Global regulators of Inflammation: Estrogen (7β-estradiol)

One of our former graduate students, Dr. Candice Brown, who is now at the University of Washington in Seattle, discovered differences in the immune responses of APOE4 and APOE3 male and female mice. Although clearly important, gender differences in immunity and neurodegeneration have largely been ignored. Compared to affected men, women with AD have slight but significant increases in global neuropathology and worsened dementia ratings. Interestingly, the gender differences can be stratified by APOE genotype. Women who express an APOE4 gene have lower hippocampal volumes, more senile plaque pathology and worsened cognitive scores than do their counterparts who do not express APOE4. While the women included in the studies were post-menopausal, the roles of sex steroid status or hormone replacement therapy in the observed results were not examined.

Part of the protective mechanism of 17β-estradiol stems from its ability to enhance neurite extension and synaptic remodeling, increase growth and survival factors, and reduce mitochondrial damage and oxidative stress. 17β-estradiol also regulates the innate immune response of macrophages and serves as an anti-inflammatory agent. For example, treatment of brain macrophages (microglia) and peripheral macrophages with physiological doses of 17β-estradiol reduces the release of pro-inflammatory factors such as nitric oxide (NO) and tumor necrosis factor (TNF-α). Furthermore, the loss of estrogen with ovariectomy or with aging results in a shift of macrophage function towards a pro-inflammatory state. Part of the brain's estrogen-mediated neuroprotection that is lost during aging and in post-menopausal women with AD may, in fact, stem from its anti-inflammatory effects.

Our data confirm previous studies that demonstrate estrogen-mediated suppression of cytokines such as TNFa, proteases and NO in immune-activated, primary microglia. However, our data also demonstrate a unique connection between APOE gentoype and estrogen regulation of microglial function. Essentially, the anti-inflammatory activity of 17β-estradiol is reduced in microglia derived from mice that express the human APOE4 gene compared to mice that express the human APOE3 gene. Treatment with 17β-estradiol over a range of physiologically meaningful concentrations significantly decreased LPS- and IFNγ-mediated NO production in APOE3/3 microglia but not in microglia from APOE4/4 mice. Our data also show that basal ERβ protein expression is higher in APOE4/4 microglia. ERβ has also been implicated in the anti-inflammatory responses mediated by 17β-estradiol.

The inability of estrogen replacement to compensate for the impaired response to conjugated equine estrogens in the presence of the APOE4 gene implies that simply adding estrogens back may not be able to re-activate the protective, anti-inflammatory effects of estrogens. Thus, the APOE genotype may be a critical component in assessing the effectiveness of estrogen's action. These data may provide insight into why the effect of estrogen on dementia in post-menopausal women is controversial. Results of the Women's Health Initiative clinical trials have recommended that hormone replacement in post-menopausal women be curtailed under most circumstances. However, this is in starkly contrasts with the obvious usefulness of estrogen as a neuroprotective agent based on a multitude of basic science studies. The discrepancy may be due, in large part, to genetic variation of estrogen responsiveness in the human population as suggested by our APOE study.

The close connection between estrogen and apolipoprotein E has led us to believe that these two global regulators of inflammation interact on a molecular basis. We are actively working on this concept and have discovered an important site of direct interaction that both alter. These data are forthcoming.

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