Effects of shear stress on protein kinase C distribution in endothelial cells

A PKC that controls polyphosphate levels, pinocytosis and exocytosis, regulates stationary phase onset in Dictyostelium

Many cells can pause their growth cycle, a topic much enriched by studies of the stationary phase (SP) of model microorganisms. While several kinases are implicated in SP onset, a possible role for protein kinase C remains unknown. We show that Dictyostelium discoideum cells lacking pkcA entered SP at a reduced cell density, but only in shaking conditions. Precocious SP entry occurs because extracellular polyphosphate (polyP) levels reach a threshold at the lower cell density; adding exopolyphosphatase to pkcA- cells reverses the effect and mimics wild type growth. PkcA's regulation of polyP depended on inositol hexakisphosphate kinase and phospholipase D. PkcA- mutants also had higher actin levels, higher rates of exocytosis and lower pinocytosis rates. Postlysosomes were smaller and present in fewer pkcA- cells, compared to the wildtype. Overall, the results suggest that a reduced PkcA level triggers SP primarily because cells do not acquire or retain nutrients as efficiently, thus mimicking, or amplifying, the conditions of actual starvation.

Hypo- and hyperglycemia impair endothelial cell actin alignment and nitric oxide synthase activation in response to shear stress

Uncontrolled blood glucose in people with diabetes correlates with endothelial cell dysfunction, which contributes to accelerated atherosclerosis and subsequent myocardial infarction, stroke, and peripheral vascular disease. In vitro, both low and high glucose induce endothelial cell dysfunction; however the effect of altered glucose on endothelial cell fluid flow response has not been studied. This is critical to understanding diabetic cardiovascular disease, since endothelial cell cytoskeletal alignment and nitric oxide release in response to shear stress from flowing blood are atheroprotective. In this study, porcine aortic endothelial cells were cultured in 1, 5.55, and 33 mM D-glucose medium (low, normal, and high glucose) and exposed to 20 dynes/cm² shear stress for up to 24 hours in a parallel plate flow chamber. Actin alignment and endothelial nitric oxide synthase phosphorylation increased with shear stress for cells in normal glucose, but not cells in low and high glucose. Both low and high glucose elevated protein kinase C (PKC) levels; however PKC blockade only restored actin alignment in high glucose cells. Cells in low glucose instead released vascular endothelial growth factor (VEGF), which translocated β-catenin away from the cell membrane and disabled the mechanosensory complex. Blocking VEGF in low glucose restored cell actin alignment in response to shear stress. These data suggest that low and high glucose alter endothelial cell alignment and nitric oxide production in response to shear stress through different mechanisms.

A non-enzymatic function of Golgi glycosyltransferases: mediation of Golgi fragmentation by interaction with non-muscle myosin IIA

The Golgi apparatus undergoes morphological changes under stress or malignant transformation, but the precise mechanisms are not known. We recently showed that non-muscle myosin IIA (NMIIA) binds to the cytoplasmic tail of Core 2 N-acetylglucosaminyltransferase mucus-type (C2GnT-M) and transports it to the endoplasmic reticulum for recycling. Here, we report that Golgi fragmentation induced by brefeldin A (BFA) or coatomer protein (β-COP) knockdown (KD) in Panc1-bC2GnT-M (c-Myc) cells is accompanied by the increased association of NMIIA with C2GnT-M and its degradation by proteasomes. Golgi fragmentation is prevented by inhibition or KD of NMIIA. Using multiple approaches, we have shown that the speed of BFA-induced Golgi fragmentation is positively correlated with the levels of this enzyme in the Golgi. The observation is reproduced in LNCaP cells which express high levels of two endogenous glycosyltransferases--C2GnT-L and β-galactoside α2,3 sialyltransferase 1. NMIIA is found to form complexes with these two enzymes but not Golgi matrix proteins. The KD of both enzymes or the prevention of Golgi glycosyltransferases from exiting endoplasmic reticulum reduced Golgi-associated NMIIA and decreased the BFA-induced fragmentation. Interestingly, the fragmented Golgi detected in colon cancer HT-29 cells can be restored to a compact morphology after inhibition or KD of NMIIA. The Golgi disorganization induced by the microtubule or actin destructive agent is NMIIA-independent and does not affect the levels of glycosyltransferases. We conclude that NMIIA interacts with Golgi residential but not matrix proteins, and this interaction is responsible for Golgi fragmentation induced by β-COP KD or BFA treatment. This is a novel non-enzymatic function of Golgi glycosyltransferases.

