Visualizing and Manipulating Growth Hormone (GH) Responses in Muscle and Other GH Target Tissues

Muscle cells, like almost all other cells in the body, show growth and metabolic responses to growth hormone (GH). Although GH actions are classically thought to be mediated via insulin-like growth factor I (IGF-I), it is now recognized that GH has direct actions via the GH receptor (GHR) expressed in many tissues, including muscle. The aim of this work was to study, at an individual cell and tissue level, those cells that respond directly to GH, and understand how their responsiveness to GH is regulated. We used immunocytochemistry to visualize the cellular expression of components of the major GH signal transduction cascades, GHR, Janus kinase 2 (JAK2), signal transducer and activator of transcription 5b (Stat5b) and suppressor of cytokine signalling 2 (SOCS2), and monitored their activation by detecting GH-induced phosphorylation after a single injection of GH in GH-deficient rodents. GHR, JAK2, Stat5b and SOCS2 proteins were all found to be present in skeletal muscle from normal and GH-deficient mice, and phosphorylated Stat5b increased in heart and skeletal muscle nuclei after a GH injection. In GH-deficient rats, continuous GH infusion increased Stat5b phosphorylation in liver but not in muscle, but blunted the further effects on Stat5b of acute GH injection in both tissues. Extracellular signal-regulated kinase (ERK), SOCS2 and cytokine-inducible SH2 domain-containing protein (CIS) are also activated by GH in different tissues, and other studies have implicated cross-talk with insulin/IGF-I transduction pathways. Activation of several of these pathways could be traced using these methods, which will shed light on the molecular mechanisms that regulate GH responsiveness in individual cells in complex tissues.