A lipoprotein secreted by cultured hepatocytes of silver or yellow eel: comparison with their plasma lipoproteins

Development and partial characterisation of an antiserum against apolipoprotein B of the short-finned eel, Anguilla australis

Despite its key role in transportation of triacylglycerides in blood, the distribution, localisation and molecular weight variants of apolipoprotein B (Apob) in teleost fish have essentially escaped study. To address this, a specific short-finned eel (Anguilla australis) Apob antiserum was produced by an immunised rabbit, purified and partially characterised. Localisation of Apob at both the mRNA (in situ hybridisation) and protein (immunohistochemistry) levels mirrored that of mammals; thus immunostaining was confined to the interstitial spaces of the liver and the vascular core of the intestinal villi. Immunostaining of proteins by Western blotting, followed by high-resolution LC-MS, indicated that peptide sequence coverage of Apob in low-density lipoproteins spanned the full-length protein. We conclude that only full-length Apob is produced by eels and that both liver and intestine are key sites for its synthesis.

Participation of high-density lipoprotein in vitellogenesis in Japanese eel hepatocytes

To investigate effect of estradiol-17beta (E(2)) treatment in vivo on binding of eel hepatocytes to HDL, we developed hepatocytes binding assay. When hepatocytes were incubated with 200 times excess of eel HDL, the binding of hepatocytes to HDL precoated on wells was inhibited competitively. This indicates that eel hepatocytes bound specifically to HDL. E(2) treatment in vivo induced vitellogenin (VTG) synthesis. Hepatocytes prepared from the same E(2) treatment eel showed 3-fold higher ability of binding to HDL compared to hepatocytes prepared from ells without E(2) treatment. We also examined effects of E(2) and HDL on VTG induction in cultured hepatocytes. VTG, determined by enzyme-linked immunosorbent assay (ELISA), induction was about two-times higher in the presence of both 10(-5) M of E(2) and 400 microg of HDL than in the presence of 10(-5) M E(2) alone. At concentrations below 10(-6) M E(2), VTG was not induced in the presence or absence of HDL. By SDS-PAGE and immunoblotting, VTG was detected only in the presence of both 10(-5) M of E(2) and HDL. Our findings strongly support the idea that HDL correlates with vitellogenesis in eel liver.

High density lipoprotein binding protein of eel (Anguilla japonica) liver with specificity of binding to apoAI as a ligand

High-density lipoprotein (HDL) binding protein (HBP) was isolated from the microsomal fraction of eel liver homogenate by affinity chromatography with a HDL-column. After SDS-PAGE and blotting, HBP on the PVDF membrane was detected by FITC-labeled HDL and apolipoprotein AI (apoAI) as a ligand. HBP in the microsomal fraction was most abundant among microsomal, mitochondrial and cytosolic fractions. The HBP isolated by a HDL-column consisted of at least three proteins with low molecular weights of 18.5, 14.5 and 13.5 kDa; the main component was 14.5 kDa. These proteins are not products of protease digestion, as the procedure was carried out in the presence of protease inhibitors including (p-aminophenyl) methansulfonyl fluoride, 4-(2-aminoethyl)-benzenesulfonyl fluoride, pepstatin A, E-64, bestatin, leupeptin, aprotinin and EDTA. The HBP specifically bound to FITC-apoAI and faintly bound or did not bind to FITC-apoAII. Furthermore, binding of HDL labeled with lipophilic fluorescence to isolated eel hepatocytes was inhibited by the antibody to apoAI, but not inhibited by the antibody to apolipoprotein AII (apoAII). These results strongly suggest that the HBP isolated from the microsomal fraction is present on the plasma membrane of eel liver and plays important roles for the lipid transport through the interaction with HDL.

The mitochondrial tricarboxylate carrier: unexpected increased activity in starved silver eels

The tricarboxylate carrier was purified to homogeneity from liver mitochondria of European eel at the silver and the yellow stage and functionally reconstituted into liposomes. Unexpectedly, the molecular activity of the tricarboxylate carrier obtained from silver eel was about twofold higher than that of the same protein from yellow eel, although eels at the silver stage stop feeding. Parallel changes were found in the activities of the lipogenic enzymes in silver eels. This suggests a functional coordination between all these proteins sequentially involved in hepatic lipogenesis. Cardiolipin added to proteoliposomes strongly stimulated the activity of the purified tricarboxylate carrier from yellow eels, whereas it slightly reduced the activity of the same protein from silver eels. The higher activity of the tricarboxylate carrier from silver eels could therefore be ascribed, at least in part, to a different composition of the lipid domain surrounding the carrier protein, possibly in response to the hormonal alterations accompanying metamorphosis from yellow to silver stage.