Peroxisomal bifunctional enzyme binds and activates the activation function-1 region of the peroxisome proliferator-activated receptor alpha

The transcriptional activity of peroxisome proliferator-activated receptors (PPARs), and of nuclear hormone receptors in general, is subject to modula tion by cofactors, However, most currently known co-activating proteins int eract in a ligand-dependent manner with the C-terminal ligand-regulated act ivation function (AF)-2 domain of nuclear receptors, Since PPAR alpha exhib its a strong constitutive transactivating function contained within an N-te rminal AF-1 region, it can be speculated that a different set of cofactors might interact with this region of PPARs, An affinity purification approach was used to identify the peroxisomal enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase (bifunctional enzyme, BFE) as a protein which strongly and s pecifically interacted with the N-terminal 92 amino acids of PPAR alpha. Pr otein-protein interaction assays with the cloned BFE confirmed this interac tion, which could be mapped to amino acids 307-514 of the BFE and the N-ter minal 70 amino acids of PPAR alpha. Moreover, transient transfection experi ments in hepatoma cells revealed a 2.2-fold increase in the basal and ligan d-stimulated transcriptional activity of PPARa in the presence of BFE. This stimulatory effect is preferentially observed for the PPAR alpha isoform a nd it is significantly stronger (4.8-fold) in nonhepatic cells, which presu mably express lower levels of endogenous BFE. Hence, the BFE represents the first known cofactor capable of activating the AF-1 domain of PPAR without requiring additional regions of this receptor. These data are compatible w ith a model whereby the PPAR-regulated BFE is able to modulate its own expr ession through an enhancement of the activity of PPAR alpha: representing a novel peroxisomal-nuclear feed-forward regulatory loop.