Cell wounding activates phospholipase D in primary
mouse keratinocytes
Senthil N. Arun,1,† Ding Xie,1,§ Amber C. Howard,** Quincy Zhong,§ Xiaofeng Zhong,§
Paul L. McNeil,** and Wendy B. Bollag2,*,†,§,**
Charlie Norwood VA Medical Center,* Augusta, GA 30904; Institute of Molecular Medicine and Genetics,†
Department of Physiology,§ and Department of Cell Biology and Anatomy,** Georgia Health Sciences
University, Augusta, GA 30912
Supplementary key words skin • phosphatidylglycerol • aquaporin-3 •
wound healing • 5-fluoro-2-indolyl des-chlorohalopemide
Keratinocytes form the epithelium of the skin, the epidermis, and comprise several cell layers. As keratinocytes
migrate up from the stratum basale, they undergo a distinct pattern of differentiation that is essential for the
function of the skin as a protective barrier. This pattern is
This project was supported by a Merit Award from the Veterans’ Administration
and by Grant AR-45212 from the National Institutes of Health/National Institute of Arthritis, Musculoskeletal and Skin Diseases (W.B.B.). Its contents are
solely the responsibility of the authors and do not necessarily represent the official
views of the National Institutes of Health or other granting agencies.
Manuscript received 2 April 2012 and in revised form 1 January 2013.
Published, JLR Papers in Press, January 2, 2013
DOI 10.1194/jlr.M027060
This article is available online at http://www.jlr.org
characterized by growth arrest and expression of the mature keratins 1 and 10 in the first differentiated layer of
the epidermis, the spinous layer. Early differentiation in the
spinous layer is followed by further differentiation in the
granular layer, which is accompanied by expression of proteins that are essential for the formation of the cornified
envelope and corneocytes. The corneocytes constitute the
outer layer of the epidermis, the stratum corneum, and
give skin its resilience to mechanical stress (as reviewed in
Ref. 1). Deficiencies in the mechanical barrier function of
the epidermis result in skin diseases. For example, epidermolysis bullosa simplex and epidermolytic hyperkeratosis
arise through mutations in keratins comprising the intermediate filaments and are characterized by extensive
blistering and epidermal sloughing as a result of the mechanical stresses encountered by routine interactions with
the environment (as reviewed in Ref. 2).
Many tissues of the body in addition to the skin are exposed to mechanical stresses that result in tearing, or disrupting, the plasma membrane of the constituent cells.
These disruptions will result in cell death if left unrepaired.
However, cells possess an active plasma membrane repair
process that can restore plasma membrane integrity if the
disruption is not too extensive (as reviewed in Ref. 3). For
example, intestinal cells in the gastrointestinal tract are
subjected to mechanical perturbations during the transit
of a food bolus; these plasma membrane disruptions can
be repaired to allow cell survival (4–6). Similarly, eccentric
contraction of skeletal muscle as a result of downhill treadmill running induces plasma membrane disruptions that
are largely repaired (7). Routine ambulation also appears
to lead to plasma membrane disruptions in the epidermis
of the digits (8). Therefore, it is critical that cells in these
mechanically active tissues be able to repair membrane
Abbreviations: 1,25(OH)2D3, 1,25-dihydroxyvitamin D3; AQP3,
aquaporin 3; FIPI, 5-fluoro-2-indolyl des-chlorohalopemide; HBSS,
Hank’s buffered salt solution; K-SFM, keratinocyte serum-free medium;
PEt, phosphatidylethanol; PG, phosphatidylglycerol; PIP2, phosphatidylinositol 4,5-bisphosphate; PLD, phospholipase D; SFKM, serum-free
keratinocyte medium; TPA, 12-O-tetradecanoylphorbol 13-acetate.
1
S. Arun and D. Xie contributed equally to this work.
2
To whom correspondence should be addressed.
e-mail:
[email protected]
Journal of Lipid Research Volume 54, 2013
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Abstract Plasma membrane disruptions occur in mechanically active tissues such as the epidermis and can lead to
cell death if the damage remains unrepaired. Repair occurs
through fusion of vesicle patches to the damaged membrane
region. The enzyme phospholipase D (PLD) is involved in
membrane traffickiing; therefore, the role of PLD in membrane repair was investigated. Generation of membrane disruptions by lifting epidermal keratinocytes from the
substratum induced PLD activation, whereas removal of
cells from the substratum via trypsinization had no effect.
Pretreatment with 1,25-dihydroxyvitamin D3, previously
shown to increase PLD1 expression and activity, had no effect on, and a PLD2-selective (but not a PLD1-selective)
inhibitor decreased, cell lifting-induced PLD activation, suggesting PLD2 as the isoform activated. PLD2 interacts functionally with the glycerol channel aquaporin-3 (AQP3) to
produce phosphatidylglycerol (PG); however, wounding resulted in decreased PG production, suggesting a potential
PG deficiency in wounded cells. Cell lifting-induced PLD
activation was transient, consistent with a possible role in
membrane repair, and PLD inhibitors inhibited membrane
resealing upon laser injury. In an in vivo full-thickness
mouse skin wound model, PG accelerated wound healing.
These results suggest that PLD and the PLD2/AQP3 signaling module may be involved in membrane repair and wound
healing.—Arun, S. N., D. Xie, A. C. Howard, Q. Zhong, X.
Zhong, P. L. McNeil, and W. B. Bollag. Cell wounding activates phospholipase D in primary mouse keratinocytes.
