Examination of the Interaction of ADAM Proteins with
Oxidized Phospholipids and their Role in Endothelial
Tridu Huynh, 1, 3
James R. Springstead,1
Judith A. Berliner1,2,4
Department of Medicine, Division of Cardiology
Department of Pathology
University of California – Los Angeles, Los Angeles, CA 90095, USA
Atherosclerosis is a chronic inflammatory disease characterized by lipid accumulation and
subsequent inflammation of the artery walls that can result in heart attacks and strokes. PAPC
is one of the major phospholipids in low-density lipoprotein (LDL), and products of its oxidation
(Ox-PAPC) interact and activate endothelial cells, which leads to the induction of chemokines,
such as IL-8. IL-8 results in the migration and retention of monocytes into the subendothelial
space, an initial step in atherogenesis. IL-8 induction is regulated by several pathways, one of
which is the ADAM-mediated HBEGF-EGFR pathway. It has previously been shown that Ox-
PAPC binds to several endothelial cell proteins, among which are some ADAMTS proteins. In
this study, using Ox-PAPE-N-biotin, a biotinylated analog of Ox-PAPC, we present evidence
that Ox-PAPC activates ADAM proteins, specifically ADAMTS1 and ADAMTS4, both of which
have been implicated in IL-8 regulation, by covalently binding to them.
Atherosclerosis, a chronic inflammatory disease of the artery wall, is the major cause of heart
attacks and strokes, which are the leading causes of death in the United States (Heron, 2007).
Atherosclerosis is characterized by the accumulation of lipids and fibrous debris in the
subendothelial space of artery walls. Early atherosclerotic lesions consist of the formation of
fatty streaks in arteries, which results from the accumulation of lipid-engorged macrophages, or
foam cells, in the subendothelial space. Although fatty streaks are not clinically significant, they
are the precursor to fibrous plaques, which arise from the migration of smooth muscle cells and
the accumulation of lipid-rich necrotic debris in these now more advanced lesions. The final
clinical complication of atherosclerosis is thrombosis, the formation of a blood clot inside a blood
vessel as a result of the rupture of the unstable atherosclerotic lesion. Such blood clot can
obstruct the blood flow through the circulatory system, resulting in a myocardial infarction or
stroke (Lusis, 2000).
Results from many clinical studies and animal models have shown that high levels of
low-density lipoprotein (LDL), a fat carrier in the bloodstream, are strongly correlated with
atherosclerotic development (Schwenke et al., 1989; Goldstein et al., 1977). More specifically, it
was noticed that LDL lipids were oxidized in the subendothelial space of arteries after retention.
These now called minimally modified LDL (MM-LDL) were seen to predict and accumulate in
atherosclerotic lesions (Witztum et al., 1991).
1-palmitoyl-2-arachidonyl-sn-glycerol-3-phosphocholine (PAPC) is one of the major
phospholipids in LDL and cell membranes. It has previously been shown that products of
oxidized PAPC (Ox-PAPC) are a major bioactive component of MM-LDL, and are present in
atherosclerotic lesions (Leitinger et al., 1997; Watson et al., 1997). Ox-PAPC contributes to
endothelial cell activation, a key initial event in atherogenesis, which enhances monocyte-
endothelial interactions partly through the induction of chemokines, such as Interleukin-8 (IL-8)
and monocyte chemotactic protein-1 (MCP-1) (Bobryshey et al., 2005). Previous research has
shown that monocyte recruitment, retention and differentiation into the subendothelial space is
an initial step in atherosclerotic plaque development. Upon entry, monocytes recruited at
atherosclerotic lesions differentiate into macrophages that take up lipids until they eventually
become lipid-laden foam cells that contribute to the formation of the fatty streak (Insull et al.,
IL-8 is regulated by many pathways, one of which is the heparin-binding epidermal
growth factor and epidermal growth factor receptor (HBEGF-EGFR) pathway. We have shown
in previous studies that Ox-PAPC activates certain ADAM proteins (a disintegrin and
metalloproteinase), and that such activated ADAMs process HBEGF on the cell surface. The
soluble HBEGF ligand then binds to the EGFR, leading to IL-8 induction in the cell (Lee et al.,
2012; Figure 1).
Using Ox-PAPE-N-biotin (Ox-PNB), a biotinylated analog of Ox-PAPC with identical
biological properties, it has previously been demonstrated that Ox-PAPC binds to several
endothelial cell proteins (Gugiu et al., 2008).Furthermore, we previously showed that Ox-PAPC
covalently binds to cysteine residues on specific ADAMs in endothelial cells (Lee et al., 2012).
