Download Fumonisins: fungal toxins that shed light on

~Fumonisins: fungal
toxins that shed
light on
sphingolipid
function
Fumonisins are sphinganine analogues produced by Fusarium
moniliforme
and related fungi. They inhibit ceramide synthase and
block the biosynthesis of complex sphingolipids, promoting
accumulation
Disruption
of sphinganine and sphinganine
of sphingolipid
l-phosphate.
metabolism by finmonisin B, alters
cell-cell interactions, the behaviour of cell-surface proteins, the
activity ofprotein
kinases, the metabolism of other lipids, and cell
growth and viability. This multitude of effects probably accounts
for the toxicity and carcinogenicity of these mycotoxins. iVaturally
occurring inhibitors of sphingolipid metabolism such as firmonisins
are proving to be powerful tools for studying the diverse roles of
sphingolipids in cell regulation and disease.
Alfred Merrill and
Dennis Liotta are
in the Depts of
Biochemistry
and
Chemistry,
Emory
University, Atlanta,
GA 30322, USA;
and Ronald Riley is
at the US Dept of
Agriculture,
Agriculture
Research Service,
Toxicology
and
Mycotoxins
Research Unit,
Athens, GA
30613, USA.
218
Fumonisins are a family of mycotoxins produced by
Fusarium monilifomne (Sheldon) and related fungi that
are common contaminants of maize (Zea mays),
sorghum and related grains throughout the worldl.
The structures of these mycotoxins (Fig. 1) were first
reported by W. F. 0. Marasas and co-workers in 1988
(Ref. 2), reflecting over two decades of research to
explain the high incidence of oesophageal cancer
in certain villages in the Transkei region of South
Africal. Early in these investigations, they noticed
that the villagers consume beer brewed from mouldy
corn, and proceeded to identify the likely culprits as
F. moniliforme and a new class of mycotoxins that
they named fumonisins. These conclusions elicited
widespread interest among toxicologists because F.
moniliforme was also associated with two devastating
and costly diseases of veterinary animals: equine
leukoencephalomalacia
and porcine pulmonary
oedema’.
Several different types of fumonisins have been
characterized2 (some of which are illustrated in Fig. 1)
and the total number is not known yet. The ‘B’ series
includes the most prevalent species, fumonisin B,
(FB,), which is both toxic and carcinogenic for animals’. Less is known about fumonisins in the ‘A
series,for which the amino group has been acetylated,
0 1996 Elsevier Science
PII: SOS&L8924(96)10021-O
Ltd
but they appear to be relatively innocuous1,3. The ‘C’
series lacks the l-methyl group and resembles another family of mycotoxins produced by AZtemaria
altematu (called the AAL toxins; Fig. 1) that are
responsible for stem canker disease in susceptible
tomatoes4. In many parts of the world, maize is
treated with alkali in the preparation of tortillas. This
hydrolyses the tricarballylic acid sidechain (the ‘R’
group in Fig. 1) and might reduce the toxicitys.
Many animals develop liver and kidney damage
after consumption of FB,, and a few manifest severe
neurotoxicity (equids) or pulmonary oedema (pigs)l.
FB, also affects embryonic development6 and is
hepatocarcinogenic3,7,8. FB, increases the ‘leakiness’
of the permeability barrier of endothelial cells9 and is
both immunostimulatory
and immunosuppressivelo.
The pathogenesis of FB, poisoning, therefore, may involve widespread disruption of cellular function.
It was unclear how these compounds could have
such diverse effects because their structures do not
have any obvious similarities to other toxins.
However, we noticed that they do resemble the
sphingoid base backbone of sphingolipids (cf. Figs 1
and 2), a class of membrane lipids that play many
important roles in cell regulationrl. This led us to
combine our knowledge of molecular toxicology and
sphingolipid biochemistry to investigate whether
the target of fumonisins might be sphingolipid
metabolism and/or cellular functions mediated by
sphingolipids. An answer was soon forthcoming: FB,
completely blocked de nova sphingolipid biosynthesis by rat hepatocytes in culture at concentrations
(1 PM) that could plausibly be achieved in vivo12.
