Analysis of Liver X Receptor target gene expression across species

Liver X Receptors (LXRs) are nuclear hormone receptors that regulate key genes involved in cholesterol and lipid metabolism. As transcription factors, LXRs turn on the gene expression of ATP-binding cassette transporters (ABCs) which mediate cholesterol efflux from cells, such as macrophage foam cells. In addition, LXRs have the ability to down regulate pro-inflammatory genes. Therefore, LXRs have been extensively investigated as potential therapeutic targets for the treatment of conditions that result from altered cholesterol and lipid homeostasis as well as increased inflammation, such as atherosclerosis and Alzheimer's disease (AD). This latter is a neurodegenerative disorder that is associated with the deposition of brain amyloid plaques, constituted by insoluble A[beta] peptides. LXR activation has been shown to promote A[beta] clearance from the brain via the ABCA1-apolipoprotein E pathway and improve cognitive functions in rodent models of AD. The ability of LXRs to promote reverse cholesterol transport (RCT) and suppress inflammation has been characterized in both human and murine in vitro systems, but mostly in rodent in vivo systems. Although the LXR signaling pathway is mostly conserved across species, LXRs can also regulate their target genes in a species-, tissue- and isoform-specific fashion. Therefore the purpose of this work is to investigate the regulation of target genes by LXRs across species and identify, if any, differences that could aid us in understanding the role of LXR modulation in higher species, such as non-human primates. In the context of inflammation, the LXR genome landscape had only been investigated in murine macrophages. Therefore, we performed a genome-wide screen in human THP-1 macrophages. This led us to the identification of a novel LXR target gene, Sphingomyelin Phosphodiesterase Acid-Like 3A Gene (SMPDL3A), which is regulated in a species- and tissue-specific fashion, being restricted to human blood cells, with no induction by LXRs in mouse cellular systems. Next, we confirmed the LXR-mediated upregulation of ABCA1 and ApoE genes in Cynomolgus monkey brains, as this had never been investigated in higher species. In addition, we also characterized the LXR transcriptome in Cynomolgus brain by RNA-sequencing in order to identify potential novel LXR target genes. For the first time in higher species, we show Apolipoprotein AI upregulation in the brain of Cynomolgus monkey upon treatment with a synthetic LXR modulator.