The Endogenous Estrogen Status Regulates Microglia Reactivity in Animal Models of Neuroinflammation

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Abstract
It has been previously demonstrated that 17β-estradiol (E2) inhibits the response of microglia, the resident brain macrophages, to acute injuries in specific brain regions. We here show that the effect of E2 in acute brain inflammation is widespread and that the hormone reduces the expression of inflammatory mediators, such as monocyte chemoattractant protein-1, macrophage inflammatory protein-2, and TNF-α, induced by lipopolysaccharide, demonstrating that microglia are a direct target of estrogen action in brain. Using the APP23 mice, an animal model of Alzheimer’s disease reproducing chronic neuroinflammation, we demonstrate that ovary ablation increases microglia activation at β-amyloid (Aβ) deposits and facilitates the progression of these cells toward a highly reactive state. Long-term administration of E2 reverts the effects of ovariectomy and decreases microglia reactivity compared with control animals. In this animal model, these events do not correlate with a reduced number of Aβ deposits. Finally, we show that E2 inhibits Aβ-induced expression of scavenger receptor-A in macrophage cells, providing a mechanism for the effect of E2 on Aβ signaling observed in the APP23 mice. Altogether, our observations reveal a substantial involvement of endogenous estrogen in neuroinflammatory processes and provide novel mechanisms for hormone action in the brain.