Transcellular transport of lipoprotein through arterial endothelial cells in monolayer culture

To study the mechanism of lipoprotein transport through arterial endothelial cells, porcine endothelial cells were cultured on gelated type I collagen supported by a dacron sheet, and the transport of low density lipoprotein (LDL) labeled with rhodamine B isothiocyanate (RB-LDL) through the cells was measured. Light and scanning electron microscopy showed that the cells on the gel were confluent. There was little RB-LDL transport through the endothelial monolayer at 0 degrees C. RB-LDL transport through the monolayer at 37 degrees C was dose-dependent saturable at 0.4 mg protein/ml. The transport was energy-dependent, since its rate was affected by temperature and was inhibited by a combination of 2-deoxyglucose (50 mM) and NaN3 (10 mM). RB-LDL was shown not to be degraded during transport.     

Transcellular transport of angiotensin II through a cultured arterial endothelial monolayer

We have studied the mechanisms of angiotensin II (A-II) transport through a cultured arterial endothelial cell monolayer. The transport of 125I-labeled A-II was inhibited by excess unlabeled A-II (50 microM) and [Sar1, Ile8]-A-II (50 microM), but was not inhibited by bradykinin (50 microM). The transport process was shown to be temperature dependent and was inhibited by 10 mM NaN3 plus 50 mM 2-deoxyglucose. Monensin (50 microM), an inhibitor of endocytotic trafficking, reduced the rate of transport of 125I-A-II. It is also shown that the specific pathway for A-II transport was unidirectional from the apical to the basolateral surface of the endothelial cell monolayer.     

Transcellular transport of CCL2 across brain microvascular endothelial cells

The means by which the chemokine CCL2 produced in the brain parenchyma can recruit leukocytes lying behind the highly impervious endothelium of the blood–brain barrier (BBB) has remained a paradox. As other chemokines have been evidenced to stimulate their own synthesis and release by peripheral microvascular endothelial cells, and/or undergo transcytosis in the abluminal-to-luminal direction, we determined whether CCL2 experiences similar fates across brain microvascular endothelial cells (BMEC). Using cultured BMEC as a paradigm of the BBB, it was observed that exogenous unlabeled CCL2 actually depressed the release of endogenous CCL2, and further caused diminished CCL2 mRNA levels in these cells. On the other hand, exogenous ¹²⁵I-labeled CCL2 exhibited transport across BMEC in a manner that was sensitive to temperature, competition by excess unlabeled CCL2 but not unlabeled CCL3, knockdown of caveolin-1/caveolae, and elimination of the cognate CCL2 receptor CCR2. These results implied a facet of CCL2 transport by a transcellular mechanism partly involving binding of CCL2 to CCR2, and subsequent transfer to caveolae vesicles for transcytosis. This notion was supported by double-label immuno-electronmicroscopy, which revealed co-localization of caveolin-1 with exogenous CCL2, during this chemokine's transit across BMEC. Collectively, these findings provide a rationale by which CCL2, deposited on the abluminal side of the brain microvasculature during inflammatory episodes, can be relayed across the BBB to foster leukocyte recruitment.     

Spray-dried mucoadhesive microspheres: Preparation and transport through nasal cell monolayer

The purpose of this research was to prepare spray-dried mucoadhesive microspheres for nasal delivery. Microspheres composed of hydroxypropyl methylcellulose (H), chitosan (CS), carbopol 934P (CP) and various combinations of these mucoadhesive polymers, and maltodextrin (M), colloidal silicon dioxide (A), and propylene glycol (P) as filler and shaper, were prepared by spray-drying technique. Using propranolol HCl as a model drug, microspheres were prepared at loadings exceedings 80% and yields between 24% and 74%. Bulky, free flowing microspheres that had median particle size between 15 and 23 μm were obtained. Their zeta potential was according to the charge of polymer. Adhesion time of mucoadhesive microspheres on isolated pig intestine was ranked, CS>CP: H>CP>H, while the rank order of swelling was CP>CS>H. Increasing the amount of CP in CP∶H formulations increased the percentage of swelling. Infrared (IR) spectra showed no interaction between excipients used except CS with acetic acid. The release of drug from CP and CP∶H microspheres was slower than the release from H and CS microspheres, correlated to their viscosity and swelling. Long lag time from the CP microspheres could be shortened when combined with H. The permeation of drug through nasal cell monolayer corresponded to their release profiles. These microspheres affected the integrity of tight junctions, relative to their swelling and charge of polymer. Cell viability was not affected except from CS microspheres, but recovery could be obtained. In conclusion, spray-dried microspheres of H, CS, CP, and CP∶H could be prepared to deliver drug through nasal cell monolayer via the opening of tight junction without cell damaging. Published: February 10, 2006     

