GalT-2 Activity Research Articles

Unraveling the Leloir Pathway of Bifidobacterium bifidum: Significance of the Uridylyltransferases

The GNB/LNB (galacto-N-biose/lacto-N-biose) pathway plays a crucial role in bifidobacteria during growth on human milk or mucin from epithelial cells. It is thought to be the major route for galactose utilization in Bifidobacterium longum as it is an energy-saving variant of the Leloir pathway. Both pathways are present in B. bifidum, and galactose 1-phosphate (gal1P) is considered to play a key role. Due to its toxic nature, gal1P is further converted into its activated UDP-sugar through the action of poorly characterized uridylyltransferases. In this study, three uridylyltransferases (galT1, galT2, and ugpA) from Bifidobacterium bifidum were cloned in an Escherichia coli mutant and screened for activity on the key intermediate gal1P. GalT1 and GalT2 showed UDP-glucose-hexose-1-phosphate uridylyltransferase activity (EC 2.7.7.12), whereas UgpA showed promiscuous UTP-hexose-1-phosphate uridylyltransferase activity (EC 2.7.7.10). The activity of UgpA toward glucose 1-phosphate was about 33-fold higher than that toward gal1P. GalT1, as part of the bifidobacterial Leloir pathway, was about 357-fold more active than GalT2, the functional analog in the GNB/LNB pathway. These results suggest that GalT1 plays a more significant role than previously thought and predominates when B. bifidum grows on lactose and human milk oligosaccharides. GalT2 activity is required only during growth on substrates with a GNB core such as mucin glycans.

ETS-1 transcription factor activates the expression of mouse UDP-Gal:GA2/GM2/GD2/GT2 galactosyltransferase gene

UDP-Gal:GA2/GM2/GD2/GT2 galactosyltransferase (Gal-T2) transfers galactose to the terminal N-acetylgalactosamine of either the neutral glycolipid GA2 or of the gangliosides GM2, GD2 and GT2. Previous studies revealed a tight regulation of Gal-T2 activity and mRNA expression during development of the rat CNS. Here, we study in PC12 cells the cis-acting elements involved in the activation of a fragment of 211 bp around the transcription initiation site of the mouse Gal-T2 promoter. Mutagenesis, competition experiments and functional assays showed that the Ets-1 transcription factor is involved in the activation of the Gal-T2 promoter.

Identification of a biologically active component in minimally oxidized low density lipoprotein (MM-LDL) responsible for aortic smooth muscle cell proliferation.

Although low concentrations (10 microg/ml) of oxidized LDL density lipoproteins (Ox-LDL) and minimally modified LDL (MM-LDL) can stimulate the proliferation of aortic smooth muscle cells the biologically active component responsible for this phenomena has not been identified. Here we report that the 1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-4-phosphocholine (m/e594.3) (POVPC) present in MM-LDL but not 1-palmitoyl-2-glutaryl-sn-glycero-3-phophochline (m/e610.2)(PGPC) can stimulate the activity of UDP-galactose:glucosylceramide (beta 1-->4) galactosyltransferase (GalT-2) and produce lactosyceramide (LacCer). LacCer, in turn, generated superoxide radicals (O(2)(.-)). This is accompanied by the phosphorylation/activation of a cytosolic transcriptional factor p(44) MAPK and the subsequent proliferation of human aortic smooth muscle cells. D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP), an inhibitor of GalT-2, impaired the induction of GalT-2 activity, O(2)(.-)generation, and cell proliferation. Thus POVPC may serve as a surrogate in MM-LDL mediated induction of aortic smooth muscle cells (A-SMC) proliferation via GalT-2 activation. The LacCer produced as a consequence of GalT-2 activation may serve as a lipid second messenger in the activation of an oxidant sensitive transcriptional pahtway that ultimately leads to cell proliferation and may contribute to the pathophysiology of atherosclerosis.

