Long-term growth hormone excess induces marked alterations in lipoprotein metabolism in mice

Fredrik Frick1, Mohammad Bohlooly-Y1, Daniel Lindén1, Bob Olsson1, Jan Törnell1,2, Staffan Edén1, and Jan Oscarsson1

1 Department of Physiology, Göteborg University, 405 30 Goteborg; and 2 AstraZeneca Research and Development, 431 83 Molndal, Sweden


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

The effects of long-term chronic growth hormone (GH) excess on lipid and lipoprotein metabolism were investigated in 8-mo-old bovine GH (bGH)-transgenic mice. Total body weight, serum cholesterol, insulin-like growth factor-I, and insulin levels were higher, whereas serum levels of glucose, free fatty acids, and triglycerides were lower in transgenic mice. Very low-density lipoprotein (VLDL) cholesterol levels were lower, and low-density lipoprotein (LDL) cholesterol levels were higher, in transgenic mice irrespective of gender, whereas only transgenic male mice had higher high-density lipoprotein cholesterol levels. Total serum apolipoprotein B (apoB) levels were not affected, but the amount of apoB in the LDL fraction was higher in transgenic mice. Hepatic LDL receptor expression was unchanged, whereas apoB mRNA editing and hepatic triglyceride secretion rate were reduced in bGH-transgenic male mice. Both lipoprotein lipase activity in adipose and heart tissue and beta -adrenergic-stimulated lipolysis were increased in transgenic male mice. The relative weight of adipose tissue was lower in transgenic mice, whereas hepatic triglyceride content was unchanged. Fat feeding of the mice equalized serum triglycerides and free fatty acids in bGH-transgenic and control mice. In summary, long-term GH excess is associated with marked alterations in lipid and lipoprotein metabolism, indicating decreased production and increased degradation of VLDL and preferential flux of fatty acids to muscle tissues.

very low-density lipoprotein; low-density lipoprotein; high-density lipoprotein; apolipoprotein B; low-density lipoprotein receptor; apolipoprotein B mRNA editing


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

TRANSGENIC MICE with chronic overexpression of growth hormone (GH) have been extensively used to investigate long-term effects of GH (19). Besides increased body size, linear growth, and organomegaly, these mice have been shown to have decreased body fat mass (13, 31, 41) and increased lean body mass (61). Metabolic studies of GH-transgenic mice have consistently shown hyperinsulinemia, normal blood glucose levels, and slightly impaired (26) or normal (19) glucose tolerance. Bovine (b)GH-transgenic mice have reduced insulin receptor concentration but increased insulin receptor substrate-1 (IRS-1) phosphorylation and phosphatidylinositol 3-kinase activity in the liver (10). However, these mice were insensitive to further stimulation with insulin with respect to insulin signaling (10). Furthermore, bGH-transgenic mice have been shown to have impaired cardiac function and bioenergetics (4) and increased locomotor activity (52). Thus many aspects of metabolism, including carbohydrate metabolism, insulin signaling, and cardiac bioenergetics, have been studied, but few studies concerning lipid and lipoprotein metabolism in the GH-transgenic mice have been performed. bGH (5, 42) and GRF-transgenic (42) mice have been shown to have increased cholesterol levels, whereas serum triglycerides were unchanged. Many treatment studies of GH-deficient patients and hypophysectomized rats have revealed that GH influences several aspects of lipid and lipoprotein metabolism. These aspects include alterations in serum lipoprotein levels (12, 16, 33-35, 51), hepatic lipoprotein production (14, 24, 49, 50), hepatic low-density lipoprotein (LDL) receptor expression (1, 45), and lipoprotein lipase (LPL) activity (35, 36) after GH treatment. However, many of the underlying mechanisms of GH action on lipoprotein metabolism, including which genes are involved, are to a large extent still unknown.

Transgenic mice overexpressing or lacking a gene have been used extensively over the last years to study the importance of various genes in lipid and lipoprotein metabolism (6, 62). Transgenic mice have also made it possible to investigate hormone regulation of human genes such as apolipoprotein a (56). The potential of using transgenic mice for the understanding of which genes are responsible for the action of GH on lipid and lipoprotein metabolism led us to investigate the effects of chronic overexpression of bGH on lipid and lipoprotein metabolism in mice.


    MATERIALS AND METHODS
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ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Generation of Transgenic Mice

bGH-transgenic mice used in this study were generated as previously described (46). The animals were housed with alternating 12-h periods of light (0700-1900) and dark (1900-0700). The animals had free access to mouse standard chow (rat/mouse standard diet, B&K Universal, Sollentuna, Sweden) and tap water. If not otherwise stated, the mice were killed at the age of 8 mo. All mice were killed at a similar time interval of the day (0900-1200) to avoid the effects of diurnal variation. In one experiment, female mice had free access to a high-fat diet for a period of 2 mo; these mice were killed at 6 mo of age. The high-fat diet contained 57.6% energy from fat, 26.6% from carbohydrates, and 15.8% from protein (Bio-Serv, Frenchtown, NJ). The animals were anesthetized with xylazine (Rompun, 9 mg/kg; Bayer, Lever-kusen, Germany) and ketamine hydrochloride (Ketalar, 77 mg/kg; Parke-Davis, Detroit, MI). Blood was collected by heart puncture using nonheparinized syringes. Adipose tissue depots, heart, and liver were excised, blotted, weighed, and instantly frozen in liquid nitrogen. Serum and tissues were kept at -70°C until assays. Littermates were used as controls. This study was approved by the Ethics Committee of Göteborg University.

Serum Analyses

Triglyceride and cholesterol concentrations were determined by enzymatic colorimetric assays (MPR2: TG/GPO-PAP and Chl/CHOD-PAP, Roche Diagnostics, Mannheim, Germany). The intra-assay coefficient of variation (CV) was 3% for the triglyceride assay and 4% for the cholesterol assay. Serum apolipoprotein B (apoB) concentrations were determined by an electroimmunoassay as previously described (33, 51). The intra- and interassay CVs for the apoB assay were 5.3 and 8.6%, respectively. Standardization of apoB measurements was made by isolation of a narrow-density cut of LDL (density 1.030-1.055 g/ml) containing only apoB (33). The protein concentration of the narrow-density cut of LDL was determined by the Bio-Rad protein assay (Bio-Rad Laboratories, Hercules, CA) using albumin as standard. Serum concentration of glucose was enzymatically determined in 15-µl samples on a YSI 2700 SELECT biochemical analyzer (Yellow Springs Instrument, Yellow Springs, OH). Serum free fatty acid (FFA) concentrations were measured with a NEFA kit (Wako Chemicals, Neuss, Germany). Serum insulin concentration was determined by a rat insulin RIA (Linco Research, St. Charles, MO) with 100% cross-reactivity to mouse insulin. Serum insulin-like growth factor I (IGF-I) concentrations were determined by a hydrochloric acid-ethanol extraction followed by RIA with the use of human IGF-I for labeling (Nichols Institute of Diagnostics, San Juan Capistrano, CA).

