demethylation, PPARg promoter leads to reduction in ovarian granulosa cells, Aromatase (Cyp19a1)

There is substantial evidence in literature supporting the KER, however the underlying mechanism are to be investigated, together with other pathways involved in aromatase down regulation. The pattern of the PPARγ expression in ovarian follicles is not steady, unlike expression of PPARs α and δ. This fact adds to the complexity to the interpretation of mechanisms involved in the pathway. The PPARγ is down-regulated in response to the LH surge (C M Komar, Braissant, Wahli, & Curry, 2001), but only in follicles that have responded to the LH surge (Carolyn M Komar & Curry, 2003). Because PPARγ is primarily expressed in granulosa cells, it may influence development of these cells and their ability to support normal oocyte maturation. PPARγ could also potentially affect somatic cell/oocyte communication not only by impacting granulosa cell development, but by direct effects on the oocyte. Modulation of the PPARγ activity/expression in the ovary therefore, could potentially affect oocyte developmental competence. Some evidence implies that the regulatory role of PPARγ might be connected to the other events in estradiol synthesis like the impairment of cholesterol transport to mitochondria (Cui et al., 2002).

PPARα The experimental data supports the parallel involvement of another member of PPAR superfamily of nuclear receptors, PPARα. PPARα was shown to be implicated in the down regulation of aromatase in rat: in vitro (Lovekamp-Swan et al., 2003); in vivo (Xu et al., 2010) and in mice in vivo (Toda, Okada, Miyaura, & Saibara, 2003). The ovarian aromatase promoter contains one half of a PPRE (peroxisome proliferator response element), which is the binding site for steroidogenic factor 1 (SF-1) (Young & McPhaul, 1997). While it is unknown whether PPARα can compete for binding on an incomplete response element, disruption of SF-1 binding to this half site would disrupt normal aromatase transcription. Studies by S. Plummer et al showed that PPARα and SF1 share a common coactivator (S. Plummer, Sharpe, Hallmark, Mahood, & Elcombe, 2007), (S. M. Plummer et al., 2013), CREB-binding protein (CBP), which is present in limiting concentrations (McCampbell, 2000). Binding of CBP to PPARα could therefore starve SF1 a cofactor essential for its transactivation functions. Surprisingly, aromatase levels were increased in ovaries of PPARα-null mice upon treatment with PPARα ligand (Toda et al., 2003).

PPARα was also reported to regulate other enzymes involved in steroidogenesis like: 17 beta-hydroxysteroid dehydrogenase type IV (HSD IV) (Corton et al., 1996), 3 beta-hydroxysteroid dehydrogenase (Wong, Ye, Muhlenkamp, & Gill, 2002) or 11beta-hydroxysteroid dehydrogenase type (Hermanowski-Vosatka et al., 2000). While PPARα/γ activators (like MEHP ) suppress aromatase, they showed no effect on Cholesterol side-chain cleavage enzyme (P450scc) in granulosa cells, demonstrating a more specific effect on steroidogenesis (Lovekamp-Swan et al., 2003). Experiments with PPARα-null mice indicate involvement of the receptor in reproductive toxicity, however cannot be entirely explained by the activation of PPARα mediated pathway as PPARα-null mice remain sensitive to DEHP-mediated reproductive toxicity (Ward et al. 1998), which implies other players including PPARγ. The above evidence supports the involvement of PPARα in regulation of steroidogenesis on its different steps. As PPARα is found primarily in the theca and stroma and the expression of PPARα in granulosa cells is very low (Carolyn M Komar, 2005) therefore it might be involved in steps in steroidogenesis upstream of aromatase.

Retinoid X Receptor (RXR)

Chemicals are able to activate RXR–PPARγ through RXR because this heterodimer interacts poorly with co-repressors in vivo and belongs to the group of so-called ‘permissive’ heterodimers, which can be stimulated by RXR ligands on their own (Germain, Iyer, Zechel, & Gronemeyer, 2002). Studies demonstrated that a PPARγ ligand and/or a RXR ligand decreased the aromatase activity in both cultured human ovarian granulosa cells (Mu et al., 2000), (Mu et al., 2001) and human granulosa-like tumor KGN cells (Kwintkiewicz et al., 2010) and combined treatment causes a greater reduction than either compound alone (Mu et al., 2000), (Mu et al., 2001).

Inconsistencies

No effect on aromatase protein expression was observed after PPARγ ligand (rosiglitazone) treatment in porcine ovarian follicles (Rak-Mardyła & Karpeta, 2014).