β-Galactoside α2,6-Sialyltransferase I Cleavage by BACE1 Enhances the Sialylation of Soluble Glycoproteins

Abstract

BACE1 (β-site amyloid precursor protein-cleaving enzyme-1) is a membrane-bound aspartic protease that cleaves amyloid precursor protein to produce a neurotoxic peptide, amyloid β-peptide, and has been implicated in triggering the pathogenesis of Alzheimer disease. We showed previously that BACE1 cleaves β-galactoside α2,6-sialyltransferase I (ST6Gal I) to initiate its secretion, but it remained unclear how BACE1 affects the cellular level of α2,6-sialylation. Here, we found that BACE1 overexpression in Hep3B cells increased the sialylation of soluble secreted glycoproteins, but did not affect cell-surface sialylation. The sialylation of soluble glycoproteins was not increased by ST6Gal I overexpression alone, but was increased by co-overexpression of ST6Gal I and BACE1 or by expression of the soluble form of ST6Gal I, suggesting that soluble ST6Gal I produced by BACE1 plays, at least in part, a role in the sialylation of soluble glycoproteins. We also found that plasma glycoproteins from BACE1-deficient mice exhibited reduced levels of α2,6-sialylation compared with those from wild-type mice. We propose a novel regulatory mechanism in which cleavage and secretion of ST6Gal I enhance the sialylation of soluble glycoprotein substrates. BACE1 (β-site amyloid precursor protein-cleaving enzyme-1) is a membrane-bound aspartic protease that cleaves amyloid precursor protein to produce a neurotoxic peptide, amyloid β-peptide, and has been implicated in triggering the pathogenesis of Alzheimer disease. We showed previously that BACE1 cleaves β-galactoside α2,6-sialyltransferase I (ST6Gal I) to initiate its secretion, but it remained unclear how BACE1 affects the cellular level of α2,6-sialylation. Here, we found that BACE1 overexpression in Hep3B cells increased the sialylation of soluble secreted glycoproteins, but did not affect cell-surface sialylation. The sialylation of soluble glycoproteins was not increased by ST6Gal I overexpression alone, but was increased by co-overexpression of ST6Gal I and BACE1 or by expression of the soluble form of ST6Gal I, suggesting that soluble ST6Gal I produced by BACE1 plays, at least in part, a role in the sialylation of soluble glycoproteins. We also found that plasma glycoproteins from BACE1-deficient mice exhibited reduced levels of α2,6-sialylation compared with those from wild-type mice. We propose a novel regulatory mechanism in which cleavage and secretion of ST6Gal I enhance the sialylation of soluble glycoprotein substrates. The generation and deposition of the amyloid β-peptide in the brain are a hallmark of Alzheimer disease and have been implicated in the pathogenesis of Alzheimer disease (1Selkoe D.J. Science. 2002; 298: 789-791Crossref PubMed Scopus (3408) Google Scholar). The amyloid β-peptide is generated from the membrane-spanning β-amyloid precursor protein via sequential proteolytic cleavage by β- and γ-secretases. BACE1 (β-site amyloid precursor protein-cleaving enzyme-1), a type I membrane-bound aspartic protease, has recently been identified as β-secretase (2Vassar R. Bennett B.D. Babu-Khan S. Kahn S. Mendiaz E.A. Denis P. Teplow D.B. Ross S. Amarante P. Loeloff R. Luo Y. Fisher S. Fuller J. Edenson S. Lile J. Jarosinski M.A. Biere A.L. Curran E. Burgess T. Louis J.C. Collins F. Treanor J. Rogers G. Citron M. Science. 1999; 286: 735-741Crossref PubMed Scopus (3307) Google Scholar, 3Wolfe M.S. Xia W. Ostaszewski B.L. Diehl T.S. Kimberly W.T. Selkoe D.J. Nature. 