We hypothesize that binding of Ox-PAPC activates the ADAMs, which would then result in an
induction of IL-8 in endothelial cells through the aforementioned ADAM-mediated HBEGF-
This study focuses on the interaction between the metalloproteinases (MPs) ADAMTS1
and ADAMTS4 (a disintegrin and metalloproteinase with thrombodspondin motifs) and Ox-
PAPC. ADAMTS4 is the subject of current study because it was previously shown to be
involved in IL-8 regulation by Ox-PAPC in past silencing studies (Lee et al., 2012). ADAMTS1 is
the subject of current study because it is known to cleave VEGFR2, which plays a role similar to
EGFR in IL-8 regulation. The ADAMTS are a group of proteases that are found both in
mammals and invertebrates. They are extracellular, multi-domain enzymes that have several
known functions, one of which is the cleavage of matrix proteoglycans aggrecan and versican
(Porter et al., 2005). Aggrecan is an extracellular matrix proteoglycan that was used in past and
present studies to assay the activity of certain ADAMTS proteins.
In this study, using Ox-PNB, we present evidence that Ox-PAPC activates ADAMTS1
and ADAMTS4’s enzymatic activity and that it covalently binds to them.
Ox-PAPC Activates ADAM Proteins
To determine whether Ox-PAPC activates ADAM proteins’ enzymatic activities, we measured
the processing of fluorogenic ADAM substrate. Human Aortic Endothelial Cells (HAECs) were
treated with either no Ox-PAPC or 50ug/mL of Ox-PAPC and substrate cleavage was assayed
at various time points (Figure 2A). Ox-PAPC is seen to clearly increase the activity of ADAM
proteins over time. To further prove the point, HAECs were treated with varying concentrations
of GM6001 or Batimastat (matrix metalloproteinase inhibitors) for 4 hours, and the amount of
ADAM cleavage was assayed through fluorescence quantification, again (Figure 2B). Increasing
concentration of GM6001 or Batimastat were seen to inhibit Ox-PAPC’s activation of ADAM
Ox-PAPC Activates Aggrecanases, ADAMTS1 and ADAMTS4 being two of them
To hone in on a specific subset of ADAM proteins for further study, exogenous aggrecan was
used as a substrate to determine whether or not aggrecanases are a subset of ADAM proteins
that undergo activation in the presence of Ox-PAPC. Also, as previously mentionned, we
previously showed that ADAMTS1 and ADAMTS4 are implicated in IL-8 regulation through Ox-
PAPC, both of which are known aggrecanases (Boeuf et al., 2012). Western blot analysis of
HAECs with aggrecan added in Ox-PAPC or control condition showed degradation of aggrecan
as early as 1 hour (Figure 3A). This show that Ox-PAPC leads to activation of aggrecanase(s).
HAECs with aggrecan added were then transfected with either ADAMTS4, ADAMTS1 or control
and treated with either no Ox-PAPC or 50ug/mL of Ox-PAPC. In this experiment, the control
condition consisted only of transfection reagent. Future experiments will consist of transfection
with a plasmid lacking an ADAM protein as a better control. Ox-PAPC is seen to increase
cleavage of aggrecan in both ADAMTS4 and ADAMTS1 transfected cells as well as
untransfected cells (Figure 3B). This shows that Ox-PAPC activates enzymatic activities of
ADAMTS4 and ADAMTS1 specifically.
Ox-PAPC Demonstrates Specificity of Binding
Given the chemically reactive nature of Ox-PAPC, it is plausible that it could have demonstrated
opportunistic binding to a variety of molecules with no relevance to the model under study. To
address this concern, HAECs were treated with different doses of Ox-PNB (10, 7, 4, 1, and 0
ug/mL) for four hours. Western blot analysis was then performed to visualize Ox-PNB bound
proteins using streptavidin-HRP. The untreated condition revealed a couple of non-specific
bands that represent endogenous proteins with avidin-binding properties (Cauli et al., 1994). A
noticeable band was detected around 90kDa, with binding seen at concentrations as low as
1ug/mL (Figure 4). This suggests that Ox-PNB has a higher binding affinity for specific proteins.