Inhibition
of ceramide
synthase
FB, and other members of the ‘B’ series of fumonisins are potent, competitive inhibitors of ceramide
synthase 12J3,the enzyme that catalyses the acylation
of sphinganine in the de novo biosynthesis of
sphingolipids and the reutilization of sphingosine
derived from sphingolipid turnover (Fig. 2). Ceramide synthase inhibition has been characterized in
vitro with liver and brain microsomes, as well as in
intact mammalian cells in culture (e.g. hepatocytes,
neurons, renal cells and macrophages inter aZia)14.FB,
blocks the incorporation of radiolabelled serine into
the sphingoid base backbone of (dihydro)ceramides
and complex sphingolipids and prevents the conversion of sphinganine to sphingosine via addition
of the 4,.5-tram double bond, which occurs after acylation of sphinganine. It also blocks the reacylation
of sphingoid bases (primarily sphingosine) released
by hydrolysis of more complex sphingolipids, as
shown in Figure 2.
As a consequence of the inhibition of ceramide
synthase, FB, causes sphinganine to accumulate12
and increases the formation of sphinganine l-phosphate and cleavage of the sphingoid base backbone
to fatty aldehydes and ethanolamine l-phosphate
(Ref. 15; Fig. 2). This route for ethanolamine phosphate synthesis has received scant attention compared with the better known decarboxylation of
phosphatidylserine. However, sphingoid base catabolism accounted for one third of the ethanolamine
trends
in CELL BIOLOGY
(Vol.
6) June 1996
in phosphatidylethanolamine
in 3774 cells treated
with FB, (Ref. 15). Therefore, catabolism of sphingoid
bases can make a significant contribution to the cellular ethanolamine pool.
Cellular
effects
CH,OH
C-CH,$HCOOH
0”
CH,COOH
of fumonisins
FB, alters cell morphology16-ls, cell-cell interactions9, the behaviour of cell-surface proteins1g-21 and
protein kinaseszz, the metabolism of other lipids15,23
and cell growth and viability16,ZP27. These changes
are not fully understood and may have multiple
causes; however, as sphingolipids are associated with
each of these processes, most (or all) of the cellular
effects of fumonisins are likely to be consequences of
the disruption of sphingolipid metabolism.
The inhibition of complex sphingolipid formation
has been shown to account for the inhibition of
neurite outgrowth by FB, because addition of a watersoluble ceramide analogue restored the normal phenotYPer7. Considerable attention has also been given
recently to the role of sphingolipids in the targeting
of proteins to the appropriate cell membrane and,
especially, to the association of glycosylphosphatidylinositol (GPI)-anchored proteins with sphingolipidand cholesterol-rich domainszO. Investigations by
several laboratories have shown that inhibitors of
sphingolipid metabolism alter membrane polarityzl
and the behaviour of GPI-anchored proteins1g,20,28,
which strongly supports this association.
The activities of numerous cellular protein kinases
and phosphoprotein phosphatases are modulated by
sphingolipidsll. Although there have been few studies of the effects of FB, on protein kinases, Jones and
co-workersz2 have reported that treatment of CV-1
(African green monkey kidney) cells with FB, caused
repression of protein kinase C activity and transcription dependent on the transcription factor APl, but
stimulated transcription from a promoter containing
a cyclic-AMP response element. Sphingoid bases are
potent inhibitors of protein kinase Ci1sz9;thus, the
accumulation of sphinganine may account for the
repression of protein kinase C. Sphingosine and
sphingosine l-phosphate have been reported to impair agonist-stimulated increases in cyclic AMP in
Swiss 3T3 cells30,31and S49 lymphoma cells32,and to
enhance APl activity in Swiss3T3 cells31.These findings are the opposite of the effects of FB, on CV-1
cells, but, as it is not uncommon for mediators to induce opposite responses in different cell types, these
effects may be related.
Effects on growth
regulation
The effects of sphingolipids on cell growth are
complex. Free sphingoid bases both stimulate and inhibit cell growth30,31,33and, at higher concentrations,
are cytotoxic34; therefore, the accumulation
of
sphinganine in FB,-treated cells is likely to play a role
in the alteration of cell growth and viability by
fumonisins. Studies with LLC-PK, cells, a renal epithelial cell line, have shown a close association
between the concentrations of FB, that inhibit
sphingolipid biosynthesis and cause growth inhibition and toxicity l6. In more recent studies, we have
found that the toxicity is partially reversed by adding
trends
CH,?
in CELL BIOLOGY
(Vol.
6) June 1996
%
Fumonisin
B1
%
Fumonisin
B2
Fumonisin
A,
Fumonisin
C1
NH,
OH OH
CH,bR
CH,
NH,
OH OH
3
CH, OR CH, OH
IYHCOCH,
OH OH
CH, OR CH, OH
NH,
OH OH
Alternaria
CH,OR
CH,
toxin
“4
[R = COCH,CH(COOH)CH,COOH]
FIGURE
Structures
1
of the major
fumonisins
and related
mycotoxins.
