Axel Ring

A New Concept of Cellular Uptake and Intracellular Trafficking of Long-Chain Fatty Acids

Fatty acids are the main structural and energy sources of the human body. Within the organism, they are presented to cells as fatty acid: albumin complexes. Dissociation from albumin represents the first step of the cellular uptake process, involving membrane proteins with high affinity for fatty acids, e.g., fatty acid translocase (FAT/CD 36) or the membrane fatty acid-binding protein (FABPpm). According to the thus created transmembrane concentration gradient, uncharged fatty acids can flip-flop from the outer leaflet across the phospholipid bilayer. At the cytosolic surface of the plasma membrane, fatty acids can associate with the cytosolic FABP (FABPc) or with caveolin-1. Caveolins are constituents of caveolae, which are proposed to serve as lipid delivery vehicles for subcellular organelles. It is not known whether protein (FABPc)- and lipid (caveolae)-mediated intracellular trafficking of fatty acids operates in conjunction, or in parallel. Channeling fatty acids to the different metabolic pathways requires activation to acyl-CoA. For this process, the family of fatty acid transport proteins (FATP 1-5/6) might be relevant because they have been shown to possess acyl-CoA synthetase activity. Their variable N-terminal signaling sequences suggest that they might be targeted to specific organelles by anchoring in the phospholipid bilayer of the different subcellular membranes. At the highly conserved cytosolic AMP-binding site of FATP, fatty acids are activated to acyl-CoA for subsequent metabolic disposition by specific organelles. Overall, fatty acid uptake represents a continuous flow involving the following: dissociation from albumin by membrane proteins with high affinity for fatty acids; passive flip-flop across the phospholipid bilayer; binding to FABPc and caveolin-1 at the cytosolic plasma membrane; and intracellular trafficking via FABPc and/or caveolae to sites of metabolic disposition. The uptake process is terminated after activation to acyl-CoA by the members of the FATP family targeted intracellularly to different organelles.

Translocation of long chain fatty acids across the plasma membrane - lipid rafts and fatty acid transport proteins

Translocation of long chain fatty acids across the plasma membrane is achieved by a concert of co-existing mechanisms. These lipids can passively diffuse, but transport can also be accelerated by certain membrane proteins as well as lipid rafts. Lipid rafts are dynamic assemblies of proteins and lipids, that float freely within the two dimensional matrix of the membrane bilayer. They are receiving increasing attention as devices that regulate membrane function in vivo and play an important role in membrane trafficking and signal transduction. In this review we will discuss how lipid rafts might be involved in the uptake process and how the candidate proteins for fatty acid uptake FAT/CD36 and the FATP proteins interact with these domains. We will also discuss the functional role of FATPs in general. To our understanding FATPs are indirectly involved in the translocation process across the plasma membrane by providing long chain fatty acid synthetase activity.

The role of dominant stenoses in bacterial infections of bile ducts in primary sclerosing cholangitis

Objective Primary sclerosing cholangitis (PSC) is characterized by progressive fibrotic inflammation and strictures of the biliary system. We studied the role of dominant stenoses in bacterial biliary infections and the effect of routine antibiotic administration with cholangiography. Design A prospective clinical trial without blinding or randomization. Setting The endoscopy unit in a university hospital. Participants Fifty patients with PSC entered and finished the study. Interventions A total of 103 endoscopic retrograde cholangiographies (ERC) was performed in 37 PSC patients with dominant stenosis and 13 controls with PSC but no dominant stenosis. After selective cannulation of the bile duct, bile samples were obtained during each procedure. All patients received systemic antibiotic treatment with ciprofloxacin for one week after ERC. Results Enteric bacteria were detected in the bile specimens of 15 out of 37 PSC patients (40.5%) with dominant stenosis but never in the absence of dominant stenosis (P=0.004). Positive cultures for enteric bacteria were associated with elevated serum C-reactive protein, high leukocyte counts in bile (P<0.05) and the deterioration of liver function assessed by increasing bilirubin levels during the follow-up period lasting a median of 7 months (P=0.06). Despite the high rate of susceptibility in vitro, ciprofloxacin treatment eradicated enteric bacteria in only two out of 12 cases. Conclusion Bacterial infection of the bile ducts with dominant stenosis is a frequent finding and may play a role in the progression of PSC. Short-course antibiotic treatment is not very effective in eradicating acteria from the bile ducts.

