Rat liver BiP/GRP78 is down-regulated by a peroxisome-proliferator, clofibrate

Analysis of the heat shock response in mouse liver reveals transcriptional dependence on the nuclear receptor peroxisome proliferator-activated receptor alpha (PPARalpha)

Background

The nuclear receptor peroxisome proliferator-activated receptor alpha (PPARα) regulates responses to chemical or physical stress in part by altering expression of genes involved in proteome maintenance. Many of these genes are also transcriptionally regulated by heat shock (HS) through activation by HS factor-1 (HSF1). We hypothesized that there are interactions on a genetic level between PPARα and the HS response mediated by HSF1.

Results

Wild-type and PPARα-null mice were exposed to HS, the PPARα agonist WY-14,643 (WY), or both; gene and protein expression was examined in the livers of the mice 4 or 24 hrs after HS. Gene expression profiling identified a number of Hsp family members that were altered similarly in both mouse strains. However, most of the targets of HS did not overlap between strains. A subset of genes was shown by microarray and RT-PCR to be regulated by HS in a PPARα-dependent manner. HS also down-regulated a large set of mitochondrial genes specifically in PPARα-null mice that are known targets of PPARγ co-activator-1 (PGC-1) family members. Pretreatment of PPARα-null mice with WY increased expression of PGC-1β and target genes and prevented the down-regulation of the mitochondrial genes by HS. A comparison of HS genes regulated in our dataset with those identified in wild-type and HSF1-null mouse embryonic fibroblasts indicated that although many HS genes are regulated independently of both PPARα and HSF1, a number require both factors for HS responsiveness.

Conclusions

These findings demonstrate that the PPARα genotype has a dramatic effect on the transcriptional targets of HS and support an expanded role for PPARα in the regulation of proteome maintenance genes after exposure to diverse forms of environmental stress including HS.

Role of the peroxisome proliferator-activated receptor alpha (PPARalpha) in responses to trichloroethylene and metabolites, trichloroacetate and dichloroacetate in mouse liver

Trichloroethylene (TCE) is an industrial solvent and a widespread environmental contaminant. Induction of liver cancer in mice by TCE is thought to be mediated by two carcinogenic metabolites, dichloroacetate (DCA) and trichloroacetate (TCA). TCE is considered to be a relatively weak peroxisome proliferator (PP), a group of rodent hepatocarcinogens that cause adaptive responses in liver through the PP-activated receptor alpha (PPARalpha). The objectives of this study were to determine whether effects of TCE, TCA and DCA in the liver associated with carcinogenesis are mediated by PPARalpha. Male wild-type and PPARalpha-null mice were given TCE by gavage for 3 days or 3 weeks; TCA or DCA were given in the drinking water for 1 week. Increases in relative liver and kidney weights by TCE were dependent on PPARalpha whereas liver weight increases by DCA were PPARalpha-independent. Dose-dependent increases in hepatocyte proliferation observed in wild-type mice after TCE exposure as determined by BrdU-labeling of hepatocytes were PPARalpha-dependent. Transcript profiling using macroarrays containing approximately 1200 genes showed that 93% (40 out of 43) of all expression changes observed in wild-type mice upon TCE exposure were dependent on PPARalpha and included known targets of PP (Cyp4a12, epidermal growth factor receptor) and additional genes involved in cell growth. Increases in enzymes that catalyze beta- and omega-oxidation of fatty acids were dependent on PPARalpha after exposure to TCE, TCA or DCA. TCE altered a unique set of genes in the livers of PPARalpha-null mice compared to wild-type mice including those that respond to different forms of stress. These data support the hypothesis that PPARalpha plays a dominant role in mediating the effects associated with hepatocarcinogenesis upon TCE exposure.

Impaired lipid metabolism in aged mice as revealed by fasting-induced expression of apolipoprotein mRNAs in the liver and changes in serum lipids

BACKGROUND

Changes in apolipoprotein (Apo) metabolism can cause an increased incidence of diseases such as cardiovascular disorders and diabetes with advancing age. Limited reports are available on this topic, however.

