Receptor-mediated pinocytosis

The cellular barriers include macrophages, eosinophils, phagocytes and natural killer (NK) cells. Some of these cells internalize macromolecules that they encounter in circulation or in tissues. This internalization takes place either by pinocytosis, receptor-mediated endocytosis or phagocytosis. The pinocytosis involves nonspecific membrane invagination. In contrast, receptor-mediated endocytosis involves specific macromolecules that are internalized after they bind to respective cell surface receptors. Endocytosis is not cell-specific and is carried out probably by all cells. 

Three processes are involved in transcellular transport across the intestinal epithelial cells simple passive trans-port, passive diffusion together with an efflux pump, and active transport and endocytosis. Simple passive transport is the diffusion of molecules across the membrane by thermodynamic driving forces and does not require direct expenditure of metabolic energy. In contrast, active transport is the movement of molecules across the mem-brane resulting directly from the expenditure of metabolic energy and transport against a concentration gradient. Endocytosis processes include three mechanisms fluid-phase endocytosis (pinocytosis), receptor-mediated endocytosis, and transcytosis (Fig. 6). Endocytosis processes are covered in detail in section Absorption of Polypeptides and Proteins, later. 

Receptor-Mediated Endocytosis. Mammalian cells have developed an assortment of mechanisms to facilitate the internalization of specific substrates and target these to defined locations inside the cytoplasm. Collectively, these processes of membrane deformations are termed "endocytosis," consisting of phagocytosis, pinocytosis, receptor-mediated endocytosis (clathrin-mediated), and potocytosis [nonclathrin (caveolin)-mediated RME]. The emphasis of this section is receptor-mediated endocytosis in the intestinal tract, but the interested student may consult alternative reviews covering the complete spectrum of endo-cytotic processes in other cell types (17,18). 

Uptake of protein by hepatocytes can occur via one of two mechanisms (a) receptor-mediated en-docytosis or (b) non-selective pinocytosis, again with subsequent protein proteolysis. Similarly, a proportion of some proteins are likely degraded within the target tissue, as binding to their functional cell surface receptors triggers endocytotic internalization of the receptor ligand complex (Figure 4.7). 

The bulk of pinocytosis in the nervous system is mediated by clathrin-mediated endocytosis (CME) [55] and this is the best-characterized pathway. More detail about clathrin-mediated pathways will be given when receptor-mediated endocytosis and the synaptic vesicle cycle pathways are considered. Pinocytosis through CME is responsible for uptake of essential nutrients such as cholesterol bound to low density lipoprotein (LDL) and transferring, but also plays a role in regulating the levels of membrane pumps and channels in neurons. Finally, CME is critical for normal synaptic vesicle recycling. 

Fig. I. Endocytic pathways used by cells to internalize soluble macromolecules [25] fluid-phase pinocytosis (1), adsorptive pinocytosis (2), and receptor-mediated endocytosis (pinocytosis) (6). Each of these processes involves a formation of a sealed vesicle formed from the plasma membrane which encloses part of the extracellular medium. The internalization of a polymer-drug conjugate (P-D), and targeted polymer-drug conjugate ( => —P-D) is shown. Other abbreviations — = cell surface receptor/antigen 1 = clathrin molecule X = lysosomal enzyme. Fluid-phase pinocytosis (1) and adsorptive pinocytosis (2) are nonspecific processes which direct the macromolecule into the lysosomal compartment of the cell. Once P-D is internalized, whether by (1) or (2), the resulting endosome (3) is ultimately fused with a primary lysosome (4) forming a secondary lysosome (5). In the latter compartment P-D is in contact with several types of lysosomal enzymes. The membrane of (5) is impermeable to macromolecules. Consequently, the structure of P-D may be designed in such...

The protective function against catabolism is illustrated in Fig. 3.5. The IgG enters a cell by receptor-mediated endocytosis (pinocytosis), and is bound to Fc-Rn. As the binding sites are limited at physiological concentrations, not all IgG molecules will be bound to the membrane-associated receptor by the Fc region. After cellular uptake, the intracellular vesicle (phagosome) fuses with a lysosome. 

