Infertility is a significant public health problem and its diagnosis and treatment are stressful, invasive and costly. Impaired fecundity is a growing health concern worldwide. Fertility predominantly depends on the maintenance of the quantity and quality of the ovarian reserve, which is determined by the extrinsic and intrinsic factors. The reserve declines with consumption of the follicle as age increases and by the imbalance in the redox activity.
Cellular redox activity is a normal mechanism of the male and female reproductive system but its inequilibrium affects the fertility by hindering attainment and maintenance of oocyte developmental potential during in vivo processes including ovarian aging. Oxidative stress (OS) has been associated with decreased female fecundity in animal and in vitro models, but no studies to date have directly assessed the relationship in women. Exposures associated with OS and pregnancy outcomes have so far been studied in relation to Poly Cystic Ovarian Syndrome (PCOS) only. Sirtuin (silent information regulator, SIRT), family of NAD dependent enzymes is evolving as main regulator of OS. They are known to repair cells damaged due to OS by stimulating the expression of antioxidants and thus preventing dysfunction of the ovarian cells. SIRT1 signaling initiates a positive ovarian cells response by catalyzing an enzymatic reaction between nicotinamide and the acetyl group of the substrate, transferring it to cleave NAD, generating a unique metabolite, O-acetyl-ADP ribose (Pillarisetti, 2008). Alterations in cellular NAD levels, or the NAD-NADH ratio, are the primary mechanisms controlling SIRT1 expression and activity(Guarente and Picard, 2005).
As far as the impact of Os on oocyte is concerned; under influence of various external and internal factors, when the oocytes start aging and generating advanced glycation end products (AGEs) in the ovarian microenvironment, there is a trigger for the generation of intracellular reactive oxygen species ROS by down regulation of NAD(P)H oxidase, mitogen-activated protein kinases (MAPKs), and the transcription factor, nuclear factor kappa B (NF-.B) (Lander et al., 1997; Xu and Kyriakis, 2003) leading to proinflammatory milieu and an increase in OS (Mohamed et al., 1999; Schmidt et al., 2001). Lipid peroxidation during OS is capable of commencing cyclic ROS dissemination via direct damage to mtDNA (mitochondrial DNA) and proteins. With increase concentrations of ROS within the oocytes, mitochondria lose their potential to culminate the state of auto-oxidation by decreased production of ATP from the electron transport chain (ETC), thus aggravating damage to mitochondrial functional components.
Mitochondria being a key player in calcium regulation in the oocytes for generation of the main bulk of ATP for the processes like apoptosis and completion of meiosis including spindle assembly, under the influence of ROS within the ovaries is considered as the main cause for chromosomal segregation disorders, maturation, and fertilization failures, or oocyte/embryo fragmentation. Presence of systemic and follicular oxidative stress markers in patients of endometriosis have also been identified (Takeuchi et al., 2005). Maintenance of the oxidative environment within the oocytes is an essential process, which needs to be guarded and carried out by the regulators.
Significant inhibition of SIRT1 mRNA has also been observed with increase in ROS, compromising the nuclear maturation and the mitotic spindles of the maturing oocytes.
An independent study conducted in Pakistan suggests that functions of SIRT1 are affected by various post-transcriptional activities; increased by phosphorylation, while glycosylation decreased it (Hanover et al., 2010). It utilizes unique pathways; its glycosylation occurs through the hexoseamine-signaling pathway (Hanover et al., 2010), furthermore, SIRT1-dependent gluconeogenesis is also intermediated by changes in the levels of NAD and pyruvate instead of employing gluconeogenic regulatory hormones (Rodgers et al., 2005). The crosstalk between various post-translational modifications regulates the proteins functionally that control the transcriptional activity of various genes(Hoessli et al., 2013). Addition of O-GlcNAc during the process of aging and development of diabetes weakens the activity of SIRT.
Metformin; (generic name; Dimethylbiguanide), widely used hypoglycemic drug utilizing its capability of inhibiting hepatic gluconeogenesis. It is recently known to inhibit cellular respiration carried out by the mitochondria in the hepatic cells, however its effect is dose-dependent and less potent on the intact cells. Among the cell signaling pathways capable of mitochondrial regulation, metformin contribute to increase in the glutathione content, thus providing aid in encountering the ROS (Figure (Figure1).1). Metformin induces increase in NAD/NADH ratio by inducing increased expression of NAMPT mRNA and protein expression, resulting in enhanced SIRT1 expression and activity in a dose-dependent fashion (Caton et al., 2010).
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Figure 1
A hypothetical view of the possible mechanisms by which Metformin helps in maintaining the microenvironment of the granulosa cells. With advancing age of the oocytes, development of oxidative stress takes place which leads to imbalance between the oxidants and the antioxidants thus elevating reactive oxygen species, lipid peroxidation and simultaneously decreasing the expression of SIRT1 via decreased NAD/NADPH ratio (STEP1). These changes lead to mitochondrial dysfunction by direct injury to the mitochondrial DNA (STEP2) depleting ATP synthesis by the electron transport chain (STEP3). Oocyte maturation failure, chromosomal segregation disorders & oocyte/embryo fragmentation occurs (STEP4) as a result causing infertility. Metformin moderates the expression of SIRT1 directly and by regulating the NAD/NADPH ratio which also in turn increases the concentration of SIRT1. It is also believed to buffer the ROS by increasing the concentration of Glutathione. NAD(P)H, Nicotinamide adenine dinucleotide phosphate; MAPKs, mitogen-activated protein kinase; ROS, reactive oxygen species; mtDNA, mitochondrial DNA; SIRT, Sirtuin; ATP, Adenosine-5-triphosphate.