Definition of the Elements Required for the Activity of the Rat Aromatase Promoter in Steroidogenic Cell Lines

Abstract

The rate limiting step in estrogen biosynthesis is catalysed by an enzyme complex that includes the aromatase cytochrome P450 (CYP19), and regulation of the synthesis of this steroidogenic P450 is the level at which estrogen synthesis is controlled. In the rat, initiation of aromatase mRNA transcription occurs immediately 5′ to the initiator methionine (proximal promoter) in the rat ovary and in two rat Leydig tumor cell lines that express high levels of aromatase (R2C and H540). Although the same site of transcription initiation is employed in both the Leydig tumor cells and in granulosa cells, the patterns of aromatase expression are distinctive. To define the mechanisms controlling aromatase expression in these model cell lines, we have studied the proximal promoter of the rat aromatase using transfection and gel mobility shift assays. These experiments indicate that the SF-1 motif is required for the expression of the reporter gene in each steroidogenic cell line, and that different combinations of CRE-like elements (particularly those located at −169 and −231) are required for full promoter activity in each steroidogenic cell line. In keeping with the results of the functional assays, we were able to demonstrate the binding of nuclear proteins from each of the Leydig tumor cells to the SF-1 motif in mobility shift assays. Nuclear proteins which bind to the CRE-like elements at −169 and −231 were detected in each of the steroidogenic cell lines, but different complexes were observed using extracts from the Leydig tumor cell lines compared to those visualized using extracts prepared from Y1 cells, an adrenocortical tumor cell line that expresses low levels of aromatase activity. Our findings suggest that, in these model steroidogenic cell lines, differences in the proteins that bind to different combinations of elements within the rat aromatase promoter are responsible for the different patterns and levels of aromatase expression which are observed.