The Hep G2 cell line in the study of growth hormone receptor/binding protein

Identification of Haptoglobin as a Readout of rhGH Therapy in GH Deficiency

BACKGROUND

GH deficiency (GHD) is characterized by a cluster of cardiovascular risk factors and subtle inflammation. We aimed to demonstrate, through a proteomic approach, molecules directly modulated by GHD and involved in the inflammatory state.

METHODS

Ten children with isolated GHD were studied before and after 1 year of treatment with rhGH and compared with 14 matched controls. A two-dimensional electrophoresis plasma proteomics analysis was performed at baseline and after GH treatment to identify the top molecules modulated by GH. In vitro studies on human hepatoma (HepG2) cells were performed to validate the data.

RESULTS

Twelve of 20 proteomic spots were predicted to be isoforms α and β of haptoglobin (Hp) and confirmed by liquid chromatography tandem mass spectrometry and Western immunoblot analyses. Hp levels were higher in patients with GHD than controls at baseline (P < 0.001) and were reduced following GH treatment (P < 0.01). In HepG2 cells, both GH and IGF-1 were able to downregulate IL-6-induced Hp secretion. Moreover, Hp secretion was restored in pegvisomant-treated HepG2 cells.

CONCLUSIONS

Hp is a molecule acting in the inflammatory state of GHD and a possible biomarker for GH treatment. Nevertheless, the contribution of other factors and the molecular pathways involved in the GH downregulation of Hp remain to be clearly defined.

Myostatin regulates pituitary development and hepatic IGF1

Circulating myostatin-attenuating agents are being developed to treat muscle-wasting disease despite their potential to produce serious off-target effects, as myostatin/activin receptors are widely distributed among many nonmuscle tissues. Our studies suggest that the myokine not only inhibits striated muscle growth but also regulates pituitary development and growth hormone (GH) action in the liver. Using a novel myostatin-null label-retaining model (Jekyll mice), we determined that the heterogeneous pool of pituitary stem, transit-amplifying, and progenitor cells in Jekyll mice depletes more rapidly after birth than the pool in wild-type mice. This correlated with increased levels of GH, prolactin, and the cells that secrete these hormones, somatotropes and lactotropes, respectively, in Jekyll pituitaries. Recombinant myostatin also stimulated GH release and gene expression in pituitary cell cultures although inhibiting prolactin release. In primary hepatocytes, recombinant myostatin blocked GH-stimulated expression of two key mediators of growth, insulin-like growth factor (IGF)1 and the acid labile subunit and increased expression of an inhibitor, IGF-binding protein-1. The significance of these findings was demonstrated by smaller muscle fiber size in a model lacking myostatin and liver IGF1 expression (LID-o-Mighty mice) compared with that in myostatin-null (Mighty) mice. These data together suggest that myostatin may regulate pituitary development and function and that its inhibitory actions in muscle may be partly mediated by attenuating GH action in the liver. They also suggest that circulating pharmacological inhibitors of myostatin could produce unintended consequences in these and possibly other tissues.

Growth hormone binding protein - physiological and analytical aspects

A significant proportion of total circulating growth hormone (GH) is bound to a high affinity growth hormone binding protein (GHBP). Several low affinity binding proteins have also been described. Significant differences between species exist with respect to origin and regulation of GHBP, but generally it resembles the extracellular domain of the GH receptor. Concentrations are associated with GH status, body composition and other factors. Although the clinical relevance of GHBP is not fully understood it is suggested that concentrations indirectly reflect GH receptor status. This is supported by cases of Laron's syndrome where a molecular defect in the extracellular domain of the GH receptor is associated with low or unmeasurable GHBP concentrations. Methods to measure GHBP have evolved from chromatographic, activity based procedures to direct immunoassays. In clinical practice, measurement of GHBP can be helpful to differentiate between GH deficiency and GH insensitivity, particularly if GHBP is absent.

Growth hormone (GH) receptors in prostate cancer: gene expression in human tissues and cell lines and characterization, GH signaling and androgen receptor regulation in LNCaP cells

