RAPID CHANGES IN NUCLEAR-PROTEIN TYROSINE PHOSPHORYLATION AFTER GROWTH-HORMONE TREATMENT IN-VIVO - IDENTIFICATION OF PHOSPHORYLATED MITOGEN-ACTIVATED PROTEIN-KINASE AND STAT91

Growth hormone (GH) plays a central role in regulating growth and inte rmediary metabolism in vertebrates, although the mechanisms by which G H initiates these actions are largely unknown. The GH receptor, a memb er of the cytokine receptor superfamily, does not demonstrate homology with any known tyrosine kinases. However, addition of GH to cells in vitro has been shown to stimulate tyrosine phosphorylation of various intracellular proteins including mitogen-activated protein kinases (MA P kinases) and the newly described Janus kinase, JAK2. Subsequent step s in GH-mediated signal transduction have not been delineated. In the present study, we have examined early events in GH action in vivo. Hyp ophysectomized juvenile male rats were treated with GH for 15, 30, or 60 min. Rat liver whole cell and nuclear extracts were prepared and an alyzed via SDS-polyacrylamide gel electrophoresis and Western blotting techniques. GH rapidly stimulated the tyrosine phosphorylation of at least 8 nuclear proteins of 205, 91, 83, 80, 65, 53, 44, and 42 kDa, a nd caused the dephosphorylation of a single similar to 149-kDa protein . Using specific antibodies, we have identified three of these nuclear phosphoproteins as 42- and 44-kDa MAP kinases, and as STAT91, a 91-kD a component of the interferon-stimulated gene factor-3 protein complex . One consequence of the activation of STAT91 in the nucleus is the ap pearance of GH-stimulated DNA binding activity, as assessed by gel-mob ility shift assay using an oligonucleotide containing a c-sis-inducibl e element from the c-fos prometer. These results show that nuclear pro tein tyrosine phosphorylation is a prominent early event in GH action in vivo and demonstrate a link between GH stimulated signal transducti on and target gene expression.