Expression of the human PDGF‐B gene is regulated by both positively and negatively acting cell type‐specific regulatory elements located in the first intron

Potential cis‐acting regulatory elements of the human platelet derived growth factor‐B (PDGF‐B) gene were identified by DNase I hypersensitive site mapping. The transcription unit was examined for the presence of hypersensitive sites in chromatin DNA isolated from human term placental cytotrophoblasts, human placental fibroblasts, the JEG‐3 choriocarcinoma cell line and the U2‐OS osteosarcoma cell line. A number of cell type‐specific hypersensitive sites were identified, all within the 1st intron. Transient transfection of JEG‐3 cells with CAT constructs containing regions of the c‐sis 1st intron linked to the basal c‐sis promoter identified a cell type‐specific positive regulatory activity within the intron, composed of at least two distinct elements. One element appeared to be specific for JEG‐3 cells, while the other was also active in U2‐OS cells. The overall positive regulatory activity of the 1st intron region was specific for JEG‐3 cells, but did not function as a classically defined enhancer, as it was orientation‐dependent (unless stably integrated into chromatin DNA). In addition, the activator appears to require interaction with the c‐sis promoter, as little or no activation was seen when either the SV40 or human beta‐globin promoters were substituted for the c‐sis promoter. The 1st intron also contained a negative regulatory element, which was specific for U2‐OS cells and silenced an abnormally high basal c‐sis promoter activity in these cells. The complexity of the transcriptional control of the PDGF‐B gene is discussed.