PROMOTER REGION OF THE HUMAN GENE CODING FOR BETA-CHAIN OF C4B BINDING-PROTEIN - HEPATOCYTE NUCLEAR FACTOR-III AND NUCLEAR FACTOR-I CTF TRANSCRIPTION FACTORS ARE REQUIRED FOR EFFICIENT EXPRESSION OF C4BPB IN HEPG2 CELLS

Differential expression of the human genes coding for the alpha and be ta polypeptides of the human C component C4b binding protein (C4BP) mo dulates the levels of C4BP molecules containing C4BP beta polypeptides , providing a mechanism to avoid the potential harmful effects of elev ated concentrations of C4BP beta in plasma. To understand how the expr ession of the C4BPB gene is controlled, we have examined, in the major promoter of the human C4BPB gene, potential regulatory elements. A re gion from nucleotide -126 to +25 was able to drive high expression of a reporter gene in the human hepatoma cell line HepG2. A small subfrag ment of this region (from -126 to -90) is responsible for more than 90 % of the promoter activity. Electrophoretic mobility shift assays reve aled that transcription factors of the hepatocyte nuclear factor-3 (HN F-3) and nuclear factor-I (NF-I/CTF) families were able to bind to thi s region in a sequence-specific manner. We have characterized binding sites for these transcription factors and determined their relative co ntribution to the activity of the C4BPB promoter. The results suggest that cooperative interaction between HNF-3 and NF-I/CTF is required to obtain a full C4BPB promoter activity. Comparison of the structures o f the C4BPA and C4BPB promoters reveals significant differences that c ould explain the differential transcription of the C4BP alpha and C4BP beta polypeptides during the acute phase response.