Hepatic Lipid Content

Frozen liver was homogenized in ice-cold distilled water (6 ml/g tissue). Total lipid content was extracted from the homogenate according to Bligh and Dyer (3). The extracted lipids were dissolved in isopropanol after evaporation of the chloroform phase by use of a stream of N2. Triglyceride and cholesterol concentrations were determined as already described.

Size Distribution of Serum Lipoproteins

Determination of size distribution of lipoproteins was performed by gel filtration with the use of fast protein liquid chromatography (FPLC) equipment (Pharmacia Upjohn, Uppsala, Sweden). Briefly, serum from six mice was pooled to a total volume of 1.5 ml, and the density was adjusted to 1.215 g/ml with KBr in 0.9% NaCl. After ultracentrifugation (35,000 g, 4°C, 24 h), the total lipoprotein fraction was recovered by aspiration, and the final volume was adjusted to 2 ml with FPLC buffer (0.15 M NaCl, 0.01% EDTA, 0.02% NaAz, pH 7.3). After filtration through a 0.45-µm low-protein filter, the sample was loaded on a 25-ml Superose 6B column (Pharmacia Upjohn) using a constant flow rate of 0.35 ml/min. Eluted samples were collected in 0.5-ml fractions, and the fractions were stored at -20°C until assay. Triglyceride and cholesterol concentrations were determined with enzymatic colorimetric assays as described in Serum Analyses. Size distribution of lipoproteins was determined in two independent samples of pooled serum. Similar differences between male and female bGH-transgenic mice and their gender controls were observed on both occasions.

In Vivo Hepatic Triglyceride Secretion Rate

Triglyceride secretion rate in vivo was measured by intravenous administration of Triton WR-1339 (22). After a 5-h period without access to mouse standard chow (0700-1200), anesthetized wild-type and bGH-transgenic male mice were injected intravenously with Triton WR-1339 (Sigma Chemical) diluted in saline (200 mg/ml) (500 mg/kg body wt) via the jugular vein by means of catheters. Blood samples (70 µl) were taken before and 30 and 60 min after Triton WR-1339 injection. The triglyceride accumulation was linear during this time period. The triglyceride concentration was analyzed as described in the previous section.

Western Blot Analyses

ApoB. Western blotting was performed using an enhanced chemiluminicence (ECL) protocol (Amersham Pharmacia Biotech, Buckinghamshire, UK). In each lane, adjacent FPLC fractions were pooled in pairs (9 µl each) and separated on 4-20% Tris-glycine polyacrylamide gels (Novex, San Diego, CA). After electrophoresis, the proteins were transferred to Hybond-P polyvinylidene difluoride transfer membrane (Amersham Pharmacia Biotech) in transfer buffer (25 mM Bis-Tris, pH 7.4, with 25 mM bicine, 0.8 mM EDTA, and 10% methanol) for 3 h at 400 mA (Transblot cell, Bio-Rad). The membrane was blocked overnight at 4°C in PBS containing 0.1% Tween-20 (PBS-T) and 5% nonfat milk before 1-h incubation with polyclonal rabbit anti-rat apoB antiserum (33) diluted 1:1,000 in PBS-T. The membrane was then incubated for 1 h with horseradish peroxidase-linked donkey anti-rabbit IgG (Amersham Life Science) diluted 1:4,000. Detection and development was performed by using ECL detection assay and Hyperfilm ECL (Amersham Pharmacia Biotech).

LDL receptor. To quantify the LDL receptor, livers were homogenized in 4 volumes of PE buffer (10 mM potassium phosphate buffer, pH 6.8, and 1 mM EDTA) containing 10 mM 3-([3-cholamidopropyl]dimethylammonio)-1-propanesulfate (18), 4 mM Pefabloc, 20 nM leupeptin, 15 nM pepstatin, and 0.15 nM aprotinin (Boehringer Mannheim, Mannheim, Germany). The homogenates were sonicated (3 × 10 s) and centrifuged (10,000 g, 10 min), and protein concentrations were determined in the supernatant by use of the Bio-Rad DC protein assay with albumin as standard. Proteins (30 µg/lane) were separated and transferred as described, but under nonreducing conditions. Primary antibody, rabbit anti-bovine LDL receptor antibody (a generous gift from Professor Joachim Herz, Dept. of Molecular Genetics, University of Texas, Dallas, TX), and secondary antibody were diluted 1:2,000 in blocking buffer. Detection and quantification were done with Fluor-S Multi Imager and Quantity One software (Bio-Rad). The detection capacity was found to be linear to 60 µg protein (data not shown). As a negative control, liver protein preparations from an LDL receptor-deficient mouse and a corresponding wild-type mouse (kindly provided by Dr. Marcela Pekna, Dept. of Medical Biochemistry, Göteborg University) were used (data not shown).

ApoB mRNA Editing

Total RNA was isolated from mouse livers according to Chomczynski and Sacchi (7). To analyze the extent of apoB mRNA editing, primer extension analysis was performed according to Sjöberg et al. (50). RNA (10-20 ng) was annealed overnight at 45°C to a 32P- and 5' end-labeled rat apoB oligonucleotide (2). The primer was labeled using a 5' end-labeling kit and 50 µCi of [32P]ATP (Amersham). The gel bands corresponding to the edited and nonedited apoB mRNA were cut out, and the radioactivity was counted after digestion of the gel. The intra-assay CV was <3%.

LPL Activity

LPL activity in heart and adipose tissue depots was determined according to the method of Peterson et al. (40). In brief, frozen tissue (-70°C) was homogenized in a detergent-containing buffer. To each gram of tissue, 9 ml of buffer were added. The homogenate was centrifuged (15,000 g, 4°C, 5 min), and the clear solution between the sediment and the floating fat layer was collected and frozen until assay of LPL. In contrast to adipose tissue, heart tissue homogenates were further diluted 10 times in buffer before being assayed. The substrate emulsion used was Intralipid labeled with [3H]triolein (Dr. Krabisch, Lund University, Lund, Sweden). Samples were incubated in triplicate at 25°C. Liberated fatty acids were extracted (53) and quantified in a liquid scintillation counter. Activity was expressed as milliunits per gram of tissue (1 mU = 1 nmol FFA released/min).