J. Lipid Res. 2013. 54: 581–591.
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Journal of Lipid Research Volume 54, 2013
to decrease cell lifting-induced PLD activity. Interestingly,
however, despite our previous finding that PLD2 activity
appears to mediate PG production in response to elevated
extracellular calcium levels (16), there was no increase in
the production of PG upon wounding. This result suggests
the possibility that the functional interaction between
PLD2 and AQP3 was disturbed and therefore suggests that
a potential deficiency in PG levels might accompany
wounding. In addition, we showed that PLD1 and PLD2
inhibitors tended to inhibit the repair of laser-induced
plasma membrane wounds, whereas the PLD inhibitor
5-fluoro-2-indolyl des-chlorohalopemide (FIPI) significantly
inhibited this process. This result indicates the importance
of PLD’s function in mediating the repair of plasma membrane disruptions caused by mechanical stresses. Because
plasma membrane wound repair is one aspect of macroscopic wound healing, we also examined the role of PG in
the healing of a full-thickness skin wound and found that
PG promoted wound healing.
MATERIALS AND METHODS
Materials
Calcium-free minimum essential medium (MEM)-␣ was from
Biologos, Inc. (Montgomery, IL), ITS+ (6.25 µg/ml insulin, 6.25
µg/ml transferrin, 6.25 ng/ml selenous acid, 1.25 mg/ml BSA,
and 5.35 µg/ml linoleic acid) from Collaborative Biomedical
Products (Bedford, MA), and bovine pituitary extract and epidermal growth factor from Gibco Invitrogen (Grand Island, NY).
DC protein assay reagents were from Bio-Rad (Hercules, CA).
BSA and FIPI were from Sigma (St. Louis, MO), and [3H]oleic
acid, [14C]glycerol, [3H]leucine, and [3H]thymidine from Perkin
Elmer NEN (Waltham, MA). Hank’s buffered salt solution (HBSS)
was obtained from Mediatech (Manassas, VA). Phosphatidic acid,
PEt, and PG derived from egg (egg PG) were obtained from
Avanti Polar Lipids (Alabaster, AL), FM1-43 from Invitrogen,
and the PLD1- (CAY10593) and PLD2-selective (CAY10594) inhibitors from Cayman Chemical (Ann Arbor, MI). Commercial keratinocyte serum-free medium (K-SFM) and the appropriate
supplements were obtained from Gibco Invitrogen. PBS contained 137.9 mM NaCl, 2.7 mM KCl, 15.2 mM Na2HPO4, 1.5 mM
KH2PO4, and 1 mM MgCl2 (with or without 1.2 mM CaCl2), all
from Sigma.
Keratinocyte culture
Mouse epidermal keratinocytes were isolated from newborn
ICR CD1 outbred mice and cultured as described previously
(19). Briefly, skins were harvested and floated overnight on 2.5%
trypsin at 4°C prior to a brief incubation at 37°C. The dermis and
epidermis were then mechanically separated, and the keratinocytes gently scraped from the underside of the epidermis. Cells
were collected by centrifugation and seeded overnight in a dialyzed serum-containing plating medium, followed by refeeding
with serum-free keratinocyte medium (SFKM) as in (19). In some
experiments, after incubation overnight in plating medium, cells
were refed with K-SFM (containing 50 µM calcium chloride) and
cultured until confluence.
PLD activity assay
Cells were prelabeled with 5 µCi/ml [3H]oleic acid in SFKM
for 20–24 h prior to refeeding with SFKM or HBSS. Ethanol
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tears in order to prevent extensive tissue damage, or disease
can result. As an example, some forms of muscular dystrophy may be caused in part by a reduced ability of skeletal
muscle cells to repair membrane disruptions. Indeed,
muscle fibers from the dysferlin knockout mouse, a mouse
model of limb-girdle muscular dystrophy and Miyoshi myopathy, fail to properly repair cell wounds (9). In addition,
patients with a form of Myoshi myopathy in which mutations in dysferlin are not observed nevertheless show a
skeletal muscle cell membrane repair defect, which is correlated with the severity of their disease (10), indicating
the importance of this process to tissue health.
The enzyme phospholipase D (PLD) hydrolyzes phospholipids, primarily phosphatidylcholine, to generate
phosphatidic acid, which can be dephosphorylated by
lipid phosphate phosphatases to yield diacylglycerol.
Two isoforms of PLD, PLD1 and PLD2, have been wellcharacterized (as reviewed in Ref. 11). Interestingly, both
PLD isoforms can also, in the presence of primary alcohols, catalyze a transphosphatidylation reaction to generate a phosphatidylalcohol. In fact, PLD utilizes alcohols
such as ethanol and butanol to yield phosphatidylethanol
(PEt) or phosphatidylbutanol (12), even at a low alcohol
concentration. PLD’s preferential use of alcohols has been
puzzling: in the absence of evolutionary pressure to utilize
these alcohols, why should PLD retain the ability to catalyze this transphosphatidylation reaction? We have proposed that PLD has retained this function in order to use
the physiological alcohol glycerol to synthesize phosphatidylglycerol (PG) (as reviewed in Refs. 13, 14). Indeed, our
data indicate that in keratinocytes, PLD2 colocalizes and
coimmunoprecipitates with the glycerol channel aquaporin-3 (AQP3) (15), and glycerol can be utilized by PLD
to generate PG in intact keratinocytes [(16) and as reviewed in Refs. 13, 14]. Thus, we have proposed that AQP3
provides glycerol to PLD2 for the production of PG via the
transphosphatidylation reaction, and that this PG acts as a
novel lipid signaling molecule to regulate early keratinocyte differentiation (as reviewed in Refs. 13, 14). In a recent study, we have shown that manipulations that alter
the function of this PLD2/AQP3/PG signaling module
can inhibit epidermal keratinocyte proliferation and promote differentiation (17).
