Loss of dopaminergic neurons and α-synuclein accumulation are the two major pathological hallmarks of Parkinson’s disease (PD). Currently, the mechanisms governing depletion of dopamine content and α-synuclein accumulation are not well understood. We showed that the oxysterol 27-hydroxycholesterol (27-OHC) reduces the expression of tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine synthesis, and increases α-synuclein levels in SH-SY5Y cells. However, the cellular mechanisms involved in 27-OHC effects were not elucidated. Here, we demonstrate that 27-OHC regulates TH and α-synuclein expression levels through the estrogen receptors (ER) and liver X receptors (LXR). We specifically show that inhibition of ERβ mediates 27-OHC-induced decrease in TH expression, an effect reversed by the ER agonist estradiol. We also show that 27-OHC and the LXR agonist GW3965 increase α-synuclein while the LXR antagonist ECHS significantly attenuated the 27-OHC-induced increase in α-synuclein expression. We further demonstrate that LXRβ positively regulates α-synuclein expression and 27-OHC increases LXRβ-mediated α-synuclein transcription. Our results demonstrate the involvement of two distinct pathways that are involved in the 27-OHC regulation of TH and α-synuclein levels. Concomitant activation of ERβ and inhibition of LXRβ prevent 27-OHC effects and may therefore reduce the progression of PD by precluding TH reduction and α-synuclein accumulation.

The etiology of Alzheimer's disease (AD) is egregiously comprehended, but epidemiological studies have posited that diets rich in the saturated fatty acid palmitic acid (palmitate) are a significant risk factor. The production and accumulation of amyloid beta peptide (Aβ) is considered the core pathological molecular event in the pathogenesis of AD. The rate-limiting step in Aβ genesis from amyloid-β precursor protein (AβPP) is catalyzed by the enzyme β-site amyloid precursor protein cleaving enzyme 1 (BACE1), the expression and enzymatic activity of which is significantly up-regulated in the AD brain. In this study, we determined the molecular mechanisms that potentially underlie the palmitate-induced up-regulation in BACE1 expression and augmented Aβ production. We demonstrate that a palmitate-enriched diet and exogenous palmitate treatment evoke an increase in BACE1 expression and activity leading to enhanced Aβ genesis in the mouse brain and SH-SY5Y-APP cells, respectively, through the activation of the transcription factor NF-κB. Chromatin immunoprecipitation (ChIP) assays and luciferase reporter assays revealed that palmitate enhances BACE1 expression by increasing the binding of NF-κB in the BACE1 promoter followed by an enhancement in the transactivation of the BACE1 promoter. Elucidation and delineation of upstream molecular events unveiled a critical role of the endoplasmic reticulum stress-associated transcription factor, C/EBP homologous protein (CHOP) in the palmitate-induced NF-κB activation, as CHOP knock-down cells and Chop mice do not exhibit the same degree of NF-κB activation in response to the palmitate challenge. Our study delineates a novel CHOP-NF-κB signaling pathway that mediates palmitate-induced up-regulation of BACE1 expression and Aβ genesis.

BackgroundAccumulation of the α-synuclein (α-syn) protein is a hallmark of a group of brain disorders collectively known as synucleinopathies. The mechanisms responsible for α-syn accumulation are not well understood. Several studies suggest a link between synucleinopathies and the cholesterol metabolite 27-hydroxycholesterol (27-OHC). 27-OHC is the major cholesterol metabolite in the blood that crosses the blood brain barrier, and its levels can increase following hypercholesterolemia, aging, and oxidative stress, which are all factors for increased synucleinopathy risk. In this study, we determined the extent to which 27-OHC regulates α-syn levels in human dopaminergic neurons, the cell type in which α-syn accumulates in PD, a major synucleinopathy disorder.ResultsOur results show that 27-OHC significantly increases the protein levels, not the mRNA expression of α-syn. The effects of 27-OHC appear to be independent of an action through liver X receptors (LXR), its cognate receptors, as the LXR agonist, GW3965, or the LXR antagonist ECHS did not affect α-syn protein or mRNA levels. Furthermore, our data strongly suggest that the 27-OHC-induced increase in α-syn protein levels emanates from inhibition of the proteasomal degradation of this protein and a decrease in the heat shock protein 70 (HSP70).ConclusionsIdentifying 27-OHC as a factor that can increase α-syn levels and the inhibition of the proteasomal function and reduction in HSP70 levels as potential cellular mechanisms involved in regulation of α-syn. This may help in targeting the correct degradation of α-syn as a potential avenue to preclude α-syn accumulation.