1999; 398: 513-517Crossref PubMed Scopus (1692) Google Scholar, 4Yan R. Bienkowski M.J. Shuck M.E. Miao H. Tory M.C. Pauley A.M. Brashier J.R. Stratman N.C. Mathews W.R. Buhl A.E. Carter D.B. Tomasselli A.G. Parodi L.A. Heinrikson R.L. Gurney M.E. Nature. 1999; 402: 533-537Crossref PubMed Scopus (1339) Google Scholar, 5Hussain I. Powell D. Howlett D.R. Tew D.G. Meek T.D. Chapman C. Gloger I.S. Murphy K.E. Southan C.D. Ryan D.M. Smith T.S. Simmons D.L. Walsh F.S. Dingwall C. Christie G. Mol. Cell. Neurosci. 1999; 14: 419-427Crossref PubMed Scopus (1001) Google Scholar). Other substrates for BACE1 have been reported, including β-galactoside α2,6-sialyltransferase I (ST6Gal I) 3The abbreviations used are: ST6Gal I, β-galactoside α2,6-sialyltransferase I; siRNAs, small interfering RNAs; LEC, Long-Evans Cinnamon; LEA, Long-Evans Agouti; SSA, S. sieboldiana agglutinin; FITC, fluorescein isothiocyanate; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; PBS, phosphate-buffered saline; Sia, sialic acid. (6Kitazume S. Tachida Y. Oka R. Shirotani K. Saido T.C. Hashimoto Y. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 13554-13559Crossref PubMed Scopus (232) Google Scholar, 7Kitazume S. Tachida Y. Oka R. Kotani N. Ogawa K. Suzuki M. Dohmae N. Takio K. Saido T.C. Hashimoto Y. J. Biol. Chem. 2003; 278: 14865-14871Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar, 8Kitazume S. Nakagawa K. Oka R. Tachida Y. Ogawa K. Luo Y. Citron M. Shitara H. Taya C. Yonekawa H. Paulson J.C. Miyoshi E. Taniguchi N. Hashimoto Y. J. Biol. Chem. 2005; 280: 8589-8595Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar), the adhesion protein P-selectin glycoprotein ligand-1 (9Lichtenthaler S.F. Dominguez D.I. Westmeyer G.G. Reiss K. Haass C. Saftig P. De Strooper B. Seed B. J. Biol. Chem. 2003; 278: 48713-48719Abstract Full Text Full Text PDF PubMed Scopus (231) Google Scholar), the β-subunit of voltage-gated sodium channels (10Wong H.K. Sakurai T. Oyama F. Kaneko K. Wada K. Miyazaki H. Kurosawa M. De Strooper B. Saftig P. Nukina N. J. Biol. Chem. 2005; 280: 23009-23017Abstract Full Text Full Text PDF PubMed Scopus (257) Google Scholar), β-amyloid precursor protein-like proteins (11Li Q. Sudhof T.C. J. Biol. Chem. 2004; 279: 10542-10550Abstract Full Text Full Text PDF PubMed Scopus (201) Google Scholar), and low density lipoprotein receptor-related protein (12von Arnim C.A. Kinoshita A. Peltan I.D. Tangredi M.M. Herl L. Lee B.M. Spoelgen R. Hshieh T.T. Ranganathan S. Battey F.D. Liu C.X. Bacskai B.J. Sever S. Irizarry M.C. Strickland D.K. Hyman B.T. J. Biol. Chem. 2005; 280: 17777-17785Abstract Full Text Full Text PDF PubMed Scopus (226) Google Scholar). BACE1 has been identified as the sole β-secretase because no measurable amyloid β-peptide is synthesized in BACE1-deficient mice (13Luo Y. Bolon B. Kahn S. Bennett B.D. Babu-Khan S. Denis P. Fan W. Kha H. Zhang J. Gong Y. Martin L. Louis J.C. Yan Q. Richards W.G. Citron M. Vassar R. Nat. Neurosci. 2001; 4: 231-232Crossref PubMed Scopus (951) Google Scholar, 14Cai H. Wang Y. McCarthy D. Wen H. Borchelt D.R. Price D.L. Wong P.C. Nat. Neurosci. 2001; 4: 233-234Crossref PubMed Scopus (954) Google Scholar). Previous reports have revealed that BACE1-deficient mice show subtle neuronal abnormalities (15Roberds S.L. Anderson J. Basi G. Bienkowski M.J. Branstetter D.G. Chen K.S. Freedman S.B. Frigon N.L. Games D. Hu K. Johnson-Wood K. Kappenman K.E. Kawabe T.T. Kola I. Kuehn R. Lee M. Liu W. Motter R. Nichols N.F. Power M. Robertson D.W. Schenk D. Schoor M. Shopp G.M. Shuck M.E. Sinha S. Svensson K.A. Tatsuno G. Tintrup H. Wijsman J. Wright S. McConlogue L. Hum. Mol. Genet. 