Female Mice Lacking Estrogen Receptor-α in Hypothalamic Proopiomelanocortin (POMC) Neurons Display Enhanced Estrogenic Response on Cortical Bone Mass

Estrogens are important regulators of bone mass and their effects are mainly mediated via estrogen receptor (ER)α. Central ERα exerts an inhibitory role on bone mass. ERα is highly expressed in the arcuate (ARC) and the ventromedial (VMN) nuclei in the hypothalamus. To test whether ERα in proopiomelanocortin (POMC) neurons, located in ARC, is involved in the regulation of bone mass, we used mice lacking ERα expression specifically in POMC neurons (POMC-ERα(-/-)). Female POMC-ERα(-/-) and control mice were ovariectomized (OVX) and treated with vehicle or estradiol (0.5 μg/d) for 6 weeks. As expected, estradiol treatment increased the cortical bone thickness in femur, the cortical bone mechanical strength in tibia and the trabecular bone volume fraction in both femur and vertebrae in OVX control mice. Importantly, the estrogenic responses were substantially increased in OVX POMC-ERα(-/-) mice compared with the estrogenic responses in OVX control mice for cortical bone thickness (+126 ± 34%, P < .01) and mechanical strength (+193 ± 38%, P < .01). To test whether ERα in VMN is involved in the regulation of bone mass, ERα was silenced using an adeno-associated viral vector. Silencing of ERα in hypothalamic VMN resulted in unchanged bone mass. In conclusion, mice lacking ERα in POMC neurons display enhanced estrogenic response on cortical bone mass and mechanical strength. We propose that the balance between inhibitory effects of central ERα activity in hypothalamic POMC neurons in ARC and stimulatory peripheral ERα-mediated effects in bone determines cortical bone mass in female mice.

Ghrelin

BACKGROUND

The gastrointestinal peptide hormone ghrelin was discovered in 1999 as the endogenous ligand of the growth hormone secretagogue receptor. Increasing evidence supports more complicated and nuanced roles for the hormone, which go beyond the regulation of systemic energy metabolism.

SCOPE OF REVIEW

In this review, we discuss the diverse biological functions of ghrelin, the regulation of its secretion, and address questions that still remain 15 years after its discovery.

MAJOR CONCLUSIONS

In recent years, ghrelin has been found to have a plethora of central and peripheral actions in distinct areas including learning and memory, gut motility and gastric acid secretion, sleep/wake rhythm, reward seeking behavior, taste sensation and glucose metabolism.

Metabolic functions of liver-derived (endocrine) insulin-like growth factor I

Until now it has been difficult to determine the relative importance of locally produced (autocrine/paracrine) versus systemically derived (endocrine) insulin-like growth factor I (IGF-I) in the intact organism. We recently eliminated IGF-I production in the livers of mice using the Cre/loxP recombination system. These mice displayed a reduction in serum IGF-I levels of more than 80%, but demonstrated normal body growth, suggesting that autocrine/paracrine-acting IGF-I, but not endocrine-acting IGF-I, regulates body growth. Long-term metabolic studies of mice in which IGF-I production had been inactivated in the liver, have shown that the mice have decreased fat mass, but increased serum levels of insulin and cholesterol. Despite the marked increase in plasma insulin following glucose administration, the glucose elimination was not altered in these animals. Thus, the mice showed an adequately compensated insulin resistance. In conclusion, liver-derived or endocrine IGF-I is not required for post-natal statural growth, but seems to be of vital importance for normal carbohydrate and lipid metabolism.

Liver-derived IGF-I regulates GH secretion at the pituitary level in mice

We have reported that liver-specific deletion of IGF-I in mice (LI-IGF-I-/-) results in decreased circulating IGF-I and increased GH levels. In the present study, we determined how elimination of hepatic IGF-I modifies the hypothalamic-pituitary GH axis to enhance GH secretion. The pituitary mRNA levels of GH releasing factor (GHRF) receptor and GH secretagogue (GHS) receptor were increased in LI-IGF-I-/- mice, and in line with this, their GH response to ip injections of GHRF and GHS was increased. Expression of mRNA for pituitary somatostatin receptors, hypothalamic GHRF, somatostatin, and neuropeptide Y was not altered in LI-IGF-I-/- mice, whereas hypothalamic IGF-I expression was increased. Changes in hepatic expression of major urinary protein and the PRL receptor in male LI-IGF-I-/- mice indicated an altered GH release pattern most consistent with enhanced GH trough levels. Liver weight was enhanced in LI-IGF-I-/- mice of both genders. In conclusion, loss of liver-derived IGF-I enhances GH release by increasing expression of pituitary GHRF and GHS receptors. The enhanced GH release in turn affects several liver parameters, in line with the existence of a pituitary-liver axis.

Possible roles of insulin-like growth factor in regulation of physiological and pathophysiological liver growth

BACKGROUND/AIMS

Almost all circulating insulin-like growth factor-1 (IGF-1) is produced and secreted from the liver. However, the possible role of IGF-1 in local regulation of liver functions including liver growth is unclear. In the present study, we investigated the role of IGF-1 on liver growth in vivo and in hepatic stellate cell function in vitro.

RESULTS

Liver-specific knock-out of the IGF-1 gene by use of the cre-loxP system caused enhanced liver growth, possibly reflecting increased growth hormone (GH) secretion due to decreased negative feedback by IGF-1. Studies on cultured rat hepatic stellate cells (HSC) showed that IGF-1 and hepatocyte-conditioned medium (PCcM) time- and dose-dependently increased hepatocyte growth factor (HGF) mRNA and HGF immunoreactivity. IGF-1 and PCcM also enhanced DNA synthesis in the HSC cultures. The PCcM did not contain bioactive IGF-1 and was also able to stimulate proliferation when prepared under serum- and hormone-free conditions.

CONCLUSION

In vivo results show that IGF-1 is not essential for normal growth of the intact liver. The in vitro results indicate that both IGF-1 and IGF- 1-independent factor(s) from hepatocytes can stimulate HGF production by HSC. It remains to be investigated whether these effects are of importance for liver regeneration or pathological conditions.

Retarded liver growth in interleukin-6-deficient and tumor necrosis factor receptor-1-deficient mice

The liver size in adult mammals is tightly regulated in relation to body weight, but the hormonal control of this is largely unknown. We investigated the roles of interleukin-6 (IL-6) and tumor necrosis factor (TNF) receptor-1 in the regulation of intact liver weight in adult mice. The relative liver wet and dry weights of older adult (5- to 10-month-old) IL-6 knockout (IL-6(-/-)) mice were decreased by 22-28%, and total contents of DNA and protein were decreased compared with those in age-matched wild-type mice. Weights of other visceral organs were unaffected. Older adult (6- to 8-month-old) TNF receptor-1 knockout (TNFR1(-/-)) mice displayed decreased relative liver weight. Treatment with a single injection of IL-6 increased liver wet and dry weights in IL-6(-/-) and wild-type mice, but not TNFR1(-/-) mice. Treatment with TNFalpha enhanced liver weight and DNA synthesis of nonparenchymal liver cells at 24 h in wild-type, but not IL-6(-/-), mice. At 48 h, TNFalpha induced DNA synthesis in nonparenchymal cells and hepatocytes of both wild-type and IL-6(-/-) mice. In conclusion, TNF receptor-1 stimulation and IL-6 production are both necessary for normal liver weight gain in older adult mice. The results of TNFalpha and IL-6 treatment further indicate that the effects of TNF receptor-1 and IL-6 depend on each other for full stimulation of liver growth.

Liver-derived IGF-I is of importance for normal carbohydrate and lipid metabolism

IGF-I is important for postnatal body growth and exhibits insulin-like effects on carbohydrate metabolism. The function of liver-derived IGF-I is still not established, although we previously demonstrated that liver-derived IGF-I is not required for postnatal body growth. Mice whose IGF-I gene in the liver was inactivated at 24 days of age were used to investigate the long-term role of liver-derived IGF-I for carbohydrate and lipid metabolism. Serum levels of leptin in these mice were increased by >100% at 3 months of age, whereas the fat mass of the mice was decreased by 25% at 13 months of age. The mice became markedly hyperinsulinemic and yet normoglycemic, indicating an adequately compensated insulin resistance. Furthermore, they had increased serum levels of cholesterol. We conclude that liver-derived IGF-I is of importance for carbohydrate and lipid metabolism.

The somatomedin hypothesis revisited in a transgenic model

Studies of insulin-like growth factor I (IGF-I) gene knockout mice models have clearly shown that IGF-I is necessary for prenatal as well as postnatal body growth in mice. Clinical studies of a patient with an IGF-I gene defect which caused complete absence of IGF-I, verified that it is important for intrauterine and postnatal growth. Recent studies of mice with liver-specific and inducible IGF-I gene knockout indicated that liver-derived IGF-I is not necessary for postnatal body growth, although serum IGF-I levels are decreased by more than 80% in these mice. Therefore, extrahepatic IGF-I is sufficient for maintenance of postnatal body growth in mice. Further investigations are needed to assess whether liver-derived circulating IGF-I is essential for other biological functions.