MECHANICS, BIOLOGY AND MEDICINE AND THE CHALLENGES OF METAMECHANICS: A PERSONAL REFLECTION

A brief retrospective of the evolution of mechanics and its reciprocal impacts on medicine and biology is offered, from the limited viewpoint of an early contributor to some aspects of biomechanics. The development of the field after World War II, and particularly in the nineteen sixties and seventies, set the foundation for today's remarkable achievements. Looking ahead, the expanding complexity and challenges of the interaction of mechanics with biology and medicine, together with the loss of centrality of the mechanistic view in the physical sciences, compel a reexamination of the role, potential and limits of mechanics in this context. Future advances call for a broader metamechanics conception encompassing forces, energies, fields, information, network and systems theory, as well as for models spanning the range of scales from atom and molecule to cell, organ and organism.

Synergistic Regulation of Angiogenic Sprouting by Biochemical Factors and Wall Shear Stress

The process of sprouting angiogenesis involves activating endothelial cells in a quiescent monolayer of an existing vessel to degrade and migrate into the underlying matrix to form new blood vessels. While the roles of biochemical factors in angiogenic sprouting have been well characterized, the roles of fluid forces have received much less attention. This review summarizes results that support a role for wall shear stress in post-capillary venules as a mechanical factor capable of synergizing with biochemical factors to stimulate pro-angiogenic signaling in endothelial cells and promote sprout formation.

Down-Regulation of Endothelial EphrinB2 Expression by Laminar Shear Stress

The EphB receptors and their ephrinB ligands are involved in vascular assembly and differentiation. In this study, the authors analyzed the regulation of ephrinB2 and EphB4 in response to laminar shear stress in human endothelial cells. In order to simulate different flow conditions in vitro, human endothelial cells were exposed to laminar shear stress (1 to 50 dyn/cm2 for up to 24 h) in a cone-and-plate viscometer. EphrinB2 mRNA expression is down-regulated by arterial, but not by venous, laminar shear stress in a dose-dependent manner in primary cultures of human umbilical vein endothelial cells (HUVECs) (maximum at 30 dyn/cm2, 24 h: 46% +/- 4%of internal control without shear stress, n = 16, p < .05). The down-regulation of ephrinB2 by arterial shear stress is blocked by the protein kinase C inhibitor RO-31-8220. A similar shear stress-dependent down-regulation of ephrin-B2 can be found in human coronary artery endothelial cells (HCAECs). Chronic application of laminar shear stress does not affect EphB4 expression in venous and arterial endothelial cells. The down-regulation of ephrinB2 in response to laminar shear stress may contribute to the differentiation of endothelial cells into a nonactivated phenotype.

eNOS activation by physical forces: from short-term regulation of contraction to chronic remodeling of cardiovascular tissues

Nitric oxide production in response to flow-dependent shear forces applied on the surface of endothelial cells is a fundamental mechanism of regulation of vascular tone, peripheral resistance, and tissue perfusion. This implicates the concerted action of multiple upstream "mechanosensing" molecules reversibly assembled in signalosomes recruiting endothelial nitric oxide synthase (eNOS) in specific subcellular locales, e.g., plasmalemmal caveolae. Subsequent short- and long-term increases in activity and expression of eNOS translate this mechanical stimulus into enhanced NO production and bioactivity through a complex transcriptional and posttranslational regulation of the enzyme, including by shear-stress responsive transcription factors, oxidant stress-dependent regulation of transcript stability, eNOS regulatory phosphorylations, and protein-protein interactions. Notably, eNOS expressed in cardiac myocytes is amenable to a similar regulation in response to stretching of cardiac muscle cells and in part mediates the length-dependent increase in cardiac contraction force. In addition to short-term regulation of contractile tone, eNOS mediates key aspects of cardiac and vascular remodeling, e.g., by orchestrating the mobilization, recruitment, migration, and differentiation of cardiac and vascular progenitor cells, in part by regulating the stabilization and transcriptional activity of hypoxia inducible factor in normoxia and hypoxia. The continuum of the influence of eNOS in cardiovascular biology explains its growing implication in mechanosensitive aspects of integrated physiology, such as the control of blood pressure variability or the modulation of cardiac remodeling in situations of hemodynamic overload.