Lipolysis

Epidydimal adipose tissue was excised from four transgenic mice and four wild-type littermates. Adipocytes were isolated with collagenase A digestion (0.5 mg/ml, Sigma) in Dulbecco's medium (Life Technologies, Paisley, Scotland, UK) at 37°C for 1 h, as described by Rodbell and Krishna (44) and with the modifications made by Gause et al. (17). After digestion, liberated adipocytes were collected by filtration through a mesh filter (250 µm) and washed three times with medium. Thereafter, a 50/50 cell suspension was prepared using isolated fat cells and incubation medium. For each incubation, 100 µl of the 50/50 cell suspension was added to plastic vials containing 2 ml of incubation medium, supplemented with 4% BSA (fraction V, Sigma), in the absence (basal lipolysis) or presence of 10-5 M isoproterenol [(-)-isoproterenol, Sigma]. In parallel, a 100-µl 50/50-cell suspension was added to Dole's solution for subsequent extraction of lipids (9). After incubation for 2 h at 37°C, cells and medium were separated by centrifugation through silicon oil (Kebo Lab, Spånga, Sweden). The glycerol content in the medium was measured enzymatically (20) and taken as an index of lipolysis. Lipolysis was expressed as nanomoles glycerol released per milligram of triglyceride.

Statistics

Values are given as means ± SE. Comparison between groups was performed with unpaired Student's t-test or one-way ANOVA followed by Student-Newman-Keul's multiple range test. P values <0.05 were considered significant. When appropriate, values were normalized by logarithmic transformation.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Chronic overexpression of bGH in transgenic mice resulted in a significant increase in body weight and absolute (not shown) and relative weights of liver and heart in both sexes (Table 1). The absolute weight of retroperitoneal adipose tissue depots was significantly lower in both male and female transgenic mice compared with wild-type mice, but there was no significant difference in absolute weights of the reproductive adipose tissue depots (not shown). The relative weights (%body wt) of retroperitoneal and reproductive adipose tissue were significantly lower in both male and female transgenic mice (Table 1).

                              
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Table 1.   Effect of bGH overexpression on body weight and relative weights of liver, heart, and AT

Serum IGF-I concentrations were higher in male bGH-transgenic mice than in their controls (Table 2). Serum insulin levels were severalfold higher, and serum levels of glucose and FFA were lower, in both male and female bGH-transgenic mice compared with their gender controls (Table 2). Serum cholesterol levels were higher in both male and female bGH-transgenic mice compared with their controls (Fig. 1A). However, serum cholesterol levels were more markedly elevated in transgenic males than in females (P < 0.05, one-way ANOVA followed by Student-Newman-Keul's test). Serum triglyceride levels were reduced to the same extent in male and female transgenic mice (Fig. 1B). Changes in size distribution of lipoproteins were determined by FPLC analysis of total lipoproteins (d < 1.215 g/ml). Size distribution of serum lipoproteins in transgenic male and wild-type male mice are shown in Fig. 2, A and B. Transgenic male mice showed a marked reduction in cholesterol and triglyceride content of the VLDL fraction and increased cholesterol content of the LDL and HDL fractions. The distribution of apoB48 and apoB100 was determined by Western blotting. As shown in Fig. 2C, the distribution of apoB was changed. The amount of B48 and B100 in VLDL was markedly reduced in transgenic male mice compared with their controls. The size distribution of serum lipoproteins in transgenic female mice and their controls was also determined, since the effect of bGH overexpression on total serum cholesterol levels was less marked than in transgenic males (Fig. 3, A and B). Compared with littermates, the cholesterol (Fig. 3A) and triglyceride content (Fig. 3B) of the VLDL fraction was lower and LDL cholesterol was higher in transgenic female mice. However, no obvious difference in HDL cholesterol content was observed between female bGH mice and their littermates. The distribution of apoB was changed in a similar way from VLDL to LDL to what was observed in males (data not shown).

                              
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Table 2.   Effects of overexpression of bGH on serum levels of IGF-I, insulin, glucose, and FFA



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Fig. 1.   Serum cholesterol (A) and triglyceride levels (B) in bovine growth hormone (bGH)-transgenic (Trg) males (n = 10) and females (n = 5), compared with wild-type (Wt) male (n = 10) and female (n = 8) littermates. At the age of 8 mo, Trg and Wt mice were killed between 0900 and 1200. Serum cholesterol and triglyceride levels were determined as described in MATERIALS AND METHODS. Values are presented as means ± SE. *P < 0.05 vs. corresponding gender (unpaired Student's t-test).



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Fig. 2.   Size distribution of serum lipoproteins in Trg male mice and Wt littermates. Pooled total lipoprotein fraction [density (d) < 1.215 g/ml] from 6 Trg and Wt mice were isolated and subjected to fast protein liquid chromatography (FPLC), as described in MATERIALS AND METHODS. A: distribution of cholesterol in serum from Wt and Trg male mice. B: distribution of triglycerides in serum from Wt and Trg male mice. C: distribution of apolipoprotein B (apoB) as determined with Western blot analysis of the FPLC fractions of Wt and Trg male mice. SeeBlue prestained standard was used as molecular size marker (NOVEX, San Diego, CA). In A and B, the distribution of apoB-containing lipoproteins is indicated schematically.



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Fig. 3.   Size distribution of serum lipoproteins in Trg female mice and Wt littermates. Pooled total lipoprotein fraction (d < 1.215 g/ml) from 6 Trg and Wt mice were isolated and subjected to FPLC, as described in MATERIALS AND METHODS. A: distribution of cholesterol in serum from Wt and Trg male mice. B: distribution of triglycerides in serum from Wt and Trg male mice. In A and B, the distribution of apoB-containing lipoproteins is indicated schematically.

