Genetic basis for the polymorphism of rat liver cytosolic aldehyde dehydrogenase

Liver aldehyde dehydrogenase (ALDH), the enzyme involved in the oxidation of aldehydes such as acetaldehyde derived from ethanol, exists in multiple forms in most mammals. Up to five separable forms have been identified from the cytosolic fraction of Wistar rat liver. We investigated the genetic basis of a particular set of three enzyme forms by selective breeding and analysis of electrophoretic patterns of liver ALDH by isoelectric focusing. The forms of liver ALDH investigated were at pI 5.8 or 6.2, or a triple form with enzymes at pI 5.8, 6.0, and 6.2. There are two alleles found at the ALDH locus which encode in homozygotes for one of two electrophoretically separable ALDH forms. A rat heterozygous at the locus forms both ALDH types plus a hybrid. The alleles are expressed codominantly, found at an autosomal locus, and remain constant postpartum. The activities associated with the triplet enzyme form were statistically indistinguishable from a 1:2:1 ratio. This suggests that the enzymes hybridize to form a set of dimers or tetramers of the form A₂, AB, B₂ or A₄, A₂B₂, B₄, respectively.     

Subcellular distribution and biosynthesis of rat liver gangliosides

Gangliosides have generally been assumed to be localized primarily in the plasma membrane. Analysis of gangliosides from isolated subcellular membrane fractions of rat liver indicated that 76% of the total ganglioside sialic acid was present in the plasma membrane. Mitochondria and endoplasmic reticulum fractions, while containing only low levels of gangliosides on a protein basis, each contained approx. 10% of total ganglioside sialic acid. Gangliosides also were present in the Golgi apparatus and nuclear membrane fractions, and soluble gangliosides were in the supernatant. Individual gangliosides were non-homogeneously distributed and each membrane fraction was characterized by a unique ganglioside composition. Plasma membrane contained only 14 and 28% of the total GD1a and GD3, respectively, but 80-90% of the GM1, GD1b, GT1b and GQ1b. Endoplasmic reticulum, when corrected for plasma membrane contamination, contained only trace amounts of GM1, GD1b, GT1b and GQ1b, but 11 and 5% of the total GD1a and GD3, respectively. The ganglioside composition of highly purified endoplasmic reticulum was similar. Ganglioside biosynthetic enzymes were concentrated in the Golgi apparatus. However, low levels of these enzymes were present in the highly purified endoplasmic reticulum fractions. Pulse-chase experiments with [3H]galactose revealed that total gangliosides were labeled first in the Golgi apparatus, mitochondria and supernatant within 10 min. Labeled gangliosides were next observed at 30 min in the endoplasmic reticulum, plasma membrane and nuclear membrane fractions. Analysis of the individual gangliosides also revealed that GM3, GM1, GD1a and GD1b were labeled first in the Golgi apparatus at 10 min. These studies indicate that gangliosides synthesized in the Golgi apparatus may be transported not only to the plasma membrane, but to the endoplasmic reticulum and to other internal endomembranes as well.     

Fibronectin binding to gangliosides and rat liver plasma membranes

Binding of fibronectins to gangliosides was tested directly using several different in vitro models. Using an enzyme-linked immunoabsorbent assay (ELISA), gangliosides were immobilized on polystyrene tubes and relative binding of fibronectin was estimated by alkaline phosphatase activity of conjugated second antibody. Above a critical ganglioside concentration, the gangliosides bound the fibronectin (GT1b congruent to GD1b congruent to GD1a greater than GM1 much greater than GM2 congruent to GD3 congruent to GM3) in approximately the same order of efficiency as they competed for the cellular sites of fibronectin binding in cell attachment assays (Kleinman et al., Proc natl acad sci US 76 (1979) 3367). Alternatively, these same gangliosides bound to immobilized fibronectin. Rat erythrocytes coated with gangliosides GM1, GD1a or GT1b bound more fibronectin than erythrocytes not supplemented with gangliosides. Using fibronectin in which lysine residues were radioiodinated, an apparent Kd for binding to mixed rat liver gangliosides of 7.8 X 10(-9) M was determined. This value compared favorably with the apparent Kd for attachment of fibronectin to isolated plasma membranes from rat liver of 3.7 X 10(-9) M for fibronectin modified on the tyrosine residue, or 6.4 X 10(-9) M for fibronectin modified on lysine residues. As shown previously by Grinnell & Minter (Biochem biophys acta 550 (1979) 92), fibronectin modified on tyrosine residues did not promote spreading and attachment of CHO cells. It did, however, bind to cells. In contrast, lysine-modified fibronectin both bound to cells and promoted cell attachment. Plasma membranes isolated from hepatic tumors in which the higher gangliosides that bind fibronectin were depleted bound 43-75% less [125I]fibronectin than did plasma membranes from control livers. The findings were consistent with binding of fibronectins to gangliosides, including the same gangliosides depleted from cell surfaces during tumorigenesis in the rat.     

