br Introduction Polycystic ovary syndrome PCOS is

Introduction Polycystic ovary syndrome (PCOS) is a common and multifactorial disease that affects approximately 4%–18% of all reproductive-aged women in the world (Moran et al., 2011). In the clinic, hyperandrogenism and insulin resistance appear to be the major etiological drivers for reproductive and metabolic abnormalities in women with PCOS (Rosenfield and Ehrmann, 2016). While it is believed that anovulation is a main reason for infertility in PCOS patients, accumulating evidence from clinical studies also indicates that the impairment of endometrial function likely causes recurrent pregnancy loss, premature delivery, endometrial hyperplasia, and carcinoma in women with PCOS (Goodarzi et al., 2011; Palomba et al., 2015; Shao et al., 2014). Additionally, several lines of evidence suggest that the systemic low-grade inflammation that often coincides with PCOS compromises multiple aspects of fertility (Repaci et al., 2011). Although the precise mechanisms of hyperandrogenism and insulin resistance-induced inflammation in the endometrium are not completely understood, in vivo and in vitro studies have demonstrated associations between the dysregulation of inflammation-related molecules in numerous endometrial cell lines and under PCOS conditions (Matteo et al., 2010; Orostica et al., 2016; Piltonen et al., 2013; Piltonen et al., 2015). Due to the clinically heterogeneous characteristics of this syndrome, its treatment remains complex with variable responses among PCOS patients (Palomba et al., 2015). Metformin, an oral biguanide insulin-sensitizing drug, is the most widely used treatment for type 2 5z and PCOS worldwide (Naderpoor et al., 2015; Nestler, 2008; Pernicova and Korbonits, 2014). The primary actions of this drug are either to increase insulin sensitivity by inhibiting gluconeogenesis and stimulating glucose uptake and utilization in the liver, skeletal muscles, adipocytes, and ovaries or to increase cellular levels of AMP-activated protein kinase (AMPK) by inhibiting mitochondrial complex 1 and subsequently activating the AMPK signaling pathway (Foretz et al., 2014; Sivalingam et al., 2014). Indeed, these postulated molecular mechanisms have been demonstrated in human endometrial carcinoma tissues in vivo and in different endometrial cancer cells in vitro (Shao et al., 2014). Moreover, previous studies by us and others have also reported that metformin can improve endometrial receptivity, enhance endometrial vascularity and blood flow, and revert endometrial hyperplasia and carcinoma into normal endometria in addition to improving hyperandrogenism and insulin resistance in some women with PCOS (Jakubowicz et al., 2001; Li et al., 2014; Palomba et al., 2006). These results clearly suggest that metformin is a promising drug for treating PCOS patients with uterine dysfunction. However, these clinical studies have provided very limited insight into the anatomical, molecular, and functional metformin-induced alterations in the uterus under physiological and pathological conditions, especially the reproductive disturbance associated with PCOS. The impairment of the androgen–androgen receptor (AR) signaling pathway is associated with PCOS patients with reproductive dysfunction (Cloke and Christian, 2012). Although previous studies have demonstrated that androgen modulates the expression of an array of genes in the mouse uterus, including Wnt4, Wnt5a, Wnt7a, Cdh1, Vcl, Igf1, Prl8a2, Prlr, Foxa2, Fgf7, and Hgf (Simitsidellis et al., 2016), it remains unclear which downstream targets of this pathway actually contribute to the uterine abnormalities associated with PCOS. Similar to our understanding of the transcriptional actions of AR activation in the uterus, the actions of insulin and insulin-like growth factor-1 – through the phosphatidylinositide-3-kinase (PI3K)–Akt signaling pathway – appear to also modulate endometrial cell survival, proliferation, and metabolism under physiological and pathological conditions, including PCOS (Li et al., 2016b; Shao et al., 2014). It has been shown that nuclear factor kappa B (NFκB) and Forkhead family of transcription factors such as Forkhead box O1 (FoxO1) are the key targets of activated Akt (Brunet et al., 1999; Dan et al., 2008). Furthermore, activation of AMPK, which interacts with alternative PI3K–Akt signaling pathways, is the hallmark of metformin action in several tissues and cell types (Foretz et al., 2014; Shao et al., 2014).