P-chloroalanine to inhibit the first enzyme of this
pathway (serine palmitoyltransferase)
and reduce
sphingoid base accumulation35.
Sphingoid bases induce dephosphorylation
of
retinoblastoma protein (pRb), which is a tumour suppressor that can cause cell-cycle arrest in the hypophosphorylated form3”. FB, addition alone did not
induce pRb dephosphorylation in MOLT-4 cells, but
FB, potentiated the dephosphorylation
of pRb induced by exogenous sphingosine33. Therefore, it is
possible that growth inhibition by FB, could involve
the promotion of pRb dephosphorylation
through
elevation of cellular sphingoid bases.
Sphingoid bases have also been reported to induce
apoptosis36, and FB, has been shown recently to induce
apoptosis in cultured keratinocytes and HET-1A cells,
a human oesophageal cell line immortalized with
SV40 T antigen”‘. This may seem odd because the
production of ceramide from the turnover of sphingomyelin is associated with apoptosis38, whereas FB,
decreases the levels of ceramide and more complex
sphingolipids. It appears that disruption of sphingolipid biosynthesis also triggers apoptosis, and this has
been demonstrated by using another inhibitor of
sphingolipid biosynthesis (ISP-l), which inhibited
cell-cycle progression and induced apoptosis in
CTLL-2 cells3g.
By contrast, the exogenous addition of sphingoid
bases stimulates DNA synthesis in growth-arrested
Swiss 3T3 cells, at least in part owing to their conversion to the 1-phosphate30,40. FB, also increases
[“Hlthymidine incorporation into DNA in this cell
line25. There are two potential causes of growth
stimulation by FB, in these cells: accumulation of
sphinganine (or a sphinganine metabolite, such as
sphinganine l-phosphate), or the loss of a growthinhibitory complex sphingolipid, such as a ganglioside, since several growth factor receptors are downregulated by gangliosides ll. It has been possible to
219
Sphingolipid
Metabolic reactions
Hexadecanal
+
Ethanolamine-P
Serine + PalmitoyCCoA
,,,,,lne
P-F
(orCWanine
1
\
OH
1 -phosphate
4 f
OH
Sphinganine
I
--4
NH2
FB, Y
OH OH
4
Dihydroceramide
(N-acyl-sphinganine)
--+
-7
NH
OH
OH
Ceramide
(N-acyl-sphingosine)
K--J,+
-TAH
R \/;_jljO-Headgroup
Sphingomyelin
and Glycolipids
-TNH
OH OH
Ceramide
(N-acyl-sphingosine)
Rw
R’,,,,,/NH
FB, --+
OH
OH
Sphingosine
4
Sphinganine
1 -phosphate
R-JYPo3H2
W
CHO
RV,J
OPO,H,
IJ
NH,
i
Hexadecenal
+
Ethanolamine-P
Bioloaical activities
FB, reduced substantially the accumulation of sphingoid bases and prevented
the stimulation of DNA synthesis by FB,
(Ref. 24). Therefore, the growth stimulation by FB, is due to the accumulation
of sphinganine rather than to a reduction in complex sphingolipids.
Disruption
metabolism
of sphingolipid
in wivo
Although the effects of fumonisins on
sphingolipid
metabolism have been
studied mostly in cell culture, sphingolipid levels in liver, kidney, lung and
other organs are altered when animals
are fed FB17J4.Sphingoid bases are able
to traverse cell membranes because the
pKa of the amino group is relatively
lowzg. Therefore, as intracellular levels of
sphingoid bases rise owing to inhibition
of ceramide synthase, the amounts of
sphinganine (and sometimes sphingosine) in serum and urine increase, and
can be used as biomarkers for exposure
of animals to fumonisins41. It is likely
that disruption of sphingolipid metabolism is fundamental to both plant and
animal diseases caused by F. moniliforme
and related fungi because FB, has been
shown to inhibit sphingolipid biosynthesis in Saccharomycescerevisiaez3and in
plants42, including corn, the primary
host for F. monilifonne (Sheldon).