Effects of growth hormone and its secretagogues on bone

The growth hormone (GH)/insulin-like growth factor-1 axis is not only of importance for linear body growth during childhood, but it is also one of the major determinants of adult bone mass. Studies show that GH treatment increases bone mass in rodents as well as in adult GH-deficient humans, but the effect of GH treatment on bone mass in healthy humans has so far not been impressive. Recently, a new class of GH secretagogues (GHSs) has been developed. In humans, GHS treatment affects biochemical markers of bone turnover and increases growth velocity in selected short children with or without GH deficiency. In rodents, GHS treatment increase bone mineral content, but it has not yet been shown that GHS treatment can affect bone mass in adult humans.

Growth hormone (GH)-independent stimulation of adiposity by GH secretagogues

Growth hormone secretagogues (GHSs) stimulate growth hormone (GH) secretion, which is lipolytic. Here we compared the effects of twice daily s.c. treatment of GH and the GHS, ipamorelin, on body fat in GH-deficient (lit/lit) and in GH-intact (+/lit and +/+) mice. In +/lit and lit/lit mice ipamorelin induced a small (15%) increase in body weight by 2 weeks, that was not further augmented by 9 weeks. GH treatment markedly enhanced body weight in both groups. Ipamorelin also increased fat pad weights relative to body weight in both lit/lit and +/lit mice. Two weeks GHS treatment (ipamorelin or GHRP-6) also increased relative body fat, quantified by in vivo dual energy X-ray absorpiometry (DEXA) in GH-intact mice. GH decreased relative fat mass in lit/lit mice and had no effect in GH-intact mice. Treatment with GHS, but not GH, increased serum leptin and food intake in GH-intact mice. Thus, GHSs increase body fat by GH-independent mechanisms that may include increased feeding.

Normal pharmacologically-induced, but decreased regenerative liver growth in interleukin-6-deficient (IL-6(-/-)) mice

BACKGROUND/AIMS

In the absence of liver damage, rapid liver growth can be induced pharmacologically by so-called primary liver growth promoters. The importance of the acute-phase cytokines interleukin-6 and tumor necrosis factor-alpha for the actions of these compounds is not clear. This study aimed to investigate the importance of IL-6 and TNF-receptor-1 in pharmacologically-induced liver growth.

METHODS

IL-6 knockout (IL-6(-/-)), TNF-receptor-1 knockout (TNFR1(-/-)) and wild-type mice were treated with the peroxisome proliferator nafenopin and the anti-androgen cyproterone acetate (CPA) in one single injection or for 6 days with daily injections, and examined at 24 or 48 h after treatment. In a control experiment, IL-6(-/-) mice were subjected to two-thirds partial hepatectomy.

RESULTS

Nafenopin treatment increased relative liver weight and DNA synthesis similarly in IL-6(-/-), TNFR1(-/-) and wild-type mice. CPA increased liver weight similarly in all groups, but did not increase DNA synthesis. Expression of peroxisome proliferator activated receptor-alpha mRNA was increased in both IL-6(-/-) and wild-type mice by nafenopin treatment, but not by CPA treatment. After hepatectomy DNA synthesis was suppressed in IL-6(-/-) mice compared to wild-type mice.

CONCLUSIONS

Liver growth induced by nafenopin and CPA was not dependent on the presence of IL-6 or TNF receptor-1, whereas liver regeneration was decreased in IL-6(-/-) mice.

Effects of growth hormone and insulinlike growth factor-I on body growth and adult bone metabolism

The anabolic action of growth hormone (GH) on bone is well demonstrated by the short stature and delayed bone maturation in children with GH deficiency and in acromegalic patients with increased cortical bone mass. The body growth is regulated by growth hormone and insulin-like growth factor-I (IGF-I). The classic somatomedin hypothesis of this regulation is that most IGF-I in the blood originates in the liver and that body growth is controlled by the concentration of IGF-I in the blood. We have recently abolished IGF-I production in the livers of mice by using the Cre/loxP recombination system. The mice, in which IGF-I production had been inactivated in the liver, displayed a more than 80% reduction in serum IGF-I. In contrast, they demonstrated a normal postnatal growth, indicating that extrahepatic, autocrine/paracrine-acting IGF-I is the main determinant of postnatal growth. GH is also important for normal adult bone remodeling. Adults with GH deficiency have reduced bone mass, and GH treatment increases bone mass in GH-deficient adults. Future clinical studies will determine whether some patients with decreased bone mass for other reasons will benefit from treatment with GH alone or in combination with other treatments.

The relative importance of endocrine versus autocrine/paracrine insulin-like growth factor-I in the regulation of body growth

Body growth is regulated by growth hormone (GH) and insulin-like growth factor-I (IGF-I). The classical somatomedin hypothesis of this regulation is that most IGF-I in the blood originates in the liver and that body growth is controlled by the concentration of IGF-I in the blood. We have recently abolished IGF-I production in the livers of mice by using the Cre/loxP recombination system. These mice displayed a more than 75% reduction in serum IGF-I associated with increased serum levels of GH. In contrast, they demonstrated a normal postnatal growth, indicating that extrahepatic, autocrine/paracrine-acting IGF-I is the main determinant of postnatal growth. Thus, the "classical" somatomedin hypothesis needs revision. We propose the "dual somatomedin hypothesis" according to which: (1) autocrine/paracrine IGF-I is the main determinant of postnatal body growth and (2) liver-derived, endocrine-acting, IGF-I exerts negative feedback on GH secretion and possibly also exerts other effects on carbohydrate and lipid metabolism.

The GH secretagogues ipamorelin and GH-releasing peptide-6 increase bone mineral content in adult female rats

Growth hormone (GH) is of importance for normal bone remodelling. A recent clinical study demonstrated that MK-677, a member of a class of GH secretagogues (GHSs), increases serum concentrations of biochemical markers of bone formation and bone resorption. The aim of the present study was to investigate whether the GHSs, ipamorelin (IPA) and GH-releasing peptide-6 (GHRP-6), increase bone mineral content (BMC) in young adult female rats. Thirteen-week-old female Sprague-Dawley rats were given IPA (0.5 mg/kg per day; n=7), GHRP-6 (0.5 mg/kg per day; n=8), GH (3.5 mg/kg per day; n=7), or vehicle administered continuously s.c. via osmotic minipumps for 12 weeks. The animals were followed in vivo by dual X-ray absorptiometry (DXA) measurements every 4th week. After the animals were killed, femurs were analysed in vitro by mid-diaphyseal peripheral quantitative computed tomography (pQCT) scans. After this, excised femurs and vertebrae L6 were analysed by the use of Archimedes' principle and by determinations of ash weights. All treatments increased body weight and total tibial and vertebral BMC measured by DXA in vivo compared with vehicle-treated controls. However, total BMC corrected for the increase in body weight (total BMC:body weight ratio) was unaffected. Tibial area bone mineral density (BMD, BMC/area) was increased, but total and vertebral area BMDs were unchanged. The pQCT measurements in vitro revealed that the increase in the cortical BMC was due to an increased cross-sectional bone area, whereas the cortical volumetric BMD was unchanged. Femur and vertebra L6 volumes were increased but no effect was seen on the volumetric BMDs as measured by Archimedes' principle. Ash weight was increased by all treatments, but the mineral concentration was unchanged. We conclude that treatment of adult female rats with the GHSs ipamorelin and GHRP-6 increases BMC as measured by DXA in vivo. The results of in vitro measurements using pQCT and Archimedes' principle, in addition to ash weight determinations, show that the increases in cortical and total BMC were due to an increased growth of the bones with increased bone dimensions, whereas the volumetric BMD was unchanged.

Characterization of hepatocyte-derived mitogenic activity on hepatic stellate cells

BACKGROUND/AIMS

Hepatic stellate cells (HSC) are located in close proximity to hepatocytes in Disse's space. Hepatocyte derived factors have earlier been implicated in the paracrine regulation of HSC proliferation. The aim of the present study was to further characterize this mitogenic activity of the parenchymal cell conditioned medium (PCcM).

METHODS

Primary rat HSC were cultured for 4 days. DNA synthesis was measured by 3H-thymidine incorporation. TGFbeta1 immunoreactivity was quantified by ELISA. PCcM was obtained from hepatocytes cultured in medium without serum or hormones for two days.

