From the Department of Zoology, Brigham Young
University, Provo, Utah 84602, the § Department of
Chemistry, the University of Illinois,
Chicago, Illinois 60607-7061, and the ¶ Department of Molecular
and Experimental Medicine, The Scripps Research Institute,
La Jolla, California 92037
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Exposure of S49 lymphoma cells to exogenous group
IIA or V secretory phospholipase A2
(sPLA2) caused an initial release of fatty acid followed by
resistance to further hydrolysis by the enzyme. This refractoriness was
overcome by exposing cells to palmitoyl lysolecithin. This effect was
specific in terms of lysophospholipid structure. Induction of membrane
susceptibility by lysolecithin involved an increase in cytosolic
calcium and was duplicated by incubating the cells with calcium
ionophores such as ionomycin. Lysolecithin also activated cytosolic
phospholipase A2 (cPLA2). Inhibition of this
enzyme attenuated the ability of lysolecithin (but not ionomycin) to
induce susceptibility to sPLA2. Lysolecithin or ionomycin
caused concurrent hydrolysis of both phosphatidylethanolamine and
phosphatidylcholine implying that transbilayer movement of phosphatidylethanolamine occurred upon exposure to these agents but
that susceptibility is not simply due to exposure of a preferred substrate (i.e. phosphatidylethanolamine) to the
enzyme. Microvesicles were apparently released from the cells upon
addition of lysolecithin or ionomycin. Both these vesicles and the
remnant cell membranes were susceptible to sPLA2. Together
these data suggest that lysolecithin induces susceptibility through
both cPLA2-dependent and -independent pathways.
Whereas elevated cytosolic calcium was required for both pathways, it
was sufficient only for the cPLA2-independent pathway. This
cPLA2-independent pathway involved changes in cell membrane
structure associated with transbilayer phospholipid migration and
microvesicle release.
Much of the research into the structure, function, and biophysics
of lipid membranes has focused on the use of artificial bilayers such
as vesicles. These studies are easily justified by the need to obtain
simple models for which all variables can be controlled or measured.
However, an important goal of these investigations is to attempt to
apply them to biological systems. Accordingly, our recent efforts have
focused on identifying means whereby studies of the relationship
between membrane structure and protein function can be applied to
cellular systems.
One system that appears amenable to this task is the control of
membrane susceptibility to the action of secretory phospholipase A2
(sPLA2).1
Normally, both cell membranes and large vesicles composed of saturated
phosphatidylcholines resist catalysis by sPLA2 (1-3). However, they become susceptible under certain conditions including exposure to specific molecules (1-5), upon oxidation of the
phospholipids (6, 7), or, in the case of cells, possibly during
pathological conditions such as inflammation, sepsis, or ischemia
(reviewed in Ref. 8).
Most of the experimental work with artificial bilayers has focused on
vesicles of defined composition and sPLA2 purified from snake venoms (groups I and IIA) or mammalian pancreas (group I). In
such systems, the induction of susceptibility appears to involve the
acquisition of specific membrane physical properties (1, 4, 5, 9, 10).
One molecule that confers susceptibility on artificial membranes is
palmitoyl-lysolecithin (lyso-PPC) which is thought to cause specific
perturbations that increase the access of membrane phospholipids to the
enzyme-active site (1, 9).
The possibility that lyso-PPC might also cause the membranes of living
cells to become susceptible to the action of sPLA2 has not
been considered previously. Nevertheless, many recent investigations
have explored potential pathological and physiological effects of the
lipid such as participation in certain inflammatory conditions (8, 11,
12), atherosclerosis (13, 14), ischemia (15), and regulation of smooth
muscle (16, 17). On a biochemical scale, lysolecithin has been reported
to regulate protein kinase C (18) and phospholipase D activities (19),
cellular calcium concentration (20, 21), gene expression (22, 23),
hormone secretion (24), and cation currents (25, 26).
The mechanisms by which susceptibility to sPLA2 is
determined in living cells are not yet known. Generally, induction of
susceptibility requires an increase in the intracellular calcium
concentration (2, 6, 27, 28). We have considered three hypotheses for
this effect of calcium as follows. Hypothesis 1, susceptibility depends
on prior activation of the high molecular weight
calcium-dependent intracellular phospholipase
A2 (29-31). Hypothesis 2, susceptibility is induced by
transbilayer migration of phospholipids normally found on the inner
leaflet of the cell membrane (3). Hypothesis 3, susceptibility depends
on the shedding of microvesicles from the plasma membrane into the
extracellular fluid (27).
We have used S49 lymphoma cells, calcium ionophores, and groups IIA and
V sPLA2 as an experimental system to examine these hypotheses. We have also determined whether lysophospholipids such as
lyso-PPC induce susceptibility and whether they share common mechanisms
with calcium ionophores. The results of these experiments have led us
to the novel conclusion that these cells are naturally susceptible
rather than resistant to the enzyme and that brief exposure to
sPLA2 causes them to become refractory to further
hydrolysis. Elevated intracellular calcium appears to circumvent the
refractoriness causing persistent susceptibility.
Materials--
S49 lymphoma cells were grown as described (2).
The various sPLA2 were expressed and/or isolated according
to published procedures (monomeric aspartate 49 (Asp-49) and lysine 49 (Lys-49) enzymes from the venom of Agkistrodon piscivorus
piscivorus, Ref. 32; human group IIA, Ref. 33; human group V, Ref.
34). Bee venom sPLA2 (group III) was obtained from Sigma.
The final concentrations sPLA2 in all experiments were 0.5 to 5 µg/ml depending on the specific activity of the individual
preparation except as specified in Fig. 1. Lysophospholipids were
purchased from Avanti (Birmingham, AL) and suspended in methanol (10 mM) and diluted in 50 mM KCl and 3 mM NaN3 to yield a 1 or 2 mM stock
for use in experiments. The amount of lyso-PPC partitioned into the
cell membrane in experiments was assessed by thin layer chromatography.