Revealing the role of phosphatidylserine in shear stress-mediated protection in endothelial cells

Previous studies have demonstrated that endothelial cells exposed to laminar shear stress are protected from apoptotic stimuli such as tumor necrosis factor (TNF)-alpha. The authors investigated the role of phosphatidylserine (PS) in this phenomenon. Western blot analysis of cleaved caspase 3 was used as an indicator of apoptosis and revealed that in the absence of serine, endothelial cells exposed to laminar shear stress were unable to protect against TNF-alpha-induced apoptosis, in contrast to sheared cells grown in regular medium. It was also found that shear-induced activation of the Akt pathway was significantly decreased in cells grown without serine. In addition, quantitation of PS using a novel isotopic labeling technique involving the use of formalin revealed that stearoyl-oleic PS (18:0/18:1) did not increase during shear treatment. These findings suggest that basal levels of PS are required to activate survival pathways in endothelial cells and thereby contribute to the overall protective mechanism initiated by shear stress.

Endothelial transcriptome profiles in vivo in complex arterial flow fields

The endothelium is highly sensitive to flow characteristics and to the accompanying hemodynamic forces particularly shear stresses. Within large arteries, atherosclerotic lesions develop at predictable sites of complicated unsteady hemodynamics where flow separation, transient flow reversals, and average lower shear forces are common characteristics. Gene expression studies (transcript profiles) of the endothelium isolated from arterial regions of different flow characteristics and susceptibility to atherogenesis are presented. Endothelial phenotype characterization in vivo is complementary to mechanistic studies of flow responses in vitro.

Flow increases superoxide production by NADPH oxidase via activation of Na-K-2Cl cotransport and mechanical stress in thick ascending limbs

Superoxide (O2−) regulates renal function and is implicated in hypertension. O2−production increases in response to increased ion delivery in thick ascending limbs (TALs) and macula densa and mechanical strain in other cell types. Tubular flow in the kidney acutely varies causing changes in ion delivery and mechanical stress. We hypothesized that increasing luminal flow stimulates O2−production by NADPH oxidase in TALs via activation of Na-K-2Cl cotransport. We measured intracellular O2−in isolated rat TALs using dihydroethidium in the presence and absence of luminal flow and inhibitors of NADPH oxidase, Na-K-2Cl cotransport, and Na/H exchange. In the absence of flow, the rate of O2−production was 5.8 ± 1.4 AU/s. After flow was initiated, it increased to 29.7 ± 4.3 AU/s ( P < 0.001). O2−production was linearly related to flow. Tempol alone and apocynin alone blocked the flow-induced increase in O2−production (3.5 ± 1.7 vs. 4.5 ± 2.8 AU/s and 8.2 ± 2.1 vs. 10.6 ± 2.8 AU/s, respectively). Furosemide decreased flow-induced O2−production by 55% (37.3 ± 5.2 to 16.8 ± 2.8 AU/s; P < 0.002); however, dimethylamiloride had no effect. Finally, we examined whether changes in mechanical forces are involved in flow-induced O2−production by using a Na-free solution to perfuse TALs. In the absence of NaCl, luminal flow enhanced O2−production (1.5 ± 0.5 to 13.5 ± 1.1 AU/s; P < 0.001), ∼50% less stimulation than when flow was increased in the presence of luminal NaCl. We conclude that flow stimulates O2−production in TALs via activation of NADPH oxidase and that NaCl absorption due to Na-K-2Cl cotransport and flow-associated mechanical factors contribute equally to this process.

