Membrane trafficking of CD98 and its ligand galectin 3 in BeWo cells − implication for placental cell fusion

CD98 heavy chain (CD98hc), expressed at high levels in developing human trophoblasts, is an integral membrane protein with multiple N-linked glycosylation sites and known to be important for cell fusion, adhesion, and amino acid transport. Western blotting and flow cytometry were used to study the effect of brefeldin A, an inhibitor of protein translocation through the Golgi, on CD98hc in the human placental trophoblast cell line BeWo. Although brefeldin A treatment caused increased cell surface expression of CD98hc, a novel partially glycosylated form of the protein was found and, concomitantly, cell fusion was reduced. Western blotting showed that CD98 and galectin 3, a proposed ligand for the glycosylated extracellular domain of CD98hc, co-immunoprecipitated, and double-label immuno-electron microscopy confirmed that CD98hc associated with galectin 3. Furthermore, cell fusion was reduced (specifically) by the disaccharide lactose, a known ligand for the carbohydrate recognition domain of galectin 3, suggesting that the association was functional. Taken together, the data suggest that N-glycosylation of CD98 and subsequent interaction with galectin 3 is critical for aspects of placental cell biology, and provides a rationale for the observation that, in the mouse, truncation of the CD98hc extracellular domain leads to early embryonic lethality [Tsumura H, Suzuki N, Saito H, Kawano M, Otake S, Kozuka Y, Komada H, Tsurudome M & Ito Y (2003) Biochem Biophys Res Commun 308, 847-851].

[Viral fusion mechanisms]

The majority of viral fusion proteins can be divided into two classes. The influenza hemagglutinin (HA) belongs to the class I fusion proteins and undergoes a series of conformational changes at acidic pH, leading to membrane fusion. The crystal structures of the prefusion and the postfusion forms of HA have been revealed in 1981 and 1994, respectively. On the basis of these structures, a model for the mechanism of membrane fusion mediated by the conformational changes of HA has been proposed. The flavivirus E and alphavirus E1 proteins belong to the class II fusion proteins and mediate membrane fusion at acidic pH. Their prefusion structures are distinct from that of HA. Last year, however, it has become evident that the postfusion structures of these class I and class II fusion proteins are similar. The paramyxovirus F protein belongs to the class I fusion proteins. In contrast to HA, an interaction between F and its homologous attachment protein is required for F to undergo the conformational changes. Since F mediates fusion at neutral pH, the infected cells can fuse with neighboring uninfected cells. The crystal structures of F and the attachment protein HN have recently been clarified, which will facilitate studies of the molecular mechanism of F-mediated membrane fusion.

Lysophosphatidylcholine enhances cytokine production of endothelial cells via induction of L-type amino acid transporter 1 and cell surface antigen 4F2

OBJECTIVE

A diverse range of lipid oxidation products detected in oxidized low-density lipoprotein (oxLDL) and atherosclerotic lesions are capable of eliciting biological responses in vascular cells. We performed DNA microarray experiments to explore novel responses of human umbilical vein endothelial cells (HUVECs) to oxLDL and its components.

METHODS AND RESULTS

cDNA microarray analysis showed that oxLDL, lysophosphatidylcholine (LysoPC), 4-hydroxy-2-nonenal, and oxysterols altered gene expression specifically, but some genes were commonly induced in HUVECs. Solute carrier family 3 member 2 and family 7 member 5, encoding the heavy chain of the cell surface antigen 4F2 (4F2hc) and the L-type amino acid transporter 1 (LAT1), respectively, were induced by oxLDL and many oxidation products. LAT1 requires 4F2hc to form a heterodimeric functional complex to transport neutral amino acids into the cell. LysoPC increased membrane protein levels of LAT1 confirmed by Western blot analysis and also uptake of L-[(14)C]leucine, which was inhibited by a competitive inhibitor for LAT1. The release of interleukin 6 (IL-6) and IL-8 was increased in LysoPC-treated cells and was attenuated by the LAT1 inhibitor.

CONCLUSIONS

These findings suggest that an increase in uptake of neutral amino acids induced by LysoPC results in enhancement of inflammatory responses of endothelial cells.

