Cytokine-Inducible SH2 Protein Up-Regulation Is Associated with Desensitization of GH Signaling in GHRH-Transgenic Mice

Abstract

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.

Increased sensitivity to GH in liver of Ames dwarf (Prop1df/Prop1df) mice related to diminished CIS abundance

“…One main broad band with an apparent molecular mass of approximately 108 kDa was observed, in accordance with previous reports from our laboratory (González et al 2002) and the expected molecular mass for the full-length form of the receptor in rodents (Frick et al 1998). Densitometric analysis of autoradiographies showed no significant differences between normal and dwarf mice (Fig.…”

“…The content of these SOCS proteins in normal and dwarf mice is similar, suggesting that the expression of these suppressors is not affected by the lack of GH. We have previously reported that transgenic mice overexpressing GH show a reduction in the levels of SOCS-3, while no changes in SOCS-2 protein content were detected, compared with normal mice (González et al 2002, Miquet et al 2004. Taken together, these results suggest that, under chronic exposure, GH does not positively regulate the protein expression of these SOCS in vivo, as the high continuous levels in GH-overexpressing mice are not mirrored by a rise of the suppressors, nor is the lack of GH accompanied by a diminution of these proteins.…”

“…We found an almost complete inhibition of GHstimulated tyrosine phosphorylation of JAK2, STAT5a and STAT5b in GH-overexpressing mice. This desensitization was mainly related to a dramatic increase of CIS protein content in transgenic animals (González et al 2002, Miquet et al 2004. CIS is an effective and specific negative regulator of GH-induced STAT5 activity.…”

“…Recently, we described that the JAK2/STAT5 pathway of GH signaling is desensitized in two different lines of transgenic mice overexpressing GH, an effect that is mainly related to the markedly high CIS levels found in the liver of these mice (González et al 2002, Miquet et al 2004. CIS acts as a negative modulator by competing with STAT5 for common phosphotyrosine-binding sites on the GHR intracellular domain (Ram & Waxman 2000).…”

Constitutive expression of suppressor of cytokine signalling-3 in skeletal muscle leads to reduced mobility and overweight in mice

“…Thus, a significant decrease in Socs-2 and Cis was found. So far, SOCS-2 and CIS have been implicated mainly in regulating postnatal growth [34,35] and very little is known about their role in skeletal muscle metabolism. However, very recent data strongly suggest that SOCS-2 is a key actor in modulating insulin signal in muscle.…”