Under conditions where an upper limit would be expected
(i.e. conditions of Table I), the cellular lysolecithin
content increased from about 1.6% of the total phospholipid to about
3.8%. Pharmacological agents were dissolved in appropriate solvents
(dimethyl sulfoxide, methyl acetate, or ethanol). Control experiments
demonstrated that these solvents did not have effects on the
experimental data at the concentrations used.
Fluorescence Spectroscopy--
Cells were harvested, washed, and
suspended to a final density of about 1-2 × 106
cells/ml in a balanced salt solution as described (2). Measurements with fluorescent probes were obtained at 37 °C using a Fluoromax (Spex Industries) photon-counting spectrofluorometer (2). Release of
fatty acids from cells was assayed with an acrylodan-labeled intestinal
fatty acid-binding protein (ADIFAB) (~0.2 µM final, excitation = 390 nm, emission = 432 and 505 nm; Refs. 2 and 35), and results were quantified by calculation of the generalized polarization (2, 36). The amount of phospholipid hydrolyzed was
estimated as described (35, 37) for oleate and arachidonate to give a
lower and upper estimate of the quantity of fatty acid produced.
Propidium iodide (37 µM final, excitation = 536 nm, emission = 617) was used to assess general cell membrane
permeability. Due to their aqueous solubility, neither ADIFAB nor
propidium iodide required previous equilibration with the sample; thus, both were added immediately prior to other experimental agents. Laurdan
(0.05 µM final, excitation = 350 nm, used to measure
bilayer polarity, Ref. 36) and indo-1 (3.75 µM final,
excitation = 350 nm, emission = 405 and 480 nm, used to
measure intracellular calcium) were incorporated into cells and assayed
as described (2). Calcium concentration was determined by calculating
the generalized polarization and comparison to a calibration curve.
BAPTA (90 µM) was equilibrated with cells for 2 h at
37 °C in cell culture medium without serum.
Light Scattering--
Cells were harvested as described above,
suspended at a density of approximately 5 × 106
cells/ml, and incubated at 37 °C in a shaking water bath. At various
time intervals following addition of experimental agents, 1 ml of cells
was removed and centrifuged 10 s at 13,000 rpm. The supernatant
was immediately removed and transferred to liquid nitrogen to quench
the reaction. An aliquot (800 µl) of the supernatant was added to 2 ml of aqueous solution, and the intensity of scattered light
(excitation = 500 nm, emission = 510 nm) was assayed.
Flow Cytometry--
To detect phosphatidylserine exposure in the
outer leaflet of the plasma membrane induced by ionomycin, cells were
stained with fluorescent-labeled annexin V and propidium iodide for 15 min at 25 °C according to instructions provided with an Apoptosis Detection Kit purchased from R & D Systems (Minneapolis, MN). Flow
cytometry data were collected and analyzed using a Coulter Epics XL
flow cytometer and the associated software.
Phospholipid Extraction and Thin Layer Chromatography--
Cells
were prepared, incubated, and centrifuged, and pellets and supernatants
were frozen in liquid nitrogen as described for light scattering
experiments. Samples were quickly thawed, and lipids were extracted
with chloroform and methanol (38). Phospholipids and lysophospholipids
were separated by thin layer chromatography in 6.5:2.5:1 (v/v)
chloroform:methanol:acetic acid and were developed in the presence of
iodine crystals. Spots were identified using standards. Lipid content
in each spot was quantified either by densitometry or by phosphate
assay (39).
Incubation of S49 lymphoma cells with the Asp-49 snake venom
sPLA2 caused a small transient release of fatty acid
followed by a return of extracellular fatty acid levels to near base
line (Fig. 1, panel A, curve
a). Similar phenomena were observed when the enzyme was added to a
variety of human white blood cell lines (Raji, HL-60, and MOLT-4 cells)
suggesting that these phenomena are not unique to S49 cells. Likewise,
this behavior was not limited to snake venom group IIA
sPLA2 since recombinant human group V sPLA2
produced a similar effect (Fig. 1, panel A, curve b). No hydrolysis was detectable with this assay upon addition of human recombinant group IIA enzyme (Fig. 1, panel A, curve c).
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
APPENDIX
REFERENCES
EXPERIMENTAL PROCEDURES
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
APPENDIX
REFERENCES
RESULTS
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
APPENDIX
REFERENCES
View larger version (20K):
[in a new window]
Fig. 1.
Effect of sPLA2 incubation on
membrane hydrolysis. Panel A, S49 cells were incubated
with 1 µg/ml snake venom Asp-49 (curve a), human group V
(curve b), or human group IIA sPLA2 (curve
c), and hydrolysis was assessed with ADIFAB as described under
"Experimental Procedures." Enzyme was added at the
dashed line. All curves are plotted on the same
relative scale and are displaced along the ordinate for
clarity of presentation. The maximum amount of hydrolysis in
curve a was estimated as 0.2-0.5
µM/106 cells (see "Experimental
Procedures"). Panel B, the effect of the second
application of sPLA2 (arrow) with cells already
exposed to one of the three types of sPLA2
(dashed line). Curve a, 1 µg/ml
snake venom Asp-49 sPLA2 and then 1 µg/ml snake venom
Asp-49 sPLA2; curve b, 100 ng/ml snake venom
Asp-49 sPLA2 and then 1 µg/ml snake venom Asp-49
sPLA2; curve c, 3 ng/ml snake venom Asp-49
sPLA2 and then 1 µg/ml snake venom Asp-49
sPLA2; curve d, 1 µg/ml human group V
sPLA2 and then 1 µg/ml human group V sPLA2;
curve e, 3 µg/ml human group V sPLA2 and then
1 µg/ml snake venom Asp-49 sPLA2; curve f, 1 µg/ml bee venom group III sPLA2 and then 1 µg/ml snake
venom Asp-49 sPLA2; curve g, 1 µg/ml human
group IIA sPLA2 and then 1 µg/ml human group V
sPLA2; curve h, 1 µg/ml snake venom Lys-49
sPLA2 and then 1 µg/ml snake venom Asp-49
sPLA2. Panel C, theoretical curves were
generated using Equation 5 (see "Appendix"). Enzyme concentrations
were the same as in the corresponding curves (a-c) of
panel B. Other parameter values were constant and are listed
under the "Appendix."