The lower triglyceride and FFA levels in the bGH-transgenic mice could be due to a decreased availability of fatty acids for hepatic VLDL production and increased uptake of fatty acids in other tissues. If a decreased hepatic fatty acid supply was important for the observed effects, an increased amount of dietary triglycerides would reduce the difference in serum triglycerides and FFA between the transgenic mice and their controls. We therefore gave a diet enriched in triglycerides to bGH-transgenic females (n = 5) and their littermate controls (n = 5) for 2 mo. In fact, there was no difference in serum triglycerides between bGH-transgenic females and gender controls after 2 mo of fat feeding (bGH-transgenic mice: 1.25 ± 0.27 mmol/l; control mice: 0.70 ± 0.11 mmol/l, not significant). Moreover, no difference in serum FFA levels was observed between transgenic and wild-type female mice fed the high-fat diet (data not shown).

Total serum apoB levels were not different between transgenic male and wild-type control mice (Table 3). Because the distribution of apoB was changed (Fig. 2C), it could be concluded that the apoB content of the LDL fraction was higher, and the apoB content of the VLDL fraction was lower, in transgenic male mice compared with controls. Because a reduced LDL receptor expression could explain the increased LDL-apoB levels, the LDL receptor expression was measured in protein extract from whole liver. However, LDL receptor expression was not different between transgenic male and wild-type male mice (Table 3).

                              
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Table 3.   Effects of overexpression of bGH on serum levels of apoB, hepatic LDL receptor expression, hepatic cholesterol, and triglyceride content, as well as apoB mRNA editing

Overexpression of bGH resulted in slightly lower editing of apoB mRNA compared with wild-type littermates (Table 3). Increased editing of apoB mRNA has been shown to be associated with increased hepatic triglyceride production in different models (2, 24). The reduced editing of apoB mRNA in bGH-transgenic mice therefore indicates decreased triglyceride synthesis and VLDL secretion in bGH-transgenic mice. In fact, the hepatic triglyceride secretion rate was decreased ~50% in transgenic male mice compared with wild-type littermates (Fig. 4). The secretion rate was related to the body weight and plasma volume estimated by Li et al. (22). Long-term GH treatment of GH-deficient humans results in an increased plasma volume (27). Therefore, the bGH-transgenic mice may have increased plasma volume per gram of body weight, leading to an underestimation of the true triglyceride secretion rate. However, it is unlikely that an increased plasma volume can explain the difference in the triglyceride secretion rate, because the plasma volume per gram of body weight would have had to be increased by ~100% to balance out the difference observed between control and bGH-transgenic mice. The 5-h period of fasting before the hepatic triglyceride secretion rate was measured did not influence the previously described difference in serum triglycerides between transgenic and wild-type male mice (0.76 ± 0.08 vs. 1.74 ± 0.09 mmol/l, P < 0.05). The hepatic triglyceride content was not different between transgenic male mice and their controls (Table 3), indicating that triglycerides were not accumulating as a result of impaired VLDL secretion. However, hepatic cholesterol content was somewhat higher in the transgenic male mice than in wild-type male mice (Table 3).


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Fig. 4.   Hepatic triglyceride secretion rate in vivo was measured by intravenous injection of Triton WR-1339 (500 mg/kg body wt). Serum triglyceride levels were measured at baseline and at 30 and 60 min, as described in MATERIALS AND METHODS. The triglyceride secretion rate was calculated from the slope of the curve and expressed as micromoles triglycerides per hour per kilogram body weight. Results are presented as means ± SE; n = 3 in each group. *P < 0.05 vs. wild type mice (unpaired Student's t-test).

Despite a marked reduction in the hepatic triglyceride secretion rate, LDL and HDL levels were higher in the transgenic male mice. This finding could be at least partly explained by an increased turnover of VLDL via increased LPL activity (39, 62). The LPL activity was therefore measured in heart and adipose tissue of male mice. The LPL activity in heart was nearly twofold higher in male transgenic mice compared with wild-type littermates (Fig. 5A). LPL activity in epididymal and retroperitoneal adipose tissue was 2- to 2.5-fold higher in both adipose tissue depots in transgenic male mice compared with wild-type controls (Fig. 5, B and C). The increased LPL activity in adipose tissue may result in increased availability of triglyceride-derived fatty acids, leading to increased triglyceride accumulation if not balanced by increased lipolysis and/or decreased lipogenesis. We therefore investigated the lipolytic response in epidydimal adipose tissue. Lipolytic response in adipose tissue was measured as glycerol release from isolated adipocytes during basal and beta -adrenergic stimulation. Basal lipolysis was not changed, but norepinephrine-stimulated lipolysis was significantly higher in transgenic males compared with their controls (Fig. 6).


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Fig. 5.   Effect of overexpression of bGH on lipoprotein lipase (LPL) activity in heart (A) and retroperitoneal (B) and epidydimal adipose tissue (C) from Trg male mice. At the age of 8 mo, Trg male mice were killed between 0900 and 1200 together with age-matched littermates. LPL activity was analyzed as described in MATERIALS AND METHODS. Results are presented as means ± SE; n = 10 in each group. *P < 0.05 vs. Wt mice (unpaired Student's t-test).



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Fig. 6.   Effect of overexpression of bGH on basal and beta -adrenergic-stimulated lipolysis in Trg male animals. Epididymal adipose tissue was freshly dissected and adipocytes acutely isolated by collagenase treatment. Incubation was then performed in the absence (basal lipolysis) or presence of isoproterenol (10-5 M). Results are presented as means ± SE; n = 4 mice in each group. *P < 0.05 vs. Wt basal values, +P < 0.05 vs. Wt isoproterenol-treated cells (one-way ANOVA followed by Student-Newman-Keul's multiple-range test).


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

This study demonstrates that chronic excess of GH leads to significant alterations in lipid and lipoprotein metabolism. The marked reduction in VLDL levels could be explained both by decreased hepatic secretion of triglycerides and by increased LPL activity. Despite an increased lipolytic response to beta -adrenergic stimuli in adipose tissue, serum levels of FFA were reduced in transgenic mice. These findings, together with reduced adipose tissue mass, reduced hepatic triglyceride secretion, and unchanged mass of hepatic triglycerides, indicate decreased triglyceride biosynthesis in both liver and adipose tissue of bGH-transgenic mice. The similar serum triglyceride levels in transgenic mice and controls after being fed a triglyceride-enriched diet indicates that the reduced triglyceride production in the liver was most likely due to a reduced fatty acid supply. Fatty acids liberated through the lipolytic effects of GH in adipose tissue and increased LPL activity in adipose tissue and heart may therefore be rerouted preferentially to heart and muscle tissue. Moreover, these findings, in combination with low glucose levels, high insulin levels, and increased locomotor activity in mice kept on a low-fat diet (52), indicate a substantial carbohydrate oxidation for energy production in the bGH-transgenic mice. In line with this assumption, subjects with acromegaly have enhanced carbohydrate oxidation after a glucose tolerance test compared with healthy controls (38). Because these effects were correlated to IGF-I levels (38) and IGF-I preferentially increases glucose uptake in skeletal muscle (15), it is suggested that increased IGF-I levels may contribute to increased glucose oxidation in muscle tissue also in bGH-transgenic mice.

