Ligand-Regulated Heterodimerization of Peroxisome Proliferator-Activated Receptor α with Liver X Receptor α

Peroxisome proliferator-activated receptor alpha (PPAR alpha) and liver X receptor alpha (LXR alpha) are members of the nuclear receptor superfamily that function to regulate lipid metabolism. Complex interactions between the LXR alpha and PPAR alpha pathways exist, including competition for the same heterodimeric partner, retinoid X receptor alpha (RXR alpha). Although data have suggested that PPAR alpha and LXR alpha may interact directly, the role of endogenous ligands in such interactions has not been investigated. Using in vitro protein-protein binding assays, circular dichroism, and co-immunoprecipitation of endogenous proteins, we established that full-length human PPAR alpha and LXR alpha interact with high affinity, resulting in altered protein conformations. We demonstrated for the first time that the affinity of this interaction and the resulting conformational changes could be altered by endogenous PPAR alpha ligands, namely long chain fatty acids (LCFA) or their coenzyme A thioesters. This heterodimer pair was capable of binding to PPAR alpha and LXR alpha response elements (PPRE and LXRE, respectively), albeit with an affinity lower than that of the respective heterodimers formed with RXR alpha. LCFA had little effect on binding to the PPRE but suppressed binding to the LXRE. Ectopic expression of PPAR alpha and LXR alpha in mammalian cells yielded an increased level of PPRE transactivation compared to overexpression of PPAR alpha alone and was largely unaffected by LCFA. Overexpression of both receptors also resulted in transactivation from an LXRE, with decreased levels compared to that of LXR alpha overexpression alone, and LCFA suppressed transactivation from the LXRE. These data are consistent with the hypothesis that ligand binding regulates heterodimer choice and downstream gene regulation by these nuclear receptors.