Refractoriness-- The fatty acid released in the time courses of Fig. 1, panel A, represented a small fraction of the cellular phospholipid (~2-5%; see legend to Fig. 1). Therefore, the transient nature of the time course could reflect a limitation in the availability of substrate for hydrolysis or some inhibitory mechanism preventing further hydrolysis. To consider these possibilities, we repeated the time courses shown in Fig. 1, panel A, with a second addition of sPLA2 following the return of fatty acid levels to base line. The second application of sPLA2 resulted in no release of fatty acid (Fig. 1, panel B, curve a). Reduction of the initial concentration of enzyme by a factor of 10 (from 1 to 0.1 µg/ml) still caused the cells to be refractory to a second addition of the higher concentration of the enzyme (i.e. 1 µg/ml, curve b). Further reduction in the initial sPLA2 concentration allowed hydrolysis to be accomplished upon the second addition (curve c). Control experiments revealed that the phenomenon was not an effect of incubation time or the solvent in which the enzyme was dissolved.
This ability of an initial dose of sPLA2 to cause resistance to hydrolysis by subsequent enzyme was independent of the amount of initial bilayer hydrolysis and was nonspecific with respect to the type of sPLA2. As shown in Fig. 1, panel B, incubation of the cells with human group V sPLA2 caused refractoriness to a second addition of either the same enzyme (curve d) or the snake venom enzyme (curve e). The less-related sPLA2 from bee venom (group III, curve f) also produced the same effect. Importantly, addition of human group IIA sPLA2 reduced the ability of the group V enzyme to hydrolyze the bilayer even though no detectable hydrolysis was observed during the initial incubation (curve g). This independence from hydrolysis was verified by incubating the cells with snake venom sPLA2 that is catalytically inactive due to a substitution of the amino acid lysine for aspartic acid at position 49 (Lys-49 sPLA2; curve h).
Effect of Lysophospholipids--
Fig.
2 demonstrates that this refractoriness
was reversed (panel A, curve a) or prevented (panel B,
curve a) by incubation with 5 µM lyso-PPC. The
increase in base-line slope upon addition of lyso-PPC (Fig. 2,
panel B, curve a) appears to reflect the activity of
cPLA2 as explained below in the description of Fig. 6. As
shown in panel C, the ability of lyso-PPC to induce
susceptibility to sPLA2 was applicable also to the human
enzymes. This effect of lyso-PPC was
concentration-dependent up to about 7-10 µM
(Fig. 3, panel A). As expected
based on previous studies (2), the calcium ionophores, A23187 and
ionomycin, each produced the same effect as lyso-PPC (see Fig. 2,
panel B, curve b, for example). Similar results
were obtained with Raji, HL-60, MOLT-4, and K-562 cells. HeLa
(epithelial) cells, however, did not respond to ionophore or
lyso-PPC.
|
|
We tested other lysophospholipids to determine whether the induction of membrane susceptibility by lyso-PPC was specific for the structure of the lipid. Lyso-PPG (Fig. 3, panel B, curve a) behaved similarly to lyso-PPC. The relative potency of the other lipids was as follows: lyso-PPG > Lyso-OPC > lyso-PPA > lyso-PPE (no detectable effect). 1-Alkyl-2-hydroxy-sn-glycero-3-phosphocholine had the same action as lyso-PPC suggesting that the effect was not due to metabolites resulting from Sn1 hydrolysis.
The dependence on lysophospholipid structure illustrated in Fig. 3,
panel B, suggested the possibility that lyso-PPC acts at
specific binding sites and that its effect involves distinct biochemical pathway(s). Neither isoproterenol, which leads to activation of the G-protein Gs and elevates cAMP levels in
S49 cells, nor phorbol myristoyl acetate, which activates protein kinase C, enhanced the ability of sPLA2 to hydrolyze S49
cells. Furthermore, the following inhibitors pertussis toxin (other
G-proteins), wortmannin (phosphatidylinositol 3-kinase), and K-252
(several protein kinases) did not reduce the ability of lyso-PPC to
render the cell membrane susceptible to sPLA2. In contrast,
cobalt, a general blocker of transmembrane calcium transport, greatly
attenuated the response (Fig. 4,
panel A, curve b) suggesting that increases in cytosolic
calcium could be responsible for the action of lyso-PPC on membrane
hydrolysis as is the case for calcium ionophores. Nickel (0.5 mM) produced a similar effect (not shown). This observation was validated by experiments demonstrating that the intracellular calcium chelator BAPTA also attenuated the response to
lyso-PPC (Fig. 4, panel B, curve b).
|
Accordingly, we used indo-1 to monitor the effect of lyso-PPC on
cytosolic calcium. As shown in Fig. 5,
panel A (curve a), the addition of
lyso-PPC caused an immediate rise in intracellular calcium. This effect
was dependent on lyso-PPC concentration and reached a maximum at 7-10
µM reminiscent of the concentration dependence displayed
in Fig. 3, panel A, for induction of susceptibility. Experiments with extracellular EDTA demonstrated that this elevation of
intracellular calcium represented an influx of the ion from the cell
exterior.
|
Even though it appeared that lyso-PPC and calcium ionophore induced membrane susceptibility by common mechanisms, important differences were found between these agents. First, the size of the calcium influx required for ionomycin to induce susceptibility was larger and more persistent than that achieved with lyso-PPC (Fig. 5, panel A, curve b). Second, although both lyso-PPC and ionophore caused immediate increases in cytosolic calcium, about 100-200 s were required for complete realization of membrane susceptibility with ionophore, whereas the effect of lyso-PPC was more rapid in onset (Fig. 5, panel B, triangles).
Calmodulin-- We examined the possible involvement of calmodulin in the process of inducing membrane susceptibility to sPLA2 using several calmodulin inhibitors as follows: W-7, calmidazolium, and ophiobolin A. In initial experiments, ophiobolin A alone was found to induce susceptibility of S49 cells to sPLA2 and therefore was not used in additional experiments. W-7 significantly reduced the rate of hydrolysis in cells incubated with lyso-PPC and sPLA2, increasing the half-time for complete hydrolysis by a factor of 100. A less profound effect was obtained with ionomycin as the stimulant of susceptibility; W-7 increased the half-time for membrane hydrolysis from 97.4 to 287.6 s. This effect of W-7 was dose-dependent (EC50 = ~30 µM). Calmidazolium had little or no reproducible effect.