Impact of convective flow on the cellular uptake and transfection activity of lipoplex and adenovirus

An in vitro cell culture model that mimics in vivo extracellular environment would be useful in developing in vivo gene delivery system. In the present study, a parallel flow model was applied to investigate the impact of convective flow on cellular uptake and transfection activity in endothelial cells. LipofectAMINE PLUS and adenovirus were used as model vectors, which bind cells via electrostatic- and ligand-receptor interactions, respectively. Whereas a convective flow increased the total amount of vector passing through the flow chamber by 3 orders of magnitude, uptake was increased by less than 10-fold, suggesting that the flow severely inhibited cellular uptake by reducing the retention time in the chamber and/or by diminishing the affinity between the cell and vector. Moreover, the uptake of both vectors was increased in a shear stress-dependent manner to a comparable extent, suggesting that the effect of flow on the cellular uptake was not significant. In contrast, transfection efficiency (TE), expressed as the transfection activity normalized by the cellular uptake of vectors was dramatically stimulated by shear stress, only when LipofectAMINE PLUS was used. Since the activities of the CMV promoter were unaffected by a shear stress, it is possible that altered intracellular trafficking may responsible for the improvement in lipoplex-mediated TE, presumably related to the cellular uptake pathway.

Molecular basis of the effects of shear stress on vascular endothelial cells

Blood vessels are constantly exposed to hemodynamic forces in the form of cyclic stretch and shear stress due to the pulsatile nature of blood pressure and flow. Endothelial cells (ECs) are subjected to the shear stress resulting from blood flow and are able to convert mechanical stimuli into intracellular signals that affect cellular functions, e.g., proliferation, apoptosis, migration, permeability, and remodeling, as well as gene expression. The ECs use multiple sensing mechanisms to detect changes in mechanical forces, leading to the activation of signaling networks. The cytoskeleton provides a structural framework for the EC to transmit mechanical forces between its luminal, abluminal and junctional surfaces and its interior, including the cytoplasm, the nucleus, and focal adhesion sites. Endothelial cells also respond differently to different modes of shear forces, e.g., laminar, disturbed, or oscillatory flows. In vitro studies on cultured ECs in flow channels have been conducted to investigate the molecular mechanisms by which cells convert the mechanical input into biochemical events, which eventually lead to functional responses. The knowledge gained on mechano-transduction, with verifications under in vivo conditions, will advance our understanding of the physiological and pathological processes in vascular remodeling and adaptation in health and disease.

Endothelial protein kinase C isoform identity and differential activity of PKCzeta in an athero-susceptible region of porcine aorta

Endothelial protein kinase C (PKC) signaling was investigated in different regions of normal porcine aorta. The locations map to differential atherosclerotic susceptibility and correlate with sites of disturbed (DF) or undisturbed (UF) local flow profiles. Endothelial lysates were isolated from the inner curvature of the aortic arch (DF; athero-susceptible) and a nearby UF region of the descending thoracic aorta (UF; athero-protected), and in some experiments a distant athero-protected UF site, the common carotid artery. Total endothelial PKC activity in the DF regions was 145% to 240% of that in both UF locations (P<0.05), whereas the UF regions were not significantly different from each other. PKC protein isoforms alpha, beta, epsilon, iota, lambda, and zeta were expressed in similar proportions in both aortic regions, suggesting that differences of kinase activity were not directly attributable to expression levels. Inhibition of members of the "conventional" and "novel" PKC families had no differential effect on regional kinase activity. However, inhibition of PKCzeta, a member of the "atypical" PKC family, reduced the DF lysate kinase activity to that of UF levels (NS P=0.35). Differential phosphorylation of PKCzeta Thr410 and Thr560, along with increased levels of PKCzeta degradation products in UF endothelial lysates, suggested posttranslational modification of PKCzeta as the basis for site-specific differences in vivo. Steady-state regional heterogeneity of an important family of regulatory proteins in intact arterial endothelium in vivo may link localized athero-susceptibility and the associated hemodynamic environment.

