Processing of growth hormone by rat adipocytes in primary culture: differentiation between release of intact hormone and degradative processing

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.

Binding and functional studies with the growth hormone receptor antagonist, B2036-PEG (pegvisomant), reveal effects of pegylation and evidence that it binds to a receptor dimer

GH actions are dependent on receptor dimerization. The GH receptor antagonist, B2036-PEG, has been developed for treating acromegaly. B2036 has mutations in site 1 to enhance receptor binding and in site 2 to block receptor dimerization. Pegylation (B2036-PEG) increases half-life and lowers immunogenicity, but high concentrations are required to control insulin-like growth factor-I levels. We examined antagonist structure and function and the impact of pegylation on biological efficacy. Unpegylated B2036 had a 4.5-fold greater affinity for GH binding protein (GHBP) than GH but similar affinity for membrane receptor. Pegylation substantially reduced membrane binding affinity and receptor antagonism, as assessed by a transcription assay, by 39- and 20-fold, respectively. GHBP reduced antagonist activity of unpegylated B2036 but did not effect antagonism by B2036-PEG. B2036 down-regulated receptors, and membrane binding sites doubled in the presence of dimerization-blocking antibodies, suggesting that B2036 binds to a receptor dimer. It is concluded that the high concentration requirement of B2036-PEG for clinical efficacy relates to pegylation, which decreases binding to membrane receptor but has the advantages of reduced clearance, immunogenicity, and interactions with GHBP. Our studies suggest that B2036 binds to a receptor dimer and induces internalization but not signaling.

Influence of obesity and body fat distribution on growth hormone kinetics in humans

We studied the kinetics of exogenous recombinant 22-kDa human growth hormone (rhGH) in premenopausal women with upper body obesity (UBO), lower body obesity (LBO), or normal body weight. A bolus of 100 mU rhGH was administered during a continuous infusion of somatostatin to suppress endogenous GH secretion. GH kinetics were investigated with noncompartmental analysis of plasma GH curves. GH peak values in response to GH infusion and plasma half-life of GH were not significantly different between normal weight and obese subjects. In contrast, GH clearance was 33% higher in LBO women and 51% higher in UBO women compared with clearance in normal weight controls. The difference in clearance between LBO and UBO was not statistically significant. Altered GH clearance characteristics contribute to low circulating GH levels in obese humans. Body fat distribution does not appear to affect GH kinetics.

Metabolic clearance rate of biosynthetic growth hormone after endogenous growth hormone suppression with a somatostatin analogue in chronic renal failure patients and control subjects

OBJECTIVE

Several disturbances in the regulation of growth hormone secretion have been reported in chronic renal failure. The general assumption is that an altered hormonal clearance is at the basis of such GH alterations. Nevertheless, details of GH elimination kinetics in uraemia are not available. To clarify the role played by the kidney in its catabolism, GH elimination kinetics were studied in uraemic and control subjects after suppression of endogenous secretion of GH.

DESIGN

In all subjects an analogue of somatostatin (octreotide 100 micrograms i.v.) was administered as a bolus before GH (-60 minutes). Sixty minutes later (0 min) biosynthetic GH (0.5 IU = 200 micrograms) was administered intravenously as a bolus.

PATIENTS

Six chronic renal failure patients before dialysis and six matched normal volunteers.

MEASUREMENTS

Plasma GH levels were measured by an immunoradiometric assay.

RESULTS

In both groups, the GH elimination curve fitted a bi-exponential model. The calculated plasma volume and GH concentration at 0 minutes were similar in both groups, while uraemic patients presented a reduced distribution volume. In all parameters measuring GH elimination, chronic renal failure patients showed an impaired clearance. In fact, the area under the curve (mU/l/150 min) was 912.8 +/- 170.6 for controls and 3524.8 +/- 642.8 for chronic renal failure patients (P < 0.005). The GH half-life was 13.8 +/- 1.6 and 26.4 +/- 2.9 minutes for control and uraemic subjects respectively (P < 0.05), and the metabolic clearance rate MCR (ml/min/m2) was 265.3 +/- 50.6 for controls and 79.9 +/- 16.4 for uraemic patients (P < 0.05). The GH mean residence time (minutes) (MRT) calculated was 12.0 +/- 0.5 for controls and 31.8 +/- 4.6 for chronic renal failure patients (P < 0.05).

