| COMMENTARIES |
Early Human Atherogenesis: Retention of Lipids in Intimal Thickenings Followed by Response of Macrophages
Yutaka Nakashima, M.D., Ph.D., Division of Pathology,
Fukuoka Red Cross Hospital, 3-1-1 Ogusu,
Minami-ku, Fukuoka 815-8555,
Japan
Tel: +81-92-521-1211
Fax: +81-92-533-9960
E-mail: y-nakashima@fukuoka-med.jrc.or.jp
Little is known as to how early human atherosclerosis develops. In the classic gross pathological study, Holman et al. showed that the fatty streak, a non-raised sudanophilic lesion, is the earliest lesion that appears in the aorta of children and adolescents and some fatty streaks convert into the advanced raised lesion in later life [1]. However, it is not clear how fatty streaks develop from normal arteries and covert into more advanced lesions. Based on microscopic findings, Virmani et al. defined the pathologic intimal thickening (PIT) as a preatheromatous lesion that is composed of extracellular lipid pools with an overlying layer of SMCs and lipid-laden macrophages [2]. PIT is thought to be an intermediate stage that represents the link from early to advanced lesions, but the nature of the early lesion and how the early lesion is converted into PIT are yet to be clarified. Williams and Tabas proposed the response-to-retention hypothesis in early atherogenesis in 1995, which states that atherogenic lipoproteins are retained in the intima by binding to extracellular proteoglycans [3]. This hypothesis further states that lipoprotein-proteoglycan complexes exhibit increased susceptibility to oxidation and leads to uptake by macrophages to form foam cells. Recent biochemical and molecular biological studies support this hypothesis. However, it is not clear whether this hypothesis is applicable to human atherogenesis, because of the lack of morphological evidence of early lesions. In this commentary, we have addressed these issues by showing our studies on human coronary arteries [4].
Normal Intima – Diffuse Intimal Thickening
In humans, normally thickened intima, which is also called adaptive intimal thickening, develops from an early age before atherosclerosis evolves [5]. There are two types of intimal thickenings i.e. eccentric intimal thickening and diffuse intimal thickening (DIT). Eccentric intimal thickening is localized around branches, and DIT is localized in the non-branching long segments of arteries and spread out circumferentially and longitudinally. However, these two types are contiguous and cannot always clearly distinguish, so that the intimal thickenings are represented by DIT in this article. So far, investigators have not been attracted so much to the presence and the role of DIT. One reason is that normal arterial intima is defined in the textbook as a very thin layer mainly composed of extracellular matrix and containing few cells. Another reason is that the fibrocellular thickening of the intima is generally considered to be a pathologic process, because in clinical situations, it is mostly encountered after PTCA and atherectomy. However, DIT is surely normal, because it is ubiquitously present in human coronary arteries and shows a well-organized structure composed of smooth muscle cells (SMCs), elastin, and proteoglycans. Collagens also present in DIT, but minimal in amount. Interestingly, DIT is developed from infants and children in atherosclerosis-prone arteries, such as coronary arteries, abdominal aorta, and iliac arteries, and gradually grows in thickness until young adults. Finally, the thickness of DIT becomes thicker than the media. DIT is not expressed in atherosclerosis-resistant arteries, such as mesenteric artery and ascending aorta. These findings suggest that DIT plays important roles in human atherogenesis.
Early Human Atherosclerosis
The initial event of human atherosclerosis is eccentric and extracellular deposition of LDL-containing lipids in the deep layer of DIT [4]. As the lipids continue to accumulate in the deep layer of the intima, macrophages are infiltrated from luminal side to reach the middle layer of the intima. These findings are compatible with those of the histologic counterpart of classic fatty streak that is defined as a nonraised sudanophilic lesion by gross inspection [1]. As the atherosclerotic lesion develops, the lipids continue to accumulate in both outer and inner layer of the intima, and macrophages are increased in number and infiltrated deeper to form PIT. Foam cell macrophages accumulate in and around the interface between infiltrating macrophages and extracellular lipids, suggesting that foam cells are formed by phagocytizing deposited lipids. Thus, PIT lesion shows three layers composed of non-foamy macrophages, foam cell macrophages, and extracellular lipids from top to bottom.
The lesion composed mainly of foam cells is defined as “fatty streak” by AHA [6]. This other type of “fatty streak” is rarely seen in humans, but typically found in experimental animals in the initial stage of atherosclerosis, showing accumulation of foam cell macrophages in the subendothelial space. No extracellular lipids are seen in histologic sections, but small lipid particles are detected in the subendothelial space prior to the macrophage infiltration in electron microscopy [7]. These findings suggest that the same molecular and cellular mechanisms occur both in experimental animals and humans in the earliest stage of atherosclerosis, but morphological expressions are considerably different. This difference between experimental animal models and humans is partly caused by differences in intimal thickness. In experimental animals, lipids deposit in the narrow subendothelial space that consists almost exclusively of ECM, so that macrophages can easily reach the lipids. However, in humans, the lipid deposits extracellularly in the deep layer of the thickened intima of DIT which contains abundant ECM and SMCs. Macrophages should infiltrate deep into the intima to reach the deposited lipids.
Mechanism of Lipid Deposition and Macrophage Infiltration
There is a lot of biochemical and molecular evidence that extracellular matrix (ECM) proteoglycans, such as biglycan and decorin, play a role in lipid deposition in the intima. However, only limited morphological evidence is available. Our data show that biglycan is concentrically expressed in the outer layer of DIT with no lipid deposits [4]. This prelesional distribution of biglycan closely ressembles that of lipid deposits in fatty streaks, which suggests that lipids bind to biglycan and deposit in the intima in the initial stage of atherosclerosis. Actually, in fatty streaks and advanced lesions with greater amount of lipid deposits, colocalizations of biglycan and lipids are demonstrated in some reports [4,8]. It is of interest that lipids deposit eccentrically, whereas biglycan is localized concentrically. It is believed that structural changes in the glycosaminoglycan (GAG) chains on proteoglycan are the initial proatherogenic step that leads to increase biding properties of proteoglycans for atherogenic lipoproteins. For example, proteoglycans produced by TGF-β1-treated SMCs show longer GAG chains and greater binding affinity to LDL than control SMCs [9]. Interestingly, TGF-β1 shows patchy, although not eccentric, distribution in the intima [10].
According to in vitro studies, chemotactic movements of macrophages are induced via modification of low-density lipoprotein (LDL) and stimulation of MCP-1 production by cells in the atherosclerotic lesion [11]. These mechanisms are expected to occur in vivo as well, because the presence of oxidized LDL (ox-LDL) and MCP-1 is revealed in human atherosclerotic lesions [4,12]. Our data show that ox-LDL is largely present intracellularly in macrophages and SMCs. However, extracellular ox-LDL is also recognized when relatively large amount of lipids were deposited in the intima. MCP-1 is present in most of the infiltrating macrophages and some SMCs.
Summary
The histological findings of human coronary arteries supports that the response-to-retention hypothesis is involved in the early phases of human atherosclerosis, and shows the importance of DIT as a reservoir for lipid retention. Only coronary arteries have been investigated so far, the same mechanisms are expected to occur in other atherosclerosis-prone arteries, such as abdominal aorta and carotid arteries, because well-developed DIT is present in their intima as well [5].
References