Modulation of peroxisome proliferator-activated receptors (PPARs) by PPAR(alpha)- and PPAR(gamma)-specific ligands and by 17beta-estradiol in isolated zebrafish hepatocytes

Abstract

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 A2 (PGA2) and prostacyclin (PGI2). PPAR(gamma) is the most selective receptor and, among others, 15-deoxy-Delta(12,14) prostaglandin J2 (PGJ2) 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 PPARgamma specific ligand (PGJ) and a peroxisome proliferator of the fibrate class (clofibrate) were selected. In addition, the female hormone 17beta-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 microM), and by PGJ2 (0.3 and 1 microM for PPAR(alpha) and 0.3 microM for PPAR(gamma)). Expression of PPARgamma was also induced at 10 microM by 17beta-estradiol. The percentage of PPAR(alpha) positive nuclei increased significantly at 1 microM HETE and the percentage of PPAR(gamma) positive cells decreased at 10 microM 17beta-estradiol. As a conclusion, clofibrate, HETE and PGJ2 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.