Another important neuroprotective role of estrogens in AD is that estrogen could reduce Aβ
levels, or prevent them from rising, in the presence of pathological triggers [149]. Estrogens
can also reduce Aβ production by favoring the non-amyloidogenic pathway through
MARK/ERK activation, reducing BACE1 levels, and promoting Aβ clearance by
stimulating microglial phagocytosis and degradation and regulating the levels of major
enzymes involved in Aβ degradation [150]. Estrogens can also be protective by regulating
the Bcl-2 protein family, increasing the expression of antiapoptotic Bcl-xL and Bcl-w and
suppressing expression of proapoptotic Bim to prevent neuronal loss from Aβ-mediated
toxicity [151]. Decreased levels of hyperphosphorylated tau (a major component of
neurofibrillary tangles) can also be mediated by estrogens through kinases and phosphatases,
such as the GSK-3β, Wnt, or PKA pathways [152].