Mouse β 1,3‐galactosyltransferase (GA1/GM1/GD1b synthase): Protein characterization, tissue expression, and developmental regulation in neural retina

The composition of cell surface gangliosides is largely dependent on the relative activities of Golgi resident glycosyltransferases. In the brain of birds and mammals, complex gangliosides (GM2, GM1, GD1a, GD1b, GT1b) abound at late stages of development and in the adult, due to the relatively high activities of the UDP-GalNAc:LacCer/GM3/GD3 β 1,4-N-acetylgalactosaminyltransferase (GalNAc-T) and the UDP-Gal: GA2/GM2/GD2 β 1,3-galactosyltransferase (Gal-T2) relative to that of CMP-NeuAc:GM3 α 2,8-sialyltransferase (Sial-T2). Unlike brain, the mature mammalian neural retina abundantly expresses the simple ganglioside GD3, in relation to complex gangliosides, due to the low activity of GalNAc-T and Gal-T2 relative to Sial-T2. Here we describe the isolation and characterization of a mouse Gal-T2 cDNA that drives the synthesis of an epitope-tagged protein of molecular mass 43 kDa, which was enzymatically active and localized to the Golgi complex in transfected cell lines. Using this cDNA as a probe, it was found that Gal-T2 is coded by a single gene located in chromosome 17, and that the coding sequence is contained in a single exon. The expression of the specific Gal-T2 mRNA (∼1.8 kb) was highest in testis, which also showed elevated Gal-T2 activity. In the postnatal neural retina, Gal-T2 mRNA increased after day 3, maintained high levels of expression by days 4–7, and then decreased to initial values by day 10. The developmental pattern of mRNA expression was temporally coincident with that of Gal-T2 activity expression, indicating that this enzyme is under transcriptional control in the neural retina. J. Neurosci. Res. 58:318–327, 1999. © 1999 Wiley-Liss, Inc.

Mouse ? 1,3-galactosyltransferase (GA1/GM1/GD1b synthase): Protein characterization, tissue expression, and developmental regulation in neural retina

The composition of cell surface gangliosides is largely dependent on the relative activities of Golgi resident glycosyltransferases. In the brain of birds and mammals, complex gangliosides (GM2, GM1, GD1a, GD1b, GT1b) abound at late stages of development and in the adult, due to the relatively high activities of the UDP-GalNAc:LacCer/GM3/GD3 beta 1,4-N-acetylgalactosaminyltransferase (GalNAc-T) and the UDP-Gal: GA2/GM2/GD2 beta 1, 3-galactosyltransferase (Gal-T2) relative to that of CMP-NeuAc:GM3 alpha 2,8-sialyltransferase (Sial-T2). Unlike brain, the mature mammalian neural retina abundantly expresses the simple ganglioside GD3, in relation to complex gangliosides, due to the low activity of GalNAc-T and Gal-T2 relative to Sial-T2. Here we describe the isolation and characterization of a mouse Gal-T2 cDNA that drives the synthesis of an epitope-tagged protein of molecular mass 43 kDa, which was enzymatically active and localized to the Golgi complex in transfected cell lines. Using this cDNA as a probe, it was found that Gal-T2 is coded by a single gene located in chromosome 17, and that the coding sequence is contained in a single exon. The expression of the specific Gal-T2 mRNA (approximately 1.8 kb) was highest in testis, which also showed elevated Gal-T2 activity. In the postnatal neural retina, Gal-T2 mRNA increased after day 3, maintained high levels of expression by days 4-7, and then decreased to initial values by day 10. The developmental pattern of mRNA expression was temporally coincident with that of Gal-T2 activity expression, indicating that this enzyme is under transcriptional control in the neural retina.

