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(Arteriosclerosis, Thrombosis, and Vascular Biology. 2007;27:600.)
© 2007 American Heart Association, Inc.

Atherosclerosis and Lipoproteins

Group V Secretory Phospholipase A₂ Promotes Atherosclerosis

Evidence From Genetically Altered Mice

Meredith A. Bostrom; Boris B. Boyanovsky; Craig T. Jordan; Marilyn P. Wadsworth; Douglas J. Taatjes; Frederick C. de Beer; Nancy R. Webb

From the Graduate Center for Nutritional Sciences (M.A.B., F.C.d.B., N.R.W.) and the Department of Internal Medicine (B.B.B., F.C.d.B., N.R.W.), University of Kentucky, Lexington; Veterans Affairs Medical Center (F.C.d.B.), Lexington, Ky; James P. Wilmot Cancer Center (C.T.J.), University of Rochester Medical Center, NY; and the Department of Pathology (M.P.W., D.J.T.), College of Medicine, University of Vermont, Burlington.

Correspondence to Nancy R. Webb, PhD, 900 South Limestone Street Rm 535, University of Kentucky, Lexington, KY 40536-0200. E-mail nrwebb1{at}uky.edu

	Abstract

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Objective— Group V secretory phospholipase A₂ (GV sPLA₂) has been detected in both human and mouse atherosclerotic lesions. This enzyme has potent hydrolytic activity towards phosphatidylcholine-containing substrates, including lipoprotein particles. Numerous studies in vitro indicate that hydrolysis of high density lipoproteins (HDL) and low density lipoproteins (LDL) by GV sPLA₂ leads to the formation of atherogenic particles and potentially proinflammatory lipid mediators. However, there is no direct evidence that this enzyme promotes atherogenic processes in vivo.

Methods and Results— We performed gain-of-function and loss-of-function studies to investigate the role of GV sPLA₂ in atherogenesis in LDL receptor–deficient mice. Compared with control mice, animals overexpressing GV sPLA₂ by retrovirus-mediated gene transfer had a 2.7 fold increase in lesion area in the ascending region of the aortic root. Increased atherosclerosis was associated with an increase in lesional collagen deposition in the same region. Mice deficient in bone marrow–derived GV sPLA₂ had a 36% reduction in atherosclerosis in the aortic arch/thoracic aorta.

Conclusions— Our data in mouse models provide the first in vivo evidence that GV sPLA₂ contributes to atherosclerotic processes, and draw attention to this enzyme as an attractive target for the treatment of atherosclerotic disease.

GV sPLA₂ has been implicated in atherosclerosis in vitro. We demonstrate in mice that overexpression of GV sPLA₂ in bone marrow cells results in increased atherosclerosis, whereas deficiency results in a reduction of atherosclerosis. We provide the first in vivo evidence that GV sPLA₂ promotes atherosclerosis.

Key Words: Group V secretory phospholipase A₂ • atherosclerosis • retrovirus-mediated gene transfer • bone marrow transplantation

	Introduction

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The secretory phospholipase A₂ (sPLA₂) family of enzymes hydrolyze the fatty acid esterified at the sn-2 position of glycerophospholipids.¹ Of the 10 sPLA₂s described in mammals, Group IIA (GIIA), Group V (GV), and Group X (GX) sPLA₂ have been detected in human and/or mouse atherosclerotic lesions.^2–4 These enzymes have been proposed to exert multiple proatherogenic effects in the arterial wall. Phospholipid hydrolysis by sPLA₂ generates potentially bioactive lipids, namely free fatty acids and lysophospholipids, which may promote various proinflammatory processes. Hydrolysis by either GV or GX sPLA₂ markedly reduces the capacity of HDL to promote cellular cholesterol efflux from lipid-loaded macrophages.⁵ Hydrolysis of LDL by sPLA₂ in vitro results in an increased affinity for extracellular matrix proteoglycans and promotes LDL aggregation.^3,6 When incubated with mouse peritoneal macrophages, LDL hydrolyzed by either GV or GX sPLA₂ induces foam cell formation.^2,3 Thus, in vitro studies suggest that sPLA₂s could promote atherogenesis by increasing the retention of LDL particles in the subendothelium and by generating potent inducers of macrophage foam cells.

