After neuronal injury and in several neurodegenerative diseases, activated microglia secrete proinflammatory molecules that can contribute to the progressive neural damage. The recent demonstration of a protective role of estrogen in neurodegenerative disorders in humans and experimental animal models led us to investigate whether this hormone regulates the inflammatory response in the CNS. We here show that estrogen exerts an anti-inflammatory activity on primary cultures of rat microglia, as suggested by the blockage of the phenotypic conversion associated with activation and by the prevention of lipopolysaccharide-induced production of inflammatory mediators: inducible form of NO synthase (iNOS), prostaglandin-E 2 (PGE 2 ), and metalloproteinase-9 (MMP-9). These effects are dose-dependent, maximal at 1 nM 17␤-estradiol, and can be blocked by the estrogen receptor (ER) antagonist ICI 182,780. The demonstration of ER␣ and ER␤ expression in microglia and macrophages and the observation of estrogen blockade of MMP-9 mRNA accumulation and MMP-9 promoter induction further support the hypothesis of a genomic activity of estrogen via intracellular receptors. This is the first report showing an anti-inflammatory activity of estrogen in microglia. Our study proposes a novel explanation for the protective effects of estrogen in neurodegenerative and inflammatory diseases and provides new molecular and cellular targets for the screening of ER ligands acting in the CNS.

Beyond the key role in reproductive and cognitive functions, estrogens have been shown to protect against neurodegeneration associated with acute and chronic injuries of the adult brain. Current hypotheses reconcile this activity with a direct effect of 17␤-estradiol (E 2 ) on neurons. Here we demonstrate that brain macrophages are also involved in E2 action on the brain. Systemic administration of hormone prevents, in a time-and dose-dependent manner, the activation of microglia and the recruitment of peripheral monocytes induced by intraventricular injection of lipopolysaccharide. This effect occurs by limiting the expression of neuroinflammatory mediators, such as the matrix metalloproteinase 9 and lysosomal enzymes and complement C3 receptor, as well as by preventing morphological changes occurring in microglia during the inflammatory response. By injecting lipopolysaccharide in estrogen receptor (ER)-null mouse brains, we demonstrate that hormone action is mediated by activation of ER␣ but not of ER␤. The specific role of ER␣ is further confirmed by comparing the effects of ERs on the matrix metalloproteinase 9 promoter activity in transient transfection assays. Finally, we report that genetic ablation of ER␣ is associated with a spontaneous reactive phenotype of microglia in specific brain regions of adult ER␣-null mice. Altogether, these results reveal a previously undescribed function for E2 in brain and provide a mechanism for its beneficial activity on neuroinflammatory pathologies. They also underline the key role of ER␣ in brain macrophage reactivity and hint toward the usefulness of ER␣-specific drugs in hormone replacement therapy of inflammatory diseases.

Background-Previous reports from our group have shown that 17␤-estradiol reduces the synthesis and activity of inducible nitric oxide synthase (iNOS) in rat aortic smooth muscle cells (SMC) in response to inflammatory mediators. In this study, we investigated the effect of 17␤-estradiol on iNOS function in aortic SMC from streptozotocin-diabetic rats. Methods and Results-Comparative analysis of NO release and of iNOS mRNA and protein content after 24-hour stimulation with a cytokine mixture revealed milder iNOS activation in diabetic than in control SMC. Furthermore, 17␤-estradiol dose-dependently blocked iNOS synthesis and activity in control but not in diabetic SMC. The defective estrogen response in diabetic SMC at 24 hours could not be attributed to reduced expression of estrogen receptors (ER). In fact, mRNA and protein levels of ER␣ and, to a greater extent, of ER␤, were increased in diabetic compared with nondiabetic SMC. Cytokines decreased ER␣ and ER␤ expression in both groups. However, 17␤-estradiol dosedependently restored the expression of ER␣ but further downregulated that of ER␤, indicating a differential regulation of ER isoforms. Conclusions-Estrogenic control of iNOS was impaired in diabetic SMC. This was associated with a larger increase of ER␤ than of ER␣ protein, whereas 17␤-estradiol regulated the two isoforms in an opposite fashion. Thus, modifications in the estrogen modulation of iNOS and in the expression pattern of ER may be involved in diabetic vascular dysfunction. (Circulation. 2003;108:211-217.)

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In mammals, estrogens have a multiplicity of effects ranging from control of differentiation of selected brain nuclei, reproductive functions, sexual behavior. In addition, these hormones influence the manifestation of disorders like depression and Alzheimer's. Study of the cells target for the hormone has shown that estrogen receptors (ERs) are expressed in all known neural cells, including microglia. In view of the potential interest in the use of estrogens in the therapy of several pathologies of the nervous system, it would be of interest to fully understand the mechanism of estrogen activity in the various neural target cells and get an insight on the molecular means allowing the hormone to display such a variety of effects. We have proposed the use of a reductionist approach for the systematic understanding of the estrogen activities in each specific type of target cell. Thus, we have generated a model system in which to study the activation of one of the known (ERs), estrogen receptor alpha. This system allowed us to identify a number of novel genes which expression may be influenced following the activation of this receptor subtype by estradiol (E(2)). We here report on data recently obtained by the study of one of these target genes, nip2, which encodes a proapoptotic protein product. We hypothesize that nip2 might be an important molecular determinant for estrogen anti-apoptotic activity in cells of neural origin and represents a potential target for drugs aimed at mimicking the E(2) beneficial effects in neural cells.

The present study stems from previous observations demonstrating that in the neuroblastoma cell line SK-ER3 the mRNA content of the pro-apoptotic gene Nip2 is decreased following treatment with estradiol. We investigate the content of Nip2 mRNA during the maturation of rat embryo brain and we show that Nip2 mRNA is very low at embryo day 15 and steadily increases up to day 20. At day 21 Nip2 mRNA is decreased almost to the low levels observed in the mature brain. Studies in neurons from rat embryo at day 18 show that Nip2 mRNA content is significantly decreased by exposure to estradiol at 1 nM concentration demonstrating that the observations previously done in the SK-ER3 neuroblastoma cell line can be reproduced in neurons in culture. The finding that estrogens may modulate the activity of Nip2 gene activity may be of relevance not only with regard to the effects of estradiol on brain maturation, but also for the understanding of the neuroprotective effects recently described for this hormone.

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