Accumulation of glycosphingolipids in human atherosclerotic plaque and unaffected aorta tissues

We have measured the levels of glycosphingolipids and the activity of glycosphingolipid glycosyltransferases in human aortic intima and media from patients who died of atherosclerosis. The effects of lactosylceramide (LacCer) and glucosylceramide (GlcCer) from plaque intima on smooth muscle cell proliferation were assessed. When the GlcCer data was expressed as (micrograms GlcCer/mg cholesterol and/mg total phospholipid, a 28-fold and 7-fold increase in plaque intima compared to normal intima was observed. Similarly, the level of LacCer was elevated 5-fold and 4-fold, respectively, compared to unaffected intima. The activity of UDP-GlcCer: ceramide beta 1-->4 glucosyltransferase (GlcT-1) was similar in unaffected tissue, fatty streaks, and plaques. However, the activity of UDP-galactose: GlcCer, beta 1-->4 galactosyltransferase (GalT-2) activity was moderately higher in plaque than in unaffected tissue. LacCer, but not GlcCer derived from plaque intima exerted a approximately 2.8-fold increase in the proliferation of human aortic smooth muscle cells grown in tissue culture compared to control presumably due to a marked increase in LacCer molecular species containing C16:0, C22:1, and C24:0 fatty acids in plaque intima compared to control. In sum, our findings provide an interesting and novel pathogenic mechanism of lactosylceramide mediated plaque formation via stimulation of aortic smooth muscle cell proliferation.

Oxidized low density lipoprotein stimulates aortic smooth muscle cell proliferation.

We have investigated the effects of oxidized low density lipoproteins (Ox-LDL) on aortic smooth muscle cell (SMC) proliferation and the biosynthesis of glycosphingolipids. We found that Ox-LDL exerted a concentration, time, and temperature dependent alteration of cell proliferation and the biosynthesis of lactosylceramide. At low concentrations (5-10 micrograms/ml medium) Ox-LDL stimulated cell proliferation measured by an increase in the incorporation of [3H]-thymidine in cells and the synthesis of lactosylceramide, but not glucosylceramide synthesis. Oxidized LDL exerted a threefold increase in the incorporation of [3H]-galactose and [3H]-serine in lactosylceramide. The activity of lactosylceramide synthetase; UDP-galactose glucosylceramide beta 1 --> 4 galactosyltransferase (GalT-2), but not glucosylceramide synthetase (GlcT-1) was stimulated by Ox-LDL. On the other hand, LDL suppressed the activity of GalT-2 in these cells. When cells were preincubated with antibody against Ox-LDL or GalT-2 it compromised the Ox-LDL mediated stimulated in cell proliferation and GalT-2 activity. Similarly, D-PDMP an inhibitor of GalT-2 compromised the Ox-LDL mediated effects in cells. In contrast, L-PDMP further stimulated the Ox-LDL mediated cell proliferation and GalT-2 activity. However, preincubation of cells with preimmune rabbit serum IgG failed to abrogate Ox-LDL mediated stimulation in cell proliferation and GalT-2 activity. In sum, we found that Ox-LDL stimulated aortic smooth muscle cell proliferation in culture. This effect resulted from Ox-LDL mediated activation of GalT-2 that produced lactosylceramide. Lactosylceramide in turn, contributed to cell proliferation. Such correlations are supportive of the notion that GalT-2 action mediates the signal transduction of Ox-LDL contributing to cell proliferation.

Glycosphingolipids during skeletal muscle cell differentiation: comparison of normal and fusion-defective myoblasts.

The regulation of glycosphingolipid (GSL) synthesis in culture by fusion-competent (E63) myoblasts and fusion-defective (fu-1) cells was examined. Upon reaching confluency E63 cells fused to form multinucleated myotubes and demonstrated many characteristics of developing skeletal muscle including induction of creatine kinase activity and a shift in creatine kinase isozymes to the MM isoform. The fu-1 cells displayed none of these characteristics, despite the fact that both cells were cloned from the same parental myoblast line (rat L8). There was a transient increase in the synthesis of total neutral GSLs by E63 cells at the time of membrane fusion. In contrast, neutral GSL synthesis by fu-1 cells gradually decreased with time in culture. The major GSLs synthesized by both cell types were lactosylceramide and ganglioside GM3, with more complex structures being observed with prolonged time in culture. Several glycosyltransferase activities were assayed at varying times in culture. Generally, the changes in activities fell into three groups. One group was maximally activated at the end of the culture period (GalT-3, GalNAcT-1 and GalT-6). Another group was maximally activated during the time of active membrane fusion (GlcT and SAT-1). A third group was maximally activated at the time of cell contact and the beginning of membrane fusion (GlcNAcT-1 and GalT-2). In terms of the times of maximal activation there were few differences between E63 and fu-1 cells, with one notable exception. The activity of GalT-2 (lactosylceramide synthase) in E63 cells increased dramatically upon contact and the beginning of membrane fusion, whereas there were no changes in GalT-2 activity in fu-1 cells during time in culture. These results support our hypothesis that membrane glycosphingolipids play an important role in the differentiation of skeletal muscle cells.