The role of the glycocalyx in reorganization of the actin cytoskeleton under fluid shear stress: a "bumper-car" model

We propose a conceptual model for the cytoskeletal organization of endothelial cells (ECs) based on a major dichotomy in structure and function at basal and apical aspects of the cells. Intracellular distributions of filamentous actin (F-actin), vinculin, paxillin, ZO-1, and Cx43 were analyzed from confocal micrographs of rat fat-pad ECs after 5 h of shear stress. With intact glycocalyx, there was severe disruption of the dense peripheral actin bands (DPABs) and migration of vinculin to cell borders under a uniform shear stress (10.5 dyne/cm2; 1 dyne = 10 microN). This behavior was augmented in corner flow regions of the flow chamber where high shear stress gradients were present. In striking contrast, no such reorganization was observed if the glycocalyx was compromised. These results are explained in terms of a "bumper-car" model, in which the actin cortical web and DPAB are only loosely connected to basal attachment sites, allowing for two distinct cellular signaling pathways in response to fluid shear stress, one transmitted by glycocalyx core proteins as a torque that acts on the actin cortical web (ACW) and DPAB, and the other emanating from focal adhesions and stress fibers at the basal and apical membranes of the cell.

Role of ET-1 in the regulation of coronary circulation

Given that circulating ET levels in heart failure, in particular, may reach physiological threshold for coronary constrictor responses, the primary objective of the present review is to consider coronary vessels as an important target for circulating and locally produced endothelin(s). In healthy vessels, ET-1 causes biphasic coronary responses characterized by a transient dilation of large and small arteries followed by a sustained constriction. ETB receptors are pivotal in the early dilation of resistance vessels, whereas dilation of conductance vessels may be a secondary phenomenon triggered by flow increases. Exogenous ET-1 causes coronary constriction almost exclusively through ETA receptor activation. Human and canine large epicardial coronary vessels display significant baseline ET-1 dependent tone in vitro and in vivo, an ETA-dependent process. In contrast, ETB receptors located on smooth muscle cells are apparently less important for producing constrictor responses. NO production may serve as an important counter-regulatory mechanism to limit ET-dependent effects on coronary vessels. Conversely, in a dysfunctional endothelium, the loss of NO may augment ET-1 production and activity. By lifting the ET-dependent burden from coronary vessels, ET receptor blockade should help to ensure a closer match between cardiac metabolic demand and coronary perfusion.

Crosstalk between endothelin and nitric oxide in the control of vascular tone

Several lines of evidence indicate that nitric oxide (NO) impairs endothelin (ET) production/action in vitro. Acute pressor responses caused by the blockade of NO formation with arginine analogues in vivo are blunted by selective ET(A) or dual ET(A)/ET(B) receptor blockade whereas blockade of NO formation magnifies ET-induced constriction of various vascular territories. Given that ET receptor blockade has normally limited effects on mean arterial pressure, the reversal of pressor responses caused by the blockade of NO formation with ET receptor blockade most likely reflects a significant crosstalk between NO and ET. Suppression of NO formation also leads to significant increases in ET production caused by agents targeting the endothelium, such as acetylcholine and thrombin. In addition, the inhibitory effect of shear stress on endothelial cells ET production also involves NO as an intermediate.Paradoxically, chronic exposure to organic nitrates which causes nitrate tolerance leads to an augmented vascular ET content. An increased angiotensin II (AII) production is apparently pivotal in this process. This article reviews observations pointing to the importance of NO/ET interactions as a fundamental and common regulatory mechanism shared across species. As a consequence of this crosstalk between NO and ET, experimental strategies designed to assess endothelial NO-dependent activity by the blockade of NO formation may be mitigated by magnified ET-dependent influences.

