TRANSCRIPTIONAL REGULATION OF INSULIN-LIKE GROWTH-FACTOR-II GENE-EXPRESSION BY CORTISOL IN FETAL SHEEP DURING LATE-GESTATION

The objective of this study was to determine the mechanisms by which c ortisol down-regulates hepatic insulin-like growth factor-II (IGF-II) gene expression in late gestation. Leader exons 6 and 7 of the ovine I GF-II gene, with their 5'-flanking regions, were first isolated. Chara cterization of transcription start sites revealed a unique site for ex on 6 and three dispersed sites for exon 7. Nuclear run-on assays showe d a 5-fold higher transcription rate of the IGF-II gene in liver of ad renalectomized fetuses compared with control animals, suggesting that regulation of IGF-II gene expression by cortisol is at the transcripti onal level. RNase protection assays demonstrated hepatic leader exon 7 expression in adrenalectomized fetuses to be more than a-fold higher than in controls, whereas it was reduced by 50% in cortisol-infused fe tuses compared with controls. There was no effect on the expression of other leader exons. Functions of the upstream regulatory region of le ader exon 7 (i.e. promoter P4) were investigated by luciferase transie nt expression. A region of -172 bases downstream relative to the first transcription site of leader exon 7 was shown to retain basal promote r activity and respond to cortisol. These results suggest that cortiso l may induce the prenatal decline in ovine hepatic IGF-II expression b y suppressing promoter P4 of the IGF-II gene.