Statins Cause Intracellular Accumulation of Amyloid Precursor Protein, β-Secretase-cleaved Fragments, and Amyloid β-Peptide via an Isoprenoid-dependent Mechanism

Abstract

The use of statins, 3-hydroxy-3-methylglutaryl-CoA reductase inhibitors that block the synthesis of mevalonate (and downstream products such as cholesterol and nonsterol isoprenoids), as a therapy for Alzheimer disease is currently the subject of intense debate. It has been reported that statins reduce the risk of developing the disorder, and a link between cholesterol and Alzheimer disease pathophysiology has been proposed. Moreover, experimental studies focusing on the cholesterol-dependent effects of statins have demonstrated a close association between cellular cholesterol levels and amyloid production. However, evidence suggests that statins are pleiotropic, and the potential cholesterol-independent effects of statins on amyloid precursor protein (APP) metabolism and amyloid ␤-peptide (A␤) genesis are unknown. In this study, we developed a novel in vitro system that enabled the discrete analysis of cholesterol-dependent and -independent (i.e. isoprenoid-dependent) statin effects on APP cleavage and A␤ formation. Given the recent interest in the role that intracellular A␤ may play in Alzheimer disease, we analyzed statin effects on both secreted and cell-associated A␤. As reported previously, low cellular cholesterol levels favored the ␣-secretase pathway and decreased A␤ secretion presumably within the endocytic pathway. In contrast, low isoprenoid levels resulted in the accumulation of APP, amyloidogenic fragments, and A␤ likely within biosynthetic compartments. Importantly, low cholesterol and low isoprenoid levels appeared to have completely independent effects on APP metabolism and A␤ formation. Although the implications of these effects for Alzheimer disease pathophysiology have yet to be investigated, to our knowledge, these results provide the first evidence that isoprenylation is involved in determining levels of intracellular A␤.A growing body of evidence suggests that the amyloid ␤-peptide (A␤) 1 plays a critical and early role in Alzheimer disease (AD) pathogenesis. AD is characterized by cerebral amyloid plaques, which are extracellular deposits of A␤ (1, 2). Overproduction of the 42-amino acid form of A␤ (A␤42) is associated with early onset familial AD, and A␤42 appears toxic to neurons in vitro and in vivo (reviewed in Refs. 3 and 4). Moreover, recent reports suggest that, in addition to extracellular A␤, the accumulation of intracellular A␤ may be involved in AD (5-15). Thus, much research has been devoted to understanding the role of A␤ in AD and to developing therapeutic strategies for reducing A␤ levels or toxicity. A␤ is cleaved from amyloid precursor protein (APP) by two proteases, the ␤-and ␥-secretases (reviewed in Refs. 4, 16, and 17). Initially, ␤-secretase (also known as BACE1) cuts APP at the N terminus of the A␤ domain to produce the membranebound APP C-terminal fragment (CTF) C99 and the secreted APP ectodomain APPs␤. C99 is the substrate of ␥-secretase, which cleaves to generate the C terminus of A␤. ␥-Secretase cleavage is heterogeneous and produces A␤ peptides of different le...

Independent Inhibition of Alzheimer Disease β- and γ-Secretase Cleavage by Lowered Cholesterol Levels

“…Our experiments have also shown BACE-1 and PS-1 protein levels not to be altered by treatment with cyclodextrin. A recent study has shown that statins cause intracellular accumulation of A␤ via a non-steroidal isoprenoid-dependent mechanism (67). In our study, however, we added mevalonate to avoid toxicity due to inhibition of these non-steroidal pathways (51,52,54).…”

Statins and the Squalene Synthase Inhibitor Zaragozic Acid Stimulate the Non-Amyloidogenic Pathway of Amyloid-β Protein Precursor Processing by Suppression of Cholesterol Synthesis

“…Several studies suggest that statins may regulate the α-secretase activity either by their cholesterol-lowering effects [23,24] or by impairment of the isoprenoid pathway [24][25][26]. Comparison of these data is complicated by the fact that studies were performed under different experimental conditions.…”

“…Several reports described the involvement of the isoprenoid pathway in AβPP proteolytic processing after statin treatment [24,25]. To investigate the involvement of the isoprenoid pathway in statin-mediated α-secretase activation, SK-N-MC cells were treated with lovastatin in the presence of 200 µM mevalonate or with farnesyltransferase (L-744,832) and geranylgeranyltransferase (GGTI-286) inhibitors.…”

“…The enzymes generating Aβ, the β-and γ-secretases, operate best in a high-cholesterol environment favoring Aβ production, whereas a reduction of cellular cholesterol leads to a decrease in Aβ production [19,[21][22][23]. Various cell culture studies showed that cholesterol reduction by statin treatment promotes the non-amyloidogenic α-secretase pathway by increasing the formation of neuroprotective sAβPPα and by decreasing Aβ production [23][24][25]. Despite of all these findings, the mechanism of statin action on secretases still remains unclear.…”

RAC1 Inhibition Targets Amyloid Precursor Protein Processing by γ-Secretase and Decreases Aβ Production in Vitro and in Vivo

“…48 for a review), inducing changes in the actin cytoskeleton, assembly of focal adhesion complexes, and decreasing efficiency of vesicular transport. Accordingly, statins induce a general, nonspecific down-regulation of the activity of small G proteins, which leads not only to a decrease of A␤ secretion but also to an intracellular accumulation of APP and of A␤ peptides, resulting in plaque formation and cellular damage (49). Our results show that inhibition of Rac1 by EHT1864 reduces both extracellular and intracellular amounts of A␤ peptides.…”