Modulation of peroxisome proliferator-activated receptors (PPARs) by PPARα- and PPARγ-specific ligands and by 17β-estradiol in isolated zebrafish hepatocytes

Peroxisome proliferation is a phenomenon occurring when responsive animals are exposed to certain compounds so-called peroxisome proliferators and is regulated through a nuclear receptor named peroxisome proliferator-activated receptor (PPAR). PPAR family members exhibit a strong binding affinity for both saturated and unsaturated fatty acids. Activators of PPAR alpha include a variety of endogenously present fatty acids, leukotrienes and hydroxyeicosatetraenoic acids (HETEs) and clinically used drugs, such as fibrates. PPAR beta activators include fatty acids, prostaglandin A(2) (PGA(2)) and prostacyclin (PGI(2)). PPAR gamma is the most selective receptor and, among others, 15-deoxy-Delta(12,14) prostaglandin J(2) (PGJ(2)) has been described to be a PPAR gamma-specific ligand. The aim of the present study was to determine if known PPAR alpha and PPAR gamma ligands were able to alter the expression of these subtypes in an in vitro model of zebrafish primary hepatocyte culture. With this purpose, a PPAR alpha specific ligand (8S-HETE), a PPAR gamma specific ligand (PGJ(2)) and a peroxisome proliferator of the fibrate class (clofibrate) were selected. In addition, the female hormone 17 beta-estradiol was also used as it is known to interact with PPARs. After cell exposure for 24 h, cells were immunohistochemically stained for both PPARs and immunolabeling was quantified as percentage of positive nuclei and cells. Levels of expression of PPARs were also measured by image analysis as grey level per cell. Expression was induced for both PPAR alpha and PPAR gamma by clofibrate (at 0.5 mM for PPAR alpha and at 1 and 2 mM for PPAR gamma), by HETE (1 mu M), and by PGJ(2) (0.3 and 1 mu M for PPAR alpha and 0.3 mu M for PPAR gamma). Expression of PPAR gamma was also induced at 10 mu M by 17 beta-estradiol. The percentage of PPAR alpha positive nuclei increased significantly at 1 mu M HETE and the percentage of PPAR gamma positive cells decreased at 10 mu M 17 beta-estradiol. As a conclusion, clofibrate, HETE and PGJ(2) are able to induce expression of both PPAR alpha and PPAR gamma in zebrafish primary hepatocyte cultures. Further studies are needed to identify how the expression of different PPAR subtypes is regulated and to elucidate the implication of PPAR subtypes in zebrafish cell functions. (c) 2005 Elsevier Ltd. All rights reserved.