Hypothesis 1--
As stated in the Introduction, three hypotheses
for the effect of calcium to cause membranes to become susceptible to
sPLA2 were considered. To test hypothesis 1, a specific
inhibitor of cPLA2 (MAFP) was used (40). As shown in Fig.
6, panel A, the release of
fatty acid from the membrane normally observed upon the addition of
lyso-PPC alone (5 µM; curve a) was blocked by MAFP (curve b) consistent with a previous observation that
lysolecithin can activate cPLA2 (41). Also, the rate of
hydrolysis upon addition of human sPLA2 was reduced in
cells treated with MAFP (Fig. 6, panel B, curve b) compared
with control cells (curve a). In contrast to lyso-PPC,
ionomycin did not stimulate measurable fatty acid release by
cPLA2 unless the incubation was prolonged (>1000-1500 s,
not shown). Accordingly, MAFP had no detectable effect on the time
course of S49 cell hydrolysis in the presence of ionomycin. These
effects were not confined to the human enzyme since they were
reproducible with the snake venom sPLA2 (Fig. 6,
panel C). A second inhibitor of cPLA2
(AACOCF3; Ref. 42) also decreased the initial rate of
hydrolysis in the presence of lyso-PPC and sPLA2. However,
in contrast to MAFP, AACOCF3 also reduced the rate of
hydrolysis in the presence of ionomycin. That these inhibitory effects
of AACOCF3 were probably due to a nonspecific action of the agent was demonstrated by the observation that an alcohol analog of
AACOCF3 (AAC(OH)CF3) that does not inhibit
cPLA2 produced the same effects on lyso-PPC and
ionomycin-stimulated susceptibility to sPLA2.
|
Hypothesis 2-- We first addressed whether significant exposure of phosphatidylserine occurred upon incubation with ionomycin on a time scale relevant to the induction of susceptibility. Cells were incubated for 15 min at 25 °C with or without ionomycin in the presence of propidium iodide and fluorescent-labeled annexin V (a phosphatidylserine binding protein, Ref. 43). The percentage of cells binding annexin V (i.e. those with phosphatidylserine exposed to the cell exterior) was then quantified by flow cytometry. Incubation with ionomycin increased the percentage of cells binding annexin from 2 to 87%. Parallel control experiments without annexin V verified that the cells were susceptible to sPLA2 under the same conditions.
Because of limitations in the time resolution of the annexin binding assay, we investigated hypothesis 2 further by monitoring the time course of hydrolysis of the major classes of phospholipid. We reasoned that if translocation were slower than and not necessary for membrane susceptibility, phosphatidylcholine would be hydrolyzed first and that subsequent hydrolysis of other phospholipids would reflect delayed translocation. Conversely, hydrolysis of phospholipids from the inner leaflet (phosphatidylethanolamine and/or phosphatidylserine) concurrent with or prior to phosphatidylcholine would reflect translocation of those lipids. Table I displays the initial rate of hydrolysis of the two most abundant phospholipids in S49 cell membranes, phosphatidylcholine and phosphatidylethanolamine, in the presence of ionomycin or lyso-PPC (at 20 µM because of the higher cell density required in the experiment). Initial hydrolysis of both lipids was rapid and concurrent for either the ionophore or lyso-PPC. When sPLA2 was present alone, the reaction was transient as was observed in the continuous fluorescence assay of total fatty acid release (Fig. 1). Control experiments revealed that the observed hydrolysis of phosphatidylethanolamine was due to sPLA2 rather than cPLA2. Reproducible hydrolysis of both phosphatidylcholine and phosphatidylethanolamine was also observed with the groups IIA and V human sPLA2, and differences among the three enzymes (human and snake venom) reflected apparent overall rates of hydrolysis rather than preferential hydrolysis of specific phospholipids (Table I). Detailed investigations into the mechanisms of differences among the human enzymes are presented elsewhere.2
|
Hypothesis 3--
Light scattering was used to detect the presence
of vesicles shed from cells. After exposing S49 cells to ionomycin or
lyso-PPC for various times, cells were separated from any microvesicles by centrifugation, and the intensity of light scattered by the supernatant was then assessed. As shown in Fig.
7, panel A, a time-dependent increase in the light scattering was
observed upon exposure of the cells to either ionomycin
(squares) or lyso-PPC (circles) suggesting that
particles had been released from the cells. Extraction of the
supernatant with chloroform and methanol followed by thin layer
chromatography revealed that these particles contained phospholipid
(phosphatidylethanolamine > phosphatidylcholine). Isolation of
the particles and subsequent exposure to sPLA2 demonstrated that they were susceptible to hydrolysis by the enzyme (40% of vesicle phospholipid hydrolyzed, estimated by ADIFAB).
|
Comparison of the data shown in Fig. 7, panel A, for ionomycin and lyso-PPC reveals that the release of the particles (presumably microvesicles) was slower for ionomycin than for lyso-PPC. Furthermore, the time course was typically biphasic in the case of lyso-PPC. To test whether the release of microvesicles stimulated by lyso-PPC was calcium-dependent, we repeated the experiment of panel A in the presence or absence of cobalt and found that the initial (but not the prolonged) phase of the time course was blocked (panel B, squares). Finally, we compared other lysophospholipids to determine whether there was a relationship between the specificity for microvesicle release and the specificity for induction of susceptibility to sPLA2 (i.e. Fig. 3, panel B). As shown in Fig. 7, panel C, the same relationship among the various lysophospholipids was observed (lyso-PPC = lyso-PPG > lyso-OPC > lyso-PPA > lyso-PPE).