2001; 10: 1317-1324Crossref PubMed Google Scholar, 16Dominguez D. Tournoy J. Hartmann D. Huth T. Cryns K. Deforce S. Serneels L. Camacho I.E. Marjaux E. Craessaerts K. Roebroek A.J. Schwake M. D'Hooge R. Bach P. Kalinke U. Moechars D. Alzheimer C. Reiss K. Saftig P. De Strooper B. J. Biol. Chem. 2005; 280: 30797-30806Abstract Full Text Full Text PDF PubMed Scopus (296) Google Scholar, 17Harrison S.M. Harper A.J. Hawkins J. Duddy G. Grau E. Pugh P.L. Winter P.H. Shilliam C.S. Hughes Z.A. Dawson L.A. Gonzalez M.I. Upton N. Pangalos M.N. Dingwall C. Mol. Cell. Neurosci. 2003; 24: 646-655Crossref PubMed Scopus (138) Google Scholar). Furthermore, BACE1 was shown recently to be required for the processing of neuregulin-1, and BACE1-deficient mice display hypomyelination (18Willem M. Garratt A.N. Novak B. Citron M. Kaufmann S. Rittger A. De Strooper B. Saftig P. Birchmeier C. Haass C. Science. 2006; 314: 664-666Crossref PubMed Scopus (608) Google Scholar). BACE1 is highly expressed in neurons, but its ubiquitous expression at lower levels has also been demonstrated (2Vassar R. Bennett B.D. Babu-Khan S. Kahn S. Mendiaz E.A. Denis P. Teplow D.B. Ross S. Amarante P. Loeloff R. Luo Y. Fisher S. Fuller J. Edenson S. Lile J. Jarosinski M.A. Biere A.L. Curran E. Burgess T. Louis J.C. Collins F. Treanor J. Rogers G. Citron M. Science. 1999; 286: 735-741Crossref PubMed Scopus (3307) Google Scholar). We showed previously that hepatic BACE1 is a major protease for secretion of ST6Gal I into the plasma (8Kitazume S. Nakagawa K. Oka R. Tachida Y. Ogawa K. Luo Y. Citron M. Shitara H. Taya C. Yonekawa H. Paulson J.C. Miyoshi E. Taniguchi N. Hashimoto Y. J. Biol. Chem. 2005; 280: 8589-8595Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar). Similar to the case for ST6Gal I, some Golgi-localized glycosyltransferases are cleaved by proteases and subsequently secreted by cells. Indeed, many glycosyltransferases have been found in body fluids such as serum, colostrum, and milk (19Kaplan H.A. Woloski B.M. Hellman M. Jamieson J.C. J. Biol. Chem. 1983; 258: 11505-11509Abstract Full Text PDF PubMed Google Scholar, 20Gerber A.C. Kozdrowski I. Wyss S.R. Berger E.G. Eur. J. Biochem. 1979; 93: 453-460Crossref PubMed Scopus (82) Google Scholar, 21Elhammer A. Kornfeld S. J. Biol. Chem. 1986; 261: 5249-5255Abstract Full Text PDF PubMed Google Scholar, 22Sarnesto A. Kohlin T. Thurin J. Blaszczyk-Thurin M. J. Biol. Chem. 1990; 265: 15067-15075Abstract Full Text PDF PubMed Google Scholar). Furthermore, such proteolytic cleavage and secretion into body fluids are affected by various pathological conditions, including malignant transformation and inflammation. Nevertheless, the biological significance of soluble glycosyltransferases has remained unclear (23Colley K.J. Glycobiology. 1997; 7: 1-13Crossref PubMed Scopus (285) Google Scholar) because the donor substrates (nucleotide sugars) are not available in body fluids. To examine how BACE1 overexpression affects cellular sialylation following ST6Gal I cleavage and secretion, we used several experimental systems. First, we utilized Hep3B hepatoma cells in which BACE1 was overexpressed or its expression was reduced by BACE1 small interfering RNAs (siRNAs) and quantified the α2,6-sialylation of both secreted and cell-surface glycoproteins. Next, we used a hepatitis rat model, the Long-Evans Cinnamon (LEC) rat, in which increased BACE1 expression has been confirmed (8Kitazume S. Nakagawa K. Oka R. Tachida Y. Ogawa K. Luo Y. Citron M. Shitara H. Taya C. Yonekawa H. Paulson J.C. Miyoshi E. Taniguchi N. Hashimoto Y. J. Biol. Chem. 2005; 280: 8589-8595Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar). The LEC rat, a model of Wilson disease, has a deletion in the copper-transporting ATPase gene (ATP7B) (24Yoshida M.C. Masuda R. Sasaki M. Takeichi N. Kobayashi H. Dempo K. Mori M. J. Hered. 