RESULTS

Incubation of 4-day-old HSC on plastic surface with PCcM for 2 days increased DNA synthesis, while no effect was seen in HSC cultured on Matrigel. Heat-, acid-, and protease-treatment of PCcM abolished its stimulatory effect. Size fractionations with spin columns indicated that the stimulatory effect was contained in the fractions of a molecular size between 30 and 100 kD. The addition of LY 294002, a phosphatidylinositol 3-kinase (PI3-K) inhibitor, dose-dependently inhibited the PCcM induced increase in DNA synthesis to about 9% of the control values. The specific MAP kinase (MAPK) inhibitor, PD 98059 only suppressed the PCcM induced DNA synthesis to 35% of control cultures at the highest dose (10 microM). DNA content in the cultures was not affected by either blocker. HSC seemed to produce immunoreactive TGFbeta1. However, addition of latency-associated peptide (LAP), a potent TGFbeta1 blocker, stimulated DNA synthesis to a much less extent than PCcM.

CONCLUSIONS

The factor(s) that stimulate DNA synthesis in HSC from hepatocytes are most likely protein(s) with a molecular size between 30-100 kD. These factor(s) rely more on PI3-K than on MAPK for their mitogenic effect and are probably not acting via TGFbeta1 inhibition.

Growth hormone induces CCAAT/enhancer binding protein alpha (C/EBPalpha) in cultured rat hepatocytes

BACKGROUND/AIMS

The transcription factor CCAAT/enhancer binding protein alpha (C/EBPalpha) is a transactivator of several genes in the liver, which are regulated by growth hormone.

METHODS

Growth hormone (100 ng/ml) was added to primary rat hepatocytes cultured on a laminin-rich matrix. C/EBP mRNA and protein levels were measured by RNase protection assay and Western blotting, respectively. DNA binding activity was measured by electrophoretic mobility shift assay (EMSA).

RESULTS

Growth hormone treatment for 6 h to 3 days increased C/EBPalpha mRNA levels. Addition of growth hormone for 24 h and 4 days also enhanced the levels of the 42 and 30 kDa isoforms of immunoreactive C/EBPalpha. EMSA showed that addition of growth hormone for 24 h enhanced the abundance of a protein complex binding to a consensus C/EBP binding DNA oligonucleotide. This protein complex was supershifted by antibodies directed against C/EBPalpha but not against C/EBPbeta. There were no consistent effects on C/EBPbeta mRNA or protein at any timepoint. The growth hormone effect on C/EBPalpha expression was not affected by simultaneous incubation with insulin or glucocorticoids, two hormones that previously have been reported to affect C/EBPs.

CONCLUSIONS

Growth hormone enhances the levels of C/EBPalpha mRNA and protein as well as the DNA binding activity of C/EBPalpha in cultured rat hepatocytes.

Overexpression of the hepatocyte growth factor (HGF) receptor (Met) and presence of a truncated and activated intracellular HGF receptor fragment in locally aggressive/malignant human musculoskeletal tumors

Enhanced hepatocyte growth factor (HGF) receptor (Met) signaling has been suggested to play an important role in the development and progression of various epithelial and nonepithelial tumors. N-terminally truncated forms of the HGF receptor have been shown to be constitutively activated and tumorigenic in animal experiments. In the present study, 102 benign and malignant human musculoskeletal tumors were examined for expression of the HGF receptor by Western blotting and/or immunohistochemistry. A clear predominance of HGF receptor expression was seen in malignant as compared to benign tumors (Western blotting, P < 0.001; immunohistochemistry, P < 0.02). For the first time we show HGF receptor expression in the following four tumor types: dermatofibrosarcoma protuberans, clear cell sarcoma of tendons, malignant primitive neuroectodermal tumor, and benign fibrous histiocytoma. In three cases of sarcoma with high HGF receptor expression by Western blotting, we found indications of a short 85-kd N-terminally truncated HGF receptor that was tyrosine phosphorylated and located in the cytoplasm. Although fragments of this length were seen in 18 of 65 tumors, most were not tyrosine-phosphorylated. Northern blotting revealed only the 7.5-kb full-length HGF receptor transcript, suggesting that the 85-kd fragment is generated by an alternative initiation of translation or by proteolytic cleavage. Southern blotting detected no amplification of the Hgfr/Met gene in the 35 tumors examined, in contrast to our recent report of Hgfr/Met gene amplification in 7, 12-dimethylbenz(a)anthracene (DMBA)-induced rat sarcomas. The present data suggest that the locally aggressive and malignant properties of human mesenchymal tumors maybe related, in part, to high levels of full-length HGF receptors, and in some cases to the occurrence of N-terminally truncated HGF receptors, activated independently of HGF.

Insulin-like growth factor signaling pathways in rat hepatic stellate cells: importance for deoxyribonucleic acid synthesis and hepatocyte growth factor production

It has been shown recently that insulin-like growth factor 1 (IGF-1) increases both DNA synthesis and hepatocyte growth factor (HGF) production in cultured hepatic stellate cells. In this study, we used selective blockers to investigate crucial signaling pathways for these effects of IGF-1 in cultured rat hepatic stellate cells. Both LY 294002 [a phosphatidylinositol 3-kinase (PI3-K) inhibitor], and rapamycin [a blocker of activation of the serine/threonine p70 S6 kinase (p70S6K), a molecule downstream from PI3-K] completely reversed the IGF-1-induced stimulation of DNA synthesis. Mitogen-activated protein kinase (MAPK) inhibition by PD 98059 had a less pronounced suppressory effect, although the used PD 98059 dose was fully effective in inhibiting MAPK phosphorylation. Both LY 294002 and PD 98059 lowered the IGF-1-induced increase of HGF in the medium by about 40%, but LY 294002 was 10 times more potent than PD 98059. Inhibition of p70S6K activation by rapamycin blocked IGF-1-induced DNA synthesis but not the increase in HGF. In conclusion, PI3-K (and, to some extent, MAPK) signaling pathways seem to be important for IGF-1-stimulated DNA synthesis and HGF production. DNA synthesis also seems to be dependent on rapamycin-sensitive activation of the PI3-K effector p70S6K.

Liver-derived insulin-like growth factor I (IGF-I) is the principal source of IGF-I in blood but is not required for postnatal body growth in mice

The body growth of animals is regulated by growth hormone and IGF-I. The classical theory of this regulation is that most IGF-I in the blood originates in the liver and that body growth is controlled by the concentration of IGF-I in the blood. We have abolished IGF-I production in the livers of mice by using the Cre/loxP recombination system. These mice demonstrated complete inactivation of the IGF-I gene in the hepatocytes. Although the liver accounts for less than 5% of body mass, the concentration of IGF-I in the serum was reduced by 75%. This finding confirms that the liver is the principal source of IGF-I in the blood. However, the reduction in serum IGF-I concentration had no discernible effect on postnatal body growth. We conclude that postnatal body growth is preserved despite complete absence of IGF-I production by the hepatocytes.

Treatment of obese subjects with the oral growth hormone secretagogue MK-677 affects serum concentrations of several lipoproteins, but not lipoprotein(a)

Obesity is associated with blunted GH secretion and an unfavorable lipoprotein pattern. The objective of this study was to investigate the effects of treatment with the oral GH secretagogue MK-677 on lipoproteins in otherwise healthy obese males. The study was randomized, double blind, and parallel. Twenty-four obese males, aged 18-50 yr, with body mass index greater than 30 kg/m2 and waist/hip ratio above 0.95 were treated with 25 mg MK-677 (n = 12) or placebo (n = 12) daily for 8 weeks. MK-677 treatment did not significantly change serum lipoprotein(a) [Lp(a)] levels. Serum apolipoprotein A-I and E (apoA-I and apoE) were increased at 2 weeks (P < 0.001 and P < 0.01 vs. placebo, respectively), but were not changed at study end. Serum total cholesterol and low density lipoprotein (LDL) cholesterol (LDL-C) levels were not significantly changed by MK-677 treatment. Serum high density lipoprotein (HDL) cholesterol (HDL-C) was increased at 2 weeks of MK-677 treatment (P < 0.01 vs. placebo), but not at 8 weeks. The LDL-C/HDL-C ratio was reduced after 8 weeks of MK-677 treatment (P < 0.05 vs. placebo). Mean LDL particle diameter was decreased at 2 weeks (P < 0.05 vs. placebo), but was unchanged compared with baseline values at 8 weeks (P = NS vs. placebo). The level of serum triglycerides was increased at 2 (P < 0.05 vs. placebo), but not at 8, weeks. Lipoprotein lipase activity in abdominal and gluteal sc adipose tissue was not affected by active treatment. In conclusion, treatment with the oral GH secretagogue MK-677 affected circulating lipoproteins. The effects on serum apoA-1, apoE, triglycerides, and mean LDL particle diameter were transient. At study end, the LDL-C/HDL-C ratio was decreased. MK-677 treatment did not significantly affect serum Lp(a) concentrations at the present dose and administration protocol.