See page 445

In this study, we directly tested the hypothesis that GV sPLA₂ promotes atherosclerosis in vivo. Using both gain-of-function and loss-of-function approaches, we demonstrate for the first time that bone marrow–derived GV sPLA₂ contributes to atherogenesis in LDL receptor–deficient mice.

	Methods

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Generation of Retroviral Vectors
Retroviral vectors expressing GV sPLA₂ and GFP or GFP only were produced in Phoenix ecotropic packaging cells (Dr G.P. Nolan, Stanford University Medical Center, Palo Alto, Calif).

Mice
Female C57BL/6 and LDL receptor–deficient (LDLR^–/–) mice in C57BL/6 background were obtained from Jackson Labs (Bar Harbor, Me). Female GV sPLA₂-deficient (GV sPLA₂^–/–) mice that had been backcrossed 11 times with the C57BL/6 strain were provided by Dr. Jonathan Arm (Brigham and Women’s Hospital, Boston, Mass).⁷ For atherosclerosis studies, mice were maintained on a high-fat diet (Harlan Teklad #TD94059) for 12 or 14 weeks, as indicated. All procedures were done in accordance with the Lexington VA Medical Center Animal Care and Use Committee.

Bone Marrow Transduction and Transplantation
Bone marrow cells were cultured for 48 hours in DMEM supplemented with 13% FBS, 5 µg/mL polybrene (Sigma H-9268), 10 ng/mL interleukin (IL)-3, 20 ng/mL IL-6, and 100 ng/mL mouse stem cell factor (mSCF). Cells were then transduced by two consecutive 24-hour incubations with retroviral supernatants. Cells (1x10⁶; 100 µL) were injected into lethally irradiated (9 Gy) female C57BL/6 mice. For atherosclerosis studies, bone marrow cells from 3 primary recipients that had >30% of peripheral white blood cells expressing GFP were injected into 15 lethally irradiated (9 Gy) female LDLR^–/– mice.

Generation of GV sPLA₂^–/– LDLR^–/– and GV sPLA₂^+/+ LDLR^–/– Mice
Female LDLR^–/– mice (6- to 8-week-old) were transplanted with 1x10⁷ bone marrow cells harvested from age-matched female GV sPLA₂^–/– or GV sPLA₂^+/+ mice.

Lipid, Lipoprotein, and Phospholipase Analyses
Plasma total cholesterol and triglyceride concentrations were measured using colorimetric assays (Wako; Thermo Electron Corporation). Plasma lipoprotein cholesterol distributions⁸ and phospholipase activity³ were determined as described previously.

Real-Time RT-PCR
RNA was isolated from bone marrow cells and cardiac tissue using the TRIzol reagent (Molecular Research Center, Inc). Semi-quantitative real-time RT-PCR was performed using the standard curve method and normalized with 18S.

Quantitation of Atherosclerosis
Atherosclerosis was quantified in the aortic arch/thoracic aorta and the aortic root as described previously.^8,9 Aortic root sections were also stained for collagen using picrosirius red and photographed under polarized light.^10,11

Further detailed materials and methods are provided in supplemental materials, available online at http://atvb.ahajournals.org.

	Results

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Generation of Chimeric Mice Expressing GV sPLA₂ and GFP
Chimeric LDL receptor–deficient (LDLR^–/–) mice overexpressing GV sPLA₂ and GFP or GFP only were generated by transducing bone marrow cells with retrovirus ex vivo, followed by two rounds of transplantation (supplemental Figure I). In a control experiment, atherosclerosis was assessed in LDLR^–/– mice transplanted with nontransduced bone marrow cells, or cells transduced with the retroviral vector expressing only GFP. Retrovirus transduction of GFP had no effect on plasma total cholesterol in LDLR^–/– mice fed normal diet (supplemental Table I). After high-fat diet, mice transduced with GFP had a slight reduction in plasma triglycerides compared with nontransduced mice. Importantly, there was no significant effect of retrovirus transduction on high-fat diet–induced hypercholesterolemia or atherosclerosis, despite persistent GFP expression in transduced mice throughout the course of the 22-week experiment (supplemental Table I, supplemental Figure II).

