We studied the effects of LPA on human immature oocytes and changes in the gene expression profiles in cumulus cells and oocytes during in vitro culture. There were significant differences in the gene expression profile between LPA treatment and control groups. The differences might explain the functional role of LPA in the culture medium to promote oocyte maturation, as well as the molecular mechanisms behind the phenotype.
LPA is present in the human follicular fluid at a concentration of 10–25 µM (Tokumura et al., 1999). In our study, we chose 10 µM of LPA as the final concentration in the culture medium based on previous studied on a mouse model and our previous study, in which we reported that oocyte maturation rates were significantly higher in 10 µM LPA-treated group than the control (Komatsu et al., 2006). Studies have shown that SMAD4 plays an important role during the development of oocytes from small antral follicles (1–3 mm in diameter) to large antral follicles (3–7 mm in diameter) (Komatsu et al., 2006). In a study by Li et al., the litter size was reduced in mice lacking Smad4 compared to the control mice (24). In the present study, the expression of this gene was upregulated in oocytes treated with LPA, indicating it may involve into the maturation of oocytes in vitro.
The biosynthetic function of ribosomes is a critical factor for the development of embryos (25). In this study, upregulated genes enriched the GO-derived items BP, CC, and MF, which are related to ribosome entry in the LPA-treated cumulus cells. In the control group, the top 20 GO-derived items enriched by upregulated genes were involved in the import, secretion, localization, and transport of eight proteins and in the biosynthesis and regulation of two chemokines. This implies that LPA treatment may stimulate the oocyte maturation in vitro.
Interleukin 6 (IL6) can mediate tyrosine kinase receptor A to regulate cumulus expansion, and IL6 knockout likewise reduces cumulus expansion (26). Insulin-like growth factor 1 (IGF1) enables cumulus cells to synthesize and accumulate hyaluronic acid, thereby promoting cumulus cell expansion (27). IL6 and IGF1 enriched eight items related to the import, secretion, localization, and transport of proteins, as well as the biosynthesis and regulation of chemokines, indicating that IL6 and IGF1 were key to the promotion of human oocyte maturation by LPA. The upregulated gene encoding heme oxygenase 1 (HMOX1) enriched two biosynthetic items related to chemokines. Studies have shown that the expression of this gene in cumulus cells can be used as a biomarker of oocyte compatibility (28). Downregulated BPs include processes relating to androgen metabolites, and androgens can promote the expression of connexin 37 (CX37) in COCs, thereby eliciting an ovarian response (29).
Studies by Gebhardt et al. have shown that high levels of PTGS2 expression are related to the rate of live birth (30). Moreover, high levels of PTGS2 expression in the cumulus cells surrounding mature oocytes are related to high-quality embryos and embryo sacs (31). In the present study, PTGS2 expression decreased in the cumulus cells of LPA treatment group compared to that in the control group, which suggested that PTGS2 negatively regulated the development of LPA-treated oocytes. However, the specific function of PTGS2 and the mechanism by which it acts require further investigation. In this study, transcriptome analysis revealed that only four differentially expressed genes were upregulated in the cumulus cells and only four were downregulated in oocytes, indicating that LPA had varying effects on the gene expression of cumulus cells and oocytes.
KEGG pathway enrichment analysis revealed upregulated and downregulated genes in the cumulus cells that enrich the TNF signaling pathway, and the cumulus cells can release soluble TNF-α to promote oocyte aging (32). In our study, the upregulated genes in the cumulus cells enriched the insulin secretion pathway. Insulin plays a central role in polycystic ovary syndrome, and it engages with the IGF1 receptor to enhance steroid production in ovaries and adrenal glands (33). Downregulated differentially expressed genes enrich signaling pathways related to CAMs and AMPK. In the cumulus cells, the downregulation of CAMs may cause polycystic ovary syndrome (34).
Among the top 20 GO-derived items enriched by upregulated genes in the oocytes of the LPA-treated group, five were related to biosynthetic processes and one was related to cell-cell adhesion. Both PLEK and PRKG1 enriched these six items. The expression of PLEK may promote cell growth and development (35), and PRKG1 may be related to early life adversity (36). The present results suggested that PLEK and PRKG1 were important for the maturation and development of oocytes in the culture medium supplemented with LPA, but there have not been any reports on these two genes regarding human oocyte maturation. We found that 47 downregulated genes enriched CC items related to the cell periphery; it is interesting to note that chromosomes separate during cell division at the cell periphery (37). Thus, our results indicated that LPA might affect cell division.
In the oocytes, both upregulated and downregulated genes enriched items related to the MAPK signaling pathway and the gap junction. MAPK signaling pathway regulates the development of oocytes (38). During the development of COCs, the gap junction pathway mediates material exchange between oocytes and cumulus cells (39). Downregulated genes enriched the estrogen signaling pathway and the RAP1 signaling pathway. Estrogen affects the physiological development of women and plays an important role in the maturation of oocytes (40). The RAP1 signaling pathway in oocytes involves multiple cellular processes, including secretion, cell adhesion, and intercellular junction formation, and regulates oocyte maturation and embryonic development (41).
There are limits in our study. Due to the limited number of participants and available immature oocytes, we had a relatively small number of cumulus cells and oocytes for RNA sequencing. If possible in clinics, we will increase the sample size in future studies.