1987; 78: 361-365Crossref PubMed Scopus (193) Google Scholar, 25Li Y. Togashi Y. Sato S. Emoto T. Kang J.H. Takeichi N. Kobayashi H. Kojima Y. Une Y. Uchino J. J. Clin. Investig. 1991; 87: 1858-1861Crossref PubMed Scopus (287) Google Scholar, 26Wu J. Forbes J.R. Chen H.S. Cox D.W. Nat. Genet. 1994; 7: 541-545Crossref PubMed Scopus Google Scholar). Golgi-localized is in secretion into the which is with and M.J. P. J. J. PubMed Scopus Google Scholar). Similar to with Wilson disease, LEC from of in the to the of hepatitis and M. M.C. N. Taniguchi N. The LEC for and Google Scholar). LEC revealed that the hepatic BACE1 level is increased the hepatitis and that plasma ST6Gal I at the (8Kitazume S. Nakagawa K. Oka R. Tachida Y. Ogawa K. Luo Y. Citron M. Shitara H. Taya C. Yonekawa H. Paulson J.C. Miyoshi E. Taniguchi N. Hashimoto Y. J. Biol. Chem. 2005; 280: 8589-8595Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar), suggesting a BACE1 expression and the level of sialylation in We also the sialylation of plasma glycoproteins from BACE1-deficient mice. LEC and from and the for and from Long-Evans by BACE1 mice and at The of the used in as and including from protein from sieboldiana and from and from or protein The available used as and I was used to soluble ST6Gal I at in was generated the and used to the of BACE1 to I was by a I which was generated by and into the of the K.J. Lee B. Paulson J.C. J. Biol. Chem. Full Text PDF PubMed Google Scholar). cells and in The cells with of BACE1 or a of in of I. The to BACE1 as and and and the was for to the the cells in I protease for The cells and for was from Hep3B cells and of the was with a to the BACE1 and and we used gene expression and the to which of the for The for BACE1 with the The by at for at for and at for and for in a of The for with at and the at The expression levels of the in and to the expression the levels of in from Hep3B cells or rat plasma by We also the levels of ST6Gal I in Hep3B which a protease and proteins from with with Nature. PubMed Scopus Google Scholar), by and to a The was with or I by or as the was used for of the (6Kitazume S. Tachida Y. Oka R. Shirotani K. Saido T.C. Hashimoto Y. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 13554-13559Crossref PubMed Scopus (232) Google Scholar). of the of Hep3B cells with and protein the was with phosphate-buffered the and and protein used for of ST6Gal I in the Hep3B the was with and with quantified with a cells in I with I protease for of the with of plasma of or from Hep3B cells with by and to a The was with in for and with in with the was with in and the quantified with the of from Hep3B cells in I for and by a was to the level of sialylation of the proteins to a and by with and by of and to of a was at with a plasma or a of with or of glycoprotein to as the with the with in at for with the with in and in at for with the with The was by the of is as the of in of glycoprotein by the cells with rat I for at with for at the cells with I and with in PBS, the cells with and with PBS, the in and with a and used to the cells with or the and for Next, the cells with in and as in and sodium at with for and in and BACE1 the of showed previously that BACE1 cleaves ST6Gal I to initiate its secretion the following experimental or cells in BACE1-deficient and BACE1 and in of BACE1 with ST6Gal I (6Kitazume S. Tachida Y. Oka R. Shirotani K. Saido T.C. Hashimoto Y. