Diabetes Volume 64, December 2015
3981
Terri J. Allen,1,2 Andrew J. Murphy,3,4 and Karin A. Jandeleit-Dahm1,5
RAGE Against the ABCs
Diabetes 2015;64:3981–3983 | DOI: 10.2337/dbi15-0015
contributes to impaired atherosclerotic lesion regression.
Subsequently, a decrease in the expression of Abca1/g1 in
the hematopoietic progenitors of diabetic mice was also
noted, and inhibition of microRNA (miR)-33 restored the
expression of Abca1/g1 (3). This was accompanied by a
reduction in blood monocytes that promoted lesion regression (3). However, the mechanism(s) contributing to
the downregulation of these key cholesterol efflux genes
in the setting of diabetes was unknown.
In this issue, Daffu et al. (12) report on an important breakthrough using a comprehensive set of studies to link the
RAGE signaling axis with the suppression of macrophage
ABCA1/G1 in the settings of diabetes (Fig. 1). This resulted in a significant impairment of macrophages to promote cholesterol efflux to HDL and apolipoprotein A1
(apoA1). A plethora of studies have documented the atherogenic role of RAGE, particularly in the setting of diabetes (13). More recently, the importance of the RAGE
pathway has been confirmed in human atherosclerotic
disease and has been linked to plaque vulnerability (14).
RAGE is classed as a pattern recognition receptor having
multiple endogenous ligands (13). It is expressed in a variety
of cells, and deletion in either hematopoietic or nonhematopoietic cells affords protection against diabetes-accelerated
atherosclerosis (15).
In vivo macrophage reverse cholesterol transport
(RCT) studies reveal diabetes significantly impairs this
process, which was restored when RAGE was deleted (12).
Supporting the downregulation of ABCA1/G1, lower HDL
levels are observed in diabetic mice and the deletion of
RAGE returns HDL levels to normal. The finding of reduced HDL levels in diabetes and restoration with RAGE
deletion is not a universal observation as studies by SoroPaavonen et al. (16) did not observe such an effect in
diabetic Apoe2/2 mice. Further, HDL can undergo modifications in diabetes resulting in impaired function (17),
suggesting a major failure of this important pathway at
1Biochemistry of Diabetic Complications Laboratory, Baker IDI Heart and Diabetes
Institute, Melbourne, Australia
2Department of Epidemiology and Preventive Medicine, Monash University, Melbourne, Australia
3Hematopoiesis and Leukocyte Biology Laboratory, Baker IDI Heart and Diabetes
Institute, Melbourne, Australia
4Department of Immunology, Monash University, Melbourne, Australia
5Department of Medicine, Monash University, Melbourne, Australia
Corresponding author: Terri J. Allen, [email protected].
© 2015 by the American Diabetes Association. Readers may use this article as
long as the work is properly cited, the use is educational and not for profit, and
the work is not altered.
See accompanying article, p. 4046.
COMMENTARY
Diabetes is an insidious disease that significantly contributes to the development of atherosclerotic cardiovascular
disease (CVD). One of the major factors contributing to
atherosclerosis is the accumulation of lipid-laden macrophages in the vascular wall (1). Removal of cholesterol from
macrophages is essential for the regression of atherosclerotic
lesions. This can be achieved by cholesterol-lowering drugs
(i.e., statins) or by enhancing the reverse cholesterol transport pathway (i.e., raising HDL). Unfortunately, in the setting
of diabetes, cholesterol-lowering drugs such as statins can be
less effective (2), and preclinical models have revealed a major defect in lesion regression even when cholesterol levels
are normalized (3–5). This suggests that a major defect in
cholesterol metabolism occurs in the setting of diabetes.
