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## Caco-2 Cell Culture

'''Caco-2 cells''' are a spontaneously immortalized human colon adenocarcinoma cell line that are widely used as an *in vitro* model of the human intestinal epithelium. Originally derived from a human colon carcinoma, these cells exhibit many of the morphological and functional characteristics of enterocytes – the absorptive cells of the small intestine. Their ability to differentiate and form tight junctions makes them particularly valuable for studying intestinal permeability, nutrient absorption, drug transport, and metabolism, as well as toxicity testing. This article provides a comprehensive overview of Caco-2 cell culture, encompassing its history, characteristics, culture methods, applications, and potential limitations. Understanding these aspects is crucial for researchers utilizing this versatile cell line.

== History and Origin ==

The Caco-2 cell line was established in 1974 by H.M. Fogh, P. Tremaine, and others from a primary colon carcinoma tissue.  ‘Caco-2’ stands for ‘Colon Adenocarcinoma 2’.  The original tumor was from an 80-year-old Caucasian male.  Unlike many other cancer cell lines, Caco-2 cells do not require exogenous growth factors for proliferation, and they exhibit a unique differentiation process when cultured under serum-free conditions. This differentiation is key to their utility as a model for the intestinal barrier.  The cells were initially deposited in the American Type Culture Collection (ATCC) in 1977 (ATCC HTB-37), becoming a widely accessible resource for the scientific community.  Initial investigations focused on their epithelial characteristics and transport properties, laying the foundation for their subsequent widespread use.

== Characteristics of Caco-2 Cells ==

Caco-2 cells exhibit numerous characteristics that make them a suitable model for the intestinal epithelium:

*   '''Morphology:''' Undifferentiated Caco-2 cells are typically small and polygonal. Upon confluence and serum restriction (usually around 7-21 days), the cells undergo morphological differentiation, becoming more columnar, developing microvilli on their apical surface, and forming tight junctions between adjacent cells. This differentiation mirrors the structure of intestinal enterocytes.
*   '''Tight Junction Formation:'''  A critical feature of Caco-2 cell monolayers is the formation of tight junctions. These junctions restrict paracellular permeability, controlling the passage of molecules between cells.  The trans-epithelial electrical resistance (TEER) increases significantly during differentiation, reflecting the strengthening of these junctions. This is important in studies of [[intestinal permeability]] and drug absorption.
*   '''Enzyme Expression:''' Differentiated Caco-2 cells express several brush border enzymes, including [[alkaline phosphatase]], [[lactase]], [[sucrase]], and [[maltase]], mirroring the digestive capabilities of the small intestine. These enzymes play a crucial role in nutrient breakdown and absorption.
*   '''Transport Activity:'''  Caco-2 cells actively transport various nutrients, drugs, and ions, utilizing both passive and active transport mechanisms. This makes them valuable for studying drug bioavailability and absorption mechanisms. They express a variety of transporters like [[P-glycoprotein]] (MDR1), which impacts drug efflux.
*   '''Differentiation:''' The differentiation process is influenced by factors such as cell density, culture time, and the presence or absence of serum.  Differentiation is typically assessed by measuring TEER, enzyme activity, and morphological changes.
*   '''Gene Expression:'''  The gene expression profile of Caco-2 cells changes significantly during differentiation, reflecting the physiological changes occurring in the intestinal epithelium.  Studies using [[microarrays]] and [[RNA sequencing]] have revealed the upregulation of genes involved in transport, metabolism, and barrier function.

== Culture Methods ==

Culturing Caco-2 cells requires careful attention to detail to ensure optimal growth and differentiation.

*   '''Media:''' Caco-2 cells are typically cultured in Dulbecco’s Modified Eagle’s Medium (DMEM) supplemented with 10% fetal bovine serum (FBS), 1% penicillin/streptomycin, and 2 mM L-glutamine.  Serum-free media formulations are used to induce differentiation.  [[DMEM]] is a common basal media for many mammalian cell lines.
*   '''Seeding Density:''' Cells are usually seeded at a density of 5 x 10^4 to 1 x 10^5 cells/cm^2.  Optimal seeding density may vary depending on the specific experimental conditions.
*   '''Incubation Conditions:''' Cells are incubated at 37°C in a humidified atmosphere containing 5% CO2. Maintaining appropriate CO2 levels is crucial for pH stability.
*   '''Passaging:''' Cells are passaged when they reach 80-90% confluence. Passaging involves detaching the cells from the culture vessel (using trypsin-EDTA), counting them, and reseeding them at the appropriate density. Frequent passaging can affect cell phenotype, so maintaining a consistent passage number is recommended.
*   '''Differentiation Protocol:''' To induce differentiation, cells are typically cultured in serum-free DMEM for 7-21 days after reaching confluence. The media is changed every 2-3 days. Monitoring TEER values is essential to assess the degree of differentiation.  TEER values typically increase over time as tight junctions form.
*   '''Maintaining Differentiation:''' After differentiation, maintaining the monolayer requires specific media and careful handling to prevent dedifferentiation. Regular monitoring of TEER and enzyme activity is crucial.

