Given the complexity of human body, it is difficulty to investigate the effect of the prenatal environment on subsequent generations in humans. High insulin resistance in PCOS patients is a hot topic in the field of reproductive endocrinology and insulin resistance appears to be the fundamental key factor within the pathophysiology (26). Metformin treatment in PCOS patients still needed to deeper investigated, especially the effect of metformin on the offspring of PCOS-IR women has been rarely reported. Continuous administration of letrozole is appropriate to be used to replicate PCOS-like phenotypes in rodents. In the current study, rats with letrozole exposure showed PCOS-like reproductive and metabolic phenotypes compared with the controls. Consistently with previous study (27), we established a rat model of PCOS-IR using continuous letrozole administration implantation to further investigate the effects of metformin on offspring of PCOS-IR. We observed that the pregnancy outcomes have been significantly improved after metformin treatment. Female F1 PCOS-IR rats are at a high risk of obesity after the sexual development period, but the metformin treatment can greatly counteract this phenomenon. Genome-wide association studies (GWAS) have identified variants in 11 genomic regions(loci) as risk factors for PCOS, but only INSR, FSHR and LHCHR, which encode receptors for insulin, FSH and LH/hCG, have clear functional relevance to the pathophysiology of PCOS (28, 29). We also investigated those PCOS candidate genes, INSR, FSHR and LHCGR, which have been significantly changed after the treatment of Metformin in female F1 offspring of PCOS-IR rats, but there was no difference in female F2 rats compared with controls. Together, these findings indicated that widespread reproductive, metabolic and endocrine changes in letrozole induced PCOS rat model, but the development of PCOS composited of multi-factors, not only due to the genetic factor.
A successful PCOS model was crucial for the continuation of the study. In our study, letrozole treated rats showed increased body weight, and PCOS-like reproductive and metabolic phenotypes, including significant alters in serum T, E2, LH, FSH and P, impaired insulin resistance and widespread metabolic abnormalities, as well as disrupted estrous cycles and poly ovaries. Compared to Maliqueo’ study (13), we also investigated the glucose impair or diabetes in letrozole induced PCOS rat model. IPGTT tests reveal that there are no significantly changes in glucose levels in 0, 15, 30, 60, 120 min between PCOS group and controls, which is indicating that those letrozole treated rats had the PCOS phenotypes of obesity, hormone levels alter and insulin resistance, but without type 2 diabetes or glucose intolerance. Abnormal insulin tolerance has been observed in our study, but the glucose levels maintained normal. Glucose levels may remain normal in PCOS despite insulin resistance because of compensatory increased pancreatic β-cell insulin production resulting in hyperinsulinemia (30). A successful PCOS-IR female rat model has been induced, more importantly, the follow-up downregulation study that based on this model was effective and valuable.
In present study, conception rate of PCOS-IR rats decreased, and metformin could improve the conception rate of PCOS-IR rats, multiple pregnancy rates in PCOS-IR rats’ groups significantly increased compared with controls, within PCOS-IR rats, it decreased after metformin treatment. Metformin increases the fetal concentration of sex hormone-binding globulin when taken during pregnancy (31) and reduces the secretion of inflammatory cytokines from trophoblast cells in vitro(32). Patients with PCOS have an increased risk for pregnancy complications (33). In present study, metformin has consistently been associated with improved ovulation and low multiple pregnancy rates. A Cochrane review on metformin concluded that ovulation and pregnancy rates were higher in women with PCOS taking metformin (34). A recent study found that in pregnant women with PCOS, metformin treatment from the late first trimester until delivery might reduce the risk of late miscarriage and preterm birth, but no substantial difference in serious adverse events in either mothers or offspring (22). In present study, we found two cases of placenta abruption among PCOS-IR rats. Palomba et al (33) found a 3- to 4-fold increase in pregnancy induced hypertension (PIH) and preeclampsia, 3-fold increase in GDM, and a 2-fold higher chance for premature delivery among PCOS patients. Hyperandrogenism, obesity, insulin resistance, and other metabolic abnormalities, may contribute to the increased risk of obstetric and neonatal complications (33). Women with PCOS have shown placental inflammation, placental thrombosis and infarction during pregnancy, added to villous immaturity and nucleated fetal red blood cells (35), which may contribute to the placenta abruption of PCOS. More probable, maternal metabolic dysfunction in PCOS mothers compromises placental function of a female fetus with a genetic susceptibility to PCOS, promoting fetal hyperinsulinemia as cause for hyperandrogenism and altered folliculogenesis in utero (33, 36).
