Adipose differentiation-related protein (ADRP) is a lipid droplet-associated protein that is expressed in various tissues. In mice treated with the peroxisome proliferator-activated receptor α (PPARα) agonist Wy14,643 (Wy), hepatic mRNA and protein levels of ADRP as well as hepatic triglyceride content increased. Also in primary mouse hepatocytes, Wy increased ADRP expression and intracellular triglyceride mass. The triglyceride mass increased in spite of unchanged triglyceride biosynthesis and increased palmitic acid oxidation. However, Wy incubation decreased the secretion of newly synthesized triglycerides, whereas apolipoprotein B secretion increased. Thus, decreased availability of triglycerides for VLDL assembly could help to explain the cellular accumulation of triglycerides after Wy treatment. We hypothesized that this effect could be mediated by increased ADRP expression. Similar to PPARα activation, adenovirus-mediated ADRP overexpression in mouse hepatocytes enhanced cellular triglyceride mass and decreased the secretion of newly synthesized triglycerides. In ADRP-overexpressing cells, Wy incubation resulted in a further decrease in triglyceride secretion. This effect of Wy was not attributable to decreased cellular triglycerides after increased fatty acid oxidation because the triglyceride mass in Wy-treated ADRP-overexpressing cells was unchanged. In summary, PPARα activation prevents the availability of triglycerides for VLDL assembly and increases hepatic triglyceride content in part by increasing the expression of ADRP.

Hepatic PPARγ expression is increased in several animal models of diabetes and obesity, and liver-specific overexpression of PPARγ induces liver steatosis. The aim of this study was to investigate the regulation of PPARγ expression in primary mouse hepatocytes. PPARγ2, but not PPARγ1, was up-regulated by insulin and to a lesser extent by oleic acid. Insulin increased transcription of the PPARγ2 gene via phosphatidylinositol 3-kinase activation. The PPARγ agonist, rosiglitazone, increased PPARγ2 expression, but not PPARγ1, only in the presence of insulin. Also aP2 mRNA expression was induced by rosiglitazone to a higher degree in the presence of insulin, while acyl-CoA oxidase was increased independently of insulin. In summary, PPARγ2 is increased in hepatocytes by oleic acid and insulin. These results may help to understand the regulation of PPARγ expression in liver, which possibly plays a role in the development of liver steatosis.

Microsomal triglyceride transfer protein (MTP) is rate-limiting in the assembly and secretion of lipoproteins containing apolipoprotein (apo) B. Previously we demonstrated that Wy 14,643 (Wy), a peroxisome proliferator-activated receptor (PPAR) alpha agonist, increases apoB-100 secretion despite decreased triglyceride synthesis. In this study, we sought to determine whether PPARalpha activation increases MTP expression and activity. Treatment with Wy increased hepatic MTP expression and activity in rats and mice and increased MTP expression in primary cultures of rat and mouse hepatocytes. Addition of actinomycin D blocked this increase and the MTP promoter (-136 to +67) containing a conserved DR1 element was activated by Wy, showing that PPARalpha activates transcription of the gene. Wy did not affect MTP expression in the intestine or in cultured hepatocytes from PPARalpha-null mice. A retinoid X receptor agonist (9-cis-retinoic acid), but not a PPARgamma agonist (rosiglitazone), increased MTP mRNA expression in cultured hepatocytes from both wild type and PPAR alpha-null mice. In rat hepatocytes incubated with Wy, MTP mRNA levels increased between 6 and 24 h, and MTP protein expression and apoB-100 secretion increased between 24 and 72 h. In conclusion, PPARalpha activation stimulates hepatic MTP expression via increased transcription of the Mtp gene. This effect is paralleled by a change in apoB-100 secretion, indicating that the effect of Wy on apoB-100 secretion is mediated by increased expression of MTP.

The peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that modulate lipid and glucose homeostasis. In the clinic, PPARalpha and PPARgamma agonists are used to treat hypertriglyceridemia and insulin resistance of diabetes, respectively. To gain further insight into the molecular mechanisms underlying the therapeutic actions of these drugs, we have by two-dimensional electrophoresis and mass spectrometry performed a comparative analysis of the hepatic protein expression profiles of lean and obese (ob/ob) mice, and obese mice treated with WY14643 (PPARalpha agonist) or rosiglitazone (PPARgamma agonist). We found that livers from obese mice displayed higher levels of enzymes involved in fatty acid oxidation and lipogenesis compared to lean mice and these differences were further amplified by treatment with both PPAR activators. WY14643 normalized the expression levels of several enzymes involved in glycolysis, gluconeogenesis and amino acid metabolism in the obese mice to the levels of lean mice, whereas rosiglitazone partially normalized levels of enzymes involved in amino acid metabolism. In summary, a classical proteomics approach was successfully used to characterize differences at the hepatic proteome level between lean and obese diabetic mice, to map metabolic pathways affected by treatment, and to discriminate between effects caused by treatment with agonists of the closely related PPARalpha and PPARgamma receptors.

The PPAR (peroxisome proliferator activated receptor) transcription factors are ligand-activated nuclear receptors that regulate genes involved in lipid metabolism and homeostasis. PPARα is preferentially expressed in liver and PPARγ preferentially in adipose tissue. Activation of PPARα leads to peroxisome proliferation and increased β-oxidation of fatty acids in rodents. PPARγ-activation leads to adipocyte differentiation and improved insulin signaling of mature adipocytes. Both PPAR receptors are believed to be functional targets for treatment of hyperlipidemia in man. We have treated obese diabetic mice (ob/ob), which have highly elevated levels of plasma triglycerides, glucose and insulin, for 1 week with WY14,643 (180 μmol/kg/day), a selective PPARα agonist, or rosiglitazone (BRL49653; 2.5 μmol/kg/day), a selective PPARγ agonist. The doses used produce a similar therapeutic effect in both treatment groups (lowering of triglycerides and glucose). High resolution two-dimensional gel electrophoresis of livers showed that WY14,643 and rosiglitazone both produced changes in expression pattern of many proteins involved in peroxisomal fatty acid β-oxidation. However, similar experiments performed in lean mice showed significant up-regulation of these proteins only with WY14,643 treatment. Furthermore, the proteins up-regulated by the drugs in obese mice had a higher basal expression in obese controls compared to the lean littermates. Liver PPARγ mRNA levels were determined and we observed that PPARγ2 mRNA levels were elevated in obese mice compared to lean littermates. As PPARα and PPARγ recognize similar DNA response elements, it is likely that the effects of rosiglitazone on PPARα responsive genes in livers of the ob/ob mice are mediated by PPARγ2.—Edvardsson, U., M. Bergström, M. Alexandersson, K. Bamberg, B. Ljung, and B. Dahllöf. Rosiglitazone (BRL49653), a PPARγ-selective agonist, causes peroxisome proliferator-like liver effects in obese mice. J. Lipid Res. 1999. 40: 1177–1184.

The PPAR (peroxisome proliferator activated receptor) transcription factors are ligand-activated receptors which regulate genes involved in lipid metabolism and homeostasis. PPARalpha is preferentially expressed in the liver and PPARgamma preferentially in adipose tissue. Activation of PPARalpha leads to peroxisome proliferation in rodents and increased beta-oxidation of fatty acids. PPARgamma-activation leads to adipocyte differentiation and improved insulin signaling of mature adipocytes. Both of these PPAR receptors are potential targets for treatment of dyslipidemia in man. Studies by others using a proteomics approach have characterized the effects of PPARalpha agonists in livers from lean healthy mice. However, we wanted to map the effects of a therapeutic dose of a PPARalpha agonist in a disease model of insulin resistance and diabetes, the obese diabetic ob/ob mouse, by proteomics. Therefore, ob/ob mice, which have highly elevated levels of plasma triglycerides, glucose and insulin, were treated for one week with WY14,643 (180 micromol/kg/day), a well-characterized selective PPARalpha agonist. Plasma triglycerides, glucose and insulin levels were determined and we found significant therapeutic effects on triglycerides and glucose levels. The liver protein compositions were investigated by high-resolution two-dimensional gel electrophoresis which showed that WY14,643 produced up-regulation of at least 16 spots. These were identified by mass spectrometry and 14 spots were found to be components of the peroxisomal fatty acid metabolism. Thus, WY14,643 at a therapeutic dose, caused induction of peroxisomal fatty acid beta-oxidation in obese diabetic mice.