A NOVEL GROWTH-HORMONE RESPONSE ELEMENT UNRELATED TO STAT (SIGNAL TRANSDUCER AND ACTIVATOR OF TRANSCRIPTION)-BINDING SITES IS A BIFUNCTIONAL ENHANCER

Regulation of gene expression by GH has so far been shown to be mediat ed by a few cis-acting elements, most of which are signal transducer a nd activator of transcription (STAT)-binding sites. Here we have chara cterized a novel GH-response element present in the promoter of rat se rine protease inhibitor (spi) genes. It consists of a 13 nucleotide-lo ng GAGA box containing two GAGGAG repeats separated by a G, structural ly unrelated to STAT-binding sites. In hepatocytes, the spi GAGA box b ehaves as a position-dependent bifunctional enhancer controlling basal and GH-dependent transcription. In addition, spi GAGA box oligonucleo tides inhibit cell-free transcription driven by GAGA box-containing as weft as GAGA box-less promoters, suggesting that the spi GAGA box int eracts directly or indirectly with component(s) of the basic transcrip tional machinery. Mobility shift assays showed that this GAGA box is s pecifically recognized by nuclear factors that are unrelated to previo usly characterized proteins binding to purine-rich elements or to OH-a ctivated STATs. Finally, experiments performed with cells expressing w ild type, truncated, or mutated forms of the GH receptor indicate that protein kinase Janus kinase 2 is involved in the OH-dependent activat ion of the spi GAGA box. These studies reveal the existence of an as y et unidentified Janus kinase-2-dependent, STAT-independent pathway in GH activation of gene expression.