19 June 2007. Alzheimer and heart disease share a common cast of villains: cholesterol, inflammation, and macrophage activation are involved in both. Could the diseases also have a common treatment? That’s one implication of a growing number of studies on the liver X receptors (LXRs), a pair of sterol-binding nuclear receptors that control cholesterol metabolism and inhibit inflammatory gene expression.
The latest contribution in the story comes from the lab of Peter Tontonoz at University of California, Los Angeles. In a paper in the June 11 online edition of PNAS, Tontonoz and colleagues show that knocking out either of the LXRα or β genes in APP/PS1 mice leaves the animals with higher amyloid plaque load.
“The LXRs play an important role in macrophage activity and inflammation, and are known to regulate cholesterol metabolism. When we looked for physiological disease states where these pathways might be important, AD was an obvious candidate,” Tontonoz said.
Their results, coupled with evidence, from their own and other studies, that activating LXRs with a synthetic agonist represses neuroinflammation while promoting Aβ clearance, provide in vivo target validation to support the idea that LXR agonists, now in development for cardiovascular disease and atherosclerosis, might also be used to treat AD.
The natural ligands of LXRs are cholesterol-derived oxysterols. When occupied with oxysterols, LXRs regulate the expression of genes in the cholesterol metabolism pathway, as well as inhibit the expression of inflammatory cytokines. Their actions in the liver and other tissues have been extensively studied, but what they do in the brain has been less clear.
The new work, done in collaboration with the lab of Gary Landreth at Case Western Reserve University School of Medicine in Cleveland, Ohio, follows on results from several investigators using LXR agonists in cell lines and AD mouse models. According to those studies, activation of LXR both inhibits Aβ production and enhances Aβ clearance (for a recent review, see Koldamova and Lefterov, 2007).
In this study, first author Noam Zelcer and colleagues took a different approach, by crossing LXR knockout mice with AD mice to gauge the effect of LXR on amyloid pathology in vivo. In 32-week-old APP/PS1 transgenic mice lacking either LXRα or LXRβ, the investigators found that cortical amyloid plaques were larger and more numerous. The increase was statistically significant in the LXRα knockouts, but not in the mice lacking LXRβ. The knockouts showed no change in APP levels or processing.
The LXRs could affect Aβ clearance by macrophages, according to the next set of experiments. LXRs are potent inhibitors of inflammation, and experiments with glial cells in vitro suggested that their effects carry over to the inflammation stimulated by fibrillar Aβ. In cultures of mixed glial cells from WT or double knockout mice, the cells responded to fibrillar Aβ with an induction of inflammatory cytokines including RANTES, TNFα, Mip1β and IL-1β. The response was inhibited by an LXR agonist and enhanced in cells from LXR knockout mice. Inflammatory cytokines have been shown to inhibit macrophage phagocytic activity, and in agreement with this idea, the researchers found that IL-1β or LPS treatment of BV2 microglia cells inhibited phagocytosis of fibrillar Aβ. An LXR agonist that inhibited the inflammatory response stimulated phagocytosis of fibrillar Aβ in the cells.
The results suggest that LXRs, by inhibiting neuroinflammation, could help pump up Aβ clearance by activated glia, which would account for the worse pathology in knockout mice. However, there could be other explanations. Loss of either LXRα or β resulted in lowered expression of the cholesterol transporter ABCA1 in young (12-week-old) APP/PS1 mice. A deficiency in ABCA1 was previously shown to result in Aβ accumulation (see ARF related news story).
Both the ABCA1 and inflammatory pathways are likely to contribute to the effects of LXRs on amyloid load, Tontonoz says. “We don’t have definite evidence which is more important, and we speculate that both are important.”
Besides ABCA1, there are many other genes that are involved in cholesterol metabolism, Tontonoz added. “LXR regulates many genes in that pathway, so it may be an advantage to modulate the whole pathway,” he said.
When it comes to LXR agonists, most researchers think of cardiovascular disease and atherosclerosis, and AD would be a new application for these compounds. The compounds are not without side effects, but several companies are fairly far along on finding agonists that have the beneficial effects of reducing cellular cholesterol without the downside of inducing fatty liver and high plasma lipids, Tontonoz says.
In the meantime, look for more data on the effects of LXR agonists in mouse AD models in the near future. Both Tontonoz and the University of Pittsburgh’s Rada Koldamova and Iliya Lefterov report they have papers submitted with much more information on the action and mechanisms of the compounds in additional AD mouse models.—Pat McCaffrey.
Zelcer N, Khanlou N, Clare R, Jiang Q, Reed-Geaghan EG, Landreth GE, Vinters HV, Tontonoz P. Attenuation of neuroinflammation and Alzheimer's disease pathology by liver x receptors. Proc Natl Acad Sci U S A. 2007 Jun 11; [Epub ahead of print] Abstract