Importance of PPARα for the effects of growth hormone on hepatic lipid and lipoprotein metabolism
Introduction
In addition to its well-known effect on longitudinal bone growth, growth hormone (GH) plays an important role in the regulation of lipid metabolism. GH influences body composition in terms of a moderate increase in lean body mass and a more marked decrease in body fat mass [1]. Decreased body fat mass is due to the lipolytic and anti-lipogenic action of GH in adipose tissue [2] that results in increased flux of fatty acids to other tissues [3]. In contrast to the effect of GH in adipose tissue, GH increases lipid synthesis in the liver. GH treatment in vivo increased hepatic triglyceride synthesis and very low-density lipoprotein (VLDL) secretion in rats [4], [5], [6] and VLDL secretion in man [7]. These effects of GH could be direct on hepatocytes or indirect via increased flux of fatty acids to the liver, since GH and oleic acid incubation of primary rat hepatocytes had similar effects on triglyceride synthesis and VLDL secretion [8]. Moreover, continuous GH infusion to hypophysectomized rats increased hepatic expression of SREBP-1c and most of its downstream target genes [9], indicating that GH also increases hepatic de novo lipogenesis. Thus, continuous GH administration could result in both increased flux of fatty acids to the liver and increased hepatic lipogenesis.
Unsaturated long-chain fatty acids and their derivatives are potent endogenous activators of the nuclear receptor peroxisome proliferator-activated receptor (PPAR)α [10]. PPARα is mainly expressed in tissues with a high degree of fatty acid metabolism, such as liver, heart, brown adipose tissue, kidney and skeletal muscle [11]. Although PPARα expression is highest in liver in rodents, this is not the case for human [12] or non-human primates [13]. PPARα agonists, i.e. fibrates, are used in the treatment of hypertriglyceridemia [14]. Activation of PPARα in the liver increases transcription of genes involved in mitochondrial, peroxisomal and microsomal fatty acid oxidation [15]. In addition, the hepatic expression of apolipoprotein (apo) A-I/A-II, apoC-III [15] and microsomal triglyceride transfer protein (MTP) [16] is regulated by PPARα. Furthermore, PPARα activation by fibrates leads to increased hepatic fatty acid oxidation and decreased VLDL-triglyceride secretion from primary rat hepatocytes [17].
There are a few studies showing that GH and PPARα interact in the regulation of hepatic metabolism. A continuous infusion of GH has been found to suppress the peroxisome proliferator induction of hepatic peroxisomal β-oxidation [18], [19] and acyl-CoA oxidase (ACO) mRNA [20] as well as cytochrome P450 (CYP) 4A-mediated ω-oxidation [19] and CYP4A mRNA [21]. Moreover, GH decreased PPARα mRNA in cultured rat hepatocytes [22], [23] and hepatic PPARα mRNA and protein expression in hypophysectomized rats [24]. GH has also been shown to decrease PPARα transcriptional activity [25] at the ligand-independent N-terminal activation function region-1 (AF-1 region) domain of PPARα [26]. Thus, GH may counteract PPARα signalling by decreasing the expression level of PPARα or interfering with PPARα signalling by other means. On the other hand, GH may increase the supply of ligands for PPARα, either via increased lipolysis in adipose tissue or via de novo synthesized fatty acids in the liver. Thus, GH and PPARα may interact in a complex manner in the regulation of hepatic lipid metabolism.
The aim of this study was to determine the importance of PPARα for the effects of GH on lipid and lipoprotein metabolism. GH was administered as a continuous infusion (5 mg/kg/day) to both PPARα-null and wild-type (wt) mice for 7 days. The effect of GH on hepatic triglyceride secretion rate, serum and liver lipid levels, as well as hepatic expression of several genes of importance for lipid and lipoprotein metabolism, was investigated.
Section snippets
Animals and hormonal treatment
Homozygous PPARα-null mice on pure Sv/129 genetic background and corresponding wt Sv/129 control mice were kindly provided by Dr. F.J. Gonzalez (NIH, Bethesda, MD, USA) [27] and kept on the Sv/129 background. The mice were maintained under standardized conditions of temperature (24–26 °C) and humidity (50–60%), with lights on between 0500 and 1900 h. The animals had free access to water and standard laboratory chow containing (w/w) 4% fat, 58% carbohydrates, 16.5% protein and 6% ashes with a
Results
In initial studies, 20 weeks old male and female mice (Sv/129) were given either vehicle or two different doses of bovine GH (2.5 mg/kg/day or 5 mg/kg/day) for 7 days via osmotic minipumps. Neither dose of GH significantly influenced PPARα mRNA expression (data not shown). The higher dose was chosen for the subsequent experiments since this dose increased plasma cholesterol levels (data not shown).
Discussion
The main finding in this study was that PPARα plays a minor role for the overall effect of GH on hepatic lipid and lipoprotein metabolism, including hepatic expression of genes involved in different aspects of fatty acid metabolism. Stimulation of hepatic triglyceride secretion or downregulation of ACO and MCAD mRNA by GH was not dependent on PPARα. However, hepatic expression of PPARα was important for GH-mediated downregulation of Cyp4a10 and upregulation of DGAT1 as well as maintenance of
Acknowledgements
We thank Lennart Svensson and co-workers at AstraZeneca R&D, Mölndal for the analyses of lipoprotein fractions. We also thank Jing Jia for excellent technical assistance and Karin Nelander for valuable help with the statistical analyses. This work was supported by Grant 14291 from the Swedish Medical Research Council, King Gustav V:s and Queen Victorias Foundation, AstraZeneca R&D and the Swedish Heart and Lung Foundation.
References (54)
- et al.
Mode of growth hormone administration influences triacylglycerol synthesis and assembly of apolipoprotein B-containing lipoproteins in cultured rat hepatocytes
J. Lipid Res.
(1996) - et al.
Human and rat peroxisome proliferator activated receptors (PPARs) demonstrate similar tissue distribution but different responsiveness to PPAR activators
J. Steroid Biochem. Mol. Biol.
(1994) - et al.
Effects of fenofibrate on lipid parameters in obese rhesus monkeys
J. Lipid Res.
(2001) - et al.
Role of the peroxisome proliferator-activated receptor (PPAR) in mediating the effects of fibrates and fatty acids on gene expression
J. Lipid Res.
(1996) - et al.
Activation of peroxisome proliferator-activated receptor alpha increases the expression and activity of microsomal triglyceride transfer protein in the liver
J. Biol. Chem.
(2005) - et al.
Influence of peroxisome proliferator-activated receptor alpha agonists on the intracellular turnover and secretion of apolipoprotein (Apo) B-100 and ApoB-48
J. Biol. Chem.
(2002) - et al.
Effects of testosterone, hypophysectomy and growth hormone treatment on clofibrate induction of peroxisomal beta-oxidation in female rat liver
Biochem. Pharmacol.
(1994) - et al.
Suppression of clofibrate-induction of peroxisomal and microsomal fatty acid-oxidizing enzymes by growth hormone and thyroid hormone in primary cultures of rat hepatocytes
Biochim. Biophys. Acta
(1995) - et al.
Hormonal modulation of peroxisomal enzyme induction caused by peroxisome proliferators: suppression by growth and thyroid hormones in cultured rat hepatocytes
Arch. Biochem. Biophys.
(1994) - et al.
Sex-dependent expression and clofibrate inducibility of cytochrome P450 4A fatty acid omega-hydroxylases. Male specificity of liver and kidney CYP4A2 mRNA and tissue-specific regulation by growth hormone and testosterone
J. Biol. Chem.
(1992)