The Hedrick and Oram laboratories originally discovered that diabetes causes a downregulation of the ATP
binding cassette transporters ABCG1 (6) and ABCA1 (7)
in macrophages, promoting foam cell formation. ABCA1
and ABCG1 provide the dominant pathway of cholesterol
removal from a number of cells in the body. Yvan-Charvet
et al. (8) made the seminal discovery that codeletion of
ABCA1/G1 results in a dramatic increase in atherosclerosis. Extensive research on the antiatherogenic role of
these transporters has revealed their importance in removing cholesterol from plaque macrophages (9). However, recent discoveries have shown that ABCA1 and
ABCG1 regulate the proliferation of hematopoietic stem
and progenitor cells to control the abundance of blood
monocytes (10). Along with promoting cholesterol efflux
from macrophages, suppressing monocyte levels can directly inhibit the progression of atherosclerotic lesions,
promote lesion regression, and provide beneficial outcomes after a myocardial infarction (11). Nagareddy
et al. (4) found that diabetes induces the production of
monocytes via neutrophil-derived S100A8/A9 binding
to the receptor for advanced glycation end products
(RAGE) on hematopoietic progenitors, which ultimately
3982
Commentary
Figure 1—The link between diabetes and impaired RCT. Red arrows indicate pathways identified by Daffu et al. (12). In the setting
of diabetes, RAGE ligands (i.e., AGE, S100A8/A9) become more
abundant and there is enhanced RAGE expression on macrophages. A signaling event occurs downstream of RAGE to stimulate
the MAPK pathway. Through an undefined mechanism, PPARg is
suppressed and inhibited from binding to PPAR response element
on the genes encoding ABCA1/G1, with subsequent downregulation of these transporters. Additionally, apoA1 and HDL can be
modified by AGEs, resulting in reduced RCT. Green arrows indicate
therapeutically targetable pathways to promote ABCA1/G1 expression, including RAGE/ligand inhibition, LXR agonists, anti–miR-33,
and, to a lesser extent, PPARg agonists (dashed green line). RXR,
retinoid X receptor.
both the plasma and cellular levels (Fig. 1). While the
experiments by Daffu et al. (12) reveal that RAGE plays
a role in suppressing ABCA1/G1 expression under normoglycemic conditions, nondiabetic Rage2/2 mice display
normal cholesterol levels, suggesting that this is perhaps
specific to macrophages, which do not significantly contribute to the circulating pool of HDL (18).
Delving deeper into the mechanism using luciferase
reporter assays, carboxymethyllysine-advanced glycation
end product (CML-AGE) was found to suppress the transcription of ABCG1 through its peroxisome proliferator–
activated receptor (PPAR)g–responsive promoter element,
at least in part via the mitogen-activated protein kinase
(MAPK) pathway (Fig. 1). CML-AGE is only one of many
RAGE ligands shown to be upregulated in the diabetes
setting such as damage-associated molecular pattern
molecules (4,5,16), which are not investigated by Daffu
et al. However, it is possible that this is a universal mechanism downstream of RAGE as S100B has been shown
to downregulate Abca1/g1 (19). Interestingly, RAGE signaling does not prevent ABCA1/G1 expression by suppressing the induction of these genes via liver X
receptors (LXRs). This could be significant as Mauldin
et al. (6) have previously shown that LXR agonists can
induce the expression of ABCA1/G1 in the setting of diabetes to reduce foam cell formation, and LXRs can also
oppose RAGE-S100B–induced suppression of Abca1/g1
(19), suggesting a utility of macrophage-selective LXR agonists to restore RCT in diabetes (Fig. 1). The relevance of
Diabetes Volume 64, December 2015
these findings to atherosclerosis was explored using
diabetic Ldlr2/2 mice compared with Ldlr2/2Rage2/2 mice.
Laser-capture microdissection of CD68 + macrophages
from atherosclerotic plaques of Ldlr2/2Rage2/2 mice displayed higher levels of Abca1, Abcg1, and Pparg mRNA
transcripts versus RAGE-expressing Ldlr2/2 mice independent of glycemic and lipid control (12). However, a recent
comparison revealed only Abca1 transcripts were suppressed
in plaque macrophages from diabetic mice (3). Moreover, the
total body knockouts and the reduction of atherosclerosis
observed here are likely not only due to changes in macrophage ABCA1/G1 expression, as there are certainly important
nonmacrophage atherogenic roles for RAGE (15). It would
have also been of interest to conduct these studies with inducers of Abca1/g1, such as LXR or PPARg agonists.
Another point of interest would be to determine the
type of macrophages within the atherosclerotic lesion in
the current study, as previous reports have shown a major
skewing toward the M1-inflammatory phenotype in the
setting of diabetes (3–5). Treating diabetic mice with the
ABC-inducing anti–miR-33 (20) does suggest that restoring the expression of these genes equates to less M1- and
more M2-like macrophages (Fig. 1) (3). Interestingly, restoration of ABCA1/G1 has no effect on the ability of
macrophages to be retained or egress from the lesion
and points to a major role in the influx of monocytes to
the atherosclerotic plaque that should be considered when
developing targets in the setting of diabetes (3). Nonetheless, Daffu et al. (12) rightly conclude that the RAGE
signaling axis may fill an important therapeutic gap in
the treatment of diabetic macrovascular complications,
particularly by this previously unexplored mechanism
linking cholesterol metabolism.
Funding. K.A.J.-D. and A.J.M. are supported by a fellowship from the
Australian National Health and Medical Research Council. A.J.M. is also supported
by a Future Fellowship (100440) from the National Heart Foundation of Australia.