Transcellular vesicular transport in epithelial and endothelial cells: Challenges and opportunities

Vesicle‐mediated transcellular transport or simply “transcytosis” is a cellular process used to shuttle macromolecules such as lipoproteins, antibodies, and albumin from one surface of a polarized cell to the other. This mechanism is in contrast to the transit of small molecules such as anions, cations and amino acids that occur via uptake, diffusion through the cytosol and release and is also distinct from paracellular leak between cells. Importantly, transcytosis has evolved as a process to selectively move macromolecules between 2 neighboring yet unique microenvironments within a multicellular organism. Examples include the movement of lipoproteins out of the circulatory system and into tissues and the delivery of immunoglobulins to mucosal surfaces. Regardless of whether the transport is conducted by endothelial or epithelial cells, the process often involves receptor‐mediated uptake of a ligand into an endocytic vesicle, regulated transit of the carrier through the cytoplasm and release of the cargo via an exocytic event. While transcytosis has been examined in detail in epithelial cells, for both historical and technical reasons, the process is less understood in endothelial cells. Here, we spotlight aspects of epithelial transcytosis including recent findings and review the comparative dearth of knowledge regarding the process in endothelial cells highlighting the opportunity for further study.      

Volume changes of an endothelial cell monolayer on exposure to anisotonic media

The osmotic process plays an important role in controlling the distribution of water across cell membranes and thus the cell volume. A system was designed to detect the volume changes of an endothelial cell monolayer when cells were exposed to media with altered osmolalities. Electrodes housed in a flow chamber measured the resistance of ionic media flowing over a cultured cell layer. Assuming the cell membrane acts as an electrical insulator, volume changes of the cell layer can be calculated from the corresponding changes in chamber resistance. The media used in the experiments had osmolalities in the range 120-630 mmol/kg. When cells were exposed to hypertonic media, there was rapid shrinkage with an approximate 30% reduction in total cell volume for a twofold increase in osmolality. On exposure to hypotonic media, the cells initially swelled with an approximate 20% volume increase for a decrease in osmolality by half. With sustained exposure to low osmolality media, there was a gradual and partial return of cell volume towards isotonic values that started 10 minutes after and was complete within 30 minutes of the osmolality alteration. This finding suggests regulatory volume decrease (RVD); however, no regulatory volume increase (RVI) was observed with the continued exposure to hypertonic media over 45 minutes.     

Peroxynitrite causes endothelial cell monolayer barrier dysfunction

Nitric oxide (·NO) attenuates hydrogen peroxide(H₂O₂)-mediated barrier dysfunction in culturedporcine pulmonary artery endothelial cells (PAEC) (Gupta MP, Ober MD,Patterson C, Al-Hassani M, Natarajan V, and Hart, CM. Am JPhysiol Lung Cell Mol Physiol 280: L116-L126, 2001). However,·NO rapidly combines with superoxide (O) to formthe powerful oxidant peroxynitrite (ONOO), which wehypothesized would cause PAEC monolayer barrier dysfunction. To testthis hypothesis, we treated PAEC with ONOO (500 µM) or3-morpholinosydnonimine hydrochloride (SIN-1; 1-500 µM).SIN-1-mediated ONOO formation was confirmed by monitoringthe oxidation of dihydrorhodamine 123 to rhodamine. BothONOO and SIN-1 increased albumin clearance(P < 0.05) in the absence of cytotoxicity and alteredthe architecture of the cytoskeletal proteins actin and -catenin asdetected by immunofluorescent confocal imaging.ONOO-induced barrier dysfunction was partially reversibleand was attenuated by cysteine. Both ONOO and SIN-1nitrated tyrosine residues, including those on -catenin and actin,and oxidized proteins in PAEC. The introduction of actin treated withONOO into PAEC monolayers via liposomes alsoresulted in barrier dysfunction. These results indicate thatONOO directly alters endothelial cytoskeletal proteins,leading to barrier dysfunction.

