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行政院國家科學委員會專題研究計畫成果報告
高膽固醇導致腦血管病變與阿茲海默氏症之關係
High Serum Cholesterol Induced Cerebrovascular Pathologies and Their
Relationships with Alzheimer’s Disease
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英文摘要
Accumulated evidences demonstrate a link between serum cholesterol levels, Aβ concentrations, and the incidence of Alzheimer’s disease. In the present report, the effects of dietary cholesterol on the brain Aβ levels were examined using diet induced hypercholesterolemic rabbit. Increased dietary cholesterol significantly increased Aβ concentrations in temporal cortex (p = 0.02). A similar trend was also observed in the frontal cortex (p = 0.06). No such difference could be detected in cerebellum (p>0.05). The regional levels of Aβ in the hypercholesterolemic rabbits correlate well with the amyloid pathology in AD. Furthermore, dietary cholesterol also induced the expression of cerebral apoE that may aggravate the accumulation of Aβ. These results suggest that higher serum cholesterol is a risk factor for Alzheimer’s disease and lowing serum cholesterol may be a useful strategy in preventing this disease.
緣由與目的
Apolipoprotein E (apoE) represents a recognized genetic risk factor for sporadic AD (1). ApoE is involved in the transport of cholesterol between tissues (1). A dose-dependency elevates the levels of amyloid deposits in cerebral cortex and vascular walls in both aging and AD brains (2,3).
A number evidences indicated that cholesterol, or cholesterol metabolisms, play a significant role in the development of AD. Clinical and epidemiological investigations demonstrated that individuals with elevated serum cholesterol have increased susceptibility to AD (4,5). The prevalence of AD is greater in countries with high fat and high-calorie diets and lower in those with diets low in fat (6). Furthermore, the increased risk of cardiovascular disease conferred by apoE4 is attributed to an associated hypercholesterolemia that can promote or exacerbate atherosclerosis,
hypertension, myocardial infarction and critical coronary artery disease (7,8). Other evidences also indicate that cardiovascular diseases increase the risk of developing AD (9-10). Several studies suggested that the production and accumulation of soluble and insoluble Aβ in Alzheimer’s disease (AD) brains are influenced directly or indirectly by lipids and factors that regulate lipid metabolism in the CNS (11-13).
In vitro and in vivo studies supported the notion that cholesterol may be directly involved in the metabolism of Aβ. Supplementation cells with cholesterol favored the βAPP processing pathway to the amyloidogenic C99 fragment and led to higher Aβ production (14). Furthermore, cells treated with cholesterol-lowering agent, lovastatin, reduced the secretion of Aβ and the βAPP C99 fragment (14). Moreover,
βAPP transgenic mice fed with high cholesterol diet not only increased the amyloid plaque numbers, but also plaque sizes (15). Finally, induced Aβ-like immunoreactivity and elevated superoxide dismutase (SOD) expression were reported in rabbits fed with high cholesterol diet (10).
In this study, we attempt to study the effects of hypercholesterolemia on the production of brain Aβ peptides in non-genetic modified animals. The levels of apoE, βAPP, and SOD were analyzed biochemically and immunohistochemically in the brains of high cholesterol fed rabbits.
結果與討論
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Table 1. Effect of diet on serum total cholesterol (TC), high-density lipoprotein (HDL), low-density lipoprotein (LDL), and triacylglycerol (TG) in rabbits.
HC Diet Basal Diet p value
TC 2419 ± 1027 68 ± 32 < 0.001 HDL 400 ± 134 31 ± 13 < 0.001 LDL 1948 ± 661 34 ± 20 < 0.001 TG 184 ± 217 63 ± 29 0.1918 Value represents mean ± standard deviation in mg/dL.
Levels of apoE in the brains of HC and control groups were compared. A significantly increase in the apoE levels (p < 0.001) were observed in the rabbits fed with high cholesterol diet (Figure 1).
