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Encyclopedia of Cancer, 2nd Edition (2008), Vol 3, 2823-1826
ISBN: 978-3-540-36847-2
M
Membrane-Lipid Therapy
PABLO V. E SCRIBÁ
Department of Biology-IUNICS, University of the
Balearic Islands, Palma de Mallorca, Spain
Synonyms
Lipid therapy
Definition
Clinical drugs that interact with membrane lipids and
that modify the composition and structure of cell
membranes can change the localization and/or activity
of membrane proteins. Several drugs used to combat
cancer and other pathologies, both in cell and animal
models or in humans, regulate ▶membrane lipid structure and they produce a concomitant alteration in the
localization and activity of signaling proteins. The net
result of these effects is the modulation of certain
signaling pathways that reverse the pathological state.
Indeed, ▶G proteins, ▶protein kinase C (PKC) and heat
shock proteins (HSPs) are among the proteins regulated
by ▶membrane-lipid therapy and the therapeutic agents
used can inhibit cell proliferation or induce ▶apoptosis
and cell differentiation.
Characteristics
Heterogeneity of Membrane Lipids and Lipid Structures
Membranes are composed of thousands of different
lipid molecules that interact dynamically to form the
transient or stable structures used by many proteins as
platforms for their activity and for their interactions
with other proteins. Usually, integral (transmembrane)
(▶Integral membrane protein) and peripheral (amphiAu1 tropic) (▶Peripheral membrane proteins) proteins show
important specificities in their interactions with lipids.
These preferences may be associated either with a
defined type of membrane lipid or a given membrane
lipid structure. Most proteins are sensitive to their lipid
environment, so that their activity can be modified by
changes in membrane lipid composition and structure.
These changes in membrane lipid composition and
structure may have a physiological basis or they may be
a response to external stimuli. An example of the former
is the change in cell membrane lipids in response to
important changes in water temperature in cold-adapted
fishes, a topic widely studied by Tibor Farkas and
colleagues. Alternatively, one example of the latter is
the variation observed in membrane lipids after the
intake of a given substance (drug, food, toxin, etc.).
Membrane lipids can organize into many more
secondary structures than proteins and nucleic acids
in vitro. Moreover, the number of lipid species exceeds
the number of different amino acids and nucleic acid
bases by various orders of magnitude. In contrast
with proteins and nucleic acids, the spatial relationship
between the lipids that form membranes is not defined
by covalent bonds, they usually move freely in this
environment. Furthermore, since some structural aspects
related to membranes still remain unclear, the behavior
of lipids is less predictable. However, this does not
mean that structure-function relationships are not established in membranes and that bulk thermodynamics
can explain all physico-chemical properties of membranes. In fact, changes in the structure of lipids
influence not only the physical behavior of membrane
lipids (membrane structure) but also the activity of
certain associated proteins (membrane function). In
general, modifications in membrane lipid structure are
reflected in changes in membrane lipid function. Thus,
oleic acid (18:1 cisΔ9) induces a large increase in the
non-lamellar (HII) phase propensity of membranes and
it alters the interaction and activity of G proteins. In
contrast, neither the trans analogue of oleic acid (elaidic
acid) nor stearic acid (18:0) markedly influence the
▶lamellar phase of membranes and accordingly, they
do not influence G protein activity.
During recent years it has become accepted that
the fluid mosaic model of membranes described more
than 30 years ago by Singer and Nicolson is a somewhat simplified model which fails to take into account
the presence of large membrane domains (e.g. the
basal, lateral and apical membrane regions of polarized cells, such as glandular, endothelial and epithelial
cells), as wall as the smaller yet highly abundant membrane structures (lipid rafts, synaptosomes, ▶caveolin
domains (caveolae), coated pits, receptor clusters, etc.).
These domains and structures are characterized by their
characteristic lipid and protein composition. In this
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Membrane-Lipid Therapy
sense, while the membrane lipid composition most likely
defines the presence of specific proteins, proteins may
also influence the lipid composition of these domains.
