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Structure and Function of
Sphingoglycolipids in
Transmembrane Signalling and
Cell-Cell Interactions
Sen-itiroh Hakomori
The Biomembrane Institute, 20 I Elliott Avenue W., Seattle, WA 98 I I9 and Department of Pathobiology,
University of Washington, Seattle, W A 98 195, U.S.A.
Primary structure and organization of
sphingoglycolipids in cell membranes
Eukaryotic cell membranes, particularly plasma
membranes, are characterized by a specific com-
Medicine, London on I6 December I992
Abbreviations used: Lac, lactose; Cer, ceramide (N-fatty
acyl sphingosine); de-NAcGM1, NH,a2 + 3Galp1- 4
GlcCer; DMS, N , N-dimethylsphingosine; DSI, disialosyl-I; GalCer, galactosylceramide (cerebroside); Gb3,
globotriaosylceramide (Gala I -4GalB1 4Cer); Gb4,
globotetraosylceramide
(globoside;
GalNAcDl
3Gala 1 4Gal/3l 4GlcCer); Gg3, gangliotriaosylceramide (GalNAcBl- 4Galp1-+4GlcCer); Gg4, gangliotetraosylceramide
(GalB1- 3GalNAcDl-t 4GalPI
4GlcCer); GM,, GalDl-3GalNAcfiI -4[NeuAca2+3]
Galfi 1 4GlcCer; G , , , sialosyl-lactosylceramide (SA2
3Gal~1-4GlcCer); H, Fucal-2Gal/31-4GlcNAc~l-+
3Galfi1+ 4GlcCer; LacCer, lactosylceramide (Gal/%
4GlcCer); LeX, Gal #
1 I4 [Fuc a 1 31-GlcNAc B 1
3Galb1+ 4GlcCer (Lewisx); Ley, Fuc a 1 2Gal/91+
4[Fuca 1 3]GlcNAc~1-3Gal#?1-4GlcCer (Lewis));
lyso-G,,, NeuAca2- 3GaI/31-4Glcj?l-t 1 sphingosine;
NAcGM~,N-acetyl-GMl (NeuAca2- 3Gal/31-4GlcCer);
NGcG,,, N-glyCOlyl-GM, (NeuGca2- 3GalPI 4GlcCer); nix,, lactoneotetraosylceramide (same as PG);
nLc,,
GalBl -4GlcNAcBI-t 3GalBl + 4GlcNAcBl
3Galj31 4GlcCer; PG, paragloboside [Galpl 4GlcNAcpl 3GalD1 4GlcCer); SPG, sialosylparagloboside (there are two types: 2 + 3SPG (NeuAc a2
3Gal/31-4GIcNAc/?l -3Gal/?1 -4GlcCer)
and
2-hSPG
(NeuAca2 -6GalBI -4GlcNAcPl+ 3GalB1 4GlcCer)l. PC, phosphatidylcholine; PE, phosphatidylethanolamine; PI. phosphatidylinositol; PIP,, phosphatidylinositol 4,s-bisphosphate; PS, phosphatidylserh e ; SM. sphingomyelin; CHO, carbohydrate; EGF, epidermal growth factor; FGF, fibroblast growth factor;
mAb, monoclonal antibody; PDGF, platelet-derived
growth factor; PKC, protein kinase C; RK, receptor kinase; SGL, sphingoglycolipid, SPG, sialosylparagloboside; SA, sialic acid.
