GROWTH HORMONE SIGNALING TO THE NUCLEUS

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Growth is associated with complex genetic programs involving changes in the expression of many genes. Growth hormone (GH), a key regulator of normal growth, elicits changes in gene expression. In examining the mechanisms by which GH regulates gene transcription using c-fos as a model, we proposed that GH regulated multiple DNA sequences in c fos, and have now identified four: the Sis-Inducible Element (SIE) which binds Signal Transducers and Activators of Transcription (Stats) 1 and 3 in response to GH, the Serum Response Element (SRE) which requires Serum Response Factor (SRF) and Elk-1 to mediate GH- promoted transcription, an AP-1 site which can bind c-Fos and c- Jun; and a CCAAT/Enhancer Binding Protein (C/EBP) site which binds C/EBPbeta and delta. By analyzing regulation by GH of the transcription factors associated with these complexes, we have so far identified three distinct signalling pathways between the GH receptor (GHR) and the nucleus, mediated by Stats by Mitogen Activated Protein Kinases (MAPKs) and by phosphatidyl inositol 3' kinase (PI3K). The present proposal takes these findings as a point of departure to investigate the hypothesis that GH- regulated transcription involves formation of higher order transcription factor complexes and related chromatin remodelling. It is proposed to examine how the transcription factors on GH- regulated genes are coordinated into higher order complexes, and how co-activators and co-repressors participate in such complexes to bring about chromatin remodelling that contributes to transcriptional activation in response to GH. The first part of this proposal delineates the regulation and importance of the phosphorylation state of C/EBPbeta in GH-regulated transcription of c fos. C/EBPbeta is particularly informative because it interacts with many transcription factors and co-activators. Thus, C/EBPbeta provides a starting point for identifying higher order complexes in GH-regulated c-fos expression. Since Stat 5 participates in the regulation of several GH-regulated genes, some of which contain C/EBPbeta binding sites, the participation of Stat 5 in higher order transcription factor complexes regulated by GH will be examined. The assembly of such higher order complexes is likely to involve co-activator and/or co- repressor molecules which can interact with many of the individual transcription factors which GH regulates. Participation of co-activators and co-repressors and the resultant chromatin remodelling in GH-regulated transcription complexes will be examined. The proposed studies will provide critical information on mechanisms for GH-regulated gene transcription, for specificity in assembly of transcription complexes in response to GH on particular genes, and on coordinated regulation of gene expression applicable to multiple genes. By adding to our understanding of fundamental mechanisms for regulation of normal growth, they provide a basis for understanding aberrant cellular growth, as in cancer, and for design of rational therapies toward growth control.