The growth hormone receptor associates with Jak1, Jak2 and Tyk2 in human liver

Studies in cell lines have shown that Jak2 is the primary tyrosine kinase involved in signal transduction by the growth hormone receptor (GHR). In addition, growth hormone (GH) stimulates tyrosine phosphorylation of Jak1 and Jak3 in certain cell lines, while the effect on Tyk2 has not been analysed. We have investigated the expression of Jak proteins in human liver and analysed their interactions with the GHR. Using Western blot analysis and immunohistochemistry, we demonstrate that Jak1, Jak2, Jak3 and Tyk2 are present in human liver. Immunoprecipitation by an antibody against the GHR (Mab 263) followed by immunoblotting with specific antibodies against Jak proteins showed that Jak1, Jak2 and Tyk2 were associated with the GHR in this tissue. We conclude that the GHR associates with Jak1, Jak2 and Tyk2 in human liver. Although experiments in vitro indicate that Jak2 mediates GH signalling, our results open the possibility that other Jak proteins may influence GHR signalling in human liver.

Amplification and overexpression of the hepatocyte growth factor receptor (HGFR/MET) in rat DMBA sarcomas

In the present study subcutaneous fibrosarcomas were induced by the carcinogen 7,12-dimethylbenz(a)anthracene (DMBA) in rats from F1 generation cross breedings of two different inbred strains. Comparative genomic hybridization (CGH) analysis, which allows detection of DNA sequence copy changes, was applied to one of the tumors and it was found that there were increased copy numbers of sequences at chromosome 4q12-q21 in this tumor. We have previously determined that the loci for the hepatocyte growth factor (Hgf) and hepatocyte growth factor receptor (Hgfr/Met), a protooncogene, are situated in this particular chromosome region. Using probes for the two genes in FISH (fluorescence in situ hybridization) and in Southern blots we found that the Hgfr/Met gene was amplified in five of the 19 sarcomas studied, and that the Hgf gene was coamplified in two of them. Northern and Western blots and tyrosine phosphorylation analysis showed that the HGF receptor was overexpressed and functional in all five tumors, as well as in two additional tumors. In summary, both amplification and overexpression of the Hgfr/Met gene was found in about 25% of DMBA-induced experimental rat sarcomas, and HGF receptor overexpression alone was seen in two additional tumors. Possibly this reflects an involvement in paracrine or autocrine stimulation of growth and invasiveness by HGF. Our finding could provide a rodent model system to increased knowledge about causality and therapy, which may be applicable to the sizeable fraction of human musculoskeletal tumors displaying MET overexpression.

Discrepancy between serum leptin values and total body fat in response to the oral growth hormone secretagogue MK-677

OBJECTIVE

Growth hormone (GH) treatment decreases total body fat while this effect has not yet been documented for the oral GH secretagogue MK-677. In the present study, the effects of MK-677 treatment on serum levels of leptin, thyroid hormones and testosterone were determined.

DESIGN

This was a randomized, double-blind, and parallel study. Twenty-four healthy obese males, 19-49 years of age, with body mass index (BMI) > 30 kg/m2 and a waist:hip ratio > 0.95, were treated with MK-677 (25 mg/day; n = 12) or placebo (n = 12) for 8 weeks.

RESULTS

MK-677 treatment increased serum leptin levels and leptin/body fat ratio at 2 weeks of treatment (P < 0.05 vs. placebo) but no significant change was observed at 8 weeks. An increase in serum free 3, 5, 3'-triiodothyronine (free T3) was not detected until 8 weeks of MK-677 treatment (P < 0.05 vs. placebo). Peak serum thyroid stimulating hormone (TSH) concentration after MK-677 administration was similar to that after placebo administration at initiation of treatment and at 2 weeks. At 8 weeks of MK-677 treatment, mean peak serum TSH concentration was increased (P < 0.05 vs. placebo) although it remained within the normal range. Serum peak values of luteinizing hormone (LH) and follicle stimulating hormone (FSH) were similar after MK-677 and placebo administration. MK-677 treatment reduced serum total testosterone (P < 0.05 vs. placebo) although total testosterone/sex hormone-binding globulin (SHBG) ratio (an index of free testosterone) was not changed.

CONCLUSION

Treatment with the oral GH secretagogue MK-677 transiently increased serum leptin levels and leptin/body fat ratio at 2 weeks of treatment, and increased serum free T3 after 8 weeks. These results indicate that MK-677 treatment is able to affect circulating factors of importance for adipose tissue mass and fuel metabolism.

Hepatocyte-stimulated expression of hepatocyte growth factor (HGF) in cultured rat hepatic stellate cells

BACKGROUND/AIMS

Hepatic stellate cells appear to be the main producers of hepatocyte growth factor of the normal liver. Insulin-like growth factors in doses over 20 ng/ml have been reported to stimulate hepatocyte growth factor production in cultured hepatic stellate cells. The aim of the present study was to investigate whether parenchymal cell conditioned medium had insulin-like growth factor-independent effects on hepatic stellate cells.

METHODS

Primary rat hepatic stellate cells were cultured for 1-7 days. DNA synthesis was measured by 3H-thymidine incorporation. Hepatocyte growth factor and transforming growth factor beta1 immunoreactivity was quantified by ELISA. Hepatocyte growth factor mRNA levels were determined with gel RNase protection assay. Parenchymal cell conditioned medium was obtained from hepatocytes cultured for 2 days in medium without added serum or hormones.

RESULTS

Incubation of 1-7-day-old hepatic stellate cells for 2 days with parenchymal cell conditioned medium enhanced the medium content of hepatocyte growth factor. Parenchymal cell conditioned medium contained less than 5.0 ng/ml immunoreactive insulin-like growth factor-1 as measured by radio immunoassay. Parenchymal cell conditioned medium did not contain any insulin-like growth factor bioactivity measured as phosphorylation of type 1 insulin-like growth factor receptor beta subunit and a protein with a size consistent with that of insulin receptor substrate-1. The stimulatory effect of parenchymal cell conditioned medium on hepatocyte growth factor was time- and dose-dependent. The effects of a high dose of parenchymal cell conditioned medium (dilution 1:2 containing less than 2.5 ng/ml insulin-like growth factor-1) were additive to that of high doses (100 ng/ml) of insulin-like growth factor-1 or des (1-3) insulin-like growth factor-1, an analogue with low affinity to insulin-like growth factor binding proteins. Neither parenchymal cell conditioned medium nor insulin-like growth factor-1 enhanced transforming growth factor beta1 immunoreactivity in the medium. Both parenchymal cell conditioned medium and insulin-like growth factor-1 stimulated DNA synthesis in hepatic stellate cells, confirming previous reports.

CONCLUSIONS

The present results indicate that both insulin-like growth factor-1 and insulin-like growth factor-1-independent factors from hepatocytes can stimulate hepatocyte growth factor production by hepatic stellate cells. Therefore, insulin-like growth factor-1 and other hepatocyte-derived factors may indirectly affect hepatocytes via a paracrine loop.

Treatment with the oral growth hormone secretagogue MK-677 increases markers of bone formation and bone resorption in obese young males

The effect of 2 months of treatment with the oral growth hormone (GH) secretagogue MK-677 on markers of bone metabolism was determined in healthy obese male subjects. This was a randomized, double-blind, parallel, placebo-controlled study. Twenty-four healthy obese males, 19-49 years of age, with body mass index > 30 kg/m2 were treated with MK-677 (25 mg/day; n = 12) or placebo (n = 12) for 8 weeks. MK-677 increased markers of bone formation; a 23% increase in the carboxy-terminal propeptide of type I procollagen levels and a 28% increase in procollagen III peptide levels were seen with as little as 2 weeks of MK-677 treatment (p < 0.01 and p = 0.001 vs. placebo, respectively) while a 15% increase in serum levels of osteocalcin was not detected until 8 weeks of treatment (p < 0.01 vs. placebo). Markers of bone resorption were induced within 2 weeks of treatment with MK-677; serum levels of the carboxy-terminal cross-linked telopeptide of type I collagen were increased 26% at 8 weeks (p = 0.001 vs. placebo), and urine hydroxyproline/creatinine and calcium/creatinine ratios at 8 weeks were increased by 23% (p < 0.05 vs. placebo) and 46% (p < 0.05 vs placebo), respectively, MK-677 increased serum insulin-like growth factor binding protein-5 (IGFBP-5) by 43-44% after 2-8 weeks of treatment (p < 0.01 vs. placebo). Serum IGFBP-4 was increased by 25% after 2 weeks of treatment (p < 0.001 vs. placebo) but no significant change from baseline was observed after 8 weeks of treatment. Plasma interleukin-6 was not significantly changed by active treatment. In conclusion, short-term treatment of healthy obese male volunteers with the GH secretagogue MK-677 increases markers of both bone resorption and formation. Large increases in serum levels of IGF-1 and IGFBP-5 and a transient increase in serum IGFBP-4 were found. Future long-term studies are needed to investigate if prolonged treatment with MK-677 increases bone mass.