== Applications of Caco-2 Cell Culture ==

Caco-2 cells are employed in a broad range of research areas:

*   '''Drug Absorption and Permeability Studies:''' This is arguably the most common application. Caco-2 monolayers provide a model to predict the intestinal absorption of drugs. The apparent permeability coefficient (Papp) is a key parameter used to assess drug absorption.  Understanding  [[drug bioavailability]] is critical in pharmaceutical development.
*   '''Nutrient Absorption Studies:'''  Caco-2 cells can be used to investigate the absorption of nutrients, such as glucose, amino acids, and vitamins.
*   '''Toxicity Testing:'''  Caco-2 cells can be used to assess the toxicity of various compounds, including pharmaceuticals, food additives, and environmental toxins.
*   '''Inflammation and Disease Modeling:''' Caco-2 cells can be used to model intestinal inflammation and diseases, such as inflammatory bowel disease (IBD) and colorectal cancer.  Co-culture systems with immune cells are frequently used to enhance the relevance of these models.
*   '''Microbiome Interactions:''' Caco-2 cells can be co-cultured with bacteria to study the interactions between the gut microbiome and the intestinal epithelium.  This is relevant to understanding the role of the microbiome in health and disease.
*   '''Gene Expression Studies:'''  Caco-2 cells are used to study the effects of various compounds on gene expression in the intestinal epithelium.  [[Gene regulation]] is a key area of research.
*   '''Transport Protein Studies:''' Investigating the function and regulation of various transport proteins expressed by Caco-2 cells.  Understanding [[P-glycoprotein]] function is particularly important.

== Assessing Caco-2 Cell Differentiation ==

Several methods are used to assess the degree of Caco-2 cell differentiation:

*   '''Trans-Epithelial Electrical Resistance (TEER):'''  Measuring TEER provides an indication of tight junction integrity. Higher TEER values indicate a more differentiated monolayer.
*   '''Alkaline Phosphatase Activity:'''  Measuring alkaline phosphatase activity is a common marker of enterocyte differentiation.
*   '''Morphological Assessment:'''  Microscopic examination of cell morphology can reveal the development of microvilli and a columnar shape, indicative of differentiation.
*   '''Immunocytochemistry:'''  Staining for specific markers of differentiation, such as villin or sucrase-isomaltase, can confirm the presence of enterocyte characteristics.
*   '''Gene Expression Analysis:'''  Measuring the expression of genes involved in intestinal function, such as transporters and enzymes, can provide a more comprehensive assessment of differentiation.

== Limitations of Caco-2 Cell Culture ==

Despite its numerous advantages, the Caco-2 cell model has limitations:

*   '''Lack of Mucus Layer:''' Caco-2 cells do not produce a mucus layer, which is an important component of the intestinal barrier *in vivo*. This can affect drug absorption and permeability studies.
*   '''Absence of Immune Cells:''' Caco-2 cells do not contain immune cells, which are important for intestinal immunity and inflammation.
*   '''Limited Metabolic Capacity:'''  Caco-2 cells have a limited metabolic capacity compared to the *in vivo* intestine.  [[CYP450]] enzyme activity may be lower.
*   '''Polarity Issues:''' Maintaining consistent polarity of the cells can be challenging.
*   '''Passage Number Effects:'''  Prolonged passaging can lead to changes in cell phenotype and function.
*   '''Species Specificity:''' The cells are human derived, which may not perfectly represent absorption in other species.

== Advanced Caco-2 Culture Models ==

To address some of the limitations of the standard Caco-2 monolayer, several advanced culture models have been developed:

*   '''Co-culture Systems:''' Co-culturing Caco-2 cells with other cell types, such as immune cells or goblet cells, can create a more complex and physiologically relevant model.
*   '''3D Culture Models:''' Culturing Caco-2 cells in 3D structures, such as spheroids or organoids, can better mimic the *in vivo* intestinal environment.
*   '''Microfluidic Devices:''' Using microfluidic devices to create a dynamic and controlled microenvironment for Caco-2 cell culture.  These devices allow for precise control of fluid flow and nutrient delivery.
*   '''Air-Liquid Interface (ALI) Culture:'''  Culturing Caco-2 cells at the air-liquid interface to promote differentiation and mucus production.

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{| class="wikitable"
|+ Key Parameters for Caco-2 Cell Culture
|-
! Parameter !! Optimal Value
|-
| Media || DMEM + 10% FBS + 1% Pen/Strep
|-
| CO2 Concentration || 5%
|-
| Temperature || 37°C
|-
| Seeding Density || 5 x 10^4 - 1 x 10^5 cells/cm^2
|-
| Differentiation Time || 7-21 days (Serum-free DMEM)
|-
| TEER (differentiated) || > 400 Ω·cm^2
|-
| Passage Number || < 20 (recommended)
|}

== Conclusion ==

Caco-2 cell culture remains a valuable *in vitro* model for studying the human intestinal epithelium. While it has limitations, ongoing advancements in culture techniques and the development of more complex models are continually improving its relevance and predictive power.  A thorough understanding of the cell line's characteristics, culture methods, and potential drawbacks is essential for researchers seeking to utilize this versatile tool.

[[Category:Cell Culture]]

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