Induction of Human Liver X Receptor α Gene Expression Via an Autoregulatory Loop Mechanism

Abstract

The liver X receptors (LXRs), members of the nuclear receptor superfamily, play an important role in controlling lipid homeostasis by activating several genes involved in reverse cholesterol transport. These include members of the ATP binding cassette (ABC) superfamily of transporter proteins ABCA1 and ABCG1, surface constituents of plasma lipoproteins like apolipoprotein E, and cholesterol ester transport protein. They also play an important role in fatty acid metabolism by activating the sterol regulatory element-binding protein 1c gene. Here, we identify human LXRalpha (hLXRalpha) as an autoinducible gene. Induction in response to LXR ligands is observed in multiple human cell types including macrophages and occurs within 2--4 h. Analysis of the hLXRalpha promoter revealed three LXR response elements (LXREs); one exhibits strong affinity for both LXRalpha:RXR and LXRbeta:RXR (a type I LXRE), and deletion and mutational studies indicate it plays a critical role in LXR-mediated induction. The other two LXREs are identical to each other, exist within highly conserved Alu repeats, and exhibit selective binding to LXRalpha:RXR (type II LXREs). In transfections, the type I LXRE acts as a strong mediator of both LXRalpha and LXRbeta activity, whereas the type II LXRE acts as a weaker and selective mediator of LXRalpha activity. Our data suggest a model in which LXR ligands trigger an autoregulatory loop leading to selective induction of hLXRalpha gene expression. This would lead to increased hLXRalpha levels and transcription of its downstream target genes such as ABCA1, providing a simple yet exquisite mechanism for cells to respond to LXR ligands and cholesterol loading.

Orphan Nuclear Receptor DAX-1 Acts as a Novel Corepressor of Liver X Receptor α and Inhibits Hepatic Lipogenesis

“…This direct interaction was increased in the presence of the LXR agonist, which is also consistent with the previous study that the interaction between SHP and LXR␣ was stronger in the presence of LXR agonist (28). This increase in interaction might be due to the increased expression and stability of the LXR␣ protein in the presence of agonist (35,36). Consistent with these reports, we have also observed an increase in LXR␣ protein level in the input panel after the treatment of synthetic agonist.…”

“…Conversely, when we performed reciprocal mapping experiments using deletion constructs of DAX-1 (Fig. 3B, upper panel), all 35 S-labeled translated DAX-1 proteins interacted with MBP-LXR␣ (Fig. 3B, lower panel), indicating that the entire DAX-1 protein is involved in the interaction with LXR␣.…”

“…MBP alone, MBP-DAX-1, and MBP-SHP were bacterially expressed and incubated with translated in vitro 35 Slabeled LXR␣ WT or LXR␣⌬AF2 proteins. LXR␣ was found to interact with MBP-DAX-1 but not with MBP alone.…”

“…Furthermore, we performed in vitro MBP competition assay to confirm competition between DAX-1 and SRC-1 for binding to LXR␣ in vitro. Increasing amounts of unlabeled full-length SRC-1 competed with 35 S-labeled DAX-1 for the binding with MBP-LXR␣. However, the weak interaction between 35 S-labeled DAX-1 and MBP-LXR␣ CT was not altered by the incubation of unlabeled SRC-1 (Fig.…”

Crosstalk of thyroid hormone receptor and liver X receptor in lipid metabolism and beyond [Review]

“…Human LXR-α autoregulates its own gene promoter through binding to the LXREs [62][63][64]. Thyroid hormone up-regulates mRNA expression of mouse LXR-α, but not LXR-β, in the liver, but cholesterol administration does not affect the LXR-α mRNA levels [65].…”