DHCR24 Knock-Down Induced Tau Hyperphosphorylation at Thr181, Ser199, Thr231, Ser262, Ser396 Epitopes and Inhibition of Autophagy by Overactivation of GSK3β/mTOR Signaling

Accumulating evidences supported that knock-down of DHCR24 is linked to the pathological risk factors of AD, suggesting a potential role of DHCR24 in AD pathogenesis. However, the molecular mechanism link between DHCR24 and tauopathy remains unknown. Here, in order to elucidate the relationship between DHCR24 and tauopathy, we will focus on the effect of DHCR24 on the tau hyperphosphorylation at some toxic sites. In present study, we found that DHCR24 knock-down significantly lead to the hyperphosphorylation of tau sites at Thr181, Ser199, Thr231, Ser262, Ser396. Moreover, DHCR24 knock-down also increase the accumulation of p62 protein, simultaneously decreased the ratio of LC3-II/LC3-I and the number of autophagosome compared to the control groups, suggesting the inhibition of autophagy activity. In contrast, DHCR24 knock-in obviously abolished the effect of DHCR24 knock-down on tau hyperphosphrylation and autophagy. In addition, to elucidate the association between DHCR24 and tauopathy, we further showed that the level of plasma membrane cholesterol, lipid raft-anchored protein caveolin-1, and concomitantly total I class PI3-K (p110α), phospho-Akt (Thr308 and Ser473) were significantly decreased, resulting in the disruption of lipid raft/caveola and inhibition of PI3-K/Akt signaling in silencing DHCR24 SH-SY5Y cells compared to control groups. At the same time, DHCR24 knock-down simultaneously decreased the level of phosphorylated GSK3β at Ser9 (inactive form) and increased the level of phosphorylated mTOR at Ser2448 (active form), leading to overactivation of GSK3β and mTOR signaling. On the contrary, DHCR24 knock-in largely increased the level of membrane cholesterol and caveolin-1, suggesting the enhancement of lipid raft/caveola. And synchronously DHCR24 knock-in also abolished the effect of DHCR24 knock-down on the inhibition of PI3-K/Akt signaling as well as the overactivation of GSK3β and mTOR signaling. Collectively, our data strongly supported DHCR24 knock-down lead to tau hyperphosphorylation and the inhibition of autophagy by a lipid raft-dependent PI3-K/Akt-mediated GSK3β and mTOR signaling. Taking together, our results firstly demonstrated that the decrease of plasma membrane cholesterol mediated by DHCR24 deficiency might contribute to the tauopathy in AD and other tauopathies.

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Abstract 365: Inhibiting Hmg Coa Reductase Reduces Palmitate-Induced Inflammation in Adipocytes

Arteriosclerosis Thrombosis and Vascular Biology ◽

10.1161/atvb.37.suppl_1.365 ◽

2017 ◽

Vol 37 (suppl_1) ◽

Author(s):

Asha K Pathak ◽

Chang Yeop Han ◽

Mohamed Omer ◽

Shari Wang ◽

Alan Chait

Keyword(s):

Gene Expression ◽

Plasma Membrane ◽

Er Stress ◽

Lipid Raft ◽

Cholesterol Content ◽

Membrane Cholesterol ◽

Hmg Coa Reductase ◽

Pre Treatment ◽

Coa Reductase ◽

Plasma Membrane Cholesterol

Adipose tissue inflammation associates with insulin resistance and increased cardiovascular disease risk. We previously observed that 3T3-L1 adipocytes exposed to palmitate become inflamed and demonstrate increased plasma membrane cholesterol and lipid raft content. It is known that palmitate induces translocation of NAPH oxidase and toll-like receptor 4 into lipid rafts, increasing adipocyte inflammation. However, it is unclear (1) how palmitate alters plasma membrane cholesterol content; and (2) whether increased cholesterol content in the plasma membrane is related to adipocyte inflammation induced by palmitate exposure. We hypothesize that mechanisms involved in increasing plasma membrane cholesterol content after palmitate treatment could be related to cholesterol synthesis and/or ER stress, and that increased cholesterol in lipid rafts is essential for induction of inflammation in adipocytes. To test these hypotheses, differentiated murine 3T3-L1 adipocytes were exposed to palmitate for 24 hours, with and without pre-treatment with HMG-CoA reductase inhibitors (statins) or HDL. RT-PCR was used to evaluate gene expression of inflammation ( Saa3 , Ccl2 ), ER stress ( Bip , Chop ), and HMG-CoA reductase ( Hmgcr ). Cholera toxin subunit β staining and flow cytometry were used to evaluate plasma membrane lipid raft content. In differentiated adipocytes, palmitate-induced inflammation neither increased expression of ER stress genes nor HMG-CoA reductase gene expression. However, treatment with 3 different statins (simvastatin, lovastatin, atorvastatin) significantly reduced palmitate-induced adipocyte inflammation as indicated by decreased gene expression of Saa3 and Ccl2 ( P <0.05). A similar effect was seen with pre-treatment with HDL. Lipid raft content induced by palmitate was decreased by HMG-CoA reductase inhibitors (difference in mean fluorescence intensity P <0.05) and also by pre-treatment with HDL. These findings indicate that ER stress was not involved in increased plasma membrane cholesterol after palmitate-induced inflammation in adipocytes. However, regulating cholesterol content in lipid rafts plays an important role in adipocyte inflammation induced by palmitate.

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