The SOCS2 Ubiquitin Ligase Complex Regulates Growth Hormone Receptor Levels

Abstract

Growth Hormone is essential for the regulation of growth and the homeostatic control of intermediary metabolism. GH actions are mediated by the Growth Hormone Receptor; a member of the cytokine receptor super family that signals chiefly through the JAK2/STAT5 pathway. Target tissue responsiveness to GH is under regulatory control to avoid excessive and off-target effects upon GHR activation. The suppressor of cytokine signalling 2 (SOCS) is a key regulator of GHR sensitivity. This is clearly shown in mice where the SOCS2 gene has been inactivated, which show 30–40% increase in body length, a phenotype that is dependent on endogenous GH secretion. SOCS2 is a GH-stimulated, STAT5b-regulated gene that acts in a negative feedback loop to downregulate GHR signalling. Since the biochemical basis for these actions is poorly understood, we studied the molecular function of SOCS2. We demonstrated that SOCS2 is part of a multimeric complex with intrinsic ubiquitin ligase activity. Mutational analysis shows that the interaction with Elongin B/C controls SOCS2 protein turnover and affects its molecular activity. Increased GHR levels were observed in livers from SOCS2−/− mice and in the absence of SOCS2 in in vitro experiments. We showed that SOCS2 regulates cellular GHR levels through direct ubiquitination and in a proteasomally dependent manner. We also confirmed the importance of the SOCS-box for the proper function of SOCS2. Finally, we identified two phosphotyrosine residues in the GHR to be responsible for the interaction with SOCS2, but only Y487 to account for the effects of SOCS2. The demonstration that SOCS2 is an ubiquitin ligase for the GHR unveils the molecular basis for its physiological actions.

SOCS proteins in regulation of receptor tyrosine kinase signaling

“…SOCS2 deficient mice display about 50% increased body size primarily due to the loss of negative regulation of growth hormone (GH) receptor function [42]. SOCS2 negatively regulates the GH receptor through ubiquitination-dependent degradation [43]. SOCS2 was also found to interact with IGF1R and INSR, and this interaction occurred only when the receptor was activated [34,44] suggesting that similar to SOCS1 the interaction between receptor and SOCS2 is mediated through receptor phosphotyrosine residues and the SOCS2 SH2 domain.…”

The SOCS box—Adapting proteins for ubiquitination and proteasomal degradation

“…SOCS2-deficient mice exhibit 30-40% increased growth when compared with wild-type littermates as a result of dysregulated growth hormone receptor (GHR) signalling (41). Deletion of the SOCS2-SOCS box abrogates regulation of GH signalling, with SOCS2 shown to have E3 ligase activity, ubiquitinating the GHR in vitro (34,42). SOCS2 has also been shown to bind phosphorylated Pyk2 in NK cells, and this interaction correlates with decreased Pyk2 levels (43).…”

Protein Interaction Screening for the Ankyrin Repeats and Suppressor of Cytokine Signaling (SOCS) Box (ASB) Family Identify Asb11 as a Novel Endoplasmic Reticulum Resident Ubiquitin Ligase

“…We have demonstrated that SOCS2 targets the growth hormone receptor for ubiquitination whereas SOCS6targets both the c-Kit and Flt3 tyrosine kinase receptors (4,5). SOCS1 has been shown to promote the ubiquitination of JAK2, and VHL is responsible for the ubiquitination and degradation of HIF1␣ (6,7).…”