     

The role of mitosis in LDL transport through cultured endothelial cell monolayers

We (7) have previously shown that leaky junctions associated with dying or dividing cells are the dominant pathway for LDL transport under convective conditions, accounting for >90% of the transport. We (8) have also recently shown that the permeability of bovine aortic endothelial cell monolayers is highly correlated with their rate of apoptosis and that inhibiting apoptosis lowers the permeability of the monolayers to LDL. To explore the role of mitosis in the leaky junction pathway, the microtubule-stabilizing agent paclitaxel was used to alter the rate of mitosis, and LDL flux and water flux (J(v)) were measured. Control monolayers had an average mitosis rate of 0.029%. Treatment with paclitaxel (2.5 μM) for 1.5, 3, 4.5, or 6 h yielded increasing rates of mitosis ranging from 0.099% to 1.03%. The convective permeability of LDL (P(e)) increased up to fivefold, whereas J(v) increased up to threefold, over this range of mitosis rates. We found strong correlations between the mitosis rate and both P(e) and J(v). However, compared with our previous apoptosis study (8), we found that mitosis was only half as effective as apoptosis in increasing P(e). The results led us to conclude that while mitosis-related leaky junctions might play a role in the initial infiltration of LDL into the artery wall, the progression of atherosclerosis might be more closely correlated with apoptosis-related leaky junctions.     

Acetyl glyceryl ether phosphorylcholine (PAF-acether) and leukotriene B4-mediated neutrophil chemotaxis through an intact endothelial cell monolayer

Human endothelial cell monolayers were grown on nucleopore filters, and used to partition the two halves of a modified Boyden chamber. Human neutrophil chemotaxis through the monolayer was studied in response to leukotriene B4 and acetyl glyceryl ether phosphorylcholine (PAF-acether). Both leukotriene B4 and PAF-acether concentration-dependently stimulated neutrophil chemotaxis through intact monolayer. The biologically inactive lyso-PAF, and leukotriene C4 and D4 were inactive as chemotactic agents. Leukotriene A4 was weakly chemotactic. In the absence of chemotaxin, little penetration of the monolayer by neutrophils was observed. Agents that elevate neutrophil cyclic AMP levels inhibit both leukotriene B4 and PAF-acether-stimulated chemotaxis through the endothelial cell monolayer. The specific 5-lipoxygenase inhibitor, 6,8-de-epoxy-6,9-(phenylimino) delta 6,8-prostaglandin I1 (U-60257), inhibits PAF-acether, but not leukotriene B4-mediated chemotaxis. These data suggest that an intact 5-lipoxygenase may be required for normal PAF-acether-mediated chemotaxis, but leukotriene B4-mediated chemotaxis is independent of 5-lipoxygenase activity. This system may prove to be a useful model for the study of neutrophil-endothelial cell interactions.     

Effects of arterial pressure on endothelial transport of macromolecules

The effects of variations in transmural pressure over a range of 0 to 200 mmHg on transendothelial transport of macromolecules were studied in the canine common carotid artery. The uptake of 125I-albumin per unit artery weight increased with rising pressure. There was no significant difference in albumin permeability per unit luminal surface area between 0 and 100 mmHg, but permeability nearly doubled when pressure was raised to 200 mmHg. The contribution of an increased rate of transendothelial vesicle diffusion, as evaluated from the experimental determination of the ratio of attached-to-free vesicles and theoretical modeling, was found to be negligible. The reduction in transendothelial vesicle diffusion distance due to pressure-induced thinning of the peripheral zone contributes to a 25% increase in permeability. With the use of colloidal Ag and Au of various sizes, vesicle loading of particles with diameters greater than or equal to 15 nm was found to be severely restricted at transmural pressure less than or equal to 100 mmHg, but it was significantly enhanced at 200 mmHg, when particles as large as 25 nm became detectable in endothelial vesicles and subendothelial space. This hypertension-induced increase in macromolecular transport across the endothelium may cause an overloading of the arterial wall with low-density lipoproteins and play a significant role in atherogenesis.     

Ascorbic acid stimulates barrier function of cultured endothelial cell monolayer

The macromolecular permeability of cultured bovine aortic, bovine venous, and human umbilical vein endothelial cell monolayers was decreased significantly in culture medium containing L-ascorbic acid (Asc Acid; 0.01–0.1 mM) and L-ascorbic acid 2-phosphate (Asc 2-P). Dithiothreitol, which shows reducing activity equivalent to that of Asc Acid, did not affect endothelial permeability. Asc Acid induced a sixfold increase in collagen synthesis by the endothelial cells. The coexistence of L-azetidine 2-carboxylic acid, an inhibitor of collagen synthesis, attenuated the effect of Asc 2-P in a dose-dependent manner. Another collagen synthesis inhibitor, ethyl-3,4-dihydroxybenzoate, also inhibited collagen synthesis and increased endothelial permeability. The decrease in permeability of the endothelial monolayer was dependent on a reduction of the permeability coefficient of the endothelial monolayer. These findings indicate that endothelial barrier function is stimulated by Asc Acid via an increase in collagen synthesis. © 1995 Wiley-Liss, Inc.     

