Evidence That Peroxisome Proliferator-activated Receptor α Is Complexed with the 90-kDa Heat Shock Protein and the Hepatitis Virus B X-associated Protein 2

Abstract

The peroxisome proliferator-activated receptor ␣ (PPAR␣) is a ligand-inducible transcription factor, which belongs to the nuclear receptor superfamily. PPAR␣ mediates the carcinogenic effects of peroxisome proliferators in rodents. In humans, PPAR␣ plays a fundamental role in regulating energy homeostasis via control of lipid metabolism. To study the possible role of chaperone proteins in the regulation of PPAR␣ activity, a monoclonal antibody (mAb) was made against PPAR␣ and designated as 3B6/PPAR. The specificity of mAb 3B6/PPAR in recognizing PPAR␣ was tested in immunoprecipitations using in vitro translated PPAR subtypes. The mAb 3B6/PPAR recognized PPAR␣, failed to bind to PPAR␤ or PPAR␥, and is efficient in both immunoprecipitating and visualizing the receptor on protein blots. The immunoprecipitation of PPAR␣ in mouse liver cytosol using mAb 3B6/PPAR has resulted in the detection of two co-immunoprecipitated proteins, which are heat shock protein 90 (hsp90) and the hepatitis B virus X-associated protein 2 (XAP2). The concomitant depletion of PPAR␣ in hsp90-depleted mouse liver cytosol was also detected. Complex formation between XAP2 and PPAR␣/FLAG was also demonstrated in an in vitro translation binding assay. hsp90 interacts with PPAR␣ in a mammalian two-hybrid assay and binds to the E/F domain. Transient expression of XAP2 co-expressed with PPAR␣ resulted in down-regulation of a peroxisome proliferator response element-driven reporter gene activity. Taken together, these results indicate that PPAR␣ is in a complex with hsp90 and XAP2, and XAP2 appears to function as a repressor. This is the first demonstration that PPAR␣ is stably associated with other proteins in tissue extracts and the first nuclear receptor shown to functionally interact with XAP2.The ability of peroxisome proliferators to activate a receptor in the steroid receptor superfamily was first discovered in 1990, and the cognate protein was designated as PPAR 1 (1). ThePPARs are soluble transcription factors that are activated by a diverse class of lipophilic compounds (2). With the activation of PPAR, a concomitant induction of a number of genes that code for peroxisomal fatty acid metabolizing enzymes was observed in mouse liver. Among these, the peroxisomal enzyme AOx is the most broadly used indicator of peroxisome proliferator action. Transcription of the AOx gene is increased by exposure to the hypolipidemic peroxisome proliferator WY-14,643, and this effect is mediated by a PPRE located 570 base pairs upstream of the transcriptional start site (3). This PPRE contains a direct repeat of the sequence motifs TGACCT and TGTCCT, which is separated by a single nucleotide. A heterodimer of PPAR and RXR binds to PPREs located in upstream regulatory regions of various target genes and the RXR ligand, 9-cis-retinoic acid, increases PPAR/RXR transcriptional activity (4). There are three PPAR subtypes, designated as PPAR␣ (NR1C1), -␤ (NR1C2), and -␥ (NR1C3); and each subtype is capable of binding to DNA after heterodimerizing with RXR (NR2B1) ...

Down Regulation of Hepatic PPAR.ALPHA. Function by AhR Ligand

“…The difference in the mode of regulation of CYP4A2 from that of CYP4A1 and CYP4A3 is well documented. CYP4A2 was reported to be male specific in the liver and kidneys [41]. There is a selective decrease in mRNA expression of CYP4A2 but not of either CYP4A1 or CYP4A3 in vitamin A-deficient rat's liver [27].…”

Peroxisome Proliferator-Activated Receptor-α Activation Inhibits Langerhans Cell Function

“…Data were analyzed using a paired t test. know whether this occurs in EC, in hepatocytes it was shown that PPAR␣ associates with heat shock protein 90 and the cochaperone, hepatitis B virus X-associated protein 2 (XAP2) (34,35). Moreover, PPAR phosphorylation within the AF1 region of domain A/B via cytosolic MAPK has been reported to occur in response to environmental changes and extracellular signals (36).…”

Mutation analysis of aryl hydrocarbon receptor interacting protein (AIP) gene in colorectal, breast, and prostate cancers

“…Aryl hydrocarbon receptor interacting protein modulates the stability and subcellular localisation of AHR by preventing its dynamic nucleocytoplasmic shuttling (Pollenz and Dougherty, 2005). Aryl hydrocarbon receptor interacting protein has also been reported to interact with phospho diesterase 4A5 (PDE4A5) (Bolger et al, 2003) and peroxisome proliferatoractivated receptor-a (PPAR-a) (Sumanasekera et al, 2003).…”