PLD also regulates vesicle trafficking and membrane fusion, as well as actin cytoskeleton rearrangements (as reviewed in Refs. 11, 18). Because both vesicle fusion and
disintegration of cortical actin are required for membrane
resealing of plasma membrane disruptions (3), we hypothesized that PLD would be activated by cell wounding and
mediate, at least in part, membrane repair. In this report,
we demonstrate that, in fact, cell wounding, induced by
lifting of the cells from the extracellular matrix, activated PLD. Pretreatment with 1,25-dihydroxyvitamin
D3[1,25(OH)2D3], which we have previously shown to increase PLD1 expression and activity (19), did not affect
PLD activity elicited by wounding, suggesting that PLD2
was the PLD isoform activated by plasma membrane disruption. This result was also consistent with the observed
ability of a PLD2-selective, but not a PLD1-selective, inhibitor
in comparison with a simultaneously photographed circle of
known size. The experiment was repeated on a second group of
mice, with the opposite side exposed to the treatment of interest
(glycerol or PG liposomes). No difference was observed between
male and female mice, so the results were pooled.
(final concentration of 1%) was added to the medium, and the
cells were immediately lifted gently from the culture dish with a
rubber policeman or exposed to 0.5% trypsin for 15 min. Reactions were terminated with 0.2% SDS containing 5 mM EDTA
and the lipids extracted using chloroform-methanol as in (16).
Phospholipids were separated by TLC and visualized with En3Hance (Perkin Elmer NEN). The radiolabeled PEt, as determined by comigration with authentic standards, was cut out, and
radioactivity was measured by a liquid scintillation spectrometer
(Beckman Coulter; Indianapolis, IN). For inhibitor experiments,
cells were pretreated with the indicated compounds for 30 min
prior to cell lifting.
Statistical analysis
All experiments were performed a minimum of three times.
Statistical analysis was performed as indicated using Graphpad
Prism or Instat (La Jolla, CA).
PG production
RESULTS
Radiolabeled PG production was measured as in (16). Briefly,
cells were lifted gently from the culture dish with a rubber policeman in the presence of [14C]glycerol for 15 min. Reactions were
terminated with 0.2% SDS containing 5 mM EDTA, and the
lipids were extracted and separated as above.
Membrane repair assay
Plasma membrane repair following cell wounding was measured as in (20, 21). Keratinocytes were incubated with PLD inhibitors or DMSO (vehicle control) in SFKM or K-SFM prior to
washing and addition of PBS with or without calcium. FM 1-43
dye (2.5 µM) was added to the cells immediately prior to wounding by a Zeiss LSM 510 confocal laser scanning microscope containing a Meta System equipped with a Coherent Mira 900
tunable Ti:Sapphire laser for multi-photon excitation, with the
laser used at full power at a wavelength of 835 nm. Intracellular
uptake of FM 1-43 after two-photon injury was measured by analyzing fluorescence intensity (FI) of each cell using Zeiss LSM
510 imaging software. Percent recovery was calculated using the
following equation:
% recovery
FI in the absence of calcium – FI of treated cells
u 100%
FI in the absence of calcium – FI in the presence of calcium
This equation sets the percent recovery in the absence of
calcium at 0% and in presence of calcium at 100%.
Cytotoxicity assay
The cytotoxicity of PLD inhibitors was monitored as effects
on protein synthesis and DNA synthesis by measuring the incorporation of radiolabeled leucine or thymidine, respectively,
into TCA-precipitable macromolecules. Cell exposure to PLD
inhibitors was set to 3 h, coniciding with the maximal length of
time that keratinocytes were incubated with PLD inhibitors during the laser cell-wounding assays. In these experiments, nearconfluent to confluent keratinocytes were incubated for 2 h
with the PLD inhibitors, followed by addition of 1 µCi/ml [3H]
leucine (or [3H]thymidine) and incubation for an additional
60 min. Reactions were terminated and macromolecules precipitated by the addition of cold 5% TCA. After washing, cells
were solubilized in NaOH, and an aliquot was counted by liquid
scintillation spectrometry.
Effect of 1,25-dihydroxyvitamin D3, an inducer of
PLD-1 expression and activity, on wounding-induced
PLD activation
In previous experiments, we have demonstrated that a
24 h pretreatment with 250 nM 1,25(OH)2D3 increases
PLD1 expression and activity (19) and can enhance PLD
activation measured in response to some agonists (23). To
determine whether the PLD isoform activated in response
to cell wounding was PLD1, we pretreated keratinocytes
In vivo wound healing
Male and female ICR CD1 mice were anesthetized with isoflurane and their flanks shaved. Two approximately 4 mm diameter
full-thickness wounds were made using a punch biopsy. The
wounds were untreated or treated daily with 2 M glycerol in water, PBS lacking divalent cations (PBS⫺), or 100 µg/ml PG, prepared as liposomes by sonication in PBS⫺. The wounds were
digitally photographed and analyzed using ImageJ imaging software
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Activation of PLD by cell wounding induced
upon cell lifting
Scraping or lifting cells from tissue culture dishes with
a rubber policeman induces plasma membrane disruptions and cell wounding (22). We tested PLD activity after
plasma membrane disruptions in epidermal keratinocytes
by monitoring changes in radiolabeled PEt levels in [3H]
oleate-prelabeled cells scraped or lifted from the culture
dish in the presence of 1% ethanol. Controls included
cells exposed to ethanol without scraping or lifting as
well as cells released from the dish by trypsinization. As
shown in Fig. 1A, we found that in SFKM cell wounding
by gently lifting the cells from the culture dish with a rubber policeman, but not trypsinization, activated PLD, resulting in increased [3H]PEt levels. This PLD activation
was not the result of growth factors present in the medium, because similar results were obtained when the
wounding was performed in HBSS (Fig. 1B). Similar results were also obtained when cells were more vigorously
scraped from the culture substratum with a plastic cell
lifter (data not shown).
