Cell-specific expression of the prolactin gene in transgenic mice is controlled by synergistic interactions between promoter and enhancer elements.

Prolactin gene expression is restricted to the lactotrophic and somatomammotrophic cells of the anterior pituitary. In transgenic mice, a fusion gene consisting of 3 kb of prolactin 5'-flanking region fused to a firefly luciferase or human growth hormone (hGH) reporter gene is expressed at high levels with the strict tissue and cell-type specificity that is characteristic of the endogenous prolactin gene. High levels of expression require two cis-acting regions: a distal enhancer (-1.8 to -1.5 kb) and a proximal region (-422 to +33 bp). Each of these regions alone can direct low levels of fusion gene expression to prolactin-producing cell types in transgenic mice, but a synergistic interaction between these regions is necessary for high levels of expression. The ontogeny of the prolactin transgene expression closely follows the appearance of high levels of a POU homeo-domain transcription factor, Pit-1, that has been shown previously to bind structurally related sequences in both the distal enhancer and proximal regions and to activate the expression of the prolactin gene in vitro. Unexpectedly, transgenes containing the distal enhancer removed from its normal context are expressed in both the prolactin-producing lactotrophs and the thyroid-stimulating hormone (TSH)-producing thyrotrophs, thereby suggesting that sequences flanking this enhancer are necessary to restrict expression to the correct cell type within the pituitary. These data indicate that distinct processes of gene activation and restriction are necessary for the fidelity of cell-type-specific expression within an organ.