Elsevier

Brain Research

Volume 990, Issues 1–2, 14 November 2003, Pages 20-27
Brain Research

Neuroprotective and neurotoxic effects of estrogens

https://doi.org/10.1016/S0006-8993(03)03380-8Get rights and content

Abstract

The ovarian hormone 17β-estradiol (E2) is neuroprotective in animal models of neurodegenerative diseases. Some studies suggest that the neuroprotective effects of 17β-estradiol are a consequence of its antioxidant activity that depend on the hydroxyl group in the C3 position of the A ring. As in other tissues, 17β-estradiol is metabolized in the brain to 2-hydroxyestradiol (2OHE2) and 2-methoxyestradiol (2MEOHE2). These two molecules present the hydroxyl group in the A ring and have a higher antioxidant activity than 17β-estradiol. To test the hypothesis that conversion to 2-hydroxyestradiol and 2-methoxyestradiol may mediate neuroprotective actions of 17β-estradiol in vivo, we have assessed whether these molecules protect hilar hippocampal neurons from kainic acid toxicity. Ovariectomized Wistar rats received an i.p. injection of 1, 10 or 100 μg 17β-estradiol, 2-hydroxyestradiol or 2-methoxyestradiol followed by an i.p. injection of kainic acid (7 mg/kg) or vehicle. Treatment with kainic acid resulted in a significant loss of hilar neurons. Only the highest dose tested of 17β-estradiol (100 μg/rat) prevented kainic acid-induced neuronal loss. 2-Hydroxyestradiol and 2-methoxyestradiol did not protect hilar neurons from kainic acid, suggesting that the mechanism of neuroprotection by 17β-estradiol in vivo is not mediated by its metabolism to catecholestrogens or methoxycatecholestrogens. Furthermore, 2-methoxyestradiol (100 μg/rat), by itself, resulted in a significant neuronal loss in the hilus that was detected 96 h after the treatment with the steroid. This finding suggests that endogenous metabolism of 17β-estradiol to 2-methoxyestradiol may counterbalance the neuroprotective effects of the hormone.

Introduction

Experimental data in animal models have provided exhaustive evidence of the neuroprotective properties of 17β-estradiol (E2). This hormone increases neuronal survival after different forms of brain injury in vivo and protects neuronal cultures from serum or growth factor deprivation, anoxia, excitotoxicity or oxidative damage [25], [28], [38], [67], [68]. E2 is thought to protect neurons against numerous traumatic or chronic neurological and mental diseases in humans as well. However, the clinical evidence is controversial [2].

To develop new therapeutic approaches using E2, it is important to determine the basis for the neuroprotective actions of the hormone. One of the possible mechanisms is related with the antioxidant properties of E2 that depend on the presence of the hydroxyl group in the C3 position on the A ring of the steroid molecule and are independent of the activation of estrogen receptors [7], [9]. In several tissues, including the brain, E2 is metabolized to 2-hydroxyestradiol (2OHE2) by a NADPH-dependent cytochrome P-450-linked monooxygenase system [6], [34], [41]. This metabolite is rapidly O-methylated by the ubiquitously present catechol-O-methyl transferase to 2-methoxyestradiol (2MEOHE2) [21], [71]. 2OHE2 and 2MEOHE2 are the most biologically active metabolites of E2 [18], [72]. Both steroids share antioxidant properties against the lipid peroxidation induced by metal-derived chemicals [36], [40], inhibit angiogenesis [23], [69] and decrease cell proliferation [37], [58], [69], [70]. Furthermore, these metabolites, in addition to their estrogenic action on reproductive tissues, induce important biological effects in the brain [18].

The transformation of E2 in 2OHE2 and 2MEOHE2 may have important consequences in the neuroprotective effects of E2. However, the potential neuroprotective effects of these estradiol metabolites have been poorly investigated in vivo. Therefore, the present study was designed to assess and compare the capacity of E2, 2OHE2 and 2MEOHE2 to protect hippocampal neurons from kainic acid toxicity in vivo. The insult produced by kainic acid on hilar hippocampal neurons has been validated as an useful experimental model of excitotoxic cell death [11], [22] and previous studies have shown that E2 is neuroprotective in this model [3], [4], [51], [62], [63].

Section snippets

Materials

All hormones and kainic acid were purchased from Sigma (St. Louis, MO), while 2,2,2,tribromoethanol and (2-hydroxypropyl)-β-cyclodextrin (cyclodextrin) were obtained from Fluka Chemika (Buchs, Switzerland). Hormones were dissolved in 20% cyclodextrin and kainic acid in phosphate-buffered saline solution, pH 7.4.

Animals

Wistar albino female rats were raised and maintained on a 12:12-h light–dark cycle, with free access to chow and water. Manipulation of the rats followed the European Union Normative

Results

The first experiment was designed to determine the optimal dose of E2 that protects hilar neurons against kainic acid. Treatment with kainic acid resulted in a significant loss of hilar neurons Fig. 1, Fig. 2. E2, at the highest dose tested (100 μg/rat), prevented hilar neuronal loss in rats treated with kainic acid Fig. 1, Fig. 2. However, lower doses of E2 were unable to protect hilar neurons (Fig. 2).

