Intestinal enterocytes

The enthusiasm for using Caco-2 cells and other epithelial cell cultures in studies of drug transport processes has been explained by the ease with which new information can be derived from these fairly simple in vitro models [7]. For instance, drug transport studies in Caco-2 cells grown on permeable supports are easy to perform under controlled conditions. This makes it possible to extract information about specific transport processes that would be difficult to obtain in more complex models such as those based on whole tissues from experimental animals. Much of our knowledge about active and passive transport mechanisms in epithelia has therefore been obtained from Caco-2 cells and other epithelial cell cultures [10-15]. This has been possible since Caco-2 cells are unusually well differentiated. In many respects they are therefore functionally similar to the human small intestinal enterocyte, despite the fact that they originate from a human colorectal carcinoma [16, 17]. 

Several active transport systems that are normally found in the small intestinal enterocytes have been characterized in the Caco-2 cell model [13]. These include transport systems for glucose [32, 33], amino acids [34-37], dipeptides [38-40], vitamins [41], and bile acids [42, 43]. 

In addition to the mechanistic simulation of absorptive and secretive saturable carrier-mediated transport, we have developed a model of saturable metabolism for the gut and liver that simulates nonlinear responses in drug bioavailability and pharmacokinetics [19]. Hepatic extraction is modeled using a modified venous equilibrium model that is applicable under transient and nonlinear conditions. For drugs undergoing gut metabolism by the same enzymes responsible for liver metabolism (e.g., CYPs 3A4 and 2D6), gut metabolism kinetic parameters are scaled from liver metabolism parameters by scaling Vmax by the ratios of the amounts of metabolizing enzymes in each of the intestinal enterocyte compart-... 

Fig. 9.1 Schematic diagram depicting drug transporters and their subcellular localization in the human small intestinal enterocyte (A), hepatocyte (B), and renal tubular cell (C).

There are several cell monolayer models that are frequently used for the evaluation of drug permeability and absorption potential (Table 18.1). For a more detailed discussion, please refer to Chap. 8. Caco-2 cells (adenocarcinoma cells derived from colon) are the most extensively characterized and frequently used of the available cell lines [5-9], A unique feature of Caco-2 cells is that they undergo spontaneous enterocyte differentiation in cell culture. Unlike intestinal enterocytes, Caco-2 cells are immortalized and replicate rapidly into confluent monolayers. When the cells reach confluency during culture on a semi-porous membrane, they start to polarize and form tight junctions, creating an ideal system for permeability and transport studies. During the past decade, use of... 

Intestines Enterocytes Dimeric IgGl Lactoferrin (enteral) Rat Rat, man Colitis, Crohn s disease... 

In a more recent work, GFJ and orange juice have been reported to reduce the availability of fexofenadine and celiprolol (22,76,107). Both drugs are substrates for P-gp and OATP, but not CYP3A4 (108,109). If P-gp had played a major role in the observed interactions, the bioavailability would have been increased instead of decreased. This led to the conclusion that a mechanism other than P-gp was involved. In theory, the inhibition of OATP could lead to a decreased absorption of OATP substrates into intestinal enterocytes. This hypothesis was tested by several in vitro studies. 

Iron crosses the luminal membrane of the intestinal mucosal cell by two mechanisms active transport of ferrous iron and absorption of iron complexed with heme (Figure 33-1). The divalent metal transporter, DMT1, efficiently transports ferrous iron across the luminal membrane of the intestinal enterocyte. The rate of iron uptake is regulated by mucosal cell iron stores such that more iron is transported when stores are low. Together with iron split from absorbed heme, the newly absorbed iron can be actively transported into the blood across the basolateral membrane by a transporter known... 

After binding, gliadin is endocytosed [52] and translocated to HLA-DR antigen-containing endocytic vesicles [53] and Golgi complexes [54] of small intestinal enterocytes in active CD. Alternatively, gliadin may be transported paracellularly across the epithelium. It was found that zonulin, a protein... 

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. 

As described above (Section 6.2.2), the Peyer s patches belong to the gut-associated lymphoid tissue (GALT) and participate in the process of antigen sampling and presentation to the immune system. Specialized antigen-presenting epithelial cells cover the patches, called M-cells (modified epithelial cells). Unlike the intestinal enterocytes, the M cells of the Peyer s patches are capable of extensive endocytic uptake of macromolecules and microparticles (Figure 6.15). 

When substantial carbohydrate maldigestion occurs in infants, it can lead to diarrhea. This is most often seen when the infant has experienced some other insult that has damaged the small intestine enterocytes, producing a secondary hypolactasia.This is more so when the infant has a rotavirus infection, and rotavirus is the most important cause of gastroenteritis in infancy. Rotavirus infects only mature enterocytes,... 

Among 20 cell lines tested, CACO-2 cells were the only onces to differentiate spontaneously to intestinal enterocytes (Chantret et al. 1988). They resemble many characteristics of small intestinal epithelial cells (Hidalgo and Borchardt 1989) and are still the most widely accepted model in industry and academia (Artursson and Karlsson 1991 Artursson et al. 1996 Audus et al. 1990 Gan and Thakker 1997 Artursson and Borchardt 1997 and many others). Reduced demands for compound volumes in cellular assays allow for screening (Stevenson et al. 1995 Kuhfeld et al. 1994). CACO-2 cells predict permeability with reasonable accuracy and are used in many different test designs. 

Final digestion of dietary carbohydrates and proteins occurs on the lumen face of small intestinal enterocytes by so-caUed brush border enzymes , including maltase, sucrose, lactase and peptidases. 

Lampen A, Meyer S, Arnhold T, Nau H. 2000. Metabolism of vitamin A and its active metabolite all-trans-retinoic acid in small intestinal enterocytes. J. Pharmacol. Exp. Ther. 295 979-85... 

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