Genotoxic stress response gene expression in the mid-organogenesis rat conceptus

The ability of the conceptus to respond to genotoxic stress may be critical for normal development, particularly after exposure to genotoxic teratogens. Members of the phosphatidylinositol 3-kinase (PI3K) superfamily are involved in controlling cell cycle activity and maintaining genomic stability. The expression of PI3K family members ATM, ATR, and DNA-PKcs, and downstream genes p53, GADD45, and p21, was examined in the mid organogenesis rat conceptus in vivo on gestational days (GD) 10 through 12 and in vitro following exposure to genotoxic stress. ATM was the most highly expressed PI3K family member in both yolk sac and embryo proper, with transcript levels increasing ~fourfold in the embryo from GD 10 to 12. Transcript concentrations for ATR, DNA-PKcs, and downstream genes were low in both tissues; all genes had increased transcript levels exclusively in the GD 12 embryo. Transient oxidative stress, induced by short-term, in vitro embryo culture, had no effect on transcript levels in either tissue. Culture for 24 or 44 h significantly decreased ATM transcript levels in both embryo and yolk sac, but downstream genes were unaffected compared to GD-11 and -12 in vivo levels, respectively. Exposure to 4-hydroperoxycyclophosphamide (4-OOHCPA), an activated form of the nitrogen mustard cyclophosphamide (CPA), had no effect on transcript levels for any of the genes examined. Therefore, while transcripts for genotoxic stress-response genes are present in the mid organogenesis rat conceptus, their expression is not regulated by exposure in culture to either transient oxidative stress or a genotoxic alkylating agent. The inability of the conceptus to upregulate transcripts in response to insult may contribute to an increased susceptibility to stressors during organogenesis.