Hematoxylin was used to identify the cell nuclei. Epi, epithelial cells; Str, stromal cells; NRS, normal rabbit serum. Scale bar, 100 μm. Different rat uterine tissue lysates were directly immunoblotted with antibodies against OCT1, OCT2, OCT3, or MATE1 as indicated in E2. Data are emerging about how the expression of different OCTs is regulated under both physiological and pathological conditions. For example, the in vitro expression of OCT1 and OCT2 decreases upon activation of the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT) signaling pathway in vitro (cell-line systems) [72, 73], and the expression of OCT1 and OCT2 decreases upon induction of diabetes in streptozotocin-inducable PF-6463922 supplier diabetic rats in
vivo [74]. Further, Hirsch and colleagues have reported in vitro results showing that the dose-dependent inhibitory regulation of androgen synthesis by metformin requires the presence of OCTs [75]. Although there is no direct evidence for a relationship between OCT expression and metformin response in the endometrium, a recent study has shown that the variations in metabolic responses observed in women with PCOS treated with metformin MK-4827 in vivo are probably due to genetic variations of OCT1 [76]. It is likely, CB-5083 mouse therefore, that the tissue-specific expression and regulation of OCTs is important for the cellular uptake of metformin and plays a role in the in vivo therapeutic efficacy of metformin in
women with PCOS. The main targets of metformin: adenosine monophosphate-activated protein kinase (AMPK), mTOR, and glucose transport protein 4 (GLUT4) Metformin has been shown to regulate multiple signaling pathways [38, 77], and at the molecular level AMPK is one of the targets for metformin action in several tissues Thalidomide and cancer cells [27, 28, 77, 78]. It has been reported that metformin decreases local androgen synthesis in human ovarian cells [79, 80], increases GLUT4 expression in endometrial cells from PCOS women with hyperinsulinemia [81], inhibits cell proliferation [36, 37], and induces cell cycle arrest and apoptosis [35] in type
I EC cells, all of which have been proposed to occur through activation of AMPK signaling [35–37, 39, 81, 82]. Although metformin has been shown to activate AMPK, which subsequently inhibits mTOR activity by phosphorylating and stabilizing the tuberous sclerosis complex-2 (TCS2) tumor suppressor [29, 31], it has also been suggested that metformin can directly inhibit mTOR signaling independently of AMPK activation [28, 77] (Figure 2). Figure 2 A schematic diagram representing the hypothetical mechanisms of the insulin-dependent systemic (I) and insulin-independent direct (II) effects of metformin in the endometrium. In the endometrium, binding of insulin and IGF-1 ligands to their receptors INSR and/or IGF-1R as homodimers or heterodimers leads to the activation of downstream signaling pathways, including the PI3K/AKT/mTOR pathway.