Decrease of Amyloid-ß Levels by Curcumin Derivative via Modulation of Amyloid-ß Protein Precursor Trafficking.

The abnormal production and deposition of amyloid-ß (Aß) peptides is a pathologic hallmark of Alzheimer's disease. Aß is generated from amyloid-ß protein precursor (AßPP) by two sequential proteolytic cleavage steps involving ß- and γ-secretases in the trans-Golgi network and endosomes. Since direct inhibition of secretase could induce undesirable side-effects due to inadvertent inhibition of unrelated secretase substrates, it is important to establish methods for inhibiting Aß production that do not affect secretase activity. It has been suggested that curcumin may have potent anti-amyloidogenic effect. In the present study, we evaluate the effect of curcumin derivatives on Aß production in human neuroblastoma SH-SY5Y cells and in CHO cells which stably express human AßPP (CHO-AßPP). We found that the curcumin derivative CU6 was more effective than curcumin itself in reducing Aß secretion. We further found that in SH-SY5Y cells CU6 inhibited neither ß- nor γ-secretase activity, and that increased amounts of immature forms of AßPP accumulated in the endoplasmic reticulum (ER). We also found that CU6 induced expression of the ER chaperone glucose-regulated protein 78 (GRP78), and enhanced formation of the AßPP/GRP78 complex. These results suggest that CU6 downregulates intracellular AßPP trafficking, resulting in suppression of Aß production independently of secretase activity.

Adaptive responses induced by 24S-hydroxycholesterol through liver X receptor pathway reduce 7-ketocholesterol-caused neuronal cell death.

Lipid peroxidation products have been known to induce cellular adaptive responses and enhance tolerance against subsequent oxidative stress through up-regulation of antioxidant compounds and enzymes. 24S-hydroxycholesterol (24SOHC) which is endogenously produced oxysterol in the brain plays an important role in maintaining brain cholesterol homeostasis. In this study, we evaluated adaptive responses induced by brain-specific oxysterol 24SOHC in human neuroblastoma SH-SY5Y cells. Cells treated with 24SOHC at sub-lethal concentrations showed significant reduction in cell death induced by subsequent treatment with 7-ketocholesterol (7KC) in both undifferentiated and retinoic acid-differentiated SH-SY5Y cells. These adaptive responses were also induced by other oxysterols such as 25-hydroxycholesterol and 27-hydroxycholesterol which are known to be ligands of liver X receptor (LXR). Co-treatment of 24SOHC with 9-cis retinoic acid, a retinoid X receptor ligand, enhanced the adaptive responses. Knockdown of LXRß by siRNA diminished the adaptive responses induced by 24SOHC almost completely. The treatment with 24SOHC induced the expression of LXR target genes, such as ATP-binding cassette transporter A1 (ABCA1) and G1 (ABCG1). The 24SOHC-induced adaptive responses were significantly attenuated by siRNA for ABCG1 but not by siRNA for ABCA1. Taken together, these results strongly suggest that 24SOHC at sub-lethal concentrations induces adaptive responses via transcriptional activation of LXR signaling pathway, thereby protecting neuronal cells from subsequent 7KC-induced cytotoxicity.