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