show a 1.4-fold increase in the amount of excreted fecal bile acids compared with wild-type mice, whereas fecal neutral sterol excretion is unchanged.

CONCLUSIONS

Our data show that elevation of PLTP activity results in rapid disposal of cholesterol from the body via increased conversion into bile acids and subsequent excretion.

Endothelial nitric oxide synthase overexpression attenuates congestive heart failure in mice

Congestive heart failure results in cardiovascular dysfunction and diminished vascular nitric oxide (NO) production. We hypothesized that overexpression of endothelial NO synthase (eNOS) within the endothelium would reduce the extent of contractile dysfunction in a murine model of infarct-induced congestive heart failure. We generated transgenic (TG) mice overexpressing the human eNOS gene. The TG mice displayed significantly enhanced eNOS protein levels and eNOS activity levels (10- to 12-fold greater) in the aorta and the coronary vasculature. Non-TG (NTg) and eNOS TG mice were subjected to permanent left anterior descending coronary artery occlusion and then observed for 1 mo. We assessed cardiac function in vivo by using echocardiography and ultraminiature ventricular pressure catheters. Myocardial infarct size was similar between study groups (approximately 70% of the risk zone). Survival was increased by 43% in the eNOS TG mice compared with NTg (P < 0.05). Fractional shortening and cardiac output were also significantly (P < 0.05) greater in the eNOS TG than in NTg. Interestingly, pulmonary edema was evident only in NTg mice, and no evidence of pulmonary edema was observed in the eNOS TG mice. Thus, targeted overexpression of the eNOS gene within the vascular endothelium in mice attenuates both cardiac and pulmonary dysfunction and dramatically improves survival during severe congestive heart failure.

Increased risk of atherosclerosis by elevated plasma levels of phospholipid transfer protein

Plasma phospholipid transfer protein (PLTP) is thought to be involved in the remodeling of high density lipoproteins (HDL), which are atheroprotective. It is also involved in the metabolism of very low density lipoproteins (VLDL). Hence, PLTP is thought to be an important factor in lipoprotein metabolism and the development of atherosclerosis. We have overexpressed PLTP in mice heterozygous for the low density lipoprotein (LDL) receptor, a model for atherosclerosis. We show that increased PLTP activity results in a dose-dependent decrease in HDL, and a moderate stimulation of VLDL secretion (</=1.5-fold). The mice were given a high fat, high cholesterol diet, which resulted in hypercholesterolemia in all animals. HDL concentrations were dramatically reduced in PLTP-overexpressing animals when compared with LDL receptor controls, whereas VLDL + LDL cholesterol levels were identical. Susceptibility to atherosclerosis was increased in a PLTP dose-responsive manner. We conclude that PLTP increases susceptibility to atherosclerosis by lowering HDL concentrations, and therefore we suggest that an increase in PLTP is a novel, long term risk factor for atherosclerosis in humans.