Previous work suggested that induction of membrane susceptibility to
sPLA2 may involve changes in physical properties of the plasma membrane detectable by laurdan and propidium iodide
fluorescence. As shown in Fig. 8,
panels A and B, incubation of S49
cells with ionomycin did not affect membrane permeability to propidium
iodide and caused only minor changes to the emission spectrum of
membrane-bound laurdan. In contrast, larger changes in the fluorescence
of both probes were observed with lyso-PPC (panels C and
D). In the presence of lyso-PPC, the propidium iodide
intensity increased substantially and the laurdan emission spectrum
shifted to shorter wavelengths. Cobalt inhibited the effect of lyso-PPC
on propidium iodide fluorescence (panel E) but had little or
no effect on the ability of lyso-PPC to shift the laurdan spectrum
(panel F). Control experiments testing whether lyso-PPC was
toxic to the cells revealed that cells exposed to lyso-PPC continued to
grow in culture similar to non-exposed cells. Consistent with their
effect on susceptibility, lyso-PPG and lyso-OPC (but not lyso-PPA or
lyso-PPE) increased propidium iodide uptake by the cells to about
0.3-0.5 of the extent shown for lyso-PPC in Fig. 8, panel
C. However, all lysophospholipids tested except lyso-OPC caused
the same changes to the laurdan emission spectrum as lyso-PPC (shown in
Fig. 8, panel D).
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
These studies revealed three novel general observations. First, S49 lymphoma cells are naturally susceptible to sPLA2 but quickly become resistant upon initial exposure to the enzyme. Second, lyso-PPC can reverse or prevent this development of resistance and thus render the cell membranes persistently susceptible to the enzyme. Third, this effect of lyso-PPC is calcium-dependent and displays many similarities with calcium ionophore treatment on vulnerability of S49 cells to sPLA2. Importantly, these observations were found to be relevant to a variety (but not all) of human cell lines and for human groups IIA and V sPLA2 in addition to snake venom group IIA enzyme. Thus, these findings are general phenomena (at least for certain leukocytes) rather than isolated experimental curiosities.
Refractoriness-- The observation that initial exposure to sPLA2 causes the cell membranes to become refractory to further hydrolysis by the enzyme was surprising. The mechanism for this phenomenon is not yet known, but several possibilities can be excluded based on the data of Fig. 1. For example, both the Lys-49 sPLA2 from snake venom and the recombinant human group IIA which did not cause detectable hydrolysis of the cell membrane were capable of protecting the membrane from hydrolysis by a more active enzyme such as the snake venom Asp-49 or the human group V sPLA2. Therefore, membrane hydrolysis is not required for the effect, and hypotheses such as physical changes to the membrane structure during hydrolysis or involvement of reaction products can be excluded as explanations for the refractoriness. The results of Fig. 1 also address whether the inactivity of the second dose of enzyme is due to limited substrate or saturation of binding sites on the membrane. This hypothesis assumes that the enzyme-binding sites responsible for refractoriness and the sites of hydrolysis are identical and that those enzymes that do not catalyze initial hydrolysis (human group IIA and venom Lys-49) still adsorb to and saturate these binding sites. If this hypothesis were true, reduction in the initial concentration of sPLA2 to the point that there is less hydrolysis (such as in curve b of Fig. 1, panel B) would correspond to a condition at which these "binding sites" would not be saturated and hydrolysis would thus occur when a second dose of sPLA2 is added. However, the contrary was observed (curve b, Fig. 1, panel B) eliminating this "saturation of binding sites" hypothesis.
A likely explanation for the data is that membrane hydrolysis and induction of refractoriness represent two separate effects of sPLA2 that involve independent binding sites with different affinities for the enzyme (the site responsible for refractoriness would have higher affinity). To consider this possibility quantitatively, we generated a mathematical model (see "Appendix") that incorporates three separate events as follows: 1) binding of enzyme to and hydrolysis of the cell membrane, 2) binding of enzyme to a distinct high affinity site that results in time-dependent inhibition of the first event, and 3) slower removal of product. As shown in Fig. 1, panel C, this model was capable of reproducing all of the results observed experimentally (Fig. 1, panel B, curves a-c) at the various doses of sPLA2 tested. Therefore, this hypothesis is a viable explanation for the data. Although an attractive candidate for the putative additional binding site is the 180-kDa M-type sPLA2 receptor (44), the observation that bee venom sPLA2 also induced refractoriness argues against this possibility since the bee venom enzyme does not to bind to the M-type receptor (44). Nevertheless, other receptors for sPLA2 have been described, and it is reasonable to propose that sPLA2 induces refractoriness through an action at one such site.
Effects of Lyso-PPC-- The presence of lyso-PPC both prevented and reversed the refractoriness of the S49 and various human cells to hydrolysis by sPLA2. This effect is reminiscent of the ability of the lysophospholipid to render phosphatidylcholine bilayers susceptible to the enzyme (1, 9). In the latter case, the induction of susceptibility is a biophysical effect involving structural alterations to the bilayer making it easier for phospholipids to migrate into the active site of enzyme adsorbed to the bilayer surface (9). This similarity between cells and artificial bilayers raises the question of whether the effect of lyso-PPC on cells is biophysical or biochemical.
Several observations suggest that the ability of lyso-PPC to make cells susceptible to sPLA2 involves specific biochemical pathways rather than direct biophysical perturbation of the bilayer. First, the concentration of lyso-PPC required for the effect was similar to or lower than that required for a variety of biochemical and physiological effects of the lipid reported for other cells and tissues (15-26). Second, as is true for other reported biochemical effects of lyso-PPC (18, 24), there was a structural specificity for the phenomenon (see Fig. 3, panel B). Third, the effect of lyso-PPC to cause the cells to be susceptible to sPLA2 was dependent on calcium influx through cobalt-sensitive pathways. Had the lipid simply been creating nonspecific holes in the membrane, it is unlikely that such would have been inhibited by the presence of cobalt or nickel. Control experiments verified that the inhibitory effect of cobalt was not due to direct inhibition of sPLA2. Also, experiments with indo-1 demonstrated that cobalt blocked the influx of calcium into the cell induced by lyso-PPC (not shown). Fourth, lyso-PPC did not cause susceptibility to sPLA2 in all cell types (e.g. HeLa cells did not respond unless damaged by excessive trypsin treatment). Also, P388D1 macrophages appear not to respond to lysophospholipids (29). Had the action of lyso-PPC been a generic nonspecific effect, one would expect it not to depend on cell type. The fact that the HeLa cells also did not respond to calcium ionophore suggests that the deficiency in their ability to respond to lyso-PPC involves one or more downstream events. Future investigations with HeLa cells may therefore help to identify further the nature of the mechanisms that govern susceptibility or resistance to sPLA2.