Possible mechanisms
and carcinogenicity
for the toxicity
of fumonisins
The toxicity of FB, in vivo may involve
the accumulation
of free sphingoid
bases as much as (or more than) the deFIGURE 2
pletion of complex sphingolipids beA summary
of the pathway
of sphingolipid
metabolism,
the sites of inhibition
of ceramide
cause the symptoms appear very rapidly;
synthase
by fumonisin
B, (FB,), and biological
activities
that have been associated
with complex
for example, ponies that consume fusphingolipids
and intermediates
of sphingolipid
biosynthesis
and turnover
(and, thus, may be
monisin-contaminated
feed can show
affected
when cells are exposed
to FB,). FB, inhibits sphingolipid
metabolism
at the sites shown.
behavioural
changes
within
days, and
Therefore,
it causes: accumulation
of sphinganine
and, sometimes,
elevation
of sphingosine
die
from
equine
leukoencephalomalacia
owing to inhibition
of the reacylation
of sphingosine
produced
by sphingolipid
turnover;
in one to two weeks43. This is consistent
elevation
of sphingoid
base 1 -phosphates;
increased
sphingoid
base turnover
to hexadecanal
with the known cytotoxicity of sphin(from sphinganine
1 -phosphate),
hexadecenal
(from sphingosine
1 -phosphate)
and
goid bases for many cell types, which
ethanolamine
phosphate;
and inhibits de nova biosynthesis
of more complex
sphingolipids.
The
has been proposed to involve the inhibiological
activities
of sphinganine
and dihydroceramide
have not been explored
as extensively
bition of protein kinase C (Ref. 34). Since
as those of sphingosine
and ceramide;
however,
sphinganine
and sphingosine
both inhibit
activation
of protein kinase C (for exprotein kinase C, whereas
dihydroceramides
are not comparable
with ceramides
in the activation
ample,
by
phorbol
esters) protects cells
of the ceramide-activated
protein
phosphatase
and in downstream
events, such as inhibition
of
against
apoptosis,
the
inhibition of progrowth
and induction
of apoptosis.
The site of inhibition
of serine palmitoyltransferase
by
tein kinase C might also explain why
P-fluoroalanine
and p-chloroalanine
is also shown.
Abbreviations:
p-F (or Cl)alanine,
sphingoid bases3(j and FB, (Ref. 37) inp-fluoroalanine
or P-chloroalanine;
ethanolamine-P,
ethanolamine
phosphate;
FB,, fumonisin
B,;
duce apoptosis. Somewhat paradoxiCPI, glycosylphosphatidylinositol;
R, CH,(CH,),,-;
R’, an alkyl chain of varying
length.
cally, liver is strongly affected by FB, in
vivo, but hepatocytes in culture are reladistinguish between these possibilities by using
tively resistant to killing by FB, even though they reP-fluoroalanine to inhibit the first enzyme of de novo tain sensitivity to inhibition of sphingolipid metabolismz7. This may be due to the many changes that
sphingolipid biosynthesis (Fig. 2). Incubation of
Swiss 3T3 cells with P-fluoroalanine blocked overall
occur when hepatocytes are placed in culture44.
sphingolipid biosynthesis but did not stimulate
The mechanism(s) for the carcinogenicity of fumogrowth; however, addition of P-fluoroalanine plus
nisins are not known; however, FB, is not genotoxicz5
220
trends
in CELL BIOLOGY
(Vol.
6) June
1996
TABLE
Cell
1 - ELUCIDATION
OF THE
FUNCTIONS
OF SPHINGOLIPIDS
Observations
type
and
BY USING
FUMONISIN
B,
conclusions
Refs
L
Hippocampal
Cultured
Purkinje
neurons
cerebellar
cells
Neuroblastoma
neurons,
17,20
FB, inhibited
de nova sphingolipid
synthesis.
There was no effect on
morphology
of cerebellar
neurons,
but Purkinje cell morphology
was
and could be partially restored
by adding a ceramide
analoguea.
The
turnover
of sphingolipids
in cerebellar
neurons
may account
for the
morphological
changes.
13,18
the
altered
slow
lack of
cells
FB, blocked
sphingosine-induced
differentiation,
which establishes
that conversion
of sphingosine
to ceramide
is required;
FB, (and other inhibitors
of sphingolipid
synthesis)
also diminished
retinoic-acid-induced
differentiation;
therefore,
de nova ceramide
synthesis
is important
in differentiation.
50
CD1 4
FB, caused CD14, a glycosylphosphatidylinositol
(GPI)-anchored
protein,
to
become
hypersensitive
to phosphatidylinositol-specific
phospholipase
C. The
hypersensitivity
was suppressed
by exogenous
sphingomyelin.
The results
indicate that sphingolipids
affect the properties
of CD14 in membranes.
