2.4. In Vitro Caco-2 Cell Monolayer Permeability Assay

Transepithelial electrical resistance (TEER) was determined using a Millicell-ERS-2 Volt-Ohm meter (Millipore) according to a previous report [22]. Briefly, Caco-2 cells (2 × 10⁵) were seeded onto Transwell plates (0.4 μm pores; Corning, USA) for 21 days to reach confluence. After that, a Caco-2 monolayer grown on the apical side of Transwell plates was treated with andrographolide (2.5 and 5 μM), followed by treatment of LPS (10 μg/mL). The effect of andrographolide on the permeability of large molecular substances across the Caco-2 cell monolayer barrier was determined using FITC-dextran. After treatment, the medium in the basolateral and apical compartment was replaced with 1.5 mL DMEM or 0.5 mL DMEM containing FITC-dextran (1 mg/mL). After 2 h incubation, the concentration of FITC-dextran in the basolateral compartment was determined by a fluorescence microplate at a wavelength of 495 nm.