The enterohepatic circulation of bile acids is maintained by Na+-dependent transport mechanisms. To better understand these processes, a full-length human ileal Na+-bile acid cotransporter cDNA was identified using rapid amplification of cDNA ends and genomic cloning techniques. Using Northern blot analysis to determine its tissue expression, we readily detected the ileal Na+-bile acid cotransporter mRNA in terminal ileum and kidney. Direct cloning and mapping of the transcriptional start sites confirmed that the kidney cDNA was identical to the ileal Na+-bile acid cotransporter. In transiently transfected COS cells, ileal Na+-bile acid cotransporter-mediated taurocholate uptake was strictly Na+ dependent and chloride independent. Analysis of the substrate specificity in transfected COS or CHO cells showed that both conjugated and unconjugated bile acids are efficiently transported. When the inhibition constants for other potential substrates such as estrone-3-sulfate were determined, the ileal Na+-bile acid cotransporter exhibited a narrower substrate specificity than the related liver Na+-bile acid cotransporter. Whereas the multispecific liver Na+-bile acid cotransporter may participate in hepatic clearance of organic anion metabolites and xenobiotics, the ileal and renal Na+-bile acid cotransporter retains a narrow specificity for reclamation of bile acids.

Perilipin A is the most abundant lipid droplet-associated protein in adipocytes and serves important functions in regulating triacylglycerol levels by reducing rates of basal lipolysis and facilitating hormonally stimulated lipolysis. We have previously shown that the central region of perilipin A targets and anchors it to lipid droplets, at least in part via three moderately hydrophobic sequences that embed the protein into the hydrophobic core of the droplet. The current study examines the roles of the amino and carboxyl termini of perilipin A in facilitating triacylglycerol storage. Amino-and carboxyl-terminal truncation mutations of mouse perilipin A were stably expressed in 3T3-L1 preadipocytes, which lack perilipins. Triacylglycerol content of the cells was quantified as a measure of perilipin function and was compared with that of cells expressing full-length perilipin A or control cells lacking perilipins. The aminoterminal sequence between amino acids 122 and 222, including four 10 -11-amino acid sequences predicted to form amphipathic ␤-strands and a consensus site for cAMP-dependent protein kinase, and the carboxyl terminus of 112 amino acids that is unique to perilipin A were critical to facilitate triacylglycerol storage. The precocious expression of full-length perilipin A in 3T3-L1 preadipocytes aided more rapid storage of triacylglycerol during adipose differentiation. By contrast, the expression of highly truncated amino-or carboxylterminal mutations of perilipin failed to serve a dominant negative function in lowering triacylglycerol storage during adipose differentiation. We conclude that the amino and carboxyl termini are critical to the function of perilipin A in facilitating triacylglycerol storage.Triacylglycerols packaged within the lipid droplets of adipocytes provide the most abundant form of stored energy in the body. Lipid droplets are spherical structures composed of a core of neutral lipids coated by a phospholipid monolayer into which specific proteins are embedded. The study of lipid dropletassociated proteins is an emerging field of inquiry. Only a few of the component proteins have been identified, and little is known about the structural properties that define lipid dropletassociated proteins and facilitate their functions in lipid metabolism. The aim of the current study is to begin to define functional domains of perilipins, the first identified mammalian lipid droplet-associated proteins.Perilipins coating the surfaces of adipocyte lipid droplets serve as gatekeeper proteins controlling both the storage and release of triacylglycerol. The translation of alternatively spliced mRNAs produces three protein isoforms of perilipin: perilipins A, B, and C (1, 2). Perilipin A is the most abundant protein located at the surfaces of lipid droplets in adipocytes and is found at lower relative abundance on lipid droplets in steroidogenic cells of the adrenal cortex, testes, and ovaries (3); perilipin B is found at lower levels in both types of cells, and perilipin C is selectively expre...

The enterohepatic circulation of bile acids is maintained by a series of membrane transport proteins. Recent studies of the cloned sodium bile acid cotransporters have provided new insights into their tissue expression, regulation, and their relationship to cholesterol homeostasis and human diseases such as primary bile acid malabsorption.