We then determined the time course of the PLD activation upon cell lifting. To do so, 1% ethanol was added to
the cultures at various times after the lifting (immediately
before and 15 min after), and PLD activity was monitored
by radiolabeled PEt levels. As previously, cell lifting in
the presence of 1% ethanol activated PLD. However,
when ethanol was added 15 min after lifting of the cells
with a rubber policeman (for 15 minutes), PLD activity
had returned to a basal, nonlifted level (Fig. 2). Because
membrane repair occurs rapidly in the presence of calcium [e.g., (20, 21) and see below], this result indicates
that upon membrane repair, PLD activity returned to
basal levels, suggesting a possible role for this enzyme in
the repair process.
Fig. 1. Cell wounding, but not trypsinization, activated PLD. A: [3H]oleate-prelabeled keratinocytes in
SFKM were treated with 1% ethanol immediately prior to gentle removal of the cells from the substratum
with a rubber policeman or trypsinization (with 0.25% trypsin) and incubation for 15 min. Reactions were
3
terminated by the addition of 0.2% SDS containing 5 mM EDTA, and [ H]PEt was extracted, separated by
3
TLC, and quantified. B: [ H]oleate-prelabeled keratinocytes in HBSS were treated, and radiolabeled PEt
levels were quantified as in A. Values represent the means ± SEM of three to four experiments performed in
duplicate and expressed as the -fold over the control level; *P < 0.05 versus the control.
Effect of PLD1- and PLD2-selective inhibitors on
wounding-induced PLD activation
The results shown in Fig. 3 suggest that PLD2 is the PLD
isoform activated by cell wounding. We, therefore, determined the effect of PLD-selective inhibitors on cell liftingelicited PLD activation. Radiolabeled cells were pretreated
with the indicated concentrations of the PLD1-selective inhibitor CAY10593 and the PLD2-selective inhibitor CAY10594
(24) prior to lifting and monitoring of PLD activity. The concentrations of the PLD inhibitors were selected based on the
data shown in (24), as determined in intact cells (i.e., Fig. 6 of
the cited reference, with CAY10593 corresponding to compound #69 and CAY10594 to compound #72 in this article).
Our results demonstrate that the PLD2-selective, but not the
PLD1-selective, inhibitor decreased radiolabeled PEt production in cells lifted from the substratum in the presence of
ethanol (Fig. 4). These data strongly suggest that PLD2 is the
PLD isoform activated by cell wounding.
Effect of wounding-induced PLD activation on PG levels
We have previously shown that the PLD2 isoform colocalizes and coimmunoprecipitates with the glycerol channel AQP3 (25). Furthermore, radiolabeled PG levels are
increased upon the addition of radiolabeled glycerol in a
PLD-mediated manner in keratinocytes stimulated to differentiate with elevated extracellular calcium levels (16),
and our data suggest that this PG increase is mediated by
PLD2 (16). Because the current study suggested that PLD2
was the isoform activated by cell wounding, we examined
the effect of cell lifting on PG levels. Despite the increase
in PLD activity, cell wounding decreased the levels of
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Journal of Lipid Research Volume 54, 2013
radiolabeled PG (Fig. 5). On the other hand, lifting the
cells from the substratum by trypsinization had no effect
on [14C]PG levels. Together, these results suggest that 1)
the functional interaction between PLD2 and AQP3 is disrupted by cell wounding, and 2) PG levels may be deficient
in wounded cells.
Effect of PLD inhibition on wound repair
Our results indicate that PLD, in particular PLD2, is activated upon plasma membrane disruptions induced by
cell lifting from the substratum. To determine the role of
this PLD activation, we first determined the effect of inhibiting PLD1 or PLD2 activity on the ability of keratinocytes
to repair plasma membrane disruptions. A laser was used
to disrupt cell membranes in the presence of a lipid-soluble fluorescent dye. This dye enters the cell and binds to
membrane-delimited cell organelles such that the fluorescence intensity of the cell continues to increase until the
membrane disruption is repaired, thereby halting dye entry (20, 21). Because membrane repair requires calcium
Fig. 2. Cell wounding, but not trypsinization, activated PLD in a
3
transient manner. [ H]oleate-prelabeled keratinocytes in SFKM
were treated with 1% ethanol immediately prior to gentle removal
of the cells from the substratum with a rubber policeman (wounding) or 15 min after lifting and incubation for 15 min (wounding
⫺ 15 min). Note that all conditions were incubated with 1% ethanol for 15 min. Reactions were terminated by the addition of 0.2%
3
SDS containing 5 mM EDTA, and [ H]PEt was extracted, separated
by TLC, and quantified. Values are expressed as -fold over the control and represent the means ± SEM from four separate experiments performed in duplicate; *P < 0.01 versus the control value.