Various hormones and growth factors have been implicated in progression of prostate cancer, but their role and the underlying molecular mechanism(s) involved remain poorly understood. In this study, we investigated the role of human growth hormone (GH) and its receptor (GHR) in human prostate cancer. We first demonstrated mRNA expression of GHR and of its exon 9-truncated isoform (GHR(tr)) in benign prostate hyperplasia (BPH) and prostate adenocarcinoma patient tissues, as well as in LNCaP, PC3 and DU145 human prostate cancer cell lines. GHR mRNA levels were 80% higher and GHR(tr) only 25% higher, in the carcinoma tissues than in BPH. Both isoforms were also expressed in LNCaP and PC3 cell lines and somewhat less so in DU145 cells. The LNCaP cell GHR protein was further characterized, on the basis of its M(r) of 120kDa, its binding to two different GHR monoclonal antibodies, its high affinity and purely somatogenic binding to (125)I-hGH and its ability to secrete GH binding protein, all characteristic of a functional GHR. Furthermore, GH induced rapid, time- and dose-dependent signaling events in LNCaP cells, including phosphorylation of JAK2 tyrosine kinase, of GHR itself and of STAT5A (JAK2-STAT5A pathway), of p42/p44 MAPK and of Akt/PKB. No effect of GH (72h) could be shown on basal or androgen-induced LNCaP cell proliferation nor on PSA secretion. Interestingly, however, GH caused a rapid (2-12h) though transient striking increase in immunoreactive androgen receptor (AR) levels (< or =5-fold), followed by a slower (24-48h) reduction (< or = 80%), with only modest parallel changes in serine-phosphorylated AR. In conclusion, the GH-induced activation of signaling pathways, its effects on AR protein in LNCaP cells and the isoform-specific regulation of GHR in prostate cancer patient tissues, suggest that GH, most likely in concert with other hormones and growth factors, may play an important role in progression of human prostate cancer.

In vitro effects of recombinant human growth hormone on growth of human gastric cancer cell line BGC823 cells

AIM: To study the effects of recombinant human growth hormone (rhGH) on growth of human gastric cancer cell line in vitro.

METHODS: Experiment was divided into control group, rhGH group, oxaliplatin (L-OHP) group and rhGH+L-OHP group. Cell inhibitory rate, cell cycle, cell proliferation index (PI) and DNA inhibitory rate of human gastric cancer line BGC823, at different concentrations of rhGH treatment were studied by cell culture, MTT assay and flow cytometry.

RESULTS: The distinctly accelerated effects of rhGH on multiplication of BGC823 cell line were not found in vitro. There was no statistical significance between rhGH group and control group, or between rhGH+L-OHP group and L-OHP group (P > 0.05). The cell growth curve did not rise. Cell inhibitory rate and cells arrested in G₀-G₁ phase were obviously increased. Meanwhile, cells in S phase and PI were distinctly decreased and DNA inhibitory rate was obviously increased in rhGH+L-OHP group in comparison with control group and rhGH group, respectively (P < 0.01). Cell inhibitory rate showed an increasing trend and PI showed a decreasing trend in rhGH+L-OHP group compared with L-OHP group.

CONCLUSION: In vitro rhGH does not accelerate the multiplication of human gastric cancer cells. It may increase the therapeutic efficacy when it is used in combination with anticancer drugs.

Growth hormone down-regulation of Interleukin-1beta and Interleukin-6 induced acute phase protein gene expression is associated with increased gene expression of suppressor of cytokine signal-3

Severe burn induces the hepatic acute phase response. We previously showed that recombinant human growth hormone (GH) treatment after burn down-regulated acute phase protein (APP) production and gene expression in vivo. In this study, we hypothesized that the inhibitory effect of GH on the hepatic acute phase response was due to increased suppressor of cytokine signaling (SOCS) gene expression. HepG2 cells were treated with Interleukin-1beta (IL-1beta; 2 ng/mL) and interleukin 6 (IL-6; 20 ng/mL) alone or combined with GH (2 microg/mL) for 15 and 30 min, and 1, 2, and 4 h. The levels of gene expression for alpha1-acid glycoprotein (AGP), alpha1-antitrypsin (ATT), and SOCS (CIS, SOCS-1, 2, and 3) were measured by reverse transcript-polymerase chain reaction (RT-PCR). APP levels in the supernatant were determined by enzyme-linked immunosorbent sandwich assay (ELISA). The gene expression of AGP and ATT were also measured in HepG2 cells transfected with pEF-Flag-l/mSOCS-3 plasmid after IL-1beta or IL-6 treatment. Data are expressed as means +/- SEM, and statistical analysis was performed by one- or two-way analysis of variance. IL-1beta and IL-6 induced AGP and ATT gene expression and protein production, respectively, which was down-regulated by GH treatment. SOCS-3 but not CIS, SOCS-1, or SOCS-2 gene expression was significantly increased by GH treatment. APP gene expression was significantly decreased in cells transfected with plasmid over expressing SOCS-3 after IL-6 and IL-1beta treatment. GH attenuates IL-1beta or IL-6 induced APP gene expression, which is associated with increased expression of SOCS-3. This study suggests that SOCS-3 plays an important role in the suppression of cytokine signaling by GH in down-regulating the acute phase response after injury.