Notwithstanding this evidence that lyso-PPC may function through specific biochemical pathways, it is clear that structural changes to the membrane do occur upon incubation with lyso-PPC. These include apparent transbilayer migration of phospholipids (Table I) and release of microvesicles (Fig. 7) as well as changes detected by the fluorescent probes propidium iodide and laurdan (Fig. 8). Experiments using fluorescence microscopy (not shown) revealed that the enhancement of propidium iodide fluorescence reflected increased permeability of the cells to the probe allowing it access to DNA (45). This result raises the question of whether lyso-PPC could be toxic to the cells since dead cells are permeable to the dye. This question is difficult to answer because some of the uptake of propidium iodide by the cells was not accompanied by a proportional increase in permeability to trypan blue. Furthermore, in a few cases, cells that stained with trypan blue were impermeable to propidium iodide. The ultimate test of cell viability is whether the cells continue to multiply. As stated under "Results," cells exposed to reasonable concentrations (similar to those of the figures) of lyso-PPC continued to divide at normal rates. Accordingly, it was clear from our results that cells death is not a prerequisite to sPLA2 susceptibility induced by lyso-PPC or ionomycin. Obviously, as suggested by the data in Table I, the presence of lyso-PPC and sPLA2 together (especially snake venom enzyme) can be lethal to the cells due to extensive membrane hydrolysis.
Although it appeared that cell death was not a prerequisite for susceptibility to sPLA2, we note that high concentrations of lyso-PPC alone can induce cell death as well as a latent nonspecific susceptibility to sPLA2 that is not sensitive to inhibitors such as cobalt. This latter observation reveals a necessary caution for interpreting investigations into the effects of lyso-PPC since many studies have been done at these higher concentrations that appeared toxic in this system. Based on our experience, we recommend that the effects of lyso-PPC on cellular biochemistry and physiology be studied at concentrations below ~5 µM/106 cells.
The membrane alteration reflected by the cobalt-sensitive increase in permeability to propidium iodide is not an absolute requirement for the induction of susceptibility to sPLA2 since ionophore treatment did not cause a similar increase in permeability. Nevertheless, the alteration may be ancillary in causing membrane susceptibility since lyso-PPC enhanced the ability of sPLA2 to hydrolyze the cells faster and at lower calcium concentration than ionophore (Fig. 5). That this and/or other effects of the lipid promote membrane hydrolysis was also seen by the observation that lyso-PPC was much more effective than ionophore at promoting membrane degradation by cPLA2 (Fig. 6).
The data with laurdan (Fig. 8) suggested that lyso-PPC caused a decrease in the interaction of water with the bilayer since the laurdan emission spectrum is sensitive to the amount and mobility of water molecules in the region of the phospholipid glycerol backbone (46, 47). Importantly, this effect of lyso-PPC was not blocked by cobalt (Fig. 8), was observed with lipids that did not induce susceptibility to sPLA2 (lyso-PPA and lyso-PPE), and was not seen with one of the lipids that did induce susceptibility (lyso-OPC). Therefore, it appeared that this membrane perturbation reflects a nonspecific interaction of these lipids with the bilayer. Nevertheless, these results do illustrate that the inability of lyso-PPA and lyso-PPE to induce susceptibility was not because these lipids were unable to bind to the cell membrane.
Does the Induction of Susceptibility to sPLA2 Involve Calmodulin?-- Although several inhibitors of calmodulin or calmodulin-dependent pathways were tested, only one, W-7, provided consistent evidence for a possible role for calmodulin. Interestingly, W-7 has also been reported to inhibit the calcium-dependent shedding of microvesicles from cell membranes (48, 49), an event associated with susceptibility to sPLA2 (see "Discussion" below and Ref. 27). If calmodulin is involved in these effects of intracellular calcium, it is difficult to understand why other calmodulin inhibitors were ineffective. One plausible explanation is that some other calcium-binding protein involved in the induction of susceptibility such as the enzymes described below shares sufficient structural similarity to calmodulin that W-7 cross-reacts with that protein. We note, for example, that W-7 also inhibited the activity of cPLA2 stimulated by lyso-PPC. Based on the results obtained in this study, we are reluctant to conclude that calmodulin plays a large role in the effect of intracellular calcium to promote cell membrane hydrolysis by sPLA2.
Hypothesis 1, cPLA2-- Data published recently by Balsinde and Dennis (29) demonstrated that blocking the activity of cPLA2 also caused a 75% reduction in the activity of extracellular sPLA2 toward P388D1 macrophages. These results suggested that membrane hydrolysis by sPLA2 is subsequent to activation of cPLA2. Similar conclusions have been reached by other investigators (30, 31). The hypothesis is that the action of cPLA2 perturbs the cell membrane in ways that make it susceptible to sPLA2 (29).
Lyso-PPC appeared to induce susceptibility by dual mechanisms. One of these mechanisms involved the apparent activation of cPLA2. This activation of cPLA2 was calcium-dependent since cobalt, nickel, and BAPTA all prevented fatty acid release in the presence of lyso-PPC alone (Fig. 4). A stimulatory effect of lyso-PPC on cPLA2 has also been reported previously (41), and the coupling of this event to hydrolysis by sPLA2 was consistent with the proposal of Balsinde and Dennis (29). The second mechanism, also requiring calcium, was independent of the action of cPLA2 and appeared to involve membrane perturbations such as transbilayer migration of phospholipids and production of microvesicles (see below). Elevation of intracellular calcium alone (i.e. through ionophore) was sufficient to mimic the latter but not the former effect of lyso-PPC. This latter mechanism could be analogous to the membrane perturbations downstream of the activation of cPLA2 leading to susceptibility to sPLA2. Alternatively, it may represent a separate mechanism by which susceptibility can be induced without requirement for cPLA2.
