Moderate increase in Mdr1a/1b expression causes in vivo resistance to doxorubicin in a mouse model for hereditary breast cancer. 
We have found previously that acquired doxorubicin resistance in a genetically engineered mouse model for BRCA1-related breast cancer was associated with increased expression of the mouse multidrug resistance (Mdr1) genes, which encode the drug efflux transporter ATP-binding cassette B1/P-glycoprotein (P-gp).  Here, we show that even moderate increases of Mdr1 expression (as low as 5-fold) are sufficient to cause doxorubicin resistance.  These moderately elevated tumor P-gp levels are below those found in some normal tissues, such as the gut.  The resistant phenotype could be completely reversed by the third-generation P-gp inhibitor tariquidar, which provides a useful strategy to circumvent this type of acquired doxorubicin resistance.  The presence of MDR1A in drug-resistant tumors with a moderate increase in Mdr1a transcripts could be shown with a newly generated chicken antibody against a mouse P-gp peptide.  Our data show the usefulness of realistic preclinical models to characterize levels of Mdr1 gene expression that are sufficient to cause resistance.