Overexpression of GH in transgenic animals produces supraphysiological levels of GH (19). The line of bGH-transgenic mice used in this study has been shown to have mean serum bGH levels of 1,057 (46) and 1,124 ng/ml (28), which are markedly higher than in normal mice (25). The difference in serum IGF-I levels between the bGH-transgenic mice and the control mice in this study is similar to that reported previously (46), indicating that the serum levels of bGH in these mice are in the same range as that reported previously (28, 46). GH is also overexpressed in several tissues that normally do not express GH, which might lead to higher concentrations of GH locally than what is observed in the circulation.

The present results confirm previous findings of decreased adipose tissue mass (13), high serum insulin (26, 42, 60), and IGF-I levels (19, 42) in GH-transgenic mice. In contrast to our findings, comparable glucose levels in GH-transgenic and wild-type mice have been observed in other studies (19, 26). The reason for the glucose levels in the bGH-transgenic mice being lower than in their controls is unclear. The mice were not fasted, indicating that the lower glucose levels were not a result of impaired glucose production but may have been due to an enhanced glucose oxidation in these mice. Serum FFA levels were reduced in the bGH-transgenic mice, suggesting that these bGH-transgenic mice possess the ability to handle liberated FFA efficiently, thereby preventing a deterioration of insulin sensitivity induced by high FFA levels (43).

The transgenic mice overexpressing GH demonstrated a marked reduction in serum triglycerides in both the fed and fasted states. This finding is in contrast to the unchanged serum triglyceride levels in GH-transgenic mice observed in other studies (5, 42) and the unchanged or slightly reduced triglyceride levels after GH treatment of hypophysectomized rats (33, 34). Treatment of GH-deficient adults results in small or no changes in serum triglycerides, but interestingly, those subjects with the highest serum triglyceride levels before treatment show a marked decrease in serum triglycerides after GH treatment (12). In acromegaly, serum triglyceride concentrations have been shown to be higher than (29) or similar to (8) the levels in a control population, but therapy of acromegaly has been shown to decrease serum triglycerides (37, 59). The reason for the difference in serum triglycerides between acromegaly and bGH-transgenic mice is not clear from the present results. However, the similar serum triglyceride levels in bGH-transgenic mice and controls after 2 mo of fat feeding indicate that a decreased supply of fatty acids to the liver may be important. FFA are regarded as rate limiting for hepatic triglyceride synthesis, and VLDL assembly and secretion (32) and FFA levels were reduced in the bGH-transgenic mice consuming a chow diet. Thus the dietary intake of fat may be a difference between the human situation and our mouse model, which may contribute to the different effects on serum triglycerides.

When consuming ordinary chow, the bGH-transgenic mice showed a markedly lower triglyceride secretion rate than controls. In contrast, acromegalic patients have an increased triglyceride production rate (29). Thus it is possible that that an increased content of fat in the diet would increase the hepatic triglyceride secretion rate also in bGH-transgenic mice above that in control mice. However, low hepatic triglyceride secretion and reduced apoB mRNA editing in bGH-transgenic mice is in contrast to the increased VLDL secretion and increased apoB mRNA editing in GH-treated, hypophysectomized rats consuming ordinary low-fat chow (14, 49, 50). Thus other explanations than the dietary intake of fat may be important for the lower triglyceride secretion. The very high serum insulin levels in the bGH-transgenic mice may have contributed to the lower triglyceride secretion. Interestingly, hyperinsulinemic ob/ob mice also have reduced hepatic triglyceride secretion and slightly lower apoB mRNA editing (23). However, long-term incubation of hepatocytes with insulin increases both triglyceride synthesis and apoB mRNA editing (24, 58). Thus hyperinsulinemia may play a role in the observed effects, but the mechanisms behind the possible importance of insulin for these effects are unclear.

In line with the present results, we have observed that heart and gastrocnemicus muscle LPL activity was increased by GH treatment of hypophysectomized rats (36). Mainly skeletal muscle and heart LPL activity has been shown to affect circulating levels of triglycerides and to be rate limiting for the uptake of triglyceride-derived fatty acids into muscle tissue (21, 62). Thus the increased LPL activity in heart may contribute to the lower VLDL levels in the bGH-transgenic mice.

LPL activity was also increased in adipose tissue in the GH transgenic mice. This finding is in contrast to the lack of effect of GH treatment of hypophysectomized rats on LPL activity in adipose tissue (36) and the decrease in LPL activity observed after GH treatment of GH-deficient adults (35). Thus the reduced adipose tissue mass in acromegaly does not need to be explained by the decreased adipose tissue LPL activity (48). LPL activity in adipose tissue, in contrast to muscle LPL activity, has been shown to increase after insulin treatment both in vivo (36) and in vitro (11). Thus it is possible that the markedly increased serum insulin levels mediate the increased LPL activity in adipose tissue of bGH-transgenic mice.

We observed increased circulating levels of LDL cholesterol and LDL-apoB, although LDL receptor expression was unchanged. Hepatic lipase is of importance for hepatic LDL uptake and degradation (47). Recent data from a DNA microarray study investigating hepatic gene expression in bGH-transgenic mice indicated a marked reduction in hepatic lipase mRNA expression in these mice (unpublished data by B. Olsson, M. Bohlooly-Y, O. Brusched, O. G. P. Isaksson, S.-O. Olofsson, J. Oscarsson, and J. Törnell). Thus reduced hepatic lipase activity may contribute to the increased LDL levels in the bGH-transgenic mice. Increased production of LDL is less likely, because the hepatic triglyceride secretion rate was decreased, indicating decreased VLDL secretion. However, we cannot exclude the possibility that the number of apoB-containing lipoprotein particles secreted from the liver was unchanged, because apoB secretion was not measured. The reduced hepatic mRNA editing in the bGH-transgenic mice may have resulted in a larger proportion of apoB100-containing particles, which may have contributed to higher LDL levels (57). It is also possible that the increased LPL activity contributed to increased production of LDL in the bGH-transgenic mice (62). In acromegaly, the LDL levels are unchanged after therapy (59), but LDL cholesterol has also been shown to be higher in acromegalic patients than in controls (8). In line with the present results, apoB levels are unchanged after therapy of acromegaly (37).