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 13554-13559Crossref PubMed Scopus (232) Google Scholar, 8Kitazume S. Nakagawa K. Oka R. Tachida Y. Ogawa K. Luo Y. Citron M. Shitara H. Taya C. Yonekawa H. Paulson J.C. Miyoshi E. Taniguchi N. Hashimoto Y. J. Biol. Chem. 2005; 280: 8589-8595Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar, S. Dohmae N. Takio K. S. K.J. Glycobiology. 1999; PubMed Scopus Google Scholar). it has remained unclear how cellular sialylation is affected by such ST6Gal I ST6Gal I is highly expressed in the H. Paulson J.C. J. Biol. Chem. 1994; Full Text PDF PubMed Google Scholar), and a of hepatic ST6Gal I is cleaved and secreted into the by the (19Kaplan H.A. Woloski B.M. Hellman M. Jamieson J.C. J. Biol. Chem. 1983; 258: 11505-11509Abstract Full Text PDF PubMed Google Scholar, M. S. J. Kobayashi L. Glycobiology. 1999; PubMed Scopus Google Scholar). R. K. S. Y. Y. H. E. N. and Y. we how BACE1 overexpression affects the sialylation of soluble glycoproteins cell-surface glycoproteins in Hep3B hepatoma cells. Hep3B cells in proteins secreted into the and by with which have been identified to ST6Gal I is to be the sole that the of in the in the and the for from Hep3B cells Hep3B cells to into the we to the that the at was from The to be of and the available BACE1 of the increased by compared with expression of the the that the proteolytic of BACE1 is required for the in in the Next, we the levels of the protein of are increased by BACE1 BACE1 no in the level of in of the and protein levels and but the was S. M. R. D.W. Science. 1999; 286: PubMed Scopus Google Scholar) showed previously that the of are not in sialylation of not affect the of via the To that the sialylation of is increased by BACE1 we BACE1 was increased revealed that BACE1 overexpression in cellular α2,6-sialylation in cells (6Kitazume S. Tachida Y. Oka R. Shirotani K. Saido T.C. Hashimoto Y. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 13554-13559Crossref PubMed Scopus (232) Google Scholar). We also such as and but did not in levels of sialylation not BACE1 we that the of ST6Gal I remained in Hep3B cells that the of BACE1 overexpression are the secretion is highly in hepatic cells to plasma glycoproteins. We of BACE1 expression have the the sialylation of soluble secreted proteins in Hep3B To we used of BACE1 as as a as a First, we confirmed that BACE1 and BACE1 expression with of and Next, we to the BACE1 also affect the sialylation of secreted by Hep3B cells. of was found to be reduced to and in and the of the glycoproteins secreted by Hep3B cells reduced by the not The that the levels of ST6Gal I and be by of BACE1 expression by that the α2,6-sialylation of soluble glycoproteins is reduced by of BACE1 expression in Hep3B cells. Next, we BACE1 expression also affects the sialylation of cell-surface glycoproteins. To Hep3B cells BACE1 or or cells with and the cell-surface by no in the levels of α2,6-sialylation Furthermore, of BACE1 expression did not the level of cell-surface α2,6-sialylation in Hep3B cells We that the in the level of ST6Gal I expression did not the α2,6-sialylation of cell-surface glycoproteins be that the level of ST6Gal I is to the cell-surface α2,6-sialylation Svensson Paulson J.C. J. Biol. Chem. Full Text PDF PubMed Google Scholar). that BACE1 expression the α2,6-sialylation of soluble glycoproteins. of ST6Gal I the of we showed previously that ST6Gal I is cleaved by BACE1 (6Kitazume S. Tachida Y. Oka R. Shirotani K. Saido T.C. Hashimoto Y. