Relationship between signal transduction and PPAR alpha-regulated genes of lipid metabolism in rat hepatic-derived Fao cells

The goal of this study was to characterize phosphorylated proteins and to evaluate the changes in their phosphorylation level under the influence of a peroxisome proliferator (PP) with hypolipidemic activity of the fibrate family. The incubation of rat hepatic derived Fao cells with ciprofibrate leads to an overphosphorylation of proteins, especially one of 85 kDa, indicating that kinase (or phosphatase) activities are modified. Moreover, immunoprecipitation of 32P-labeled cell lysates shows that the nuclear receptor, PP-activated receptor, alpha isoform, can exist in a phosphorylated form, and its phosphorylation is increased by ciprofibrate. This study shows that PP acts at different steps of cell signaling. These steps can modulate gene expression of enzymes involved in fatty acid metabolism and lipid homeostasis, as well as in detoxication processes.

Effect of the nonpeptide neurotrophic compound SR 57746A on the phenotypic survival of purified mouse motoneurons

1. Neurotrophic factors have been used for the treatment of several neurodegenerative diseases. However, their use is limited by their inability to cross the blood-brain barrier, their short half life and their side effects. SR 57746A is a new orally active compound that exhibits in vivo and in vitro neurotrophic effects in several experimental models. 2. We show here that SR 57746A (1 microM) increases the phenotypic survival of embryonic purified mouse motoneurons in vitro to the same extent as brain-derived neurotrophic factor (100 ng ml-1), and increases the outgrowth and number of their neurites. It acts in a dose-dependent manner up to 1 microM which is the optimal concentration. Above this concentration, its neurotrophic effect decreases. 3. Genistein (10 microM), a protein tyrosine kinase inhibitor, also increases the phenotypic survival and differentiation of mouse motoneurons. It does not act in a synergistic or additive manner with SR 57746A. However, at concentrations equal or superior to 25 microM, it decreases the survival of motoneurons. This suggests that the neurotrophic effect of genistein is due to a favourable alteration of equilibrium between phosphorylated and dephosphorylated states of proteins involved in survival and differentiation of motoneurons. 4. Like genistein, SR 57746A should be used at a critical concentration (1 microM) to exert its optimal effects. Since SR 57746A does not act synergistically with genistein, it is likely that its mechanism of action involves a pathway similar to that affected by this tyrosine kinase inhibitor. 5. At the present time, SR 57746A is the only orally active compound and the only synthetic compound shown to be active on motoneurons in vitro. It should thus be considered as a good candidate for the treatment of motoneuron diseases.

Phosphorylation of peroxisome proliferator-activated receptor alpha in rat Fao cells and stimulation by ciprofibrate

The basic mechanism(s) by which peroxisome proliferators activate peroxisome proliferator-activated receptors (PPARs) is (are) not yet fully understood. Given the diversity of peroxisome proliferators, several hypotheses of activation have been proposed. Among them is the notion that peroxisome proliferators could activate PPARs by changing their phosphorylation status. In fact, it is well known that several members of the nuclear hormone receptor superfamily are regulated by phosphorylation. In this report, we show that the rat Fao hepatic-derived cell line, known to respond to peroxisome proliferators, exhibited a high content of PPARalpha. Alkaline phosphatase treatment of Fao cell lysate as well as immunoprecipitation of PPARalpha from cells prelabeled with [32P] orthophosphate clearly showed that PPARalpha is indeed a phosphoprotein in vivo. Moreover, treatment of rat Fao cells with ciprofibrate, a peroxisome proliferator, increased the phosphorylation level of the PPARalpha. In addition, treatment of Fao cells with phosphatase inhibitors (okadaic acid and sodium orthovanadate) decreased the activity of ciprofibrate-induced peroxisomal acyl-coenzyme A oxidase, an enzyme encoded by a PPARalpha target gene. Our results suggest that the gene expression controlled by peroxisome proliferators could be mediated in part by a modulation of the PPARalpha effect via a modification of the phosphorylation level of this receptor.