Two-month treatment of obese subjects with the oral growth hormone (GH) secretagogue MK-677 increases GH secretion, fat-free mass, and energy expenditure

Obesity is associated with blunted GH secretion, unfavorable body composition, and increased cardiovascular mortality. The objective of this study was to investigate the effects of oral treatment with the GH secretagogue MK-677 on GH secretion and body composition in otherwise healthy obese males. The study was randomized, double blind, parallel, and placebo controlled. Twenty-four obese males, aged 18-50 yr, with body mass indexes greater than 30 kg/m2 and waist/hip ratios greater than 0.95, were treated with MK-677 25 mg (n = 12) or placebo (n = 12) daily for 8 weeks. Serum insulin-like growth factor I (IGF-I) increased approximately 40% with MK-677 treatment (P < 0.001 vs. placebo). Serum IGF-binding protein-3 was also significantly increased (P < or = 0.001 vs. placebo). GH and PRL (peak and area under the curve values) were significantly increased after the initial dose of MK-677. Significant increases, with the exception of peak PRL, persisted at 2 and 8 weeks of treatment. The increases in GH and PRL after the initial dose were significantly greater than the increase seen after multiple doses. Serum and urinary concentrations of cortisol were not increased at 2 and 8 weeks (P = NS, vs. placebo). Fat-free mass increased significantly in the MK-677 treatment group when determined with dual energy x-ray absorptiometry (P < 0.01) or using a four-compartment model (P < 0.05). Total and visceral fat were not significantly changed with active therapy. The basal metabolic rate was significantly increased at 2 weeks of MK-677 treatment (P = 0.01) but not at 8 weeks (P = 0.1). Fasting concentrations of glucose and insulin were unchanged, whereas an oral glucose tolerance test showed impairment of glucose homeostasis at 2 and 8 weeks. We conclude that 2-month treatment with MK-677 in healthy obese males caused a sustained increase in serum levels of GH, IGF-I, and IGF-binding protein-3. The effects on cortisol secretion were transient. Changes in body composition and energy expenditure were of an anabolic nature, with a sustained increase in fat-free mass and a transient increase in basal metabolic rate. Further studies are needed to evaluate whether a higher dose of MK-677 or a more prolonged treatment period can promote a reduction in body fat.

Acromegaly and Cushing's syndrome due to ectopic production of GHRH and ACTH by a thymic carcinoid tumour: in vitro responses to GHRH and GHRP-6

A 50-year-old male presented with diabetes mellitus and Cushing's syndrome associated with a large mediastinal mass. The levels of serum cortisol were high (1500-1800 nmol/l) without diurnal variation. Plasma ACTH levels (200-250 ng/l) and urinary excretion of cortisol were also increased. The levels of these hormones did not change in response to stimulation with corticotrophin releasing hormone (CRH) or suppression with high doses of dexamethasone. The patient had an elevated baseline GH level (7.3 mU/l), and the levels of immunoreactive GH-releasing hormone (GHRH) in eight plasma samples were markedly increased (600-1500 ng/l). Circulating levels of IGF-1, chromogranin A and neuropeptide Y (NPY) were also increased. Computer-assisted tomography and octreotide scintigraphy revealed a large mediastinal tumour and metastases in the left supraclavicular fossa. During treatment with octreotide, the baseline GH level was decreased (to 4.4 mU/l), while the GH pulse height was unchanged. Surgical removal of most of the tumour tissue resulted in a further decrease in the baseline serum GH level to a value (1.6 mU/l) about 20% of that before treatment, while the pulse height and mean GH were affected to a lesser extent. Postoperatively, circulating levels of cortisol and IGF-1 decreased, and the patient exhibited clinical improvement. Histological examination showed a neuroendocrine tumour with characteristics consistent with a foregut carcinoid of thymic origin. Immunoreactive GHRH, ACTH and NPY, but not immunoreactive GH, were detected in 80-90% of the tumour cells and the three peptides appeared to be co-localized. In primary culture, cells from this tumour displayed calcium influx in response to GHRH or GH releasing peptide-6 (GHRP-6), while there were not such responses by cells from another carcinoid not producing GHRH, ACTH or NPY. These results demonstrate a rare case of ectopic production of GHRH, ACTH and NPY, and indicate that the tumour cells were responsive to GHRH and GHRP-6 as well as octreotide.

Insulin-like growth factors stimulate expression of hepatocyte growth factor but not transforming growth factor beta1 in cultured hepatic stellate cells

Hepatic stellate cells (HSC) are located adjacent to hepatocytes and produce hepatocyte growth factor (HGF) in the normal liver, whereas transformed HSC in fibrotic livers produce transforming growth factor beta1 (TGFbeta1), an inhibitor ofhepatocyte proliferation. In addition to the endocrine actions of hepatic insulin-like growth factor-I (IGF-I), it also stimulates the proliferation of HSC. In this study we found that addition of IGF-1 (20-500 ng/ml) for 48 h to 2- to 7-day-old primary cultures of rat HSC resulted in a time- and dose-dependent increase by 50-190% of the concentrations of immunoreactive HGF in the medium. The levels of HGF as well as DNA synthesis measured as thymidine incorporation were also enhanced by IGF-II and des(1-3)IGF-I, which has reduced binding to IGF binding proteins. There was no consistent effect of the IGFs on the levels of immunoreactive TGFbeta1 or on the total DNA content of the cultures. There was no effect of human GH on medium levels of HGF or TGFbeta1, thymidine incorporation, or total DNA content. IGF-I increased the abundance of HGF messenger RNA, as measured by the RNase protection/solution hybridization technique, whereas there was no effect on TGFbeta1 or glyceraldehyde phosphate dehydrogenase messenger RNA. The results suggest that IGFs stimulate the production of HGF but not TGFbeta1 by HSC in vitro.

Changes in expression of CCAAT/enhancer binding protein alpha (C/EBP alpha) and C/EBP beta in rat liver after partial hepatectomy but not after treatment with cyproterone acetate

BACKGROUND/AIMS

The proliferation rate of adult rat liver is normally very low. It is markedly enhanced during compensatory regeneration, e.g. after partial hepatectomy, or after administration of certain growth promoters, e.g. cyproterone acetate. These two types of liver cell proliferation appear to differ, since the expression of immediate early genes is induced during compensatory regeneration but not after cyproterone acetate treatment. The transcription factor C/EBP alpha, which has been associated with hepatocyte differentiation and growth arrest, is suppressed during compensatory regeneration. In contrast, C/EBP beta, associated with acute phase reaction, is increased during regeneration. We have investigated the effects of the liver growth promoter cyproterone acetate on the hepatic expression of C/EBP alpha and C/EBP beta.

METHODS

Adult male rats received either cyproterone acetate treatment or were subjected to partial hepatectomy. Livers were obtained at different time intervals for measurement of C/EBP alpha and C/EBP beta mRNA with solution hybridization/RNAse protection assay, and C/EBP alpha and C/EBP beta content with immunoblotting.

RESULTS

The levels of both C/EBP alpha and C/EBP beta mRNA and the corresponding immunoreactivities were unchanged 2-48 h after injection of cyproterone acetate. The levels of C/EBP alpha mRNA and immunoreactivity were significantly suppressed 10-18 h and 18-26 h after partial hepatectomy, respectively. The levels of C/EBP beta mRNA and immunoreactivity were enhanced during compensatory regeneration 2 h after partial hepatectomy.

CONCLUSIONS

Liver cell proliferation during regeneration, but not in response to cyproterone acetate treatment, is associated with changes in C/EBP alpha and C/EBP beta expression. This further supports the notion that changes in expression of transcription factors during liver growth in vivo are dependent on the growth inducer.

Chromosomal localization of rat hepatocyte growth factor (Hgf) and HGF receptor (Met) and characterization of HGF receptor cDNA

The Met protooncogene encodes the tyrosine kinase receptor for the hepatocyte growth factor (HGF), a potent mitogen for hepatocytes and other epithelial cells produced by mesenchymal cells. Many of the studies on the physiologic and neoplastic growth of the liver, as well as other organs, have been performed in the rat. Therefore, chromosomal mapping of the rat Hgf gene and the gene of its receptor is of particular value. To achieve this, a probe of the coding part of rat HGF cDNA was used to isolate four genomic probes from a lambda phage rat genomic library. These probes were used to map the Hgf gene to Chromosome (Chr) 4q12 by the FISH technique. To obtain a probe for the mapping of the HGF receptor/Met gene, we cloned the complete coding region of the rat HGF receptor mRNA. Complementary DNA (cDNA) was synthesized with reverse transcriptase from total RNA for use as a template for the PCR. The two PCR primers were designed based on human and mouse sequences and were located in the flanking regions of the open reading frame of the HGF receptor mRNA. Amplification resulted in a band of an estimated size of 4.1 kb, which was cloned and sequenced. The nucleotide sequence showed about 93% and 85% homology compared with mouse and human HGF receptor sequences, respectively. A full-length probe of the coding part of the cDNA was used to map the rat HGF receptor/Met gene to Chr 4q21 by the FISH technique. Therefore, the rat Hgf and HGF receptor/Met genes are located relatively close to each other, in a way similar to humans but not mice.