Duality of Interest. No potential conflicts of interest relevant to this article
were reported.
References
1. Randolph GJ. Mechanisms that regulate macrophage burden in atherosclerosis. Circ Res 2014;114:1757–1771
2. Hiro T, Kimura T, Morimoto T, et al; JAPAN-ACS Investigators. Diabetes
mellitus is a major negative determinant of coronary plaque regression during
statin therapy in patients with acute coronary syndrome–serial intravascular
ultrasound observations from the Japan Assessment of Pitavastatin and Atorvastatin in Acute Coronary Syndrome Trial (the JAPAN-ACS Trial). Circ J 2010;74:
1165–1174
3. Distel E, Barrett TJ, Chung K, et al. MiR33 inhibition overcomes deleterious
effects of diabetes mellitus on atherosclerosis plaque regression in mice. Circ
Res 2014;115:759–769
4. Nagareddy PR, Murphy AJ, Stirzaker RA, et al. Hyperglycemia promotes
myelopoiesis and impairs the resolution of atherosclerosis. Cell Metab 2013;17:
695–708
5. Parathath S, Grauer L, Huang LS, et al. Diabetes adversely affects macrophages during atherosclerotic plaque regression in mice. Diabetes 2011;60:
1759–1769
diabetes.diabetesjournals.org
6. Mauldin JP, Srinivasan S, Mulya A, et al. Reduction in ABCG1 in type 2
diabetic mice increases macrophage foam cell formation. J Biol Chem 2006;281:
21216–21224
7. Tang C, Kanter JE, Bornfeldt KE, Leboeuf RC, Oram JF. Diabetes reduces
the cholesterol exporter ABCA1 in mouse macrophages and kidneys. J Lipid Res
2010;51:1719–1728
8. Yvan-Charvet L, Ranalletta M, Wang N, et al. Combined deficiency of ABCA1
and ABCG1 promotes foam cell accumulation and accelerates atherosclerosis in
mice. J Clin Invest 2007;117:3900–3908
9. Murphy AJ, Westerterp M, Yvan-Charvet L, Tall AR. Anti-atherogenic
mechanisms of high density lipoprotein: effects on myeloid cells. Biochim Biophys Acta 2012;1821:513–521
10. Yvan-Charvet L, Pagler T, Gautier EL, et al. ATP-binding cassette transporters and HDL suppress hematopoietic stem cell proliferation. Science 2010;
328:1689–1693
11. Swirski FK, Nahrendorf M. Leukocyte behavior in atherosclerosis, myocardial infarction, and heart failure. Science 2013;339:161–166
12. Daffu G, Shen X, Senatus L, et al. RAGE suppresses ABCG1-mediated
macrophage cholesterol efflux in diabetes. Diabetes 2015;64:4046–4060
13. Barlovic DP, Soro-Paavonen A, Jandeleit-Dahm KA. RAGE biology, atherosclerosis and diabetes. Clin Sci (Lond) 2011;121:43–55
Allen, Murphy, and Jandeleit-Dahm
3983
14. Hanssen NM, Wouters K, Huijberts MS, et al. Higher levels of advanced
glycation endproducts in human carotid atherosclerotic plaques are associated
with a rupture-prone phenotype. Eur Heart J 2014;35:1137–1146
15. Koulis C, Kanellakis P, Pickering RJ, et al. Role of bone-marrow- and nonbone-marrow-derived receptor for advanced glycation end-products (RAGE) in a
mouse model of diabetes-associated atherosclerosis. Clin Sci (Lond) 2014;127:
485–497
16. Soro-Paavonen A, Watson AMD, Li J, et al. Receptor for advanced glycation
end products (RAGE) deficiency attenuates the development of atherosclerosis in
diabetes. Diabetes 2008;57:2461–2469
17. Hoang A, Murphy AJ, Coughlan MT, et al. Advanced glycation of apolipoprotein A-I impairs its anti-atherogenic properties. Diabetologia 2007;50:
1770–1779
18. Westerterp M, Murphy AJ, Wang M, et al. Deficiency of ATP-binding cassette transporters A1 and G1 in macrophages increases inflammation and accelerates atherosclerosis in mice. Circ Res 2013;112:1456–1465
19. Kumar P, Raghavan S, Shanmugam G, Shanmugam N. Ligation of RAGE
with ligand S100B attenuates ABCA1 expression in monocytes. Metabolism
2013;62:1149–1158
20. Rayner KJ, Suárez Y, Dávalos A, et al. MiR-33 contributes to the regulation
of cholesterol homeostasis. Science 2010;328:1570–1573