Biosynthesis of gangliosides from asialogangliosides in rat liver Golgi vesicles

Biosynthesis of glycolipids GA2, GA1, GM1b, and GD1c was studied in Golgi vesicles isolated from rat liver. Sequential addition of N-acetylgalactosamine, galactose and two sialic acid residues to lactosylceramide led to the endproduct GD1c. Activities of the corresponding glycosyltransferases were shown to be present in isolated Golgi vesicles and their respective kinetic data were determined. The products of each reaction were characterized by their mobility on thin-layer chromatography, by enzymic degradation to their respective precursors, and in case of GM1b by FAB mass spectrometry.     

Expression of c-series gangliosides in rat hepatocytes and liver tissues

C-series gangliosides in rat hepatocytes and liver tissues were analyzed by thin-layer chromatographic (TLC) immunostaining with the specific monoclonal antibody A2B5. Primary cultures of hepatocytes isolated from adult rats were immunostained positively by A2B5. TLC immunostaining with A2B5 of gangliosides from the cells suggested that rat hepatocytes express c-series gangliosides including GT3, GT1c, GQ1c, and GP1c. Expression of c-series gangliosides in cultured hepatocytes was modulated by growth conditions of cells. The amount of GT3 was increased significantly by epidermal growth factor, while the contents of polysialo species such as GT1c, GQ1c, and GP1c were enhanced by higher cell density in culture. Examination of c-series gangliosides in rat liver tissues showed a unique developmental profile with a shift from GT3-dominant to polysialo species-dominant composition in late embryonic stages. These results suggest that the expression of c-series gangliosides in rat hepatocytes is regulated in a growth- and development-dependent manner.     

Alterations in gangliosides of rat liver cells in hyperlipidemic state

The possibility that liver cell membrane is modified in hyperlipidemic state was studied using nephrotic hyperlipidemic rats. Liver cells of normal and nephrotic rats were isolated and subjected to labeling of cell surface components using lactoperoxidase catalyzed radioiodination. The labeling of total surface lipids of hepatocytes of nephrotic rats was about five times higher than that of normal ones and was particularly higher in glycosphingolipids. Cultivation of the isolated hepatocytes as primary cultures reduced drastically labeling of surface lipids in liver cells of both nephrotic and normal rats and abolished the differences observed in liver cells of the two types. Determination of cell associated gangliosides, showed that the level in nephrotic rat hepatocytes was only 35% higher than that of normal rats. Yet, in both types of liver cells 24 h cultivation decreased markedly the ganglioside content. However, similar to the effect observed in hyperlipidemic rats, supplementing the culture medium with very low density lipoproteins (VLDL) increased considerably the ganglioside level of cultured hepatocytes. These treatments did not affect the activity of enzymes involved in the synthesis of gangliosides. It is suggested that ganglioside content in liver cell membrane is modulated in the hyperlipidemic state.     

Specific gangliosides increase rapidly in rat liver following partial hepatectomy

Rat liver gangliosides (sialic acid containing glycosphingolipids) were analyzed by HPTLC and HPLC following either partial hepatectomy or sham operation. Analysis of whole liver gangliosides by HPTLC demonstrated that within 6 h after partial (68%) hepatectomy, there was a significant increase in GM1 compared to both sham and control animals. By 48 h, GM1 was further increased and the polysialylgangliosides GD1a, GD1b and GT1b had also risen significantly, whereas changes in GM3 were negligible. Gangliosides associated with the plasma membrane were increased up to 3.5-fold in regenerating liver compared to sham-hepatectomized controls as assessed by HPLC. Although elevations in membrane gangliosides were associated with hepatocyte proliferation, they did not closely follow the growth curve. The time course of changes in ganglioside biosynthesis suggests differential upregulation of GM3 synthase and GD3 synthase in regenerating livers.     

Distribution of gangliosides in parenchymal and non-parenchymal cells of rat liver

Parenchymal and non-parenchymal cells were isolated from rat liver with purities of more than 90%. Total and ganglioside sialic acid contents were higher in non-parenchymal cells than in parenchymal cells. Thin-layer chromatography of gangliosides showed that the main component in rat liver was ganglioside GM3 and that this was abundant in non-parenchymal cells. Parenchymal cells had ganglioside GD1b as the main component and less GM3 than non-parenchymal cells. These results suggested that the main ganglioside of rat liver, GM3, arises mainly from non-parenchymal cells.     