CONCLUSIONS

Contrary to previous estimates, GH elimination kinetics follows a bi-exponential model and in normal subjects the GH half-life of the second phase is 13.8 +/- 1.6 minutes. Uraemic patients have impaired clearance of GH, suggesting that the kidney plays a role in GH disposal. However, the degree of impairment does not fully explain the alterations in GH secretion previously described in chronic renal failure.

The role and mechanism of growth hormone in the regulation of sexually dimorphic P450 enzymes in rat liver

The determination of sexually dimorphic hepatic steroid metabolism in rat liver has been shown to involve growth hormone. However, the mechanisms by which growth hormone controls the cytochrome P450 enzymes responsible for this dimorphic steroid metabolism is largely unknown. In this review we discuss different levels of growth hormone signal transduction, including receptor binding, signal transduction and activation of target genes by growth hormone.

Characterization of retroendocytosis in rat liver parenchymal cells and sinusoidal endothelial cells

After receptor-mediated endocytosis, internalized ligands may be recycled to the cell surface instead of being routed to lysosomes for degradation, a process termed retroendocytosis. We have investigated the kinetics and extent of retroendocytosis of neoglycoproteins after internalization via two carbohydrate-specific receptors in rat liver cells: galactose receptors in parenchymal cells (PC) and mannose receptors in sinusoidal endothelial cells (EC). Retroendocytosis in both cell types occurred with first-order kinetics, and the rate of recycling of internalized ligands was about 4 times higher in EC than in PC. As the length of the internalization pulse was increased, the extent of subsequent retroendocytosis decreased, indicating that retroendocytosis takes place from a relatively early stage in the endocytic pathway. Furthermore, as the degree of carbohydrate substitution of the neoglycoprotein ligands increased, the affinities of the receptors for the ligands and the extent of ligand retroendocytosis increased. In the EC, the relationship between degree of substitution and extent of retroendocytosis was not immediately apparent, as some of the neoglycoprotein ligands used may also bind to and be internalized by scavenger receptors on the EC, causing a decreased apparent retroendocytosis. However, when this interaction was inhibited, this relationship was restored. We conclude that retroendocytosis mainly occurs because of incomplete dissociation of ligands from receptors before receptor recycling to the cell surface and that the affinities of a receptor for its ligand at the cell surface and in the endosomal environment are major factors in determining the extent of retroendocytosis.

Regulation of human growth hormone receptor gene expression by human growth hormone in a human hepatoma cell line

We have investigated the effects of recombinant human growth hormone (r-hGH) on the expression of hGH-receptor in a human hepatoma cell line (HuH 7). Levels of hGH-receptor mRNA in HuH 7 cells treated with different doses of r-hGH were measured by means of an RNase protection assay. Treatment with r-hGH at physiological concentrations (12.5, 25 and 50 ng/ml) resulted in an increase in hGH-receptor mRNA levels within 1 h of addition of the hormone. A steady state was reached after 3-4 h and maintained for at least 48 h. In contrast, treatment with supraphysiological r-hGH concentrations (150 and 500 ng/ml) led to a down-regulation of hGH-receptor mRNA levels during the first 3 h after hormone addition followed by an increase in hGH-receptor mRNA levels thereafter. Nuclear run-off assays demonstrated that these changes in hGH-receptor mRNA levels were a result of changes in the rate of transcription of the hGH-receptor gene. Cycloheximide (10 micrograms/ml) did not affect these changes in hGH-receptor gene transcription significantly, indicating that they are mediated by pre-existing factors and do not require new protein synthesis. These data demonstrate that r-hGH specifically regulates the rate of transcription of the hGH-receptor gene in a human hepatoma cell line.

Isolated adipocytes from growth hormone-treated obese (ob/ob) mice exhibit insulin resistance

The genetically obese (ob/ob) mouse is a useful model for the study of the diabetogenic action of growth hormone (GH), because treatment of these animals with GH results in decreased responsiveness of their adipose tissue to insulin in vitro. Studies of the mechanisms involved in GH-induced insulin resistance using isolated adipocytes of ob/ob mice have not been possible, however, because of their extreme fragility and the lack of an adequate system for the maintenance of these cells. This study describes a new method for the isolation of ob/ob mouse adipocytes. The isolated cells are stable, viable and metabolically responsive to insulin. In addition, these adipocytes have been maintained in primary culture, in serum-free medium, for up to 3 days. During culture, the cells exhibit large increases in 125I-hGH binding (10-20-fold) and porcine 125I-insulin binding (5-10-fold). The induction of insulin resistance by GH has also been demonstrated in these freshly isolated ob/ob mouse adipocytes. The studies to date indicate that the ob/ob mouse adipocyte system should provide a useful model for detailed studies of the cellular and molecular mechanisms of GH induced insulin resistance.