-
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SEN-ITIROH HAKOMORI
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position and enrichment of sphingolipids and
sphingoglycolipids (SGLs) [ 11. SGLs are abbreviated
according to the recommendations of the IUPACIUB COfnmiSSiOn on Biochemical Nomenclature
1977 (Lipids 12, 455-463); however the suffix
-0seCer is omitted. Full definitions are given in the
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Biochemical Society Transactions
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list of abbreviations. Four major series of SGLs
have been identified, based on the chemical structure of the core carbohydrate (CHO): ganglio-,
globo-, type 1 lacto- and type 2 lacto-series (Table
1). So far, over 150 molecular species of SGL have
been identified, based on differences in their CHO
chains. There are a few exceptions that show a
‘hybrid’ structure between two or three different
series, for example, a ‘lacto-ganglio’hybrid [Z] and
a ‘globo-lacto-ganglio’hybrid [31. Most cells display
high concentrations of SGL on the plasma membrane. In some cells, SGLs are also abundant in
intracellular membranes [4], but we know little
about the exact distribution of SGLs in these cells
and even less about their organization. One interesting suggestion is that some intracellular ‘cytoplasmic’ SGLs are closely associated with
cytoskeletal proteins. For example, galactosylceramide (GalCer) and gangliotetraosylceramide)
(Gb4Cer) are associated with intermediate-filament
proteins in hepatoma cells, and the pattern of intra-
cellular distribution of a given SGI, varies considerably in different cells [5].
Electron-microscopic studies, based on the
freeze-etch technique using ferritin-labelled antibodies or their Fab fragments, have revealed that
SGLs are present at the external surface of the lipid
bilayer and form large clusters, rather than being
distributed homogeneously. These SGI, clusters are
located independently from the clusters of transmembrane glycoproteins that are labelled by lectins
[6-81. A conceptual model of SGI, organization in
the membrane is shown in Figure 1.
According to minimum-energy conformational models, based on hard-sphere exanomeric
calculations, the axis of the CHO chain in SGLs is
oriented perpendicular to the axis of ceramide,
which is supposedly inserted into the lipid bilayer of
plasma membranes [9-111. In these models, the
outer surface of the CHO chain, exposed at the cell
surface, constitutes a hydrophobic domain surrounded by a hydrophilic area (Figure 2). Various
Table I
Basic core structures of the four SGL series
Symbols used indicate possible substitutions: (+) sialosyl; (m) galactosyl; ( 0 )fucosyl
Series
Abbreviations
Structure
Gal/?(I
Ganglio-series
GalNAc/?( I --L 4)Gal/?(I
-t
-
3)GalNAc/?(I
3)GalNAc/?(I
Globo-series
rn
GalNAc/?( I -3)Gala(
GalNAc/?( I -3)Gala(
Lacto-seriestype I
Gal/?(I -,3)GlcNAc/?(
.*
.*
.*
Gal/?(I -4)GlcNAc/?(
.
-4)GlcNAc/?( I -,3)Gal/?(I -4)GlcNAc/?(
-4)GlcNAc/?( I
Gal/?(I
.*
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-
-
3)
Gal/?(I -4)GIcNAc/?( I
4)GlcNAc/?( I -+ 6)
-
--
4)Gal/?(I
4)Gal/?(I
-
4)GIc/?( I
-
I )Cer GgOs,
4)GIc/?(I
-
I )Cer GgOs,
4)Gal/?( I 4)Glc/?(
3)Gal/?(I -4)Glc/?(
)Cer GbOs,
)Cer GbnOs,
-
3)Gal/?(I -4)Glc/?(
)Cer LcOs,
-
3)Gal/?(I -4)Glc/?(
+
-
-
I)Cer LcnOs,
4)GIc/?(
-
I)Cer LcOs,
3)Gal/?(I -4)Glc/?( I
-
I)Cer LciOs,
3)Gal/?(I
-
Morton Lecture
Figure I
Organization and distribution pattern of SGLs and glycoproteins a t the cellsurface membrane
( a ) Proposed clustering of SGLs and glycoproteins (Gp), based on ( b ) . ( b ) Freeze-etch
electron micrograph of a human-erythrocyte membrane double-labelled with
ferritin-wheat germ lectin and rabbit anti-globoside-staphylococcal-protein-A-colloidalgold (from [7]). Ferritin-labelled areas (f) are well separated from the gold sol-labelled
area (large black dot; labelled g) at the external surface (E), indicating that these t w o
major glycoconjugates form separate clusters. Abbreviations used: i, surrounding ice; P,
intramembranous particle surface, 1.e. P face. The arrowhead indicates the fracture line.
