Decreased intramolecular turnover of L-fucose in membrane glycoproteins of rat liver during liver regeneration

Endotoxin-induced inflammation down-regulates L-type amino acid transporter 1 (LAT1) expression at the blood-brain barrier of male rats and mice

Background

We recently reported that bacterial lipopolysaccharide (LPS)-induced inflammation decreases the expression of the primary thyroid hormone transporters at the blood–brain barrier, organic anion-transporting polypeptide 1c1 (OATP1c1) and monocarboxylate transporter 8 (MCT8). l-type amino acid transporters 1 and 2 (LAT1 & LAT2) are regarded as secondary thyroid hormone transporters, and are expressed in cells of the blood–brain or blood-cerebrospinal fluid barrier and by neurons. The purpose of this study was to examine the effect of LPS-induced inflammation on the expression of LAT1 and LAT2, as these may compensate for the downregulation of OATP1c1 and MCT8.

Methods

LPS (2.5 mg/kg body weight) was injected intraperitoneally to adult, male, Sprague–Dawley rats and C57Bl/6 mice, which were euthanized 2, 4, 9, 24 or 48 h later. LAT1 and LAT2 mRNA expression were studied on forebrain sections using semiquantitative radioactive in situ hybridization. LAT1 protein levels in brain vessels were studied using LAT1 immunofluorescence. Statistical comparisons were made by the non-parametric Kruskal–Wallis and Dunn’s tests.

Results

In both species, LAT1 mRNA decreased in brain blood vessels as soon as 2 h after LPS injection and was virtually undetectable at 4 h and 9 h. During recovery from endotoxemia, 48 h after LPS injection, LAT1 mRNA in brain vessels increased above control levels. A modest but significant decrease in LAT1 protein levels was detected in the brain vessels of mice at 24 h following LPS injection. LPS did not affect LAT1 and LAT2 mRNA expression in neurons and choroid plexus epithelial cells.

Conclusions

The results demonstrate that LPS-induced inflammation rapidly decreases LAT1 mRNA expression at the blood–brain barrier in a very similar manner to primary thyroid hormone transporters, while changes in LAT1 protein level follow a slower kinetics. The data raise the possibility that inflammation may similarly down-regulate other blood–brain barrier transport systems at the transcriptional level. Future studies are required to examine this possibility and the potential pathophysiological consequences of inflammation-induced changes in blood–brain barrier transport functions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12987-015-0016-8) contains supplementary material, which is available to authorized users.

Transporters on demand: intrahepatic pools of canalicular ATP binding cassette transporters in rat liver

ABC transporter trafficking in rat liver induced by cAMP or taurocholate and [(35)S]methionine metabolic labeling followed by subcellular fractionation were used to identify and characterize intrahepatic pools of ABC transporters. ABC transporter trafficking induced by cAMP or taurocholate is a physiologic response to a temporal demand for increased bile secretion. Administration of cAMP or taurocholate to rats increased amounts of SPGP, MDR1, and MDR2 in the bile canalicular membrane by 3-fold; these effects abated after 6 h and were insensitive to prior treatment of rats with cycloheximide. Half-lives of ABC transporters were 5 days, which suggests cycling of ABC transporters between canalicular membrane and intrahepatic sites before degradation. In vivo [(35)S]methionine labeling of rats followed by immunoprecipitation of (sister of P-glycoprotein) (SPGP) from subcellular liver fractions revealed a steady state distribution after 20 h of SPGP between canalicular membrane and a combined endosomal fraction. After mobilization of transporters from intrahepatic sites with cAMP or taurocholate, a significant increase in the amount of ABC transporters in canalicular membrane vesicles was observed, whereas the decrease in the combined endosomal fraction remained below detection limits in Western blots. This observation is in accordance with relatively large intracellular ABC transporter pools compared with the amount present in the bile canalicular membrane. Furthermore, trafficking of newly synthesized SPGP through intrahepatic sites was accelerated by additional administration of cAMP but not by taurocholate, indicating two distinct intrahepatic pools. Our data indicate that ABC transporters cycle between the bile canaliculus and at least two large intrahepatic ABC transporter pools, one of which is mobilized to the canalicular membrane by cAMP and the other, by taurocholate. In parallel to regulation of other membrane transporters, we propose that the "cAMP-pool" in hepatocytes corresponds to a recycling endosome, whereas recruitment from the "taurocholate-pool" involves a hepatocyte-specific mechanism.

Oligosaccharide reprocessing and recycling of a cell surface glycoprotein in cultured rat hepatocytes

The metabolism of the cell surface glycoprotein dipeptidyl peptidase IV(DPPIV) was studied in cultured rat hepatocytes. In pulse-chase labelling experiments using L-[35S]methionine a 100-kDa high-mannose precursor polypeptide is converted into the mature complex-type 110-kDa glycoprotein. Digestion with exo- and endoglycosidases and metabolic labelling with radioactive sugars demonstrate that the 110-kDa form contains about 6 complex-type oligosaccharides which are fucosylated and sialylated. About 25 min after the beginning of the pulse-labelled glycoprotein appears in the sinusoidal membrane. Physiologically only the 110-kDa form is found in the cell surface. If cell surface DPP IV was desialylated by sialidase at 4 degrees C, it is resialylated during incubation at 37 degrees C. This oligosaccharide reprocessing indicates that the surface glycoprotein has been recycled to the cell compartment containing terminal glycosyltransferases (presumably the trans Golgi system). Two different methods demonstrate internalization of cell surface DPP IV: 1) The complex cell surface DPPIV -anti-DPP IV-antibody -L-[35S]methionine-labelled secondary goat-anti-mouse-antibody formed at 4 degrees C becomes less accessible to trypsin during incubation at 37 degrees C. 2) Part of the complex plasma membrane DPP IV-anti-DPP IV-antibody formed in the cold cannot be recognized by the radioactive secondary antibody after rewarming. Internalization is not blocked by inhibition of protein synthesis with cycloheximide. During internalization of plasma membrane DPP IV its concentration in the membrane remains constant.(ABSTRACT TRUNCATED AT 250 WORDS)

Content, pattern and metabolic processing of rat-liver gangliosides during liver regeneration

During rat liver regeneration, the ganglioside content and distribution undergo significant changes after partial hepatectomy; total liver gangliosides increase remarkably till the 4th day after surgery, thereafter progressively decreasing to reach the values of sham-operated controls at the 12th day. The qualitative pattern is characterized by the 95% relative increase of GD1a at the 4th day and the 40% relative decrease of GD1b. In order to investigate the processes of ganglioside penetration into cells, degradation and biosynthesis, radiolabelled GM1 ([Sph-3H] GM1) was administered. One day after hepatectomy the liver uptake and metabolism of exogenous ganglioside were significantly reduced. Three days post-surgery these parameters were restored to control values; however an increased radioactivity incorporation was found in GD1a, thus suggesting an enhancement of its biosynthesis around the 4th day. The data reported here suggest that in the first two days after partial hepatectomy, the ganglioside degradation is reduced with a consequent increase of ganglioside content; later on the catabolic routes normalize and some biosynthetic processes leading to GD1a are enhanced. GD1a seems to be a marker of a peculiar transition phase of liver regeneration.