Regulation of synthesis of lactosylceramide in normal and tumor proximal tubular cells

We have measured the binding and degradation of low-density lipoprotein (LDL) and LDL-mediated effects on cholesteryl ester (CE) synthesis in cultured normal and tumor proximal tubular (PT) cells. The effects of LDL on the regulation of glycosphingolipid metabolism in cells was pursued employing radioactive precursors, e.g., [ 3H]serine, and [ 3H]glucose and by measuring the activity of UDP-galactose:glucosylceramide:B1-4 galactosyltransferase (GalT-2). In normal PT cells, there was a saturable and displaceable binding and degradation of 125I-LDL and a LDL mediated 14-fold stimulation of cholesteryl ester (CE) synthesis. This was accompanied by a suppression (70–80%) of incorporation of [ 3H]glucose and [ 3H]serine into GlcCer, LacCer, GbOse 3Cer and GbOse 4Cer and suppression (70–80%) of GalT-2 activity. In tumor PT cells, displaceable binding and degradation of 125I-LDL was not observed and LDL failed to stimulate CE synthesis. In such cells, LDL exerted a concentration-dependent stimulation of [ 3H]glucose and [ 3H]serine incorporation into GSL. Maximum stimulation (250%) of GalT-2 activity in tumor PT cells occurred with 25 μg LDL/ml medium. We conclude that LDL taken up via receptor mediated pathway decreases GaIT-2 activity in normal PT cells. In contrast, LDL not taken up via the LDL receptor pathway in tumor PT cells failed to suppress the incorporation of [ 3H]glucose and [ 3H]serine into glycosphingolipids and GalT-2 activity leading to a stimulation of lactosylceramide synthesis.

Topography of glycosyltransferases involved in the initial glycosylations of gangliosides.

We attempted to establish within which organelle UDP-Glc:ceramide beta 1----1'glucosyltransferase (GlcT) is located and moreover to obtain information about its orientation on intracellular membranes as well as that of UDP-Gal:glucosylceramide beta 1----4galactosyltransferase (GalT-2) and CMP-NeuAc:lactosylceramide alpha 2----3sialyltransferase (SAT-1). An extremely purified Golgi apparatus fraction was the only liver fraction where a ceramide-dependent formation of glucosylceramide could be demonstrated. This Golgi fraction, mainly constituted by stacks of intact cisternae which retained the same topographical orientation as in vivo, was then incubated with liposomal dispersions of glycosphingolipid-glycosyltransferase acceptors in reaction mixtures containing all the requirements for enzyme activity but no detergent. Under such conditions, SAT-1 and other late acting glycosyltransferases were over 90% latent, while both GlcT and GalT-2 were just as active as in the detergent-containing assay; they were still inhibited by EDTA. Sepharose-immobilized ceramide and Sepharose-immobilized glucosylceramide were found to be suitable acceptors for GlcT and GalT-2, respectively, still using intact Golgi cisternae as the enzyme source. Moreover, a part of GlcT and GalT-2 activity was released from intact Golgi cisternae upon cathepsin D treatment. These results provide strong evidence that GlcT and GalT-2 face the cytoplasmic side of the Golgi apparatus, whereas SAT-1 and the other late acting enzymes face the luminal side.