19
MDCK II cell clones exhibiting
polarized
distributions
of marker proteins
of the
apical and basal-lateral
membranes
FB, caused Na+/K+-ATPase
and CP-2, a GPI-anchored
protein,
to be delivered
to both apical and basal-lateral
membranes
in MDCK II/C cells; addition
of
exogenous
ceramide
restored
the normal apical membrane
sorting.
These
results suggest
that sphingolipids
are involved
in the generation
of
cell-surface
polarity.
21
P388 and U937
FB, blocked
daunorubicin-induced
suggests
that the biosynthesis
by daunorubicin.
51
CHO
(NeuroZa)
FB, inhibited
ganglioside
synthesis
and reduced
axon length and branching.
Addition
of a ceramide
analogue”
restored
both, which suggests
that
sphingolipids
play a role in the formation
or stabilization
of axonal branches.
cells transfected
with
cells
ceramide
elevation
and apoptosis,
which
de nova of ceramide
is involved
in cell killing
Rat hepatocytes
FB, inhibited
sphingolipid
secretion
as part of very-low-density
lipoproteins
(VLDL) but not the secretion
of apoB, cholesterol,
or phosphatidylcholine,
which suggests
that sphingomyelin
synthesis
is not required
for secretion
of VLDL, in contrast
to the requirement
for phosphatidylcholine.
44
MOLT-4
cells
FB, potentiated
the induction
of dephosphorylation
product
(pRb) by sphingosine;
therefore,
metabolism
ceramides
is not necessary
for them to affect pRb.
33
Xenopus
laevis oocytes
FB, blocked
sphingosine-induced
germinal-vesicle
that ceramide,
rather than sphingosine,
is active
cell cycle.
Saccharomyces
cerevisiae
which suggests
of the meiotic
lipids,
and the
pathway,
and glycerolipid
52
23
,3-diazol-4-yl)-aminocaproyllsphingosine.
and appears to behave mostly as a tumour promoter’.
Many tumour promoters are mitogens, and, as discussed above, FB, stimulates DNA synthesis in growtharrested Swiss3T3 cellsz5and, therefore, can be classified as a mitogen. The cytotoxicity of FB, might also
be involved since some tumour promoters are toxic
for normal cells, while stimulating the proliferation
of cells that have escaped the normal mechanisms of
growth regulation’.
in CELL BIOLOGY
breakdown,
in reinitiation
FB, decreased
growth
and the synthesis
of sphingolipids,
neutral
major phospholipids
synthesized
via the CDP-diacylglycerol-dependent
which suggests
that there is coordinate
regulation
of sphingolipid
synthesis.
a6-[N-(7-nitrobenz-2-oxa-l
trends
of the retinoblastoma
gene
of sphingoid
bases to
(Vol.
6) June
1996
Use of FE, to elucidate
sphingolipids
the cellular
functions
of
If used properly, FB, can provide valuable information about the functions of sphingolipids. Examples of how it has been used are given in Table 1. As
far as we are aware, FB, has inhibited sphingolipid
biosynthesis in every eukaryotic cell type that has
been tested. Therefore, it can be used to investigate
whether complex sphingolipids are involved in a
221
variety of cell functions. The dose dependence and
time course of inhibition vary among different cell
types (as does the extent of depletion of sphingolipid
mass, which is a function of the rate of synthesis versus turnover), so the conditions must be optimized
for each cell type. Furthermore, because the levels of
sphinganine and sphinganine l-phosphate are elevated, and these are highly bioactive compounds,
care must be taken in concluding that depletion of
complex sphingolipids is responsible for the observed
cell changes. FB, has also proved useful in determining whether exogenously added sphingoid bases
must be metabolized to ceramides to elicit their cellular effects, since this conversion can be blocked
with FB, (Ref. 33).
The possibility that FB, may have effects other than
inhibition of sphingolipid metabolism must always
be considered, and a recent report has described the
inhibition of protein phosphatases45, but at much
higher concentrations than are required for inhibition
of ceramide synthase. As described in several of the
examples in the preceding section, a definitive link
between the disruption of sphingolipid metabolism
and changes in cell behaviour can be established by
using additional inhibitors of sphingolipid biosynthesis. That is, inhibitors of serine palmitoyltransferase (such as B-fluoroalanine or B-chloroalanine)
should be able to mimic the effects of FB, if inhibition of overall sphingolipid synthesis is responsible
for the downstream event, but they reverse the effects
of FB, when the accumulation of free sphingoid bases
(or their metabolites, such as the l-phosphates) is responsible. If the inhibition of complex sphingolipid
synthesis is implicated, this can also be confirmed by
addition of short-chain ceramides to bypass the inhibition of ceramide synthesis.