Binding specificity t o
ligands (Abs, lectins.
GSLs)
Organization of GSLs in
membranes
ligands with a complementary structure are capable
of binding to this exposed hydrophobic domain.
Examples of ligands are antibodies [ 121, lectins
(including selectins) [ 13, 141, bacterial adhesin [ 151
585
Figure 2
Minimum-energy conformational model of globoside
(Gb4Cer)
Note that the CHO chain is oriented perpendicular t o the axis
of the ceramide. The outer surface of the CHO chain, exposed
at the cell surface, consists of a hydrophobic domain
surrounded by hydrophilic groups, and a specific binding site
for antibodies, ledins and complementary CHOs. The antibody
reactivity is ranked: short-chain fatty acid < long-chain fatty
acid 4 a-hydroxy-fatty acid [28, 291. The ceramide portion
defines the organization of the SGLs in the membrane. This
model was provided by courtesy of P.-G. Nyholm and I.
Pascher (University of Goteborg, Sweden).
and complementary CHO molecules (specific
CHO-CHO interactions) [ 16-18].
Dramatic changes in SGL composition and
metabolism have been documented for many years
I993
Biochemical Society Transactions
586
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Morton Lecture
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Biochemical Society Transactions
588
in association with differentiation, development and
oncogenesis [ 191. Our knowledge of the functional
role of SGLs in these processes is very fragmentary,
except for their clear identification as cell-typespecific antigens in certain cases (see the following
section). In this article, I will summarize evidence
from our laboratory that suggests that SGLs function in: (i) cell-cell recognition, and (ii) regulation of
cell growth through functional modulation of key
molecules that are essential in transmembrane signalling.
SGLs as cell-type-specific and tumourassociated antigens
Much of our research effort from the 1960s to the
1980s was directed to this area, in which the structural basis for antigenic specificity is well defined. A
large variety of antigens in normal cells (for
example, histo-blood group ABH, Lewis and I/i
antigens) are present as SGLs (they are also present
in peripheral regions of glycoprotein side chains).
This research area has been reviewed several times
[20-221. Since the introduction of monoclonalantibody (mAb) techniques in tumour immunology,
many tumour-associated antigens have also been
identified as SGLs. Some of these, particularly lactoseries type 1 and type 2 chain structures, appear as
side chains of glycoproteins. This area has also
been reviewed repeatedly [ 23-25]. Some typical
tumour-associated carbohydrate antigens and their
defining mAbs are listed in Table 2.
The antigenicity of cellular SGLs is defined
not only by their primary CHO structure, but also
by their organizational status. Some mAbs are
capable Of recognizing the latter as
as the
former. For example, mAb M2590 is claimed to be
highly specific to mouse, hamster and human
melanoma cells, and is claimed not to react with
normal cells. Surprisingly, the epitope of M2590 is
the ganglioside GM3, which is present ubiquitously
in most normal cells [26]. We found that M2590
reacts with cells only when the GM3density is above
a certain threshold value. At high density, GM3 may
assume a different conformation or clustering
pattern that allows reactivity with M2590 [27].
The structure of ceramide (that is, the chain
length of fatty acids and the hydroxylation of fatty
acids and sphingosine) strongly affects the reactivities between SGLs and their antibodies or other
ligands, via some unknown mechanisms [28,29].
SGLs as ligands in cell-cell recognition
In some cases, endogenous lectins (including selectins) define CHO-dependent cell recognition. This
Volume 21
mechanism is not universal, however, because the
surface expression of lectins or selectins is
restricted to a limited variety of cell types, and the
scope of their target CHO structures is very limited.
In contrast, the surface expression of a variety of
SGLs is known to change dramatically and continuously, in terms of quantity and structure, during
ontogenesis and oncogenesis. The functions of
SGLs or CHOs in cell adhesion occurring at
defined stages of these processes can be inhibited
by specific SGL or CHO structures. If lectins were
universally involved in cell recognition, we would
expect to see continuous changes in lectin-binding
specificity during ontogenesis and oncogenesis.
