This project focuses on human growth hormone receptor (GHR) activation and deactivation as a model for understanding the mechanisms and components which play a role in signaling through this receptor class (including the receptors for several cytokines and growth factors). Growth hormone (GH), as well as many other cytokines, regulates gene expression by the tyrosine phosphorylation of DNA binding factors known as STATs (Signal Transducers and Activators of Transcription). The activated STATs then bind to enhancers present in the promoters of specific genes. Our studies have shown that the treatment of human peripheral blood monocytes with interleukin-3 (IL-3), interleukin-5 (IL-5), interleukin-10 (IL-10), granulocyte-macrophage colony stimulating factor, prolactin and the IFNs, as well as GH, differentially activates DNA-binding proteins which recognize the same IFN gamma response region (GRR) located in the promoter of the high affinity Fcgamma receptor (FcgammaRI) gene. Furthermore, we have shown that the activation of STATs and receptor- associated tyrosine kinases occurs through the receptor. For my studies, I have used murine cell lines which are stably transfected with the full length or mutated GHR to identify sequences in the cytoplasmic domain of this receptor which are required for the activation of both the GRR binding complex and the GHR-associated tyrosine kinase, Jak2. Further immunological studies with these cell lines and antibodies to various putative components of the GH signaling cascade were used to map the sequences of the cytoplasmic domain of the GHR which are required for growth hormone signaling. Most recently, I have delineated a region of the GHR cytoplasmic domain which is needed to turn-off GH signaling. In addition, I have identified a protein tyrosine phosphatase which specifically associates with this receptor sequence. These studies are providing a broader understanding of the components involved in signaling through cytokine receptors and, consequently, the mechanisms of cytokine stimulated gene activation.