Since FB, inhibits sphingolipid synthesis in vivo,
it should be useful in establishing the roles of
sphingolipids in development and other complex
biological events that are not easily reproduced by
cells in culture.
Perspectives
The discovery that fumonisins inhibit ceramide
synthase has broadened the range of diseasesthat are
known to involve sphingolipids to include not only
genetic defects in sphingolipid hydrolases but also a
wide variety of diseases caused by fungal toxins.
A major challenge for sphingolipid research is to
identify the ‘switch’ that determines whether sphingoid bases are growth stimulatory, growth inhibitory
or cytotoxic. When found, this will most likely help
explain the diverse effects of fumonisins, aswell. Just
a few years ago, the only naturally occurring inhibitor of sphingolipid metabolism was cycloserine,
which is a relatively non-specific inhibitor of pyridoxal5’-phosphate-dependent
enzymes. But, as often
happens in research, the list of microbial agents that
affect sphingolipid metabolism has grown rapidly to
include: fumonisins (as described in this review);
the Altemaria toxins4; another family of structurally
unrelated inhibitors of ceramide synthase, the australifungins, which are made by Sporonniella ausiralis46;
and, several naturally occurring inhibitors of serine
222
palmitoyltransferase
(lipoxamycin47, sphingofungins48, and ISP-1 or myriocin4g). Evolution has obviously given considerable attention to the development of compounds
that target sphingolipid
metabolism. The prevalence of such compounds in
nature raises interesting questions concerning their
possible roles in disease. It also presents numerous
opportunities to utilize them as tools for basic research and, possibly, in the development of new
classesof pharmaceutical agents.
References
1 MARASAS, W. F. 0. (1994) in Foodborne Disease Handbook.
Diseases caused by Viruses, Parasites, and Fungi (Vol. 2)
(Hui, Y. H., Corham, 1. R., Murrell, K. D. and Cliver, D. O., eds),
2
3
4
5
6
7
8
9
10
11
pp. 521-573,
Marcel Dekker
BEZUIDENHOUT,
C. S. et al. (1988) 1. Cbem. Sot. Cbem.
Commun. 743-745
CELDERBLOM, W. C. A., KRIEK, N. P. I., MARASAS, W. F. 0.
and THIEL, P. C. (1991) Carcinogenesis 12, 1247-l 251
BOTTINI, A. T., BOWEN, J. R. and GILCHRIST, D. G. (1981)
Tetrahedron Lett. 22, 2723-2726
SYNDENHAM,
E. W., STOCKENSTROM,
S., THIEL, P. G.,
SHEPHARD, G. S., KOCH, K. R. and MARASAS, W. F. 0. (1995)
1. Agric. Food Chem. 43, 1198-l 201
GROSS, S. M., REDDY, R.V., ROTTINGHAUS,
G. E., JOHNSON, G.
and REDDY, C. S. (1994) Mycopathologia
128,ll
l-l 18
RILEY, R. T., VOSS, K. A., YOO, H-S., GELDERBLOM, W. C. A.
and MERRILL, A. H., Jr (1994) 1. Food Protect. 57, 638-645
GELDERBLOM, W. C. A. et a/. (1988) Carcinogenesis 9, 1405-l 409
RAMASAMY, S., WANG, E., HENNIG, B. and MERRILL, A. H., Jr
(1995) Toxicol. Appl. Pharmacol. 133, 343-348
MARTINOVA,
E. A. and MERRILL, A. H., Jr (1995)
Mycopathologia
130, 163-l 70
MERRILL, A. H., Jr, HANNUN, Y. A. and BELL, R. M. (1993)