There has been no evidence of such a phenomenon.
In 1984, we found that ‘tight’ cell adhesion
(compaction) of the morula-stage mouse embryo is
mediated by the Lewis” antigen (Le”) [30]. During a
search for the Le”-binding lectin that is expressed
on F9 teratocarcinoma cells [31], we discovered
that the Le”-recognizing molecule is Le” itself; that
is Le”-Le” interaction occurs in the presence of a
bivalent cation [ 161. In subsequent systematic
studies, we incorporated various SGLs into
[ 14C]cholesterol-labelledliposomes and observed
their adhesion to SGL-coated plastic plates. Various
other methods were also used to confirm the specificity and intensity of SGL-SGL interactions (see
Figure 3). Strong interaction was observed
Figure 3
CHO-CHO interaction as a basis of cell recognition
This model is based on findings from a variety of experimental
methods, including adhesion of SGL-liposomes t o solid-phase
SGL; homotypic or heterotypic aggregation of SGL-liposomes
or of SGL-coated beads; affinity chromatography of multivalent
oligosaccharides on SGL-columns. All interactions require the
presence of a bivalent cation, such as Ca”.
Morton Lecture
Figure 4
Interaction of various SGLs with the Le" antigen ( a ) and N-acetyl-G,, ( b )
S indicates a strong interaction; M, a moderately strong interaction and W, a weak interaction. The dotted lines indicate no interaction
and the arrows a negative or repellant interaction. Based on data from the adhesion of SLG-liposomes t o solid-phase SGL or from
multivalent SGL-oligosaccharide adhesion t o SGL bound to a column [ 16, 17, 321.
-
-
I )Cer
-
I)Cer
GalB( I -4)GlcNAc/?( I -3)Gal( I -4)Glc( I
-
I)Cer
GalNAc/?( I -3)Gala( I ~ 4 ) G a l / ?I (-4)Glc( I
-
I)Cer
-4)Glc( I
-
I)Cer
-4)Glc( I
-
I)Cer
3)Gal/?(I -4)Glc( I
-
I)Cer
Gal/?(I -3)GalNAcB( I -4)Gal/?( I -4)Glc( I
-
I)Cer
Gal/?(I -4)GlcNAc/?( I 3)Gal( I -4)Glc( I
GalB( I -4)GlcNAc/?( I -3)Gal( I -4)Glc( I
2
:I
/=/
Gal/?(I -4)GlcNAc/?( I -3)Gal( I -4)Glc( I
-
t
I)Cer
I
Fuca
I
I
M
>
w\
31
\
\
Fuca
I
Gal/?(I -4)GlcN!c/?(
2
t
I
Fuca
\
\
,
\
I -3)Gal( I -4)Glc(
\
-
NeuAca (2 *3)Gal/?(I -4)Glc( I
-
m-
/
I)Cer
-
- NeuGca(2-3)Gal/?( I -4)G
\
\
NeuAca(2
\
-
\
\
for H-H and GM,-Gg3, and moderate interaction
for H-Lewis' (Ley) and G,,-LacCer (Lac, lactose;
for structures of SGLs, see Figure 4 and list of
abbreviations). Typical cases of interaction between
'
two structures are shown in Figure 4. W e also
demonstrated that (i) adhesion of B16 melanoma
variants expressing different degrees of G,, to
mouse or human endothelial cells that express Lac-
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Biochemical Society Transactions
590
Cer or Gg3 is correlated with the degree of metastatic potential of the melanoma cells [32], and (ii)
adhesion of B16 cells to mouse leukaemia L5178
cells is based on G,,-Gg3 interaction [ 171.
Models of cell-cell interaction based on
SGL-SGL interaction involve several factors, as
follows. (i) The presence of a complementary
surface structure between two CHO chains. (ii) The
requirement for a bivalent cation, particularly Ca2+ .