In contrast to E2, 2OHE2 was unable to protect hilar neurons from kainic acid, even at the

Discussion

As shown previously [3], [4], [51], [62], [63], our study confirms that E2 is able to protect hilar neurons of ovariectomized rats against kainic acid in vivo. Furthermore, our findings indicate that the neuroprotective effect of E2 in this model is obtained only with a high dose of the hormone that results in high physiological proestrus concentration of E2 in plasma by 24 h, followed by a decrease to low levels by 48 h [57].

It has been proposed that E2 exerts neuroprotection by the

Acknowledgements

This study has been carried out with financial support from Ministerio de Ciencia y Tecnologı́a (SAF 2002-00652), Instituto de Salud Carlos III (01/1214) and the Commission of the European Communities, specific RTD programme “Quality of Life and Management of Living Resources” (QLK6-CT-2000-00179). OP thanks the support for this study of CSIC, CONACyT, CEGEPI-IPN and COFAA.

References (72)

  • S. Goodenough et al.

    Estrogen-induced cell signalling in a cellular model of Alzheimer's disease

    J. Steroid Biochem. Mol. Biol.

    (2003)
  • Y. Kuroki et al.

    Putative membrane-bound estrogen receptors possibly stimulate mitogen-activated protein kinase in the rat hippocampus

    Eur. J. Pharmacol.

    (2000)
  • A. Maran et al.

    2-Methoxyestradiol induces interferon gene expression and apoptosis in osteosarcoma cells

    Bone

    (2002)
  • C.S. Markides et al.

    Concentration dependence of prooxidant and antioxidant properties of catecholestrogens

    Arch. Biochem. Biophys.

    (1998)
  • C.P. Martucci et al.

    P450 enzymes of estrogen metabolism

    Pharmacol. Ther.

    (1993)
  • P. Mendez et al.

    Estrogen receptor alpha forms estrogen-dependent multimolecular complexes with insulin-like growth factor receptor and phosphatidylinositol 3-kinase in the adult rat brain

    Mol. Brain Res.

    (2003)
  • G.R. Merriam et al.

    Comparative properties of the catechol estrogens: I. Methylation by catechol-O-methyltransferase and binding to cytosol estrogen receptors

    Steroids

    (1980)
  • Y. Nakagawa-Yagi et al.

    The endogenous estrogen metabolite 2-methoxyestradiol induces apoptotic neuronal cell death in vitro

    Life Sci.

    (1996)
  • N. Parvizi et al.

    Autoradiographic determination of catechol estrogen binding sites in brain, pituitary and uterus

    Brain Res.

    (1985)
  • L.R. Qadan et al.

    2-Methoxyestradiol induces G2/M arrest and apoptosis in prostate cancer

    Biochem. Biophys. Res. Commun.

    (2001)
  • S. Reibel et al.

    Neuroprotective effects of chronic estradiol benzoate treatment on hippocampal cell loss induced by status epilepticus in the female rat

    Neurosci. Lett.

    (2000)
  • J.C. Seegers et al.

    The mammalian metabolite, 2-methoxyestradiol, affects P53 levels and apoptosis induction in transformed cells but not in normal cells

    J. Steroid Biochem. Mol. Biol.

    (1997)
  • A. Tsukamoto et al.

    2-Methoxyestradiol, an endogenous metabolite of estrogen, enhances apoptosis and beta-galactosidase expression in vascular endothelial cells

    Biochem. Biophys. Res. Commun.

    (1998)
  • M.J. West

    Regionally specific loss of neurons in the aging human hippocampus

    Neurobiol. Aging

    (1993)
  • M.E. Wilson et al.

    Estradiol protects against injury-induced cell death in cortical explant cultures: a role for estrogen receptors

    Brain Res.

    (2000)
  • M.E. Wilson et al.

    Estradiol enhances Akt activation in cortical explant cultures following neuronal injury

    Mol. Brain Res.

    (2002)
  • P.M. Wise et al.

    Estradiol is a protective factor in the adult and aging brain: understanding of mechanisms derived from in vivo and in vitro studies

    Brain Res. Rev.

    (2001)
  • I. Azcoitia et al.

    Are gonadal steroid hormones involved in disorders of brain aging?

    Aging Cell

    (2003)
  • I. Azcoitia et al.

    Gonadal hormones affect neuronal vulnerability to excitotoxin-induced degeneration

    J. Neurocytol.

    (1999)
  • I. Azcoitia et al.

    Estradiol prevents kainic acid-induced neuronal loss in the rat dentate gyrus

    NeuroReport

    (1998)
  • I. Azcoitia et al.

    Neuroprotective effects of estradiol in the adult rat hippocampus: interaction with insulin-like growth factor-I signalling

    J. Neurosci. Res.

    (1999)
  • P. Ball et al.

    Comparative studies on the metabolism of oestradiol in the brain, the pituitary and the liver of the rat

    Acta Endocrinol. (Copenh.)

    (1978)
  • C. Behl et al.

    Neuroprotective activities of estrogen: an update

    J. Neurocytol.

    (2000)
  • C. Behl et al.

    Neuroprotection against oxidative stress by estrogens: structure-activity relationship

    Mol. Pharmacol.

    (1997)
  • P.S. Buckmaster et al.

    Neuron loss, granule cell axon reorganization, and functional changes in the dentate gyrus of epileptic kainate-treated rats

    J. Comp. Neurol.

    (1997)
  • J.A. Chowen et al.

    Trophic effects of estradiol on fetal rat hypothalamic neurons

    Neuroendocrinology

    (1992)
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