Overexpression of GV sPLA₂ in Bone Marrow–Derived Cells of LDLR^–/– Mice
The expression of retroviral vector-encoded genes in transduced mice was assessed by several methods. First, transduction rates in GFP LDLR^–/– and GV sPLA₂ + GFP LDLR^–/– mice were quantified by determining the number of peripheral white blood cells that express GFP (Table). Flow cytometric analysis of mice 6 weeks and 18 weeks after transplantation (ie, before initiation of atherogenic diet and at the termination of the experiment) indicated that transduction rates were similar among mice within each group, and persisted throughout the course of the experiment. Mean transduction rates in GFP LDLR^–/– mice (10%) were considerably lower compared with GV sPLA₂ + GFP LDLR^–/– mice (52%). However, as noted above, we established that GFP expression in bone marrow–derived cells does not influence the extent of atherosclerosis in mice.

View this table: [in this window] [in a new window]   Gene Transduction Rates, Plasma Total Cholesterol Concentrations, and Phospholipase Activity in GFP LDLR–/–, GV sPLA2 + GFP LDLR–/–, GV sPLA2+/+ LDLR–/–, and GV sPLA2–/– LDLR–/– mice

GFP could also be detected by indirect immunofluorescent staining in atherosclerotic lesions of GFP LDLR^–/– and GV sPLA₂ + GFP LDLR^–/– mice (green fluorescence, supplemental Figure IIIA). Staining of the same aortic root sections with a GV sPLA₂ specific antibody provided strong evidence that GV sPLA₂ expression was induced in GV sPLA₂ + GFP LDLR^–/– mice above the endogenous levels expressed in GFP LDLR^–/– mice (red fluorescence, supplemental Figure IIIA). Consistent with the immunostaining data, we determined that GV sPLA₂ mRNA was significantly increased both in bone marrow cells and in cardiac tissue encompassing the aortic root region of GV sPLA₂ + GFP LDLR^–/– mice compared with GFP LDLR^–/– mice (supplemental Figure IIIB). Taken together, our data clearly show that retroviral vector-encoded genes were persistently expressed in the transduced mice, and GV sPLA₂ + GFP LDLR^–/– mice had higher levels of GV sPLA₂ expression in bone marrow–derived cells compared with GFP LDLR^–/– mice.

Plasma Phospholipase Activity and Lipids/Lipoproteins
To determine whether overexpression of GV sPLA₂ in bone marrow–derived cells alters plasma phospholipase activity or lipid/lipoprotein concentrations, we assessed these parameters in GFP LDLR^–/– and GV sPLA₂ + GFP LDLR^–/– mice both before the initiation of atherogenic diet (6 weeks after transplantation) and at the end of the experiment (18 weeks after transplantation). Plasma phospholipase activity was similar for all groups of mice, both before and after atherogenic diet feeding (Table). Plasma total cholesterol concentrations (Table) and lipoprotein cholesterol distributions (Figure 1A) were similar in GFP LDLR^–/– and GV sPLA₂ + GFP LDLR^–/– mice six weeks after bone marrow transplantation. Twelve weeks of high fat diet feeding resulted in a substantial increase in plasma total cholesterol that was not altered by GV sPLA₂ overexpression. Fractionation by size exclusion chromatography revealed a modestly reduced amount of LDL-associated cholesterol in GV sPLA₂ + GFP LDLR^–/– mice compared with GFP LDLR^–/– mice (Figure 1B). This difference in lipoprotein profiles did not appear to be associated with a difference in LDL particle size.

View larger version (19K): [in this window] [in a new window]   Figure 1. Plasma from GFP LDLR–/– () and GV sPLA2 + GFP LDLR–/– (•) mice was separated on a Superose 6 column and eluted fractions were analyzed for cholesterol content before (A) and after (B) 12-week high-fat diet feeding. Plasma from GV sPLA2+/+ LDLR–/– mice (•) and GV sPLA2–/– LDLR–/– mice () was separated on a Superose 6 column and eluted fractions were analyzed for cholesterol content before (C) and after (D) 14-week high-fat diet feeding. Values in A and C are the mean (±SEM) from the analysis of 3 pools of plasma per group, with 2 mice per pool. Values in B and D are the mean (±SEM) from the analysis of plasma from 5 individual mice per group.