Receptor-mediated endocytosis is a form of pinocytosis that is actively initiated. The trigger is the binding of a large extracellular molecule (e. g., a mAb) to a... 

Considerable evidence has accumulated indicating that macromolecules and microparticulates can be taken up by the intestinal enterocytes, generally via pinocytosis. In some cases, transcytosis, i.e. passage through the cells, has been observed, with microparticles subsequently gaining access to the lymphatics of the mucosa. For example, studies have shown that receptor-mediated endocytosis via enterocytes is a major pathway for the internalization of certain antisense oligonucleotides. 

Cell-surface receptors are involved in both phagocytosis and pinocytosis. At least four distinct mechanisms of pinocytosis have been characterized macropinocytosis, clathrin-mediated endocytosis, raft/caveolae-mediated endocytosis, and clathrin-and caveolae-independent endocytosis (1). Selected receptor-mediated aspects of these mechanisms are outlined below. 

Pinocytosis is a non-specific process whereby a substrate enters a cell by invagination to form an intracellular vesicle. Receptor-mediated endocytosis occurs when substrate binds to a specific membrane receptor. Substrates ingested by cells in this way are stored in... 

The epithelial membrane of the GI tract consists of a continuous barrier of cells, which allows the transport of low-molecular-weight molecules by simple diffusion or various carrier processes. Macromolecules such as proteins may be absorbed from the intestinal lumen by cellular vesicular processes, through fluid-phase endocytosis (pinocytosis), or by receptor-mediated endocytosis or transcytosis (Fig. 6). In pinocytosis, extracellular fluid is captured within an epithelial membrane vesicle. It begins with the formation of a pocket... 

Fig. 8. Schematic diagram illustrating various pathways of endocytosis (1) phagocytosis, (2) receptor-mediated phagocytosis, (3) pinocytosis, and (4) receptor-mediated pinocytosis...

Toxin B is an intracellularly acting cytotoxin and enters the cell via a receptor-mediated endocytosis pathway to reach the endosomes, from which the toxin is translocated to the cytoplasm (Florin and The-lestam, 1986 Henriques etal., 1987). Because of this specific mode of entry, the toxin concentration needed for intoxication of cells is low (lOOng/ml for about 4h). In contrast, Clostridium bofulinum exoenzyme C3 (23.5 kDa), which ADP-ribosylates the Rho subtype proteins RhoA, B and C only, enters the cells by a non-specific uptake process, possibly by pinocytosis. Therefore, C3 has to be applied in high concentrations (about 30 g/ml) for 24 h or longer. 

The receptor-mediated form of endocytic uptake, compared with nonspecific pinocytosis, provides a more rapid means of cellular uptake (24,25). It has been identified for a wide variety of physiological ligands, such as metabolites, hormones, immunoglobulins, and pathogens (e.g., virus and bacterial and plant toxins). Several endosomotropic receptors identified in cells are listed in Table 2 (5). 

Many therapeutic proteins, particularly protein hormones and monoclonal antibodies, exhibit saturable clearance mechanisms that appear to be receptor mediated (28, 29). This form of clearance is usually directly linked to the pharmacological activity of the drug and plays an intricate role in removal of the protein from circulation. The mechanism usually involves binding of the therapeutic agent followed either by internalization (pinocytosis) and breakdown of the protein or... 

Stahl, P Schlesinger, P. H., Sigardson, E., Rodman, J. S., Lee, Y. C. (1980). Receptor-mediated pinocytosis of mannose glycoconjugates by macrophages characterisation and evidence for receptor recycling. Cell, 19,207-215. 

In addition to high-affinity receptor-mediated uptake, cultured fibroblasts can take up LDL by receptor-independent bulk fluid pinocytosis (for a review of endocytosis, see ref. 72). LDL uptake by this process is non-saturable and is directly proportional to the concentration of LDL in the medium. Unlike receptor-depen-dent uptake, receptor-independent uptake of LDL is protease-insensitive and does not require divalent cations [28]. In addition, LDL entering cells through non-specific pinocytosis do not increase cellular cholesterol content or regulate cellular cholesterol synthesis or cholesterol esterification [28,73], possibly because the amount of LDL taken up may be insufficient to significantly expand cellular cholesterol pools. 

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