Peroxisome proliferators such as clofibric acid, nafenopin, and WY-14,643 have been shown to activate PPAR (peroxisome proliferator-activated receptor), a member of the steroid nuclear receptor superfamily. We have cloned the cDNA from the rat that is homologous to that from the mouse [Issemann, I. & Green, S. (1990) Unsaturated fatty acids induce peroxisomal proliferation and lower blood triglyceride levels (1-3). Similar effects are evoked by a number of man-made compounds which are either considered for therapy of hyperlipidemia-e.g., clofibric acid, nafenopin, WY-14,643, or sulfur-substituted fatty acids-or are in use as industrial plasticizers (4-6). The steroid dehydroepiandrosterone (DHEA) also induces peroxisomal proliferation but increases blood triglyceride and cholesterol levels (7,8). Two main hypotheses have been developed to explain the complex response of peroxisomal proliferation to this wide variety of inducers. According to one theory the intracellular accumulation of fatty acids is the key stimulus for triggering peroxisomal proliferation (4, 9). The other theory postulates the involvement of a receptor protein (4, 10) and an as-yet-unknown intracellular messenger-e.g., the ligand for this receptor.The latter idea gained substantial support from the discovery of mouse peroxisome proliferator-activated receptor (mPPAR), a member of the steroid nuclear receptor superfamily (11,12). The gene encoding PPAR belongs to a number of genes cloned in the last few years by means of their homology with steroid receptors. In general, ligands or physiologically occurring activators have been identified for only a few of these so-called orphan receptors (13-15). In the case of mPPAR, transactivation studies using chimeric proteins composed of the putative ligand-binding domain of the novel receptor and DNA-binding domains of known steroid receptors showed that mPPAR could be activated by peroxisome proliferators (11). However, the identity ofthe ultimate ligand of the receptor protein, the nature of physiological activators, and how the receptor might relate to the concept of fatty acids as inducers of peroxisomal proliferation remain unclear.We now describe the cloning from rat liver of a gene homologous to that encoding mPPAR.

The rat peroxisome-proliferator-activated receptor (PPAR) was expressed in insect cells and was shown to bind to a cognate PPAR response element (PPRE) from the acyl-CoA oxidase gene. Upon purification, PPAR was no longer able to bind DNA, although binding could be restored by addition of insect cell extracts. We investigated whether the retinoid X receptor (RXR) could supplement for this accessory activity. The rat RXRa cDNA was cloned and it was found that addition of in vitro-translated RXRa to purified PPAR facilitated binding of PPAR to a PPRE. Furthermore, an additional activity, which appeared to be distinct from rRXRa, was found in COS cell nuclear extracts that enabled binding of PPAR to a PPRE. Transient expression ofRXRa in CHO cells was found to be essential for the response of a chloramphenicol acetyltransferase reporter construct containing PPREs to activators of PPAR. These results raise the possibility of convergence of the PPAR and retinoid-dependent signaling pathways on promoters containing PPRE-like responsive elements.

The human major histocompatibility complex contains the genes for at least three different types of class II antigens, DR, DC and SB (DR, DQ and DP). They are all composed of an alpha and a beta chain. We have cloned a chromosomal region of 70 kb containing the SB (DP) gene family in overlapping cosmid clones. This segment contains two alpha genes and two beta genes, located in the order SB alpha 1, SB beta 1, SB alpha 2 and SB beta 2. The orientation of the alpha genes is reversed compared with that of the beta genes. This organisation suggests that the SB region has arisen by duplication of a chromosomal segment encompassing one alpha and one beta gene. Partial nucleotide sequences of the SB alpha 1 and SB beta 1 exons demonstrate that the genes correspond to SB alpha and beta cDNA clones. Consequently these genes are expressed. In contrast nucleotide sequence determination of the SB alpha 2 gene shows that it is a pseudogene.

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The peroxisome proliferator activated receptor is a member of the steroid receptor gene superfamily, sharing amino acid sequence homology with other receptors and also showing similarities at the level of gene structure. This receptor is activated both by xenobiotic compounds that induce peroxisome proliferation and by fatty acids at physiological concentrations. Upon activation the receptor mediates transcription of responsive genes through binding to peroxisome proliferator response elements. These genes include those encoding peroxisomal enzymes and members of the cytochrome P450 family of drug metabolizing enzymes. It is therefore possible that the peroxisome proliferator activated receptor may play a crucial role in regulating lipid homeostasis.