Intestinal absorption barriers as modelled by p-glycoprotein and cytochrome P450 3A4 in Caco-2 cells
The passage number and origin of two populations of Caco-2 cells influence their
enterocyte-like characteristics. Caco-2 cells of passage number >90 from
Novartis pharmaceutical company possess higher levels of expression of alkaline
phosphatase and P-glycoprotein and a greater cellular uptake of Gly-L-Pro than
those ofpassage number <40 from the American Type Tissue Culture collection.
High P-gp expressing Caco-2 cells have been developed through stepwise
selection of the cells with doxorubicin. This newly-developed cell line (hereafter
referred to as Type I) possesses approximately twice as much P-gp protein than
non-exposed cells, restricts the transepithelialtransport of vincristine in the apicalto-
basolateral direction whilst facilitating its transport in the reverse direction and
accumulates less vincristine than non-exposed cells. There is no apparent
evidence of the co-existence of the multidrug resistance protein (MRP) in Type I
cells to account for the above-listed observations. Stopping the exposure for more
than 28 days decreases the P-gp protein expression in previously doxorubicinexposed
Type I Caco-2 cells and reduces the magnitude of vincristine
transepithelial fluxes in both directions to the levels that are almost similar to
those ofnon-exposed cells.
Exposing Caco-2 cells to 0.25 ~ la., 25-dihydroxyvitamin D3 induces their
expression of cytochrome P450 3A4 protein to the level that is equivalent to that
from isolated human jejunal cells. Under the same treatment, doxorubicinexposed
(Type I) cells metabolise midazolam poorly and less extensively
compared to non-exposed cells, suggesting that there is no such co-regulation of
P-gp and CYP3A4 in Caco-2 cells. However, there is evidence which suggests
CYP3A metabolises midazolam into 1- and 4-hydroxymidazolam, the latter may
possibly be a P-gp substrate and is transported extracellularly by P-gp, supporting
the hypothesis of P-gp-CYP3A4 synergistic roles in keeping xenobiotics out of
Doxorubicin-exposed (Type I) cells are less effective in translocating L-proline
and glycyl-L-proline across the cell monolayers.
Caco-2, P-glycoprotein (P-gp), Vincristine sulphate, Cytochrome P450 3A4
(CYP3A4), Midazolam, L-Proline, Glycyl-L-proline.