Ger Strous: Cell biology of growth hormone receptor

Aim
Understanding how cellular metabolism is regulated and how this connects to the regulation of longevity, cell cycle, apoptosis and immunity are major challenges of modern biology. A major regulator of cellular metabolism is the growth hormone receptor (GHR). Although all cells of the body contain GHRs, the liver is the most important director of the system, where GHR acts upstream of the IGF1 and insulin signalling pathways. Upon GH binding these receptors initiate signal transduction that results in the expression of genes involved in anabolic processes including protein synthesis, lipid degradation, immune function, muscle mass, bone turnover, and tooth development.
Our aim is to elucidate how cells regulate their GH-sensitivity.

Overview of growth hormone receptor transport pathways.

Summary of Results (2001-2006)
GH binding activates Jak2, via a proline-rich motif (box 1), and initiates Stat activation. Deviant from receptor tyrosine kinases (EGF, NGF, Met, c-kit, and insulin) where ligand binding triggers both signal transduction and swift receptor degradation, box 1-related activities in GHR do neither contribute to GHR endocytosis nor to its degradation. We have found that the GH sensitivity is regulated by the ubiquitination machinery via the Ubiquitin-dependent Endocytosis (UbE) motif. Mutation of this motif leads to a dramatic drop in endocytosis and lysosomal degradation, rendering the cells more GH-sensitive. This principle enables the cells to react fast on stress and other changing growth conditions, and opens new therapeutic possibilities to regulate hormone sensitivity without interfering with delicate (GH/IGF1/insulin) hormone balances.
In the last 5 years we have discovered that the ubiquitin system regulates GHR degradation both at the cell surface and in endosomes. Recently, we identified the SCF(TrCP) as the factor that binds the UbE motif and controls endocytosis. As this finding can be exploited to control the GH sensitivity of cells, we have filed a European Patent Application together with Drug Discovery Factory BV. Bussum. In another study we described the folding and dimerisation characteristics of the GHR in the rough ER. The results show that GHR folds very rapidly, probably during and immediately after completion of translation, dependent on the proper formation of three S-S bonds. Subsequent dimerisation is slower and is probably a requisite for ER exit.

Highlights: (2001-2006)

1. GHR dimerisation occurs in the rough ER, not via GH binding at the cell surface; Jak2 activation is initiated by a conformational change, induced by GH binding
2. Cargo selection and concentration at the multivesicular endosomes takes place in specialized coated areas and is clathrin-dependent.
3. Ubiquitin system regulates the GH sensitivity of cells, presumably via the SCF(TrCP) E3.
   

Key Publications: (2001-2006)

1. van Kerkhof P, Putters J, Strous GJ. 2007.The ubiquitin ligase SCF(beta TrCP) regulates the degradation of the growth hormone receptor. J. Biol Chem, 282:20475-83.
2. Van den Eijnden MJM, Lahaye LL, Strous GJ. 2006. Disulfide bonds determine Growth Hormone Receptor folding, dimerisation and ligand binding. J Cell Sci 119:3078-3086
3. Gent J, Van Kerkhof P, Roza M, Bu G, and Strous GJ. 2002. GHR dimerization occurs in the ER and is essential for ubiquitin-dependent endocytosis. Proc Natl Acad Sci USA 99:9858-9863.
4. Sachse M, Urbé S, Oorschot V, Strous GJ, and Klumperman J. 2002. Bi-layered clathrin coates on endosomal vacuoles are involved in protein sorting towards lysosomes. Mol Biol Cell 13:1313-1328.
5. Strous GJ and Schantl JA. 2001. ß-Arrestin and Mdm2, unsuspected partners in signalling from the cell surface. Science's STKE, PE41.