Increased expression of RXRα in dementia: an early harbinger for the cholesterol dyshomeostasis?

Abstract

Background: Cholesterol content of cerebral membranes is tightly regulated by elaborate mechanisms that balance the level of cholesterol synthesis, uptake and efflux. Among the conventional regulatory elements, a recent research focus has been nuclear receptors, a superfamily of ligand-activated transcription factors providing an indispensable regulatory framework in controlling cholesterol metabolism pathway genes. The mechanism of transcriptional regulation by nuclear receptors such as LXRs involves formation of heterodimers with RXRs. LXR/RXR functions as a sensor of cellular cholesterol concentration and mediates cholesterol efflux by inducing the transcription of key cholesterol shuffling vehicles namely, ATP-binding cassette transporter A1 (ABCA1) and ApoE.

Results: In the absence of quantitative data from humans, the relevance of expression of nuclear receptors and their involvement in cerebral cholesterol homeostasis has remained elusive. In this work, new evidence is provided from direct analysis of human postmortem brain gene and protein expression suggesting that RXRα, a key regulator of cholesterol metabolism is differentially expressed in individuals with dementia. Importantly, RXRα expression showed strong association with ABCA1 and ApoE gene expression, particularly in AD vulnerable regions.

Conclusions: These findings suggest that LXR/RXR-induced upregulation of ABCA1 and ApoE levels may be the molecular determinants of cholesterol dyshomeostasis and of the accompanying dementia observed in AD.

Figure 1

Schematic diagram of LXR/RXR activation mechanism (adapted from the references cited above). In the absence of a ligand, LXR/RXR heterodimers are bound to the LXREs in the promoter region of target genes and in complex with corepressors (SMRT or NCoR). Ligand binding (e.g., oxysterols) induces a dissociation of corepressors and recruitment of coactivators and the target gene expression is induced.

Figure 2

RXRα (Black bars) and LXRβ mRNA (white bars) expression in individuals with and without dementia or AD-associated neuropathology in area 20. Mean values ± SEM are shown. *, p < 0.05; ****, p < 0.0001. Number within the parentheses indicates the individuals within each group.

Figure 3

Normalized RXRα (Black bars) and LXRβ mRNA (white bars) expression in area 20 plotted against CDR scores, Braak neuropathological stages and NP density. ANCOVA was used to compare gene expression in individuals with varying degree of dementia (CDR 0.5-5) and AD associated neuropathology (Braak stage I-VI, NP density 2-5) relative to the control group. Mean values ± SEM are shown. *, p < 0.05; **, p < 0.01. Number within the parentheses indicates the individuals within each disease severity group.

Figure 4

RXRα (Black bars) and LXRβ mRNA (white bars) expression in individuals with and without dementia or AD-associated neuropathology in hippocampus. Mean values ± SEM are shown. **, p < 0.01. Number within the parentheses indicates the individuals within each group.

Figure 5

Normalized RXRα (Black bars) and LXRβ mRNA (white bars) expression in hippocampus plotted against CDR scores, Braak neuropathological stages and NP density groups. ANCOVA was used to compare gene expression in individuals with varying degree of dementia (CDR 0.5-5) and AD-associated neuropathology (Braak stage I-VI, NP density 2-5) relative to the control group. Mean values ± SEM are shown. *, p < 0.05; **, p < 0.01. Number within the parentheses indicates the individuals within control groups and each of the disease severity group.

Figure 6

Western blot analysis of RXRα in the hippocampus of cognitively intact controls and subjects with varying severity of dementia. A, Representative immunoblots of RXRα protein expression are shown. Total tissue homogenates were separated by reducing SDS-PAGE and probed with rabbit anti-RXRα and mouse anti-VCP antibodies. Tissue lysate from each subjects were loaded in triplicate and pooled tissue lysate (first 4 lanes) were run in quadruplicates. B, Protein quantification was done by assessing the ratio of RXRα and VCP signal. Mean values ± SEM are shown. *, p < 0.05; **, p < 0.01. Number within the parentheses indicates the individuals within each group.

Figure 7

Association between RXRα and target gene expression in area 20. A. RXRα gene expression is strongly associated with ABCA1 gene expression (r = 0.531, df = 83, p < 0.0001). B. RXRα gene expression is strongly associated with ApoE gene expression (r = 0.622, df = 84, p < 0.0001).

Figure 8

Association between RXRα and LRP gene expression. A. RXRα gene expression is strongly associated with LRP gene expression in area 20 (r = 0.89, df = 84, p < 0.0001). B. RXRα gene expression is strongly associated with LRP gene expression in hippocampus (r = 0.70 df = 69, p < 0.0001).