Ox-PAPC Binds to ADAMTS4 and ADAMTS1
To test the hypothesis that ADAMTS4 and ADAMTS1 are enzymatically activated through
covalent binding to Ox-PAPC, human embryonic kidney 293 (HEK293) cells were transfected
with either ADAMTS4-HA or ADAMTS1-HA, treated with 50ug/mL of either unoxidized PNB or
Ox-PNB for 30 minutes, immunoprecipitated with streptavidin beads, and blotted with anti-HA-
HRP. ADAMTS4 and ADAMTS1 have an expected molecular weight of 90 and 105kDa,
respectively. There is a clear increase in band intensity in the according bands for both
ADAMTS1 and ADAMTS4 going from treatment with PNB to treatment with Ox-PNB,
suggesting that Ox-PNB, and therefore Ox-PAPC, binds to ADAMTS4 and ADAMTS1.
However, the increase in band intensity is much greater for ADAMTS4 than ADAMTS1,
suggesting that Ox-PAPC binds to ADAMTS4 more strongly (Figure 5).
Ox-PAPC Promotes Cleavage of ADAMTS4 into Mature Form
HEK293 cells were transfected with either ADAMTS1-HA or ADAMTS4-HA, treated with either
phosphate buffered saline (PBS) or 50ug/mL of Ox-PAPC for an hour, immunoprecipitated with
anti-HA beads, and blotted with anti-HA-HRP. There is a clear increase in band intensity for
what is supposedly the cleaved, mature form of ADAMTS4 around 68kDa going from PBS to
Ox-PAPC treatment. On the other hand, ADAMTS1’s cleaved, mature form around 85kDa does
not seem to show such an increase (Figure 6). This suggests that Ox-PAPC promotes cleavage
of ADAMTS4 into its active, mature form, but that ADAMTS1 does not undergo the same
PCSK3 is Implicated in IL-8 Regulation by Ox-PAPC
Given the results obtained in figure 5, we naturally became interested in the mechanism by
which Ox-PAPC might lead to increased production of the mature form of ADAMTS4. Proprotein
convertase subtilisin/kexin (PCSK) is a family of enzymes that perform cleavage and conversion
of immature, target proteins into their biologically active forms (Turpeinen et al., 2011). PCSK3
(FURIN) is known to proteolytically process pro-ADAMTS4 into its mature form. We then first
tested whether or not PCSKs had a role in IL-8 regulation by Ox-PAPC using silencing
techniques. HAECs were transfected with either scrambled siRNA or one of two siRNAs against
PCSK3. The second siRNA against PCSK3 resulted in a 40-50% knockdown of the protein with
a corresponding ~30% knockdown of IL-8 induction in the cells (Figure 7). This is modest
evidence that PCSKs might play a role in IL-8 regulation by Ox-PAPC.
This study provides evidence for the activation of ADAM proteins’ enzymatic activities
through Ox-PAPC, specifically the aggrecanases ADAMTS4 and ADAMTS1, which we have
previously shown to be implicated in IL-8 upregulation in endothelial cells by Ox-PAPC.
Furthermore, using Ox-PNB, we show that Ox-PAPC clearly binds to ADAMTS4, with modest if
not negligible binding to ADAMTS1 (Figure 5). Taken together, several hypotheses can be put
forward as to how Ox-PAPC activates ADAMTS’s enzymatic activity. Other groups have shown
that covalent interaction of metalloproteinases (MPs) with electrophiles caused enhancement of
enzyme activity (Rajagopalan et al., 1996). A plausible mechanism of activation could be
through Ox-PAPC displacing what is known as the “cysteine switch” from the zinc-containing
catalytic domain of the MP. The cysteine switch is a cysteine-containing consensus sequence in
the N-terminal pro-peptide domain that coordinates with the zinc ion in the catalytic site. The
MP’s activity is suppressed as a result of zinc-cysteine coordination and pro-peptide domain
occlusion of the active site (Rosenblum et al., 2007). This mechanism is of particular interest
and the subject of further study to us because we previously showed that Ox-PAPC binds to
cysteine residues in some ADAM and ADAMTS proteins, ADAMTS4 being one of them (Lee et
al., 2012). Determination of the specific cysteines bound by Ox-PAPC is the subject of further
study. Mutation of putative cysteine binding sites on ADAMTSs in an attempt to determine
actual binding sites as well as to confirm whether binding is the actual mechanism of ADAMTS
activation are the aims of future studies.