Sequential activation of protein kinase C (PKC)-alpha and PKC-epsilon contributes to sustained Raf/ERK1/2 activation in endothelial cells under mechanical strain

Endothelial cells (ECs) are constantly subjected to hemodynamic forces including cyclic pressure-induced strain. The role of protein kinase C (PKC) in cyclic strain-treated ECs was studied. PKC activities were induced as cyclic strain was initiated. Cyclic strain to ECs caused activation of PKC-alpha and -epsilon. The translocation of PKC-alpha and -epsilon but not PKC-beta from the cytosolic to membrane fraction was observed. An early transient activation of PKC-alpha versus a late but sustained activation of PKC-epsilon was shown after the onset of cyclic strain. Consistently, a sequential association of PKC-alpha and -epsilon with the signaling molecule Raf-1 was shown. ECs treated with a PKC inhibitor (calphostin C) abolished the cyclic strain-induced Raf-1 activation. ECs under cyclic strain induced a sustained activation of extracellular signal-regulated protein kinases (ERK1/2), which was inhibited by treating ECs with calphostin C. ECs treated with a specific Ca(2+)-dependent PKC inhibitor (Go 6976) showed an inhibition in the early phase of ERK1/2 activation but not in the late and sustained phase. ECs transfected with the antisense to PKC-alpha, the antisense to PKC-epsilon, or the inhibition peptide to PKC-epsilon reduced strain-induced ERK1/2 phosphorylation in a temporal manner. PKC-alpha mediated mainly the early ERK1/2 activation, whereas PKC-epsilon was involved in the sustained ERK1/2 activation. Strained ECs increased transcriptional activity of Elk1 (an ERK1/2 substrate). ECs transfected with the antisense to each PKC isoform reduced Elk1 and monocyte chemotactic protein-1 promotor activity. Our findings conclude that a sequential activation of PKC isoform (alpha and epsilon) contribute to Raf/ERK1/2 activation, and PKC-epsilon appears to play a key role in endothelial adaptation to hemodynamic environment.

Shear stress regulation of endothelial NOS in fetal pulmonary arterial endothelial cells involves PKC

We have shown that increased pulmonary blood flow at birth increases the activity and expression of endothelial nitric oxide (NO) synthase (eNOS). However, the signal transduction pathway regulating this process is unclear. Because protein kinase C (PKC) has been shown to be activated in response to shear stress, we undertook a study to examine its role in mediating shear stress effects on eNOS. Initial experiments demonstrated that PKC activity increased in response to shear stress. NO production in response to shear stress was found to be biphasic, with an increase in NO release up to 1 h, a plateau phase until 4 h, and another increase between 4 and 8 h. PKC inhibition reduced the initial rise in NO release by 50% and the second increase by 70%. eNOS mRNA and protein levels were also increased in response to shear stress, whereas PKC inhibition prevented this increase. The stimulation of PKC activity with phorbol ester increased eNOS gene expression without increasing NO release. These results suggest that PKC may play different roles in shear stress-mediated release of NO and increased eNOS gene expression.

Intracellular calcium response of endothelial cells exposed to flow in the presence of thrombin or histamine

Endothelial cells line the vasculature and are exposed to mechanical shear stress because of blood motion. Previous studies have shown that endothelial cells respond to shear stress by altering their metabolism and genetic expression, but the mechanism for shear stress signal transduction remains unclear. In the current study, we investigated the role of intracellular Ca(2+) increases as a part of the shear stress signal transduction cascade. Primary human umbilical vein endothelial cells were loaded with the calcium-sensitive dye fura-2 and exposed to fluid flow in a parallel plate flow chamber in the presence of the inflammatory mediator histamine or the proteolytic enzyme thrombin. The initiation of shear stress (in the range of 0.2-20 dyne/cm(2)) in the absence of either agonist caused no increase in intracellular Ca(2+) levels. Cells exposed to either histamine (10(-9) to 10(-7) mol/l) or thrombin (0.02-0.2 u/ml) showed an intracellular calcium increase (20-150 nmol/L) that was dependent on the magnitude of the shear stress and on the concentration of agonist. In cells exposed to histamine and shear stress, the magnitude of the intracellular calcium increase was not altered, except at 10(-7) mol/L histamine. The time course of the response was significantly faster for arterial than for venous levels of shear stress at histamine concentrations from 10(-9) to 10(-7) mol/L. The magnitude of the [Ca(2+)](i) response was dependent on both the magnitude of the shear stress and the concentration of thrombin. At a thrombin concentration of 0.2 U/mL, the increase in intracellular Ca(2+) was significantly greater at arterial levels of shear stress (6-20 dyne/cm(2)) than at venous levels of shear stress (0.2-1 dyne/cm(2)). Because we solved the governing mass balance equation to precisely determine the effect of flow on local agonist concentration, the alterations in the [Ca(2+)](i) response were not due to differences in mass transfer characteristics. These results demonstrate that even in a system in which the initiation of shear stress without agonist causes no detectable change in intracellular Ca(2+), the calcium response to agonists is changed, which suggests that the signal transduction pathway for shear stress acts synergistically with the thrombin and histamine signal transduction pathways.