(iii) Interaction has been typically demonstrated
between SGL-liposomes, between SGL-liposomes
and solid-phase SGL or between multivalent SGL
oligosaccharide and SGL bound to a column [ 16,
171. Since SGLs form clusters at the membrane
surface, effective cell adhesion would require the
interaction of a large number of homotypic SGL
molecules in the cluster. (iv) Cell binding based on
SGL-SGL interaction is greatly enhanced by
synergy with integrin-dependent adhesion [33]. (v)
SGL-SGL interaction is a more rapid process than
integrin-dependent adhesion. In a dynamic laminarflow adhesion system, the former was more prominent than the latter [34]. Comparative cell adhesion
based on SGL-SGL interaction as against integrindependent interaction in dynamic and static experimental systems is shown in Figure 5.
Although the exact mechanism of SGL
involvement in cellular adhesion is unknown, two
model systems can be visualized: one based on
interaction between clusters of glycoprotein CHO
and SGL CHO, and the other between two SGL
clusters. In either model, cell adhesion based on
CHO-CHO interaction is the earliest event in cell
recognition, followed by the involvement of adhesive proteins and of integrin receptors (Figure 6).
Figure 5
Adhesion of BL6 cells to mouse endothelial cells is
based on the interaction of G,, (expressed on BL6
cells) with Gg3 or LacCer (expressed on endothelial
cells)
(0,b) Represent a dynamic adhesion system. Adhesion based
on Gg3 or LacCer predominated over that based on fibronectin (FN) or laminin (LN), regardless of the shear stress. (c)
Represents a static adhesion system. FN- or LN-dependent
adhesion became obvious only after approx. 30 min of incubation. In contrast, Gg3- or LacCer-dependent adhesion were
obvious at 10-20 min. These results suggest that there is a
longer 'lag time' for integrin-based cell adhesion, compared
with adhesion based on CHO-CHO interaction, in a static
system. Abbreviation used: PG, paragloboside.
RA\I
\
LN
Regulation of cell growth and
transmembrane signalling by SGLs or
their derivatives
The growth of most eukaryotic cells is regulated by
three major transmembrane signalling mechanisms:
(i) protein kinases associated with growth-factor
receptors, (ii) protein kinase C (PKC), and (iii) the
G-protein family (which involves protein kinase A).
Many studies since 1982 from our laboratory and
others have shown clearly that the former two
mechanisms are modulated by SGLs or SGL
derivatives [35-481. Furthermore, the role of
integrin-receptor function in cell adhesion has been
shown recently to be modulated by gangliosides
[49]. This is a complex subject in which our knowledge is still very fragmentary. In this section, I will
try to focus on relatively well-established facts and
avoid speculative theories.
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=g3
Wall shear stress (dyneskm')
20
40
Time (min)
60
For the growth of baby-hamster kidney
(BHK) cells in a chemically defined medium, the
only growth factor required is fibroblast growth
factor (FGF) [SO]. The FGF receptor function is
Morton Lecture
Figure 6
Proposed cell-adhesion mechanisms based on the organization of SGL and
glycoprotein
Compare with Figure I . ( a ) The SGL cluster interacts with the glycoprotein cluster (GP);
simultaneously, adhesive protein (AP) interacts with the integrin receptor (I). ( b ) Interaction between SGL clusters on neighbouring cells, subsequently reinforced by adhesive
protein-integrin receptor interaction. Abbreviation used: S, selectin.
(4
59 I
I
I
Figure 7
Ganglioside-dependent downregulation of
receptor-associated protein
kinase
( a ) Insulin-dependent tyrosine phosphorylation of a 95 kDa protein, and its specific
inhibition by (2 -3)SPG. Specific phosphorylation is induced by the addition of insulin
(lane 2), the phosphorylation is slightly inhibited by the addition of G,, (lane 3), and
almost completely inhibited by the addition of (2-3)SPG (lane 4). The insulin receptor
was immunoprecipitated from IM7 cells, as described in 1381. ( b )Specificity of the inhibition of the tyrosine phosphorylation of the 95 kDa protein by various lacto-series
gangliosides. Strong inhibition was produced by (2- 3)SPG (IV3NeuAcnLc4)but not by
(2- 6)SPG (WNeuAcnLc,), (2- 3)SnHC (VI3NeuAcnLc,), nor by DSI (disialosyl-I;
VI3NeuAclll6NeuAca2- 3GalB I -4GlcNAcnLc,).