Carcinogenic aspect of xenobiotic molecules belonging to the peroxisome proliferator family

It is known that a short-term exposure of rat, mice or incubation of hepatic cells with fibrate molecules leads to increase in peroxisome number and cell hyperplasia. Further, long-term incubation of cells (at least a year) show transformed characteristics with foci and nodules. To explain the hepatocarcinogenic effect of peroxisome proliferators in rodents we studied the effect of peroxisome proliferators on rat liver oncogenes expression. Earlier, we reported an increase in liver and kidney mRNA level of c-myc and N-myc. Since several metabolic genes are activated by PPAR (peroxisome proliferators activated receptor) through a PPRE (peroxisome proliferator response element), we suggest the involvment of PPAR in oncogene activation, because of the presence of PPRE in the N-myc 5'-upstream region. We showed by flow cytometric analysis that ciprofibrate increased the size of rat Fao derived cell line and the activity of palmitoyl CoA oxidase, a peroxisome proliferation enzyme marker for studying peroxisome proliferation was increased. The above effects which can contribute to hepatocarcinogenesis seem to be restricted to rat and mice, which show strong response to peroxisome proliferators. Indeed, no changes are observed in weak responsive species such as humans (using hepatic derived cell lines) and guinea pig. These data provide arguments for the non-carcinogenic effect of this xenobiotic class in human especially when sensitive, or normal individuals are exposed either to hypolipidaemic agents of the fibrate family.

Loss of response of carnitine palmitoyltransferase I to okadaic acid in transformed hepatic cells

The specific activity of carnitine palmitoyltransferase I (CPT-I) was similar in mitochondria isolated from rat Fao and human HepG2 hepatoma cells and from rat hepatocytes, but almost twofold higher in permeabilized hepatoma cells than in permeabilized hepatocytes. Short-term exposure to okadaic acid induced a ca. 80% stimulation of CPT-I in hepatocytes, whereas no significant response of the enzyme from hepatoma cells was evident. Thus, the high CPT-I activity displayed by hepatoma cells may be reached by hepatocytes upon challenge to okadaic acid. Reconstitution experiments with purified mitochondrial and cytoskeletal fractions showed that the cytoskeleton of hepatocytes produced a more remarkable inhibition of CPT-I than the cytoskeleton of Fao cells. The present data may be explained by a disruption of interactions between CPT-I and cytoskeletal components in tumor cells that may be involved in the okadaic acid-induced activation of hepatic CPT-I as previously suggested.

Expression of putative fatty acid transporter genes are regulated by peroxisome proliferator-activated receptor alpha and gamma activators in a tissue- and inducer-specific manner

Regulation of gene expression of three putative long-chain fatty acid transport proteins, fatty acid translocase (FAT), mitochondrial aspartate aminotransferase (mAspAT), and fatty acid transport protein (FATP), by drugs that activate peroxisome proliferator-activated receptor (PPAR) alpha and gamma were studied using normal and obese mice and rat hepatoma cells. FAT mRNA was induced in liver and intestine of normal mice and in hepatoma cells to various extents only by PPARalpha-activating drugs. FATP mRNA was similarly induced in liver, but to a lesser extent in intestine. The induction time course in the liver was slower for FAT and FATP mRNA than that of an mRNA encoding a peroxisomal enzyme. An obligatory role of PPARalpha in hepatic FAT and FATP induction was demonstrated, since an increase in these mRNAs was not observed in PPARalpha-null mice. Levels of mAspAT mRNA were higher in liver and intestine of mice treated with peroxisome proliferators, while levels in hepatoma cells were similar regardless of treatment. In white adipose tissue of KKAy obese mice, thiazolidinedione PPARgamma activators (pioglitazone and troglitazone) induced FAT and FATP more efficiently than the PPARalpha activator, clofibrate. This effect was absent in brown adipose tissue. Under the same conditions, levels of mAspAT mRNA did not change significantly in these tissues. In conclusion, tissue-specific expression of FAT and FATP genes involves both PPARalpha and -gamma. Our data suggest that among the three putative long-chain fatty acid transporters, FAT and FATP appear to have physiological roles. Thus, peroxisome proliferators not only influence the metabolism of intracellular fatty acids but also cellular uptake, which is likely to be an important regulatory step in lipid homeostasis.