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 13554-13559Crossref PubMed Scopus (232) Google Scholar, 7Kitazume S. Tachida Y. Oka R. Kotani N. Ogawa K. Suzuki M. Dohmae N. Takio K. Saido T.C. Hashimoto Y. J. Biol. Chem. 2003; 278: 14865-14871Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar, 8Kitazume S. Nakagawa K. Oka R. Tachida Y. Ogawa K. Luo Y. Citron M. Shitara H. Taya C. Yonekawa H. Paulson J.C. Miyoshi E. Taniguchi N. Hashimoto Y. J. Biol. Chem. 2005; 280: 8589-8595Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar), we that the of soluble ST6Gal I be in the increased α2,6-sialylation of soluble glycoproteins. ST6Gal I was overexpressed in Hep3B the α2,6-sialylation of secreted and soluble glycoproteins remained compared with that in cells Next, we overexpressed ST6Gal I with that soluble ST6Gal I be produced by Indeed, the the α2,6-sialylation of soluble glycoproteins by To the of soluble ST6Gal I in the we a soluble ST6Gal I protein in which a of ST6Gal I was with soluble ST6Gal I increased the α2,6-sialylation of soluble glycoproteins. Next, we soluble ST6Gal I also the α2,6-sialylation of membrane-bound glycoproteins. shown in soluble ST6Gal I ST6Gal I overexpression affected the level of α2,6-sialylation of cell-surface glycoproteins. that in soluble ST6Gal I, but not membrane-bound ST6Gal I, are for the α2,6-sialylation of soluble glycoproteins in Hep3B cells. ST6Gal I is cleaved and subsequently secreted by the ST6Gal I is to show because the donor is not available of the We that soluble ST6Gal I the its the of in which ST6Gal I and BACE1 in the (6Kitazume S. Tachida Y. Oka R. Shirotani K. Saido T.C. Hashimoto Y. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 13554-13559Crossref PubMed Scopus (232) Google Scholar), we that soluble ST6Gal I be the To examine the of soluble ST6Gal I, we used that the soluble form of rat ST6Gal I at BACE1 (6Kitazume S. Tachida Y. Oka R. Shirotani K. Saido T.C. Hashimoto Y. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 13554-13559Crossref PubMed Scopus (232) Google Scholar, 7Kitazume S. Tachida Y. Oka R. Kotani N. Ogawa K. Suzuki M. Dohmae N. Takio K. Saido T.C. Hashimoto Y. J. Biol. Chem. 2003; 278: 14865-14871Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar). Hep3B cells rat ST6Gal I, soluble rat ST6Gal I was the and with in the that a of soluble ST6Gal I was the cells revealed that soluble ST6Gal I was in Hep3B of the suggesting the of soluble ST6Gal I the cells Furthermore, soluble ST6Gal I was increased by BACE1 overexpression in Hep3B cells. the level of soluble ST6Gal I was increased the cells at because of the of synthesized soluble ST6Gal I in BACE1 with the of of in we increased BACE1 expression enhance the α2,6-sialylation of glycoproteins in First, we used LEC model that in the and hepatic We previously that the hepatic BACE1 level in LEC at of is the level at of the hepatic ST6Gal I level and that the level of plasma ST6Gal I with hepatic BACE1 expression (8Kitazume S. Nakagawa K. Oka R. Tachida Y. Ogawa K. Luo Y. Citron M. Shitara H. Taya C. Yonekawa H. Paulson J.C. Miyoshi E. Taniguchi N. Hashimoto Y. J. Biol. Chem. 2005; 280: 