(0)
Insulin
Gangliosides
(50 P M )
kDa
m
(2 3)SPG
200 116.392.566.2 45.0 -
Oh?-----
I50
250
[Gangliosides] (yM)
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Biochemical Society Transactions
Figure 8
Co-operative effect of lyso-PC and GM3on the EGF-RK activity
(0)EGF-RK
activity was specifically enhanced by lyso-PC, but not by lysophosphatidylethanolamine (lyso-PE), lysophosphatidylserine (lyso-PS) or lysophosphatidylinositol (lysoPI). This effect of lyso-PC was observed in the absence of detergent. ( b ) In the presence
of 0.05% (w/v) Triton X-100, G
,, inhibited EGF-RK activity. This inhibitory effect of GM,
was not observed in the absence of Triton X-100, but was clearly observed in the presence of lyso-PC (c), Abbreviation used: CDH, ceramide dihexoside. (d) EGF (E), when
bound t o its receptor (ER). activates a receptor-associated protein kinase (PK) by an asyet-unidentified mechanism - probably di- or oligomerization of the receptor [63]. Based
on data that is summarized in the upper panels [39], and the observed effects of lyso-G,,
on protein kinases [41], it seems likely that the protein-kinase activity associated with the
EGF receptor is modulated positively by lyso-PC (route I), negatively by lyso-G,, (route
2), and is co-operatively strongly downregulated by G,, in addition t o lyso-PC (route 3).
Free G, which is not accessible t o lyso-PC, is not capable of modulating protein-kinase
activity.
592
L
4 2000
l-7
LYSO-PC(200 pM)
v
pkl
n
Y
m
Lysophospholipid ( p M )
n
m
clearly downregulated by GM, (but not by GM,),
although GM3 does not bind to FGF [35]. Similarly,
the growth of human-ovarian-epidermoid carcinoma Kl3 cells and A431 cells is highly dependent
on epidermal growth factor (EGF), and its receptor
function is also downregulated by GM,, through
inhibition of tyrosine-kinase activity associated with
the receptor [Sl].
GM,-dependent regulation of cell growth was
further substantiated using mutant cell lines that
display positive or negative endogenous G,, synthesis. A galactose-4-epimerase-less mutant of
Volume 2 I
0 50 100 I50200
Concentration ( p M )
Chinese-hamster ovary cells is incapable of synthesizing LacCer (and hence GM3) unless cultured
in galactose-containing medium, and also lacks the
EGF receptor [36]. When the mutant cells were
transfected to express the EGF receptor, they displayed EGF-dependent growth in the absence of
galactose, when no GM3 synthesis took place. Addition of galactose induced GM, synthesis, leading to
the inhibition of EGF-dependent cell growth.
The growth of Swiss 3T3 cells is exclusively
dependent on platelet-derived growth factor
(PDGF). PDGF-dependent 3T3 cell growth and
Morton Lecture
Figure 9
Proposed co-operative modulatory effects of SGLs, phospholipids and their
metabolites on the key regulators of cell proliferation
Membrane SGLs (G,,
paragloboside [PG]), phospholipids (PC, sphingomyelin [SM]),
phosphatidylinositol [PI]) and their derivatives (diacylglycerol [DAG]), sphingosine [SPN],
DMS, lyso-G,,,
de-N-acetyl-G,,, (2- 3)SPG, ceramide) function as modulators of PKC,
EGF-RK, and insulin-RK, some of them possibly affecting src and ras oncoproteins.