Topical zinc oxide treatment increases endogenous gene expression of insulin-like growth factor-1 in granulation tissue from porcine wounds

Application of zinc oxide has been shown to accelerate the healing of both chronic and acute wounds, but the mechanisms are unknown. We quantified the gene expression (mRNA) for one important growth factor, insulin-like growth factor-1 (IGF-1) in 12 full-thickness wounds in each of three domestic pigs treated with or without topical zinc oxide. We used a RNAase protection/solution hybridisation technique to measure IGF-1 mRNA concentrations, which were 50% higher in the granulation tissue in wounds treated with zinc oxide compared with control wounds on days 3-4 (p < 0.05), but not thereafter (up to postoperative day 11). Topical zinc oxide increased the healing rate of wounds compared to the control group (p < 0.01). The cell composition of the granulation tissue was similar in the two groups. The increased gene expression of IGF-1 may be one mechanism by which topical zinc oxide enhances wound healing.

Increased gene expression of scatter factor-hepatocyte growth factor and basic fibroblast growth factor in granulation tissue in the rat

Scatter factor-hepatocyte growth factor is a protein secreted by fibroblasts which disperses colonies of epithelial cells and keratinocytes in culture. The factor is also a patent mitogen for hepatocytes, synthesized in the liver. Basic fibroblast growth factor, another heparin-binding factor, is most abundant in the brain but also plays a role in wound healing. Using a solution hybridization/RNAase protection assay, we have measured the abundance of messenger RNA for scatter factor-hepatocyte growth factor and basic fibroblast growth factor in granulation tissue obtained from subcutaneously Hunt-Schilling wound cylinders. The levels of scatter factor-hepatocyte growth factor messenger RNA increased after weeks 2 through 4 to a twofold higher level in weeks 5 through 7 after implantation of the cylinders, whereas no changes in basic fibroblast growth factor messenger RNA levels were noticed. At week 3 after implantation of the cylinders, scatter factor-hepatocyte growth factor messenger RNA levels in granulation tissue were more than threefold higher than in skin dermis fibroblasts but markedly lower than in the liver. The abundance of basic fibroblast growth factor messenger RNA was also significantly increased in granulation tissue compared with dermis but, as expected, markedly lower than in the brain. In conclusion, the gene expression of the scatter factor-hepatocyte growth factor, as well as basic fibroblast growth factor, is increased in granulation tissue. Because there was a time-dependent increase in the expression of scatter factor-hepatocyte growth factor, it is hypothesized that scatter factor-hepatocyte growth factor acts as a signal from fully developed granulation tissue to stimulate skin epithelial cells to scatter over the wound.

Growth hormone promotes early initiation of hepatocyte growth factor gene expression in the liver of hypophysectomized rats after partial hepatectomy

GH accelerates hepatic regeneration in the rat. Hepatocyte growth factor (HGF), a potent hepatocyte mitogen in vitro, is considered to be a major regulator of hepatic regeneration. In the present study, the effects of GH and insulin-like growth factor-I (IGF-I) on HGF gene expression in regenerating rat liver was investigated. In hypophysectomized rats treated with GH, hepatic HGF mRNA levels were increased 3 h after partial hepatectomy and reached peak levels after 5 h. In rats with intact pituitaries and in hypophysectomized rats not given GH treatment, HGF mRNA levels in liver were unchanged during the first 5 h following hepatectomy and reached peak levels after 10-18 h. DNA synthesis in the liver of GH-treated rats increased from low levels 10 h after hepatectomy to peak levels after 18 h. In rats without GH treatment the synthesis of DNA was still low 18 h after hepatectomy and was increased after 26 h. Treatment of hypophysectomized rats with IGF-I promoted increases in hepatic HGF mRNA levels and DNA synthesis 3.5 h and 15 h after hepatectomy respectively. HGF mRNA levels were constantly lower after sham-hepatectomy than after partial hepatectomy. In summary, in hypophysectomized rats the responses of hepatic HGF gene expression and DNA synthesis to partial hepatectomy were both accelerated by treatment with GH or IGF-I.

Roles of pulsatility and continuity of growth hormone (GH) administration in the regulation of hepatic GH-receptors, and circulating GH-binding protein and insulin-like growth factor-I

GH secretion in the male rat is characterized by regular GH peaks of high amplitude, and their GH receptors and GH-binding protein (BP) are low. Female rats have a more continuous secretion, and their GH receptors and GH-BP are high. To determine how the pattern of GH delivery may affects the physiological responses to the hormone, hypophysectomized male rats were infused with human GH (1.5 U/kg.day) for 5 days through an implanted iv cannula in either a pulsatile manner (for 5 min every 3 h) or continuously. We then measured free and total (after MgCl2 treatment) somatogenic and lactogenic receptors in hepatic cell membranes, and GH-BP and immunoreactive insulin-like growth factor-I (IGF-I) in serum. The concentrations of occupied, but not free, somatogenic and lactogenic binding sites were higher 5 min after a 5-min infusion of human GH than 90 min after the infusion. Consequently, the total levels (free plus ligand binding) of somatogenic and lactogenic receptors were also higher 5 min after a GH pulse than after 90 min. The levels of circulating GH-BP were higher 90 min than 5 min after a GH pulse, and the serum IGF-I levels did not differ between these time points. The concentrations of free and total somatogenic and free lactogenic receptors in the liver, as well as GH-BP and IGF-I in circulation were higher and body weight gain was lower in rats given continuous GH infusion than after pulsatile infusions. It is concluded that the nature of GH-levels profile has a marked influence on the interrelationship of GH, GH receptors, GH-BP, serum IGF-I and growth, whereby the later is stimulated maximally by pulsatile GH-levels, while the receptor and GH-BP are enhanced maximally by continuous levels. The basis for this discrepancy may lay in the competitive power of GH-BP toward GH receptor binding. A pulsatile GH pattern induced cyclicity in the levels of hepatic cell membrane GH receptors and circulating GH-BP. It is suggested that these factors are regulated in a similar way by the plasma GH pattern in pituitary intact rats.

Gene expression of insulin-like growth factor-I and IGF-I receptor during wound healing in rats

OBJECTIVE

To measure the time course of changes in the concentrations of insulin-like growth factor-I (IGF-I), IGF-I messenger RNA (mRNA), and IGF-I receptor mRNA during wound healing.

DESIGN

Open experimental study.

MATERIAL

40 male Sprague-Dawley rats, each weighing 300 g.

INTERVENTION

Stainless steel wire mesh cylinders implanted in the subcutaneous tissue of the back. Five rats killed at each time point (1.5, 2, 3, 4, 5, 7, 11, and 18 weeks).

MAIN OUTCOME MEASURES

Wet weight and concentrations of IGF-I mRNA, and IGF-I receptor mRNA of granulation tissue, and concentration of IGF-I in wound fluid.

RESULTS

The wet weight of granulation tissue increased significantly between week 1.5 and weeks 4-5, and then decreased. IGF-I mRNA concentration (amol/microgram DNA) increased significantly (threefold) between week 1.5 and weeks 3-5, and decreased between weeks 5 and 7. The concentration of IGF-I receptor RNA remained constant throughout the study, and the concentration of IGF-I in wound fluid remained constant until week 8 and then increased to a higher level at weeks 11-18.

CONCLUSION

There is a transient rise in the gene expression of IGF-I but not of IGF-I receptor mRNA during wound healing.

Growth hormone stimulates granulation tissue formation and insulin-like growth factor-I gene expression in wound chambers in the rat

It has been reported that GH stimulates fibroblast growth and wound healing. In the present study we measured the effect of locally applied GH on insulin-like growth factor (IGF-I) mRNA concentrations and granulation tissue formation in wound cylinders, implanted s.c. Four stainless-steel wiremesh cylinders were implanted s.c. in the back of male rats (280 g). Each cylinder was then injected every day with either 0.014 or 0.14 U human GH, or vehicle only. Ingrown granulation tissue and wound fluid was obtained on day 17 after implantation. The wet weight of granulation tissue was determined and concentrations of IGF-I mRNA in the tissue were measured by solution hybridization/RNAase protection assay. Similar assays were used to measure the levels of IGF-I receptor mRNA and GH receptor mRNA, while the IGF-I concentration in wound fluid and serum was determined by radioimmunoassay (RIA) after acid-ethanol extraction. The concentrations of IGF-I mRNA in ingrown granulation tissue as well as the wet weight of this tissue were significantly higher in the GH-treated cylinders. There was no significant effect of GH on IGF-I receptor mRNA and GH receptor mRNA levels. Consistent with the results of previous studies, wound fluid IGF-I levels were lower than serum IGF-I levels, but no significant difference was found between the GH-treated cylinders and the control cylinders. The results of the present study show that GH stimulates granulation tissue formation and increases the concentration of IGF-I mRNA in the ingrown granulation tissue.

