Pit-1 Binding Sites at the Somatotrope-specific DNase I Hypersensitive Sites I, II of the Human Growth Hormone Locus Control Region Are Essential for in Vivo hGH-N Gene Activation

Abstract

The human growth hormone gene cluster is composed of five closely related genes. The 5'-most gene in the cluster, hGH-N, is expressed exclusively in somatotropes and lactosomatotropes of the anterior pituitary. Although the hGH-N promoter contains functional binding sites for multiple transcription factors, including Sp1, Zn-15, and Pit-1, predictable and developmentally appropriate expression of hGH-N transgenes in the mouse pituitary requires the presence of a previously characterized locus control region (LCR) composed of multiple chromatin DNase I hypersensitive sites (HS). LCR determinant(s) necessary for hGH-N transgene activation are largely conferred by two closely spaced HS (HS I,II) located 14.5 kilobase pairs upstream of the hGH-N gene. The region sufficient to mediate this activity has recently been sublocalized to a 404-base pair segment of HS I,II (F14 segment). In the present study, we identified multiple binding sites for the pituitary POU domain transcription factor Pit-1 within this segment. Using a transgenic founder assay, these sites were shown to be required for high level, position-independent, and somatotrope-specific expression of a linked hGH-N transgene. Because the Pit-1 sites in the hGH-N gene promoter are insufficient for such gene activation in vivo, these data suggested a unique chromatin-mediated developmental role for Pit-1 in the hGH LCR.