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with 1,25(OH)2D3 for 24 h before monitoring PLD activation
in lifted cells as in Fig. 1. Although PLD was still activated
by cell lifting in the 1,25(OH)2D3-pretreated keratinocytes,
radiolabeled PEt levels were not enhanced in these cells;
in fact, PEt levels were not even increased to as great an
extent with 1,25(OH)2D3 pretreatment as without (Fig. 3).
This result suggests that PLD2, rather than PLD1, is the
isoform activated upon cell wounding induced by lifting of
keratinocytes from the culture dish.
calcium) caused a dose-dependent inhibition of plasma
membrane resealing upon laser-induced cell wounding.
Cumulative data from three experiments showed a trend
toward inhibition with the 0.1 µM dose of the PLD1 inhibitor and the 1 µM concentration of the PLD2 inhibitor;
thus, the fluorescence intensity under these two conditions was not significantly different from that measured in
the absence of calcium (Fig. 6B).
Fig. 3. Pretreatment with 1,25(OH)2D3 had no enhancing effect
on PLD activation induced by cell wounding. Cells were pretreated
3
with or without 250 nM 1,25(OH)2D3 and prelabeled with [ H]
oleate for 24 h in SFKM prior to assay of PLD activity upon cell lifting as in Fig. 1. Values are expressed as -fold over the control (with
or without 1,25(OH)2D3 pretreatment) and represent the means
±SEM from four separate experiments performed in duplicate;
*P < 0.01 versus the control value.
(3), monitoring fluorescence intensity in the presence
and absence of extracellular calcium allows comparison of
a repairing and nonrepairing condition (20, 21). In addition, if fluorescence intensity is monitored in the presence
of calcium with and without PLD inhibitor exposure, any
potential effect of PLD inhibition on membrane repair
can be determined. As shown in Fig. 6A, both the PLD1
inhibitor and the PLD2 inhibitor (in the presence of
PLD inhibitors exerted little or no cytotoxic effect
Despite the prior report of the lack of toxicity of FIPI
(26), it was important to exclude the possibility that the
inhibitory effects of the compound on membrane resealing were the result of a generalized cytotoxicity. Because of
similar concerns about potential toxicity of the other PLD
inhibitors as well, protein and DNA synthesis after exposure
to the PLD inhibitors were monitored as measures of cell
health. A total incubation of 3 h with the PLD inhibitors
was used, because at no time were cells exposed to inhibitors for longer than this time period prior to monitoring of
Fig. 4. A PLD2-selective but not a PLD1-selective inhibitor inhib3
ited PLD activation induced by cell wounding. [ H]oleate-prelabeled
cells were pretreated with or without the indicated concentrations
of each inhibitor for 30 min in K-SFM prior to assay of PLD activity
upon cell lifting as in Fig. 1. Values are expressed as -fold over the
lifted control and represent the means ± SEM of three separate
experiments performed in duplicate; *P < 0.05, **P < 0.01 versus
the lifted control value.
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The PLD inhibitor FIPI decreased PLD activity in
keratinocytes stimulated with the phorbol ester
12-O-tetradecanoylphorbol 13-acetate
Our results with the PLD1- and PLD2-selective inhibitors suggested that both isoforms could play a role in
membrane repair, although our earlier results (Figs. 3, 4)
suggested that PLD2 is the isoform activated by cell lifting.
We therefore wished to use an inhibitor of PLD that targets both isoforms. FIPI has previously been shown to
completely inhibit PLD activity in intact Chinese hamster
ovary cells overexpressing PLD isoforms in the 750 nM to
7.5 µM concentration range, as well as PLD-mediated cellular responses in human embryonic kidney, COS-7, fMLP-treated HL-60 (neutrophil), and Min-6 (pancreatic )
cells (26). We determined the ability of FIPI, an agent that
inhibits both PLD1 and PLD2 (26), to inhibit PLD activity
in intact keratinocytes by measuring the effect of FIPI
pretreatment on PLD activation in response to the phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA). As
shown previously (27), TPA stimulated PLD activity in keratinocytes, as measured by an approximate 4.5-fold increase in radiolabeled PEt levels (Fig. 7). Whereas FIPI
alone had no significant effect on PEt levels, pretreatment
with this compound inhibited the TPA-induced PLD activation, indicating the inhibitory efficacy of FIPI in intact
keratinocytes.
We then performed the laser-wounding membrane repair assay in keratinocytes pretreated with 750 nM or 7.5
µM FIPI. As shown in Fig. 8A, B, FIPI inhibited membrane
repair, such that the rate of increase in fluorescence intensity in the cells incubated with extracellular calcium and
FIPI was intermediate between cells incubated in the absence of calcium and cells incubated in the presence of
calcium without FIPI. Data analysis of multiple separate
experiments to determine the degree of membrane resealing indicates that FIPI inhibited membrane repair in a
dose-dependent manner (Fig. 8C), indicating the importance of PLD in this process.
DISCUSSION
Fig. 5. Cell wounding inhibited PG production. Cells were
14
treated with [ C]glycerol immediately prior to, or 15 minutes
after, cell lifting and incubation for 15 min. Reactions were terminated by the addition of 0.2% SDS containing 5 mM EDTA, and
14
[ H]PG was extracted, separated by TLC, and quantified. Values are
expressed as -fold over the control and represent the means ± SEM
of three separate experiments performed in duplicate; *P < 0.05, versus the control value. Similar results were obtained when cells were
more forcefully lifted from the dish using a plastic cell lifter.