The plasma pattern of growth hormone in conscious rats during late pregnancy

The plasma GH levels of female rats during late pregnancy were determined using an automatic method for repetitive blood sampling from conscious animals. The plasma GH patterns were analysed by a pulse analysis computer program (PULSAR). The mean plasma GH levels were about twofold higher in pregnant females on days 15, 18 and 22 of gestation than in age-matched non-pregnant females. The basal plasma GH levels were also increased, while there was no change in GH pulse amplitude or frequency. The augmentation of GH release was even more pronounced on day 20 of gestation, with a fourfold increase in mean plasma GH levels compared with those in non-pregnant females. This increase reflected an increase in both basal plasma GH levels and GH pulse amplitude, but there was no increase in pulse frequency. In female rats that delivered on day 22 of gestation, the basal and mean plasma GH levels increased during parturition. Pregnant females consistently responded to multiple i.v. infusions of 1 microgram human GH-releasing factor analogue (hGRF(1-29)-NH2) given at 45-min intervals on day 18 of gestation. Both basal and GRF analogue-stimulated plasma GH levels were undetectable after hypophysectomy of pregnant rats. The present study demonstrates an increase in basal plasma GH levels during late pregnancy and a marked increase in both basal plasma GH levels and GH pulse amplitude on day 20 of gestation. Furthermore, hypophysectomy of pregnant rats results in undetectable GH levels, indicating that the high levels of GH during pregnancy are derived from the pituitary.

Endogenous growth hormone (GH) secretion in male rats is synchronized to pulsatile GH infusions given at 3-hour intervals

The feedback effects of GH on its own secretion were studied in conscious male rats receiving intermittent iv infusions of human GH. Male Sprague-Dawley rats (150-180 g) were implanted with double bore iv cannula. Infusions of human GH (hGH) or buffer were given for up to 32 h, while frequent microsamples (20 microliters) of blood were withdrawn simultaneously using an automatic blood-sampling system. The endogenous GH pulses became synchronized to pulsatile hGH infusions (2.1 U/kg.infusion) given at 3-h intervals. After two or more hGH infusions episodic GH release was present in most rats, and all endogenous pulses occurred concomitantly with the hGH infusions. After 24 h of hGH treatment the endogenous pulses were still synchronized to the every 3 h hGH infusions. In addition, the pulse amplitude was lower than that in vehicle-treated animals (74 +/- 12 vs. 215 +/- 35 ng/ml; P less than 0.01). At this time a complete (1.5-h) phase shift of the 3-hourly hGH infusions markedly suppressed endogenous GH pulses in all rats. In another experiment where the same daily dose of hGH was given in iv infusions every 1.5 h instead of every 3 h, the endogenous GH pulses were irregular, infrequent, and suppressed. Infusions at 3-h intervals of a lower dose of hGH (0.42 U/kg.infusion) did not affect the timing or amplitude of endogenous GH pulses compared to those in buffer-infused animals. The endogenous GH pulses were not synchronized between animals given 0.42 U/kg hGH or buffer at 3-h intervals. It is concluded that the endogenous GH pulses in male rats became synchronized to intermittent infusions of hGH at 3-h (but not 1.5-h) intervals. The fact that there were no endogenous pulses between the 3-hourly infusions suggests that the feedback effect of a GH pulse lasts for approximately 3 h. This mechanism may be involved in the control of the GH secretory pattern in male rats.

Plasma growth hormone pattern regulates epidermal growth factor (EGF) receptor messenger ribonucleic acid levels and EGF binding in the rat liver

It has recently been shown that GH increases the number of available hepatic receptors for epidermal growth factor (EGF). In the present study the effects of the sexually dimorphic plasma GH pattern (higher pulsatility in male rats) on hepatic EGF binding and EGF receptor mRNA concentration were investigated. The specific binding of [125I]EGF to purified liver membranes was about 2-fold higher in male rats than in females on days 35, 50, and 80 of life. EGF receptor mRNA levels, as determined by an RNase protection solution hybridization assay, were higher in males only on days 47-50. Hypophysectomy on day 50 reduced the EGF receptor mRNA concentration to a level that did not differ between male and female rats. In hypophysectomized rats of both sexes, intermittent GH treatment (sc injections every 12 h for 7 days) enhanced hepatic EGF receptor mRNA concentrations to normal male levels, while continuous GH administration was less effective. Northern blot analysis indicated that transcripts with apparent sizes of 9.5 and 6.6 kilobases were dependent on the plasma GH pattern. Intermittent iv GH replacement therapy for 5 days given at 3-h intervals by an automatic iv infusion system increased the hepatic EGF receptor mRNA concentration as well as specific EGF binding, whereas continuous iv GH infusion was ineffective. These results show that a pulsatile plasma GH pattern, similar to that of male rodents, is markedly more effective in enhancing hepatic EGF receptor mRNA levels and EGF binding than a continuous feminine GH pattern. These results are consistent with a pretranslatory stimulation of EGF receptor synthesis by pulsatile GH.

Plasma growth hormone pattern and androgens influence the levels of corticosteroid-binding globulin in rat serum

The serum concentration of corticosteroid-binding globulin (CBG) is higher in female rats than in males. Combined hypophysectomy and gonadectomy of female rats reduced the serum concentration of CBG as measured by steady-state polyacrylamide gel electrophoresis, whereas hypophysectomy of male rats increased serum CBG. These effects were seen despite replacement therapy with thyroxine and glucocorticoids. Moreover, neither androgen nor oestrogen treatment affected the serum concentrations of CBG in hypophysectomized rats. Continuous infusions of human or bovine GH (1.4 U/kg per day), by means of osmotic minipumps for 1 week, increased serum concentrations of CBG in both hypophysectomized male and female rats. In contrast, intermittent GH replacement therapy by s.c. injections at 12-h intervals either had no effect or suppressed serum CBG levels. In male rats, neonatal (days 1-2) gonadectomy increased CBG levels more than did prepubertal (day 25) gonadectomy, and testosterone replacement therapy reversed these effects. It is concluded that GH increases the serum CBG levels of hypophysectomized rats when it is given in a continuous manner, but not when given intermittently. The sex difference in serum CBG levels of normal rats may, therefore, be attributed to the more continuous secretory pattern of GH previously observed in female rats.

Pulsatile intravenous growth hormone (GH) infusion to hypophysectomized rats increases insulin-like growth factor I messenger ribonucleic acid in skeletal tissues more effectively than continuous GH infusion

In this study we have investigated a possible functional role of the plasma pattern of GH in regulation of insulin-like growth factor I (IGF-I) mRNA in liver, skeletal muscle, and rib growth plate of the rat. Hypophysectomized male rats given T4 and glucocorticoid replacement therapy were equipped with indwelling jugular venous cannulae attached via swivels to a multichannel pumping system programmed to deliver human GH in a continuous or pulsatile (one pulse per 3 h) pattern for 5 days. At the end of the experiment, skeletal muscle, rib growth plates, and liver from intact and hypophysectomized rats were homogenized, and total nucleic acid was prepared. IGF-I mRNA was quantitated by solution hybridization assay using a RNA probe radiolabeled with [35S]UTP. Pulsatile treatment with GH in a dose of 1.5 U/kg.day induced a 3- to 5-fold increase in the levels of IGF-I mRNA in skeletal muscle and rib growth plates. In contrast, continuous infusion with GH was much less effective in these tissues. In the liver both continuous and pulsatile GH infusion significantly elevated the amount of IGF-I mRNA, and there was no significant difference between these two treatments. In the tissues studied similar results were obtained with a higher dose of GH (3.0 U/kg.day). Pulsatile GH treatment stimulated longitudinal bone growth more effectively than continuous GH treatment, confirming earlier studies. It is concluded that pulsatile GH treatment is more effective than continuous GH infusion in increasing IGF-I mRNA levels in rib growth plate and skeletal muscle, i.e. two major target organs for the anabolic effects of GH.

Growth hormone enhances hepatic epidermal growth factor receptor concentration in mice

The effect of growth hormone (GH) on binding of epidermal growth factor (EGF) to liver membrane preparations was investigated in hypophysectomized mice and partially GH-deficient, genetic mutant "little" (lit/lit) mice. The EGF binding of normal male mice and testosterone-treated females was higher than in normal females. Due to diminished receptor concentration, hepatic EGF binding was decreased in male and female lit/lit mice to a level that was unaffected by gender or androgen treatment. GH replacement therapy by intermittent injections and continuous infusion restored the EGF binding of hypophysectomized mice to normal male and female levels, respectively, suggesting a role for the more pulsatile GH secretion in normal males. In lit/lit mice, however, both continuous and intermittent GH resulted in EGF binding levels comparable to those in normal females. In normal males continuous GH suppressed EGF binding. In conclusion, endogenous GH secretion induces EGF receptors in mice and this effect may be modulated by sex differences in GH secretion.

