The protective role of melatonin in porcine oocyte meiotic failure caused by the exposure to benzo(a)pyrene

Abstract

Study question: Does melatonin restore the benzo(a)pyrene (BaP)-induced meiotic failure in porcine oocytes?

Summary answer: Melatonin effectively inhibits the increased reactive oxygen species (ROS) level and apoptotic rate in BaP-exposed porcine oocytes to recover the meiotic failure.

What is known already: BaP, a widespread environmental carcinogen found in particulate matter, 2.5 µm or less (PM2.5), has been shown to have toxicity at the level of the reproductive systems. BaP exposure disrupts the steroid balance, alters the expression of ovarian estrogen receptor and causes premature ovarian failure through the rapid depletion of the primordial follicle pool. In addition, acute exposure to BaP has transient adverse effects on the follicle growth, ovulation and formation of corpora lutea, which results in transient infertility.

Study design, size, duration: Porcine oocytes were randomly assigned to control, BaP-exposed and melatonin-supplemented groups. BaP was dissolved in dimethylsulphoxide and diluted to a final concentration of 50, 100 or 250 μM with maturation medium, respectively. Melatonin was dissolved in the absolute ethanol and diluted with maturation medium to a final concentration of 1 nM, 100 nM, 10 μM and 1 mM, respectively. The in vitro cultured oocytes from each group after treatment were applied to the subsequent analysis.

Participants/materials, setting, methods: Acquisition of oocyte meiotic competence was assessed using immunostaining, fluorescent intensity quantification and/or immunoblotting to analyse the cytoskeleton assembly, mitochondrial integrity, cortical granule dynamics, ovastacin distribution, ROS level and apoptotic rate. Fertilization ability of oocytes was examined by sperm binding assay and IVF.

Main results and the role of chance: BaP exposure resulted in the oocyte meiotic failure (P = 0.001) via impairing the meiotic apparatus, showing a prominently defective spindle assembly (P = 0.003), actin dynamics (P < 0.001) and mitochondrion integrity (P < 0.001). In addition, BaP exposure caused the abnormal distribution of cortical granules (P < 0.001) and ovastacin (P = 0.003), which were consistent with the observation that fewer sperm bound to the zona pellucida surrounding the unfertilized BaP-exposed eggs (P < 0.001), contributing to the fertilization failure (P < 0.001). Conversely, melatonin supplementation recovered, at least partially, all the meiotic defects caused by BaP exposure through inhibiting the rise in ROS level (P = 0.015) and apoptotic rate (P = 0.001).

Limitations, reasons for caution: We investigated the negative impact of BaP on the oocyte meiotic maturation in vitro, but not in vivo.

Wider implications of the findings: Our findings not only deeply clarify the potential mechanisms of BaP-induced oocyte meiotic failure, but also extend the understanding about how environmental pollutants influence the reproductive systems in humans.

Study funding/competing interests: This study was supported by the National Natural Science Foundation of China (31571545) and the Natural Science Foundation of Jiangsu Province (BK20150677). The authors have no conflict of interest to disclose.

Keywords: benzo(a)pyrene; cortical granule; meiotic apparatus; melatonin; oocyte meiotic competence; ovastacin; oxidative stress.