| COMMENTARIES |
At the Crossroads of Hyperlipidemia and Diabetes: Aldose Reductase and the Polyol Pathway in Atherosclerosis
Christian A. Gleissner, M.D., Division of Inflammation Biology, La Jolla Institute for Allergy and Immunology, 9420 Athena Circle, La Jolla, Ca 92037, U.S.A., and Department of Cardiology, University of Heidelberg, Im Neuenheimer Feld 410, 69120 Heidelberg, Germany, Email: christian@liai.org
Macrophages and Foam Cells in Atherogenesis
Today, it is broadly accepted that atherosclerosis is an inflammatory disease of the arterial wall [1]. Hyperlipidemia and diabetes mellitus represent two of the most important yet treatable risk factors for the development of atherosclerotic disease. They enhance recruitment of monocytes into the endothelial wall and promote formation of the atherosclerotic lesion by various mechanisms [2,3].
Macrophages represent the most abundant leukocyte subtype within the atherosclerotic plaque. They differentiate from blood monocytes that enter the arterial wall and are exposed to growth factors like macrophage colony-stimulating factor (M-CSF) [4] or the chemokine CXCL4 [5]. These monocyte-derived macrophages take up low density lipoprotein (LDL) trapped and modified within the subendothelial space and become foam cells [6]. By producing inflammatory cytokines and chemokines, these foam cells attract other cells to the atherosclerotic lesion thereby sustaining the inflammation process. They finally undergo apoptosis leading to formation of the necrotic core and promoting plaque instability [7].
Gene Expression during Foam Cell Formation
In order to develop strategies to prevent or treat the development of atherosclerotic disease it is crucial to understand the exact mechanisms of macrophage foam cell formation. To this end, we recently did an Affymetrix gene expression screen of peripheral blood monocytes which were differentiated to macrophages under the influence of M-CSF or CXCL4 for six days and then exposed to native or modified forms of LDL for 48 hours [8]. The expression of about 2,000 genes (out of 22,000 genes on the entire gene chip) was significantly regulated upon uptake of native of modified LDL. Among them were genes for chemokines, cytokines, their corresponding receptors, and also genes characteristic for dendritic cell differentiation. One gene significantly upregulated by oxidized LDL (oxLDL) was AKR1B1, the gene coding for the enzyme aldose reductase.
Aldose Reductase and the Polyol Pathway
Aldose reductase is the rate-limiting enzyme of the polyol pathway, which as a second enzyme includes sorbitol dehydrogenase. The physiological role of the polyol pathway is ambivalent. It is thought to detoxify reactive aldehydes [9] and therefore be beneficial; on the other hand, there are data which indicate that lack of AR leads to less damage, e.g. after brain ischemia [10]. Furthermore, it is known that with increasing glucose levels, AR can metabolize glucose leading to the formation of sorbitol, which is further metabolized to fructose by sorbitol dehydrogenase [3,11]. For several reasons, this is thought to be harmful: AR activation results in reduced levels of NAPDH, a cofactor necessary for regeneration of glutathione which protects the cell from oxidative stress. Fructose is about five times more likely than glucose to generate advanced glycation end products (AGE), a process which leads to dysfunction of many cellular processes and thereby impairs their function. Also, in some tissues like the ocular lens, formation of sorbitol increases oxidative stress.
Clinically, aldose reductase and the polyol pathway are linked to a number of microvascular diabetic complications including cataract, nephropathy, or neuropathy [11]. Furthermore, pharmacological inhibition or aldose reductase in patients with diabetic neuropathy has been demonstrated to significantly improve nerve conduction velocity and relieve symptoms. [12].
Relevance of the Polyol Pathway in Atherosclerosis
Early evidence for a role for the polyol pathway in atherosclerosis was presented in LDLR-/- mice, an atherosclerotic mouse model, which over-expressed human AR [13]. Normally, LDLR-/- mice only express very low levels of AR. As expected, fed a high fat diet, these AR overexpressing LDLR-/- mice developed atherosclerotic lesions to a similar extent as LDLR-/- that only expressed physiologically low AR levels. However, induction of diabetes mellitus by selectively killing the insulin-producing beta cells of the pancreas using streptocotozin (STZ), led to significantly increased atherosclerotic lesions in the AR overexpressing mice as compared to diabetic non-AR over-expressing LDLR-/- mice. This finding suggests that the harmful effects of the polyol pathway are related to hyperglycemia.
Prompted by the results of our initial foam cell gene expression screen, we investigated the relevance of AR upregulation for atherogenesis in macrophages exposed to oxLDL [14]. We found that AR gene expression and activity were upregulated by oxLDL and could be effectively inhibited by the pharmacological AR inhibitor epalrestat. oxLDL-induced AR upregulation could be attributed to 4-hydroxynonenal, a major component of oxLDL. Furthermore, AR upregulation was dependent on the scavenger receptor CD36, which was demonstrated by the fact that it was almost completely abrogated in the presence of blocking antibodies against CD36.
High glucose levels (30 mM) also resulted in AR upregulation in macrophages, a finding consistent with previously published data from other tissues. Interestingly, hyperglycemia and hyperlipidemia had synergistic effects on AR expression and activity suggesting that activation of the polyol pathway represents a link between those two risk factors for atherosclerosis. Under hyperglycemic conditions, inhibition of AR partly, but significantly, reduced oxLDL-induced oxidative stress in macrophages, an effect that was not seen in a normoglycemic environment.
The in vivo significance of our findings was supported by immunohistochemistry data from post mortem human atherosclerotic plaques. Thus, colocalization of AR with the macrophage marker CD68 could be demonstrated within the lesions, but also within the pericoronary fat tissue. Interestingly, not all CD68 positive macrophages also expressed aldose reductase supporting the concept of macrophage heterogeneity within the atherosclerotic lesion.
Implications and Potential Clinical Role
Our findings indicate that hyperglycemia and hyperlipidemia synergistically upregulate expression and activity of aldose reductase and the polyol pathway thereby increasing oxidative stress in macrophage-derived foam cells. The expression of aldose reductase in foam cells within human atherosclerotic lesions suggests that this mechanism integrates two important risk factors for atherosclerosis and contributes to atherogenesis. It not only underlines the importance of treating hyperlipidemia and diabetes in patients at risk, but also opens a potential new therapeutic target. In diabetic neuropathy, pharmacological inhibition of aldose reductase has proven to be an effective treatment [12]. Whether this therapy is an option to prevent or reduce atherosclerosis in diabetic patients remains to be investigated.
References