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

Abstract

To investigate the influence of chronic GH deficiency on GH signaling in vivo, we have analyzed Janus kinase (JAK) 2/signal transducers and activators of transcription (STAT) 5 GH signaling pathway, and its regulation by the suppressors of the cytokine signaling SOCS and by the JAK2-interacting protein SH2-B , in liver of Ames dwarf (Prop1 df /Prop1 df ) mice, which are severely deficient in GH, prolactin and TSH, and of their normal littermates. Prop1 df /Prop1 df mice displayed unaltered GH receptor, JAK2 and STAT5a/b protein levels. No significant differences in the basal tyrosine-phosphorylation levels of JAK2 and STAT5a/b were found between both groups of animals. After in vivo administration of a high GH dose (5 µg/g body weight (BW)), the tyrosine-phosphorylation levels of JAK2 and STAT5a/b increased significantly, reaching similar values in normal and dwarf mice.However, after stimulation with lower GH doses (50 and 15 ng/g BW) the tyrosine-phosphorylation level of STAT5a/b was higher in dwarf mice. The protein content of CIS, a SOCS protein that inhibits STAT5 signaling, was approximately 80% lower in dwarf mice liver, while SOCS-2 and SOCS-3 levels were unaltered. The content of SH2-B , a modulator of JAK2 activity, was reduced by approximately 30% in dwarf mice, although this was associated with normal JAK2 response to a high GH dose. In summary, Prop1 df /Prop1 df mice display increased hepatic sensitivity to GH, an effect that could be related to the lower abundance of CIS in this tissue. Furthermore, the lower CIS content found in this model of GH deficiency suggests that CIS protein levels are regulated by GH in vivo.

Ames dwarf (Prop1df/Prop1df) mice display increased sensitivity of the major GH-signaling pathways in liver and skeletal muscle

“…Furthermore, in dwarf mice, STAT5 became phosphorylated upon GH stimulation with lower hormone doses than those that were required to increase phosphorylation in control siblings. This correlated with diminished levels of the suppressor of cytokine signaling CIS, which in Ames dwarfs were 20% of those found in normal mice liver [14]. In this study we have extended our observations to other GH-signaling mediators and, in addition, to another GH-effector tissue, the skeletal muscle.…”

“…Therefore, we have determined the abundance of GHR in liver and in muscle but found no significant differences between normal and dwarf mice in either tissue. Diminished levels of CIS in liver could be responsible for the higher sensitivity found for the STAT5 signaling pathways studied in Ames dwarf mice [14]. Since CIS may negatively modulate GH-signal by acting as an adaptor protein that targets GHR for internalization and degradation, rather than by competing with STAT5 binding to phosphorylated GHR [45,46], decreased levels of CIS found in liver would affect not only STAT5 signaling but also the other pathways activated by GH.…”

“…Moreover, STAT5b, the STAT5 predominantly expressed in liver, has been related to IGF-I gene expression in this tissue [26,27]. We have previously shown that Ames dwarf mice exhibit diminished or unaffected hepatic GH-receptor levels, with unaltered hepatic content of JAK2 and STAT5a/b proteins [28,14]. Furthermore, in dwarf mice, STAT5 became phosphorylated upon GH stimulation with lower hormone doses than those that were required to increase phosphorylation in control siblings.…”

“…In a previous work we have reported that GH-induced STAT5 activation was enhanced in dwarf mice liver, suggesting that these mice have increased sensitivity to GH in this tissue [14]. However, GH exerts its actions on many other organs and tissues; moreover, GH also triggers other signaling pathways beyond the JAK2/STAT5 cascade.…”

Differential regulation of membrane associated-growth hormone binding protein (MA-GHBP) and growth hormone receptor (GHR) expression by growth hormone (GH) in mouse liver

“…In Mt-GHRH and PEPCK-bGH transgenic mice we have observed that overexpression of GH levels is associated with desensitization of GH signal transduction, evidenced by the inability of exogenous high doses of GH to induce STAT5 phosphorylation, and with overexpression of the SOCS protein CIS, a specific inhibitor of STAT5 [44,45]. On the contrary, Ames dwarf mice exhibit hypersensitibity to exogenously administred GH, which is associated with reduced levels of CIS [46]. Therefore, the desensitization of GH signaling would be another compensatory mechanism of high GH levels.…”