Peroxisome proliferator-activated receptors (ppars) and other nuclear receptors in aquatic organismscloning and gene expression studies in relation to environmental contamination

Resumen

Marine organisms are exposed to a variety of chemicals that may interfere with lipid homeostasis and the endocrine system. In vertebrates, such interference is modulated by transcription factors belonging to the superfamily of nuclear receptors. Peroxisome proliferator compounds are agonists of peroxisome proliferator-activated receptors (PPARs) that act through heterodimerization with the retinoid X receptor (RXR). Many xenoestrogens on the other hand act through the estrogen receptor (ER). With the purpose of deepening our understanding of the mechanisms of action of pollutants on marine organisms and of developing new tools to assess the effect of those pollutants, the aims of this PhD were first to clone genes encoding nuclear receptors in marine species widely used as sentinels in pollution monitoring and of commercial interest, the mussel Mytilus galloprovincialis, the thicklip grey mullet Chelon labrosus, and the European hake Merluccius merluccius, and second, to investigate their expression pattern in various tissues at different seasons and in response to xenobiotic exposure. PCR-based screening of cDNA with degenerate primers resulted in amplification, cloning and sequencing of cDNA fragments encoding PPARalfa PPARy, RXR and ERalfa in C. labrosus and M. merluccius. In M. galloprovincialis, 6 genes of the NR1 subfamily where PPAR belongs, RXR and ER were cloned and sequenced. For expression studies 18S rRNA, ß-actin and elongation-factor 1 ¿ were cloned when not available to be used as housekeeping genes. PPARalfa, PPARy, RXR and ERalfa of C. labrosus and M. merluccius are broadly expressed in a variety of tissues, and differentially expressed at the transcriptional level depending on periods of the year and degree of maturity and/or gender. In M. galloprovincialis, the six PPAR family members, RXR and ER are expressed in all tissues tested at different periods of the year. Short term exposure to organic xenobiotics such as Prestige-like heavy fuel oil modulates the expression of PPARalfa, PPARy and RXRalfa in C. labrosus, and of the 6 PPAR family members, RXR and ER in M. galloprovincialis. Variations in expression of PPARy and RXR in M. merluccius sampled during monitoring campaigns after the Prestige oil spill could not be related to fuel oil exposure. Taken as a whole, the results obtained in the present work in both laboratory and field-based studies constitute the basis of further investigations aiming to understand mechanisms of action of organic pollutants and to develop tools to assess the effects of those pollutants on marine organisms.