PLoS One. 2016 Mar 24;11(3):e0150862. doi: 10.1371/journal.pone.0150862.
Pericoronary Adipose Tissue as Storage and Supply Site for Oxidized Low-Density Lipoprotein in Human Coronary Plaques.
Yasumi Uchida1, 2, Yasuto Uchida 3, Ei Shimoyama4,Nobuyuki Hiruta5, Toshihiko Kishimoto6, Soichiro Watanabe7.
- Japanese Foundation for Cardiovascular Research, Funabashi, Japan. 274-0063. E-mail: firstname.lastname@example.org Tel: +81-47-462-2159
- Department of Cardiology, Tokyo Jikei University School of Medicine, Tokyo. Japan.
- Department of Cardiology, Tsukuba Memorial Hospital, Tsukuba, Japan..305-0032. E-mail:email@example.com; Tel: +81-29-864-1212.
- Department of Pathology, Funabashi-Futawa Hospital, Funabashi, Japan. 274-0078. E-mail: firstname.lastname@example.org; Tel: +81-47-443-7111.
- Department of Pathology, Toho University Sakura Medical Center, Sakura, Japan.285-8741, E-mail: email@example.com; Tel: +81-43-462-8811.
- Department of Biomolecular Science, Faculty of Science, Toho University. Okubo, Funabashi, Japan 274-8501. E-mail: firstname.lastname@example.org Tel: +81-47-472-1836.
- Department of Biomolecular Science, Faculty of Science, Toho University. Okubo, Funabashi, Japan 274-8501. E-mail: email@example.com. Tel: +81-47-472-1836.
Address for Correspondence:
Yasumi Uchida, M.D.
Japan Foundation for Cardiovascular Research,
2-30-17, Narashinodai, Funabashi, Japan, 274-0063.
Tel: +81-47-462-2159. E-mail: firstname.lastname@example.org
Backgrounds: Oxidized low-density lipoprotein (oxLDL) plays a key role in atherosclerosis. It is generally believed that low-density lipoprotein enters the vascular intima from its lumen and becomes oxLDL.
Methods and Results: Localization of oxLDL was examined by immunohistochemical techniques in excised human coronary arteries and their surrounding pericoronay adipose tissues (PCAT). OxLDL deposited in adipocytes in all PCAT and in 35 to 69% of 25 normal segments and 44 plaques. OxLDL deposited in either dotted or diffuse pattern. The former occurred when the oxLDL was contained in CD68(+)-macrophages that were observed traversing from PCAT, adventitia and media to intima. The localization of diffusely deposited oxLDL coincided with that of intimal vasa vasorum.
Conclusions: Contrary to the generally believed mechanism, oxLDL is stored in PCAT and conveyed either by macrophages or vasa vasorum to the coronary intima. Therefore, therapies targeting the PCAT could be effective in preventing human coronary atherosclerosis.
General belief on atherosclerosis
It is generally believed that (1) low-density lipoprotein (LDL) enters the vascular wall from the lumen and is oxidized (oxLDL), after which it plays a key role in the initiation, progression and destabilization of atherosclerotic plaques, and monocytes migrate into the vascular wall from the lumen, and become macrophages, that in turn accumulate oxLDL and become foam cells, while producing collagen-degrading enzymes such as metalloproteases and collagenases which destroy collagen fibers, and also hypochlorous acid (OHCl) which damages endothelial cells, resulting in vulnerable plaques. Based on this general belief and using the plasma levels of lipoproteins and apolipoproteins as a marker, lipid-lowering therapies are performed in clinical situation, however with limited preventive effects on ischemic cardiovascular events.
Figure 1. OxLDL and HDL in PCAT. OxLDL and HDL co-localize in the cytoplasm (arrows in A – A-2) and plasma membrane of the same adipocytes (arrowheads in A – A-2). Scale bars= 5 µm. Also, CD68(+)-macrophages that contain oxLDL are observed in the interstitial space between the adipocytes (AC; arrows in B- B-2). Scale bars= 20 µm. HDL: high-density lipoprotein. OxLDL: oxidized low-density lipoprotein. PCAT: pericoronary adipose tissue.
Figure 2. CD68(+)-macrophages that contain oxLDL in the media and intima. OxLDL-containing CD68(+)-macrophages were observed in the media (arrows in A – A-2) and intima (B-B-2). Scale bars= 20 µm. Arrowheads: in B- B-2: foam cells.
We considered that there are other hitherto unrecognized storage and supply site(s) for pro-atherogenic substances and they are not reflected on their plasma levels.
