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Regulation of aryl hydrocarbon receptor activity in porcine cumulus–oocyte complexes in physiological and toxicological conditions: the role of follicular fluid

¹ Department of Anatomy and Cell Biology, Faculty of Medicine, Martin Luther University, Halle (Saale), Germany and ² Department of Anatomy of Domestic Animals, Faculty of Veterinary Medicine, University of Milano, Milano, Italy

Correspondence should be addressed to P Pocar who is now at Istituto di Anatomia degli Animali Domestici, Facoltá di Medicina Veterinaria, Universitá degli studi di Milano, Via Celoria, 10, I-20133 Milano, Italy; Email: paola.pocar{at}unimi.it

The arylhydrocarbon receptor (AhR) mediates the adverse effects of dioxin-like compounds. However, it has also been reported that the AhR may exert a role in ovarian physiology. In the present study, porcine cumulus–oocyte complexes (COCs) were matured in vitro in the presence of 10% follicular fluid. Expression of AhR and its partner, AhR nuclear translocator occurs in immature COCs. After in vitro maturation (IVM), an up-regulation of AhR and cytochrome P450 1A1 (CYP1A1; the main AhR-target gene) was observed. To explore the role of the AhR during IVM, we exposed the COCs to 50 µM ß-napthoflavone (ßNF). The treatment induced a marked up-regulation of CYP1A1 mRNA, indicating both constitutive and inducible AhR activity. However, in contrast to what was observed in other cell types, no sign of toxicity was observed in COCs. To investigate if components of porcine follicular fluid may exert a protective role against AhR ligands, we exposed porcine COCs to ßNF, in the absence of follicular fluid. In these conditions, a marked decrease in the percentage of matured oocytes, concomitant with an increase in oocyte degeneration, was observed. Furthermore, ßNF increased apoptosis in cumulus cells in the absence of follicular fluid, whereas ßNF has no effects when COCs were treated in the presence of porcine follicular fluid (pFF). In conclusion, these results suggest the presence of unknown endogenous AhR-ligand(s) during porcine IVM and that a dysregulation of this mechanism may result in ovotoxicity by inducing apoptosis in cumulus cells. However, this phenomenon is interrupted by the presence of follicular fluid, indicating a putative protective role for follicular fluid components against exogenous insults.