BCL6 encodes a transcription factor that represses genes necessary for the terminal differentiation of lymphocytes within germinal centers, and the misregulated expression of this factor is strongly implicated in several types of B cell lymphoma. The homodimeric BTB domain of BCL6 (also known as the POZ domain) is required for the repression activity of the protein and interacts directly with the SMRT and N-CoR corepressors that are found within large multiprotein histone deacetylase-containing complexes. We have identified a 17 residue fragment from SMRT that binds to the BCL6 BTB domain, and determined the crystal structure of the complex to 2.2 A. Two SMRT fragments bind symmetrically to the BCL6 BTB homodimer and, in combination with biochemical and in vivo data, the structure provides insight into the basis of transcriptional repression by this critical B cell lymphoma protein.

To examine whether fatty acid transport is abnormal in obesity, the kinetics of [ 3 H]oleate uptake by hepatocytes, cardiac myocytes, and adipocytes from adult male Wistar (؉/؉), Zucker lean (fa/؉) and fatty (fa/fa), and Zucker diabetic fatty (ZDF) rats were studied. A tissuespecific increase in oleate uptake was found in fa/fa and ZDF adipocytes, in which the V max was increased 9-fold (p < 0.005) and 13-fold (p < 0.001), respectively. This increase greatly exceeded the 2-fold increase in the surface area of adipocytes from obese animals, and did not result from trans-stimulation secondary to increased lipolysis. Adipocyte tumor necrosis factor-␣ mRNA levels, assayed by Northern hybridization, increased in the order ؉/؉ < fa/fa < ZDF. Oleate uptake was also studied in adipocytes from 20 -24-day-old male ؉/؉, fa/؉, and fa/fa weanlings. These animals were not obese, and had equivalent plasma fatty acid and glucose levels. Tumor necrosis factor-␣ mRNA levels in ؉/؉ and fa/fa cells also were similar. Nevertheless, V max was increased 2.9-fold (p < 0.005) in fa/fa compared ؉/؉ cells. These studies indicate 1) that regulation of fatty acid uptake is tissuespecific and 2) that up-regulation of adipocyte fatty acid uptake is an early event in Zucker fa/fa rats. These findings are independent of the role of any particular fatty acid transporter. Adipocyte mRNA levels of three putative transporters, mitochondrial aspartate aminotransferase, fatty acid translocase, and fatty acid transporting protein (FATP) were also determined; mitochondrial aspartate aminotransferase and FATP mRNAs correlated strongly with fatty acid uptake.

June 19, 1987) ABSTRACT A portion of the hepatocellular uptake of nonesterified long-chain fatty acids is mediated by a specific 40-kDa plasma membrane fatty acid binding protein, which has also been isolated from the gut. To investigate whether a similar transport process exists in other tissues with high transmembrane fatty acid fluxes, initial rates (Vd) of [3H]-

The hepatic plasma membrane fatty acidbinding protein (h-FABPpM) [14C]taurocholate. The inhibition of oleate uptake produced by anti-h-FABPpM can be eliminated by preincubation of the antiserum with mGOT; similarly, the plasma membrane immunofluorescence produced by either antiserum can be elimiated by preincubation with the other antigen. These data suggest that h-FABPpM and mGOT are closely related.

Uptake of V3Hjoleate by canine or rat cardiac myocytes is saturable, displays the countertransport phenomenon, and is inhibited by phloretin and trypsin. Cardiac myocytes contain a basic (pI 9.1) 40-kD plasma membrane fatty atid binding protein (FABPpM) analogous to those recently isolated from liver, adipose tissue, and gut, unrelated to the 12-14-kD cytosolic FAIRP in these same tissues. An antibody to rat liver FABPpM selectively inhibits specific uptake of [3Hfoleate by rat heart myocytes at 370C, but has no influence on nonspecific VHoleate uptake at 40C or on specific uptake of VHjglucose. Uptake of long-chain free fatty acids by cardiac muscle cells, liver, and adipose tissue and absorption by gut epithelial cells is a facilitated process mediated by identical or closely related plasma membrane FABPs.

