Cumulus cells (CC) play a critical role in oocyte maturation and fertilization via gap junctions. The oocyte itself maintains CC health to favour oocyte maturation via the secretion of paracrine growth factors. However, the antiapoptotic effects of oocyte-secreted factors follow a gradient from the site of the oocytes. Moreover, degrees of CC apoptosis are inversely related to the in vitro embryo development. Therefore, inhibition of CC apoptosis is important for efficient in vitro embryo development. The beneficial effects of retinoic acid (RA) during in vitro embryo production are well known in different species. However, the effect of RA on CC apoptosis is yet to be elucidated. All-trans RA and 9-cis RA are the natural components of retinoids, and all-trans RA are metabolized to 9-cis RA for physiological function. Therefore, the objective of the present study was to evaluate the effect of 9-cis RA on the mechanism for inhibition of apoptosis in CC. Slaughterhouse cumulus–oocyte complexes (COC) were matured in vitro in TCM-199-based in vitro maturation medium containing 0 or 5 mM 9-cis RA for 23 to 24 h (15 COC/100 μL droplet) at 38.5°C and 5% CO2 in air with maximum humidity. Following in vitro maturation, COC of a droplet were fixed in 4% paraformaldehyde for TUNEL staining using In Situ Cell Death Detection Kit (Roche, Budapest, Hungary). The proportion of apoptotic cells was estimated using Olympus Soft Imaging Solutions GmBH (Olympus, Münster, Germany). The COC of the remaining droplet were denuded. The CC were frozen and stored at –80°C. The CC of 3 different cultures were pooled, and total RNA was extracted using RNeasy Mini Kit (Qiagen, Valencia, CA, USA). Total RNA was reverse transcribed into cDNA using Omniscript Reverse Transcription kit (Qiagen). Relative expression of candidate genes was quantified using SYBER green real-time PCR with ΔΔ CT method. The expression was normalized against β-actin, glyceraldehyde 3-phosphate dehydrogenase, and 18s rRNA genes expression. The PCR efficiencies were calculated using relative calibration curves following 10-fold dilution series at 5 measuring points. Data were analysed for one-way ANOVA. The proportion of apoptotic cells was low in the 9-cis RA group (1.3 v. 3.3% of total CC; P < 0.05). Expression of tumor necrosis factor-α (11.1 v. 1.0; P < 0.001), caspase9 (2.0 v. 1.0; P < 0.01), and caspase3 (2.1 v. 1.0; P < 0.001) genes was down-regulated in the 9-cis RA group, whereas expression of Bcl2 gene was increased (1.0 v. 2.6 fold; P < 0.05). Moreover, the expression of c-fos gene of AP-1 pathway was down-regulated (1.9 v. 1 fold; P < 0.05) in the 9-cis RA group. Retinoic acid suppressed the expression of NF-kB, which in turn inhibits tumor necrosis factor-α-mediated caspase activity. However, the expression of NF-kB in CC was not affected by 9-cis RA (1.1 v. 1.0; P > 0.05). In conclusion, the present study indicated that 9-cis RA may inhibit cumulus cell apoptosis through suppression of AP-1 pathway.
This work was partly supported by a scholarship from the BK21 program, the KRF (KRF-2008-211-F00011), the IPET (108068-03-1-SB010), and the KOSEF (10525010001-05N2501-00110).