Regulation of STAT5 activity by growth hormone

Sammanfattning: The growth hormone (GH) receptor is a member of the cytokine receptor superfamily. This receptor family has certain common characteristics, among them the absence of a canonical tyrosine kinase consensus sequence and the association with the Janus family of cytosolic tyrosine kinases (JAK). The predominant JAK utilized by the GH receptor is JAK2. GH has been reported to stimulate activation of several intracellular signal mediators, among them signal transducer and activator of transcription (STAT) factors and MAP kinases. STAT proteins are latent cytoplasmic transcription factors. Six mammalian STAT proteins have been described. Upon tyrosine phosphorylation, the STAT molecules dimerize, translocate to the nucleus, bind to their appropriate DNA response elements and stimulate transcription. GH has been shown to utilize STAT1, 3 and 5 for regulation of a variety of genes. Activation of STAT molecules has also been demonstrated to require serine phosphorylation, in addition to tyrosine phosphorylation, for full transcriptional activity. STAT1[alpha], 3 and 5a possess putative MAP kinase recognition consensus sequences in their C- terminal part. MAP kinases are serine/threonine kinases with important roles in regulating several different cellular events evoked by extracellular signals, e.g. stimulation by polypeptide growth factors. MAP kinases are involved in serine phosphorylation of several transcription factors. We have investigated the interaction of STAT5 with JAK2, the involvement of MAPK pathway in serine phosphorylation of STAT5 and the effect of serine phosphorylation on STAT5 activity. Furthermore, we have analysed the effect of GH on the actin cytoskeleton and the importance of this cellular structure for STAT5 activity. Finally, we have studied nuclear STAT5 deactivating mechanism. We found that interaction between STAT5 and JAK2 only occurs between non-tyrosine phosphorylated STAT5 and active JAK2. We found that phosphorylation of serine 780 of STAT5a by the MAP kinase, extracellular signal regulated kinase (ERK), is important for full transcriptional activity of STAT5a. We also demonstrated that STAT5a and ERK form a complex prior to GH stimulation of cells. We could show that GH induces rapid reorganisation of actin cytoskeleton and that this cytoskeletal GH-response is not required for STAT5-mediated transcriptional response to GH. We also detected constitutive active nuclear tyrosine phosphatase activity which deactivates STAT5 by dephosphorylation. We could also show that this phosphatase activity is transiently enhanced by GH.