Caco-2 (from Cancer coli, "colon cancer") is an immortalized cell line of human colorectal adenocarcinoma cells. It is primarily used as a model of the intestinal epithelial barrier. [1] In culture, Caco-2 cells spontaneously differentiate into a heterogeneous mixture of intestinal epithelial cells. [1] It was developed in 1977 by Jorgen Fogh at the Sloan-Kettering Institute for Cancer Research. [2]
The line was developed in 1977 by Jorgen Fogh at the Sloan-Kettering Institute for Cancer Research. [2] The research application of Caco-2 cells was developed during the 1980s by Alain Zweibaum group at INSERM, France as well as Ismael Hidalgo, at the Borchardt laboratory, University of Kansas and Tom Rauband at the Upjohn Company. The first publication of the discovery of the spontaneous enterocyte like differentiation was published by Alain Zweibaum group in 1983. [3]
Although derived from a colon (large intestine) carcinoma, when cultured under specific conditions the cells become differentiated and polarized such that their phenotype, morphologically and functionally, resembles the enterocytes lining the small intestine. [3] [4] Polarized Caco-2 cells express tight junctions, microvilli, and a number of enzymes and transporters that are characteristic of such enterocytes: peptidases, esterases, P-glycoprotein, uptake transporters for amino acids, bile acids, carboxylic acids, etc.
Microscopically, Caco-2 cell cultures show obvious heterogeneity likely reflecting the complex mixture of cells found in the epithelial lining of the large and small intestine i.e. enterocytes, enteroendocrine cells, goblet cells, transit amplifying cells, paneth cells and intestinal stem cells. [5] Over time, the characteristics of the cells used in different laboratories have diverged, introducing inter-laboratory variation. [6] Despite such heterogeneity, Caco-2 cells are used in cell invasion studies, viral transfection research, and lipid transport. [7]
Caco-2 cells may be used as a confluent monolayer on a cell culture insert filter (e.g., Transwell). In this format, Caco-2 cells form a polarized epithelial cell monolayer that provides a physical and biochemical barrier to the passage of ions and small molecules. [4] [8] The Caco-2 monolayer can be used as an in vitro model of the human small intestinal mucosa to predict the absorption of orally administered drugs. Kits, such as the CacoReady, have been developed to simplify this procedure. [9] A correlation between the in vitro apparent permeability across Caco-2 monolayers and the in vivo fraction absorbed has been reported. [10] Transwell diagram
Enterocytes, or intestinal absorptive cells, are simple columnar epithelial cells which line the inner surface of the small and large intestines. A glycocalyx surface coat contains digestive enzymes. Microvilli on the apical surface increase its surface area. This facilitates transport of numerous small molecules into the enterocyte from the intestinal lumen. These include broken down proteins, fats, and sugars, as well as water, electrolytes, vitamins, and bile salts. Enterocytes also have an endocrine role, secreting hormones such as leptin.
Gut-associated lymphoid tissue (GALT) is a component of the mucosa-associated lymphoid tissue (MALT) which works in the immune system to protect the body from invasion in the gut.
An organoid is a miniaturised and simplified version of an organ produced in vitro in three dimensions that mimics the key functional, structural, and biological complexity of that organ. It is derived from one or a few cells from a tissue, embryonic stem cells, or induced pluripotent stem cells, which can self-organize in three-dimensional culture owing to their self-renewal and differentiation capacities. The technique for growing organoids has rapidly improved since the early 2010s, and The Scientist named it one of the biggest scientific advancements of 2013. Scientists and engineers use organoids to study development and disease in the laboratory, for drug discovery and development in industry, personalized diagnostics and medicine, gene and cell therapies, tissue engineering, and regenerative medicine.
Intestinal permeability is a term describing the control of material passing from inside the gastrointestinal tract through the cells lining the gut wall, into the rest of the body. The intestine normally exhibits some permeability, which allows nutrients to pass through the gut, while also maintaining a barrier function to keep potentially harmful substances from leaving the intestine and migrating to the body more widely. In a healthy human intestine, small particles can migrate through tight junction claudin pore pathways, and particles up to 10–15 Å can transit through the paracellular space uptake route. There is some evidence abnormally increased intestinal permeability may play a role in some chronic diseases and inflammatory conditions. The most well understood condition with observed increased intestinal permeability is celiac disease.
In histology, an intestinal gland is a gland found in between villi in the intestinal epithelial lining of the small intestine and large intestine. The glands and intestinal villi are covered by epithelium, which contains multiple types of cells: enterocytes, goblet cells, enteroendocrine cells, cup cells, tuft cells, and at the base of the gland, Paneth cells and stem cells.
TRPV6 is a membrane calcium (Ca2+) channel protein which is particularly involved in the first step in Ca2+absorption in the intestine.
Hephaestin, also known as HEPH, is a protein which in humans is encoded by the HEPH gene.
