Abstract
The purpose of this study was to investigate the correlation between mRNA and protein levels for P‐glycoprotein (P‐gp) expressed in various cell lines to validate the estimation of P‐gp activity from its mRNA levels. P‐gp expression levels in various cell monolayers, normal, P‐gp‐induced, P‐gp‐highly induced, (multidrug resistance, MDR) MDR1‐knockdown (A2‐2) and MDR1‐knockdown (B2‐2) Caco‐2 cells and MDCKII/MDR1 cells, were quantified by real‐time quantitative polymerase chain reaction (PCR) and western blot analysis. Both mRNA and protein levels of P‐gp were lowest in the MDR1‐knockdown (B2‐2) Caco‐2 cells, followed by the MDR1‐knockdown (A2‐2) Caco‐2, normal Caco‐2, P‐gp‐induced Caco‐2 and P‐gp‐highly induced Caco‐2 cells, and highest in the MDCKII/MDR1 cells. Except for the MDCKII/MDR1 cells, the protein levels of P‐gp in all Caco‐2 cell lines showed a linear correlation with its mRNA levels; however, although the MDR1 mRNA level in MDCKII/MDR1 cells was much higher than in the P‐gp‐highly induced Caco‐2 cells, the protein levels were almost the same in both cells. From these findings, it was suggested that P‐gp activity in MDCKII/MDR1 cells could not be estimated from its mRNA levels. Copyright © 2009 John Wiley & Sons, Ltd.