Negative regulation of growth hormone (GH) signaling

Abstract

Growth Hormone (GH) regulates postnatal longitudinal growth. It also has several other functions. It decreases cholesterol and lipoproteins levels, reduces fat tissue and increases lean body mass. GH actions are mediated by the JAK2/STAT5 signaling pathway. The manner whereby this pathway is activated has been extensively studied. There are, however, gaps in our knowledge how the pathway is silenced or negatively regulated.

Down regulation of GH signaling is related to several interconnected mechanisms mainly involving the internalization of GH receptor (GHR) and the action of negative regulators such as tyrosine phosphatases and suppressors of cytokine signaling (SOCS). SOCS proteins have recently received attention as important regulators of the JAK/STAT pathway. In relation to GH, it is interesting to note that mice lacking SOCS2 (SOCS2-/-) are 40% larger that their littermates. While this evidence suggests that SOCS2 controls GH and/or IGF-I signaling, the exact mechanism behind this action has not been defined yet. One hypothesis is that GHR turnover is critical and that SOCS proteins take part in this process by regulating protein breakdown through ubiquitination.

The aim of this thesis was to characterize pathways that regulate GH sensitivity in liver and to evaluate the mechanism of action of SOCS proteins, particularly SOCS2, as negative regulators for GH signaling pathway.

We demonstrated that the duration of GH-activated JAK2/STAT5 can be prolonged by other signaling pathways, such as the UPR (Unfolded Protein Response) and those regulating the actin cytoskeleton. These two pathways target different steps in the down regulatory pathway involving GHR ubiquitination and endocytosis. Induction of UPR decreases the degree of GHR ubiquitination while actin cytoskeleton disruption results in accumulation of ubiquitinated GHR. Mice lacking SOCS2 (SOCS2-/-) were analyzed using transcript profiling and other means to evaluate their phenotype.

We demonstrated that SOCS2-/- mice have limited similarity with GH-overexpression models. These mice are therefore not an exact mimic of GH overproduction and instead they represent 'a state of their own' in terms of gigantic phenotypes. Crossing SOCS2-/- mice with GH deficient dwarf mice resulted in a dwarf phenotype, indicating that the gigantism of SOCS2-/- mice depends on endogenous GH secretion. When the two dwarf models were compared, lack of SOCS2 resulted in hypersensitivity to GH treatment, as was demonstrated by increased growth rate and exaggerated responsiveness of hepatic GH-regulated genes. The mechanism of SOCS2 action involves its binding to GHR through the tyrosines 487 and 595 and its association to Elongin BC, probably forming an ubiquitin ligase complex that could mediate GHR ubiquitination and degradation.

In conclusion, our studies indicate that GH signaling down regulation is a tightly regulated process, involving several steps. It is modulated by UPR, actin cytoskeleton network, SOCS proteins (particularly SOCS2), the rate of GHR ubiquitination and might include the action of some still unidentified proteins that could contribute to the negative action of SOCS2.