Effects of sex steroids on growth hormone responses to clonidine and GHRH in reserpine pretreated rats

Administration of reserpine in a dose causing depletion of brain monoamines led to a complete suppression of the pulsatile secretory pattern of growth hormone (GH) in gonadectomized (GX) as well as in sham-operated male and female rats. In GX animals of both sexes treated with estradiol, but not in those treated with testosterone or dihydrotestosterone (DHT), the reserpine induced inhibition of GH release was partially antagonized. Administration of the alpha 2-adrenoceptor agonist clonidine caused secretion of GH in reserpine pretreated, sham-operated rats. In GX male rats GH responses to clonidine were blunted, while in GX males treated with testosterone or estradiol, but not in those treated with DHT, the responses were restored. In female rats gonadectomy did not significantly affect the GH releasing effect of clonidine. However, administration of estradiol to GX females led to enhanced responses to the alpha 2-agonist. Administration of the GH releasing hormone (GHRH) induced pronounced GH secretion in reserpine pretreated animals of both sexes; this effect was not significantly affected by gonadectomy. In GX males, however, GH responses to GHRH were enhanced by replacement with estradiol or testosterone, while in GX females, estradiol, but not testosterone, had the same effect.

Plasma pattern of growth hormone regulates sexual differentiation of phosphatidylcholine in rat plasma

The effects of continuous and intermittent administration of human growth hormone (hGH) in the regulation of plasma phosphatidylcholine fatty acid composition was studied in adult hypophysectomized male and female Sprague-Dawley rats. Human GH was given for 7 days, either continuously by means of Alzet osmotic minipumps implanted s.c. or intermittently by means of s.c. injections at 12-h intervals. The 'masculinizing' effect of hypophysectomy on the fatty acid composition of plasma phosphatidylcholine in female rats, i.e. an increase in the proportions of palmitic and linoleic acids and a decrease in the proportions of stearic and arachidonic acids, was to a large extent reversed by continuous administration of hGH, while intermittent administration had no consistent effect. This effect of continuous hGH administration similar effect of continuous administration of hGH was also observed in hypophysectomized male rats. Prolactin had no effect on the fatty acid composition of phosphatidylcholine. Replacement therapy with thyroid hormones and glucocorticoids had a small effect on the proportions of palmitic and stearic acids in plasma phosphatidylcholine. It is concluded that the more continuous secretory pattern of GH in female rats is involved in the sexual differentiation of the fatty acid composition of plasma phosphatidylcholine.

Inhibitory effect of the ovaries on neonatal androgen imprinting of growth hormone secretion in female rats

The effect of neonatal androgen treatment on the GH secretory pattern was examined in intact and ovariectomized adult female rats. Neonatal ovariectomy or sham operation was performed at 1-2 days of age; thereafter, the animals were immediately given testosterone propionate (250 micrograms) or vehicle. Other rats, also treated neonatally with testosterone, were ovariectomized 15-22 days before blood sampling. Plasma GH was measured in blood samples obtained from indwelling intraatrial cannulae every 20 min for 8 h when the animals were 100-140 days old. Plasma GH secretory patterns were analyzed by a pulse analysis computer program (PULSAR). Neonatal testosterone treatment did not affect the GH secretory pattern of female rats with intact ovaries. In contrast, neonatal androgen treatment enhanced GH pulse height as well as mean GH concentration in neonatally ovariectomized female rats to levels comparable to those in intact male rats. Neonatal testosterone administration also significantly increased GH pulse height and mean plasma GH concentration in female rats that were ovariectomized during adulthood. However, the GH secretory pattern of ovariectomized female rats given testosterone neonatally still differed markedly from that of normal males, in that GH pulses occurred less regularly and baseline levels were higher. Pituitary GH content and concentration in neonatally ovariectomized female rats were increased to levels indistinguishable from those in male rats by neonatal testosterone treatment. No significant effect of neonatal testosterone was observed in sham-operated females. Neonatal ovariectomy decreased basal plasma GH levels, but did not affect plasma GH pulse height or pituitary GH levels. The serum estradiol concentration was markedly decreased in ovariectomized female rats, but was unchanged in sham-operated rats given neonatal testosterone, raising the possibility that serum estradiol secretion mediated the antagonistic effect of the ovaries on neonatal androgen imprinting. These results indicate that the presence of ovaries can prevent the stimulatory effect of neonatal androgen exposure on GH storage and secretion in adult female rats.

The influence of gonadal steroids and the pituitary on the levels and composition of plasma phospholipids in the rat

Gonadal steroids have been shown to influence plasma phospholipids. In the present study, the possible interaction between gonadal steroids and the pituitary in the regulation of plasma phospholipids was studied in rats. The total phospholipid concentration (higher in females) and the fatty acid composition of plasma lecithin was different in male compared to female rats. Gonadectomy resulted in a "feminization" of plasma phospholipids (total concentration and fatty acids in lecithin) in male rats but had no effect in females. Testosterone treatment of gonadectomized males or intact females resulted in a "masculinization" of plasma phospholipids, whereas estrogen treatment of intact males resulted in a "feminization." Hypophysectomy resulted in a marked decrease in plasma phospholipid concentration and the fatty acid composition of lecithin showed a "masculine" pattern in both males and females. Neither testosterone nor estrogen treatment had any effects on plasma phospholipids in hypophysectomized male and female rats, respectively. It is concluded that gonadal steroids and the hypothalamic-pituitary axis interact in the regulation of the synthetic and perhaps also degradative pathways controlling plasma phospholipids.

Endogenous testosterone enhances growth hormone (GH)-releasing factor-induced GH secretion in vitro

The influence of endogenous gonadal steroids in male and female rats on basal and growth hormone-releasing factor (GRF)-stimulated GH secretion from perifused anterior pituitaries was studied. After 75 min of perifusion with basal medium, freshly dissected pituitaries were exposed to human GRF(1-44) (10 nmol/l) for 15 min. Neonatal (day 1-2) or prepubertal (day 25) gonadectomy of male rats suppressed baseline GH release (ng/min per mg dry weight) as well as GRF-stimulated GH release by 40-70%. This effect was slightly more pronounced in neonatally gonadectomized animals. In prepubertally gonadectomized male rats, the suppression of GH release was completely reversed by testosterone replacement therapy. In female rats, prepubertal gonadectomy did not affect GH secretion from perfused pituitaries. However, treatment of ovariectomized female rats with oestradiol reduced baseline and GRF-induced GH release to levels lower than those observed in sham-operated or vehicle-treated ovariectomized animals. The data suggest that testicular androgen secretion in adult male rats increases the pituitary GH release in response to GRF in vitro, whereas ovarian oestrogen secretion is of less importance for the GRF responsiveness of female rat pituitaries.

Differential effects of neonatal and adult androgen exposure on the growth hormone secretory pattern in male rats

The interactive effects of androgen exposure during neonatal and adult life on the pattern of GH secretion in adult male rats was investigated. Neonatal rats were orchidectomized or sham-operated on days 1-2 of life and injected immediately postoperatively with testosterone propionate (250 micrograms, sc) or vehicle. At 90-130 days of age the rats were bled every 20 min between 9 and 17 h from an indwelling intraatrial catheter. Some neonatally gonadectomized, testosterone- or vehicle-treated rats were also given depot testosterone (15 mg/kg, im) 5-10 days before blood sampling. Plasma GH concentrations were measured by RIA, and the pulsatile secretory patterns were analyzed by the PULSAR computer program. Neonatal orchidectomy resulted in a marked suppression (50-75%) of both the height and duration of GH secretory episodes, while baseline GH levels were higher in neonatally gonadectomized males than in sham-operated controls. Neonatal testosterone replacement therapy restored high amplitude GH pulses. However, the GH pulses of these animals were of significantly shorter duration and occurred more frequently, and baseline GH levels were markedly higher than those in intact male rats. In contrast, neonatally gonadectomized rats treated with testosterone both neonatally and during adulthood exhibited a GH pattern indistinguishable from that in normal males, with high amplitude and long-lasting (103 +/- 8 min) pulses at regular intervals (178 +/- 9 min). A similar masculine GH pattern was seen in neonatally gonadectomized rats given testosterone only during adult life. The present results indicate that high amplitude GH pulses can be induced by either neonatal or adult androgen exposure. However, while neonatal androgens irreversibly cause stimulation of overall GH secretion, only the continuous presence of androgens during adult life can induce a GH secretory pattern, consisting of large surges at regular 3-h intervals separated by a low baseline that is characteristic of normal male rats.