The same structural specificity of lysophospholipid species identified for the induction of sPLA2 susceptibility (Fig. 3, panel B) was observed for the activation of cPLA2 and probably reflects the ability of each species to promote calcium entry into the cells. This structural specificity is important for preventing possible positive feedback. If lysophosphatidylethanolamine were capable of stimulating calcium uptake and activation of cPLA2, positive feedback would result from the hydrolysis of phosphatidylethanolamine on the interior of the cell membrane. Likewise, the natural membrane asymmetry protects phosphatidylcholine from attack by cPLA2 and the cell from the positive feedback that would result from that event.
Hypothesis 2, Transbilayer Migration of Phospholipids-- One of the effects of increased intracellular calcium is translocation of phospholipids normally found on the extracellular face (such as phosphatidylcholine) to the interior of the cell membrane and export of phospholipids normally found on the intracellular face (phosphatidylethanolamine and phosphatidylserine) to the extracellular face (reviewed in Ref. 50). These transbilayer migrations may involve at least two calcium-dependent events. First, the enzyme normally responsible for maintaining bilayer asymmetry, aminophospholipid translocase, is inhibited by elevated intracellular calcium that results in a progressive loss of membrane phospholipid asymmetry (50). A second enzyme, scramblase, is activated by a rise in cytosolic calcium and responds by catalyzing migration of phospholipids across the membrane (50, 51). It has been hypothesized that phosphatidylethanolamine is a better substrate for sPLA2 than phosphatidylcholine and that the simple act of exposing the aminophospholipid to the exterior might be sufficient to account for the effect of intracellular calcium on membrane hydrolysis by extracellular sPLA2 (3). However, the specificity of sPLA2 for phospholipid head groups appears controversial. Reports of preference for phosphatidylcholine, phosphatidylethanolamine, or neither exist in the literature (e.g. Ref. 52-55). One possible resolution to that controversy is that specificity is determined not by the structure of the head group per se, but by the physical state of the phospholipids within the bilayer and that an altered distribution of phospholipids across the bilayer could affect that physical state (52). Another explanation for the controversy may lie in differential specificity of the various types of sPLA2 (venom versus mammalian, group II versus group V; see Ref. 34).
Measurements of annexin binding illustrated that exposure of phosphatidylserine on the outer leaflet of S49 cell membranes occurs at a dose of ionomycin and time scale consistent with the induction of susceptibility. Also, rapid transbilayer movement of phospholipids stimulated by calcium (t1/2 in the range of minutes or less) has been observed (56). Therefore, this hypothesis appears feasible. The data of Table I also support rapid phospholipid translocation since the initial rate of phosphatidylethanolamine hydrolysis was comparable to that of phosphatidylcholine. Moreover, these data do not support the idea that translocation of a preferred substrate is the mechanism by which calcium induces susceptibility. Nevertheless, translocation of phospholipids may be important for other physical changes to the bilayer that result in susceptibility (see below). Also, since sPLA2 binding to bilayers is promoted by negative charge (4, 5), the exposure of phosphatidylserine may help recruit enzyme to the bilayer surface for catalysis.
Hypothesis 3, Membrane Vesiculation-- Several cells have been shown to shed vesicles upon stimulation with ionophores (27, 56, 57), and these vesicles appear to be susceptible to sPLA2 (27). This shedding of microvesicles relates to calcium levels by two pathways. First, it is promoted by the loss of membrane asymmetry described in the previous section (56, 57). Second, it also involves hydrolysis of cytoskeletal elements by calcium-dependent proteases (58). Fig. 7 suggests that an increase in intracellular calcium causes S49 cells to shed vesicles. Two lines of evidence support the idea that the release of these vesicles is associated with the induction of susceptibility to sPLA2 in S49 cells. First, vesicle release was more rapid when induced by lyso-PPC than by ionophore (Fig. 7, panel A). This time dependence coincided with the relative time dependence for the two agents to induce susceptibility (Fig. 5, panel B). Second, the same dependence on lysophospholipid structure for release of the vesicles was found as for induction of susceptibility (Fig. 3, panel B).
Interestingly, cobalt blocked rapid release of microvesicles stimulated by lyso-PPC but did not block slower subsequent release (Fig. 7, panel B). The rapid, cobalt-sensitive phase also appeared to coincide with the rapid, transient rise in intracellular calcium observed with lyso-PPC (Fig. 5, panel A). The reason why the intensity of light scattering during the rapid phase of vesicle release decreased is also not known but would be consistent with a time-dependent reduction in vesicle size. The origin of the slower phase of vesicle release is not known, but we did observe in some experiments at excessive lyso-PPC concentrations a cobalt-insensitive induction of susceptibility that was much slower in onset consistent with slower vesicle release.
These correlations between vesicle release and susceptibility suggest that the vesicles could be the source of membrane hydrolysis as has been proposed (27). These vesicles would have high curvature (the reported size of vesicles shed from the plasma membrane of cells is around 50-200 nm in diameter; Ref. 59) which is known from studies in artificial membranes to promote increased hydrolysis by sPLA2 (60). Indeed, these vesicles were susceptible when isolated and incubated alone with sPLA2. However, further experiments revealed that vesicle hydrolysis alone cannot account for all of the phospholipid catalysis observed when cells were made susceptible. In thin layer chromatography experiments such as those described in Table I, cells were separated from microvesicles by centrifugation. Although a significant proportion of the cellular pellet could contain vesicles, nearly complete hydrolysis of extractable lipids was observed suggesting that both cells and vesicles were substrates of the enzyme and that changes in cellular membrane structure per se must be part of the induction of susceptibility.
