Growth hormone (GH) is the major regulator of postnatal growth and an important metabolic regulator. It modulates a variety of processes in many tissues, ranging from adipogenesis to neurogenesis and immune function.
GH is currently thought to activate its receptor through hormone-induced dimerization, resulting in activation of the Janus kinase, JAK2. However, we find that the receptor exists as a dimer in the plasma membrane before hormone binding, based on co-immunoprecipitation of receptors with dual tags, on FRET, and on BRET analysis. We have also solved the crystal structure of the unliganded receptor extracellular domain, and observe a rotation of receptor extracellular domains in response to hormone binding. As forced rotation of the receptor intracellular domain by mutagenesis results in constitutive activation of JAK-STAT signalling, we propose that subunit rotation within a preexisting receptor dimer is the mechanism of activation, rather than dimerization.
We have observed nuclear accumulation of GH receptor in response to hormone addition to cells, and in regenerating liver and in some cancers. In vitro studies indicate that the uptake mechanism involves importins. Constitutive nuclear localization of the receptor with an introduced NLS sequence renders the BaF proB line factor independent, and is associated with a set of regulated genes evident in microarray analysis, some of which relate to proliferation and survival. A potential mechanism for this will be presented.
Finally, we have created targeted knockin mice with defined truncations to the cytoplasmic signalling domains of the GH receptor, and have defined sets of genes which are regulated by the different regions of the receptor. These mice will be useful for defining the physiological roles of the signalling pathways used by the GH receptor, and may assist in defining those genes associated with longevity. Supported by NHMRC (Australia).