Quantitative aspects of endocytosis in cultured endothelial cells

Atherogenic levels of low-density lipoprotein increase hydrogen peroxide generation in cultured human endothelial cells: possible mechanism of heightened endocytosis

Cultured human umbilical vein endothelial cells(EC) exposed to atherogenic low-density lipoprotein (LDL) levels have augmented reactive oxygen species generation. Confluent EC were incubated with 30-330 mg/dl LDL cholesterol and cellular hydrogen peroxide (H2O2) generation measured. EC incubated with 30 and 90 mg/dl LDL cholesterol showed similar low level H2O2 production. In contrast, EC exposed to 180 and 330 mg/dl LDL cholesterol have a marked, dose-related elevation in H2O2 generation. Subsequent studies have explored if direct EC exposure to H2O2 promotes cellular functional changes similar to those induced by high LDL levels (> 160 mg/dl cholesterol). Confluent EC were incubated with 0.1-10 mM H2O2 for 30 minutes and endocytosis measured and cytoskeletal structure examined. H2O2 exposure (0.5 and 1 mM) promoted heightened EC endocytosis, which similarly occurs with high LDL exposure. Likewise, cytoskeletal examination of EC perturbed with 1 mM H2O2 reveals structural remodeling with a marked increase in stress fibers, which similarly happens with high LDL levels. The above observations that high LDL levels cause increased EC H2O2 production, and direct H2O2 exposure promotes cellular functional changes similar to those induced by high LDL concentrations, suggest a modulatory role for reactive oxygen species. Thus LDL-induced reactive oxygen species generation may contribute mechanistically to endothelial perturbation, which has been hypothesized to be a major contributing factor in the pathogenesis of atherosclerosis.

A re-evaluation of the tissue distribution and physiology of xanthine oxidoreductase

Xanthine oxidoreductase is an enzyme which has the unusual property that it can exist in a dehydrogenase form which uses NAD+ and an oxidase form which uses oxygen as electron acceptor. Both forms have a high affinity for hypoxanthine and xanthine as substrates. In addition, conversion of one form to the other may occur under different conditions. The exact function of the enzyme is still unknown but it seems to play a role in purine catabolism, detoxification of xenobiotics and antioxidant capacity by producing urate. The oxidase form produces reactive oxygen species and, therefore, the enzyme is thought to be involved in various pathological processes such as tissue injury due to ischaemia followed by reperfusion, but its role is still a matter of debate. The present review summarizes information that has become available about the enzyme. Interpretations of contradictory findings are presented in order to reduce confusion that still exists with respect to the role of this enzyme in physiology and pathology.

Adsorptive endocytosis and membrane recycling by cultured primary bovine brain microvessel endothelial cell monolayers

The dynamics of membrane recycling were examined in primary cultures of brain microvessel endothelial cells (BMECs). Because the BMEC surface was dominated by galactosylated glycoconjugates, ricin agglutinin (RCAI) was used as a tracer to follow the endocytosis and recycling of RCAI binding sites. These binding sites accounted for 75% of the iodinatable or most externally disposed plasma membrane proteins. Because greater than 90% of the RCAI that had bound to BMECs was removed by a brief, nontoxic treatment with galactose, the amounts and kinetics for internalization and efflux of [125I]RCAI were measured. Both endocytosis and efflux were energy dependent. By using pseudo-first-order kinetics, the t1/2 values for RCAI binding, internalization and efflux were 5, 18 and 13-14 min, respectively. By comparing efflux with and without galactose present, we found that 60% of the RCAI binding sites that had been internalized were returned to the cell surface and reinternalized. Quantifying the distribution of gold-RCAI following internalization showed kinetics consistent with that obtained using radiolabeled RCAI. Both horseradish peroxidase (HRP) and gold-conjugated RCAI that had bound BMEC at 4 degrees C became localized within more caveolae within 2.5 min of warming to 37 degrees C to permit endocytosis. With time, RCAI appeared within endosomes and tubules and vesicles of which some were located in the trans-Golgi network (TGN). The distribution of HRP-RCAI contrasted with that of free HRP, which was not routed to the TGN. The absence of RCAI conjugates in association with the basolateral membrane domain suggested the presence of functional tight junctions and maintenance of polarity throughout the duration of these experiments. These results showed that membrane recycling was more extensive and much slower than fluid-phase endocytosis in cultured BMECs. Moreover, we found that endocytosis of membrane by BMECs in culture was similar to that reported for brain endothelium in vivo in that a fraction of the cell surface membrane was routed to the TGN.

Cytoplasmic calcium response to fluid shear stress in cultured vascular endothelial cells

Vascular endothelial cells modulate their structure and functions in response to changes in hemodynamic forces such as fluid shear stress. We have studied how endothelial cells perceive the shearing force generated by blood flow and the substance(s) that may mediate such a response. We identify cytoplasmic-free calcium ion (Ca++), a major component of an internal signaling system, as a mediator of the cellular response to fluid shear stress. Cultured monolayers of bovine aortic endothelial cells loaded with the highly fluorescent Ca++-sensitive dye Fura 2 were exposed to different levels of fluid shear stress in a specially designed flow chamber, and simultaneous changes in fluorescence intensity, reflecting the intracellular-free calcium concentration [( Ca++]i), were monitored by photometric fluorescence microscopy. Application of shear stress to cells by fluid perfusion led to an immediate severalfold increase in fluorescence within 1 min, followed by a rapid decline for about 5 min, and finally a plateau somewhat higher than control levels during the entire period of the stress application. Repeated application of the stress induced similar peak and plateau levels of [Ca++]i but at reduced magnitudes of response. These responses were observed even in Ca++-free medium. Thus, a shear stress transducer might exist in endothelial cells, which perceives the shearing force on the membrane as a stimulus and mediates the signal to increase cytosolic free Ca++.