Quantification of Atherosclerosis
Atherosclerotic lesion area was measured on the intimal surface of the aortic arch and thoracic aorta. There was no significant difference in atherosclerotic lesion area in the arch and thoracic regions of GV sPLA₂ + GFP LDLR^–/– mice (mean=2.2±0.3%) compared with GFP LDLR^–/– mice (mean=2.7±0.5%). Atherosclerotic lesion area in aortic root sections was quantified after oil red O staining for neutral lipid (Figure 2A). GV sPLA₂ + GFP LDLR^–/– mice had significantly more lesion area in the aortic root when compared with GFP LDLR^–/– mice (Figure 2B and 2C). In the ascending region of the aortic root (defined as the region anterior to the aortic valves), average lesion area was 2.7-fold greater in mice overexpressing GV sPLA₂.

View larger version (39K): [in this window] [in a new window]   Figure 2. A, Oil Red O staining of representative aortic root sections from GFP LDLR–/– and GV sPLA2 + GFP LDLR–/– mice. Sections are located 200 µm above the disappearance of the aortic valves. Images were taken at 50x under light microscopy. The box indicates the approximate region of a nearby section (within 64 µm) shown in D. B, Atherosclerotic lesion area in the aortic root. Values are mean lesion areas (±SEM) per section for sections 64 µm apart; n=6. The transition zone between the aortic sinus and the ascending aorta, defined by disappearance of the valve cusps, is 0 on the x axis. C, Mean lesion area (±SEM) in the ascending aorta (n=6; *P<0.05). D, Picrosirius red staining of aortic root sections from GFP LDLR–/– and GV sPLA2 + GFP LDLR–/– mice. Images were photographed under polarized light (magnification, 400x). Blue areas delineate regions stained with oil red O; pink areas delineate regions stained with picrosirius red. Regions outside the lesions have been cropped from the image. E, Mean (±SEM) collagen area, represented as the percent of atherosclerotic lesion area, for aortic root sections 64 µm apart (n=6). Numbers on the x axis correspond to values depicted in B. F, Mean collagen area (±SEM) as a percent of lesion area in the ascending region of the aortic root (n=6; *P<0.05).

Inflammatory Gene Expression in Aortic Tissue
To assess inflammatory gene expression in lesions, real time RT-PCR was used to measure COX-2, tumor necrosis factor (TNF)-, and IL-6 mRNAs in cardiac tissue containing the aortic root, where GV sPLA₂ mRNA was shown to be significantly induced in GV sPLA₂ + GFP LDLR^–/– mice (supplemental Figure IIIB). There were no significant differences in mRNA levels of any of these genes for the 2 groups of mice (supplemental Figure IVA through IVC), although there was a trend for increased TNF- and IL-6 mRNA in mice overexpressing GV sPLA₂.

Quantification of Lesional Collagen
Two previous studies have reported increased collagen deposition in atherosclerotic lesions of mice with macrophage-specific expression of human GIIA sPLA₂.^12,13 Thus, it was of interest to determine whether GV sPLA₂ overexpression similarly promotes collagen deposition. Lesional collagen was visualized by staining aortic root sections with picrosirius red followed by polarized light microscopy (Figure 2D). Collagen area, calculated as a percentage of atherosclerotic lesion area, was significantly increased (2-fold) in the ascending region of the aortic root in mice that overexpressed GV sPLA₂ (Figure 2E and 2F). This increase in collagen area was not associated with any detectable difference in matrix metalloproteinase (MMP)-9 or MMP-13 mRNA expression (supplemental Figure IVD and IVE).