8589-8595Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar). of plasma glycoproteins revealed that of the in LEC at of at of did not show in the the the which is was increased at of the level of protein remained of the plasma revealed that some of the proteins increased in LEC at of Nevertheless, did not to the increased of the Next, we the plasma levels of glycoproteins in BACE1-deficient mice. To we used recently by which the plasma levels of glycoproteins be from ST6Gal mice did not in that the is for ST6Gal I plasma of to in the we quantified the of in the plasma of BACE1-deficient mice. BACE1-deficient mice of the of in plasma compared with the in mice that BACE1 the α2,6-sialylation of plasma proteins in We showed previously that BACE1 cleaves ST6Gal I to initiate its secretion (6Kitazume S. Tachida Y. Oka R. Shirotani K. Saido T.C. Hashimoto Y. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 13554-13559Crossref PubMed Scopus (232) Google Scholar, 7Kitazume S. Tachida Y. Oka R. Kotani N. Ogawa K. Suzuki M. Dohmae N. Takio K. Saido T.C. Hashimoto Y. J. Biol. Chem. 2003; 278: 14865-14871Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar, 8Kitazume S. Nakagawa K. Oka R. Tachida Y. Ogawa K. Luo Y. Citron M. Shitara H. Taya C. Yonekawa H. Paulson J.C. Miyoshi E. Taniguchi N. Hashimoto Y. J. Biol. Chem. 2005; 280: 8589-8595Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar). we have demonstrated that BACE1 affects the α2,6-sialylation of proteins secreted from Hep3B cells. BACE1 overexpression in Hep3B the α2,6-sialylation of soluble secreted glycoproteins was that of cell-surface glycoproteins remained We have also demonstrated that BACE1-deficient mice reduced levels of α2,6-sialylation of plasma glycoproteins. The BACE1 did not enhance the that cleavage of ST6Gal I, is required for the sialylation. shown in we that soluble ST6Gal I, from because of the of its the sialylation of soluble glycoproteins in the or is by that soluble ST6Gal I protein of and a of ST6Gal I also the sialylation of soluble glycoproteins, ST6Gal I did for the generated of soluble ST6Gal I, we in its in the a of the a of glycoproteins is synthesized in the and subsequently secreted into the is of the mechanism from or a H. J. 1994; Full Text PDF PubMed Scopus Google Scholar). plasma glycoproteins are by hepatic ST6Gal I, and the of is a for in the Indeed, the of glycoproteins, are by and is by G. A.G. Mol. Biol. Google Scholar). the hepatic ST6Gal I and plasma ST6Gal I are both be to the sialylation of plasma glycoproteins and subsequently plasma proteins from by the hepatic LEC which from revealed that hepatic BACE1 but not ST6Gal I is increased the hepatitis (8Kitazume S. Nakagawa K. Oka R. Tachida Y. Ogawa K. Luo Y. Citron M. Shitara H. Taya C. Yonekawa H. Paulson J.C. Miyoshi E. Taniguchi N. Hashimoto Y. J. Biol. Chem. 2005; 280: 8589-8595Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar). Here, we found that the sialylation of plasma glycoproteins was increased in LEC at the The biological significance of soluble glycosyltransferases in body has been a for many the of soluble ST6Gal I for the α2,6-sialylation of soluble glycoproteins. soluble ST6Gal I in plasma be a for (8Kitazume S. Nakagawa K. Oka R. Tachida Y. Ogawa K. Luo Y. Citron M. Shitara H. Taya C. Yonekawa H. Paulson J.C. Miyoshi E. Taniguchi N. Hashimoto Y. J. Biol. Chem. 2005; 280: 8589-8595Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar). We are the plasma ST6Gal I levels and various rat plasma was a from with