OBJECTIVE

To investigate age-related changes in mobilization of stored lipid, we studied the effects of fasting on the gene expression of Apos in the liver as well as serum triglyceride (TG) and cholesterol levels in the serum.

METHODS

Using young (6- to 8-month-old) and old (24- to 28-month-old) fasted and re-fed mice, Northern blots of hepatic mRNAs for Apos A-I, A-IV, C-II, C-III, and liver-type fatty acid-binding protein and HPLC analyses of serum lipids were conducted.

RESULTS

Fasting induced 4- and 20-fold increases in the mRNA of Apo C-II and A-IV, respectively, in young mice while only 1.1- and 7-fold increases, respectively, were detected in old mice. In contrast, the Apo C-III gene expression was significantly reduced by fasting in the young mice but the reduction was small in the old. In view of the stimulating effect of Apo C-II and A-IV and the inhibiting effect of C-III on lipoprotein lipase (LPL), these findings suggest that the fasting-induced activation of LPL may be considerably decreased in old mice. The amount of TG in very low-density lipoprotein (VLDL), a major form of the transport of TG to peripheral tissues, was significantly greater in the young than in the old mice. Despite possible activation of LPL by fasting, the amount of TG in VLDL, a major form of the transport of TG to peripheral tissues, was significantly greater in the young mice than in the old. It is indicated that the synthesis of VLDL in the liver is high in the young but low in the old mice, which also may be true for the rate of transport of TG.

CONCLUSION

The present findings suggest that mobilization of lipids is impaired in old animals due to decreased gene expression of Apos, possibly leading in the long run to excessive lipid accumulation in tissues such as the liver, adipose tissues and blood vessels even in normal feeding, and resulting in an increased incidence of age-related diseases.

Effect of a peroxisome proliferator on 3 beta-hydroxysteroid dehydrogenase

To better understand the changes that occur following exposure to peroxisome proliferators, we utilized mRNA differential display and microarray to screen for peroxisome proliferator target genes apart from those involved in lipid metabolism in male C57B6 mice by using the ubiquitous plasticizer, di(2-ethylhexyl)phthalate (DEHP). One noted change was the dose-dependent suppression of the mouse hormone metabolizing 3 beta-hydroxysteroid dehydrogenase V (HSD3b5), which is specifically expressed in the male mouse liver. Northern analysis showed that HSD3b5 mRNA levels decreased dramatically upon one-day exposure to 2.0% dietary DEHP, and were nearly undetectable by one week of treatment. Food restriction also significantly suppressed HSD3b5 expression; however, in this case the suppression was delayed and to a lesser extent. Another mouse 3 beta-hydroxysteroid dehydrogenase, HSD3b4, predominantly expressed in kidneys, was also regulated by DEHP and food restriction. The sex-specific gene, HSD3b5, was affected more by DEHP and food restriction than the tissue-specific gene, HSD3b4.

Proteomic analysis of differential protein expression in primary hepatocytes induced by EGF, tumour necrosis factor alpha or the peroxisome proliferator nafenopin

Peroxisome proliferators are nongenotoxic rodent-liver carcinogens that have been shown to cause both an induction of hepatocyte proliferation and a suppression of apoptosis. Both epidermal growth factor (EGF) and the peroxisome proliferator nafenopin induce DNA replication in primary rat hepatocyte cultures, but apparently through different signalling pathways. However, both EGF and nafenopin require tumour necrosis factor alpha (TNFalpha) signalling to induce DNA replication. By examining proteins isolated from rat primary hepatocyte cultures using two-dimensional gel electrophoresis and mass spectrometry, we found that proteins showing an altered expression pattern in response to nafenopin differed from those showing altered expression in response to EGF. However, many proteins showing altered expression upon stimulation with TNFalpha were common to both the EGF and nafenopin responses. These proteome profiling experiments contribute to a better understanding of the molecular mechanisms involved in the response to peroxisome proliferators. We found 32 proteins with altered expression upon stimulation with nafenopin, including muscarinic acetylcholine receptor 3, intermediate filament vimentin and the beta subunit of the ATP synthase. These nonperoxisomal protein targets offer insights into the mechanisms of peroxisome proliferator-induced carcinogenesis in rodents and provide opportunities to identify toxicological markers to facilitate early identification of nongenotoxic carcinogens.