Effect of provision of PG on wound healing in vivo
The involvement of PLD activity in membrane repair
suggests that a lipid signal generated by the enzyme could
mediate the process. The data shown in Fig. 5 suggest that
PG levels will be reduced upon keratinocyte wounding,
suggesting the possibility that if PG is important in cell
membrane repair, increasing the levels of PG might accelerate the process. However, the hydrophobicity of the FM
1-43 dye used for assessment of plasma membrane repair
in the laser wounding assay prevented an investigation
of the effect of PG liposomes on plasma membrane repair in vitro using this technique. Nevertheless, repair of
cell wounding is one aspect of macroscopic wound repair;
therefore, we sought to determine the importance of PG
in repair by examining the effect of PG on skin wound
healing, with the expectation that increasing the levels of
PG might accelerate the process. To test this idea, we made
two full-thickness skin punch biopsies on either flank of
two groups of mice. For one group, the wounds were
either not treated or were treated with 2 M glycerol (in
water, a positive control); for the other group, the wounds
were treated with either PBS lacking divalent cations
(PBS⫺) or 100 µg/ml PG liposomes in PBS⫺. Wound
healing was followed over 4 days using digital photography
and computer image analysis. Figure 10A, B show a representative mouse from each group, and Fig. 10C represents
the cumulative results from eight mice per group, expressed as the percent of wound healing at day 4 relative
to day 1 (to control for possible slight differences in the
size of the initial wounds and wound contraction). Glycerol is known to improve skin function, and, as anticipated
based on the literature (as reviewed in Ref. 28), glycerol
treatment accelerated wound healing. More importantly,
PG liposomes also significantly increased the rate of wound
healing to a comparable degree.
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membrane resealing. As shown in Fig. 9, no significant effect
was found for any of the PLD inhibitors, such that protein
and DNA synthesis remained at control levels after a 3 h
exposure to the compounds.
In this study, we demonstrate that plasma membrane
disruptions, as induced by cell lifting from the substratum,
resulted in the activation of PLD in a time-dependent
manner (Figs. 1, 2). This activation did not appear to be
the result of specific factors in the medium, inasmuch as
similar activation was observed in HBSS (Fig. 1B) or SFKM
(Fig. 1A). In the presence of calcium, plasma membrane
disruptions are known to repair within minutes [e.g., (20,
21)], if the wound is not too extensive. The time course
data in Fig. 2, therefore, are consistent with a role for this
PLD activation in membrane repair, inasmuch as PLD activity returned to a basal value within 15 min of the wounding by cell lifting.
The generation of radiolabeled PEt in the presence of
small amounts of ethanol is a measure of PLD activation;
however, this assay does not distinguish between the PLD1
and PLD2 isoforms. Our previous results indicated that a
24 h treatment with 250 nM 1,25-(OH)2D3 induced an increase in PLD1 expression and activity (19). In the current
study, 1,25(OH)2D3 pretreatment actually slightly inhibited the PLD activity induced upon cell wounding (Fig. 3),
suggesting that PLD1 was not the isoform activated. Although we have previously shown that 1,25(OH)2D3 increases PLD1 expression and activity (19), both PLD1
and PLD2 require phosphatidylinositol 4,5-bisphosphate
(PIP2) such that decreasing PIP2 levels or availability can
inhibit PLD activity [(29) and as reviewed in Ref. 30]. 1,25(OH)2D3 is known to increase PIP2 hydrolysis in keratinocytes (31, 32); therefore, the reduced cell lifting-induced
PLD activation may be the result of decreased PIP2 levels
inhibiting PLD2 activity. The idea that PLD2 is the isoform
activated by cell wounding is consistent with the intracellular distribution and function of PLD2 versus PLD1, in
that PLD2 is reported to localize predominantly to the
plasma membrane in many cell types (33), whereas PLD1
is typically found on intracellular membrane compartments (34). The involvement of PLD2 was further indicated by the data showing that a PLD2-selective but not a
PLD1-selective inhibitor inhibited cell wounding-induced
PLD activation (Fig. 4).
We have also previously proposed that the PLD-mediated increase in radiolabeled PG levels in response to elevated calcium concentrations is related to PLD2 activity (16).
This view was based in part on the inability of 1,25-(OH)2D3
pretreatment to influence calcium-induced changes in PG
levels and in part on the inhibitory (rather than stimulatory) effect on PG levels of the phorbol ester TPA, which is
purportedly a better activator of PLD1 than PLD2 (35). In
the current study, however, PLD activation of cell wounding was not accompanied by increased PG levels (Fig. 5)
despite evidence from the 1,25-(OH)2D3 pretreatment
and PLD isoform-selective inhibitor experiments (Figs. 3, 4)
suggesting that PLD2 rather than PLD1 was the isoform
activated. In fact, cell wounding induced a decrease in
PG levels (Fig. 5). The inhibitory effect of wounding on
PG levels suggests two possibilities: the first of these is that
the functional association between PLD2 and AQP3 may
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Fig. 6. The isoform-selective PLD inhibitors tended to delay membrane repair following laser-induced
membrane disruption. Keratinocytes were treated with the PLD1-selective inhibitor CAY10593 (0.1 or 0.3 µM),
the PLD2-selective inhibitor CAY10594 (0.3 or 1 µM), or 0.1% DMSO (vehicle control) for approximately
30 min and then wounded using a sapphire laser at a wavelength of 835 nm. The control cells were wounded
2+
2+
in PBS in the presence (Ca ) and absence (no Ca ) of calcium as positive and negative controls, respectively. The inhibitor-treated cells were wounded in PBS containing calcium. A: Shown are the means ± SEM
of the fluorescence intensity of a minimum of eight cells from a representative experiment; *P < 0.05 versus
the values in PBS in the absence of calcium; †P < 0.05 versus the values of PBS in the presence of calcium
determined using a Tukey’s multiple comparison test. B: Shown are the cumulative means ± SEM from three
experiments in which the fluorescence intensity at 165 s after wounding was monitored under each condition in a minimum of eight cells; *P < 0.05 versus the values obtained in PBS lacking calcium.