HDL cholesterol (59) and apoA-I levels (37) have been shown to increase after therapy of acromegaly, indicating lower HDL levels in acromegaly. However, there are some studies that did not detect a difference in HDL levels between acromegalic patients and a control population (8). HDL cholesterol levels were higher in male transgenic mice than in their controls, but no difference was observed in female mice. We have previously observed that the higher HDL levels in female rats than in male rats were due to the more continuous GH secretion in female rats compared with male rats (33, 34). The secretion of GH is sexually dimorphic (25) and regulates sex differences in hepatic metabolism also in mice (30). Therefore, the effect of a continuous secretion of GH in the transgenic mice may affect the HDL levels to a larger extent in males than in females, because the secretion is more profoundly shifted from a pulsatile to a continuous secretion in male mice.

In conclusion, the present study shows that there are marked changes in lipid and lipoprotein metabolism in mice overexpressing GH. However, these changes differ in several respects from the effects of acromegaly (29, 37, 48, 55, 59) on lipoprotein metabolism. In Table 4, the effects of overexpression of bGH in mice on certain aspects of lipoproteins are compared with the effects of acromegaly. The differences between acromegaly and bGH-transgenic mice may have several reasons. In humans, increased activity of cholesteryl ester transfer protein (55) and absence of editing of apoB mRNA in the liver may contribute to differences in HDL and LDL plus VLDL metabolism. Moreover, LPL is differently regulated in acromegaly and in bGH-transgenic mice. The tendency toward higher serum triglycerides in bGH-transgenic mice compared with control mice after fat feeding indicates that the different amount of dietary fat in laboratory chow and human diet may at least partly contribute to the observed differences between acromegaly and bGH-transgenic mice.

                              
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Table 4.   Comparison between lipoprotein metabolism in acromegaly and in bGH-transgenic mice


    ACKNOWLEDGEMENTS

We thank Birgitta Odén and Britt-Marie Larsson for excellent technical assistance.


    FOOTNOTES

This study was supported by grants from the Swedish Medical Research Council (8269), Novo Nordisk Foundation, Handledaren Hjalmar Svensson Foundation, Magnus Bergvall Foundation, Åke Wiberg Foundation, King Gustav V's and Queen Victoria's Foundation, The Swedish Medical Society, The Swedish Cancer Society, and AstraZeneca R&D, Molndal, Sweden.

Address for reprint requests and other correspondence: F. Frick, Dept. of Physiology, Endocrinology unit, Göteborg Univ., Box 434, S-405 30 Goteborg, Sweden (E-mail: Fredrik.Frick{at}fysiologi.gu.se).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 5 March 2001; accepted in final form 17 July 2001.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Angelin, B, and Rudling M. Growth hormone and hepatic lipoprotein metabolism. Curr Opin Lipidol 5: 160-165, 1994[Medline].

2.   Baum, CL, Teng BB, and Davidson NO. Apolipoprotein B messenger RNA editing in the rat liver. Modulation by fasting and refeeding a high carbohydrate diet. J Biol Chem 265: 19263-19270, 1990[Abstract/Free Full Text].

3.   Bligh, EG, and Dyer WJ. A rapid method of total lipid extraction and purification. Can J Biochem Physiol 37: 911-917, 1959[ISI].

4.   Bollano, E, Omerovic E, Bohlooly-Y M, Kujacic V, Madhu B, Törnell J, Isaksson O, Soussi B, Schulze W, Fu ML, Matejka G, Waagstein F, and Isgaard J. Impairment of cardiac function and bioenergetics in adult transgenic mice overexpressing the bovine growth hormone gene. Endocrinology 141: 2229-2235, 2000[Abstract/Free Full Text].

5.   Brem, G, Wanke R, Wolf E, Buchmuller T, Muller M, Brenig B, and Hermanns W. Multiple consequences of human growth hormone expression in transgenic mice. Mol Biol Med 6: 531-547, 1989[ISI][Medline].

6.   Breslow, JL. Transgenic mouse models of lipoprotein metabolism and atherosclerosis. Proc Natl Acad Sci USA 90: 8314-8318, 1993[Abstract/Free Full Text].

7.   Chomczynski, P, and Sacchi N. Single-step method of RNA isolation by acid guanidinium thiocyanate-phenol-chloroform extraction. Anal Biochem 162: 156-159, 1987[ISI][Medline].

8.   Colao, A, Ferone D, Marzullo P, Cappabianca P, Cirillo S, Boerlin V, Lancranjan I, and Lombardi G. Long-term effects of depot long-acting somatostatin analog octreotide on hormone levels and tumor mass in acromegaly. J Clin Endocrinol Metab 86: 2779-2786, 2001[Abstract/Free Full Text].

9.   Dole, VP, and Meinerz H. Microdetermination of long-chain fatty acids in plasma and tissues. J Biol Chem 235: 2595-2599, 1960[ISI][Medline].

10.   Dominici, FP, Cifone D, Bartke A, and Turyn D. Loss of sensitivity to insulin at early events of the insulin signaling pathway in the liver of growth hormone-transgenic mice. J Endocrinol 161: 383-392, 1999[Abstract/Free Full Text].

11.   Eckel, RH, Prasad JE, Kern PA, and Marshall S. Insulin regulation of lipoprotein lipase in cultured isolated rat adipocytes. Endocrinology 114: 1665-1671, 1984[Abstract].

12.   Edén, S, Wiklund O, Oscarsson J, Rosén T, and Bengtsson BÅ. Growth hormone treatment of growth hormone-deficient adults results in a marked increase in Lp(a) and HDL cholesterol concentrations. Arterioscler Thromb 13: 296-301, 1993[Abstract].

13.   Eisen, EJ, Peterson CB, Parker IJ, and Murray JD. Effects of zinc ion concentration on growth, fat content and reproduction in oMT1a-oGH transgenic mice. Growth Dev Aging 62: 173-186, 1998[ISI][Medline].

14.   Elam, MB, Wilcox HG, Solomon SS, and Heimberg M. In vivo growth hormone treatment stimulates secretion of very low density lipoprotein by the isolated perfused rat liver. Endocrinology 131: 2717-2722, 1992[Abstract].