Human Monocyte-derived Multinucleated Giant Cells

Multinucleated giant cells (MGC) are characteristic cells in granulomatous disorders such as sarcoidosis and leprosy. There are two types of MGC; foreign body-type and Langhans-type cells. The exact mechanisms of the formation and the functional significance of MGC are not determined, although their morphological features are well understood. MGC are also formed in vitro from peripheral blood mononuclear cells by stimulation with cytokines and lectins. Particularly IFN-gamma is considered to play a pivotal role in monocyte fusion. IL-3, IL-4, IL-13, and GM-CSF are other reported cytokines involved in MGC formation. In addition to such inflammatory mediators, a factor derived from the pathogens of granulomatous disorders may be necessary for MGC formation. Muramyl dipeptide, a peptidoglycan portion of bacterial cell walls, is one of the candidates and can preferentially induce Langhans-type cells in in vitro MGC formation system. Although the exact mechanisms of in vitro MGC formation remains unknown, cell surface molecules such as P2X7 receptor, integrins, CD98, and macrophage fusion protein are considered to be involved in fusion process. Monocytes of sarcoidosis patients expressed higher levels of P2X7 and had a higher ability to induce MGC than those of healthy controls. Effective agents for sarcoidosis such as tranilast, alloprinol, and captopril inhibited in vitro MGC formation, suggesting their therapeutic effects through the direct effects on monocytes. Thus, an in vitro MGC formation model would be a useful tool to understand the relevance of MGC in granulomatous disorders.

Activation of CEA-CAM-1-mediated cell adhesion via CD98: involvement of PKCδ

CD98 is a multifunctional protein involved in amino acid transport and regulation of integrin-mediated cell adhesion. Herein, we demonstrated that CD98 stimulation by anti-CD98 antibodies induced CEA-CAM-1-mediated cell adhesion in BaF3 cells expressing CEA-CAM-1, and suggest that this might be responsible for compact clumping of F9 embryonic carcinoma cells by CD98 stimulation. CEA-CAM-1 was co-immunoprecipitated by anti-CD98 antibody. CD98 stimulation induced the translocation of cytoplasmic protein kinase Cdelta (PKCdelta) to the cell adhesion sites, and rottlerin that inhibited the PKCdelta translocation abolished the cell aggregation without affecting integrin activation. The results suggested that CD98 stimulation could activate CEA-CAM-1-mediated cell adhesion independently of integrins.

CD98 and intracellular adhesion molecule I regulate the activity of amino acid transporter LAT-2 in polarized intestinal epithelia

We have previously shown that the heterodimer CD98/LAT-2 (LAT-2: amino acid transporter) is expressed in the basolateral membrane of intestinal epithelia and is associated with beta1 integrin (Merlin, D., Sitaraman, S., Liu, X., Easterburn, K., Sun, J., Kucharzik, T., Lewis, B., and Madara, J. L. (2001) J. Biol. Chem. 276, 39282-39289). In the present study we examined the interaction of CD98/LAT2 with intracellular adhesion molecule I (ICAM-1) and the potential of such interaction on the activation of intracellular signal in Caco2-BBE cell monolayers. ICAM-1 was found to be expressed to the basolateral domain and to selectively coimmunoprecipitate with CD98/LAT-2 in Caco2-BBE monolayers. Using antibodies as ligands to CD98 and ICAM-1, we demonstrate that the basolateral cross-linking of CD98 and ICAM-1 differentially affects the intrinsic activity of the LAT-2 transporter. Whereas CD98 ligation decreases the Km and Vm of the LAT-2 transporter, ICAM-1 ligation increases Km and Vm of the amino acid transporter LAT-2. In addition, basolateral cross-linking of CD98 or ICAM-1 induces threonine phosphorylation of an approximately 160-kDa supramolecular complex that is consistent with CD98/LAT-2-ICAM-1 complex. Together these findings demonstrate that (i). CD98/LAT-2 interacts with ICAM-1 in Caco2-BBE cell monolayers, and (ii). CD98 and ICAM-1 ligands generate intracellular signals that regulate the amino acids transporter (LAT-2) activity. Our data provide a novel mechanism by which events such as adhesion may be integrated by amino acid transport activity resulting from the direct interaction of cell surface molecules such as CD98 and ICAM-1.

Heteromeric amino acid transporters: biochemistry, genetics, and physiology

The heteromeric amino acid transporters (HATs) are composed of two polypeptides: a heavy subunit (HSHAT) and a light subunit (LSHAT) linked by a disulfide bridge. HSHATs are N-glycosylated type II membrane glycoproteins, whereas LSHATs are nonglycosylated polytopic membrane proteins. The HSHATs have been known since 1992, and the LSHATs have been described in the last three years. HATs represent several of the classic mammalian amino acid transport systems (e.g., L isoforms, y(+)L isoforms, asc, x(c)(-), and b(0,+)). Members of the HAT family are the molecular bases of inherited primary aminoacidurias cystinuria and lysinuric protein intolerance. In addition to the role in amino acid transport, one HSHAT [the heavy subunit of the cell-surface antigen 4F2 (also named CD98)] is involved in other cell functions that might be related to integrin activation. This review covers the biochemistry, human genetics, and cell physiology of HATs, including the multifunctional character of CD98.