The results of figure 6 were unexpected and hint at a possible involvement of
PCSKs in IL-8 regulation by Ox-PAPC. There is a clear increase in what seems to be the
cleaved, mature form of ADAMTS4 going from control to Ox-PAPC condition, showing that Ox-
PAPC leads to activation of ADAMTS4’s processing. The same cannot be said of ADAMTS1,
however, suggesting that ADAMTS1 does not undergo the same process. We hypothesized that
Ox-PAPC binding to ADAMTS4 leads to a conformational change that would predispose it to
processing by PCSK3 (FURIN), which is known to process pro-ADAMTS4 into its mature form
(Wang et al., 2004). Silencing PCSK3 resulted in a modest (~30%) reduction in IL-8 levels in
HAECs (Figure 7). The low impact of PCSK3’s silencing on IL-8 could be attributed to the fact
that the silencing only reduced PCSK3’s level by 40-50%. It could also be due to the fact that IL-
8 is regulated by many pathways. Nevertheless, PCSK3 still shows some evidence of being
involved in IL-8 upregulation by Ox-PAPC. Replication of PCSK3’s silencing is required to firmly
determine that. We would also broaden our silencing study to other members of the PCSK
family that might be involved in processing of ADAM proteins involved in IL-8 regulation by Ox-
Preparation of Ox-PAPC
PAPC (1-palmitoyl-2-arachidonyl-sn-glycerol-3-phosphocholine) was purchased from Avanti
Polar Lipids and was oxidized by exposure to air for 48 hrs. Oxidation was monitored by
electrospray ionization-mass spectrometry (ESI-MS).
A solution 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphatidylethanolamine (PAPE) in dry
dichloromethane was added drop-wise to a magnetically stirred solution of biotin,
dicyclohexylcarbodiimide, and dimethylaminopyridine under argon at room temperature. The
solution was mixed for 12 h at room temperature. The solvent was evaporated and the lipid was
separated by reverse-phase high-performance liquid chromatography (HPLC) with ESI-MS
detection in negative mode to produce 1-palmitoyl-2-arachidonoyl-snglycero-3-phosphatidyl-(N-
Cell Culture and Treatment
Plates for Human Aortic Endothelial Cells (HAECs) or Human Embryonic Kidney 293 (HEK293)
cells were coated with 0.1% gelatin-PBS. HAECs were cultured in
MCDB-131 complete media (VEC technologies) alone or M199 medium supplemented with
20% FBS (Hyclone), 100U/mL penicillin, 100ug/mL streptomycin, 1mmol/L sodium pyruvate,
65ug/mL heparin (Sigma), and 50ug/mL endothelial cell growth supplement (ECGS) (BD
Biosciences). HEK293 cells were cultured in DMEM (Dulbecco's Modified Eagle Medium)
containing 4.5 g/L glucose supplemented with 10% FBS (Hyclone), 100U/mL penicillin,
100ug/mL streptomycin, 1mmol/L sodium pyruvate. Ox-PAPC in chloroform (stock: 10mg/ml)
was dried to a lipid residue and resuspended in M199 medium plus 1% FBS for cell treatment.
Generally, cells were changed to M199 medium containing 1% serum for 30min before cell
treatment. Cells were then incubated with or without Ox-PAPC in medium containing 1% serum.
ADAM Substrate Cleavage Assay
The activity of endogenous ADAMs in HAECs were determined using a fluorogenic ADAM
substrate (Enzo BML-P235, Dabcyl-Leu–Ala-Gln–Ala-Homophe–Arg-Ser—Lys[5-FAM]-NH2).
The product formation was determined by fluoroscence measurement using excitation at 485
nm and emission at 520 nm.
Transfection of Plasmids or siRNAs
90% and above confluent cells were treated with plasmid or siRNA complexes with
Lipofectamine 2000 (Invitrogen) for 4-6 hours in OPTIMEM media (Invitrogen) with fungizone at
37°C. The OPTIMEM media was then removed, washed with 1x PBS without calcium and
magnesium, and replaced with 4.5g/L DMEM with 10% FBS media. Cells were used for
experiments after 2 days of cell growth. The specific silencing of target genes was confirmed by
qRT-PCR and Western blotting.
Anti-HA resins or Neutravidin beads (Roche) were used for immunoprecipitation. 1mL of lysate
was mixed with 50uL of either beads in 1.5mL eppendorf tubes. The tubes were then sealed
and incubated with gentle-end-over-end mixing in 4°C room overnight. Following the incubation,
the lysate was centrifuged at 4,000g and the supernatant was removed. Resins were washed
with 500uL of Tris-buffered saline containing 0.1% Tween 20 (TBST) three times. 45uL of
sample buffer consisting of 2x SDS sample buffer with β-mercaptoethanol in a 19:1 ratio was
added to the tubes and then boiled for 5 minutes. The tubes were then centrifuged for 2 minutes
at 4000rpm and the eluent was collected and ready to be loaded on a gel.
