Patricia Passilly

Phosphorylation of peroxisome proliferator-activated receptor α 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.

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.

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, α 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.

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.

Peroxisome proliferation in rodents and human: A model of cell organelle biogenesis☆

ASSAKA L.,PACOTC.,BARDOTO.elLATRUFFE N.(1991). Cell.Mo[. Biol., 37, 123-133). This peroxisomal proliferation is characterized by enhanced peroxisomes biogenesis and specific genes activation. In one hand, our current knowledges on biogenesis indicate that peroxisomal number increase derives from the fission of pre-existing peroxisomes provoked by the import in the organelle of newly svnthesized oroteins and lioids. The oolvueptides neo-svnthesized on free cytosolic poiysomes are addressed io pyroxisomes, either by C-terminal label: the SKL amino acids motif, or by N-terminal signal. Two membrane receptors for polypeptides translocation have been already identified (MANNAERTS G., Belgium and SUBRAMANlS.,USA). On the other hand. neroxisome oroliferation is associated with eenes activation under n’u’clear recep’tors control: PPAR (Peroxisome Proliferator Activated Recentor) and RXR (9-cis Retinoid Acid Receptor)