Deficiency of GV sPLA₂ in Bone Marrow–Derived Cells of LDLR^–/– Mice
Given the significant proatherogenic effect of GV sPLA₂ overexpression in transduced LDLR^–/– mice, it was of interest to investigate whether endogenous GV sPLA₂ in bone marrow–derived cells plays a significant role in atherogenesis. LDLR^–/– mice were transplanted with bone marrow harvested from either GV sPLA₂^+/+ or GV sPLA₂^–/– mice.⁷ Six weeks after transplantation, plasma phospholipase activity and total cholesterol levels were not different between GV sPLA₂^+/+ LDLR^–/– and GV sPLA₂^–/– LDLR^–/– mice (Table). There was also no detectable difference in the lipoprotein-associated cholesterol distribution between the two groups (Figure 1C).

Mice were fed a high-fat diet for 14 weeks to accelerate atherosclerotic lipid deposition. We chose to maintain the mice on the atherogenic diet for a somewhat longer period than the overexpression study, because we anticipated that this would help to define a protective effect of GV sPLA₂ depletion. After high fat diet feeding, plasma phospholipase activity, total cholesterol, and lipoprotein cholesterol distributions were similar in GV sPLA₂^+/+ LDLR^–/– and GV sPLA₂^–/– LDLR^–/– mice (Table; Figure 1D).

The distribution of GV sPLA₂ in atherosclerotic lesions of GV sPLA₂^+/+ LDLR^–/– and GV sPLA₂^–/– LDLR^–/– mice was assessed by indirect immunofluorescence and confocal microscopy. GV sPLA₂ was detected in GV sPLA₂^+/+ LDLR^–/– mice, associated with lesional macrophages and to a lesser extent, vascular smooth muscle cells (Figure 3B and 3C, left panels). In contrast, there was a notable absence of GV sPLA₂ colocalized with macrophages in lesions of GV sPLA₂^–/– LDLR^–/– mice (Figure 3B, right panel).

View larger version (55K): [in this window] [in a new window]   Figure 3. A, Aortic root sections from GV sPLA2+/+ LDLR–/– (left) and GV sPLA2–/– LDLR–/– mice (right) stained with oil red O (ORO) to visualize atherosclerotic lesions (50x). The boxes indicate the approximate regions of nearby sections (within 64 µm) depicted in B and C. B, Indirect immunofluorescent staining of GV sPLA2 (green) and CD68 (M; red) (confocal image using a 20x objective). White arrows indicate colocalization of GV sPLA2 and macrophages. C, Indirect immunofluorescent staining of GV sPLA2 (green) and smooth muscle cell actin (SMC; red) (confocal image using a 20x objective). White arrows indicate colocalization of GV sPLA2 and smooth muscle cells.

Atherosclerosis was quantified by en face analysis of the aortic tree, and in serial sections throughout the aortic root. Compared with GV sPLA₂^+/+ LDLR^–/– mice, there was a 36% reduction in atherosclerotic lesion area in the aortic arch and thoracic aorta of GV sPLA₂^–/– LDLR^–/– mice (Figure 4A). However, there was no significant difference in atherosclerotic lesion area in the aortic root between the two groups (Figure 4B).

View larger version (9K): [in this window] [in a new window]   Figure 4. A, Mean percent atherosclerotic lesion area (±SEM) in the aortic arch/thoracic aorta of GV sPLA2+/+ LDLR–/– and GV sPLA2–/– LDLR–/– mice (n=12; *P<0.05). B, Atherosclerotic lesion area in the aortic root. Values shown are mean lesion areas (±SEM) for sections located 64 µm apart (n=6). Numbers on the x axis correspond to those described in Figure 2B Legend.

	Discussion

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An abundance of data implicates sPLA₂s as mediators of atherosclerosis.^14,15 Notably, expression of human GIIA sPLA₂ in bone marrow–derived cells of LDLR^–/– mice promotes atherosclerosis in the absence of alterations in plasma lipoproteins,^8,12,13 providing compelling evidence that increased sPLA₂ activity within the vessel wall is proatherogenic. Recently, other members of the sPLA₂ family in addition to GIIA have been speculated to play a role in atherosclerosis.^14,15 Although GV sPLA₂ has been detected in atherosclerotic lesions,^3,5 there is no direct evidence that this enzyme contributes to atherogenesis in vivo. Results from this study provide the novel finding that GV sPLA₂ promotes vascular lipid deposition in LDLR^–/– mice.