Peroxisome proliferator-activated receptor (PPAR)-dependent and -independent transcriptional modulation of several non-peroxisomal genes by peroxisome proliferators

To better characterize peroxisome proliferator-induced non-peroxisomal responses, we searched the mRNAs of which the levels were modulated by proliferators. We used the PCR-based methods including differential display. The mRNAs were divided into at least four groups by their time-courses of induction and repression: group 1 very rapidly increased then decreased; group 2 increased after a time lag (well-characterized peroxisomal mRNAs belonged to this group); group 3 decreased reciprocally compared with group 2 mRNAs; group 4 increased after group 2 mRNAs, with a much longer lag period. All of these modulations cannot be explained by peroxisome proliferator action through PPAR and RXR dimerization on the target genes to activate transcription. Another unidentified transcription factor may be involved in some of these modulations. It will also be important to consider PPAR-independent pathways when studying the diverse effects of peroxisome proliferators.

PPAR alpha mediates peroxisome proliferator-induced transcriptional repression of nonperoxisomal gene expression in mouse

The strain difference, peroxisome proliferator specificity and role of PPAR alpha in peroxisome proliferator-induced transcriptional repression of nonperoxisomal transthyretin and alpha2u-globulin genes were examined. The genes were repressed by four peroxisome proliferators in all seven mouse strains tested. The extent of repression was strongly dependent on both the mouse strains and type of proliferator, although the mRNA levels of PPAR alpha and its partner in heterodimerization, RXR alpha were not different. The role of PPAR alpha in repression was confirmed by the finding that PPAR alpha-null mice were not responsive to transcriptional repression. These results indicate that PPAR alpha plays an obligatory role in transcription of various genes, some of which are not related to lipid metabolism.

Peroxisome proliferator-activated receptors A link between endocrinology and nutrition?

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily like the steroid, thyroid, or retinoid hormone receptors, which are ligand-activated transcription factors regulating gene expression. PPARs mediate the induction of the enzymes of the peroxisomal and microsomal fatty-acid oxidation pathways by hypolipidemic drugs such as clofibrate and are probably also involved in the gene expression of other lipid-metabolism-associated proteins that are controlled by fibrate hypolipidemic drugs. That PPARs play an important role in the regulation of lipid metabolism is reinforced by the discovery of their activation by physiologic concentrations of fatty acids. This observation raises the question of whether fatty acids are ligands of PPARs, which would imply that nutritional fatty acids can act like hormones.

A protein histidine kinase induced in rat liver by peroxisome proliferators. In vitro activation by Ras protein and guanine nucleotides

A novel protein kinase is induced in rat liver plasma membrane by the administration of peroxisome proliferators. A 36 kDa protein (P36) on the membrane was rapidly phosphorylated in vitro by the kinase and the phosphorylated amino acid was identified as phosphohistidine. Histidine phosphorylation of P36 was activated in vitro by recombinant Ras protein and GTP; both decreased Michaelis constant (Km) for ATP from 1.25 to 0.25 microM. The novel histidine kinase, products of which have been overlooked due to their acid lability, may participate in cellular signaling and peroxisome proliferators may perturb the pathway.

Specific repression of transthyretin gene expression in rat liver by a peroxisome proliferator clofibrate

The effect of clofibrate, a hypolipidemic drug and a potent peroxisome proliferator, on expression of a nonperoxisomal transthyretin (prealbumin) gene was studied using rats fed clofibrate for various periods. Upon feeding a diet containing clofibrate, the level of transthyretin mRNA was down-regulated, reaching 20% of the control level, in an almost reciprocal time course to that of increases in the levels of peroxisomal mRNAs. Studies on expression of other steroid hormone regulated genes suggest that clofibrate may down-regulate several but not all types of steroid hormone regulated mRNA expression.