be disrupted upon cell wounding. The mechanism by
which the functional interaction between PLD2 and AQP3
is disrupted by cell wounding is unclear, but we have
demonstrated an apparent protein kinase C-mediated
decrease in AQP3 activity/glycerol transport (16). Thus,
protein kinase C activation in response to the increase in
cytosolic calcium levels induced by cell wounding could
potentially trigger a reduction in AQP3-mediated glycerol
transport and thus PG production. Second, the decreased
radiolabeled PG levels with cell lifting suggested that PG
levels in wounded cells might also be deficient.
This latter possibility prompted us to examine the effect
of PG on skin wound healing in vivo. Our results showed
that PG, at a fairly low dose (100 µg/ml), promoted healing of a full-thickness skin wound (Fig. 10). Hara, Ma, and
Verkman (36) have observed that AQP3 knockout mice
exhibit delayed wound healing as well as other epidermal abnormalities, and we have hypothesized that one
mechanism underlying the abnormalities in this mouse
model might be the lack of generation of the PG lipid signal.
Hara and Verkman (37) also reported that they could correct the phenotype of the AQP3 knockout by topical application of glycerol. Although aquaglyceroporins such as
AQP3 facilitate the entry of glycerol into the cell, they are
not strictly required, and even in the absence of AQP3,
glycerol can gain entrance and presumably serve as a precursor for the generation of PG. However, if the functional
coupling between PLD2 and AQP3 is lacking, pharmacological doses of glycerol would probably be necessary. In
this regard, the concentration of 100 µg/ml PG used in
the wound-healing experiments shown in Fig. 10 is roughly
equivalent to a concentration of 100 µM, which is approximately 10,000-fold less than the 2 M glycerol concentration that yielded an essentially equal acceleration of wound
healing. This result implies that direct provision of PG might
be more effective and/or potent in terms of stimulating
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Cell wounding activates phospholipase D in keratinocytes
587
Fig. 7. The PLD inhibitor FIPI decreased PLD activity stimulated
by TPA. [3H]oleate-prelabeled keratinocytes in SFKM were pretreated for 20 min with 750 nM FIPI prior to stimulation with or
without 100 nM TPA in the presence of 0.5% ethanol for 30 min.
Reactions were terminated by the addition of 0.2% SDS containing
3
5 mM EDTA, and [ H]PEt was extracted, separated by TLC, and
quantified. Values represent the means ± SEM of three separate
experiments performed in triplicate; ***P < 0.001 versus the control; †††P < 0.01 versus TPA alone.
wound healing than providing glycerol, inasmuch as the
ability of AQP3 to “feed” the glycerol to PLD2 for PG production would seem to be impaired. Dermal fibroblasts
also appear to express AQP3, the expression of which is induced by epidermal growth factor and modulates fibro-
blast migration (38), as do immune cells (39, 40).
Therefore, dermal fibroblasts and/or immune cells may
also produce PG through the PLD2/AQP3 signaling module, suggesting that PG could potentially contribute to fullthickness skin wound healing by affecting the function of
multiple cell types in the skin.
Conversely, PLD activity results in second messengers
in addition to PG, including phosphatidic acid and, indirectly, diacylglycerol and lysophosphatidic acid (and
FFAs). Unfortunately, FM 1-43 interacts with lipids such
as PG, and we were not able to examine the effect of PG
liposomes on plasma membrane wound repair using the
laser wounding assay. Thus, the exogenous, extracellular
PG could not be removed from the cells once added,
even upon extensive washing, leading to a high background fluorescence that made assay of membrane repair
impossible (data not shown). Nevertheless, experiments
using the PLD isoform-selective inhibitors as well as the
nonisoform-selective PLD inhibitor FIPI indicated that PLD
activity was important in membrane repair. Both the PLD1and PLD2-selective inhibitors reduced plasma membrane
resealing in a representative experiment (Fig. 6A) and
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Fig. 8. The PLD inhibitor FIPI delayed membrane repair following laser-induced membrane disruption. Keratinocytes were treated with FIPI
(750 nM or 7.5 µM), or 0.1% DMSO (vehicle control) for 30 min and then wounded using a sapphire laser at a wavelength of 835 nm. The control
2+
2+
cells were wounded in PBS in the presence (Ca ) or absence (no Ca ) of calcium as positive and negative controls, respectively. The FIPI-treated
cells were wounded in PBS containing calcium. A: Shown are the means ± SEM of the fluorescence intensity of a minimum of six cells from a
representative experiment; *P < 0.05 versus the values in PBS in the absence of calcium; †P < 0.05 versus the values in PBS in the presence of
calcium determined using a Tukey’s multiple comparison test. B: Shown are the cumulative means ± SEM from six experiments in which the fluorescence intensity at 164 s after wounding was monitored under each condition in a minimum of six cells; *P < 0.05, **P < 0.01, ***P < 0.001 versus
the values obtained in PBS lacking calcium; †P < 0.05, ††P < 0.01 versus in PBS without calcium. In panel C the data are replotted as the percentage
resealing, with the recovery in the presence of calcium set to 100% and that in the absence of calcium to 0%; *P < 0.05, **P < 0.01, ***P < 0.001
versus the values obtained in PBS lacking calcium; †P < 0.05, ††P < 0.01, †††P < 0.001 versus in PBS without calcium.
