Fluid-Phase Pinocytosis in Arterial Smooth Muscle Cells
Pinocytosis is the process by which cells take up materials from the extracellular environment. This can involve the specific recognition of a macromolecule by a receptor on the plasma membrane, followed by the interiorisation of the receptor-ligand complex (receptor-mediated pinocytosis, or the non-specific uptake of material present in the fluid entrapped within the forming pinosomes (non-specific fluid-phase pinocytosis). The functions of these processes include the uptake of nutrients and hormones, and possibly membrane surveillance. They may also be of significance in the pathological-overload of cells with extracellular substances. In atherosclerosis overload of arterial smooth muscle cells (SMC) with lipid, predominantly cholesterol and cholesteryl esters, is a characteristic feature of the foam cell (1). SMC take up low density lipoprotein (LDL) by both receptor-mediated and non-specific fluid-phase pinocytosis (2). Uptake by the receptor-mediated pathway, however, leads to the down regulation of receptors. It has been calculated that even under normal conditions, the concentrations of LDL to which SMC are exposed, would cause maximal suppression of the LDL receptor. Under such conditions the non-specific route may contribute significantly to the uptake of LDL (3). This route would become even more important under conditions in which the concentration of lipoproteins in the interstitial fluid is raised, for example hypercholesterolemia or following loss of the endothelium. In view of the potential significance of the fluid-phase pathway in atherogenesis we have developed a method for its measurement in arterial SMC in culture, investigated the basic mechanisms involved, and begun to explore ways in which it may be modified.
KeywordsCholesterol Sucrose Foam Fluoride Choline
Unable to display preview. Download preview PDF.
- 1.Coltoff-Schiller B, Goldfischer, S, Adamany AM, Wolinsky, H (1976) Endocytosis by vascular smooth muscle in vivo and in vitro. Am J Path 83: 45–60Google Scholar
- 4.Muir EM (1981) Ph.D Thesis (Cambridge University)Google Scholar
- 5.Laake DS, Bowyer DE (1977) Quantitative studies of pinocytic uptake of 125 I-labelled PVP by pig aortic smooth muscle cells in culture. Biochem Soc 5: 130–133Google Scholar
- 8.Duncan R, Pratten MK (1971) Membrane economics in endocytic systems. J Theor Biol 66: 729–739Google Scholar
- 15.Roberts AVS, Nicholls SDE, Williams KT, Lloyd JB (1976) Pinocytosis and intracellular proteolysis in experimentally induced lysosomal storage. Biochem Soc Trans 2: 1096–1098Google Scholar