Arylhydrocarbon receptor (AhR) activity during in vitro oocyte maturation (IVM)

The AhR is a ligand activated transcription factor mediating the adverse effects of a variety of environmental toxicant. However, recent reports suggest that the AhR may play a central role in ovarian physiology.

We recently demonstrated that the AhR activity (as indicated by the up-regulation of the target genes cytochrome p450 1a1 and 1b1– CYP1A1/CYP1B1) is stimulated during bovine oocyte in vitro maturation in absence of exogenous ligands and that he exposure to specific AhR antagonists, significantly impairs the ability of the oocyte to complete the maturation. These data strongly suggest that the AhR may play a predominant role during oocyte maturation.

In the present study, we analyzed the effects of the broad-spectrum serine/threonine kinase inhibitor 6-dimethylaminopurine (6-DMAP) on the induction of CYP1A1 during IVM. Besides inducing meiotic arrest and down-regulating the phosphorylation status of ERK 1 and 2, exposure to 6-DMAP was able to induce a significant decrease in the expression level of CYP1A1 and CYP1B1. The phenomenon was partially reversible after a period of further 24h of culture in absence of 6-DMAP. In these conditions, beside a recover of oocyte maturation and phosphorylation status of ERK 1 and 2 to levels comparable to control, a significant up-regulation of CYP1A1 mRNA was observed compared to 6-DMAP treated group. To confirm the role of serine/threonine kinases in modulating the activity of AhR during resumption of meiosis, we exposed the oocytes to cycloheximide, a protein synthesis inhibitor, also known to arrest oocyte maturation. We observed that, although cycloheximide exposure induced meiotic arrest, no significant differences on the expression level of AhR or its target genes were observed compared to control.

In conclusion, our results strongly suggest that the activity of AhR is modulated during oocyte maturation and that a 6-DMAP-sensitive kinase(s) is (are) involved in the regulation of the receptor in this model system. Further analyses are necessary to understand the biological significance of these observations.