Using retroviral vector mediated gene transfer, we investigated whether increased GV sPLA₂ expression in bone marrow–derived cells modulates atherosclerosis. By coexpressing GFP in the transduced mice, we were able to specifically monitor transduction rates, and verify that expression of retroviral vector-encoded genes was maintained throughout the course of the 20-week experiment. Although GFP has been used previously as a control in atherosclerosis studies using gene transfer,^16–19 to our knowledge the effect of GFP on atherosclerosis has not been specifically addressed. Given the known effect of GFP to stimulate immune responses,^20,21 it was important to confirm that GFP expression in bone marrow–derived cells does not alter the extent of atherosclerosis, because this could confound the interpretation of our results. Using this gene transfer approach, we unequivocally showed that mice expressing GV sPLA₂ and GFP by retroviral vector had significantly increased lesion area compared with mice expressing only GFP.

We also showed that deficiency of GV sPLA₂ in bone marrow–derived cells protects against atherosclerosis. We reported previously that GV sPLA₂ is present in atherosclerotic lesions of both apoE^–/– and LDLR^–/– mice; however, the cellular source of the secreted enzyme was not established.³ By analyzing the lesional distribution of GV sPLA₂ in mice transplanted with GV sPLA₂^+/+ and GV sPLA₂^–/– cells, we determined that macrophages are the major source of this enzyme in mouse lesions. Colocalization of GV sPLA₂ with smooth muscle cells was also detected; however, this represented only a minor fraction of the total GV sPLA₂ present in lesions.

An interesting aspect of our results is the finding that GV sPLA₂ overexpression had region-specific effects on atherosclerosis that were different from GV sPLA₂ deficiency. Whereas GV sPLA₂ overexpression produced increased lipid deposition in the ascending aorta, GV sPLA₂ deficiency resulted in decreased lesion area that was limited to the aortic arch/thoracic aorta. Although the reason for this discrepancy is unclear, studies investigating the effect of scavenger receptor A overexpression and deficiency likewise yielded regional differences in the modulation of atherosclerosis.^22,23 It is possible that the regional differences in the effect of GV sPLA₂ were due to the amount of time animals were fed the atherogenic diet for the two studies. Because we anticipated that GV sPLA₂^–/– LDLR^–/– mice would have less lesion area compared with GV sPLA₂^+/+ LDLR^–/– mice, we maintained the mice on the atherogenic diet for 2 weeks longer than the overexpression study to more easily discern a protective effect. As expected, control mice fed the atherogenic diet for 14 weeks had considerably larger lesions throughout the aorta compared with mice fed the diet for 12 weeks. Thus, the protective effect of GV sPLA₂ deficiency in the ascending aorta may have been obscured in the more advanced lesions. The effect of an intervention has also been shown to be dependant on the extent of atherosclerosis for SR-BI²⁴ and 15-lipoxygenase.²⁵ An alternate possibility to explain the regional differences in our study is regional differences in endogenous GV sPLA₂ expression. The potential role of other sPLA₂s must also be considered. GX sPLA₂ has been detected in mouse atherosclerotic lesions and has similar hydrolytic activity as GV sPLA₂.^2,26 Our data indicated that modulation of GV sPLA₂ expression in bone marrow–derived cells did not alter GX sPLA₂ expression (data not shown). However, we cannot rule out the possibility that GX sPLA₂ has a redundant effect in promoting atherosclerosis that varies throughout the length of the aorta, attributable to regional differences in endogenous GX sPLA₂ expression.

Based on in vitro data, there are several possible mechanisms by which GV sPLA₂ may contribute to atherosclerosis. It has been postulated that sPLA₂s initiate and amplify inflammatory cascades by generating arachidonic acid and other proinflammatory lipid mediators. To study potential etiologies of the proatherogenic effect of GV sPLA₂, we evaluated inflammatory cytokine expression in the mice. Our data indicate that TNF- and IL-6 mRNA levels were similar for control mice and mice overexpressing GV sPLA₂. Despite extensive literature that increased sPLA₂ activity can upregulate COX-2^27–29 we found that COX-2 expression was unaltered in GV sPLA₂-overexpressing mice. For our analyses, RNA was extracted from cardiac tissue containing the aortic root and analyzed by real-time RT-PCR. Although we expect that the primary source of IL-6, TNF-, and COX-2 mRNA is derived from lesions within this tissue, it is possible that localized differences in these proinflammatory mediators were not detected in our assays because of a dilution effect by the surrounding cardiac tissue. However, it should be noted that for these same tissue samples we measured an almost 7-fold increase in GV sPLA₂ mRNA in mice overexpressing GV sPLA₂ compared with control mice.

