Double-Edged Roles of Nitric Oxide Signaling on APP Processing and Amyloid-β Production In Vitro: Preliminary Evidence from Sodium Nitroprusside

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is thought to be caused in part by the age-related accumulation of amyloid-beta (A beta) in the brain. Recent findings have revealed that nitric oxide (NO) modulates the processing of amyloid-beta precursor protein (APP) and alters A beta production; however, the previously presented data are contradictory and the underlying molecular mechanisms are still incomplete. Here, using human SH-SY5Y neuroblastoma cells stably transfected with wild-type APPwt(695), we found that NO, derived from NO donor sodium nitroprusside (SNP), bi-directionally modulates APP processing in vitro. The data from ELISA and Western blot (WB) tests indicated that SNP at lower concentrations (0.01 and 0.1 mu M) inhibits BACE1 expression, thus consequently suppresses APP beta-cleavage and decreases A beta production. In contrast, SNP at higher concentrations (10 and 20 mu M) biases the APP processing toward the amyloidogenic pathway as evidenced by an increased BACE1 but a decreased ADAM10 expression, together with an elevated A beta secretion. This bi-directional modulating activity of SNP on APP processing was completely blocked by specific NO scavenger c-PTIO, indicating NO-dependent mechanisms. Moreover, the anti-amyloidogenic activity of SNP is sGC/cGMP/PKG-dependent as evidenced by its reversal by sGC/PKG inhibitions, whereas the amyloidogenic activity of SNP is peroxynitrite-related and can be reversed by peroxynitrite scavenger uric acid. In summary, these present findings predict a double-edged role of NO in APP processing in vitro. Low (physiological) levels of NO inhibit the amyloidogenic processing of APP, whereas extra-high (pathological) concentrations of NO favor the amyloidogenic pathway of APP processing. This preliminary study may provide further evidence to clarify the molecular roles of NO and NO-related signaling in AD and supply potential molecular targets for AD treatment.