Orientation of endothelial cells in shear fields in vitro

Vascular endothelial cells subjected to fluid shear stress change their shape from polygonal to ellipsoidal and become uniformly oriented with the flow. In order to study the mechanisms of this response, we have measured the relaxation of bovine aortic endothelial cells that were grown on glass coverslips and exposed to fluid shear stress for 72 hours. An image analysis system was developed to quantify the cell shape relaxation that occurs following the cessation of shear stress. This method provides two different quantitative measures of relaxation: the loss of elongated shape by the cells and the change in cell direction with time. After equilibration to a fluid shear stress level of 8 dynes/cm2, cells immersed in static medium relax their shape in about 20 hours. After 72 hours in this static condition, the cell elongation is comparable to that of unstressed control cells but vestiges remain of the original orientation in the flow direction. This relaxation process contributes to our understanding of the response of vascular endothelium to fluid shear stress.

Influence of hemodynamic forces on vascular endothelial function. In vitro studies of shear stress and pinocytosis in bovine aortic cells

The relationships between fluid shear stress, a physiologically relevant mechanical force in the circulatory system, and pinocytosis (fluid-phase endocytosis) were investigated in cultured bovine aortic endothelial cells using a specially designed apparatus. Continuous exposure to steady shear stresses (1-15 dyn/cm2) in laminar flow stimulated time- and amplitude-dependent increases in pinocytotic rate which returned to control levels after several hours. After 48 h continuous exposure to steady shear stress, removal to static conditions also resulted in a transient increase in pinocytotic rate, suggesting that temporal fluctuations in shear stress may influence endothelial cell function. Endothelial pinocytotic rates remained constant during exposure to rapidly oscillating shear stress at near physiological frequency (1 Hz) in laminar flow. In contrast, however, a sustained elevation of pinocytotic rate occurred when cells were subjected to fluctuations in shear stress amplitude (3-13 dyn/cm2) of longer cycle time (15 min), suggesting that changes in blood flow of slower periodicity may influence pinocytotic vesicle formation. As determined by [3H]thymidine autoradiography, neither steady nor oscillating shear stress stimulated the proliferation of confluent endothelial cells. These observations indicate that: (a) alterations in fluid shear stress can significantly influence the rate of formation of pinocytotic vesicles in vascular endothelial cells, (b) this process is force- and time-dependent and shows accommodation, (c) certain patterns of fluctuation in shear stress result in sustained elevation of pinocytotic rate, and (d) shear stresses can modulate endothelial pinocytosis independent of growth stimulation. These findings are relevant to (i) transendothelial transport and the metabolism of macromolecules in normal endothelium and (ii) the role of hemodynamic factors in the localization of atherosclerotic lesions in vivo.

Modification of low density lipoprotein metabolism by growth factors in cultured vascular cells and human skin fibroblasts. Dependence upon duration of exposure

The influences of specific growth factors upon binding, internalization and degradation of low density lipoproteins (LDL) were investigated in cultured vascular smooth muscle cells and skin fibroblasts. Platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) significantly stimulated the binding of LDL to high affinity receptors of smooth muscle cells and fibroblasts. The effects of enhanced binding were reflected in elevated rates in internalization and degradation of LDL. FGF and PDGF elicited a mitogenic response as measured by [3H]thymidine autoradiography, indicating that altered LDL metabolism was associated with entry into the cell cycle. When fibroblasts were exposed to mitogen for periods long enough to commit the cells to the growth cycle, after which growth factor was removed before addition of LDL however, enhanced LDL binding to cycling fibroblasts appeared to be dependent upon the length of the period of exposure to growth factors in the early part of the cell cycle. LDL binding was stimulated in the presence of PDGF or FGF but not after their removal within 8 h of entry into the cell cycle. Exposure to the growth factors for 16 h or longer resulted in stimulation of LDL metabolism whether or not mitogens were present at the cell surface. PDGF and FGF, therefore, appear to exert a direct influence upon LDL receptor expression in addition to that mediated via the cell cycle.