Lipid-Protein Interactions in Membranes
Membrane proteins are classified as integral (transmembrane, intrinsic) and peripheral (amphitropic,
extrinsic) proteins and both these types of proteins are
very sensitive to changes in their lipid environment. For
example, the exchange of sodium ions through the
nicotinic acetylcholine receptor (▶Integral protein) is
modified by changes in ▶membrane fluidity that can
be achieved by altering the membrane lipid composition. On the other hand, PKC (a ▶peripheral protein)
translocates from the cytosol to the membrane upon
different types of stimuli. One such stimulus is an
increase in the ▶non-lamellar phase propensity of membranes. Thus, a high non-lamellar HII-phase propensity
(i.e. elevated negative curvature strain) favors the
binding of PKC to membranes and its subsequent
activation. Diacylglycerol induces PKC activation not
only by binding to the enzyme but also by promoting
the HII-phase. In line with this, phorbol esters, which
induce a marked activation of PKC enzyme and that
are tumor promoters, also induce these non-lamellar
phases. For this reason, it has been suggested that inhibitors of PKC could potentially behave as anticancer
compounds. However, the anticancer drug ▶minerval
(▶2-Hydroxyoleic acid) also increases the non-lamellar
propensity of membrane lipids, activating PKC. An
explanation for this apparently paradoxical behavior is
that phorbol esters induce massive activation of PKC,
followed by rapid enzyme depletion within minutes
of the treatment. In contrast, minerval produces a
mild (about two-fold) and sustained activation and
overexpression of PKC. Likewise, some signaling
pathways regulated by G proteins are also involved in
the control of cell proliferation. In this context, the
localization and activity of G proteins is also regulated
by the non-lamellar propensity of membranes.
One important question regarding the regulation of
protein-lipid interactions is how lipid structure can
control the membrane translocation, cellular localization and the membrane sorting of these and other
▶peripheral signaling proteins. Different mechanisms
could be involved in these phenomena based on the
formation of membrane “defects”. Non-lamellar prone
lipids (e.g. phosphatidylethanolamine and minerval,
Fig. 1) reduce phospholipid surface packing (i.e. the
lateral pressure) at the interface and the polar regions
of the lipid bilayer. This allows hydrophobic domains of
peripheral proteins to interact with deep hydrophobic
regions of the membrane and/or fatty acid moieties of
phospholipids that exit out of the bilayer plane (Fig. 1).
In addition, electrostatic interactions with phospholipid
headgroups and interactions with other proteins also
Membrane-Lipid Therapy. Figure 1 Lipids with bulky
polar head, like phosphatidylcholine (yellow) and
phosphatidylserine (red), have a ▶molecular shape
similar to a cylinder. In turn, phosphatidylethanolamine
(green) has a small polar head and increases the
negative curvature strain (hexagonal phase propensity)
of membranes. Minerval (blue) also favors non-lamellar
phases. The scheme also shows a peripheral
(amphitropic) protein interacting with fatty acid moieties
of phospholipids (A) or deep hydrophobic regions of the
membrane (B) in non-lamellar prone regions.
Electrostatic interactions with the polar head of charged
phospholipids (C) also participate in the binding of this
protein to membrane.
participate in the binding of peripheral proteins to
membranes. The effect of lipids in regulating the
interaction of these proteins with membranes has been
demonstrated using synthetic lipids and purified proteins (G proteins, PKC).
Changes in membrane lipid composition have been
reported in several pathologies, including cancer. These
alterations may be associated with the etiology of the
disease or they may reflect an adaptive response.
Therefore, lipid interventions could be effective in
reversing pathological processes. Despite the potential
use of this new therapeutic approach, three important
issues will require further study during the following
years. First, an in-depth study of the molecular basis
underlying the interaction of numerous relevant proteins with membranes will be necessary. Second, the
influence of lipid changes on protein activity, pathophysiological processes and therapeutic actions must
be defined. Finally, the specificity of clinical strategies
based on this principle will have to be assessed.
Conventional chemotherapy is based on the design
of drugs to target specific proteins and the elucidation of their structure. From the molecular point of
view, conventional chemotherapy and membrane-lipid
therapy are quite different, although the final result
is the regulation of protein activity. The fact that all
membranes contain lipids could question the potential
specificity of this therapy. However, there is a huge
diversity of membrane lipids, a wide variety of membrane
lipid compositions and structures, and tremendous variety
among the protein-lipid interactions, which establishes
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Membrane-Lipid Therapy
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a suitable context to design specific therapies (see above).
Obviously, drugs acting through membrane-lipid therapy
not only reach pathological cells but also healthy cells,
as do drugs acting on a given protein through conventional chemotherapy. However, compounds used in
conventional chemotherapy often interact with proteins
other than their original target, causing side effects of
diverse importance. Thus, the degree of specificity of
both approaches could be similar. In this context, the
specificity of membrane-lipid therapy is more directly
associated with the effects promoted in cancer and other
types of pathological cells than with its interaction with
a given cell. An example of this is the activity of minerval
in tumor cells where its inhibitory effects display an IC50
in the range of 50–100 μM in comparison with the
IC50 value of >5,000 μM in non-tumor human fibroblasts
(IMR90 cells). In addition, it has been found that minerval
strongly induces the expression or repression of fewer
genes than most drugs, further supporting the specificity
of membrane-lipid therapy.
