HMG-CoA reductase inhibitors suppress macrophage growth induced by oxidized low density lipoprotein

Abstract

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors ameliorate atherosclerotic diseases in several models of vascular disease. This is largely due to their ability to reduce plasma cholesterol levels in vivo. Proliferation of cellular components is one of the major events in the development and progression of atherosclerotic lesions. We recently demonstrated that oxidized low density lipoprotein (Ox-LDL), a likely atherogenic lipoprotein present in vivo, is capable of inducing macrophage growth in vitro. In the present study, we investigated the effect of HMG-CoA reductase inhibitors, simvastatin and pravastatin, on Ox-LDL-induced macrophage growth. Our results demonstrated that these inhibitors effectively suppressed Ox-LDL-induced macrophage growth with concentrations required for 50% inhibition by simvastatin and pravastatin being 0.1 and 80 μM, respectively, and that this inhibitory effect was reversed by mevalonate but not by squalene. Under these conditions, simvastatin did not affect the endocytic degradation of Ox-LDL, nor subsequent accumulation of intracellular cholesteryl esters. Our results suggest that a non-cholesterol metabolite(s) of mevalonate pathway may play an important role in Ox-LDL-induced macrophage growth. Since it is well known that macrophage-derived foam cells are the key cellular element in the early stage of atherosclerosis, a significant inhibition of Ox-LDL-induced macrophage growth by HMG-CoA reductase inhibitors in vitro, particularly simvastatin, may also explain, at least in part, their anti-atherogenic action in vivo.

Introduction

Proliferation of cellular components, particularly foam cells in atherosclerotic lesions, is a major event in the development of atherosclerosis 1, 2. Recent immunohistochemical and pathological studies have established that foam cells observed in the advanced stage of atherosclerotic lesions are derived from smooth muscle cell (SMC), whereas those observed in the early phase of atherosclerotic lesions are derived from blood monocytes/macrophages 3, 4, 5. Macrophages are known to endocytose oxidized low density lipoprotein (Ox-LDL) through the scavenger receptor pathway and become foam cells in vitro [6]. These cells in turn produce various bioactive molecules, such as growth factors and cytokines, which play an important role in the progression of the early stage of atherosclerosis [1]. In this regard, we have recently demonstrated that Ox-LDL induces mouse 7, 8, rat [9]and human [10]macrophage growth in vitro. Since Ox-LDL is present in atherosclerotic lesions and is expected to interact with the macrophage scavenger receptor, it is possible that Ox-LDL may serve as a growth factor for macrophages in vivo, thereby enhancing the progression of atherosclerosis.

Recent reports suggested that metabolites of the mevalonate pathway play a crucial role in cell proliferation 11, 12, 13, 14, 15. Mevalonate is synthesized intracellulary from 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) by the action of HMG-CoA reductase [16]. A series of isoprenoid compounds, such as isopentenyl pyrophosphate, geranyl pyrophosphate and farnesyl pyrophosphate, are generated during mevalonate metabolism until squalene formation which is finally converted to cholesterol. Isopentenyladenine, farnesylated proteins and other end-products derived from these isoprenoid compounds are believed to be essential for cell growth [16]. Thus, it is thought that inhibition of HMG-CoA reductase by simvastatin or pravastatin leads to a total loss or significant reduction of mevalonate, subsequently inhibiting DNA synthesis as well as cell proliferation in several types of cells, such as SMC [11], mesangial cells [12], lymphocytes [13], Swiss 3T3 cells [14]and human glioma cells [15].

In the present study, we investigated the effect of HMG-CoA reductase inhibitors, simvastatin and pravastatin, on the Ox-LDL-induced macrophage growth. The results demonstrated that these inhibitors suppressed Ox-LDL-induced macrophage growth. Moreover, this inhibition was effectively prevented by mevalonate but not by squalene. Since these inhibitors had no effect on the endocytic uptake of Ox-LDL by macrophages nor on subsequent intracellular accumulation of cholesteryl esters in foam cells, it is likely that mevalonate itself or its related non-cholesterol metabolite(s) may play an essential role in the Ox-LDL-induced macrophage growth. The capacity of these inhibitors, particularly simvastatin, to inhibit Ox-LDL-induced macrophage growth in vitro may, at least in part, explain their in vivo anti-atherogenic effect 17, 18, 19, 20, 21.