Endocytic vesicles and surface invaginations in cultured vascular endothelium: a morphometric comparison

Ruthenium red staining of plasma membrane glycoproteins of confluent cultured arterial endothelial cells revealed that the limiting membrane of many apparently discrete cytoplasmic vesicles was continuous with the plasmalemma. Surface invaginations accessible to ruthenium red appeared as vesicles when sectioned out of the plane of attachment to the cell surface, Morphometric analysis of ruthenium red-positive (RR+) and ruthenium red-negative vesicles (RR-) indicated that 47.2% of the total apparent vesicle population was RR+ and that those infoldings accounted for 19.6 +/- 1.4% of the cell surface in transverse sections. Whereas 14.9% of the true vesicles (ruthenium red-negative) were coated vesicles, only 1.1% of RR+ "vesicles" were coated pits. These studies show that although many deep infoldings of the cell surface may be misinterpreted as vesicles, almost all are uncoated. The existence of discrete coated vesicles (independent of coated pits) in vascular endothelium in vitro is readily apparent.

The dynamic response of vascular endothelial cells to fluid shear stress

We have developed an in-vitro system for studying the dynamic response of vascular endothelial cells to controlled levels of fluid shear stress. Cultured monolayers of bovine aortic endothelial cells are placed in a cone-plate apparatus that produces a uniform fluid shear stress on replicate samples. Subconfluent endothelial cultures continuously exposed to 1-5 dynes/cm2 shear proliferate at a rate comparable to that of static cultures and reach the same saturation density (congruent to 1.0-1.5 X 10(5) cells/cm2). When exposed to a laminar shear stress of 5-10 dynes/cm2, confluent monolayers undergo a time-dependent change in cell shape from polygonal to ellipsoidal and become uniformly oriented with flow. Regeneration of linear "wounds" in confluent monolayer appears to be influenced by the direction of the applied force. Preliminary studies indicate that certain endothelial cell functions, including fluid endocytosis, cytoskeletal assembly and nonthrombogenic surface properties, also are sensitive to shear stress. These observations suggest that fluid mechanical forces can directly influence endothelial cell structure and function. Modulation of endothelial behavior by fluid shear stresses may be relevant to normal vessel wall physiology, as well as the pathogenesis of vascular diseases, such as atherosclerosis.

Influence of molecular charge upon the endocytosis and intracellular fate of peroxidase activity in cultured arterial endothelium

The molecular charge of the macromolecule, horseradish peroxidase (HRPase, 40 000 mol. wt), was modified to yield highly anionic (PI less than 3.68) and cationic (PI = 9.5-10.5) derivatives. The effects upon the interactions between HRPase and arterial endothelium were then studied in vitro. The net rate of uptake of HRPase into endocytic vesicles and vacuoles of confluent endothelium was influenced by its molecular charge, there being less internalization of the anionic HRPase than of the native (pI = 7.9-8.2) and cationic derivatives. The molecular diameter was not significantly different between the cationic (Ae = 28.8 A), anionic (Ae = 31.2 A) and native (Ae = 29.6 A) HRPase. The rate of uptake of [U-14C]sucrose, a tracer of bulk fluid endocytosis, was unaffected by the presence of the differently charged HRPase, indicating that the volume of vesicles formed per cell per hour remained constant. The intracellular fate of HRPase of different charge was investigated biochemically and morphologically. The rate of loss of internalized HRPase activity in the endothelial cells approximated first-order kinetics. The rate of disappearance of intracellular HRPase activity was much greater for cationic (t1/2 = 8 h) and native (t1/2 = I 8 h) than for anionic HRPase (t1/2 = 80–100 h). By electron microscopy, all 3 forms of HRPase were restricted to intracellular membrane-bounded vesicles and vacuoles consistent with a vesicle-lysosomal pathway. Studies with purified lysosomal cathepsin D indicated that the differences in the intracellular half-lives of HRPase may be attributable in small part to decreased and increased rates of lysosomal proteolysis of anionic and cationic HRPase, respectively, in comparison with native HRPase. Pre-labelling of endothelial secondary lysosomes by inhibitors of phagosome-lysosome fusion (dextran sulphate, polyglutamate) lengthened the intracellular half-life of native HRPase, while introduction of cationic ferritin to cells pulsed with anionic HRPase greatly decreased its half-life. Thus an influence of molecular charge upon endosome-lysosome fusion cannot be excluded. The studies indicate that the net charge carried by exogenous HRPase influences both its internalization in endocytic vesicles and its subsequent intracellular fate, which in turn may be modified by the introduction of other differently charged macromolecules. These results are discussed in relation to macromolecular transport by vascular endothelium in vivo.

