Signalling pathways in vascular endothelium activated by shear stress: relevance to atherosclerosis.

Major advances in our understanding of how endothelial cells sense and respond to haemodynamic forces and, more specifically, to fluid shear stress have been achieved during the past 3 years. These include definition of potential shear stress receptors and multiple signalling pathways that mediate shear stress regulation of gene expression. A few studies have also pointed to the unique effects of complex shear stress on endothelial activation, thus leading to better understanding of the mechanisms that lead to the development of atherosclerosis.

Endothelial gene regulation by laminar shear stress.

Endothelial cells, because of their unique localization, are constantly exposed to fluid mechanical forces derived by the flowing blood. These forces, and more specifically shear stresses; affect endothelial structure and function, both in vivo and in vitro, and are implicated as contributing factors in the development of cardiovascular diseases. We have demonstrated earlier that the shear stress selectively induces the transcription of several endothelial genes, and have defined a shear stress response element (SSRE) in the promoter of platelet-derived-growth-factor B (PDGF-B), that is shared by additional endothelial shear stress responsive genes. Here we further characterize this SSRE and the nuclear factors that bind to it, and imply the possible role of the endothelium cytoskeleton in transducing shear stress, leading to the expression of PDGF-B/SSRE constructs in transfected endothelial cells exposed to shear stress. We also present, yet a new shear stress response element in the Platelet Derived Growth Factor A promoter, that contains a binding site to the transcription factors egr1/sp1. These results further demonstrate the complexity of gene regulation by hemodynamic forces, and support the important part that these forces have in the physiology and pathophysiology of the vessel wall.

Secretion of two different flowing masses by lymphokine-activated killer cells.

When grown on mesenchyme-fibroblastoid monolayers made of 16-day-old embryos, lymphokine-activated killer (LAK) cells in clones derived from nude mouse lymph node cells are signaled to synthesize and secrete two mucoid masses. The first is made of chondroitin sulfate, as determined by the degradation of 35S- and [3H]glucosamine-labeled macromolecules in the extracellular matrix, by hyaluronidase, and by chondroitin sulfate lyase AC. This determination correlates with the distinctive blue staining by periodic acid-Schiff/alcian blue (PAS-Ab) at pH 1.0. In the present study, two different masses were identified when methanol-fixed and dried LAK cells and their secretions were examined prior to staining. The chondroitin-sulfate-containing mass appeared as an optically bright structure. It also produced a positive fluorescence with rabbit anti-mouse perforin. The second structure, which appeared as a flowing material or as filling holes in the first, could be identified by its high optical density. However, it was not stained by PAS-Ab and was not blackened by osmium tetroxide. The biochemical nature of the second mass has yet to be determined. Both masses seemed eventually to mix, producing pools, in lacunae, or to spread into the culture space.

Secretion of mucoid material by lymphokine-activated killer cells: study by light and electron microscopy.

When lymph node cells from nude mice were grown on embryonic fibroblast monolayers together with rat interleukin-2, only one type of colonies developed. These colonies were composed of cytotoxic cells termed "granular/lymphokine-activated killer/mucus-secreting cells" (LAK-GM). An extensive differentiation course, in which all the cellular components were involved, ended with a population of short-lived, mature, nondividing large cells that apparently synthesized and deposited a flowing mucoid material (FMM) that stained distinctly blue with periodic acid-Schiff/alcian blue (PAS-Ab) at pH 1 and distinctly red by the naphthol AS-D-chloracetate method for specific esterase. So far, the best monolayers to trigger the FMM synthesis were those prepared from 16- to 18-day-old whole embryos. These cells were compared with LAK cells that developed on monolayers (such as embryonic skin or adult kidney) that did not trigger FMM synthesis. They were also compared with other cell types that differentiated in colonies on the fibroblast monolayers: histiocytes (fixed macrophages), mixed granulocytes/monocytes, mucosal mast cells; and with populations of mature rat T-killer cells developed on same mouse monolayers. Features distinctive to the secreting LAK-GM cells were presence of masses of membrane-limited vesicles that were strictly confined to the surface of the cells in FMM-containing colonies. All transitional forms of budding activity could be seen on the cell surface facing the masses. Within the same cells, many granules displayed varying degrees of degradation, the granular material being transformed into flocculent material that formed small pools facing each degraded surface. Other characteristics of the LAK-GM lineage were the accumulation of glycogen prior to the appearance of the FMM, the presence of several structures of a ribosome-lamella complex in the LAK-GM in colonies that did not accumulate FMM, and filopodia commonly emerging from the pole proximal to the nucleus. Of various fixation methods tried, only after treatment with absolute alcohol and subsequent drying was the FMM stained with PAS-Ab. By subsequent wetting, the capacity to be stained was irreversibly lost. After incubation of the living cultures with the enzymes hyaluronidase or chondroitinases AC or ABC, the FMM disappeared. These observations suggest a triggering mechanism by the embryonic mesenchymal fibroblastoid cells for synthesis and secretion of mucous material that is a proteoglycan of the chondroitin sulfate group.