Development
It was proposed that the cytotoxic effects of anthracyclines on cancer cells may be exclusively exerted
through their interaction with membranes. Indeed, it
was later demonstrated that they acted by regulating
membrane lipid structure which altered the interaction
between peripheral signaling proteins and the plasma
membrane. Subsequently, a potential anticancer drug
under study, hexamethylene bisacetamide, was found
to have a very similar effect on cell membranes, which
was associated with the regulation of gene expression.
In the knowledge that the mechanism of action was
based on the regulation of membrane lipid structure,
a number of lipid-interacting molecules were subsequently studied and accordingly, it was demonstrated
that oleic acid and its derivatives are potent regulators
of the membrane structure. Subsequently, minerval
(the α-hydroxyl-derivative of oleic acid) was found to
be a potent antitumor agent without displaying any
relevant side effects.
Minerval is not the only drug used against cancer
that interacts with membranes. As mentioned above,
anthracyclines and hexamethylene bisacetamide also
regulate membrane structure and peripheral proteinassociated signaling. In addition, certain molecules that
readily bind to membranes have been shown to have
important anticancer effects. For instance, edelfosine
(Et-18-OCH3 (1-O-octadecyl-2-O-methyl-rac-glycero3-phosphocholine)) and miltefosine (HePC (hexadecyl
phosphocholine)) have an important hydrophobic moiety
with long hydrocarbon chains (18 and 16 C atoms,
respectively), and a polar region comprised of a phosphate group and a choline moiety (Fig. 2). This polarapolar hybrid structure appears to be a common feature
of these anticancer drugs that appear to be active in
Membrane-Lipid Therapy. Figure 2 Structures of the
semi-empirical RHF calculations for minerval (1), HMBA
(2), edelfosine (3), miltefosine (4), ▶daunorubicin (5),
propofol-DHA (6) and NEO6002 in gas phase. Carbon
atoms are shown in gray, hydrogen in light gray,
phosphorus in orange, oxygen in red, nitrogen in light
blue and fluorine.
membrane-lipid therapy. This physico-chemical property
of these drugs may allow them to interact with both the
surface-interface of the membrane and with the hydrophobic core, facilitating more stable and long-lasting
interactions, as well as inducing the relevant effects on
membrane lipid structure. Thus, an interesting class of
new anticancer drugs are those compounds known to
bind to lipid molecules, such as the molecule NEO6002
(Fig. 2). This drug results from the combination of
▶gemcitabine with cardiolipin, a phospholipid typical
of mitochondrial membranes, and it appears to be less
toxic and more effective than gemcitabine alone. The
lipid modification of gemcitabine induces the membranemediated internalization of the compound, which is not
blocked by nucleoside transporter inhibitors. Another
type of lipid-interacting compound is propofol-DHA
(Fig. 2), which combines a well-known anesthetic (propofol) with a polyunsaturated fatty acid (docosahexaenoic acid, DHA) that is present in membranes. The
resulting compound has been shown to induce apoptosis
in MDA-MB-231 breast cancer cells.
Most cell functions are localized in or around
membranes, and lipids control the interaction and
activity of many proteins. The relevance of lipids in
the treatment of cancer is also highlighted by the lipid
abnormalities identified in the membranes of patients
with cancer. Thus, changes in the type or abundance of
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Membrane-Lipid Therapy
lipids and other types of membrane components may
produce either positive or negative effects on health.
Hence, membrane-lipid therapy is a new therapeutic
approach that could be used in the treatment of cancer
and other pathologies.
References
1. Escriba PV, Sastre M, García-Sevilla JA (1995) Disruption
of cellular signaling pathways by daunomycin through
destabilization of nonlamellar membrane structures. Proc
Natl Acad Sci USA 92:7595–7599
2. Escribá PV, Ozaita A, Ribas C et al (1997) Role of
lipid polymorphism in G protein-membrane interactions:
nonlamellar-prone phospholipids and peripheral protein
binding to membranes. Proc Natl Acad Sci USA
94:11375–11380
3. Martínez J, Vögler O, Casas J et al (2005) Membrane
structure modulation, protein kinase Cα activation, and
anticancer activity of Minerval. Mol Pharmacol 67:531–540
4. Martínez J, Gutiérrez A, Casas J et al (2005) The
repression of E2F-1 is critical for the activity of Minerval
against cancer. J Pharmacol Exp Ther 315:466–474
5. Escribá PV (2006) Membrane-lipid therapy: a new
approach in molecular medicine. Trends Mol Med
12:34–43