The Growth Hormone-binding Protein Is a Location-dependent Cytokine Receptor Transcriptional Enhancer

Abstract

In the rat, a growth hormone-binding protein (GHBP) exists that is derived from the growth hormone (GH) receptor gene by an alternative mRNA splicing mechanism such that the transmembrane and intracellular domains of the GH receptor are replaced by a hydrophilic carboxyl terminus. In isolation, the GHBP is inactive, although it does compete with the receptor for ligand binding in the extracellular space and therefore inhibits the cellular response to GH. The GHBP is also located intracellularly and is translocated to the nucleus upon ligand stimulation. We show here that endogenously produced GHBP, in contrast to exogenous GHBP, was able to enhance the STAT5-mediated transcriptional response to GH. Interestingly, when the GHBP was targeted constitutively to the nucleus by the addition of the nuclear localization sequence of the SV40 large T antigen, greater enhancement of STAT5-mediated transcription was obtained. The transcriptional enhancement did not require GH per se and was not specific to the GH receptor, since similar enhancement of STAT5-mediated transcription by nuclear localized GHBP was obtained with specific ligand stimulation of both prolactin and erythropoietin receptors. Thus, the GHBP exerts divergent effects on STAT5-mediated transcription depending on its cellular location. The use of a soluble cytokine receptor as a location-dependent transcriptional enhancer and the ligand-independent involvement of the extracellular domain of a polypeptide ligand receptor in intracellular signal transduction provide additional novel mechanisms of transcriptional control. Growth hormone (GH)1 is the major regulator of postnatal body growth and initiates its biological actions, including the induction of a number of RNA species in mammalian tissues, by interaction with a specific membrane-bound receptor (1, 2).The GH receptor was the first cloned member of the now extensive cytokine receptor superfamily, which includes the receptors for prolactin (PRL), erythropoietin (EPO), granulocyte colony-stimulating factor, granulocyte-macrophage colony stimulating factor, ciliary neutrophic factor, thrombopoietin, leptin, cardiotrophin I, and the ␤-chain of interleukin (IL)-2 through IL-7, IL-9, and IL-11 to IL-13 (3). Most receptors of the cytokine receptor superfamily exist in a soluble and transmembrane form (4 -7). The function of the transmembrane forms is well documented and includes signal transduction predominantly but not exclusively through the JAK-STAT pathway, resulting in gene transcription (8, 9). The role of the soluble cytokine receptors, with the notable exception of the soluble forms of the IL-6 and ciliary neurotropitir factor (CNTF) receptors (10, 11), appears confined to ligand sequestration in the extracellular space with a consequent impairment of the cellular response to exogenous ligand (4).A soluble rat growth hormone-binding protein (GHBP) exists that is derived from the GH receptor gene by an alternative mRNA splicing mechanism such that the transmembrane and intracellular domains of the GH r...

Trafficking and signaling pathways of nuclear localizing protein ligands and their receptors

“…(72) In the context of nuclear translocation as it relates to signal transduction, several studies have shown in rat cell lines that all three molecules, GH, GHR and GHBP (the splice variant forms), are translocated to the nucleus. (73)(74)(75)(76) Both GHR and GHBP require GH stimulation to translocate to the nucleus. Further, nuclear rat GHBP appears to act as a transcriptional inducer of STAT5.…”

JAK-STAT pathway in carcinogenesis: Is it relevant to cholangiocarcinoma progression?

“…Thus, nuclear accumulation of Prl receptors as well as their overexpression in intrahepatically transplanted RS1 cholangiocarcinoma cells and in hepatocytes of tumor bearing animals can serve as a specific marker of cholangiocarcinoma. Since we have used monoclonal antibodies U5 to the extracellular domain of rat Prl receptor, it was revealed that nuclear accumulation may be due to both full length Prl receptor or Prl binding protein as have been found for GH binding protein [9,172] . Appearance of Prl receptors in the nucleus is associated with Prl participation in the regulation of the proliferation process.…”

“…Rat growth hormone binding protein (GHBP) that is a product of alternative splicing of GH receptor gene with an intact extracellular domain and without transmembrane and intracellular domains replaced by short carboxyterminal sequences, is also localized in cell nuclei in in vitro and in vivo experiments upon GH stimulation. Nuclear localized GHBP acts as a potent enhancer of STAT5 mediated transcription presumably by binding to PIAS proteins and thereby enhances the action of both GH and other members of the cytokine receptor superfamily [9,172] .…”

The growth hormone receptor mediated oncogenesis

“…Importantly, the internalisation and nuclear uptake of GH was not observed in the absence of an intact GHR. Graichen et al have reported that GH and GHR co-translocate to the nucleus and receptor phosphorylation may be the stimulus for nuclear translocation (Graichen et al, 2003).…”

“…Analysis of a number of clinical cancer samples has shown GHR localised to the nucleus, including B-cell lymphomas, mammary carcinoma, melanoma and prostate tumours (Lincoln et al, 1998;Mertani et al, 1996). The GHBP, and by implication the nuclear GHR, has been shown to act as a transcription factor capable of direct transactivation in a mammalian β-galactosidase reporter assay (Graichen et al, 2003;Conway-Campbell et al, 2008).…”