Suppression of Growth Hormone (GH) Janus Tyrosine Kinase 2/Signal Transducer and Activator of Transcription 5 Signaling Pathway in Transgenic Mice Overexpressing Bovine GH

Abstract

High continuous GH levels in vivo produce desensitization of the Janus tyrosine kinase 2 (JAK2)/signal transducer and activator of transcription 5 (STAT5) pathway of GH signaling in the liver. To evaluate the mechanisms involved in this desensitization, transgenic mice overexpressing bovine GH were used. In these animals, GH receptor and membrane-associated JAK2 kinase are increased 4.5- and 6-fold, respectively. However, JAK2. STAT5a and -5b do not become tyrosine phosphorylated in response to GH stimulus, nor are these STAT proteins recruited to membranes, suggesting that they cannot bind to the receptor. The content of the suppressor cytokine-inducible src homology 2 (SH2)-containing protein (CIS), both total and membrane-associated, is markedly increased in the liver of GH transgenic mice. This could account for the inhibition of STAT5 activation, because CIS competes with STAT5 for GH receptor docking sites. Existence of an alternative mechanism of negative regulation of this signaling pathway by chronically elevated GH levels is suggested by the low level of JAK2 phosphorylation that transgenic mice exhibit. Whereas total SH2-containing phosphatase 2 (SHP-2) content is the same in both kinds of mice, membrane-associated SHP-2 protein levels increase 4.5-fold in GH transgenic animals. This could explain the dramatic inhibition of JAK2 phosphotyrosine level, thus contributing to the suppression of GH signaling observed in these transgenic mice.

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

“…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.…”

“…05-495) were obtained from Upstate Laboratories (Lake Placid, NY, USA). Anti-GHR antiserum ( GHR) was produced in our laboratory (Miquet et al 2004), and anti-SH2-B antibody ( SH2-B) was kindly provided by Dr D D Ginty (Johns Hopkins University School of Medicine, Baltimore, MD, USA) (Qian et al 1998). All other chemicals were of reagent grade.…”

“…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).…”

Growth hormone (GH) modulation of Epidermal growth factor (EGF) signalling in human mammary epithelial cells

“…Cell membrane P P P P Raf main GH signalling pathway JAK-2/STAT5 was desensitized, assuming that the JAK-2/STAT-5 pathway is not involved in proliferative effects of overexpressed GH (Gónzalez et al, 2002, Miquet et al 2004.…”