To explore the relationship between mitochondrial aspartate aminotransferase (mAspAT; EC 2.6.1.1) and plasma membrane fatty acid-binding protein (FABPpm) and their role in cellular fatty acid uptake, 3T3 fibroblasts were cotransfected with plasmid pMAAT2, containing a fulllength mAspAT cDNA downstream of a Zn2+-inducible metal-

To reexamine the role of albumin in cellular uptake of long chain fatty acids, we measured I3HIoleate uptake by isolated hepatocytes, adipocytes, and cardiac myocytes from incubations containing oleate/albumin complexes at molar ratios from 0.01:1 to 2:1. For each ratio the uptake was studied over a wide range of albumin concentrations. In all three cell types and at any given oleate/albumin ratio, the uptake appeared saturable with increasing concentrations of oleate:albumin complexes despite the fact that the unbound oleate concentration for each molar ratio is essentially constant. However, the "K."' but not the "V,,,,C" of these pseudosaturation curves was influenced by substrate availability. At low albumin concentrations, uptake velocities did not correlate with unbound oleate concentrations. However, observed and expected uptake velocities coincided at albumin concentrations approaching physiologic levels and were a saturable function of the oleate/albumin ratios and the consequent unbound oleate concentrations employed. Hence, under the experimental conditions employed in this study using a variety of suspended cell types, oleate uptake kinetics were consistent with the conventional theory at physiologic concentrations of albumin.

Long chain fatty acid transport is selectively up-regulated in adipocytes of Zucker fatty rats, diverting fatty acids from sites of oxidation toward storage in adipose tissue. To determine whether this is a general feature of obesity, we studied [ 3 H]oleate uptake by adipocytes and hepatocytes from 1) homozygous male obese (ob), diabetic (db), fat (fat), and tubby (tub) mice and from 2) male Harlan Sprague-Dawley rats fed for 7 weeks a diet containing 55% of calories from fat. V max and K m were compared with controls of the appropriate background strain (C57BL/6J or C57BLKS) or diet (13% of calories from fat). V max for adipocyte fatty acid uptake was increased 5-6-fold in ob, db, fat, and tub mice versus controls (p < 0.001), whereas no differences were seen in the corresponding hepatocytes. Similar changes occurred in fat-fed rats. Of three membrane fatty acid transporters expressed in adipocytes, plasma membrane fatty acid-binding protein mRNA was increased 9 -11-fold in ob and db, which lack a competent leptin/leptin receptor system, but was not increased in fat and tub, i.e. in strains with normal leptin signaling capability; fatty acid translocase mRNA was increased 2.2-6.5-fold in tub, ob, and fat adipocytes, but not in db adipocytes; and only marginal changes in fatty acid transport protein 1 mRNA were found in any of the mutant strains. Adipocyte fatty acid uptake is generally increased in murine obesity models, but up-regulation of individual transporters depends on the specific pathophysiology. Leptin may normally down-regulate expression of plasma membrane fatty acid binding protein.In normal man and most mammalian species, body weight is maintained within narrow limits through regulation of both caloric intake and energy expenditure (1, 2). If caloric intake persistently exceeds energy expenditure, obesity is an inevitable consequence. However, there are obvious differences in the tendency to obesity among individuals with seemingly equivalent caloric intake and similar degrees of physical activity (3). Likewise, there are differences among rat strains in the propensity to develop obesity on high fat diets (4). Finally, a number of single-gene mutations that lead to obesity in mice and rats have been identified and cloned (5-13), leading in several instances to elucidation of the mechanisms underlying phenotypic expression. Studies in these animal models and in obese humans have led to the concepts of energy efficiency and of nutrient partitioning as being important physiological mechanisms underlying individual or strain differences in the propensity to become obese (1,14,15).Individuals with high energy efficiency require fewer calories to meet basal metabolic needs and accomplish a given level of physical work. Thus, on a given caloric intake, more calories are, in essence, left over, and are stored as fat. Individuals with low energy efficiency utilize more of their caloric intake for basal metabolism and physical work, leaving fewer calories for storage as fat. The factors responsible for dif...