Enhanced rates of fluid pinocytosis during exponential growth and monolayer regeneration by cultured arterial endothelial cells

Rates of fluid pinocytosis by bovine aortic endothelial cells were measured during various manipulations of growth status in vitro. Sparsely seeded cultures grew exponentially until a confluent monolayer was formed, at which time growth slowed. This change in growth rate coincided with a decline in the rate of pinocytosis to about one-third that in the growing cultures. During the subsequent attainment of maximal cell density in the confluent monolayer, the pinocytic rate remained constant. There was close correlation between 3H-thymidine labelling indices, as measured by autoradiography, and the rates of pinocytosis. Mechanical "wounding" of the confluent monolayer resulted in cell migration and proliferation. Twenty-four hours after "wounding," rates of pinocytosis per mg. cell protein were significantly enhanced. When regeneration of the monolayer was blocked by cytochalasin B, pinocytosis remained at the same rate as in the uninjured, confluent monolayer. These experiments support, and extend to endothelium, earlier observations that in growing cells pinocytosis proceeds at a higher rate than in non-growing, quiescent cells. Furthermore, they raise the possibility that the transendothelial transport of macromolecules such as lipoproteins by receptor-independent fluid pinocytosis in vivo may be altered by the growth status of the endothelium.

Cell-cell contact and growth regulation of pinocytosis in 3T3 cells

In sub-confluent cultures of Balb/c-3T3 cells, pinocytosis rates were increased after exposure to specific growth factors (serum; platelet-derived growth factor, PDGF; epidermal growth factor, EGF). Conversely, as cells became growth-inhibited with increasing culture density, there was a corresponding decline in pinocytosis rate per cell. In order to test whether density-inhibition of pinocytosis was influenced either by the growth cycle or by cell contact independently of growth, cells were induced into a quiescent state at a range of subconfluent and confluent densities. Under such conditions, cell density did not significantly inhibit pinocytosis rate. When confluent quiescent cultures in 2.5% serum were exposed to 10% serum, the resulting round of DNA synthesis was accompanied by enhanced pinocytosis per cell, even though the cells were in contact with one another. Furthermore, in a SV40-viral transformed 3T3 cell line, both the growth fraction and the pinocytosis rate per cell remained unchanged over a wide range of culture densities. These studies indicate that density-dependent inhibition of pinocytosis in 3T3 cells appears to be secondary to growth-inhibition rather than to any direct physical effects of cell-cell contact.

Mediation of pinocytosis in cultured arterial smooth muscle and endothelial cells by platelet-derived growth factor

Pinocytosis was measured in monkey aortic smooth muscle cells (SMC), bovine aortic endothelial cells, and Swiss 3T3 cells in culture as cellular uptake of [U-(14)C]sucrose and horseradish peroxidase (HRP) from the tissue culture medium. Monkey arterial SMC and Swiss 3T3 cells were maintained in a quiescent state of growth at low cells density in medium containing 5 percent monkey plasma-derived serum (PDS). Replacement of PDS with 5 percent monkey whole blood serum (WBS) from the same donor, or addition to PDS of partially purified platelet-derived growth factor(s) (PF), resulted in a marked stimulation of pinocytosis as well as of cellular proliferation. In SMC, enhancement of the rate of pinocytosis occurred 4-6 h after exposure to WBS or PF, and the rate was up to twofold higher than the rate in medium containing PDS. In contrast, [(3)H]thymidine uptake by SMC did not increase until 12-16 h after exposure to PF. In endothelial cells the presence of PF or WBS did not enhance either the rate of pinocytosis or the rate of proliferation over that in PDS. Thus, endothelial cells did not become quiescent at subconfluent densities in PDS but maintained rates of proliferation and pinocytosis that were equivalent to those in WBS. By autoradiography, the fraction of labeled nuclei in SMC cultures 24 h after change of medium increased from 0.061 +/- 0.004 in quiescent cultures to 0.313 +/- 0.028 after exposure to WBS or PF. In contrast, labeling indices of endothelial cells were similar for cultures grown in PDS, WBS, or PF at any single time point after change of medium. These findings suggest that the rate of pinocytosis maybe be coupled in some fashion to growth regulation, which may be mediated in part by specific growth factors, such as that derived from the thrombocyte.

