Atherosclerosis 153 2000 37 – 46
Group-II phospholipase A
2
enhances oxidized low density lipoprotein-induced macrophage growth through enhancement of
GM-CSF release
Kengo Kaneko, Masakazu Sakai , Takeshi Matsumura, Takeshi Biwa, Noboru Furukawa, Tetsuya Shirotani, Shinsuke Kiritoshi, Yoshichika Anami,
Kohji Matsuda, Takayuki Sasahara, Motoaki Shichiri
Department of Metabolic Medicine, Kumamoto Uni6ersity School of Medicine,
1
-
1
-
1
Honjo, Kumamoto
860
-
8556
, Japan Received 26 July 1999; received in revised form 19 November 1999; accepted 19 January 2000
Abstract
Inflammatory process plays an important role in the development and progression of atherosclerotic lesions. Recently, group-II phospholipase A
2
PLA
2
, an inflammatory mediator, was reported to exist in human atherosclerotic lesions and to enhance the development of murine atherosclerotic lesions. Oxidized low density lipoprotein Ox-LDL stimulates the growth of several types
of macrophages in vitro. Since proliferation of macrophages occurs in atherosclerotic lesions, it is possible to assume that the Ox-LDL-induced macrophage proliferation might be involved in the progression of atherosclerosis. In this study, the role of
group-II PLA
2
in the Ox-LDL-induced macrophage growth was investigated using thioglycollate-elicited mouse peritoneal macrophages. Thioglycollate-elicited macrophages significantly expressed group-II PLA
2
and released it into the culture medium. The Ox-LDL-induced thymidine incorporation into thioglycollate-elicited macrophages was three times higher than that into
resident macrophages, whereas under the same conditions, granulocytemacrophage colony-stimulating factor GM-CSF equally induced thymidine incorporation into both types of macrophages. Moreover, the Ox-LDL-induced GM-CSF release from
thioglycollate-elicited macrophages was significantly higher than that from resident macrophages. In addition, the Ox-LDL-in- duced thymidine incorporation into macrophages obtained from human group-II PLA
2
transgenic mice and the GM-CSF release from these cells were significantly higher than those from their negative littermates, and the Ox-LDL-induced thymidine
incorporation into human group-II PLA
2
transgenic macrophages was significantly inhibited by a polyclonal anti-human group-II PLA
2
antibody. These results suggest that the expression of group-II PLA
2
in thioglycollate-elicited macrophages may play an enhancing role in the Ox-LDL-induced macrophage growth through the enhancement of the GM-CSF release. © 2000 Elsevier
Science Ireland Ltd. All rights reserved.
Keywords
:
Oxidized LDL; Macrophage growth; Atherosclerosis; Inflammation; Phospholipase A
2
; GM-CSF www.elsevier.comlocateatherosclerosis
1. Introduction
The presence of a massive cluster of macrophage- derived foam cells in the subendothelial spaces of arter-
ies is one of the characteristic features of the early stages of atherosclerotic lesions [1]. Macrophages take
up chemically modified low density lipoproteins LDL, such as oxidized LDL Ox-LDL and acetylated LDL
Ac-LDL, through the scavenger receptor pathways and become foam cells in vitro [2 – 4]. Macrophage-
derived foam cells are believed to play an important role in the development and progression of atheroscle-
rosis through the production of various cytokines and growth factors [1]. Previous studies have reported that
macrophages and macrophage-derived foam cells pro- liferate in atherosclerotic lesions [5 – 7]. Recent studies
Abbre6iations
:
GM-CSF, granulocytemacrophage colony-stimulat- ing factor; LDL, low density lipoprotein; Ox-LDL, oxidized LDL;
PKC, protein kinase C; PLA
2
, phospholipase A
2
; WHHL, Watanabe heritable hyperlipidemic; ELISA, enzyme-linked immunosorbent as-
say. Corresponding author. Tel.: + 81-96-3735169; fax: + 81-96-
3668397. E-mail address
:
osakaigpo.kumamoto-u.ac.jp M. Sakai. 0021-915000 - see front matter © 2000 Elsevier Science Ireland Ltd. All rights reserved.
PII: S 0 0 2 1 - 9 1 5 0 0 0 0 0 3 9 1 - 9
from the laboratory as well as by other investigators [8 – 17] have demonstrated that Ox-LDL could induce
macrophage growth in vitro. Based on these findings, it was postulated that Ox-LDL might be involved in
macrophage growth in vivo and linked to the progres- sion of atherosclerotic process.
One of the pathological and immunohistochemical features of atherosclerotic lesions is accumulation of
inflammatory cells and cytokines [1]. Moreover, dexam- ethasone, an anti-inflammatory agent, is known to ex-
hibit an anti-atherogenic effect in animal models of experimental atherosclerosis, such as Watanabe herita-
ble hyperlipidemic WHHL rabbit [18], cholesterol-fed rabbit [19] and rat balloon angioplasty model [20].
Based on these studies, it seems reasonable to consider atherosclerosis as a chronic inflammatory disease, or
that the inflammatory process plays an important role in the development and progression of atherosclerosis.
Moreover, recent reports using human group-II phos- pholipase A
2
PLA
2
transgenic mouse demonstrated that group-II PLA
2
, one of the inflammatory media- tors, played an enhancing role in the development of
atherosclerotic lesions [21,22]. Thus, to further elucidate the pathophysiological significance of macrophage
growth in atherosclerosis, it seems reasonable to investi- gate the growth promoting effect of Ox-LDL on inflam-
matory macrophages, and the effect of group-II PLA
2
on the Ox-LDL-induced macrophage growth. Here, the growth stimulating effects of Ox-LDL on the thiogly-
collate-induced non-infectious
inflammatory mouse
peritoneal macrophages was examined. The results demonstrated that the responsiveness of thioglycollate-
elicited macrophages to the growth-stimulating activity of Ox-LDL was significantly greater than that of resi-
dent macrophages, and the expression of group-II PLA
2
might play, at least in part, an enhancing role in the
growth of
thioglycollate-elicited macrophage
through the enhancement of granulocytemacrophage colony-stimulating factor GM-CSF release.
2. Methods
