Growth hormone (GH) initiates biological actions by interacting with GH receptor (GHR), which activates the cytoplasmic tyrosine kinase, JAK2, and signaling molecules including signal transducer and activator of transcription (STAT)-5. GH-induced STAT5 activity enhances transcription of GH-responsive genes, including insulin-like growth factor (IGF)-1. IGF-1 interacts with cell surface IGF-1 receptor (IGF-1R), a disulfide-linked heterotetramer, causing activation of its tyrosine kinase, autophosphorylation, and intracellular signals. The somatomedin hypothesis of GH action holds that GH-induced IGF-1 mediates many of GH's somatogenic actions. Our proposal addresses the idea that, in addition to the linear GH? GHR ?IGF-1 ?GF-1R pathway, IGF-1R also subserves GH signaling in several novel ways. Our preliminary data in osteoblasts, preadipocytes, and human cancer cells indicate that: 1) deletion of IGF-1R diminishes acute GH signaling and reconstitution with kinase-deficient IGF-1R undoes this defect; 2) cotreatment with IGF-1 plus GH enhances GH responsiveness; and 3) GH promotes association of IGF-1R with the GHR-JAK2 complex. Collectively, these data suggest novel molecular mechanisms by which GH and IGF-1 collaborate. Exploring these mechanisms is of fundamental significance, especially as GH and IGF-1 signaling pathways serve both independent and overlapping growth functions in vivo and disruption of the GH-IGF-1 axis reduces incidence and aggressiveness of experimental cancers. We hypothesize: 1) GH action is facilitated by IGF-1R's presence in GH target cells such that, even without IGF-1, IGF-1R acts a proximal element in GH signaling; further, IGF-1 augments GH signaling, likely by decreasing the threshold for GH-induced GHR triggering. 2) These effects of IGF-1R and IGF-1 on GH signaling relate to the GH-induced formation of a GHR/JAK2/IGF-1R complex. Our specific aims are to: 1.Define IGF-1R determinants that allow enhanced GH signaling. 2. Investigate determinants of GH-induced GHR/JAK2/IGF-1R complex formation. 3. Define molecular determinants and mechanisms of IGF-1 enhancement of GH signaling. 4. Examine consequences of GHR-IGF-1R collaboration in in vivo systems. These studies pursue a novel hypothesis about how GHR and IGF-1R interact. Completion of the aims will allow new insights into how GH uses the IGF-1R to enable GH actions and how GH and IGF-1 collaborate to optimize GH effects. These insights have potential relevance to the endocrinology of growth and the behavior of human cancers.