Effect of concentration of perfusing free fatty acid on arterial lipid synthesis in perfused normal and atherosclerotic rabbit aortas

Normal and atherosclerotic rabbit aortas were perfused at physiological pressure for 1 hour with media containing various concentrations of [3H]oleic acid, between 0.5 and 2.0 mmoles/l, complexed to a fixed concentration 40 g/l of bovine serum albumin (BSA). The mass of free fatty acid (FFA), which entered the arterial wall and was subsequently utilised for lipid synthesis, was calculated from the measured specific activities of FFA in the perfusates. In normal tissue, at all concentrations of FFA in the perfusate, the highest rates of utilisation of perfusate FFA for arterial lipid synthesis were for phospholipids (PL) and triglycerides (TG), with only about 2% in cholesteryl esters (CE). In atherosclerotic tissue, at both low and high concentrations of perfusate FFA, about 25% of fatty acid entering arterial lipids was in CE. When the concentration of FFA in the perfusion medium was raised, the mass of FFA from the medium that was incorporated in the total arterial lipids, increased in both normal and atherosclerotic tissue. The increase was due in normal tissue, to significant increases in incorporation into FFA, lecithin (PC), phosphatidyl inositol (PI), phosphatidyl ethanolamine (PE), TG and CE, whilst in atherosclerotic tissue it was due to increased incorporation into PC, PI, TG and CE. The results suggest that raised concentrations of FFA in blood may increase the rate of synthesis of lipids in normal and atherosclerotic tissue and thus exacerbate the accumulation of certain lipids such as cholesteryl esters, in fatty streak lesions of atherosclerosis.

Scanning electron microscopy in the evaluation of endothelial integrity of the fatty lesion in atherosclerosis

The luminal surface of fatty lesions of atherosclerosis was viewed by scanning electron microscopy (SEM). Endothelial cells were outlined by staining intercellular junctions with silver and the aortas were fixed in situ at physiological pressure. When aortas were dehydrated by passage through organic solvents followed by critical point drying from liquid CO2, there was considerable disruption of the luminal surface and it was not possible to correctly interpret the morphological integrity of the endothelium. In contrast, simple air-drying of aortas, without solvent dehydration after fixation, allowed the integrity of the cell layer overlying the lesion to be evaluated. The success of this technique was attributed to the retention of arterial lipids during dehydration of the tissue.

Scanning electron microscopy: arterial endothelial integrity after fixation at physiological pressure

The luminal surface of normal rabbit aorta was examined by scanning electron microscopy (SEM) after outlining endothelial cells by staining intercellular junctions with silver. When aortas were fixed in situ at physiological pressure before processing for SEM, a reliable assessment of the morphological integrity of the endothelium was possible. In contrast, when aortas were excised and placed in fixative, contraction of the sub-endothelial structures made interpretation of endothelial integrity difficult.

Evidence for the mediatory role of brown adipose tissue during nonshivering thermogenesis in the cold-acclimated mouse

The increased rate of oxygen consumption by cold-acclimated, adult mice after subcutaneous injection of noradrenaline has been measured for intact individuals and for those with the arterial supply to their interscapular brown adipose tissue ligated. An immediate reduction of 40% of this calorigenic response was noted in mice thus operated. Dissection of the total brown fat of the body indicated that the interscapular deposits comprise 43% by weight of the total brown adipose tissue, which in turn forms only 1% of the body weight. Since a 40% reduction in calorigenic response to noradrenaline cannot be ascribed to the loss of an amount of brown fat constituting less than 0.5%) of the body weight, the results support the hypothesis that brown fat can mediate calorigenic responses of other tissues.