BACE-1 inhibition prevents the γ-secretase inhibitor evoked Aβ rise in human neuroblastoma SH-SY5Y cells

Background: Accumulation of amyloid beta-peptide (A beta) in the plaques is one of the major pathological features in Alzheimer's disease (AD). Sequential cleavage of amyloid precursor protein (APP) by beta-site APP cleaving enzyme 1 (BACE-1) and gamma-secretase results in the formation of A beta peptides. Preventing A beta formation is believed to attenuate AD progression and BACE-1 and gamma-secretase are thus attractive targets for AD drug development. Methods: Combining BACE-1 and gamma-secretase inhibition on A beta secretion from human neuroblastoma SH-SY5Y cells was evaluated in this study. Secreted A beta 40 and A beta 42 levels were measured from SH-SY5Y cells stably transfected with APPwt or APPswe genes. A selective BACE inhibitor and the gamma-secretase inhibitor LY450139 (semagacestat) were used to inhibit respective secretase. Results: LY450139 increased A beta 40 and A beta 42 secretion from SH-SY5Y APPwt cells at low concentrations (by 60% at 3 nM) followed by subsequent inhibition at higher concentrations (IC(50) 90 nM). Washout studies showed that the A beta increase evoked by 3 nM LY450139 was not due to enhanced cleavage following substrate accumulation but rather to activation of A beta formation. By contrast, LY450139 inhibited A beta formation from SH-SY5Y APPswe in a monophasic manner (IC(50) 18 nM). The BACE inhibitor per se inhibited A beta secretion from both SH-SY5Y APPwt and SH-SY5Y APPswe cells with IC(50)s ranging between 7 - 18 nM and also prevented the increased A beta secretion evoked by 3 nM LY450139. Combining the BACE inhibitor with higher inhibitory concentrations of LY450139 failed to demonstrate any clear additive or synergistic effects. Conclusion: BACE-1 inhibition attenuates the A beta increase evoked by LY450139 while not providing any obvious synergistic effects on LY450139-mediated inhibition.