Laemmeli buffer (2x, Bio-rad) containing both protease and phosphatase inhibitors and PMSF
(1mM) was used for protein-samples preparation for SDS-PAGE. The samples were loaded
onto wells of a 4-20% Tris-glycine SDS gel (NuGel). Blots were transferred overnight. The blots
were incubated with primary and secondary antibodies in 5% milk or 1% BSA in TBST. They
were then developed and analyzed using enhanced chemiluminescence (ECL) prime kit
(Amersham). VersaDoc Imaging System (BioRad) and Quantity One® program were used for
image acquirement and band density analysis.
Quantitative Real-Time PCR (qRT-PCR)
Total RNAs and cDNAs were isolated and prepared using RNA extraction and cDNA synthesis
kits from Bio-Rad. SYBR® green master mixture and PCR amplification system from Roche
Diagnostics were used for PCR amplification and quantification procedure. The transcriptional
level of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was determined for each cDNA
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Figure 1. Hypothesized induction of IL-8 expression through Ox-PAPC pathway.
Figure 2. Ox-PAPC activates ADAM proteins.
HAECs in 50mM Tris – pH 7.5 and 100mM NaCl buffer with 8uM of ADAM fluorogenic substrate
(BML-P235 from Enzo).
(A) Cells were treated with either no Ox-PAPC or 50ug/mL Ox-PAPC. Cleavage of the
fluorogenic substrate was assayed at various time points as described in the Methods section
(30, 60, 90, 120, 180, 240 mins).
(B) Cells were treated with either no Ox-PAPC or 50ug/mL Ox-PAPC and varying
concentrations of GM6001 or Batimastat (matrix metalloproteinase inhibitors) for 4 hours and
the amount of ADAM cleavage was assayed through quantification of fluorescence.
Figure 3. Ox-PAPC activates Aggrecanases.
(A) HAECs in 50mM Tris – pH 7.5, 100 mM NaCl and 10mM calcium buffer with or without
50ug/mL Ox-PAPC. Exogenous aggrecan fragments were added to the cells due to the large
size of endogenous aggrecan, and supernatant was collected at different time points (1, 2, 4
hours and overnight). Western blot analysis with anti-aggrecan shows that Ox-PAPC leads to
degradation of aggrecan as early as 1 hour.
(B) HAECs transfected with either vehicle or ADAMTS4 or ADAMTS1, treated with no Ox-
PAPC or 50ug/mL of Ox-PAPC, and blotted with anti-aggrecan. Top band represents
undigested aggrecan fragments, bottom band represents digested fragments.
Figure 4. Ox-PNB demonstrates specificity of binding.
HAECs treated with different doses of Ox-PNB for 4 hours. Western blot analysis with
streptavidin-HRP was performed to visualize Ox-PNB bound proteins. A couple of non-specific
avidin-binding proteins can be seen in the untreated lane. A band around 90kDa can be seen at
concentrations as low as 1ug/mL.
Figure 5. Ox-PAPC binds to ADAMTS4 and ADAMTS1.
HEK293 cells transfected with ADAMTS4 or ADAMTS1, treated with 50ug/mL of PNB or Ox-
PNB for 30 minutes, immunoprecipitated with streptavidin resins, and blotted with anti-HA-HRP.
Arrows represent the size of each ADAMTS protein.
Figure 6. Ox-PAPC promotes cleavage of ADAMTS4 into mature form.
HEK293 cells transfected with ADAMTS4 or ADAMTS1, treated with PBS or Ox-PAPC for an
hour, immunoprecipitated with anti-HA resins, and blotted with anti-HA-HRP. Arrows indicate
the size of each ADAMTS’s mature size.
Figure 7. PCSK3 is involved in IL-8 regulation by Ox-PAPC
HEK293 cells transfected with PCSK3 siRNA or scrambled for 4 hours and grown in VEC media
for 3.5 days before treatment with Ox-PAPC for 4 hours. Values normalized to GAPDH levels.
siFURIN2 knocked down PCSK3 expression by 40-50% with a corresponding 30% knockdown
of IL-8 induction.