Cytokine-inducible SH2 protein up-regulation is associated with desensitization of GH signaling in GHRH-transgenic mice

The effects of continuous high GH levels on GH signal transduction through the GH receptor (GHR)/Janus kinase 2 (JAK2)/signal transducer and activator of transcription 5 (STAT5) pathway as well as the desensitization of this pathway by suppressors of cytokine signaling (SOCS) were studied in transgenic mice overexpressing GHRH. In transgenic mice, hepatic GHR levels were 4.5-fold higher than in normal animals, whereas the protein contents of JAK2, STAT5a, and STAT5b did not vary. This same pattern was found for basal tyrosine phosphorylation (PY-): PY-GHR was 4.5-fold increased in transgenic mice, whereas there were no differences in PY-JAK2 and PY-STATs between normal and transgenic animals. After GH administration, tyrosine phosphorylation of GHR, JAK2, and STAT5s increased 3- to 7-fold in normal mice, but no significant changes were found in transgenic mice, indicating a decreased GH sensitivity in these animals. The content of cytokine-inducible SH2 protein, a member of the SOCS family, was 18-fold higher in GHRH-transgenic than in normal mice. Conversely, SOCS-3, present in normal mice, was hardly seen in transgenic animals, whereas SOCS-2 levels did not vary. These findings suggest that cytokine-inducible SH2 protein, significantly induced by continuously elevated GH levels, may be the SOCS protein responsible for the GH signaling desensitization in transgenic animals.