Pinocytosi endocytosis

Fig. 43.10. Synthesis of the thyroid hormones (T3 and T4). The protein thyroglobulin (Tgb) is synthesized in thyroid follicular cells and secreted into the colloid, lodination and coupling of tyrosine residues in Tgb produce T3 and T4 residues, which are released from Tgb by pinocytosis (endocytosis) and lysosomal action. The coupling of a monoiodotyrosine with a diiodotyrosine (DIT) to form triiodothyronine (Tj) is not depicted here.

With animal cells, the whole virus particle penetrates the cell, being carried inside by endocytosis (phagocytosis or pinocytosis), an active cellular process. We describe some of these processes in detail later in this chapter. 

We might also note another important difference between animal and bacterial cells. Bacterial cells have rigid cell walls containing peptidoglycan and associated substances. Animal cells, on the other hand, lack cell walls. This difference is important for the way by which the virus genome enters and exits the cell. In bacteria, the protein coat of the virus remains on the outside of the cell and only the nucleic acid enters. In animal viruses, on the other hand, uptake of the virus often occurs by endocytosis (pinocytosis or phagocytosis), processes which are characteristic of animal cells, so that the whole virus particle enters the cell. The separation of animal virus genomes from their protein coats then occurs inside the cell. 

Pinocytosis is a type of endocytosis that is responsible for the transport of large molecules such as proteins and colloids. Some cell types (e.g., endothelial cells) employ this transport mechanism extensively, but its importance in drug action is uncertain. 

Endocytosis for degradation of macromolecules and uptake of nutrients involves phagocytosis, pinocytosis and autophagy 151 The constitutive pathway is also known as the default pathway because it does not require any type of signal to enter 154... 

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. 

The cellular uptake of AS-ODN is an energy-dependent process and takes place in a saturable and sequence-independent manner [120,121]. The exact mechanism of uptake remains controversial. From in vitro experiments, some authors have proposed that the uptake is endocytic and mediated by membrane receptor proteins. The receptor responsible for the cellular uptake of AS-ODNs was reported to consist of both a 30-kDa protein [122] and an 80-kDa membrane protein [121]. However, other workers have argued that AS-ODN binding to membrane proteins is relatively non-specific and is mostly charge associated, consistent with adsorptive endocytosis or fluid-phase pinocytosis [101]. As a result of these conflicting reports, it is unlikely that in vitro data can be safely extrapolated to what occurs in the intact organism. 

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...

Endocytosis (phagocytosis/pinocytosis requires energy large, insoluble molecules). 

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. 

Endocytosis. Transport takes the form of pinocytosis for liquids and phagocytosis for solids. 

Substances ingested into cells by any of these three mechanisms either are stored in vesicles inside the cell or are degraded by intracellular enzymes, and only certain cells are capable of either process. The endothelial cells of capillaries are capable of pinocytosis. Neutrophils and macrophages (specific types of white blood cells) are capable of phagocytosis. Phagocytosis, endocytosis, and pinocytosis are slow and inefficient when compared to the other processes by which substances enter cells. 

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... 

Pinocytosis Unlike phagocytosis, which is a regulated form of endocytosis carried out by 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. 

Endocytosis — A process in which a substance gains entry into a eukaryotic cell without passing through the cell membrane. In this process particles (phagocytosis) or droplets (pinocytosis) are accumulated at the cell surface and enclosed by the cell membrane. The so-formed - vesicles are then delivered into the cell. Opposite process -> exocytosis. Endo- and exocytosis are fundamental processes of membrane fusion in living cells. See also - membrane. 

Figure 29.10. Postulated pathways of aminoglycoside-induced cellular injury. On the left, aminoglycoside (AG) enters the cell by pinocytosis and endocytosis, subsequently fusing with a primary lysosome (L). Aminoglycosides may interfere with normal lysosomal function, forming myeloid bodies (center). Additionally, aminoglycosides may destabilize lysosomes, leading to release of intralysosomal enzymes (lower left). Intracellular aminoglycosides may produce direct injury to intracellular organelles such as mitochondria. (Adapted from G.J. Kaloyanides and E. Pastoriza-Munoz, Kidney Int. 18, 571-582,1980.)...

Advantages of nanostructure-mediated drug delivery include the ability to deliver drug molecules directly into cells and the capacity to target tumors within healthy tissue [50]. The mechanisms of cellular uptake of external particulates include calthrin- and caveoli-mediated endocytosis, pinocytosis, and phagocytosis. However, phagocytosis may not play a role in the uptake of nanoscale particles because of the small size of such particles. 

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... 

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