EPITHELIAL CELL MONOLAYERS
Investigations of the integrity and transport characteristics of 2/4/A1 cells have been done in this report. The cell line was isolated from rat fetal intestinal epithelial cells and transfected with thermolabile SV40 large T antigen.
These cells proliferated at 33 °C, but eliminated the antigen and ceased proliferating at a non-permissive temperature (39°C). At 39°C 2/4/A1 cells started to differentiate but simultaneously the cells also underwent massive cell death.
When cultured at 37°C these cells formed confluent and tight monolayers that seemed to have paracellular transport characteristics similar to that of the human intestine.
Transmission electron microscopy confirmed the development of multilayers at 33°C, monolayers at 37°C and defects in the cell layer due to apoptosis at 39°C.
Different immunostainings of ZO-1, E-cadherin and vinculin confirmed formation of tight and adherence junctions. Transepithelial resistance reached a plateau of 25-35 Ohm.cm2, which was similar to the small intestine. In transport studies 2/4/A1 cell line monolayers selectively restricted the permeation of hydrophilic permeability markers proportional to molecular weight and discriminated more accurately between the molecules of intermediate molecular weight compared to Caco-2 cells.
These results indicated that 2/4/A1 cells could be used as a model for hydrophilic drug absorption.
The small intestine plays a crucial role in the absorption of drugs and nutrients. Exogenous substances cross a series of barriers during the process of intestinal absorption: (1) the aqueous boundary/mucus layer, (2) a single layer of epithelial cells, and (3) the lamina propria, which contains the blood and lymph vessels that then transport the absorbed drugs to other parts of the body (Artursson 1991).
The cell monolayer is comprised of two parallel barriers: the cell membrane and the tight junctions. Most drugs are absorbed by a passive diffusion across the cell membrane by the transcellular route, or across the tight junctions between the cells - the paracellular route. Drug transport can also be carrier mediated, when the drug utilizes transporters located in the cellular membrane. Transcytosis is another kind of active transport, in which macromolecules can be transported across the intestinal epithelial cell in endocytosed vesicles.
The hydrophilic and charged drugs are absorbed after passing through the paracellular route, the water-filled channels between the cells (Artursson 1991). Rates and extent of the paracellular transport are, therefore, highly influenced by the structure and size of the tight junctions as well as by the size of the molecules. Only small and hydrophilic drugs can pass between the cells rapidly and completely; permeation of larger molecules can be limited proportionally to their size and lipophilicity (Hillgren et al. 1995).
Simple assay methods are needed for drug absorption studies. Excised intestinal tissue, isolated cells, membrane vesicles and in vivo models have distinct limitations, which have been previously discussed in detail (Audus et al. 1990; Artursson 1991; Hillgren et al. 1995). The most suitable method for the study of drug intestinal transport appeared to be the use of cultured intestinal epithelial cells. This model has several advantages over conventional drug absorption models: (a) it is less time-consuming; (b) it enables rapid evaluation of methods for improving drug absorption; (c) it allows an opportunity to use human rather than animal tissues; (d) it can minimize expensive and sometimes controversial animal studies.
Human colorectal carcinoma cell line Caco-2 is nowadays the most widely used and the best explored model for drug intestinal transport (Hidalgo et al. 1989; Artursson 1990; Artursson & Karlsson 1991). This cell line displays spontaneous enterocytic differentiation in...