Genetic polymorphism of ceruloplasmin in the rat

The genetic heterogeneity of ceruloplasmin in serum was studied in the progeny of the (LEW X BN)F1 X (LEW X BN)F1 rats. The results of the statistical analysis showed that the Hbb and c loci were linked. However, the autosomal Ces gene was not linked to the Hbb or c loci. The Ces gene was expressed in normal Mendelian pattern and had two alleles that manifested a low (Cesl) or high (Cesh) level of the enzyme in serum. The Cesl gene was expressed as a dominant in the F1 generation. Two phenotypes CES-H and CES-L were found in the F2 rats; the females in the parental strain and hybrid had higher ceruloplasmin concentration than the male rats.     

Effect of rat liver gangliosides on the interaction of liposomes with rat hepatocytes in vitro

The influence of rat liver GM1, GM2, GD1 and GT1 gangliosides on the interaction of liposomes with rat hepatocytes was investigated. It was shown that liposomes coated with GM1 and GT1 are effectively bound and captured by hepatocytes. Preincubation of hepatocytes with N-acetylglucosamine and D-galactose reduced the binding of GM1- and GT1-liposomes by those cells. The data obtained suggest that there are binding sites for some gangliosides on the surface of rat hepatocytes.     

Biosynthesis and excretion of gangliosides by the isolated perfused rat liver.

De novo synthesis and excretion into perfusate and bile fluid of hepatic gangliosides were studied in isolated perfused rat livers. Addition of N-acetyl-[6-3H(n)]D-mannosamine to the perfusate resulted in radioactive synthesis of at least eight gangliosides labeled in their sialic acid residues. About 10% of total de novo synthesized gangliosides were excreted into the perfusate, less than 1% into the bile fluid. Labeled gangliosides were tentatively identified by cochromatography with known standards. All of them are known to occur in rat liver and sera. The results indicate that most, if not all, normal serum gangliosides are synthesized in the liver; excretion with bile fluid is negligible. They explain previous observations, and indicate clinical implications, which are discussed.     

Genetic polymorphism of tear proteins in the rat

Polymorphism of tear proteins was found by agarose gel electrophoresis among inbred strains of rats. The proteins (RTP-1) are inherited as a single autosomoal trait. The locus was designated Rtp-1 (rat tear protein-1) and it had two codominant alleles (Rtp-1a, Rtp-1b). Although we did not find any recombinant between the Rtp-1 and the Mup-1 loci among 67 backcross progeny, we found 3 strains with the recombinant type between them in 33 inbred strains tested. The results suggest that the Rtp-1 locus is very closely linked with the Mup-1 locus, which belongs to rat linkage group II. RTP-1 proteins strongly reacted with anti-MUP-1 A serum on agarose gel electrophoretograms.     

Extensive precursor-product relationship between gangliosides formed from exogenous glucosylceramide in rat liver

Glucosylceramide, radiolabelled on the glucose residue, was administered to rats and the radioactive gangliosides formed at different time points were chemically characterized. They were identified as GM3, GM1, GD1a and GD1b, each one carrying only radioactive glucose. The time course of each individual ganglioside showed that the simpler gangliosides were formed earlier but were consumed earlier than the more complex ones, resulting in radioactivity patterns that were different at each time point. Only 30 h after injection did it resemble that of endogenous rat liver gangliosides. These results indicate that an extensive precursor-product relationship actually exists in the course of ganglioside biosynthesis.     

Growth- and development-dependent expression of gangliosides in rat hepatocytes and liver tissues.

Expression of gangliosides in the liver was examined in primary cultures of hepatocytes from adult rats and liver tissues from rats of different ages. Hepatocytes were isolated from 7-week-old rat liver and cultured in L-15 medium containing insulin, dexamethasone and 10% fetal bovine serum. Hepatocytes proliferated only on the first day, and then ceased proliferation. The content of GD3 and GD1a increased during the period of active proliferation and reached a nearly constant level, whereas GM1, GD1b, GT1b, and GQ1b gradually increased throughout culture. Addition of EGF to the culture medium caused significant increases in the content of GD3, and to a lesser degree of GM3, but exhibited little effect on the expression of other ganglioside species. The specific induction of GD3 and GM3 expression by EGF was reproduced under serum-free conditions, despite the lack of hepatocyte proliferation. Expression of gangliosides in cultured hepatocytes was also modulated by cell density; higher cell density brought about increased content of GM1, GD1a, GD1b, GT1b, and GQ1b with concomitant reduction of GM3 in cells. The composition of gangliosides in liver tissues demonstrated a unique developmental pattern. GD3 and GD1a were strongly expressed in E-16 embryonic tissue and rapidly decreased with increasing age. GD1b, GT1b, and GQ1b were found only in postnatal liver tissues. These findings suggest that the expression of gangliosides in rat hepatocytes and liver tissues are regulated by growth- and development-dependent factors.     

