Aromatic hydrocarbon receptor-driven Bax gene expression is required for premature ovarian failure caused by biohazardous environmental chemicals

Polycyclic aromatic hydrocarbons (PAHs) are toxic chemicals released into the environment by fossil fuel combustion. Moreover, a primary route of human exposure to PAHs is tobacco smoke(1,2). Oocyte destruction and ovarian failure occur in PAH-treated mice(1,2), and cigarette smoking causes early menopause in women(1,3). In many cells, PAHs activate the aromatic hydrocarbon receptor (Ahr), a member of the Per-Arnt-Sim family of transcription factors(4,5). The Ahr is also activated by dioxin, one of the most intensively studied environmental contaminants. Here we show that an exposure of mice to PAHs induces the expression of Bax in oocytes(6), followed by apoptosis. Ovarian damage caused by PAHs is prevented by Ahr or Bax inactivation. Oocytes microinjected with a Bax promoter-reporter construct show Ahr-dependent transcriptional activation after PAH, but not dioxin, treatment, consistent with findings that dioxin is not cytotoxic to oocytes. This difference in the action of PAHs versus dioxin is conveyed by a single base pair flanking each Ahr response element in the Bax promoter. Oocytes in human ovarian biopsies grafted into immunodeficient mice also accumulate Bax and undergo apoptosis after PAH exposure in vivo. Thus, Ahr-driven Bax transcription is a novel and evolutionarily conserved cell-death signaling pathway responsible for environmental toxicant-induced ovarian failure.