15.   Frick, F, Oscarsson J, Vikman-Adolfsson K, Ottosson M, Yoshida N, and Edén S. Different effects of IGF-I on insulin-stimulated glucose uptake in adipose tissue and skeletal muscle. Am J Physiol Endocrinol Metab 278: E729-E737, 2000[Abstract/Free Full Text].

16.   Friedman, M, Byers SO, and Elek SR. Pituitary growth hormone essential for regulation of serum cholesterol. Nature 5231: 464-467, 1970[Medline].

17.   Gause, I, Edén S, Jansson JO, and Isaksson O. Effects of in vivo administration of antiserum to rat growth hormone on body growth and insulin responsiveness in adipose tissue. Endocrinology 112: 1559-1566, 1983[Abstract].

18.   Hjelmeland, LM. A nondenaturing zwitterionic detergent for membrane biochemistry: design and synthesis. Proc Natl Acad Sci USA 77: 6368-6370, 1980[Abstract].

19.   Kopchick, JJ, Bellush LL, and Coschigano KT. Transgenic models of growth hormone action. Annu Rev Nutr 19: 437-461, 1999[ISI][Medline].

20.   Laurell, S, and Tibbling G. An enzymatic fluorometric micromethod for the determination of glycerol. Clin Chim Acta 13: 317-322, 1966[ISI][Medline].

21.   Levak-Frank, S, Hofmann W, Weinstock PH, Radner H, Sattler W, Breslow JL, and Zechner R. Induced mutant mouse lines that express lipoprotein lipase in cardiac muscle, but not in skeletal muscle and adipose tissue, have normal plasma triglyceride and high-density lipoprotein-cholesterol levels. Proc Natl Acad Sci USA 96: 3165-3170, 1999[Abstract/Free Full Text].

22.   Li, X, Catalina F, Grundy SM, and Patel S. Method to measure apolipoprotein B-48 and B-100 secretion rates in an individual mouse: evidence for a very rapid turnover of VLDL and preferential removal of B-48- relative to B-100-containing lipoproteins. J Lipid Res 37: 210-220, 1996[Abstract].

23.   Li, X, Grundy SM, and Patel SB. Obesity in db and ob animals leads to impaired hepatic very low density lipoprotein secretion and differential secretion of apolipoprotein B-48 and B-100. J Lipid Res 38: 1277-1288, 1997[Abstract].

24.   Lindén, D, Sjöberg A, Asp L, Carlsson L, and Oscarsson J. Direct effects of growth hormone on production and secretion of apolipoprotein B from rat hepatocytes. Am J Physiol Endocrinol Metab 279: E1335-E1346, 2000[Abstract/Free Full Text].

25.   MacLeod, JN, Pampori NA, and Shapiro BH. Sex differences in the ultradian pattern of plasma growth hormone concentrations in mice. J Endocrinol 131: 395-399, 1991[Abstract].

26.   McGrane, MM, Yun JS, Moorman AF, Lamers WH, Hendrick GK, Arafah BM, Park EA, Wagner TE, and Hanson RW. Metabolic effects of developmental, tissue-, and cell-specific expression of a chimeric phosphoenolpyruvate carboxykinase (GTP)/bovine growth hormone gene in transgenic mice. J Biol Chem 265: 22371-22379, 1990[Abstract/Free Full Text].

27.   Möller, J, Fisker S, Rosenfalck AM, Frandsen E, Jörgensen JO, Hilsted J, and Christiansen JS. Long-term effects of growth hormone (GH) on body fluid distribution in GH deficient adults: a four months double blind placebo controlled trial. Eur J Endocrinol 140: 11-16, 1999[ISI][Medline].

28.   Morberg, PH, Isaksson OG, Johansson CB, Sandstedt J, and Törnell J. Improved long-term bone-implant integration. Experiments in transgenic mice overexpressing bovine growth hormone. Acta Orthop Scand 68: 344-348, 1997[ISI][Medline].

29.   Nikkilä, EA, and Pelkonen R. Serum lipids in acromegaly. Metabolism 24: 829-838, 1975[ISI][Medline].

30.   Norstedt, G, and Palmiter R. Secretory rhythm of growth hormone regulates sexual differentiation of mouse liver. Cell 36: 805-812, 1984[ISI][Medline].

31.   Oberbauer, AM, and Murray JD. Consequences of limited exposure to elevated growth hormone in the mature oMt1a-oGH transgenic mouse. Growth Dev Aging 62: 87-93, 1998[ISI][Medline].

32.   Olofsson, SO, Asp L, and Borén J. The assembly and secretion of apolipoprotein B-containing lipoproteins. Curr Opin Lipidol 10: 341-346, 1999[ISI][Medline].

33.   Oscarsson, J, Olofsson SO, Bondjers G, and Edén S. Differential effects of continuous versus intermittent administration of growth hormone to hypophysectomized female rats on serum lipoproteins and their apoproteins. Endocrinology 125: 1638-1649, 1989[Abstract].

34.   Oscarsson, J, Olofsson SO, Vikman K, and Edén S. Growth hormone regulation of serum lipoproteins in the rat: different growth hormone regulatory principles for apolipoprotein (apo) B and the sexually dimorphic apo E concentrations. Metabolism 40: 1191-1198, 1991[ISI][Medline].

35.   Oscarsson, J, Ottosson M, Johansson JO, Wiklund O, Mårin P, Björntorp P, and Bengtsson BA. Two weeks of daily injections and continuous infusion of recombinant human growth hormone (GH) in GH-deficient adults. II. Effects on serum lipoproteins and lipoprotein and hepatic lipase activity. Metabolism 45: 370-377, 1996[ISI][Medline].

36.   Oscarsson, J, Ottosson M, Vikman-Adolfsson K, Frick F, Enerbäck S, Lithell H, and Edén S. GH but not IGF-I or insulin increases lipoprotein lipase activity in muscle tissues of hypophysectomised rats. J Endocrinol 160: 247-255, 1999[Abstract/Free Full Text].

37.   Oscarsson, J, Wiklund O, Jakobsson KE, Petruson B, and Bengtsson BÅ. Serum lipoproteins in acromegaly before and 6-15 months after transsphenoidal adenomectomy. Clin Endocrinol (Oxf) 41: 603-608, 1994[ISI][Medline].

38.   O'Sullivan, AJ, Kelly JJ, Hoffman DM, Baxter RC, and Ho KK. Energy metabolism and substrate oxidation in acromegaly. J Clin Endocrinol Metab 80: 486-491, 1995[Abstract].