Proteasome inhibitor enhances growth hormone-binding protein release

We used murine Ba/F3 cells transfected with human growth hormone receptor (hGHR) cDNA to investigate the regulatory mechanisms of human growth hormone-binding protein (hGH-BP) release. The extracellular domain of hGHRs were cleaved and released as hGH-BPs (a soluble form of hGHR). The hGH-BP release was enhanced by phorbol 12,13-dibutyrate (PDBu), and suggested to be mediated by activation of PKC, the same as in human IM-9 cells. Thus, Ba/F3 cells have hGH-BP-releasing pathways similar to those of human cells. The proteasome inhibitors MG-132 and clasto-lactacystin beta-lactone also increased hGH-BP release from Ba/F3-hGHR cells, and MG-132 and PDBu synergistically increased hGH-BP release. The results obtained by using three PKC inhibitors Gö 6976, GF 109203X and Gö 6983 suggest that the enhancement of hGH-BP release by MG-132 and PDBu is mediated by different mechanisms probably involving different PKC isozymes.

Role of ecto-kinase in phorbol ester-enhanced growth hormone-binding protein release from human IM-9 cells

Previously we reported that a phorbol ester, phorbol 12, 13-dibutyrate (PDBu), increased the release of human growth hormone-binding protein (hGH-BP) in IM-9 cells, and that this phorbol ester-enhanced release was mediated by protein kinase Ca (PKCalpha). In the present study, the mechanisms of the phorbol ester-enhanced hGH-BP release were further investigated. Treatment of IM-9 cells with PDBu did not increase hGH-BPs (55-60 kDa) in the intracellular soluble fraction. When the cells were treated with trypsin to remove human growth hormone receptors (hGHRs) on the cell surface after stimulation, no hGH-BPs were detected in the culture supernatants, nor did treatment with bafilomycin A1 or chloroquine affect the PDBu-enhanced hGH-BP release. These results suggest that hGH-BPs released by PDBu stimulation are derived from cell surface hGHRs and not generated within the cells. Protein kinase inhibitors with broad specificities, K-252a and K-252b, inhibited the PDBu-enhanced release with almost the same dose-dependency, although only a trace amount of K-252b was incorporated into IM-9 cells than K-252a, suggesting that K-252b probably inhibits an ecto-kinase extracellularly. PDBu actually enhanced the phosphorylation of several extracellular proteins, and this enhanced phosphorylation was completely inhibited by K-252b treatment. Moreover, the PKCalpha-specific inhibitor bisindolylmaleimide III which inhibits PDBu enhanced hGH-BP release inhibited the PDBu-enhanced phosphorylation of extracellular proteins. On the other hand, the impermeable PKC inhibitor PKC inhibitor peptide 19-31 did not inhibit PDBu-enhanced release, suggesting that the target PKCalpha for PDBu is not present on the extracellular surface. Taken together, these results suggest that, in addition to intracellular PKCalpha, activation of an undefined ecto-kinase may also be involved in the PDBu-enhanced hGH-BP release.

Activation of protein kinase C alpha enhances human growth hormone-binding protein release

The effect of phorbol ester on human growth hormone-binding protein (hGH-BP) release was investigated. The hGH-BP release from human IM-9 cells measured by immunoblotting was dose-dependently enhanced by a phorbol ester, phorbol 12, 13-dibutyrate (PDBu), and reached plateau at 100 nM. The increased hGH-BP release was shown after 10 min incubation with PDBu and reached a plateau at 60 min after stimulation. Similarly, a diacylglycerol analogue, 1-oleoyl-2-acetyl-sn-glycerol, enhanced hGH-BP release. The enhancement was not inhibited by cycloheximide pretreatment, suggesting that the enhanced hGH-BP release does not require de novo protein synthesis. The PDBu-enhanced hGH-BP release was strongly inhibited by extracellular EDTA, and was dose-dependently inhibited by protein kinase C (PKC)-specific inhibitor, Ro 31-8220. These results suggest that activation of PKC mediates the PDBu-enhanced hGH-BP release. Of the 11 known PKC isoforms in human cells, PKCalpha, delta, mu and iota were detected in IM-9 cells by immunoblotting. Of these isoforms, PKCalpha, delta and mu were present in the membrane fraction, which is a known activation marker of PKC. Furthermore, when several PKC-specific inhibitors (Gö 6976, GF 109203X or bisindolylmaleimide III) with different specificities for each isoform were used, there was a good correlation between inhibition of the enhancement of hGH-BP release and inhibition of the phosphorylation of PKC isoforms, another activation marker of PKC, in PKCalpha but not in PKCdelta and mu. These results suggest that activation of PKCalpha is involved in PDBu-enhanced hGH-BP release.