In present study, the data revealed that the body weight of the F1 female rats in the group PCOS-IR was higher than both group PCOS-IR + Met and group Control after sexual development period, it showed that the female F1 offspring, which exposed to metformin, demonstrates higher weight gain compared to the offspring of mothers not treated with the drug. For F2 generation, no significant difference of body weight was detected in either group, which demonstrated that the phenotype of obesity is unstable to inherit to F2 generation in PCOS. Metformin use in PCOS pregnancies increases the risk of offspring overweight at 4 years of age compared to placebo-exposed children(37), but in their study, the clinical implications, body composition and metabolic health of these children are not known, which still should be subject to further investigation with long-term follew up of children. The T level was increased after letrozole administration, as usually observed in these murine models (13, 38). Our data revealed that glucometabolic indexes of both F1 and F2 female rats among all the three groups in both sexual development period and sexual maturity period showed no significant difference, as well as the serum T level.
Fetal nutritional and endocrine programming in utero may affect the neuroendocrine systems with long-term health consequences, such as hypertension, hypercholesterolemia, impaired glucose tolerance (39). It is believed that both genetic and early-life environmental factors in the uterus may contribute to the development of PCOS (40). The effect of the insulin resistance of PCOS on fetal growth and in utero programming, independent of either obesity or gestational diabetes, has yet needed to be elucidated. Hyperinsulinemia in PCOS could possibly be due to defects in the expression and/or activity of insulin receptor (INSR) (15). In our study, drastic decreased expression of INSR in pancreas was observed in PCOS-IR F1 rats compared to controls. The number of INSR is a main determinant of cellular response to circulatory insulin, any decrease significantly reducing insulin sensitivity, and functional INSR are critical for insulin binding and signal transduction, any alterations in INSR markedly impairs insulin binding and subsequent signaling pathways (41). Presence of hyperinsulinemia in PCOS-IR has also been reported earlier, which may be due to some defect in downstream of INSR (42, 43). The cellular insulin resistance in polycystic ovary syndrome has been further shown to involve a novel post-binding defect in insulin signal transduction, treatment of insulin resistance with a diabetes drug, such as metformin has become mainstream therapy in PCOS women (30). In present study, metformin could significantly increase the INSR expression of PCOS-IR F1 rats, which may contribute to improve hyperinsulinemia and insulin resistance, and reduce the incidence of PCOS-IR F1 correspondingly. Nevertheless, after mating with normal male rats, the INSR expression of F2 rats has no differences between PCOS-IR rats and controls, no matter with or without metformin treatment. However, metformin treatment can significantly reduce the expression level of INSR gene after sexual maturation of PCOS-IR rats’ F1 female rats, which may reduce the incidence of PCOS correspondingly. Metformin has been shown to confer protective effects on mouse pancreas exposed to fatty-acid induced stress, and chronic high glucose exposure(44). Hence, the effects of metformin on metabolic risks in human pancreas remain to be determined.
Metformin treatment in PCOS has been associated with increased hepatic synthesis of sex hormone-binding globulin and decreased ovarian and adrenal androgens. FSH is an important endocrine hormone in regulating ovarian function, which is must bind to special receptor, FSHR, stimulates follicular development via FSHR activation and induces granule cell proliferation. The interaction of FSH and FSHR is the key of follicle development during the process of follicle development. LH also plays a critical role in folliculogenesis. Studies find significantly higher LHR and lower FSHR levels in PCOS compared to controls (45–48). This study also revealed alterations of FSHR and LHCGR expression levels in offspring of PCOS-IR rats. Our results show that the expression of FSHR decreased and LHCGR increased both in F1 rat ovary of PCOS-IR and PCOS-IR + Met group, which suggests that FSHR and LHCGR dysfunction may promote the development of PCOS. Our findings differ from those of Rice(49) et al who showed a reduction basal levels of FSHR mRNA by metformin. With the treatment of metformin, the expression of FSHR and LHCGR have greatly improved. The expression of PCOS related genes-both INSR gene and FSHR gene in F2 offspring of PCOS-IR rats were still abnormal, but there was no difference in the offspring of PCOS-IR rats’ F2 when comparing to the normal rats. PCOS may be a genetic disease, but genetic factors may not be the only reason for the development of PCOS, the non-genetic factors play an important role in the pathogenesis of PCOS too.
However, the investigation is still limited to its preliminary stage, and further functional studies of these mediators in future studies should be investigated. Further clinical studies may be performed to determine the effect of metformin on offspring of PCOS patients, and to evaluate the risk of PCOS development in PCOS offspring.