39.   Patsch, JR, Gotto AM, Jr, Olivercrona T, and Eisenberg S. Formation of high density lipoprotein2-like particles during lipolysis of very low density lipoproteins in vitro. Proc Natl Acad Sci USA 75: 4519-4523, 1978[Abstract].

40.   Peterson, J, Olivecrona T, and Bengtsson-Olivecrona G. Distribution of lipoprotein lipase and hepatic lipase between plasma and tissues: effect of hypertriglyceridemia. Biochim Biophys Acta 837: 262-270, 1985[ISI][Medline].

41.   Pomp, D, Oberbauer AM, and Murray JD. Development of obesity following inactivation of a growth hormone transgene in mice. Transgenic Res 5: 13-23, 1996[ISI][Medline].

42.   Quaife, CJ, Mathews LS, Pinkert CA, Hammer RE, Brinster RL, and Palmiter RD. Histopathology associated with elevated levels of growth hormone and insulin-like growth factor I in transgenic mice. Endocrinology 124: 40-48, 1989[Abstract].

43.   Randle, PJ. Regulatory interactions between lipids and carbohydrates: the glucose fatty acid cycle after 35 years. Diabetes Metab Rev 14: 263-283, 1998[ISI][Medline].

44.   Rodbell, M, and Krishna G. Preparation of isolated fat cells and fat cell "ghosts"; methods for assaying adenylate cyclase activity and levels of cyclic AMP. Methods Enzymol 31: 103-114, 1974[Medline].

45.   Rudling, M, Olivecrona H, Eggertsen G, and Angelin B. Regulation of rat hepatic low density lipoprotein receptors. In vivo stimulation by growth hormone is not mediated by insulin-like growth factor I. J Clin Invest 97: 292-299, 1996[Abstract/Free Full Text].

46.   Sandstedt, J, Ohlsson C, Norjavaara E, Nilsson J, and Törnell J. Disproportional bone growth and reduced weight gain in gonadectomized male bovine growth hormone transgenic and normal mice. Endocrinology 135: 2574-2580, 1994[Abstract].

47.   Santamarina-Fojo, S, Haudenschild C, and Amar M. The role of hepatic lipase in lipoprotein metabolism and atherosclerosis. Curr Opin Lipidol 9: 211-219, 1998[ISI][Medline].

48.   Simsolo, RB, Ezzat S, Ong JM, Saghizadeh M, and Kern PA. Effects of acromegaly treatment and growth hormone on adipose tissue lipoprotein lipase. J Clin Endocrinol Metab 80: 3233-3238, 1995[Abstract].

49.   Sjöberg, A, Oscarsson J, Borén J, Edén S, and Olofsson SO. Mode of growth hormone administration influences triacylglycerol synthesis and assembly of apolipoprotein B-containing lipoproteins in cultured rat hepatocytes. J Lipid Res 37: 275-289, 1996[Abstract].

50.   Sjöberg, A, Oscarsson J, Boström K, Innerarity TL, Edén S, and Olofsson SO. Effects of growth hormone on apolipoprotein-B (apoB) messenger ribonucleic acid editing, and apoB 48 and apoB 100 synthesis and secretion in the rat liver. Endocrinology 130: 3356-3364, 1992[Abstract].

51.   Sjöberg, A, Oscarsson J, Olofsson SO, and Edén S. Insulin-like growth factor-I and growth hormone have different effects on serum lipoproteins and secretion of lipoproteins from cultured rat hepatocytes. Endocrinology 135: 1415-1421, 1994[Abstract].

52.   Söderpalm, B, Ericson M, Bohlooly-Y M, Engel JA, and Törnell J. Bovine growth hormone transgenic mice display alterations in locomotor activity and brain monoamine neurochemistry. Endocrinology 140: 5619-5625, 1999[Abstract/Free Full Text].

53.   Spooner, PM, Garrison MM, and Scow RO. Regulation of mammary and adipose tissue lipoprotein lipase and blood triacylglycerol in rats during late pregnancy. Effect of prostaglandins. J Clin Invest 60: 702-708, 1977[ISI][Medline].

54.   Takeda, R, Tatami R, Ueda K, Sagara H, Nakabayashi H, and Mabuchi H. The incidence and pathogenesis of hyperlipidaemia in 16 consecutive acromegalic patients. Acta Endocrinol 100: 358-362, 1982[ISI][Medline].

55.   Tan, KC, Shiu SW, Janus ED, and Lam KS. LDL subfractions in acromegaly: relation to growth hormone and insulin-like growth factor-I. Atherosclerosis 129: 59-65, 1997[ISI][Medline].

56.   Tao, R, Acquati F, Marcovina SM, and Hobbs HH. Human growth hormone increases apo(a) expression in transgenic mice. Arterioscler Thromb Vasc Biol 19: 2439-2447, 1999[Abstract/Free Full Text].

57.   Teng, B, Blumenthal S, Forte T, Navaratnam N, Scott J, Gotto AM, Jr, and Chan L. Adenovirus-mediated gene transfer of rat apolipoprotein B mRNA-editing protein in mice virtually eliminates apolipoprotein B-100 and normal low density lipoprotein production. J Biol Chem 269: 29395-29404, 1994[Abstract/Free Full Text].

58.   Thorngate, FE, Raghow R, Wilcox HG, Werner CS, Heimberg M, and Elam MB. Insulin promotes the biosynthesis and secretion of apolipoprotein B-48 by altering apolipoprotein B mRNA editing. Proc Natl Acad Sci USA 91: 5392-5396, 1994[Abstract].

59.   Tsuchiya, H, Onishi T, Mogami H, and Iida M. Lipid metabolism in acromegalic patients before and after selective pituitary adenomectomy. Endocrinol Jpn 37: 797-807, 1990[Medline].

60.   Valera, A, Rodriguez-Gil JE, Yun JS, McGrane MM, Hanson RW, and Bosch F. Glucose metabolism in transgenic mice containing a chimeric P-enolpyruvate carboxykinase/bovine growth hormone gene. FASEB J 7: 791-800, 1993[Abstract/Free Full Text].

61.   Wolf, E, Wanke R, Schenck E, Hermanns W, and Brem G. Effects of growth hormone overproduction on grip strength of transgenic mice. Eur J Endocrinol 133: 735-740, 1995[ISI][Medline].

62.   Zechner, R. The tissue-specific expression of lipoprotein lipase: implications for energy and lipoprotein metabolism. Curr Opin Lipidol 8: 77-88, 1997[ISI][Medline].


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