Impaired hepatic amyloid-beta degradation in Alzheimer's disease.

ABSTRACT

Extensive research strongly suggests that amyloid beta (Aß) aggregates in the brain have a central role in Alzheimer's disease (AD) pathogenesis. Pathological Aß deposition is likely due to an altered balance between overproduction and elimination. Rodent studies have suggested that the liver has a major role in Aß degradation. It is possible alterations of liver function could affect brain Aß levels through changes in blood Aß concentration. In this study, we hypothesized hepatic Aß degradation to be impaired in AD subjects. To test our hypothesis, an Aß degradation assay was developed using synthetic fluorescein-labeled Aß40 and Aß42 spiked into human liver homogenates. Aß degradation rates were lower in AD-derived homogenates as compared with those from non-demented (ND) control subjects, even after accounting for such covariates as age, sex, and APOE genotype. The protein expression of potential Aß-degrading enzymes were also examined. Neprilysin levels were not different in AD liver samples, while cathepsin D and insulin-degrading enzyme were significantly altered in AD subjects. The results support the possibility that impaired hepatic Aß degradation could be a factor contributing to increased brain Aß accumulation and AD.