MODELLI SPERIMENTALI DI SOVRACCARICO DIETETICO DI FERRO: EFFETTI CENTRALI E PERIFERICI SU METABOLISMO E FUNZIONE RIPRODUTTIVA

Background and Aim. Iron is an essential micronutrient, which is involved as a cofactor in fundamental biochemical activities, and it is necessary for proper brain development in the fetal and early neonatal period. However, cellular iron overload produces toxic build-up in many organs, including brain, and, under aerobic conditions, catalyses the propagation of reactive oxygen species and the generation of highly reactive radicals through Fenton Chemistry. Association between metabolic and reproductive impairment has been proved in patients affected by dysmetabolic iron overload syndrome (DIOS). In particular, iron is the most important factor afflicting the hypothalamic-pituitary axis in a dose-dependent fashion leading to hypogonadotropic hypogonadism (HH). Our previous studies in a mouse model of DIOS showed the association between dietary iron overload, visceral adipose tissue insulin resistance and hypertriglyceridemia. Aim of this thesis was to assess whether and how iron overload may affect (a) the reproductive axis (mainly at the hypothalamic-pituitary levels) in a mouse model of DIOS; (b) the migratory feature and GnRH secretory pattern in GN-11 and GT1-7 cells, in vitro models of immature/migratory and mature/GnRH-secreting neurons, respectively. Results. In male mice, dietary-iron overload (IED) led to: a) an increment in testis iron content, b) a reduction in testicular weight and length, c) no changes in hypothalamic iron content c) no changes in mRNA levels of iron-responsive genes, transferrin receptor (TfR) and ferritin H (FtH), in testes and hypothalamus d) an up-regulation of hypothalamic GnRH mRNA levels, e) no changes in hypothalamic Kiss1 and GPR54 gene expression, e) a reduction in pituitary LHβ gene expression. Moreover, the hypothalamic increment of TNFα gene expression along with the phosphorylation/activation of AMPK protein suggested the presence of an inflammatory condition. Increased hypothalamic CHOP mRNA levels also confirmed the endoplasmic reticulum stress feature. IED mice gained less weight than controls showing a reduction in VAT mass and in serum leptin levels, whereas hypothalamic NPY mRNA levels were increased and POMC gene expression was reduced. Western blot analysis showed that the pAkt/Akt ratio was up-regulated in the hypothalamus of IED mice, whereas phosphorylation of ERK1/2 (pERK) protein resulted unchanged in both groups. As far as GN-11 and GT1-7 cells are concerned, a 24-hour treatment with 200 µM Ferric Ammonium Citrate (FAC, source of ferric iron) induced an increment in the intracellular specific iron content of both cell-based models without affecting the cell viability and morphology. Gene expression analysis showed that both cell lines express TfR and FtH, whose mRNA levels were modulated by iron overload. Exposure of GN-11 cells to FAC resulted in the dose (200–1000 µM FAC for 24 hours)- and time (24-72 hours with 200 µM FAC)-dependent inhibition of FBS-induced chemomigration, as assessed by Boyden chamber assay. Pre-treatment with 200 µM deferoxamine (DFO, a specific iron chelator) reverted the above reported iron-driven effect on cell migration. Time-course experiments showed that 200 μM FAC was associated with increased pERK1/2 and pAkt protein levels and with decreased pAMPK ones. Chemomigration assays carried out with the specific inhibitors of ERK1/2, Akt and AMPK highlighted that only Akt pathway seems involved in FAC-mediated inhibition of GN-11 cell migration. In GN-11 cells, iron treatment increased IL-6 gene expression in a dose-dependent mode, whereas NF-kB nuclear translocation and activation was not affected. Up-regulated SOD2 mRNA levels confirmed a condition of activated oxidative stress. Conclusions. The present data show that dietary-iron overload impairs the reproductive axis, probably leading to HH, but further experiments are needed to understand the anatomic site mainly involved in iron-driven damage. Iron treatment negatively affects the migration of GN-11 neuronal cells by the activation of Akt signaling pathway. Hence, iron overload may impair the migration of GnRH neurons from the olfactory placode into forebrain and hypothalamus, where these neurons promote the reproductive competence.