GLUT5 is a fructose transporter expressed on the apical border of enterocytes in the small intestine. GLUT5 allows for fructose to be transported from the intestinal lumen into the enterocyte by facilitated diffusion due to fructose's high concentration in the intestinal lumen. GLUT5 is also expressed in skeletal muscle, testis, kidney, fat tissue (adipocytes), and brain.
Homeobox protein CDX-2 is a protein that in humans is encoded by the CDX2 gene. The CDX-2 protein is a homeobox transcription factor expressed in the nuclei of intestinal epithelial cells, playing an essential role in the development and function of the digestive system. CDX2 is part of the ParaHox gene cluster, a group of three highly conserved developmental genes present in most vertebrate species. Together with CDX1 and CDX4, CDX2 is one of three caudal-related genes in the human genome.
Peptide transporter 1 also known as solute carrier family 15 member 1 (SLC15A1) is a protein that in humans is encoded by SLC15A1 gene. PepT 1 is a solute carrier for oligopeptides. It functions in renal oligopeptide reabsorption and in the intestines in a proton dependent way, hence acting like a cotransporter.
Homeobox protein CDX-1 is a protein in humans that is encoded by the CDX1 gene. CDX-1 is expressed in the developing endoderm and its expression persists in the intestine throughout adulthood. CDX-1 protein expression varies along the intestine, with high expression in intestinal crypts and diminishing expression along intestinal villi.
Ectonucleotide pyrophosphatase/phosphodiesterase family member 7 also known as alkaline sphingomyelin phosphodiesterase (Alk-SMase) or intestinal alkaline sphingomyelinase is an enzyme that in humans is encoded by the ENPP7 gene.
Sodium-coupled monocarboxylate transporter 1 (i.e., SMCT1) and sodium-coupled monocarboxylate transporter 2 (i.e., SMCT2) are plasma membrane transport proteins in the solute carrier family. They transport sodium cations in association with the anionic forms (see conjugated base) of certain short-chain fatty acids (i.e., SC-FAs) through the plasma membrane from the outside to the inside of cells. For example, propionic acid (i.e., CH
3CH
2CO
2H) in its anionic "propionate" form (i.e., CH
3CH
2CO−
2) along with sodium cations (i.e., Na+) are co-transported from the extracellular fluid into a SMCT1-epxressing cell's cytoplasm. Monocarboxylate transporters (MCTs) are also transport proteins in the solute carrier family. They co-transport the anionic forms of various compounds into cells in association with proton cations (i.e. H+). Four of the 14 MCTs, i.e. SLC16A1 (i.e., MCT1), SLC16A7 (i.e., MCT22), SLC16A8 (i.e., MCT3), and SLC16A3 (i.e., MCT4), transport some of the same SC-FAs anions that the SMCTs transport into cells. SC-FAs do diffuse into cells independently of transport proteins but at the levels normally occurring in tissues far greater amounts of the SC-FAs are brought into cells that express a SC-FA transporter.
The intestinal epithelium is the single cell layer that forms the luminal surface (lining) of both the small and large intestine (colon) of the gastrointestinal tract. Composed of simple columnar epithelium its main functions are absorption, and secretion. Useful substances are absorbed into the body, and the entry of harmful substances is restricted. Secretions include mucins, and peptides.
Transepithelial potential difference (TEPD) is the voltage across an epithelium, and is the sum of the membrane potentials for the outer and inner cell membranes.
In medicinal chemistry, parallel artificial membrane permeability assay (PAMPA) is a method which determines the permeability of substances from a donor compartment, through a lipid-infused artificial membrane into an acceptor compartment. A multi-well microtitre plate is used for the donor and a membrane/acceptor compartment is placed on top; the whole assembly is commonly referred to as a “sandwich”. At the beginning of the test, the drug is added to the donor compartment, and the acceptor compartment is drug-free. After an incubation period which may include stirring, the sandwich is separated and the amount of drug is measured in each compartment. Mass balance allows calculation of drug that remains in the membrane.
HT-29 is a human colon cancer cell line used extensively in biological and cancer research.
Calu-3 is a human lung cancer cell line commonly used in cancer research and drug development. Calu-3 cells are epithelial and can act as respiratory models in preclinical applications.
In medicinal chemistry, Drug Permeability is an empirical parameter that indicates how quickly a chemical entity or an active pharmaceutical ingredient crosses a biological membrane or another biological barrier to become bioavailable in the body. Drug permeability, together with drug aqueous solubility are the two parameters that define the fate of the active ingredient after oral administration and ultimately define its bioavailability. When drug permeability is empirically measured in vitro, it is generally called apparent permeability (Papp) as its absolute value varies according to the method selected for its measurement. Papp is measured in vitro utilizing cellular based barriers such as the Caco-2 model or utilizing artificial biomimetic barriers, such as the Parallel Artificial Membrane Permeation Assay (PAMPA) or the PermeaPad. All these methods are built on an acceptor compartment where the drug solution is placed, a biomimetic barrier and an acceptor compartment, where the drug concentration is quantified over time. By maintaining sink condition, a steady state is reached after a lag time.