New concepts of cellular fatty acid uptake: role of fatty acid transport proteins and of caveolae

Efficient uptake and channelling of long-chain fatty acids (LCFA) are critical cell functions. Evidence is emerging that proteins are important mediators of LCFA-trafficking into cells and various proteins have been suggested to be involved in this process. Amongst these proteins is a family of membrane-associated proteins termed fatty acid transport proteins (FATP). So far six members of this family, designated FATP 1-6, have been characterized. FATP 1, 2 and 6 show a highly-conserved AMP-binding region that participates in the activation of very-long-chain fatty acids (VLCFA) to form their acyl-CoA derivatives. The mechanisms by which FATP mediate LCFA uptake are not well understood, but several studies provide evidence that uptake of LCFA across cellular membranes is closely linked to acyl-CoA synthetase activity. It is proposed that FATP indirectly enhance LCFA uptake by activating VLCFA to their CoA esters, which are required to maintain the typical structure of lipid rafts in cellular membranes. Recent work has shown that the structural integrity of lipid rafts is essential for cellular LCFA uptake. This effect might be exerted by proteins, e.g. caveolin-1 and FAT/CD36, that use lipid rafts as platforms and bind or transport LCFA. The proposed molecular mechanisms await further experimental investigation.

Group B streptococcal β-hemolysin induces mortality and liver injury in experimental sepsis

New Zealand White rabbits were challenged with the wild-type (wt) group B streptococci (GBS) serotype III strain (COH1) and its isogenic nonhemolytic (NH) and hyperhemolytic (HH) mutants. Mortality differed significantly between rabbits infected with the HH mutant IN40 (67%), compared with rabbits infected with the wt COH1 strain (27%) and the NH strains COH1-20 and COH1:cylEDeltacat (13% and 0%, respectively; P<.05). Histopathologically, disseminated septic microabscesses surrounded by necrotic foci were found exclusively in the livers of HH mutant IN40-infected animals. Serum transaminase levels were 20-fold higher in the HH-infected group, compared with rabbits infected with the other strains. Positive TUNEL (in situ terminal deoxynucleotide transferase-mediated dUTP nick end labeling) staining and activation of caspase-3 in hepatocytes were more frequent in HH-infected than in wt-infected animals and absent in the NH mutant COH1-20-infected group, indicating that GBS beta-hemolysin triggers apoptotic pathways in hepatocytes. This work provides the first evidence that GBS beta-hemolysin plays a crucial role in the pathophysiology of GBS sepsis by inducing liver failure and high mortality.

Synergistic Action of Nitric Oxide Release from Murine Macrophages Caused by Group B Streptococcal Cell Wall and β-Hemolysin/Cytolysin

Group B streptococcus (GBS) is the leading cause of sepsis in neonates. Nitric oxide (NO) release plays a role in the hypotension that characterizes septic shock. It has been shown that GBS beta-hemolysin/cytolysin (beta-h/c) stimulates the transcription of inducible NO synthase (iNOS) in murine macrophages via intracellular pathways similar to those that mediate lipopolysaccharide-induced iNOS activation. Here, it is demonstrated that the GBS cell wall and beta-h/c act synergistically to induce iNOS in interferon (IFN)-gamma-primed [corrected] RAW 264.7 murine macrophages. In nonprimed macrophages, combined activation by the GBS cell wall plus beta-h/c is necessary to induce an NO response, which indicates that both virulence factors cooperate to substitute for the priming signal typically provided by IFN-gamma [corrected].

Role of FATP in parenchymal cell fatty acid uptake

Long-chain fatty acids (LCFAs) represent key metabolites for energy generation and storage. Transport and metabolism of LCFA are believed to be regulated by membrane-associated proteins that bind and transport LCFA. Identifying the postulated fatty acid transporters is of considerable interest since altered fatty acid uptake has been implicated in disease such as insulin resistance and obesity. Recently, a family of membrane associated proteins, termed fatty acid transport proteins (FATPs), have been described that enhance uptake of LCFAs. Until today, six members of this family, designated FATP1-6, have been characterized. This review will focus on FATP structure, expression patterns, regulation, mechanism of transport and clinical implications.