Sphingomyelin and Re-esterification-- One other hypothesis to explain promotion of susceptibility is that sphingomyelin inhibits the action of sPLA2 and that susceptibility could be induced by removing sphingomyelin from the outer leaflet by translocation across the membrane, by hydrolysis, or by sequestration into domains segregated from sPLA2 substrate. Indeed, some cells appear to be more readily hydrolyzed by sPLA2 following pre-hydrolysis of the plasma membrane with sphingomyelinase (27, 55, 61). We examined this hypothesis briefly by incubating S49 cells with 1 unit/ml sphingomyelinase for various times at 37 °C and then assaying the time course of membrane hydrolysis in the presence of sPLA2. Regardless of the length of treatment with sphingomyelinase (up to 90 min), there was no evidence that it increased the amount of membrane hydrolyzed upon subsequent exposure to sPLA2. However, the time course of restoration of fatty acid levels to base line normally observed with sPLA2 alone (Fig. 1) was absent or reduced in several of the experiments. This result may explain the previous reports of promotion of membrane hydrolysis by sPLA2 after treatment with sphingomyelinase since those data were collected following lengthy incubation with sPLA2 after which the restoration to base line after initial hydrolysis would already be completed.
It is likely that this restoration to base line that occurs in the absence of ionophore represents re-esterification of phospholipid after hydrolysis catalyzed by sPLA2 has ceased. Thin layer chromatography experiments supported this interpretation since it was observed that lysophospholipid levels fall and phospholipid levels rise over the same time course as those shown in Fig. 1. The results with sphingomyelinase described above, then, raises the possibility that the re-esterification reaction is regulated by sphingomyelin and/or its hydrolysis product (ceramide).
Concluding Remarks-- In a biological setting, excess lysolecithin is produced as a consequence of hydrolysis of oxidized lipoproteins (13, 14). A broad range of responses to this lipid have been explored in an effort to understand the relationship between it and the pathology of diseases such as atherosclerosis (8, 11-17). Based on this study, a potential action of lysolecithin to render cells vulnerable to extracellular sPLA2 must now be added to the list. The relationship of these observations to long term prostaglandin synthesis induced by certain proinflammatory agents is more difficult to predict (31). It appears likely that the action of cPLA2 is involved in priming cells for the action of sPLA2 during long term prostaglandin synthesis (29-31) as it was in the response to lyso-PPC reported here. Nevertheless, the additional calcium-dependent and cPLA2-independent effects of the lysophospholipid appear more substantial in promoting membrane hydrolysis by sPLA2 compared with stimulation by agents such as platelet-activating factor and lipopolysaccharide (29). Consequently, the response to lyso-PPC probably relates more to pathological conditions that result in larger scale membrane hydrolysis and tissue damage. We emphasize that this response is relevant to other investigations attempting to understand the action of lysolecithin since it occurs at concentrations similar to or less than other biological and pathological effects of the lipid.
In summary, we propose the following as a testable hypothesis for this
action of lysolecithin and sPLA2 on mammalian white blood
cells (Fig. 9). First, cells are
initially susceptible to the enzyme but respond by quickly becoming
resistant. This phenomenon is a response to the enzyme involving
binding to sites separate from the loci of membrane hydrolysis.
Lyso-PPC supersedes this resistance by elevating intracellular calcium.
Part of the effect involves a combined action of calcium and lyso-PPC
to activate cPLA2 which then leads to susceptibility to
sPLA2 as proposed by Balsinde and Dennis (29). The
remainder of the effect of lyso-PPC requires only the elevation of
intracellular calcium. The calcium then acts on enzymes such as
scramblase, aminophospholipid translocase, and specific proteases that
lead to phospholipid translocation across the bilayer and release of
microvesicles from the plasma membrane into the extracellular fluid.
Changes associated with the plasma membrane as these vesicles are
released create a membrane susceptible to sPLA2.
|
![]() |
ACKNOWLEDGEMENTS |
---|
We gratefully acknowledge the technical assistance of Michael Schillen and Samantha Smith at the Department of Zoology, Brigham Young University, and Steven Wood, Department of Chemistry and Biochemistry, Brigham Young University. We thank Drs. Michael Gelb and Rao Koduri at the Department of Chemistry and Biochemistry, University of Washington, for providing group IIA sPLA2, inactive analogs of AACOCF3, and for helpful discussions.
![]() |
FOOTNOTES |
---|
* This work was supported by U. S. Public Health Service Grants GM49710 (to J. D. B.), HL36946 (to P. J. S.), and GM52598 (to W. C.) from the National Institutes of Health and by funds from Brigham Young University (to J. D. B.) and the Arthritis Foundation (to W. C).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
To whom correspondence should be addressed: Dept. of Zoology,
Brigham Young University, Provo, UT 84602. Tel.: 801-378-8160; Fax:
801-378-7499; john_bell{at}byu.edu.
2 W. Cho and M. H. Gelb, manuscript in preparation.
![]() |
ABBREVIATIONS |
---|
The abbreviations used are: sPLA2, secretory phospholipase A2; lyso-PPC, 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine; ADIFAB, acrylodan-derivatized fatty acid binding protein; laurdan, 6-dodecanoyl-2-dimethylaminonaphthalene; lyso-PPG, 1-palmitoyl-2-hydroxy-sn-glycero-3-[phospho-rac-(1-glycerol)]; lyso-OPC, 1-oleoyl-2-hydroxy-sn-glycero-3-phosphocholine; lyso-PPA, 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphate; lyso-PPE, 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphoethanolamine; W-7, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide; cPLA2, cytosolic calcium-dependent phospholipase A2; MAFP, methyl arachidonyl fluorophosphonate; AACOCF3, arachidonyltrifluoromethyl ketone; BAPTA, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid.
![]() |
APPENDIX |
---|
The disposition of fatty acid during the initial interaction of sPLA2 is described by three processes as follows: production through hydrolysis of phospholipids, removal of fatty acid, and time-dependent inhibition of the production dependent on sPLA2 binding to a separate site with higher affinity for the enzyme than the loci of hydrolysis. The rate of change of fatty acid concentration (P) is given by Equation 1.
![]() |
(Eq. 1) |
![]() |
![]() |
(Eq. 2) |
![]() |
(Eq. 3) |
![]() |
(Eq. 4) |
![]() |
![]() |
(Eq. 5) |
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|