Retroendocytosis of insulin in a cultured kidney epithelial cell line

It has been generally accepted that in renal tubular epithelium endocytosed proteohormones are transported to lysosomes where they undergo complete hydrolysis. En route, as endosomal pH falls, the proteohormone uncouples from the endocytosed membrane binding site, which recycles to the cell surface. However, studies in other tissues have uncovered alternate intracellular pathways for proteins. One such pathway is retroendocytosis (endocytosis then exocytosis). To determine whether a retroendocytotic pathway exists for insulin in renal epithelium, a study was carried out with confluent monolayers of a proximal-like opossum kidney cell line that exhibits receptor-mediated endocytosis of insulin. Cells were preloaded with 125I-labeled insulin (4 X 10(-10) M) for 30 min, surface-bound insulin was then removed by acid washing, and over the next 60 min the release of intracellular radioactivity into the medium was monitored. At 37 degrees C, control cells released on average 7-15% of the intracellular radioactivity as intact insulin [trichloroacetic acid (TCA)-precipitable radioactivity] and approximately 62% as TCA-soluble degradation products. In the presence of 0.1 mM chloroquine (an acidotropic agent) the release of intact insulin increased approximately twofold while degradation fell by nearly one-half. With Sephadex G-50 chromatography we found that the released radioactivity included insulin-size material that increased in the presence of chloroquine. High-performance liquid chromatography revealed that 53 (controls) and 81% (chloroquine treatment) of this latter material consisted of intact insulin. We conclude that, in addition to a major degradative pathway, cultured kidney epithelial cells exhibit a retroendocytotic pathway for insulin. Chloroquine inhibits degradation and appears to divert insulin from the degradative into the retroendocytotic pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

The half-life of exogenous growth hormone after suppression of endogenous growth hormone secretion with somatostatin

We have estimated the half-life of serum growth hormone (GH) in six subjects on 14 occasions following an intravenous bolus injection of either 50 or 500 mU of biosynthetic human growth hormone (B-hGH) while endogenous GH secretion was suppressed by a continuous infusion of somatostatin. The disappearance curve of serum GH was mono-exponential and the mean half-life was 8.9 min (SD 1.5). This is less than previously reported and has important implications for the performance of GH profiles, which should be performed with 10-15 min sampling intervals, and the calculation of pituitary GH secretion rates.

Intracellular processing of growth hormone receptors by adipocytes in primary culture

Rat adipocytes in primary culture have been used to study the intracellular processing of growth hormone (GH) receptors. Pretreatment of adipocytes with 20 micrograms/ml cycloheximide resulted in a rapid decline (t1/2 approximately 45 min) of the 125I-human growth hormone (hGH) binding capacity of the cells. This decline occurred at a faster rate in the presence of extracellular unlabeled hGH (400 ng/ml) and was not due to receptor occupancy. These data suggest that GH receptors turn over rapidly and constitutively on the plasma membrane and in the absence of protein synthesis are not replaced. Dissociation of GH-receptor complexes was shown not to occur at pH 5.5, the pH encountered in the acidic pre-lysosomal compartments (endosomes) where intracellular dissociation of many hormone-receptor complexes takes place. These data, together, suggest that the majority of GH receptors are not recycled but instead suffer the same fate as the majority of GH, i.e. degradation. To determine the rate of appearance of GH receptors at the cell surface, adipocytes were first treated with trypsin and then incubated at 37 degrees C to permit incorporation of any available GH receptors into the plasma membrane. Binding of 125I-hGH recovered to pre-trypsin levels by 2 h. This recovery was completely blocked by concomitant treatment with monensin, cytochalasin B, colchicine and 2,4-dinitrophenol. NH4Cl had no effect on receptor recovery. These data suggest that once GH receptors are synthesized in the rough endoplasmic reticulum, they travel via the Golgi apparatus to the plasma membrane (by processes involving both microfilaments and microtubules) and are then inserted into the plasma membrane in an energy-dependent step.