Alternatively spliced forms in the cytoplasmic domain of the human growth hormone (GH) receptor regulate its ability to generate a soluble GH-binding protein

The mechanism underlying the generation of soluble growth hormone binding protein (GHBP) probably differs among species. In rats and mice, it involves an alternatively spliced mRNA, whereas in rabbits, it involves limited proteolysis of the membrane-bound growth hormone receptor (GHR). In humans, this latter mechanism is favored, as no transcript coding for a soluble GHR has been detected so far. To test this hypothesis, we analyzed COS-7 cells transiently expressing the full-length human (h) GHR and observed specific GH-binding activity in the cell supernatants. Concomitantly, an alternatively spliced form in the cytoplasmic domain of GHR, hGHR-tr, was isolated from several human tissues. hGHR-tr is identical in sequence to hGHR, except for a 26-bp deletion leading to a stop codon at position 280, thereby truncating 97.5% of the intracellular domain of the receptor protein. When compared with hGHR, hGHR-tr showed a significantly increased capacity to generate a soluble GHBP. Interestingly, this alternative transcript is also expressed in liver from rabbits, mice, and rats, suggesting that, in these four species, proteolysis of the corresponding truncated transmembrane GHR is a common mechanism leading to GHBP generation. These findings support the hypothesis that GHBP may at least partly result from alternative splicing of the region encoding the intracellular domain and that the absence of a cytoplasmic domain may be involved in increased release of GHBP.

Molecular basis of inherited growth hormone resistance in childhood

The growth hormone receptor (GHR), a member of the cytokine receptor superfamily that gives rise to a soluble and circulating counterpart (GHBP), is the main target of Laron syndrome (LS), a severe autosomal recessive dwarfism characterized by complete GH insensitivity. Genetic and mutation analyses have attested to the high molecular heterogeneity of this syndrome, and, to date, more than 30 different GHR mutations including deletion, frameshift, nonsense, missense and splicing defects have been described. However, among them, missense mutations are of particular interest in potentially providing critical information on the structure-function relationship of the GHR and related molecules. The study of the recently described forms of atypical LS is now very promising. These patients display detectable plasma GH binding activity associated with complete or partial GH insensitivity. Molecular analysis of such a phenotype with positive GHBP and complete GH insensitivity has revealed the existence of a missense mutation abolishing receptor homodimerization, thereby providing in vivo evidence for the critical role of the dimerization process in the growth-promoting action of GH. Similarly, mutations in the cytoplasmic region, which are expected to be associated with normal GH binding activity, should contribute to the identification of other functionally important domains. Partial GH insensitivity syndromes may theorically encompass a wide range of distinct phenotypes with variable degrees of GH resistance. Missense GHR mutations and a quantitative GHR mRNA defect have been identified in some cases belonging to this heterogeneous group. Interestingly, exclusion of linkage between the Laron phenotype and the GHR locus was demonstrated in one affected family. This latter situation may indicate the existence of other genes controlling GHR expression or required at different steps of the signal transduction pathway. In this regard, the availability of a possible animal model for LS should offer new prospects in the identification of GH-inducible genes.

Growth hormone receptor synthesis and release in tumorous somatolactotrophs

The growth hormone (GH) receptor (GHR) gene is expressed in pituitary somatotrophs and lactotrophs, in which GHR/GH-binding protein (GHBP) immunoreactivity is primarily located in secretory granules. The possibility that the GHR gene may be similarly expressed in GH(3) cells was therefore investigated, since these tumorous pituitary cells are used as models of GH- and prolactin-secreting cells but lack secretory granules. GHR/GHBP gene transcripts were detected in GH(3) cells and were homologous to hepatic GHR/GHBP transcripts. Immunoreactive GHRs were also detected by Western Blotting and GHBP/GHR immunoreactivity was localized by immunogold electron microscopy within each intracellular compartment. Although the immunoreactive GHR/GHBP content of these cells was less than that in rat tissues, GHBP release from GH(3) cells into incubation media (relative to tissue content) was much greater than GHBP release from rat tissue explants. These results demonstrate GHR expression in GH(3) cells, comparable with that in normal pituitary cells. These tumorous somatolactotrophs could thus provide a model for studies on GHR/GHBP synthesis and release and for studying GH effects on pituitary function. The release of GHBP from pituitary cells also indicates, for the first time, an extra-hepatic source of the plasma GHBP.