A cDNA for an adipocyte membrane protein, implicated in the transport of long-chain fatty acids, was isolated by screening with a synthetic oligonucleotide derived from the amino terminal sequence of the protein. The 88-kDa adipocyte membrane protein was previously identified by covalent labeling with N-sulfosuccinimidyl esters of long-chain fatty acids which irreversibly inhibited fatty acid transport by 75% (Harmon, C. M., and Abumrad, N.A. (1993) J. Membr. Biol. 124, 261-268). The cDNA (FAT, 2432 base pairs (bp)) contained 70 bp of 5'-untranslated sequence, an open reading frame encoding a 472-amino acid protein with a predicted molecular mass of 52466, and 940 bp of 3'-untranslated sequence with two polyadenylation signal sequences but with no polyadenylation tail. The deduced protein sequence predicted two transmembrane segments and 10 potential N-linked glycosylation sites. Extensive glycosylation most likely explains why the molecular mass of the isolated protein (88 kDa) is different from that deduced from the cDNA sequence (53 kDa). The sequence of FAT is 85% homologous with that of glycoprotein IV (CD36) identified in human platelets and in lactating mammary epithelium. Consistent with this, a polyclonal antibody against CD36 reacted with adipocyte plasma membranes and detected a single band at 88 kDa. Northern blot analysis of RNA obtained from rat adipose tissue and probed with the cDNA identified two major transcripts of 4.8 and 2.9 kilobases which were abundant in heart, intestine, fat, muscle, and testis. The mRNAs were not detectable in cultured adipose cell lines (Ob1771, 3T3F442A) at the fibroblastic stage but were strongly induced during the differentiation process and by treatment of preadipocytes with dexamethasone, conditions that were also associated with an increase in oleate transport. In contrast, the fibroblastic cell lines 3T3-C2 and L929, which do not differentiate, did not express the mRNAs at all stages of culture. The data suggest that FAT and CD36 belong to a family of proteins that bind/transport long-chain fatty acids or function as regulators of these processes.

The ability of mammals to resist body fat accumulation is linked to their ability to expand the number and activity of “brown adipocytes” within white fat depots. Activation of β-adrenergic receptors (β-ARs) can induce a functional “brown-like” adipocyte phenotype. As cardiac natriuretic peptides (NPs) and β-AR agonists are similarly potent at stimulating lipolysis in human adipocytes, we investigated whether NPs could induce human and mouse adipocytes to acquire brown adipocyte features, including a capacity for thermogenic energy expenditure mediated by uncoupling protein 1 (UCP1). In human adipocytes, atrial NP (ANP) and ventricular NP (BNP) activated PPARγ coactivator-1α (PGC-1α) and UCP1 expression, induced mitochondriogenesis, and increased uncoupled and total respiration. At low concentrations, ANP and β-AR agonists additively enhanced expression of brown fat and mitochondrial markers in a p38 MAPK–dependent manner. Mice exposed to cold temperatures had increased levels of circulating NPs as well as higher expression of NP signaling receptor and lower expression of the NP clearance receptor (Nprc) in brown adipose tissue (BAT) and white adipose tissue (WAT). NPR-C^–/– mice had markedly smaller WAT and BAT depots but higher expression of thermogenic genes such as Ucp1. Infusion of BNP into mice robustly increased Ucp1 and Pgc-1α expression in WAT and BAT, with corresponding elevation of respiration and energy expenditure. These results suggest that NPs promote “browning” of white adipocytes to increase energy expenditure, defining the heart as a central regulator of adipose tissue biology.

Exposure of preadipocytes to long chain fatty acids induces expression of several gene markers of adipocyte differentiation. This report describes the cloning, from a preadipocyte library, of a cDNA encoding a fatty acid-activated receptor, FAAR. The cDNA had the characteristics and ligand-binding domains of nuclear hormone receptors and encoded a 440 amino acid protein related to peroxisome proliferator-activated receptors, PPAR. The deduced protein sequence was 88% homologous to that of hNUC I, isolated from human osteosarcoma cells. FAAR mRNA was abundant in adipose tissue, intestine, brain, heart, and skeletal muscles and less abundant in kidney, liver, testis, and spleen. The mRNA was undetectable in growing Ob1771 and 3T3-F442A preadipocytes, was strongly induced early during differentiation, and was increased by fatty acid. Transcription assays using hybrid receptor showed strong stimulation by fatty acid and weaker induction by fibrates. Transfection of 3T3-C2 fibroblasts, with a FAAR expression vector, conferred fatty acid inducibility of the adipocyte lipid-binding protein and the fatty acid transporter. Transcriptional induction of these genes exhibited inducer specificity identical to that described in preadipocytes. In summary, the data indicate that FAAR is likely a mediator of fatty acid transcriptional effects in preadipocytes.