Deficiency of GV sPLA₂ in bone marrow–derived cells had no detectable effect on plasma lipoproteins or phospholipase activity, indicating that the protective effect was mediated within the vascular intima. That there was no detectable increase in plasma phospholipase activity in GV sPLA₂ + GFP LDLR^–/– compared with GFP LDLR^–/– mice suggests that systemic effects of retroviral vector-mediated expression of GV sPLA₂ were minimal. Nevertheless, we were able to detect a modest decrease in LDL-sized particles in mice overexpressing GV sPLA₂ after atherogenic diet feeding when plasma lipoproteins were separated by size exclusion chromatography that was not observed in mice before high-fat diet feeding. We have shown previously that LDL particles hydrolyzed by GV sPLA₂ are significantly smaller than native LDL.³ Small dense LDL particles are associated with increased atherogenecity.³⁰ However, there was no evidence from fast protein liquid chromatography (FPLC) data that overexpression of GV sPLA₂ resulted in the accumulation of smaller LDL particles. GV sPLA₂ binds extracellular matrix proteoglycans and thus has the potential to hydrolyze LDL retained in the subendothelium. Rosengren et al recently showed that binding to proteoglycans significantly enhances GV sPLA₂ hydrolysis of LDL, and in turn, sPLA₂ hydrolysis increases LDL–proteoglycan complex formation.³¹ We have shown that hydrolysis by GV sPLA₂ alters the interaction of LDL particles with proteoglycans expressed on the surface of macrophages, leading to foam cell formation.³ The new finding that GV sPLA₂ expression in bone marrow–derived cells is directly correlated with atherosclerotic lipid deposition in vivo is consistent with these in vitro results.

Transgenic expression of human GIIA sPLA₂ promotes collagen deposition in atherosclerotic lesions of LDLR^–/– mice.^12,13 In the current study, mice overexpressing GV sPLA₂ had significantly increased collagen area (normalized for lesion area) in the ascending region of the aortic root, the same region where GV sPLA₂ overexpression increased lipid deposition. Although the molecular mechanisms are unknown, the possibility that sPLA₂ may regulate the signaling pathway that leads to collagen deposition is intriguing. A recent study reported that pharmacological inhibition of GIIA sPLA₂ prevents collagen deposition in the left ventricle that normally occurs during the development of hypertension in young spontaneously hypertensive rats.³² Because there is evidence that collagen content is in part regulated by matrix metalloproteinases (MMPs; reviewed in³³), it is possible that increased GV sPLA₂ activity leads to generation of specific arachidonate metabolites which have been shown to modulate MMP expression.^34,35 However, we found no evidence that MMP-9 or MMP-13 transcripts are altered in mice overexpressing GV sPLA₂.

In summary, using gain-of-function and loss-of-function approaches, we demonstrate for the first time that GV sPLA₂ mediates atherosclerosis in vivo, consistent with abundant in vitro data. As with GIIA sPLA₂, overexpression of GV sPLA₂ in bone marrow cells leads to increased collagen deposition in atherosclerotic lesions. Future studies will clarify the mechanisms by which GV sPLA₂ modulates atherosclerotic lesion development.

	Acknowledgments

 
The authors gratefully acknowledge Kathy Forrest and Preetha Shridas for assistance with mouse dissections and real-time RT-PCR.

Sources of Funding

This work was supported by National Institutes of Health Grants HL-071098 (N.R.W.) and T32 HL072743 (M.A.B.). Support was also provided by American Heart Association Pre-doctoral Training Grant 0315079B (M.A.B.).

Disclosures

None.

	Footnotes

 
Original received September 26, 2006; final version accepted December 13, 2006.

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