Identification of a key role for permeability glycoprotein in enhancing the cellular defense mechanisms of fertilized oocytes

- Oocytes and pronuclear stage zygotes differ in their sensitivity to etoposide.
- Upregulation of PGP associated with fertilization is of maternal origin.
- PGP activity and expression is associated with cellular sensitivity.
- Drug exclusion by permeability glycoprotein (PGP) is a rapid process.
- PGP's efflux capacity is regulated by intracellular Ca2+, translation and serine/threonine phosphorylation.

Double strand breaks (DSBs) are highly damaging DNA lesions that can destabilize the genome and generate a suite of adverse physiological outcomes in the oocyte and early embryo. While it is therefore likely that these cells possess a sophisticated suite of protective mechanisms to ameliorate such damage, the precise nature of these defense systems are yet to be fully elucidated. This study characterizes the sensitivity of the oocyte to etoposide, a chemotherapeutic agent with the ability to elicit DSBs. We demonstrate significant developmental changes in etoposide vulnerability, with fertilization of the oocyte leading to an enhancement of its cellular defense machinery. Using a parthenogenic model we show that this response is mediated, at least in part, by permeability glycoprotein (PGP), an endogenous multidrug efflux transporter that is up-regulated, translocated to the oolemma and phosphorylated upon oocyte activation. Moreover, evidence from dye exclusion assays in the presence of a specific PGP pharmacological inhibitor (PSC833), illustrates that these events effectively increase oocyte efflux activity, thereby enhancing the ability of these cells to exclude genotoxicants capable of eliciting DSB formation.