Implications of cholesterol and cholesterol-lowering therapy in Alzheimer's disease

Abstract

BACKGROUND: Alzheimer's disease (AD) is a severe neurodegenerative disease that mainly afflicts elderly persons, with a characteristic progressive decline of cognitive functions and dementia. It is believed that the majority of all AD patients are affected by the sporadic form, thus caused by the combined effects of several risk factors, such as elevated cholesterol levels in midlife and deficiencies in the lipoprotein transporters apolipoprotein E (ApoE). Cholesterol play an essential role in the central nervous system (CNS) were it maintains normal physiological conditions, and is a requirement for the ability of neurons to communicate. It has previously been demonstrated that the cholesterol homeostasis in the CNS is maintained by a slow conversion of brain cholesterol into 24(S)-hydroxycholesterol, with a net flux of 27-hydroxycholesterol from the circulation into the brain. The importance of a preserved cholesterol homeostasis has been supported by the observation that dietary cholesterol may induce an inflammation in the CNS, and increase the levels of pro-inflammatory mediators. It has, in direct association with these observations, been demonstrated that plasma levels of pro-inflammatory proteins, such as interleukin-6, are increased before the clinical onset of AD, after which a chronic state of inflammation often is found. Accordingly, it has been suggested that memory and cognitive functions could benefit from a cholesterol-lowering therapy.

STUDIES: The research presented in this thesis has examined the effect of cholesterol and cholesterol-lowering therapy with rosuvastatin, on key factors associated with AD. The experimental setup consisted of in vitro-models with human neuroblastoma SH-SY5Y cells (paper I II), and in vivo-models with wild type (WT) and ApoE knockout (ApoE-/-) mice, provided with high levels of dietary cholesterol for 18 weeks (paper III IV). The hydrophilic compound rosuvastatin is an effective inhibitor of cholesterol synthesis and has been demonstrated to be an effective treatment for hypercholesterolemia, with an indirect effect on microglial activation and inflammation through isoprenoid depletion.

RESULTS: Paper I describes the selective non-amyloidogenic processing (alpha-secretase activity) of the amyloid precursor protein (APP), induced by 24(S)-hydroxycholesterol, with a subsequent increased ratio of alpha-/beta-secretase activity, and increased levels of soluble APPalpha and total soluble sAPP; effects significantly decreased by the presence of 27-hydroxycholesterol. Paper II describes the effect of pre-treating in vitro-cultures with rosuvastatin prior exposure to Abeta oligomers. The treatment was found to decrease caspase-3 activity and promote cell survival, with a selective non-amyloidogenic processing of APP. However, no influence was observed on cell viability, with a potential explanation in a downregulated metabolism. Paper III IV describes the effect of high levels of dietary cholesterol in vivo, with increased microglial activation in WT mice, increased gliosis in ApoE-/- mice and subsequent increased plasma IL-6 levels in WT and ApoE-/- mice. The elevated levels of total cholesterol (TC) were found to be caused by increased levels of HDL and LDL in WT mice, whereas only LDL where increased in ApoE-/- mice. Plasma TC levels were correlated with body weight gain in WT and ApoE-/- mice. Furthermore, the selective and ApoE-associated influence induced by the rosuvastatin therapy, on the effects of dietary cholesterol, is described. We observed a decreased microglial activation and gliosis in WT and ApoE-/- mice, with plasma IL-6 levels decreased by 45% in WT mice, without reaching significance. The elevated levels of TC and LDL were decreased in WT mice, whereas no effect was observed on the levels of TC, HDL or LDL in ApoE-/- mice. Body weight gain was decreased in WT mice, with a previously unpublished age- and ApoE-associated declining response to the therapy observed in ApoE-/- mice.

SUMMARY: The presented thesis has examined the connection between Alzheimer's disease and cholesterol, and demonstrated a selection of detrimental effects which may be induced by high levels of dietary cholesterol. Furthermore, the thesis has demonstrated the potential for a cholesterol-lowering therapy with rosuvastatin to exercise a preventive influence, with an age- and ApoE-associated response. The latter is an important observation, which indicates that rosuvastatin may be an effective therapy up to a certain point in the progression of the disease, after which the effects decline. In conclusion, the results discussed in this thesis offer a hypothetical explanation, at least in part, for the discrepancies observed in clinical trials on statins. In addition, this thesis have been able to support the theoretical connection between AD and plasma cholesterol levels with the observation that 27-hydroxycholesterol may exhibit an inhibitory influence on 24(S)-hydroxycholesterol and non-AD processes. It is suggested that persons with high risk of developing AD may benefit from a rosuvastatin therapy, in combination with lifestyle precautions taken in early midlife. The approach could be an effective lifestyle-preventive strategy, with preserved cognitive functions and quality of life at high age.