MEDIATION OF GROWTH HORMONE-DEPENDENT TRANSCRIPTIONAL ACTIVATION BY MAMMARY-GLAND FACTOR STAT-5

Previous observations have shown that binding of growth hormone to its receptor leads to activation of transcription factors via a mechanism involving phosphorylation on tyrosine residues. In order to establish whether the prolactin-activated transcription factor Stat 5 (mammary gland factor) is also activated by growth hormone, nuclear extracts we re prepared from COS-7 cells transiently expressing transfected Stat 5 and growth hormone receptor cDNA. Gel electrophoresis mobility shift analyses revealed the growth hormone-dependent presence of specific DN A-binding proteins in these extracts, The complexes formed could be su pershifted by polyclonal anti-Stat 5 antiserum. In other experiments n uclear extracts from growth hormone-treated Chinese hamster ovary cell s stably expressing transfected growth hormone receptor cDNA and liver from growth hormone-treated hypophysectomized rats were used for gel electrophoresis mobility shift analyses. These also revealed the prese nce of specific DNA-binding proteins sharing antigenic determinants wi th Stat 5. Stat 5 cDNA was shown to be capable of complementing the gr owth hormone-dependent activation of transcription of a reporter gene in the otherwise unresponsive COS-7 cell line. This complementation wa s dependent on the presence of Stat 5 tyrosine 694, suggesting a role for phosphorylation of this residue in growth hormone-dependent activa tion of DNA-binding and transcription.