Regulation of glycosphingolipid glycosyltransferase by low density lipoprotein receptors in cultured human proximal tubular cells.

We have shown previously that low density lipoproteins (LDL) suppressed the synthesis of lactosylceramide in normal human proximal tubular cells, but stimulated such synthesis in proximal tubular cells from LDL receptor negative subjects (Chatterjee, S., Clarke, K., and Kwiterovich, P.O., Jr. (1986) J. Biol. Chem. 261, 13474-13479). To understand the mechanism(s) of this effect of LDL, we have studied here the effects of LDL on the activity of UDP-GalCer:beta-galactosyltransferase (GalT-2). Maximum suppression (70-80%) of the activity of GalT-2 in normal proximal tubular cells at 37 degrees C occurred at a LDL concentration of 25 micrograms/ml medium. Such suppression was not observed either when the cells were incubated with LDL at 4 degrees C, or when the cells were preincubated with leupeptin, followed by incubation with LDL at 37 degrees C. High density lipoproteins and fetuin did not suppress the activity of GalT-2 in normal proximal tubular cells. In contrast LDL modified by reductive methylation (M-LDL, 100 micrograms/ml) stimulated the activity of GalT-2, approximately 3-fold. The effects of LDL and M-LDL were not related to their glycosphingolipid content. Much less suppression and stimulation of the activity of GalT-2 in proximal tubular cells by LDL and M-LDL, respectively, was found in normal human skin fibroblasts, Chinese hamster ovary cells, and bovine smooth muscle cells, suggesting that the LDL-mediated effect may be tissue-specific. In cells grown to very high density, the activity of the LDL receptor is decreased, and there was less suppression of GalT-2 activity by LDL. In normal proximal tubular cells, LDL stimulated the activity of UDP-Gal:LacCer, alpha-galactosyltransferase activity, UDP-Gal:LcOse3Cer, beta-galactosyltransferase, and CMP-NeuAc:LacCer,alpha-sialyltransferase activity but did not alter the activity of sulfotransferase. In conclusion, LDL that entered the normal proximal tubular cells via the LDL receptor-mediated pathway decreased GalT-2 activity, an effect that was dependent upon the binding, internalization, and degradation of receptor-bound LDL. In contrast LDL that entered normal or LDL receptor-negative proximal tubular cells via an LDL receptor-independent pathway failed to suppress GalT-2 activity, and led to a stimulation of LacCer synthesis.

UDPgalactose:glucosylceramide beta 1----4-galactosyltransferase activity in human proximal tubular cells from normal and familial hypercholesterolemic homozygotes.

The activity of a galactosyltransferase (GalT-2) that catalyzes the transfer of galactose from uridinediphosphogalactose to glucosylceramide in cultured normal human proximal tubular (PT) cells was characterized with respect to substrate saturation and metal ion requirements. Using a membrane-bound enzyme source, optimum activity was obtained in the presence of 1.0 mM Mn2+/Mg2+ (1:1) and a detergent mixture, Triton X-100/Cutscum (1:2, v/v), 0.1 mg/ml. The apparent Km values for glucosylceramide and UDP[14C]galactose were 3 microM and 0.5 microM, respectively. The Vmax values for glucosylceramide and UDP[U-14C]galactose were 0.12 nmol/mg protein per 2 h and 173 nmol/mg protein per 2 h, respectively. The purified 14C-labelled product comigrated with authentic lactosylceramide (LacCer) on TLC and HPLC analysis. The presence of a terminal beta-[14C]galactosyl group in the enzymatic product was proved by its cleavage (79%) by beta-galactosidase. Following the development of optimal assay conditions in normal PT cells, GalT-2 activity was next measured in urinary PT cells from homozygous familial hypercholesterolemic (FH) patients previously shown to accumulate large amounts of lactosylceramide. Urinary PT cells from familial hypercholesterolemic homozygous patients contained 35% higher GalT-2 activity as compared to control cells. We speculate that elevated GalT-2 activity may contribute to the storage of LacCer in FH-PT cells.