TISSUE DISTRIBUTION OF THE HUMAN MDR3 P-GLYCOPROTEIN

BACKGROUND: P-glycoproteins (Pgps) belong to a family of well conserve d plasma membrane proteins with two members in humans: MDR1 and MDR3. The MDR1 Pgp can transport drugs; the murine homologue of MDR3, mdr2, was recently shown by us to be involved in transport of the phospholip id phosphatidylcholine (lecithin) into bile. EXPERIMENTAL DESIGN: We h ave determined the MDR3 mRNA levels in a panel of human tissues by RNa se protection. We have also generated polyclonal antibodies specific f or the MDR3 Pgp. Detection of the MDR3 Pgp in human tissues with these antibodies was by a streptavidin-ABC procedure. RESULTS: The RNase pr otection results show that expression of the MDR3 gene has a more rest ricted distribution than that of MDR1. A high level of MDR3 mRNA was d etected in the liver and in low levels in the adrenal gland, heart, st riated muscle, spleen, and tonsil. In all of these tissues, some of th e previously described splice variants of MDR3 were abundantly express ed. No indications were found for a tissue-specific regulation of alte rnative splicing of the MDR3 pre-mRNA. Two MDR3 Pgp-specific antibodie s stained the bile canalicular membrane of hepatocytes across the enti re liver lobule. No staining was found in the epithelial cells of the bile ductules and gall bladder, indicating that the staining at these sites with C219, a monoclonal antibody that recognizes both MDR1 and M DR3 Pgp, (mainly) represents the MDR1 Pgp. No MDR3 was detected by spe cific antibodies in the adrenal gland, spleen, and muscle. Since no st aining was reported with MDR1-specific antibodies in muscle either, ou r results indicate that the C219 staining in some fibers of striated m uscle represents a cross-reaction with another protein. One of the hum an MDR3-specific antibodies cross-reacted with the highly homologous m ouse mdr2 Pgp. Staining with this antibody showed that the distributio n of this protein in mouse liver and striated muscle is very similar t o that of MDR3 Pgp in human tissues. CONCLUSIONS: The highest expressi on of the MDR3 Pgp was found in liver in the canalicular membranes of hepatocytes. This is in agreement with a role for MDR3 in the transpor t of phospholipid into bile.