A role for A/T-rich sequences and Pit-1/GHF-1 in a distal enhancer located in the human growth hormone locus control region with preferential pituitary activity in culture and transgenic mice.

A region located remotely upstream of the human pituitary GH (GH-N) gene and required for efficient GH-N gene expression in the pituitary of transgenic mice was cloned as a 1.6-kb Bg/II (1.6G) fragment. The 1.6G fragment in the forward or reverse orientation increased -496GH-N promoter activity significantly in pituitary GC and GH3 cells after gene transfer. The 1.6G fragment was also able to stimulate activity from a minimal thymidine kinase (TK) promoter which, unlike -496GH-N, lacked any Pit-1/GHF-1 element. Enhancer activity was localized by deletion analysis to a 203-bp region in the 3'-end of the 1.6G fragment and was characterized by the presence of a diffuse 136-bp nuclease-protected site, observed with pituitary (GC) but not nonpituitary (HeLa) cell nuclear protein. A major low-mobility complex was observed by electrophoretic mobility shift assay (EMSA) with GC cell nuclear protein, and the pattern was distinct from that seen with a HeLa cell extract. The nuclease-protected region contains three A/T-rich Pit-1/ GHF-1-like elements, and their disruption, in the context of the 203-bp region fused to the TK promoter, reduced enhancer activity significantly in pituitary cells in culture. A mutation in this region was also shown to decrease enhancer activity in transgenic mice and correlated with a decrease in the 203-bp enhancer region complex observed by EMSA. The participation of Pit-1/GHF-1 in this complex is indicated by competition studies with Pit-1/GHF-1 elements and antibodies, and direct binding of Pit-1/GHF-1 to the A/T-rich sequences was shown by EMSA using recombinant protein. These studies link the A/T-rich sequences to the distal enhancer activity associated with the GH locus control region in vitro and in vivo.     

Phylogenetic specificity of prolactin gene expression with conservation of Pit-1 function.

In mammals, the pituitary POU homeodomain protein, Pit-1, binds to proximal and distal 5'-flanking sequences of the PRL gene that dictate tissue-specific expression. These DNA sequences are highly conserved among mammals but are dramatically different from PRL 5' sequences in the teleost species, Oncorhynchus tschawytscha (chinook salmon). To analyze the molecular basis for pituitary-specific gene expression in a distantly related vertebrate, we transfected CAT reporter gene constructs containing 2.4 kilobases (kb) 5'-flanking sequence from the salmon PRL (sPRL) gene into various cell types. Expression of the sPRL gene was restricted to pituitary cells, but in rat pituitary GH4 cells levels of expression were at least 90-fold lower than those obtained with a -3 kb rat PRL (rPRL) construct. Conversely, in primary teleost pituitary cells, -2.4 kb sPRL/CAT was expressed at levels about 10-fold higher than -3 kb rPRL/CAT. To determine whether species-specific transactivation by Pit-1 was sufficient to explain these species differences in PRL gene expression, we isolated a cDNA clone encoding the salmon Pit-1 POU domain and constructed a rat Pit-1 expression vector that contained salmon Pit-1 POU domain sequences substituted in frame. The chimeric Pit-1 encoded 14 amino acids unique to salmon. Coexpression of rat Pit-1 with salmon or rat PRL/CAT in transfected HeLa cells resulted in specific and strikingly comparable levels of promoter activation. Moreover, the specificity and efficacy of the chimeric salmon/rat Pit-1 was similar to wild type rat Pit-1 in activating salmon and rat PRL/CAT.(ABSTRACT TRUNCATED AT 250 WORDS)