Subcellular biosynthesis and transport of gangliosides formed from exogenous lactosylceramide in rat liver.

In order to clarify the mechanisms of ganglioside biosynthesis and transport we intravenously administered a liposomal dispersion of radiolabelled lactosylceramide (LacCer) to rats and then followed the time course of the individual gangliosides which became radioactive in the Golgi-apparatus and plasma-membrane fractions prepared from the liver. After administration of radiolabelled LacCer the liver retained a substantial amount of radioactivity, which was distributed among an organic phase (mainly residual LacCer), a fraction containing low-Mr substances (mainly 3H2O) and a ganglioside fraction. The hepatocytes were found to provide the bulk of gangliosides biosynthesized from exogenous LacCer. After subcellular fractionation, the total radioactive gangliosides increased in the Golgi apparatus up to 8 h, to then decrease and practically disappear at 24 h; in the plasma membranes they were progressively concentrated, accounting for high absolute values. Ganglioside patterns were greatly modified with time in both the Golgi apparatus and plasma membrane, but without significant differences between them. Biosynthesis in the Golgi apparatus and accumulation in the plasma membrane of each individual ganglioside followed a precursor-product relationship. The obtained results indicated that once a ganglioside is biosynthesized in the Golgi apparatus, it is in part made available for translocation to the plasma membrane, which rapidly occurs, and is in part retained in the Golgi apparatus, where it acts as a precursor for the biosynthesis of more glycosylated gangliosides.     

The biosynthesis of gangliosides. Labelling of rat brain gangliosides in vivo

1. After injection of [6-(3)H]glucosamine into 8-day-old rats it was found that all the major brain gangliosides and their sialyl groups were labelled at essentially the same rate, except the hematoside, which was the least labelled. In 18-day-old rats it was found that the two major gangliosides with the sialyl (2-->8)-sialyl linkage, and their sialyl groups were more labelled than the hematoside, the Tay-Sachs ganglioside, the other two major gangliosides and their respective sialyl groups. 2. No difference was found in any of the cases studied between the specific radioactivities of the neuraminidase-resistant and -labile sialyl groups belonging to the same ganglioside. The same was found for the specific radioactivities of the galactosyl groups proximal and distal to the ceramide moiety of total brain gangliosides from rats injected with [U-(14)C]glucose. From this it was concluded that partial turnover of the ganglioside molecule does not occur. 3. A model for the synthesis of gangliosides is presented that accounts for results from previous experiments in vitro and the lack of precursor-product relationships observed in experiments in vivo.     

Recycling of glucosylceramide and sphingosine for the biosynthesis of gangliosides and sphingomyelin in rat liver.

It was previously shown that sphingomyelin and gangliosides can be biosynthesized starting from sphingosine or sphingosine-containing fragments which originated in the course of GM1 ganglioside catabolism. In the present paper we investigated which fragments were specifically re-used for sphingomyelin and ganglioside biosynthesis in rat liver. At 30 h after intravenous injection of GM1 labelled at the level of the fatty acid ([stearoyl-14C]GM1) or of the sphingosine ([Sph-3H]) moiety, it was observed that radioactive sphingomyelin was formed almost exclusively after the sphingosine-labelled-GM1 administration. This permitted the recognition of sphingosine as the metabolite re-used for sphingomyelin biosynthesis. Conversely, gangliosides more complex than GM1 were similarly radiolabelled after the two treatments, thus ruling out sphingosine re-utilization for ganglioside biosynthesis. For the identification of the lipid fragment re-used for ganglioside biosynthesis, we administered to rats neutral glycosphingolipids (galactosylceramide, glucosylceramide and lactosylceramide) each radiolabelled in the sphingosine moiety or in the terminal sugar residue. Thereafter we compared the formation of radiolabelled gangliosides in the liver with respect to the species administered and the label location. After galactosylceramide was injected, no radiolabelled gangliosides were formed. After the administration of differently labelled glucosylceramide, radiolabelled gangliosides were formed, regardless of the position of the label. After lactosylceramide administration, the ganglioside fraction became more radioactive when the long-chain-base-labelled precursors were used. These results suggest that glucosylceramide, derived from glycosphingolipid and ganglioside catabolism, is recycled for ganglioside biosynthesis.