Human HepG2 and rat Fao hepatic-derived cell lines show different responses to ciprofibrate, a peroxisome proliferator: analysis by flow cytometry

Peroxisome proliferators, and especially hypolipidemic drugs such as ciprofibrate, are known to be hepatocarcinogens in rodents, but their effect in humans is controversial. In an attempt to investigate the effects of ciprofibrate at a cellular level, the analysis of individual whole cells was performed by flow cytometry on samples from two hepatic-derived cell lines: the rat Fao cell line and the human HepG2 cell line. The increase of light scatter signals in rat Fao cells treated for 3 days with ciprofibrate at 250 microM was related to modifications of intrinsic cellular parameters, such as size and cytoplasmic granularity. Conversely, no variations appeared in human HepG2-treated cells. Moreover, the study of the cell cycle distribution of asynchronously growing cells showed an increase in the percentage of proliferative cells in Fao-treated cells, but not in HepG2-treated cells. In order to give a simultaneous assessment of changes in cellular parameters and cell metabolism, these flow cytometric experiments were completed with the measurements of the palmitoyl-CoA oxidase activity, used as a marker of peroxisome proliferation. The cellular modifications in the rat Fao cell line were accompanied by a great increase in this enzymatic activity, whereas the human HepG2 cell line, which failed to exhibit changes of cytometric data, presented no, or weak, increase in this oxidase activity. The cellular modifications observed in the rat Fao cell line may be related to the well-known hepatocarcinogenicity of ciprofibrate in rodents, whereas the absence of response of HepG2 cells is in favor of the noncarcinogenicity of this drug in humans. This report validates another methodological approach for the investigation of the safety of peroxisome proliferators in humans.

Influence of peroxisome proliferators on phosphoprotein levels in human and rat hepatic-derived cell lines

To elucidate the effect of peroxisome proliferators on the signal-transduction pathway, we have compared the effect of ciprofibrate, an hypolipaemic agent, on the overall phosphoprotein level between rat and human well differentiated hepatic derived cell lines. The phosphorylation status of several phosphoproteins in the rat Fao cell line was increased by the drug while no changes were observed in the human HepG2 cell line. In rat Fao cells, this increase, which is concentration and time dependent, can be as much as eightfold for 20-kDa and 22-kDa proteins. Wy-14,643, a non-fibrate molecule and a more potent peroxisome proliferator than ciprofibrate, increased the phosphorylation status of the same phosphoproteins. Peroxisome proliferators may act by activating kinases inactive in control cells, by amplifying kinases already active in control cells or by inactivating phosphatases. The phosphoamino acid residues affected are essentially serine and threonine. This modification of the signal-transduction pathway by the peroxisome proliferators in rodent cells appears to be an early event or an independent mechanism of the peroxisome proliferation. These results support the accumulating evidence that the perturbation of this pathway may be a major cause of the hepatomegaly and the hepatocarcinogenesis induced by peroxisome proliferators in rodent species. In contrast, the lack of phosphorylation changes in the human HepG2 cell line supports the non-toxic effect of peroxisome proliferators also used as hypolipaemic agents in humans.

The analysis of modified peroxisome proliferator responsive elements of the peroxisomal bifunctional enzyme in transfected HepG2 cells reveals two regulatory motifs

Peroxisome proliferators (PPs) are non-genotoxic carcinogens in rodents. They can induce the expression of numerous genes via the heterodimerization of two members of the steroid hormone receptor superfamily, called the peroxisome proliferator-activated receptor (PPAR) and the 9-cis retinoic acid receptor (RXR). Many of the PP responsive genes possess a peroxisome proliferator response element (PPRE) formed by two TGACCT-related motifs. The bifunctional enzyme (HD) PPRE contains 3 such motifs, creating DR1 and DR2 sequences. PPAR and RXR regulate transcription via the DR1 element while DR2 modulates the expression of the gene via auxiliary factors in HepG2 cells.

Histidyl phosphorylation of P36 in rat hepatoma Fao cells in vitro and in vivo

We have previously shown that a membrane-associated P36 from rat liver was in vitro phosphorylated at His residue(s) with a phosphoric amide bond (FEBS Lett., 319:75-79, 1993), and the activity was solubilized and partially purified (J. Biol. Chem., 269:9030-9037, 1994). The present study demonstrates that the P36 histidyl phosphorylation occurs in rat hepatoma cells under normal conditions. Phosphorylation and dephosphorylation of histidine as well as those of serine, threonine and tyrosine residues may also play an important role in animal cells.