BESSESEN, DANIEL H, CONNIE L RUPP AND ROBERT H ECKEL. Dietary fat is shunted away from oxidation, toward storage in obese zucker rats.

BESSESEN, DANIEL H, CONNIE L RUPP AND ROBERT H ECKEL. Trafficking of dietary fat in lean rats. Obes Res. 1995;3:191-203. Despite increasing interest in the role that fuel partitioning plays in determining body composition, the relative importance of oxidative versus storage pathways in the clearance of dietary fat remains unclear. A widely held view is that the primary destination of chylomicron triglyceride fatty acids (TGFA) is adipose tissue, and the primary source of lipid fuel for skeletal muscle is non-esterified fatty acids (NEFA).An alternate view is that muscle, not adipose tissue, is the primary site of TGFA clearance. This view is supported by estimates of the total lipoprotein lipase content of muscle and adipose tissue. To directly study the partitioning of dietary fat between oxidation and storage, 14C-labeled oleic acid was fed to Sprague Dawley rats and its metabolic fate followed over 30 days. Two hours after ingestion, more than 3.5 times as much label was found in skeletal muscle tissue (2.42 f 0.45 nmols) and C 0 2 (0.25 f 0.01 nmols) than was found in adipose tissue (0.71 f 0.14 nmols). Intramuscular triglyceride was the lipid class most extensively labeled. After skeletal muscle, liver was the next most important site of TGFA clearance. Surprisingly a substantial quantity of label remained associated with the GI tract even 24 hours after ingestion. Between 2 and 10 days following ingestion there was a net decline in the 14C content of muscle, liver and GI tract, associated with a net rise in the 14C content of adipose tissue. These findings demonstrate: 1) the importance of skeletal muscle and liver in whole organism TGFA clearance, 2) the importance of intramuscular partitioning of lipid fuels between direct oxidation and storage as TG, 3) the potentially important role of the GI tract in the delivery of dietary fat to the circulation 10-24 hours following ingestion, and 4) the stability of adipose tissue as a storage site. The complex nature of the tissue-specific clearance of TGFA over time is perhaps better described by the term "trafficking' than by the more commonly used term "partitioning." Future studies of TGFA clearance combined with sampling of relevant tissues over time will provide insight into the specific roles that abnormalities in liver, muscle and adipose tissue TGFA metabolism play in the development of hypertriglyceridemic disorders and states of increased or reduced body weight.

Thyrotropin (TSH) beta is a subunit of TSH, the expression of which is limited to the thyrotrope cells of the anterior pituitary gland. We have utilized the thyrotrope-derived TtT-97 thyrotropic tumors to investigate tissue-specific expression of the TSH beta promoter. TSH beta promoter activity in thyrotropes is conferred by sequences between -270 and -80 of the 5'-flanking region. We have recently reported that the proximal region from -133 to -100 (P1) is required for promoter expression in thyrotropes. This region interacts with the pituitary-specific transcription factor Pit-1. While Pit-1 appears necessary for TSH beta promoter activity in thyrotropes, this transcription factor is not alone sufficient for promoter activity in pituitary-derived cells. In this report, we have generated a series of promoter mutations in the P1 region to identify additional protein-DNA interactions and determine their functional significance. We have found that Pit-1 interacts with the distal portion of the P1 region, and a second protein interacts with the proximal segment of this region. Each protein is able to independently interact with the TSH beta promoter, but neither alone can maintain promoter activity. Both proteins appear to be necessary for full promoter activity in thyrotropes. Southwestern analysis with the proximal segment of the P1 region (-117 to -88) reveals interaction with a 50-kDa protein. Interestingly, this protein is not found in the pituitary-derived GH3 cells and may represent a thyrotrope-specific transcription factor. Further characterization of this newly identified DNA-binding protein will further our understanding of the tissue-specific expression of the TSH beta gene.