Two patterns of lipid deposition in the cholesterol-fed rabbit

A central feature of arterial lipid deposition is its nonuniform and variable distribution. In immature human and rabbit aortas, spontaneous lesions occur most frequently downstream of branch points, but they tend to occur upstream of the same branches at later ages. In cholesterol-fed rabbits, the juvenile pattern has been seen regardless of age. These distributions may be determined by transport properties of the arterial wall, because uptake of plasma macromolecules is elevated downstream of aortic branches in immature rabbits and upstream in mature ones, except during cholesterol feeding, when the juvenile pattern is seen in adult vessels. The effect of cholesterol could reflect its inhibitory influence on the nitric oxide (NO) pathway because the adult transport pattern is NO dependent. Using protocols expected to preserve NO function and the mature pattern of transport during hypercholesterolemia, we made 2 attempts to induce upstream disease in rabbits. In trial I, plasma concentrations of cholesterol were kept within the normal human range for 15 weeks by using dietary levels of 0.05% to 0.2%. Although disease patterns reverse with age in human vessels exposed to these concentrations, lesions in both immature and mature rabbits occurred downstream of intercostal branch ostia. Trial II used older rabbits, a different base diet containing more vitamin E (96 mg/kg rather than 57 mg/kg), and higher levels of cholesterol (1%, administered for 8 weeks). For some animals, extra vitamin E (2000 mg/kg) was added to the diet. The mature pattern of lipid deposition was apparent around intercostal branches in the first group and was accentuated by the additional vitamin E, a change that was associated with a significant increase in the plasma concentration of NO metabolites. Spontaneous lesions, assessed on the base diet, were too rare to have influenced these distributions. This is the first report of upstream disease in the cholesterol-fed rabbit. The results support but do not prove the view that NO and transport are important in atherogenesis.

Mechanotransduction in response to shear stress. Roles of receptor tyrosine kinases, integrins, and Shc

Shear stress, the tangential component of hemodynamic forces, activates many signal transduction pathways in vascular endothelial cells. The conversion of mechanical stimulation into chemical signals is still unclear. We report here that shear stress (12 dynes/cm2) induced a rapid and transient tyrosine phosphorylation of Flk-1 and its concomitant association with the adaptor protein Shc; these are accompanied by a concurrent clustering of Flk-1, as demonstrated by confocal microscopy. Our results also show that shear stress induced an association of alphavbeta3 and beta1 integrins with Shc, and an attendant association of Shc with Grb2. These associations are sustained, in contrast to the transient Flk-1. Shc association in response to shear stress and the transient association between alphavbeta3 integrin and Shc caused by cell attachment to substratum. Shc-SH2, an expression plasmid encoding the SH2 domain of Shc, attenuated shear stress activation of extracellular signal-regulated kinases and c-Jun N-terminal kinases, and the gene transcription mediated by the activator protein-1/12-O-tetradecanoylphorbol-13-acetate-responsive element complex. Our results indicate that receptor tyrosine kinases and integrins can serve as mechanosensors to transduce mechanical stimuli into chemical signals via their association with Shc.