Quantitative analysis of albumin uptake and transport in the rat microvessel endothelial monolayer

We determined the concentration dependence of albumin binding, uptake, and transport in confluent monolayers of cultured rat lung microvascular endothelial cells (RLMVEC). Transport of (125)I-albumin in RLMVEC monolayers occurred at a rate of 7.2 fmol. min(-1). 10(6) cells(-1). Albumin transport was inhibited by cell surface depletion of the 60-kDa albumin-binding glycoprotein gp60 and by disruption of caveolae using methyl-beta-cyclodextrin. By contrast, gp60 activation (by means of gp60 cross-linking using primary and secondary antibodies) increased (125)I-albumin uptake 2.3-fold. At 37 degrees C, (125)I-albumin uptake had a half time of 10 min and was competitively inhibited by unlabeled albumin (IC(50) = 1 microM). Using a two-site model, we estimated by Scatchard analysis the affinity (K(D)) and maximal capacity (B(max)) of albumin uptake to be 0.87 microM (K(D1)) and 0.47 pmol/10(6) cells (B(max1)) and 93.3 microM (K(D2)) and 20.2 pmol/10(6) cells (B(max2)). At 4 degrees C, we also observed two populations of specific binding sites, with high (K(D1) = 13.5 nM, 1% of the total) and low (K(D2) = 1.6 microM) affinity. On the basis of these data, we propose a model in which the two binding affinities represent the clustered and unclustered gp60 forms. The model predicts that fluid phase albumin in caveolae accounts for the bulk of albumin internalized and transported in the endothelial monolayer.     

Enhancement of pancreatic islet cell monolayer growth by endothelial cell matrix and insulin

Summary The roles of glucose and insulin in the promotion of DNA synthesis in pancreatic islet cell monolayers were assessed using a variety of in vitro conditions. Several substrates including collagen, poly-l-lysine, Matrigel, and the extracellular matrix produced by cultured bovine endothelial cells (BCEM) were compared for their ability to promote monolayer growth. Islets grown on BCEM in combination with medium RPMI 1640 supplemented with 22.2 mM glucose or 10 μg/ml insulin gave the best results as determined by new DNA synthesis. The new-form monolayers were free of contaminating, fibroblasts. These results suggest that insulin is critical to pancreatic islet growth when the cells are attached to biocompatible matrices.     

Iron transport kinetics through blood-brain barrier endothelial cells

Background

Transferrin and its receptors play an important role during the uptake and transcytosis of iron through blood-brain barrier (BBB) endothelial cells (ECs) to maintain iron homeostasis in BBB endothelium and brain. Any disruptions in the cell environment may change the distribution of transferrin receptors on the cell surface, which eventually alter the homeostasis and initiate neurodegenerative disorders. In this paper, we developed a comprehensive mathematical model that considers the necessary kinetics for holo-transferrin internalization and acidification, apo-transferrin recycling, and exocytosis of free iron and transferrin-bound iron through basolateral side of BBB ECs.

A Riemannian manifold analysis of endothelial cell monolayer impedance parameter precision

Endothelial cell adhesion and barrier function play a critical role in many biological and pathophysiological processes. The decomposition of endothelial cell adhesion and barrier function into cell–cell and cell–matrix components using frequency dependent cellular micro-impedance measurements has, therefore, received widespread application. Few if any studies, however, have examined the precision of these model parameters. This study presents a parameter sensitivity analysis of a representative cellular barrier function model using a concise geometric formulation that includes instrumental data acquisition settings. Both model state dependence and instrumental noise distributions are accounted for within the framework of Riemannian manifold theory. Experimentally acquired microimpedance measurements of attached endothelial cells define the model state domain, while experimentally measured noise statistics define the data space Riemannian metric based on the Fisher information matrix. The results of this analysis show that the sensitivity of cell–cell and cell–matrix impedance components are highly model state dependent and several well defined regions of low precision exist. The results of this study further indicate that membrane resistive components can significantly reduce the precision of the remaining parameters in these models. This work was supported by a National Science Foundation CAREER Award (AE), BES-0238905, and in part by the American Heart Association under Grant 0265029B (AE).