588
Journal of Lipid Research Volume 54, 2013
tended to decrease repair in multiple experiments (Fig. 6B)
without cytotoxicity (Fig. 9). Interestingly, even though we
detected no effect of the PLD1 inhibitor on cell liftinginduced PLD activation, nevertheless, on the basis of these
data, PLD1 appeared to play a role in membrane repair. It
is not clear whether this result suggests a slightly different
mechanism by which membrane tears are repaired versus
membrane holes induced by a laser, or if it represents an
off-target effect of the PLD1 inhibitor other than cytotoxicity, inasmuch as no toxicity was observed (Fig. 9).
The nonisoform-selective PLD inhibitor FIPI also inhibited the process of membrane repair in a dose-dependent
fashion (Fig. 8). It should be noted that FIPI has previously
been shown to be effective in the 750 nM- to 7.5 µM-dose
range, but is irreversible (26); and indeed, we demonstrated the ability of 750 nM FIPI to inhibit TPA-induced
PLD activation (Fig. 7). This inhibition of membrane
resealing by FIPI also did not appear to be the result
of nonspecific cytotoxicity, because neither DNA nor protein synthesis was affected by the compound (Fig. 9),
consistent with previous reports of its lack of toxicity
(26). Inhibition of protein synthesis has previously been
reported to be a measure of the cytotoxic effects of
chemical agents [e.g., (41, 42)] and, in some cases, has
been found to be at least as sensitive as or more sensitive
than the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay (43). Thus, our data indicate that
the activation of PLD in response to cell wounding probably plays a key role in allowing the cell to repair plasma
membrane disruptions.
On the other hand, the relationship between plasma
membrane wound healing and skin wound healing is not
obvious. From studies in skeletal muscle, it is known that
the ability to repair plasma membrane wounding is important for the health of these cells (9, 10). Thus, a defect in
plasma membrane wound healing in skeletal muscle cells
underlies some forms of muscular dystrophy, in which the
cells die, leading to progressive muscle weakness (as reviewed in Ref. 44). Similarly, some skin disorders, such as
epidermolysis bullosa simplex and epidermolytic hyperkeratosis, are caused by mutations in the keratins comprising intermediate filaments (as reviewed in Ref. 2). These
mutations decrease the resistance of the cells to mechanical perturbations such that mechanical stress triggers cell
rupture and blistering of the skin (2). The common characteristic of these diseases is that the death of the mechanically wounded cells results in the appearance of
macroscopic wounds. Thus, these defective wound healing
experiments of nature suggest a connection between plasma
membrane wound healing and tissue wounding.
In summary, our results demonstrate that plasma membrane disruptions triggered by lifting keratinocytes from
the substratum induce transient PLD activation, with a
time course suggestive of a potential role for this activity in
membrane repair. Further, the data suggest that PLD2
rather than PLD1 is the isoform activated. However, in
contrast to the PLD2 activity elicited in keratinocytes by
elevated extracellular calcium concentrations (16), this
PLD2 activation does not result in enhanced PG production, suggesting a possible interruption of the AQP3 supply of glycerol to PLD2 and thus a potential disruption
of the functional association between PLD2 and AQP3.
This disturbance would predict PG deficiency in wounded
cells, and our studies in vivo supported the ability of PG
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Cell wounding activates phospholipase D in keratinocytes
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Fig. 9. The PLD inhibitors had no effect on protein or DNA synthesis, suggesting a lack of cytotoxicity.
Keratinocytes were pretreated for 2 h with K-SFM containing vehicle (0.1% DMSO), 0.3 µM PLD1-selective
3
inhibitor CAY10593, 1 µM PLD2-selective inhibitor CAY10594, or 750 nM or 7.5 µM FIPI. A: [ H]leucine or
3
B: [ H]thymidine, at a final concentration of 1 µCi/ml, was then added for an additional 1 h. Macromolecules were precipitated by the addition of cold 5% trichloroacetic acid and, following washing, were solubilized with NaOH and counted by liquid scintillation spectroscopy. Results represent the means ± SEM of five
separate experiments performed in duplicate and expressed relative to the vehicle control. The value for
treatment with any of the PLD inhibitors at the indicated concentrations was not significantly different from
the control.
REFERENCES
liposomes, at a relatively low dose, to promote epidermal
wound healing. Our results thus suggest that further studies
are warranted to explore the role of PLD, the PLD2-AQP3
signaling module, and PG in wound healing.
The authors greatly appreciate the expert technical
assistance of Mr. Peter Parker, Ms. Mariya George, and Mrs.
Purnima Merai.
590
Journal of Lipid Research Volume 54, 2013
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Fig. 10. PG liposomes accelerated wound healing of full-thickness
punch biopsies of mouse skin. Two full-thickness skin punch biopsies
of ⵑ4 mm were made on the backs of ICR CD-1 mice. For each
mouse, one wound was either (A) untreated (left) or treated with
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The rate of wound healing was then monitored. Shown is the extent
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relative to day 1 for each of the four groups is shown. The experiment was repeated on a second group of mice, with the opposite side
exposed to glycerol or PG liposomes. No difference was observed
between male and female mice, so the results were pooled. Results
represent the means ± SEM of eight mice for each condition; *P <
0.02 versus treatment with PBS; **P < 0.001 versus no treatment.
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