in Advances in lipid Research: Sphingolipids and their Metabolites
(Vol. 25) (Bell, R. M., Hannun, Y. A. and Merrill, A. H., Jr, eds),
pp. l-24, Academic Press
WANG, E., NORRED, W. P., BACON, C. W., RILEY, R. T. and
MERRILL, A. H., Jr (1991) 1. Biol. Chem. 266, 14486-I 4490
13 MERRILL, A. H., Jr, VAN ECHTEN, G., WANG, E. and
SANDHOFF,
K. (1993) I. Biol. Cbem. 26827299-27306
14 MERRILL, A. H, Jr, WANG, E., SCHROEDER, J. J., SMITH, E. R.,
YOO, H-S. and RILEY, R. T. (1995) in Molecular Approaches to
Food Safety. issues Involving Toxic Microorganisms
(Eklund, M.,
Richards, J, and Mise, K., eds), pp. 429-443,
Alaken Press
15 SMITH, E. R. and MERRILL, A. H., Jr (1995) 1. Biol. Chem. 270,
18749-l 8758
16 YOO, H-S., NORRED, W. P., WANG, E., MERRILL, A. H., Jr and
RILEY, R. T. (1992) Toxicol. Appl. Pharmacol. 114, 9-l 5
17 HAREL, R. and FUTERMAN, A. H. (1993) 1. Biol. Chem. 268,
14476-l 4481
18 FURUYA, A., ONO, K. and HIRABAYASHI, Y. (1995)
1. Neurocbem. 65,1551-l
561
19 HANADA, K., IZAWA, K., NISHIJIMA, M. and AKAMATSU, Y.
(1993) 1. Biol. Chem. 268,13820-l
3823
20 FUTERMAN, A. H. (1995) Trends Cell Biol. 5, 377-380
21 MAYS, R. W., SIEMERS, K. A., FRITZ, B.A., LOWE, A. W.,
VAN MEER, G. and NELSON, W. J. (1995) 1. Cell Biol. 130,
1105-1115
22 HUANG, C., DICKMAN,
M., HENDERSON, G. and JONES, C.
(1995) Cancer Res. 55, 1655-l 659
23 WU, W. I. et al. (1995) 1, Biol. Chem. 270, 13171-I 3178
24 SCHROEDER, J. J., CRANE, H. M., XIA, J., LIOTTA, D. C. and
12
trends
in CELL BIOLOGY
(Vol.
6) June 1996
25
26
27
28
29
30
37
32
33
34
35
36
37
38
MERRILL, A. H., Jr (1994) 1. Biol. Chem. 269, 3475-3481
NORRED, W. P., PLAl-TNER, R. D., VESONDER, R. F.,
BACON, C. W. and VOX, K. A. (1992) food. Chem. Toxicol. 30,
233-237
CAWOOD,
M. E., GELDERBLOM, W. C., ALBERTS, J. F. and
SNYMAN, S. D. (1994) food Chem. Toxicol. 32, 627-632
CELDERBLOM, W. C., SNYMAN, S. D., VAN DERWESTHUIZEN,
L.
and MARASAS, W. F. 0. (1995) Carcinogenesis 16, 625-631
HORVATH, A., SUTTERLIN, C., MANNING-KRIEC,
U.,
MOWA,
N. R. and RIEZMAN, H. (1994) EMBO]. 13, 3687-3695
MERRILL, A. H., Jr et al. (1989) Biochemistry28,
3138-3145
ZHANC, H., DESAI, N. N., MURPHEY, J. M. and SPIEGEL, S.
(1990) 1. Biol. Chem. 265,21309-21316
GOODEMOTE,
K. A., MATTIE, M. E., BERGER, A. and SPIEGEL, 5.
(1995) 1, Biol. Chem. 270,10272-l
0277
JOHNSON, 1. A. and CLARK, R. B. (1990) 1. Viol. Chem. 265,
9333-9339
PUSHKAREVA, M. et al. (1995) Biochemistry 34, 1885-l 892
STEVENS, V. L., NIMKAR, S., JAMISON, W. C., LIOl-TA, D. C.
and MERRILL, A. H., Jr (1990) Biocbim. Biophys. Acta 1051, 37-45
YOO, H-S., NORRED, W. P., SHOWKER, J. and RILEY, R. E.