Metabolic pathways (a), (b), (c), (d) and (e) are important in the inhibition of EGF-RK,
PKC and the insulin-RK; therefore, factors (enzymes) involved in these pathways could
be anti-oncogenic. In addition, the PI derivatives PIP, (phosphatidylinositol 4,sbisphosphate), IP, (inositol t ,4,5-trisphosphate), created by pathway (f), and SPN-P
(sphingosin- I -phosphate), created by pathway (h), act t o induce Ca2+ mobilization, and
consequently t o promote cell growth. Dashed lines indicate processes that consist of
multiple steps. Psychosine (lyso-CMH) in the presence of GM3 inhibits src and rasassociated kinase activity, while sphingosine and DMS enhance this activity 1641.
593
PG
e
(2
-
3)SPG
Y
tyrosine phosphorylation of PDGF receptor was
inhibited specifically by GM, (and to a lesser degree
by GM3).However, PDGF did not interact directly
with the gangliosides, and the gangliosides did not
alter the Kd of PDGF binding to the cell-surface
receptor and they did not change the receptor
number [37].
W e recently found a similar correlation
between insulin-dependent cell growth and the
presence of a ganglioside [381. Insulin-dependent
growth of IM9, HL60 and K562 cells was specifically inhibited by the exogenous addition of
(2 3)-sialosylparagloboside (SPG), and tyrosine
phosphorylation of the 95 kDa subunit was specifically inhibited by (2 3)SPG, but not by other
gangliosides (Figure 7). The inhibition by
(2 + 3)SPG induced the differentiation of HL60
cells into myelomonocytes, as evidenced by morphological and surface-marker changes.
+
+
The above three examples involve the effects
of intact gangliosides on cell-growth regulation.
Two additional mechanisms have been considered.
(i) The inhibitory effect of intact gangliosides on the
EGF-receptor kinase (RK) was greatly enhanced in
the presence of lysophosphatidylcholine (lyso-PC),
but not other lysophospholipids [39]. Lyso-PC
itself promotes the activity of the EGF-RK. This
suggests a possible co-operative effect of lyso-PC
and gangliosides in the modulation of protein
kinases (Figure 8). (ii) Catabolites of gangliosides,
for example I~so-GM~and de-N-acetyl-GM3, have
been detected in A431 cells and in other cell types
140, 411. Whereas lyso-G,, inhibits the EGF-RK,
de-N-acetyl-G,, promotes the EGF-RK. Lyso-G,,
strongly inhibits PKC [42]. Although our knowledge of the modulatory effects of SGL catabolites is
very limited, this is clearly an important research
area. Psychosine has been reported to inhibit PKC
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Biochemical Society Transactions
[43], but is completely absent in normal neuronal
594
tissues. Plasmalopsychosine, however, is a major
component of white matter in brain, and completely
absent from grey matter. Plasmalopsychosine is
capable of modulating neuronal functions such as
neuritogenesis, although its effect is weaker than
that of psychosine [44].
As with other SGI, catabolites, sphingosine
was initially claimed to be an inhibitor of PKC, in
contrast with diacylglycerol, which promotes PKC
activity [45]. W e showed that exogenously added
sphingosine is converted into N, N-dimethylsphingosine (DMS) [46], or is phosphorylated to
yield sphingosine- 1-phosphate by the sphingosine
kinase (a classical catabolic pathway). DMS displays
strong stereospecific inhibition of PKC [47] and
stereospecific enhancement of EGF-RK [48].
Sphingosine- 1-phosphate has no effect on PKC or
EGF-RK, but strongly inhibits cell motility (not cell
growth) through some unknown mechanism [52]. It
is possible that some crucial molecule affected by
sphingosine- 1-phosphate is involved in the interaction between the integrin receptor and a cytoskeletal protein.
A conceptual scheme for the co-operative
modulatory effects of SGI,s, phospholipids and
their metabolites on the key regulators of cell
growth and proliferation is shown in Figure 9.
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Received 5 April 1993
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