Differential expression of transporter proteins involved in drug export in human macrophages compared to liver cells

We recently showed that human blood macrophages express a cell type specific pattern of extrahepatic cytochrome P450 isoenzymes indicating the ability of mononuclear phagocytes to metabolize a variety of drugs including allergens. Here we analyzed the mRNA expression of transport proteins involved in the export of drugs (multidrug resistance associated proteins 1 to 6, MRP) in blood monocyte derived macrophages compared to primary liver cells by using RT-PCR. We found that all known MRP transport proteins were present in macrophages as well as in the liver except for MRP2, which seems to be specific for the liver tissue. These results show that human macrophages express various transport-associated and detoxifying metabolic enzymes. These findings are interesting because previous studies have revealed that CYP-enzymes and transport-associated proteins play complementary roles in drug disposition by biotransformation (phase I) and antitransport (phase III).