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AROMATIC HYDROCARBON RECEPTOR ACTIVATION INDUCES APOPTOSIS IN MURINE PREIMPLANTATION EMBRYOS.
Jurisicova, Andrea1,2, Oh, Jaymin1, Perez, Gloria2, Tilly, Jonathan2, 1 2
ABSTRACT- The high miscarriage rates observed in women smokers suggest that chemicals present in cigarette smoke, such as polycyclic aromatic hydrocarbons (PAH), are cytotoxic to preimplantation embryos (PE). In many cells, PAH bind with and activate the aromatic hydrocarbon receptor (AHR), a member of the PAS family of transcription factors. Previous studies have established that PAH are toxic to oocytes and that the AHR is expressed in PE; however, very little is known of the effects of PAH on PE. Recently, we identified the gene encoding Bax, a pro-apoptotic member of the Bcl-2 family of cell death regulators, as a target for transcriptional up-regulation by the PAH-activated AHR in murine and human oocytes. Herein we tested if PAH induce bax gene expression and apoptosis in murine PE as a potential mechanism underlying cigarette smoking-induced pregnancy loss. When compared with PE exposed to vehicle around the time of compaction PE exposed to 1 
M of a prototypical PAH (7,12-dimethylbenz[a]anthracene) in parallel displayed a higher incidence of apoptosis at 24 h 21±3%(n=35) versus 14±2% (n=25) and 48 h 29±5% (n=27) versus 15±2% (n=26). Both inner cell mass cells and trophectoderm were affected by PAH since apoptosis was observed in both cell lineages. Increased levels of mRNA encoding two pro-apoptotic Bcl-2 family members, Bax (>20-fold) and Bok/Mtd (3.5-fold), accompanied the induction of cell death. No changes were observed in the levels of p53, bcl-x or caspase-2 mRNAs. Moreover, using immunocytochemistry coupled with deconvolution microscopy, we also detected markedly increased levels of Bax protein in PAH-treated PE. We conclude that exposure of PE to PAH reduces the allocation of cells to the embryonic and placental lineages by inducing apoptosis, possibly via increasing expression of pro-apoptotic Bcl-2 family members. (Supported by the Canadian Toxic Substances Research Initiative and NIH R01-ES08430).
KEY WORDS: blastocyst, apoptosis, gene expression, embryo development