OxLDL in human pericoronary adipose tissue (PCAT) and its supply routes to coronary plaques
Using immunohistochemical techniques, we demonstrated that, contrary to these generally held beliefs, oxLDL is stored in epicardial pericoronary adipose tissues (PCAT) and is conveyed either by CD68(+)- macrophages or the vasa vasorum (penetrating microvessels) to the adjacent epicardial coronary intima, which is the site of atherosclerosis (Figs. 1B-B-2, 2). The findings that vasa vasorum arising from the adventitial side and extending into the intima(Fig. 3A) and macrophages traversing in series from the adventitial side into the intima (Fig. 3A-1) support this possibility.
In addition, we demonstrated that the incidence of oxLDL deposits in the intima of normal coronary segments is low, increases in growth stage of coronary plaques, but decreases with plaque maturation. Therefore, we suggested that molecular therapy targeting oxLDL should be started before plaque maturation1.
Figure 3. Vasa vasorum and CD68(+)-macrophages in the intima. Vasa variorum arising from the adventitial side extended into the intima (arrows in A) and CD(+)-macrophages traversing in series from the adventitia side to the intima (arrows in B) are observed.
Figure 4. Co-localization of OxLDL and HDL with vasa vasorum. Three adjacent sections were stained in the order of oxLDL, HDL and vasa vasorum (CD31). When vasa vasorum was localized to the outer half of the intima, i.e., medial side (arrow in A-2), oxLDL and HDL deposited in the same portion (arrows in A, A-1). When vasa vasorum was distributed to both inner and outer halves of the intima (arrow in B-2), both oxLDL and HDL deposited in both halves of the intima (arrows in B, B-1). Scale bars =100µ m.
High-density lipoprotein (HDL) co-deposits with oxLDL in PCAT
It is also generally believed that HDL enters from the vascular lumen into the vascular wall. Based on this belief, therapies to elevate the plasma levels of HDL are attempted using fibrates or HDL –mimetic or apolipoprotein-mimetic substances, but their preventive effects on ischemic coronary events are currently inconsistent in clinical setting.
Using immunohistochemical techniques, we found that HDL and oxLDL co-deposit in human PCAT, irrespective of the presence or absence of atherosclerotic plaques in the adjacent coronary artery or underlying disease (Fig. 1A-A-2)2. It seems that HDL in the PCAT is supplied by vasa vasorum but not by macrophages (Fig. 4).
Different to oxLDL, the incidence of HDL in the coronary intima increased with plaque growth and increased further with plaque maturation.
Especially in the plaques with necrotic core, the necrotic core was filled with HDL in all preparations. This finding suggested that administration of HDL- or apolipoprotein A1-mimetics should be started before plaque growth because matured plaques are filled with native HDL and they have no space to accept the externally administrated HDL or apolipoprotein A1-mimetics.
Figures 5 and 6 represents the generally believed mechanisms of ahteorslcerosis3,4 and our hypothesis on the storage and supply routes of oxLDL and HDL, respectively.
Figure 5. General belief on the supply routes of oxLDL and HDL, and the mechanisms of atherosclerosis. Low-density lipoprotein (LDL) enters the vascular wall from the lumen and is oxidized (oxLDL), after which it plays a key role in the initiation, progression and destabilization of atherosclerotic plaques, and monocytes migrate into the vascular wall from the lumen, and become macrophages, that in turn accumulate oxLDL and become foam cells, while producing collagen-degrading enzymes such as metalloproteases and collagenases which destroy collagen fibers, and also hypochlorous acid (OHCl) which damages endothelial cells, resulting in vulnerable plaques.
Figure 6. Possible mechanisms of the storage and supply site for oxLDL and HDL in human coronary intima (plaques). Schematic representation of a hitherto unrecognized supply route of oxLDL and HDL to the coronary intima. OxLDL and HDL are stored in PCAT. The former is conveyed by either CD68(+)-macrophages or neovascularized vasa vasorum and the latter by vasa variorum to the intima. EC: endothelial cells. MMPs: matrix metalloproteases.
- Uchida Y, Uchida Y, Shimoyama E, Hiruta N, Kishimoto T, Watanabe S. Pericoronary adipose tissue as storage and supply Site for oxidized low-density lipoprotein in human coronary plaques. PLoS One. 2016 Mar 24;11(3):e0150862
- Uchida Y, Uchida Y, Shimoyama E, Hiruta N, Kishimoto T, Watanabe S. Human pericoronary adipose tissue as storage and possible supply site for oxidized low-density lipoprotein and high-density lipoprotein in coronary artery. J Cardiol. 2016 May 18. pii: S0914-5087(16)30042-9. doi: 10.1016/j.jjcc.2016.03.015.
- Libby P (2015) The vascular biology of atherosclerosis. In: Mann DL, Zipes DP, Libby P, Bonow RO, Braunwald E (editors) Heart disease 10th Edition. Elsevier Saunders Ltd, Philadelphia 873–890.
- Yoshida H, Kisugi R (2011) Mechanisms of LDL oxidation. Chimica Acta 411: 1875–1882.