Figure 1. Rabbit brain apoE levels increases
in response to high cholesterol diet. A) Immunoblot of apoE extracted from cortex of rabbits fed either basal control (Control) or high cholesterol (HC) diets. B) Densitometric analysis of bands for apoE relative units in control and HC groups. Bars represent mean
± SE and were adjusted by actin. P < 0.001. The determination of the brain’s relative levels of APP demonstrated no significant difference between the two groups in terms of relative units of density (p = 0.438). Neither was the brain SOD levels (p = 0.194).
The quantifications of the Aβ peptides by EuIA were conducted in three different brain regions, temporal, fronal, and cerebellar cortices. The levels of Aβ in the temporal cortex were significantly higher in the HC group than the controls (Fig. 2A). A similar trend was also observed in the frontal cortex, however, the difference did not reach significance (Fig. 2B). In contrast,
hypercholesterolemia had no effect on the levels of Aβ in the cerebellum (Fig. 2C).
Figure 2. Aβ levels in three brain regions of control and high cholesterol diets fed rabbits. Aβ from A) temporal, B) frontal, and C) cerebellum regions of rabbit brains was extracted and quantified as described in Materials and Methods. Each bar represents mean ± SE. T-test probabilities were indicated accordingly.
Brain sections from HC and basal control diet fed rabbits stained for Aβ, SOD and apoE utilizing their specific antibodies revealed no obvious changes between the two groups. Positive SOD reactions were occasionally observed around perivascular regions in the brain of one of four examined HC fed rabbits, and none in any control animals.
In this study, we showed that hypercholesterolemia increased brain Aβ accumulation was in a regional specific manner. In the three examined brain regions, temporal cortex had the highest, followed by frontal cortex, and the cerebellum had the lowest Aβ levels in the cholesterol fed rabbit. Significantly, these findings correlate with the severity of Aβ pathology in AD brains. To our knowledge, these results are the first demonstration that hypercholesterolemia increases brain Aβ accumulation in a AD related regional specific manner.
4 cortex. Moreover, apoE was shown to increase the sulfation of glycosaminoglycans which possess a high affinity for both apoE and Aβ (18). Elevations in apoE induced by hypercholesterolemia may together with Aβ entrapped in the sulfated extracellular matrix leading to the deposition of Aβ.
In conclusion, our findings support the notion that dietary cholesterol increases brain Aβ levels and hence raises the risk of developing AD. Temporal cortex appears to be more sensitive to the hypercholesterolemia in elevating Aβ than frontal cortex, and cerebellum is spared, that is correlated well with AD pathology. High serum cholesterol induced brain apoE may aggravate such effects. Finally, lowing serum cholesterol may be a useful strategy in preventing Alzheimer’s disease.
參考文獻
1. Roses, A. D. 1997. Neurogenetics 1: 3-11.
2. Kuo, et al. 2000. Am. J. Pathol. 156: 797-805.
3. Premkumar, et al. 1996. Am. J. Pathol.
148: 2083-2095.
4. Roher, et al. 1999. Amyloid 6: 136-145. 5. Jarvik, et al. 1994. Genet. Epidemiol. 11:
375-384.
6. Grant, W B. 1997. Alz. Dis. Rev. 2: 42-55.
7. Ciruzzi, et al. 1997. Am. J. Cardiol. 80: 122-127.
8. Kivipelto, et al. 2001. BMJ 322: 1447-1451.
9. Ott, et al. 1997. Stroke 28: 316-321. 10. Sparks, D. L. 1997. Ann. N. Y. Acad. Sci.
826: 128-146.
11. Fassbender, et al. 2001. Proc. Natl. Acad. Sci. USA 98: 5856-5861.
12. Kojro, et al. 2001. Proc. Natl. Acad. Sci. USA 98: 5815-5820.
13. Simons, et al. 1998. Proc. Natl. Acad. Sci. USA 95: 6460-6464.
14. Racchi, et al. 1997. Biochem. J. 322:
893-898.
15. Refolo, et al. 2001. Neurobiol. Dis. 8: 890-899.
16. Howland, et al. 1998. J. Biol. Chem. 273: 16576-16582.
17. Kuo, et al. 2000. Mol. Med. 6: 430-439. 18. Paka, et al. 1999. J. Biol. Chem. 274:
