CHARACTERIZATION OF THE PROMOTER REGION OF THE HUMAN MDR3 P-GLYCOPROTEIN GENE

The human MDR3 (or MDR2) P-glycoprotein is probably involved in the tr ansport of phospholipids from liver hepatocytes into bile (Smit et al. (1993) Cell 75, 451-462). In accordance with this function, MDR3 is h ighly expressed in human liver, but lower mRNA levels were also found in adrenal, heart, muscle and cells of the B-cell compartment. We have cloned and analyzed the MDR3 promoter region. It is GC-rich, and cont ains nether a TATA nor a CAAT box, but it does contain multiple putati ve SP1 binding sites, features also found in so-called housekeeping ge nes. RNase protection and primer extension analyses indicate that the MDR3 gene has multiple transcription start sites in a GC-rich region w ith considerable homology to the putative mouse mdr2 promoter. A 3 kb genomic fragment containing the MDR3 start sites directs transcription of a chloramphenicol acetyltransferase (CAT) reporter gene upon trans ient transfection in the human hepatoma cell line HepG2. This transcri ption is orientation dependent, and stimulated by a SV40 enhancer, ind icating that the 3 kb insert contains the core promoter elements of th e MDR3 gene. The promoter region contains several consensus sequences where known or putative liver-specific (C/EBP, HNF5) or lymphoid speci fic (Pu.1, ets-1) transcription factors may bind.