Toxicol. Appl. Pharmacol. (in press)
OHTA, H., SWEENEY, E. A., MASAMUNE, A., YATOMI, Y.,
HAKOMORI,
S-l. and IGARASHI, Y. (1995) Cancer Rex 55,
691-697
TOLLESON, W. H. et a/. (1996) Carcinogenesis 17, 239-249
HANNUN, Y. A. and OBEID, L. M. (1995) Trends
The existence of a cell is dependent intimately on the
presence of iron - too much or too little results in cell
death. Cells from all tissues have an essential requirement for iron in order to proceed from Gl to
S phase of the cell cycle. Iron-containing enzymes
catalyse many reactions involving energy metabolism, respiration and DNA synthesis’. In general,
iron metabolism is highly regulated, and iron is
found complexed mainly with either specific transport proteins or storage proteins. Iron is rarely found
in the blood plasma in the free state since it is highly
toxic to sensitive tissues such as the liver, pancreas,
heart and pituitary2. Owing to its toxicity, mechanisms have evolved to control closely the absorption
and distribution of iron within the body. Intracellular iron is stored predominantly in the form of ferritin within the liver and is supplied to other tissues
through the circulation of the serum iron-transport
protein, transferrin (Tf). The main role of transferrin
is to mop up free iron and to shuttle iron in a soluble
non-toxic form among the organs and cells of the
body. Although there is extensive literature concerning cellular iron uptake and regulation, the picture is
still far from complete and there is considerable evidence that the process is more complex than thought
previously, involving a number of defined and undefined transporters, receptor-mediated endocytosis
(RME) and other uptake mechanisms.
Biocbem. Sci. 20, 73-77
NAKAMURA, S., KOZUTSUMI, Y., SUN, Y., MIYAKE, Y., FUJITA, Y.
and KAWASAKI, T. (1996) 1. Biol. Chem. 271, 1255-l 257
40 ZHANG, H., DESAI, N. N., OLIVERA, A., SEKI, T., BROOKER, G.
and SPIEGEL, S. (1991)/.
CellBiol. 114, 155-167
41 RILEY, R. T., WANG, E. and MERRILL, A. H., Jr (1993) 1. AOAC
Int. 77,533~540
42 ABBAS, H. K. et al. (1994) P/ant Physiol. 106, 1085-l 093
43 WANG, E., ROSS, P. F., WILSON, T. M., RILEY, R. T. and
MERRILL, A. H., Jr (1992) 1. Nutr. 122, 1706-l 716
44 MERRILL, A. H., Jr, LINGRELL, S., WANG, E.,
NIKOLOVA-KARAKASHIAN,
M., VALES, T. R. and VANCE, D. E.
(1995) 1. No/. Gem. 270, 13834-I 3841
45 FUKUDA, H. et al. Biochem. Biophys. Res. Commun. (in press)
46 MANDALA, S. M. et a/. (1995) 1. Antibiot. 48, 349-356
47 MANDALA, S. M. et al. (1994) /. Antibiot. 47, 376-379
48 ZWEERINK, M. M., EDISON, A. M., WELLS, G. B., PINTO, W.
and LESTER, R. L. (1992) /, Biol. Chem. 267,25032-25038
49 MIYAKE, Y., KOZUTSUMI,
Y., NAKAMURA, S., FUJITA, T. and
KAWASAKI, T. (1995) Biochem. Biophys. Res. Commun. 211,
396-403
50 RIBONI, L., PRINETTI, A., BASSI, R., CAMINITI, A. and
TETTAMANTI,
G. (1995) 1. Biol. Chem. 270,26868-26875
51 BOSE, R., VERHEIJ, M., HAIMOVITZ-FRIEDMAN,
A., SCOTTO, K.,
FUKS, Z. and KOLESNICK, R. (1995) Cell 82,405414
52 STRUM, J. C., SWENSON, K. I., TURNER, J. E. and BELL, R. M.
(1995) 1. Biol. Chem. 270,13541-l
3547
39
Acknowledgements
We are grateful to
the graduate
students,
postdoctoral
fellows and
collaborators
who
have been
instrumental
to
these studies.
Our research on
fumonisins
has
been supported
by
funds from the
NIH, the US Dept
of Agriculture
and
the American
Institute for Cancer
Research.
Pumping iron
in the ’90s
The role of iron in cell division, cell death and human disease has
recently gained increased attention.
iron uptake into mammalian
The best studied process for
cells involves transferrin and its
receptor. This review discusses evidence supporting the existence of
other routes by which iron can enter mammalian
cells. Specifically,
iron uptake by the cell-surface GPI-linked transferrin homologue,
melanotransfenin
orp97, is described and possible functions of
this trans fen-m-independent pa thway are proposed.
The predominant pathway by which cells acquire
iron from Tf is by RME (Fig. 1) of diferric-Tf complexed
to the transferrin receptor (TR)3. After Tf has bound
free iron, it is able to bind to the TR at the cell surface. The complex is internalized into endosomes
where acidification releases the iron. Tf remains
bound to the TR and recycles back to the cell surface.
trends
0 1996 Elsevier Science
Transferrin
and the uptake
in CELL BIOLOGY
(Vol.
of iron
6) June
1996
PII: SO962-8924(96)10019-2
Ltd
223